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Structuring porous “sponge-like” BiVO4 film for efficient photocatalysis under visible light illumination
Journal of Colloid and Interface Science 393 (2013) 126–129
Contents lists available at SciVerse ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Structuring porous ‘‘sponge-like’’ BiVO4 film for efficient photocatalysis under visible light illumination Lu Dong, Xiufang Zhang ⇑, Xiaoli Dong ⇑, Xinxin Zhang, Chun Ma, Hongchao Ma, Mang Xue, Fei Shi School of Light Industry and Chemical Engineering, Dalian Polytechnic University, Dalian 116034, China
a r t i c l e
i n f o
Article history: Received 17 August 2012 Accepted 3 November 2012 Available online 28 November 2012 Keywords: Photocatalysis BiVO4 Porous structure Visible light
a b s t r a c t The porous ‘‘sponge-like’’ BiVO4 films were prepared with the polystyrene (PS) as pore forming material and F-doped SnO2 (FTO) glass as substrate. SEM observation displayed that ‘‘sponge-like’’ BiVO4 film with interconnect pore structure was successfully obtained. DRS analysis indicated the light absorption ability of BiVO4 film was enhanced by constructing porous structure. The measurement of surface area showed that porosity could elevate the surface area of the BiVO4 film. The experiment of PEC degradation of phenol showed that the degradation rate on the porous BiVO4 film (with 200 lL) was 2.68 times as much as that on the BiVO4 film. The enhanced PEC performance was attributed to the increased photo absorption ability, elevated surface area, and more efficient reactant transfer. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction During the past decades, semiconductor photocatalytic (PC) process has been proven a promising method in the destruction of organic pollutants in wastewater due to its strong oxidation power, moderate operation temperature, and relative ‘‘green’’ final products [1–3]. Recently, considerable research was focused on the development of photocatalysts with visible light response in order to obtain high utilization efficiency of solar energy [4–8]. Monoclinic scheelite-type BiVO4, one kind of visible light photocatalyst with the energy of band gap (Eg) of 2.4 eV, has also attracted a great deal of attention for water splitting and pollutant elimination [9,10]. However, low efficiency is the bottle neck that holds up the application progress. Some efforts have been pursued to enhance the PC efficiency such as deposition of noble metals and constructing the heterojunction [11,12]. Despite all the advances in the modifications, a nonporous photocatalyst still suffers from limited light penetration, which partly accounts for its poor photoactivity [13]. One solution to solve this problem is to construct the porous material as the photocatalyst [14–17]. The channels in the porous photocatalyst could serve as light-transfer paths for the distribution of photon energy onto the large surface of inner photocatalyst particles. As a result, the light penetration distance could be lengthened, and the efficiency of photo absorption, one of the most important factors of the photocatalysis, could be enhanced. Furthermore, porosity enhances diffusion process and decreases diffusion resistance of the pollutant throughout the ⇑ Corresponding authors. Fax: +86 411 86323736. E-mail addresses: [email protected] (X. Zhang), [email protected] (X. Dong). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.11.009
channels in the photocatalyst [15]. At last, appropriate pore diameter can elevate the surface area of the photocatalyst, which is a basic requirement for an efficient photocatalyst, both to enhance the adsorption of reactants and to offer a large number of reactive sites [18,19]. Otherwise, applying a bias potential to the reaction compartment is a promising way to improve the efficiency of PC process [20]. The photogenerated electron can be drawn from work electrode (photo-anode) to the counter electrode, and the separation efficiency of photogenerated electron-hole pairs can be enhanced. Template procedures are an ideal way to control material structure including the outer morphology and