The Rational Design of a Single‐Component Photocatalyst for Gas‐Phase CO₂ Reduction Using Both UV and Visible Light

The solar-to-chemical energy conversion of greenhouse gas CO2 into carbon-based fuels is a very important research challenge, with implications for both climate change and energy security. Herein, the key attributes of hydroxides and oxygen vacancies are experimentally identified in non-stoichiometric indium oxide nanoparticles, In2O3-x(OH)y, that function in concert to reduce CO2 to CO under simulated solar irradiation. The emerging field of solar fuels centers on storing radiant solar energy in the form of chemicals that can be used as an alternative to fossil fuels. A major goal in this field is to realize an “artificial leaf” – a material that converts light energy in the form of solar photons into chemical energy – using CO2 as a feedstock to generate useful chemical species. Enabling this technology will allow the greenhouse gas, CO2, emitted from energy production and manufacturing exhaust streams to be converted into valuable products (such as solar fuels or chemical feedstocks), thereby creating huge economic and environmental benefits by simultaneously addressing energy security and climate change issues.1-4 While the global research effort with respect to the artificial leaf has focused on H2O splitting, the photocatalytic reduction of CO2 remains a significant challenge and thus form the focus of our work.5 This artificial leaf can exist in multiple configurations, of which gas-phase photocatalysis has been identified as the most practical and economically feasible option for large-scale CO2 reduction.5 Thus, the envisioned artificial leaf will be a multi-component system that intakes large quantities of gaseous CO2 and pipes out large volumes of carbon-based fuels. Clearly, the key component of this artificial leaf system is a functional material that utilizes the energy from absorbed solar photons to drive the complex multi-electron and proton transfer reactions involved in reducing CO2 to fuels. As a result, there is growing interest in synthesizing semiconductor nanomaterials, which have the surface, optical, and electronic properties that can enable photocatalytic reduction of gas-phase CO2 to generate solar fuels.5-12 However, despite the growing interest and investment in the field, there are few examples of successful gas-phase photocatalysts – particularly those active in the visible region of the solar spectrum – suggesting that new approaches to materials discovery are necessary.13 A class of materials capable of photocatalytically reducing CO2 are oxygen deficient metal oxides. Oxygen vacancies can function as active catalytic sites and enhance both the absorption of visible light and the photocatalytic activity of the material.14, 15 The most notable example of this is black titania, TiO2-xHx, which exhibits a substantial increase in light absorption and photoactivity for water splitting after hydrogen treatment.16, 17 Another effective approach to increasing the photocatalytic activity of metal oxide nanomaterials is to improve the CO2 capture capacity of the nanoparticle surface. Several groups have demonstrated that surface hydroxides can enhance the affinity of CO2 for a photocatalytic surface, which can have a significant effect on the photocatalytic activity and CO2 reduction rates.18-20 Clearly, the surface, optical, and electronic properties of metal oxide nanoparticles must work in concert for photocatalytic reduction of CO2 to occur; understanding this relationship is critical for the advancement towards a practical global scale solar fuels technology.13, 17, 19, 21-23 Indium oxide is a material with surface, optical, and electronic properties that make it a compelling choice as a CO2 reduction photocatalyst. For example, its conduction band (CB) and valence band (VB) positions on an energy band diagram straddle the H2O oxidation and CO2 reduction half reaction energies required to drive artificial photosynthetic production of hydrocarbons and carbon monoxide.4, 24 Furthermore, In2O3 has a direct “forbidden” band gap where the lowest-energy optical transition from the top of its VB to the bottom of its CB and vice-versa is forbidden by symmetry.25 This “forbidden” transition has been shown in other materials to provide a built in mechanism for decreasing photo-excited electron-hole pair recombination rates and prolonging their lifetime, thereby greatly increasing their chances of carrying out useful surface chemistry.26 In addition to these beneficial optical and electronic properties, the surface properties of In2O3 have garnered interest in the field of thermally driven heterogeneous catalysis. Sun et al. have demonstrated the high activity of In2O3 towards the reverse water gas shift (RWGS) reaction at high temperatures, specifically