Synthesis of Nanoparticulate In-Doped BiVO 4 for Enhanced Visible-Light Photocatalytic Degradation of Dye

Synthesis of Nanoparticulate In-doped BiVO4 for EnhancedVisible-Light Photocatalytic Degradation of Dye

Chockalingam Karunakaran* and Selvaraj Kalaivani

Department of Chemistry, Annamalai University, Annamalainagar 608002, Tamilnadu, India

Indium (2.9, 3.5, and 4.9 at. %)-doped and pristine monoclinic BiVO4 nanoparticles were synthesized by hydrothermalmethod. They were characterized by high-resolution scanning electron, field emission scanning electron, transmission electronand high-resolution transmission electron microscopies, powder X-ray and selected-area electron diffractometries, energy-disper-sive X-ray, Raman, UV-visible diffuse reflectance, photoluminescence, and solid-state impedance spectroscopies. The band gap

and near-band-gap emission of 4.9% In-doped BiVO4 nanoparticles are larger than those of the rest of the nanomaterials.The charge-transfer resistance of 4.9% In-doped BiVO4 is the least. In-doping enhances visible-light photocatalytic activity.

Introduction

Heterogeneous photocatalysis is a green route to min-eralize organic pollutants, and utilization of natural sun-light for the same makes the process attractive. BiVO4 is apromising visible-light photocatalyst due to its favorableband structure and reasonable stability in water.1–3 It existsin three crystalline states: monoclinic scheelite, tetragonalzircon, and tetragonal scheelite. Among the three crystal-line phases, monoclinic BiVO4 is the best visible-light-dri-ven photocatalyst.4 It is an n-type intrinsic semiconductorwith band gap energy of about 2.4 eV. The electronicexcitation from the VB contributed by the O 2p orbitals toCB of V 3d orbitals in VO4

3� tetrahedron leads to theUV absorption. Because of high effective nuclear charge,the Bi 6s orbitals hybridize less effectively with the non-bonding molecular orbitals of the vanadate group. At thesame time, the Bi 6p orbitals hybridize more effectivelywith the antibonding orbitals of the VO4

3� group. Thevisible-light absorption is due to excitation of electronsfrom the VB made of the hybridized Bi 6s and O 2p orbi-tals to the CB comprised of V 3d orbitals of VO4

3�. Thelone-pair distortion causes the pronounced overlap of O2p and Bi 6s orbitals of monoclinic scheelite BiVO4.

5 Thegreater overlap between Bi 6s and O 2p orbitals in mono-clinic BiVO4 than that in tetragonal BiVO4 enhances themobility of photogenerated holes.5 But the short carrierdiffusion lengths and significant recombination of photo-formed electron–hole pairs limit the photocatalytic activityof BiVO4.

6 An important step in semiconductor photoca-talysis is that the electron–hole pair generated by lightabsorption is to be separated and transferred to the surfaceof the semiconductors.7 Hence the photocatalytic activity

of BiVO4 may be enhanced by suppression of the elec-tron–hole recombination. This is possible by the creationof heterojunction or by doping BiVO4. Recently,Ag3PO4/BiVO4,

8 BiOCl/BiVO4,9 core–shell BiVO4@c-

Bi2O3,10 ZnFe2O4/BiVO4,

11 BiPO4/BiVO4,12 Bi2WO6/

BiVO4,13 and WO3/BiVO4

6 coupled semiconductors havebeen prepared for enhanced visible-light photocatalysis.Noble metal or nonmetal-deposited BiVO4 nanoparticlessuch as Ag/BiVO4,

14 carbon quantum dot/BiVO415 also

show enhanced photocatalysis. Although BiVO4 isreported to be photostable, the photocorrosion of theother semiconductor constituting the heterojunction leadsto a serious problem for industrial application of the syn-thesized nanocomposite for mineralization of pollutants.However, such contingency does not occur with the dopedBiVO4, and the photocatalytic activities of doped BiVO4

nanoparticles such as Mo- or W-doped BiVO4,16 Er-

doped BiVO4,17 C-doped BiVO4 with hierarchical struc-

ture18 are larger than that of the undoped BiVO4. Wepresent here the hydrothermal synthesis of In-dopedBiVO4 nanoparticles; doping BiVO4 with indiumenhances the visible-light photocatalytic activity. In3+ ismuch smaller than Bi3+ and doping is feasible. The ionicradii of In3+ and Bi3+ in crystals are 62 and 103 pm,respectively,19 and replacement of Bi3+ in the BiVO4 lat-tice by In3+ is sterically feasible.

