Letter pubs.acs.org/NanoLett
Photocatalysis Deconstructed: Design of a New Selective Catalyst for Artificial Photosynthesis Vivek Singh,† Ignacio J. Castellanos Beltran,† Josep Casamada Ribot,† and Prashant Nagpal*,†,‡,§ †
Department of Chemical and Biological Engineering, ‡Materials Science and Engineering, and §Renewable and Sustainable Energy Institute (RASEI), University of Colorado Boulder, Boulder, Colorado 80303, United States S Supporting Information *
ABSTRACT: A rapid increase in anthropogenic emission of greenhouse gases, mainly carbon dioxide, has been a growing cause for concern. While photocatalytic reduction of carbon dioxide (CO2) into solar fuels can provide a solution, lack of insight into energetic pathways governing photocatalysis has impeded study. Here, we utilize measurements of electronic density of states (DOS), using scanning tunneling microscopy/spectroscopy (STM/STS), to identify energy levels responsible for photocatalytic reduction of CO2−water in an artificial photosynthetic process. We introduce desired states in titanium dioxide (TiO2) nanoparticles, using metal dopants or semiconductor nanocrystals, and the designed catalysts were used for selective reduction of CO2 into hydrocarbons, alcohols, and aldehydes. Using a simple model, we provide insights into the photophysics governing this multielectron reduction and design a new composite photocatalyst based on overlapping energy states of TiO2 and copper indium sulfide (CIS) nanocrystals. These nanoparticles demonstrate the highest selectivity for ethane (>70%) and a higher efficiency of converting ultraviolet radiation into fuels (4.3%) using concentrated sunlight (>4 Sun illumination), compared with platinum-doped TiO2 nanoparticles (2.1%), and utilize hot electrons to tune the solar fuel from alkanes to aldehydes. These results can have important implications for the development of new inexpensive photocatalysts with tuned activity and selectivity. KEYWORDS: Artificial photosynthesis, hot electrons, quantum dots, selective catalysis
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following photogeneration of the electron−hole pair, has also prevented rational design of new photocatalysts with higher efficiency and/or selectivity. Titanium dioxide (TiO2) has been extensively studied for several photocatalysis, photoelectrochemical, and catalytic reactions.7−13 There have been several experimental and theoretical investigations into modified TiO2 semiconductors for enhancing light absorption or improving the adsorption of reactant species.7−13 While most of the studies focused on enhancing the catalytic rates on TiO2, improving the selectivity of the products obtained can play a crucial role in their viable application, especially due to the high costs of separation processes. The difference in reduction potentials of solar fuels (alkanes, alcohols, aldehydes, Figure 1a,d) provides an opportunity to carefully align these respective products with “molecule-like” energy-levels (instead of bulk-like energy bands) of quantum-confined semiconductors nanocrystal states,
he simultaneous reduction of CO2 and water using sunlight has been an important step in the life cycle on earth.1 This single reaction performed by plants in an energetically frugal, albeit inefficient process,2 allows simultaneous balance of CO2 gas and energy harvesting from primary source of energy on earth, the Sun. Several strategies are being investigated currently for converting sunlight into viable renewable source of energy,3 to address the growing emission of greenhouse gases4−6 and depleting sources of cheap energy for burgeoning human population. Developing an efficient artificial photocatalyst to carry out a simultaneous reduction of CO2 and water can address this issue of rising level of greenhouse gas emission and provide an alternative source of renewable energy (Figure 1a). While several studies have recently addressed developing modified TiO2-based photocatalysts for water splitting or CO2−water reduction,7−13 the effect of chemical modification on the electronic structure of the TiO2 semiconductor has been largely unexplored. Moreover, lack of insight into photophysics governing recombination pathways, defect mediated photocatalytic reduction, and energetic levels participating in electronic reduction of CO2 © 2014 American Chemical Society
