Photocatalytic CO2 Reduction by Carbon-Coated Indium-Oxide

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Photocatalytic CO2 Reduction by Carbon-Coated Indium-Oxide Nanobelts Yun-Xiang Pan, Ya You, Sen Xin, Yutao Li, Gengtao Fu, Zhiming Cui, Yu-Long Men, Fei-Fei Cao, Shu-Hong Yu, and John B. Goodenough J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b00266 • Publication Date (Web): 19 Feb 2017 Downloaded from http://pubs.acs.org on February 19, 2017

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Journal of the American Chemical Society

Photocatalytic CO2 Reduction by Carbon-Coated IndiumOxide Nanobelts †

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Yun-Xiang Pan, Ya You, Sen Xin,* Yutao Li, Gengtao Fu, Zhiming Cui, Yu-Long Men, FeiFei Cao,# Shu-Hong Yu,*,§ and John B. Goodenough*,‡ †

School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, P. R. China.

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Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX 78712, United States.

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Division of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, University of Science and Technology of China, Hefei 230026, P. R. China. #

College of Science, Huazhong Agricultural University, Wuhan 430070, P. R. China.

Supporting Information Placeholder ABSTRACT: Indium-oxide (In2O3) nanobelts coated by a 5nm-thick carbon layer provide an enhanced photocatalytic reduction of CO2 to CO and CH4, yielding CO and CH4 evo-1 lution rates of 126.6 and 27.9 µmol h , respectively, with water as reductant and Pt as co-catalyst. The carbon coat promotes the absorption of visible light, improves the separation of photoinduced electron-hole pairs, increases the chemisorption of CO2, makes more protons from water splitting participate in CO2 reduction, and thereby facilitates the photocatalytic reduction of CO2 to CO and CH4.

INTRODUCTION The photocatalytic reduction by sunlight of CO2 to useful chemicals is a desirable target for a sustainable energy economy,1-3 but the rate of reduction of CO2 to, for example, CO and CH4 does not exceed tens of micromoles per hour.2 The low efficiency of CO2 reduction by sunlight is the result of poor separation of photoinduced electron-hole pairs and the inadequate adsorption of the CO2 at the catalytic site.2,6,7 Thus, catalysts with abilities to facilitate electron-hole separation and CO2 adsorption are desired for the photocatalytic reduction of CO2. Water is the most widely used reductant for photocatalytic CO2 reduction.1-9 However, H2 formation via the reduction of protons from the photocatalytic water splitting inevitably occurs in photocatalytic CO2 reduction with water.2,6,7 The proton reduction into H2 is the main competitive process of CO2 reduction; it limits the amount of photoinduced electrons and protons for reducing CO2, thus suppressing CO2 reduction to carbon-containing chemicals.2,6,7 Improving the photocatalytic reduction of CO2 with water requires suppression of H2 formation.

Figure 1. SEM image (upper), TEM image (inset upper) and EDX elemental mappings of In (lower left) and C (lower right) on the C-In2O3.

Indium-oxide (In2O3) is of particular interest for the photocatalytic CO2 reduction.10-16 First, In2O3 has a good chemical stability under illumination.10,13 Second, In2O3 shows a high activity to generate CO2− from CO2, a key species in the photocatalytic CO2 reduction.11,14,16 Third, In2O3 has a reduction potential (-0.62 V vs. NHE at pH=7) which can meet the value needed for converting CO2 to CO (-0.53 V vs. NHE at pH=7) and CH4 (-0.24 V vs. NHE at pH=7).16 However, in the photocatalytic CO2 reduction with water, the In2O3-based catalysts exhibit a high activi-

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autoclave at 140 °C for 16 h before being cooled to room temperature. The obtained precipitate was washed with DI water and ethanol several times, dried at 60 °C for 12 h, and calcined at 600 °C for 2 h in Ar with a heating rate of 5 °C min-1. The synthesis procedure for C-In2O3 is similar to that of P-In2O3, except that 6 mmol of In(NO3)3 and 0.8 g of glucose were simultaneously added into the starting solution for hydrothermal synthesis. The glucose was used for coating carbon on In2O3; it leads to the highest CO and CH4 evolutions from the photocatalytic reduction of CO2 (Figure S1).

