Photochemical Carbon Dioxide Reduction on Mg-Doped Ga(In)N

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Letter

Photochemical Carbon Dioxide Reduction on Mg-doped Ga(In)N Nanowire Arrays under Visible Light Irradiation Bandar AlOtaibi, Xianghua Kong, Srinivas Vanka, Steffi Yee-Mei Woo, Alexandre Pofelski, Fatma Oudjedi, Shizhao Fan, Md Golam Kibria, Gianluigi A. Botton, Wei Ji, Hong Guo, and Zetian Mi ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00119 • Publication Date (Web): 08 Jun 2016 Downloaded from http://pubs.acs.org on June 14, 2016

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ACS Energy Letters

Photochemical Carbon Dioxide Reduction on Mg-doped Ga(In)N Nanowire Arrays under Visible Light Irradiation

B. AlOtaibi1, X. Kong2,3, S. Vanka1, S. Y. Woo4, A. Pofelski4, F. Oudjedi2, S. Fan1, M.G. Kibria1,4, G. A. Botton4, W. Ji3, H. Guo2, and Z. Mi1* 1

Department of Electrical and Computer Engineering, McGill University, 3480 University Street, Montreal, Quebec H3A 0E9, Canada

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Department of Physics, McGill University, 3600 University Street, Montreal, Quebec H3A 2T8, Canada

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Department of Physics and Beijing Key Laboratory of Optoelectronic Functional Materials and Micro-Nano Devices, Renmin University of China, Beijing 100872, China

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Department of Materials Science and Engineering, Canadian Centre for Electron Microscopy, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4M1, Canada *

E-mail: [email protected]; Phone: 1 514 398 7114

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Abstract

The photochemical reduction of carbon dioxide (CO2) into energy-rich products can potentially address some of the critical challenges we face today, including energy resource shortages and greenhouse gas emission. Our ab initio calculations show that CO2 molecules can be spontaneously activated on the clean nonpolar surfaces of wurtzite metal-nitrides, e.g., Ga(In)N. We have further demonstrated the photoreduction of CO2 into methanol (CH3OH) with sunlight as the only energy input. A conversion rate of CO2 into CH3OH (~ 0.5 mmol gcat-1h-1) is achieved under visible light illumination (> 400 nm). Moreover, we have discovered that the photocatalytic activity for CO2 reduction can be enhanced by incorporating a small amount of Mg-dopant. The definite role of Mg-dopant in Ga(In)N, at both the atomic and device level, has been identified. This study reveals the potential of IIInitride semiconductor nanostructures in solar-powered reduction of CO2 into hydrocarbon fuels.

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The photochemical reduction of CO2 into energy-rich products with the use of sunlight as the only energy input has received considerable attention1-5, because it can convert solar energy directly into storable chemical fuels and, at the same time, can potentially mitigate greenhouse gas emission into the atmosphere. Recent reports on the use of a photovoltaic-electrolyzer technique have benchmarked solar-to-CO conversion efficiency at 6.5%6. However, methanol is by far the most desired product from CO2 reduction since it can be used as a liquid fuel-like renewable energy source and a chemical feedstock for other useful chemicals. Unlike the twoelectron process for CO2 conversion to CO, CO2 conversion to methanol is a multiple protoncoupled electron (PCET) process which is kinetically more challenging. Various metal-oxide photocatalysts have been proposed and demonstrated for the photochemical reduction of CO2 to methanol7. The first and foremost step for CO2 reduction is the adsorption and deformation of the relatively inert CO2 molecules on the photocatalyst surface. It has been well recognized that the adsorption of CO2 on conventional metal-oxide surfaces is dominated by defect sites (oxygen vacancies)8, 9. The oxygen vacancies, however, also enhances the adsorption of O2 molecule, which can subsequently fill the vacancy sites and lead to an inactive surface. Moreover, metaloxide semiconductors generally exhibit a large bandgap, which limits the absorption of the visible and infrared solar spectrum10. Therefore, it is of both fundamental and practical interests to explore alternative photocatalysts that can harvest a large part of the solar spectrum and can lead to spontaneous activation of CO2 molecule. Metal-nitride semiconductors, e.g., InGaN, have recently emerged as highly promising photocatalysts for solar fuel applications11-18. This is because the energy bandgap of InGaN can be tuned to absorb nearly the entire solar spectrum. The conduction and valence band edges of InGaN straddle the redox potentials of water. Moreover, InGaN with an indium composition up 4


