**Nanostructured TiO2 Layers for Photovoltaic and Gas Sensing Applications**

André Decroly, Arnaud Krumpmann, Marc Debliquy and Driss Lahem

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/62316

#### **Abstract**

Titanium dioxide (TiO2) has been an important material for decades, combining numerous attractive properties in terms of economy (low price, large availability) or ecology (nontoxic), as well as broad physical and chemical possibilities. In the last few years, the development of nanotechnologies offered new opportunities, not only in an academic perspective but also with a view to many applications with particular reference to the environment. This chapter focuses on the many ways that allow to tailor and organize TiO2 crystallites at the nanometre scale to make the most of this amazing material in the field of photovoltaics and gas sensing.

**Keywords:** titanium dioxide, solar cell, gas sensing, nanotube, anodization

#### **1. Introduction**

Titanium dioxide (TiO2), also known as titania, has attracted a great deal of interest over the past decades because of its ability to create electron-hole pairs when absorbing light in an ade‐ quate wavelength range, taking into account that the valence and conduction bands are separated by a *ca*. 3.2 eV gap. Such a charge separation capacity makes TiO2 a very convenient material for photovoltaic (see Section **2**) and photocatalytic applications. Basically, once an electron is excited by absorption of a photon of sufficient energy, either of the following phenomena can occur: the electron-hole pair dissociates and both charge carriers migrate toward opposite electrodes

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Figure 1.** Schematization of either photocatalytic or photovoltaic effect following photon absorption by TiO2.

generating a voltage that constitutes the photovoltaic effect or both charge carriers act like very active reagents in many chemical reactions what constitutes a photocatalytic effect (**Figure 1**).

Another interesting characteristic of crystalline TiO2 is the variation of its semiconductivity because of charge transfer induced by the chemisorption of gaseous molecules onto the solid surface, making this material also very interesting for gas sensing applications (see Section **3**).

The photocatalysis field has already a very rich literature, hence it is not discussed in this chapter, despite its extreme importance, to focus on photovoltaic and gas sensing applications.

TiO2 may crystallize in different allotropic structures, mainly anatase, rutile and brookite. Anatase is generally considered to have the best properties for the above-mentioned applica‐ tions. It is metastable (or "kinetically stable") up to 550°C. Above this temperature, it trans‐ forms into the equilibrium rutile allotrope though anatase nanocrystallites (about 10 nm in size) appear to be metastable over a wider temperature range.

Of course, the manufacturing and optimization of a TiO2 layer will be conducted in different ways depending on the intended application, but there is an important parameter that must be maximized in any case, which is the specific surface area. Therefore, such layers will always consist of nanocrystallites or nanostructures.

From a practical point of view, TiO2 is also cheap, thermally and chemically stable and nontoxic (food additive E171, stated Kosher and Halal). However, in 2006, the *International Agency for Research on Cancer* (IARC) classified the titanium dioxide as possibly carcinogenic for humans.

## **2. TiO2 for dye sensitized solar cells**

#### **2.1. Generalities and operating principle of a DSSC**

In 1991, Professor Michael Grätzel from the *École Polytechnique Fédérale de Lausanne* (EPFL) published an article [1] describing the functioning of a new type of solar cell (called of the third generation) based on the use of a TiO2 layer sensitized with a dye (dye-sensitized solar cell [DSSC]). This cell differs from the massive (single crystal or polycrystalline) silicon cells based on a *p-n* junction that allows an easy dissociation of the electron-hole pairs generated following photons absorption by the silicon itself (first-generation cells) or from other solar cells also based on a *p-n* junction but using thin layers of various semiconductor materials such as CdS-CdTe or CdS-CuInSe2 (second-generation cells) [2].

Probably to increase their eco-attractiveness, DSSCs are now often presented as biomimetic devices inspired by photosynthesis though the comparison seems rather misleading if one looks closely. Indeed, when chlorophyll absorbs a photon, an electron-hole pair is created, which established the energy quantum needed for the photosynthetic reaction to occur. This electron-hole pair has to diffuse randomly over a long distance (and therefore with a high recombination probability) along the chlorophyll molecule antenna until it reaches the hemelike porphyrin group, which is the chemical reaction locus. Of course, neither voltage nor electrical current is produced. The role of the dye in a DSSC is completely different and is discussed thoroughly below.

Like in any solar cell, the photogenerated electron-hole pairs must readily dissociate. This is the reason why each type of solar cell is made up of two media of different conduction modes, which prevents the charge carriers once separated to recombine (too easily) so that a voltage can be generated. The TiO2 layer is an electron conductor, and the electrolytic solu‐ tion acts as an ionic conductor (**Figure 2**). An electric current is generated when the dye mol‐ ecules anchored onto the surface of the TiO2 nanoparticles absorb the incoming photons and inject the photoexcited electrons into the TiO2 layer.

**Figure 2.** Functional scheme of a DSSC (thermodynamic point of view).

The function of the ionic redox couple (traditionally I3 − /I−−) in solution is to reduce the oxidized dye molecules back to their ground state to enable continuous electron production, a process in which the reduced form of the redox couple (I<sup>−</sup> ) gets oxidized itself. The electrons are collected from the TiO2 (photoanode) and transferred through an external circuit to the counter-electrode (cathode) where they complete the operating cycle of the cell by reducing the oxidized redox species (I3 − ) back to its original state.

Kinetic aspects are of utmost importance: electron injection from the dye into the TiO2 conduction band must be much faster than the dye relaxation, electron transport into the TiO2 must be faster than electron trapping or recombination and the reduction of the dye by the reduced ion in solution must not be a rate-limiting step (**Figure 3**).

**Figure 3.** Functional scheme of a DSSC (kinetic point of view).

Nevertheless, since TiO2 has a wide band gap (3.2 eV), only the most energetic photons (λ < 400 nm, i.e., about 5 % of the total amount of photons) are able to induce a charge separa‐ tion, thereby leading to a very poor conversion efficiency if TiO2 was used alone. The dye, thus, allows to harvest photons on a wider wavelength range (up to 920 nm, for instance) [3] so as to raise the photocurrent, and therefore the efficiency of the cell. So, unlike the cells of first and second generations, the vast majority of electron-hole pairs are not generated by the material (here TiO2) itself, but following the dye excitation. As outlined above, a very large specific surface area is therefore needed to increase the amount of dye molecules chemically grafted onto the TiO2 surface.

Each component of the cell has undergone many developments since 1991 [4]:

**•** The electrodes: both electrodes must obviously be conducting but at least one must be transparent, and it is best to choose the TiO2 photoanode to be exposed directly to the light so as to avoid great absorption of photons by the counter electrode (cathode) and the electrolytic solution. So the TiO2 layer should always be deposited onto a transparent conducting oxide (TCO) like an indium-tin oxide (ITO) coating onto a glass substrate. Nowadays, the classical ITO is progressively replaced by FTO (fluorine-doped tin oxide), which presents a lower sheet resistance, a better thermal resistance and avoids the use of indium whose long-term availability is uncertain. The ultimate goal would be to use flexible electrodes (conductive polymer films), but their low long-term stability, especially when exposed to UV-light, is a drawback;


Nevertheless, the rest of this section focuses on the TiO2 layer. The first prototypes were using TiO2 nanoparticles (10–20 nm) in liquid suspensions and deposited by some simple processes such as the "doctor blade" technique. Sometime later, the dip-coating and the sol-gel techni‐ ques starting from a titanium alkoxide aqueous solution were combined to produce wellcontrolled TiO2 layers made up of anatase nanocrystallites after thermal treatment. Nevertheless, these deposits were completely disorganized and of rather poor conductivity because of high contact resistance at the grain boundaries. This electrical resistance favored the electrons recombination leading to great loss of efficiency. An improvement was brought by combining the templating techniques to the previous ones (**Figure 4**): surfactant molecules were used to organize the TiO2 layer and, therefore, to create higher conductivity channels inside the layer (**Figure 5**). However, this process was still not sufficient and the conductivity of the TiO2 layer remained one of the main weak spots of DSSC.

**Figure 4.** Scheme of the dip-coating + sol-gel + templating technique.

**Figure 5.** TiO2 layer constituted by nanocrystallites obtained by the dip-coating + sol-gel + templating technique (with kind permission of the GREENMAT-LCIS Department, ULg, Belgium).

The last step is currently to produce "one-dimensional" (1-D) crystallites in the form of TiO2 nanotube array (TNA or TiO2 – NTA) [5] (**Figure 6**). Such a structure combines good specific surface area and high conductivity. It can be obtained by anodization of metallic titanium in well-defined experimental conditions that are discussed later.

#### **2.2. Techniques and recent developments**

#### *2.2.1. Overview*

Anodization is the oxidation of a metal, forced by a potential difference willingly induced between an anode (the metal to oxidize) and a cathode (classically Pt), in an electrolytic bath. The positive potential set at the anode induces the metal oxidation, whereas the cathode reaction is water reduction that leads to hydrogen evolution.

Anodization can lead to either dense or porous structures, depending on the anodization conditions (especially the bath composition). Nanotubular structures are well-known for anodized aluminum oxide, but anodization also makes it possible to elaborate nanostructured layers of several metal oxides (WO3, FexOy, ZrO2, etc.) [6], including TiO2. The obtained structure depends on numerous parameters such as applied potential, bath composition, temperature and anodization time. Therefore, a good knowledge of the impact of these parameters allows a precise morphological control of the anodized layer.

As explained before, nanotubular structures are of particular interest in various fields, including photovoltaics. To obtain those porous nanostructures, anodization has to be carried out in a medium with the ability to dissolve the formed oxide. For titanium, fluoride ions are the most efficient, forming a TiF6 2− complex. Initially, aqueous solutions of HF or fluoride salts were used, but because of the high reactivity of these compounds with TiO2, only small nanotube lengths were achieved. Anodization in organic electrolytes has then been investi‐ gated to get longer nanotubes and smoother aspect of the layers. In this case, a small amount of water (at least 0.18% [7]) is added as a source of oxygen. Classical organic solvents such as glycerol, dimethyl sulfoxide or ethylene glycol have been used to produce nanotube layers of dozens (even hundreds) of micrometres on a Ti foil.