ty towards H2 formation, but low activity towards production of carbon-containing chemicals.10-16 The efficiency of the In2O3-based catalysts in producing carbon-based chemicals from the photocatalytic CO2 reduction with water requires further improvement. In this paper, we report that In2O3 nanobelts coated by a 5-nm-thick carbon layer (C-In2O3) show an enhanced photocatalytic reduction of CO2 to CO and CH4 with water as the reductant, triethanol amine (TEOA, C6H15NO3) as the sacrificial electron donor and Pt nanoparticles as the co-catalyst compared with catalysts based on carbon-free pure In2O3 (PIn2O3) and other catalysts studied in literature (commercial TiO2(P25) and those listed in Table S1).2,7,17-32 Experiments and density functional theory (DFT) calculations indicate that the thin carbon layer coating on In2O3 nanobelts improves the efficiency of absorbing visible light, separating the photoinduced electron-hole pairs, adsorbing CO2 and suppressing the proton reduction to H2. In this way, it promotes the photocatalytic reduction of CO2 to CO and CH4.

For comparisons, we prepared two pure carbon materials without In2O3. One of the carbon materials was hydrothermally-synthesized by using a similar method to C-In2O3, except that only glucose (0.8 g) was dissolved in the starting solution for hydrothermal synthesis. For clarity, the hydrothermally synthesized carbon material was denoted as C(H). The other carbon material was produced by washing the as-prepared C-In2O3 in aqua regia under vigorous stirring at room temperature. After washing, the In2O3 was dissolved and the obtained black precipitate was centrifuged and further washed with DI water and ethanol for several times, and then dried at 60 °C for 12 h. This carbon material was denoted as C(W).

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Figure 2. (a) XRD patterns of P-In2O3, C-In2O3 and Pt/CIn2O3. (b) XPS spectra of carbon species on C-In2O3.

RESULTS Materials Synthesis. The P-In2O3 was prepared by dissolving 6 mmol In(NO3)3 and 24 mmol urea in 40 mL deionized (DI) water; the solution was kept in a 45 mL polytetrafluoroethylene-lined stainless-steel

Figure 3. (a) HRTEM image of Pt/C-In2O3. (b) XPS spectrum of Pt on Pt/C-In2O3.

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g-1, which is slightly larger than that of P-In2O3 (54.9 m2 g-1).

To add the Pt co-catalyst on the supports, P-In2O3, C-In2O3, C(H), C(W) and TiO2(P25) were separately dispersed in an aqueous solution with a stoichiometric amount of H2PtCl6 to yield 2 wt% Pt in the final Pt/P-In2O3, Pt/C-In2O3, Pt/C(H), Pt/C(W) and Pt/TiO2(P25) samples. The solution was illuminated for 2 h under vigorous stirring for photocatalytically reducing the Pt4+ to Pt0; the resultant precipitate was dried in a vacuum freeze drier. The mass content of Pt in the catalysts was selected to be 2 wt% as it was the optimum Pt content for CO and CH4 evolution from the photocatalytic CO2 reduction (Figure S2).

The Raman spectrum of C-In2O3 in Figure S4 shows a D band at 1367 cm-1 from disordered sp2 carbon and a G band at 1582 cm-1 from ordered sp2 carbon.35 The higher intensity of the D band indicates the carbon coating on C-In2O3 is amorphous. The Xray photoelectron spectroscopy (XPS) peak, which is calibrated by C1s peak, for the carbon species on CIn2O3 (Figure 2b) can be fit to four peaks corresponding to C=C, C−C, C–O−C, and O−C=O bonds, respectively.36 The thermogravimetric analyses (TGA) data in Figure S5 show a loss of bound water below 200 °C and 8 wt% loss of carbon from C-In2O3 in the temperature interval from 200 °C to 500 °C.

