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Novel synthesis of highly ordered BiVO4 nanorod array for photoelectrochemical water oxidation using a facile solution process
Journal of Power Sources 436 (2019) 226842
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Novel synthesis of highly ordered BiVO4 nanorod array for photoelectrochemical water oxidation using a facile solution process Yu-Shiang Chen a, 1, Lu-Yin Lin a, b, *, 1 a b
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 1 Sec. 3, Zhongxiao E. Rd., Taipei, 10608, Taiwan Research Center of Energy Conservation for New Generation of Residential, Commercial, and Industrial Sectors, Taiwan
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Highly ordered BiVO4 nanocone is made on conductive substrate via a solution process. � The BiVO4 nanocone has the high crys tallinity and the preferable (040) crystal plane. � The possible growth mechanism of the BiVO4 nanocone array is proposed in this work. � Highest photocurrent density of 1.00 mA/cm2 at 1.23 VRHE is got for BiVO4 electrode. � BiVO4 nanocone electrode has small charge-transfer resistance/high carrier density. A R T I C L E I N F O
A B S T R A C T
Keywords: Bismuth vanadate Growth mechanism Nanorod array Photoelectrochemical Solution process Water oxidation
Bismuth vanadate (BiVO4) with the suitable band gap and band edges is one of the efficient photocatalysts for water oxidation, but the short carrier diffusion path greatly reduces its photoelectrochemical performance. Improving intrinsic properties of BiVO4 is the basic requirement to solve serious recombination and establish efficient photocatalysts. To refine the intrinsic property of BiVO4, the morphology design is the primary strategy rarely investigated in previous works. In this study, the highly ordered cone-shape BiVO4 nanorod array is firstly fabricated on the conductive substrate via a facile solution process. Reaction time of the solution process is varied to fabricate different BiVO4 nanostructures and the possible growth mechanism of BiVO4 nanocone array is proposed. The highest photocurrent density of 1.00 mA/cm2 at 1.23 V versus reversible hydrogen electrode is obtained for the BiVO4 nanocone electrode, owing to the highest crystallinity, preferable (040) crystal plane, the smallest charge-transfer resistance and the highest carrier density. Main contribution of this study is the fundamental improvement on photocatalytic ability of BiVO4 toward water oxidation. Incorporating other ma terials with BiVO4 to realize heteroatom doping or heterojunction are expected to improve the photocatalytic ability of the ordered BiVO4 nanocone array and fabricate efficient photocatalysts toward water oxidation.
* Corresponding author. Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 1 Sec. 3, Zhongxiao E. Rd., Taipei, 10608, Taiwan. E-mail address: [email protected] (L.-Y. Lin). 1 The authors contributed equally. https://doi.org/10.1016/j.jpowsour.2019.226842 Received 10 May 2019; Received in revised form 17 June 2019; Accepted 1 July 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Y.-S. Chen and L.-Y. Lin
Journal of Power Sources 436 (2019) 226842
1. Introduction
effective morphology of BiVO4 for catalyzing the water oxidation. This highly ordered one-dimensional nanostructure is also benefit on combining with other materials for doping heteroatoms or constructing heterojunction to further improve the photoelectrochemical catalytic ability of the BiVO4-based catalysts toward water oxidation.
Photoelectrochemical water splitting is one of the effective ways to produce hydrogen with high energy densities [1–7]. Accelerating the four-hole process of water oxidation is significant to improve the pho toelectrochemical water splitting efficiency, since it is considered to be the rate-limiting step in the water splitting process [8–10]. Therefore, fabricating the useful photocatalysts such as BiVO4 for promoting the water oxidation process is necessary [11,12]. BiVO4 is one of the effective photocatalysts for water oxidation, due to the suitable band gap and band edges [13]. Nevertheless, the reported photocurrent density of the pristine BiVO4 electrode is usually less than 1 mA/cm2, because the short carrier diffusion length (around 70 nm) in the BiVO4 photocatalyst would cause the serious charge recombination [14]. Numerous methods were proposed to solve this problem, such as designing well-defined morphology [15–19], doping charge-abundant heteroatoms [20–22], and constructing heterojunctions [23–25]. These methods can be used separately or combined with each other to enhance the photoelectrochemical catalytic ability of BiVO4. However, the intrinsic property of the pristine BiVO4 is requisite to be improved and therefore incorporating other materials could be more effective to enhance the photoelectrochemical performance of BiVO4 electrodes with the high catalytic ability of its pristine state. With this regard, designing the well-defined BiVO4 nanostructure without incorporating other materials is the basic requirement for establishing efficient pho tocatalysts. Kim et al. applied the electrospray method to synthesize BiVO4 nanopillars on conductive glasses for water oxidation catalysis. The BiVO4 nanopillar electrode showed a photocurrent density of 0.82 mA/cm2 at 1.2 VAg/AgCl [15]. Huang et al. fabricated BiVO4 single crystals with twin structure for photoelectrochemical catalyzing water oxidation. A photocurrent density of 3.1 mA/cm2 at 1.23 VRHE was attained [16]. Jeong et al. fabricated BiVO4 polycrystalline thin films by pulsed laser deposition for water oxidation catalysis. The photocurrent density of 3.0 mA/cm2 at 1.23 VRHE under AM 1.5G illumination was obtained [17]. Haibo et al. deposited BiVO4 thin films by reactive co-magnetron sputtering from metallic Bi and V targets as the photo catalyst for water oxidation. With the optimized thickness of BiVO4 film, the photocurrent density of 0.42 mA/cm2 at 1.23 VRHE was achieved [18]. Although several studies tried to establish well-defined BiVO4 nanostructures by using numerous methods, the fabrication of the most efficient charge-transfer routes, the one-dimensional nanostructure, is limited. There is almost no study proposing the synthesis and discussing the growth mechanism of the well-defined one-dimensional BiVO4 nanostructure as the photocatalyst for water oxidation. In this work, the novel highly ordered cone-shape BiVO4 nanorod array was successfully synthesized on the transparent conductive glass via the facile solution process, which was also reported in our previous report [26]. It was found that by simply tuning the reaction time for carrying out the solution process, the highly ordered BiVO4 nanorod array can be facilely obtained. To understand the growth of the highly ordered nanorod array more clearly, the reaction times of 1, 2, 3, 4, 5 and 6 h were applied to synthesize the BiVO4 nanostructures on conductive glasses. By analyzing the morphologies of the BiVO4 nano structures prepared using different reaction times, the growth mecha nism of the BiVO4 nanorod array was proposed in this work. The BiVO4 nanorod array electrode prepared using 5 h shows the highest photo current density of 1.00 mA/cm2 at 1.23 VRHE under AM 1.5G illumina tion. The best photoelectrochemical catalytic ability for this case is primarily owing to the most highly ordered one-dimensional structure for providing efficient routes for charge transfer. The smallest charge-transfer resistance and the highest carrier density for this case also confirm this inference. The co-catalyst of NiOOH was deposited on the highly ordered BiVO4 nanorod array to enhance the long-term sta bility. The photocurrent retention of 78.2% was obtained for the opti mized NiOOH/BiVO4 electrode after continuously illuminating the sample for 900 s. This work successfully synthesized the novel and
2. Experimental 2.1. Synthesis of BiVO4 nanostructures using the solution process The BiVO4 nanostructures were grown on the fluorine-doped tin oxide (FTO) glass (Nippon Sheet Glass, 8–10 Ω/□, 2.2 mm-thick), which was cleaned by using neutral cleaner, deionized water (DIW), acetone, and acetonitrile for 15 min under sonication, sequentially. According to our previous study [26], the BiVO4 nanostructure was grown on FTO glasses via a two-step process. The first step is the seed layer growth, and the second step is the BiVO4 nanostructure growth on the top of the seed layer. In a typical synthesis, a spin-coating method was used to deposit BiVO4 precursor on the FTO glass. The solution for depositing the seed layer is referred to our previous work [26]. The spin-coating process includes the first step with 500 rpm for 10 s and the second step with 4000 rpm for 40 s. After the deposition of the precursor solution, the electrode was dried at 60� C for 15 min in vacuum oven and then annealed at 400� C for 5 h in air with the heating rate of 10� C/min. Subsequently, the BiVO4 nanostructure was synthesized on the BiVO4 seed layer using the solution process. The suspension for carrying out the solution process contains 2 mmol Bi(NO3)3 (Acros Organics, 98%), 2 mmol NH4VO3 (Acros Organics, 99.5%) and 14.3 g NaHCO3 (Showa chemical, 99.5%) in 50 mL of 23.3% HNO3 aqueous solution (Honey well, 65%). The seed layer-deposited FTO glass was placed side-up in an Erlenmeyer flask with the suspension, which was put in a temperature controlled water bath connected to a condenser as a reflux system. The synthesis of BiVO4 nanostructure was carried out at 60� C for different reaction times under stirring. After the reaction, the BiVO4 electrodes were calcined at 500� C for 30 min in air with the heating rate of 10� C/min. 2.2. Material characterization and electrochemical measurements The morphology of BiVO4 nanostructures was examined using the field-emission scanning electron microscopy (FE-SEM, Nova NanoSEM 230, FEI, Oregon, USA). The composition of BiVO4 nanostructures was analyzed using the X-ray diffraction (XRD, X’Pert 3 Powder, PAN alytical) pattern). The light absorption of BiVO4 nanostructures was evaluated using ultraviolet–visible (UV–vis) spectroscopy (V-750, Jasco). The photoelectrochemical performance of BiVO4 electrodes was carried out using a potentiostat/galvanostat (PGSTAT 204, Autolab, Eco–Chemie, the Netherlands) with the three-electrode electrochemical system. The working electrode is the BiVO4 electrode, the counter electrode is the Pt wire, and the reference electrode is the Ag/AgCl electrode. The aqueous solution of 0.5 M Na2SO4 (Showa chemical, 99%) and 0.1 M phosphate buffer solution [26] (Acros Organics, 99%, PBS) (pH~7.2) was used as the electrolyte. All potential reported was calculated vs. RHE using Equation (1). E (vs. RHE) ¼ E (vs. Ag/AgCl) þ 0.05916 � 7 þ 0.197 ¼ E (vs. Ag/AgCl) þ 0.611 (1) The light source for simulated sunlight 300-Watt xenen lamp equipped with an air mass 1.5-global (AM 1.5G) filter (Newport). The Nyquist plot and the Mott-Schottky plot were measured using the elec trochemical impedance spectroscopy (EIS) respectively with the fre quency range from 0.1 to 15,000 Hz and 100 Hz at 1.23 VRHE under illumination, which was carried out using the potentiostat/galvanostat (PGSTAT 204, Autolab, Eco–Chemie, the Netherlands) equipped with an FRA2 module. 2
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Journal of Power Sources 436 (2019) 226842
Fig. 1. The top-view SEM images for the BiVO4 nanostructures synthesized using the reaction time of (a) 1, (b) 2, (c) 3, (d) 4, (e) 5, and (f) 6 h. The scale bar is 2 μm.