citing CO2 capture as a key factor in enhancing the activity.27 Ye et al. have suggested from computational modeling that surface oxygen vacancies could act as active sites to promote thermally driven methanol synthesis.28 In this paper, hydroxylated indium oxide nanoparticles (In2O3-x(OH)y), populated with surface hydroxides and oxygen vacancies, are investigated as a gas-phase CO2 reduction photocatalyst. We use a temperature-programmed thermal dehydration reaction to make In2O3-x(OH)y nanoparticles from In(OH)3. This simple and “green” fabrication method has numerous advantages including high atom economy, ease of scale-up, and negligible residual carbon contamination, which can block active sites and lower the overall gas-phase adsorption capacity and catalytic activity.29 Moreover, since it has been reported that the sample calcination temperature has an effect on the incident photon-to-electron conversion efficiency (IPCE) of In2O3 films for photoelectrochemical water splitting30 as well as the photocatalytic degradation of dyes,31 we produced, characterized and evaluated the photocatalytic performance of In2O3-x(OH)y nanoparticles prepared via thermal dehydration reactions at 250 °C, 350 °C, and 450 °C, in addition to crystalline In(OH)3 nanoparticles prepared from the same precursor. Although minimal amounts of organics are present in our synthesis, we still took precaution by using 13C-labelled CO2 (13CO2) as a reactant while testing the photocatalytic performance of these nanoparticles for CO2 photocatalytic activity. Light-driven CO2 conversion rates reported in the literature are often low and the ubiquitous carbon contamination from carbon-containing precursors, organic solvents, and organic additives that are used to control the size and morphology of the nanostructure can create false positive results, calling into question the validity of previously reported photoactivity.29 In fact, until recently few studies provided this type of evidence to support their claims, however this practice is becoming increasingly more common due to increased recognition of the importance of these tests.32 The overall reaction between CO2 and H2O is highly endergonic, with the majority of the energy consumed in splitting water; but vast improvements in H2O splitting systems have opened the possibilities for a sustainable and economically competitive supply of H2 as a reactant. Reactions with H2 and CO2 are thermodynamically favourable relative to those between H2O and CO2. Thus, H2 generated separately via solar-driven water splitting can be used in the subsequent photocatalytic reduction of CO2 – such as the one described herein – to maximize the potential of the harvested sunlight. Therefore, CO2 reduction photocatalyts that operate in a H2 environment at reasonably elevated temperatures provide valuable insights into CO2 reduction mechanisms and increase the opportunity for researchers to discover components for a scalable artificial leaf. The majority of CO2 reduction photocatalysts reported in the literature operate at room temperature or 80 °C for gas- and aqueous-phase reactions, respectively.13, 17 A key insight presented in this study is that although a photocatalyst may show little or no activity at these low temperatures, by slightly elevating the reaction temperature the material can be activated and function as a CO2 reduction photocatalyst. These moderate temperatures can easily be reached by using simple solar trough concentrators,33 meaning that no external energy input is required to heat the samples. In this work we report gas-phase photocatalytic conversion of 13CO2 in the presence of H2 to generate 13CO at a rate as high as 0.25 μmol gcat−1 h−1 in a batch reactor at 150 °C under simulated solar illumination intensities of 2200 W m−2 on hydroxylated indium oxide nanoparticle films. We then perform the isotope tracing experiments with coupled gas chromatography-mass spectroscopy (GC-MS) analysis to confirm – with complete certainty – that the observed gaseous products originate from 13CO2 feedstock rather than adventitious carbon sources.34 Furthermore, under only visible light irradiation (λ > 420 nm) we find that our indium oxide nanoparticles photocatalytic reduce 13CO2 at a rate of 70 nmol gcat−1 h−1 at the same light intensity. Finally, by using a tubular flow reactor under similar conditions but with flowing CO2 and H2, the observed CO production rate can be further increased to 15 μmol gcat−1 h−1. Our results show that by combining the favourable optical and electronic properties inherent to indium oxide with a judiciously tailored surface, In2O3-x(OH)y nanoparticles can function as an active photocatalyst for gas-phase CO2 reduction. This study provides valuable insight about key parameters for the composition selection, materials design and performance