size and the inner pore size and distribution [21,22]. Organic materials are commonly used as template, which are easily removed with solvent or heating procedures. Polystyrene (PS) ball has been proved useful for the formation of porous materials [23]. Herein, porous BiVO4 films were produced with the PS as the template for enhanced PC performance. The preparation and the photoelectrochemical property, especially their photoelectrocatalytic (PEC) ability in pollution controlling, was described. Phenol, a common pollutant in the industry wastewater, was chosen as a test substance to evaluate the PEC performance of the porous BiVO4 films under visible light. 2. Experimental 2.1. Preparation of porous BiVO4 films Vanadium (V) tri-i-propoxy oxide was purchased from Strem Chemicals, USA. All of the other reagents (analytical grade purity) were bought from Tianjin Kermel Chemical Reagents Development
127
Centre, China, and were used without further purification. F-doped SnO2 (FTO) glass with a thickness of 2.2 mm, which was purchased from Geao Co., China, was employed as a substrate. The porous BiVO4 films were prepared with PS as pore forming material. The procedure for preparing porous BiVO4 films was illustrated in Scheme 1. In detail, Bi(NO3)55H2O was dissolved in acetic acid (CH3COOH), and the solution was named as A, Triisopropoxyvanadium (V) Oxide was dissolved in acetylacetone (C5H8O2), and the solution was named as B. Then, B was added into A drop by drop with a 1:1 stoichiometric ratio of V to Bi, and the concentration of Bi (or V) in the obtained sol was 0.02 mol/L. And then, 25 lL, 50 lL, 100 lL, 200 lL, and 300 lL of PS balls (240 nm) were added to 10 ml sol and recorded by S25, S50, S100, S200, and S300, respectively. The sol was vigorously stirred for 0.5 h. The BiVO4 film and porous BiVO4 films were obtained by coating certain quantity of the above sol onto FTO glass and annealed in air at 500 °C for 2 h. The BiVO4 film was prepared by the same process without PS balls and was recorded by S0.
Intensity (a.u.)
L. Dong et al. / Journal of Colloid and Interface Science 393 (2013) 126–129
S300 S200 S100 S50 S25 S0 20
30
40
50
60
70
2 Theta (deg.) Fig. 1. XRD patterns of BiVO4 film and porous BiVO4 films.
BiVO4 film, it was obvious that no characteristic peaks were observed in the patterns of porous BiVO4 films.
2.2. Characterization of the prepared samples 3.2. SEM The morphology of the samples was observed using a scanning electron microscopy (SEM, Hitachi S-4800); The crystal structure of the films was investigated by XRD (Rigaku D/MAX-2400) with Cu Ka radiation, accelerating voltage of 40 kV, current of 30 mA, and UV–vis absorption spectra of the samples were recorded on a UV–vis spectrophotometer (Shimadzu, UV-2450) in the range of 300–600 nm. The specific surface area was determined by an adsorption instrument (Tristar 3000) and calculated using the linear portion of the Brunauer–Emmett–Teller (BET) model. 2.3. PEC degradation of phenol The experimental cell was a standard three-electrode configuration with the porous BiVO4 film electrode as photo-anode, a platinum foil as counter electrode, and a saturated calomel electrode (SCE) as reference electrode. The light from a 500 W Xe lamp (Shanghai Jiguang Light, China) was passed through a glass filter, which allowed wavelength above 400 nm to be incident on the photo-anode at a measured intensity of 50 mW cm 2. The initial concentration of phenol was 5 mg L 1. The potential applied to the experimental cell was 0.5 V. The concentration of phenol was determined by high-performance liquid chromatography (HPLC, Waters 2695, Photodiode Array Detector 2996) with a Sun-Fire C 18 (5 lm) reverse-phase column at 30 °C. Methanol and water (v:v = 0.55:0.45) at a flow rate of 1.0 mL min 1 served as the mobile phase.
3. Results and discussion 3.1. XRD The XRD patterns of BiVO4 film and porous BiVO4 film were shown in Fig. 1. The crystal form of the BiVO4 film could be identified to the monoclinic scheelite type by the diffraction pattern according to JCPDS Card No. 14-0688. Compared with that of the
Scheme 1. Schematic procedure for preparing porous the BiVO4 film.