Fig. 2. The side-view SEM images for the BiVO4 nanostructures synthesized using the reaction time of (a) 1, (b) 2, (c) 3, (d) 4, (e) 5, and (f) 6 h. The scale bar is 1 μm.
3. Results and discussion
become denser and some of them even connected with each other to form a concreted film. It is inferred that the new ingredients tended to fill in the gaps between the rods at the fourth hour of the solution pro cess. At the reaction time of 5 h, the perfect cone-shape nanorods with the bottom diameter of around 500 nm were obtained. As observed in the inserted figure, the side surface of the cone even shows small pro trusions. The rough surface with several protrusions is expected to provide larger surface area for carrying out more water oxidation re actions. Finally, when the reaction was carried out for 6 h, the much denser aggregations were observed for the BiVO4 nanostructure with shallow gaps in-between. To more clearly observe the morphology variation of the BiVO4 nanostructure synthesized using different reac tion times, the side-view SEM images were shown in Fig. 2. Fig. 2(a)–(f) presents the side-view SEM images for the BiVO4 nanostructures pre pared using the reaction times of 1, 2, 3, 4, 5, and 6 h, respectively. The very flat surface and the uniform thickness of around 350 nm were ob tained for the nanostructure prepared using 1 h. The nanostructure shows some protrusions at the surface and the thickness of 500 nm was obtained when 2 h of the reaction time was used. Furthermore, the morphology of the BiVO4 nanostructure varied in a large extent when the reaction time of 3 h was applied for the synthesis. The very long protrusions with the average length of around 2 μm and the bottom layer with the thickness of 900 nm were found for this case. At this stage, not only the total thickness of the BiVO4 nanostructure increases quickly, but also several nanorods with much longer sizes were formed. It is inferred that the growth of the BiVO4 nanostructure could be restricted
3.1. The material characterizations of BiVO4 nanostructures To understand the growth of BiVO4 nanostructures, different reac tion times were applied to fabricate BiVO4 nanostructures on the FTO glass using the solution process. The morphology was first examined by using the SEM images. Fig. 1(a)–(f) respectively shows the SEM images for the BiVO4 nanostructures prepared using the reaction times of 1, 2, 3, 4, 5, and 6 h. When the reaction time of only 1 h was applied for the synthesis, the flat surface with several densely packed particles was observed. Compared to the seed layer shown in the insert of Fig. 1(a), the pores in the seed layer are invisible when the 1 h solution process was carried out for growing the nanostructure. Also, the size of the packed BiVO4 nanoparticles obtained using 1 h is found to be much larger than the size of the seeds. Therefore, the growth of the nanostructure is inferred to be fast and complete in the first hour. Further increasing the reaction time to 2 h, several smaller nanoparticles were grown on the surface of the packed nanostructure obtained at the first 1 h. It is inferred that the core centers may form at the high energy regions on the surface. The smaller particles with the size of around one fourth of those ob tained at the first 1 h may grow on the core centers to reduce the surface energy. For the BiVO4 nanostructure prepared using 3 h, more small nanoparticles kept growing on the core centers, and several protrusions and long rods were formed in the high energy regions. For the BiVO4 nanostructure synthesized using 4 h, the distribution of the nanorods 3
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Journal of Power Sources 436 (2019) 226842
Scheme 1. Illustration for the growth of the BiVO4 nanostructure synthesized using the solution process.
and the core centers could become more active to attract new in gredients growing on these regions when the thickness of the BiVO4 nanostructure reaches 900 nm. In addition, the BiVO4 nanostructure changes greatly at the reaction time ranged from 3 to 4 h. The thickness of the BiVO4 nanostructure increases to 2 μm when 4 h were applied for the synthesis. The thickness of 2 μm is similar to the average length of the protruding nanorods for the BiVO4 nanostructure prepared using 3 h. Although there are still several protrusions observed in the sample prepared using 4 h, the thickness become more uniform for this case compared to that obtained using 3 h as the reaction time. Hence, it is very possible that the new ingredients are tended to fill in the gaps be tween the nanorods and form a film with the more uniform thickness at the reaction time ranged from 3 to 4 h. Further increasing the reaction time to 5 h, the highly ordered core-shape nanorod array was found to grow on the FTO layer. The thickness of the highly ordered nanorod array is only slightly larger than the thickness of the BiVO4 nano structure synthesized using 4 h. This phenomenon suggests that the growth of the film thickness is somehow restricted at this moment. Instead of increasing the thickness of the BiVO4 film, the BiVO4 crys talline seems to reorganize and form the highly ordered nanorods with the wide bottom and the sharp top. Eventually, when 6 h was used for synthesizing the BiVO4 nanostructure, the thickness of the film is slightly reduced to be the similar size of the thickness for the BiVO4 film pre pared using 4 h. This result indicated that after 4 h the thickness of the film stops to change greatly. The new ingredients in the solution may be used to fill in the gaps and reorganize the nanostructure. For the BiVO4 nanostructure synthesized using 6 h, the perfect cone-shape rods are hard to observe. Instead, the rods became larger and aggregated with neighbor rods to form a continuous film. After carefully discussing the
morphology variation of the BiVO4 nanostructure prepared using different reaction times, the growth mechanism of the highly order nanorod array was proposed, as shown in Scheme 1. The ions of Bi3þ, BiOþ and VO3 are supplied in the solution for growing the BiVO4 nanostructure on the FTO glass. At the first hour of the solution process, the precursor ions were deposited on the FTO glass. Since the entire surface of the FTO glass possesses the same electrical conductivity, the deposition of precursor ions is inferred to be uniform. After the pre cursor ions fully depositing on the FTO glass and forming a densely aggregated film, some high energy regions started to act as the cores in the second hour of the solution process for further growing the nano structure in the vertical direction. At the third hour of the solution process, the ions kept grew at the high energy regions and formed very long rods on the densely packed film. It is inferred that at this moment the crystallized monoclinic BiVO4 could be formed along with the growth of the long rods. At the fourth hour of the solution process, the growth of rods at the high energy region is saturated. The more ions complemented in the solution tend to fill in the gaps between long rods. At this time rods are composed of small nanoparticles, which would generate several grain boundaries in-between. Further increasing the solution process time to 5 h, the heat energy supplied by the hot plate and the extra precursor ions cause the reorganization of the nanoparticle-assembled nanorods. The perfect highly ordered coneshape nanorod array was obtained for this case with much less grain boundaries in-between. More ions supplied at the sixth hour are inferred to destroy the cone-shape nanorod array and form the aggregated nanorod array. The composition of the BiVO4 nanostructures prepared using different reaction times was further confirmed by using the XRD
Fig. 3. The XRD patterns for the (a) BiVO4 nanostructures synthesized using different reaction times and (b) the BiVO4 seed layer. 4
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Journal of Power Sources 436 (2019) 226842
Fig. 4. (a) The UV–vis spectra and (b) the Tauc plot for the BiVO4 electrodes synthesized using different reaction times.
patterns, as shown in Fig. 3(a). The pound signs indicate the peaks for FTO, which is contributed from the conductive substrate. The standard pattern of monoclinic BiVO4 (JCPDS #01-083-1699) was also presented in this figure for comparison. The patterns of the BiVO4 nanostructures prepared using 3, 4, 5, and 6 h present consistent peaks to those in the standard pattern, indicating the successful growth of monoclinic BiVO4 synthesized using the solution process with the reaction times longer than 3 h. No obvious peaks corresponding to BiVO4 were obtained in the patterns of the BiVO4 nanostructures prepared using 1 and 2 h. This phenomenon indicates that the formation of BiVO4 is not complete with the reaction time of only 2 h. This inference could also be revealed by their relatively flat structures as observed in the SEM images. The peaks in the patterns of the BiVO4 nanostructure prepared using 1 and 2 h are consistent to those of the peaks in the pattern of the BiVO4 seed layer (Fig. 3(b)). This phenomenon suggests that at the first 2 h of the solution process no crystallized semiconductor was formed on the seed layercoated FTO glass, so the same XRD patterns were obtained for the seed layer/FTO electrode and those prepared using 1 and 2 h of the solution process. On the other hand, the peak ratio of (040) to (121) is a significant index to evaluate the catalytic ability of the BiVO4 photo catalyst [27–29]. Among the monoclinic BiVO4 electrodes, the highest (040) to (121) peak ratio of 28 was obtained for the BiVO4 electrode prepared using 5 h, inferring the best photocatalytic ability for this case.
