Semiconductor Surface Chemistry as Holy Grail in Photocatalysis

Commentary pubs.acs.org/accounts

Semiconductor Surface Chemistry as Holy Grail in Photocatalysis and Photovoltaics Published as part of the Accounts of Chemical Research special issue “Holy Grails in Chemistry”. Prashant V. Kamat* Radiation Laboratory, Department of Chemistry and Biochemistry University of Notre Dame, Notre Dame, Indiana 46556, United States ABSTRACT: The trail of semiconductor surface photochemistry during the past four decades has led to the emergence of new areas in chemistry (e.g., photocatalysis, solar cells, solar fuels). How can one now exploit the richness of surface chemistry of hybrid architectures and make a transformative leap in light energy conversion and other applications?

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INTRODUCTION Surface chemistry as recognized in catalytic processes plays an important role toward enhancing the selectivity and the yield of a desired chemical reaction. The ability of surface atoms to promote the bond forming or bond breaking reaction with thermal activation has resulted in many industrial processes. During the last couple of decades, semiconductor surfaces have been extensively studied to tailor the chemical processes activated by light. More importantly, these semiconductor materials have enabled us to harvest light energy and convert it into electrical or chemical energy. In addition to the influence of the size and shape of semiconductor nanostructures, the surface of these materials can be tuned to tailor the interfacial charge transfer processes (commonly referred to as photocatalysis). During the early time period of 1970−1990, many interesting aspects of semiconductor single crystals as well as particulate (colloidal) systems were explored.1 Most notably, the reactivity of semiconductor surfaces was realized through photocatalytic oxidation of water, mineralization of organic contaminants from aqueous and gaseous systems, and photoelectrochemical cells. Metal oxides such as TiO2 and ZnO, and metal chalcogenides such as CdS and CdSe served as model systems for chemists to study photocatalytic properties. The discovery of the hot injection method in synthesizing quantized CdSe nanocrystals led to a new era of nanoscience in which chemists, material scientists, physicists and engineers collaborated to advance the field.2 The availability of high resolution transmission microscopy, new surface characterization tools, and ultrafast transient spectroscopy enabled a better understanding of the photoinduced surface chemical processes of nanostructured © 2017 American Chemical Society

semiconductor systems. A few representative examples discussed in this Account provide the holy grails of chemical processes at the semiconductor surfaces and challenges they offer in making transformative advances.

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PHOTOCATALYTIC PROCESSES AT TIO2 SURFACE

Although the photocatalytic aspects of metal oxides date back to 1950s,3 it was not until 1970s when the field of photoelectrochemistry emerged as the semiconductor interfacial chemistry and became the foundation of light energy conversion.4,5 During the same time period, the ability to capture photogenerated electrons and holes following bandgap excitation was recognized through the demonstration of metal ion reduction6 and hydroxyl radical formation7 at TiO2 surface. The mechanistic and kinetic details of electron and hole transfer at the TiO2 interface were established through transient absorption spectroscopy using colloidal suspensions. The initial demonstration of the photocatalytic activity of TiO2 particles in the 1970s has now evolved into a major discipline. The surface processes at the atomic scale are now being investigated based on theoretical and experimental approaches. The initial TiO2 colloid work was later extended to metal chalcogenide quantum dots (QDs) and 2-D semiconductors. As we pursue a more complex semiconductor architecture with two or more semiconductor/catalyst systems, there is need for better understanding of the excited processes and the factors dictating interfacial electron transfer. Received: October 20, 2016 Published: March 21, 2017 527

DOI: 10.1021/acs.accounts.6b00528 Acc. Chem. Res. 2017, 50, 527−531

Commentary

Accounts of Chemical Research

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PHOTOCATALYTIC DEGRADATION OF ORGANIC CONTAMINANTSA BOON OR A BUST? When suspended in water, hydroxyl groups dominate the surface of TiO2. These hydroxyl groups quickly capture the photogenerated holes to produce highly reactive hydroxyl radicals (Figure 1). These hydroxyl radicals are capable of

Figure 1. Photoinduced charge separation, charge trapping, and interfacial charge transfer following the band gap excitation of the semiconductor nanoparticle.

degrading organic compounds leading to complete mineralization.8 This property was explored as a potential strategy for abating pollutants from air and water. In addition direct transfer of holes and reactive oxygen species at the semiconductor surface also contributes to the photocatalytic transformation. The mechanistic insights into the oxidative steps leading to the mineralization of the organic compounds have been discussed elsewhere.9 The electron and hole transfer to the adsorbed species occurs over ps-ns time scale as revealed by the transient absorption spectroscopy.10 Surface modification as well as medium effects are influential in determining the overall photocatalytic activity. Although various semiconductors have been tested for photocatalytic remediation of organic contaminants, TiO2 remains a champion because of long-term stability and high UV-activity. Efforts to extend its response into the visible through doping or coupling with sensitizers have produced moderate success.11 The removal of gas phase contaminants from air is a success story leading to the development of products such as selfcleaning glass, odor removing tiles, and indoor air purifiers. On the other hand, the photocatalytic remediation of contaminants from wastewater has mostly remained a laboratory exercise. Coexisting contaminants such as chloride ions preferentially get oxidized and foul the photocatalytic activity of semiconductor systems or generate chlorinated intermediates.