Experimental Section

Bismuth nitrate pentahydrate (Sigma Aldrich, St.Louis, MO), ammonium metavanadate (Sigma Aldrich),indium oxide (Sigma Aldrich), nitric acid (70%; Merck,Bangalore, India), sodium hydroxide (SD Fine-Chem,Mumbai, India), and methylene blue (SD Fine) of LR

*[email protected]

© 2014 The American Ceramic Society

Int. J. Appl. Ceram. Technol., 1–11 (2014)DOI:10.1111/ijac.12299

grade were used as supplied. Distilled deionized waterwas employed throughout the investigation.

Bi(NO3)3.5H2O (1 mmol, 0.485 g) was dissolvedin 20 mL of 1M HNO3 at room temperature. Alkalinesolution of NH4VO3 was obtained by dissolving 1 mmol(0.117 g) of NH4VO3 in 20 mL of 0.5 M NaOH atroom temperature. In2O3 (0.007 or 0.035 g) was dis-solved in a few drops of 70% HNO3 and added drop-wise to Bi(NO3)3 solution under stirring for 30 min.NH4VO3 solution was added dropwise to the solutioncontaining In3+ and Bi(NO3)3 with stirring to get atransparent homogeneous solution. Then the pH of thesolution was adjusted to 0.5 or 5 for 0.007 g addition ofIn2O3 and 0.5 for 0.035 g addition of In2O3 with 2 MNaOH to get precursor. It was transferred into a 100-mL Teflon-lined stainless steel autoclave, sealed andheated at 120°C for 12 h under autogeneous pressure,and allowed to cool to room temperature. The obtainedsamples were filtered, washed with deionized distilledwater followed by absolute ethanol. Finally, the obtainedsolid samples were dried at 50°C for 12 h in a hot airoven. Similar procedure was followed without the addi-tion of In2O3 to prepare pristine BiVO4 nanoparticles;the pH was adjusted to 5.

The high-resolution scanning electron micrographs(HR-SEM) of the synthesized nanomaterials wereobtained with a FEI Quanta FEG 200 HR-SEM in highvacuum mode. Field emission scanning electron micro-scopic (FE-SEM) images were recorded with a HitachiSU6600 FE-SEM (Tokyo, Japan). The samples were dis-persed in acetone and spread on aluminum grids forimaging. The energy-dispersive X-ray (EDX) spectrawere obtained with either an FEI Quanta FEG 200 HR-SEM or a Hitachi SU6600 FE-SEM. A Hitachi H-7650transmission electron microscope (TEM) was employedto record the TEM images of the synthesized nanomate-rials. The samples were dispersed in acetone and spreadon copper grids for imaging. A FEI Tecnai G2F30 S-Twin TEM was used to record the TEM images at highresolution. The nanoparticles were dispersed in acetoneand spread on carbon grids for imaging. The selected-area electron diffraction (SAED) patterns of the synthe-sized samples were obtained using a Hitaci H-7650TEM as well as with a FEI Tecnai G2F30 S-TwinTEM. The powder X-ray diffractograms (XRD) of thesynthesized nanoparticles were obtained in a 2h range of10–80° with a PANalytical X’Pert PRO diffractometerusing Cu Ka rays of wavelength 1.5406

��A with a tubecurrent of 30 mA at 40 kV. A Princeton Action SP2500Raman spectrometer (Trenton, NJ) was used to recordthe Raman spectra of the synthesized nanomaterials. Thesamples were excited at 514.5 nm. A Shimadzu UV-