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to ~ 50% meets the thermodynamic requirements for reducing CO2 to CO and hydrocarbons in solutions with a wide pH range11,

19

. Recent studies have further demonstrated that nonpolar

metal-nitride surfaces are extremely reactive15, 16, 20-24, producing H2 directly from water under UV and visible light illumination, which is essentially required for CO2 hydrogenation. To date, however, the interaction between metal-nitride surface and CO2 has remained unknown, and there has been no report on the photoreduction of CO2 on InGaN. In this study, we aim to provide a fundamental understanding, both theoretically and experimentally, of the interaction between CO2 molecules and Ga(In)N photocatalyst surfaces. Our ab initio calculations reveal that CO2 molecules can be spontaneously adsorbed and deformed on the clean nonpolar surfaces of wurtzite Ga(In)N. Experimentally, with the use of multi-band p-InGaN/GaN nanowire arrays, we have demonstrated the photochemical reduction of CO2 to CH3OH, CO and CH4 with sunlight as the only energy input. Under visible light irradiation (> 400 nm), the average CO2 reduction rates into CH3OH, CO and CH4 are ~ 0.5, 0.1 and 0.25 mmol gcat-1h-1, respectively. It is further discovered that, with the incorporation of Mgdopant, the rate of CO2 reduction to CH3OH, CH4, and CO is enhanced by nearly 50-fold, due to the reduced surface potential barrier and the enhanced adsorption of CO2 molecules on the surface of the nanowires. The distinct surface properties offered by III-nitride nanowire photocatalysts for adsorbing and deforming CO2 molecules, coupled with their tunable energy bandgap and scalable manufacturing process, provides a new approach for realizing highefficiency solar-powered artificial photosynthesis for a methanol-based economy. In order to gain an atomic level understanding CH3OH formation, we have first investigated the adsorption characteristics of CO2 on wurtzite GaN surface using density function theory (DFT) calculations (see the Supporting Info. for details). Figures 1a (top view) and 1b (side 5


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view) show the most energetically stable CO2 adsorption configuration on the GaN(1010) surface, which was derived based on detailed simulation studies (see Supplementary Info.). Tables S1 and S2 summarize all geometrically related parameters and the change of surface energies upon the adsorption of CO2 molecules. Intriguingly, the CO2 molecule preferably adsorbs on the surface Ga−N dimer site. Illustrated in Figs. 1a and 1b, one O atom (O1) attaches to the Ga atom (Ga1) of the dimer underneath with the bond length of 2.10 Å and the other O atom (O2) approaches to the Ga atom (Ga2) of the adjacent dimer across the dimer row with a longer bond length of 2.24 Å. In addition, the C atom strongly binds to the N atom underneath with the bond length of 1.41 Å. Such a strong bonding between CO and the GaN surface results in the original linear CO2 molecule heavily deformed with an O−C−O angle (ɵ in Fig. 1a) of 128.27°, thus forming a tridentate carbonate species. In addition, the chemisorption weakens the two C-O bonds such that the C-O1 bond elongates from the original 1.18Å to 1.31Å and the CO2 bond is stretched from 1.18Å to 1.28Å. The weakened C-O bonds suggest a significant spontaneous activation of CO2 molecules upon the chemisorption on the clean GaN m-plane surface. CO2 The adsorption energy ( E ad ) and deformation energy ( Edef ) for a CO2 molecule on GaN m-

plane surface are further summarized in Table S225, 26. Here the adsorption energy represents the net energy gained upon adsorption. The deformation energy, which denotes the energy increase from a linear CO2 molecule to a buckled one, describes the degree of activation of the surface CO2 molecule. The calculation details are shown in the Supplementary Info. For the adsorption CO2 on a pristine GaN surface, E ad and Edef are -1.76 eV and 2.55 eV, respectively. The negative

adsorption energy indicates the adsorption of CO2 on the GaN surface is a highly favored process. The positive deformation energy of CO2 confirms that the adsorbed CO2 is activated 6


