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Manipulation of Charge Transfer Across Semiconductor Interface. A Criterion That Cannot Be Ignored in Photocatalyst Design Prashant V. Kamat*

J. Phys. Chem. Lett. 2012.3:663-672. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/28/19. For personal use only.

Radiation Laboratory, Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States ABSTRACT: The Perspective focuses on photoinduced electron transfer between semiconductor−metal and semiconductor−semiconductor nanostructures and factors that influence the rate of electron transfer at the interface. The storage and discharge properties of metal nanoparticles play an important role in dictating the photocatalytic performance of semiconductor−metal composite assemblies. Both electron and hole transfer across the interface with comparable rates are important in maintaining high photocatalytic efficiency and stability of the semiconductor assemblies. Coupled semiconductors of well-matched band energies are convenient to improve charge separation. Furthermore, semiconductor and metal nanoparticles assembled on reduced graphene oxide sheets offer new ways to design multifunctional catalyst mat. The fundamental understanding of charge-transfer processes is important in the future design of light-harvesting assemblies.

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status and issues of QD solar cells and efforts to improve solar cell efficiency have been discussed in other reviews.2,19−24

emiconductor nanoparticles and nanostructures have been considered as building blocks of light harvesting systems. Of particular interest is their role in capturing incident photons to induce charge separation in quantum dot (QD) solar cells1−5 and photocatalyst systems.6−13 Efficient transfer of charges across the semiconductor interface is the key for converting light energy into electricity or fuels. Metal oxides such as TiO2 have been the subject of extensive investigation for last three decades. The other class of semiconductors are metal chalcogenides (CdS, CdSe, PbS, and PbSe), which offer a significant advantage because of their tunable response to visible light.14−18 Most of the recent effort has focused on designing semiconductor nanostructures for QD solar cells and photocatalysis applications (Figure 1). Details of the current

Efficient transfer of charges across the semiconductor interface is the key for converting light energy into electricity or fuels. The recent thrust in designing semiconductor nanostructure assemblies for solar fuel production (e.g., H2 from water splitting reaction) has further drawn interest in understanding dynamics and kinetic details of interfacial electron and hole transfer.25−27 Whereas most of the photocatalytic studies focus on the net photoconversion efficiency, understanding of various electron-transfer steps at the fundamental level is still lacking. Early studies have shown that photogenerated electrons and holes in a semiconductor nanoparticle can be transferred across the interface in picoseconds.28−37 One of the ways to boost the efficiency of photocatalytic reaction is to couple semiconductor nanoparticles to noblemetal cocatalysts.38−48 In particular, noble-metal cocatalysts enhance the quantum yield of photoinduced electron-transfer processes by: (1) improving charge separation within the semiconductor particle, (2) discharging photogenerated electrons across the interface, and (3) providing a redox pathway with low overpotential. Previous studies have shown that metal Received: December 12, 2011 Accepted: February 9, 2012 Published: February 9, 2012

Figure 1. Photoinduced charge-transfer processes at semiconductor interface. (A) Quantum dot solar cells and (B) photocatalytic splitting of water with Pt/TiO2/IrO2 photocatalyst assemblies. © 2012 American Chemical Society

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dx.doi.org/10.1021/jz201629p | J. Phys. Chem. Lett. 2012, 3, 663−672

