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Facile synthesis of porous Bi2O3-BiVO4 p-n heterojunction composite microrods with highly efficient photocatalytic degradation of phenol
Journal of Alloys and Compounds 688 (2016) 1080e1087
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Facile synthesis of porous Bi2O3-BiVO4 p-n heterojunction composite microrods with highly efficient photocatalytic degradation of phenol Mengmeng Mao a, Fang Chen a, Changcheng Zheng b, e, Jiqiang Ning c, Yijun Zhong a, Yong Hu a, d, * a
Institute of Physical Chemistry, Zhejiang Normal University, Jinhua, 321004, PR China Mathematics and Physics Centre, Department of Mathematical Sciences, Xi'an Jiaotong-Liverpool University, Suzhou, 215123, PR China Vacuum Interconnected Nanotech Workstation, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, 215123, PR China d State Key Lab of Silicon Materials, College of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, PR China e State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, CAS, 865 Changning Road, Shanghai, 200050, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 March 2016 Received in revised form 22 June 2016 Accepted 14 July 2016 Available online 15 July 2016
In the present work, high-quality porous Bi2O3-BiVO4 composite microrods (CMRs) with a uniform size distribution have been successfully prepared through a facile solvothermal method followed by an annealing treatment, in which Bi(OH)C2O4-BiVO4 precursors were first synthesized using Na2C2O4 and NaVO3 as starting materials and then thermally decomposed to produce porous Bi2O3-BiVO4 p-n heterojunction CMRs. The as-prepared Bi2O3-BiVO4 product was characterized by UVeVis diffuse reflectance spectroscopy (DRS) and valence-band X-ray photoelectron spectroscopy (XPS). Significantly enhanced photocatalytic activity in degrading colorless organic phenol under visible-light illumination (l > 420 nm) was observed with the porous CMRs, more than 48 times higher than that of the mixture of BiVO4 and Bi2O3, and 192 and 160 times higher than pure BiVO4 and Bi2O3, respectively. A direct Zscheme mechanism was employed to describe the transfer of photogenerated electrons and holes in the Bi2O3-BiVO4 CMR system, and the dramatically enhancement of the observed photoactivity may be attributed to the high separation efficiency of electron-hole pairs, resulting from the p-n heterojunction formed in the porous structure. © 2016 Elsevier B.V. All rights reserved.
Keywords: Composite microrods Bi2O3-BiVO4 Porous structure p-n heterojunction Solvothermal method Z-scheme mechanism
1. Introduction Nowadays, semiconductor photocatalysis has been regarded as a cost-effective and sustainable green chemical technology to eliminate organic pollutants by utilizing solar energy [1e7]. Unfortunately, most semiconductors have weak photocatalytic activity because of their poor photo-quantum efficiency [8,9]. It has been widely accepted that heterostructured semiconductors could improve visible light responsive efficiency due to the hybrid nature of the composite semiconductors which can effectively reduce the recombination rate of photogenerated electrons and holes [10e14]. Among various heterogeneous photocatalysts, those with p-n heterojunctions generally exhibit significantly enhanced
* Corresponding author. Institute of Physical Chemistry, Zhejiang Normal University, Jinhua, 321004, PR China. E-mail address: [email protected] (Y. Hu). http://dx.doi.org/10.1016/j.jallcom.2016.07.128 0925-8388/© 2016 Elsevier B.V. All rights reserved.
photocatalysis performances, resulting from expanded visible-light absorption and improved charge separation [15,16]. The internal electric field formed within the p-n junction is believed to play a decisive role [17,18], which has been thoroughly studied in the field of modern semiconductor devices, including diodes, photodiodes, and solar cells [19e21]. There can be two types of underlying mechanisms that govern carrier separation and transfer processes with considering the effect of a p-n junction (Type II). One is a double-charge transfer model, in which the photogenerated electrons transfer to the conductor with a more positive conduction band (CB) while holes transfer to the other one with a more negative valence band (VB). The other model is Z-scheme type, in which the charges recombine directly between the weaker oxidative holes and reductive electrons, leaving more oxidative holes and more reductive electrons in the heterostructure system [22,23]. Recently, bismuth-based photocatalysts have attracted tremendous attention because of their unique crystalline structures and superior optical properties [24e26]. For example, monoclinic
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scheelite bismuth vanadate (m-BiVO4), with a narrow band gap of about 2.40 eV, is an outstanding visible-light responsive photocatalyst, and has been widely applied to photodegradation of organic contaminants and investigation of photocatalytic evolution of O2 [27,28]. However, despite the advantage of visible light response, the photocatalytic ability of BiVO4 can not meet practical application needs due to its poor migration of photogenerated electrons and holes [29]. To overcome this shortcoming, the strategy of combining BiVO4 with another semiconductor to form a p-n heterojunction has been proposed, aiming at improving electronhole pair separation and interfacial charge transfer efficiency, to promote the photocatalytic activity [30]. Up to now, a series of BiVO4-based p-n heterojunction photocatalysts with various morphologies have been reported, including BiOCl-BiVO4 [31], Cu2OBiVO4 [32,33], CuO-BiVO4 [34], Co3O4-BiVO4 [35], as well as Bi2O3BiVO4 [36,37], which were prepared via different methods and exhibited enhanced photocatalytic performance. In this study, we demonstrate a facile and novel synthetic strategy developed for controlled fabrication of a particular type of bismuth-based p-n heterojunction system made of porous Bi2O3BiVO4 composite microrods (CMRs). The as-prepared porous Bi2O3BiVO4 CMRs exhibit significantly enhanced photocatalytic activity in degrading phenol under visible-light illumination. As an example, after degrading phenol for 1 h, a rate of 96.3% was achieved, which is more than 48 times greater than what was produced by the mechanical mixture of BiVO4 and Bi2O3, and 192 and 160 times better than pure BiVO4 and Bi2O3, respectively. The dramatically enhanced photocatalytic activity can be attributed to the unique porous p-n heterojunction structure which is expected to have high separation efficiency of electron-hole pairs. The band structure of the Bi2O3-BiVO4 composite system is characterized by UVeVis diffuse reflectance spectra (DRS) and valence-band X-ray photoelectron spectroscopy (XPS), and a direct Z-scheme for electrons and holes transfer mechanism is proposed to explain the enhanced photocatalytic activity. Our experiments demonstrate that the as-prepared porous Bi2O3-BiVO4 CMR is a promising photocatalyst which can be applied to water pollution. 2. Experimental section All the reagents employed in this work were of analytical grade, purchased from the Shanghai Chemical Reagent Factory and used as received, without further purification. 2.1. Synthesis of porous Bi2O3-BiVO4 CMRs To prepare Bi(OH)C2O4-BiVO4 CMR precursors, 0.35 mmol of Bi(NO3)3$5H2O and 1 g of polyvinylpyrrolidone (PVP, MW~ 58 K) were first dissolved in 15 mL of ethylene glycol (EG) under stirring to achieve mixture A. 0.28 mmol of Na2C2O4 and 0.07 mmol of NaVO3 were added into 25 mL of distilled water, to obtain solution B. Solution B was then added dropwise into solution A and kept stirring for 20 min at room temperature. The resulting mixture was sealed in a 50 mL PTFE-lined stainless-steel autoclave and heated at 180 C for 10 h. The products were collected by centrifugation and washed with ethanol and distilled water, respectively, for three times before dried at 80 C for 4 h to obtain Bi(OH)C2O4-BiVO4 CMRs. The CMR precursors were heated to 350 C in an air flow with a heating rate of 1.5 C min1 and maintained for 4 h for the aim of calcination which finally leads to the formation of porous Bi2O3-BiVO4 CMRs. For comparison, pure BiVO4 product was also prepared under the same solvothermal condition except that Na2C2O4 was not added. In addition, pure Bi2O3 product was also prepared and shown in Supporting Information.
