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rGO hybrid with enhanced photoelectrochemical performance
Applied Surface Science 510 (2020) 145447
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Facile preparation of Cu2O nanoparticles/Bi2WO6/rGO hybrid with enhanced photoelectrochemical performance
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Xuan Tanga, Xinli Guoa, , Zhongtao Chena, Yuanyuan Liua, Weijie Zhanga, Yixuan Wanga, ⁎ Yanmei Zhenga, Ming Zhanga, Zhengbin Penga, Rui Lia, Yuhong Zhaob, a b
Jiangsu Key Laboratory of Advanced Metallic Materials, School of Materials Science and Engineering, Southeast University, Nanjing 211189, China College of Materials Science and Engineering, North University of China, Taiyuan 030051, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Bi2WO6 (BWO) Cu2O nanoparticles/BWO/Reduced graphene oxide (Cu2O NPs/BWO/rGO) hybrid Solvothermal synthesis Chemical reduction Photoelectrochemical (PEC) performance
Bi2WO6 (BWO) has recently received extensive interest for water splitting application due to its attractive properties of non-toxicity, high stability and narrow band gap (~2.7 eV). However, the photoelectrochemical (PEC) performance of pure BWO is limited by the inefficiency visible light absorption, and the low electron transport efficiency. Herein we prepared a novel Cu2O nanoparticles/BWO/reduced graphene oxide (Cu2O NPs/ BWO/rGO) hybrid by a facile solvothermal synthesis followed by a chemical reduction. The Cu2O NPs and rGO decorate homogeneously on the surface of hydrangea-like BWO. This novel hybrid exhibits a significantly enhanced PEC performance with a ~14 times high photocurrent density than that of pure BWO under simulated sunlight irradiation. This enhancement of the PEC performance is attributed to the significant improvement of coupling Cu2O NPs and rGO, which proves a direct Z-scheme as the main mechanism in Cu2O NPs/BWO, and rGO as a pivotal charge transmission bridge for the efficient photogenerated charge separation, and the increased visible light absorption. The results have provided a new way for the practical water splitting application of BWO.
1. Introduction For the past few years, PEC water splitting has become one of the efficient strategy to solve the problems of energy shortage and environmental pollution by converting solar energy into the clean, storable chemical fuels [1]. Recently, some bismuth-based semiconductors such as BiVO4, Bi2O3, and BWO, etc. have received extensive interest in the study of PEC water splitting [2–5]. Among them, BWO shows attractive properties of non-toxicity, high stability and narrow band gap (~2.7 eV) [6]. However, the PEC performance of pure BWO is limited by the inefficiency visible light absorption, the low electron transport efficiency and the fast recombination of the photogenerated electronhole pairs. To conquer these drawbacks, plenty of efforts have been developed, such as hybridizing with the carbon materials [7], coupling with other semiconductors, etc. Graphene, as one of the two dimensional (2D) monolayer carbonaceous materials, could effectively not only boost the charge transfer but also inhibit the recombination of photogenerated carriers when merging with semiconductors due to its unique conjugated carbon network structure [8]. Meanwhile, cuprous oxide (Cu2O), as a nonnoble metal oxide semiconductor, has been widely used in various ⁎
applications such as photocatalysis [9], photoelectrical devices [10–12], and other areas because of its nontoxicity, low cost and narrow band gap. Liu prepared the Cu2O nanodots/BWO hybrid in which Cu2O nano-structure extended the visible light absorption and the lower charge transfer resistance for degradation of methylene blue [13]. The Cu2O/BWO composites synthesized by Zheng et al. for production of alkanes and hydrogen gas [14]. Nevertheless, the study about Cu2O NPs modified BWO composites for PEC property have been rarely reported. In this work, we prepared a novel Cu2O NPs/BWO/rGO hybrid via a facile solvothermal synthesis followed by a chemical reduction. The asprepared Cu2O NPs/BWO/rGO hybrid exhibits a significantly enhanced PEC performance in Na2SO4 solution under simulated sunlight irradiation. 2. Experimental section 2.1. Synthesis of hydrangea-like BWO The hydrangea-like BWO was synthesized by a solvothermal method. 1 mmol of Na2WO4·2H2O and 0.45 g of hexadecyl trimethyl
Corresponding authors. E-mail addresses: [email protected] (X. Guo), [email protected] (Y. Zhao).
https://doi.org/10.1016/j.apsusc.2020.145447 Received 16 September 2019; Received in revised form 29 December 2019; Accepted 16 January 2020 Available online 20 January 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved.
Applied Surface Science 510 (2020) 145447
X. Tang, et al.
ammonium bromide (CTAB) were dissolved into 20 mL of ethylene glycol under magnetic stirring at 40 °C. Then the above solution was dropwise added to 50 mL of Bi(NO3)3·3H2O (0.05 M) followed by stirring for 2 h, heating at 180 °C for 12 h and freeze-drying for 12 h. 2.2. Synthesis of Cu2O NPs/BWO/rGO hybrid The graphene oxide was synthesized by the modified Hummers method [15].The Cu2O NPs/BWO/rGO hybrid was prepared by a facile solvothermal synthesis followed by a chemical reduction. 0.2 g of BWO powder and 0.4 mL GO solution (10 mg/mL) were dispersed in mixed solution of C2H5OH:H2O = 1:2 (v:v, 60 mL) by ultrasonic treatment for 60 min. The obtained suspension was transferred to a Teflon-lined stainless steel autoclave (100 mL) and maintained at 180 °C for 4 h. Then the precipitate was centrifuged, washed several times with deionized water and ethanol and freeze dried for 12 h followed by a chemical reduction to obtain Cu2O NPs/BWO/rGO hybrid. 0.2 g of obtained BWO/rGO power was ultrasonically dispersed in deionized water, 0.1 g of polyvinylpyrrolidone (PVP) and 1 mL of CuCl2 (0.1 M) solution were added to the above solution by magnetically stirred for 5 min. Then, 2.5 mL of NaOH (0.2 M) was dropwise added to the above solution and magnetically stirred for 5 min. Then 2.5 mL of ascorbic acid (0.1 M) was dropwise added to the solution. After stirred for 30 min, the obtained hybrid was washed with ethanol several times and dried under vacuum. For comparison, the BWO/rGO and Cu2O NPs/ BWO hybrid were also prepared by the same process except adding Cu2O and rGO, respectively.