2600 spectrophotometer (Kyoto, Japan) with ISR-2600integrating sphere attachment was used to record theUV-visible diffuse reflectance spectra (DRS). The photo-luminescence (PL) spectra at room temperature wereobtained with a PerkinElmer LS 55 fluorescence spec-trometer (Waltham, MA). The nanoparticles were dis-persed in carbon tetrachloride under sonication andexcited using light of wavelength 435 nm. The solid-state electrochemical impedance spectra (IS) at roomtemperature were recorded with a CH Instrument Elec-trochemical Analyzer 604C in the frequency range0.1 MHz to 1 Hz. The disk area was 0.5024 cm2, andthe thickness of the pellets of pristine BiVO4, 2.9 (at.)% In-doped BiVO4 and 4.9 (at.) % In-doped BiVO4

were 2.50, 1.37, and 1.19 nm, respectively.The visible–light photocatalytic activity was tested

with a 150-W tungsten-halogen lamp fitted into a dou-ble-walled borosilicate vessel with an inlet and outlet forwater circulation. Sodium nitrite solution was employedas UV light cut-off filter. Circulation of 2 M NaNO2

solution in place of water curtails 99% UV light ofwavelength between 320 and 400 nm. The light inten-sity was measured (415 W/m2) using a Daystar solarmeter (Fairbanks, AK). The photocatalytic degradationof methylene blue was made in a wide cylindrical boro-silicate glass vessel of uniform diameter (7.0 cm), placedunderneath (13 cm) the visible-light source. One hun-dred milliliter of the freshly prepared dye solution(25 ppm) along with accurately weighted 0.100 g of thephotocatalyst was taken in the reaction vessel. It was sat-urated with oxygen by bubbling air using a micro airpump. The solution was stirred continuously using amagnetic stirrer. The photocatalytic degradation of thedye was followed by determining the concentration ofmethylene blue spectrophotometrically at 662 nm. A cal-ibration curve was constructed by measuring the absor-bance of methylene blue at different ppm.

Results and Discussion

The high-resolution scanning electron micrographs(HR-SEM) or field emission scanning electron micro-graph (FE-SEM) of hydrothermally synthesized indium-doped BiVO4 and pristine BiVO4 nanoparticles is dis-played Fig. 1. The images show the synthesized dopedand pristine BiVO4 as nanoparticles. They are in nano-scale and lack definite shape or structure. Furthermore,they agglomerate. Figure 2 presents the transmissionelectron micrographs (TEM) of hydrothermally synthe-sized indium-doped BiVO4 and pristine BiVO4.The TEM images confirm the synthesized materials as

2 International Journal of Applied Ceramic Technology—Karunakaran and Kalaivani 2014

nanoparticles. Also, the TEM images show that the syn-thesized indium-doped and pristine BiVO4 nanoparticlesdo not have any regular shape or structure.

Figure 3 displays the energy-dispersive X-ray (EDX)spectra of the synthesized indium-doped BiVO4 nano-particles and pristine BiVO4 nanoparticles. The EDXspectrum of pristine BiVO4 shows the presence of bis-muth, vanadium, and oxygen. The absence of any otherelement confirms the purity of the synthesized material.The EDX spectra of the doped BiVO4 nanoparticlesconfirm the indium doping. In addition, the absence ofany element except bismuth, vanadium, indium, andoxygen shows the purity of the synthesized doped mate-rials. The unlabelled peak at 1.5 keV (in one of theEDX spectra, Fig. 3, top right) is that of the aluminumfoil. The atomic percentages of the dopant indium,determined by EDX analysis, in the three synthesizeddoped samples are 2.9, 3.5, and 4.9. The TEM imagesof In-doped and undoped BiVO4 nanoparticles, pre-sented in Fig. 4, were recorded at high resolution. They