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2.2. Characterizations Powder X-ray diffraction (XRD) measurements were performed with a Philips PW3040/60 X-ray diffractometer using Cu Ka radiation with a scanning rate of 0.06 deg s1. Scanning electron microscopy (SEM) was carried out for morphology characterization, on a Hitachi S-4800 scanning electron micro-analyzer with an accelerating voltage of 15 kV. The attached energy dispersive X-ray spectrometry (EDS) was employed for elemental analysis. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were also conducted by using a JEM-2100F field-emission TEM with samples prepared by dispersing the products in ethanol and dropping the suspension on a holey carbon net supported on copper grids. UVeVis diffuse reflectance spectra (UVeVis DRS) of the as-prepared samples were measured by using a Thermo Nicolet Evolution 500 UVeVis spectrophotometer in an absorption mode over the spectral range of 200e800 nm and absorption spectra were also measured at room temperature with a PerkinElmer Lambda 900 UVeVis spectrophotometer. N2 adsorption-desorption isotherms were obtained at 77 K on a Micrometrics ASAP 2020 surface area and porosity analyzer. And the samples were degassed in vacuum at 160 C for 4 h before the measurement. Liqui TOCII (ELEMENTAR Corporation) system was used for the measurement of total organic carbon (TOC) concentration. X-ray photoelectron spectroscopy (XPS) measurements were carried out at room temperature on a ESCALAB 250Xi X-ray photoelectron spectrometer. The photoluminescence (PL) spectra were measured using an excitation wavelength of 325 nm on a FLS 920 fluorescence spectrophotometer. The GC-MS spectra were measured using a GCMS-QP2020. The experiments were carried out under room conditions.
2.3. Photocatalytic test The photocatalytic activities of the as-prepared samples were evaluated by the degradation of phenol under visible light irradiation which was provided by a 500 W Xe lamp with a 420 nm cutoff filter. The reaction cell was placed in a sealed black box with the top open. In a typical process, 15 mg of the as-prepared sample, as the photocatalyst, was added into 20 mL of phenol solution (concentration: 25 mg/L). The solution was first dispersed in an ultrasonic bath for 5 min, then stirred for 1 h in the dark to reach adsorption equilibrium between the catalyst and the solution, and finally exposed to visible light irradiation. The resultant suspension was collected by centrifugation at a series of given times, and the phenol degradation concentrations were measured by the method of UVeVis spectroscopy.
2.4. Radical-trapping experiment In order to identify the major active species in degrading phenol, radical-trapping experiments were conducted by using three chemicals, benzoquinone (a superoxide anion radical scavenger, O 2 ), Na2C2O4 (a hole scavenger) and tert-butanol (an $OH radical scavenger). Similar to photocatalytic tests, 15 mg of the as-prepared porous Bi2O3-BiVO4 CMRs and scavengers were added into 20 mL of phenol solution (concentration: 25 mg/L). Dispersed in an ultrasonic bath for 5 min, the mixture was also stirred for 1 h in the dark to reach adsorption equilibrium between the catalyst and the solution. After irradiated with visible light, the suspensions were collected by centrifugation at given time intervals and then the concentration of phenol was measured by the UVeVis absorption method.
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3. Results and discussion The fabrication of porous Bi2O3-BiVO4 CMRs is schematically illustrated in Fig. 1. The porous Bi2O3-BiVO4 CMRs are obtained via a two-step process, in which Bi(OH)C2O4-BiVO4 CMRs are first prepared, and then the CMR precursors are transformed into porous Bi2O3-BiVO4 CMRs by thermal treatment. The Bi(OH)C2O4-BiVO4 CMR precursors are produced via a simple solvothermal method, using Bi(NO3)3$5H2O, Na2C2O4 and NaVO3 as the starting materials. They are mixed with EG and water and maintained at 180 C for 10 h to form Bi(OH)C2O4-BiVO4 CMRs. The Bi(OH)C2O4-BiVO4 CMR product is kept at 350 C for 4 h so that thermal decomposition occurs, releasing CO2 to produce Bi2O3, and porous Bi2O3-BiVO4 CMRs are therefore obtained. The morphology of the precursors is completely retained and the final products are highly porous due to the creation and release of CO2. The crystallographic structure and phase purity of the asprepared BiVO4 and porous Bi2O3-BiVO4 CMR were examined by XRD analysis, Fig. 2 shows the XRD pattern of the as-prepared porous Bi2O3-BiVO4 CMRs, which obviously consists of two sets of diffractions. In addition to the obvious body-centered monoclinic phase of BiVO4 (a ¼ 5.195 Å, b ¼ 11.70 Å, c ¼ 5.092 Å, JCPDS card no. 14-0688) diffraction peaks, the other peaks marked by triangles are well matched with the tetragonal phase of b-Bi2O3 (a ¼ b ¼ 7.742 Å, c ¼ 5.631 Å, JCPDS card no. 27-0050) [38], indicating the complete transformation of Bi2O3-BiVO4 hetero-nanostructures from Bi(OH) C2O4-BiVO4 CMRs precursor. Additionally, the XRD patterns of the as-prepared pure BiVO4 and Bi(OH)C2O4-BiVO4 CMRs precursor are shown in Fig. S1 (see Supporting Information) [39]. The morphology and microstructure of the porous Bi2O3-BiVO4 CMRs, as well as the as-prepared Bi(OH)C2O4-BiVO4 CMRs precursor, are examined by SEM as shown in Fig. 3. The low-magnification SEM image (Fig. 3a) reveals that the as-obtained Bi(OH)C2O4-BiVO4 precursor is completely composed of numerous uniform microrods (3.5e4.5 mm in length and about 1 mm in diameter), and the highmagnification image (Fig. 3b) exhibits that the surface of the Bi(OH) C2O4-BiVO4 precursor is more smooth than that of Bi2O3-BiVO4 CMR. The SEM images of the as-obtained Bi2O3-BiVO4 CMRs (Fig. 3c and d) show that the product well maintains the shape of the Bi(OH)C2O4-BiVO4 precursor, except that the surface of the microrods has been significantly modified. In contrast to the precursor microrods, the Bi2O3-BiVO4 CMRs contain a lot of wormhole-like pores. This is a result of the chemical transformation of Bi2O3BiVO4 hetero-microstructures from Bi(OH)C2O4-BiVO4 precursor
Fig. 2. XRD pattern of the as-prepared porous Bi2O3-BiVO4 CMRs.
