Significant Enhancement of Photocatalytic Reduction of CO2 with H2O

Jun 19, 2017 - †Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin Key Laboratory of Applied Catalysis Science a...
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Significant Enhancement of Photocatalytic Reduction of CO2 with H2O over ZnO by the Formation of Basic Zinc Carbonate Chunyu Xin,† Maocong Hu,§ Kang Wang,*,‡ and Xitao Wang*,† †

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin Key Laboratory of Applied Catalysis Science and Technology, College of Chemical Engineering and Technology, and ‡Chemical Engineering Research Center, College of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China § Department of Chemical, Biological and Pharmaceutical Engineering, New Jersey Institute of Technology, Newark, New Jersey 07102, United States S Supporting Information *

ABSTRACT: Electron−hole pair separation efficiency and adsorption performance of photocatalysts to CO2 are the two key factors affecting the performance of photocatalytic CO2 reduction with H2O. Distinct from conventional promoter addition, this study proposed a novel approach to address these two issues by tuning the own surface features of semiconductor photocatalyst. Three ZnO samples with different morphologies, surface area, and defect content were fabricated by varying preparation methods, characterized by XRD, TEM, and room-temperature PL spectra, and tested in photoreduction of CO2 with H2O. The results show that the as-prepared porous ZnO nanosheets exhibit a much higher activity for photoreduction of CO2 with H2O when compared to ZnO nanoparticles and nanorods attributed to the existence of more defect sites, that is, zinc and oxygen vacancies. These defects would lower the combination rate of electron− hole pair as well as promote the formation of basic zinc carbonate by Lewis acid−base interaction, which is the active intermediate species for photoreduction of CO2. ZnO nanoparticles and ZnO nanorods with few defects show weak adsorption for CO2 leading to the inferior photocatalytic activities. This work provides new insight on the CO2 activation under light irradiation.

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INTRODUCTION CO2 emission from the consumption of fossil fuels is widely accepted as the main contributor to global warming. One of the most attractive strategies to address this issue is photocatalytic CO2 reduction with H2O while simultaneously producing high value-added products (CO, methane, ethane, methanol, etc.) due to its low cost, cleanliness, and environmental friendliness.1 However, the intrinsic high standard Gibbs free energy (−394.39 kJ mol−1) makes the activation of CO2 difficult. Therefore, developing high performance photocatalysts to activate CO2 molecule becomes the most critical target in photocatalytic CO2 reduction. One well-established way is by using catalyst with the appropriate band gap while its conduction and valence bands locate at a suitable position leading to generating electrons and holes with strong redox ability, which can finally catalyze adsorbed CO2 to varying products. To date, a large number of photocatalysts have been designed and applied for this purpose, including TiO2 and modified TiO2,2 non-TiO2 metal oxides (ZnO, WO3, CuO, NiO), 3−6 perovskite-type compound oxides (SrTiO 3 , LaCoO3),7,8 metal sulfides (ZnS, CdS),9,10 organic-metallic coordination compounds (Ru(bpy) 3(PF6)2, Fe(bpy) 3+2/ rGO),11,12 and other photocatalysts (g-C3N4, Ag@AgBr/ carbon nanotubes, [Zn 1 . 5 Cu 1 . 5 Ga(OH) 8 ] 2 + (CO 3 ) 2 · mH2O).13−15 Although many efforts have been taken to improve photocatalytic performance of photocatalysts and © XXXX American Chemical Society

considerable advances have been achieved, challenges such as low separation efficiency of electron−hole pair and limited understanding of the reaction mechanism still remain.5,9 Moreover, adsorption performance of photocatalysts to CO2, an important factor significantly affecting the efficiency of photocatalytic CO2 reduction, is often under insufficient consideration. CO2 reduction is essentially an oxidation− reduction process under the photoinduced, which includes two basic processes: CO2 absorption on the active sites of the photocatalysts and conversion process between adsorbed CO2 (or derived intermediates species) and photogenerated electron−hole. 16 Thus, the strength and state of CO 2 adsorption on the photocatalyst surface are of crucial importance for photoreduction of CO2. Previous studies suggest that two types of surface species for the adsorption of CO2 exist on the surface of semiconductor photocatalysts such as TiO2, which are molecularly adsorbed CO2 and surface carbonates.17,18 Both have weak interaction with surface, and are easily desorbed at room temperature due to low adsorption energy,17 leading to a low conversion and inferior photocatalytic efficiency. In consideration of the intrinsic acidic feature of CO2, one common approach to improve the Received: February 23, 2017 Revised: June 17, 2017 Published: June 19, 2017 A