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spontaneously. The C-O bond can then be readily broken if the molecule is further excited by photo-generated electrons. For comparison, the adsorption of CO2 on conventional metal oxide surface is often dominated by the presence of defects (oxygen vacancies)8,

9, 27-33

, which

inevitably lead to carrier loss, induce instability related issues, and become inactive with the adsorption of oxygen molecules. Differential charge density (DCD) was plotted in Fig. 1c to understand the bonding mechanism between CO2 and the GaN surface. There is a significant charge accumulation around the oxygen atoms (indicated by the salmon pink color) and charge reduction near the Ga atoms (indicated by the light blue color); this implies an ionic like Ga-O bonding. If we inspect the C-N region, the formation of a covalent bond is evident by the charge reduction near C and N and charge accumulation in between these two atoms. In addition, for the CO2 molecule itself, substantial charge reduction was found around the C-O bonds, which is most likely the reason that the C-O bonds are weakened and elongated significantly. As a consequence, the charge transfer activates the CO2 molecule upon the adsorption on pristine GaN surfaces. The results of Bader charge analysis34, 35 before and after the adsorption are listed in Table S2, which provides a quantitative description of the charge changes. The charge redistribution is in agreement with the afore-described structural analysis (see Supplementary Information). Experimentally, we have investigated the photoreduction of CO2 on multi-band InGaN/GaN nanowire arrays with sunlight as the only energy input. Schematically shown in Fig. 2a, the InGaN/GaN nanowires are grown on Si substrate by molecular beam epitaxy (Supporting Info.). Each nanowire consists of multiple segments of InGaN/GaN. The insertion of a thin (~ 10 nm) GaN can suppress indium phase separation. Detailed characterization of similar InGaN/GaN nanowire arrays can be found in previous publications15,

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. The nanowires are doped p-type

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using Mg, with an average doping concentration in the range of 5×1020 cm-3. Shown in Fig. 2b is the scanning electron microscope (SEM) image of p-InGaN/GaN nanowire arrays, which are vertically aligned on Si substrate with their lateral surfaces being nonpolar planes. The nanowires exhibit a high degree of size and height uniformity, with diameters in the range of ~100–120 nm and lengths ~ 600–800 nm. The optical properties of p-InGaN/GaN nanowire arrays were further studied using photoluminescence (PL) spectroscopy, shown in Fig. 2c. The PL emission peak is at ~ 533 nm. Such nanowire arrays can harvest green, blue, and UV photons. It is worthwhile mentioning that InGaN and GaN materials have very large absorption coefficient, and the majority photons with energy above the bandgap of InGaN can be effectivley absorbed for a thickness in the range of 0.5 to 1 µm36-38. For this study, Pt nanoparticles were also incorporated as co-catalysts on the nanowire surfaces using a one-step photo-deposition process (Supporting Info.). Structural properties of p-InGaN/GaN nanowires were further characterized using highresolution scanning transmission electron microscopy (STEM) (Supporting Info.). Figure 2d (top) shows the high-angle annular dark-field (HAADF) image of the InGaN segments. Pt nanoparticles of 2-3 nm in diameter are uniformly distributed on the nanowire side surfaces and on surfaces viewed in projection, as evident in the Pt elemental map in Fig. 2d (bottom) with direct correlation to the brightest features in Fig. 2d (top). The Ga-map in Fig. 2e (top) shows the presence of a thin GaN shell at the InGaN segments; the In-map (Fig. 2e (bottom)) shows a strong localization of signal only within the InGaN segments. The In-content projected through the specimen thickness is determined using various methods39, and ranges between x = 0.1 – 0.25 for the centrally-embedded InxGa1-xN segments, illustrated in Fig. 2f. With increasing In content (decreasing energy bandgap) towards the substrate, such InGaN nanowire photocatalysts can 8


