**1. Introduction**

As a photosensitive semiconductor material with good long-term stability, nontoxicity, low cost and abundance, TiO2 has been widely used in photocatalysis and photovoltaics [1]. However, due to the wide band gap (i.e., 3.0 eV for rutile and 3.2 eV for anatase TiO2 , respectively), pristine TiO2 only responds to the irradiation in UV region.It is inefficient for capturing the majority

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of photons lying in the visible range of the normal solar irradiation spectrum. Decorating visible-light-excitable compounds, so-called photo-sensitizers or dyes, on TiO2 can effectively overcome this issue, which has been adopted to develop dye-sensitized solar cells (DSSCs) [2].

The operation principle of DSSCs is illustrated in **Figure 1**. Organic dyes anchored on TiO2 surface are excited by absorbing visible light in the specific wavelength range. The charge separation occurs at the sensitizer/TiO2 interface by injecting electrons from the excited state of the dye into the TiO2 conduction band to generate free electrons, which are then diffused through the sintered TiO2 nanoparticle layer and external circuit to the cathode to generate a photocurrent. Concurrently, the oxidized dye is reduced to its ground state by oxidation of the redox mediator I¯ into I3 ¯, with I3 ¯ ions then diffusing through the electrolyte to the cathode and reduced back to I¯ by accepting electrons combing back from the external circuit to complete the whole regeneration process. Overall, this system converts solar energy into electricity without any net consumption of chemicals, and thus, the DSSC can continuous supply power under irradiation by sun light. In 2011, a porphyrin sensitized DSSC incorporated with CoII/III tris(bipyridyl) redox electrolyte achieved a record-high power conversion efficiency (PCE) of 12.3% [4]. Recently, a new emerging perovskite solar cell using solid-state mesoscopic TiO2 photoanode sensitized with lead halide perovskite (CH<sup>3</sup> NH3 PbX3 ) was reported to achieve an exciting PCE of more than 15% [5], and it quickly approached to 20% by posttreatment of mesoporous TiO2 photoanodes with lithium salts [6].

The prototype of DSSC is analogue to the photosynthesis in plants taking place at thylakoid membrane in the chloroplast composed of various photosynthetic proteins. **Figure 2** depicts the light-dependent reactions in the photosynthesis. Briefly, solar energy is absorbed by exciting the light harvesting complexes (i.e., LHCI and LHCII), a kind of proteins binding a lot of chlorophylls (Chls) as major pigments in the photosystems (PSI and PSII). The excitation

**Figure 1.** Structure and operating mechanism of a DSSC. Reprinted with permission of Ref. [3].

TiO2: A Critical Interfacial Material for Incorporating Photosynthetic Protein Complexes and Plasmonic... http://dx.doi.org/10.5772/intechopen.68744 209

of photons lying in the visible range of the normal solar irradiation spectrum. Decorating visible-light-excitable compounds, so-called photo-sensitizers or dyes, on TiO2 can effectively overcome this issue, which has been adopted to develop dye-sensitized solar cells (DSSCs) [2].

The operation principle of DSSCs is illustrated in **Figure 1**. Organic dyes anchored on TiO2 surface are excited by absorbing visible light in the specific wavelength range. The charge

photocurrent. Concurrently, the oxidized dye is reduced to its ground state by oxidation of

ode and reduced back to I¯ by accepting electrons combing back from the external circuit to complete the whole regeneration process. Overall, this system converts solar energy into electricity without any net consumption of chemicals, and thus, the DSSC can continuous supply power under irradiation by sun light. In 2011, a porphyrin sensitized DSSC incorporated with CoII/III tris(bipyridyl) redox electrolyte achieved a record-high power conversion efficiency (PCE) of 12.3% [4]. Recently, a new emerging perovskite solar cell using solid-state meso-

to achieve an exciting PCE of more than 15% [5], and it quickly approached to 20% by post-