The Journal of Physical Chemistry Letters

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growth, and confined chambers to carry out fast, efficient chemical reactions.58 AOT is the preferred choice of surfactant because of its ability to form spherical droplets of controlled sizes.37,59−64 The size of the water pool is directly related to the water−surfactant molar ratio, wo.65 Particle size, however, depends on an array of variables including precursor ratios, temperature, and pH. The synthesis of nanoparticles in reverse micelle provides a quick, low-temperature synthetic approach. More importantly, it allows one to directly control the surrounding medium of the particles and introduce other species into the pools while retaining uncapped particle surfaces. Figure 3A shows the time-resolved transient absorption spectra recorded following the excitation of CdSe nanoparticles in reverse micelles at several delay times. The bleaching seen at 510 nm corresponds to the depletion of excitonic absorption band of the CdSe. As shown in the previous study,64,67−72 this bleaching arises from the charge separation following the laser pulse excitation of CdSe QDs. With increasing delay times, these charge carriers recombine, and the observed bleaching recovers. These Pt nanoparticles enhance the rate of bleaching recovery at 510 nm (Figure 3B) as the photogenerated electrons are transferred from excited CdSe into Pt nanoparticles. The corresponding rate constants derived from the traces are 1.80 ± 0.16 × 109 s−1 (ka) and 3.02 ± 0.11 × 109 s−1 (kb), respectively. The rate of electron transfer (kET) from excited CdSe into Pt was 1.22 ± 0.19 × 109 s1. The relatively high rate constant of the electron transfer reaffirms that the interaction between the CdSe and Pt nanoparticles is static. Additionally, we can estimate the maximum attainable efficiency of electron transfer during the initial encounter from the rate constant of recovery of the CdSe and CdSe-Pt systems. From this analysis, it is determined that 40% of the electrons generated following the pulse excitation are scavenged by Pt nanoparticles. Recently, Majima and coworkers have employed single-particle spectroscopy to probe size-dependent photocatalytic activity in Au/TiO2 nanocomposites.73 Electron Storage and Discharge at Semiconductor/Metal Interface. The metal nanoparticles often act as an electron sink and thus improve charge separation within the semiconductor−metal composite photocatalyst system.40 These electrons can then be discharged to acceptor molecules present at the interface with relatively lower overpotential of reduction. The efficiency of electron transfer to metal nanoparticles depends upon its ability to compete with other acceptor molecules for photogenerated electrons at the semiconductor

nanoparticles such as Pt promote hydrogen generation by intercepting photochemically reduced methyl viologen.49−52 The interfacial charge-transfer processes are influenced by the presence of a metal cocatalyst or surface-bound molecular relays. For example, if metal nanoparticles are coupled to the semiconductor nanoparticle, then they readily accept and shuttle electrons to an acceptor molecule at the interface quite efficiently. The storage/discharge capacity of metal nanoparticles plays an important role in dictating Fermi level equilibration between semiconductor−metal nanoparticles.53−57 This account aims at answering questions that often arise in understanding photocatalytic processes. (1) How fast are the electrons transferred from an excited semiconductor nanoparticle to the metal nanoparticles and holes to cocatalysts such as IrO2? (2) How does the electron storage in metal nanoparticles and charge equilibration influence the overall energetics and the charge transfer across the interface? (3) How can these interfacial processes be further manipulated to improve the photocatalytic efficiency? Electron Transfer between Semiconductor and Metal Nanoparticles. The semiconductor−metal interactions on the nanoscale can be easily probed by confining CdSe QDs and Pt nanoparticles in reverse micelles of heptane/dioctyl sulphosuccinate (AOT)/water (Figure 2). Reverse micelles are stable,

Figure 2. CdSe and Pt coupled in a heptane/AOT/water reverse micelle. From ref 66.

dynamic structures composed of a polar core surrounded by surfactant molecules in a nonpolar bulk solvent. As inert “nanocages”, reverse micelles act as both templates for particle

Figure 3. (A) Transient absorption spectra of 4 μM CdSe in heptane-AOT-water reverse micelles following 387 nm laser excitation at various pump−probe delay times. (B). Absorbance-time (kinetic) profiles of (a) CdSe alone (e-h recombination) and (b) in the presence of 4 μM Pt (recombination + electron transfer). From ref 66. 664



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Figure 4. (A) Absorption spectra of deaerated suspension of TiO2 colloid (0.86 mM) and AgNO3 (86 μM) recorded (a) before and (b) after UVirradiation for 12 min. The spectra (c−j) were recorded after stopping the irradiation at intervals of 20 s, 40 s, 60 s, 2 min, 4 min, 6 min, 8 min, and 10 min, respectively. (B) The shift in the plasmon absorption peak during on−off cycles of irradiation. All experiments were conducted under a nitrogen atmosphere. From ref 83.

