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Cu2−xSe Modification onto Monoclinic BiVO4 for Enhanced Photocatalytic Activity Under Visible Light
Nanoscale Research Letters volume 14, Article number: 95 (2019)
The rapid recombination of electron-hole pairs in BiVO4 has limited its performance as a photocatalysis. In this paper, BiVO4 is combined with Cu2−xSe semiconductor to slow down the recombination process, and thus improve its photocatalytic activity. This is enabled by careful band structure design. The work function of Cu2−xSe is larger than that of BiVO4. Therefore, electrons flow to Cu2−xSe from BiVO4 after the composition. Accordingly, an inner field could be built, which facilitates the separation of electrons and holes. The experimental result shows that the photocatalytic efficiency of the 3 wt% Cu2−xSe/BiVO4 composite is 15.8 times than that of pure BiVO4.
With the developing of modern industry, environmental pollution has become more and more severe. Utilizing solar energy, photocatalytic decomposition of organic matter is an environmentally friendly and efficient technology to solve pollution [1,2,3,4,5,6]. The Bi-based semiconductor photocatalytic material has a suitable band gap, which enables it to absorb visible light sufficiently and possess superior photocatalytic performance [7,8,9,10]. Among them, monoclinic BiVO4 has a narrow band gap of 2.4 eV and good photocatalytic activity, which has been nominated as an efficient material for decomposing organic pollutions [11,12,13,14,15]. The rapid electron-hole recombination rate, however, leads to a low photocatalytic activity for pure BiVO4 [16,17,18]. An effective approach to slow down the recombination of electrons and holes is to combine two different semiconductor materials, given the band structures of the two combined materials match a specific condition.
As a p-type semiconductor, Cu2−x Se has an indirect bandgap of 1.4 eV, which is beneficial to absorb visible light [19,20,21]. When BiVO4 semiconductor is compounded with Cu2−xSe, redistribution of charges is caused. The work function of Cu2−xSe is larger than that of BiVO4, and the Fermi energy is lower than that of BiVO4 [22, 23]. Therefore, electrons flow to Cu2−xSe from BiVO4 while holes flow the other way around. Accordingly, an inner field could be built pointing from BiVO4 to Cu2−xSe, which facilitates the separation of electrons and holes. When under illumination, the photo-generated electrons in BiVO4 and photo-generated holes in Cu2−xSe will recombine preferentially, due to the band bending and inner field, leaving useful holes in BiVO4. The useful holes possess higher energy level, which can benefit the generation of •OH species. These •OH species can break down long chains of organic matter into small molecules. Hence, the Cu2−xSe/BiVO4 composites are expected to have high visible light photocatalytic activity.
In this work, we have fabricated Cu2−x Se/BiVO4 composites and made use of it for the degradation of RhB under visible light irradiation (> 420 nm) for the first time. After compounding with Cu2−x Se, the photocatalytic activity becomes much higher than pure BiVO4. Specifically, the photocatalytic efficiency of 3 wt% Cu2−xSe/BiVO4 composite is 15.8 times that of pure BiVO4. Furthermore, after adding low concentration H2O2 into the organic solution, RhB completely degraded within 50 min. This work provides evidence that Cu2−xSe is an effective co-catalysis for the development of new composite semiconductor photocatalysts.
Preparation of Cu2−xSe/BiVO4 Composites
BiVO4 was synthesized through a chemical precipitation method [24, 25]. The preparation method of Cu2−xSe can be found in our previously reported paper . Then Cu2−xSe/BiVO4 composites were fabricated by a co-precipitation approach. The schematic illustration of the preparation progress is shown in Fig. 1. Firstly, the pre-prepared Cu2−xSe and BiVO4 powders were dispersed in ethanol with constant stirring for 4 h under 60 °C. Secondly, the suspension of the mixture was continuously stirred at 80 °C to remove the ethanol solvent. Finally, the obtained powdery sample was heated at 160 °C for 6 h under a flowing nitrogen atmosphere to form the Cu2−xSe/BiVO4 composite.