However, further photoelectrochemical performance measurement should be carried out for testing the photocatalytic ability for these samples to confirm this inference. Other than the morphology and composition, the optical property is another significant factor for an efficient photocatalyst. Fig. 4(a) shows the absorption spectra for the BiVO4 nanostructures synthesized using different reaction times. The light absorption edge of around 520 nm was obtained for the electrodes prepared using 3, 4, 5, and 6 h, indi cating their monoclinic BiVO4 features as also observed from the XRD analysis. The electrodes prepared using 1 and 2 h present almost no absorption in the visible region. This phenomenon could be attributed to the morphology and the composition. The flat structure for the samples prepared using 1 and 2 h is not benefit on light absorption, and the lack of monoclinic BiVO4 feature also leads to the worse light absorption for these samples. On the other hand, the band gaps of the photocatalysts were examined by using the Tauc plots (Fig. 4(b)), which were obtained by using the absorption spectra and the Tauc equation [30]. The x-intercept of the tangent line is index to the band gap of the photo catalyst. The band gaps of around 2.4 eV were obtained for the BiVO4 electrodes prepared using 3, 4, 5, and 6 h, indicating the monoclinic BiVO4 features for these samples. The samples prepared using 1 and 2 h show larger band gaps, which again indicate the lack of monoclinic BiVO4 feature for these cases.
Fig. 5. (a) The transient photocurrent plot, (b) the LSV plot, (c) the photoconversion efficiency curves at 1.23 VRHE, (d) the Nyquist plot, and (e) the Mott-Schottky plot of the BiVO4 electrodes synthesized using different reaction times; (f) the current measurement for the stability test for the BiVO4 electrodes synthesized using 5 h with the NiOOH co-catalyst deposition. The inserted figure is the SEM image for the NiOOH/BiVO4 electrode after stability test. 5
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Journal of Power Sources 436 (2019) 226842
3.2. Photoelectrochemical performances of BVO electrodes made using different reaction times
the BiVO4 electrode prepared using 5 h. To understand the causes of the photocatalytic ability variation for the BiVO4 electrode prepared using different reaction times more clearly, the charge-transfer resistances of the electrodes was further examined by using the EIS technique. The Nyquist plot for the BiVO4 electrodes prepared using different reaction times was shown in Fig. 5 (d). The equivalent circuit was inserted in this figure to fit the chargetransfer resistance (Rct), which could be estimated by using the diam eter of the semicircle at the high frequency region [35]. The BiVO4 electrodes prepared using the reaction times of 1 and 2 h show the largest semicircle diameters respectively corresponding to the highest Rct values of 2.50 and 1.57 kΩ, mainly due to the failure of fabricating monoclinic BiVO4 using the reaction times less than 2 h and probably to the compact structure for transferring charges in random ways. On the other hand, the semicircle diameter in the Nyquist curve decreases for the BiVO4 electrodes prepared using longer reaction times from 3 to 5 h. The Rct values of 0.73, 0.56 and 0.40 kΩ were respectively obtained for the BiVO4 electrodes prepared using 3, 4, and 5 h. The smallest Rct value for the BiVO4 electrode prepared using 5 h is owing to the highly ordered nanorod array structure to provide efficient charge-transfer path and the high crystallinity with the high (040) to (121) peak ratio to display the high photocatalytic ability toward water oxidation. In addition, among the curves of the BiVO4 electrodes, the BiVO4 electrode prepared using 6 h presents the largest semicircle along with the largest Rct value of 0.80 kΩ. This result may be due to the random growth of the nano structure with several rods aggregated with each other, which could hinder the effective transfer of charges and hence increase the charge-transfer resistance for this case. Effective charge transfer is one of the important abilities for an efficient electrode in the photoelectrochemical system. Similarly, the number of charges generated in the photocatalyst is of the great sig nificance to influence the photoelectrochemical performance of the electrode. The carrier densities of the BiVO4 electrodes were further estimated by using the Mott-Schottky plot, as shown in Fig. 5(e). The carrier density was calculated by using the slope of the fitting lines in the Mott-Schottky plot and the Mott-Schottky equation [36]. The carrier density of 1.87 � 1017, 2.56 � 1017, 3.78 � 1017, 5.62 � 1017, 8.33 � 1017 and 5.95 � 1017 cm 3 were respectively obtained for the BiVO4 electrodes prepared using 1, 2, 3, 4, 5 and 6 h. The highest carrier density was obtained for the BiVO4 electrode prepared using 5 h. This carrier density is higher than 4-fold to the smallest carrier density ob tained for the BiVO4 electrode prepared using 1 h. It is significant to emphasize that the carrier density could be largely enhanced with the well-defined structure and well-designed composition. The highly or dered nanorod structure could allow efficient light absorption, and the charge recombination possibility could be largely reduced in the highly ordered and highly crystallized structure of the BiVO4 electrode pre pared using 5 h. This result also supports the best photocatalytic ability of BiVO4 electrode prepared using 5 h. Also, the trend of the carrier density in the electrodes is the same as their corresponding photocurrent densities, suggesting that the carrier density is one of the dominated factors to determine the photoelectrochemical catalytic ability of the photocatalysts toward water oxidation in our cases. Last but not least, the long-term stability of the optimized BiVO4 electrode was further examined to understand the endurance of the photocatalytic ability. To enhance the long-term stability, the cocatalyst of NiOOH was deposited on the BiVO4 electrode by using the method referred to our previous work [37]. The long-term stability of the electrode was measured under the continuous light illumination for 900 s at the potential of 1.23 VRHE. Fig. 5(f) shows the plot for the photocurrent density of the BiVO4 electrode as a function of time. The increased current at the beginning of the stability test is due to the sudden separation of charges at the light-on moment. The subsequent current decreases with longer time is infected to be caused by the recombination of charges. The photocurrent density of the NiOOH-decorated BiVO4 electrode maintained 78.2% of the initial
After examining the physical properties of the BiVO4 nanostructures, the catalytic ability of the materials under light illumination was further investigated. Fig. 5(a) shows the transient current measurements for the BiVO4 electrodes prepared using different reaction times. All the elec trodes present sudden enhancements on the photocurrent when the light was turned on. Then the current reduced to zero suddenly when the light was turned off. This phenomenon suggests the photocatalytic ability for all the electrodes. It is found that the BiVO4 electrodes prepared using 4 and 5 h presents abnormal shapes for their transient current curves, which were also observed in the previous literature [31–34]. The photocurrent increases at the light-on moment and reaches a steady-state with the continuous illumination. The photocurrent density of the electrode may probably decrease again when longer light illu mination was applied. This phenomenon could be attributed to the dy namics of the faster carrier and the subsequent slow rise to the dynamics of the slower carrier [33]. After confirming the photocatalytic feature of the electrodes, it is important to evaluate the photoelectrochemical catalytic ability of the BiVO4 electrodes by using the LSV curves, as shown in Fig. 5(b). The photocurrent density of 0.02, 0.11, 0.34, 0.81, 1.00 and 0.29 mA/cm2 at 1.23 V vs. RHE were respectively obtained for the BiVO4 electrodes synthesized using 1, 2, 3, 4, 5 and 6 h for the so lution process. The photocurrent densities of the electrodes prepared using 1 and 2 h are much smaller than those of the BiVO4 electrodes prepared using 4–6 h, since the successful synthesis of monoclinic BiVO4 can only be achieved using the reaction times longer than 3 h. For the BiVO4 electrodes, the photocurrent density increases for the samples prepared using longer reaction times, until the optimized reaction time of 5 h was reached. The BiVO4 electrode prepared using the reaction time of 4 h shows a higher photocurrent density than that of the BiVO4 electrode prepared using the reaction time to 3 h. This phenomenon is primarily due to the thicker film and the more protrusions in the nanostructure for the BiVO4 electrode prepared using the reaction time of 4 h. These properties could promote the light absorption and electron excitation. Furthermore, the highest photocurrent density of the BiVO4 electrode prepared using reaction time of 5 h is primarily due to the highly ordered nanorod array, which is able to reduce the grain boundaries and the charge recombination possibilities. However, the photocurrent density of the BiVO4 electrode prepared using 6 h is smaller than that of the BiVO4 electrode prepared using 5 h. This result could be attributed to the structure variation. As observed in the SEM images, the well-defined core-shape BiVO4 nanorod array can only be obtained by using 5 h as the reaction time. The aggregations happened again when 6 h were applied for fabricating the BiVO4 electrode. On the other hand, the onset potentials of 0.54, 0.39, 0.15, 0.18, 0.16 and 0.02 VRHE were obtained for the BiVO4 electrodes prepared using 1, 2, 3, 4, 5, and 6 h. The smaller onset potential suggests the smaller driving force required for starting the water oxidation process, which was attained for the BiVO4 electrodes prepared using longer reaction times. For the layer with the thickness smaller than the charge diffusion length, the thicker BiVO4 layer could catalyze more water oxidation reactions. Also, the more well-defined nanostructure could provide more efficient charge-transfer routes for reducing the driving force required to initiate the oxidation reaction. Hence, the reduced onset potentials for the BiVO4 electrodes prepared using longer reaction times is reasonable except for the sample prepared using 6 h. The reason for the smallest onset potential of the BiVO4 electrode prepared using 6 h is not clear and will be investigated in our future work. Furthermore, the photo conversion efficiency was evaluated using the LSV curves, as presented in Fig. 5(c). The maximum photoconversion efficiency increases for the BiVO4 electrode prepared using longer reaction times, and achieves the highest maximum photoconversion efficiency of 0.67% for the BiVO4 electrode prepared using 5 h. This result is consistent with their photo current densities, and again indicates the best photocatalytic ability for 6
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transfer resistance and the highest carrier density were also obtained for the optimized BiVO4 electrode. The excellent long-term stability was also achieved for the optimized BiVO4 electrode with the co-catalyst NiOOH deposition. The photocurrent density retention of 78.2% was achieved after 900 s continuous light illumination. Exclusive of incor porating other materials to enhance the photocatalytic ability of BiVO4, the performance achieved in this work is promising. It is expected that by adding other materials in this system, the better photocatalytic per formance can be achieved in our future works.
