Efficient Photosynthesis of Organics from Aqueous Bicarbonate Ions

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Efficient Photosynthesis of Organics from Aqueous Bicarbonate Ions by Quantum Dots using Visible Light Biswajit Bhattacharyya, AMIT KUMAR SIMLANDY, Arunavo Chakraborty, Guru Pratheep Rajasekar, Nagaphani B. Aetukuri, Santanu Mukherjee, and Anshu Pandey ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00886 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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ACS Energy Letters

Efficient Photosynthesis of Organics from Aqueous Bicarbonate Ions by Quantum Dots using Visible Light Biswajit Bhattacharyya,1 Amit Kumar Simlandy,2 Arunavo Chakraborty,1 Guru Pratheep Rajasekar,1 Nagaphani Aetukuri,1 Santanu Mukherjee2 and Anshu Pandey1*

1

2

Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012

Department of Organic Chemistry, Indian Institute of Science, Bangalore 560012

Corresponding Author * [email protected]

ABSTRACT: We synthesized CuAlS2/ZnS quantum dots (QDs) composed of biocompatible, earth abundant elements that can reduce salts of carbon dioxide under visible light. The use of an asymmetric morphology at a type-II CuAlS2/ZnS heterointerface balances multiple requirements

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of a photoredox agent by providing a low optical bandgap (~1.5 eV), a large optical cross section (>10-16 cm2 above 1.8 eV), spatial proximity of both semiconductor components to the surface as well as photochemical stability. CuAlS2/ZnS QDs thus have an unprecedented photochemical activity in terms of reducing carbon dioxide in the form of aqueous sodium bicarbonate under visible light, without the need of a co-catalyst, promoter or a sacrificial reagent while maintaining large turnover numbers in excess of 7x104 per QD. Devices based on these QDs exhibit energy conversion efficiencies as high as 20.2 ± 0.2%. These observations are rationalized through our spectroscopic studies that show a short 550 fs electron dwell times in these structures. The high energy efficiency and the environmentally friendly composition of these materials suggest a future role in solar light harvesting.

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ACS Energy Letters

Direct solar carbon dioxide reduction is the conceptual reverse of fuel combustion, and holds the potential of not just mitigating, but rather reversing climate change. While extensive efforts have been made towards electrically assisted CO2 reduction, few demonstrations of exclusively photocatalytic reduction exist in literature.1-5 The potential of wide gap semiconductors such as TiO26-10 ZnO11-14 and ZnS15-16 in promoting light driven reactions has been widely acknowledged. However, since none of these materials absorb visible light, their utility in driving direct solar carbon dioxide reduction reactions has been limited. Indeed, intricate schemes2, 9, 12-13, 16-22 need to be developed to enable practical visible light photocatalysis with these materials. For example, TiO2 and ZnO have low lying conduction bands that have been successfully sensitized by other, stronger visible light absorbers.9, 12 In contrast, while the high conduction band offset of ZnS bestows upon it the potential of carbon dioxide reduction18, it also ensures that this material cannot be readily sensitized by most commonly available quantum dots (QDs) such as CdSe19, 23-27. Demonstrations of photoredox reactions using ZnS have thus employed short wavelength ultraviolet sources that cannot be extended to efficiently harvest sunlight.15-16 Here we develop asymmetric type-II QDs composed exclusively of inexpensive, earth abundant, relatively non-toxic elements. These CuAlS2/ZnS QDs exhibit an optical gap as low as 1.5 eV, much narrower than the bulk band gaps of either semiconductor.28-30 We find that CuAlS2/ZnS QDs can thus efficiently absorb visible light, and thereby photoreduce sodium bicarbonate in aqueous solution. This process involves successive reduction events that eventually give rise to sodium format as a major product. The surprisingly high turnover number >7x104 and efficiencies (as high as 20.2 ± 0.2% with a mean of 17.8 ±0.2%) are rationalized


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through our spectroscopic studies that show a short 550 fs electron dwell times in these structures.