The formation mechanism of these nanotubes is quite well known and has been described on several occasions [6, 8]. Focusing on the electrochemical reactions, it can be summarized as follows: at the anode, the main reaction is the oxidation of titanium:

$$\text{Ti} \rightarrow \text{Ti}^{4+} + 4\,\text{e}^- \tag{1}$$

This oxidation leads to the formation of titanium dioxide, by a hydrated intermediate:

$$\text{Ti}^{4+} + 4\,\text{OH}^- \rightarrow \text{Ti}(\text{OH})\_4 \tag{2}$$

$$\text{Ti(OH}\text{)}\_{4} \rightarrow \text{TiO}\_{2} + \text{H}\_{2}\text{O} \tag{3}$$

Equation (2) shows that titanium oxidation goes with a decreasing pH, which favours the formation of nanotubes by increasing the dissolution rate at the bottom of the tubes. It is worth noting the presence of a side reaction of oxygen evolution taking place at the anode, which can impact the quality of the structure (especially for the anodization of thin films, as shown below):

$$2H\_2O \to O\_2 + 4H^+ + 4e^- \tag{4}$$

At the cathode, the reaction is water reduction, with hydrogen evolution:

$$4\,H^{+} + 4\,e^{-} \to 2\,H\_{2} \tag{5}$$

And the overall equation is:

$$\text{Ti} + 2\,\,H\_2O \to \text{TiO}\_2 + 2\,\,H\_2\tag{6}$$

To get a porous structure, a compound that can chemically dissolve the formed titanium dioxide is needed. This is achieved by fluoride ions forming a complex:

$$\text{TiO}\_2 + 6\, F^- + 4\, H^+ \to \text{TiF}\_6^{2-} + 2\, H\_2O \tag{7}$$

It is the competition between reactions (3) and (7), that is, between the electrochemical formation of the oxide and its chemical dissolution by the fluorides that mostly determine the structure of the anodized layer. The anodized TiO2 is generally amorphous, and this is detrimental to the charge transport properties of the nanotubular layer. Therefore, a thermal treatment around 450°C is commonly used to crystallize it into anatase (the most suited TiO2 polymorph for photovoltaics).

#### *2.2.2. Anodization parameters*

The first important parameter to consider is the bath composition. As said previously, it mainly consists of an organic solvent with usually 1–5 wt% of water as a source of oxygen and 0.1–5 wt% of fluorides from NH4F or HF.

The anodization process is mostly carried out in potentiostatic mode because the control of the morphological properties is easier than in galvanostatic mode [9]. The applied voltage has an important impact on the nanotube diameter. Several studies have shown a linear relationship between the tube diameter and the applied voltage in the 10 to 60 V range [10, 11], allowing to tune the nanotube diameter from approximately 20 nm (10 V) to 120 nm (60 V).

In organic electrolytes, the nanotube length increases with the anodization duration, as the chemical dissolution is much lower than the electrochemical growth of the tubes. Therefore, nanotubes of several hundreds of micrometres can be achieved on Ti foil by a very long anodization time [12]. However, very long nanotubes are not especially suited for photovoltaic applications because the electron diffusion length in dye-sensitized solar cells is commonly a few micrometres. Longer nanotubes only increases charge recombination and finally decreases the cell efficiency.

The anodization temperature is another key parameter in the anodization process. In the case of titanium anodization, most studies are carried out at room temperature (20–25°C). On the one hand, a higher temperature obviously increases the kinetics of the reactions (which means a higher growth rate), but this is not the only effect. It also favours water oxidation (side reaction) and an increase of the tube diameter, which is disadvantageous. On the other hand, a low temperature anodization provokes an important increase of the wall thickness of the nanotubes and, consequently, reduces the porosity of the structure (and hence the specific surface area). This is also undesirable as a high specific surface area is required for the dye to adsorb and get high efficiency DSSCs. The structures obtained at 5, 20 and 30°C are shown in **Figure 6**.

**Figure 6.** TNAs obtained by anodization at 40 V in a bath at 5°C (top left), 20°C (top right) and 30°C (bottom).

## *2.2.3. Anodizing a TiO2 thin film*

In order to get a transparent photoelectrode made of TNA on a conducting substrate, a Ti thin film is sputtered onto a FTO glass substrate. This film is then anodized to fully convert the dense titanium into titania nanotubes. The need for an adherent, homogeneous and dense film requires the use of magnetron (or sometimes rf) sputtering, since a simple evaporation leads to insufficient adherence and a delamination of the Ti film during anodization [13]. Some studies have pointed out that substrate preheating allows a better adherence of the film, therefore this kind of thin film is often deposited with a substrate preheated around 400°C [14, 15]. Another method is to apply a bias, so the growing Ti film is hit by more energetic ions [16]. Using these methods, several micrometres thick Ti layers can be obtained on FTO glass.

When anodization is performed on a thin Ti film on a transparent substrate to make a trans‐ parent electrode, the nanotube length is no longer linked to the anodization duration but to the thickness of the Ti film, as the anodization is carried out until the complete metal oxidation and hence the full transparency of the film are obtained. It is worth noting that the expansion factor (the ratio between the TNA film thickness and the initial Ti film thickness) varies from 1.3 to 2.5 approximately, depending on the anodization parameters (especially the applied voltage).

Another specificity of anodizing thin films on FTO glass is the role of the substrate. In this case, the anodization is carried out until the full oxidation of the Ti film is achieved, which means that there can be interactions between the electrolyte and the substrate at the end of the anodization process. The main problem is the side reaction of water oxidation (Equation (4)) taking place at the substrate–electrolyte interface, evolving oxygen. This reaction is known as a side reaction even on Ti foils but remains secondary compared with the oxidation of Ti. In this case, at the substrate–electrolyte interface, it is the only reaction and it can lead to a sufficiently intense bubbling to destructure the TNA film and provoke local delamination, even at low anodization potentials.

Knowing this side reaction is not a problem on Ti foils or on isolating substrates (such as conventional glass), there are mainly two ways to address the delamination problem. The first one is to stop anodization as soon as any contact between the substrate and the electrolyte is detected (bubbles appears), thus leaving a few nanometres of non-oxidized Ti that are oxidized during the thermal treatment of crystallization. The second is to prevent any contact between the FTO glass and the electrolyte by adding a compact underlayer of TiO2 on which oxygen evolution is less favoured than on a highly conducting substrate such as FTO, but that is conducting enough to transport the electrons in the solar cell. Consequently, the thickness of that underlayer is chosen similar to the thickness of conventional blocking layers in DSSCs (50–100 nm). This compact underlayer is deposited by reactive sputtering just before the metallic Ti deposition, the transition between TiO2 and Ti being triggered by switching off the oxygen admission in the vacuum chamber. The process of anodization of thin films with and without an underlayer is shown in **Figure 7**.

**Figure 7.** Schematized process to elaborate TNAs on conducting glass with and without underlayer.

## *2.2.4. Impact of a TiO2 underlayer*

A first impact of sputtering a compact TiO2 underlayer is a better adherence of the film, allowing to sputter the Ti without preheating the substrate. This is a significant gain of productivity, probably by ensuring a higher chemical affinity between the metallic Ti and the substrate. Another impact is on the regularity of the nanostructure. Indeed, from a microscopic point of view, the introduction of this underlayer has little impact on the nanotubes morphol‐ ogy. However, from a macroscopic point of view, it induces considerable improvements of homogeneity (reducing edge effects, for example). This is especially important for samples of larger size and as pointed out earlier, it prevents delamination, allowing a complete anodiza‐ tion at different applied potentials. SEM cross-sectional view of nanotubes on FTO glass with a TiO2 underlayer is shown in **Figure 8a** and top views at different potentials of anodization (with tube diameters from 30 to 60 nm) are shown in **Figure 8b**, **8c** and **8d**.

**Figure 8.** SEM views of TNAs: (top left) cross-sectional, (top right and bottom) top views at 20, 30 and 40 V, respective‐ ly.

Another important parameter for a photoelectrode is its transparency, as any absorption by the defects in the TiO2 is parasitic and will not be converted in electricity (recombination). The transmittance of the photoelectrodes based on TNAs are shown in **Figure 9** and compared with classical photoelectrodes with the same TiO2 thickness (about 1 μm). If the transmittance of commercial nanoparticles is the highest (around 80% in average in the 450–850 nm range), it is worth noting the great difference between the TNA elaborated with and without an underlayer (73–75% and 63–68% in average, respectively). The lower transmittance observed without an underlayer indicates that the residual Ti left to avoid delamination at the end of the anodization process is not completely oxidized by the thermal treatment or it creates other absorbing defects. A complete oxidation by anodization is therefore encouraged, and this can only be done with an underlayer to prevent delamination. Moreover, this oxidation by the thermal treatment is known for creating a resistive layer under the nanotubes (especially on Ti foils). A complete anodization allows avoiding this resistive layer, and therefore an improvement in the electron transport properties can be observed for the samples with a TiO2 underlayer.

**Figure 9.** Transmittance of the photoelectrodes based on TNAs.

## **3. TiO2 for gas sensing**

#### **3.1. Overview and principles**

TiO2 is a member of the large family of the semiconducting metal oxides used for chemical gas sensors applications [17, 18]. The detection principle is based on the conductivity change of those materials when they are in contact with certain gases. When a gas is adsorbed onto the surface, and if the interaction is strong enough (meaning chemisorption), an electron transfer between the semiconductor and the adsorbed species takes place resulting in an increase (decrease) of the charge carrier concentration in the semiconductor and as a consequence an increase (decrease) of the conductivity. As there is an equilibrium between the gas concentra‐ tion in the atmosphere and the adsorbed quantity, a direct relation is expected between the conductivity change and the gas concentration. This phenomenon can be observed with organic or inorganic semiconductors.

The general scheme of such a sensor is presented in **Figure 10**.

**Figure 10.** Scheme of a semiconductor sensor.

The sensor acts just like a variable resistance and is often called chemoresistive sensor or chemoresistor.

As the detection principle deals with chemisorption and surface reactions with the gas, an activation energy is generally needed. Otherwise, the response to the gas and the recovery are too slow or even no response is observed. That is why a heating element is usually added on the substrate (very often in the form of a coil).

Historically, the effect of gases on semiconductors was discovered by Brattain and Bardeen. Already in 1953, they observed that Ge samples modified their resistance, depending on the atmosphere they were in contact with [19]. Later in the 1960s, Seiyama [20], using a zinc oxide (ZnO) thin film, was able to demonstrate that gas sensing is possible with simple electrical devices. He studied a simple chemoresistive device sensitive to propane based on ZnO thin films, operating at 485°C. Taguchi [21] fabricated and patented the first chemoresistive gas sensor device for practical applications using tin dioxide (SnO2) as the sensitive material. This led to the foundation of the company *Figaro Inc* whose first main product was alarms for explosive gases. This paved the way to intense researches to extend the principle to numerous applications of the semiconductor gas sensors.