Fig. 1. Schematically shown the preparation process and mechanism of Cu2O NPs/BWO/rGO hybrid.
2.3. Characterizations Scanning electron microscopy (SEM, Sirion) and a Transmission electron microscopy (TEM, Titan 80-300) were carried out to study the morphologies and microstructures of the samples. X-ray diffractometer (XRD, D8-Discover) was used to identify the crystal structures and phase constitution of the samples with a Cu-Kα radiation at the 2θ angles of 20°~90°. The chemical constitution was characterized by Xray photoelectron microscope (XPS, PHI 500). The UV–vis diffuse reflectance spectra (DRS) of the samples were examined by a Shimadzu UV-2550 UV–vis spectrophotometer with BaSO4 as the background at 200–800 nm. Fig. 2. XRD patterns of BWO, BWO/rGO, Cu2O NPs/BWO and Cu2O NPs/BWO/ rGO hybrid.
2.4. Photoelectrochemical measurements PEC performance was measured by electrochemical workstation (CHI660E, Shanghai, China) using a three-electrode system under 300 W xenon lamp irradiation, using an Ag/AgCl electrode as the reference electrode, a Pt foil as the counter electrode and 0.1 M of Na2SO4 solution as the electrolyte. The working electrode was prepared by dispersing 1 mg of as-obtained samples and 1 mL of N,NDimethylformamide (DMF) containing 20 uL of Nafion with ultrasonic treatment. The as-obtained solution of 40 uL was dropped onto ITO glass (20 × 20 × 2.3 mm3, 10 Ωsq−1) with an area of 1 × 1 cm2. Finally, the samples were dried at 60 °C for 12 h.
prepared samples at 2θ of 28.3°, 32.7°, 47.0° and 55.9° are corresponded to (1 3 1), (0 0 2), (2 6 0), and (3 3 1) crystal planes of orthorhombic BWO (JCPDS 39-0256), respectively. The diffraction peaks of Cu2O and rGO are too weak to be detected due to their low contents, and their high dispersion in the hybrids [16,17]. The morphology and crystal structure of as-obtained samples were employed by SEM, TEM and HRTEM, respectively (see Fig. 3). Fig. 3(a)-(b) show SEM images of pristine BWO and Cu2O NPs/ BWO/rGO hybrid. The pristine BWO is consituted by ultra-thin nanosheets assembled layered structure and exhibits a uniform hydrangea-like structure with an average spherical diameter of 3–5 μm. This unique hydrangea-like structure can effectively increase the specific surface area and provide a channel for electron orientation transfer. The spherical structure of BWO does not change and still keep unbroken after coupling with rGO and Cu2O NPs. Fig. 3(c)-(d) show the EDS spectrum and corresponding elemental mapping of the Cu2O NPs/BWO/rGO hybrid. The Bi, W, O, Cu and C elements are observed and uniformly distributed on the surface of hybrid. Fig. 3(e) and (f) exhibit some nanoparticles and ultrathin nanoplates distributed on hollow microsphere structure. The (1 1 3) and (0 0 2) plane of orthorhombic BWO with lattice spacing distance of 0.315 nm and 0.27 nm, respectively can be observed in HRTEM images
3. Results and discussion The preparation process and mechanism of Cu2O NPs/BWO/rGO hybrid was schematically shown in Fig. 1. As shown in Fig. 1, the hydrangea-like BWO with layered spherical structure was formed by a solvothermal treatment. Then, the Cu2O NPs/BWO/rGO hybrid was fabricated through mixing with a certain amount of GO and BWO by a solvothermal treatment, and following by a chemical reduction. The purity, crystallinity and phase structure of the material was measured by XRD, see Fig. 2. As shown in Fig. 2, the diffraction characteristic peaks of as2
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Fig. 3. (a)-(b) SEM images of BWO and Cu2O NPs/BWO/rGO hybrid; (c) The EDS spectrum of Cu2O NPs/BWO/rGO hybrid; (d) The mapping of Cu2O NPs/BWO/rGO hybrid; (e)-(f) TEM and (g)-(h) HRTEM images of Cu2O NPs/BWO/rGO hybrid.
