**3. Energy state coupling and charge transfer between photosynthetic proteins and TiO<sup>2</sup>**

on a APTES grafted FTO substrate via electrostatic interaction between the anionic residues

**Figure 7.** (a) Bioengineered modification of PS I by substitution of native PsaE with PsaE-ZnO. (b) Schematic electron

Moreover, adsorption of the photosynthetic proteins onto internal surface of the mesoporous

 Chlsa(*a* + *b*) = 17.6 A646.6 + 7.34 A663.6 (1) A is the absorbance at certain wavelength. By this means, the amount of LHCII trimers

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>

face, it is evident that nanoscale LHCII trimers were able to penetrate into the 3D TiO<sup>2</sup>

thin film, TiO<sup>2</sup>


 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

groups [14]. We adopted this approach and used

nanotrees was determined to be 2.5 folds of that on bare TiO<sup>2</sup>

nanotree array photoanodes and

sur-

nanotree

+

transfer path in PS I-based biophotovoltaic cells. Adapted with permission of Ref. [35].

on the stromal side with cationic ─NH3

modified photoanode (**Figure 8a**).

adsorbed on APTES-treated TiO<sup>2</sup>

array and adsorb on a large surface area.

TiO2

214 Application of Titanium Dioxide

TiO2

APTES to functionalize the surface of TiO<sup>2</sup>

in **Figure 8b** based on the following equation [37]:

#### **3.1. General electron transfer in the biophotovoltaic cells with photosynthetic protein sensitized TiO<sup>2</sup> photoanodes**

Similar to DSSCs, photocurrent generation by biophotovoltaic cells based on photosynthetic proteins sensitized TiO2 photoanode is inextricably linked with the charge separation at the photosynthetic protein/TiO2 interface. The energy state matching among photosynthetic proteins, semiconductive TiO2 , and redox mediators is crucial to enable the electron injection from photosynthetic proteins to TiO2 as well as the electron refill from redox mediators to the proteins. The photoinduced electrons originate from Q band excitation of Chls. Since Chl is the major pigment contained in photosynthetic proteins, the energy levels of the photosynthetic proteins can be represented by the ground and excitation states of Chls Q band, and they can be determined by measuring the oxidation potential and the absorption spectrum of the photosynthetic proteins. **Figure 9** shows the electron transfer and energy level scheme of the biophotovoltaic cell based on LHCII aggregates as the sensitizer on thin film TiO<sup>2</sup> photoanode. Potentials are relative to the normal hydrogen electrode (NHE) [38]. Similar to DSSCs, when the chlorophylls in the photosynthetic proteins are photoexcited to the higher energy level Chl\*, the electrons are able to inject into the less negative conduction band of TiO<sup>2</sup> . In

**Figure 9.** Electron transfer and energy level scheme of a photovoltaic device based on aggregated LHCII complexes. Reprinted with permission from Ref [38].

this step, the harvested solar energy is converted into anodic photocurrent. Meanwhile, the redox potential of the mediator should be more negative (i.e., in lower position in **Figure 9**) than the hole left at the ground state of Chlorophyll Ch<sup>+</sup> so that it can resupply an electron to the oxidized Chl, thus regenerating the sensitizer. The *VOC* (maximum output voltage of solar cell) corresponds to the difference between the redox potential of the redox mediator and the Fermi level of the FTO current collector.

#### **3.2. Effect of charge transfer state in LHCs**

Comparing to artificial DSSCs, the biophotovoltaics involving photosynthesis complexes as sensitizers have two distinct features. First, the captured photon may go through a rapid internal energy transfer process to the charge-separation states. For example, **Figure 10A** shows that the excitation energy of LHCII at 496.5 nm is quickly transferred to the lower-energy Q band around 650–690 nm before giving fluorescence or producing charge separation. Second, the charge transfer process of densely assembled chlorophylls in photosynthetic protein complexes depends on the specific protein environments involving photosynthesis regulation through a photoprotective mechanism called non-photochemical quenching (NPQ) [9, 39–44]. Excess energy in the photo-excited chlorophylls was dissipated through specific LHCII protein aggregation [45]. The Chl excited states in the aggregated LHCII, unlike in isolated LHCII trimers, are severely quenched due to the formation of chlorophyll-chlorophyll coupled charge transfer (CT) states, which has been observed by the high-resolution hole-burning spectroscopy [46].