Because of their simplicity, low cost, small size and ability to be integrated into electronic devices, chemical sensors have been the object of an extensive work as they have a big potential in all kinds of applications: industrial emission control, household security, vehicle emission control and environmental monitoring, agricultural, biomedical, and so on [22–27].

Although organic materials are potentially very attractive because they can be more easily modified than inorganic materials and thereby their performances "tailored", they encoun‐ tered considerably less success than the inorganic semiconductors, especially metal oxides. The main reasons are the bad long-term stability, and it is often impossible to use them at temperatures at which gas–solid interactions proceed rapidly and reversibly.

The detection mechanism for metal oxide is represented in **Figure 11** [18–28].

**Figure 11.** Detection mechanism for metal oxide sensors.

Taking the example of an *n*-type semiconductor, in air, oxygen will be adsorbed at the surface of the oxide. An electron transfer occurs from the oxide to the oxygen leading to the formation of oxygen ions O2 − , O<sup>−</sup> or O2−. The nature of the ion depends on the temperature. The conse‐ quence is that the surface of the crystals is depleted, a potential barrier appears and the conductivity is decreased. Very often, the sensitive films are constituted of grains in contact and the potential barrier will modulate the transfer of electrons between the grains and so the overall conductivity.

So for *n*-type semiconductors, the global conductance can be expressed as follows [29, 30]:

$$G = k \cdot \sigma\_b \cdot \exp\left(\frac{-e \cdot V\_s}{k\_b \cdot T}\right).$$

with


This formula is established assuming the electron transfer limitation by the surface potential.

Adsorption of the gases can modify this potential leading to conductivity changes. For instance, reduction of the oxygen pressure in the atmosphere means reduction of the adsorbed oxygen surface concentration leading to an increase of the conductivity.

In general, for *n*-type semiconductors, the contact with reducing gases (CO, H2, CH4, etc.) will increase the conductivity. Indeed, reducing gases can react with adsorbed oxygen, thereby releasing the trapped electrons that increase the charge carrier concentration. For oxidizing gases (NO2, Cl2, O3, etc.), their adsorption will cause an increase of the barrier because of the trapping of electrons. For *p*-type semiconductors, the effects are opposite.

The nature of the oxide is a key factor for the choice of the sensitive layer for a given gas but the defects at the surface of the material play an important role as they are adsorption centres for gases. So the control of the surface is a major concern for gas sensing.

The most studied metal oxides for gas sensing as *n*-type semiconductors are SnO2, ZnO, In2O3, WO3 and TiO2, whereas NiO, Cr2O3 and CuO are the most studied as *p*-type semicon‐ ductors.

The main drawbacks of these sensors remain the lack of selectivity because of the fact that the interaction between the semiconductor and the target gas is not always specific, and the need to heat the sensitive layer to rather high temperatures leading to increased power consump‐ tion. The current developments strive to reduce these drawbacks by studying a lot of other materials and by exploiting the opportunities provided by the new nanoscale technologies. Nanoscience, enabling controllable manipulation of matter at the molecular level, has become a precious tool for innovations in materials processing. A smaller size and so lower power consumption, greater sensitivity and better selectivity are expected. For instance, it is quite clear that grain-size reduction at nanometric scale can enhance the detection properties of metal oxides gas sensors.

**Figure 12.** Spill over mechanism with hydrogen enhancing the oxidation rate.

Adding suitable promoters (metal particles, foreign metal oxide, ions, etc.) on the metal oxide layer is a common way of enhancing the sensing characteristics of metal oxide gas sensors [31, 32]. For instance, Pt promotes the gas-sensing reaction by the spill-over mechanism (massively exploited for heterogeneous catalysis). Pt clusters catalyse the dissociation of the gases, favouring the reactions with the adsorbed oxygen species. **Figure 12** shows the mechanism with hydrogen. The result is an increase of the sensitivity to hydrogen and a lowering of the working temperature.

TiO2 is particularly attractive for gas sensors because of its cross-sensitivity to humidity lower than other metal oxides than other metal oxides [33]. Among the other applications, TiO2 has been largely investigated as a sensing layer in resistive oxygen gas sensors operating at medium-high temperatures for automotive air/fuel ratio (A/F) control. The first titania gas sensors were developed in the late 1970s and early 1980s. The first application was the detection of the stoichiometric A/F ratio for engines [34]. Indeed, the sensor resistance was increased by orders of magnitude around the stoichiometric A/F, making it a very useful device for these applications. It was a competitor to the classical *Lambda* probe based on a solid state electro‐ chemical cell.

Depending on the application of interest and availability of fabrication methods, different surface morphology and configurations of the metal oxides have been achieved, including single crystals, thin films, thick films and one dimensional (1-D) nanostructures [35–37]. Among all these, following the same trend as for photovoltaics, 1-D nanostructures have recently attracted much attention because of their potential applications in gas sensors [38]. 1-D nanostructures are particularly suited to this application because of their high surface-tovolume ratio as well as their good chemical and thermal stabilities [39, 40]. This chapterfocuses on these materials and in particular, TiO2.

#### **3.2. Techniques and recent developments**

The purpose is not to perform a comprehensive research survey in this chapter. There are already many excellent review articles on the topic of chemoresistive gas sensors [17, 41–45], but the aim is to give an overview of the use of nanostructured TiO2 in the gas sensing field.

The development of fabrication methods for producing 1-D nanostructures has been the object of an intense research in the field of nanoscience and nanotechnology [37, 44, 46]. Several metal oxides such as ZnO, SnO2, TiO2, In2O3, WO3, AgVO3, CdO, MoO3, CuO, TeO2 and Fe2O3 have been investigated for gas sensing. However, according to our findings [47], 1-D structures are not always preferable for all gases. Several routes (chemical or physical) have been investigated for 1-D metal-oxide nanostructures for gas sensing applications. Arafat et al. [36] give a good summary of these routes. The most important ones are explained in this chapter focusing on TiO2.

The sensor's response to a given gas can be enhanced by the modification of both surface states and bulk properties of the 1-D metal-oxide nanostructures. These modifications can be achieved by either depositing nanoparticles on the nanostructure's surface or coating and doping with impurities or even cover them with organic molecules [48, 49]. Sensors utilizing these types of surface and bulk property modifications showed higher sensitivity compared with unmodified systems. The functionalization can be performed in a second step after the synthesis of the 1-D structures or in one step during their formation.

Another trend is to use organized nanostructures as porous templates for the deposition of a sensitive material. This was done for instance with polypyrrole on TiO2 for NH3 detection [50] and was studied in our lab for the fabrication of molecularly imprinted polymers (MIP) based on polypyrrole electropolymerized in TiO2 nanotubes for aldehydes detection.

## **3.3. Growth of TiO2 nanostructures**

The synthesis methods of 1-D TiO2 nanostructures can be divided into two groups: solid-state etching and wet processing routes. The solid-state etching process includes nanocarving by H2 gas, UV lithography and dry plasma etching [51, 52]. The wet processing route is, by far, more popular as it does not need complex equipment. The wet processing route includes hydrothermal synthesis, electrospinning and anodization.

Different surface morphologies such as nanotube arrays, branched nanotubes, coated nano‐ tubes, nanoparticle added nanotubes, nanobelts, nanofibres and nanowires of TiO2 can be obtained, depending on the synthesis method and the process parameters. Generally, the process ends with an annealing phase to define and stabilize the crystal structure. Depending on the starting materials and process conditions, the crystal structure of TiO2 nanostructures varies from anatase, rutile and brookite to lepidocrocite.

The morphology of the nanostructures can be altered by combining two different processes. As an example, branched nanotubes can be obtained by the combination of anodization and hydrothermal processes [53].

As an example of wet processing route to obtain pure TiO2, let us cite Rout et al. [54] describing TiO2 nanowires obtained by hydrothermal process using TiCl3 solution in HCl and saturated NaCl. The mixture was put in a *Teflon*-lined autoclave and heated at 200°C for 2 hours. After washing with water and alcohol and drying in vacuum, 1-D nanostructures with diameters of 20–80 nm and lengths of 100–800 nm were obtained. The crystal structure was found to be rutile.

A one-step functionalization and synthesis method of TiO2 nanotubes by hydrothermal processing covered with Pd and Pt nanoparticles was detailed by Han et al. [48]. In this case, commercial anatase TiO2 powder and PdCl2 or H2PtCl6 were dispersed in an aqueous solution of NaOH and charged into a *Teflon*-lined autoclave at 150°C for 12 hours. The precipitates were separated by filtration, washed with dilute HCl and water and finally dried at 120°C, yielding Pd-Pt-TiO2. The resulting nanotubes were 100 nm in diameter with a lepidocrocite-type phase.

A two-step method is illustrated by Hu et al. [55] with Ag clusters on TiO2 nanobelts. The nanobelts were prepared via an alkaline hydrothermal process using commercial TiO2 powders, NaOH, HCl and deionized water. The obtained H2Ti3O7 nanobelts were annealed at 600°C for 1 hour to obtain crystalline TiO2 nanobelts. The surface of the TiO2 nanobelts was coarsened by adding H2SO4 into H2Ti3O7 aqueous solution under magnetic stirring followed by heating at 100°C for 12 hours. The obtained phase of TiO2 nanobelt was anatase. For the preparation of Ag nanoparticle-TiO2 nanobelts, the as-prepared TiO2 nanobelts obtained by hydrothermal route were dispersed into AgNO3 and ethanol solution. Taking advantage of a photoreduction process, the solution was illuminated with a 20 W ultraviolet lamp under magnetic agitation.

Electrospinning was exploited by Landau et al. [56] to synthesize TiO2 nanofibres. Biao et al. [49] describe a method to prepare Cu-doped TiO2 nanofibres by electrospinning in one step.

A very elegant method that is used for both gas sensing and photovoltaics is anodization as described earlier (see Section **2.2**).

Varghese et al. [57] used a platinum foil as a cathode and titanium foil as an anode at an anodization potential of 12 V and 20 V between the electrodes. The electrolyte consisted of 0.5% hydrofluoric acid in water. The samples were then annealed at 500°C in pure oxygen for 6 hours. The nanotubes were approximately 400 nm in length with a 46–76 nm diameter. A barrier layer with a thickness of 50 nm was formed in between the nanotubes and foil. Nanotubes fabricated using 20 V had an average pore diameter of 76 nm with a wall thickness of 27 nm, while samples anodized at 12 V had an average pore diameter of 46 nm with a wall thickness 17 nm. Anatase concentrated on the walls of the nanotubes and rutile in the barrier layer [58].