due to the release of CO2 in thermal decomposition process. The geometrical structure of the porous Bi2O3-BiVO4 CMRs is further elucidated by TEM measurement (Fig. 3e), revealing a porous structure of the final product, in a good agreement with the SEM result. The associated EDS pattern (inset in Fig. 3e) further confirms the existence of Bi, O and V elements in the Bi2O3-BiVO4 product. Furthermore, the element contents show in Supporting Information (see Fig. S2). Fig. 3f shows a high-resolution TEM (HRTEM) image of the edge section of one porous Bi2O3-BiVO4 CMR, which clearly reveals the heterojunction region between BiVO4 and Bi2O3. The corresponding interplanar spacings are about 0.32 and 0.31 nm, which correspond to the (201) plane of tetragonal Bi2O3 and (121) plane of monoclinic BiVO4, respectively. Interestingly, it has been also found that the mixed solvent (EG and water) has an important effect on the formation of microrod morphology. No intact rod-structure was observed in our experiments when only pure EG or H2O is used as solvent (SEM images, Fig. S3, see Supporting Information). EG is one of the most frequently used organic solvents because of its amphiphilic aggregation due to its high cohesive energy, dielectric constant, and considerable hydrogen bonding ability, and EG is also a reducing agent with a relatively high boiling point [40,41]. In this case, only Bi spheres can be obtained in pure EG reaction system. Thus, the mixed solvent (EG and water) was chosen as the reaction solvent. UVeVis DRS is used to characterize the optical response of the as-obtained samples. As shown in Fig. 4a, all the Bi2O3-BiVO4 CMRs, pure BiVO4 and Bi2O3 samples exhibit light absorption in the visible-light range, whose absorption edges are located at ca.570, 580 and 520 nm, respectively. Compared with pure BiVO4, the asprepared porous Bi2O3-BiVO4 CMRs exhibit a little increased photoresponse in the whole visible-light region, suggesting that the product can utilize solar energy to obtain enhanced photocatalytic activity [42,43]. The band gap energy of a semiconductor can be evaluated by the following equation [44]:
ahn ¼ A hn Eg
Fig. 1. Schematic illustration of the formation of porous Bi2O3-BiVO4 CMRs via a solidphase reaction process.
n=2
(1)
where a, h, n, A and Eg represent the absorption coefficient, Planck's constant, light frequency, energy-independent constant, and band gap, respectively. The value of the index n depends on the characteristics of the materials: for direct band-gap semiconductors, n ¼ 1; for indirect band-gap semiconductors, n ¼ 4. For pure BiVO4,
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Fig. 3. (a, b) SEM images of the as-prepared Bi(OH)C2O4-BiVO4 CMRs; (c, d) SEM, (e) TEM and (f) HRTEM images of the as-prepared porous Bi2O3-BiVO4 CMRs. The inset in (e) shows an EDS spectrum.
n is 4, for Bi2O3, n is 1 [38]. So, the Eg values of BiVO4 and b-Bi2O3 are about 2.15 and 2.50 eV (Fig. 4b), respectively. Furthermore, valence band XPS (VB-XPS) spectra were measured to study the band structure of the two components, which were shown in Fig. 4c. The positions of the VB edge of BiVO4 and Bi2O3 are located at 2.78 eV and 2.04 eV, respectively. According to the VB edge of them, and combined with the band gap derived from DRS, the band positions of the BiVO4 and Bi2O3 can be calculated by the following empirical formula: [45]
ECB ¼ EVB Eg
(2)
where EVB is the VB potential and ECB is the CB potential. The position of the CB edges of BiVO4 and Bi2O3 are located at 0.63 eV and 0.46 eV, respectively. N2 adsorption-desorption measurement was employed to further characterize the porous structure of the as-prepared porous Bi2O3-BiVO4 CMRs. The N2 adsorptiondesorption isotherm result (Fig. 4d) can be classified as type IV isotherm with a distinct hysteresis loop, indicating the existence of abundant mesopores in the products [46,47]. The Barrett-JoynerHalenda (BJH) pore size distribution curve (inset in Fig. 4d) shows that the average size of the pores is centered around 30 nm, and the BET surface area and pore volume are calculated to be about 16.7 m2 g1 and 0.063 cm3 g1, respectively. These porous channels are anticipated to facilitate the diffusion of pollutants inside the
heterostructured catalyst, which may greatly improve the photocatalytic activity of the hybrid semiconductor system. The photocatalytic activity of the as-obtained porous Bi2O3BiVO4 CMR sample is investigated by degrading the refractory organic pollutant of phenol, and it is found that the porous Bi2O3BiVO4 CMRs also exhibit superior catalytic performance. Fig. 5a shows the absorption evolution of the degraded phenol at a series of reaction time. After irradiated with visible light for 60 min, nearly 96.3% phenol is degraded by the porous Bi2O3-BiVO4 CMRs, while pure BiVO4, Bi2O3, the mechanical mixture of BiVO4 and Bi2O3, exhibit lower degradation rates of about 0.5%, 0.6%, 2.06% (Fig. 5b), respectively. To further studied reusability of the asprepared porous Bi2O3-BiVO4 CMR photocatalyst, we collected and reused the photocatalyst for 4 cycles, and the results are shown in Fig. S4 (in the supporting information). Only a small amount loss in photocatalytic activity is observed, which might be partly caused by incomplete collection of the photocatalyst during each cycle. Additionally, the porous Bi2O3-BiVO4 CMR also exhibits highly photocatalytic efficiency for the degradation of an organic dye methyl orange (MO) (Fig. S5, in the supporting information). After irradiated for 60 min, nearly 93.2% of MO is degraded. In order to identify the major active species produced in the photodegradation process, radical-trapping measurements were also performed. Fig. 6a shows the photodegradation rate of phenol reduced by the addition of 1 mM tert-butanol under visible-light
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Fig. 4. (a) UVeVis DRS of different samples, (b) the plots of (ahn)n vs. (hn) of BiVO4, Bi2O3, (c) VB-XPS spectra of BiVO4 and Bi2O3, and (d) N2 adsorption-desorption isotherm and pore-size distribution curve of the as-obtained Bi2O3-BiVO4 CMRs.