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function effectively as a multi-band photocatalyst, enabling efficient conversion of solar energy into fuels. Other structural characterization of the nanowire photocatalysts is described in the Supplementary Info. The photochemical CO2 Reduction experiments were subsequently performed (Supporting Info.). The major products from CO2 photoreduction on p-InGaN/GaN nanowires observed over several hours were CO, CH4, and CH3OH. Shown in Fig. 3a, the average production rates of CO, CH4, and CH3OH are ~ 5, 0.86, and 0.5 mmol gcat-1h-1, respectively. This is in agreement with the afore-mentioned DFT calculations which revealed that the C−O1 bond was elongated and could be broken first, leading to the formation of CO. However, the selective production of CO also depends on the energy states of photo-excited electrons40, 41. Under visible light illumination (> 400 nm), the generation rates for CO and CH4 are substantially reduced, shown in the inset of Fig. 3b. Intriguingly, the production of CH3OH becomes dominant, with an average production rate approximately 0.5 mmol gcat-1h-1. The CH3OH evolution over 10 hours from the photochemical CO2 reduction under this illumination condition is shown in Fig. 3b. The nearly constant reduction rate of CO2 to CH3OH over 10 hours suggests the stability of the pInGaN/GaN nanowires during CO2 photoreduction. The dominance of CH3OH and CO production under visible and UV light illumination, respectively, has been confirmed by repeated experiments. These studies suggest that the main contribution to CO production is likely due to the photo-excited electrons in the GaN layer40, 42. Under visible light illumination, only InGaN layers can be excited. Photo-excited electrons in InGaN can readily tunnel through the GaN barrier layer to the surface for CO2 reduction. The associated electron energy states, however, may be more favorable for CH3OH generation, considering the electrochemical potentials of CO2 to form CO, CH4, and CH3OH being –0.53, –0.23, and –0.38 V, respectively, at pH = 7. In 9


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addition, CH3OH formation is a six-electron process, whereas CH4 formation requires eight electrons and protons. The dominance of CH3OH generation under visible light illumination has been reported previously30,

42, 43

. The conversion rates into CH3OH in the presented study

outperformed those of the previously reported photocatalyst systems using the reduction rate as a catalyst-based evaluation measure42-46, which were previously reported in the range of 0.1-10 µmol gcat-1h-1 under visible light illumination47. In order to confirm that the product formation was actually occurring through CO2 reduction, a

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CO2 isotope reactant was used in a set of

photochemical experiments. Figure 3c shows a gas sample of the Pt nanoparticle-decorated pInGaN/GaN nanowires under visible light illumination, as recorded using GC-MS. A distinct peak associated with 3c. Also, the

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CH3OH (m/z = 33.1) instead of

CH4 and

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CH3OH (m/z = 32.1) is shown in Fig.

CO peaks are indicated in Fig. 3c. These data confirmed that the

products were produced directly from the photocatalytic reduction of CO2. In this study, formaldehyde was not measured. It is noticed that one of the pathways for CO2 reduction is the carbene pathway, in which methanol, methane and CO are final products, and the formation of Formaldehyde is not expected in this process48. In these experiments, the overall mechanism for CO2 photoreduction to hydrocarbons is the result of a sequential combination of H2O oxidation and CO2 reduction. InGaN/GaN has been previously reported to have excellent water oxidation and reduction activity15,

16, 21

. Water

oxidation occurred on the GaN nanowire surfaces, providing protons for the formation of hydrocarbons. H+ species can also be formed through the dissociation of hydrogen molecules (H2). This H+ formation on the surface plays an important role in these reactions, providing the feedstock for the formation of hydrocarbons. Another essential property that played a vital role in the enhancement of the CO2 reduction is the excellent CO2 adsorption ability of GaN surface. 10