Fig. 2 showed SEM images of the BiVO4 film and porous BiVO4 films. From Fig. 2a, the particle of S0 was big, and the size of them was micron grade. The particle size of all the porous BiVO4 films was decreased to the nanometer level (Fig. 2b–f). The particle size reduced demonstrated that the introduction of PS prevented the generation of the particle in the anneal process. It could be clearly seen that the PS quantity added to the sample affected the pore quantity and structure. The pores of S25 and S50 did not connect and separated to each other. But, the pores of S100 and S200 interconnected due to the increased PS quantity in sol. S200 showed the porous ‘‘sponge-like’’ morphology. This interconnected pore structure may provide convenient and efficient path for the photo and reactant transfer and expose more active sites on the surface of the BiVO4 particle to the object substance, and these two excellent features were the basic advantage to efficient PC performance. When 300 lL of PS was added, and some pores in the BiVO4 film collapsed. From the morphology of films, there was not clear template effect. The reason that the size of BiVO4 gel was larger than PS balls was speculated [24]. The balls were supposed to be present separated from the gel domains or present between the interlamellar spaces formed by the gel. After burning off the PS balls, the spaces maintained by the spheres are mostly lost, leaving only the stacks of crumpled BiVO4 particles. 3.3. DRS It is well known that the optical absorption property, which can exhibit the spectrum range the photocatalyst can absorb and the value of absorption coefficient of the photocatalyst, is recognized as the key factor in determining the PC performance of the photocatalyst. The UV–vis diffuse reflectance spectra of the BiVO4 film and porous BiVO4 films were illustrated in Fig. 3. All the films showed intense absorption in the visible light region (from 400 to 500 nm). The absorption value of all the porous films was bigger than that of the BiVO4 film, indicating that the porous structure in the film was benefit to the absorption of the light. This enhanced light-trapping effect was attributed to the multiple scattering by pores in BiVO4 films. Furthermore, the channels in the porous BiVO4 film could serve as the light-transfer paths for the distribution of more photons onto the large surface of the photocatalyst particles and promote the next photon absorption. A similar effect was reported in other porous materials [15,25,26]. The band gap absorption edges of all the films were nearly same.
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L. Dong et al. / Journal of Colloid and Interface Science 393 (2013) 126–129
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 2. SEM images (50,000) of BiVO4 films of S0 (a), S25 (b), S50 (c), S100 (d), S200 and inset is the image (100,000) (e) and S300 (f).
1.0 0.9
A.
0.8 0.7
Phenol Concentration (mg/L)
S0 S25 S50 S100 S200 S250
0.6 0.5
5
S25 S50 S100 S200 S300
4 3 2 1
0.4
0 300
350
400
450
500
550
600
Wavelength (nm) Fig. 3. UV–vis diffuse reflectance spectra of the BiVO4 film and the porous BiVO4 films.
3.4. BET surface area The BET surface area of S0, S25, S50, S100, S200, and S300 were 2.41, 4.18, 9.17, 18.7, and 8.55 m2 g 1, respectively. The BET surface area of the BiVO4 film was smaller than that of all the porous BiVO4 films, indicating that constructing porous structure could enhance the BET surface area of the BiVO4 film. Among the porous BiVO4 films, with the increase in the PS quantity from 25 lL to 200 lL in the sol, the BET surface area was enhanced. However, the BET surface area of sample S300 decreased compared to that of S200. From SEM image of S300, a part of the pores collapsed, and this may decrease the surface area.