Received: October 10, 2013 Revised: January 4, 2014 Published: January 20, 2014 597
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Figure 1. Energy levels responsible for photocatalysis in unmodified TiO2 nanoparticles. (a) Schematic of the proposed artificial photosynthesis in TiO2-QD based catalysts for selectively producing alkanes, alcohols, and aldehydes. (b) CO2−water photocatalytic reduction data, for unmodified TiO2 nanoparticles, under different intensities of simulated solar irradiation (AM1.5 spectrum = 1 Sun). The chemical composition of different catalytic products (hydrogen H2, methane CH4, ethane C2H6, acetaldehyde CH3CHO, and higher hydrocarbons including propane, butane, and paraffins CxHy) is indicated using color coded bar graphs, on a per electron mole basis. The colored asterisk sign indicates the highest yields for respective products obtained in our measurements. (c) Internal quantum efficiency and selectivity of solar fuels (for ethane production) under different intensities of solar irradiance. The internal quantum efficiency (QE, percent) is computed based on the fraction of sunlight above the energy bandgap of TiO2 nanoparticles and the total catalytic yield of different products obtained, per electron mole basis (since different products require different number of electrons in the multielectron reduction process). Unmodified TiO2 exhibits low activity and selectivity for CO2−water reduction. The symbols represent the average internal QE over several experiments, and the lines represent the highest and lowest yields measured. (d) STS data showing defect levels (∼0.5−1 eV below conduction band) in TiO2 nanoparticles, likely responsible for photocatalytic activity (see discussion on advantages of ambient measurements in the Supporting Information). The solid line shows the first derivative of current with voltage (dI/dV) or the density of electronic states, the dashed line represents the current−voltage spectrum, and levels indicated on the right represent the relative reduction potential values for different fuel products formed from CO2−water reduction.15 The work function of the Pt−Ir tip and the electron affinity of TiO2 semiconductor were used to represent the data on an absolute energy level, with respect to vacuum. Standard hydrogen electrode potential was used to represent NHE and energetic levels (w.r.t. vacuum) on the same scale. (e) Cross-section and 3D map of STS data collected on 15 nm unmodified TiO2 nanoparticles. The current−voltage spectrum (I−V) shown here is used to obtain the density of electronic states (dI/dV) based on tunneling probability (or current) from the Pt−Ir tip. The work function of the tip and the electron affinity of TiO2 semiconductor were used to obtain energy values with respect to vacuum level (0 eV). 598
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Figure 2. Effect of platinum metal doping on TiO2 energy levels and photocatalytic rates. (a) CO2−water photocatalytic reduction products obtained using doped TiO2−Pt (1 wt %, see Methods in the Supporting Information) catalyst illuminated with different intensities of AM1.5 simulated solar irradiation. The solar irradiance was increased from half up to 4.6 times the incident radiation. The increase in sunlight illumination results in change in product yields due to filling up of lower energy levels and likely slower cooling of hot carriers, resulting in products with higher reduction potentials. (b) Internal quantum efficiency (percent) and selectivity (for ethane hydrocarbon fuel) obtained using TiO2−Pt (1 wt %) photocatalyst. (c) STS spectrum collected with 0.5 wt % Pt metal dopant in TiO2 nanoparticles and (d) 1 wt % Pt doped TiO2 semiconductor nanoparticle. While the 0.5 wt % Pt metal dopant does not cause significant changes in the electronic DOS of the TiO2 semiconductor (compared to Figure S1 in Supporting Information), 1 wt % platinum clearly shows increased DOS between the bandgap, extending all of the way through the bandgap. This caused likely trapping, and eventually recombination, of the photogenerated electron and hole excited in the doped semiconductor (as shown by arrows in d). This enhanced recombination on platinum sites is likely responsible for reduced photocatalytic yields on increasing the light intensity (as shown in b).
