Influence of precursor aging time period on physical and photocatalytic properties of nebulizer spray coated BiVO4 thin films

Solid State Sciences 92 (2019) 36–45

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Influence of precursor aging time period on physical and photocatalytic properties of nebulizer spray coated BiVO4 thin films

T

C. Ravi Dhasa,∗, D. Arivukarasana, R. Venkatesha, A. Juliat Josephinea,b, K.C. Mercy Gnana Malara, S. Esther Santhoshi Monicaa, B. Subramanianc a

PG and Research Department of Physics, Bishop Heber College (Autonomous), Tiruchirappalli, 620 017, India Department of Physics, Holy Cross College (Autonomous), Tiruchirappalli, 620 002, India c Electrochemical Materials Science Division, CSIR- Central Electrochemical Research Institute, Karaikudi, 630 003, India b

A R T I C LE I N FO

A B S T R A C T

Keywords: BiVO4 Precursor aging Jet nebulizer Photocatalyst

BiVO4 thin films were deposited on glass substrates using nebulizer spray deposition route for different precursor solution aging period. The effect of the solution aging on structural, optical and chemical composition was investigated through different analytical techniques such as X-ray diffraction (XRD), scanning electron microscope (SEM), Raman, UV–Visible spectrometer, photoluminescence (PL) and Energy Dispersive X - ray analysis (EDX). It was observed that aging time of precursor solution strongly affects the growth of preferred orientation and morphology of as-deposited BiVO4 thin films. The photocatalytic performance of the deposited BiVO4 films was studied under visible light dye degradation experiments with rhodamine B (RhB) as test effluent. The photocatalytic experiments revealed that BiVO4 thin films deposited from aged solution exhibited better activity. The recycle tests were also conducted to analyze the chemical stability and reusability of the optimum BiVO4 film in dye solution.

1. Introduction Organic dyes are commonly used as coloring agents in textile, pharmaceutical, food, cosmetics, plastics, photographic and paper industries [1]. More than ten thousand varieties of complex organic dyes and pigments are available which are highly toxic when mixed with water resources causing severe hazards to human and aquatic life [2]. Research community has paid more attention to removing organic pollutants through various physical, chemical, physicochemical, adsorption, advanced oxidation process (AOP), coagulation, and biochemical processes [3]. Among the available methods photocatalysis is more attractive since it requires less capital for instrumentation and utilizes sunlight for dye degradation [4]. TiO2 [5,6] is the highly explored semiconductor for photocatalytic process; however, the band gap lies in the ultraviolet region which is a limiting factor in its utilization under direct sunlight. The prime focus is to develop visible light responsive photocatalytic materials with broad absorption range. Recently bismuth vanadate (BiVO4) is more promising and especially mBiVO4 which has a band gap of 2.4 eV is a potential candidate for organic dye removal and water - splitting reactions [7,8]. Photocatalytic reaction process is highly sensitive to size and shape of the semiconducting materials [9,10]. It is often a great challenge to adopt a ∗

suitable chemical method to design appropriate size of semiconductor crystals without altering its optical properties and chemical stability. So far, BiVO4 thin films have been synthesized by traditional methods such as pulsed laser deposition (PLD), spin coating, electron beam evaporation, spray pyrolysis, chemical bath deposition, etc. [7,11,12]. Jet nebulizer spray pyrolysis technique has many advantages compared to other solution-based methods such as cost-effective, facile method to fabricate large area coatings and chemical composition of the films being easily controlled for alloy and doped films. Particularly, it is a unique method which has the ability to produce controlled droplet-size distributions, ensuing strongly adherent, well crystalline quality with different morphological features thin films being formed [13]. The role of aging time period of precursor solution has profound impact on the nucleation and growth of thin film coating. The solution aging provides better chemical bonding between complex species of precursors and the solvent which in turn determines the size and shape of the crystalline grains in thin film layers. Li et al. have studied the physical and optical properties of sol-gel processed ZnO thin films using aged precursor solution for hours and ZnO thin film prepared at 24 h aged solution displayed better physical properties [14]. Ananthi et al. has reported that the 4 day aged ZnO thin film shows minimum resistivity prepared by spray pyrolysis technique [15]. Toubane and their

Corresponding author. Head PG and Research Department of Physics Bishop Heber College (Autonomous), Tiruchirappalli, 620 017, Tamil Nadu, India. E-mail address: [email protected] (C.R. Dhas).

https://doi.org/10.1016/j.solidstatesciences.2019.04.006 Received 8 February 2019; Received in revised form 17 April 2019; Accepted 19 April 2019 Available online 25 April 2019 1293-2558/ © 2019 Elsevier Masson SAS. All rights reserved.