3.5. PEC degradation of phenol Fig. 4 displayed the phenol concentration versus reaction time for PEC degradation of phenol on S0, S25, S50, S100, S200, and S300. It was found that the S200 exhibited the most excellent PEC ability. Fig. 5 showed the phenol concentration (a) and ln(C0/Ct) (b) versus reaction time for PEC and PC degradation of phenol on S0 and S200. It was directly seen from Fig. 5a that the phenol removal rate on S0 in PEC process was faster than that in PC process. In 6 h, 8.01% of phenol was degraded in PC process, while 65.5% of phenol was degraded in PEC process. The enhancement confirmed that the bias
0
1
2
3
4
5
6
Time (h) Fig. 4. The phenol concentration versus reaction time for PEC degradation of phenol on S0, S25, S50, S100, S200, and S300 under visible light.
potential applied in PEC process could efficiently separate the photogenerated electron-hole pairs by transporting the photogenerated electron to the counter electrode and therefore inhibit the combination of holes and electrons. In the mean time, the phenol removal rate on S200 (96.4%) was faster than that on S0 (65.5%) in PEC process. Under illumination with energy larger than the band gap, electrons and holes are generated. Driven by the applied potential, the electrons are transferred from the anode to the counter electrode via the external circuit, while the holes are transferred to the surface of the BiVO4 and react with phenol. The higher phenol removal rate indicated that more electrons were transferred from the porous BiVO4 films to the counter electrode, and the same quantity of holes, which would subsequently participate in the oxidation reaction, was left on the surface of the BiVO4 film. The phenol removal rates of porous BiVO4 films were all increased compared to that of the BiVO4 film, suggesting that constructing porous BiVO4 film could enhance the PEC performance of the BiVO4 film. The enhanced PEC ability could firstly attributed to the increased light absorption ability. The more photons were absorbed, the more photo-excited holes and electrons were generated and subsequently participated in the oxidation reaction, and the increased PEC ability was obtained. Furthermore, suitable pore structure in porous BiVO4 film may enhance diffusion process and decrease diffusion resistance of the pollutant in the photocatalyst.
129
3.5
(a)
5 4
S0 in PC S0 in PEC S200 in PEC
3 2
(b)
3.0
S0 in PC S0 in PEC S200 in PEC
2.5
ln(C0/Ct)
Phenol concentration (mg/L)
L. Dong et al. / Journal of Colloid and Interface Science 393 (2013) 126–129
2.0 1.5 1.0 0.5
1
0.0 0 0
1
2
3
4
5
6
0
1
2
Time (h)
3
4
5
6
Time (h)
Phenol Concentration (mg/L)
Fig. 5. The phenol concentration (a) and the variation of ln(C0/Ct) (b) versus reaction time for PEC and PC degradation of phenol on S0 and S200 under visible light.
3
photon absorption ability, elevated surface area, and more efficient reactant transfer. It is rationally confirmed that constructing porous structure is a versatile method to prepare photocatalyst with high PC ability, and porous BiVO4 film with interconnected pore network is a potential candidate for water purification application for its good PEC performance under visible light.
2
Acknowledgments
1
This work was supported by the National Science Fund China (Project No. 21107007), program for Liaoning excellent talents in university (LJQ2012049) and Program for Key Science & Technology Platform in Universities of Liaoning Province.
5 4
0 0
6
12
18
24
30
Time (h) Fig. 6. Cycling runs in the PEC degradation of phenol on S200 film under visible light.