∼1.5−1.7 eV, when compared to CO2 reduction without proton (H+) assistance):15
for the selective generation of solar fuels. Therefore, these semiconductor photocatalysts with different band alignments and/or bandgap energies can yield selective products on illumination with band-edge photons. Moreover, careful tuning of energy states can help in utilizing hot electrons (especially with solar concentrators) to change the selective solar fuel from low reduction potential alkane to aldehyde (which requires higher energy electrons for reduction). Therefore, we can utilize modifications on TiO2 semiconductors, including the attachment of semiconductor quantum dots, to tune the energetic alignment of band-edge states. Since chemical modification of the Fermi level (FL) in semiconductor nanoparticles can have important consequences on recombination dynamics of photogenerated charges14 and density of electronic states which participate in charge transfer for photoreduction of adsorbed species, we carried out careful photophysical studies on unmodified (Figure 1b,c) and modified TiO2 semiconductor nanoparticles (Figures 2−5). The energetic position of these states can affect the reduction potential (Figure 1d) and hence likely product of simultaneous reduction of CO2−water (because simultaneous or synergistic reduction of CO2−water does not require a large overpotential
H 2O + h+ → H+ + ·OH CO2 + 8H+ + 8e− → CH4 + 2H 2O CO2 + 6H+ + 6e− → CH3OH + H 2O CO2 + 4H+ + 4e− → HCHO + H 2O
where e− and h+ represents photogenerated electrons and holes, respectively. Moreover, since different solar fuels (alkanes, alcohols, and aldehydes) require a different number of photogenerated electrons, their respective efficiencies are normalized to the number of electron moles of fuel produced (see Methods in the Supporting Information). We used correlated STM and scanning tunneling spectroscopy (STS)16,17 for measurement of single nanoparticle electronic density of states (see Methods in Supporting Information). Unmodified TiO2 nanoparticles demonstrated low catalytic rates for reducing CO2−water under simulated AM1.5 illumination (represented as 1 Sun) and very low selectivity toward specific solar fuels resulting in a mixture of 599
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Figure 3. TiO2−Cu2−xO composite photocatalyst. (a) Chemical analysis of photocatalytic products obtained using TiO2−Cu2−xO (0 < x < 1) composite catalyst (see Methods in Supporting Information), for CO2−water reduction, on illuminating with simulated solar radiation. (b) TiO2− Cu2−xO photocatalyst shows higher yields (internal QE) for CO2−water reduction than unmodified TiO2, but variation in oxidation state of copper results in change in semiconductor bandgap (Figure S2 in the Supporting Information) and traps on the surface, leading to enhanced recombination, lower yields, and unselective product formation. Due to the large number of defect states present on the surface, the highest catalytic yields are obtained under low light intensities (here 0.5 Sun illumination). On increasing the photogeneration rate (or solar irradiance), while the yield decreases (explained using our photophysical model, Figure 5c), the filling of low energy electron states (c) leads to more products with a higher reduction potential (acetaldehyde and higher hydrocarbons CxHy), thereby increasing the observed selectivity. (c) STS data of the composite photocatalyst showing the overlapping energy levels of Cu2−xO and the TiO2 semiconductor.
Chemical doping of nanoparticle can be used to selectively tune the FL and introduce energetic states with-in the bandgap of a semiconductor.17,19 To utilize this, we introduced some well-studied metal dopants in the TiO2 semiconductor to understand their role in photocatalysis. We first introduced varying amounts of platinum (Pt) dopant in TiO2 to optimize the Pt loading and obtain highest catalytic rates for solar fuel production. While there has been variability in literature on the amount of Pt metal doping to obtain the highest photocatalytic rates,7,12,20 we obtained the highest yield (23.1%, Figure 2a,b) for 1 wt % (percentage by weight) Pt under 1 Sun (AM1.5) simulated irradiation. On studying the effect of metal doping on energetic states, we did not observe any additional states in the electronic DOS of 0.5 wt % Pt-doped TiO2 nanoparticles, when compared to unmodified TiO2 (Figure 2c compared to Figure 1d,S1). However, the analysis of STS data on 1 wt % Pt-doped TiO2 clearly revealed bandtail states within the TiO2 semiconductor bandgap (Figure 2d). Since the photocatalytic activity of CO2−water reduction was enhanced several fold due to the introduction of Pt metal (Figure 2a), it is likely caused due to an increase in adsorption of oxygen atom on the Pt interface.12 But as we increased the photogeneration rate by increasing the light intensity to enhance the photocatalytic conversion, we saw a decrease in catalytic yields (Figure 2b). This observation can be explained since the bandtail states extended throughout the entire Pt-doped TiO2 bandgap, likely facilitating the trapping and recombination of both photogenerated electron and holes (Figure 2d). Therefore, while the