optimization of photocatalysts suited for large-scale solar fuels production. Hydroxylated indium oxide nanoparticles were produced via a thermal dehydration of In(OH)3 (Figure 1). As the transition from In(OH)3 to In2O3-x(OH)y does not occur until approximately 210 °C, only samples heated above this temperature undergo dehydration to form indium oxide.35, 36 The transmission electron microscopy (TEM) images in Figure 1 illustrate the change in nanostructure morphology with increasing calcination temperature. The In(OH)3 sample calcined at 185 °C (Figure 1a) consists of large porous sheet-like structures. As the calcination temperature is increased to 250 °C, the sheet-like structures decompose into clusters of fused nanoparticles approximately 5 nm in diameter (Figure 1b and 1f). The overall clusters are similar in size to the In(OH)3 sheets, indicating that the observed porosity is likely a result of water molecules being released from the as the In(OH)3 As the calcination temperature further to 350 °C (Figure and and 450 °C (Figure and the size and overall porosity of the clusters The high field images and the in Figure confirm that sample consists of a crystalline The sample at 185 °C to form while other samples form In2O3 with no In(OH)3 crystalline For the of In2O3 samples prepared at the calcination temperatures of 250 °C, 350 °C, and 450 °C will be to as and The optical properties of sample were from the shown in Figure As the absorption of In2O3-x(OH)y is in to In(OH)3. These were with a (Figure to the optical band gap of as in Figure these with the valence band and energy from spectroscopy we can the band relative to the (Figure which well to has been reported in the The of the energy the conduction band that the In2O3-x(OH)y samples are and the overall band that samples may have reducing to photocatalytically drive gas-phase CO2 reduction In to confirm the photocatalytic activity of the In2O3-x(OH)y carbon (13CO2) is used as a to products from the photocatalytic reaction in the presence or of irradiation. This is an important that the carbon of the products from CO2 or from adventitious carbon contamination on the of reaction at 150 °C under both light W m−2 using a W metal and produced H2O and amounts of that is produced at an rate of nmol gcat−1 under and produced in the of irradiation. observed that the production rate with subsequent batch The using (Figure that the of the of is while the of the 17 of is above This is produced by the or reaction of adventitious carbon on the surface and not from the CO2 In it that CO is a of CO2 photocatalytic activity and is produced only under light irradiation at an rate of 0.25 μmol gcat−1 in batch Figure the relative intensities of the of and the of 13CO under both and light The of the in the and the significant increase in its under irradiation that the conversion of 13CO2 to 13CO is light This is further by a of Figure that CO production with only under while production remains at under both and light studies have that sample calcination temperature can have a effect on the aqueous-phase photoelectrochemical performance of indium oxide In to the calcination temperature the photocatalytic performance in the we the CO production rates under light and conditions for a of In2O3-x(OH)y sample with of sample and calcined at temperatures and as well as a In(OH)3 control sample The CO production rates for these samples under 80 of irradiation using a W metal at a of reaction temperatures are shown for sample in Figure In indium oxide samples calcined at lower temperatures produced CO at rates for the of reaction temperatures The photocatalytic CO2 reduction rate at 150 °C for is that of and approximately an of than that of The In(OH)3 control produced no CO at temperature. and an increase in photocatalytic CO production with increasing reaction a at 150 °C and decreasing in activity at on the other an increase in reaction activity at °C relative to 150 °C, in to and to on the temperature of the the reaction not investigated at temperatures than reactor are being investigated to these at The effect of irradiation on CO production rates for sample at reaction temperatures is shown in Figure As samples produced very little CO under with a of nmol gcat−1 h−1 at °C for that the observed gas CO2 reduction is a In to the effect of light on the CO production a with a W with an to the solar The photocatalytic activity of the sample at 150 °C under light intensities from to Figure a increase in CO production rate with increasing light which further that the CO2 to CO conversion is a A sample used for the of these the of this photocatalyst. The of CO production investigated (Figure A with a with an or an with a 420 nm or a nm The light to W using a to the and a to the the sample with the a CO production rate of μmol gcat−1 The – with the 420 