The prototype of DSSC is analogue to the photosynthesis in plants taking place at thylakoid membrane in the chloroplast composed of various photosynthetic proteins. **Figure 2** depicts the light-dependent reactions in the photosynthesis. Briefly, solar energy is absorbed by exciting the light harvesting complexes (i.e., LHCI and LHCII), a kind of proteins binding a lot of chlorophylls (Chls) as major pigments in the photosystems (PSI and PSII). The excitation

photoanodes with lithium salts [6].

photoanode sensitized with lead halide perovskite (CH<sup>3</sup>

**Figure 1.** Structure and operating mechanism of a DSSC. Reprinted with permission of Ref. [3].

¯, with I3

interface by injecting electrons from the excited state

¯ ions then diffusing through the electrolyte to the cath-

NH3 PbX3

) was reported

conduction band to generate free electrons, which are then diffused

nanoparticle layer and external circuit to the cathode to generate a

separation occurs at the sensitizer/TiO2

of the dye into the TiO2

208 Application of Titanium Dioxide

scopic TiO2

through the sintered TiO2

the redox mediator I¯ into I3

treatment of mesoporous TiO2

**Figure 2.** Photoreactions in photosynthesis at the thylakoid membrane of plant cells. Inset: the energy flow in PSII and the structure of LHCII trimer. Adapted with permission of Refs. [7, 8].

energy is resonantly transferred to the associated reaction centers (RCs) where the energy is converted into electrons by exciting a special pair of Chls, triggering a series of chemical reactions, such as water splitting in PSII, reduction of NADP<sup>+</sup> to NADPH in PSI and ATP synthesis. Inset of **Figure 2** shows components of PSII and the energy flow therein. The largest PSII supercomplex, C<sup>2</sup> S2 M2 , consists of a dimeric core complex (C<sup>2</sup> ) containing RCs, 4 monomeric minor antenna complexes (CP29), 4 strongly attached LHCII trimers (S<sup>2</sup> and M2 ), and 3–4 loosely attached LHCII trimers [9]. LHCII trimer is the most abundant Chl-protein complex in nature and the major antenna complex in PSII. The LHCII trimer consists of three monomers each of which comprises a polypeptide of about 232 amino-acid residues, 8 Chl *a* and Chl *b* molecules, 3–4 carotenoids and one phospholipid [10].

It should be noted that both photosynthesis systems and DSSCs utilize separate media for photon capture and energy transfer (executed by the excitation of LHCs and photosensitizers, respectively) and charge separation (occurs in RCs and the dye/TiO<sup>2</sup> interface, respectively). This mechanism has an advantage to reduce the possibility of charge recombination [11]. Owing to such similarity, various DSC architectures have been explored to directly use natural extracted pigments [12, 13] and photosynthetic LHCs [14–17] as photosensitizers to replace synthetic dyes in developing biophotovoltaic cells. Although the biophotovoltaic cells have much lower PCE than normal DSSCs, these hybrid systems serve as a unique platform to study the crucial processes including charge separation and transport at the interface of TiO2 photoanode with natural photosensitizers and provide insights into the limiting factors. In this chapter, we will summarize our recent research and other related literature on incorporating photosynthetic proteins and plasmonic nanoparticles (PNPs) onto anatase TiO2 photoanodes as a means to tap into the charge separation, electron and energy transfer processes, and plasmonic enhancement in the biophotovoltaics. The following aspects will be involved: (i) configuration and surface modification of TiO<sup>2</sup> photoanode, (ii) energy state coupling and charge transfer between photosynthetic proteins and TiO2 , (iii) plasmonic effects on biophotovoltaics, and (iv) hot electrons across Schottky barrier at Au/TiO<sup>2</sup> interface.

