- Email: [email protected]
Photocatalytic reduction of CO2 to methane over PtOx-loaded ultrathin Bi2WO6 nanosheets
Applied Surface Science 470 (2019) 832–839
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
Photocatalytic reduction of CO2 to methane over PtOx-loaded ultrathin Bi2WO6 nanosheets
T
⁎
Qianqian Wang, Kefu Wang, Ling Zhang, Haipeng Wang, Wenzhong Wang
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: CO2 photoreduction Bi2WO6 nanosheets PtOx nanoparticles Carrier separation Water oxidation
Solar CO2 photoreduction into hydrocarbons is promising and significative. However, many conventional catalysts reported usually suffer from poor photocatalytic activities. Herein, ultrathin Bi2WO6 nanosheets with a thickness of about 4.8 nm have been synthesized by hydrothermal method, which exhibited a CH4 production rate of 19 ppm g−1 h−1 under a low CO2 concentration of 400 ppm. PtOx nanoparticles with a size of about 2 nm were then loaded on the Bi2WO6 nanosheets as excellent co-catalysts by photoreduction in aqueous solution, and an optimal CH4 yield of 108.8 ppm g−1 h−1 was achieved, which was about 5.7 times than that of pristine Bi2WO6 nanosheets. Further analyses of photocurrent curves, electrochemical impedance spectroscopy and polarization curves of water oxidation indicated that the improved photocatalytic activity was suggested to result from the enhanced carrier separation and accelerated water oxidation by PtOx nanoparticles. The work will likely give a deeper insight of PtOx nanoparticles and provide a new idea to design catalysts for CO2 photoreduction to CH4.
1. Introduction The atmospheric CO2 concentration has been growing steadily over the past century, inducing the severe global warming problem [1–4]. Artificial photosynthesis, employing semiconductor photocatalysts to convert CO2 into valuable fuels utilizing sunlight, is probably one of the most sustainable and economical methods to cut down CO2 emission and utilize solar energy simultaneously [5–9]. To date, various photocatalysts have been investigated for CO2 photoreduction, such as TiO2 [10,11], ZnO [12], C3N4 [13]. Although progresses have been made in this field, the photocatalytic activity remains pretty low, possibly due to the limited exposed active sites and the fast recombination rate of photo-induced carriers. The catalysts of ultrathin structure and loading co-catalysts seem to be a good solution to address the problems. Bi2WO6 has caught a lot of attention for its good photocatalytic performance [14–20]. Bi2WO6 is comprised of alternating perovskitelike (Bi2O2)2+ and fluorite-like (WO4)2− layers, which is conducive to construct ultrathin Bi2WO6 nanosheets due to the special laminated structure. Considering the fast recombination rate of photo-induced carriers, loading of co-catalysts is an effective method to facilitate carrier separation [21,22]. Recently, it has been reported that PtOx nanoparticles are inclined to capture photo-induced holes and serve as active sites for water oxidation, implying that PtOx nanoparticles could ⁎
not only enhance the carrier separation efficiency but also promote the process of water oxidation [23]. Therefore, PtOx-loaded ultrathin Bi2WO6 nanosheets are supposed to be ideal catalysts for CO2 photoreduction. Herein, we investigated the photoreaction of CO2 and water on PtOx-loaded ultrathin Bi2WO6 nanosheets. The ultrathin Bi2WO6 nanosheets with a thickness of 4.8 nm were synthesized by a hydrothermal method, while PtOx nanoparticles of 2 nm were subsequently deposited on the surface of Bi2WO6 nanosheets through photoreduction. PtOx-loaded ultrathin Bi2WO6 nanosheets exhibited an efficient CH4 production rate of 108.8 ppm g−1 h−1, which was about 5.7 times the yield of pristine Bi2WO6 nanosheets, manifesting that PtOx nanoparticles could greatly improve the efficiency of CO2 photoreduction. Photocurrent experiments and electrochemical impedance spectroscopy characterizations indicated that PtOx nanoparticles contributed to the separation of photo-generated carriers, while the O2 evolution experiments and polarization curves of water oxidation demonstrated that PtOx nanoparticles could also promote water oxidation, which might account for the improved CH4 production.
Corresponding author. E-mail address: [email protected] (W. Wang).
https://doi.org/10.1016/j.apsusc.2018.11.197 Received 28 August 2018; Received in revised form 19 November 2018; Accepted 25 November 2018 Available online 27 November 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
Applied Surface Science 470 (2019) 832–839
Q. Wang et al.
2. Experimental section 2.1. Catalyst preparation All chemical regents of analytical purity were utilized without any further treatment. The ultrathin Bi2WO6 nanosheets were prepared employing a method provided by Sun et al. [24]. Typically, Bi (NO3)5·5H2O (2.425 g) was dissolved in HNO3 solution (5 mL, 4 M). When it was clarified, 0.824 g of Na2WO4 was added into it. Then the NaOH solution (pH = 2) was used to regulate the pH value of the suspension weakly acidic. After vigorous stirring for 20 min, the suspension was transferred to 50 mL Teflon-lined autoclave, which was then sealed into a stainless-steel tank and heated at 160 °C for 27 h. The autoclave was allowed to cool down naturally. The final production was collected, washed with water for several times and then freeze-dried. The PtOx/Bi2WO6 was prepared by photoreduction method. Firstly, 0.1 g of Bi2WO6 was dispersed into the mixture solution with 100 mL of water and 1 mL of 1 mg/mL H2PtCl6 at pH around 6. The suspension was stirring for several minutes and then irradiated under 500 W Xe lamp for 1 h. Next, the sample was centrifuged with water for several times and freeze-dried.
Fig. 1. XRD patterns of as-prepared Bi2WO6 and PtOx/Bi2WO6 samples.
naturally, the FTO glass was utilized as the working electrode. The flatband potentials (Efb) of catalysts were obtained at 1000 Hz with 5 mV amplitude, while the polarization curves of water oxidation were conducted at 20 mV/s.
2.2. Characterization 3. Result and discussion Powder X-ray diffraction (XRD) characterization was conducted on a D/MAX 2250 diffractometer (Rigaku, Japan) employing monochromatized Cu Kα (λ = 0.15418 nm) radiation under 40 kV and 100 mA, while the 2θ ranged from 5° to 70°. The microstructures and morphologies of catalysts were determined by Transmission electron microscopy (TEM). Brunauer-Emmett-Teller (BET) specific surface area and Barrett-Joyner-Halenda (BJH) pore size distribution were obtained by a surface area and pore size analyzer (V-sorb 2008P, Gold APP). Xray photoelectron spectroscopy (XPS) employing an ESCALAB 250Xi (ThermoFischer) was implemented by irradiating of aluminum Kα Xrays at 1253.6 eV under ultrahigh-vacuum conditions. Raman spectra were obtained utilizing a microscopic confocal Raman spectrometer (Renishaw 1000NR) with an excitation of 514 nm laser light at room temperature. Fourier transformed infrared (FTIR) spectra were recorded with a Nicolet iS10 FTIR spectrometer. UV–vis diffuse reflectance spectra (DRS) of catalysts were conducted utilizing a Hitachi UV-3010PC UV–vis spectrophotometer. The photoluminescence (PL) spectra were obtained employing a Hitachi F4600 fluorescence spectrophotometer (excitation wavelength = 280 nm) at room temperature in air.