Fig. 5. (a) UVeVis absorption spectra of phenol using porous Bi2O3-BiVO4 CMRs under visible-light irradiation. (b) Photodegradation of phenol in the presence of different samples under visible-light irradiation for 60 min.
irradiation [48], indicating that $OH radicals are not the main active oxidizing species in the photoreaction process. At the meantime, the phenol concentration remains almost similar with the use of Na2C2O4 and benzoquinone. We can therefore conclude that the photogenerated holes and O2 are the dominant reactive species contributing to the oxidative degradation of phenol. A TOC test was also performed to further investigate the activity of the porous Bi2O3-BiVO4 CMRs. Fig. 6b shows that the mineralization yield reaches a value of 51.6% after irradiating for 60 min. Obviously, the rate of TOC reduction of porous Bi2O3-BiVO4 CMRs is slower than that of the degradation, indicating that the phenol is partially decomposed into H2O and CO2. The GC-MS chromatogram was used to identify the intermediate products of degradation process [49,50]. Fig. S6 (see Supporting Information) shows the GC-MS of different sample with reaction time of 0, 30 and 60 min,
respectively, degraded by Bi2O3-BiVO4 CMR photocatalyst. At first, phenol occupied the main position. After 30 min of photocatalysis, the phenol was partly degraded to acetone, acetic acid, 1,2-ethanediol and so on. After 60 min, phenol was almost degraded completely, leaving the intermediate products. To further understand the mechanism of the enhanced photocatalytic performance, PL and electrochemical impedance spectra (EIS) were investigated for the as-obtained samples. Generally, a lower PL intensity reveals a lower recombination rate of electronhole pairs, resulting in a higher photocatalytic performance [7,51]. From Fig. S7 (see Supporting Information), it can be seen that all the samples feature a broad emission at around 432 nm under an excitation wavelength of 320 nm. The PL intensity of Bi2O3-BiVO4 CMRs is lower than that of the pure BiVO4 and Bi2O3, indicating that the as-obtained Bi2O3-BiVO4 CMR shows higher photo-induced
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Fig. 6. (a) Plots of active species trapped in the system for the photodegradation of phenol using porous Bi2O3-BiVO4 CMRs as photocatalyst, and (b) TOC degradation as a function of time for the decomposition of phenol using porous Bi2O3-BiVO4 CMRs as photocatalyst.
charge separation efficiency than that of pure samples. Fig. S8 (see Supporting Information) shows the arc radius of the Nyquist plot of the Bi2O3-BiVO4 CMRs electrode is smaller than that of the BiVO4
electrode, which suggests a better charge transfer efficiency of the Bi2O3-BiVO4 CMRs. The enhanced separation and transfer efficiency of the photogenerated carriers may account for the high
Fig. 7. Schematic diagram representing the (a) double-charge transfer mode, and (b) Z-scheme type mode in Bi2O3-BiVO4 p-n heterojunction CMRs.
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degradation activity of Bi2O3-BiVO4 CMRs [42]. Furthermore, the photocurrent transient response measurements of pure BiVO4, pure Bi2O3 and Bi2O3-BiVO4 CMRs were also carried out in a 0.1 M Na2SO4 solution under visible-light irradiation. Fig. S9 (in Supporting Information) displays the rapid and consistent photocurrent responses for each switch-on and -off event in multiple 40 s on-off cycles. It is worth to note that the photocurrent density of the Bi2O3-BiVO4 CMRs electrode (ca. 0.12e0.15 mA cm2) is higher than that of the pure BiVO4, Bi2O3 electrode (ca. 0.04e0.05 mA cm2). The enhanced photocurrent response of the as-prepared Bi2O3-BiVO4 CMRs also indicates higher separation efficiency of the photoinduced electron-hole pairs in such CMR structures [6,28]. Based on the measured bandgap structures of BiVO4 and b-Bi2O3 and the trapping experiments, a possible direct Z-scheme mechanism is proposed to explain the enhanced photocatalytic performance of the Bi2O3-BiVO4 CMRs, which is schematically illustrated in Fig. 7b. BiVO4 is known as an intrinsic n-type semiconductor with a band gap of 2.15 eV, and its CB and VB energy levels are positioned at ca. 0.63 and 2.78 eV (vs standard hydrogen electrode (NHE)). And Bi2O3 is an intrinsic p-type semiconductor with a band gap of 2.50 eV, with the CB at ca. 0.46 eV and VB at ca. 2.04 eV. After the two materials get in contact, a p-n heterojunction will form when their Fermi levels reach equilibrium state [18,52], and an inner electric field, pointing from the BiVO4 side to Bi2O3, is built across the interface. The Fermi level of p-type Bi2O3 moves up, at the meantime, that of n-type BiVO4 moves down until the equilibrium state is established, resulting in that the whole energy band of Bi2O3 is raised up and BiVO4 is descended. Under visible light irradiation (l > 420 nm), both BiVO4 and Bi2O3 could be excited, creating photogenerated electron and hole pairs. Thus, the photoexcited electrons on the CB of Bi2O3 possess a higher reducibility and photoexcited holes on the VB of BiVO4 have a stronger oxidizability in this heterostructure system. According to the conventional double-charge transfer mode, the photoinduced electrons on the CB of Bi2O3 transfer to the CB of BiVO4, while holes on the VB of BiVO4 transfer to the VB of Bi2O3. However, since O2 is one of the main active species, the CB potential of BiVO4 is lower than the standard reduction potential of O 2 /O2 (O2 þ e /O2 , E(O2/ O2) ¼ 0.33 eV). It is impossible to produce the dominant reactive species O2 through double-charge transfer mechanism in this photocatalytic process (Fig. 7a). As a result, a direct Z-scheme electrons and holes transfer mode is proposed to explain the enhanced photocatalytic activity [22,53]. In this hetero-structure system, both BiVO4 and Bi2O3 can be excited under visible-light irradiation, the holes on the VB of Bi2O3 and the electrons on the CB of BiVO4 recombine directly at their interfacial region. The electrons on the CB of Bi2O3 are captured by the surface chemisorbed O2 to give O2, and the remaining holes in BiVO4 can directly oxidize phenol. In this case, the Bi2O3-BiVO4 CMRs are a type of direct Z-scheme photocatalysts under our experimental conditions and the photoactivity is dramatically improved based on this charge transfer mode. 4. Conclusions In summary, we have demonstrated a facile solvothermal method followed by an annealing process to synthesize highquality porous Bi2O3-BiVO4 composite microrods (CMRs) with uniform size distribution. Due to high separation efficiency of electron-hole pairs caused by the p-n heterojunction, intrinsically porous structure nature, the as-prepared Bi2O3-BiVO4 CMRs exhibit excellent photocatalytic activity for the degradation of colorless organic phenol under visible-light irradiation. A Z-scheme transfer mechanism is proposed and further confirmed by the reactive species trapping experiments. We believe that this new synthesis
strategy is not restricted to the specific materials discussed in this work and can be extended to the preparation of a wide range of porous microstructures for various applications. Acknowledgements Financial support from the Natural Science Foundation of China (21171146, 21371152), Zhejiang Provincial Natural Science Foundation of China (LR14B010001), the Zhejiang Provincial Public Welfare Project (2016C31015) and the State Key Laboratory of Silicon Materials at Zhejiang University (2015-10) are gratefully acknowledged. CCZ acknowledges partial support by an open project from State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.07.128. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]
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Facile synthesis of porous Bi2O3-BiVO4 p-n heterojunction composite microrods with highly efficient photocatalytic degradation of phenol Mengmeng Mao a, Fang Chen a, Changcheng Zheng b, e, Jiqiang Ning c, Yijun Zhong a, Yong Hu a, d, * a