electron-hole pairs and promote internal charge separation due to the addition of rGO [25]. The increased absorption edge is attributed to the electronic interactions between the components, i.e. rGO and BWO or Cu2O [26,27]. The color of the as-prepared materials will also changes with the addition of rGO (see Fig. S2), while the Cu2O NPs/BWO/rGO hybrid color is changed to charcoal gray. Fig. 6(a) shows the linear sweep voltammetry (LSV) curves, in which Cu2O NPs/BWO/rGO hybrid presents the highest photocurrent density than other samples. Transient photocurrent response curves are shown in Fig. 6(b). The BWO/rGO, Cu2O NPs/BWO and Cu2O NPs/BWO/rGO hybrids exhibit an increased photocurrent response of ~4, 7 and 14 times higher photocurrent response than that of pure BWO, respectively. In order to further explain the reason for the improvement of PEC performance of hybrid, the electrochemical impedance spectra (EIS) were measured to analyze the interface electron conduction and transfer capabilities. Typically, the arc radius on EIS Nyquist plot is primarily proportional to the charge transfer resistance. It can be seen from Fig. 6(c) that the Cu2O NPs/BWO/rGO hybrid shows the lowest arc radius, indicating a lower charge transfer resistance. BWO/rGO and Cu2O NPs/BWO also exhibit smaller arc radius than that of pure BWO. But BWO/rGO displays a dramatically decreased arc radius due to the high conductivity of graphene, which accelerates electron transfer and improves the efficiency of interface charge transfer. The rGO acts a pivotal part in the process of electron transfer between the electrode and the electrolyte. Fig. 6(d) represents the semiconductor properties of the samples measured by using the Mott-Schottky method. All the samples exhibit the ntype semiconductor characteristics. In particular, the Cu2O NPs/BWO/ rGO hybrid has the smallest tangent slope, which also verifies the fastest charge transfer rate as indicate by the EIS results. The photocurrent stability of BWO, BWO/rGO, Cu2O NPs/BWO, and Cu2O NPs/BWO/rGO were measured and shown in Fig. 7. The Cu2O NPs/BWO/rGO hybrid exhibits a remarkable higher photocurrent than those of BWO, Cu2O NPs/BWO after the initial instantaneous photocurrent decreased. The Cu2O NPs/BWO shows a large decrease photocurrent with a decrease about 52% from 10 min to 60 min for irradiation. In contrast, the photocurrent of BWO/rGO and Cu2O NPs/
of Fig. 3(g)-(h). The nanoparticles on the surface of Cu2O NPs/BWO/ rGO hybrid has the lattice spacing of 0.245 nm corresponding to the cubic Cu2O (1 1 1) plane and the lattice spacing of 0.34 nm corresponding to Graphene, which indicates that the Cu2O NPs/BWO/rGO hybrid is synthesized successfully. Fig. 4 represents the XPS spectra to further confirm the chemical element composition and the interaction of each component in as-prepared Cu2O NPs/BWO/rGO hybrid. As shown in the full spectrum of XPS in Fig. 4(a), the elemental signals of W, Bi, Cu, C, and O are detected, which match with the element mapping data. The Bi 4f has the strongest signal intensity, which displays that Bi3+ is a predominantly valence state of the Bi element [18]. In Fig. 4(b), the main characteristic peaks of C 1s at 284.4 eV, 285.4 eV and 287.5 eV are attributed to the CeC and C]C in non-oxidative carbon and the CeO in epoxy and hydroxy, respectively [19]. The other peak intensity of CeO is significantly lower than that of GO as reported [20], which might be due to the reduction of the organic functional groups and GO is efficiently reduced to rGO via solvothermal process. Fig. 4(d) represents the Cu 2p spectrum. The double peaks at 932.2 eV and 951.7 eV correspond to Cu 2p3/2 and Cu 2p1/2, respectively. The Cu 2p3/2 photoelectrons detected at 932.2 eV indicates the existence of Cu+ [21]. The O 1s spectrum can be resolved into three peaks (Fig. 4c). The peak at 531.6 eV was well in accordance with O 1s state in rGO [22], and the peaks at 529.4 eV and 530.0 eV can be assigned to the oxygen atoms bound to metal [23,24].Therefore, the XPS spectras give another experimental support for the successful preparation of Cu2O NPs/BWO/rGO hybrid. Fig. 5 represents the light absorption properties of as-prepared samples measured by the UV–visible diffuse reflectance spectroscopy. The absorption range of light plays an important role in the PEC water splitting, especially the utilization for the visible light. As shown in Fig. 5(a), the pure BWO exhibits typical semiconductor absorption with optical absorption edge of about 462 nm. Fig. 5(b) shows that the light absorption efficiency of Cu2O NPs/BWO/rGO hybrid at 700 nm (77.8%) is 73% higher than that of pristine BWO (44.7%). It can be seen that Cu2O NPs/BWO/rGO hybrid exhibits a significantly increased absorption edge, which can excite to produce more photogenerated 3
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Fig. 4. XPS spectra of Cu2O NPs/BWO/rGO hybrid.
2.1 eV, respectively. The Mott-Schottky plots can be employed to assess the CB value (ECB) of BWO and VB value (EVB) of Cu2O, respectively. Fig. 5(d) shows the Mott-Schottky plots of BWO. The positive slope represents the n-type semiconductor. The ECB for BWO is −0.5 V vs. Ag/AgCl electrode obtained by measuring the intercept along with X axis. As well, Fig. S4 displays the negative slope of the Mott-Schottky plots of Cu2O. The EVB is +0.95 V vs. Ag/AgCl electrode. The negative slope of Cu2O exhibits the p-type semiconductor characteristics. According to the formula for the band gap [28], the calculated CB energy of Cu2O and VB energy of BWO are −1.15 V and +2.25 V, respectively.