interface. Additionally, they can undergo charge equilibration with reduced molecules. Therefore, the metal nanoparticles in direct contact with a photoexcited semiconductor would either store the electrons or quickly discharge them to the adsorbed species (e.g., to H+ ions to produce hydrogen). Whereas metals such as Pt and Pd provide an ohmic contact, metals such as Ag and Au exhibit capacitive properties. When they are small (2−5 nm in diameter), these coinage metals are capable of storing electrons.53,54 Such electron storage causes a shift in the Fermi level. For example, previous studies have shown that the shift in Fermi level could be as high as 0.1 V per stored electron in a nanoparticle.74−77 The electron storage property can be beneficially used to improve the photoelectrochemical performance of nanostructured semiconductor films. In previous studies, it has been shown that Au nanoparticles deposited on TiO2 and CdSe can directly participate in the Fermi level equilibration under UV irradiation, thereby shifting the Fermi level of the composite to more negative potentials.55−57,78−81 The stored electrons can be readily estimated by titrating with a known redox couple such as thionine or C60. Smaller size particles were found to induce a maximum shift in the Fermi level of the semiconductor−metal composite. Another convenient way to probe the electron storage in metal nanoparticles is with its plasmon frequency. The addition of electrons to silver and gold nanoparticles or nanorods causes a blue shift in the absorption spectrum due to the increasing surface plasmon frequency of the electron gas.80,82 The correlation between the number of stored electrons and shift in the plasmon frequency was compared in these studies. If one continues UV irradiation, then the plasmon band intensity increases with a shift in the absorption peak position from 430 to 415 nm. This shift of ∼15 nm in the plasmon absorption peak is representative of excess electrons stored within the Ag nanoparticle. Upon stopping the illumination, the plasmon peak slowly reverts back to 430 nm, indicating possible discharge of excess electrons at the electrolyte interface (Figure 4A, spectra c−j). This behavior can be attributed to the existence of two separate equilibria under illuminated and dark conditions, respectively (Figure 5). Figure 4B shows the response of the plasmon peak position during on−off cycles of UV irradiation of TiO2−Ag system after completing the process of UV reduction of Ag+ ions. Nitrogen gas was continuously purged to maintain an inert atmosphere in the TiO2/Ag suspension. It is interesting to note that the shift in

Figure 5. Charge equilibration scenarios between TiO2 and Ag nanoparticles during continuous UV irradiation and in dark. From ref 83.

the plasmon peak observed during UV-irradiation of TiO2/Ag composite is reversible.

The electron storage in metal nanoparticles shifts the apparent Fermi level of the semiconductor−metal composite to more negative potential, and this in turn makes the photocatalyst more reductive.

The electron storage in metal nanoparticles shifts the apparent Fermi level of the semiconductor−metal composite to more negative potential, and this in turn makes the photocatalyst more reductive. With decreasing metal particle size, the shift in Fermi level is greater. A size-dependent enhancement in photocatalytic degradation has been previously established for Au-TiO2 composites.57 Metal-Mediated Electron Transfer and Its Implication in Photocatalysis. A major application of photoinduced electron transfer at the semiconductor and semiconductor−metal composite system is in the water splitting reaction to produce hydrogen. Given the current thrust of solar fuel production, it is important to understand the kinetic and mechanistic aspects of interfacial charge-transfer processes. Methyl viologen (MV2+) is an excellent probe for monitoring photoinduced electron transfer at the semiconductor interface because of its favorable redox potential, pH independence, and unique spectral characteristics. The MV2+ cation readily undergoes one-electron reduction (E0= −0.445 V vs NHE) to 665



