applied sciences Article
Flower-like BiVO4 Microspheres and Their Visible Light-Driven Photocatalytic Activity Arini Nuran Zulkifili 1 , Akira Fujiki 2 and Shinji Kimijima 2, * 1 2
*
Graduate School of Engineering, Shibaura Institute of Technology, Saitama 337-8570, Japan;
[email protected] Department of Machinery and Control Systems, Shibaura Institute of Technology, Saitama 337-8570, Japan;
[email protected] Correspondence:
[email protected]; Tel.: +81-48-687-5124
Received: 13 November 2017; Accepted: 22 January 2018; Published: 31 January 2018
Abstract: A flower-like BiVO4 microsphere photocatalyst was synthesized with a simple template-free homogeneous precipitation method at 60 ◦ C for 24 h. The purpose of this study is to explore a low-cost, simple method of synthesizing the self-assembled 3D structure in order to enhance photocatalytic performance under visible light irradiation (λ > 420 nm). In this study, the morphology, structure, and photo-absorption of flower-like BiVO4 microspheres were characterized, and the effects of photocatalysis were analyzed. The results indicate that the size of the flower-like microspheres was about 2 µm to 4 µm and they were composed of several nanosheets. The mechanism of hierarchical microsphere formation has been proposed as the Ostwald ripening process and the self-assembled process. The obtained samples were calcined under different temperatures (300 ◦ C, 400 ◦ C, 500 ◦ C and 600 ◦ C) to study the effects of calcination on the structure and on photocatalysis. The photocatalytic process was then evaluated by decolorization of methylene blue dye under visible-light irradiation. Keywords: bismuth vanadate; low-cost method; methylene blue dye; microspheres; photocatalyst; visible light
1. Introduction In recent years, photocatalysts have attracted much attention due to their wide range of applications, from generating hydrogen by splitting water to decomposing organic contaminants in wastewater [1]. In the decomposition of organic contaminants such as dyes in wastewater, the photocatalyst uses visible light or ultraviolet (UV) light as an irradiating source to mineralize the contaminants in order to produce the end products such as carbon dioxide and water. TiO2 has been the subject of early study in photocatalytic materials. However, due to its wide band gap, it can only work under ultraviolet irradiation for photocatalytic activation, which leads to very low energy efficiency in utilizing solar light. This is because UV light is only utilizing 5% solar energy, a small fraction compared to visible light, which is 45% [2]. Therefore, research has been focused on finding a method or materials that can work efficiently under visible light irradiation. Among the widely studied materials are bismuth-based photocatalysts, which have been proven to be suitable in the degradation of organic contaminants under visible light irradiation [3]. This is because bismuth compound such as Bi2 O3 , BiVO4 , NaBiO3 has a band gap range less than 3.0 eV and is environmentally friendly. Various approaches in synthesizing BiVO4 have been reported. A monoclinic BiVO4 has successfully been prepared by solid state reaction (SSR) and by melting at a high temperature [4], whereas, a tetragonal BiVO4 has been synthesized by a precipitation method at a room temperature [5]. In addition, a simple aqueous process in preparing both monoclinic and tetragonal phase was also reported [6]. In addition, the use of hydrothermal process in creating various
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tetragonal phase was also reported [6]. In addition, the use of hydrothermal process in creating various structural, morphologies and phase were also being reported. Since BiVO4 can be prepared structural, anditphase alsosuitable being reported. Since BiVO beaprepared using a 4 canof by using amorphologies simple method, is onewere of the candidates in the study low-costby synthesis simple method, it is one of the suitable candidates in the study of a low-cost synthesis method [7]. method [7]. Besides have been proven to have a high degradation rate Besides that, that,nano-size nano-sizephotocatalytic photocatalyticmaterials materials have been proven to have a high degradation in degrading organic contaminants, due to their having a greater surface area, which provides more rate in degrading organic contaminants, due to their having a greater surface area, which provides fine amongamong catalyst particles and organic substrate, good dispersion, and abundant active morecontact fine contact catalyst particles and organic substrate, good dispersion, and abundant sites. the recovery process process becomesbecomes a drawback for this nano-size photocatalyst, since the activeHowever, sites. However, the recovery a drawback for this nano-size photocatalyst, powder-like nano-photocatalyst often suffers from deactivation, agglomeration, and difficulty settling since the powder-like nano-photocatalyst often suffers from deactivation, agglomeration, and for recycling after for therecycling initial run. Thisthe makes hardThis to recycle photocatalyst, nonrecyclable difficulty settling after initialit run. makesthe it hard to recycle and the photocatalyst, photocatalytic materials will increase operating which significantly limits their application in and nonrecyclable photocatalytic materials will cost, increase operating cost, which significantly limits wastewater treatment. In most wastewater treatment, photocatalysts are suspended in aqueous their application in wastewater treatment. In most wastewater treatment, photocatalysts are solution. Separation of solution. photocatalysts must of