display the lattice fringes of monoclinic BiVO4. Theobserved d-spacing of 0.31 and 0.47 nm are those of -13 0- and 1 1 0-planes of monoclinic BiVO4 lattice,respectively. This shows that the lattice parameters donot change on doping monoclinic BiVO4 with indiumhydrothermally. The SAED patterns of the indium-doped BiVO4 nanoparticles, recorded at two differentarea of the sample with two different instruments, arepresented in Fig. 5; the images on the left are those withFEI Tecnai G2F30 S-Twin TEM, and patterns on theright were those with Hitachi H-7650 TEM. The SAEDof pristine BiVO4 is also presented in Fig. 5, for com-parison. The clearly observed concentric rings of theSAED patterns support the crystalline nature of the syn-thesized doped and undoped BiVO4. They index to theplanes of monoclinic BiVO4 corresponding to JCPDScard no. 14-0688. The results of the SAED patternsobtained at two different area of the sample with twodifferent machines indicate the homogeneity of thesynthesized In-doped BiVO4 nanoparticles. The powder

3.5% In-BiVO4 4.9% In-BiVO4

65 nm

56 nm

67 nm

71 nm

65 nm

BiVO4 2.9% In-BiVO4

49 nm

53 nm

47 nm

55 nm

51 nm

Fig. 1. HR-SEM of BiVO4, 2.9 and 4.9% In-doped BiVO4 and FE-SEM of 3.5% In-doped BiVO4 nanoparticles.

www.ceramics.org/ACT Synthesis of Nanoparticulate In-Doped BiVO4 3

X-ray diffractograms (XRD) of hydrothermally synthe-sized indium-doped BiVO4 and pristine BiVO4 nanopar-ticles are displayed in Fig. 6. The XRD of pristineBiVO4 and those of indium-doped BiVO4 are inagreement with JCPDS card no. 14-0688 revealing themonoclinic crystalline structure of the synthesizedpristine as well as indium-doped BiVO4 nanoparticles.The crystal constants are as follows: a = 0.5195 nm,b = 1.1701 nm, c = 0.5092 nm, a = 90°, b = 90.38°,and c = 90°. Examination of the reflection angles of theindium-doped BiVO4 samples and those of pristineBiVO4 shows that the former are not shifted in compari-son with the latter. This shows that doping BiVO4 withIn3+ does not affect the d-spacing. This is because In3+ ismuch smaller (62 pm) than Bi3+ (103 pm)19 andreplacement of Bi3+ by In3+ will not enlarge the unit cell.The average crystallite sizes (L) of the synthesizedindium-doped BiVO4 and pristine BiVO4 nanoparticleshave been obtained from the full width at half maximum(FWHM, b) of the most intense peak of the respective

crystals using Scherrer’s formula L = kk/bcosh, where kis the shape factor, k is the wavelength of X-raysemployed, h is the diffraction angle, and the full widthat half maximum of the peak is in radians. The value ofthe dimensionless shape factor varies with the actualshape of the crystallite, and for spherical crystals, it has atypical value of about 0.94. The mean crystallite size of2.9, 3.5, and 4.9% indium-doped BiVO4 and pristineBiVO4 are 32, 29, 20, and 29 nm, respectively. Thesizes shown by HR-TEM, FE-TEM, and TEM are thoseof the particles, whereas the same deduced from FWHMof the XRD peaks are those of the crystallites. The aver-age crystallite size will be the same or smaller than theparticle size. Generally, crystallites agglomerate to formparticles resulting in increase of size. If there is noagglomeration, each crystallite acts as a particle and theparticle size and the crystallite size do not differ signifi-cantly. In this study, the crystallites agglomerate and themean crystallite sizes of the synthesized nanoparticles aresmaller than the corresponding particle size. The specific

4.9% In-BiVO4

50 nm100 nm

3.5% In-BiVO4

36 nm

29 nm

27 nm

39 nm

100 nm

2.9% In-BiVO4BiVO4

50 nm

49 nm

91 nm

56 nm

27 nm

Fig. 2. TEM images of BiVO4 and In-doped BiVO4 nanoparticles.


surface areas of the synthesized nanoparticles have beendeduced using the relationship S = 6/qL, where S is thesurface area and q is the material density. The obtainedsurface area of 2.9, 3.5, and 4.9% indium-doped BiVO4

and pristine BiVO4 are 27, 30, 42, and 30 m2/g, respec-tively. However, these values may not correspond to thesurface area under illumination in photocatalytic degra-dation of dye as the nanocrystallites may agglomerate inthe dye solution.