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Figure 6. Absorption−time profiles at (A) 510 nm (bleaching recovery of CdSe) and (B) 605 nm (formation and decay of MV+•) recorded following the 387 nm excitation of 4 μM CdSe confined in a heptane/AOT/water reverse micelle: (a) CdSe alone; (b) CdSe + 4 μm Pt NP; (c) CdSe + 1 mM MV2+; and (d) CdSe +1 mM MV2+ + 4 μM Pt NP. The scheme on the right illustrates competing electron transfer to Pt nanoparticles and MV2+. The MV+• formed during reduction step also transfers electrons to Pt, whereas the electrons transferred to Pt are discharged to reduce H+. From ref 66.

form the cationic radical, MV+•, with a characteristic blue color (ε = 13 700 M−1 cm−1 at 605 nm).84 Furthermore, MV+• is capable of transferring electrons to Pt nanoparticles, which in turn reduces surface adsorbed H+ ions to produce hydrogen.85 Figure 6 A,B shows the changes in absorbance versus delay time monitored at the excitonic peak of CdSe (510 nm) and at the absorption peak of the MV+• (605 nm) following the excitation of the CdSe and CdSe/Pt systems. The bleaching recovery of CdSe (trace c) is significantly faster when MV2+ is present and parallels the formation of MV+• radical with absorption in the 605 nm region. The rate constant of the exciton bleaching recovery of CdSe in the presence of MV2+ is estimated to be 1.7 ± 0.11 × 1010 s−1 and is similar to the rate constant of electron transfer between CdSe and MV2+ (trace c) kET = 1.52 × 1010 s−1. The appearance of 605 nm absorption within the time scale of 10 ps confirms that the electron transfer to MV2+ from excited CdSe (both in the absence and presence of Pt) is an ultrafast event. Achieving fast electron transfer across the interface is an important criterion to maximize the efficiency of photoconversion. Similar fast electron transfer between CdSe QDs and polymeric viologen has been observed.86 The rates of photoinduced electron transfer from CdSe QDs to poly(viologen) within thin films were found to be influenced by the length of the ligands passivating the QDs. An interesting difference in the transient absorption of MV+• is seen on longer time scales (traces c and d in Figure 6B). In the presence of Pt, the MV+• exhibits a sharp decay within 100 ps. In the absence of Pt nanoparticles, the MV+• remains stable during the same time period. The decay of the MV+•

absorbance indicates fast oxidation of MV+• as it transfers electrons to Pt (kET = 3.1 × 109 s−1). These spectroscopic results establish the role of Pt as a mediator to capture and discharge electrons. The mediating role of cocatalysts such as Pt is important to manipulate the photocatalytic reaction. The Hole Transfer at Irradiated Semiconductor Nanoparticles. The hole-transfer process at an irradiated semiconductor nanoparticle has important implications in photocatalytic remediation of organic contaminants from air and water. For example, in UV-irradiated TiO2 nanoparticles, the surfacebound hydroxyl groups scavenge the photogenerated holes to produce hydroxyl radicals.87,88 Pulse radiolysis measurements have identified the coexistence of holes and hydroxyl radicals near the surface and their contribution in the oxidation of organics.89 Hole transfer from excited CdSe into p-phenylenediamine has also been established from emission and transient absorption measurements.90 The surface interaction between the organic molecules and the semiconductor is an important player as it dictates overall effectiveness of the photocatalytic process. Along with pulse radiolysis, laser flash photolysis has also been found to be useful in identifying mechanistic and kinetic aspects of interfacial charge transfer processes.87−92 Another important contribution of hole reactivity is in the hole oxidation of water to produce oxygen. Proton-coupled multielectron transfer processes to produce hydrogen peroxide and oxygen have been identified as the possible pathways in the water oxidation.93,94 Metal oxides such as IrO2,95,96 RuO2,97−99 and cobalt phosphate100,101 promote water oxidation at the semiconductor/electrolyte interface with high turnover rates. 666