In this view, the PEC performance may be increased. At last, the enhanced surface area of the porous BiVO4 films, which could elevate the adsorption of reactants and offer more quantity of reactive sites, was benefit to the PEC performance. The phenol degradation in PEC and PC processes is fitted for pseudo-first kinetics (Fig. 5b). The kinetic constant of phenol oxidation on S0 sample in PEC process (0.187 h 1) was 13.0 times the value in PC process (0.0144 h 1). And, in PEC process, the kinetic constants of phenol oxidation on S200 sample (0.501 h 1) were 2.68 times as much as that on S0 sample. This enhancement confirms the contribution of constructing porous structure to improving the PC ability of BiVO4. The enhancement could attribute to the increased absorption ability, elevated surface area, and efficient reactant diffusion. To evaluate the stability of the porous BiVO4 film, repeated experiments were carried out, and result was shown in Fig. 6. After five recycles for the photodegradation of phenol, no significant loss of activity was found, confirming the porous BiVO4 film was photocorroded, and the porous structure in the film was stable. 4. Conclusions The ‘‘sponge-like’’ BiVO4 film with interconnect pore network was successfully produced. The PEC degradation rate of phenol on porous BiVO4 film was higher than that on the BiVO4 film. The enhanced PEC performance was attributed to the increased
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Contents lists available at SciVerse ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Structuring porous ‘‘sponge-like’’ BiVO4 film for efficient photocatalysis under visible light illumination Lu Dong, Xiufang Zhang ⇑, Xiaoli Dong ⇑, Xinxin Zhang, Chun Ma, Hongchao Ma, Mang Xue, Fei Shi School of Light Industry and Chemical Engineering, Dalian Polytechnic University, Dalian 116034, China
a r t i c l e
i n f o
Article history: Received 17 August 2012 Accepted 3 November 2012 Available online 28 November 2012 Keywords: Photocatalysis BiVO4 Porous structure Visible light
a b s t r a c t The porous ‘‘sponge-like’’ BiVO4 films were prepared with the polystyrene (PS) as pore forming material and F-doped SnO2 (FTO) glass as substrate. SEM observation displayed that ‘‘sponge-like’’ BiVO4 film with interconnect pore structure was successfully obtained. DRS analysis indicated the light absorption ability of BiVO4 film was enhanced by constructing porous structure. The measurement of surface area showed that porosity could elevate the surface area of the BiVO4 film. The experiment of PEC degradation of phenol showed that the degradation rate on the porous BiVO4 film (with 200 lL) was 2.68 times as much as that on the BiVO4 film. The enhanced PEC performance was attributed to the increased photo absorption ability, elevated surface area, and more efficient reactant transfer. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction During the past decades, semiconductor photocatalytic (PC) process has been proven a promising method in the destruction of organic pollutants in wastewater due to its strong oxidation power, moderate operation temperature, and relative ‘‘green’’ final products [1–3]. Recently, considerable research was focused on the development of photocatalysts with visible light response in order to obtain high utilization efficiency of solar energy [4–8]. Monoclinic scheelite-type BiVO4, one kind of visible light photocatalyst with the energy of band gap (Eg) of 2.4 eV, has also attracted a great deal of attention for water splitting and pollutant elimination [9,10]. However, low efficiency is the bottle neck that holds up the application progress. Some efforts have been pursued to enhance the PC efficiency such as deposition of noble metals and constructing the heterojunction [11,12]. Despite all the advances in the modifications, a nonporous photocatalyst still suffers from limited light penetration, which partly accounts for its poor photoactivity [13]. One solution to solve this problem is to construct the porous material as the photocatalyst [14–17]. The channels in the porous photocatalyst could serve as light-transfer paths for the distribution of photon energy onto the large surface of inner photocatalyst particles. As a result, the light penetration distance could be lengthened, and the efficiency of photo absorption, one of the most important factors of the photocatalysis, could be enhanced. Furthermore, porosity enhances diffusion process and decreases diffusion resistance of the pollutant throughout the ⇑ Corresponding authors. Fax: +86 411 86323736. E-mail addresses: [email protected] (X. Zhang), [email protected] (X. Dong). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.11.009