Raman spectroscopy is useful to probe the localstructure. The Raman frequencies are sensitive to short-range order.20–25 They yield useful information regardingthe bonding states in the coordination polyhedra. Fig-ure 7 displays the Raman spectra of indium-doped andpristine BiVO4 nanoparticles. The most intense Ramanband of 2.9, 3.5, and 4.9% indium-doped BiVO4 nano-particles at 814 per cm is assigned to the symmetricV-O stretching mode of monoclinic BiVO4 with Agsymmetry. The observed intense Raman frequency is inagreement with the literature values (818–827 per

cm).20–24,26–34 The small shift to lower wavenumber sug-gests that the average short-range symmetry of theVO4

3� tetrahedron is more regular compared to thosestudied already.26,30 The corresponding peak of pristineBiVO4 nanoparticles occurs at 805 per cm. The symmet-ric V-O stretching frequency of pristine BiVO4 nanopar-ticles is shifted to lower wavenumber by 13–22 per cmcompared to the widely reported value of 818–827 percm.20–24,26–34 This shift of Raman band indicates thatthe average short-range symmetry of the VO4

3� tetrahe-dron in pristine BiVO4 is highly regular than those syn-thesized earlier.26,30 The peak of 2.9, 3.5, and 4.9% In-doped BiVO4 nanoparticles at 360, 351, and 341 percm is attributed to the symmetric deformation mode ofthe VO4

3� tetrahedron.21–26,31,32,35 The shift of thefrequency to lower wavenumber with increase of thedopant concentration could also be because of the short-range symmetry of the VO4

3� tetrahedron becomingmuch more regular with increase of In-doping. PristineBiVO4 nanoparticles show the symmetric deformation

2.9% In-BiVO4BiVO4

3.5% In-BiVO44.9% In-BiVO4

Fig. 3. EDX spectra of BiVO4 and In-doped BiVO4 nanoparticles.


mode of the VO43� tetrahedron at 346 per cm which

also suggests highly regular short-range symmetry of theVO4

3� tetrahedron in pristine BiVO4 nanoparticles.The UV-visible diffuse reflectance spectra (DRS) of

indium-doped BiVO4 and pristine BiVO4 nanoparticlesare displayed in Fig. 8. The DRS are presented in termsof F(R), obtained from the recorded reflectance (R) bythe application of Kubelka–Munk algorithm [F(R) = (1–R)2/2R]. The DRS do not show any significant shift ofthe absorption edge on doping BiVO4 with indium upto 3.5%. However, the absorption edge is blue shiftedon doping BiVO4 with indium at 4.9%. The visible-light absorption is due to excitation of electrons fromthe VB made of the hybridized Bi 6s and O 2p orbitalsto the CB comprised of V 3d orbitals of VO4

3�.36 Theelectronic excitation from the VB contributed by the O2p orbitals to CB of V 3d orbitals in VO4

3� tetrahedronresults in the UV absorption. The Bi 6s orbitals, becauseof high effective nuclear charge, hybridize less effectively

with the nonbonding molecular orbitals of the vanadategroup. At the same time, the Bi 6p orbitals hybridizemore effectively with the antibonding orbitals of theVO4

3� group. The modified KM plots are presented inFig. 9. They provide the direct and indirect band gapsof the synthesized nanoparticles. The indirect electronictransition occurs because of the breaking of the symme-try of the VO4

3� tetrahedron by the stereoactive lonepair of Bi3+.37 Plots of [F(R)hm]2 and [F(R)hm]½ versusphoton energy give the direct38,39 and indirect37 bandgaps of the nanoparticles. The direct band gaps of 2.9and 3.5% indium-doped BiVO4 and pristine BiVO4

nanoparticles do not differ significantly (2.51 eV) butthat of 4.9% indium-doped BiVO4 nanoparticles isslightly larger (2.65 eV), which is in accordance with thedisplayed KM plots. Similar results are observed with theindirect band gap. The indirect band gap of 4.9%indium-doped BiVO4 nanoparticles is 2.45 eV and thatof other nanoparticles is 2.34 eV.