The XRD diffraction patterns of Bi2WO6 and PtOx/Bi2WO6 are displayed in Fig. 1. All of the diffraction peaks could be indexed to the standard Bi2WO6 (JCPDS 73-2020) and no other peaks of possible impurities appeared in the XRD pattern. The broad diffraction peaks implied the nanocrystalline nature of Bi2WO6 sample. There were no diffraction peaks of PtOx for the PtOx/Bi2WO6 sample, which could be ascribed to the fact that the amount of PtOx (0.5%) was lower than the detection limit of XRD. Fig. 2 and Table 1 show the N2 adsorptiondesorption isotherm curves and corresponding pore size distributions of Bi2WO6 and PtOx/Bi2WO6. It can be found that the surface area became slightly larger after the loading of PtOx, increasing from 24.3 to 27.8 m2/g, which could be in favor of improving CO2 photoreduction. The TEM images of Bi2WO6 in Fig. 3a and b show that the sample possessed a square sheet-like morphology with an average lateral size of about 60 nm and a thickness of about 4.8 nm. The high-resolution TEM image of Bi2WO6 is displayed in Fig. 3c. Lattice distances of 0.258 nm and 0.315 nm can be allotted to (0 2 2) and (1 1 3) planes of Bi2WO6, respectively, indicating that ultrathin Bi2WO6 nanosheets were successfully synthesized. Besides, from the TEM images of PtOx/Bi2WO6 in Fig. 3d, it can be seen that PtOx nanoparticles with a size of about 2 nm are uniformly loaded on the Bi2WO6 nanosheets. To investigate and reveal the elemental compositions and corresponding chemical valence states of the samples, XPS analysis was conducted. As shown in Fig. 4a, the O 1s peaks of Bi2WO6 and PtOx/ Bi2WO6 appear around 530.0 eV and 530.2 eV, respectively. The O 1s peak of Bi2WO6 can be de-convoluted into three peaks at 532.0 eV, 530.5 eV, and 529.7 eV by Gaussian fitting, corresponding to the surface adsorbed oxygen, W-O, Bi-O, respectively, while the peaks centered at 532.0 eV, 530.8 eV and 529.9 eV are attributed to relevant O 1s peaks of PtOx/Bi2WO6 [25]. It can be found that the O 1s peak position shifted to higher binding energy after the deposition of PtOx nanoparticles. The binding energy of Bi 4f peak in Fig. 4b showed a similar increase as well when PtOx was loaded, implying that PtOx might be closely contacted with Bi2WO6 nanosheets and an interaction existed between them, which could contribute to the carrier separation. The high-resolution W 4f peaks of Bi2WO6 and PtOx/Bi2WO6 are illustrated in Fig. 4c. The peaks concentrated at 37.8 eV and 35.7 eV correspond to the W+6 peaks of W4f 5/2 and W 4f 7/2, respectively [26]. The binding energy peak of Pt 4f (Fig. 4d) can be de-convoluted into four peaks at 78.1 eV, 76.3 eV, 74.8 eV, and 72.9 eV. The peaks at 78.1 eV and 74.8 eV are attributed to Pt4+, while the peaks at 76.3 eV and 72.9 eV
2.3. Catalytic tests CO2 reduction experiments were conducted in an airtight system with a 500 W Xe lamp as light source. 20 mg of catalysts were evenly spread on the bottom of the reactor (600 mL) with a quartz window for illumination, and then de-ionized water (1 mL) was added into the system, which was degassed by nitrogen thoroughly. Finally, CO2 of 400 ppm was injected into the closed system. Gas chromatograph (GC, Tianmei 7890) with a flame ionization detector (FID) was employed to detect CH4 yield in the reactor. 2.4. Electrochemical tests Electrochemical experiments were carried out on a CHI 660D electrochemical workstation (Shanghai Chenhua, China) employing a standard three-electrode system with a working electrode, a platinum slice counter electrode, and a saturated calomel electrode (SCE) as the reference electrode, while the light source is a Xe lamp (CHF-XM500). For the preparation of working electrode, typically, catalyst powder of 10 mg was dispersed in 1 mL nafion-ethanol (1 wt%) solution. 100 μL of above suspension was then coated on the FTO glass. After dried 833
Applied Surface Science 470 (2019) 832–839
Q. Wang et al.
Fig. 2. N2 adsorption-desorption isothermal curves of Bi2WO6 and PtOx/Bi2WO6 with corresponding pore-size distributions.
600–1000 cm−1 are mainly assigned to WeO stretching vibrations. In detail, the peaks at 794 and 821 cm−1 are allotted to the antisymmetric and symmetrical Ag vibrations of the terminal OeWeO, respectively, while the peak at 724 cm−1 conforms to the oxygen stretching vibration in WO6 octahedrons.[16,17] The peak at 309 cm−1 could be classified as translational modes involving simultaneous vibrations of Bi and WO4 [31]. It can be found that the vibrational peaks of Bi2WO6 nanosheets are weakened when PtOx was loaded, which could be possibly due to the absorption of light by PtOx. Moreover, corresponding peak positions (Fig. 5b and c) shift to higher vibrational frequencies from 309 to 310 cm−1 and from 794 to 796 cm−1, respectively, implying that the vibrational states of pristine Bi2WO6 has been adjusted by PtOx due to the close contact. FTIR spectra were further utilized to analyze the surface species of
Table 1 Surface area, pore volume, and average pore size of Bi2WO6 and PtOx/Bi2WO6. Catalyst
BET surface area (m2/g)
Pore volume (cm3/g)
Average pore size (nm)
Bi2WO6 PtOx/Bi2WO6
24.3 27.8
0.075 0.066
12.3 9.5
appertain to Pt2+, which clearly indicates that the Pt existed in the form of PtOx on Bi2WO6 surface [25,27]. Raman spectra of Bi2WO6 and PtOx/Bi2WO6 samples are displayed in Fig. 5. Those samples exhibit two major peak regions located around 100–400 cm−1 and 600–1000 cm−1, which is consistent with literatures reported [28–30]. Among them, the peaks in the band of
Fig. 3. (a and b) TEM images and (c) HRTEM image of Bi2WO6 (d) TEM image of Bi2WO6 sample with 0.5% Pt. 834
Applied Surface Science 470 (2019) 832–839
Q. Wang et al.
Fig. 4. The XPS of Bi2WO6 and PtOx/Bi2WO6 samples: (a) O 1s, (b) Bi 4f, (c) W 4f, (d) Pt 4f.
water bending mode. The appearance of NeO vibrations at 1381 cm−1 in the spectra might result from the residual nitrate in the synthesizing process [33]. It is worth noting that there are no peaks of organic carbon in the FIIR spectra, excluding the interference of foreign organic
Bi2WO6 and PtOx/Bi2WO6 samples (Fig. 6a). The bands at 400–1000 cm−1 are likely pertained to WeO stretching modes [32]. The broad peak around 3529 cm−1 can be attributed to the stretching vibrations of OH groups, while the peak at 1625 cm−1 derives from
Fig. 5. Raman spectra of Bi2WO6 and PtOx/Bi2WO6 samples. 835
Applied Surface Science 470 (2019) 832–839
Q. Wang et al.
Fig. 6. (a) FTIR spectra and (b) UV–Vis absorption spectra of Bi2WO6 and PtOx/Bi2WO6 samples.