bephotocatalysts done by settling with aidby ofsettling gravity with or mechanical suspended in aqueous Separation must bethe done the aid of centrifugation. Settling of ultrafine photocatalyst particles with the aid of gravity has been commonly gravity or mechanical centrifugation. Settling of ultrafine photocatalyst particles with the aid of observed to been proceed very slowly. gravity has commonly observed to proceed very slowly. Therefore, in this study, composed of of nanosheets to Therefore, in this study,we wedeveloped developeda amicrostructure microstructurephotocatalyst photocatalyst composed nanosheets enhance the process by using a simple, low-cost method that results in the formation of a flower-like to enhance the process by using a simple, low-cost method that results in the formation of a flowerbismuth vanadate microstructure that has a greater surface surface area, more stability, and higher efficiency like bismuth vanadate microstructure that has a greater area, more stability, and higher in organic dye degradation under visible light irradiation. As for the dye contaminant, we chose efficiency in organic dye degradation under visible light irradiation. As for the dye contaminant, we methylene blue (MB) dye because it is a frequently used industrial dye. MB dye has the chemical chose methylene blue (MB) dye because it is a frequently used industrial dye. MB dye has the structure of C16 H18of N3C SCl is shown Figurein1.Figure 1. chemical structure 16Hand 18N3SCl and isinshown
Figure 1. Structure blue dye. dye. Figure 1. Structure of of methylene methylene blue
2. Materials and Methods 2. Materials and Methods Bismuth nitrate pentahydrate (Bi(NO3)3∙5H2O) crystal and nitric acid (HNO3) were purchased Bismuth nitrate pentahydrate (Bi(NO3 )3 ·5H2 O) crystal and nitric acid (HNO3 ) were purchased from Wako Pure Chemical Industries (Osaka, Japan). Sodium hydroxide (NaOH) granule was from Wako Pure Chemical Industries (Osaka, Japan). Sodium hydroxide (NaOH) granule was purchased from Kanto Chemical Co., Inc., (Tokyo, Japan) vanadium (III) acetylacetonate (C10H14O5V) purchased from Kanto Chemical Co., Inc., (Tokyo, Japan) vanadium (III) acetylacetonate (C10 H14 O5 V) crystal from Strem Chemicals Inc., (Newburyport, MA, USA) and methylene blue dye powder from crystal from Strem Chemicals Inc., (Newburyport, MA, USA) and methylene blue dye powder Waldeck GmbH & Co. KG (Münster, Germany). Ultrapure water was used in all experiments. All from Waldeck GmbH & Co. KG (Münster, Germany). Ultrapure water was used in all experiments. chemicals were analytical grade and used as received, without further purification. All chemicals were analytical grade and used as received, without further purification. Flowerlike BiVO4 microspheres were synthesized by a simple template-free precipitation Flowerlike BiVO4 microspheres were synthesized by a simple template-free precipitation method method at 60 °C for 24 h. Then, 4.2 g of Bi(NO3)3∙5H2O and 0.27 g of C10H14O5V were dissolved in 100 at 60 ◦ C for 24 h. Then, 4.2 g of Bi(NO3 )3 ·5H2 O and 0.27 g of C10 H14 O5 V were dissolved in 100 mL mL of HNO3 at a concentration of 4 mol–1L–1. Ultrapure water was added until it reached a final volume of HNO3 at a concentration of 4 mol L . Ultrapure water was added until it reached a final volume of 500 mL. The solution was stirred until a clear light blue solution was formed. Then, the pH of the of 500 mL. The solution was stirred until a clear light blue solution was formed. Then, the pH of the solution was adjusted to 12 by adding NaOH. The solution changes color from clear light blue to solution was adjusted to 12 by adding NaOH. The solution changes color from clear light blue to bluish bluish precipitate and brownish precipitate, which indicates the change from acidic to neutral to precipitate and brownish precipitate, which indicates the change from acidic to neutral to alkaline alkaline state. The suspension was inserted into an oil bath at 60 °C for 24 h without stirring. The state. The suspension was inserted into an oil bath at 60 ◦ C for 24 h without stirring. The resulting resulting yellow precipitate was retrieved by centrifugation, and washed several times with distilled yellow precipitate was retrieved by centrifugation, and washed several times with distilled water and water and absolute ethanol. The yellow precipitate was then dried at 60 °C for 24 h and was calcined absolute ethanol. The yellow precipitate was then dried at 60 ◦ C for 24 h and was calcined at different at different temperatures (300 °C, 400 °C, 500 °C and 600 °C) for further study. temperatures (300 ◦ C, 400 ◦ C, 500 ◦ C and 600 ◦ C) for further study. The samples were analyzed with an X-ray diffractometer (XRD; RINT 2000, Rigaku Corp., The samples were analyzed with an X-ray diffractometer (XRD; RINT 2000, Rigaku Corp., Tokyo, Japan) operated at 30 kV and 40 mA with Cu Kα radiation (λ = 1.54178 Å). Data were collected Tokyo, Japan) operated at 30 kV and 40 mA with Cu Kα radiation (λ = 1.54178 Å). Data were in the angular range of 2θ = 15–80°. Field◦ emission scanning electron microscopy images were collected in the angular range of 2θ = 15–80 . Field emission scanning electron microscopy images obtained from a Hitachi FE-SEM SU8000 (Tokyo, Japan). UV-visible diffusion reflectance spectra were obtained from a Hitachi FE-SEM SU8000 (Tokyo, Japan). UV-visible diffusion reflectance were measured at room temperature with a JASCO V-7200 UV-Vis-NIR Spectrophotometer (Jasco spectra were measured at room temperature with a JASCO V-7200 UV-Vis-NIR Spectrophotometer