5 nm

130 d = 0.31

BiVO42.9% In-BiVO4

130 d = 0.31

5 nm

3.5% In-BiVO4

110 d = 0.47 nm

5 nm

4.9% In-BiVO4

110 d = 0.47 nm

5 nm

Fig. 4. Lattice fringes in pristine BiVO4 and In-doped BiVO4 nanoparticles.


The efficiency of recombination of photogeneratedcharge carriers in nanocrystalline semiconductors could bestudied by photoluminescence (PL) spectroscopy. Fig-ure 10 displays the room temperature PL spectra ofindium-doped BiVO4 and pristine BiVO4 nanoparticles.The doped BiVO4 and undoped BiVO4 nanoparticles exhi-bit blue, green, and yellow emissions. They display near-band-gap emission (NBE) and deep level emission (DLE),and the observed emissions are in agreement with the litera-ture.39–41 The luminescence at ~483 nm correspondsto the direct band gap energy and is because of the

recombination of hole formed in the O 2p orbitals of theVB with the electron in the V 3d orbitals of the CB. Theemissions at about 532 and 580 nm (DLE) are likely to bedue to crystal defects. Strong NBE reflects the large recom-bination of photogenerated electron–hole pair. Figure 10shows that recombination charge carriers are more in 4.9%In-doped BiVO4 than in other synthesized nanoparticles.

The dopant, its concentration, and the method ofdoping influence the electrical properties of semiconductornanocrystals. Solid-state impedance spectroscopy (IS) isuseful to study the electrical properties of semiconductor

I55

141051222123

130 042 020 011200231

50 1/nm 4.9% In-BiVO4

200141

020

040

112123

130

231

3.5% In-BiVO450 1/nm

042202020

132121013123

130

2.9% In-BiVO450 1/nm

BiVO450 1/nm

013002130110220231

141

040

202

123

112220

222

130

2 1/nm3.5% In-BiVO4

2 1/nm4.9% In-BiVO4

220040222

123

202

2 1/nm2.9% In-BiVO4

152

013 040

123202

Fig. 5. SAED patterns obtained at two different area of the sample.


nanocrystals. The dynamics of the mobile and boundcharges in the bulk or interfacial region of the semiconduc-tor could be investigated. In polycrystalline materials, the

overall crystal resistance is a combination of intragranularresistance and intergranular resistance. The impedancedata are analysed in terms of an equivalent circuit model.An electrode interface undergoing an electron-transfer pro-cess is typically analogous to an electric circuit consisting

Inte

nsity

, a.u

.

20 30 40 50 60 70 802θ, 0

( ) monoclinicBiVO4

(042

)

(051

)

(002

)

(161

)

(123

)

(240

)

(251

)(200

)

(121

)

(011

)

(040

)

(211

)(110

)

(321

)

(202

)

( ) monoclinic2.9% In-BiVO4

(042

)

(150

)

(002

)

(161

)

(123

)

(240

)

(251

)

(200

)

(121

)(0

40)

(211

)

(110

)

(132

)

(202

) (321

)


(042

)

(150

)

(002

)

(161

)

(123

)

(240

)

(251

)

(200

)

(121

)(0

40)

(211

)

(110

)

(132

)(011

)

(321

)

(202

)


(011

)

(121

)(0

40)

(200

)

(051

)(1

32)

(240

)(2

02)

(161

)

(123

)

(112

)

Fig. 6. XRDs of pristine BiVO4 and In-doped BiVO4 nanoparticles.

Inte

nsity

, a.u

.