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Zn(NO3)2·6H2O at 500 °C for 4 h. The obtained sample is marked as ZnO-NP. ZnO nanorods were synthesized via a solvothermal route by adjusting the pH of ZnCl2 aqueous solution. For a typical preparation, a certain amount of ZnCl2 was dissolved in deionized water to form a 0.1 mol L−1 solution. The pH of solution was adjusted to 6.5 by dropping a 1 mol L−1 NaOH into this solution. After being stirred for 1 h, the solution was transferred into a 100 mL Teflon-lined stainless steel autoclave. Subsequently, the autoclave was kept at 150 °C for 16 h and then cooled to room temperature. The white precipitate was collected after centrifugal separation, washed with deionized water and ethanol three times, respectively, and dried at 80 °C overnight. Finally, the sample was ground and calcined at 500 °C for 2 h, which was designed as ZnO-NR. ZnO nanosheets were fabricated by annealing ZnS(en)0.5 (en = ethylenediamine) complex precursor. ZnS(en)0.5 composites were synthesized via a solvothermal route according to the literature.27 In a typical procedure, a certain amount of Zn(CH3OO)2 and thiourea was dissolved in 75 mL of ethylenediamine under constant stirring. Subsequently, the solution was transferred into a 100 mL Teflon-lined stainless steel. The autoclave then was kept at 200 °C for 12 h. As the autoclave was cooled to room temperature naturally, the white precipitate was collected by centrifugation, washed several times with ethanol and deionized water alternately, and dried in a vacuum at 60 °C overnight. ZnO nanosheets were prepared by annealing the as-synthesized ZnS(en)0.5 composites at 700 °C for 1 h in air. This ZnO sample is labeled as ZnO-NS. Characterization. The phase composition of the as-prepared products was determined by an X-ray diffractometer (XRD), which was recorded on a D/MAX-2500 automatic powder diffract meter equipped with the graphite monochromatized Cu Kα radiation flux (λ = 0.15418 nm) at a scanning rate of 0.2° s−1 in the 2θ range of 20− 80°. N2 adsorption−desorption measurements were carried out at 77 K using Quantachrome Autosorb. The samples were degassed under vacuum at 473 K for 6 h before measurements were made. The Brunauer−Emmett−Teller (BET) equation was used to calculate the specific surface area. The Barrett−Joyner−Halenda (BJH) method was used to determine the pore-size distribution. The morphologies of samples were characterized by a Hitachi S-4800 scanning electron microscope (SEM, 5 kV). Transmission electron microscopy (TEM) and higher-magnification transmission electron microscopy (HRTEM) were obtained with JEOL-2100F system (200 kV). Specimens for TEM and HRTEM measurements were prepared via drop-casting a droplet of ethanol suspension onto a copper grid, coated with a thin layer of amorphous carbon film, and dried in air. Ultraviolet and visible diffusive reflectance spectra (UV−vis DRS) were taken on a UV−vis spectrometer (PerkinElmer, Lambda 750), the scanned range being 250−600 nm against barium sulfate standard. The Fourier transform infrared (FT-IR) spectra were recorded with Thermo Nicolet Nexus using the KBr disk method. In situ IR spectra for the coadsorption of CO2 and H2O were also taken with the same spectrophotometer. First, the sample was annealed in an IR cell at 473 K in Ar for 2 h, and then CO2 and H2O vapor were introduced into the cell. In situ IR spectra were recorded at different adsorption times. TPD of the coadsorption of CO2 and H2O over ZnO-NS sample was performed to confirm the formation of the basic zinc carbonate and the stability of photocatalyst. In this experiment, 100 mg of catalyst was loaded in a quartz reactor, and the two ends were stuffed with asbestos fiber. Before the adsorption of CO2 and H2O, the catalyst was treated by heating to 648 K in N2 for 2 h, and then the temperature of the reactor was cooled to 473 K. After that the feed stream was switched to a mixture of CO2 and H2O vapor with a molar ratio of 5:1 at the flow rate of 30 mL/min for 30 min. After being cooled to room temperature, the reactor was heated to 673 K with a rate of 5 K/min in a N2 flow of 30 mL/min. The desorbed gases were analyzed by an infrared gas analyzer. Photoluminescence (PL) spectra were tested through using a Fluorolog 3 photoluminescence spectrometer (Horiba Jobin Yvon, Japan). The spectra were obtained in the range of 350−750 nm using a 325 nm laser excitation. Photoelectrochemical measurements were performed in an electrochemistry workstation (CHI 660, CH Instrument, Austin, TX) with a three-electrode system using a three-

adsorption of CO2 as well as enhance the chemical interaction of CO2 and photocatalysts is by addition of alkali and alkaline earth metal promoter. Wang et al.18 demonstrated that the presence of CeO2-containing Ce3+ strengthens the bonding of CO2 with catalyst surfaces and increases the formation of bidentate b-CO32− and b-HCO3− species, which are readily transformed to surface CO2− in the presence of H2O under simulated sunlight irradiation, that was considered as the key intermediate for CO2 photo reduction. Li et al.19 and Wang et al.20 proved that addition of MgO can enhance CO2 and H2O adsorption on TiO2 surface, and improve the activity of CO2 photoreduction. Surface modification of TiO2 with alkali (Na2CO3, NaOH) was also found to be an effective way for the CO2 adsorption, activation, and leads to highly effective conversion of CO2 into CH4 without any noble metal cocatalyst loading.21,22 The above studies indicate that the enhanced CO2 chemisorption plays an important role in the effective activation of CO2 and photoreduction. However, alkali such as Na2CO3, NaOH, and MgO is an insulator (Eg = 8−9 eV), which may prohibit the charge transfer between photocatalyst and active intermediate species and accordingly decrease the photocatalytic activity. Although the ultrathin layers of these alkali may have little effect on the charge transferring, the preparation cost and controllable synthesis may be another issue. Therefore, the direct formation of carbonate or hydroxy-carbonate by coadsorption of CO2 and H2O on the surface of semiconductor photocatalyst but not by adsorption via a second additive would be a more promising and effective way for activation and photoreduction of CO2. In this work, we proposed a facile method to tune the formation of the active carbonate and its content on a single semiconductor photocatalyst surface by changing its morphology, surface area, and defect site content with ZnO as a model. ZnO, as an important direct wide band gap semiconductor (3.37 eV), has drawn attention for photocatalytic application due to its high exciton binding energy (60 meV), rapid generation of photoexcited electron−hole pairs, and high photocatalytic activity under UV-light irradiation. 23−26 Although the intrinsic feature of amphoteric oxide allows it possible to chemically adsorb CO2 and further form hydroxycarbonate in the presence of H2O, the weaker reactivity of ZnO with CO2 causes slow formation of carbonates, which leads to unsatisfactory efficiency for CO2 photoreduction. Three types of ZnO semiconductors with various morphologies, surface areas, and defect concentrations were prepared using different preparation methods, and characterized by XRD, SEM, TEM, HR-TEM, N2 adsorption−desorption, UV−vis spectroscopy, photoluminescence spectra, and photoelectrochemical measurements. The reaction performance of three different ZnO catalysts for photoreduction of CO2 with H2O was evaluated. Furthermore, one novel CO2 activation mechanism by the formation of basic zinc carbonate was illustrated by integrating reaction data with characterizations of XRD, FT-IR, TG-DTG, and in situ light irradiation-mass Spectra. This work not only demonstrates a strategy to promote CO2 photoreduction performance by tuning the own surface features of semiconductor photocatalyst, but also provides new insight on CO2 activation under light irradiation.