#### **2. Configuration and surface modification of TiO<sup>2</sup> photoanode**

#### **2.1. Fabrication of TiO<sup>2</sup> photoanode**

Conventional photoanode in DSSCs is composed of a 10-µm-thick nanostructured TiO<sup>2</sup> film prepared from deposition and sintering of spherical TiO2 nanoparticles (NPs) on conducting fluorine doped tin oxide (FTO) glass. This mesoporous layer has a large surface area for dye adsorption while maintaining a percolation network for electron transport. Later, a variety of TiO2 nanorods, nanowires and networks were designed to replace the spherical NPs in photoanode [18–20]. These structures are regarded with more efficient electron transport pathway owing to the inherent well-aligned crystalline domains and the greater electron diffusion length [21, 22]. Typically, the TiO2 nanomaterials are synthesized by sol-gel method, then dispersed with surfactants into a paste and coated on FTO glass via doctor-blade casting, spin coating or screen printing. This multi-step route is tedious and induces large variances in prepared TiO2 layers. The electron diffusion in the layer is restrained by large boundary. Instead, direct growth of highly ordered architectures on substrates is among the most exciting developments for novel photoanodes. Vertically aligned TiO2 nanotube arrays have been successfully produced by potentiostatic anodization of titanium metal in a fluoride containing electrolyte and exhibit larger electron diffusion length [23]. A forest-like photoanode combining efficient light trapping and high surface area for dye absorption was synthesized via fine control of pulse laser deposition, which consists of hierarchical assemblies of nanocrystalline particles of anatase TiO2 [24]. This hierarchical architecture was demonstrated to suppress electron recombination with tri-iodide along with increase of electron lifetime and perform no hindering in mass transport using ionic liquid electrolyte. Recently, we employed a similar anatase TiO2 nanotree array as photoanode scaffold for the LHCII sensitized biophotovoltaic cells [25]. This TiO2 nanotree array can be simply grown on TiO2 coated FTO glass by one-pot hydrothermal reaction without necessity of high-tech equipment [26]. The transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images in **Figure 3** confirm the morphology of the TiO2 nanotrees prepared by this method. Each nanotree is composed of 6-µm-long TiO<sup>2</sup> nanowire trunk covered by short and thinner branches extending sideway. The growing model is illustrated in **Figure 4**, showing a hierarchical assembly of TiO2 nanostructure in which the number and length of branches on the TiO2 nanowire trunk can be increased with the longer reaction time.

TiO2: A Critical Interfacial Material for Incorporating Photosynthetic Protein Complexes and Plasmonic... http://dx.doi.org/10.5772/intechopen.68744 211

to study the crucial processes including charge separation and transport at the interface of

toanodes as a means to tap into the charge separation, electron and energy transfer processes, and plasmonic enhancement in the biophotovoltaics. The following aspects will be involved:

Conventional photoanode in DSSCs is composed of a 10-µm-thick nanostructured TiO<sup>2</sup>

ing fluorine doped tin oxide (FTO) glass. This mesoporous layer has a large surface area for dye adsorption while maintaining a percolation network for electron transport. Later, a vari-

in photoanode [18–20]. These structures are regarded with more efficient electron transport pathway owing to the inherent well-aligned crystalline domains and the greater electron dif-

then dispersed with surfactants into a paste and coated on FTO glass via doctor-blade casting, spin coating or screen printing. This multi-step route is tedious and induces large variances

Instead, direct growth of highly ordered architectures on substrates is among the most excit-

successfully produced by potentiostatic anodization of titanium metal in a fluoride containing electrolyte and exhibit larger electron diffusion length [23]. A forest-like photoanode combining efficient light trapping and high surface area for dye absorption was synthesized via fine control of pulse laser deposition, which consists of hierarchical assemblies of nanocrystalline

electron recombination with tri-iodide along with increase of electron lifetime and perform no hindering in mass transport using ionic liquid electrolyte. Recently, we employed a similar

hydrothermal reaction without necessity of high-tech equipment [26]. The transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images in **Figure 3** confirm

nanotree array can be simply grown on TiO2

The growing model is illustrated in **Figure 4**, showing a hierarchical assembly of TiO2

structure in which the number and length of branches on the TiO2

nanotree array as photoanode scaffold for the LHCII sensitized biophotovoltaic

nanorods, nanowires and networks were designed to replace the spherical NPs

layers. The electron diffusion in the layer is restrained by large boundary.