XRD (X-ray diffraction) measurement of the as-prepared samples was performed by a PANalytical X’pert Pro diffractometer with Cu Kα radiation. The morphology of the sample was obtained by an SEM (scanning electron microscope) Hitachi S-4800. XPS (X-ray photoelectron spectroscopy) of the samples was characterized on a Pekin Elmer PHI-5300 instrument. The photoluminescence emission spectra of the samples were committed using a Cary Eclipse fluorescence spectrophotometer.
The photocatalytic performance was characterized by an XPA photochemical reactor. Additionally, a Xe lamp with a power of 500 W and a cut-off wavelength of 420 nm is utilized to simulate natural light, while a solution of test dye RhB is used to mimic organic solutions. During the degradation process, 60 mg Cu2−xSe composite powder was placed in a 60-mL RhB solution. The suspension was stirred in a dark environment for 2 h before light irradiation to realize an adsorption-desorption balance. Then, light illumination is added with stirring remaining and about 6 mL of the suspension was taken out at intervals of 10 min. Subsequently, the suspension was centrifuged twice. The absorbance spectrum of the solution was characterized on a Shimadzu UV-2450 spectrometer.
The photocurrent is measured by a CHI 660E electrochemical workstation. To make the illumination consistent with that in the degradation process, the light source is still selected as a Xe lamp with a power of 500 W and a cut-off wavelength of 420 nm. The photoelectrochemical measurement is detailed as follows. First, 10 mg of the photocatalyst and 20 μL of Nafion solution were ultrasonically dispersed in 2 mL of ethyl alcohol. Then, 40 μL of the above solution was deposited on an ITO conductive glass with 0.196 cm2, which was sequentially heated at 200 °C for 1 h to obtain the working electrode. Besides, Pt foil is chosen as the counter electrode. A saturated solution of mercury and mercurous chloride in an aqueous solution of potassium chloride as the reference electrode, and 0 .5M Na2SO4 solution is used for the electrolyte.
Results and Discussion
We used photodegradation of RhB to examine the photocatalytic properties of the samples. Figure 2a shows the photocatalytic degradation of RhB over Cu2−xSe/BiVO4. When BiVO4 is combined with Cu2−xSe, its photocatalytic performance is significantly improved. The optimum composite ratio is 3%, and the photocatalytic efficiency at this ratio reaches the maximum. Figure 2b shows the degradation rate of the Cu2−xSe/BiVO4 composites, corresponding to the concentration of Cu2−xSe with 0, 2, 3, and 4 wt%, respectively. In Fig. 2b, the slope value of degradation lines is 0.0011, 0.0118, 0.0174, and 0.0045 min−1, respectively. Therefore, the photocatalytic efficiency of the 3 wt% Cu2−xSe/BiVO4 composite is 15.8 times than that of pure BiVO4. Figure 2c shows the recycle runs of photocatalytic degradation of RhB over 3 wt% Cu2−xSe/BiVO4 composite with added H2O2 under visible light irradiation. When a small amount of H2O2 is added (103 μL/100 mL), the 3 wt% Cu2−xSe/BiVO4 composites can degrade RhB completely in 50 min under visible light excitation. It can also be seen from Fig. 2c that the degradation efficiency is not attenuated after 3 cycles.
In order to analyze the microscopic morphology and grain size of the samples, the samples were characterized by SEM. As shown in Fig. 3a, BiVO4 is a hexagonal bulk with a particle size of 0.2–1 μm. In Fig. 3b, the area circled by the red solid line exhibits a Cu2−xSe sheet with a thickness of 300 nm and a length of 4 μm. After compounding, the Cu2−xSe sheets are randomly distributed on the surface of BiVO4 bulk. The XPS results also reveal the presence of Cu2−xSe (shown below).
Figure 4a shows the XRD data for BiVO4 and 3 wt% Cu2−xSe/BiVO4 composite, which exhibits that the BiVO4 has a monoclinic crystal structure. It can be seen that the crystal structure of BiVO4 does not change when BiVO4 is combined with Cu2−xSe. This may be due to the fact that the content of Cu is relatively too small to be detected by XRD. Photoluminescence measurement is a general way to explore the separation and combination of electrons and holes. The relatively low luminescence intensity means a high electron-hole separation efficiency [27, 28]. Figure 4b shows the PL spectra for BiVO4 and Cu2−xSe/BiVO4 composites. After BiVO4 is combined with Cu2−xSe, the relative luminescence intensity of the Cu2−xSe/BiVO4 composite is lower than that of BiVO4, which indicates that the Cu2−xSe/BiVO4 composite has higher electron-hole separation efficiency after the combination of BiVO4 and Cu2−xSe.