Institute of Physical Chemistry, Zhejiang Normal University, Jinhua, 321004, PR China Mathematics and Physics Centre, Department of Mathematical Sciences, Xi'an Jiaotong-Liverpool University, Suzhou, 215123, PR China Vacuum Interconnected Nanotech Workstation, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, 215123, PR China d State Key Lab of Silicon Materials, College of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, PR China e State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, CAS, 865 Changning Road, Shanghai, 200050, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 March 2016 Received in revised form 22 June 2016 Accepted 14 July 2016 Available online 15 July 2016
In the present work, high-quality porous Bi2O3-BiVO4 composite microrods (CMRs) with a uniform size distribution have been successfully prepared through a facile solvothermal method followed by an annealing treatment, in which Bi(OH)C2O4-BiVO4 precursors were first synthesized using Na2C2O4 and NaVO3 as starting materials and then thermally decomposed to produce porous Bi2O3-BiVO4 p-n heterojunction CMRs. The as-prepared Bi2O3-BiVO4 product was characterized by UVeVis diffuse reflectance spectroscopy (DRS) and valence-band X-ray photoelectron spectroscopy (XPS). Significantly enhanced photocatalytic activity in degrading colorless organic phenol under visible-light illumination (l > 420 nm) was observed with the porous CMRs, more than 48 times higher than that of the mixture of BiVO4 and Bi2O3, and 192 and 160 times higher than pure BiVO4 and Bi2O3, respectively. A direct Zscheme mechanism was employed to describe the transfer of photogenerated electrons and holes in the Bi2O3-BiVO4 CMR system, and the dramatically enhancement of the observed photoactivity may be attributed to the high separation efficiency of electron-hole pairs, resulting from the p-n heterojunction formed in the porous structure. © 2016 Elsevier B.V. All rights reserved.
Keywords: Composite microrods Bi2O3-BiVO4 Porous structure p-n heterojunction Solvothermal method Z-scheme mechanism
1. Introduction Nowadays, semiconductor photocatalysis has been regarded as a cost-effective and sustainable green chemical technology to eliminate organic pollutants by utilizing solar energy [1e7]. Unfortunately, most semiconductors have weak photocatalytic activity because of their poor photo-quantum efficiency [8,9]. It has been widely accepted that heterostructured semiconductors could improve visible light responsive efficiency due to the hybrid nature of the composite semiconductors which can effectively reduce the recombination rate of photogenerated electrons and holes [10e14]. Among various heterogeneous photocatalysts, those with p-n heterojunctions generally exhibit significantly enhanced
* Corresponding author. Institute of Physical Chemistry, Zhejiang Normal University, Jinhua, 321004, PR China. E-mail address: [email protected] (Y. Hu). http://dx.doi.org/10.1016/j.jallcom.2016.07.128 0925-8388/© 2016 Elsevier B.V. All rights reserved.
photocatalysis performances, resulting from expanded visible-light absorption and improved charge separation [15,16]. The internal electric field formed within the p-n junction is believed to play a decisive role [17,18], which has been thoroughly studied in the field of modern semiconductor devices, including diodes, photodiodes, and solar cells [19e21]. There can be two types of underlying mechanisms that govern carrier separation and transfer processes with considering the effect of a p-n junction (Type II). One is a double-charge transfer model, in which the photogenerated electrons transfer to the conductor with a more positive conduction band (CB) while holes transfer to the other one with a more negative valence band (VB). The other model is Z-scheme type, in which the charges recombine directly between the weaker oxidative holes and reductive electrons, leaving more oxidative holes and more reductive electrons in the heterostructure system [22,23]. Recently, bismuth-based photocatalysts have attracted tremendous attention because of their unique crystalline structures and superior optical properties [24e26]. For example, monoclinic
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scheelite bismuth vanadate (m-BiVO4), with a narrow band gap of about 2.40 eV, is an outstanding visible-light responsive photocatalyst, and has been widely applied to photodegradation of organic contaminants and investigation of photocatalytic evolution of O2 [27,28]. However, despite the advantage of visible light response, the photocatalytic ability of BiVO4 can not meet practical application needs due to its poor migration of photogenerated electrons and holes [29]. To overcome this shortcoming, the strategy of combining BiVO4 with another semiconductor to form a p-n heterojunction has been proposed, aiming at improving electronhole pair separation and interfacial charge transfer efficiency, to promote the photocatalytic activity [30]. Up to now, a series of BiVO4-based p-n heterojunction photocatalysts with various morphologies have been reported, including BiOCl-BiVO4 [31], Cu2OBiVO4 [32,33], CuO-BiVO4 [34], Co3O4-BiVO4 [35], as well as Bi2O3BiVO4 [36,37], which were prepared via different methods and exhibited enhanced photocatalytic performance. In this study, we demonstrate a facile and novel synthetic strategy developed for controlled fabrication of a particular type of bismuth-based p-n heterojunction system made of porous Bi2O3BiVO4 composite microrods (CMRs). The as-prepared porous Bi2O3BiVO4 CMRs exhibit significantly enhanced photocatalytic activity in degrading phenol under visible-light illumination. As an example, after degrading phenol for 1 h, a rate of 96.3% was achieved, which is more than 48 times greater than what was produced by the mechanical mixture of BiVO4 and Bi2O3, and 192 and 160 times better than pure BiVO4 and Bi2O3, respectively. The dramatically enhanced photocatalytic activity can be attributed to the unique porous p-n heterojunction structure which is expected to have high separation efficiency of electron-hole pairs. The band structure of the Bi2O3-BiVO4 composite system is characterized by UVeVis diffuse reflectance spectra (DRS) and valence-band X-ray photoelectron spectroscopy (XPS), and a direct Z-scheme for electrons and holes transfer mechanism is proposed to explain the enhanced photocatalytic activity. Our experiments demonstrate that the as-prepared porous Bi2O3-BiVO4 CMR is a promising photocatalyst which can be applied to water pollution. 2. Experimental section All the reagents employed in this work were of analytical grade, purchased from the Shanghai Chemical Reagent Factory and used as received, without further purification. 2.1. Synthesis of porous Bi2O3-BiVO4 CMRs To prepare Bi(OH)C2O4-BiVO4 CMR precursors, 0.35 mmol of Bi(NO3)3$5H2O and 1 g of polyvinylpyrrolidone (PVP, MW~ 58 K) were first dissolved in 15 mL of ethylene glycol (EG) under stirring to achieve mixture A. 0.28 mmol of Na2C2O4 and 0.07 mmol of NaVO3 were added into 25 mL of distilled water, to obtain solution B. Solution B was then added dropwise into solution A and kept stirring for 20 min at room temperature. The resulting mixture was sealed in a 50 mL PTFE-lined stainless-steel autoclave and heated at 180 C for 10 h. The products were collected by centrifugation and washed with ethanol and distilled water, respectively, for three times before dried at 80 C for 4 h to obtain Bi(OH)C2O4-BiVO4 CMRs. The CMR precursors were heated to 350 C in an air flow with a heating rate of 1.5 C min1 and maintained for 4 h for the aim of calcination which finally leads to the formation of porous Bi2O3-BiVO4 CMRs. For comparison, pure BiVO4 product was also prepared under the same solvothermal condition except that Na2C2O4 was not added. In addition, pure Bi2O3 product was also prepared and shown in Supporting Information.