[24]. This hierarchical architecture was demonstrated to suppress

nanotrees prepared by this method. Each nanotree is composed

nanowire trunk covered by short and thinner branches extending sideway.

(i) configuration and surface modification of TiO<sup>2</sup>

charge transfer between photosynthetic proteins and TiO2

tovoltaics, and (iv) hot electrons across Schottky barrier at Au/TiO<sup>2</sup>

**2. Configuration and surface modification of TiO<sup>2</sup>**

 **photoanode**

prepared from deposition and sintering of spherical TiO2

ing developments for novel photoanodes. Vertically aligned TiO2

fusion length [21, 22]. Typically, the TiO2

 photoanode with natural photosensitizers and provide insights into the limiting factors. In this chapter, we will summarize our recent research and other related literature on incorporating photosynthetic proteins and plasmonic nanoparticles (PNPs) onto anatase TiO2

pho-

film

photoanode, (ii) energy state coupling and

interface.

 **photoanode**

nanomaterials are synthesized by sol-gel method,

, (iii) plasmonic effects on biopho-

nanoparticles (NPs) on conduct-

nanotube arrays have been

coated FTO glass by one-pot

nanowire trunk can be

nano-

TiO2

210 Application of Titanium Dioxide

**2.1. Fabrication of TiO<sup>2</sup>**

ety of TiO2

in prepared TiO2

particles of anatase TiO2

the morphology of the TiO2

increased with the longer reaction time.

anatase TiO2

cells [25]. This TiO2

of 6-µm-long TiO<sup>2</sup>

**Figure 3.** (A) TEM image of the TiO<sup>2</sup> nanotree scraped off from the FTO substrate and (B) the cross-sectional view of TiO<sup>2</sup> nanotree array by SEM. Reprinted with permission of Ref. [27].

In addition, post-synthesis thermal treatment can be applied to attain highly crystalline TiO<sup>2</sup> photoanodes. Due to higher dye loading and faster electron transport rate, the TiO2 with pure anatase phase is more favorable than rutile phase in photoanode applications [28]. The crystallization strongly depends on annealing process. It normally yields the anatase phase if the annealing temperature is below 550°C but tends to form the thermodynamically stable rutile phase at higher temperatures [29]. The XRD (**Figure 5A**) and Raman (**Figure 5B**) characterizations confirmed that single crystalline anatase phase was attained for the TiO<sup>2</sup> nanotrees subjected to 500 °C calcination for 30 min.

**Figure 4.** Schematic formation process of the hierarchical anatase TiO<sup>2</sup> nanotree arrays on FTO substrates. Reprinted with permission of Ref. [26].

**Figure 5.** (A) XRD pattern and (B) Raman spectrum of the TiO<sup>2</sup> nanotrees after 500 °C thermal annealing. The standard XRD peak position of anatase TiO2 (JCPDS card No 71–1166) is indicated as the vertical red lines in (A). Reprinted with permission of Ref. [26].

#### **2.2. TiO<sup>2</sup> barrier layer**

In the photoanode of DSSCs, besides using TiO<sup>2</sup> for charge separation and electron transport, a thin compact TiO2 layer of tens to hundreds of nanometers is usually deposited between mesoporous TiO2 nanoparticle film and transparent conductive oxide (TCO) coated glass as a barrier layer. This barrier layer was found to be critical in impairing the electron backflow at the TCO/electrolyte interface, increasing the shunt resistance, and therefore increasing the fill factor and overall cell efficiency [30, 31], as depicted in **Figure 6A**.