We synthesized CuAlS2 QDs as well as core/shell structures of CuAlS2/ZnS using colloidal air-free shlenck-line techniques. (Figures S1-S2) exemplify the optical and structural properties of tetragonal chalcopyrite CuAlS2 QD materials. CuAlS2/ZnS structures were subsequently synthesized by starting from the initial CuAlS2 QDs and further overgrowing ZnS shells by addition of zinc and sulphur precursors. To confirm the presence of a core/shell structure, we have performed electron microscopy, x-ray diffraction (XRD) as well as x-ray photoelectron spectroscopy (XPS) of the QDs. Figure 1a shows a low resolution transmission electron microscope (TEM) image of elongated CuAlS2/ZnS QDs with a mean aspect ratio of 1:2.2. This elongation is explained by Scanning Transmission Electron Microscopy (STEM) images that show a phase contrast within each QD thereby implying the existence of distinct regions of ZnS and CuAlS2 (Figure 1b). The high and low contrast regions were assigned to ZnS and CuAlS2 respectively using elemental mapping and High Annular Aperture Dark Field (HAADF) imaging (Figure 1c-e). This is further summarized in Figure 1f that shows a HAADF image of a single QD overlaid with the elemental distributions of Zn (blue) and Cu (red). X-ray Photoelectron Spectroscopy (XPS) was used to develop an understanding of the QD surface. Firstly we note that the surfaces of as-prepared CuAlS2 QDs are found to contain copper, aluminium and sulphur atoms consistent with their composition. This is shown in (Figure 1g) for copper and (Figure 1h) for aluminium (red lines). ZnS growth removes the signatures corresponding to aluminium and copper as shown in (Figure 1g and 1h) (black lines). The


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ACS Energy Letters

photoelectron spectra of CuAlS2/ZnS QDs further show the presence of zinc and sulphur on the surface. These data imply a continuous ZnS surface coating (Figure 1i and 1j). We engineered asymmetric ZnS growth on CuAlS2 through the epitaxy of two different crystal structure types. In particular, for the CuAlS2/ZnS system, it is firstly important to note that ensemble level XRD (Figure 2a, black line) shows the presence of the hexagonal variant of ZnS (red line) in CuAlS2/ZnS QDs. As discussed earlier, CuAlS2 itself has a tetragonal lattice structure. The epitaxial growth of these two different lattice types is however possible under certain directions. In particular, the [002] plane of ZnS has an interplanar separation identical to the interplanar separation of [1 1 2] planes of CuAlS2 (0.32 nm). The epitaxial growth of ZnS on CuAlS2 can thus occur by continuity of [002] and [1 1 2] planes at the hetero-interface. These data confirm that CuAlS2/ZnS QDs may be described as core/shell materials where the CuAlS2 core is completely embedded in an asymmetric ZnS coating (Figure 2b).This is also shown in the high resolution TEM (HRTEM) image (Figure 2c) as well its selected area Fourier transform (Figure 2d). Collectively these images confirm a well-defined alignment of planes of ZnS and CuAlS2 across the entire structure. This point is further illustrated in the method section where epitaxy is inferred from high resolution STEM images (Figure S3). In contrast to our approach, most previously reported synthetic approaches have relied upon oriented attachment or strain to produce anisotropy or asymmetry31-33. The asymmetric morphology of CuAlS2/ZnS offers several advantages. The proximity of both electron and hole containing phases to the QD surface is important for photochemistry. At the same time, the CuAlS2 core is insulated from the surface by a thin but significant ZnS shell that protects it from degradation (Figure S4-S5). Most notably, this morphology is also associated


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with large optical absorption cross sections in the visible (for example >10-16 cm2 for photon energies >1.85 eV in Figure 3a), making it suitable for light harvesting. In particular, while both CuAlS2 and ZnS have optical band gaps in the ultraviolet region (3.5 and 3.9 eV respectively)24, 28-30, 34-35, the CuAlS2/ZnS heterointerface has a type-II character, with an effective gap of 1.4 eV (Figure S6) 24, 26, 34-37 thereby allowing nanostructures to absorb visible light. As already noted in the introduction, the lowest bandgap observed in this work is slightly higher, 1.5 eV, presumably due to quantum confinement effects. As a further consequence of this staggered offset, the electrons relax to the ZnS conduction band edge, while the holes relax into the CuAlS2 layer. This is in contrast to most other ZnS containing core/shell structures where the electron is expelled from the ZnS layer after relaxation38, and leads to important differences in the optical properties of CuAlS2/ZnS QDs. For example, CuAlS2/ZnS QDs exhibit lower emission quantum yields (e.g. < 3% in Figure S7a) than typical type-I ZnS containing QDs such as CdSe/ZnS39 40-41. This distinction is linked to electron residence times which can be measured by transient absorption spectroscopy. Figures 3b and 3c show pump induced bleach features observed in type-I CdSe/ZnS (black line) and type-II CuAlS2/ZnS (red dots) respectively. The features correspond to 0.6 (0.5) excitons per CuAlS2/ZnS QD (CdSe/ZnS), and correspond to the filling of the lowest lying electronic levels. Linear spectra of each sample are shown by the dashed lines and also in Figure S8. Consistent with previous studies38, CdSe/ZnS QDs show long lived electron populations that do not decay significantly over a 400 ps time window (Figure 3d). In contrast, the band edge bleach of CuAlS2/ZnS QDs decays with a time constant of 550 fs (Figure 3e). The rapid depopulation of CuAlS2/ZnS electronic levels could occur because of exciton recombination or else by electron expulsion from the conduction band. Because of the high