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rhodamine B (RhB) in distilled water (5 mg/L−1) and continuously stirred for 30 min. The dye solution to be degraded was poured into sample holders and the prepared BiVO4 thin film photocatalyst was immersed in the dye solution and kept in darkness to achieve adsorption –desorption equilibrium between dye and the catalyst. For every 30 min interval, 3 ml of suspension was taken out from the reactor to measure the degradation rate of RhB using UV-VIS-NIR spectrophotometer (AnTech UV 7000). The total duration of 180 min was kept constant for the entire photocatalytic reaction process.

group have reported the effect of aging and number of layers on photocatalytic properties of ZnO nanorods [16]. The effect of aging time period on precursor solution in preparation of BiVO4 thin films has not been reported to the best of our knowledge. Keeping all the above - mentioned valuable points in our mind, in this present study, we have reported for the first time the effect of different precursor solution aging time (fresh solution, 3, 6, 9, 12 and 15 day) on the physical properties of Bismuth vanadate (BiVO4) thin films deposited by versatile jet nebulizer spray pyrolysis route for photocatalytic degradation of rhodamine B.

3. Result and discussion 2. Experimental 3.1. XRD analysis 2.1. Material Fig. 2 shows the XRD patterns of BiVO4 thin films deposited for different aged precursor solutions (Fresh, 3, 6, 9, 12 and 15 days) using nebulizer spray technique. All BiVO4 films show sharp diffraction peaks which signify the better crystallinity of the deposited samples. The diffraction peaks observed at 2θ = 18.80°, 28.89°, 30.53°, 42.46°, 46.98° and 53.24° were indexed with (110), (−121), (020), (150), (240) and (161) planes respectively which is matched with standard monoclinic BiVO4 (JCPDS card no. 14–0688). An interesting observation is that the fresh and 3 days aged precursor solution sprayed films display random orientation of (110), (−121) and (020) planes whereas samples prepared using 6, 9 and 12 days aged precursor solutions show prominent orientation along (−121) plane. The above inference suggests that films prepared using aged precursor solution prefer c-axis orientation [21]. The aging of starting solution causes better condensation of active cationic species over the substrate and might lead to significant crystallite growth along predominant orientation plane. Similar observation have been made by other researchers [22]. No peaks related to mixed phases and impurities were detected for the aging period of 3–12 days which indicates the phase purity of the prepared films. Furthermore, extending the aging time to 15 days, sample transformation from the monoclinic scheelite BiVO4 phase has been transformed into tetragonal phase (JCPDS card no. 78-1535). This result is attributed to the chemical reaction between bismuth and vanadium ions in precursor solution over a prolonged aging period causing structural rearrangement during thermal decomposition [9]. The crystallite size (D) was calculated using Debye Scherer equation [5],

Bismuth (III) nitrate pentahydrate (Bi(NO3)3·5H2O), ammonium metavanadate (NH4VO3), nitric acid (HNO3) and deionized water were purchased from Merck (99% pure). The chemicals were directly utilized without further purification. 2.2. Preparation of BiVO4 photocatalytic thin films BiVO4 thin films were prepared by the low - cost and efficient nebulizer spray pyrolysis equipment. An equimolar concentration of bismuth nitrate (Bi (NO3)3·5H2O) and ammonium metavanadate (NH4VO3) were dissolved separately in diluted nitric acid (HNO3). Then vanadate - containing solution was slowly added into bismuth nitrate solution and subjected to vigorous stirring for 30 min to dissolve the additives completely. BiVO4 thin films were prepared with the following conditions: (i) using a fresh solution and (ii) using solution with different aging period such as 3, 6, 9, 12 and 15 days respectively. The prepared solutions were kept in dark at room temperature during the aging process. Recently our group has reported the working principle and mechanism of nebulizer spray pyrolysis technique and demonstrated the better efficiency of metal oxide and sulfide thin films for different applications such as self - cleaning [17], photovoltaic [18], electrochromic [19] and photocatalytic degradation [20]. The jet nebulizer spray working mechanism is a more advanced form than the conventional spray pyrolysis. Precursor chemicals were converted into aerosols with the help of jet. The schematic illustration of nebulizer spray coating unit is shown in Fig. 1. The optimum values of deposition parameters were fixed based on our earlier report [20]. The deposition parameters are listed in Table 1.