Figure 2. Top: Ligand exchange between dodecylamine and 3-mercaptopropionic acid leading to the tuning of emission. Bottom: Corresponding changes in the emission spectra showing the quenching of band-edge emission and increasing trap emission for a solution of QDs treated with (a) 0 M, (b) 50 μM, (c) 100 μM, (d) 500 μM, (e) 1 mM, (f) 5 mM, and (g) 10 mM 3-MPA. Reprinted from ref 12. Copyright 2010 American Chemical Society.

Since the surface bound molecules influence the emissive properties of semiconductor QDs, it is possible to use them for sensing contaminants from gas and liquid systems. There is a an immediate need to develop strategies for utilizing stable smart semiconductor assemblies and use them for simultaneous detection and degradation of contaminants. A better understanding of excited state interactions in such multifunctional catalysts could lead to design of practical devices.

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DYE SENSITIZED SOLAR CELLS Using carboxylic acid as linker, dye molecules with absorption in the visible can be attached to mesoscopic TiO2 films. Although initial sensitization of TiO2 single crystal with dyes such as Ru(II)polypyridyl complex was demonstrated in mid-1970,18,19 it was not until 1991 when mesoscopic TiO2 film caused a big leap in dye-sensitized solar cells (DSSCs) performance.20 The electron injection from the surface bound sensitizer into TiO2 under visible excitation followed by the capture of electrons at the electrode surface provides the basis for conversion of light energy into electricity in DSSC. Estimation of electron injection rate constants as high as 1011 s−1 has been established through spectroscopic measurements.21 In order to maintain the stability, it is important that the redox couple present in the cell has the ability to quickly regenerate the sensitizer.22 Extensive efforts to design new sensitizers and manipulate charge recombination at the semiconductor surface have led to power conversion efficiencies greater than 12%. Challenges still remain to further boost the efficiency of DSSCs.

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SEMICONDUCTOR QUANTUM DOTS AND SURFACE LIGAND CHEMISTRY The size quantization effects of QDs led to the exploration of size and shape dependent chemical processes.13 Metal chalcogenides such as CdSe, CdS, PbSe, and PbS served as model systems to explore the excited state dynamics of nanostructures with size quantization in 0, 1, and 2 dimensions.14−16 Given the large surface/volume ratio these nanostructure properties are dictated by the surface defects and the attached ligands. For example, electron rich functional groups such as amines passivate sulfur vacancies while the binding of the thiols create additional vacancies in metal chacogenide systems. These effects can be visualized through the radiative charge recombination processes (Figure 2). Furthermore, by carefully tuning the surface with desired ligands it is possible to tailor the interfacial charge transfer processes.12 Multinary semiconductors such as CuInS2 and CuInSe2 now offer unique properties of size quantized nanoparticles with an environmentally friendly touch.17

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QUANTUM DOT SOLAR CELLS The chemistry of bifunctional surface ligands enables coupling of two different semiconductor nanostructures. For example, 528

DOI: 10.1021/acs.accounts.6b00528 Acc. Chem. Res. 2017, 50, 527−531

Commentary

Accounts of Chemical Research

Furthermore, theoretical understanding of long-range charge transport in QD assemblies is also needed to improve the performance of quantum dot solar cells. Recent efforts with CuInS2 nanostructure electrodes have exhibited efficiencies greater than 11%.27 With the emergence of perovskite solar cells28 with efficiencies greater than 22%, it is important that new strategies are developed to achieve a competitive edge for other types of quantum dot solar cells.

harvesting applications.29 Surface modification of semiconductor surface with bifunctional ligands makes it possible to design hybrid assemblies. A major interest of designing such systems is to mimic photosynthesis by inducing energy transfer coupled with electron transfer processes. Energy transfer through both long-range dipole-based Förster resonance energy transfer (FRET) and short-range Dexter energy transfer (DET) mechanisms has been demonstrated in a CdSe quantum dotsquaraine dye linked system.31 The majority of the spectroscopy work on semiconductor QDs carried out to date establishes the ground rules for interparticle electron transfer processes. In a comprehensive examination of electron transfer between a series of CdSe QD donors (sizes 2.8, 3.3, 4.0, and 4.2 nm) and metal oxide nanoparticle acceptors (SnO2, TiO2, and ZnO), a strong dependence of electron transfer rate on the change in system free energy was established.26 The electron transfer between CdSe QD and metal oxide semiconductors followed Marcus many-state electron transfer theory with a sharp rise at small driving forces followed by a modest rise further away from the characteristic reorganization energy.26 The observed trend mimicked 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. Figure 4 shows a strategy of combining semiconductor QDs and NIR absorbing dye, thus providing new opportunities to harvest photons from different regions of the solar spectrum. CdSe QDs and a squaraine molecule (SQQH) when deposited sequentially onto a mesoscopic TiO2 film, enable energy and electron transfer processes following visible excitation. The synergy of coupling energy and electron transfer process is confirmed from the photocurrent measurements in a hybrid solar cell. Such new opportunities to harvest photons from selective regions of the solar spectrum in an efficient and orderly manner offer new areas to explore.