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The adverse discharge of holes to reduced species can be suppressed if the anodic and cathodic processes are separated (e.g., through a photoelectrolysis cell). Such a configuration allows the separation of the products (viz: H2 and O2) of the water-splitting reaction.104 Utilization of small bandgap semiconductors (e.g., metal chalcogenides) for photocatalytic production of hydrogen from water requires a sacrificial electron donor system. Redox couples such as S2−/Sn2− are quite effective in scavenging the holes from CdS and CdSe and help to maintain the stability of the semiconductor. The emission of CdSe deposited on an inert oxide such as SiO2 is useful to monitor the hole transfer at the CdSe interface.105 In the absence of Na2S, the CdSe exhibits its natural decay as it undergoes charge recombination. In the presence of Na2S, the hole transfer to S2− competes with the charge recombination processes. A decrease in the emission yield and emission lifetime with increasing concentration of S2− allows one to monitor the hole-transfer process. The apparent rate constant obtained from the emission decay yields a value of 8.5 × 107 s−1 for the hole transfer to S2−. This value is two to three orders of magnitude lower than the electron injection from excited CdSe into TiO2 and other oxide semiconductors.106,107 Disparity between the electron and hole scavenging rate could lead to increased charge carrier recombination and thus decrease the photocatalytic efficiency. Electron Transfer Between Coupled Semiconductor Nanoparticles. The early findings of the improved charge separation were made with CdS-ZnO and CdS-TiO2 coupled semiconductors.28,108 The matching of the band alignment of the two semiconductors facilitates desired functionality. When semiconductor nanocrystals are implemented into a practical device (e.g., light-emitting diodes or QD solar cells), photoinduced electron-transfer reactions are intimately involved in dictating the overall functionality. Transient absorption spectroscopy provides a convenient way to monitor photoinduced electron transfer between two semiconductor nanoparticles. Because the size quantization varies the band energies of semiconductor nanocrystals such as CdSe and CdS, one can study the energy gap dependence of photoinduced electron transfer between two semiconductor nanocrystals.37,107 A comprehensive examination of electron transfer at CdSe QDs and metal oxide junctions (SnO2, TiO2, and ZnO) was conducted using a series of CdSe QD donors (diameters of 2.8, 3.3, 4.0, and 4.2 nm).106 Apparent electrontransfer rate constants showed strong dependence on change in system free energy, exhibiting a sharp rise at small driving forces, followed by a modest rise further away from the characteristic reorganization energy (Figure 8).106 The observed trend mimics the predicted behavior of electron transfer from a single quantum state to a continuum of electron accepting states, such as those present in the conduction band of a metal oxide nanoparticle. Utilizing ultrafast transient absorption spectroscopy, we measured electron-transfer rates from four different sizes of CdSe QDs to three unique metal oxide species. Electron transfer rates ranged from 1.9 × 1010 to 4.6 × 1011 s−1, and trends generally agreed with Marcus theory.109 Such agreement highlights the accuracy of the manystate Marcus model, in conjunction with our determination of change in free energy for QD to nanoparticulate metal oxide electron transfer, over a range of CdSe QD sizes and metal oxide accepting species. Another aspect to consider is the molecular linkage between the particles, which can slow down the electron transfer based on the alkyl chain length.110−112

In addition, direct photoexcitation of these cocatalysts (e.g., IrO2) may also play an important role in the oxidation of water.102 Yet very little information exists on the kinetics of hole transfer to such cocatalysts. The spectral fingerprint of trapped holes with absorption that is around 360 nm enables monitoring of the transfer of trapped holes to IrO2.103 Figure 7A shows transient absorption spectra

Figure 7. (A) Transient absorption spectra recorded 2 μs after 308 nm excimer laser pulse excitation of 23 mM TiO2 (5% acetic acid/95% ethanol) containing (a) 0, (b) 0.02, (c) 0.04, and (d) 0.06 mM IrO2. (B) Normalized absorption−time profiles recorded at 380 nm following the excitation of 23 mM TiO2 (a) without IrO2 and (b) with 0.06 mM IrO2. From ref 103.