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EXPERIMENTAL SECTION

Preparation of ZnO with Different Morphologies. All of the chemical reagents were of analysis grade and used without further purification. ZnO particles were prepared by directly calcining B

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Langmuir compartment glass cell. The working electrode was obtained by dropcasting a drop of as-prepared product on glassy carbon electrode. A platinum wire ring and a saturated calomel electrode were used as the counter electrode and reference electrode, respectively. The experiments were operated at room temperature with Na2SO4 solution (0.1 mol L−1) as electrolyte. Photocatalytic Activity Test. CO2 photoreduction experiments were conducted under irradiation of high-pressure mercury lamp (125 W) in a stainless steel tank reactor equipped with a top quartz glass (100 mm in diameter) and a heating platform. The temperature of platform was monitored by a thermocouple. In a typical experiment, 0.05 g of photocatalyst was loaded, and the temperature was maintained at 150−250 °C. Prior to the irradiation, the reactor was flushed with CO2 at the rate of 30 mL/min for 2 h to remove the air in the reactor. The valves of gas inlet and outlet then were closed, and 0.5 mL of deionized water was injected into reactor. After 4 h of irradiation, the products of the reaction were analyzed by an online gas chromatograph using a thermal conductivity detector (Agilent 4890, nitrogen as a carrier gas).

through each nanosheet with an ultrahigh density, which would obviously increase the surface area of ZnO nanosheets. As seen in the HRTEM images (Figure 2c, f, and i), welldefined crystal structures are revealed for three ZnO samples. The interplanar spacing of 0.28 nm for ZnO-NP and ZnO-NR consistent with the d-spacing of (100) plane of wurtzite structure ZnO was detected,29 whereas the interplanar spacing of 0.26 nm for ZnO-NS was detected, which is related to the dspacing of (002) lattice planes of the wurtzite ZnO, suggesting that the nanosheets grow along the c-axis, the [0001] direction.28 These results are in agreement with the XRD patterns. The specific surface area and porosity of ZnO samples were examined by N2 adsorption/desorption analysis. As depicted in Figure 3A, N2 adsorption/desorption isotherms of the ZnO-NS sample show typical irreversible type-IV curves,30 with a distinct condensation step and a hysteresis loop, revealing the characteristics of mesoporous materials. Furthermore, the pore-size distribution curves of ZnO-NS exhibited mesoporous peaks with the range of 5−30 nm (Figure 3B). However, much smaller hysteresis loops in N2 sorption/desorption isotherms and weak peaks in pore size distribution curves were observed for either ZnO-NP or ZnO-NR samples, which confirmed that ZnO-NP and ZnO-NR samples contain less pores. The BET specific surface areas, average diameters, and pore volumes of three ZnO samples were listed in Table S1. BET surface areas of ZnO-NP and ZnO-NR samples are 1.3 and 12.1 m2/g, respectively, which are much lower than that of ZnO-NS (82.1 m2/g). Pore volumes and average pore sizes for ZnO-NP, ZnONR, and ZnO-NS samples were 0.0087, 0.0093, and 0.52 cm3/g and 4.7, 4.0, and 17.3 nm, respectively, further proving that the ZnO-NS sample possesses more pore structure. The large surface area of ZnO-NS could be attributed to the porous structure of nanosheet, which may originate from the decomposition of ethylenediamine in the precursor. Optical and Electronic Properties of ZnO Semiconductors. The UV−vis absorption spectra of the asprepared ZnO samples are shown in Figure 4A. The strong characteristic absorption below 400 nm from all samples indicated the existence of crystalline ZnO, which is consistent with the above XRD results. When compared to ZnO-NP, ZnO-NS and ZnO-NR displayed a stronger absorption in the UV range, which might be due to specific crystallinity of these two samples. More importantly, ZnO-NS showed much more obvious absorption in the visible region (λ = 500−700 nm) than ZnO-NP and ZnO-NR (inset of Figure 4A), indicating that the ZnO-NS sample has more surface defects than ZnONP and ZnO-NR.31 In this work, ZnS(en)0.5 was used as the precursor for ZnO-NS synthesis via thermal treatment in air where the substitution of S atoms by O atoms would result in the distortion of the crystal lattice in consideration of the smaller radius of O atoms than S atoms.27 Consequently, considerable tensile stress was formed and further would be released through the cracking of the crystal lattice leading to the formation of defect sites. For their direct band gap feature, the band gaps of ZnO samples were calculated according to the following formula:32