channels in the photocatalyst [15]. At last, appropriate pore diameter can elevate the surface area of the photocatalyst, which is a basic requirement for an efficient photocatalyst, both to enhance the adsorption of reactants and to offer a large number of reactive sites [18,19]. Otherwise, applying a bias potential to the reaction compartment is a promising way to improve the efficiency of PC process [20]. The photogenerated electron can be drawn from work electrode (photo-anode) to the counter electrode, and the separation efficiency of photogenerated electron-hole pairs can be enhanced. Template procedures are an ideal way to control material structure including the outer morphology and size and the inner pore size and distribution [21,22]. Organic materials are commonly used as template, which are easily removed with solvent or heating procedures. Polystyrene (PS) ball has been proved useful for the formation of porous materials [23]. Herein, porous BiVO4 films were produced with the PS as the template for enhanced PC performance. The preparation and the photoelectrochemical property, especially their photoelectrocatalytic (PEC) ability in pollution controlling, was described. Phenol, a common pollutant in the industry wastewater, was chosen as a test substance to evaluate the PEC performance of the porous BiVO4 films under visible light. 2. Experimental 2.1. Preparation of porous BiVO4 films Vanadium (V) tri-i-propoxy oxide was purchased from Strem Chemicals, USA. All of the other reagents (analytical grade purity) were bought from Tianjin Kermel Chemical Reagents Development
127
Centre, China, and were used without further purification. F-doped SnO2 (FTO) glass with a thickness of 2.2 mm, which was purchased from Geao Co., China, was employed as a substrate. The porous BiVO4 films were prepared with PS as pore forming material. The procedure for preparing porous BiVO4 films was illustrated in Scheme 1. In detail, Bi(NO3)55H2O was dissolved in acetic acid (CH3COOH), and the solution was named as A, Triisopropoxyvanadium (V) Oxide was dissolved in acetylacetone (C5H8O2), and the solution was named as B. Then, B was added into A drop by drop with a 1:1 stoichiometric ratio of V to Bi, and the concentration of Bi (or V) in the obtained sol was 0.02 mol/L. And then, 25 lL, 50 lL, 100 lL, 200 lL, and 300 lL of PS balls (240 nm) were added to 10 ml sol and recorded by S25, S50, S100, S200, and S300, respectively. The sol was vigorously stirred for 0.5 h. The BiVO4 film and porous BiVO4 films were obtained by coating certain quantity of the above sol onto FTO glass and annealed in air at 500 °C for 2 h. The BiVO4 film was prepared by the same process without PS balls and was recorded by S0.
Intensity (a.u.)
L. Dong et al. / Journal of Colloid and Interface Science 393 (2013) 126–129
S300 S200 S100 S50 S25 S0 20
30
40
50
60
70
2 Theta (deg.) Fig. 1. XRD patterns of BiVO4 film and porous BiVO4 films.
BiVO4 film, it was obvious that no characteristic peaks were observed in the patterns of porous BiVO4 films.
2.2. Characterization of the prepared samples 3.2. SEM The morphology of the samples was observed using a scanning electron microscopy (SEM, Hitachi S-4800); The crystal structure of the films was investigated by XRD (Rigaku D/MAX-2400) with Cu Ka radiation, accelerating voltage of 40 kV, current of 30 mA, and UV–vis absorption spectra of the samples were recorded on a UV–vis spectrophotometer (Shimadzu, UV-2450) in the range of 300–600 nm. The specific surface area was determined by an adsorption instrument (Tristar 3000) and calculated using the linear portion of the Brunauer–Emmett–Teller (BET) model. 2.3. PEC degradation of phenol The experimental cell was a standard three-electrode configuration with the porous BiVO4 film electrode as photo-anode, a platinum foil as counter electrode, and a saturated calomel electrode (SCE) as reference electrode. The light from a 500 W Xe lamp (Shanghai Jiguang Light, China) was passed through a glass filter, which allowed wavelength above 400 nm to be incident on the photo-anode at a measured intensity of 50 mW cm 2. The initial concentration of phenol was 5 mg L 1. The potential applied to the experimental cell was 0.5 V. The concentration of phenol was determined by high-performance liquid chromatography (HPLC, Waters 2695, Photodiode Array Detector 2996) with a Sun-Fire C 18 (5 lm) reverse-phase column at 30 °C. Methanol and water (v:v = 0.55:0.45) at a flow rate of 1.0 mL min 1 served as the mobile phase.
3. Results and discussion 3.1. XRD The XRD patterns of BiVO4 film and porous BiVO4 film were shown in Fig. 1. The crystal form of the BiVO4 film could be identified to the monoclinic scheelite type by the diffraction pattern according to JCPDS Card No. 14-0688. Compared with that of the
Scheme 1. Schematic procedure for preparing porous the BiVO4 film.