Bi2WO6 should be about −0.33 V vs NHE, while the associated valence band is around 2.37 eV vs NHE based on the band gap (2.7 eV). As shown in Fig. 7d, the band structure of Bi2WO6, compared with the potentials of CO2 reduction and water oxidation, indicates the viability of CO2 photoreduction on Bi2WO6. In order to investigate the transfer efficiency of photo-induced carriers in the samples, photocurrent measurements of Bi2WO6 and PtOx/Bi2WO6 were then carried out. As shown in Fig. 8a, compared with pristine Bi2WO6, the photocurrent of PtOx/Bi2WO6 has been significantly improved, indicating that the loading of PtOx could promote the separation efficiency of photo-generated charge carriers. Electrochemical impedance spectroscopy was further implemented. The Nyquist plot (Fig. 8b) of PtOx/Bi2WO6 shows a smaller semicircle than that of Bi2WO6, manifesting boosted charge separation and transport for PtOx/Bi2WO6. It was expected that the generated holes could be transferred from the valence band of Bi2WO6 to PtOx nanoparticles, which acted as the capture centers of holes. The CO2 photoreduction performances of Bi2WO6 and PtOx/Bi2WO6 samples are displayed in Fig. 9. All the reactions were performed under closed condition employing CO2 and H2O as reactants without adding
carbon sources during the catalytic reduction of CO2. Fig. 6b shows the UV–Vis diffuse reflectance spectra of Bi2WO6 and PtOx/Bi2WO6. After the photodeposition of PtOx nanoparticles, a noticeably enhanced light absorption is observed in the visible region, which could be due to the light absorption of PtOx and is beneficial to CO2 photoreduction. The band gap of Bi2WO6 can be calculated by the following equations:
α hν = A(hν−Eg) n/2 where α represents the light absorption coefficient, hν represents the photon energy, A represents the proportionality constant, and Eg refers to the band gap energy. The transition characteristics of semiconductors are determined by n. According to previous reports, the value of n is 1 for Bi2WO6 [32]. As seen from Fig. 7a, the band gap of Bi2WO6 sample is about 2.7 eV. To characterize the conduction band of Bi2WO6, electrochemical measurement of Mott-Schottky curve was implemented. As displayed in Fig. 7b, the flat-band potential (Efb) of Bi2WO6 is about −0.37 V vs SCE (−0.13 V vs NHE) at pH 7. In general, Efb is at least 0.2 V above the conduction band for semiconductors which possess a large electrical resistivity (Fig. 7c) [34]. Thus, the conduction band of
Fig. 7. (a) Tauc plot of Bi2WO6 sample. (b) Mott-Schottky plot of Bi2WO6 sample. (c) Electrochemical impedance spectroscopy of Bi2WO6 sample. (d) Band structure of Bi2WO6 sample. 836
Applied Surface Science 470 (2019) 832–839
Q. Wang et al.
Fig. 8. (a) Transient current curves of Bi2WO6 and PtOx/Bi2WO6 samples. (b) Electrochemical impedance spectroscopy of Bi2WO6 and PtOx/ Bi2WO6 samples.
shown in Fig. 10a, PtOx/Bi2WO6 displayed a much higher yield of oxygen evolution compared with the pristine Bi2WO6, implying the property of PtOx for accelerating the process of water oxidation, which is consistent with previous study [23]. Besides, polarization curves of water oxidation for Bi2WO6 and PtOx/Bi2WO6 samples were carried out. It can be found the current of water oxidation for PtOx/Bi2WO6 is above that of Bi2WO6 under same bias voltages, manifesting the enhanced property for water oxidation. Based on above results, a proposed mechanism is illustrated in Scheme 1. First, CO2 molecules are adsorbed on the surface of Bi2WO6. Electrons and holes are generated on the conduction band and the valence band of Bi2WO6 under illumination, respectively. The photo-induced holes are then tranferred from Bi2WO6 to PtOx, while water is subsequently oxidized by the holes in PtOx, accompanied with the release of protons. Finally, CO2 molecules are further activated and reduced to produce CH4 combining with the photo-induced electrons and the released protons via the proton-assisted multi-electron transfer mechanism.
any sacrifices. To exclude the possible organic contaminations, controlled experiments with adding CO2 reactant or not were conducted. As shown in Fig. 9a, negligible CH4 generated when no CO2 was added, indicating that CO2 was the carbon source. Bi2WO6 and PtOx/Bi2WO6 exhibited relatively steady CH4 productions as a function of irradiation time (Fig. 9b). It can be found that the CH4 production rate increased significantly after PtOx was loaded, indicating that PtOx could effectively improve the performance of Bi2WO6 for CO2 photoreduction. In order to optimize the performance, different amounts of PtOx loading were investigated. As displayed in Fig. 9c, the yield of CH4 raised at first and then decreased with the increase of PtOx amount, while 0.5% PtOx/ Bi2WO6 exhibited the optimal CH4 production rate of 108.8 ppm g−1 h−1, which was about 5.7 times the yield of pristine Bi2WO6 nanosheets. In addition, cycling tests of 0.5% PtOx/Bi2WO6 were implemented. As shown in Fig. 9d, the CH4 yield remained almost constant, implying the good stability of PtOx/Bi2WO6. Although the strategy of PtOx loading indeed improves the activity of CO2 photoreduction for Bi2WO6 nanosheets, further analysis is demanded to investigate the mechanism. To confirm that PtOx was active for the promotion of water oxidation, photocatalytic activities of asprepared samples toward oxygen evolution were evaluated in an aqueous solution containing silver nitrate as the electron scavenger. As
4. Conclusion In summary, ultrathin Bi2WO6 nanosheets loaded with PtOx Fig. 9. (a) Time courses of CH4 evolution on PtOx/Bi2WO6 under normal condition and without adding CO2. (b) Time courses of CH4 evolution on Bi2WO6 and PtOx/Bi2WO6 samples. (c) Average yields of CH4 in 6 h with different amounts of PtOx loading. (d) Cycling curves of photocatalytic production of CH4 on 0.5%PtOx/ Bi2WO6 sample.
837
Applied Surface Science 470 (2019) 832–839
Q. Wang et al.