The surface chemical state plays an important role in determining photocatalytic performance. So XPS is used to analyze the surface element valence of the Cu2−xSe/BiVO4 composite. Figure 5a is the XPS survey spectrum of the Cu2−xSe/BiVO4 composite and pure BiVO4, from which characteristic energy of Bi, V, O, Cu, and Se can be observed for Cu2−xSe/BiVO4, and characteristic energy of Bi, V, and O can be observed for BiVO4. The peaks of 159.1 and 164.1 eV can be attributed to the binding energies of Bi 4f7/2 and Bi 4f5/2, respectively (Fig. 5b), which are derived from Bi3+ in BiVO4 . The peaks of 517.0 eV and 525.0 eV correspond to V 2p3/2 and V 2p1/2 band respectively (Fig. 5c), which are derived from the V5+ of BiVO4. The peak of 530.2 eV can be attributed to O 1 s in BiVO4 (Fig. 5d) [30, 31]. The two peaks of 58.6 eV and 53.8 eV correspond to Se 3d3/2 and Se 3d5/2, respectively (Fig. 5e) . The Cu 2p3/2 peak located at 931.9 eV corresponds to Cu0 or CuI (Fig. 5f) .
To further illustrate the separation efficiency of photo-generated electrons and holes, the sample was subjected to EIS analysis. As shown in Fig. 6, the EIS Nyquist diagram of Cu2−xSe/BiVO4 has a smaller arc radius than Cu2−xSe, indicating that Cu2−xSe/BiVO4 composites have smaller charge transfer resistance and faster interface electron transfer. [34, 35]
The reason why Cu2−xSe/BiVO4 composite exhibits high efficiency is explained as follows. As illustrated in Fig. 7, the Fermi level of Cu2−xSe and BiVO4 disagrees with each other. As a result, after the BiVO4 semiconductor surface is compounded with CuSe, the charges will be redistributed. Cu2−xSe has larger work function and lower Fermi energy, so electrons flow to Cu2−xSe from BiVO4 while holes flow the other way around. As a result, the Cu2−xSe is negatively charged and BiVO4 is positively charged until the Fermi level is equal. Meanwhile, the band structure of both materials will bend corresponding to the movement of Fermi levels. Another effect of the redistribution of carriers is the building of an inner field pointing from BiVO4 to Cu2−xSe. Both the Fermi level movement and inner field form the so-called S-scheme heterojunction between Cu2−xSe and BiVO4 . Under illumination, electrons and holes are excited in both materials. In this type of heterojunction, however, the photo-generated electrons in BiVO4 and photo-generated holes in Cu2−xSe will recombine preferentially, due to the band bending and inner field, leaving useful holes in BiVO4. The useful holes possess higher energy level, which can benefit the generation of •OH species. These •OH species can break down long chains of organic matter into small molecules. The above results indicate that loading Cu2−xSe on the surface of BiVO4 can enhance the visible light photocatalytic activity.
In summary, the Cu2−xSe/BiVO4 composites have been successfully prepared and examined for degrading organic pollutions. Experimental data shows that the photocatalytic activity is largely improved after the combination. The photocatalytic efficiency of 3 wt% Cu2−xSe/BiVO4 composite is 15.8 times that of pure BiVO4. Furthermore, after adding low concentration H2O2, RhB can be completely degraded within 50 min. The SEM and XPS results confirm the presence of Cu2−xSe in the Cu2−xSe/BiVO4 composites. The results of photoluminescence indicate that the Cu2−xSe/BiVO4 composites have higher electron-hole separation efficiency. The results of EIS indicate that Cu2−xSe/BiVO4 composites have smaller charge transfer resistance and faster interface electron transfer. This work shows that Cu2−xSe is an effective co-catalysis for the development of new composite semiconductor photocatalysts.