Table 1 A partial list of the morphology, the synthesis method, and the photocurrent density of the BiVO4 electrode reported in the previous literature and obtained in this work. Morphology
Synthesis Method
Photocurrent density
Onset potential
Ref.
Twin structure
Ionic exchange
0.23 VRHE
[16]
Polycrystalline
0.31 VRHE
[17]
Nanofern
Pulsed laser deposition Electrospray
0.92 VRHE
[15]
Nanopillar
Electrospray
0.87 VRHE
[15]
Porous film
0.40 VRHE
[19]
Microsphere
Comagnetron sputtering Solid-state synthesis
0.69 VRHE
[13]
Nanorod array
Solution process
3.10 mA/cm2 @1.23 VRHE 3.00 mA/cm2 @1.23 VRHE 1.23 mA/cm2 @1.2 VAg/AgCl 0.82 mA/cm2 @1.2 VAg/AgCl 0.42 mA/cm2 @1.23 VRHE 0.02 mA/cm2 @1.23 VRHE 1.00 mA/cm2 @1.23 VRHE
0.12 VRHE
This work
Acknowledgements This work is partially supported by the Ministry of Science and Technology, Taiwan under grant number of 107-2636-E-027-003. This work was financially supported by the “Research Center of Energy Conservation for New Generation of Residential, Commercial, and In dustrial Sectors” from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan.
photocurrent density after 900 s continuous light illumination. The inserted figure is the SEM image of the NiOOH-decorated BiVO4 elec trode after the long-term stability test. The morphology remained similar to that without conducting the long-term stability test (Fig. 1(e)). This result suggests the excellent long-term stability for the optimized BiVO4 electrode proposed in this study. In this work, the novel BiVO4 structure of the highly ordered nanorod array was proposed and applied as the photocatalyst for efficient water oxidation. To have fair and complete comparison, the morphology of the BiVO4 photocatalyst, the method for synthesizing the photocatalyst, and the resulting photocurrent density for evaluating the photocatalytic ability reported in the previous literature and obtained in this work were listed in Table 1. It is worthy to mention that there are many reports tried to design efficient morphology of BiVO4 by incorporating other materials for doping or establishing heterojunction. However, the main purpose in this work is to fabricate the pristine BiVO4 electrode with the excellent photocatalytic ability. Hence, the literature lists in Table 1 only include the information of the pristine BiVO4 electrodes. Lots of the photocurrent densities cannot achieve the value higher than 1 mA/cm2 in the previous literature. The photocurrent density reported in this work is competing to most of the values reported in previous literature although some of the higher photocurrent densities have been previ ously proposed. Also, the highly ordered nanorod array is firstly pro posed in this work and the solution process is facile for fabricating the highly ordered BiVO4 nanorod array in this work. It is expected that the higher photocurrent density and hence the better photoelectrochemical catalytic ability of the BiVO4 electrode can be further achieved in our future works by tuning the precursor composition, the precursor con centration, the reaction temperature and even the system configuration for carrying out the solution process.
References [1] T.-R. Kuo, Y.-C. Lee, H.-L. Chou, S.M.G., C.-Y. Wei, C.-Y. Wen, Y.-H. Chang, X.Y. Pan, D.-Y. Wang, Chem. Mater. 31 (2019) 3722–3728. [2] Y. Zhu, H.-C. Chen, C.-S. Hsu, T.-S. Lin, C.-J. Chang, S.-C. Chang, L.-D. Tsai, H. M. Chen, ACS Energ. Lett. 4 (2019) 987–994. [3] S.-F. Hung, Y.-T. Chan, C.-C. Chang, M.-K. Tsai, Y.-F. Liao, N. Hiraoka, C.-S. Hsu, H. M. Chen, J. Am. Chem. Soc. 140 (2018) 17263–17270. [4] S.-C. Lin, C.-S. Hsu, S.-Y. Chiu, T.-Y. Liao, H.M. Chen, J. Am. Chem. Soc. 139 (2017) 2224–2233. [5] M. Ahmed, I. Dincer, Int. J. Hydrogen Energy 44 (2019) 2474–2507. [6] S.D. Tilley, Adv. Energy. Mater 9 (2019) 1802877. [7] T.J. Jacobsson, Energy Environ. Sci. 11 (2018) 1977–1979. [8] K. Afroz, M. Moniruddin, N. Bakranov, S. Kudaibergenov, N. Nuraje, J. Mater. Chem. 6 (2018) 21696–21718. [9] J. Gan, X. Lu, Y. Tong, Nanoscale 6 (2014) 7142–7164. [10] Z. Wang, C. Li, K. Domen, Chem. Soc. Rev. 48 (2019) 2109–2125. [11] K.R. Tolod, S. Hern� andez, N. Russo, Catalysts 7 (2017) 13. [12] Z.-F. Huang, L. Pan, J.-J. Zou, X. Zhang, L. Wang, Nanoscale 6 (2014) 14044–14063. [13] J.-S. Ma, L.-Y. Lin, Y.-S. Chen, Int. J. Hydrogen Energy 44 (2019) 7905–7914. [14] H.-Q. Chen, L.-Y. Lin, S.-L. Chen, ACS Appl. Energy Mater. 1 (2018) 6089–6100. [15] M.-W. Kim, E. Samuel, K. Kim, H. Yoon, B. Joshi, M.T. Swihart, S.S. Yoon, J. Alloy. Comp. 769 (2018) 193–200. [16] M. Huang, C. Li, L. Zhang, Q. Chen, Z. Zhen, Z. Li, H. Zhu, Adv. Energy. Mater. 8 (2018) 1802198. [17] S.Y. Jeong, K.S. Choi, H.-M. Shin, T.L. Kim, J. Song, S. Yoon, H.W. Jang, M.H. Yoon, C. Jeon, J. Lee, S. Lee, ACS Appl. Mater. Interfaces 9 (2017) 505–512. [18] H. Gong, N. Freudenberg, M. Nie, R. van de Krol, K. Ellmer, AIP Adv. 6 (2016), 045108. [19] B.-C. Xiao, L.-Y. Lin, J.-Y. Hong, H.-S. Lin, Y.-T. Song, RSC Adv. 7 (2017) 7547–7554. [20] B. Zhang, H. Zhang, Z. Wang, X. Zhang, X. Qin, Y. Dai, Y. Liu, P. Wang, Y. Li, B. Huang, Appl. Catal. B Environ. 211 (2017) 258–265. [21] W.J. Jo, J.-W. Jang, K.-J. Kong, H.J. Kang, J.Y. Kim, H. Jun, K.P.S. Parmar, J.S. Lee, Angew. Chem. Int. Ed. 51 (2012) 3147–3151. [22] X. Zhao, J. Hu, S. Chen, Z. Chen, Phys. Chem. Chem. Phys. 20 (2018) 13637–13645. [23] B. Jin, E. Jung, M. Ma, S. Kim, K. Zhang, J.I. Kim, Y. Son, J.H. Park, J. Mater. Chem. 6 (2018) 2585–2592. [24] H. He, Y. Zhou, G. Ke, X. Zhong, M. Yang, L. Bian, K. Lv, F. Dong, Electrochim. Acta 257 (2017) 181–191. [25] B.-Y. Cheng, J.-S. Yang, H.-W. Cho, J.-J. Wu, ACS Appl. Mater. Interfaces 8 (2016) 20032–20039. [26] Y.-S. Chen, L.-Y. Lin, Electrochim. Acta 285 (2018) 164–171. [27] L. Chen, J. Wang, D. Meng, Y. Xing, C. Wang, F. Li, Y. Wang, X. Wu, Mater. Lett. 147 (2015) 1–3. [28] M. Guo, Y. Wang, Q. He, W. Wang, W. Wang, Z. Fu, H. Wang, RSC Adv. 5 (2015) 58633–58639. [29] G. Tan, L. Zhang, H. Ren, S. Wei, J. Huang, A. Xia, ACS Appl. Mater. Interfaces 5 (2013) 5186–5193. [30] Y.-T. Song, L.-Y. Lin, Y.-S. Chen, H.-Q. Chen, Z.-D. Ni, C.-C. Tu, S.-S. Yang, RSC Adv. 6 (2016) 49130–49137. [31] L. Xi, Z. Jin, Z. Sun, R. Liu, L. Xu, Appl. Catal. Gen. 536 (2017) 67–74. [32] H. Huang, Y. He, X. Du, P.K. Chu, Y. Zhang, ACS Sustain. Chem. Eng. 3 (2015) 3262–3273. [33] Z. Li, W. Wang, N.C. Greenham, C.R. McNeill, Phys. Chem. Chem. Phys. 16 (2014) 25684–25693. [34] I. Hwang, C.R. McNeill, N.C. Greenham, J. Appl. Phys. 106 (2009), 094506.
4. Conclusion The highly ordered BiVO4 nanorod array was successfully grown on the conductive glass using a facile solution process at the first time. The reaction time of the solution process was varied from 1 to 6 h to examine the growth of the BiVO4 nanostructure and to find the optimized con dition for synthesizing efficient photocatalysts toward water oxidation. The possible growing mechanism of the highly ordered BiVO4 nanorod array was proposed to explain the fabrication of the efficient photo catalyst in the scientific way. The highly ordered BiVO4 nanorod array was obtained by using 5 h for carrying out the solution process. This efficient BiVO4 electrode also presents high crystallinity with the high (040) to (121) ratio. The photocurrent density of 1.0 mA/cm2 was ob tained for the optimized BiVO4 electrode measured at the potential of 1.23 VRHE under 100 mW/cm2 light illumination. The smallest charge7
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[35] H.-S. Lin, L.-Y. Lin, Electrochim. Acta 252 (2017) 235–244. [36] Y. Ma, S.R. Pendlebury, A. Reynal, F. Le Formal, J.R. Durrant, Chem. Sci. 5 (2014) 2964–2973.