0.94λ βcosθ

D=

(1)

where ‘λ’ is the wavelength of Cu-Kα radiation, ‘β’ is the full width halfmaximum (FWHM) in radians, and ‘θ’ is the angle of incident radiation. The dislocation density (δ), texture coefficient, strain (ε) and lattice parameters (a, b, & c) of BiVO4 thin films were calculated from the following relations,

2.3. Characterization techniques The structural characterization of the BiVO4 thin films were carried out using a XPERT- PRO advanced diffractometer equipped with Cu-Kα radiation (1.5406 Å). The surface morphology of thin films was examined by ZEISS ULTRA - High Resolution Scanning Electron Microscope (HR -SEM). The chemical composition of the samples was determined by FEI Quanta X – ray energy dispersive analysis (EDAX). Optical absorbance was recorded in the range of 200–1000 nm using AnTech UV 7000 UV–VIS–NIR spectrophotometer. Room temperature photoluminescence emission spectra were recorded using FLUOROLOG FL-311 at the excitation wavelength (λ exc) of 383 nm. Raman spectra of BiVO4 thin films were obtained using Raman spectrophotometer (Reinshaw invia make) using Ar+ ion laser (532 nm).

δ=

1 D2

(2)

ε=

β cos θ 4

(3)

Tc =

I(hkl)/Io (hkl) (1/ n) ∑n (I(hkl)/Io (hkl))

2hl cos β h2 sin2 β 1 1 h2 l2 = ( 2 + + 2 − ) 2 2 2 d sin β a b c ac

2.4. Photocatalytic studies

(4)

(5)

where ‘d’ is the inter planar spacing I(hkl), I0(hkl) are the measured and standard intensities of the (hkl) plane and n is the total number of diffraction peaks present in the XRD. The obtained values are listed in Table 2. The crystallite size values were obtained in the range of 31–42 nm. The crystallite size values increased with aging of precursor solution

The photocatalytic activity of the as-prepared BiVO4 thin films was examined by Heber annular visible light photoreactor. A high-pressure tungsten halogen lamp of 300 W was used as a source, positioned within the double wall tube under continuous water circulation to avoid thermal effect. Dye solution was prepared by dissolving 37

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Fig. 1. Schematic diagram of jet nebulizer spray equipment. Table 1 Deposition parameters used for jet nebulizer sprayed BiVO4 thin film. Parameter

Condition

Solution aging time period Substrate temperature Spray volume Carrier gas pressure Area of the substrate Nozzle to substrate distance

Fresh, 3, 6, 9 12 and 15 days 350 °C 15 ml 1 kg/cm2 2.5 × 2 cm2 5 cm

which may be due to the improvement of chemical bonding between bismuth and vanadium precursor complexes and thereby favoring high thermal decomposition over the substrate [23]. This process facilitates better nucleation and growth of grains. However, for 12 day aged solution the crystallite size value decreases which indicates that larger aging duration of precursor solution is not beneficial for crystallite growth. Larger crystallite size with minimum dislocation density and microstrain has been observed for BiVO4 film deposited using 9 day aged solution. The lattice parameters observed for BiVO4 films (Table 2) are found to be in good agreement with standard values (a = 5.195 Å, b = 11.701 Å and c = 5.092 Å). The lattice parameter ‘c’ is found to be slightly increased for films prepared from aged solution. This shows the length of c-axis is found be extended upon solution aging which indicates that the preferential growth of crystalline grains due to minimization of crystal surface energy along this direction [24]. 3.2. SEM analysis The surface properties of the deposited films were characterized using scanning electron microscopy. The SEM images of BiVO4 thin films for different precursor solution aging period with two different magnifications are displayed in Fig. 3. It is observed that the BiVO4 film deposited using the fresh solution shows the uniform spherical shaped particles whereas the surface of BiVO4 films followed progressive growth of crystalline grains upon solution aging (3, 6, 9 and 12 days). The films prepared from aged solution show that the individual spherical shaped particles begin to agglomerate and overgrowth of rod-like structure was observed particularly for the films deposited using 6, 9,

Fig. 2. XRD patterns of BiVO4 thin films for different precursor aging time fresh, 3, 6, 9 and 15 days respectively.

and 12 days aged solution. The hierarchical overgrowth of rod shape on spherical grains will be beneficial to optical absorption and adsorption of dye molecules which might lead to high photocatalytic activity [25]. The length of micro-rods increased with respect to increase in solution 38

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films. This optical behaviour will be playing a vital role in photocatalytic degradation of organic pollutants.