TiO2: A Critical Interfacial Material for Incorporating Photosynthetic Protein Complexes and Plasmonic... http://dx.doi.org/10.5772/intechopen.68744 217

**Figure 10.** Absorption and emission properties of LHCII aggregates associated with the formation of CT states. (A) Absorption and fluorescence emission spectra (*λ*ex = 496.5 nm) of small- and large-size LHCII aggregate in tricine buffer. (B) Normalized steady-state fluorescence emission spectra (*λ*ex = 663 nm) of small- and large-size LHCII aggregate in solutions and deposited on the APTES-TiO<sup>2</sup> -FTO photoanode surface, respectively. (C) Schematic illustration of the CT states (dots) formed in small and large aggregates. Reprinted with permission of Ref. [38].

Previous study revealed that the photovoltaic performance of the biophotovoltaic cell was correlated with strong coupling between the extensive CT states formed between the aggregated LHCIIs (schematically depicted in **Figure 10C**) and the TiO2 conduction band. The CT states have slightly lower oxidation potential (i.e., less negative in energy level in **Figure 9**) than the excited state of chlorophylls due to their more reddish absorption and red tail in UV spectra (**Figure 10A**). The CT states couple with the TiO<sup>2</sup> conduction band more effectively, convinced by severe quench on the fluorescence emission of the CT states when the large LHCII aggregates were anchored on TiO<sup>2</sup> surface (as shown in **Figure 10B**). This strong coupling facilitated more efficient electron injection across LHCII/TiO<sup>2</sup> interface and resulted in larger photocurrent generation in the corresponding biophotovoltaic cells.

#### **3.3. Effect of plasmonic nanoparticles**

this step, the harvested solar energy is converted into anodic photocurrent. Meanwhile, the redox potential of the mediator should be more negative (i.e., in lower position in **Figure 9**)

**Figure 9.** Electron transfer and energy level scheme of a photovoltaic device based on aggregated LHCII complexes.

the oxidized Chl, thus regenerating the sensitizer. The *VOC* (maximum output voltage of solar cell) corresponds to the difference between the redox potential of the redox mediator and the

Comparing to artificial DSSCs, the biophotovoltaics involving photosynthesis complexes as sensitizers have two distinct features. First, the captured photon may go through a rapid internal energy transfer process to the charge-separation states. For example, **Figure 10A** shows that the excitation energy of LHCII at 496.5 nm is quickly transferred to the lower-energy Q band around 650–690 nm before giving fluorescence or producing charge separation. Second, the charge transfer process of densely assembled chlorophylls in photosynthetic protein complexes depends on the specific protein environments involving photosynthesis regulation through a photoprotective mechanism called non-photochemical quenching (NPQ) [9, 39–44]. Excess energy in the photo-excited chlorophylls was dissipated through specific LHCII protein aggregation [45]. The Chl excited states in the aggregated LHCII, unlike in isolated LHCII trimers, are severely quenched due to the formation of chlorophyll-chlorophyll coupled charge transfer (CT) states, which has been observed by the high-resolution hole-burning

so that it can resupply an electron to

than the hole left at the ground state of Chlorophyll Ch<sup>+</sup>

Fermi level of the FTO current collector.

Reprinted with permission from Ref [38].

216 Application of Titanium Dioxide

spectroscopy [46].

**3.2. Effect of charge transfer state in LHCs**

While the biophotovoltaic cells based on interfacing the artificial DSSC platform with the photosynthetic proteins provide useful insight into the fundamental photon capture and charge separation processes, their PCE is much lower than the conventional DSSCs using organic dye molecules (such as N719 [47]) as light harvesting antennas. Noble metal nanoparticles, that is, gold or silver nanoparticles, have been explored to enhance the solar cell performance utilizing their surface plasmonic resonance (SPR) effects that can enormously alter the optical absorption and emission of photosynthetic proteins near the nanoparticle surface [48]. An example of such solar cells is shown in **Figure 11** [25]. Enhancement of light absorption was observed for

**Figure 11.** The structure of the plasmonic biophotovoltaic cell. The enlarged portion (in square) shows the binding of different PNPs on LHCII-sensitized TiO<sup>2</sup> nanotrees and the electron injection from LHCII to TiO<sup>2</sup> (curve arrow). Reprinted with permission of Ref. [25].