[37] S.-L. Chen, L.-Y. Lin, Y.-S. Chen, Electrochim. Acta 295 (2019) 507–513.
8
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Novel synthesis of highly ordered BiVO4 nanorod array for photoelectrochemical water oxidation using a facile solution process Yu-Shiang Chen a, 1, Lu-Yin Lin a, b, *, 1 a b
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 1 Sec. 3, Zhongxiao E. Rd., Taipei, 10608, Taiwan Research Center of Energy Conservation for New Generation of Residential, Commercial, and Industrial Sectors, Taiwan
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Highly ordered BiVO4 nanocone is made on conductive substrate via a solution process. � The BiVO4 nanocone has the high crys tallinity and the preferable (040) crystal plane. � The possible growth mechanism of the BiVO4 nanocone array is proposed in this work. � Highest photocurrent density of 1.00 mA/cm2 at 1.23 VRHE is got for BiVO4 electrode. � BiVO4 nanocone electrode has small charge-transfer resistance/high carrier density. A R T I C L E I N F O
A B S T R A C T
Keywords: Bismuth vanadate Growth mechanism Nanorod array Photoelectrochemical Solution process Water oxidation
Bismuth vanadate (BiVO4) with the suitable band gap and band edges is one of the efficient photocatalysts for water oxidation, but the short carrier diffusion path greatly reduces its photoelectrochemical performance. Improving intrinsic properties of BiVO4 is the basic requirement to solve serious recombination and establish efficient photocatalysts. To refine the intrinsic property of BiVO4, the morphology design is the primary strategy rarely investigated in previous works. In this study, the highly ordered cone-shape BiVO4 nanorod array is firstly fabricated on the conductive substrate via a facile solution process. Reaction time of the solution process is varied to fabricate different BiVO4 nanostructures and the possible growth mechanism of BiVO4 nanocone array is proposed. The highest photocurrent density of 1.00 mA/cm2 at 1.23 V versus reversible hydrogen electrode is obtained for the BiVO4 nanocone electrode, owing to the highest crystallinity, preferable (040) crystal plane, the smallest charge-transfer resistance and the highest carrier density. Main contribution of this study is the fundamental improvement on photocatalytic ability of BiVO4 toward water oxidation. Incorporating other ma terials with BiVO4 to realize heteroatom doping or heterojunction are expected to improve the photocatalytic ability of the ordered BiVO4 nanocone array and fabricate efficient photocatalysts toward water oxidation.
* Corresponding author. Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 1 Sec. 3, Zhongxiao E. Rd., Taipei, 10608, Taiwan. E-mail address: [email protected] (L.-Y. Lin). 1 The authors contributed equally. https://doi.org/10.1016/j.jpowsour.2019.226842 Received 10 May 2019; Received in revised form 17 June 2019; Accepted 1 July 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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1. Introduction
effective morphology of BiVO4 for catalyzing the water oxidation. This highly ordered one-dimensional nanostructure is also benefit on combining with other materials for doping heteroatoms or constructing heterojunction to further improve the photoelectrochemical catalytic ability of the BiVO4-based catalysts toward water oxidation.
Photoelectrochemical water splitting is one of the effective ways to produce hydrogen with high energy densities [1–7]. Accelerating the four-hole process of water oxidation is significant to improve the pho toelectrochemical water splitting efficiency, since it is considered to be the rate-limiting step in the water splitting process [8–10]. Therefore, fabricating the useful photocatalysts such as BiVO4 for promoting the water oxidation process is necessary [11,12]. BiVO4 is one of the effective photocatalysts for water oxidation, due to the suitable band gap and band edges [13]. Nevertheless, the reported photocurrent density of the pristine BiVO4 electrode is usually less than 1 mA/cm2, because the short carrier diffusion length (around 70 nm) in the BiVO4 photocatalyst would cause the serious charge recombination [14]. Numerous methods were proposed to solve this problem, such as designing well-defined morphology [15–19], doping charge-abundant heteroatoms [20–22], and constructing heterojunctions [23–25]. These methods can be used separately or combined with each other to enhance the photoelectrochemical catalytic ability of BiVO4. However, the intrinsic property of the pristine BiVO4 is requisite to be improved and therefore incorporating other materials could be more effective to enhance the photoelectrochemical performance of BiVO4 electrodes with the high catalytic ability of its pristine state. With this regard, designing the well-defined BiVO4 nanostructure without incorporating other materials is the basic requirement for establishing efficient pho tocatalysts. Kim et al. applied the electrospray method to synthesize BiVO4 nanopillars on conductive glasses for water oxidation catalysis. The BiVO4 nanopillar electrode showed a photocurrent density of 0.82 mA/cm2 at 1.2 VAg/AgCl [15]. Huang et al. fabricated BiVO4 single crystals with twin structure for photoelectrochemical catalyzing water oxidation. A photocurrent density of 3.1 mA/cm2 at 1.23 VRHE was attained [16]. Jeong et al. fabricated BiVO4 polycrystalline thin films by pulsed laser deposition for water oxidation catalysis. The photocurrent density of 3.0 mA/cm2 at 1.23 VRHE under AM 1.5G illumination was obtained [17]. Haibo et al. deposited BiVO4 thin films by reactive co-magnetron sputtering from metallic Bi and V targets as the photo catalyst for water oxidation. With the optimized thickness of BiVO4 film, the photocurrent density of 0.42 mA/cm2 at 1.23 VRHE was achieved [18]. Although several studies tried to establish well-defined BiVO4 nanostructures by using numerous methods, the fabrication of the most efficient charge-transfer routes, the one-dimensional nanostructure, is limited. There is almost no study proposing the synthesis and discussing the growth mechanism of the well-defined one-dimensional BiVO4 nanostructure as the photocatalyst for water oxidation. In this work, the novel highly ordered cone-shape BiVO4 nanorod array was successfully synthesized on the transparent conductive glass via the facile solution process, which was also reported in our previous report [26]. It was found that by simply tuning the reaction time for carrying out the solution process, the highly ordered BiVO4 nanorod array can be facilely obtained. To understand the growth of the highly ordered nanorod array more clearly, the reaction times of 1, 2, 3, 4, 5 and 6 h were applied to synthesize the BiVO4 nanostructures on conductive glasses. By analyzing the morphologies of the BiVO4 nano structures prepared using different reaction times, the growth mecha nism of the BiVO4 nanorod array was proposed in this work. The BiVO4 nanorod array electrode prepared using 5 h shows the highest photo current density of 1.00 mA/cm2 at 1.23 VRHE under AM 1.5G illumina tion. The best photoelectrochemical catalytic ability for this case is primarily owing to the most highly ordered one-dimensional structure for providing efficient routes for charge transfer. The smallest charge-transfer resistance and the highest carrier density for this case also confirm this inference. The co-catalyst of NiOOH was deposited on the highly ordered BiVO4 nanorod array to enhance the long-term sta bility. The photocurrent retention of 78.2% was obtained for the opti mized NiOOH/BiVO4 electrode after continuously illuminating the sample for 900 s. This work successfully synthesized the novel and
2. Experimental 2.1. Synthesis of BiVO4 nanostructures using the solution process The BiVO4 nanostructures were grown on the fluorine-doped tin oxide (FTO) glass (Nippon Sheet Glass, 8–10 Ω/□, 2.2 mm-thick), which was cleaned by using neutral cleaner, deionized water (DIW), acetone, and acetonitrile for 15 min under sonication, sequentially. According to our previous study [26], the BiVO4 nanostructure was grown on FTO glasses via a two-step process. The first step is the seed layer growth, and the second step is the BiVO4 nanostructure growth on the top of the seed layer. In a typical synthesis, a spin-coating method was used to deposit BiVO4 precursor on the FTO glass. The solution for depositing the seed layer is referred to our previous work [26]. The spin-coating process includes the first step with 500 rpm for 10 s and the second step with 4000 rpm for 40 s. After the deposition of the precursor solution, the electrode was dried at 60� C for 15 min in vacuum oven and then annealed at 400� C for 5 h in air with the heating rate of 10� C/min. Subsequently, the BiVO4 nanostructure was synthesized on the BiVO4 seed layer using the solution process. The suspension for carrying out the solution process contains 2 mmol Bi(NO3)3 (Acros Organics, 98%), 2 mmol NH4VO3 (Acros Organics, 99.5%) and 14.3 g NaHCO3 (Showa chemical, 99.5%) in 50 mL of 23.3% HNO3 aqueous solution (Honey well, 65%). The seed layer-deposited FTO glass was placed side-up in an Erlenmeyer flask with the suspension, which was put in a temperature controlled water bath connected to a condenser as a reflux system. The synthesis of BiVO4 nanostructure was carried out at 60� C for different reaction times under stirring. After the reaction, the BiVO4 electrodes were calcined at 500� C for 30 min in air with the heating rate of 10� C/min. 2.2. Material characterization and electrochemical measurements The morphology of BiVO4 nanostructures was examined using the field-emission scanning electron microscopy (FE-SEM, Nova NanoSEM 230, FEI, Oregon, USA). The composition of BiVO4 nanostructures was analyzed using the X-ray diffraction (XRD, X’Pert 3 Powder, PAN alytical) pattern). The light absorption of BiVO4 nanostructures was evaluated using ultraviolet–visible (UV–vis) spectroscopy (V-750, Jasco). The photoelectrochemical performance of BiVO4 electrodes was carried out using a potentiostat/galvanostat (PGSTAT 204, Autolab, Eco–Chemie, the Netherlands) with the three-electrode electrochemical system. The working electrode is the BiVO4 electrode, the counter electrode is the Pt wire, and the reference electrode is the Ag/AgCl electrode. The aqueous solution of 0.5 M Na2SO4 (Showa chemical, 99%) and 0.1 M phosphate buffer solution [26] (Acros Organics, 99%, PBS) (pH~7.2) was used as the electrolyte. All potential reported was calculated vs. RHE using Equation (1). E (vs. RHE) ¼ E (vs. Ag/AgCl) þ 0.05916 � 7 þ 0.197 ¼ E (vs. Ag/AgCl) þ 0.611 (1) The light source for simulated sunlight 300-Watt xenen lamp equipped with an air mass 1.5-global (AM 1.5G) filter (Newport). The Nyquist plot and the Mott-Schottky plot were measured using the elec trochemical impedance spectroscopy (EIS) respectively with the fre quency range from 0.1 to 15,000 Hz and 100 Hz at 1.23 VRHE under illumination, which was carried out using the potentiostat/galvanostat (PGSTAT 204, Autolab, Eco–Chemie, the Netherlands) equipped with an FRA2 module. 2
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Fig. 1. The top-view SEM images for the BiVO4 nanostructures synthesized using the reaction time of (a) 1, (b) 2, (c) 3, (d) 4, (e) 5, and (f) 6 h. The scale bar is 2 μm.