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ACS Energy Letters

crystallinity of CuAlS2/ZnS structures, ultrafast recombination is a less likely eventuality in comparison to electron expulsion. Electron expulsion is further supported by Upconversion Photoluminescence (UPL) as well as low temperature photoluminescence. In Figure 4a, two different lifetimes with different emission band have been observed. In particular at 2.53 eV the average lifetime is 3.7 ns but at 1.91eV, it has a short lifetime of 7 ns with a 42 ns long tail. To understand the nature of this band we correlated the TA and UPL studies. Figure 4b shows conduction band depopulation (TA) with a 550 fs lifetime (grey dotted line). The UPL signal is associated with a similar buildup time (590 fs) (red dot solid). It is thus inferred that luminescence from the sample begins after electron removal from the conduction band, implying that PL originates only from defects. As the sample emission occurs in the energetic vicinity of the absorption, we deduce that the trap states are shallow. This is further confirmed by examining the temperature dependence of material luminescence (Figure 4c). In particular, while cooling the samples does slightly enhance the emission intensity on the blue side of the emission band, the continued luminescence of the material at low temperature is consistent with shallow surface electron traps. Indeed, past studies have shown sub-picosecond expulsion of hot electrons from CdSe/ZnSe QDs. This effect was observed when electrons were nearly isoenergetic with the bottom of the zinc chalcogenide conduction band42. Our data are qualitatively similar, except that the lowest lying electron levels are derived from the zinc chalcogenide conduction band in the case of CuAlS2/ZnS structures. We therefore considered the possibility of electron expulsion in greater detail, and examined the propensity of this material to drive photochemical processes such as bicarbonate reduction43. As shown in (Figure S6), an electron in the ZnS conduction band has a


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large -1.68 V electrochemical potential that is sufficient to drive a range of electrochemical reduction reactions, including bicarbonate reduction that is considered here. For these experiments, a visible light source was used to illuminate CuAlS2/ZnS QDs immersed in an aqueous sodium bicarbonate solution. It is observed that QDs indeed give rise to formate under these conditions (Figure S9-S11). Insignificant amounts of other reduction products (Figure S9c) were detected in reactions run for very long periods of time. It was inferred that these products formed only after the buildup of a substantial concentration of formate. Consequently, only reaction conditions optimized for formate synthesis alone were used in further quantitative studies, and thus subsequent formate reactions were prevented in all quantitative data. Oxidation products are described in Figures S13-S14. No products are observed in control experiments where any one or more of the factors (QDs/light/bicarbonate) are missing (Table S1 and Figure S15). We employed 1H nuclear magnetic resonance (NMR) spectroscopy to detect the reaction products. The occurrence of this reaction was independently confirmed by using powder XRD (Figure S16). Our results were further confirmed through 13C solid state NMR (Figure S17). To confirm the identity of the oxidation side of the reaction, we attempted to look for oxidized species such as peroxide as well as oxygen. As shown in Figure S14, oxygen evolution was clearly observed during the photochemical reduction of bicarbonate. In Figure 5a, we have shown the formation of both oxidizing and reducing products with the course of time where it is clear that a formate:oxygen stoichiometry of ~2:1 is maintained during the course of the reaction. Further, an averaged formate to oxygen ratio of 1.97:1 was verified to persist even over longer times by quantitative 1H NMR and volumetric oxygen measurements (please see product analysis in the Supporting Info. as well as Figures S13-S14). Therefore we can conclude that the reductive and oxidative photochemical reactions are respectively:


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ACS Energy Letters

‫ܱܥܪ‬ଷି + 2݁ ି ሺܳ‫ ∗ܦ‬ሻ + ‫ܪ‬ଶ ܱ → ‫ ିܱܱܥܪ‬+ 2ܱ‫ ି ܪ‬, and 4ܱ‫ → ି ܪ‬2‫ܪ‬ଶ ܱ + ܱଶ + 4݁ ି ሺܳ‫ܦ‬ሻ, where ܳ‫ ∗ܦ‬represents a photoexcited QD, implying an overall reaction ௛௩, ொ஽, ௪௔௧௘௥