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RESULTS AND DISCUSSION Morphologies and Structures of ZnO Semiconductors. Figure 1 shows the XRD patterns of three ZnO samples

Figure 1. X-ray diffraction patterns of the ZnO samples prepared with different methods: (a) ZnO-NP, (b) ZnO-NR, and (c) ZnO-NS.

prepared with different methods. It can be seen clearly that all patterns exhibit almost the same diffraction lines, which can be ascribed to wurtzite ZnO (JCPDS36-1451). This indicates that the difference of preparation methods did not alter the crystal structure of ZnO. The diffraction peaks of ZnO-NP and ZnONR samples are distinctly stronger and sharper when compared to those of ZnO-NS, revealing that ZnO-NP and ZnO-NR have much larger particle size and higher crystallinity than ZnO-NS. In addition, the ZnO-NS sample displays relatively higher diffraction intensity of (002) plane, while the other two samples possess stronger peaks of (100) and (101) plane. This result shows that the preferred orientation of ZnO-NS was along the c-axis.28 The morphologies of the obtained ZnO samples were investigated by SEM and TEM, as shown in Figure 2. ZnO-NP exhibits polyhedral structure and rod-like shape formed by the accumulation of polyhedral ZnO particles, and the particle sizes are in the range of 200−900 nm (Figure 2a and b). For the ZnO-NR sample, the hexagonal prisms were observed, which have an average diameter of ca. 50−60 nm and length of 300− 400 nm. ZnO fabricated by annealing ZnS(en)0.5 are porous planar nanosheets with a size of 800 × 1000 nm2 and a thickness of 80−100 nm, which were formed by the accumulating of small particles as presented in Figure 2g and h. Numerous pores with different size distribution are found all

αhν = A(hν − Eg )1/2

(1)

where α is the linear absorption, A is a constant, hν is the photon energy, and Eg is the direct band gap. Extrapolating the linear section of the (hν)−(αhν/A)2 to (αhν/A)2 = 0, the C

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Figure 2. SEM images of (a) ZnO-NP, (d) ZnO-NR, and (g) ZnO-NS; TEM images of (b) ZnO-NP, (e) ZnO-NR, and (h) ZnO-NS; and HRTEM images of (c) ZnO-NP, (f) ZnO-NR, and (i) ZnO-NS.

Figure 3. N2 adsorption−desorption isotherms (A) and pore size distributions (B) of ZnO samples prepared with different methods.

intercept value is Eg. The obtained plots were presented in Figure 4B, and the estimated Eg values of ZnO were listed in Table S1. The Eg values increase in the order of ZnO-NP, ZnONR, and ZnO-NS, which can be related to the particle size of ZnO samples as revealed by XRD, SEM, and TEM results in Figures 1 and 2. The room-temperature PL spectrum was used to investigate the state of photogenerated electron−hole pairs and the defects

in the photocatalyst, as shown in Figure 4C. The PL spectra of ZnO-NP and ZnO-NR exhibited a broad UV emission band with a peak around 402 nm, which is considered as near band edge emission due to recombination of free excitons.33 It is notable that ZnO-NS showed two emission bands, a relative low UV emission band with a peak around 390 nm and a much stronger defect emission band with a peak around 508 nm. The UV emission band showed a blue shift of 12 nm when D

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Figure 4. (A) UV−vis diffuse reflectance spectra, (B) plot of the transformed Kubelka−Munkfunction versus the photon energy from where the band gap is determined, (C) room-temperature PL spectra, and (D) photocurrent response of ZnO samples prepared with different methods.

CO2 in the reaction system, suggesting that the formed carboncontaining products originated from CO2 but not contamination. In addition to oxygen, H2, CO, and CH4 were found to be the major CO2 reduction products, in accord with previous papers.38,39 Except for a trace amount of C2H6, other products such as CH3OH, CH2O, and HCOOH were not detected during the reaction. Figure 5 shows the evolution of the main products (H2, CO, and CH4) of CO2 reduction with H2O over ZnO photocatalysts prepared with different methods under UV light irradiation.

compared to that of ZnO-NP and ZnO-NR. The blue shift of UV emission band can be explained by the larger band gap of ZnO-NS. The defect emission was far stronger than UV emission, indicating that ZnO-NS contains a large number of surface defects. This broad visible emission from 425 to 650 nm can be attributed to the transition from native point defects in ZnO nanosheets, such as oxygen vacancy VO, zinc vacancy VZn, interstitial zinc Zni, and interstitial oxygen Oi, as discussed earlier.33−35 However, blue light emission around 460 nm from Zni centers and red-orange emission around 657 nm from the intrinsic defects of Oi are observed at very low scale in roomtemperature PL measurements. Therefore, the origin of this broad visible emission may be due to contributions from VZn and VO.36 Furthermore, the morphology and structure of ZnO have a significant influence on the intensity of fluorescence emission.37 Among these three ZnO, ZnO-NS possesses the smallest emission band, implying the lowest combination rate of photogenerated electrons and holes. To provide additional evidence for the above suggested observation, the transient photocurrent responses of ZnO series were recorded for several on−off cycles under Xe lamp as shown in Figure 4D. ZnO-NR and ZnO-NS samples exhibit much higher photocurrents as compared to ZnO-NP. This enhancement could be attributed to the improved photo absorption in the range of UV light, large surface area, and low recombination rate of photogenerated charge carriers. Photocatalytic Activity. The photocatalytic activities of three ZnO catalysts were evaluated by the reduction of CO2 with H2O vapor at 200 °C under UV light irradiation. In the control experiments, without irradiation or photocatalysts, no product was detected, indicating that the CO2 reduction requires the coexistence of photocatalyst and light irradiation. Besides, only H2 was detected when the high-purity N2 replaced