Table 2 Crystalline parameters of BiVO4 thin films as a function of different precursor aging time are given below. Lattice parameter (Å)

Aging time (Day)

a

b

c

Fresh 3 6 9 12

5.19 5.19 5.19 5.19 5.19

11.67 11.68 11.70 11.68 11.69

5.09 5.10 5.11 5.12 5.10

Crystallite size D (nm)

31 33 36 42 36

Strain (ε)

0.0017 0.0016 0.0012 0.0010 0.0014

3.5. Raman analysis

Dislocation density δ x 1015 (lines/m2)

Raman spectroscopy is more useful to understand the local structure, electronic properties and nature of the bonds in any material [33]. Fig. 7 shows the Raman spectra of BiVO4 thin films prepared by the jet nebulizer technique for different aged precursor solutions. The as-deposited BiVO4 thin films exhibit five characteristic Raman vibrational bands at 212, 330, 367, 717 and 828 cm−1 corresponding to monoclinic scheelite BiVO4 which is in accordance with previously reported data [33,34]. The high intense peak observed around 828 cm−1 is attributed to the symmetric V- O stretching mode (Ag). A weak shoulder peak appearing at 717 cm−1 could be assigned to the anti symmetric V- O stretching mode. The small peaks were observed at 330 and 367 cm−1 due to symmetric (Ag) and antisymmetric bending vibrational modes (Bg) of vanadium ion. The peak obtained at 212 cm−1 can be attributed to the rotational/translation vibrational mode [35,36]. No other peaks were observed related to other impurity phases. This confirms that precursor aging period (3–12 days) does not encourage the formation of secondary/impurity phases which supports X-ray diffraction data. The intensity of Ag mode indicates better crystalline quality of BiVO4 films using aged precursor solution. The dominant intense peak 828 cm−1 has shifted marginally towards the lower wave number for films prepared from aged solutions when compared to the fresh solution sprayed BiVO4 film. Such results indicate that the short range symmetry of V – O tetrahedral may be affected by the following factors such as morphology and crystalline defects [37].

1.04 0.91 0.77 0.62 0.77

aging period which could be correlated with the c-axis growth of (−121) plane from X-ray diffraction studies. The schematic illustration describing the growth of BiVO4 films as a function of solution aging period is given in Fig. 4. It is well established that chemical growth of thin films inevitably involves the precipitation of molecular complex species over the substrate which is followed by several stages of nucleation process. The theory behind the growth mechanism of films from aged solution is still under research. However, from the existing literature, it can be understood that chemical reactivity between the metal complex species, (in the present case vanadium and bismuth ions) improved upon solution aging period [23,26–28]. The solution aging also alters the surface energy of crystallographic orientation of the resultant films on the basis of atomic species in precursor which in turn strongly determines the growth patterns. The surface overgrowth may be ascribed to the coalescence of smaller crystallite grains to form larger grains [28]. 3.3. EDX analysis

3.6. Photoluminescence analysis

EDX spectrum (Fig. 5) depicts the presence of elements such as Bismuth (Bi), Vanadium (V) and Oxygen (O) in coated BiVO4 thin film with 9 day aged precursor. The chemical compositions are given in the inset table. It is observed that the BiVO4 thin films exhibits better elemental stoichiometric ratio.

The efficiency of a semiconductor photocatalyst can be estimated from the rate of photogenerated electron – hole (e−/h+) pairs. Photoluminescence (PL) is a more useful tool in probing the charge separation and recombination of the photogenerated e−/h+ pair in semiconductors [38]. Fig. 8 shows the room temperature PL emission spectra of BiVO4 thin films deposited using precursor solution with different aging period. The strong intense peak observed at 548 nm (2.26 eV) corresponds to the near band edge emission (NBE) which is in good agreement with the band gap obtained from optical studies. The NBE peak is attributed to excitonic recombination of the hole formed in the Bi6s and O2p hybrid orbitals and the electron in V3d orbitals [39]. The PL emission peak is more sensitive to factors such as particle size, crystalline defects, and morphology in semiconducting thin films. It is found that rod - shaped growth appeared in BiVO4 thin films prepared using aged solutions and display low PL emission intensity compared with spherical grain structure of BiVO4 prepared from fresh solution. It is anticipated that unlike spherical grains, the vertically oriented growth of rod-like structure above the spherical grains acts as channels for effective charge transport avoiding recombination at grain boundary interfaces. Among the samples, BiVO4 films deposited using 9 day aged precursor solution show lower PL intensity which implies that it could be more suitable for photocatalytic applications. However, extending the precursor aging time to 12 days causes severe agglomeration of grains as shown in Fig. 3(e) which leads to trapping or recombination of photogenerated charge carriers at grain boundaries resulting in high PL intensity [40].