Fig. 10. (a) Oxygen production performances and (b) Polarization curves of water oxidation for Bi2WO6 and PtOx/Bi2WO6 samples. [8] W. Dai, J. Yu, Y. Deng, X. Hu, T. Wang, X. Luo, Facile synthesis of MoS2 /Bi2WO6 nanocomposites for enhanced CO2 photoreduction activity under visible light irradiation, Appl. Surf. Sci. 403 (2017) 230–239. [9] S. Cao, B. Shen, T. Tong, J. Fu, J. Yu, 2D/2D heterojunction of ultrathin MXene/ Bi2WO6 nanosheets for improved photocatalytic CO2 reduction, Adv. Funct. Mater. 28 (2018) 1800136. [10] Y. Sohn, W. Huang, F. Taghipour, Recent progress and perspectives in the photocatalytic CO2 reduction of Ti-oxide-based nanomaterials, Appl. Surf. Sci. 396 (2017) 1696–1711. [11] G. Yang, D. Chen, H. Ding, J. Feng, J.Z. Zhang, Y. Zhu, S. Hamid, D.W. Bahnemann, Well-designed 3D ZnIn2S4 nanosheets/TiO2 nanobelts as direct Z-scheme photocatalysts for CO2 photoreduction into renewable hydrocarbon fuel with high efficiency, Appl. Catal. B: Environ. 219 (2017) 611–618. [12] T. Wang, L. Shi, J. Tang, V. Malgras, S. Asahina, G. Liu, H. Zhang, X. Meng, K. Chang, J. He, O. Terasaki, Y. Yamauchi, J. Ye, A Co3O4-embedded porous ZnO rhombic dodecahedron prepared using zeolitic imidazolate frameworks as precursors for CO2 photoreduction, Nanoscale 8 (2016) 6712–6720. [13] J. Qin, S. Wang, H. Ren, Y. Hou, X. Wang, Photocatalytic reduction of CO2 by graphitic carbon nitride polymers derived from urea and barbituric acid, Appl. Catal. B: Environ. 179 (2015) 1–8. [14] L. Liang, F. Lei, S. Gao, Y. Sun, X. Jiao, J. Wu, S. Qamar, Y. Xie, Single unit cell bismuth tungstate layers realizing robust solar CO2 reduction to methanol, Angew. Chem. Int. Ed. Engl. 54 (2015) 13971–13974. [15] S. Girish Kumar, K.S.R. Koteswara Rao, Tungsten-based nanomaterials (WO3 & Bi2WO6): modifications related to charge carrier transfer mechanisms and photocatalytic applications, Appl. Surf. Sci. 355 (2015) 939–958. [16] L. Zhang, Y. Zhu, A review of controllable synthesis and enhancement of performances of bismuth tungstate visible-light-driven photocatalysts, Catal. Sci. Technol. 2 (2012) 694–706. [17] J. Zhang, T. Chen, H. Lu, Z. Yang, F. Yin, J. Gao, O. Liu, Y. Tu, Hierarchical Bi2WO6 architectures decorated with Pd nanoparticles for enhanced visible-light-driven photocatalytic activities, Appl. Surf. Sci. 404 (2017) 282–290. [18] Z. Yang, L. Huang, Y. Xie, Z. Lin, Y. Fan, D. Liu, L. Chen, Z. Zhang, X. Wang, Controllable synthesis of Bi2WO6 nanoplate self-assembled hierarchical erythrocyte microspheres via a one-pot hydrothermal reaction with enhanced visible light photocatalytic activity, Appl. Surf. Sci. 403 (2017) 326–334. [19] X. Zhang, S. Yu, Y. Liu, Q. Zhang, Y. Zhou, Photoreduction of non-noble metal Bi on the surface of Bi2WO6 for enhanced visible light photocatalysis, Appl. Surf. Sci. 396 (2017) 652–658. [20] S. Yu, Y. Zhang, M. Li, X. Du, H. Huang, Non-noble metal Bi deposition by utilizing Bi2WO6 as the self-sacrificing template for enhancing visible light photocatalytic activity, Appl. Surf. Sci. 391 (2017) 491–498. [21] R. Shi, Z. Li, H. Yu, L. Shang, C. Zhou, G.I.N. Waterhouse, L.-Z. Wu, T. Zhang, Effect of nitrogen doping level on the performance of N-doped carbon quantum dot/TiO2 composites for photocatalytic hydrogen evolution, ChemSusChem 10 (2017) 4650–4656. [22] G. Zhao, W. Zhou, Y. Sun, X. Wang, H. Liu, X. Meng, K. Chang, J. Ye, Efficient photocatalytic CO2 reduction over Co(II) species modified CdS in aqueous solution, Appl. Catal. B: Environ. 226 (2018) 252–257. [23] G. Zhang, Z.A. Lan, L. Lin, S. Lin, X. Wang, Overall water splitting by Pt/g-C3N4 photocatalysts without using sacrificial agents, Chem. Sci. 7 (2016) 3062–3066. [24] S. Sun, W. Wang, L. Zhang, E. Gao, D. Jiang, Y. Sun, Y. Xie, Ultrathin {001}-oriented bismuth tungsten oxide nanosheets as highly efficient photocatalysts, ChemSusChem 6 (2013) 1873–1877. [25] H. Zheng, P. Niu, Z. Zhao, Carbon quantum dot sensitized Pt@Bi2WO6/FTO electrodes for enhanced photoelectro-catalytic activity of methanol oxidation, RSC Adv. 7 (2017) 26943–26951. [26] Y. Peng, M. Yan, Q.-G. Chen, C.-M. Fan, H.-Y. Zhou, A.-W. Xu, Novel one-dimensional Bi2O3–Bi2WO6 p–n hierarchical heterojunction with enhanced photocatalytic activity, J. Mater. Chem. A 2 (2014) 8517–8524. [27] P. Rupa kasturi, R.K. Selvan, Y.S. Lee, Pt decorated artocarpus heterophyllus seed derived carbon as an anode catalyst for DMFC application, RSC Adv. 6 (2016) 62680–62694. [28] H. Fu, L. Zhang, W. Yao, Y. Zhu, Photocatalytic properties of nanosized Bi2WO6 catalysts synthesized via a hydrothermal process, Appl. Catal. B: Environ. 66 (2006) 100–110.
Scheme 1. Proposed mechanism of the photocatalytic CO2 reduction by PtOx/ Bi2WO6 sample.
nanoparticles were successfully synthesized through hydrothermal and photodeposition methods. The performance of CO2 photoreduction to CH4 has been greatly improved via PtOx loading. Further analysis indicates that the introduction of PtOx nanoparticles could not only accelerate the separation of photo-induced carriers but also promote the process of water oxidation, so as to improve CO2 photoreduction efficiency. The work helps to give a deeper insight of PtOx nanoparticles and provide a new idea to design catalysts for CO2 photoreduction to CH4. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51772312, 51472260). References [1] X. Chang, T. Wang, J. Gong, CO2 photo-reduction: insights into CO2 activation and reaction on surfaces of photocatalysts, Energy Environ. Sci. 9 (2016) 2177–2196. [2] Y. Ma, Z. Wang, X. Xu, J. Wang, Review on porous nanomaterials for adsorption and photocatalytic conversion of CO2, Chin. J. Catal. 38 (2017) 1956–1969. [3] X. Li, J. Wen, J. Low, Y. Fang, J. Yu, Design and fabrication of semiconductor photocatalyst for photocatalytic reduction of CO2 to solar fuel, Sci. China Mater. 57 (2014) 70–100. [4] J. Low, B. Cheng, J. Yu, Surface modification and enhanced photocatalytic CO2 reduction performance of TiO2: a review, Appl. Surf. Sci. 392 (2017) 658–686. [5] X. Liu, S. Inagaki, J. Gong, Heterogeneous molecular systems for photocatalytic CO2 reduction with water oxidation, Angew. Chem. Int. Ed. Engl. 55 (2016) 14924–14950. [6] F. Wang, Y. Zhou, P. Li, L. Kuai, Z. Zou, Synthesis of bionic-macro/microporous MgO-modified TiO2 for enhanced CO2 photoreduction into hydrocarbon fuels, Chin. J. Catal. 37 (2016) 863–868. [7] W. Dai, H. Xu, J. Yu, X. Hu, X. Luo, X. Tu, L. Yang, Photocatalytic reduction of CO2 into methanol and ethanol over conducting polymers modified Bi2WO6 microspheres under visible light, Appl. Surf. Sci. 356 (2015) 173–180.
838
Applied Surface Science 470 (2019) 832–839
Q. Wang et al.
[32] M. Qamar, R.B. Elsayed, K.R. Alhooshani, M.I. Ahmed, D.W. Bahnemann, Highly efficient and selective oxidation of aromatic alcohols photocatalyzed by nanoporous hierarchical Pt/Bi2WO6 in organic solvent-free environment, ACS Appl. Mater. Interface 7 (2015) 1257–1269. [33] H. Huang, K. Liu, K. Chen, Y. Zhang, Y. Zhang, S. Wang, Ce and F comodification on the crystal structure and enhanced photocatalytic activity of Bi2WO6 photocatalyst under visible light irradiation, J. Phys. Chem. C 118 (2014) 14379–14387. [34] Y. Matsumoto, Energy positions of oxide semiconductors and photocatalysis with iron complex oxides, J. Solid State Chem. 126 (1996) 227–234.
[29] Y. Huang, S. Kang, Y. Yang, H. Qin, Z. Ni, S. Yang, X. Li, Facile synthesis of Bi/ Bi2WO6 nanocomposite with enhanced photocatalytic activity under visible light, Appl. Catal. B: Environ. 196 (2016) 89–99. [30] H. Ma, J. Shen, M. Shi, X. Lu, Z. Li, Y. Long, N. Li, M. Ye, Significant enhanced performance for rhodamine B, phenol and Cr(VI) removal by Bi2WO6 nancomposites via reduced graphene oxide modification, Appl. Catal. B: Environ. 121 (2012) 198–205. [31] Y. Wu, G. Zhang, W. Guan, X. Zhang, Visible light-assisted synthesis of Pt/Bi2WO6 and photocatalytic activity for ciprofloxacin, Nano Lett. 9 (2014) 119–122.