ENERGY AND ELECTRON TRANSFER IN HYBRID SYSTEMS Fundamental understanding of energy transfer mechanisms between different sized QDs as well as QD-sensitizer systems is crucial in enhancing the light harvest capability of QD sensitized solar cells.30 Extending the absorption of a semiconductor nanocrystal by coupling with another semiconductor nanocrystal or a dye molecule with complementary absorption properties is useful in designing assemblies for light energy

SOLAR FUELS A major application of interfacial electron transfer of semiconductor systems under bandgap irradiation is the production of solar fuels.33 Of particular interest is the photocatalytic water splitting process to produce H2 and O2 and reduction of CO2 to produce C1 products (CO to CH4). The thermodynamic barrier of 1.23 V for water splitting reaction and the electrode overpotential collectively limit the choice of photocatalyst for water oxidation. For example the energetic barrier of water

bifunctional surface modifiers such as mercaptopropionic acid allow linking of CdSe QDs to mesoscopic TiO2 films.24 These QD modified TiO2 films exhibit photocurrent when employed as a photoanode in a liquid junction or solid state solar cell.25 The size dependent electron transfer from excited CdSe into TiO2 was found to be in the range of 109−1011 s−1 following Marcus many-state theory.26 On the other hand, hole extraction at the semiconductor interface is remarkably slower (109 s−1), thus allowing the accumulation of holes within QDs.23 Such slow hole transfer is a major limiting factor in boosting the efficiency of quantum dot solar cells and efforts are needed to overcome this kinetic barrier (Figure 3).

Figure 3. Schematic diagram illustrating various charge-transfer processes that follow the excitation of the semiconductor QD in a quantum dot solar cell. Reprinted from ref 23. Copyright 2014 American Chemical Society.

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Figure 4. (A) Energy (red arrow) and electron (blue arrow) transfer processes in TiO2/SQSH/CdSe hybrid assembly. (B) Energy level diagram illustrating sequential energy and electron processes in TiO2/SQSH/CdSe under visible light irradiation. Reproduced from ref 29. Copyright 2012 American Chemical Society. 529

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Accounts of Chemical Research

Figure 5. Left: Schematic diagram of the tandem BiVO4−CH3NH3PbI3 device for solar fuels generation. The perovskite solar cell harnesses transmitted photons above 500 nm, and the resulting photogenerated electrons drive H2 production. Right: Energy level diagram showing the flow of charge carriers in a tandem photoelectrolysis cell. Reprinted from ref 32. Copyright 2015 American Chemical Society.

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ACKNOWLEDGMENTS This work was supported the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through award DE-FC0204ER15533. The author would also like to thank past and present members of the group for their valuable contributions to our research program. This is contribution number NDRL No. 5148 from the Notre Dame Radiation Laboratory.

oxidation demands the photocatalyst (semiconductor) to have a bandgap energy of >2.0 eV. In addition the conduction and valence bands need to be energetic enough to drive the proton coupled electron and hole transfer processes at the semiconductor interface. This energetic barrier becomes even greater for CO2 reduction. Another hurdle for CO2 reduction is its low concentration in the atmosphere and low solubility in common solvents.34 Despite more than four decades of research, a practical photocatalytic system that can split water without the aid of an external bias or use of a sacrificial electron donor is yet to be realized.35 As pointed out in recent studies, it is important to consider tandem structures with two or more semiconductors capturing different parts of the visible spectrum (see for example, Figure 5) and to overcome the energetic barrier in a water splitting process.32,36 Suitable surface modification of semiconductors with an oxygen evolution catalyst will be the key in overcoming the barrier of the water splitting process. The examples discussed above highlight the importance of semiconductor surface chemistry in the conversion of photons into charge carriers (e.g., photovoltaics) or chemical transformation of molecules (e.g., solar fuels). Significant advances have been made in recent years to understand excited state dynamics and interfacial charge transfer processes of semiconductor nanostructures. As we now design semiconductor photocatalysts with complex architectures, it is important to develop both theoretical and experimental understanding to manipulate their surface chemistry. Specifically, surface chemistry of tandem or hybrid structures with two or more semiconductor nanostructures will play an important role in achieving the desired optical and electronic properties. Such complex structures are also likely to aid in tackling the problems of multiphoton absorption and proton coupled multielectron transfer processes (e.g., water splitting or CO2 reduction). The design of large scale practical devices with long-term stability remains the holy grail of photocatalysis.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. URL: http://www.nd.edu/~kamatlab. ORCID

Prashant V. Kamat: 0000-0002-2465-6819 Notes

The author declares no competing financial interest. 530

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Commentary

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