BWO/rGO hybrid are decreased about 36% and 41%, respectively. This indicates that the Cu2O NPs/BWO/rGO hybrid can not only significantly enhance the photoelectric performance but also weak the photo-corrosion of Cu2O [9]. The PEC performance enhancement mechanism of as-prepared Cu2O NPs/BWO/rGO hybrid is proposed as shown in Fig. 8 In order to investigate the enhancement mechanism for the PEC performance, the band gap energy (Eg) of BWO and Cu2O were determined by the classical Kubelka-Munk function as shown in Fig. S3. The band gaps values of BWO and Cu2O are identified as 2.75 eV and
Fig. 5. (a) UV–visible diffuse reflectance spectroscopy; (b) light absorption efficiency of BWO, BWO/rGO, Cu2O NPs/BWO, and Cu2O NPs/BWO/rGO samples. 4
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Fig. 6. (a) LSV curves (b) Transient photocurrent responses (the bias potential at 0.6 V vs. Ag/AgCl); (c) EIS Nyquist plots; (d) Mott-Schottky plots of BWO, BWO/rGO, Cu2O NPs/BWO, and Cu2O NPs/BWO/rGO samples.
reason that the edge potential of CB in BWO is a bit positive than that of O2 reduction (O2%−/O2 = −0.33 V vs. NHE) [28]. Meanwhile, the VB edge potential of Cu2O NPs is more negative than that of H2O oxidation (O2/H2O = 1.23 V vs. NHE) and the production of O2 cannot be realize [31]. According to the promoted PEC activity results, the separation of photogenerated electron-hole pairs is followed by the direct Z-scheme mechanism [32,33]. In this mechanism, the photogenerated electrons in the CB of BWO are moved to the VB of Cu2O NPs and combine with the holes. The electrons in the CB of Cu2O NPs and the holes in the VB of BWO transfer to the surface for PEC water splitting. Hence, the Cu2O NPs/BWO system is efficient separation and inefficient recombination for the electron-hole pairs and function as a direct Z-scheme heterojunction. The enhancement of the PEC performance is attributed to the recombination of Cu2O NPs and rGO, which proves a direct Z-scheme as the main mechanism in Cu2O NPs/BWO. The rGO acts as a pivotal charge carrier transmission bridge to enhance the separation and mobility of photogenerated electron-hole pairs and inhibits the recombination of electrons and holes during the above process. The Cu2O NPs/BWO/rGO hybrid can not only excite to produce more photogenerated electron-hole pairs because of its increased visible light absorption ability but also provide a channel for electron orientation transfer due to this unique hydrangea-like structure.
Fig. 7. Photocurrent time dependence curves of BWO, BWO/rGO, Cu2O NPs/ BWO, and Cu2O NPs/BWO/rGO samples in 0.1 M Na2SO4 solution at 0.6 V vs. Ag/AgCl.
The measure results are consistent with the previous reports [29,30]. As shown in Fig. 8, upon irradiation by the simulated sunlight, both BWO and Cu2O NPs are excited to generate the electrons and the holes, respectively. Based on the traditional P-N heterojunction formation mechanism, the electrons of CB in Cu2O NPs will be transferred to that of BWO. Simultaneously, the holes will be transferred from the VB of BWO to that of Cu2O NPs. However, it is regrettably that the accumulated electrons of CB in BWO cannot convert O2 to O2%− due to the
4. Conclusion In this study, we prepared a novel Cu2O nanoparticles/BWO/reduced graphene oxide (Cu2O NPs/BWO/rGO) hybrid by a facile solvothermal synthesis followed by a chemical reduction. The Cu2O NPs and rGO decorate homogeneously on the surface of hydrangea-like 5
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Fig. 8. The PEC performance enhancement mechanism of Cu2O NPs/BWO/rGO hybrid.
BWO. This novel hybrid exhibits a significantly enhanced PEC performance with a ~14 times high photocurrent density than that of pure BWO under simulated sunlight irradiation. The enhancement of the PEC performance is attributed to the significant improvement for decorating with Cu2O NPs, and rGO, which proves a direct Z-scheme as the main mechanism in Cu2O NPs/BWO, and rGO as a pivotal charge transmission bridge for the efficient photogenerated charge separation, and its increased visible light absorption. The results have provided a new way for the practical water splitting application of BWO.