PSI attached to plasmonic nanoparticles (PNPs) [49]. The LHCs anchored on plasmonic gold or silver island substrates were able to generate 10- to 20-fold of fluorescence emission [50–52]. A theoretical model for SPR-enhanced free electron production and photocurrent generation was proposed based on PSI-RCs bound to Au and Ag nanocrystals [53]. The internal photosynthetic efficiency of PSI-RC was found to be strongly enhanced by the metal nanoparticles, which involved two competing effects, that is, plasmon enhanced light absorption of Chl molecules and energy transfer from Chl to metal nanoparticles [53]. These studies provide useful insights into energy-conversion devices involving the interplay between photosynthetic proteins and PNPs.

PNPs have already been widely employed in photovoltaic devices, including DSSCs [54–56] and emerging Perovskite solar cells [57, 58], to enhance the performance. However, the explanation of the interplays between PNPs and different light harvesting antennas is still ambiguous in each specific cases. Unlike single chromatic synthetic dye, photosynthetic proteins contain multiple pigments whose light harvesting and conversion involve intrinsic energy transfer among pigments and cofactors, inducing additional complexity to understand the plasmonic effects on the whole photovoltaic processes of the bio-hybrid systems. Recently, a bio-solar cell using natural extract graminoids coupled with silver nanoparticles (Ag-NPs) has been reported to achieve larger photocurrent [59]. First, incorporating plasmonic Ag-NPs (~13.8 nm in diameter) enables quenching the emission of the natural graminoid sensitizers and thus enhancing the electron collection efficiency. Meanwhile, the small size of the Ag-NPs barely takes up surface area for attachment of light-harvesting sensitizers. Second, a TiO2 (001) nanosheet structure provides a good surface for collection of solar-driven electrons from graminoids. Third, the ligand tethering enables good attachment between graminoids and Ag-NPs on the (100) face of TiO2 nanosheet. The enhanced performance of the plasmonic biophotovoltaic cell in this case was attributed to the emission quenching by Ag-NPs for efficient collection of photoinduced electrons from graminoid complexes, as well as the efficient light trapping due to the plasmonic enhanced local electromagnetic field.

We proposed another enhancement mechanism for plasmonic biophotovoltaics with the design shown in **Figure 12** [25]. Core-shell PNPs with a 2–5 nm TiO<sup>2</sup> shell and a plasmonic silver or gold core were hybridized with LHCII and incorporated into the aforementioned 3D TiO2 nanotree photoanode for the plasmonic enhanced biophotovoltaic cells. Compared with the bare Ag NPs used in the above-discussed work [59], this core-shell structure has multiple functions. First, the hydrophilic nature of TiO<sup>2</sup> shell makes the PNP surface compatible for protein attachment [60]. Second, the semiconductive TiO<sup>2</sup> shell serves as an energy barrier to prevent electron recombination on the metallic core due to unwanted electron flow from the attached proteins to the metallic core [61]. Third, the TiO2 shell acts as a protective armor to ensure the stability of the metallic core in the corrosive iodide electrolytes in solar cells [62]. These PNPs with different plasmonic resonance bands were able to enhance and manipulate the photon capture of LHCII at specific wavelength ranges. The photocurrent

PSI attached to plasmonic nanoparticles (PNPs) [49]. The LHCs anchored on plasmonic gold or silver island substrates were able to generate 10- to 20-fold of fluorescence emission [50–52]. A theoretical model for SPR-enhanced free electron production and photocurrent generation was proposed based on PSI-RCs bound to Au and Ag nanocrystals [53]. The internal photosynthetic efficiency of PSI-RC was found to be strongly enhanced by the metal nanoparticles, which involved two competing effects, that is, plasmon enhanced light absorption of Chl molecules and energy transfer from Chl to metal nanoparticles [53]. These studies provide useful insights into energy-conversion devices involving the interplay between photosynthetic proteins and PNPs. PNPs have already been widely employed in photovoltaic devices, including DSSCs [54–56] and emerging Perovskite solar cells [57, 58], to enhance the performance. However, the explanation of the interplays between PNPs and different light harvesting antennas is still ambiguous in each specific cases. Unlike single chromatic synthetic dye, photosynthetic proteins contain multiple pigments whose light harvesting and conversion involve intrinsic energy transfer among pigments and cofactors, inducing additional complexity to understand the plasmonic effects on the whole photovoltaic processes of the bio-hybrid systems. Recently, a bio-solar cell using natural extract graminoids coupled with silver nanoparticles (Ag-NPs) has been reported to achieve larger photocurrent [59]. First, incorporating plasmonic Ag-NPs (~13.8 nm in diameter) enables quenching the emission of the natural graminoid sensitizers and thus enhancing the electron collection efficiency. Meanwhile, the small size of the Ag-NPs barely takes up surface area for attachment of light-harvesting sensitizers. Second, a

**Figure 11.** The structure of the plasmonic biophotovoltaic cell. The enlarged portion (in square) shows the binding

nanotrees and the electron injection from LHCII to TiO<sup>2</sup>

(curve arrow).