Material Characterization. Figures 1 and S3 show the scanning electron microscopy (SEM) images of the C-In2O3 and P-In2O3, respectively. Both materials are belt-like, with lengths of 1-2 µm and widths of 50-100 nm. The carbon layer coating on CIn2O3 is 5-nm thick according to the transmission electron microscopic (TEM) image (inset of upper Figure 1). Energy dispersive X-ray elemental (EDX) mappings obtained by TEM confirm the coexistence of C and In on C-In2O3. The crystal form of In2O3 in the P-In2O3, C-In2O3 and Pt/C-In2O3 (Pt-loaded CIn2O3) are assigned to the cubic In2O3 (PDF#06-0416) according to the X-ray powder diffraction (XRD) patterns in Figure 2a.33,34 The C-In2O3 holds a BrunauerEmmett-Teller (BET) specific surface area of 61.3 m2

The XRD pattern of Pt/C-In2O3 in Figure 2a shows two small peaks at 40.4 ° and 46.2 ° that match well with the (111) and (200) planes of Pt (JCPDS 040802).37 The presence of Pt on the Pt/C-In2O3 is also confirmed by the 0.22 nm lattice fringe in the highresolution TEM (HRTEM) image of Figure 3a and the XPS spectrum of Figure 3b.34,37 The two small XPS peaks at 72.8 and 75.8 eV in the XPS spectrum of Pt/C-In2O3 come from Pt2+ and Pt5+, respectively.37-40 Pt2+ and Pt5+ could be due to Pt-O or Pt-C bonds, indicating that the Pt nanoparticles interact with C-In2O3 through Pt-O or/and Pt-C bonds.

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Figure 4. (a) H2, CO and CH4 evolution rates from the photocatalytic CO2 reduction on Pt/C-In2O3 and Pt/P-In2O3. (b) Product distribution during the photocatalytic CO2 reduction on Pt/C-In2O3 (4 h). (c) Stability of Pt/C-In2O3 during the photocatalytic CO2 reduction. (d) Stability of Pt/P-In2O3 during the photocatalytic CO2 reduction. Reaction conditions: 200 mL aqueous solution with 10 vol% TEOA (pH=9), 200 mg catalyst, Xe-lamp (300 W).


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Photocatalytic Reduction of CO2. The evolution rates of CO and CH4 are 126.6 and 27.9 µmol h-1 for Pt/C-In2O3, respectively. The results are highly enhanced, as compared with Pt/P-In2O3 on which the CO and CH4 evolution rates are only 26.7 and 4.1 µmol h-1, respectively (Figure 4a). In addition to CO and CH4, other carbon-containing products, such as methanol (CH3OH), formaldehyde (HCHO) and formic acid (HCOOH), were also detected in the photocatalytic CO2 reduction on Pt/C-In2O3, yet their amounts are relatively small (Figure 4b). Under the same reaction conditions, the optimal CO and CH4 evolution rates on the Pt/TiO2(P25) catalyst are 12.1 and 1.8 µmol h-1, respectively, which are much lower than those on the Pt/C-In2O3. Also, the photocatalytic performance of Pt/C-In2O3 is much better than the catalysts reported in the former literatures (Table S1).2,7,17-32

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production drops by 12.3%, from 106.8 µmol to 93.7 µmol, and the CH4 production drops by 12.8%, from 16.4 µmol to 14.3 µmol (Figure 4d). The total TONs obtained at the end of five runs are, respectively, 24.1 and 4.3 for CO and CH4, based on the amount of Pt in the Pt/P-In2O3. DISCUSSION Light Absorption. As shown in Figures 5, S7 and S8, the energy gap between the O-2p band and the bottom of the In-4s band of P-In2O3 is 2.53 eV. Therefore, in the absence of a carbon coat, only the high-energy visible and ultraviolet portion of the sunlight spectrum is absorbed. The Fermi level of carbon lies about 0.05 eV below the bottom of the In-4s band. At the interface of In2O3 with the carbon coat, excitation of electrons from the O-2p band of In2O3 to the carbon band allows access to a larger percentage of the sunlight spectrum, but access to this excitation restricts the optimal thickness of the carbon coat to 5 nm. Lowering the bottom of the conduction band provides a better match to the chemical reduction of CO2 by water and the electron photoexcited by sunlight.