2‫ܱܥܪ‬ଷି ሱۛۛۛۛۛۛۛۛۛۛሮ 2‫ ିܱܱܥܪ‬+ ܱଶ Isotopic labeling studies with

13

C labeled (98%) sodium bicarbonate are presented in

Figure 5b. 13C enrichment of the products is confirmed through the observation of enhanced 1H13

C coupling peaks The observation of

13

C enrichment in 1H NMR confirms that sodium

bicarbonate is the only carbon source. In order to determine the feasibility of photoreduction for large scale usage, we studied the turnover numbers associated with CuAlS2/ZnS QDs. An estimate was made as follows. 100 mg of CuAlS2/ZnS QDs were used to reduce 25 ml of an aqueous solution containing 2 g of sodium bicarbonate. The solution was exposed to light for 48 hours. 1H NMR spectra of the resultant substance were determined using the procedure described in Figure S10-S12. A known quantity of DMSO was added to the product solution only after the completion of the reaction (Fig. 5c), allowing us to determine the quantity of sodium formate using a calibration curve (Figure S18). 1H NMR confirmed the formation of 704.5 mg of sodium formate after reaction. The ratio of moles of formate produced to the moles of QDs used was thus determined to be 7.8 × 10ସ providing a lower estimate of the turnover number. It must be noted that the turnover number thus obtained is limited by the amount of time for which the reaction has been carried out and only represents a lower bound. While QDs do eventually degrade after prolonged usage (Figure S19), the reduction of 7.8 × 10ସ molecules of bicarbonate per QD is nevertheless consistent with stability of QDs towards the photoredox cycle itself.


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The high turnover numbers are readily rationalized by the remarkably high photon to formate conversion efficiencies of these materials. Indeed, we determined that CuAlS2/ZnS QDs have internal conversion efficiencies as high as 84% for 2.33 eV photons (Figure S20). As shown in the action spectrum, the material exhibits similar activities across a wide range of photon energies (Figure S21). Collectively these data suggest a remarkable potential for solar to chemical energy conversion. We therefore turn to evaluate the efficiency of these materials towards harvesting sunlight. This quantity was measured by constructing a reactor where 56 mg of CuAlS2/ZnS QDs were deposited onto ~1 mm sized glass particles. The reactor was shaped as a cylinder of inner diameter 2.2 cm, and the glass particles fill up a height of ~1.5 cm from the reactor base. The reactor was loaded with 0.8 ml of a solution containing 87 mg of sodium bicarbonate. The reactor top is covered with a quartz lid. Such devices typically yielded an external quantum efficiency of 82% for 2.33 eV photons (Figure S20), close to the thermodynamic limit. This assembly was irradiated with AM1.5 sunlight derived from a solar simulator from the top lid (see Figure 5d inset for a schematic representation of the reaction setup). After 30 minutes of light exposure, the amount of sodium formate was determined using 1H NMR spectroscopy with an internal standard (dimethylsulfoxide, DMSO) in a D2O medium. Briefly, the water soluble products were separated from QDs by decantation, and the water was the removed by evaporation. The solid residue obtained was then redissolved in D2O followed by addition of DMSO. The areas under the proton signals in NMR were determined by integration. These were converted into moles by using a calibration curve (Figure S18) constructed out of known amounts of both reagents. In the example shown in (Fig. 5d), 20µL of DMSO was added to the reaction products in D2O. The relative areas under the peaks associated with each reagent was used to estimate the