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(Jasco Products Company, Oklahoma City, OK, USA). The reflectance was then converted to absorbance Products Company, Oklahoma City, OK, USA). The reflectance was then converted to absorbance by by using the Kubelka–Munk method. The size of the particles was recorded by zeta potential and an using the Kubelka–Munk method. The size of the particles was recorded by zeta potential and an Otsuka Electronics ELSZ-2000 particle analyzer (Otsuka Electronics Co., Ltd., Osaka. Japan) at room Otsuka Electronics ELSZ-2000 particle analyzer (Otsuka Electronics Co., Ltd., Osaka. Japan) at room temperature, Furtheranalysis analysiswas wasconducted conducted with a Thermo temperature,using usingultrapure ultrapurewater water as as aa solution. solution. Further with a Thermo Scientific Nicolet 4700 Fourier-transform infrared spectroscopy analyzer (Thermo Fisher Scientific, Scientific Nicolet 4700 Fourier-transform infrared spectroscopy analyzer (Thermo Fisher Scientific, Waltham, MA, USA), Ramanspectrometer spectrometer(HORIBA (HORIBA Jobin Yvon Waltham, MA, USA),a aHoriba-Jovin Horiba-JovinYvon Yvon T64000 T64000 micro micro Raman Jobin Yvon IBH Ltd., Paris, France), and a JASCO FP8500 fluorescence spectrometer. IBH Ltd., Paris, France), and a JASCO FP8500 fluorescence spectrometer. Photocatalysis as follows: follows:44mL mLofofmethylene methylene blue solution Photocatalysisevaluation evaluationtest test was was conducted conducted as blue solution at aat a concentration ofof 0.30.3mg/100 100 mL mLPyrex Pyrexvessel. vessel.Then, Then,9696 mL ultrapure water concentration mg/100mL mLwas was placed placed in aa 100 mL ofof ultrapure water was added toto the aqueous ofBiVO BiVO44powder powderwas was added each experiment. was added the aqueoussolution. solution.After After that, that, 0.3 g of added forfor each experiment. The experimentswere wereconducted conductedunder under the the presence presence of solution was The experiments of visible visiblelight light(λ(λ>>400 400nm). nm).The The solution was magnetically stirred to ensure uniformity. The samples werefiltered then filtered bya syringe using a membrane syringe magnetically stirred to ensure uniformity. The samples were then by using membrane filter (Millex; Millipore Corp., Burlington, MA, USA) to remove the photocatalyst filter (Millex; Millipore Corp., Burlington, MA, USA) to remove the photocatalyst particles. Samples particles. Samples were taken from the solution every 30 min and placed inside a UV-Vis were taken from the solution every 30 min and placed inside a UV-Vis spectrophotometer cell in spectrophotometer in order to measure the maximum absorption of wavelength forofthe dye and order to measure thecell maximum absorption of wavelength for the dye and the rate degradation. the rate of degradation. The percentage of degradation was calculated by the×formula [1 − (C/C The percentage of degradation was calculated by the formula [1 − (C/C0)] 100%, where C00)]is ×the 100%, where C 0 is the initial concentration of the dye solution and C is the concentration of the dye initial concentration of the dye solution and C is the concentration of the dye at each 30-min interval. at each 30-min interval. 3. Result and Discussion 3. Result and Discussion 3.1. Material Characterization 3.1. Material Characterization XRD was used to characterize the phase structure of the obtained samples. Figure 2 shows XRD was used to BiVO characterize the phase structure of the obtained samples. Figure 2 shows the the XRD pattern of the 4 microspheres prepared for each calcination temperature. The XRD XRD pattern of the BiVO 4 microspheres prepared for each calcination temperature. The XRD pattern ◦ pattern at 400 C indicates that the sample was mainly in the monoclinic phase, which exhibits high at 400 °C indicates that the sample was mainly in the monoclinic phase, which exhibits high photocatalytic activity compared to other phases. In addition, a well-defined sharp peak can be seen photocatalytic activity compared to other phases. In addition, a well-defined sharp peak can be seen after calcination at 300 ◦ C. This indicates that the sample achieved a well-defined organizational after calcination at 300 °C. This indicates that the sample achieved a well-defined organizational structure, which leads to better photocatalytic activity. structure, which leads to better photocatalytic activity.
Figure X-ray diffractometer analysis BiVO4 microspheres under different Figure2.2. X-ray diffractometer (XRD)(XRD) analysis for BiVOfor 4 microspheres under different calcination calcination temperatures. temperatures.
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AsAsshown microscopy (SEM) observation images Figure 3a–c, after As in the scanningelectron electron microscopy (SEM) observation images in Figure 3a–c, shownin inthe thescanning scanning electron microscopy (SEM) observation images inin Figure 3a–c, after ◦ C, calcination at 400 °C, the products formed were uniform. The SEM images of commercial BiVO 4 can after calcination at 400 the products formed were uniform. The SEM images of commercial calcination at 400 °C, the products formed were uniform. The SEM images of commercial BiVO4 can bebe seen asasbulky, not area is available. available. BiVO bebulky, seenand as bulky, and surface not much surface area is available. 4 can seen and notmuch much surface area is
Figure3.3.(a–c) (a–c)Fourier-transform Fourier-transform infrared infrared spectroscopy images ofofflower-like BiVO 4; and (c) Figure spectroscopy imagesimages flower-like BiVO 4; and (c) Figure 3. (a–c) Fourier-transform infrared spectroscopy of flower-like BiVO4 ; commercial BiVO 4. commercial BiVO 4. and (c) commercial BiVO4 .