839
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
Photocatalytic reduction of CO2 to methane over PtOx-loaded ultrathin Bi2WO6 nanosheets
T
⁎
Qianqian Wang, Kefu Wang, Ling Zhang, Haipeng Wang, Wenzhong Wang
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: CO2 photoreduction Bi2WO6 nanosheets PtOx nanoparticles Carrier separation Water oxidation
Solar CO2 photoreduction into hydrocarbons is promising and significative. However, many conventional catalysts reported usually suffer from poor photocatalytic activities. Herein, ultrathin Bi2WO6 nanosheets with a thickness of about 4.8 nm have been synthesized by hydrothermal method, which exhibited a CH4 production rate of 19 ppm g−1 h−1 under a low CO2 concentration of 400 ppm. PtOx nanoparticles with a size of about 2 nm were then loaded on the Bi2WO6 nanosheets as excellent co-catalysts by photoreduction in aqueous solution, and an optimal CH4 yield of 108.8 ppm g−1 h−1 was achieved, which was about 5.7 times than that of pristine Bi2WO6 nanosheets. Further analyses of photocurrent curves, electrochemical impedance spectroscopy and polarization curves of water oxidation indicated that the improved photocatalytic activity was suggested to result from the enhanced carrier separation and accelerated water oxidation by PtOx nanoparticles. The work will likely give a deeper insight of PtOx nanoparticles and provide a new idea to design catalysts for CO2 photoreduction to CH4.
1. Introduction The atmospheric CO2 concentration has been growing steadily over the past century, inducing the severe global warming problem [1–4]. Artificial photosynthesis, employing semiconductor photocatalysts to convert CO2 into valuable fuels utilizing sunlight, is probably one of the most sustainable and economical methods to cut down CO2 emission and utilize solar energy simultaneously [5–9]. To date, various photocatalysts have been investigated for CO2 photoreduction, such as TiO2 [10,11], ZnO [12], C3N4 [13]. Although progresses have been made in this field, the photocatalytic activity remains pretty low, possibly due to the limited exposed active sites and the fast recombination rate of photo-induced carriers. The catalysts of ultrathin structure and loading co-catalysts seem to be a good solution to address the problems. Bi2WO6 has caught a lot of attention for its good photocatalytic performance [14–20]. Bi2WO6 is comprised of alternating perovskitelike (Bi2O2)2+ and fluorite-like (WO4)2− layers, which is conducive to construct ultrathin Bi2WO6 nanosheets due to the special laminated structure. Considering the fast recombination rate of photo-induced carriers, loading of co-catalysts is an effective method to facilitate carrier separation [21,22]. Recently, it has been reported that PtOx nanoparticles are inclined to capture photo-induced holes and serve as active sites for water oxidation, implying that PtOx nanoparticles could ⁎
not only enhance the carrier separation efficiency but also promote the process of water oxidation [23]. Therefore, PtOx-loaded ultrathin Bi2WO6 nanosheets are supposed to be ideal catalysts for CO2 photoreduction. Herein, we investigated the photoreaction of CO2 and water on PtOx-loaded ultrathin Bi2WO6 nanosheets. The ultrathin Bi2WO6 nanosheets with a thickness of 4.8 nm were synthesized by a hydrothermal method, while PtOx nanoparticles of 2 nm were subsequently deposited on the surface of Bi2WO6 nanosheets through photoreduction. PtOx-loaded ultrathin Bi2WO6 nanosheets exhibited an efficient CH4 production rate of 108.8 ppm g−1 h−1, which was about 5.7 times the yield of pristine Bi2WO6 nanosheets, manifesting that PtOx nanoparticles could greatly improve the efficiency of CO2 photoreduction. Photocurrent experiments and electrochemical impedance spectroscopy characterizations indicated that PtOx nanoparticles contributed to the separation of photo-generated carriers, while the O2 evolution experiments and polarization curves of water oxidation demonstrated that PtOx nanoparticles could also promote water oxidation, which might account for the improved CH4 production.
Corresponding author. E-mail address: [email protected] (W. Wang).
https://doi.org/10.1016/j.apsusc.2018.11.197 Received 28 August 2018; Received in revised form 19 November 2018; Accepted 25 November 2018 Available online 27 November 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
Applied Surface Science 470 (2019) 832–839
Q. Wang et al.
2. Experimental section 2.1. Catalyst preparation All chemical regents of analytical purity were utilized without any further treatment. The ultrathin Bi2WO6 nanosheets were prepared employing a method provided by Sun et al. [24]. Typically, Bi (NO3)5·5H2O (2.425 g) was dissolved in HNO3 solution (5 mL, 4 M). When it was clarified, 0.824 g of Na2WO4 was added into it. Then the NaOH solution (pH = 2) was used to regulate the pH value of the suspension weakly acidic. After vigorous stirring for 20 min, the suspension was transferred to 50 mL Teflon-lined autoclave, which was then sealed into a stainless-steel tank and heated at 160 °C for 27 h. The autoclave was allowed to cool down naturally. The final production was collected, washed with water for several times and then freeze-dried. The PtOx/Bi2WO6 was prepared by photoreduction method. Firstly, 0.1 g of Bi2WO6 was dispersed into the mixture solution with 100 mL of water and 1 mL of 1 mg/mL H2PtCl6 at pH around 6. The suspension was stirring for several minutes and then irradiated under 500 W Xe lamp for 1 h. Next, the sample was centrifuged with water for several times and freeze-dried.
Fig. 1. XRD patterns of as-prepared Bi2WO6 and PtOx/Bi2WO6 samples.
naturally, the FTO glass was utilized as the working electrode. The flatband potentials (Efb) of catalysts were obtained at 1000 Hz with 5 mV amplitude, while the polarization curves of water oxidation were conducted at 20 mV/s.
2.2. Characterization 3. Result and discussion Powder X-ray diffraction (XRD) characterization was conducted on a D/MAX 2250 diffractometer (Rigaku, Japan) employing monochromatized Cu Kα (λ = 0.15418 nm) radiation under 40 kV and 100 mA, while the 2θ ranged from 5° to 70°. The microstructures and morphologies of catalysts were determined by Transmission electron microscopy (TEM). Brunauer-Emmett-Teller (BET) specific surface area and Barrett-Joyner-Halenda (BJH) pore size distribution were obtained by a surface area and pore size analyzer (V-sorb 2008P, Gold APP). Xray photoelectron spectroscopy (XPS) employing an ESCALAB 250Xi (ThermoFischer) was implemented by irradiating of aluminum Kα Xrays at 1253.6 eV under ultrahigh-vacuum conditions. Raman spectra were obtained utilizing a microscopic confocal Raman spectrometer (Renishaw 1000NR) with an excitation of 514 nm laser light at room temperature. Fourier transformed infrared (FTIR) spectra were recorded with a Nicolet iS10 FTIR spectrometer. UV–vis diffuse reflectance spectra (DRS) of catalysts were conducted utilizing a Hitachi UV-3010PC UV–vis spectrophotometer. The photoluminescence (PL) spectra were obtained employing a Hitachi F4600 fluorescence spectrophotometer (excitation wavelength = 280 nm) at room temperature in air.