Figure 5. Production rates of CO, H2, and CH4 over ZnO samples prepared with different methods for CO2 photoreduction by H2O vapor at 200 °C. E

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the water splitting and the CO2 reduction rates equally (or at least in similar order of magnitudes), which is not true in the above reaction test. Integrating the above discussion, it is reasonable to make the hypothesis that different active sites for CO2 chemisorption leading to new chemisorbed state of CO2 exist in ZnO-NS, which leads to a completely different product distribution and promotion effect on reactants’ conversion as compared to ZnO-NP and ZnO-NR. To clarify the assumption that the new chemisorbed state of CO2 formed on ZnO-NS, several techniques were adopted. XRD patterns of fresh, exposed to air for 72 h, and spent ZnONS sample are shown in Figure 6A. As compared to fresh ZnONS, sample placed in air for 3 days and those spent for 4 and 16 h exhibit new diffraction peaks in addition to those belonging to the wurtzite ZnO, which can be ascribed to the Zn5(OH)6(CO3)2 phase (JCPDS no. 72-1100). This result indicates that ZnO-NS would react with CO2 and H2O to form basic zinc carbonate due to the reaction:

After 4 h of irradiation, the ZnO-NP sample presents a relatively higher H2 production rate of 84.30 μmol/gcat/h, but those of CO and CH4 are pretty low (8.68 and 0.32 μmol/gcat/ h). The H2 production rate of ZnO-NR is slightly lower than that of ZnO-NP, which is about 66.80 μmol/gcat/h, whereas its production rates of CO (57.96 μmol/gcat/h) and CH4 (0.52 μmol/gcat/h) are obviously higher than those of ZnO-NP. As compared to the other two samples, ZnO-NS exhibits the highest photocatalytic activity, and H 2 , CO, and CH 4 production rates of 112.69 μmol/gcat/h, 406.77 μmol/gcat/h, and 20.16 μmol/gcat/h are achieved. The accumulated yield of 4 h for H2, CO, and CH4 over ZnO-NS reached 450.7, 1627.08, and 80.6 μmol/gcat, respectively. The effects of reaction temperature on photocatalytic reduction of CO2 over ZnONS photocatalyst were also investigated, and the production rates of the main products at 150, 200, and 250 °C are shown in Figure S2. It can be seen clearly that the increase of temperature significantly improves the rates of main products evolution. Reaction temperature was raised from 150 to 200 °C, and the production rates of all main products were almost doubled. Further increase in reaction temperature to 250 °C could increase the photocatalytic activity by 50%. This obvious improvement may be due to the effect of temperature on “dark” reaction steps, such as the adsorption−desorption equilibrium of reactants and products, diffusion of adsorbed species, and other thermal catalytic reaction steps occurring on the semiconductor surface. Table 1 presents the distributions of main products and total CO2 conversion rates for all ZnO samples at 200 °C.

5ZnO + 2CO2 + 3H 2O → Zn5(OH)6 (CO3)2

Further evidence for the formation of Zn5(OH)6(CO3)2 over ZnO-NS sample was obtained by FT-IR of three ZnO samples exposed to air for 72 h, as shown in Figure 6B. For the FT-IR spectrum of ZnO-NS, the absorption band at 3420 cm−1 related to hydroxyl groups was detected, while 1525, 1388, 1047, 837, and 713 cm−1 corresponding to the vibration of CO32− were observed, which were consistent with t hose o f Zn5(OH)6(CO3)2 reported in previous papers.40 These absorption bands reveal the formation of Zn5(OH)6(CO3)2 over ZnO-NS sample, which further confirmed the XRD results. However, ZnO-NP and ZnO-NR only displayed very weak absorption bands at 3420, 1525, and 1388 cm−1, while other peaks due to Zn5(OH)6(CO3)2 were not detected, which suggests that the amount of basic zinc carbonate is in a very low level over ZnO-NR and ZnO-NP samples. Furthermore, the peak centered at 1626 cm−1, which was contributed from bidentate bicarbonate (b-HCO3−), the common adsorbed intermediate due to CO2 adsorption on semiconductor,18 was observed on ZnO-NR and ZnO-NP with weak signals, while it is not obvious over ZnO-NS. However, it is worth noting that the peak of 1525 cm−1 in ZnO-NS is very broad, which may cover other weak signals such as bidentate bicarbonate. In addition, in situ FT-IR spectra for the coadsorption of CO2 and H2O at different adsorption times under reaction conditions were also taken, as shown in Figure 8B. It could be seen clearly that the strong bands ascribed to Zn5(OH)6(CO3)2 were detected in 20 min, and the intensity of these bands increased gradually with the increase of adsorption time. This result further indicated that Zn5(OH)6(CO3)2 was able to form under the reaction conditions in a short time. Figure 6C shows the TG analytic result of ZnO-NS exposed to air for 72 h at a heating rate of 10 °C min−1. There is an obvious weight loss from 225 to 275 °C, resulting from the decomposition of Zn5(OH)6(CO3)2.41 The weight loss in this stage is about 20.5%, indicating that close to 62% of ZnO was converted to Zn5(OH)6(CO3)2 according to the reaction equation, which confirmed that Zn5(OH)6(CO3)2 can be formed in a large number over ZnO-NS sample. The difference between the intermediates formed on the three ZnO catalysts can be attributed to different surface defect conditions. As revealed by UV/vis spectra and PL spectra, the ZnO-NS sample possesses more defect sites (i.e., VO and VZn) when compared to ZnO-NP and ZnO-NR, which can