recorded immediately after the 308 nm laser pulse excitation of TiO2 colloids in ethanol/acetic acid. The difference absorption spectrum recorded in the absence of IrO2 shows a peak at 380 nm and a broad absorption in the near IR. Previous studies have established that these peaks correspond to trapped holes and trapped electrons, respectively.89,92 Figure 7B shows absorbance− time profiles monitored at 380 nm in the absence and presence of IrO2. The absorbance at 380 nm remains steady during the time scale of 15 μs in the absence of IrO2. However, a rapid decay in the absorbance (380 nm) is seen in the presence of IrO2. The lifetime of 1.5 μs obtained from the first-order kinetics yields a rate constant of hole transfer of 6 × 105 s−1. Whereas IrO2 is quite effective for transfer of holes at a UV-irradiated semiconductor system, it also catalyzes the recombination of trapped holes with reduced oxygen species. 667



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Figure 9. Designing multifunctional catalyst mat with spatially separated semiconductor and metal nanoparticles. The schematic illustrates catalytic selectivity in water splitting reaction. From ref 114.

(e.g., TiO2) absorbs the light and induces the oxidation reaction. The reduced graphene oxide can capture electrons and shuttle them across the 2-D carbon network to the Pt site to facilitate hydrogen reduction. Use of such multifunctional photocatalyst assembly in a water splitting reaction has been demonstrated recently.10,115 Recent spectroscopic studies have confirmed the ability of graphene oxide to accept electrons from excited semiconductor nanoparticles and reduce Ag+ ions at a site that is spatially different than the semiconductor nanoparticles.114,116 Such multifunctional catalyst assemblies can be tailored to tune the selectivity and efficiency of photocatalytic reduction and oxidation processes

By incorporating two or more catalyst particles on a single graphene or reduced graphene oxide sheet, it should be possible to carry out selective catalytic processes at separate sites.

Figure 8. (Top) Band energy diagram of different size CdSe nanoparticles and metal oxides. (Bottom) Global plot of electron transfer rate constant versus free energy for all CdSe (donor) to metal oxide (acceptor). The trace is a theoretical fit based on Marcus manystate model with reorganization energy, λ = 30 meV and defect distribution, Δ = 50 meV. Reprinted with permission from ref 106. Copyright 2011 National Academy of Sciences.

Future Outlook. Significant effort has been made in recent years to design new photocatalytic materials and hybrid composites to improve the efficiency of photoconversion efficiencies. Despite extensive effort to design nanostructures of various shapes and sizes, the improvement in the overall efficiency is marginal. The beauty of displaying TEM images (often mimicking the shapes of flowers and fruits) has not translated into better photocatalyst systems. The basic limitations of suppressing charge recombination within the semiconductor nanostructure remain a challenge. The same scenario is true for the design of dye-sensitized and QDsensitized solar cells. Despite all major thrusts to use different support nanoarchitectures in the form of rods, tubes, nanoforest, and so on, the best-performing dye-sensitized and QD solar cells employ mesoscopic TiO2 films that were designed more than two decades ago.117 The kinetic and spectroscopic highlights of this account show that the manipulation of interfacial charge transfer is the most important factor that can assist in improving the photocatalytic and solar cell conversion efficiency. Future efforts need to focus on further manipulation of the interfacial chargetransfer processes with surface modification and/or with a cocatalyst. A better understanding of the charge-transfer dynamics between semiconductor and cocatalyst is needed to improve the photocatalyst design. Additionally, one needs to consider two photon processes to induce reduction and oxidation processes on two separate semiconductors using Z-scheme or a tandem configuration.8,118 Currently, a trend is emerging to hype the photocatalytic conversion efficiencies of solar fuel production (H2 formation, CO2