Scanning electron microscope
Fu JW, Yu JG, Jiang CJ, Cheng B (2018) g-C3N4-based heterostructured photocatalysts. Adv Energy Mater 8:1701503
Yang HJ, Geng L, Zhang YT, Chang G, Zhang ZL, Liu X, Lei M, He YB (2019) Graphene-templated synthesis of palladium nanoplates as novel electrocatalyst for direct methanol fuel cell. Appl Surf Sci 466:385
Xu AJ, Feng SS, Shen SJ, Liu YP, Zhong WW (2018) Enhanced visible light-responsive photocatalytic properties of ag/BiPbO2Cl nanosheet composites. Nanoscale Res Lett 13:292
Wang H, Liu RP, Li YT, Lü XJ, Wang Q, Zhao SQ, Yuan KJ, Cui ZM, Li X, Xin S, Zhang R, Lei M, Lin ZQ (2018) Durable and efficient hollow porous oxide spinel microspheres for oxygen reduction. Joule. 2:337
Zhang LY, Gong Y, Wu D, Li Z, Li Q, Zheng L, Chen W (2019) Palladium-cobalt nanodots anchored on graphene: in-situ synthesis, and application as an anode catalyst for direct formic acid fuel cells. Appl Surf Sci 469:305–311
Wang X, Cui Y, Li T, Lei M, Li J, Wei Z (2019) Recent advances in the functional 2D photonic and optoelectronic devices. Adv Opt Mater 7:1801274
Pan CS, Xu J, Wang YJ, Li D, Zhu YF (2012) Dramatic activity of C3N4/BiPO4 photocatalyst with core/shell structure formed by self-assembly. Adv Funct Mater 22:1518
Hu JL, Fan WJ, Ye WQ, Huang CJ, Qiu XQ (2014) Insights into the photosensitivity activity of BiOCl under visible light irradiation. Appl Catal B Environ 158-159:182
Fu HB, Pan CS, Yao WQ, Zhu YF (2005) Visible-light-induced degradation of rhodamine B by nanosized Bi2WO6. J Phys Chem B 109:22432
Liu B, Wang S, Mo Q, Peng L, Cao S, Wang J, Wu C, Li C, Guo J, Liu B, Chen W, Lin Y (2018) Epitaxial MoS2 nanosheets on nitrogen doped graphite foam as a 3D electrode for highly efficient electrochemical hydrogen evolution. Electrochim Acta 292:407–418
Zhong DK, Choi S, Gamelin DR (2011) Near-complete suppression of surface recombination in solar photoelectrolysis by “Co-Pi” catalyst-modified W: BiVO4. J Am Chem Soc 133:18370
Yu JQ, Kudo A (2006) Effects of structural variation on the photocatalytic performance of hydrothermally synthesized BiVO4. Adv Funct Mater 16:2163
Tokunaga S, Kato H, Kudo A (2001) Selective preparation of monoclinic and tetragonal BiVO4 with Scheelite structure and their photocatalytic properties. Chem Mater 13:4624
Wu C, Liu B, Wang J, Su Y, Yan H, Ng C, Li C, Wei J (2018) 3D structured Mo-doped Ni3S2 nanosheets as efficient dual-electrocatalyst for overall water splitting. Appl Surf Sci 441:1024–1033
Liang C, Tao N, Wang B, Lin S, Lei M (2018) Graphite carbon coated TiN nanoparticles as high durable oxygen reduction reaction catalyst in alkaline electrolyte. Micro Nano Lett 13:1260–1264
Tan HL, Amal R, Ng YH (2017) Alternative strategies in improving the photocatalytic and photoelectrochemical activities of visible light-driven BiVO4: a review. J Mater Chem A 5:16498
Tan HL, Amal R, Ng YH (2016) Exploring the different roles of particle size in photoelectrochemical and photocatalytic water oxidation on BiVO4. ACS Appl Mater Interfaces 8:28607
Guo DY, Su YL, Shi HZ, Li PG, Zhao N, Ye JH, Wang SL, Liu AP, Chen ZW, Li CR, Tang WH (2018) Self-powered ultraviolet photodetector with superhigh photoresponsivity (3.05 A/W) based on the GaN/Sn:Ga2O3 pn junction. ACS Nano 12:12827–12835
Zheng XW, Hu QT (2009) Facile synthesis and phase control of copper chalcogenides with different morphologies. Appl Phys A Mater Sci Process 94:805