The XRD diffraction patterns of Bi2WO6 and PtOx/Bi2WO6 are displayed in Fig. 1. All of the diffraction peaks could be indexed to the standard Bi2WO6 (JCPDS 73-2020) and no other peaks of possible impurities appeared in the XRD pattern. The broad diffraction peaks implied the nanocrystalline nature of Bi2WO6 sample. There were no diffraction peaks of PtOx for the PtOx/Bi2WO6 sample, which could be ascribed to the fact that the amount of PtOx (0.5%) was lower than the detection limit of XRD. Fig. 2 and Table 1 show the N2 adsorptiondesorption isotherm curves and corresponding pore size distributions of Bi2WO6 and PtOx/Bi2WO6. It can be found that the surface area became slightly larger after the loading of PtOx, increasing from 24.3 to 27.8 m2/g, which could be in favor of improving CO2 photoreduction. The TEM images of Bi2WO6 in Fig. 3a and b show that the sample possessed a square sheet-like morphology with an average lateral size of about 60 nm and a thickness of about 4.8 nm. The high-resolution TEM image of Bi2WO6 is displayed in Fig. 3c. Lattice distances of 0.258 nm and 0.315 nm can be allotted to (0 2 2) and (1 1 3) planes of Bi2WO6, respectively, indicating that ultrathin Bi2WO6 nanosheets were successfully synthesized. Besides, from the TEM images of PtOx/Bi2WO6 in Fig. 3d, it can be seen that PtOx nanoparticles with a size of about 2 nm are uniformly loaded on the Bi2WO6 nanosheets. To investigate and reveal the elemental compositions and corresponding chemical valence states of the samples, XPS analysis was conducted. As shown in Fig. 4a, the O 1s peaks of Bi2WO6 and PtOx/ Bi2WO6 appear around 530.0 eV and 530.2 eV, respectively. The O 1s peak of Bi2WO6 can be de-convoluted into three peaks at 532.0 eV, 530.5 eV, and 529.7 eV by Gaussian fitting, corresponding to the surface adsorbed oxygen, W-O, Bi-O, respectively, while the peaks centered at 532.0 eV, 530.8 eV and 529.9 eV are attributed to relevant O 1s peaks of PtOx/Bi2WO6 [25]. It can be found that the O 1s peak position shifted to higher binding energy after the deposition of PtOx nanoparticles. The binding energy of Bi 4f peak in Fig. 4b showed a similar increase as well when PtOx was loaded, implying that PtOx might be closely contacted with Bi2WO6 nanosheets and an interaction existed between them, which could contribute to the carrier separation. The high-resolution W 4f peaks of Bi2WO6 and PtOx/Bi2WO6 are illustrated in Fig. 4c. The peaks concentrated at 37.8 eV and 35.7 eV correspond to the W+6 peaks of W4f 5/2 and W 4f 7/2, respectively [26]. The binding energy peak of Pt 4f (Fig. 4d) can be de-convoluted into four peaks at 78.1 eV, 76.3 eV, 74.8 eV, and 72.9 eV. The peaks at 78.1 eV and 74.8 eV are attributed to Pt4+, while the peaks at 76.3 eV and 72.9 eV
2.3. Catalytic tests CO2 reduction experiments were conducted in an airtight system with a 500 W Xe lamp as light source. 20 mg of catalysts were evenly spread on the bottom of the reactor (600 mL) with a quartz window for illumination, and then de-ionized water (1 mL) was added into the system, which was degassed by nitrogen thoroughly. Finally, CO2 of 400 ppm was injected into the closed system. Gas chromatograph (GC, Tianmei 7890) with a flame ionization detector (FID) was employed to detect CH4 yield in the reactor. 2.4. Electrochemical tests Electrochemical experiments were carried out on a CHI 660D electrochemical workstation (Shanghai Chenhua, China) employing a standard three-electrode system with a working electrode, a platinum slice counter electrode, and a saturated calomel electrode (SCE) as the reference electrode, while the light source is a Xe lamp (CHF-XM500). For the preparation of working electrode, typically, catalyst powder of 10 mg was dispersed in 1 mL nafion-ethanol (1 wt%) solution. 100 μL of above suspension was then coated on the FTO glass. After dried 833
Applied Surface Science 470 (2019) 832–839
Q. Wang et al.
Fig. 2. N2 adsorption-desorption isothermal curves of Bi2WO6 and PtOx/Bi2WO6 with corresponding pore-size distributions.
600–1000 cm−1 are mainly assigned to WeO stretching vibrations. In detail, the peaks at 794 and 821 cm−1 are allotted to the antisymmetric and symmetrical Ag vibrations of the terminal OeWeO, respectively, while the peak at 724 cm−1 conforms to the oxygen stretching vibration in WO6 octahedrons.[16,17] The peak at 309 cm−1 could be classified as translational modes involving simultaneous vibrations of Bi and WO4 [31]. It can be found that the vibrational peaks of Bi2WO6 nanosheets are weakened when PtOx was loaded, which could be possibly due to the absorption of light by PtOx. Moreover, corresponding peak positions (Fig. 5b and c) shift to higher vibrational frequencies from 309 to 310 cm−1 and from 794 to 796 cm−1, respectively, implying that the vibrational states of pristine Bi2WO6 has been adjusted by PtOx due to the close contact. FTIR spectra were further utilized to analyze the surface species of
Table 1 Surface area, pore volume, and average pore size of Bi2WO6 and PtOx/Bi2WO6. Catalyst
BET surface area (m2/g)
Pore volume (cm3/g)
Average pore size (nm)
Bi2WO6 PtOx/Bi2WO6
24.3 27.8
0.075 0.066
12.3 9.5
appertain to Pt2+, which clearly indicates that the Pt existed in the form of PtOx on Bi2WO6 surface [25,27]. Raman spectra of Bi2WO6 and PtOx/Bi2WO6 samples are displayed in Fig. 5. Those samples exhibit two major peak regions located around 100–400 cm−1 and 600–1000 cm−1, which is consistent with literatures reported [28–30]. Among them, the peaks in the band of
Fig. 3. (a and b) TEM images and (c) HRTEM image of Bi2WO6 (d) TEM image of Bi2WO6 sample with 0.5% Pt. 834
Applied Surface Science 470 (2019) 832–839
Q. Wang et al.
Fig. 4. The XPS of Bi2WO6 and PtOx/Bi2WO6 samples: (a) O 1s, (b) Bi 4f, (c) W 4f, (d) Pt 4f.
water bending mode. The appearance of NeO vibrations at 1381 cm−1 in the spectra might result from the residual nitrate in the synthesizing process [33]. It is worth noting that there are no peaks of organic carbon in the FIIR spectra, excluding the interference of foreign organic
Bi2WO6 and PtOx/Bi2WO6 samples (Fig. 6a). The bands at 400–1000 cm−1 are likely pertained to WeO stretching modes [32]. The broad peak around 3529 cm−1 can be attributed to the stretching vibrations of OH groups, while the peak at 1625 cm−1 derives from
Fig. 5. Raman spectra of Bi2WO6 and PtOx/Bi2WO6 samples. 835
Applied Surface Science 470 (2019) 832–839
Q. Wang et al.
Fig. 6. (a) FTIR spectra and (b) UV–Vis absorption spectra of Bi2WO6 and PtOx/Bi2WO6 samples.