Lu et al. [59] also synthesized TiO2 nanotube arrays by anodization of a titanium foil in a NH4F and (NH4)2SO4 aqueous solution. The titanium foil was used as an anode under a constant potential of 20 V (for 2 hours at room temperature) and the cathode was a platinum foil too. The as-prepared amorphous TiO2 nanotube arrays were annealed at 450°C in air for 2 hours to obtain anatase phase. The resulting nanostructure had an outer and inner diameter of 150 nm and 110 nm, respectively, with length of approximately 2.3 μm. As usual, the nanotube dimensions could be varied in the anodization process by changing both the pH of the electrolyte and the electrode voltage [60]. Paulose et al. [60] prepared nanotube arrays by anodization of titanium foils in an aqueous solution containing sodium hydrogen sulfate monohydrate, potassium fluoride and sodium citrate tribasic dihydrate. It was seen that the pore diameter depended on the anodization voltage, whereas the nanotube length depended on both the electrolyte pH and anodization voltage. Hu et al. [53] also synthesized TiO2 nanotube arrays by the anodization approach. A titanium foil was cleaned by soap, acetone and isopropanol and used as an anode. The cathode was a platinum foil. The anodization solution contained NH4F and dimethyl sulfoxide. The anodization was conducted at a 45 V constant potential for 9 hours. The obtained amorphous TiO2 nanotube arrays were annealed at 400°C for 1.5 hours. The resulting nanotubes were 350 nm in diameter and 3.5 μm in length with a wall thickness of 10 nm. The branched TiO2 nanotubes were obtained through a modification process on TiO2 nanotubes array by hydrothermal methods [53]. The as-prepared TiO2 nanotube arrays were immersed in a solution containing HCl with constant stirring at 25°C for 15 minutes. Titanium (IV) isopropoxide was dropped into the solution under constant stirring for 1 hour, and then the beaker was sealed and heated at 95°C for 9 hours with slight stirring. After the reaction, the reactant was cooled to room temperature and washed with ethanol and distilled water. The as-prepared branched TiO2 nanotube arrays were annealed in a muffle furnace at 400°C for 2 hours. It was observed that TiO2 nanocrystal nucleus formed on the rough surfaces of the TiO2 nanotubes with special bamboo structures with a larger and rougher surface area. Similarly, P25 (a commercial photocatalyst from *Degussa*, Germany) coated TiO2 nanotube arrays were synthesized by the hydrothermal approach on the uncoated TiO2 nanotube arrays [53]. In this process P25 was added to distilled water and then mixed vigorously by magnetic stirring and ultrasonicating, followed by transferring into a Teflonlined autoclave. The treatment at 80–120°C for 12 hours was carried out to coat P25 on the TiO2 nanotube arrays. After washing with distilled water, the P25 coated TiO2 nanotube arrays were annealed at 400°C for 2 hours in air.

#### **3.4. Sensing performance of TiO2 1-D nanostructures**

The sensitivity of nanowire arrays on silica fabricated by Francioso et al. [52] according to a solid-state process was studied for ethanol sensing. It was seen that the sensitivity of the sensor (defined as the ratio IEtOH/Iair) was approximately 50 at 550°C for 2% ethanol. Comparing these results with the response of TiO2 thin film, the nanowire array showed higher sensitivity towards ethanol. The response is less than 10 in the case of TiO2 thin film for the same concentration.

Gönüllü et al. [61] studied the impact of doping the surface of TiO2 nanotubes. TNA for NO2 detection at high temperature were obtained by anodization. They showed the cross-sensitiv‐ ity with CO was almost cancelled after doping compared with pure TiO2 nanotubes.

Hu et al. [55] synthesized four types of TiO2 nanobelts (TiO2 untreated nanobelts, TiO2 surfacecoarsened nanobelts, Ag nanoparticles-TiO2 untreated nanobelts and Ag nanoparticles-TiO2 surface-coarsened nanobelts) for the detection of ethanol vapour according to the abovementioned hydrothermal process. The best performance is obtained for Ag nanoparticles-TiO2 surface-coarsened nanobelts. The response (defined as the ratio Rair/Rgas) was about 100 at 200°C for 500 ppm ethanol. The optimum working temperature was in the range of 200– 250°C. The response and recovery for ethanol were very short (a few seconds).

Landau et al. [56] measured the sensitivity of TiO2 nanofibres obtained by electrospinning towards NO2 gas. These nanofibres showed a good sensitivity at low concentrations of CO and NO2 in air. The sensor was more sensitive at 300°C than at 400°C (sensitivity to 250 ppb NO2 (defined as the ratio Rgas/Rair) was 74.3 at 300°C and 3.3 at 400°C).

Biao et al. [49] compared the sensitivity of Cu-doped and undoped TiO2 nanofibres also by electrospinning for CO detection. It was observed that Cu-doped TiO2 nanofibres showed much higher sensitivity compared with pure TiO2 nanofibres. The sensitivity of Cu-doped TiO2 nanofibres was approximately 21, which is 17 times larger than pure TiO2 at 300°C for 100 ppm CO (temperature of maximum sensitivity). The response and recovery times are 4 and 8 seconds, respectively. Moreover, the Cu-doped TiO2 presented a higher selectivity towards CH4, CH3OH, C2H5OH, H2 and NO.

Lu et al. [59] synthesized an amorphous TiO2 nanotube array for the detection of O2 using the anodization process. The sensitivity of amorphous TNA approximately increased with increasing temperature, but exhibited non-reproducible behaviour with a very poor recovery above 180°C. However, at 100°C, high sensitivity, excellent recovery and a linear relationship with oxygen concentration was observed. Other metal-oxide sensors such as Ga2O3 thin film (∼1.5) [62], nanoscale TiO2 thick film (∼1.5) [63] and SrTiO3 thick film (∼6.5) [64] showed lower responses towards oxygen at 100°C. Crystalline TiO2 is highly advantageous for H2, CO, NO2 and CH4 detection [65–67] but for oxygen, crystalline TiO2 exhibits a very poor recovery [57, 59]. That is why, as for photovoltaics, TiO2 is usually annealed in air or oxygen atmosphere at high temperature to form a crystalline structure more favourable for gas sensing.

TNA on Ti foil obtained by the anodization process described by Varghese et al. [57] were tested for H2 sensing. They were able to detect H2 at temperatures as low as 180°C. The sensitivity of TiO2 nanotubes increased with increasing temperature showing a variation of three orders in magnitude of resistance to 0.1% of H2 at 400°C. As always, the response time decreased with increasing temperature. At 290°C the response time was typically 3 min. The sensors showed high selectivity to H2 compared with CO, CO2 and NH3. TiO2 nanotubes with a smaller pore diameter (46 nm) had higher sensitivity than larger pore diameters (76 nm) towards H2 gas. The high sensitivity of the nanotubes to H2 was due to the highly active surface states on the nanotube walls, high surface area of the nanotube architecture and the ordered geometry of the tube-to-tube electrical connections. Han et al. [48] showed that Pt and Pd nanoparticles on TiO2 nanotubes had a response almost twice that of pure TiO2 nanoparticles or nanotubes. As usual, the sensitivity goes through a maximum at 25°C. The rate of reaction on the catalytic surface is probably the fastest at this temperature, resulting to a large change of conductivity. This also suggests that the higher response of Pt and Pd nanoparticle-TiO2 is due to the higher number of adsorption sites or the catalytic surface area. Another possible reason for the enhanced response of Pt and Pd nanoparticle-TiO2 nanotubes is due to increased adsorption of hydrogen on the TiO2 nanotube surface that facilitates the hydrogen oxidation reaction by the Pd and Pt catalysts.

Comparing the sensitivity of anatase and rutile nanostructures, it was stated that anatase shows high sensitivity towards reducing gases such as H2 and CO [68–70]. The reason would be that the diffusing hydrogen atoms go to the interstitial sites [70, 71] and as the c/a (lattice parameters along c-axis and a-axis) ratio of anatase is almost four times that of rutile, anatase lattice accommodates hydrogen more easily and hence has a higher sensitivity to hydrogen.

## **3.5. TiO2 used as a porous template**

TiO2 can also be used as a porous support for the deposition of an organic sensitive layer as described by Wang et al. [50] for the case of ammonia detection with polypyrrole. Polypyrrole (PPy) is a well-known organic semiconductor that was studied for gas detection, in particular NH3 and VOC. The problem with PPy films is that they are rather non-porous leading to low surface-to-volume ratio and low sensitivity.

To increase this surface-to-volume ratio, Wang et al. [50] proposed to synthesize PPy on a very porous TiO2/ZnO nanofibre network (diameter of the fibres ~100 nm). The nanofibres were obtained by electrospinning. The TiO2/ZnO nanofibre network was dipped into a FeCl3 alcoholic solution for 30 minutes. After drying, it was exposed to saturated pyrrole vapours at room temperature for 5 hours, resulting in a thin PPy covering the nanofibres. The obtained material showed a good sensitivity for NH3 in the ppm concentration range although the response time was still long (15 minutes).

TiO2 nanotube arrays can be used as a sacrificial template for the preparation of nanowires or nanotubes materials. This is inspired by the new trends in photovoltaic applications of TNA.

In the example detailed below, it was done with polypyrrole for formaldehyde detection. The sensitive polymer is a molecularly imprinted polymer (MIP) based on the polymerization of a mixture of pyrrole and polypyrrole-3-carboxylic acid. Formaldehyde, the target gas, is used as the template for the formation of the MIP. The formation of MIP films by electropolymeri‐ zation is simple but leads to low porosity films that show a weak sensitivity to formaldehyde. To increase the specific surface area of the polypyrrole, it was synthesized by electrodeposition in an anodized TiO2 matrix. After getting rid of the TNA by dissolution in NH4F solution, porous polypyrrole is obtained with high specific surface area.

The anodization was performed on Ti foils cleaned with isopropanol and treated with 1M nitric acid for 30 minutes. The foil was used as the anode and a platinum electrode as the cathode in an electrochemical bath filled with 500 mL ethylene glycol, 10 mL water and 1.7 g NH4F. A 40 V voltage is applied for 4 hours. After washing with water and drying, the foil is annealed in air at 475°C for 2 hours. Regularly spaced nanotubes with diameter 80–90 nm and wall thickness 7–9 nm were obtained (**Figure 13**).