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ACS Energy Letters

formation of 0.48 mmol (32.6 mg) of sodium formate after 30 min of light exposure. The enthalpy of formation of sodium formate is known to be -666.5 kJ/mol. Further, sodium bicarbonate has an enthalpy of formation of -950.8 kJ/mol44. The conversion of 1 mole of solid sodium bicarbonate to solid sodium formate thus implies the storage of 284.3 kJ of energy. From this, we estimate the chemical energy stored into the reactor contents to be 138.48 J. This corresponds to a 20.2% solar to chemical energy conversion efficiency that is significantly greater than biomass accumulation through terrestrial photosynthesis. As the error in estimation of the formate concentration is known to be 0.7%, the error in this particular efficiency measurement is estimated to be 0.2%. Efficiencies of a representative set of devices are shown in the extended data section (Figure S22-S26). An average of 17.8% is observed. This high efficiency is consistent with the ability of CuAlS2/ZnS QDs to utilize visible light for driving bicarbonate reduction. We note that the equation employed by us corresponds to the conversion of a pure solid bicarbonate salt into a pure, solid formate salt. Due to the use of enthalpies, the obtained efficiency corresponds to the energy efficiency of conversion of solar energy into stored chemical energy by an overall process that starts from solid sodium bicarbonate and ends in sodium formate. It is further possible to evaluate the efficiency of the catalyst to convert solar energy into useful non-pressure-volume work that becomes stored into the aqueous formate species. As shown in the supporting section, our most efficient device exhibits an energy to available chemical work conversion efficiency of 16.8%, while the average across devices is14.7%. We note that these numbers may be estimated from other device data already presented. In particular, through the quantum efficiency data we estimate that our devices utilize 25.6% of all incoming photons. Using average solar photon energy of 1.4 eV and the electrochemical work required of 1.04 eV,


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we obtain an expected energy to photoelectrochemical work conversion efficiency of 17.6%, in excellent agreement with the observed value of 16.8% for the most efficient device. In conclusion, we synthesized CuAlS2/ZnS structures that can reduce aqueous bicarbonate ions to acetate and formate under visible light. Sodium acetate and sodium formate were photosynthesized from aqueous sodium bicarbonate. These QDs were further shown to exhibit extremely short 550 fs electron dwell times. The high turnover numbers (>7x104 molecules of sodium formate produced per QD), solar to chemical energy conversion efficiencies of the total bicarbonate to formate process(20.2 +/- 0.2%) and stability (Figure S27) prove the potential of these materials as solar light harvesters. We note that these high energy efficiencies correspond to a 16.8 % efficiency of conversion of solar energy input into chemical work stored into the formate species. The quest to employ solar based renewable energy has intensified in the past few years due to increased awareness regarding CO2 induced climate change. As photoreductive solar energy harvesters, CuAlS2/ZnS QDs offer unprecedented advantages: these are composed of completely biocompatible, earth abundant, inexpensive elements; these exhibit very high solar to chemical energy conversion efficiencies and finally, light harvesting via these materials may be set up to reduce the carbon dioxide already present within the earth’s atmosphere45.


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Figure 1.Structural Properties of CuAlS2/ZnS QDs. (a) TEM image of CuAlS2/ZnS QDs. (b) STEM image of CuAlS2/ZnS showing ZnS (bright) and CuAlS2 (dim) regions. (c) HAADFSTEM mapping of CuAlS2/ZnS QDs. (d) Elemental mapping showing Cu distribution and (e) Elemental mapping showing Zn distribution. (f) Reconstructed image showing abundances of copper (red) and Zinc (blue) distributed across a single QD with scale bar of 5 nm. (g) Surface sensitive XPS spectrum of Cu levels in CuAlS2 (red) and CuAlS2/ZnS (black) QDs. (h) Surface sensitive XPS spectrum of Al levels in CuAlS2 (red) and CuAlS2/ZnS (black) QDs. (i) XPS spectrum of Zn levels in CuAlS2/ZnS QDs. (j) XPS spectrum of S levels in CuAlS2/ZnS QDs


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Figure 2. Structural Properties of CuAlS2/ZnS QDs. (a) XRD pattern of CuAlS2/ZnS QDs (thick black line) along with the patterns for tetragonal CuAlS2 (top, blue) and hexagonal ZnS (bottom, red). (b) Schematic of CuAlS2/ZnS QDs highlighting asymmetry and complete capping of the inner CuAlS2 core as inferred from the techniques described above. (c) HRTEM image of a CuAlS2/ZnS QD. (d) Fourier transform of the region marked in red in panel m showing coincidence of spots corresponding to ZnS [002] and CuAlS2 [112] planes. Note that the image generated by the transform has been cropped and scaled. The red circle corresponds to a CuAlS2 [112] plane, while both CuAlS2 [112] and the ZnS [002] planes give rise to the maximum that is indicated by the blue circle, implying a common directionality.