Bi2WO6 should be about −0.33 V vs NHE, while the associated valence band is around 2.37 eV vs NHE based on the band gap (2.7 eV). As shown in Fig. 7d, the band structure of Bi2WO6, compared with the potentials of CO2 reduction and water oxidation, indicates the viability of CO2 photoreduction on Bi2WO6. In order to investigate the transfer efficiency of photo-induced carriers in the samples, photocurrent measurements of Bi2WO6 and PtOx/Bi2WO6 were then carried out. As shown in Fig. 8a, compared with pristine Bi2WO6, the photocurrent of PtOx/Bi2WO6 has been significantly improved, indicating that the loading of PtOx could promote the separation efficiency of photo-generated charge carriers. Electrochemical impedance spectroscopy was further implemented. The Nyquist plot (Fig. 8b) of PtOx/Bi2WO6 shows a smaller semicircle than that of Bi2WO6, manifesting boosted charge separation and transport for PtOx/Bi2WO6. It was expected that the generated holes could be transferred from the valence band of Bi2WO6 to PtOx nanoparticles, which acted as the capture centers of holes. The CO2 photoreduction performances of Bi2WO6 and PtOx/Bi2WO6 samples are displayed in Fig. 9. All the reactions were performed under closed condition employing CO2 and H2O as reactants without adding
carbon sources during the catalytic reduction of CO2. Fig. 6b shows the UV–Vis diffuse reflectance spectra of Bi2WO6 and PtOx/Bi2WO6. After the photodeposition of PtOx nanoparticles, a noticeably enhanced light absorption is observed in the visible region, which could be due to the light absorption of PtOx and is beneficial to CO2 photoreduction. The band gap of Bi2WO6 can be calculated by the following equations:
α hν = A(hν−Eg) n/2 where α represents the light absorption coefficient, hν represents the photon energy, A represents the proportionality constant, and Eg refers to the band gap energy. The transition characteristics of semiconductors are determined by n. According to previous reports, the value of n is 1 for Bi2WO6 [32]. As seen from Fig. 7a, the band gap of Bi2WO6 sample is about 2.7 eV. To characterize the conduction band of Bi2WO6, electrochemical measurement of Mott-Schottky curve was implemented. As displayed in Fig. 7b, the flat-band potential (Efb) of Bi2WO6 is about −0.37 V vs SCE (−0.13 V vs NHE) at pH 7. In general, Efb is at least 0.2 V above the conduction band for semiconductors which possess a large electrical resistivity (Fig. 7c) [34]. Thus, the conduction band of
Fig. 7. (a) Tauc plot of Bi2WO6 sample. (b) Mott-Schottky plot of Bi2WO6 sample. (c) Electrochemical impedance spectroscopy of Bi2WO6 sample. (d) Band structure of Bi2WO6 sample. 836
Applied Surface Science 470 (2019) 832–839
Q. Wang et al.
Fig. 8. (a) Transient current curves of Bi2WO6 and PtOx/Bi2WO6 samples. (b) Electrochemical impedance spectroscopy of Bi2WO6 and PtOx/ Bi2WO6 samples.
shown in Fig. 10a, PtOx/Bi2WO6 displayed a much higher yield of oxygen evolution compared with the pristine Bi2WO6, implying the property of PtOx for accelerating the process of water oxidation, which is consistent with previous study [23]. Besides, polarization curves of water oxidation for Bi2WO6 and PtOx/Bi2WO6 samples were carried out. It can be found the current of water oxidation for PtOx/Bi2WO6 is above that of Bi2WO6 under same bias voltages, manifesting the enhanced property for water oxidation. Based on above results, a proposed mechanism is illustrated in Scheme 1. First, CO2 molecules are adsorbed on the surface of Bi2WO6. Electrons and holes are generated on the conduction band and the valence band of Bi2WO6 under illumination, respectively. The photo-induced holes are then tranferred from Bi2WO6 to PtOx, while water is subsequently oxidized by the holes in PtOx, accompanied with the release of protons. Finally, CO2 molecules are further activated and reduced to produce CH4 combining with the photo-induced electrons and the released protons via the proton-assisted multi-electron transfer mechanism.
any sacrifices. To exclude the possible organic contaminations, controlled experiments with adding CO2 reactant or not were conducted. As shown in Fig. 9a, negligible CH4 generated when no CO2 was added, indicating that CO2 was the carbon source. Bi2WO6 and PtOx/Bi2WO6 exhibited relatively steady CH4 productions as a function of irradiation time (Fig. 9b). It can be found that the CH4 production rate increased significantly after PtOx was loaded, indicating that PtOx could effectively improve the performance of Bi2WO6 for CO2 photoreduction. In order to optimize the performance, different amounts of PtOx loading were investigated. As displayed in Fig. 9c, the yield of CH4 raised at first and then decreased with the increase of PtOx amount, while 0.5% PtOx/ Bi2WO6 exhibited the optimal CH4 production rate of 108.8 ppm g−1 h−1, which was about 5.7 times the yield of pristine Bi2WO6 nanosheets. In addition, cycling tests of 0.5% PtOx/Bi2WO6 were implemented. As shown in Fig. 9d, the CH4 yield remained almost constant, implying the good stability of PtOx/Bi2WO6. Although the strategy of PtOx loading indeed improves the activity of CO2 photoreduction for Bi2WO6 nanosheets, further analysis is demanded to investigate the mechanism. To confirm that PtOx was active for the promotion of water oxidation, photocatalytic activities of asprepared samples toward oxygen evolution were evaluated in an aqueous solution containing silver nitrate as the electron scavenger. As
4. Conclusion In summary, ultrathin Bi2WO6 nanosheets loaded with PtOx Fig. 9. (a) Time courses of CH4 evolution on PtOx/Bi2WO6 under normal condition and without adding CO2. (b) Time courses of CH4 evolution on Bi2WO6 and PtOx/Bi2WO6 samples. (c) Average yields of CH4 in 6 h with different amounts of PtOx loading. (d) Cycling curves of photocatalytic production of CH4 on 0.5%PtOx/ Bi2WO6 sample.
837
Applied Surface Science 470 (2019) 832–839
Q. Wang et al.