At 500 °C (Figure 6), the mixture of flower-like structure and agglomerates started to form, At 500◦ °C (Figure 6), the mixture of flower-like structure and agglomerates started to form, while, at 600 °C, the particles startedof toflower-like form only agglomerates, no flower-like structures At 500 C (Figure 6), the mixture structure and and agglomerates started to form,were while, while,◦at 600 °C, the particles started to form only agglomerates, and no flower-like structures were found. at 600 C, the particles started to form only agglomerates, and no flower-like structures were found. found. further studythe themorphologic morphologicstructure, structure, we observed magnification ToTo further study observedthe thesamples samplesunder underhigh high magnification To further study the morphologic structure, we observed the samples under high magnification SEM (Figure 4a,b), the flower-like structures of the sample were composed of numerous SEM (Figure 4a,b), and and the flower-like structures of the sample were composed of numerous nanosheets SEM (Figure with 4a,b),a thickness and the flower-like structures of the sample were composed of numerous nanosheets of about 100 nm. The nanosheets intercrossed to form a flower-like with a thickness of about 100 nm. The nanosheets intercrossed to form a flower-like microsphere via nanosheets with a thickness of about 100 nm. The nanosheets intercrossed to form a flower-like microsphereand viathe self-assembly and the Ostwald ripening process. These flower-like structures self-assembly Ostwald ripening process. These flower-like structures resulted in an increased microsphere viaincreased self-assembly and the Ostwald ripening process. These flower-like structures resulted in an surface area, which leads to better photocatalytic performance compared to surface area, which leads to better photocatalytic performance compared to commercial BiVO4 , which resulted in an increased surface area, which leads to better photocatalytic performance compared commercial BiVO4, which has a bulky structure and low surface area. Besides that, the large surface to has a bulky structure and low surface area. Besides that, the large surface area provided by the commercial BiVOby 4, which has a bulky structure lowmore surface area. Besidesofthat, large surface area provided the flower-like structure canand allow rapid diffusion the the reactants and flower-like structure can allow more rapid diffusion of the reactants and products during the reaction area provided by the thereaction flower-like structure can allow diffusion of thetoreactants products during process. If we increased the more pH of rapid the synthesis condition more thanand process. If we increased the pH of the synthesis condition to more than 12, the flower-like structure products during thestructure reaction could process. we increased the pH of the synthesis condition more than 12, the flower-like notIfform, and only microspheres/microcubes would be to observed. could not form, and only microspheres/microcubes would be observed.
12, the flower-like structure could not form, and only microspheres/microcubes would be observed.
Figure 4. (a,b) High magnification of flower-like BiVO4 and the results obtained under synthesis conditions of pH > 12.
Figure 4. (a,b) High magnification of flower-like BiVO4 and the results obtained under synthesis Figure 4. (a,b) High magnification of flower-like BiVO4 and the results obtained under synthesis conditions of pH > 12.the formation mechanism of the flower-like BiVO4 microspheres, as shown in We also of proposed conditions pH > 12.
Figure 5. A precipitation process occurred by the nucleation and growth of the second phase from a We also proposed the With formation mechanism of the flower-like 4 microspheres, as shown in supersaturated solution. a two-phase mixture composed of BiVO a dispersed second phase in a We5.also proposed theprocess formation mechanism of the flower-like BiVO4 of microspheres, as shown Figure A precipitation occurred by the nucleation and growth second phase fromina matrix, the mixture does not satisfy thermodynamic equilibrium. This is due the to small particles with Figure 5. A precipitation process occurred by the nucleation and growth of the second phase from supersaturated solution. With a two-phase mixture composed a dispersed phasenot in aa a higher surface ratio and a higher energy state leading to excessofsurface energy,second which does supersaturated solution. With a two-phase mixture composed of a dispersed second phase in a matrix, matrix, does not satisfy thermodynamic equilibrium. This is dueoftothis small particles with satisfythe themixture requirement of minimum energy configuration. The total energy system can be the mixture does not satisfy thermodynamic equilibrium. This is due to small particles with a higher a higher surface and in a higher leading excess surface which does decreased via anratio increase the scaleenergy of the state second phase, to thus decreasing theenergy, total interfacial area.not surface ratiorequirement and ainhigher state leading togrowth excess process surface whichof does satisfy satisfy the ofenergy minimum energy configuration. The energy, total aenergy this not system canthe be This will result a continuous nucleation and until single precipitate particle requirement of minimum energy configuration. The total energy of this system can be decreased via forms, which introduces a given size class,thus which, in this case, flower-like decreased via an increase ainnew the particle scale of of the second phase, decreasing the was totalthe interfacial area. an increase in the scale of[8,9]. the second phase, thus decreasing the totaluntil interfacial area.precipitate This will result in microsphere structure This will result in a continuous nucleation and growth process a single particle aforms, continuous nucleation and growth process until a single precipitate particle forms, which introduces which introduces a new particle of a given size class, which, in this case, was the flower-like amicrosphere new particlestructure of a given size class, which, in this case, was the flower-like microsphere structure [8,9]. [8,9].