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The amount of charge transfer among the substrate, O1, C atoms was larger than that between substrate, O2, and C atoms, which suggests that the C−O1 bond may be broken easier and earlier than the C−O2 bond. In addition, the relatively strong covalent bond formed between C−N1, evidenced by the 0.2586 e charge-electrons moved from the C atom to the N1 atom, may also assist in breaking the C−O bond. The charge-electrons on N1 atom increased by 1.3945 e while the electrons on Ga1 atom decreased by 1.5667 e, shown in Table S2. Meanwhile, Ga2 atom lost 1.5475 e charge-electrons and N2 atom gained 1.4659 e charge-electrons. As a result, dimer bonds on the surface flipped over from time to time, with this surface-mediated electron transfer occurring constantly under light excitation. These phenomena likely play significant roles in the CO2 reduction reaction that occurs on GaN nanowire surfaces. Further studies have also been performed to investigate the effect of doping of InGaN/GaN nanowires on the photocatalytic CO2 reduction activity. In this regard, two samples with a similar structure as the Mg-doped InGaN/GaN nanowires were grown using MBE (see Supporting Info.). One InGaN/GaN sample was n-type doped with Ge, whereas no doping species were introduced in the other InGaN/GaN sample. The average CO2 photoreduction rate into CO and CH4 on the undoped, n-type, and p-type doped InGaN/GaN nanowire photocatalysts are shown in Figs. 4a and b, respectively. The average production rates of CO and CH4 were ~ 0.4 and ~ 0.18 mmol gcat-1h-1 on the undoped InGaN/GaN nanowires, whereas the average production rates of CO and CH4 on n-type InGaN/GaN were ~ 0.09 and ~ 0.08 mmol gcat-1h-1, respectively. In addition, only traces of CH3OH were detected during the course of the experiments for both samples. These values are nearly a factor of 10 to 50 times smaller, compared to those of InGaN/GaN nanowire photocatalysts with optimum p-type (Mg) doping. The underlying mechanism for the drastically enhanced activity of Mg-doped nanowire arrays is 11


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investigated. Given the relatively small nanowire diameters (~ 100 nm, or less) compared to the carrier diffusion length (> 200 nm), the separation and transport of photo-generated charge carriers (electrons and holes) is largely governed by the presence of surface band bending. For instance, depending upon the incorporated impurities, upward and downward band bending has been commonly measured on n-type and p-type semiconductor surfaces, respectively. The upward surface band bending of n-type nanowires, schematically shown in Fig. 4c (left panel), creates an energy barrier that inhibits the transport of photo-excited electrons to the photocatalyst surface, thereby significantly suppressing reduction reactions. The downward band bending associated with p-type semiconductors, on the other hand, leads to a hole depletion in the nearsurface region, shown in Fig. 4c (right panel). We have recently discovered that the surface potential of InGaN nanowire photocatalysts can be tuned over a wide range (up to 2 eV) through controlled Mg-dopant incorporation17, 21. The resulting near-flat surface band structure, shown as the dotted lines in Fig. 4c (right panel), can significantly enhance the photocatalytic activity17, 21, due to the efficient transfer of charge carriers (both electrons and holes) to the photocatalyst surface. This finding is also consistent with the improved photocatalytic activity of GaN powder after divalent metal ion doping49. p-Type doping has also been found to be beneficial in other material systems for CO2 photoreduction50. The presence of dopants, such as Ge and Mg atoms on the adsorption and deformation of CO2 molecules was also investigated using DFT. It was found that the presence of Mg atoms can further enhance the activation of CO2 molecules on GaN surfaces. Figure 4d shows the most energetically stable CO2 adsorption sites on GaN surfaces with the presence of Mg and Ge doping. The geometric and electronic structures and related CO2 adsorption and deformation energy values are summarized in Tables S1 and S2. Compared to a clean GaN surface, the C-O2 12


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bond length on the Mg-doped surface is further stretched to 1.32 Å. It is seen that the Mg-doped surface has the lowest CO2 adsorption energy of -2.48 eV and largest CO2 deformation energy of 2.86 eV among all these three GaN surfaces, i.e. undoped, Ge-doped, and Mg-doped (Supplementary Info.). More importantly, more surface-mediated charge, namely 0.4785 e, on Mg-doped surface is transferred to the CO2 molecule. The Ge-doped surface, on the other hand, do not offer any clear advantage, compared to the undoped surface (Supplementary Info.). These studies, together with the reduction of surface potential barrier, offer critical insights on the microscopic origins for the significantly enhanced CO2 reduction activities on Mg-doped InGaN/GaN nanowires. In summary, we have demonstrated that the nonpolar m-plane surfaces of wurtzite InGaN/GaN nanowire photocatalysts are highly reactive with CO2 molecules and offer unique advantages for CO2 photoreduction. Moreover, the incorporation of Mg-dopant can further enhance the photocatalytic activities by nearly 50-fold, compared to the Ge-doped nanowire arrays, due to the reduced surface potential barrier and the enhanced adsorption and deformation of CO2 molecules. The reaction was found to exhibit relatively high conversion rates into CH3OH (ca. 0.5 mmol gcat-1h-1) and high stability over 10 hours under visible light illumination (> 400 nm). Compared to the conventional metal-oxide photocatalysts, such wafer-scale nanostructured metal-nitride photocatalysts offer an alternative path for the deployment of solarpowered artificial photosynthesis. The present work also offers a new avenue for the solid adsorption of CO2 to overcome some critical challenges of the conventional chemical absorption process, including low CO2 loading and severe absorbent corrosion.