nm high that with energy than 420 nm – produced CO at a rate of 70 nmol gcat−1 CO a nm Finally, a of the using only the the rate of μmol gcat−1 These results that not only is the capable of gaseous CO2 to CO using only visible which well with the but that In2O3-x(OH)y is under these reaction conditions and can rates with the after being for In an to more photocatalytic rate were out in a tubular flow reactor with a This of the In2O3-x(OH)y nanoparticles that under similar conditions to those in the batch 150 °C and under flowing CO2 and H2, CO is photocatalytically produced at a rate of 15 μmol gcat−1 h−1 with 24 of the nanoparticle These rates are than previously reported CO production rates for other component metal for μmol gcat−1 h−1 for and μmol gcat−1 h−1 for This increased activity is the focus of a in to in this the photocatalytic rate presented are to those from batch This by in size between and and to more make between these samples. in size of in a can result in substantial in between samples more In to further the of calcination temperature on the photocatalytic were Figure that the to a lower energy as the calcination temperature is indicating an increase in the In as a result of the of The in Figure a between In(OH)3 and In2O3-x(OH)y samples. is an approximately shift to lower energy of the from for In(OH)3 to for a in the of the In2O3-x(OH)y indicating that there is more than one chemical of oxygen present in the the for the In2O3-x(OH)y samples can be into the oxide at and at and (Figure The at is to the presence of oxygen vacancies in the is with the observed of the relative to the conduction (Figure which is a result of This is to a energy relative to the oxide This is a result of the change in with an In that is more in it is by than to the oxygen The at has been to surface and well with the for the In(OH)3 these in Figure it is that the – with from both vacancies and surface hydroxides – with increasing calcination temperature. were on samples to reaction conditions and results a change in the of the research to this material in to the surface may change The of the samples investigated further by both spectroscopy and The of the in the (Figure with increasing calcination that samples at temperatures have lower In to the of synthesis, In2O3-x(OH)y sample in a The observed to to form oxides. Figure the of sample after calcination under flow for is that lower calcination temperatures to overall and result in at a relative these to the hydroxides are converted to it is that have no hydroxides and on the other have above the which we to the this and the surface described we that the surface is on the of μmol m−2 for and μmol m−2 for is that the of both groups and oxygen vacancies is a key of our In2O3-x(OH)y nanoparticle photocatalysts and that both of these are present and work in concert at active the observed in CO production rates between In2O3-x(OH)y the CO2 capture capacity for sample at 150 °C, the reaction temperature at which the CO production rates for samples were Furthermore, in to more the CO2 capture capacity and the both are to the surface of using the The surface for the and were similar at and a lower surface at which is likely a result of nanoparticle at the calcination Figure 5 the CO2 capture for sample with their CO production is a notable between CO2 reduction rates and the CO2 capture The affinity of a surface for CO2 has been identified in this as well as in as a critical factor photocatalytic activity. As Figure 5 the CO2 capture capacity of the In2O3-x(OH)y nanoparticles very well with indicating that CO2 adsorption an important in the CO2 molecules must be to approach and with the surface for a of in for electron transfer to hydroxides have a affinity for the This could the positive between CO2 capture capacity and However, while the In(OH)3 control sample has the and a similar surface to that of it has a lower CO2 capture This that surface hydroxides are not to CO2 capture and photocatalytic reduction of CO2. In addition to the surface of the In2O3-x(OH)y nanoparticles is populated with oxygen The presence of these oxygen vacancies in the In2O3-x(OH)y samples is by both the of the (Figure as well as the of the relative to the conduction (Figure which is a result of is from both that has the with oxygen vacancies as well as the a of vacancies to the other In2O3-x(OH)y samples. The increase in oxygen vacancies for may result from the increase in surface sites as the size oxygen vacancies may from the of The In2O3-x(OH)y samples have a which can be as the with of the sites This in the may result in more at the nanoparticle surface, for more in the In2O3 is a and these oxygen vacancies may increase the of the surface the material to under reaction the In(OH)3 with its does not have a significant of surface oxygen The of surface hydroxides and