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2.2. Characterizations Powder X-ray diffraction (XRD) measurements were performed with a Philips PW3040/60 X-ray diffractometer using Cu Ka radiation with a scanning rate of 0.06 deg s1. Scanning electron microscopy (SEM) was carried out for morphology characterization, on a Hitachi S-4800 scanning electron micro-analyzer with an accelerating voltage of 15 kV. The attached energy dispersive X-ray spectrometry (EDS) was employed for elemental analysis. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were also conducted by using a JEM-2100F field-emission TEM with samples prepared by dispersing the products in ethanol and dropping the suspension on a holey carbon net supported on copper grids. UVeVis diffuse reflectance spectra (UVeVis DRS) of the as-prepared samples were measured by using a Thermo Nicolet Evolution 500 UVeVis spectrophotometer in an absorption mode over the spectral range of 200e800 nm and absorption spectra were also measured at room temperature with a PerkinElmer Lambda 900 UVeVis spectrophotometer. N2 adsorption-desorption isotherms were obtained at 77 K on a Micrometrics ASAP 2020 surface area and porosity analyzer. And the samples were degassed in vacuum at 160 C for 4 h before the measurement. Liqui TOCII (ELEMENTAR Corporation) system was used for the measurement of total organic carbon (TOC) concentration. X-ray photoelectron spectroscopy (XPS) measurements were carried out at room temperature on a ESCALAB 250Xi X-ray photoelectron spectrometer. The photoluminescence (PL) spectra were measured using an excitation wavelength of 325 nm on a FLS 920 fluorescence spectrophotometer. The GC-MS spectra were measured using a GCMS-QP2020. The experiments were carried out under room conditions.
2.3. Photocatalytic test The photocatalytic activities of the as-prepared samples were evaluated by the degradation of phenol under visible light irradiation which was provided by a 500 W Xe lamp with a 420 nm cutoff filter. The reaction cell was placed in a sealed black box with the top open. In a typical process, 15 mg of the as-prepared sample, as the photocatalyst, was added into 20 mL of phenol solution (concentration: 25 mg/L). The solution was first dispersed in an ultrasonic bath for 5 min, then stirred for 1 h in the dark to reach adsorption equilibrium between the catalyst and the solution, and finally exposed to visible light irradiation. The resultant suspension was collected by centrifugation at a series of given times, and the phenol degradation concentrations were measured by the method of UVeVis spectroscopy.
2.4. Radical-trapping experiment In order to identify the major active species in degrading phenol, radical-trapping experiments were conducted by using three chemicals, benzoquinone (a superoxide anion radical scavenger, O 2 ), Na2C2O4 (a hole scavenger) and tert-butanol (an $OH radical scavenger). Similar to photocatalytic tests, 15 mg of the as-prepared porous Bi2O3-BiVO4 CMRs and scavengers were added into 20 mL of phenol solution (concentration: 25 mg/L). Dispersed in an ultrasonic bath for 5 min, the mixture was also stirred for 1 h in the dark to reach adsorption equilibrium between the catalyst and the solution. After irradiated with visible light, the suspensions were collected by centrifugation at given time intervals and then the concentration of phenol was measured by the UVeVis absorption method.
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3. Results and discussion The fabrication of porous Bi2O3-BiVO4 CMRs is schematically illustrated in Fig. 1. The porous Bi2O3-BiVO4 CMRs are obtained via a two-step process, in which Bi(OH)C2O4-BiVO4 CMRs are first prepared, and then the CMR precursors are transformed into porous Bi2O3-BiVO4 CMRs by thermal treatment. The Bi(OH)C2O4-BiVO4 CMR precursors are produced via a simple solvothermal method, using Bi(NO3)3$5H2O, Na2C2O4 and NaVO3 as the starting materials. They are mixed with EG and water and maintained at 180 C for 10 h to form Bi(OH)C2O4-BiVO4 CMRs. The Bi(OH)C2O4-BiVO4 CMR product is kept at 350 C for 4 h so that thermal decomposition occurs, releasing CO2 to produce Bi2O3, and porous Bi2O3-BiVO4 CMRs are therefore obtained. The morphology of the precursors is completely retained and the final products are highly porous due to the creation and release of CO2. The crystallographic structure and phase purity of the asprepared BiVO4 and porous Bi2O3-BiVO4 CMR were examined by XRD analysis, Fig. 2 shows the XRD pattern of the as-prepared porous Bi2O3-BiVO4 CMRs, which obviously consists of two sets of diffractions. In addition to the obvious body-centered monoclinic phase of BiVO4 (a ¼ 5.195 Å, b ¼ 11.70 Å, c ¼ 5.092 Å, JCPDS card no. 14-0688) diffraction peaks, the other peaks marked by triangles are well matched with the tetragonal phase of b-Bi2O3 (a ¼ b ¼ 7.742 Å, c ¼ 5.631 Å, JCPDS card no. 27-0050) [38], indicating the complete transformation of Bi2O3-BiVO4 hetero-nanostructures from Bi(OH) C2O4-BiVO4 CMRs precursor. Additionally, the XRD patterns of the as-prepared pure BiVO4 and Bi(OH)C2O4-BiVO4 CMRs precursor are shown in Fig. S1 (see Supporting Information) [39]. The morphology and microstructure of the porous Bi2O3-BiVO4 CMRs, as well as the as-prepared Bi(OH)C2O4-BiVO4 CMRs precursor, are examined by SEM as shown in Fig. 3. The low-magnification SEM image (Fig. 3a) reveals that the as-obtained Bi(OH)C2O4-BiVO4 precursor is completely composed of numerous uniform microrods (3.5e4.5 mm in length and about 1 mm in diameter), and the highmagnification image (Fig. 3b) exhibits that the surface of the Bi(OH) C2O4-BiVO4 precursor is more smooth than that of Bi2O3-BiVO4 CMR. The SEM images of the as-obtained Bi2O3-BiVO4 CMRs (Fig. 3c and d) show that the product well maintains the shape of the Bi(OH)C2O4-BiVO4 precursor, except that the surface of the microrods has been significantly modified. In contrast to the precursor microrods, the Bi2O3-BiVO4 CMRs contain a lot of wormhole-like pores. This is a result of the chemical transformation of Bi2O3BiVO4 hetero-microstructures from Bi(OH)C2O4-BiVO4 precursor
Fig. 2. XRD pattern of the as-prepared porous Bi2O3-BiVO4 CMRs.