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Figure Schematic illustration ofof the proposed formation ofof flower-like BiVO microspherestructure. structure. Figure 5.5.5. Schematic illustration of the proposed formation of flower-like BiVO 44microsphere Figure Schematic illustration the proposed formation flower-like BiVO 4 microsphere structure.
The The mean particle size for the obtained samples was also studied using the zeta potential and Themean meanparticle particlesize sizefor forthe theobtained obtainedsamples sampleswas wasalso alsostudied studiedusing usingthe thezeta zetapotential potentialand and ◦ particle analyzer. As shown in Figure 6, the smallest size was at 400 °C, which was 2566 particle nm with particleanalyzer. analyzer.As Asshown shownininFigure Figure6,6,the thesmallest smallestsize sizewas wasatat400 400 C, °C,which whichwas was2566 2566nm nmwith withaa a ◦ Cand ◦ C,the slight error value. At 60 °C 300 °C, sample consisted of impurities, which led to aalarger mean slight error value. At 6060 300 sample consisted ofof impurities, which led toto mean slight error value. At °Cand and 300 °C,the the sample consisted impurities, which led alarger larger mean ◦ ◦ particle size. However, at 500 °C, the mean particle size was larger than at 400 °C, because a mixture particle at at 500 thethe mean particle size was larger than at 400 C, °C, because a mixture of particlesize. size.However, However, 500C,°C, mean particle size was larger than at 400 because a mixture of structure started totoform. The size the 3D microsphere structure andand agglomerates started to form. The mean particle size became the largest, of3D 3Dmicrosphere microsphere structure andagglomerates agglomerates started form. Themean meanparticle particle sizebecame became the ◦ C due largest, with a substantial error value at 60 °C due to most of the sample being made up of large with a substantial error value at 60 to most of the sample being made up of large agglomerates, largest, with a substantial error value at 60 °C due to most of the sample being made up of large agglomerates, totopoor contributing tocontributing poor photocatalytic performance. agglomerates, contributing poorphotocatalytic photocatalyticperformance. performance.
Figure 4 microspheres under calcination temperatures of 60 °C, Figure6. 6.Particle Particlesize sizeanalysis analysisofofBiVO BiVO 4 microspheres under calcination temperatures of 60 °C, Figure 6. Particle size analysis of BiVO4 microspheres under calcination temperatures of 60 ◦ C, 300 ◦ C, 300 °C, 400 °C, 500 °C and 600 °C. 300◦ °C, 400◦ °C, 500 °C ◦and 600 °C. 400 C, 500 C and 600 C.
3.2. 3.2.Optical OpticalProperties Properties 3.2. Optical Properties The 4 microspheres was measured and TheUV-Visible UV-Visibleoptical opticalabsorption absorptionspectrum spectrumofofBiVO BiVO 4 microspheres was measured and The UV-Visible optical absorption spectrum of BiVO microspheres wasasmeasured and compared compared with commercial BiVO 4 and synthesized BiVO 4 microspheres, shown in Figure 7. A 4 compared with commercial BiVO4 and synthesized BiVO 4 microspheres, as shown in Figure 7. A with commercial BiVO and synthesized BiVO microspheres, as shown in Figure 7. A broad absorption broad could 4 4 300 broadabsorption absorptionspectrum spectrum couldbebeseen seenfrom from 300toto400 400nm. nm.AtAtaround around420 420nm, nm,ananabrupt abruptcutoff cutoff spectrum could be seen from 300 to 400 nm. At around 420 nm, an abrupt cutoff absorption edge absorption edge was observed. absorption edge was observed. was The observed. absorption band contains a tail extending rightward until about 800 nm. This may bebe a result The absorption band contains a tail extending rightward until about 800 nm. This may a result The defects absorption band contains a tail extending rightward until about 800 nm.gap This may was be a ofofcrystal formation during the growth of the BiVO 4 microspheres. The band energy crystal defects formation during the growth of the BiVO4 microspheres. The band gap energy was result of crystal defects formation during(Figure the growth ofband the BiVO microspheres. Theisband gap determined by Kubelka–Munk derivation 7). The gap of this photocatalyst 2.60 4 determined by Kubelka–Munk derivation (Figure 7). The band gap of this photocatalyst is 2.60eV, eV, energy was determined by Kubelka–Munk derivation (Figure 7). The band gap of this photocatalyst and it has yellowish colored powder. This shows that this photocatalyst can work efficiently under and it has yellowish colored powder. This shows that this photocatalyst can work efficiently under is 2.60 light eV, and it has yellowish colored presence powder. This shows that this photocatalyst can work visible visible lightirradiation. irradiation.InInaddition, addition,the the presenceofofthe themonoclinic monoclinicphase phasewhich whichhas hassuperior superior efficiently under visiblecontributed light irradiation. Inenhancement addition, the of presence of the monoclinic phaseofwhich photocatalytic activity to the the photocatalytic character photocatalytic activity contributed to the enhancement of the photocatalytic character ofthis this has superior photocatalytic activity contributed to the enhancement of the photocatalytic character of photocatalyst. photocatalyst. this photocatalyst.
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Figure 7. 7. UV-Vis UV-Vis diffuse diffuse reflectance reflectance spectra microspheres. Figure spectra of of BiVO BiVO44 microspheres.