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CRediT authorship contribution statement Xuan Tang: Conceptualization, Methodology, Writing - original draft. Xinli Guo: Writing - review & editing. Zhongtao Chen: Investigation. Yuanyuan Liu: Data curation. Weijie Zhang: Visualization. Yixuan Wang: Validation. Yanmei Zheng: Validation. Ming Zhang: Validation. Zhengbin Peng: Validation. Rui Li: Validation. Yuhong Zhao: Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The most financial support of this work was under Grant Number 2017YFA0205800, National Natural Science Foundation of China (21173041) and Open Project of State Key Laboratory of Advanced Metallic Materials, China Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2020.145447. References [1] C. Jiang, S.J.A. Moniz, A. Wang, T. Zhang, J. Tang, Photoelectrochemical devices for solar water splitting - materials and challenges, Chem. Soc. Rev. 46 (2017) 4645–4660. [2] S. Zhou, P. Yue, J. Huang, L. Wang, H. She, Q. Wang, High-performance photoelectrochemical water splitting of BiVO4@Co-MIm prepared by a facile in-situ deposition method, Chem. Eng. J. 371 (2019) 885–892. [3] Y. Pang, Y. Li, G. Xu, Y. Hu, Z. Kou, Q. Feng, J. Lv, Y. Zhang, J. Wang, Y. Wu, Zscheme carbon-bridged Bi2O3/TiO2 nanotube arrays to boost photoelectrochemical
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Full Length Article
Facile preparation of Cu2O nanoparticles/Bi2WO6/rGO hybrid with enhanced photoelectrochemical performance
T
⁎
Xuan Tanga, Xinli Guoa, , Zhongtao Chena, Yuanyuan Liua, Weijie Zhanga, Yixuan Wanga, ⁎ Yanmei Zhenga, Ming Zhanga, Zhengbin Penga, Rui Lia, Yuhong Zhaob, a b
Jiangsu Key Laboratory of Advanced Metallic Materials, School of Materials Science and Engineering, Southeast University, Nanjing 211189, China College of Materials Science and Engineering, North University of China, Taiyuan 030051, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Bi2WO6 (BWO) Cu2O nanoparticles/BWO/Reduced graphene oxide (Cu2O NPs/BWO/rGO) hybrid Solvothermal synthesis Chemical reduction Photoelectrochemical (PEC) performance
Bi2WO6 (BWO) has recently received extensive interest for water splitting application due to its attractive properties of non-toxicity, high stability and narrow band gap (~2.7 eV). However, the photoelectrochemical (PEC) performance of pure BWO is limited by the inefficiency visible light absorption, and the low electron transport efficiency. Herein we prepared a novel Cu2O nanoparticles/BWO/reduced graphene oxide (Cu2O NPs/ BWO/rGO) hybrid by a facile solvothermal synthesis followed by a chemical reduction. The Cu2O NPs and rGO decorate homogeneously on the surface of hydrangea-like BWO. This novel hybrid exhibits a significantly enhanced PEC performance with a ~14 times high photocurrent density than that of pure BWO under simulated sunlight irradiation. This enhancement of the PEC performance is attributed to the significant improvement of coupling Cu2O NPs and rGO, which proves a direct Z-scheme as the main mechanism in Cu2O NPs/BWO, and rGO as a pivotal charge transmission bridge for the efficient photogenerated charge separation, and the increased visible light absorption. The results have provided a new way for the practical water splitting application of BWO.
1. Introduction For the past few years, PEC water splitting has become one of the efficient strategy to solve the problems of energy shortage and environmental pollution by converting solar energy into the clean, storable chemical fuels [1]. Recently, some bismuth-based semiconductors such as BiVO4, Bi2O3, and BWO, etc. have received extensive interest in the study of PEC water splitting [2–5]. Among them, BWO shows attractive properties of non-toxicity, high stability and narrow band gap (~2.7 eV) [6]. However, the PEC performance of pure BWO is limited by the inefficiency visible light absorption, the low electron transport efficiency and the fast recombination of the photogenerated electronhole pairs. To conquer these drawbacks, plenty of efforts have been developed, such as hybridizing with the carbon materials [7], coupling with other semiconductors, etc. Graphene, as one of the two dimensional (2D) monolayer carbonaceous materials, could effectively not only boost the charge transfer but also inhibit the recombination of photogenerated carriers when merging with semiconductors due to its unique conjugated carbon network structure [8]. Meanwhile, cuprous oxide (Cu2O), as a nonnoble metal oxide semiconductor, has been widely used in various ⁎
applications such as photocatalysis [9], photoelectrical devices [10–12], and other areas because of its nontoxicity, low cost and narrow band gap. Liu prepared the Cu2O nanodots/BWO hybrid in which Cu2O nano-structure extended the visible light absorption and the lower charge transfer resistance for degradation of methylene blue [13]. The Cu2O/BWO composites synthesized by Zheng et al. for production of alkanes and hydrogen gas [14]. Nevertheless, the study about Cu2O NPs modified BWO composites for PEC property have been rarely reported. In this work, we prepared a novel Cu2O NPs/BWO/rGO hybrid via a facile solvothermal synthesis followed by a chemical reduction. The asprepared Cu2O NPs/BWO/rGO hybrid exhibits a significantly enhanced PEC performance in Na2SO4 solution under simulated sunlight irradiation. 2. Experimental section 2.1. Synthesis of hydrangea-like BWO The hydrangea-like BWO was synthesized by a solvothermal method. 1 mmol of Na2WO4·2H2O and 0.45 g of hexadecyl trimethyl
Corresponding authors. E-mail addresses: [email protected] (X. Guo), [email protected] (Y. Zhao).
https://doi.org/10.1016/j.apsusc.2020.145447 Received 16 September 2019; Received in revised form 29 December 2019; Accepted 16 January 2020 Available online 20 January 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved.
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ammonium bromide (CTAB) were dissolved into 20 mL of ethylene glycol under magnetic stirring at 40 °C. Then the above solution was dropwise added to 50 mL of Bi(NO3)3·3H2O (0.05 M) followed by stirring for 2 h, heating at 180 °C for 12 h and freeze-drying for 12 h. 2.2. Synthesis of Cu2O NPs/BWO/rGO hybrid The graphene oxide was synthesized by the modified Hummers method [15].The Cu2O NPs/BWO/rGO hybrid was prepared by a facile solvothermal synthesis followed by a chemical reduction. 0.2 g of BWO powder and 0.4 mL GO solution (10 mg/mL) were dispersed in mixed solution of C2H5OH:H2O = 1:2 (v:v, 60 mL) by ultrasonic treatment for 60 min. The obtained suspension was transferred to a Teflon-lined stainless steel autoclave (100 mL) and maintained at 180 °C for 4 h. Then the precipitate was centrifuged, washed several times with deionized water and ethanol and freeze dried for 12 h followed by a chemical reduction to obtain Cu2O NPs/BWO/rGO hybrid. 0.2 g of obtained BWO/rGO power was ultrasonically dispersed in deionized water, 0.1 g of polyvinylpyrrolidone (PVP) and 1 mL of CuCl2 (0.1 M) solution were added to the above solution by magnetically stirred for 5 min. Then, 2.5 mL of NaOH (0.2 M) was dropwise added to the above solution and magnetically stirred for 5 min. Then 2.5 mL of ascorbic acid (0.1 M) was dropwise added to the solution. After stirred for 30 min, the obtained hybrid was washed with ethanol several times and dried under vacuum. For comparison, the BWO/rGO and Cu2O NPs/ BWO hybrid were also prepared by the same process except adding Cu2O and rGO, respectively.