Fig. 2 showed SEM images of the BiVO4 film and porous BiVO4 films. From Fig. 2a, the particle of S0 was big, and the size of them was micron grade. The particle size of all the porous BiVO4 films was decreased to the nanometer level (Fig. 2b–f). The particle size reduced demonstrated that the introduction of PS prevented the generation of the particle in the anneal process. It could be clearly seen that the PS quantity added to the sample affected the pore quantity and structure. The pores of S25 and S50 did not connect and separated to each other. But, the pores of S100 and S200 interconnected due to the increased PS quantity in sol. S200 showed the porous ‘‘sponge-like’’ morphology. This interconnected pore structure may provide convenient and efficient path for the photo and reactant transfer and expose more active sites on the surface of the BiVO4 particle to the object substance, and these two excellent features were the basic advantage to efficient PC performance. When 300 lL of PS was added, and some pores in the BiVO4 film collapsed. From the morphology of films, there was not clear template effect. The reason that the size of BiVO4 gel was larger than PS balls was speculated [24]. The balls were supposed to be present separated from the gel domains or present between the interlamellar spaces formed by the gel. After burning off the PS balls, the spaces maintained by the spheres are mostly lost, leaving only the stacks of crumpled BiVO4 particles. 3.3. DRS It is well known that the optical absorption property, which can exhibit the spectrum range the photocatalyst can absorb and the value of absorption coefficient of the photocatalyst, is recognized as the key factor in determining the PC performance of the photocatalyst. The UV–vis diffuse reflectance spectra of the BiVO4 film and porous BiVO4 films were illustrated in Fig. 3. All the films showed intense absorption in the visible light region (from 400 to 500 nm). The absorption value of all the porous films was bigger than that of the BiVO4 film, indicating that the porous structure in the film was benefit to the absorption of the light. This enhanced light-trapping effect was attributed to the multiple scattering by pores in BiVO4 films. Furthermore, the channels in the porous BiVO4 film could serve as the light-transfer paths for the distribution of more photons onto the large surface of the photocatalyst particles and promote the next photon absorption. A similar effect was reported in other porous materials [15,25,26]. The band gap absorption edges of all the films were nearly same.
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(a)
(b)
(c)
(d)
(e)
(f)
Fig. 2. SEM images (50,000) of BiVO4 films of S0 (a), S25 (b), S50 (c), S100 (d), S200 and inset is the image (100,000) (e) and S300 (f).
1.0 0.9
A.
0.8 0.7
Phenol Concentration (mg/L)
S0 S25 S50 S100 S200 S250
0.6 0.5
5
S25 S50 S100 S200 S300
4 3 2 1
0.4
0 300
350
400
450
500
550
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Wavelength (nm) Fig. 3. UV–vis diffuse reflectance spectra of the BiVO4 film and the porous BiVO4 films.
3.4. BET surface area The BET surface area of S0, S25, S50, S100, S200, and S300 were 2.41, 4.18, 9.17, 18.7, and 8.55 m2 g 1, respectively. The BET surface area of the BiVO4 film was smaller than that of all the porous BiVO4 films, indicating that constructing porous structure could enhance the BET surface area of the BiVO4 film. Among the porous BiVO4 films, with the increase in the PS quantity from 25 lL to 200 lL in the sol, the BET surface area was enhanced. However, the BET surface area of sample S300 decreased compared to that of S200. From SEM image of S300, a part of the pores collapsed, and this may decrease the surface area.