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Supporting Information Experimental methods and computational details, CO2 reduction, density function theory (DFT) calculations, Pt nanoparticles cocatalyst, structural characterization of Pt-decorated pInGaN/GaN photocatalyst, undoped and Ge-doped InGaN/GaN nanowires, lamp spectrum, additional figures, and additional references.

Acknowledgement This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Climate Change and Emissions Management (CCEMC) Corporation. Part of the work was performed in the Micro-fabrication Facility at McGill University. Electron microscopy images and analysis were carried out at the Canadian Center for Electron Microscopy, a National facility supported by NSERC and McMaster University. The authors would like to acknowledge Mr. F.A. Chowdhury for his assistance with the MBE operation. B.A. acknowledges King Abdullah Scholarship Program. X.K. thanks the Chinese Scholarship Council for support. We thank CalcuQuebec and Compute Canada for computation facilities. Competing financial interests. The authors declare no competing financial interests.

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Figure 1: Calculated Structural and electronic properties GaN m-plane and CO2 adsorption. (a) and (b) are a side and a top view, respectively, of the most stable fully relaxed configuration of CO2 adsorbed on GaN m-plane. (c) Differential charge density of the adsorbed CO2 molecule. The color of salmon pink suggests electroncharge gain while the color of light blue indicates electron-charge reduction. Isosurface contours of electron density differences are drawn at ±0.005 e/Å3.

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Figure 2: Structural and optical properties of p-InGaN/GaN nanowires. (a) Schematic of pInGaN/GaN nanowire photocatalyst decorated with Pt nanoparticles. (b) A 45 °C tilted scanning electron microscope image of p-InGaN/GaN nanowires grown on Si(111) substrate. (c) Photoluminescence spectrum of as-grown p-InGaN/GaN nanowire photocatalyst showing the PL peak at ~ 533 nm. (d) Top: STEM-HAADF image of the p-InGaN/GaN nanowire structure. Bottom: Pt map extracted from the selected region marked in red in Fig. 2d (top) displayed in pseudo-color scale. (e) Top: elemental Ga map from the selected region marked in green from Fig. 2d (top). Bottom: thickness-projected In-content map obtained from the selected region marked in green Fig. 2d (top). x in InxGa1-xN is presented in colour-scale shown on the left. (f) Extracted line-profile (representing x in InxGa1-xN) from the In-content EELS map from the selected region in Fig. 2e (bottom).

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Figure 3: Photochemical activity of p-InGaN/GaN nanowires. (a) Measured CO, CH4 and CH3OH evolution rates on Pt decorated p-InGaN/GaN nanowire photocatalysts under full spectrum of a Xe lamp equipped with AM1.5G filter. (b) CH3OH evolution over Pt-decorated p-InGaN/GaN nanowires as a function of time under visible light illumination (> 400 nm). The inset shows CO and CH4 generation rate on Ptdecorated nanowires. (c) Mass spectrum indicating the presence of and

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CH3OH, 13CH4

CO peaks of a sample taken after 7 hr from a photochemical CO2 reduction

experiment on Pt-decorated p-InGaN/GaN under visible light irradiation (> 400 nm).

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Figure 4: Photocatalytic activities and surface charge properties of InGaN/GaN nanowires with different dopants. Measured evolution rates of (a) CO and (b) CH4 on Ptdecorated Mg-doped InGaN/GaN, undoped InGaN/GaN and Ge-doped InGaN/GaN nanowires, under full spectrum of a Xe lamp equipped with an AM1.5G filter. (c) Schematic of charge carrier transfer illustrated on n- (p-) doped semiconductors with upward- (downward-) surface band bending showing the impact of surface band bending on the CO2 reduction reaction. (d) The most relaxed stable configurations of CO2 molecules adsorbed on Mg- and Ge-doped GaN m-plane surfaces.

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Photochemical Carbon Dioxide Reduction on Mg-Doped Ga(In)N

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