Photoluminescence wavelength λex λ=ex325 nm. nm. As Photoluminescence spectroscopy spectroscopywas wasstudied studiedatatthe theexcitation excitation wavelength = 325 shown in Figure 8, a strong peak can be seen at around 560 nm for both synthesized microspheres As shown in Figure 8, a strong peak can be seen at around 560 nm for both synthesized microspheres and commercial commercialBiVO BiVO Thestrong strong emission at 560 nm corresponds to recombination of the hole emission at 560 nm corresponds to recombination of the hole formed 4 .4.The formed from the hybrid orbitals of Bi 6s and O 2p and the electron generated from the V 3d orbitals. from the hybrid orbitals of Bi 6s and O 2p the electron generated from the V 3d orbitals. In addition, In emission bands of BiVO4–m have a blue shift compared to commercial BiVO4, which theaddition, emissionthe bands of BiVO 4 –m have a blue shift compared to commercial BiVO4 , which shows that it shows that itrecombination has a lower recombination rate. has a lower rate. 600
BiVO44
Intensity Intensity (a.u) (a.u)
BiVO44 - m 400ooC 400
200
0 450
500
550
600
Wavelength (nm) Figure 8. Photoluminescence spectra at the excitation wavelength λex = 325 nm for BiVO4 microspheres Figure at the wavelength λλex == 325 nm for BiVO4 microspheres Figure 8. 8. Photoluminescence Photoluminescence spectra spectra at the excitation excitation wavelength ex 325 nm for BiVO4 microspheres at 400 °C. ◦ at 400 °C. at 400 C. −1 Raman spectroscopy spectroscopy was was also also investigated, investigated, as as shown shown in in Figure Figure 9. 9. The The Raman Raman bands bands at at 313 313 cm cm−1 Raman Raman was 4also in Figure 9. symmetric The Raman bands at are the the typicalspectroscopy vibrations of of BiVO BiVO and investigated, can be assignedastoshown the asymmetric and deformation are typical vibrations 4 and can be assigned to the asymmetric and symmetric deformation −1 are the 3− −1 313 cm typical vibrations of BiVO and can be assigned to the asymmetric and symmetric modes of of the the VO VO443− tetrahedron. tetrahedron. It It has has been been 4reported reported that that there there were were two two Raman Raman bands, bands, at at 710 710 cm cm−1 modes 3 − −1modes deformation the VO tetrahedron. It has been reported thatdifferent there were twoofRaman bands, and 805 cm cm−1 whichofwere were attributed to the the stretching stretching modes of two two types Vanadium– 4 and 805 ,, which to modes of different types of Vanadium– −1 and −1 ,attributed −1 at 710 cm 805 cm which were attributed to the stretching modes of two different types of Oxygen bands bands in in the the Raman Raman spectrum spectrum of of BiVO BiVO44.. However, However, only only one one band band around around 805 805 cm cm−1 was was found found Oxygen −1 −1 Vanadium–Oxygen bands in the Raman spectrum of BiVO . However, only one band around 805 cm in our our case. case. The The absence absence of of aa 710 710 cm cm−1 band band might might be be attributed attributed to the the local local structure structure of of porous porous 4 in to −1 was found in our case.were The absence of aof710 cm−1 bandAdditionally, might be attributed to theband local at structure microspheres, which composed nanosheets. the Raman 517 cmof −1 microspheres, which were composed of nanosheets. Additionally, the Raman band at 517 cm −1 porous microspheres, which were composed of nanosheets. Additionally, the Raman band at 517 cm indicates the the characteristics characteristics of of monoclinic monoclinic scheelite scheelite BiVO BiVO44 that that made made up up these these microsphere microsphere structures. structures. indicates indicates the characteristics of monoclinic scheelite BiVO4 that made up these microsphere structures.
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Figure 9. Raman analysis of BiVO4 microspheres at 400 ◦°C. Figure9.9.Raman Ramananalysis analysisof ofBiVO BiVO44microspheres microspheresatat400 400°C. C. Figure
From the Fourier-transform infrared spectroscopy analysis we conducted (Figure 10), we can From the Fourier-transform infrared analysis we From infraredspectroscopy spectroscopy weconducted conducted(Figure (Figure10), 10),we wecan can understandthe theFourier-transform bonding group presence in these BiVOanalysis 4 microspheres. At 1021 cm−1, there is an −1 − 1 understand the bonding group presence in these BiVO 4 microspheres. At 1021 cm , there isisan understand the bonding group presence in these BiVO microspheres. At 1021 cm , there −1 4 unshared V = O stretching vibration, followed by 833 cm−1 , which shows the presence of an an −1 , which unshared V ==OO stretching vibration, followed cm , shows which the shows the of presence of an unshared V stretching vibration, followed 833by cm833 presence anatoms asymmetric asymmetric stretching vibration of the boundbyoxygen that is shared by two vanadium (V–O– asymmetric stretching vibration of the bound that is shared by two vanadium atoms (V–O– stretching vibration the bound oxygen that isoxygen sharedmode by two atoms −1, a symmetric V). Furthermore, at of 523 cm stretching ofvanadium the V–O–V unit (V–O–V). is present,Furthermore, and finally, −1, a symmetric stretching mode of the V–O–V unit is present, and finally, − 1 V). Furthermore, at 523 cm at 523 cm , a symmetric stretching of the V–O–V unit is present, and finally, at molecules 3402 and at 3402 and 1627 cm−1−1, it shows an O–Hmode stretching and bending vibration of the lattice water − 1 at 3402 and 1627 cm , it shows an O–H stretching and bending vibration of the lattice water molecules 1627 [10]. cm , it shows an O–H stretching and bending vibration of the lattice water molecules [10]. [10].