3.4. Optical properties The optical absorbance spectra of BiVO4 thin films deposition using different precursor aging time are shown in Fig. 6(a). The UV absorption bands observed at 344 and 374 nm are attributed to the electronic charge transfer between O2p and V3d orbitals and the visible absorption is due to electronic transition between Bi6s to V3d orbitals [29]. BiVO4 films deposited using aged solution exhibited strong absorption in the visible region compared to the film prepared from fresh solution. The steep absorption edge for the BiVO4 films is evident with high degree of crystallinity and improved bonding between metal complex species upon solution aging [30,31]. In addition, the red shift was observed with increase in the aging time and it could be correlated with the variation in the absorption edge. The optical band gap values of the prepared films were estimated from the following relation [7]:

(αhν)2 = A(hν − Eg)

(6)

where ‘Eg’ is the optical band gap, ‘α’ is the absorption coefficient of the film. ‘ν’ refers to the incident frequency of radiation and ‘A’ is a constant. Fig. 6(b) shows the plot of (αhν)2 vs. (hν) for different precursor aging time. The calculated band gap values were found to be 2.42, 2.40, 2.37, 2.27, and 2.34 eV respectively for fresh, 3, 6, 9 and 12 day aged solution processed BiVO4 thin films. The rod-like shape observed in the SEM micrographs increases the scattering of light within the grain boundaries and thereby improving the visible light absorption of BiVO4 thin films [32]. Therefore, precursor aging treatment is more favourable for tuning the optical absorption and band gap of the BiVO4 thin

3.7. Photocatalytic studies The photocatalytic activity of nebulizer - sprayed BiVO4 thin films for different solution aging period was examined by degradation of rhodamine B (RhB) as a test effluent under visible light illumination. The characteristic absorption peak of RhB (λmax = 552 nm) was used to analyze the photocatalytic degradation process. Fig. 9 (a) - (e) shows 39

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Fig. 3. SEM images of BiVO4 thin films prepared with various precursor aging time (a) fresh, (b) 3, (c) 6, (d) 9 and (e) 12 days respectively.

Fig. 4. Schematic representation of BiVO4 growth patterns on the glass substrates.

where ‘C0’ is the initial RhB dye concentration and ‘C’ is the concentration of dye at specific time interval t. The degradation efficiency of RhB was estimated to be 80, 83, 87, 96, and 78% for the BiVO4 thin films prepared using fresh and different aged solutions 3, 6, 9 and 12 days respectively. From the results, it is observed that the photocatalytic activity shows an increasing trend for aged BiVO4 films which is far better than the film deposited using fresh solution. The poor photocatalytic activity of film deposited using 12 day aged solution might be attributed to the agglomerated morphology observed from SEM analysis which reduces the surface to volume ratio and restricts the amount of incoming light radiation [41]. The maximum degradation efficiency of RhB was observed for BiVO4 film deposited using 9 days aged solution. The obtained spectral data obey first order kinetics and can be fitted using Langmuir - Hinshelhood model [5],

Fig. 5. EDX spectra of the BiVO4 thin film prepared with 9 days aged precursor solution.

ln(C / C0) = k appt the time dependent absorption profiles of RhB dye using BiVO4 thin film photocatalysts under visible light illumination. The linear decrement in the intensity of the absorption spectra of RhB is related to their breaking of chromophores rings, which are responsible for the color of the dyes. The dye degradation percentage was calculated using the following relation [36]:

Degradation percentage(%) = [

C0 − C ] × 100 C0

(8)

where kapp is the pseudo first order reaction rate constant, t is the reaction time, C0 and C refers to initial concentration and final concentration after reaction at specific time interval ‘t’. The apparent reaction rate constants have been determined from the slope of the plots (Inset Fig. 9) drawn between C/C0 and illumination time. The apparent reaction rate (Kapp) estimated using this model is given in inset of Fig. 9 (a) - (e) for films deposited using fresh and aged solution. The maximum apparent reaction rate with high linear regression (R2 = 0.99) was observed for the film deposited using 9 days

(7) 41

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Fig. 6. (a) Optical absorbance spectra and (b) Tauc plot of BiVO4 thin films as a function of precursor aging time (fresh, 3, 6, 9 and 12 day).