In order to verify that the CO and CH4 was evolved from CO2, four control experiments and an isotopic experiment with 13CO2 were conducted: (i) experiment with Pt/C-In2O3, CO2 and water, but without light irradiation; (ii) experiment with CO2, water and light irradiation, but without Pt/C-In2O3; (iii) experiment with CO2, water, light irradiation and C-In2O3, but without Pt; and (iv) experiment with Pt/C-In2O3, water and light irradiation, but without CO2. No appreciable amounts of CO and CH4 are generated in all of the four control experiments, implying that CO and CH4 are generated by the photocatalytic reduction of CO2 on the Pt/C-In2O3. In the isotopic experiment using 13CO2, the generated CO and CH4 were analyzed by gas chromatography-mass spectrometry (GC-MS). According to the results, the m/z at 29 and 17 on the GC-MS spectra are due to 13CO and 13CH4 (Figure S6), which further demonstrates that CO and CH4 are formed from the photocatalytic CO2 reduction on Pt/C-In2O3.

Figure 5. Conduction bands, valence bands and band gap energies of C-In2O3 and P-In2O3 relative to the CO2 reduction potentials to CO, CH4, HCOOH, HCHO and CH3OH, respectively, vs. the NHE at pH=7.

The stability of Pt/C-In2O3 was demonstrated by five consecutive runs with each run of 4 h (Figure 4c). After each run, the light irradiation was stopped, the reactor was evacuated and then refilled with CO2 at 1.0 bar, but no catalyst washing or introduction of fresh aqueous solution was performed. After five runs, the CO production decreases by 3.3%, from 506.4 to 489.5 µmol, and the CH4 production decreases by 4.0%, from 111.6 to 107.1 µmol. The total turnover numbers (TONs) obtained at the end of five runs are, respectively, 121.5 and 26.6 for CO and CH4, based on the amount of Pt in the Pt/C-In2O3. The stability and total TONs of the Pt/P-In2O3 are much lower than those of the Pt/C-In2O3 under the same reaction conditions. On the Pt/P-In2O3, the CO

Reduction Potential. Equations (1)-(5) show the potentials for reducing CO2 relative to the NHE in water at a pH value of 7, in which the protons come from the photocatalytic water splitting. CO2 + 2H+ + 2e- → CO + H2O

E0redox = −0.53 V (1)

CO2 + 8H+ + 8e- → CH4 + 2H2O E0redox = −0.24 V (2) CO2 + 2H+ + 2e- → HCOOH

E0redox = −0.61 V (3)

CO2 + 4H+ + 4e- → HCHO + H2O E0redox = −0.48 V (4) CO2 + 6H+ + 6e- → CH3OH + H2O E0redox = −0.38 V (5)


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The relative energies and the electrons required for the reduction reactions show that CO could be the preferred product of the photocatalytic CO2 reduction. Experimentally, Figures 4a and 4b show that, on Pt/C-In2O3, the photocatalytic evolution of CO is strongly preferred over that of H2 and then CH4. Also, it is noted that the evolution rates of the heavier products (HCOOH, HCHO and CH3OH) are significantly lower than those of CO and CH4, which may be ascribed to a slower kinetics of their reduction reactions.41