oxygen vacancies could provide an for the in CO2 capture capacity and photocatalytic activity of In(OH)3 and alternative for the observed on temperature is the adsorption and of molecules at the surface. temperatures, molecules such as which can block active may more at these active it is observed that samples a efficiency at 150 °C, this may that 150 °C is a combining CO2 adsorption and CO and H2O A functional component CO2 reduction photocatalyst must have surface, optical, and electronic properties in concert for photocatalytic reduction of CO2 to occur in the gas In this study the In2O3-x(OH)y nanoparticles activity for the photocatalytic reduction of CO2 in the presence of H2 at temperatures as low as °C using both and visible Our work that the observed activity of In2O3-x(OH)y samples is with surface of oxygen vacancies and which may act in concert as active sites for CO2 adsorption and transfer under simulated solar irradiation. We have produced a of In2O3-x(OH)y materials via a temperature thermal dehydration of In(OH)3. 13CO2 as a light and photocatalytic reduction of gaseous 13CO2 to 13CO is in the presence of The surface and oxygen with both an increase in 13CO2 capture capacity and an increase in photocatalytic activity for 13CO production. combining the favourable surface, electronic and optical properties of In2O3-x(OH)y with the and its CO2 capture we have demonstrated a of key components to be in the and of new and gas-phase CO2 reduction photocatalysts for solar fuels production. of In2O3-x(OH)y In(OH)3 and into In2O3 nanoparticles a of a previously chemicals were used as further In a synthesis, of in a of of and water In a a of and prepared by combining to with and of The were in the of a control the the in a at 80 °C and for The then from the and to to room temperature. The via and with The between to of and then at 80 °C in a The with a and and calcined for in at 185 °C, 250 °C, 350 °C, and 450 films were prepared for photocatalytic testing by of sample – via in water – on top of a that under very This sample to CO production as in Figure on a using at adsorption were at using a morphology using a high transmission electron spectroscopy using a with a with a of the samples using a from and an with a diameter of 150 the calcination by approximately of indium in a to the temperature of 250 °C, 350 °C or 450 °C and for under a flow of The sample using the The morphology and characterized by electron microscopy using a 250 The were used as a to provide increased surface as well as Figure of a In2O3 sample on the Figure a of the indicating its is approximately The shown in Figure that the sample its high an important factor for gas-phase spectroscopy using a in an with of 1 The an at 15 and The samples used in were prepared by in the of the In2O3 samples and oxide in the of the In(OH)3 were out using the and the energies were to the and the of photocatalytic rate were in a batch reactor with a fused with The were using an to being with the reactant H2 and CO2 at a flow rate of and a of for or for reverse water gas shift the were been heated to the temperature. The reactor temperatures were by an temperature with a in with the The the reactor reaction the reaction using an were with a W metal is shown in Figure for a of were by a and thermal in a with a and tracing experiments were using 13CO2 The were to being with by were using an gas (GC-MS) with a to the The of the photoactivity of the In2O3 nanoparticles investigated using a W is shown in Figure with a of 420 nm and nm the irradiation the focused using to an irradiation of The with a by a and photocatalytic rate were out in a diameter and reactor with 24 of In2O3-x(OH)y nanoparticle used to support The reactor in a from a heated block the The top of the reactor in to allow light irradiation from a W a of and a light of The reactor with H2 and CO2 at a flow rate of and a of for reverse water gas shift The reactor temperatures were by an temperature A in with the top of the reactor that the reactor a temperature of 150 CO2 The CO2 capture capacity of sample by analysis with a A out under flow at a rate of with a temperature of °C to 150 the temperature at 150 °C for the of CO2 the gas then to CO2 at a flow rate of the temperature then at 150 °C for The observed this adsorption used to the CO2 capture capacity of the of CO2 by the gas flow to flow for 5 while the temperature at 150 and to this is of 1 in and The and support of this solar fuels research by the of and the of and the and of and the of are We are for the of and at the of at for the and for and the The of the in 1 and after The of the not As a to our and this provides by the materials are and may be for but are not or support from than be to the The is not for the or of by the than be to the for the

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