**Figure 13.** TiO2 nanotubes used for the synthesis of polypyrrole based MIP.

The obtained anodized Ti is then used as anode for the electrodeposition of the conducting polymer.

The electropolymerization is carried out as follows:

Working electrode: TNA

Counter electrode: Platinum

Reference electrode: Ag/AgCl

Composition of the bath: 50 mL acetonitrile | 0.614 g NaClO4 | 0.17 g sodium dodecylsulfate (SDS) | 0.335 g pyrrole | 55 mg pyrrole-3 carboxylic acid | 10 mL formaldehyde solution (37% in water).

Deposition mode: pulsed 1000 cycles 2 V pulses for 0.1 second | −0.3 V for 0.04 seconds.

By so doing, the nanotubes are covered with the polymer, as shown in **Figure 14**.

**Figure 14.** Growth of the polymer in the TiO2 nanotubes.

After dissolution of the TiO2 matrix, the porous polymer is obtained (**Figure 15**).

**Figure 15.** Obtained sensitive polymer after removal of the TiO2 matrix.

The crushed polymer is deposited by drop coating on a polyimide substrate fitted with interdigitated silver electrodes (electrode width 100 μm, spacing 100 μm, length 20 mm and 20 fingers).

**Figure 16** shows the evolution of the resistance of the sensitive polymer after exposure to formaldehyde in humid air (60% at 22°C).

**Figure 16.** Evolution of the resistance of the porous MIP film when exposed to formaldehyde in humid air.

The same substrate fitted with a continuous film deposited by electropolymerization in the same conditions gives a response three times lower.

#### **4. Conclusion**

As can be seen in this chapter, the potentialities of TiO2 and, in particular, the 1-D nanostruc‐ tures are very attractive. Due to its unique combination of chemical, electronic and optical properties, TiO2 has become one of the major components in the third generation of solar cells and various possible variations (doping, hydrothermal synthesis, hybrid materials, etc.) expand the horizon. On the other hand, the applications of TiO2 in gas detection are numerous and the interest for this material is growing. One can expect a lot of developments with TiO2 as sensitive material for chemoresistors, leading to better performances in terms of sensitivity, selectivity and durability.

## **Acknowledgements**

The authors thank the *Région Wallonne* and the *Fédération Wallonie-Bruxelles* for funding (RW: *Photocell*, *Nanorod* and *Captindoor* projects; FWB: Madsscells project). A.K. also thanks FRIA for a grant.

## **Author details**

André Decroly1\*, Arnaud Krumpmann1 , Marc Debliquy1 and Driss Lahem2

\*Address all correspondence to: andre.decroly@umons.ac.be

1 University of Mons, Mons, Belgium

2 Materia Nova, Mons, Belgium

## **References**


## **Anodic Nanostructures for Solar Cell Applications**

Jia Lin, Xiaolin Liu, Shu Zhu and Xianfeng Chen

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/62396

#### **Abstract**

As a versatile, straightforward, and cost-effective strategy for the synthesis of selforganized nanomaterials, electrochemical anodization is nowadays frequently used to synthesize anodic metal oxide nanostructures for various solar cell applications. This chapter mainly discusses the synthesis of various anodic TiO2 nanostructures on foils and as membranes or powders, and their potential use as the photoanode materials based on foils, transparent conductive oxide substrates, and flexible substrates in dye-sensitized solar cell applications, acting as dye-loading frames, light-harvesting enhancement assembly, and electron transport medium. Through the control and modulation of the electrical and chemical parameters of electrochemical anodization process, such as applied voltages, currents, bath temperatures, electrolyte composition, or post-treatments, anodic nanostructures with controllable structures and geometries and unique optical, electronic, and photoelectric properties in solar cell applications can be obtained. Compared with other types of nanostructures, there are several major advantages for anodic nanostruc‐ tures to be used in solar cells. They are (1) optimized solar cell configuration to achieve efficient light utilization; (2) easy fabrication of large size nanostructures to enhance light scattering; (3) precise modulation of the electrochemical processes to realize periodic nanostructured geometry with excellent optical properties; (4) unidirectional electron transport pathways with suppressed charge recombination; and (5) large surface areas by modification of nanostructures. Due to the simple fabrication processes and unique properties, the anodic nanostructures will have a fascinating future to boost the solar cell performances.

**Keywords:** anodic nanostructures, membranes, scattering, photonic crystals, thermal stability, surface area

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

## **1. Introduction**

#### **1.1. Anodic oxides**

The electrochemical anodization of anodic oxides is conducted with two-electrode or threeelectrode configurations in an electrochemical cell, with the targeted metals as the working electrode (anode) and usually Pt/carbon as the counter electrode (cathode). The electrolytes usedaretypicallyfluoride-containingorganicoraqueoussolutions.Avoltageisappliedbetween the two electrodes to fabricate various nanostructures on the anodic metal surface. The anodic oxides featuretheself-organizedconfigurationswithregularandopenstructures, suchas tubes, pores, channels, bundles, powders, and various tailored shapes. The formation mechanism of oxide nanostructures is of great concern. For tube arrays, the possible processes are as fol‐ lows:(1)Thefirst stepis thetubeinitiation.Thetubegrowthinitiatesat somepreferablepositions on the metal surface that can provide both high local electric field and narrow channel-like surface morphology. It has been indicated that the morphological instability of the initial oxide surface layer causes the formation of pores [1]. Hence the surface state of the starting metal substrate can significantly influence the surface morphology of the anodic oxides, such as pore size distribution, regularity, and shapes. (2) The second step is the tube growth. One most accepted view is that the oxidation of the metal with the assistance of electric field forms a compact thin oxide layer (barrier layer) on the surface [2]. The thin oxide layer is also partially dissolved under the assistance of electric field. The oxidation and dissolution happen simulta‐ neously. When the oxide growth rate at the metal/oxide interface equals to the oxide dissolu‐ tionattheoxide/electrolyte interface,the thicknessofthe compactoxide layerkeepsunchanged. Then the compact layer at the oxide/metal interface moves towards the inner part of the metals and the oxide nanostructures form into the metals.

Due the advantages such as simplicity, high efficiency, and low cost of electrochemical anodization, it has been utilized to fabricate various wide band gap metal oxide nanostruc‐ tures, such as titanium oxide (TiO2), aluminum oxide (Al2O3), hafnium oxide, zirconium ox‐ ide, niobium oxide, tantalum oxide, tungsten oxide, and their alloys. Among them, the most widely studied anodic oxides are Al2O3 and TiO2. For Al2O3, usually nanoporous structures can be obtained, which can be used as filters and templates. TiO2 nanotube structure was first reported by Zwilling in 1999 by electrochemical anodization of Ti metal in aqueous flu‐ oride containing electrolyte [3, 4]. Recent studies have indicated that the nanotube morphol‐ ogy is in fact converted from nanopores by dissolution of the oxides at the interpore region. Due to the unique properties, TiO2 nanotubes have been utilized in dye-sensitized solar cells (DSSCs), perovskite solar cells, quantum dot solar cells, photocatalysis, batteries, supercapa‐ citors, electrochromic devices, sensors, drug release, and cell differentiation applications. In this chapter, we mainly discuss the fabrication of TiO2 nanotubes and the design and im‐ provement of DSSCs.

The electrochemical anodization is a highly controllable technique. The growth of anodic TiO2 nanostructures can be influenced by many key anodization parameters, such as ap‐ plied voltages, currents, bath temperatures, post treatments, and kinds of electrolyte. The control of the complex oxidation formation and chemical etching of the oxides can be realiz‐ ed during the anodization process, to establish the balance between the oxidation and disso‐ lution. As a result, the morphology, regularity, growth rate, size, length, wall smoothness, and single or double walled nanotubes can be artificially designed. For example, fast growth of nanotubes can be realized in an electrolyte-containing lactic acid, which is desirable for industry production [5]. The nanotubes with lengths ranged from several 100 nm to 1000 μm have been fabricated [6]. Furthermore, by in-situ doping with various non-metal or met‐ al elements in the electrolytes containing additives, visible light response of the anodic ox‐ ides can be obtained. The control of anodic nanostructures will greatly influence their physical and chemical properties, and their performances in various devices.

#### **1.2. Solar cell applications**

The development of low-cost new-generation solar cells has attained broad attention recent‐ ly. Dye-sensitized solar cell (DSSC) is a kind of photoelectrochemical cell with the advantag‐ es of low cost, simple synthesis, large area, and high stability. Since the first report by O'regan and Grätzel with 7.1% efficiency [7], the highest efficiency of above 13% has been achieved [8]. In DSSCs, typically TiO2 nanoparticles are used as the photoanode material, which are coated on transparent conductive oxide (TCO) substrates to form porous net‐ works. The dye sensitizers are adsorbed on the surface of nanoparticles to harvest incident light. The dyes are surrounded by liquid redox electrolytes, which can reduce the oxidized dyes and accept electrons from the counter electrode. In the photon-to-electricity conversion process, the light absorption and the charge transport are two separate processes. The elec‐ tron-hole pairs are separated by the heterojunctions with different work functions. No obvi‐ ous built-in electric field exists in DSSCs, and the electrons go through the TiO2 network by diffusion. Inspired by the development of DSSCs, other types of sensitized solar cells such as quantum dot (QD)/semiconductor sensitized solar cells and perovskite solar cells have been introduced. For QD systems, by adjusting the size of QDs, the band gaps can be tuned [9]. Furthermore, by using QDs, multiple exciton generation effect exists [10]. The perovskite solar cells are a new type of solar cell devices. Due to the high absorption efficiency and broad absorption range of perovskite materials, the efficiency has been above 20% recently [11].

For solar cell applications, the anodic nanostructured materials are usually used to replace TiO2 nanoparticles to fabricate the photoanodes. In DSSCs, the photoanodes mainly play two important roles. First, the nanostructures provide large specific internal surface areas for the anchoring of dyes. Second, the nanostructures provide the charge diffusion path‐ ways to transport the injected charges to the outer circuit. As a result, the morphology, structure, crystal structure, and surface state of anodic nanostructures can determine the performance of DSSCs based on these photoanodes, including loading of dyes, light harvest‐ ing, charge transport, recombination, and collection, and finally power conversion efficien‐ cy.