alkanes, aldehydes, and alcohols (Figure 1b,c). However, the presence of some defect states in TiO2 nanoparticles ∼0.5−1 eV below the conduction band (Figure 1d) accelerated the yield of CO2−water reduction reaction (Figure 1b,c). The role of these defect states has also been studied by other groups,7,9−12 and nominally unmodified TiO2 nanoparticles with a larger number of defect states showed higher catalytic activity (see Figure S1 in the Supporting Information). These defect states are likely electron traps14 since they are energetically closer to the conduction band (∼0.5−1 eV) than the valence band (∼2.2−2.7 eV, Figure 1d). Therefore, multiphonon emission leading to relaxation of photogenerated charges in these sub-bandgap states18 supports their role as electron traps. While adsorption of oxygen (or CO2 here) is the rate-limiting step in photocatalytic reactions and oxygen vacancies with similar energies (∼0.5−1 eV below conduction band) have been predicted to catalyze the reaction,7 our measurements suggest that photogenerated electron trapping can also lead to similar adsorption of reactant species. Similar findings also reported by other groups7−13 can likely explain the improved photocatalytic activity due to TiO2 modifications, leading to visible light absorption and improved catalytic activity in TiO2 nanostructures.7,9,13 Therefore, three-dimensional mapping of electronic-DOS in TiO2 nanoparticles (using individual current−voltage scans as shown in Figure 1e) helped us in spatially correlating these electronic states responsible for accelerated photocatalytic yields for CO2−water reduction. 600
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Figure 4. Characterization of our new photocatalyst TiO2−CIS. (a) Chemical composition and (b) photocatalytic yields and selectivity for CO2− water reduction obtained using our designed TiO2−CIS photocatalyst. The new TiO2−CIS catalyst (see Methods in Supporting Information) combines high selectivity (>70% selectivity for ethane production) with high yield (>5% for 5 Sun illumination) and represents an important step toward rational design of new catalyst using energy level tuning, as shown in this photocatalysis data. (c) Electronic DOS tuning can be easily achieved by tuning the energy levels of the quantum confined CIS nanoparticle, as shown in STS data of the composite nanoparticle. 1.7 eV bandgap CIS nanocrystals (∼3.8 nm in size) were attached to TiO2 nanoparticles resulting in desired energy levels below the conduction band of TiO2 semiconductor and possible tuning the products formed from photocatalytic reduction of CO2−water.
Pt dopant may increase the adsorption of reactant molecules,12 enhancing the photocatalysis rates (Figure 2a), it also increases the recombination of photogenerated electrons and holes, leading to an overall reduction in observed photogenerated hydrogen yields. This kind of metal-mediated enhanced recombination is not uncommon in semiconductor−metal interfaces.21 To prevent metal dopants from acting as recombination sites, we incorporated other inexpensive copper-based nanoparticles, with staggered energy levels (Figure 3c, Figure 4c), to add the desired electronic energy states in TiO2 composite photocatalyst. This can also tune the energetic pathway and hence the likely products formed from CO2−water reduction. To develop a robust and stable photocatalyst (see stability measurements in Supporting Information), we utilized copper-oxide nanoparticles grown on surface of TiO2 as “dopants”. The new composite nanoparticle has combined energetic states from the TiO2 nanoparticle and the Cu2−xO (0 < x < 1) semiconductor (Figure 3c). On illumination with AM1.5 light spectrum, there is a rapid increase in CO2−water reduction, as compared to unmodified TiO2 photocatalyst (Figure 3a,b compared to Figure 1b,c). However, the overall incident photon to fuel conversion efficiency (Figure 3c) is low because of variable oxidation state of copper-oxide nanoparticles. Since Cu2O (bandgap 2.1 eV) nanoparticles can be oxidized to CuO (bandgap 1.2 eV) nanoparticles, we have observed Cu2−xO nanoparticles with bandgap ranging from 1.87 to 1.37 eV (Supplementary Figure S2). This change in oxidation state of the Cu2−xO semiconductor can give rise to a charged defect or