(001) nanosheet structure provides a good surface for collection of solar-driven electrons from graminoids. Third, the ligand tethering enables good attachment between graminoids

biophotovoltaic cell in this case was attributed to the emission quenching by Ag-NPs for efficient collection of photoinduced electrons from graminoid complexes, as well as the efficient

light trapping due to the plasmonic enhanced local electromagnetic field.

nanosheet. The enhanced performance of the plasmonic

TiO2

and Ag-NPs on the (100) face of TiO2

of different PNPs on LHCII-sensitized TiO<sup>2</sup>

Reprinted with permission of Ref. [25].

218 Application of Titanium Dioxide

**Figure 12.** Schematic diagram of the energy and electron pathways in LHCII-PNP hybrid system. The LHCII trimers are excited by strong absorption of Chls' Soret and Q bands (i). The excited Chls go through an ultrafast excitation energy transfer (EET) from Soret band to Q band (ii) and then give fluorescence emission (iii) to return to the ground level. With LHCII attached to TiO<sup>2</sup> surface, a charge transfer process occurs, leading to injection of excited electrons in Q band to the conduction band (CB) of TiO<sup>2</sup> (iv). Thus, the fluorescence intensity is reduced. In the presence of the metallic core, further excitation to LHCII may occur due to plasmon-induced resonance energy transfer (PIRET) from PNPs to LHCII (v), resulting in larger electron injection from LHCII to TiO<sup>2</sup> . At the meantime, the injection of hot electrons from the metal core of PNPs across the Schottky barrier (vi) leads to higher charge carrier density in TiO2 . Reprinted with permission from Ref. [25].

and incident photon-to-current efficiency (IPCE) of the plasmonic biophotovoltaic cell were achieved, while the fluorescence emission of excited LHCII was quenched along with shortened lifetime. Obviously, the electrons in the excited LHCII state flow efficiently through the TiO2 network.

Cushing et al. [63, 64] elaborated that three mechanisms are involved charge generation in a semiconductor incorporated with plasmonic metal NPs, including light trapping based on scattering, hot electron/hole transfer, and plasmon-induced resonance energy transfer (PIRET) based on near-field. These are also applicable to the LHCII-PNP hybrids, with the possible mechanisms illustrated in **Figure 12**. During the excitation process, electrons are pumped from the ground state to the excited states of Soret band or Q band of LHCII. However, the excited Soret band quickly goes through an ultrafast excitation energy transfer (EET) to the Q band, as verified by a theoretical modeling [65]. Thus, all fluorescence emission from LHCIIs is at 683 nm, corresponding to a radiative relaxation for the excited electrons to return to the ground state of Q band. When LHCII is adsorbed on the TiO<sup>2</sup> surface, upon excitation a charge transfer process occurs, that is, the excited electrons in Q band are injected to the conduction band (CB) of TiO<sup>2</sup> , resulting in the reduced fluorescence intensity. In the presence of the metallic core, the incident photons by plasmonic absorption generate a strong near-field oscillation with ~10 nm decay length, which can affect all LHCII adsorbed on the surface of the ~2–3-nm-thick TiO2 shell. A strong PIRET is enabled by the strong dipole-dipole coupling between the plasmon and LHCII, leading to enhanced LHCII excitations at Soret and Q bands. More efficient electron injection from LHCII to TiO<sup>2</sup> is also facilitated by the near-field and thus quenches the fluorescence emission though PIRET induces higher LHCII excitation. These effects collectively enhance the photocurrent of the corresponding plasmonic biophotovoltaic cells. In addition, the plasmonic hot electrons excited at the metal core of PNPs may overcome the Schottky barrier at the metal-TiO<sup>2</sup> interface, raise the charge carrier density in TiO2 shell, and therefore the charge collection efficiency. Details about such interfacial activity are unravelled in the next section.