**Figure 6.** (A) Charge recombinations in DSSC due to the electron back flow from TCO to oxidized dye and redox electrolyte. (B) Illustration of the differences of the TiO<sup>2</sup> barrier layers formed by (a) TiCl<sup>4</sup> treatment and (b) Ti sputtering followed by thermal annealing. Adapted with permission of Ref. [32].

To establish a reliable planar photoanode for biophotovoltaics, we systematically studied the TiO2 barrier layer deposited by two distinct methods and correlated the TiO2 structure with its barrier properties [32]. **Figure 6B** schematically shows that a porous dye-penetrable TiO2 film was attained by the TiCl<sup>4</sup> solution treatment, while a conformal compact TiO2 film was obtained by sputtering-annealing Ti. The latter seemed to be an ideal barrier layer since dye molecules can only adsorb on the external TiO2 surface. However, the performance of DSSCs made with the sputtering-annealing method was worse than those by TiCl<sup>4</sup> treatments due to the lower electrical conductivity since anatase structure is mixed with amorphous and rutile phases in the film. In this work, the DSSCs fabricated with photoanodes by 20 min TiCl<sup>4</sup> treatment showed the best performance, likely due to the formation of desired anatase crystallites with the optimum thickness. Such thin-film DSSC was used as a model system to test the photovoltaic effects of photosynthetic proteins that cannot easily access the interior pores of traditional mesoporous DSSCs.

#### **2.3. Biosensitizations on surface modified TiO<sup>2</sup> photoanodes**

**2.2. TiO<sup>2</sup>**

 **barrier layer**

XRD peak position of anatase TiO2

a thin compact TiO2

mesoporous TiO2

permission of Ref. [26].

212 Application of Titanium Dioxide

In the photoanode of DSSCs, besides using TiO<sup>2</sup>

**Figure 5.** (A) XRD pattern and (B) Raman spectrum of the TiO<sup>2</sup>

electrolyte. (B) Illustration of the differences of the TiO<sup>2</sup>

followed by thermal annealing. Adapted with permission of Ref. [32].

factor and overall cell efficiency [30, 31], as depicted in **Figure 6A**.

for charge separation and electron transport,

nanotrees after 500 °C thermal annealing. The standard

treatment and (b) Ti sputtering

layer of tens to hundreds of nanometers is usually deposited between

(JCPDS card No 71–1166) is indicated as the vertical red lines in (A). Reprinted with

nanoparticle film and transparent conductive oxide (TCO) coated glass as a

barrier layer. This barrier layer was found to be critical in impairing the electron backflow at the TCO/electrolyte interface, increasing the shunt resistance, and therefore increasing the fill

**Figure 6.** (A) Charge recombinations in DSSC due to the electron back flow from TCO to oxidized dye and redox

barrier layers formed by (a) TiCl<sup>4</sup>

One complication to fabricate biophotovoltaic devices is to integrate biophotosensitizers with artificial semiconductors in photoanode. Unlike synthetic dyes that can easily use various anchoring groups, for example, carboxylate (─COOH), phosphonate (─H2 PO3 ) or siloxy moiety (─O─SiR<sup>3</sup> ), through molecular engineering to increase binding affinity to metal oxide semiconductor [33, 34], chemical modifications on natural extracted photosynthetic protein complexes would cause unfavorable structural changes that impair their intrinsic photoelectric properties. Although the photosynthetic protein complexes contain carboxylate groups in their polypeptide matrix, they are usually extracted and dispersed in aqueous buffer solution in which the strong polar water solvent and the surfactants tend to break the adsorption equilibrium causing desorption from the metal oxide surface. Addition of binding agents is desired to conjugate photosynthetic protein complexes to artificial photoanodes. Mershin et al. bioengineered PSI with a designed peptide surfactant that contains an amino acid sequence with specific high binding affinity to ZnO (**Figure 7**) [35]. In the study, the native electron acceptor subunit PsaE within PSI was substituted with the ZnO-binding peptide tag: RSNTRMTARQHRSANHKSTQRARS to promote attachment and orientation of the PSI on ZnO nanowires. Thus, the modified PSI was preferentially bound to ZnO nanowires by the electron acceptor side, minimizing the electron traveling distance between electron acceptor and electrode and maximizing the electron transfer.