trap which is likely responsible for an enhancement in recombination rate of photogenerated electron and results in lower photocatalytic yields at higher photogeneration rates (Figure 3c). To develop a more stable catalyst (see stability measurements in Supporting Information) which can produce a specific product by using the energetic alignment of states, we synthesized copper−indium sulfide (CIS) nanocrystals22 attached to TiO2 nanoparticles. We studied the electronic DOS of our new composite nanoparticle photocatalyst (TiO2−CIS, using single nanoparticle scanning tunneling spectroscopy (Figure 4c). The Fermi level in the TiO2−CIS quantum dots was expected to favor multielectron reduction to alkane hydrocarbon fuels (Figure 4c). When we conducted photocatalytic reduction of CO2−water, we observed evolution of hydrocarbon gases like ethane, propane, and so forth, as shown in Figure 4a. However, the most striking result was that CIS nanoparticles with a bandgap ∼1.7 eV demonstrated very high selectivity (>70%) for the formation of ethane gas (Figure 4a,b), as predicted by our study (Figure 4c). Moreover, when we increased the photon flux and the lower energy states get filled up, hot electrons favor formation of acetaldehyde, and the predominant product switches from alkanes to aldehyde (Figure 4a,b). With high selectivity, hot-electron tunability of the reaction product, combined with yield of ∼5% for this multielectron reduction process using a small concentrator (∼5 Sun illumination, Figure 4b), this new inexpensive photocatalyst shows a lot of promise for improved catalytic reduction of CO2−water into selective solar fuels (see stability measurements in the Supporting 601
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Figure 5. Photophysics in TiO2−CIS nanoparticle catalyst. (a) Current sensing atomic force microscope (CS-AFM) spectrum of a single TiO2−CIS nanoparticle in two regions marked “1” (red curve, TiO2 showing electron injection) and “2” (black curve, CIS showing presence of holes). The inset shows comparable current spectrum of unmodified TiO2 nanoparticle using gold coated current sensing AFM tip. (b) CS-AFM image of a single photocatalyst nanoparticle. The image is acquired by applying a small bias between the gold coated AFM tip, and the nanoparticles dispersed on conductive indium−tin−oxide (ITO) substrate. Cross-section slices of the 3D spectrum obtained clearly shows spatial separation of injected electrons and holes in the composite TiO2−CIS photocatalyst. (c) Schematic of the proposed model explaining the photophysics of the multielectron reduction on photocatalyst. n and p represent photogenerated electron and hole concentration per nanoparticle, on illumination with photons above the TiO2 bandgap. Nd represents the total number of defects/dopants per nanoparticle, and the dashed line represents recombination pathways for photogenerated (with rate constant α) and trapped electrons (with rate constant α) with bandedge photogenerated holes in the photocatalyst. (d) Relationship between photocatalytic rate of solar fuel produced by CO2−water reduction and the rate of photogeneration (expressed as comparable solar irradiance). The decrease in photocatalytic rate of TiO2−Pt doped nanoparticles and the superlinear increase in TiO2−CIS solar fuel production rate is explained well using our simple model.
(rate rt) in dopant/defect centers and back to conduction band (rate rdt), and recombination between electrons in dopant/ defect centers and photogenerated holes (rate constant β) change the steady state concentration of electrons and hence rate of photocatalytic reduction (R = r × n, since reduction is the rate determining step). Using charge neutrality (p = n + nd) and charge balance for electrons (dp/dt = g − αpn − rtn(1 − nd/Nd) + rdtnd − rn = 0 at steady state) and holes (dp/dt = g − αpn − βpnd − rn)), we get a relationship between the photogeneration rate (g) and the rate of photocatalysis (R). Since the photocatalysis rate depends on rate of recombination (α,β), trapping (rt,rdt), and fraction of electrons in defects (nd, compared to n), we identified four regimes for change in photocatalysis rate (r × n) with rate of photogeneration (g = αn(n + nd) + βnd(n + nd) + rn). In the first regime, we identified when α ≫ β (recombination between photogenerated electron and hole) and n ≫ nd, g ∝ n2, and R(= r × n) ∝ g0.5 (bimolecular recombination) or expected square root dependence between rate of photocatalysis and photogeneration rate. In photocatalysis mediated by TiO2 nanoparticles, since traps and defects play an important role, we also modeled (β ≫ α, g ≈ βnd(n + nd)) trap mediated recombination. The second class of results predict when n > nd (Nd is small and/or rt is slow), a linear relationship exists between photocatalytic rate (R = r × n) and incident photon rate (g ≈ βndn, nd is constant, unimolecular recombination, Figure 1c observed in TiO2 nanoparticles). In cases when Nd is large (compared to n,p)