Table 1. Product Distribution and CO2 Conversion for All of the Samples at 200 °C under UV Radiation product distribution (%)

conversion (μmol/gcat/h)

samples

H2

CO

CH4

CO2

H2O

(CO+CH4)/H2

ZnO-NP ZnO-NR ZnO-NS

90.3 53.3 20.8

9.3 46.2 75.4

0.34 0.49 3.73

9.0 58.5 426.9

84.94 67.84 153.01

0.11 0.88 3.80

(2)

Obviously, ZnO-NP shows a relative high evolution rate for H2, but those for CO and CH4 are very low (the (CO+CH4)/ H2 molar ratio = 0.11), revealing that the photocatalytic water splitting to H2 preferentially occurs over the ZnO-NP sample rather than photocatalytic reduction of CO2. For ZnO-NR nanorod, the formation rate of H2 is close to that of CO and CH4, suggesting it had comparable activity for water splitting and CO2 reduction. The ZnO-NS sample exhibited a far higher production rate for CO and CH4 than H2, on which the molar ratio of (CO+CH4)/H2 of 3.80 was achieved. This result indicates that the ZnO-NS sample is a more efficient photocatalyst for CO2 reduction. Moreover, it can be observed that H2O conversion over ZnO-NS is around twice those on ZnO-NP and ZnO-NR, while the CO2 conversion on ZnO-NS is about 50 and 7 times those over ZnO-NP and ZnO-NR, respectively, suggesting that the promoting effects of ZnO-NS for H2O and CO2 conversion rates are completely different. Although high surface area, good photoabsorption, and low recombination rate of photogenerated electron−hole pairs of ZnO-NS can improve the photocatalytic activity, these variables are not major factors for remarkably changing product distribution and resulting in different promotion effects for H2O and CO2 conversion because they should promote both F

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Figure 6. (A) XRD pattern of ZnO-NS: (a) fresh, (b) exposed to air for 72 h, (c) used for 4 h, and (d) used for 16 h. (B) FT-IR of three ZnO samples exposed to air for 72 h: (a) ZnO-NP, (b) ZnO-NR, and (c) ZnO-NS. (C) TG-DTG curves of ZnO-NS exposed to air for 72 h. (D) Schematic diagram showing H2O and CO2 adsorption mechanism on ZnO-NS surface.

Figure 7. (A) In situ light irradiation-MS spectra of ZnO-NS exposed to air at 170 °C and (B) band gap structure of ZnO-NS and the possible process for the photoreduction of CO2 with H2O.

chemisorb H2O and CO2, respectively, to form OH and CO32−. H2O is a Lewis base donating a pair of nonbonding electrons while the oxygen vacancies (VO) on the ZnO-NS surface serve as active Lewis acidic sites, leading to the formation of Lewis acid−base interactions. Accordingly, H2O would dissociatively adsorb on oxygen vacancy and further transfer one proton to a nearby oxygen atom, resulting in forming two hydroxyl groups for each vacancy. On the other hand, CO2 is a Lewis acid, and the zinc vacancies on the surface play the role of active Lewis basic sites for the Lewis acid−base interactions. Consequently, CO2 can be chemisorbed on zinc vacancies VZn to form CO32− species. In this manner, the novel intermediate Zn5(OH)6(CO3)2 was produced on the ZnO-NS surface. In

contrast, due to the lack of oxygen vacancies VO and zinc vacancies VZn, the common intermediates bidentate bicarbonate (b-HCO3−) originating from CO2 adsorption were formed on the surface of ZnO-NR and ZnO-NP. Figure 6D presents a proposed possible model for chemisorption of H2O and CO2 on ZnO-NS surface. On the basis of the above analysis, we could conclude that the formation of novel intermediates basic zinc carbonate might be responsible for the enhancement of photoreduction of CO2 with H2O over ZnO-NS sample. However, before accepting this assumption, it should be considered whether the adsorbed Zn5(OH)6(CO3)2 can be converted to CO and CH4 under light irradiation. Therefore, two experiments were performed. G

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Figure 8. (A) Photocatalytic production rates of CO, H2, and CH4 in four cycles over ZnO-NS; (B) in situ FT-IR of cosorption of CO2 and H2O over ZnO-NS at different adsorption times: (a) 0 min, (b) 20 min, (c) 40 min, and (d) 60 min; (C) TPD of coadsorption of CO2 and H2O over ZnO-NS; and (D) TEM image of the ZnO-NS sample used for 16 h.