Deka S, Genovese A, Zhang Y, Miszta K, Bertoni G, Krahne R, Giannini C, Manna L (2010) Phosphine-free synthesis of p-type copper(I) selenide nanocrystals in hot coordinating solvents. J Am Chem Soc 132:8912
Zheng LX, Han SC, Liu H, Yu PP, Fang XS (2016) Hierarchical MoS2 nanosheet@TiO2 nanotube array composites with enhanced photocatalytic and photocurrent performances. Small 12:1527
Ghosh A, Kulsi C, Banerjee D, Mondal A (2016) Galvanic synthesis of Cu2-XSe thin films and their photocatalytic and thermoelectric properties. Appl Surf Sci 369:525
Su JZ, Guo LJ, Bao NZ, Grimes CA (2011) Nanostructured WO3/BiVO4 heterojunction films for efficient photoelectrochemical water splitting. Nano Lett 11:1928
Booshehri AY, Goh SC-K, Hong JD, Jiang RR, Xu R (2014) Effect of depositing silver nanoparticles on BiVO4 in enhancing visible light photocatalytic inactivation of bacteria in water. J Mater Chem A 2:6209
Ke D, Peng T, Ma L, Cai P, Jiang P (2008) Photocatalytic water splitting for O2 production under visible-light irradiation on BiVO4 nanoparticles in different sacrificial reagent solutions. Appl Catal A 350:111
Zhong WW, Shen SJ, Feng SS, Lin ZP, Wang ZP, Fang BZ (2018) Facile fabrication of alveolate Cu2-xSe microsheets as a new visible-light photocatalyst for discoloration of rhodamine B. CrystEngComm 20:7851
Jiang J, Zhao K, Xiao XY, Zhang LZ (2012) Synthesis and facet-dependent photoreactivity of BiOCl single-crystalline nanosheets. J Am Chem Soc 134:4473
Lin S, Bai X, Wang H, Wang H, Song J, Huang K, Wang C, Wang N, Li B, Lei M, Wu H (2017) Roll-to-roll production of transparent silver-nanofiber-network electrodes for flexible electrochromic smart windows. Adv Mater 29:1703238
Zhang KF, Liu YX, Deng JG, Xie SH, Zhao XT, Yang J, Han Z, Dai HX (2018) Co-Pd/BiVO4: high-performance photocatalysts for the degradation of phenol under visible light irradiation. Appl Catal B Environ 224:350
Guo DY, Shi HZ, Qian YP, Lv M, Li PG, Su YL, Liu Q, Chen K, Wang SL, Cui C, Li CR, Tang WH (2017) Fabrication of beta-Ga2O3/ZnO heterojunction for solar-blind deep ultraviolet photodetection. Semicond Sci Technol 32:03LT01
Guo D, Liu H, Li P, Wu Z, Wang S, Cui C, Li C, Tang W (2017) Zero-power-consumption solar-blind photodetector based on β-Ga2O3/NSTO heterojunction. ACS Appl Mater Interfaces 9:1619–1628
Shenasa M, Sainkar S, Lichtman D (1986) XPS study of some selected selenium compounds. J Electron Spectrosc Relat Phenom 40:329
Silvester EJ, Grieser F, Sexton BA, Healy TW (1991) Spectroscopic studies on copper sulfide sols. Langmuir 7:2917
Jiang ZY, Liu YY, Jing T, Huang BB, Zhang XY, Qin XY, Dai Y, Whangbo M-H (2016) Enhancing the photocatalytic activity of BiVO4 for oxygen evolution by Ce doping: Ce3+ ions as hole traps. J Phys Chem C 120:2058
Huang QW, Tian SQ, Zeng DW, Wang XX, Song WL, Li YY, Xiao W, Xie CS (2013) Enhanced photocatalytic activity of chemically bonded TiO2/graphene composites based on the effective interfacial charge transfer through the C-Ti bond. ACS Catal 3:1477
Fu J, Xu Q, Lou J, Jiang C, Yu J (2018) Highly porous carbon nitride by supramolecular preassembly of monomers for photocatalytic removal of sulfamethazine under visible light driven. Appl Catal B Environ 243:556
We acknowledge financial support from the National Natural Science Foundation of China (Grant No. 51572183 and 51502188).
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