measurements of C-In2O3 and P-In2O3 show that the C-In2O3 has an electronic conductivity (σe) of 1.7×10-1 S cm-1, which is about 15 times higher than that of PIn2O3 (with a σe of 1.1×10-2 S cm-1). The electrochemical impedance spectra of C-In2O3 vs. P-In2O3 in Figure 6 further confirm a higher σe of C-In2O3 due to electron transfer to the conduction band of carbon at the C/In2O3 interface. The higher conductivity of C-In2O3 provides a faster electron transfer than the P-In2O3, which is beneficial for achieving an efficient separation of the photoinduced electron-hole pairs in the photocatalytic process on C-In2O3. The separation-recombination of the photoinduced electron-hole pairs on C-In2O3 and P-In2O3 was studied by photoluminescence (PL) spectra and photocurrent measurements. A strong emission at about 440 nm on the PL spectrum of P-In2O3 (Figure 6b) is produced by the recombination of the photoinduced electron-hole pairs.33, 42-44 However, in the PL spectrum of C-In2O3, the emission at 440 nm almost vanishes, indicating a more efficient electronhole separation on C-In2O3. Figure 6c shows that the photocurrents are consistently higher on C-In2O3 than that on P-In2O3. A more efficient electron-hole separation makes more photoinduced electrons participate in reducing CO2 and thereby enhances the photocatalytic CO2 reduction to CO and CH4.

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CO2 Adsorption. Another important factor affecting the photocalalysis performance is the adsorption capacity of CO2 on the catalysts. Although the protons from the photocatalytic water splitting react with CO2 in the catalytic reduction reactions, the retention time of CO2 on the catalyst should be long enough for the reaction to occur. Figure 7a displays the CO2 adsorption capacities of the C-In2O3-, PIn2O3-, C(H)- and C(W)-based catalysts. The larger CO2 adsorption capacity on the C-In2O3-based catalyst than those on the pure In2O3 and on the pure carbon indicates a possible synergistic effect between the In2O3 and the thin carbon layer on the CIn2O3. Figure 7b shows that the CO2 adsorption capacity on the C-In2O3-based catalyst reaches a maximum value with use of 0.8 g of glucose for coating a 5-nm C layer on the In2O3 nanobelts. Hence, the synergistic effect between the In2O3 and the thin carbon layer on CO2 adsorption may be related to a narrow electric double layer at the C/In2O3 interface.

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Figure 6. (a) Electrochemical impedance spectra of In2O3 and C-In2O3. (b) PL spectra of P-In2O3 and C-In2O3. (c) Photocurrent-time profiles of In2O3 and C-In2O3.

Charge Separation. Recombination of the photoinduced electron-hole pairs should be slow relative to the rate of pair separation that enables the electrons to reduce adsorbed molecules, and this separation can be facilitated by a higher mobility of the electron than the holes. Four-point conductivity


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The Bader charge of the adsorbed CO2 on the Pt2/C-In2O3(110) and Pt2/P-In2O3(110) were DFTcalculated to be −0.42 |e| and −0.21 |e|, respectively. This result is in good agreement with the FTIR results and implies electron transfer from the substrates to the adsorbed CO2. On the Pt2/C-In2O3(110), the Bader charges on the surface In atoms are reduced during the CO2 adsorption process (from +2.11 |e| before CO2 adsorption to +1.85 |e| after CO2 adsorption). The CO2 adsorption also reduces the total Bader charges on the surface carbon atoms. This indicates that the Bader charges on the adsorbed CO2 come from both the In atoms of In2O3 and C atoms coated outside. As such, there is a synergistic effect between In2O3 and the thin carbon layer for CO2 adsorption, which agrees with the results in Figure 7.

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Both experiment and theory thus support a stronger CO2 chemisorption on the C-In2O3-based catalyst than that on the P-In2O3-based catalyst. The improved chemisorption increases the probability of proton capture by the CO2- and promotes the photocatalytic reduction of CO2 to CO and CH4. This could also be an origin for the enhanced CO and CH4 evolutions on the C-In2O3-based catalyst. 2H+ + 2e- → H2

Figure 7. (a) CO2 adsorption capacities of Pt/C-In2O3, Pt/-PIn2O3, Pt/C(H) and Pt/C(W). (b) CO2 adsorption capacity at 1.01 bar as a function of the glucose amount used for preparing the thin-carbon-layer-coated In2O3 nanobelts. The black and red stars represent the CO2 adsorption capacities of Pt/C(H) and Pt/C(W), respectively.