(b) growth of micro-rod formation on spherical grains which improves the overall photo-activity by providing active catalytic sites for adsorption of dye molecules and (c) narrow band gap with low recombination rate observed from PL and optical studies results in enormous generation of electron-hole pairs. Another important factor is the shift in optical absorption edge towards longer wavelength region. This shift indicates the changes in the conduction band edge (ECB) and valence band edge (EVB) position of the BiVO4. The position of conduction and valence band edge in BiVO4 at the point of zero charge is calculated from following relations [42]:

1 [Af + I1] 2

(9)

ECB = χ − E0 − 0.5Eg

(10)

χ=

where, χ is the bulk electronegativity of BiVO4, Af and I1 refers to electron affinity and first ionization potential, E0 is the energy of free electron on the hydrogen scale (4.5eV) and Eg is the energy band gap of the semiconductor. According to the values of Eg, the calculated ECB and EVB of fresh BiVO4 films were 0.32 eV and 2.75eV respectively. For 3 day aged sample the values were found to be 0.33 eV and 2.74eV and for 6 day aged sample the values were estimated to be 0.35 eV and 2.72 eV respectively for ECB and EVB. For 9 day aged sample the values were calculated to be 0.36 eV and 2.70 eV respectively. The conduction band edge of BiVO4 film deposited using 9 day aged solution was more negative in NHE potential scale compared to other films and hence there is a possibility of direct oxidation of RhB molecules by photogenerated holes [43]. The chemical stability of BiVO4 thin film photocatalyst (9 day aged precursor solution) in RhB dye was verified using recycle tests. The photocatalytic efficiency of the films was estimated from four successive cycles. After every reaction cycle, the thin film catalyst was washed with distilled water and dried in air which was reused for the next cycle. Fig. 10(a) shows the photodegradation efficiency of BiVO4 films for four cycles. There was no pronounced variation observed in photodegradation efficiency of the nebulizer spray - deposited BiVO4 film deposited using 9 day aged solution indicating that it could function as a stable photocatalyst for organic dye removal. The film stability was once again tested by analyzing the X-ray diffraction patterns before and after the photocatalytic reaction (4 cycles) which is displayed in Fig. 10 (b). It is worth mentioning that the crystal structure was retained even after 4 cycles. The slight variation in intensity could be attributed to the small fraction of dye molecules attached to the film even after washing which could be eliminated through proper heat treatment [6].

Fig. 7. Raman spectra of BiVO4 thin films prepared for different precursor aging time (fresh, 3, 6, 9 and 12 day).

Fig. 8. Photoluminescence spectra of BiVO4 thin films as a function of precursor aging time (fresh, 3, 6, 9 and 12 day).

of aged precursor solution. The enhancement observed in photocatalytic activity of 9 days aged BiVO4 film may be ascribed to the following reasons: (a) highly reactive c-axis growth of (−121) plane,

4. Conclusion The influence of precursor aging period on the physical properties of 42

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Fig. 9. Temporal evolution of RhB as a function of BiVO4 thin films prepared using different precursor aging time (a) Fresh, (b) 3 day, (c) 6 day, (d) 9 day, (e) 12 day and the inset graph denotes degradation kinetics of BiVO4 thin films.

Fig. 10. (a) Recycle test for the 9 day aged precursor sprayed BiVO4 film. (b) XRD pattern of 9 day aged BiVO4 film before and after photocatalysis reaction.

jet-nebulizer - sprayed BiVO4 thin films has been studied. It was found that structural, morphological, and optical properties of BiVO4 films improved upon precursor solution aging time. The XRD results reveal that the preferential (−121) growth orientation with different solution aging time could be owing to the improvement in chemical bonding between complex atomic species of precursors. From the SEM micrographs it can be confirmed that variation in morphology of the BiVO4 thin films from spherical structure into one dimensional BiVO4 rod-like structure and the length of the rods increased by extending the solution aging time. The better photocatalytic degradation of RhB dye exhibited by BiVO4 films deposited using 9 days aged solution with high chemical stability and reusability. The present study encourages the study of precursor aging effect of similar alloy compound thin films by employing nebulizer spray technique in future.

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