## **2. Solar cell configuration**

## **2.1. 1D TiO2 nanotubes**

The nanoparticle photoanodes in DSSCs have the randomly distributed sizes, and loosely and irregularly packed structures. As a result, the injected electrons encounter a large amount of nanoparticles (about 103 to 106 as estimated) when diffuse through the photoanodes. The real path for electrons to travel before reaching the substrate is very long. This increases the probability of electron recombination loss at the oxide/electrolyte interface. One-dimensional (1D) nanostructures, such as nanotubes, nanowires, nanofibers, and nanorods, can provide the directional electron diffusion, which shortens the electron pathways, and are proved to have better collection efficiency [12]. By electrochemical anodization, various 1D nanostructures have been developed, and one very important nanostructure is TiO2 nanotubes. The first attempt to use TiO2 nanotubes to replace nanoparticles in DSSCs is reported by Schmuki et al. [13], and afterwards various reports have been emerging. The electron recombination rate in TiO2 nanotubes is found to be very slow, with much larger electron lifetime (more than 10 times) than TiO2 nanoparticles [14]. Thus, anodic TiO2 nanotubes are very promising for high performance solar cells.

#### **2.2. Free-standing membranes**

Usually, the anodic nanotubes are grown on the Ti metal foil substrate, which is opaque. When directly utilizing TiO2 nanotubes on foils in DSSCs, back-side illumination cell configuration is needed [15]. That is to say, the solar light enters the solar cell from the counter electrode. As a result, the light would be reflected by counter electrode that is coated with a thin layer of catalyst, and also be absorbed by dark redox electrolyte before it can be absorbed by dyes loaded on the anodic nanostructures. It has been estimated that about approximately 30–40% light energy would be lost by using this back-side illumination configuration [16]. To realize the front-side light illumination and to fabricate transparent electrode for broad applications, the anodic TiO2 nanotubes should be fabricated on the TCO substrates.

One strategy is the direct growth of TiO2 nanotubes by anodizing the sputtered or thermal evaporated thick Ti metal films on TCO substrates. However, there exist two problems. One is that the synthesis of thick and high-quality metal films on TCO substrate is very difficult, with complex and expensive procedures. The other is that the anodization oxidation process would lead to the increase of the TCO substrate resistance, the weak connection between the anodic oxide films and the substrate, and even the peeling off of the oxide films from the substrate. The low electric conductivity will result in the efficiency loss of solar cells.

The more promising method is to peel the anodic oxide layer off the metal substrate to obtain free-standing membranes, and transfer them onto TCO substrate. Various strategies such as ultrasonication separation, solvent evaporation, selective metal dissolution, and chemicalassisted separation have been proposed to peel off the films, but the procedure is complex and need careful handling. Furthermore, to obtain open bottom structure for effective tube filling and flow through application, usually additional chemical etching steps of tube bottoms are needed.

To obtain high-quality free-standing membranes with simple synthesis procedures and tunable bottom morphologies, the self-detaching method has been proposed [17]. The asformed anodic oxide layer is thermally treated at a certain temperature, and anodized again in the same electrolyte. After a short time, the oxide layer can be peeled off from the substrate without any post-treatment procedure. If the as-formed oxide layer is subjected to heat treatment at a low temperature (e.g., 200°C), the detached layer is amorphous and shows a black color. After annealing, the layer becomes crystallized and transparent (**Figure 1a**). For high temperature-treated layer (e.g., 400°C), the tubes are crystallized in the anatase phase before detachment. As a result, the detached layer is already transparent without subsequent annealing (**Figure 1b**). The detachment is probably because of the difference in mechanical and chemical stability between the top anodic layer (which we try to peel off) and the newly formed layer underneath the top layer [18]. Furthermore, according to the heat treatment temperatures during the detachment, the free-standing oxide layer can have open tube bottoms (200°C, **Figure 1c-f**) or closed bottom ends (400°C). It should also be noted that during the detaching anodization, elevating the bath temperature can facilitate the layer detachment and promote the formation of open bottoms. This elevated temperature can reduce the electrolyte viscosity and enhance the filed-assisted chemical dissolution at the tube bottom. Also, the strategy is a green technology without the use of corrosive solution and ensures continuous production [19].

**Figure 1.** The photographs of (a) the detached 200°C-treated oxide layer with open bottoms before and after annealing and (b) the detached 400°C-treated oxide layer with closed bottoms. SEM images of the detached 200°C-treated oxide layer with (c) the cross-sectional view, (d) the top view, (e) the bottom view showing the open morphology, and (f) the enlarged bottom view. Reprinted with permission from Lin et al. [17].

#### **2.3. Front-side illumination**

The above-mentioned detached free-standing membranes can be used in DSSCs to achieve front-side illumination solar cell configuration, improving the light utilization efficiency. The detached oxide layer is transferred and adhered onto the TCO substrate by a thin layer of TiO2 nanoparticles, and then sintered to enhance the connectivity (**Figure 2a**). For this detach‐ ing and transfer method, the solar cell efficiency is much higher (about 1.75 times) than on foil, due to the improvement of illumination configuration [20]. Furthermore, it has been found that tube bottom morphology also affects the solar cell efficiency. The solar cell based on tubes with open bottoms shows a better performance than that with closed bottoms, with an efficiency improvement of 17.7%. The photographs of the two kinds of photoanodes before annealing can be seen in **Figure 2b**. The removal of the tube bottom and the barrier layer can induce more dye loading of the tubes and less light scattering of the bottom caps. Furthermore, the open bottoms can facilitate the diffusion of redox electrolyte and thus reduce the recom‐ bination probability of electrons with the oxidized ions in the electrolyte [21]. Other solar cell configurations utilizing detached membranes include the bottom down or bottom-up struc‐ tures and combination of nanoparticle/nanotube layers, which can be designed as required. Besides the free-standing films, nanotube powders can also be used to achieve the front-side illumination configuration, which is discussed in Section 3.

**Figure 2.** (a) Scheme of the fabrication procedure of front-side illuminated DSSCs using detached nanotube layers. (b) The photographs of the oxide layers with open and closed bottoms adhered onto FTO substrates. Reprinted with per‐ mission from Lin et al. [20].

## **3. Light scattering**

#### **3.1. Scattering effect**

The widely used dyes in DSSCs commonly have high absorption efficiencies only within a narrow wavelength region, with low response for the red- and near-infrared light. Simply increasing the thickness of photoelectrode film to enhance the absorption of photons will cause the increase of resistance and recombination loss. Therefore, it is essential to develop light scattering structures to increase the photon absorption opportunity by dye molecules in the weak absorption regions. As a common optical phenomenon, light scattering effect could extend the optical traveling length of incident light, so as to improve the light-harvesting efficiency and to achieve high performances. Based on Mie theory, to achieve efficient light scattering, the size of scattering centers should be comparable to the wavelength of the incident light. As a consequence, scattering structures with various morphologies have been intro‐ duced.

In general, there are three different photoelectrode configurations to combine light scatter‐ ing centers. The first one is the mixed structure, for which large particles such as scattering centers are embedded into the photoelectrode films (**Figure 3a**). The scattering centers in such structure could introduce multiple scattering in the light absorption layer. However, the large particles would unavoidably cause the loss of dye adsorption because of the low surface area. The second one is the double layered structure, for which the light scattering layer is placed on the top of the nanocrystalline film (**Figure 3b**). Adding the scattering layer on the top could ensure sufficient dye loading. However, the light scattering would be weaker than in the mixed structure. The third one is the photoelectrode composed of hier‐ archical nanostructures with dual functions (**Figure 3c**). There have been many studies on the fabrication of such hierarchical nanocrystallite aggregates. The intensive light scattering could be guaranteed without much loss of specific surface area, but the preparation process is much more complicated. As a result, to achieve prominent light scattering and thus high DSSC performance, we should overcome the drawbacks of these structures.

**Figure 3.** The schematic and light chart of the light scattering structures.

#### **3.2. Conventional scattering centers**

The large TiO2 particles with diameters of about 200–400 nm are commonly used as light scattering centers, which can be simply mixed into the photoelectrode films to increase the light harvesting efficiency. In the early studies, Grätzel et al. have found that when TiO2 nanoparticles are hydrothermally prepared at 250°C (normally below 230°C), the nanoparti‐ cle film would become translucent because of the spontaneous particle agglomeration [22]. Subsequently, the fabrication of large TiO2 particles with high crystallinity has been paid more and more attention.

Besides nanoparticles, solid spheres fabricated by facile hydrothermal method also exhibit light scattering effect. However, the large particles or spheres usually suffer from low sur‐ face area for dye loading, which would affect the light harvesting. To overcome the weak‐ ness of low surface area, TiO2 spheres with rough surfaces have been synthesized. Also, mesoporous spheres with dual functions have been fabricated by nanocrystallite clustering, which are dominant in light scattering without the loss of surface area for dye-uptake. The hollow spheres are an alternative candidate for light scattering centers because of their larg‐ er surface area and better infiltration of electrolyte. Recently, core-shell microspheres have gradually emerged and become very promising scattering center. The core-shell structure could not only provide large light scattering, but also confine the light inside the spheres.

## **3.3. TiO2 nanotubes powders**

Apart from the above structures, 1D nanostructures could also serve as light scattering cen‐ ters, such as nanorods, nanofibers, nanotubes, and their aggregates. For example, the lightto-electricity conversion yield of 6.08% has been achieved by blending of large TiO2 nanorods (800 nm) and small nanorods (20–40 nm) on the top of small nanorod films, bene‐ fit from the low charge transport resistance and high light scattering effect [23]. TiO2 nano‐ fibers have been readily fabricated by simple electrospinning and applied in DSSCs as photoanode films [24]. The mixture of 1D nanostructures with nanoparticles could make a significant improvement of the performance of DSSCs also because of the combined effect of strong light scattering, abundant dye-loading amounts, and the improvement of electron transport properties.

TiO2 nanotube arrays with 1D nanostructure have been found to have prominent light scat‐ tering effect, with the combination of superior electron transport when applied in DSSCs [14, 25]. Due to the formation of large crystalline grains and existence of nanotube bundles, the TiO2 nanotubes could generate effective light scattering. Lee et al. [26] obtained TiO2 nanotube powders and coated the powders on the top of the nanocrystalline film, and at‐ tained noticeable increment of light harvesting.