Jin and Lian have probed electron-transfer dynamics from single CdSe/ZnS (core/shell) nanoparticles to TiO2 nanoparticles by single-particle fluorescence spectroscopy and compared them with ensemble average fluorescence decay.113 Compared with QDs on glass, the presence of the interparticle electron-transfer pathway on TiO2 led to smaller on-state and larger off-state probability densities as well as a shortened lifetime of the on-state. The average electron-transfer rate from CdSe/ZnS to TiO2 was estimated to be 3.2 × 107 s−1. The slower electron-transfer rate observed in this example is likely to arise from ZnS shell. A careful design of semiconductor heterostructures is likely to play an important role in tapping hot electrons and multiple electrons generated in semiconductor nanocrystals. Multif unctional Photocatalytic Carbon Mat. Carbon nanostructures such as single and multiwall carbon nanotubes and graphene oxide provide a convenient platform to anchor semiconductor and metal nanoparticles. The ability of carbon nanotubes and graphene-based systems to capture and shuttle electrons through the π−π network has been previously demonstrated.114 By incorporating two or more catalyst particles on a single graphene or reduced graphene oxide sheet, it should be possible to carry out selective catalytic processes at separate sites. The illustration in Figure 9 shows an example of incorporating a semiconductor and Pt nanoparticle in a catalyst mat for the water splitting reaction. The semiconductor nanoparticle 668



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ACKNOWLEDGMENTS I would like to thank past and present members of the group and collaborators for their research contributions and helpful discussions. The research described herein was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through award DE-FC02-04ER15533. This is contribution number NDRL 4908 from the Notre Dame Radiation Laboratory.

Future efforts need to focus on further manipulation of the interfacial charge-transfer processes with surface modification and/or with a cocatalyst.

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reduction) in a photocatalytic process. For example, the photoconversion efficiency of water splitting reactions at TiO2 and other semiconductor electrodes under applied bias fails to account for the power drawn from the potentiostat to maintain a certain bias.119 Use of a sacrificial reagent is a good strategy to investigate selective reduction or oxidation processes. However, one cannot use such experimental configurations to claim efficiency breakthroughs. Similarly, photocatalytic reduction of CO2 needs to be carefully scrutinized. The claims made for photocatalytic reduction usually involve formation of products such as methanol and carboxylic acids. First of all, these organic products if formed in the system are readily oxidized at the irradiated TiO2 surface. The hydroxyl radical-mediated oxidation of organics is well-known and is the basis of thousands of papers published in the literature during last two decades. Another important point to consider is the thermodynamic requirements. With conduction band energy less than −0.5 V versus NHE (pH 7), TiO2 is not energetic enough to reduce CO2. The one-electron reduction requires potentials greater than −1.95 V versus NHE. Although twoelectron reduction appears to be thermodynamically favorable, such a multielectron-reduction accompanied by proton transfer is yet to be realized. To date, no spectroscopic evidence exists for multielectron-transfer process in a semiconductor photocatalyst system. The complexity of proton-coupled electron transfer and difficulty in achieving multielectron-transfer process makes direct reduction of CO2 by semiconductor-assisted photocatalysis thermodynamically challenging. Photocatalytic reduction with isotope-enriched CO2 can provide conclusive mechanistic evidence of solar fuel generation. In addition, probing the CO2 interaction with photocatalyst surface and identification of reaction intermediates using surface science measurements are needed to establish the validity of the CO2 reduction on a photocatalyst surface. Coupling a semiconductor catalyst to a homogeneous catalyst120,121 would be an interesting approach to overcome thermodynamic limitations for multielectron-transfer processes.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Biography Prashant V. Kamat is a John A. Zahm Professor of Science in the Department of Chemistry and Biochemistry and the Radiation Laboratory and a Concurrent Professor in the Department of Chemical and Biomolecular Engineering, University of Notre Dame. His major research interests are in the areas of photoinduced charge-transfer processes in semiconductor nanocrystal-based architectures, photocatalytic aspects of metal nanoparticles and carbon nanostructures, and designing lightharvesting assemblies for next generation solar cells. See http://www.nd. edu/∼pkamat for further details. 669



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