Fourier transform infrared (FTIR) spectroscopy of the CO2 adsorption on both the C-In2O3- and PIn2O3-based catalysts (Figure S9) indicate the presence of chemisorbed CO2- by accepting electrons from the substrate.45,46 The CO2 adsorption on the C-In2O3- and P-In2O3-based catalysts were also calculated by density functional theory (DFT), with Pt2/C-In2O3(110) and Pt2/P-In2O3(110) as models since the (110) surface is the most stable surface of In2O3.14 The metal dimer is the smallest unit allowing to probe both metal-metal and metal-support interactions, and has been widely used in theoretical investigations of the interaction of metal with metal oxide or carbon-based materials and for exploring the reactions on metal-oxide-supported metal catalysts.4749 The calculated CO2 adsorption energies on the Pt2/C-In2O3(110) and Pt2/P-In2O3(110) are 1.63 and 1.35 eV, respectively (Figure S10), indicating a stronger CO2 binding on the Pt2/C-In2O3(110). Such a strong binding is responsible for the higher CO2 adsorption capacity of the C-In2O3-based catalyst than the P-In2O3-based catalyst.

E0redox = −0.41 V (6)

Figure 8. DFT-calculated relative energy profiles for the proton reduction into H2 and proton transfer to an O atom of CO2 on Pt2/P-In2O3(110) and Pt2/C-In2O3(110).

Proton Reduction and Transfer. Eq. (6) shows the H2 formation from the proton reduction by the photoinduced electrons.2 The potential required for H2 formation is plotted in Figure 5. The conduction band of both the P-In2O3 and C-In2O3 can meet the potential required for Equation (6). Hence, during the photocatalytic CO2 reduction, H2 evolution via the reduction of protons from water splitting becomes the main competitive process of CO2 reduction to CO and CH4, as it consumes protons and photoinduced electrons for CO2 reduction. According to Figure 4a, the H2 evolution rate on the CIn2O3-based catalyst is 45.5 µmol h-1, which is much lower than that on the P-In2O3-based catalyst (171.3


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µmol h-1). This result implies that the proton reduction into H2 is efficiently suppressed on the C-In2O3based catalyst, which may be linked to its higher CO2 adsorption capacity (Figure 7a) and more negative charge of the adsorbed CO2 (Bader charges) on the catalyst.

to yield TEOA+ (Eq. 10), so that the surplus electrons can transfer through the carbon coat to the catalytic active sites (i.e., Pt particles) and participate in the CO2 reduction reaction (Eq. (1) – Eq. (2)) with the adsorbed CO2 and the as-generated protons. The Pt nanoparticles accept the photogenerated electrons from In2O3 (or C-In2O3), and provide the sites for the photocatalytic reactions. The positively charged TEOA+ molecules could interact with hydroxyl ions, yielding electrically neutral TEOA and continuing to serve as the electron donor. Herein, both CO and CH4 are produced from the photocatalytic CO2 reduction. In our previous studies, it has been shown that OH group on the catalysts enhances the CO selectivity in CO2 conversion.50 We thus expect that fabricating OH group on the catalysts with a proper content may increase the selectivity of CO2 conversion to CO.