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ACS Energy Letters

Figure 3. Optical properties of CuAlS2/ZnS QDs. (a) Optical cross section of a CuAlS2/ZnS QDs (In inset schematic of a type-II band alignment of CuAlS2/ZnS). (b) Absorbance (red, dashed) and Bleach (solid, black) of a sample of CdSe/ZnS. The horizontal line corresponds to the zero of the y axis. Bleach feature recorded 6 ps after initial photoexcitation and represents a cooled 1S exciton. (c) Band edge bleach feature observed in the case of CuAlS2/ZnS QDs (red dots) and absorbance (dashed blue). Bleach feature has been recorded immediately after photoexcitation. (d) Bleach dynamics at 2.17 eV (572 nm) in CdSe/ZnS QDs in the single exciton regime. Bleach is stable over the 400 ps duration of the experiment. (e) Transient bleach of CuAlS2/ZnS at 2.25 eV (550 nm) decays with a lifetime of 550 fs.


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Figure 4. Optical properties of CuAlS2/ZnS QDs. (a) Lifetimes at different emission maxima like 2.53 eV (490 nm) lifetime is fast (average lifetime 3.7 ns) and slower in 1.91 eV (650 nm) (biexponential decay with an extended tail 7 ns and 42 ns). (b) TA (grey, dashed line) and Upconversion Photoluminescence (UPL) (red solid dot) decay of CuAlS2/ZnS QDs at 2.25 eV (550 nm). (c) Low temperature photoluminescence of CuAlS2/ ZnS QDs.


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Figure 5. Photo reduction of sodium bicarbonate. (a) Formation of formate and oxygen with time. Product formation rates are nearly constant with time, with an excellent agreement to the ideal stoichiometric ratio of 2:1. (b) 1H NMR of photo reduction product of 13C labelled sodium bicarbonate is shown above (Black thick line). For comparison (in inset) a typical NMR for formate produced from unlabelled bicarbonate is also shown in the inset (thin green curve). This confirms bicarbonate to the only source of carbon. (c) 1H NMR of a reaction mixture used to estimate turnover numbers. DMSO has been added to enable determination of concentration. (d) 1

H NMR of the products formed upon illuminating a light harvesting device based on

CuAlS2/ZnS QDs with an AM 1.5 sunlight for 30 minutes. Inset: Schematic of the device.

ASSOCIATED CONTENT Supporting Information. Detail method of synthesis and characterization with efficiency calculation. All supporting figure stated in the main text (S1-S27) with supporting Table S1.


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AUTHOR INFORMATION Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We thank Prof. Satish Patil (Solid State and Structural Chemistry Unit, Indian Institute of Science) for allowing us to use their laboratory equipment. We also thank Prof. G. Mohan Rao, Instrumentation and Applied Physics Department, Indian institute of science for providing the XPS measurements. We further thank Prof. H. S. Atreya (NMR Center) for helpful discussions. A.P. acknowledges DST Nano Mission [Grant No. SR/NM/NS-1117/2012] for funding. Spectroscopic studies were performed on equipment purchased and maintained under an IRHPA grant [IR/S2/PU-0005/2012]. SM thanks Science and Engineering Research Board (SERB) [EMR/2016/005045]. NBA thanks Indian Institute of Science for seed funding. B.B. thanks Indian Institute of science for fellowship. REFERENCES (1) Zhou, X.; Liu, R.; Sun, K.; Chen, Y.; Verlage, E.; Francis, S. A.; Lewis, N. S.; Xiang, C. Solar-Driven Reduction of 1 atm of CO2 to Formate at 10% Energy-Conversion Efficiency by Use of a TiO2-Protected III–V Tandem Photoanode in Conjunction with a Bipolar Membrane and a Pd/C Cathode. ACS Energy Lett. 2016, 1, 764-770. (2) Pan, Y.-X.; You, Y.; Xin, S.; Li, Y.; Fu, G.; Cui, Z.; Men, Y.-L.; Cao, F.-F.; Yu, S.-H.; Goodenough, J. B. Photocatalytic CO2 Reduction by Carbon-Coated Indium-Oxide Nanobelts. J. Am. Chem. Soc. 2017, 139, 4123-4129. (3) Gao, S.; Lin, Y.; Jiao, X.; Sun, Y.; Luo, Q.; Zhang, W.; Li, D.; Yang, J.; Xie, Y. Partially Oxidized Atomic Cobalt Layers for Carbon Dioxide Electroreduction to Liquid Fuel. Nature 2016, 529, 68-71. (4) Behrens, M.; Studt, F.; Kasatkin, I.; Kühl, S.; Hävecker, M.; Abild-Pedersen, F.; Zander, S.; Girgsdies, F.; Kurr, P.; Kniep, B.-L.; Tovar, M et al. The Active Site of Methanol Synthesis Over Cu/ZnO/Al2O3 Industrial Catalysts. Science 2012, 336, 893-897.


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ACS Energy Letters

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