Fig. 2. The side-view SEM images for the BiVO4 nanostructures synthesized using the reaction time of (a) 1, (b) 2, (c) 3, (d) 4, (e) 5, and (f) 6 h. The scale bar is 1 μm.
3. Results and discussion
become denser and some of them even connected with each other to form a concreted film. It is inferred that the new ingredients tended to fill in the gaps between the rods at the fourth hour of the solution pro cess. At the reaction time of 5 h, the perfect cone-shape nanorods with the bottom diameter of around 500 nm were obtained. As observed in the inserted figure, the side surface of the cone even shows small pro trusions. The rough surface with several protrusions is expected to provide larger surface area for carrying out more water oxidation re actions. Finally, when the reaction was carried out for 6 h, the much denser aggregations were observed for the BiVO4 nanostructure with shallow gaps in-between. To more clearly observe the morphology variation of the BiVO4 nanostructure synthesized using different reac tion times, the side-view SEM images were shown in Fig. 2. Fig. 2(a)–(f) presents the side-view SEM images for the BiVO4 nanostructures pre pared using the reaction times of 1, 2, 3, 4, 5, and 6 h, respectively. The very flat surface and the uniform thickness of around 350 nm were ob tained for the nanostructure prepared using 1 h. The nanostructure shows some protrusions at the surface and the thickness of 500 nm was obtained when 2 h of the reaction time was used. Furthermore, the morphology of the BiVO4 nanostructure varied in a large extent when the reaction time of 3 h was applied for the synthesis. The very long protrusions with the average length of around 2 μm and the bottom layer with the thickness of 900 nm were found for this case. At this stage, not only the total thickness of the BiVO4 nanostructure increases quickly, but also several nanorods with much longer sizes were formed. It is inferred that the growth of the BiVO4 nanostructure could be restricted
3.1. The material characterizations of BiVO4 nanostructures To understand the growth of BiVO4 nanostructures, different reac tion times were applied to fabricate BiVO4 nanostructures on the FTO glass using the solution process. The morphology was first examined by using the SEM images. Fig. 1(a)–(f) respectively shows the SEM images for the BiVO4 nanostructures prepared using the reaction times of 1, 2, 3, 4, 5, and 6 h. When the reaction time of only 1 h was applied for the synthesis, the flat surface with several densely packed particles was observed. Compared to the seed layer shown in the insert of Fig. 1(a), the pores in the seed layer are invisible when the 1 h solution process was carried out for growing the nanostructure. Also, the size of the packed BiVO4 nanoparticles obtained using 1 h is found to be much larger than the size of the seeds. Therefore, the growth of the nanostructure is inferred to be fast and complete in the first hour. Further increasing the reaction time to 2 h, several smaller nanoparticles were grown on the surface of the packed nanostructure obtained at the first 1 h. It is inferred that the core centers may form at the high energy regions on the surface. The smaller particles with the size of around one fourth of those ob tained at the first 1 h may grow on the core centers to reduce the surface energy. For the BiVO4 nanostructure prepared using 3 h, more small nanoparticles kept growing on the core centers, and several protrusions and long rods were formed in the high energy regions. For the BiVO4 nanostructure synthesized using 4 h, the distribution of the nanorods 3
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Scheme 1. Illustration for the growth of the BiVO4 nanostructure synthesized using the solution process.
and the core centers could become more active to attract new in gredients growing on these regions when the thickness of the BiVO4 nanostructure reaches 900 nm. In addition, the BiVO4 nanostructure changes greatly at the reaction time ranged from 3 to 4 h. The thickness of the BiVO4 nanostructure increases to 2 μm when 4 h were applied for the synthesis. The thickness of 2 μm is similar to the average length of the protruding nanorods for the BiVO4 nanostructure prepared using 3 h. Although there are still several protrusions observed in the sample prepared using 4 h, the thickness become more uniform for this case compared to that obtained using 3 h as the reaction time. Hence, it is very possible that the new ingredients are tended to fill in the gaps be tween the nanorods and form a film with the more uniform thickness at the reaction time ranged from 3 to 4 h. Further increasing the reaction time to 5 h, the highly ordered core-shape nanorod array was found to grow on the FTO layer. The thickness of the highly ordered nanorod array is only slightly larger than the thickness of the BiVO4 nano structure synthesized using 4 h. This phenomenon suggests that the growth of the film thickness is somehow restricted at this moment. Instead of increasing the thickness of the BiVO4 film, the BiVO4 crys talline seems to reorganize and form the highly ordered nanorods with the wide bottom and the sharp top. Eventually, when 6 h was used for synthesizing the BiVO4 nanostructure, the thickness of the film is slightly reduced to be the similar size of the thickness for the BiVO4 film pre pared using 4 h. This result indicated that after 4 h the thickness of the film stops to change greatly. The new ingredients in the solution may be used to fill in the gaps and reorganize the nanostructure. For the BiVO4 nanostructure synthesized using 6 h, the perfect cone-shape rods are hard to observe. Instead, the rods became larger and aggregated with neighbor rods to form a continuous film. After carefully discussing the
morphology variation of the BiVO4 nanostructure prepared using different reaction times, the growth mechanism of the highly order nanorod array was proposed, as shown in Scheme 1. The ions of Bi3þ, BiOþ and VO3 are supplied in the solution for growing the BiVO4 nanostructure on the FTO glass. At the first hour of the solution process, the precursor ions were deposited on the FTO glass. Since the entire surface of the FTO glass possesses the same electrical conductivity, the deposition of precursor ions is inferred to be uniform. After the pre cursor ions fully depositing on the FTO glass and forming a densely aggregated film, some high energy regions started to act as the cores in the second hour of the solution process for further growing the nano structure in the vertical direction. At the third hour of the solution process, the ions kept grew at the high energy regions and formed very long rods on the densely packed film. It is inferred that at this moment the crystallized monoclinic BiVO4 could be formed along with the growth of the long rods. At the fourth hour of the solution process, the growth of rods at the high energy region is saturated. The more ions complemented in the solution tend to fill in the gaps between long rods. At this time rods are composed of small nanoparticles, which would generate several grain boundaries in-between. Further increasing the solution process time to 5 h, the heat energy supplied by the hot plate and the extra precursor ions cause the reorganization of the nanoparticle-assembled nanorods. The perfect highly ordered coneshape nanorod array was obtained for this case with much less grain boundaries in-between. More ions supplied at the sixth hour are inferred to destroy the cone-shape nanorod array and form the aggregated nanorod array. The composition of the BiVO4 nanostructures prepared using different reaction times was further confirmed by using the XRD
Fig. 3. The XRD patterns for the (a) BiVO4 nanostructures synthesized using different reaction times and (b) the BiVO4 seed layer. 4
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Fig. 4. (a) The UV–vis spectra and (b) the Tauc plot for the BiVO4 electrodes synthesized using different reaction times.
patterns, as shown in Fig. 3(a). The pound signs indicate the peaks for FTO, which is contributed from the conductive substrate. The standard pattern of monoclinic BiVO4 (JCPDS #01-083-1699) was also presented in this figure for comparison. The patterns of the BiVO4 nanostructures prepared using 3, 4, 5, and 6 h present consistent peaks to those in the standard pattern, indicating the successful growth of monoclinic BiVO4 synthesized using the solution process with the reaction times longer than 3 h. No obvious peaks corresponding to BiVO4 were obtained in the patterns of the BiVO4 nanostructures prepared using 1 and 2 h. This phenomenon indicates that the formation of BiVO4 is not complete with the reaction time of only 2 h. This inference could also be revealed by their relatively flat structures as observed in the SEM images. The peaks in the patterns of the BiVO4 nanostructure prepared using 1 and 2 h are consistent to those of the peaks in the pattern of the BiVO4 seed layer (Fig. 3(b)). This phenomenon suggests that at the first 2 h of the solution process no crystallized semiconductor was formed on the seed layercoated FTO glass, so the same XRD patterns were obtained for the seed layer/FTO electrode and those prepared using 1 and 2 h of the solution process. On the other hand, the peak ratio of (040) to (121) is a significant index to evaluate the catalytic ability of the BiVO4 photo catalyst [27–29]. Among the monoclinic BiVO4 electrodes, the highest (040) to (121) peak ratio of 28 was obtained for the BiVO4 electrode prepared using 5 h, inferring the best photocatalytic ability for this case.