due to the release of CO2 in thermal decomposition process. The geometrical structure of the porous Bi2O3-BiVO4 CMRs is further elucidated by TEM measurement (Fig. 3e), revealing a porous structure of the final product, in a good agreement with the SEM result. The associated EDS pattern (inset in Fig. 3e) further confirms the existence of Bi, O and V elements in the Bi2O3-BiVO4 product. Furthermore, the element contents show in Supporting Information (see Fig. S2). Fig. 3f shows a high-resolution TEM (HRTEM) image of the edge section of one porous Bi2O3-BiVO4 CMR, which clearly reveals the heterojunction region between BiVO4 and Bi2O3. The corresponding interplanar spacings are about 0.32 and 0.31 nm, which correspond to the (201) plane of tetragonal Bi2O3 and (121) plane of monoclinic BiVO4, respectively. Interestingly, it has been also found that the mixed solvent (EG and water) has an important effect on the formation of microrod morphology. No intact rod-structure was observed in our experiments when only pure EG or H2O is used as solvent (SEM images, Fig. S3, see Supporting Information). EG is one of the most frequently used organic solvents because of its amphiphilic aggregation due to its high cohesive energy, dielectric constant, and considerable hydrogen bonding ability, and EG is also a reducing agent with a relatively high boiling point [40,41]. In this case, only Bi spheres can be obtained in pure EG reaction system. Thus, the mixed solvent (EG and water) was chosen as the reaction solvent. UVeVis DRS is used to characterize the optical response of the as-obtained samples. As shown in Fig. 4a, all the Bi2O3-BiVO4 CMRs, pure BiVO4 and Bi2O3 samples exhibit light absorption in the visible-light range, whose absorption edges are located at ca.570, 580 and 520 nm, respectively. Compared with pure BiVO4, the asprepared porous Bi2O3-BiVO4 CMRs exhibit a little increased photoresponse in the whole visible-light region, suggesting that the product can utilize solar energy to obtain enhanced photocatalytic activity [42,43]. The band gap energy of a semiconductor can be evaluated by the following equation [44]:
ahn ¼ A hn Eg
Fig. 1. Schematic illustration of the formation of porous Bi2O3-BiVO4 CMRs via a solidphase reaction process.
n=2
(1)
where a, h, n, A and Eg represent the absorption coefficient, Planck's constant, light frequency, energy-independent constant, and band gap, respectively. The value of the index n depends on the characteristics of the materials: for direct band-gap semiconductors, n ¼ 1; for indirect band-gap semiconductors, n ¼ 4. For pure BiVO4,
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Fig. 3. (a, b) SEM images of the as-prepared Bi(OH)C2O4-BiVO4 CMRs; (c, d) SEM, (e) TEM and (f) HRTEM images of the as-prepared porous Bi2O3-BiVO4 CMRs. The inset in (e) shows an EDS spectrum.
n is 4, for Bi2O3, n is 1 [38]. So, the Eg values of BiVO4 and b-Bi2O3 are about 2.15 and 2.50 eV (Fig. 4b), respectively. Furthermore, valence band XPS (VB-XPS) spectra were measured to study the band structure of the two components, which were shown in Fig. 4c. The positions of the VB edge of BiVO4 and Bi2O3 are located at 2.78 eV and 2.04 eV, respectively. According to the VB edge of them, and combined with the band gap derived from DRS, the band positions of the BiVO4 and Bi2O3 can be calculated by the following empirical formula: [45]
ECB ¼ EVB Eg
(2)
where EVB is the VB potential and ECB is the CB potential. The position of the CB edges of BiVO4 and Bi2O3 are located at 0.63 eV and 0.46 eV, respectively. N2 adsorption-desorption measurement was employed to further characterize the porous structure of the as-prepared porous Bi2O3-BiVO4 CMRs. The N2 adsorptiondesorption isotherm result (Fig. 4d) can be classified as type IV isotherm with a distinct hysteresis loop, indicating the existence of abundant mesopores in the products [46,47]. The Barrett-JoynerHalenda (BJH) pore size distribution curve (inset in Fig. 4d) shows that the average size of the pores is centered around 30 nm, and the BET surface area and pore volume are calculated to be about 16.7 m2 g1 and 0.063 cm3 g1, respectively. These porous channels are anticipated to facilitate the diffusion of pollutants inside the
heterostructured catalyst, which may greatly improve the photocatalytic activity of the hybrid semiconductor system. The photocatalytic activity of the as-obtained porous Bi2O3BiVO4 CMR sample is investigated by degrading the refractory organic pollutant of phenol, and it is found that the porous Bi2O3BiVO4 CMRs also exhibit superior catalytic performance. Fig. 5a shows the absorption evolution of the degraded phenol at a series of reaction time. After irradiated with visible light for 60 min, nearly 96.3% phenol is degraded by the porous Bi2O3-BiVO4 CMRs, while pure BiVO4, Bi2O3, the mechanical mixture of BiVO4 and Bi2O3, exhibit lower degradation rates of about 0.5%, 0.6%, 2.06% (Fig. 5b), respectively. To further studied reusability of the asprepared porous Bi2O3-BiVO4 CMR photocatalyst, we collected and reused the photocatalyst for 4 cycles, and the results are shown in Fig. S4 (in the supporting information). Only a small amount loss in photocatalytic activity is observed, which might be partly caused by incomplete collection of the photocatalyst during each cycle. Additionally, the porous Bi2O3-BiVO4 CMR also exhibits highly photocatalytic efficiency for the degradation of an organic dye methyl orange (MO) (Fig. S5, in the supporting information). After irradiated for 60 min, nearly 93.2% of MO is degraded. In order to identify the major active species produced in the photodegradation process, radical-trapping measurements were also performed. Fig. 6a shows the photodegradation rate of phenol reduced by the addition of 1 mM tert-butanol under visible-light
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Fig. 4. (a) UVeVis DRS of different samples, (b) the plots of (ahn)n vs. (hn) of BiVO4, Bi2O3, (c) VB-XPS spectra of BiVO4 and Bi2O3, and (d) N2 adsorption-desorption isotherm and pore-size distribution curve of the as-obtained Bi2O3-BiVO4 CMRs.