Fig. 1. Schematically shown the preparation process and mechanism of Cu2O NPs/BWO/rGO hybrid.
2.3. Characterizations Scanning electron microscopy (SEM, Sirion) and a Transmission electron microscopy (TEM, Titan 80-300) were carried out to study the morphologies and microstructures of the samples. X-ray diffractometer (XRD, D8-Discover) was used to identify the crystal structures and phase constitution of the samples with a Cu-Kα radiation at the 2θ angles of 20°~90°. The chemical constitution was characterized by Xray photoelectron microscope (XPS, PHI 500). The UV–vis diffuse reflectance spectra (DRS) of the samples were examined by a Shimadzu UV-2550 UV–vis spectrophotometer with BaSO4 as the background at 200–800 nm. Fig. 2. XRD patterns of BWO, BWO/rGO, Cu2O NPs/BWO and Cu2O NPs/BWO/ rGO hybrid.
2.4. Photoelectrochemical measurements PEC performance was measured by electrochemical workstation (CHI660E, Shanghai, China) using a three-electrode system under 300 W xenon lamp irradiation, using an Ag/AgCl electrode as the reference electrode, a Pt foil as the counter electrode and 0.1 M of Na2SO4 solution as the electrolyte. The working electrode was prepared by dispersing 1 mg of as-obtained samples and 1 mL of N,NDimethylformamide (DMF) containing 20 uL of Nafion with ultrasonic treatment. The as-obtained solution of 40 uL was dropped onto ITO glass (20 × 20 × 2.3 mm3, 10 Ωsq−1) with an area of 1 × 1 cm2. Finally, the samples were dried at 60 °C for 12 h.
prepared samples at 2θ of 28.3°, 32.7°, 47.0° and 55.9° are corresponded to (1 3 1), (0 0 2), (2 6 0), and (3 3 1) crystal planes of orthorhombic BWO (JCPDS 39-0256), respectively. The diffraction peaks of Cu2O and rGO are too weak to be detected due to their low contents, and their high dispersion in the hybrids [16,17]. The morphology and crystal structure of as-obtained samples were employed by SEM, TEM and HRTEM, respectively (see Fig. 3). Fig. 3(a)-(b) show SEM images of pristine BWO and Cu2O NPs/ BWO/rGO hybrid. The pristine BWO is consituted by ultra-thin nanosheets assembled layered structure and exhibits a uniform hydrangea-like structure with an average spherical diameter of 3–5 μm. This unique hydrangea-like structure can effectively increase the specific surface area and provide a channel for electron orientation transfer. The spherical structure of BWO does not change and still keep unbroken after coupling with rGO and Cu2O NPs. Fig. 3(c)-(d) show the EDS spectrum and corresponding elemental mapping of the Cu2O NPs/BWO/rGO hybrid. The Bi, W, O, Cu and C elements are observed and uniformly distributed on the surface of hybrid. Fig. 3(e) and (f) exhibit some nanoparticles and ultrathin nanoplates distributed on hollow microsphere structure. The (1 1 3) and (0 0 2) plane of orthorhombic BWO with lattice spacing distance of 0.315 nm and 0.27 nm, respectively can be observed in HRTEM images
3. Results and discussion The preparation process and mechanism of Cu2O NPs/BWO/rGO hybrid was schematically shown in Fig. 1. As shown in Fig. 1, the hydrangea-like BWO with layered spherical structure was formed by a solvothermal treatment. Then, the Cu2O NPs/BWO/rGO hybrid was fabricated through mixing with a certain amount of GO and BWO by a solvothermal treatment, and following by a chemical reduction. The purity, crystallinity and phase structure of the material was measured by XRD, see Fig. 2. As shown in Fig. 2, the diffraction characteristic peaks of as2
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Fig. 3. (a)-(b) SEM images of BWO and Cu2O NPs/BWO/rGO hybrid; (c) The EDS spectrum of Cu2O NPs/BWO/rGO hybrid; (d) The mapping of Cu2O NPs/BWO/rGO hybrid; (e)-(f) TEM and (g)-(h) HRTEM images of Cu2O NPs/BWO/rGO hybrid.