However, further photoelectrochemical performance measurement should be carried out for testing the photocatalytic ability for these samples to confirm this inference. Other than the morphology and composition, the optical property is another significant factor for an efficient photocatalyst. Fig. 4(a) shows the absorption spectra for the BiVO4 nanostructures synthesized using different reaction times. The light absorption edge of around 520 nm was obtained for the electrodes prepared using 3, 4, 5, and 6 h, indi cating their monoclinic BiVO4 features as also observed from the XRD analysis. The electrodes prepared using 1 and 2 h present almost no absorption in the visible region. This phenomenon could be attributed to the morphology and the composition. The flat structure for the samples prepared using 1 and 2 h is not benefit on light absorption, and the lack of monoclinic BiVO4 feature also leads to the worse light absorption for these samples. On the other hand, the band gaps of the photocatalysts were examined by using the Tauc plots (Fig. 4(b)), which were obtained by using the absorption spectra and the Tauc equation [30]. The x-intercept of the tangent line is index to the band gap of the photo catalyst. The band gaps of around 2.4 eV were obtained for the BiVO4 electrodes prepared using 3, 4, 5, and 6 h, indicating the monoclinic BiVO4 features for these samples. The samples prepared using 1 and 2 h show larger band gaps, which again indicate the lack of monoclinic BiVO4 feature for these cases.
Fig. 5. (a) The transient photocurrent plot, (b) the LSV plot, (c) the photoconversion efficiency curves at 1.23 VRHE, (d) the Nyquist plot, and (e) the Mott-Schottky plot of the BiVO4 electrodes synthesized using different reaction times; (f) the current measurement for the stability test for the BiVO4 electrodes synthesized using 5 h with the NiOOH co-catalyst deposition. The inserted figure is the SEM image for the NiOOH/BiVO4 electrode after stability test. 5
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3.2. Photoelectrochemical performances of BVO electrodes made using different reaction times
the BiVO4 electrode prepared using 5 h. To understand the causes of the photocatalytic ability variation for the BiVO4 electrode prepared using different reaction times more clearly, the charge-transfer resistances of the electrodes was further examined by using the EIS technique. The Nyquist plot for the BiVO4 electrodes prepared using different reaction times was shown in Fig. 5 (d). The equivalent circuit was inserted in this figure to fit the chargetransfer resistance (Rct), which could be estimated by using the diam eter of the semicircle at the high frequency region [35]. The BiVO4 electrodes prepared using the reaction times of 1 and 2 h show the largest semicircle diameters respectively corresponding to the highest Rct values of 2.50 and 1.57 kΩ, mainly due to the failure of fabricating monoclinic BiVO4 using the reaction times less than 2 h and probably to the compact structure for transferring charges in random ways. On the other hand, the semicircle diameter in the Nyquist curve decreases for the BiVO4 electrodes prepared using longer reaction times from 3 to 5 h. The Rct values of 0.73, 0.56 and 0.40 kΩ were respectively obtained for the BiVO4 electrodes prepared using 3, 4, and 5 h. The smallest Rct value for the BiVO4 electrode prepared using 5 h is owing to the highly ordered nanorod array structure to provide efficient charge-transfer path and the high crystallinity with the high (040) to (121) peak ratio to display the high photocatalytic ability toward water oxidation. In addition, among the curves of the BiVO4 electrodes, the BiVO4 electrode prepared using 6 h presents the largest semicircle along with the largest Rct value of 0.80 kΩ. This result may be due to the random growth of the nano structure with several rods aggregated with each other, which could hinder the effective transfer of charges and hence increase the charge-transfer resistance for this case. Effective charge transfer is one of the important abilities for an efficient electrode in the photoelectrochemical system. Similarly, the number of charges generated in the photocatalyst is of the great sig nificance to influence the photoelectrochemical performance of the electrode. The carrier densities of the BiVO4 electrodes were further estimated by using the Mott-Schottky plot, as shown in Fig. 5(e). The carrier density was calculated by using the slope of the fitting lines in the Mott-Schottky plot and the Mott-Schottky equation [36]. The carrier density of 1.87 � 1017, 2.56 � 1017, 3.78 � 1017, 5.62 � 1017, 8.33 � 1017 and 5.95 � 1017 cm 3 were respectively obtained for the BiVO4 electrodes prepared using 1, 2, 3, 4, 5 and 6 h. The highest carrier density was obtained for the BiVO4 electrode prepared using 5 h. This carrier density is higher than 4-fold to the smallest carrier density ob tained for the BiVO4 electrode prepared using 1 h. It is significant to emphasize that the carrier density could be largely enhanced with the well-defined structure and well-designed composition. The highly or dered nanorod structure could allow efficient light absorption, and the charge recombination possibility could be largely reduced in the highly ordered and highly crystallized structure of the BiVO4 electrode pre pared using 5 h. This result also supports the best photocatalytic ability of BiVO4 electrode prepared using 5 h. Also, the trend of the carrier density in the electrodes is the same as their corresponding photocurrent densities, suggesting that the carrier density is one of the dominated factors to determine the photoelectrochemical catalytic ability of the photocatalysts toward water oxidation in our cases. Last but not least, the long-term stability of the optimized BiVO4 electrode was further examined to understand the endurance of the photocatalytic ability. To enhance the long-term stability, the cocatalyst of NiOOH was deposited on the BiVO4 electrode by using the method referred to our previous work [37]. The long-term stability of the electrode was measured under the continuous light illumination for 900 s at the potential of 1.23 VRHE. Fig. 5(f) shows the plot for the photocurrent density of the BiVO4 electrode as a function of time. The increased current at the beginning of the stability test is due to the sudden separation of charges at the light-on moment. The subsequent current decreases with longer time is infected to be caused by the recombination of charges. The photocurrent density of the NiOOH-decorated BiVO4 electrode maintained 78.2% of the initial
After examining the physical properties of the BiVO4 nanostructures, the catalytic ability of the materials under light illumination was further investigated. Fig. 5(a) shows the transient current measurements for the BiVO4 electrodes prepared using different reaction times. All the elec trodes present sudden enhancements on the photocurrent when the light was turned on. Then the current reduced to zero suddenly when the light was turned off. This phenomenon suggests the photocatalytic ability for all the electrodes. It is found that the BiVO4 electrodes prepared using 4 and 5 h presents abnormal shapes for their transient current curves, which were also observed in the previous literature [31–34]. The photocurrent increases at the light-on moment and reaches a steady-state with the continuous illumination. The photocurrent density of the electrode may probably decrease again when longer light illu mination was applied. This phenomenon could be attributed to the dy namics of the faster carrier and the subsequent slow rise to the dynamics of the slower carrier [33]. After confirming the photocatalytic feature of the electrodes, it is important to evaluate the photoelectrochemical catalytic ability of the BiVO4 electrodes by using the LSV curves, as shown in Fig. 5(b). The photocurrent density of 0.02, 0.11, 0.34, 0.81, 1.00 and 0.29 mA/cm2 at 1.23 V vs. RHE were respectively obtained for the BiVO4 electrodes synthesized using 1, 2, 3, 4, 5 and 6 h for the so lution process. The photocurrent densities of the electrodes prepared using 1 and 2 h are much smaller than those of the BiVO4 electrodes prepared using 4–6 h, since the successful synthesis of monoclinic BiVO4 can only be achieved using the reaction times longer than 3 h. For the BiVO4 electrodes, the photocurrent density increases for the samples prepared using longer reaction times, until the optimized reaction time of 5 h was reached. The BiVO4 electrode prepared using the reaction time of 4 h shows a higher photocurrent density than that of the BiVO4 electrode prepared using the reaction time to 3 h. This phenomenon is primarily due to the thicker film and the more protrusions in the nanostructure for the BiVO4 electrode prepared using the reaction time of 4 h. These properties could promote the light absorption and electron excitation. Furthermore, the highest photocurrent density of the BiVO4 electrode prepared using reaction time of 5 h is primarily due to the highly ordered nanorod array, which is able to reduce the grain boundaries and the charge recombination possibilities. However, the photocurrent density of the BiVO4 electrode prepared using 6 h is smaller than that of the BiVO4 electrode prepared using 5 h. This result could be attributed to the structure variation. As observed in the SEM images, the well-defined core-shape BiVO4 nanorod array can only be obtained by using 5 h as the reaction time. The aggregations happened again when 6 h were applied for fabricating the BiVO4 electrode. On the other hand, the onset potentials of 0.54, 0.39, 0.15, 0.18, 0.16 and 0.02 VRHE were obtained for the BiVO4 electrodes prepared using 1, 2, 3, 4, 5, and 6 h. The smaller onset potential suggests the smaller driving force required for starting the water oxidation process, which was attained for the BiVO4 electrodes prepared using longer reaction times. For the layer with the thickness smaller than the charge diffusion length, the thicker BiVO4 layer could catalyze more water oxidation reactions. Also, the more well-defined nanostructure could provide more efficient charge-transfer routes for reducing the driving force required to initiate the oxidation reaction. Hence, the reduced onset potentials for the BiVO4 electrodes prepared using longer reaction times is reasonable except for the sample prepared using 6 h. The reason for the smallest onset potential of the BiVO4 electrode prepared using 6 h is not clear and will be investigated in our future work. Furthermore, the photo conversion efficiency was evaluated using the LSV curves, as presented in Fig. 5(c). The maximum photoconversion efficiency increases for the BiVO4 electrode prepared using longer reaction times, and achieves the highest maximum photoconversion efficiency of 0.67% for the BiVO4 electrode prepared using 5 h. This result is consistent with their photo current densities, and again indicates the best photocatalytic ability for 6
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Journal of Power Sources 436 (2019) 226842
transfer resistance and the highest carrier density were also obtained for the optimized BiVO4 electrode. The excellent long-term stability was also achieved for the optimized BiVO4 electrode with the co-catalyst NiOOH deposition. The photocurrent density retention of 78.2% was achieved after 900 s continuous light illumination. Exclusive of incor porating other materials to enhance the photocatalytic ability of BiVO4, the performance achieved in this work is promising. It is expected that by adding other materials in this system, the better photocatalytic per formance can be achieved in our future works.
Table 1 A partial list of the morphology, the synthesis method, and the photocurrent density of the BiVO4 electrode reported in the previous literature and obtained in this work. Morphology
Synthesis Method
Photocurrent density
Onset potential
Ref.