Fig. 5. (a) UVeVis absorption spectra of phenol using porous Bi2O3-BiVO4 CMRs under visible-light irradiation. (b) Photodegradation of phenol in the presence of different samples under visible-light irradiation for 60 min.
irradiation [48], indicating that $OH radicals are not the main active oxidizing species in the photoreaction process. At the meantime, the phenol concentration remains almost similar with the use of Na2C2O4 and benzoquinone. We can therefore conclude that the photogenerated holes and O2 are the dominant reactive species contributing to the oxidative degradation of phenol. A TOC test was also performed to further investigate the activity of the porous Bi2O3-BiVO4 CMRs. Fig. 6b shows that the mineralization yield reaches a value of 51.6% after irradiating for 60 min. Obviously, the rate of TOC reduction of porous Bi2O3-BiVO4 CMRs is slower than that of the degradation, indicating that the phenol is partially decomposed into H2O and CO2. The GC-MS chromatogram was used to identify the intermediate products of degradation process [49,50]. Fig. S6 (see Supporting Information) shows the GC-MS of different sample with reaction time of 0, 30 and 60 min,
respectively, degraded by Bi2O3-BiVO4 CMR photocatalyst. At first, phenol occupied the main position. After 30 min of photocatalysis, the phenol was partly degraded to acetone, acetic acid, 1,2-ethanediol and so on. After 60 min, phenol was almost degraded completely, leaving the intermediate products. To further understand the mechanism of the enhanced photocatalytic performance, PL and electrochemical impedance spectra (EIS) were investigated for the as-obtained samples. Generally, a lower PL intensity reveals a lower recombination rate of electronhole pairs, resulting in a higher photocatalytic performance [7,51]. From Fig. S7 (see Supporting Information), it can be seen that all the samples feature a broad emission at around 432 nm under an excitation wavelength of 320 nm. The PL intensity of Bi2O3-BiVO4 CMRs is lower than that of the pure BiVO4 and Bi2O3, indicating that the as-obtained Bi2O3-BiVO4 CMR shows higher photo-induced
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Fig. 6. (a) Plots of active species trapped in the system for the photodegradation of phenol using porous Bi2O3-BiVO4 CMRs as photocatalyst, and (b) TOC degradation as a function of time for the decomposition of phenol using porous Bi2O3-BiVO4 CMRs as photocatalyst.
charge separation efficiency than that of pure samples. Fig. S8 (see Supporting Information) shows the arc radius of the Nyquist plot of the Bi2O3-BiVO4 CMRs electrode is smaller than that of the BiVO4
electrode, which suggests a better charge transfer efficiency of the Bi2O3-BiVO4 CMRs. The enhanced separation and transfer efficiency of the photogenerated carriers may account for the high
Fig. 7. Schematic diagram representing the (a) double-charge transfer mode, and (b) Z-scheme type mode in Bi2O3-BiVO4 p-n heterojunction CMRs.
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degradation activity of Bi2O3-BiVO4 CMRs [42]. Furthermore, the photocurrent transient response measurements of pure BiVO4, pure Bi2O3 and Bi2O3-BiVO4 CMRs were also carried out in a 0.1 M Na2SO4 solution under visible-light irradiation. Fig. S9 (in Supporting Information) displays the rapid and consistent photocurrent responses for each switch-on and -off event in multiple 40 s on-off cycles. It is worth to note that the photocurrent density of the Bi2O3-BiVO4 CMRs electrode (ca. 0.12e0.15 mA cm2) is higher than that of the pure BiVO4, Bi2O3 electrode (ca. 0.04e0.05 mA cm2). The enhanced photocurrent response of the as-prepared Bi2O3-BiVO4 CMRs also indicates higher separation efficiency of the photoinduced electron-hole pairs in such CMR structures [6,28]. Based on the measured bandgap structures of BiVO4 and b-Bi2O3 and the trapping experiments, a possible direct Z-scheme mechanism is proposed to explain the enhanced photocatalytic performance of the Bi2O3-BiVO4 CMRs, which is schematically illustrated in Fig. 7b. BiVO4 is known as an intrinsic n-type semiconductor with a band gap of 2.15 eV, and its CB and VB energy levels are positioned at ca. 0.63 and 2.78 eV (vs standard hydrogen electrode (NHE)). And Bi2O3 is an intrinsic p-type semiconductor with a band gap of 2.50 eV, with the CB at ca. 0.46 eV and VB at ca. 2.04 eV. After the two materials get in contact, a p-n heterojunction will form when their Fermi levels reach equilibrium state [18,52], and an inner electric field, pointing from the BiVO4 side to Bi2O3, is built across the interface. The Fermi level of p-type Bi2O3 moves up, at the meantime, that of n-type BiVO4 moves down until the equilibrium state is established, resulting in that the whole energy band of Bi2O3 is raised up and BiVO4 is descended. Under visible light irradiation (l > 420 nm), both BiVO4 and Bi2O3 could be excited, creating photogenerated electron and hole pairs. Thus, the photoexcited electrons on the CB of Bi2O3 possess a higher reducibility and photoexcited holes on the VB of BiVO4 have a stronger oxidizability in this heterostructure system. According to the conventional double-charge transfer mode, the photoinduced electrons on the CB of Bi2O3 transfer to the CB of BiVO4, while holes on the VB of BiVO4 transfer to the VB of Bi2O3. However, since O2 is one of the main active species, the CB potential of BiVO4 is lower than the standard reduction potential of O 2 /O2 (O2 þ e /O2 , E(O2/ O2) ¼ 0.33 eV). It is impossible to produce the dominant reactive species O2 through double-charge transfer mechanism in this photocatalytic process (Fig. 7a). As a result, a direct Z-scheme electrons and holes transfer mode is proposed to explain the enhanced photocatalytic activity [22,53]. In this hetero-structure system, both BiVO4 and Bi2O3 can be excited under visible-light irradiation, the holes on the VB of Bi2O3 and the electrons on the CB of BiVO4 recombine directly at their interfacial region. The electrons on the CB of Bi2O3 are captured by the surface chemisorbed O2 to give O2, and the remaining holes in BiVO4 can directly oxidize phenol. In this case, the Bi2O3-BiVO4 CMRs are a type of direct Z-scheme photocatalysts under our experimental conditions and the photoactivity is dramatically improved based on this charge transfer mode. 4. Conclusions In summary, we have demonstrated a facile solvothermal method followed by an annealing process to synthesize highquality porous Bi2O3-BiVO4 composite microrods (CMRs) with uniform size distribution. Due to high separation efficiency of electron-hole pairs caused by the p-n heterojunction, intrinsically porous structure nature, the as-prepared Bi2O3-BiVO4 CMRs exhibit excellent photocatalytic activity for the degradation of colorless organic phenol under visible-light irradiation. A Z-scheme transfer mechanism is proposed and further confirmed by the reactive species trapping experiments. We believe that this new synthesis
strategy is not restricted to the specific materials discussed in this work and can be extended to the preparation of a wide range of porous microstructures for various applications. Acknowledgements Financial support from the Natural Science Foundation of China (21171146, 21371152), Zhejiang Provincial Natural Science Foundation of China (LR14B010001), the Zhejiang Provincial Public Welfare Project (2016C31015) and the State Key Laboratory of Silicon Materials at Zhejiang University (2015-10) are gratefully acknowledged. CCZ acknowledges partial support by an open project from State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.07.128. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]
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