The thermodynamic and kinetic properties of the proton reduction into H2 and proton transfer to the adsorbed CO2 on the C-In2O3- and P-In2O3-based catalysts were also studied by DFT calculation with Pt2/C-In2O3(110) and Pt2/P-In2O3(110) as models. The results are plotted in Figure 8. On Pt2/P-In2O3(110), the H2 formation is exothermic by 0.25 eV with an activation energy barrier (Ea) of 0.68 eV, while the proton transfer to an O atom of the adsorbed CO2 is endothermic by 0.11 eV with an Ea of 0.95 eV. On Pt2/C-In2O3(110), the H2 formation is exothermic by 0.21 eV with an Ea of 0.74 eV, while the proton transfer to an O atom of the adsorbed CO2 is exothermic by 0.32 eV with an Ea of 0.52 eV. As such, the H2 formation on Pt2/P-In2O3(110) is preferred over the proton transfer to the adsorbed CO2, while on Pt2/CIn2O3(110), the proton transfer to the adsorbed CO2 becomes easier than the H2 formation. The calculation results are in good agreement with the experimental data that shows the H2 formation rate is higher than the rate of CO2 reduction to CO and CH4 on the P-In2O3-based catalyst, but lower on the CIn2O3-based catalyst (Figure 4a). Both the experimental and calculation results indicate that, in the presence of C-In2O3, the protons from water splitting prefer to react with CO2, and the proton reduction into H2 is efficiently suppressed. Consequently, the CO and CH4 evolutions on the CIn2O3-based catalyst are improved.

Figure 9. Schematic of the mechanism of the photocatalytic CO2 reduction with water on Pt/C-In2O3, with TEOA as the electron donor to bind with the hole.

Reaction Mechanism. Based on the above discussion, the possible mechanism for photocatalytic CO2 reduction on the C-In2O3-based catalyst, with water as the reductant, TEOA as the electron donor, and Pt as the co-catalyst is proposed as shown in Figure 9. +

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In2O3 + hv → h + e

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(8)

+

+

(9)

h + TEOA → TEOA +

-

In summary, with water as reductant and Pt as cocatalyst, coating a 5-nm-thick carbon layer on In2O3 nanobelts can trigger highly enhanced photocatalytic reduction of CO2 to CO and CH4, with CO and CH4 evolution rates of 126.6 and 27.9 µmol h-1, respectively. The thin carbon layer induces excitation of electrons from the O-2p band of In2O3 to the carbon band within an electric double layer at the interface between the carbon layer and In2O3, thereby increasing the absorption of sunlight and sharply decreasing the electron-hole recombination rate. The thin carbon layer also increases the CO2 chemisorption on the catalyst to allow longer time for cap-

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e + H2O → H + OH

CONCLUSIONS

TEOA + OH → TEOA + OH

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4OH → 2H2O + O2

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As described by the above Equations, light absorption by In2O3 generates the electron-hole pairs (Eq. (7)), which participate in the photocatalytic water splitting reaction to yield protons (Eq. (8) – Eq. (9)). Meanwhile, the holes can also bind with the TEOA


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ture of the protons from water splitting. In addition, on the catalyst with the carbon-coated In2O3 nanobelts, H2 evolution via the reduction of the protons from water splitting, which is the main competitive process of CO2 reduction, is suppressed, making more protons participate in CO2 reduction. The extended absorption to sunlight, decreased electronhole recombination rate, improved chemisorption of CO2 and increased amount of protons participating in CO2 reduction are responsible for the enhanced photocatalytic reduction of CO2 to CO and CH4 in the presence of the carbon-coated In2O3 nanobelts.

ASSOCIATED CONTENT Supporting Information Experimental details, SEM image, Raman spectra, TGA curves, GC-MS spectra, Influence of the amount of glucose and Pt content on the photocatalytic activity, UV-visible spectra, Mott-Schottky plots, FTIR spectra, DFT-calculated CO2 adsorption configurations. This material is available free of charge via the internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors [email protected] (S. Xin); [email protected] (S.-H. Yu); [email protected] (J. B. Goodenough) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT S This work is supported by the National Basic Research Program of China (Grants 2014CB931800, 2013CB933900), the National Natural Science Foundation of China (Grant No. 21431006, 21503062, 21403050, U1662138) and Anhui Provincial Natural Science Foundation (1608085MB32). The theoretical calculation work in Austin, TX, was supported by the Lawrence Berkeley National Laboratory BMR Program (Grant 7223523).

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Artwork for TOC


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Photocatalytic CO2 Reduction by Carbon-Coated Indium-Oxide

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