electron-hole pairs and promote internal charge separation due to the addition of rGO [25]. The increased absorption edge is attributed to the electronic interactions between the components, i.e. rGO and BWO or Cu2O [26,27]. The color of the as-prepared materials will also changes with the addition of rGO (see Fig. S2), while the Cu2O NPs/BWO/rGO hybrid color is changed to charcoal gray. Fig. 6(a) shows the linear sweep voltammetry (LSV) curves, in which Cu2O NPs/BWO/rGO hybrid presents the highest photocurrent density than other samples. Transient photocurrent response curves are shown in Fig. 6(b). The BWO/rGO, Cu2O NPs/BWO and Cu2O NPs/BWO/rGO hybrids exhibit an increased photocurrent response of ~4, 7 and 14 times higher photocurrent response than that of pure BWO, respectively. In order to further explain the reason for the improvement of PEC performance of hybrid, the electrochemical impedance spectra (EIS) were measured to analyze the interface electron conduction and transfer capabilities. Typically, the arc radius on EIS Nyquist plot is primarily proportional to the charge transfer resistance. It can be seen from Fig. 6(c) that the Cu2O NPs/BWO/rGO hybrid shows the lowest arc radius, indicating a lower charge transfer resistance. BWO/rGO and Cu2O NPs/BWO also exhibit smaller arc radius than that of pure BWO. But BWO/rGO displays a dramatically decreased arc radius due to the high conductivity of graphene, which accelerates electron transfer and improves the efficiency of interface charge transfer. The rGO acts a pivotal part in the process of electron transfer between the electrode and the electrolyte. Fig. 6(d) represents the semiconductor properties of the samples measured by using the Mott-Schottky method. All the samples exhibit the ntype semiconductor characteristics. In particular, the Cu2O NPs/BWO/ rGO hybrid has the smallest tangent slope, which also verifies the fastest charge transfer rate as indicate by the EIS results. The photocurrent stability of BWO, BWO/rGO, Cu2O NPs/BWO, and Cu2O NPs/BWO/rGO were measured and shown in Fig. 7. The Cu2O NPs/BWO/rGO hybrid exhibits a remarkable higher photocurrent than those of BWO, Cu2O NPs/BWO after the initial instantaneous photocurrent decreased. The Cu2O NPs/BWO shows a large decrease photocurrent with a decrease about 52% from 10 min to 60 min for irradiation. In contrast, the photocurrent of BWO/rGO and Cu2O NPs/
of Fig. 3(g)-(h). The nanoparticles on the surface of Cu2O NPs/BWO/ rGO hybrid has the lattice spacing of 0.245 nm corresponding to the cubic Cu2O (1 1 1) plane and the lattice spacing of 0.34 nm corresponding to Graphene, which indicates that the Cu2O NPs/BWO/rGO hybrid is synthesized successfully. Fig. 4 represents the XPS spectra to further confirm the chemical element composition and the interaction of each component in as-prepared Cu2O NPs/BWO/rGO hybrid. As shown in the full spectrum of XPS in Fig. 4(a), the elemental signals of W, Bi, Cu, C, and O are detected, which match with the element mapping data. The Bi 4f has the strongest signal intensity, which displays that Bi3+ is a predominantly valence state of the Bi element [18]. In Fig. 4(b), the main characteristic peaks of C 1s at 284.4 eV, 285.4 eV and 287.5 eV are attributed to the CeC and C]C in non-oxidative carbon and the CeO in epoxy and hydroxy, respectively [19]. The other peak intensity of CeO is significantly lower than that of GO as reported [20], which might be due to the reduction of the organic functional groups and GO is efficiently reduced to rGO via solvothermal process. Fig. 4(d) represents the Cu 2p spectrum. The double peaks at 932.2 eV and 951.7 eV correspond to Cu 2p3/2 and Cu 2p1/2, respectively. The Cu 2p3/2 photoelectrons detected at 932.2 eV indicates the existence of Cu+ [21]. The O 1s spectrum can be resolved into three peaks (Fig. 4c). The peak at 531.6 eV was well in accordance with O 1s state in rGO [22], and the peaks at 529.4 eV and 530.0 eV can be assigned to the oxygen atoms bound to metal [23,24].Therefore, the XPS spectras give another experimental support for the successful preparation of Cu2O NPs/BWO/rGO hybrid. Fig. 5 represents the light absorption properties of as-prepared samples measured by the UV–visible diffuse reflectance spectroscopy. The absorption range of light plays an important role in the PEC water splitting, especially the utilization for the visible light. As shown in Fig. 5(a), the pure BWO exhibits typical semiconductor absorption with optical absorption edge of about 462 nm. Fig. 5(b) shows that the light absorption efficiency of Cu2O NPs/BWO/rGO hybrid at 700 nm (77.8%) is 73% higher than that of pristine BWO (44.7%). It can be seen that Cu2O NPs/BWO/rGO hybrid exhibits a significantly increased absorption edge, which can excite to produce more photogenerated 3
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Fig. 4. XPS spectra of Cu2O NPs/BWO/rGO hybrid.
2.1 eV, respectively. The Mott-Schottky plots can be employed to assess the CB value (ECB) of BWO and VB value (EVB) of Cu2O, respectively. Fig. 5(d) shows the Mott-Schottky plots of BWO. The positive slope represents the n-type semiconductor. The ECB for BWO is −0.5 V vs. Ag/AgCl electrode obtained by measuring the intercept along with X axis. As well, Fig. S4 displays the negative slope of the Mott-Schottky plots of Cu2O. The EVB is +0.95 V vs. Ag/AgCl electrode. The negative slope of Cu2O exhibits the p-type semiconductor characteristics. According to the formula for the band gap [28], the calculated CB energy of Cu2O and VB energy of BWO are −1.15 V and +2.25 V, respectively.