Beyond introducing the specific linkers through delicate bioengineering on photosynthetic protein complexes, another simpler method to improve the protein attachment is to perform surface modifications on photoanode materials with binding molecules. Dihydroxyacetone phosphate was reported as a suitable linker between PSI and metal oxide. The indium-tin oxide (ITO) and titanium suboxide (TiO*<sup>x</sup>* , *x* = 1, 2) substrates covered with a self-assembled monolayer of dihydroxyacetone phosphate can immobilize a densely packed PSI layer by electrostatic and hydrogen bond interactions with the polar stroma and lumen faces of PSI [36]. LHCII of PSII can also be isolated and appointed as photosensitizers in biophotovoltaic cells. However, the physisorption of LHCII on the TiO<sup>2</sup> photoanode was found to be very weak and unstable, as indicated by the long incubation time (96 hours) required to reach saturated adsorption [45]. It was recently reported that strong LHCII attachment can be obtained

**Figure 7.** (a) Bioengineered modification of PS I by substitution of native PsaE with PsaE-ZnO. (b) Schematic electron transfer path in PS I-based biophotovoltaic cells. Adapted with permission of Ref. [35].

on a APTES grafted FTO substrate via electrostatic interaction between the anionic residues on the stromal side with cationic ─NH3 + groups [14]. We adopted this approach and used APTES to functionalize the surface of TiO<sup>2</sup> thin film, TiO<sup>2</sup> nanotree array photoanodes and TiO2 -encapsulated plasmonic NPs in LHCII sensitized solar cells [25]. Clear improvement in protein attachment is indicated by the more intense and uniform greenish color on the APTES modified photoanode (**Figure 8a**).

Moreover, adsorption of the photosynthetic proteins onto internal surface of the mesoporous TiO2 anode is also hindered by its much larger size (4–20 nm) than dye molecules (<1 nm), researchers strive to increase their loading capacity by engineering more open three-dimensional (3D) electrode architecture. The amount of the adsorbed proteins can be extracted into buffer solutions and quantitatively assessed from the absorption spectra of the extracted Chls in **Figure 8b** based on the following equation [37]:

$$\text{Chlsa}(a+b) = 17.6 \text{ A}^{\ast 4k\text{\'\'\text{s}}} + 7.34 \text{ A}^{\ast 63\text{\'\'\text{s}}} \tag{1}$$

 A is the absorbance at certain wavelength. By this means, the amount of LHCII trimers adsorbed on APTES-treated TiO<sup>2</sup> nanotrees was determined to be 2.5 folds of that on bare TiO<sup>2</sup> nanotrees. It can be derived from basic calculations that LHCII trimers containing 0.2 µg Chls are required to form a hexagonal close-packed monolayer on a flat 1 × 1 cm<sup>2</sup>surface. Since the adsorbed LHCII trimers were equivalent to 6.6 µg Chls, 33 times of that on the flat TiO<sup>2</sup> surface, it is evident that nanoscale LHCII trimers were able to penetrate into the 3D TiO<sup>2</sup> nanotree array and adsorb on a large surface area.

TiO2: A Critical Interfacial Material for Incorporating Photosynthetic Protein Complexes and Plasmonic... http://dx.doi.org/10.5772/intechopen.68744 215

**Figure 8.** (a) The digital photographs of the LHCII-sensitized region of an APTES-treated TiO<sup>2</sup> nanotree array (top) and a bare TiO2 nanotree array (bottom) (b) UV-Vis absorption of the chlorophylls pigment extracted from the LHCII trimers adsorbed on TiO2 nanotree photoanodes with and without APTES functionalization. Reprinted with permission of Ref. [25].