Fig. 10. (a) Oxygen production performances and (b) Polarization curves of water oxidation for Bi2WO6 and PtOx/Bi2WO6 samples. [8] W. Dai, J. Yu, Y. Deng, X. Hu, T. Wang, X. Luo, Facile synthesis of MoS2 /Bi2WO6 nanocomposites for enhanced CO2 photoreduction activity under visible light irradiation, Appl. Surf. Sci. 403 (2017) 230–239. [9] S. Cao, B. Shen, T. Tong, J. Fu, J. Yu, 2D/2D heterojunction of ultrathin MXene/ Bi2WO6 nanosheets for improved photocatalytic CO2 reduction, Adv. Funct. Mater. 28 (2018) 1800136. [10] Y. Sohn, W. Huang, F. Taghipour, Recent progress and perspectives in the photocatalytic CO2 reduction of Ti-oxide-based nanomaterials, Appl. Surf. Sci. 396 (2017) 1696–1711. [11] G. Yang, D. Chen, H. Ding, J. Feng, J.Z. Zhang, Y. Zhu, S. Hamid, D.W. Bahnemann, Well-designed 3D ZnIn2S4 nanosheets/TiO2 nanobelts as direct Z-scheme photocatalysts for CO2 photoreduction into renewable hydrocarbon fuel with high efficiency, Appl. Catal. B: Environ. 219 (2017) 611–618. [12] T. Wang, L. Shi, J. Tang, V. Malgras, S. Asahina, G. Liu, H. Zhang, X. Meng, K. Chang, J. He, O. Terasaki, Y. Yamauchi, J. Ye, A Co3O4-embedded porous ZnO rhombic dodecahedron prepared using zeolitic imidazolate frameworks as precursors for CO2 photoreduction, Nanoscale 8 (2016) 6712–6720. [13] J. Qin, S. Wang, H. Ren, Y. Hou, X. Wang, Photocatalytic reduction of CO2 by graphitic carbon nitride polymers derived from urea and barbituric acid, Appl. Catal. B: Environ. 179 (2015) 1–8. [14] L. Liang, F. Lei, S. Gao, Y. Sun, X. Jiao, J. Wu, S. Qamar, Y. Xie, Single unit cell bismuth tungstate layers realizing robust solar CO2 reduction to methanol, Angew. Chem. Int. Ed. Engl. 54 (2015) 13971–13974. [15] S. Girish Kumar, K.S.R. Koteswara Rao, Tungsten-based nanomaterials (WO3 & Bi2WO6): modifications related to charge carrier transfer mechanisms and photocatalytic applications, Appl. Surf. Sci. 355 (2015) 939–958. [16] L. Zhang, Y. Zhu, A review of controllable synthesis and enhancement of performances of bismuth tungstate visible-light-driven photocatalysts, Catal. Sci. Technol. 2 (2012) 694–706. [17] J. Zhang, T. Chen, H. Lu, Z. Yang, F. Yin, J. Gao, O. Liu, Y. Tu, Hierarchical Bi2WO6 architectures decorated with Pd nanoparticles for enhanced visible-light-driven photocatalytic activities, Appl. Surf. Sci. 404 (2017) 282–290. [18] Z. Yang, L. Huang, Y. Xie, Z. Lin, Y. Fan, D. Liu, L. Chen, Z. Zhang, X. Wang, Controllable synthesis of Bi2WO6 nanoplate self-assembled hierarchical erythrocyte microspheres via a one-pot hydrothermal reaction with enhanced visible light photocatalytic activity, Appl. Surf. Sci. 403 (2017) 326–334. [19] X. Zhang, S. Yu, Y. Liu, Q. Zhang, Y. Zhou, Photoreduction of non-noble metal Bi on the surface of Bi2WO6 for enhanced visible light photocatalysis, Appl. Surf. Sci. 396 (2017) 652–658. [20] S. Yu, Y. Zhang, M. Li, X. Du, H. Huang, Non-noble metal Bi deposition by utilizing Bi2WO6 as the self-sacrificing template for enhancing visible light photocatalytic activity, Appl. Surf. Sci. 391 (2017) 491–498. [21] R. Shi, Z. Li, H. Yu, L. Shang, C. Zhou, G.I.N. Waterhouse, L.-Z. Wu, T. Zhang, Effect of nitrogen doping level on the performance of N-doped carbon quantum dot/TiO2 composites for photocatalytic hydrogen evolution, ChemSusChem 10 (2017) 4650–4656. [22] G. Zhao, W. Zhou, Y. Sun, X. Wang, H. Liu, X. Meng, K. Chang, J. Ye, Efficient photocatalytic CO2 reduction over Co(II) species modified CdS in aqueous solution, Appl. Catal. B: Environ. 226 (2018) 252–257. [23] G. Zhang, Z.A. Lan, L. Lin, S. Lin, X. Wang, Overall water splitting by Pt/g-C3N4 photocatalysts without using sacrificial agents, Chem. Sci. 7 (2016) 3062–3066. [24] S. Sun, W. Wang, L. Zhang, E. Gao, D. Jiang, Y. Sun, Y. Xie, Ultrathin {001}-oriented bismuth tungsten oxide nanosheets as highly efficient photocatalysts, ChemSusChem 6 (2013) 1873–1877. [25] H. Zheng, P. Niu, Z. Zhao, Carbon quantum dot sensitized Pt@Bi2WO6/FTO electrodes for enhanced photoelectro-catalytic activity of methanol oxidation, RSC Adv. 7 (2017) 26943–26951. [26] Y. Peng, M. Yan, Q.-G. Chen, C.-M. Fan, H.-Y. Zhou, A.-W. Xu, Novel one-dimensional Bi2O3–Bi2WO6 p–n hierarchical heterojunction with enhanced photocatalytic activity, J. Mater. Chem. A 2 (2014) 8517–8524. [27] P. Rupa kasturi, R.K. Selvan, Y.S. Lee, Pt decorated artocarpus heterophyllus seed derived carbon as an anode catalyst for DMFC application, RSC Adv. 6 (2016) 62680–62694. [28] H. Fu, L. Zhang, W. Yao, Y. Zhu, Photocatalytic properties of nanosized Bi2WO6 catalysts synthesized via a hydrothermal process, Appl. Catal. B: Environ. 66 (2006) 100–110.
Scheme 1. Proposed mechanism of the photocatalytic CO2 reduction by PtOx/ Bi2WO6 sample.
nanoparticles were successfully synthesized through hydrothermal and photodeposition methods. The performance of CO2 photoreduction to CH4 has been greatly improved via PtOx loading. Further analysis indicates that the introduction of PtOx nanoparticles could not only accelerate the separation of photo-induced carriers but also promote the process of water oxidation, so as to improve CO2 photoreduction efficiency. The work helps to give a deeper insight of PtOx nanoparticles and provide a new idea to design catalysts for CO2 photoreduction to CH4. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51772312, 51472260). References [1] X. Chang, T. Wang, J. Gong, CO2 photo-reduction: insights into CO2 activation and reaction on surfaces of photocatalysts, Energy Environ. Sci. 9 (2016) 2177–2196. [2] Y. Ma, Z. Wang, X. Xu, J. Wang, Review on porous nanomaterials for adsorption and photocatalytic conversion of CO2, Chin. J. Catal. 38 (2017) 1956–1969. [3] X. Li, J. Wen, J. Low, Y. Fang, J. Yu, Design and fabrication of semiconductor photocatalyst for photocatalytic reduction of CO2 to solar fuel, Sci. China Mater. 57 (2014) 70–100. [4] J. Low, B. Cheng, J. Yu, Surface modification and enhanced photocatalytic CO2 reduction performance of TiO2: a review, Appl. Surf. Sci. 392 (2017) 658–686. [5] X. Liu, S. Inagaki, J. Gong, Heterogeneous molecular systems for photocatalytic CO2 reduction with water oxidation, Angew. Chem. Int. Ed. Engl. 55 (2016) 14924–14950. [6] F. Wang, Y. Zhou, P. Li, L. Kuai, Z. Zou, Synthesis of bionic-macro/microporous MgO-modified TiO2 for enhanced CO2 photoreduction into hydrocarbon fuels, Chin. J. Catal. 37 (2016) 863–868. [7] W. Dai, H. Xu, J. Yu, X. Hu, X. Luo, X. Tu, L. Yang, Photocatalytic reduction of CO2 into methanol and ethanol over conducting polymers modified Bi2WO6 microspheres under visible light, Appl. Surf. Sci. 356 (2015) 173–180.
838
Applied Surface Science 470 (2019) 832–839
Q. Wang et al.
[32] M. Qamar, R.B. Elsayed, K.R. Alhooshani, M.I. Ahmed, D.W. Bahnemann, Highly efficient and selective oxidation of aromatic alcohols photocatalyzed by nanoporous hierarchical Pt/Bi2WO6 in organic solvent-free environment, ACS Appl. Mater. Interface 7 (2015) 1257–1269. [33] H. Huang, K. Liu, K. Chen, Y. Zhang, Y. Zhang, S. Wang, Ce and F comodification on the crystal structure and enhanced photocatalytic activity of Bi2WO6 photocatalyst under visible light irradiation, J. Phys. Chem. C 118 (2014) 14379–14387. [34] Y. Matsumoto, Energy positions of oxide semiconductors and photocatalysis with iron complex oxides, J. Solid State Chem. 126 (1996) 227–234.
[29] Y. Huang, S. Kang, Y. Yang, H. Qin, Z. Ni, S. Yang, X. Li, Facile synthesis of Bi/ Bi2WO6 nanocomposite with enhanced photocatalytic activity under visible light, Appl. Catal. B: Environ. 196 (2016) 89–99. [30] H. Ma, J. Shen, M. Shi, X. Lu, Z. Li, Y. Long, N. Li, M. Ye, Significant enhanced performance for rhodamine B, phenol and Cr(VI) removal by Bi2WO6 nancomposites via reduced graphene oxide modification, Appl. Catal. B: Environ. 121 (2012) 198–205. [31] Y. Wu, G. Zhang, W. Guan, X. Zhang, Visible light-assisted synthesis of Pt/Bi2WO6 and photocatalytic activity for ciprofloxacin, Nano Lett. 9 (2014) 119–122.
839