oxygen vacancy VO to form OH. Accordingly, the novel strongly adsorbed intermediate basic zinc carbonate was produced, which facilitated the photocatalytic reduction of CO2 to CO and CH4. However, the normal ZnO conductor band possesses weak adsorption for CO2, leading to an inferior efficiency for photoreduction of CO2 with H2O, which is the case on ZnO-NR and ZnO-NP. The other active sites for H2O adsorption over the three catalysts are on the conductor band of ZnO (zinc ion) due to the acidic−basic interaction, which is responsible for the H2 production. This explains why ZnO-NP and ZnO-NR samples exhibit fairly high H2 production rates while holding low CO production rates. We also did the stability test over the ZnO-NS catalyst for four cycles (16 h), and the results are shown in Figure 8A. It is clear that the ZnO-NS catalyst remained stable for the photocatalytic reduction of CO2 with H2O after running four cycles under experimental condition. The production rate of CO, H2, and CH4 was observed to be ∼88%, ∼100%, and >90%, respectively, of that from fresh sample. The high stability may be attributed to the good recoverability of active species (i.e., defect sites, VO and VZn). To clarify this assumption, TPD over the sample with different number of cycles by coadsorption for H2O and CO2 was conducted, and the results were exhibited in Figure 8C. Obviously, as compared to the fresh ZnO-NS, catalysts with different numbers of cycles displayed similar results. The highly symmetrical peak at around 270 °C can be ascribed to the decomposition of basic zinc carbonate, which is consistent with the results of TG-DTG (Figure 6C). Especially, as compared to the peak from the first run, the two peaks of the next two cycles exhibit almost the

One is photocatalytic reaction using ZnO-NS with the formation of Zn5(OH)6(CO3)2 as photocatalyst, in which feed gases were changed to high-purity N2 and H2O. After light irradiation for 4 h at 200 °C, CO, CH4, CO2, along with H2 were detected, showing that Zn5(OH)6(CO3)2 can be converted to CO, CH4, and CO2. To rule out the possibility that the obtained products were converted from the adsorbed gas phase CO2 on catalyst surface, in situ light irradiation-mass spectrograph (MS) experiment was employed using Ar as carrier gas. Prior to the light irradiation, 50 mg of ZnO-NS with the formation of Zn5(OH)6(CO3)2 was loaded into a reactor, and treated at an Ar flow of 50 mL/min at 170 °C for 2 h to remove CO2 and H2O absorbed on the surface. After the UV light was turned on, the formed products were brought into the mass spectrograph by Ar carrier gas. The obtained MS spectra were shown in Figure 7A. It can be seen that obvious CO, CH4, and CO2 signals were detected, and the CO signal was significantly stronger than CH4, which agreed with photocatalytic reaction results, further proving that Zn5(OH)6(CO3)2 is a reactive intermediate for photoreduction of CO2 with H2O. Moreover, it is notable that no H2 product was observed, which was due to the fact that oxygen vacancies are electron acceptors, and OH absorbed on oxygen vacancies cannot be reduced to H2. On the basis of the above discussion, the band gap structure and possible mechanism for the photoreduction of CO2 with H2O over ZnO-NS were proposed as illustrated in Figure 7B. The zinc vacancy VZn served as electron donors under light irradiation to chemisorb CO2 and form CO32− by Lewis acid− base interactions, while the H2O would dissociatively adsorb on H