The mixed structures of nanotubes and nanoparticles have also been explored in many stud‐ ies. Embedding nanotube powders inside the nanoparticle film can (1) promote the permea‐ tion of liquid electrolyte, (2) increase the electron transport in the photoanode, and (3) introduce the scattering effect of nanotubes, which would be profitable for high perform‐ ance DSSCs. Lin et al. have fabricated TiO2 nanotubes by anodic growth and involved ultra‐ sonic separation of the resulted nanotubes to obtain TiO2 nanotube powders (**Figure 4a**) [27]. To reduce the electron trap states, the nanotube powders are thermally treated at a high an‐ nealing temperature (650°C) to enhance crystallinity (the details are discussed in Section 5). The hybrid photoanodes can be formed by mixing these highly crystallized nanotube pow‐ ders with TiO2 nanoparticles (**Figure 4b**). By adjusting the weight ratio of the nanotube pow‐ ders, the performance of DSSCs could be optimized and the highest efficiency of 6% has been achieved.

**Figure 4.** The schematic of the fabrication procedures of nanotube powders, nanotube electrode, and hybrid electrode. (b, c) The cross-sectional views of the hybrid photoanode with nanotubes embedded in nanoparticles. Reprinted with permission from Lin et al. [27].

#### **3.4. Large nanotubes**

Generally, the diameter of nanotubes fabricated by conventional electrochemical anodization is about 100 nm, which is obviously smaller than the wavelength of visible light. The large diameter nanotubes of size comparable to the visible light wavelength (500 nm or above) can be synthesized under high-voltage anodization. For example, by anodization at a high voltage of 180 V in an organic electrolyte containing lactic acid (aged for 10 hours), large nanotubes with top diameter of 300 nm and bottom diameter of 500 nm have been fabricated (**Figure 5a,b**) [28]. The resulting nanotube membranes are transferred onto a thick TiO2 nanocrystalline film to act as the light scattering layer (**Figure 5c,d**). The large diameter nanotubes show a superior scattering property, and the photoanode incorporated with large nanotubes on top is nearly opaque in visible light (**Figure 5e**). By introducing the large diameter nanotube membranes, the performance of DSSCs has been greatly improved, which is 19% higher than that without scattering layer, and 11% higher than that using normal nanotubes (100 nm) as the scattering layer (**Figure 5f**).

**Figure 5.** SEM images of large diameter nanotubes with (a) the top view and (b) the cross-sectional view. (c) SEM im‐ age and (d) schematic of double-layered photoanode using large nanotubes as the scattering layer. (e) The transmit‐ tance spectra of the photoanodes without scattering layer (TP(3L)), with normal nanotubes (TP(3L) + STNA), and with large nanotubes (TP(3L) + LTNA) as scattering layers. (f) Photocurrent-voltage curves of the DSSCs based on these photoanodes. Reprinted with permission from Liu et al. [28].

To enhance the functionality of the photoanodes of DSSCs, the multi-stacked photoanodes have also been developed by integration of three or more various TiO2 architectures [29, 30]. For TiO2 nanotubes, the similar multi-layered structures have been fabricated (**Figure 6a**), based on the study of large diameter nanotubes [31]. First, bi-layered TiO2 nanotube membrane with top large nanotubes (~540 nm) and underlayer normal nanotubes (~130 nm) has been fabricated by two-step anodization in different kinds of electrolytes (**Figure 6b**). Secondly, the bi-layered membrane is transferred onto a layer of TiO2 nanoparticles for building photoano‐ des. The three layers are stacked together with gradually decreased sizes from top to bottom. For this type of multi-layered photoelectrode, layers with different tailored nanostructures play different roles on the performance of the DSSCs. By optimizing the thickness and synergistic effects of each layer (**Figure 6c**), large amounts of dye adsorption, reduced recombination during the electron diffusion, and efficient light scattering can be simultane‐ ously guaranteed. DSSCs based on the multi-functional photoanode shows a photoelectric efficiency as high as 6.52%.

**Figure 6.** (a) The schematic of the multi-layered photoanode with top layer large diameter nanotubes (LNT), second layer normal nanotubes (SNT), and bottom layer nanoparticles (NP). (b) SEM image of the bilayered nanotube mem‐ brane. (c) The solar cell efficiencies based on multi-layered photoanodes with different layer thicknesses. Reprinted with permission from Liu et al. [31].

## **4. Photonic crystal structures**

#### **4.1. Photonic crystal effect**

As illustrated above, the improvement of light harvesting is very important to enhance the solar cell performances. Besides light scattering centers, the photonic crystals (PCs), including one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) structures, can also be used to enhance the light harvesting. PCs are the optical materials with periodically changed refractive index. By adding PCs on the top of nanocrystalline film, PCs will greatly influence the light behavior inside the photoanode, to reflect the light, which would transmit through the photoanode back to the photoanode if the light is in the band gap of PCs, reabsorbed by dye molecules. The possible mechanisms of PC effect are photon localization, special photon behavior at the band edge, and light reflection.

The PC structures can be fabricated by various strategies, such as self-assembly, lithography, and laser writing. For example, TiO2–SiO2 nanoparticle alternate layers have been deposited onto TiO2 nanoparticle mesoporous layer by spin-coating to act as 1D PCs [32]. The fabrication of such multi-layers, however, needs complex processing procedures. Using soft-lithographic technique, 2D PCs with high periodicity have been produced, to enhance the photocurrent generation in DSSCs [33]. 3D PCs usually consist of opal/inverse opal structures from PS spheres [34–36], which can provide a complete photonic band gap and large enhancement in light harvesting.

#### **4.2. Anodic photonic crystals**

For PC applications, the excellent regularity of anodic nanostructures is required. The porous and tube structures by electrochemical anodization have the inherent 2D periodicity. Due to the high periodicity, porous Al2O3 nanostructures have been applied as 2D PCs for applications such as lasing and light-emitting diode [37, 38]. However, for TiO2 nanotubes, due to the randomness of tube initiation, the regularity is not so satisfactory. To obtain highly ordered arrays, focused ion beam (FIB) sculpting has been used to achieve the patterned metal surface prior to anodization, which can guide the subsequent nanotube growth. This technique has been largely studied by Lu et al. [39], both on Ti and Al metals, and various kinds of patterns have been produced. The regularity can well meet the requirement of PCs, but the main problem is that the fabrication procedure is complex and time-consuming, which is not suitable for large-scale production. By using the so-called two-step or multi-step anodization [40], the dips formed by the first-step anodization on the metal surface can also act as the template to guide the tube growth, improving the tube regularity to a certain extent.

#### **4.3. TiO2 nanotube 3D photonic crystals**

Usually, the anodized TiO2 nanotubes have the smooth tube walls (**Figure 7a**). By the constant voltage anodization in certain electrolytes, the random and spontaneous current oscillation can lead to the formation of small ripples on the tube walls (**Figure 7b**). Inspired by this phenomenon, the fabrication of regular 1D nanostructures along the tube axis has been proposed. For Al2O3, Lee et al. [41, 42] tried to fabricate 3D ordered nanoporous structure by mild and hard anodization in two different electrolytes. Periodic voltage anodization (cyclic anodization or pulse voltage anodization) has also been used to fabricate branched nanostruc‐ tures for PCs [43]. The key point to induce the structure change along the longitudinal direction is the abruption of the established steady growth state of anodic nanostructures. TiO2 possesses higher refractive index (n ~ 2.7) than Al2O3 (n ~ 1.7), more suitable to be used as structural color materials to realize the complete bandgap. Hence, the strategies to induce ordered periodic structure along the TiO2 tube axis have been greatly concerned. The bamboo-type TiO2 nanotubes have been achieved by the periodic or pulse voltage anodization [44], which have certain longitudinal periodicity. However, due to the unstable nanotube growth rate under constant voltage anodization, the periodic voltage can only fabricate the structures with short range regularity. Furthermore, the structural parameters and thus PC characteristics cannot be preciously adjusted.

To realize the strict control of regularity, the periodic structures along the longitudinal direction can be fabricated by periodic current anodization [45]. Because the applied current is directly related to the growth rate of anodic oxide, the steady current can ensure the uniformity of the oxide growth and the control of the interrupt of current can lead to the periodic structures along the axis. During the current pause, the oxide growth stops, but the chemical dissolution continues, resulting in the structural difference on the tube walls (**Figure 7c**). Unlike the smooth or rippled tube wall morphologies, the tube segments can be clearly observed with the concave shaped interfaces. In the range of about 20 periods from top to bottom, the distance between segments (period length) is almost the same, revealing excellent long-range regularity (**Figure 7d**).

**Figure 7.** The cross-sectional SEM image of the periodic structures with (a) smooth, (b) rippled, and (c) periodic tubes with concave interfacial morphologies. (d) The periodic structures along the tube axis with long-range regularity (peri‐ od length: 292 nm). Reprinted with permission from Lin et al. [45].

As discussed above, due to the higher reflective index, TiO2 is more suitable photonic material than Al2O3. By using periodic current-pulse sequences, the photonic features of TiO2 nano‐ tubes, including period length, interfacial morphology, period number, and type of period (periodic, quasiperiodic, or aperiodic), can be precisely and continuously modulated by the anodization parameters. The periodic nanotube films (after detachment) with different period lengths show different colors and transparencies (**Figure 8a**), and corresponding different reflection spectra (**Figure 8b**). Furthermore, the structural color of the nanostructured film is not static. The film with fixed period structure shows different colors with different light incident and viewing angles (**Figure 8c**).

**Figure 8.** The optical images of the colorful films, from purple to red. (b) The reflection spectra of the colorful films, showing the bandgaps. (c) The change of film colors with different incident and viewing angles and the flexibility of the colorful films. Reprinted with permission from Lin et al. [45].

TiO2 nanotube PCs have been coupled directly to nanotube layers in DSSCs by a single step [46]. Also the thin PC membrane can be placed on the top of nanocrystalline TiO2 layers as semi-transparent photoanode for DSSCs [47]. During the design of the solar cells, we should consider the match between the bandgap of PC structures and the incident light spectrum, to maximize the solar cell light harvesting. Thus, the different PCs with different band structures are designed and coupled to DSSCs. When using N719 dye as light absorber, it is found that DSSCs coupled with 150 nm periodic structure show the best performance. The advantages of the strategy using the above 3D nanotube PCs in solar cells are (1) tunable photonic structure, (2) controllable fabrication process, (3) easy integration by direct fabrication of both light absorbing layer and PC layer tubes by a single-step anodization or by membrane transfer, (4) tight mechanical and electrical connection between the light absorbing layer and PC layer, facilitating the charge transport, (5) interconnected two layers at the interface region, facilitat‐ ing the electrolyte infiltration, and (6) easy fabrication of large area, transparent, and flexible PC films.