BWO/rGO hybrid are decreased about 36% and 41%, respectively. This indicates that the Cu2O NPs/BWO/rGO hybrid can not only significantly enhance the photoelectric performance but also weak the photo-corrosion of Cu2O [9]. The PEC performance enhancement mechanism of as-prepared Cu2O NPs/BWO/rGO hybrid is proposed as shown in Fig. 8 In order to investigate the enhancement mechanism for the PEC performance, the band gap energy (Eg) of BWO and Cu2O were determined by the classical Kubelka-Munk function as shown in Fig. S3. The band gaps values of BWO and Cu2O are identified as 2.75 eV and
Fig. 5. (a) UV–visible diffuse reflectance spectroscopy; (b) light absorption efficiency of BWO, BWO/rGO, Cu2O NPs/BWO, and Cu2O NPs/BWO/rGO samples. 4
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Fig. 6. (a) LSV curves (b) Transient photocurrent responses (the bias potential at 0.6 V vs. Ag/AgCl); (c) EIS Nyquist plots; (d) Mott-Schottky plots of BWO, BWO/rGO, Cu2O NPs/BWO, and Cu2O NPs/BWO/rGO samples.
reason that the edge potential of CB in BWO is a bit positive than that of O2 reduction (O2%−/O2 = −0.33 V vs. NHE) [28]. Meanwhile, the VB edge potential of Cu2O NPs is more negative than that of H2O oxidation (O2/H2O = 1.23 V vs. NHE) and the production of O2 cannot be realize [31]. According to the promoted PEC activity results, the separation of photogenerated electron-hole pairs is followed by the direct Z-scheme mechanism [32,33]. In this mechanism, the photogenerated electrons in the CB of BWO are moved to the VB of Cu2O NPs and combine with the holes. The electrons in the CB of Cu2O NPs and the holes in the VB of BWO transfer to the surface for PEC water splitting. Hence, the Cu2O NPs/BWO system is efficient separation and inefficient recombination for the electron-hole pairs and function as a direct Z-scheme heterojunction. The enhancement of the PEC performance is attributed to the recombination of Cu2O NPs and rGO, which proves a direct Z-scheme as the main mechanism in Cu2O NPs/BWO. The rGO acts as a pivotal charge carrier transmission bridge to enhance the separation and mobility of photogenerated electron-hole pairs and inhibits the recombination of electrons and holes during the above process. The Cu2O NPs/BWO/rGO hybrid can not only excite to produce more photogenerated electron-hole pairs because of its increased visible light absorption ability but also provide a channel for electron orientation transfer due to this unique hydrangea-like structure.
Fig. 7. Photocurrent time dependence curves of BWO, BWO/rGO, Cu2O NPs/ BWO, and Cu2O NPs/BWO/rGO samples in 0.1 M Na2SO4 solution at 0.6 V vs. Ag/AgCl.
The measure results are consistent with the previous reports [29,30]. As shown in Fig. 8, upon irradiation by the simulated sunlight, both BWO and Cu2O NPs are excited to generate the electrons and the holes, respectively. Based on the traditional P-N heterojunction formation mechanism, the electrons of CB in Cu2O NPs will be transferred to that of BWO. Simultaneously, the holes will be transferred from the VB of BWO to that of Cu2O NPs. However, it is regrettably that the accumulated electrons of CB in BWO cannot convert O2 to O2%− due to the
4. Conclusion In this study, we prepared a novel Cu2O nanoparticles/BWO/reduced graphene oxide (Cu2O NPs/BWO/rGO) hybrid by a facile solvothermal synthesis followed by a chemical reduction. The Cu2O NPs and rGO decorate homogeneously on the surface of hydrangea-like 5
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Fig. 8. The PEC performance enhancement mechanism of Cu2O NPs/BWO/rGO hybrid.
BWO. This novel hybrid exhibits a significantly enhanced PEC performance with a ~14 times high photocurrent density than that of pure BWO under simulated sunlight irradiation. The enhancement of the PEC performance is attributed to the significant improvement for decorating with Cu2O NPs, and rGO, which proves a direct Z-scheme as the main mechanism in Cu2O NPs/BWO, and rGO as a pivotal charge transmission bridge for the efficient photogenerated charge separation, and its increased visible light absorption. The results have provided a new way for the practical water splitting application of BWO.
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CRediT authorship contribution statement Xuan Tang: Conceptualization, Methodology, Writing - original draft. Xinli Guo: Writing - review & editing. Zhongtao Chen: Investigation. Yuanyuan Liu: Data curation. Weijie Zhang: Visualization. Yixuan Wang: Validation. Yanmei Zheng: Validation. Ming Zhang: Validation. Zhengbin Peng: Validation. Rui Li: Validation. Yuhong Zhao: Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The most financial support of this work was under Grant Number 2017YFA0205800, National Natural Science Foundation of China (21173041) and Open Project of State Key Laboratory of Advanced Metallic Materials, China Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2020.145447. References [1] C. Jiang, S.J.A. Moniz, A. Wang, T. Zhang, J. Tang, Photoelectrochemical devices for solar water splitting - materials and challenges, Chem. Soc. Rev. 46 (2017) 4645–4660. [2] S. Zhou, P. Yue, J. Huang, L. Wang, H. She, Q. Wang, High-performance photoelectrochemical water splitting of BiVO4@Co-MIm prepared by a facile in-situ deposition method, Chem. Eng. J. 371 (2019) 885–892. [3] Y. Pang, Y. Li, G. Xu, Y. Hu, Z. Kou, Q. Feng, J. Lv, Y. Zhang, J. Wang, Y. Wu, Zscheme carbon-bridged Bi2O3/TiO2 nanotube arrays to boost photoelectrochemical
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