Wavenumber, cm-1300 600 900 1200

4.9% In-BiVO4

3.5% In-BiVO4

2.9% In-BiVO4

BiVO4

Fig. 7. Raman spectra of pristine BiVO4 and In-doped BiVO4

nanoparticles.


of a specific combination of capacitors and resistors. By fit-ting the IS data to a model or an equivalent circuit, theelectrical properties are inferred. The uncompensated resis-tance (RΩ) corresponds to the intergranular resistance, andthe charge-transfer resistance (RCT) refers to the intragran-ular resistance; the latter is related to the Warburg resis-tance, which is the resistance to mass transfer. It iscontrolled by the specific conductance (r). The constantphase element is associated with a nonuniform distributionof current due to material heterogeneity and is equivalentto double-layer capacitance (C). The solid-state compleximpedance spectra of indium-doped BiVO4 and pristineBiVO4 at room temperature are displayed in Fig. 11. Thedoped and undoped BiVO4 nanoparticles exhibit a perfectsemicircular Nyquist plots, which is the expected responseof the simple circuit. The equivalent circuit consists ofohmic resistance (RΩ) and charge-transfer resistance (RCT)in parallel to the constant phase element (CPE). Thediameter of the semicircle yields the RCT representing theefficiency of charge transfer. As there is no study on thesolid-state impedance spectroscopy of pristine BiVO4 aswell as doped BiVO4 nanocrystals, the present resultscould not be compared with the literature. However, thecurrent results are compared with those of three-electrodesolution impedance studies, which provide information onelectrode–electrolyte interface but not on solid state.While FTO/WO3/BiVO4/Au electrode in an aqueousNa2SO4 solution under illumination displays a perfectsemicircular Nyquist plot, FTO/WO3/BiVO4 exhibits ahalf-semicircular arc and FTO/BiVO4 shows a quasi-linearrelationship between ZIm and ZRe, all under identical

K-M

Wavelength, nm

0

1.25

2.5

3.75

5

300 500400 600

4.9% In-BiVO43.5% In-BiVO4

2.9% In-BiVO4

BiVO4

Fig. 8. The diffuse reflectance spectra.

h , eV2 3 4 5 61

[F(R

)hν]

2

0

100

200

300

400

500

2.51 eV 2.65 eV

4.9% In-BiVO43.5% In-BiVO42.9% In-BiVO4BiVO4

[F(R

)hν]

0.5

0

1

2

3

4

2.45 eV2.34 eV


Fig. 9. The direct (bottom) and indirect (top) band gaps.

Inte

nsity

, a.u

.

Wavelength, nm450 500 550 600 650

0

20

40

60


Fig. 10. The PL spectra.


experimental conditions.42 BiVO4 thick film electrode inNaOH solution under visible-light illumination with Pt ascounter electrode and saturated calomel electrode as refer-ence electrode exhibits a quasi-linear ZIm–ZRe correla-tion.43 However, microwave synthesized44 BiVO4 and alsohydrothermally synthesized45 BiVO4 show perfect semicir-cular Nyquist plots, both under three-electrode systemwith saturated calomel electrode as the reference electrodeand Pt wire and Pt foil, respectively, as the indicator andcounter electrodes; 0.01 M Na2SO4 and 1 M H2SO4 werethe supporting electrolyte employed, respectively. Fig-ure 11 shows that the charge-transfer resistance of 4.9%indium-doped BiVO4 nanoparticles is less than that ofpristine BiVO4 as well as 2.9% indium-doped BiVO4

nanoparticles.Figure 12 displays the time profile of dye degrada-

tion photocatalysed by indium-doped BiVO4 nanoparti-cles. Methylene blue was the test dye used in this study.The profile of dye degradation with pristine BiVO4

nanoparticles as photocatalyst is also displayed for com-parison. The dye does not degrade in the absence ofnanoparticulate photocatalyst. The experimental condi-tions are given in the experimental section. The visible-light photocatalytic activity of indium-doped BiVO4

nanoparticles is larger than that of pristine BiVO4

nanoparticles. Furthermore, there is no large differencein the photocatalytic activities of the three synthesizedIn-doped BiVO4 nanoparticles.

Although the band gap and NBE of 4.9% indium-doped BiVO4 nanoparticles are larger than the otherdoped as well as pristine BiVO4 nanoparticles, the pho-tocatalytic activity of 4.9% In-doped BiVO4 is not less

than the other doped BiVO4 nanoparticles. A possiblereason is its least charge-transfer resistance (Fig. 11); lowcharge-transfer resistance favors migration of charge carri-ers to the crystal surface, a prerequisite for semiconduc-tor photocatalysis.