## **5. Electron properties**

#### **5.1. Substrate effect**

In DSSCs, the widely used model describing the diffusion process in the TiO2 electrode is the multiple trapping model. During the electron transport, the electron undergoes the trapping and detrapping processes by the trap states. To enhance the charge collection and thus efficiency, the fast transport and slow recombination of electrons are required. It has been reported that the order of the nanomaterials can greatly influence the electron property. For disordered nanoparticles, there exist numerous particle–particle interfaces. While for nano‐ tubes, electrons transport along the tube axis, which would lead to higher electron transport rate [48, 49]. However, in anodic tubes, the transport is not as fast as expected. Various studies have been devoted to investigating the possible underlying mechanism. The widely accepted view is that there exist a large amount of trap states in tubes, as compared with nanoparticles, which suppress the electron transport [14, 50, 51]. The trap states, usually oxygen vacancies or Ti3+ ions, are most likely originated from non-crystallized amorphous regions, grain bounda‐ ries, and the impurities induced during the anodization process.

The improvement of tube crystallinity by high temperature annealing is supposed to be useful for the reduction of the trap densities. However, usually the substrate effect exists during annealing of anodic oxides on metal substrate. That is to say, the metal substrate can greatly affect the crystal phase and nanostructure of the oxide film layer during the annealing process. For TiO2 nanotubes on Ti, rutile phase can be detected at a low annealing temperature of 400– 450°C. Furthermore, the nanotubes are gradually condensed at high temperatures, and finally the porous structure is fully destroyed at about 700–800°C. This is due to the fact that the Ti foil can be directly oxided to the rutile phase during high temperature annealing, forming a thin and compact oxide layer at the oxide/metal interface region, and gradually becomes thicker [52]. This rutile layer will initiate the crystal phase and structure transformation of the upper TiO2 layer, from the bottom to the top. The substrate effect exists at different annealing conditions, and even more severe when the annealing temperature increases [53–55]. Thus, anodic oxide films on metal foils can only be crystallized at a relatively low annealing temperature for DSSC applications. The high temperature annealing would cause the destruc‐ tion of nanostructure and increase of resistance, both of which deteriorate the solar cell performance.

#### **5.2. Highly crystallized nanotubes**

To realize the high temperature annealing and thus highly crystallized TiO2 nanotubes for DSSCs, the substrate effect should be eliminated. This can be realized by annealing of freestanding TiO2 nanotube membranes before the attachment of the membrane onto the TCO substrate. For membranes without metal substrate, the substrate effect does not exist, and the crystallization behavior is completely different. There have been several attempts to anneal membranes at high temperatures [56–58]. The initiation temperature of phase transformation and structure densification becomes much higher. The main problem is that during the high temperature annealing, the membrane is inclined to curling and cracking, due to the low quality of the membranes fabricated by various strategies. By optimizing the self-detaching anodization process as discussed in Section 2, high-quality TiO2 nanotube membranes can be obtained. The membranes can withstand the high temperature annealing up to 700°C and the hollow, porous, and ordered structure is maintained (**Figure 9a–e**) [59], although the crystal‐ lites in the tube walls gradually become larger (**Figure 9f**).

**Figure 9.** TEM images of tube membranes annealed at (a) 400, (b) 500, (c) 600, (d) 700, and (e) 800°C. (f) HRTEM view of the grain in the tube walls annealed at 700°C. Reprinted with permission from Lin et al. [59].

The TiO2 nanotube membranes with high crystallinity have been used as the photoanode in DSSCs (**Figure 10a**). The electron transport is found to be enhanced significantly because of the reduction of the impurity and defects and thus the electron trap states. As a result, the electron diffusion length is much longer in the highly crystallized nanotube membranes (**Figure 10b**). Despite the lower surface area and thus lower dye loading amount, a significantly improved solar cell efficiency of 7.81% has been obtained for 700°C annealed sample. The high temperature annealing for enhancement of crystallinity can also be used to fabricate flexible DSSCs. For such type of solar cells, the high temperature annealing is also not applicable because the flexible substrate (usually PET or PEN) cannot withstand high temperatures. For membranes, the high temperature annealing is completed before the transfer of nanotubes to the flexible substrate. Thus, the conductivity and quality of flexible substrate are not influenced by the high temperature annealing process. The only problem is how to keep the membrane adhered tightly to the flexible substrate, ensuring the electron transport pathways [60].

**Figure 10.** (a) The photographs of photoanodes consisting nanotube membranes annealed at 400 and 700°C. (b) The variation of diffusion length in DSSCs as a function of annealing temperature. Reprinted with permission from Lin et al. [59].

## **6. Surface areas**

#### **6.1. Small size nanotubes**

Anodic TiO2 nanotubes usually have low specific surface areas (BET surface area ~20 m2 /g), as compared with other nanomaterials. For normal hexagonal closely packed nanotube arrays, due to the compact and ordered structure, large tube size, and smooth tube wall, the internal surface area is limited. When the nanotubes are applied in DSSCs as the photoanode material, the light cannot be fully absorbed, leading to low light harvesting efficiency. To increase the surface area, the most common strategy is the decoration of small diameter nanoparticle on the hollow tube inner or outer surface by TiCl4 treatment or immersion filling, to fabricate tube/ particle mixed structure. The synthesis of bi-layered structure consisting of both tube layer and nanoparticle layer, double-walled nanotubes, and bamboo type tubes with rings or ripples on tube outer walls are all benefit for the improvement of surface areas.

Fabrication of small diameter but large thickness tube layer is another strategy to increase the surface area. Typically the small diameter tubes are very short [61], and thus the surface area per electrode area is still very low. The attempt has been focused on the fabrication of high aspect ratio tubes with small diameters and large thicknesses, increasing the dye loading amount per electrode area. In usual anodization conditions, it has been found that the outer diameter of TiO2 nanotubes is proportional to the anodization voltage within a certain range, but inversely proportional to the bath temperature. By anodization at a low voltage and a high temperature, the diameter can be greatly reduced. When the bath temperature increases from 20 to 50°C, the outer diameter can be decreased from 93 to 75 nm [62]. On the other hand, the high bath temperature can promote the growth of anodic nanotubes, leading to the small tubes with large thicknesses, which can provide enough surface area for dye anchoring.

#### **6.2. Water immersion treatment**

Besides the in-situ anodization strategies, post-treatment method can also be used to tune the geometry and structure of nanotubes and guarantee the sufficient surface area for dye adsorption. By simple water immersion of as-grown TiO2 nanotubes at room temperature (about 1–3 days) or hot water immersion, the tube wall morphology can be changed in a controllable way [63]. This is only useful for as-formed tubes before annealing, which are amorphous. The water treatment leads to the formation of hybrid-walled tubes with outer wall unchanged (smooth tubes) while inner wall converted to small nanoparticles (**Figure 11a,b**). The TEM image clearly shows that in the hybrid structure, the tube outer wall consists elongurated nanocrystallites along the tube axis with lengths of dozens of nanometers to several hundred nanometers (**Figure 11c**). The small nanoparticles existing in the inner wall have the average crystal size of about 11 nm. The hybrid structure of the tube wall after water treatment appears to be similar to the filling or decoration of tube inner part with small nanoparticles. But it is apparently different than after water treatment, the solid tube wall appears to become thinner. This can be explained by the fact that in hybrid tubes, the nano‐ particles in the inner wall are, in fact, converted from the tube wall. The formation of particles consumes the tube wall.

**Figure 11.** (a) The cross-sectional SEM images of the hybrid tubes by water immersion treatment. The insert is the top view. (b, c) TEM images of the hybrid structure consisting tubes and particles. Reprinted with permission from Lin et al. [63].

During the water immersion process, the inner wall is inclined to be converted to nanoparticles. Normally the tube wall fabricated in organic electrolyte consists of inner and outer shells. The inner shell contains a large amount of carbon, which is originated from the anodization electrolyte in the anodization process [64]. Due to the different chemical compositions of the two shells, the inner walls are more easily to be converted by water to nanoparticles. This is a simple but effective way to enhance the tube roughness. Also along with the structure change, partial phase transition of nanotubes from amorphous to anatase has been observed. But if the tubes are annealed (>200°C), the tubes are stable, and the water immersion cannot cause any structural change even for a long time. It has been reported that the TiO2 dissolution/precipitate process in water would cause the rearrangement of the construction unit of TiO2, leading to the spontaneous variation of crystal structure and morphology [65, 66].

As discussed above, the conversion of the inner tube wall to nanoparticles can enhance the tube roughness and lead to the full utilization of the tube hollow space, increasing the internal surface area of anodic nanotubes, while keeping the tubular morphology unchanged. That is to say, the unique properties of tubes can be maintained, such as more convenient dye adsorption and electrolyte infiltration, short electron diffusion path, and slow electron recombination. After the subsequent annealing, the hybrid nanostructures are applied in DSSCs. The BET surface area increases with increasing immersion time. Before treatment, the BET surface area is about 20.1 m2 /g, and increases to 39.9 and 42.7 m2 /g after 2 and 3 days immersion, respectively. The 3 days water treated sample has a BET surface area about 2.1 times larger than normal tubes. Accordingly, the photoanode dye loading amount increases by 38.9% and 57.8% as compared with normal tubes. An optimized efficiency of 6.06% is obtained, improved by about 33%. Due to the maintenance of tubular structure, the water treatments do not affect the electron recombination property. This is beneficial for the im‐ provement of the DSSC efficiency as compared with previous report that the nanotubes decorated with nanoparticles showed decreased electron lifetimes [67, 68].

## **7. Conclusion remarks**

This chapter discusses about the fabrication of anodic nanostructures by electrochemical anodization and their application in dye-sensitized solar cells to enhance the power conversion efficiency. The solar cell configurations can be optimized by free-standing anodic membranes to maximize the light utilization. By using large-sized nanotubes, due to the light scattering effect, the full interaction of the incident light with the absorbing layer can enhance the light harvesting of the solar cells. For periodic structures, the interface of the period is typically voids, resulting in the required periodic modulation of the refractive index for photonic crystals, which show the tunable photonic bandgaps, a property very attractive for light harvesting. By fabrication of high-quality membranes, highly crystallized nanotube membrane can be obtained, which can provide superior electron collection properties in solar cells. The surface areas of nanotubes can be increased by using high-aspect-ratio nanotubes and tube wall modification. The solar cells equipped with the proposed anodic nanostructures are expected to show excellent device performances, valuable for practical applications.

## **Author details**

Jia Lin, Xiaolin Liu, Shu Zhu and Xianfeng Chen\* \*Address all correspondence to: xfchen@sjtu.edu.cn Shanghai Jiao Tong University, Shanghai, China

## **References**