3.5. PEC degradation of phenol Fig. 4 displayed the phenol concentration versus reaction time for PEC degradation of phenol on S0, S25, S50, S100, S200, and S300. It was found that the S200 exhibited the most excellent PEC ability. Fig. 5 showed the phenol concentration (a) and ln(C0/Ct) (b) versus reaction time for PEC and PC degradation of phenol on S0 and S200. It was directly seen from Fig. 5a that the phenol removal rate on S0 in PEC process was faster than that in PC process. In 6 h, 8.01% of phenol was degraded in PC process, while 65.5% of phenol was degraded in PEC process. The enhancement confirmed that the bias
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Time (h) Fig. 4. The phenol concentration versus reaction time for PEC degradation of phenol on S0, S25, S50, S100, S200, and S300 under visible light.
potential applied in PEC process could efficiently separate the photogenerated electron-hole pairs by transporting the photogenerated electron to the counter electrode and therefore inhibit the combination of holes and electrons. In the mean time, the phenol removal rate on S200 (96.4%) was faster than that on S0 (65.5%) in PEC process. Under illumination with energy larger than the band gap, electrons and holes are generated. Driven by the applied potential, the electrons are transferred from the anode to the counter electrode via the external circuit, while the holes are transferred to the surface of the BiVO4 and react with phenol. The higher phenol removal rate indicated that more electrons were transferred from the porous BiVO4 films to the counter electrode, and the same quantity of holes, which would subsequently participate in the oxidation reaction, was left on the surface of the BiVO4 film. The phenol removal rates of porous BiVO4 films were all increased compared to that of the BiVO4 film, suggesting that constructing porous BiVO4 film could enhance the PEC performance of the BiVO4 film. The enhanced PEC ability could firstly attributed to the increased light absorption ability. The more photons were absorbed, the more photo-excited holes and electrons were generated and subsequently participated in the oxidation reaction, and the increased PEC ability was obtained. Furthermore, suitable pore structure in porous BiVO4 film may enhance diffusion process and decrease diffusion resistance of the pollutant in the photocatalyst.
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L. Dong et al. / Journal of Colloid and Interface Science 393 (2013) 126–129
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Fig. 5. The phenol concentration (a) and the variation of ln(C0/Ct) (b) versus reaction time for PEC and PC degradation of phenol on S0 and S200 under visible light.
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photon absorption ability, elevated surface area, and more efficient reactant transfer. It is rationally confirmed that constructing porous structure is a versatile method to prepare photocatalyst with high PC ability, and porous BiVO4 film with interconnected pore network is a potential candidate for water purification application for its good PEC performance under visible light.
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Acknowledgments
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This work was supported by the National Science Fund China (Project No. 21107007), program for Liaoning excellent talents in university (LJQ2012049) and Program for Key Science & Technology Platform in Universities of Liaoning Province.
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Time (h) Fig. 6. Cycling runs in the PEC degradation of phenol on S200 film under visible light.
In this view, the PEC performance may be increased. At last, the enhanced surface area of the porous BiVO4 films, which could elevate the adsorption of reactants and offer more quantity of reactive sites, was benefit to the PEC performance. The phenol degradation in PEC and PC processes is fitted for pseudo-first kinetics (Fig. 5b). The kinetic constant of phenol oxidation on S0 sample in PEC process (0.187 h 1) was 13.0 times the value in PC process (0.0144 h 1). And, in PEC process, the kinetic constants of phenol oxidation on S200 sample (0.501 h 1) were 2.68 times as much as that on S0 sample. This enhancement confirms the contribution of constructing porous structure to improving the PC ability of BiVO4. The enhancement could attribute to the increased absorption ability, elevated surface area, and efficient reactant diffusion. To evaluate the stability of the porous BiVO4 film, repeated experiments were carried out, and result was shown in Fig. 6. After five recycles for the photodegradation of phenol, no significant loss of activity was found, confirming the porous BiVO4 film was photocorroded, and the porous structure in the film was stable. 4. Conclusions The ‘‘sponge-like’’ BiVO4 film with interconnect pore network was successfully produced. The PEC degradation rate of phenol on porous BiVO4 film was higher than that on the BiVO4 film. The enhanced PEC performance was attributed to the increased
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