Information). Moreover, this new class of quantum dot (QDTiO2) based composite photocatalysts developed here allows careful tuning of electronic DOS in the composite photocatalyst, by utilizing quantum confinement effect and hotelectron states.23−25 To improve our insights into the mechanism governing high efficiency of photocatalytic yields in the new TiO2−CIS nanoparticle, we utilized current sensing atomic force microscopy (CS-AFM) (Figure 5a,b,S3 see Methods in Supporting Information). We observed a clear spatial separation of electron and hole, as shown in Figure 5a and b. This spatial separation of photogenerated charges can lead to longer lifetimes (due smaller overlap of electron and hole wave functions) and therefore likely higher photocatalytic yields. We developed a simple model (Figure 5c) to explain the photophysics governing the photogeneration, recombination, charge trapping, and the multielectron photocatalytic reduction of adsorbed species in these dopant/defect centers on TiO2 nanoparticles. Upon irradiation with light above the TiO2 nanoparticle bandgap (photogeneration rate g), photogenerated electrons (n) and holes (p) are excited in the conduction and valence “band” of the nanoparticle. A fraction of photogenerated electrons is transferred/trapped (nd) in different dopant/defect centers (total number Nd) in these nanoparticles. Different photophysical events like recombination between photogenerated electrons and holes (rate constant α), transfer/trapping of photogenerated electrons 602
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and trapping rate (rtn(1 − nd/Nd)) of photogenerated electrons is significant, we observed inverse relationship between bandedge photogenerated electrons (n) and trapped electrons (nd ∝ n−x, 0 < x < 1) depending on trapping rates (rt, rdt), total available defects/dopants per nanoparticle (Nd), and the rates of recombination (α,β). In this third regime, we expect to observe superlinear relationship between photocatalytic rate (r × n) and rate of photogeneration (g ∝ n1−x or n ∝ g1/(1−x)). This superlinear dependence (Figure 4b and 5d observed in our TiO2−CIS photocatalyst) implies the rate of multielectron photocatalytic reduction (R) to obtain solar fuels can be enhanced by increasing the photon flux or concentrating sunlight. This can have important implications for design of inexpensive photocatalysts by using solar concentrators, than employing precious metal dopants in new photocatalyst design. However, this increase cannot occur indefinitely (Figure 4b observed in TiO2−CIS), and as the defect states get filled on increasing the rate of photogeneration (rtn(1 − nd/Nd)), our model predicts the slope turns to linear or unimolecular recombination regime. The fourth regime of results obtained from our model include cases when nd > n and hence a possible decrease in multielectron photocatalytic reduction of CO2− water can occur (g ≈ βnd2 ≈ βn−2x) on increasing the rate of photogeneration (Figure 2b and 3b observed in TiO2−Cu2−xO and TiO2−Pt). The experimental results on photocatalysis and the measurements of electronic DOS can explain the observed dependence of catalytic yields and selectivity in different TiO2-based photocatalysts. The insights obtained into the energetic states involved in photocatalysis led a rational design of a new composite TiO2−CIS catalyst, with high selectivity and yields for CO2−water reduction. These results, combined with our model for photocatalytic reduction, explain the photophysics behind this important multielectron reduction reaction. The model also clearly explains the observed relationship between photocatalytic rate and solar concentration for different unmodified (Figure 1c) and modified (Figure 2b, 3b, and 4b) TiO2 photocatalysts. Therefore, this new class of QD-TiO2 photocatalysts developed here, and the photophysics governing multielectron reduction of CO2-water, can lead to development of new inexpensive and selective photocatalysts.
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ACKNOWLEDGMENTS This work was supported by start-up funds from University of Colorado and NSF CAREER Award CBET-1351281. The authors would like to thank Prof. John Falconer and Hans Funke for help with photocatalysis measurements.
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ASSOCIATED CONTENT
S Supporting Information *
General methods, measuring photocatalytic activity, X-ray diffraction, X-ray photoelectron spectroscopy, STM and STS, CSAFM, TEM and optical characterization for copper indium sulfide nanoparticles, advantage of STM/CSAFM characterization in air, photocorrosion studies of our TiO2−CIS catalyst, and supplementary figures. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
V.S. and I.J.C.B. contributed equally. Notes
The authors declare no competing financial interest. 603
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