Furthermore, the mean crystallite size of 4.9%In-doped BiVO4 (20 nm) is the smallest among the syn-thesized In-doped and undoped BiVO4 nanoparticles.Smaller is the crystal size lesser is the recombination ofphotoformed electron–hole pairs. The average time (s)taken by the randomly generated charge carrier to diffusefrom the bulk to the surface is given by s = r2/pD,where r is the grain radius and D is the diffusion coeffi-cient of the charge carrier.41 Decrease of the grain sizedecreases the recombination opportunities of the photo-generated electron–hole pairs, which are to diffuse effec-tively to the crystal surface to generate reactive oxygenspecies. The low charge-transfer resistance and small crys-tallite size of 4.9% In-doped BiVO4 photocatalyst out-weigh its enhanced NBE and increased band gap toprovide large photocatalytic activity. Although the (i)average crystallite size of some of the In-doped BiVO4 isnot smaller than that of pristine BiVO4, (ii) the intensityof NBE of the In-doped BiVO4 is not less than that ofpristine BiVO4, (iii) the band gap of the In-dopedBiVO4 is not smaller than that of undoped BiVO4, and(v) the charge-transfer resistance of some of the In-dopedBiVO4 is not less than that of pristine BiVO4, the pho-tocatalytic activities of all the In-doped BiVO4, underidentical experimental conditions, are larger than that ofpristine BiVO4. A possible reason may be enhanced (i)adsorption of the reactant molecules on the illuminated

ZRe, MΩ

-ZIm

, MΩ

0.2

0.4

0

0.6

0 0.3 0.6 0.9 1.2

BiVO4

2.9% In-BiVO4

4.9% In-BiVO4

Fig. 11. Solid-state impedance spectra of pristine BiVO4 and In-doped BiVO4 nanoparticles.

[Met

hyle

ne b

lue]

, ppm

Time, min0 30 60 90 120 150 180

25

0

10

15

20

5

4.9% In-BiVO43.5% In-BiVO42.9% In-BiVO4

BiVO4

No Catalyst

Fig. 12. Visible-light photocatalytic activities of pristine BiVO4

and In-doped BiVO4 nanoparticles.


surface of In-doped BiVO4, and (ii) desorption of thereaction products from the illuminated In-doped BiVO4

surface, both could not be determined by experiments.

Conclusions

Hydrothermally synthesized indium (2.9, 3.5 and4.9 at. %)-doped monoclinic BiVO4 nanoparticles showlarger visible-light photocatalytic activity than hydrother-mally obtained pristine monoclinic BiVO4 nanoparticles.Doping BiVO4 with indium does not change the latticeparameters of BiVO4. The band gap and NBE of 4.9%In-doped BiVO4 are larger than those of the other syn-thesized nanoparticles; In-doping up to 3.5% does notmodify the band gap and NBE. The charge-transferresistance of 4.9% In-doped BiVO4 is less than those ofthe other synthesized materials. The observed photocata-lytic activities have been analysed in terms of particlesize, band gap, NBE, charge-transfer resistance, etc. Theresults show that the enhanced visible-light photocatalyticactivity is not significantly affected by the variation ofthe dopant concentration between 2.9 and 4.9 at. % andhence is suitable for product development; a small varia-tion in the percentage of doping during the manufactur-ing process may not affect the product performance.

Acknowledgments

Prof. C. Karunakaran is thankful to the Council ofScientific and Industrial Research (CSIR), New Delhi [21(0887)/12/EMR-II] and the Science and EngineeringResearch Board (SERB), Department of Science andTechnology (DST), New Delhi (SR/S1/PC-41/2011), forproviding funds to this study. The authors also thank Dr.J. Jayabharathi, Annamalai University for the PL facility.

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Documents

Synthesis of Nanoparticulate In-Doped BiVO 4 for Enhanced Visible-Light Photocatalytic Degradation of Dye