Figure C. Figure 10. 10. Fourier-transform Fourier-transform infrared infrared spectroscopy spectroscopy analysis analysis of of BiVO BiVO44 microspheres microspheres at at 400 400 ◦°C. Figure 10. Fourier-transform infrared spectroscopy analysis of BiVO4 microspheres at 400 °C.
We also also conducted conducted aa zeta zeta potential potential analysis analysis to to study study the the stability stability of of this this flower-like flower-like BiVO BiVO4 in in We We also conducted a zeta potential analysis to study the stability of this flower-like BiVO44 in terms of the colloidal dispersion, as shown in Figure 11. This shows that it has high stability compared termsof ofthe thecolloidal colloidaldispersion, dispersion,as asshown shownin inFigure Figure11. 11.This Thisshows showsthat thatitithas hashigh highstability stabilitycompared compared terms to commercial BiVO 4, since it possesses a higher zeta potential value. When the potential is small, tocommercial commercialBiVO BiVO4,4 ,since sinceititpossesses possessesaahigher higherzeta zetapotential potentialvalue. value.When Whenthe thepotential potentialisissmall, small, to attractive forces forces can can exceed exceed this this repulsion repulsion and and the the dispersion dispersion can can break break and and flocculate. flocculate. Therefore, Therefore, attractive attractive forces can exceed this repulsion and the dispersion can break and flocculate. Therefore, colloids with with high high zeta zeta potential potential (negative (negative or or positive) positive) are are electrically electrically stabilized, stabilized, while while colloids colloids with with colloids colloids with high zeta potential (negative or positive) are electrically stabilized, while colloids with low zeta potential tend to coagulate or flocculate [11,12]. lowzeta zetapotential potentialtend tendto tocoagulate coagulateor orflocculate flocculate[11,12]. [11,12]. low
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Figure 11.0 Zeta potential analysis of BiVO4 microspheres. Figure potentialanalysis analysisofofBiVO BiVO 4 microspheres. Figure 11. 11. Zeta Zeta60potential 4 microspheres. Bi O C 300 C 400 C 500 C 600 C o
o
o
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3.3. Photocatalytic Properties 3.3. 3.3.Photocatalytic PhotocatalyticProperties Properties Figure 11. Zeta potential analysis of BiVO4 microspheres. Photocatalysis of the samples was evaluated by decolorization of MB aqueous solution under Photocatalysis ofProperties theand samples was MB aqueous solution under Photocatalysis of the samples was evaluated evaluated by decolorization of MB aqueous under 3.3. Photocatalytic visible light irradiation, the results are shownby indecolorization Figures 12 andof13. Absorption ofsolution MB aqueous visible light irradiation, and the results are shown in Figures 12 and 13. Absorption of MB aqueous visible light andused the to results arethe shown in Figures 12 and 13.process. Absorption of MB aqueous solution atirradiation, λ = 664 nm was monitor MB dye Thesolution decolorization Photocatalysis of the samples was evaluated bydecolorization decolorization rate of MB aqueous under solution at λ = 664 nm was to monitor the MB dye decolorization rate process. The decolorization solution at λ = 664 nm was used to monitor the MB dye decolorization rate process. The decolorization ratevisible sequence be concluded asresults 400 °C,are 500 °C, 60 and 300 °C, 13. followed by 600 Improved lightcan irradiation, and the shown in°C Figures 12 and Absorption of °C. MB aqueous ◦ C, 500 ◦ C,60 ◦ Cand ◦ C, rate sequencecan can be concluded asto 400 °C, °C, °C 300 °C, followed bythe 600presence °C.◦ C. Improved rate sequence concluded as 400 500 60 and 300 followed by Improved performance was observed 60 °C, 300 °C, 400 °Cdye and 500 °C. However, due to of only solution at λ = be 664 nm wasfor used monitor the MB decolorization rate process. The600 decolorization ◦ ◦ ◦ ◦ performance was observed for 60 °C, 300 °C, 400 °C and 500 °C. However, due to the presence of only performance wasand observed for 60 C,as C, C and However, dueby to600 the °C. presence of only rate sequence canbulk be concluded 400 500 60 °C500 andC. 300 °C, followed Improved agglomeration structures at300 600°C, °C,400 the°C, reaction rate declined. ◦ C, agglomeration and bulk structures at°C, 600 °C, rate declined. performance was observed for 60at 300 °C,the 400reaction °C and 500 °C. However, due to the presence of only agglomeration and bulk structures 600 the reaction rate declined. agglomeration and bulk structures at 600 °C, the reaction rate declined.
Figure 12. Rate of decolorization of methylene blue dye for BiVO4 microspheres calcined at 400 °C. Figure12. 12.Rate Rateof ofdecolorization decolorization of of methylene blue dye for BiVO4 microspheres calcined at 400 °C. ◦ Figure microspheres calcined at 400 Figure 12. Rate of decolorization methylene of methyleneblue bluedye dyefor for BiVO BiVO44microspheres calcined at 400 °C. C.