Twin structure
Ionic exchange
0.23 VRHE
[16]
Polycrystalline
0.31 VRHE
[17]
Nanofern
Pulsed laser deposition Electrospray
0.92 VRHE
[15]
Nanopillar
Electrospray
0.87 VRHE
[15]
Porous film
0.40 VRHE
[19]
Microsphere
Comagnetron sputtering Solid-state synthesis
0.69 VRHE
[13]
Nanorod array
Solution process
3.10 mA/cm2 @1.23 VRHE 3.00 mA/cm2 @1.23 VRHE 1.23 mA/cm2 @1.2 VAg/AgCl 0.82 mA/cm2 @1.2 VAg/AgCl 0.42 mA/cm2 @1.23 VRHE 0.02 mA/cm2 @1.23 VRHE 1.00 mA/cm2 @1.23 VRHE
0.12 VRHE
This work
Acknowledgements This work is partially supported by the Ministry of Science and Technology, Taiwan under grant number of 107-2636-E-027-003. This work was financially supported by the “Research Center of Energy Conservation for New Generation of Residential, Commercial, and In dustrial Sectors” from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan.
photocurrent density after 900 s continuous light illumination. The inserted figure is the SEM image of the NiOOH-decorated BiVO4 elec trode after the long-term stability test. The morphology remained similar to that without conducting the long-term stability test (Fig. 1(e)). This result suggests the excellent long-term stability for the optimized BiVO4 electrode proposed in this study. In this work, the novel BiVO4 structure of the highly ordered nanorod array was proposed and applied as the photocatalyst for efficient water oxidation. To have fair and complete comparison, the morphology of the BiVO4 photocatalyst, the method for synthesizing the photocatalyst, and the resulting photocurrent density for evaluating the photocatalytic ability reported in the previous literature and obtained in this work were listed in Table 1. It is worthy to mention that there are many reports tried to design efficient morphology of BiVO4 by incorporating other materials for doping or establishing heterojunction. However, the main purpose in this work is to fabricate the pristine BiVO4 electrode with the excellent photocatalytic ability. Hence, the literature lists in Table 1 only include the information of the pristine BiVO4 electrodes. Lots of the photocurrent densities cannot achieve the value higher than 1 mA/cm2 in the previous literature. The photocurrent density reported in this work is competing to most of the values reported in previous literature although some of the higher photocurrent densities have been previ ously proposed. Also, the highly ordered nanorod array is firstly pro posed in this work and the solution process is facile for fabricating the highly ordered BiVO4 nanorod array in this work. It is expected that the higher photocurrent density and hence the better photoelectrochemical catalytic ability of the BiVO4 electrode can be further achieved in our future works by tuning the precursor composition, the precursor con centration, the reaction temperature and even the system configuration for carrying out the solution process.
References [1] T.-R. Kuo, Y.-C. Lee, H.-L. Chou, S.M.G., C.-Y. Wei, C.-Y. Wen, Y.-H. Chang, X.Y. Pan, D.-Y. Wang, Chem. Mater. 31 (2019) 3722–3728. [2] Y. Zhu, H.-C. Chen, C.-S. Hsu, T.-S. Lin, C.-J. Chang, S.-C. Chang, L.-D. Tsai, H. M. Chen, ACS Energ. Lett. 4 (2019) 987–994. [3] S.-F. Hung, Y.-T. Chan, C.-C. Chang, M.-K. Tsai, Y.-F. Liao, N. Hiraoka, C.-S. Hsu, H. M. Chen, J. Am. Chem. Soc. 140 (2018) 17263–17270. [4] S.-C. Lin, C.-S. Hsu, S.-Y. Chiu, T.-Y. Liao, H.M. Chen, J. Am. Chem. Soc. 139 (2017) 2224–2233. [5] M. Ahmed, I. Dincer, Int. J. Hydrogen Energy 44 (2019) 2474–2507. [6] S.D. Tilley, Adv. Energy. Mater 9 (2019) 1802877. [7] T.J. Jacobsson, Energy Environ. Sci. 11 (2018) 1977–1979. [8] K. Afroz, M. Moniruddin, N. Bakranov, S. Kudaibergenov, N. Nuraje, J. Mater. Chem. 6 (2018) 21696–21718. [9] J. Gan, X. Lu, Y. Tong, Nanoscale 6 (2014) 7142–7164. [10] Z. Wang, C. Li, K. Domen, Chem. Soc. Rev. 48 (2019) 2109–2125. [11] K.R. Tolod, S. Hern� andez, N. Russo, Catalysts 7 (2017) 13. [12] Z.-F. Huang, L. Pan, J.-J. Zou, X. Zhang, L. Wang, Nanoscale 6 (2014) 14044–14063. [13] J.-S. Ma, L.-Y. Lin, Y.-S. Chen, Int. J. Hydrogen Energy 44 (2019) 7905–7914. [14] H.-Q. Chen, L.-Y. Lin, S.-L. Chen, ACS Appl. Energy Mater. 1 (2018) 6089–6100. [15] M.-W. Kim, E. Samuel, K. Kim, H. Yoon, B. Joshi, M.T. Swihart, S.S. Yoon, J. Alloy. Comp. 769 (2018) 193–200. [16] M. Huang, C. Li, L. Zhang, Q. Chen, Z. Zhen, Z. Li, H. Zhu, Adv. Energy. Mater. 8 (2018) 1802198. [17] S.Y. Jeong, K.S. Choi, H.-M. Shin, T.L. Kim, J. Song, S. Yoon, H.W. Jang, M.H. Yoon, C. Jeon, J. Lee, S. Lee, ACS Appl. Mater. Interfaces 9 (2017) 505–512. [18] H. Gong, N. Freudenberg, M. Nie, R. van de Krol, K. Ellmer, AIP Adv. 6 (2016), 045108. [19] B.-C. Xiao, L.-Y. Lin, J.-Y. Hong, H.-S. Lin, Y.-T. Song, RSC Adv. 7 (2017) 7547–7554. [20] B. Zhang, H. Zhang, Z. Wang, X. Zhang, X. Qin, Y. Dai, Y. Liu, P. Wang, Y. Li, B. Huang, Appl. Catal. B Environ. 211 (2017) 258–265. [21] W.J. Jo, J.-W. Jang, K.-J. Kong, H.J. Kang, J.Y. Kim, H. Jun, K.P.S. Parmar, J.S. Lee, Angew. Chem. Int. Ed. 51 (2012) 3147–3151. [22] X. Zhao, J. Hu, S. Chen, Z. Chen, Phys. Chem. Chem. Phys. 20 (2018) 13637–13645. [23] B. Jin, E. Jung, M. Ma, S. Kim, K. Zhang, J.I. Kim, Y. Son, J.H. Park, J. Mater. Chem. 6 (2018) 2585–2592. [24] H. He, Y. Zhou, G. Ke, X. Zhong, M. Yang, L. Bian, K. Lv, F. Dong, Electrochim. Acta 257 (2017) 181–191. [25] B.-Y. Cheng, J.-S. Yang, H.-W. Cho, J.-J. Wu, ACS Appl. Mater. Interfaces 8 (2016) 20032–20039. [26] Y.-S. Chen, L.-Y. Lin, Electrochim. Acta 285 (2018) 164–171. [27] L. Chen, J. Wang, D. Meng, Y. Xing, C. Wang, F. Li, Y. Wang, X. Wu, Mater. Lett. 147 (2015) 1–3. [28] M. Guo, Y. Wang, Q. He, W. Wang, W. Wang, Z. Fu, H. Wang, RSC Adv. 5 (2015) 58633–58639. [29] G. Tan, L. Zhang, H. Ren, S. Wei, J. Huang, A. Xia, ACS Appl. Mater. Interfaces 5 (2013) 5186–5193. [30] Y.-T. Song, L.-Y. Lin, Y.-S. Chen, H.-Q. Chen, Z.-D. Ni, C.-C. Tu, S.-S. Yang, RSC Adv. 6 (2016) 49130–49137. [31] L. Xi, Z. Jin, Z. Sun, R. Liu, L. Xu, Appl. Catal. Gen. 536 (2017) 67–74. [32] H. Huang, Y. He, X. Du, P.K. Chu, Y. Zhang, ACS Sustain. Chem. Eng. 3 (2015) 3262–3273. [33] Z. Li, W. Wang, N.C. Greenham, C.R. McNeill, Phys. Chem. Chem. Phys. 16 (2014) 25684–25693. [34] I. Hwang, C.R. McNeill, N.C. Greenham, J. Appl. Phys. 106 (2009), 094506.
4. Conclusion The highly ordered BiVO4 nanorod array was successfully grown on the conductive glass using a facile solution process at the first time. The reaction time of the solution process was varied from 1 to 6 h to examine the growth of the BiVO4 nanostructure and to find the optimized con dition for synthesizing efficient photocatalysts toward water oxidation. The possible growing mechanism of the highly ordered BiVO4 nanorod array was proposed to explain the fabrication of the efficient photo catalyst in the scientific way. The highly ordered BiVO4 nanorod array was obtained by using 5 h for carrying out the solution process. This efficient BiVO4 electrode also presents high crystallinity with the high (040) to (121) ratio. The photocurrent density of 1.0 mA/cm2 was ob tained for the optimized BiVO4 electrode measured at the potential of 1.23 VRHE under 100 mW/cm2 light illumination. The smallest charge7
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[35] H.-S. Lin, L.-Y. Lin, Electrochim. Acta 252 (2017) 235–244. [36] Y. Ma, S.R. Pendlebury, A. Reynal, F. Le Formal, J.R. Durrant, Chem. Sci. 5 (2014) 2964–2973.
[37] S.-L. Chen, L.-Y. Lin, Y.-S. Chen, Electrochim. Acta 295 (2019) 507–513.
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