DOI: 10.1021/acs.langmuir.7b00620 Langmuir XXXX, XXX, XXX−XXX

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(3) Leung, Y. H.; Chen, X. Y.; Ng, A. M. C.; Guo, M. Y.; Liu, F. Z.; Djurišić, A. B.; Chan, W. K.; Shi, X. Q.; Van Hove, M. A. Green Emission in ZnO NanostructuresExamination of the Roles of Oxygen and Zinc Vacancies. Appl. Surf. Sci. 2013, 271, 202−209. (4) Hu, H.; Deng, C.; Xu, J.; Zheng, Q.; Chen, G.; Ge, X. Facile Synthesis of Hierarchical WO3 Nanocakes Displaying the Excellent Visible Light Photocatalytic Performance. Mater. Lett. 2015, 161, 17− 19. (5) Villani, M.; Alabi, A. B.; Coppedè, N.; Calestani, D.; Lazzarini, L.; Zappettini, A. Facile Synthesis of Hierarchical CuO Nanostructures with Enhanced Photocatalytic Activity. Cryst. Res. Technol. 2014, 49, 594−598. (6) Wu, J.; Luo, C.; Li, D.; Fu, Q.; Pan, C. Preparation of Au Nanoparticle-Decorated ZnO/NiO Heterostructure via Nonsolvent Method for High-Performance Photocatalysis. J. Mater. Sci. 2017, 52, 1285−1295. (7) Xian, T.; Yang, H.; Di, L. J.; Dai, J. F. Enhanced Photocatalytic Activity of SrTiO3 Particles by Surface Decoration with Ag Nanoparticles for Dye Degradation. Phys. Scr. 2015, 90, 055801. (8) Yamada, Y.; Yano, K.; Hong, D.; Fukuzumi, S. LaCoO3 Acting as An Efficient and Robust Catalyst for Photocatalytic Water Oxidation with Persulfate. Phys. Chem. Chem. Phys. 2012, 14, 5753−5760. (9) Zhang, J.; Wang, Y.; Zhang, J.; Lin, Z.; Huang, F.; Yu, J. Enhanced Photocatalytic Hydrogen Production Activities of Au-loaded ZnS Flowers. ACS Appl. Mater. Interfaces 2013, 5, 1031−7. (10) Chen, Z.; Liu, S.; Yang, M. Q.; Xu, Y. J. Synthesis of Uniform CdS Nanospheres/Graphene Hybrid Nanocomposites and Their Application as Visible Light Photocatalyst for Selective Reduction of Nitro Organics in Water. ACS Appl. Mater. Interfaces 2013, 5, 4309− 19. (11) Lackner, G. L.; Quasdorf, K. W.; Pratsch, G.; Overman, L. E. Fragment Coupling and the Construction of Quaternary Carbons Using Tertiary Radicals Generated from tert-Alkyl N-Phthalimidoyl Oxalates by Visible-light Photocatalysis. J. Org. Chem. 2015, 80, 6012− 24. (12) Kumar, A.; Kumar, P.; Paul, S.; Jain, S. L. Visible Light Assisted Reduction of Nitrobenzenes Using Fe(bpy)3+2/rGO Nanocomposite as Photocatalyst. Appl. Surf. Sci. 2016, 386, 103−114. (13) Bai, X.; Wang, L.; Zong, R.; Zhu, Y. Photocatalytic Activity Enhanced via g-C3N4 Nanoplates to Nanorods. J. Phys. Chem. C 2013, 117, 9952−9961. (14) Abou Asi, M.; Zhu, L.; He, C.; Sharma, V. K.; Shu, D.; Li, S.; Yang, J.; Xiong, Y. Visible-Light-Harvesting Reduction of CO2 to Chemical Fuels with Plasmonic Ag@AgBr/CNT Nanocomposites. Catal. Today 2013, 216, 268−275. (15) Morikawa, M.; Ahmed, N.; Yoshida, Y.; Izumi, Y. Photoconversion of Carbon Dioxide in Zinc−copper−gallium Layered Double Hydroxides: The Kinetics to Hydrogen Carbonate and Further to CO/Methanol. Appl. Catal., B 2014, 144, 561−569. (16) Handoko, A. D.; Li, K. F.; Tang, J. W. Recent Progress in Artificial Photosynthesis: CO2 Photoreduction to Valuable Chemicals in a Heterogeneous System. Curr. Opin. Chem. Eng. 2012, 2, 200−206. (17) Liu, L.; Zhao, C.; Pitts, D.; Zhao, H.; Li, Y. CO2 Photoreduction with H2O Vapor by Porous MgO−TiO2 Microspheres: Effects of Surface MgO Dispersion and CO2 Adsorption−desorption Dynamics. Catal. Sci. Technol. 2014, 4, 1539−1546. (18) Wang, Y.; Zhao, J.; Wang, T.; Li, Y.; Li, X.; Yin, J.; Wang, C. CO2 Photoreduction with H2O Vapor on Highly Dispersed CeO2/ TiO2 Catalysts: Surface Species and Their Reactivity. J. Catal. 2016, 337, 293−302. (19) Li, H.; Wu, X.; Wang, J.; Gao, Y.; Li, L.; Shih, K. Enhanced Activity of Ag-MgO-TiO2 Catalyst for Photocatalytic Conversion of CO2 and H2O into CH4. Int. J. Hydrogen Energy 2016, 41, 8479−8488. (20) Wang, F.; Zhou, Y.; Li, P.; Kuai, L.; Zou, Z. Synthesis of BionicMacro/Microporous MgO-Modified TiO2 for Enhanced CO2 Photoreduction into Hydrocarbon Fuels. Chinese Journal of Catalysis 2016, 37, 863−868.

same areas, revealing that the defect sites (leading to the formation of basic zinc carbonate) were completely recovered after the decomposition of basic zinc carbonate. The TEM image of the spent catalyst used for 16 h was illustrated in Figure 8D. The spent ZnO-NS remained in the same nanosheet structure as the fresh one, further explaining the high stability of this photocatalyst. These results confirm that ZnO-NS is highly efficient and stable for photocatalytic reduction of CO2 with H2O under the irradiation of UV lamp.

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CONCLUSIONS This work demonstrates a novel strategy to promote CO2 photoreduction performance. Three ZnO samples were fabricated by varying preparation methods. The morphologies, surface area, photoluminescence properties, and photocatalytic activities of those three ZnO samples were investigated. The results show that the as-prepared porous ZnO nanosheets with the existence of more defect sites that is, zinc and oxygen vacancies, exhibit a much higher activity for photoreduction of CO2 with H2O when compared to ZnO nanoparticles and nanorods. The production rate for H2, CO, and CH4 can reach 112.69, 406.77, and 20.16 μmol/gcat/h at reaction temperature of 200 °C over ZnO nanosheets. Basic zinc carbonate strongly adsorbed on the ZnO nanosheets surface is believed to be the active intermediate species, which were formed due to Lewis acid−base interactions and finally facilitated the photocatalytic reduction of CO2 to CO and CH4. On the contrary, due to the lack of defect sites, ZnO nanoparticles and ZnO nanorods showed weak adsorption for CO2 while holding a high combination rate of photogenerated electrons and holes, leading to inferior photocatalytic activities. This work provides new insight on the CO2 activation under light irradiation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b00620. Table S1, physicochemical properties of ZnO samples; and Figure S2, production rates of the main products (H2, CO, CH4) at 150, 200, and 250 °C using ZnO-NP nanosheet as photocatalyst (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the National Natural Science Foundation of China (nos. 21276190 and 20806059).

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REFERENCES

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DOI: 10.1021/acs.langmuir.7b00620 Langmuir XXXX, XXX, XXX−XXX