Figure 13. 13. Rate of of absorbance undercalcination calcination Figure Rate absorbanceofofmethylene methyleneblue bluedye dye for for BiVO BiVO44 microspheres microspheres under Figure 13. Rate of absorbance of methylene blue dye for BiVO 4 microspheres under calcination Figure 13. Rate of absorbance of methylene blue dye for BiVO microspheres under calcination 4 temperatures of 60 300300 °C,°C, 400 °C, 500 °C°Cand temperatures of °C, 60 °C, 400 500 and600 600°C. ◦ C,°C, ◦C ◦°C. temperaturesofof60 60◦°C, 300 ◦°C, 500 and temperatures C, 300 C, 400 400 °C, 500°C and600 600°C. C.
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Inaddition, addition,we we conducted conducted aa Total Total Organic Organic Carbon Carbon (TOC) (TOC) study study (Figure (Figure 14) 14) on on this photocatalyst, In In addition, wedegrade conducted Total Organic (TOC)BiVO study (Figure 14) onbetter this photocatalyst, showing that itit can can degrade MBadye dye better thanCarbon commercial BiVO 4 . Thus, it has better efficiency in in showing that MB better than commercial . Thus, it has efficiency 4 showing that it can degrade MB dye better than commercial BiVO 4 . Thus, it has better efficiency in decolorizingand andmineralizing mineralizingthe thedye. dye. decolorizing decolorizing and mineralizing the dye.
Figure 14. Total Organic Carbon (TOC) percentage of methylene blue dye for BiVO4 microspheres Figure Total Organic Carbon (TOC) percentage of methylene blue dye for BiVO4 4microspheres Figure14. 14. Total microspheres calcined at 400 °C.Organic Carbon (TOC) percentage of methylene blue dye for BiVO ◦ C. calcined at 400 calcined at 400 °C.
The recyclability of this photocatalyst was also studied (Figure 15). We found that this material The ofof this also studied (Figure Wecycles found this material has Therecyclability recyclability this photocatalyst was also studied (Figure 15). We found this material has good stability, since it photocatalyst only declinedwas from 94% to 88% after15). four ofthat thethat activity, which good stability, since it only declined from 94% to 88% after four cycles of the activity, which indicates has goodthat stability, since stability it only declined from 94% to 88% after four cycles of the activity, which indicates it has good in this recyclability study. that it has good instability this recyclability study. indicates that itstability has good in this recyclability study.
Figure 15. Recyclability of methylene blue dye degradation rate for BiVO4 microspheres calcined at Figure 15.Recyclability Recyclability methyleneblue bluedye dyedegradation degradationrate ratefor forBiVO BiVO4microspheres microspherescalcined calcinedatat 400 °C 15. for four cycles. ofofmethylene Figure 4 400 °C for four cycles. ◦ 400 C for four cycles.
4. Conclusions 4. Conclusions 4. Conclusions In conclusion, by adding vanadium (III) acetylacetonate to the system, the flower-like structure In conclusion, by adding vanadium (III) acetylacetonate to photocatalysis the system, theinflower-like structure can be at aby low temperature of 60 °Cacetylacetonate for 24 h. Although decolorization just In achieved conclusion, adding vanadium (III) to the system, the flower-like structure can be achieved at a low temperature of 60 °C for 24 h. Although photocatalysis in decolorization ◦ slightly improved compared to commercial BiVO 4, it can mineralize the dye better, as can be seenjust in can be achieved at a low temperature of 60 C for 24 h. Although photocatalysis in decolorization just slightlyofimproved compared to believe commercial BiVOfurther 4, it can study, mineralize the dyetime better, asphotocatalytic can be seen in results the TOC analysis. We that, with the reaction and slightly improved compared to commercial BiVO4 , it can mineralize the dye better, as can be seen in results of thecan TOC analysis. WeThus, believe that, with further study, theto reaction time and photocatalytic performance improved. this work open the way possible new varied lowresults of the TOCbe analysis. We believe that, withmay further study, the reaction time andand photocatalytic performance can be improved. methods. Thus, this work may open the way to possible new and varied lowcost self-assembly synthesizing performance can be improved. Thus, this work may open the way to possible new and varied low-cost cost self-assembly synthesizing methods. self-assembly synthesizing methods. Acknowledgments: We are grateful for the acknowledgement support from the National Institute of Material Acknowledgments: We are grateful for the acknowledgement support from the National Institute of Material Science. Acknowledgments: We are grateful for the acknowledgement support from the National Institute of Science. Author Contributions: Arini Nuran Zulkifili conceived, designed, performed and analyzed the experiments and Material Science. Author Contributions: Arini Nuran Zulkifili conceived, designed, performed and analyzed the experiments and data; Akira Fujiki and Shinji contributed reagents/materials/analysis tools. Author Contributions: Arini Kimijima Nuran Zulkifili conceived, designed, performed and analyzed the experiments and data; Akira Fujiki and Shinji Kimijima contributed reagents/materials/analysis tools. data; AkiraofFujiki and Shinji Kimijimasponsors contributed tools. in the collection, analysis, Conflicts Interest: The founding had reagents/materials/analysis no role in the design of the study; Conflicts of Interest: The sponsors had no role or in in thethe design of the study; in the collection, analysis, or interpretation of data; infounding the writing of the manuscript; decision to publish the results. or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
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Conflicts of Interest: The founding sponsors had no role in the design of the study; in the collection, analysis, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
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