Synthesis of Spherical Bi 2 WO 6 Nanoparticles by a Hydrothermal Route and Their Photocatalytic Properties

Spherical Bi 2 WO 6 nanoparticles were synthesized by a hydrothermal route. SEM observation shows that the size of the particles ranges from 60 to 120 nm and the average particle size is ∼85 nm. TEM investigation shows that the particles are made up of subgrainswith size of 5–10 nm.Thebandgap energy of the particles ismeasured to be 2.93 eVbyultraviolet-visible diffuse reflectance spectroscopy. RhB was chosen as the target pollutant to evaluate the photocatalytic activity of the particles under irradiation of simulated sunlight, revealing that they exhibit an obvious photocatalytic activity. The effects of ethanol, KI, and BQ on the photocatalytic efficiency of Bi 2 WO 6 particles towards the RhB degradation were investigated. It is observed that ethanol has no effect on the photocatalytic degradation of RhB, whereas KI and BQ exhibit a substantial suppression of RhB degradation. No hydroxyl (∙OH) is found, by the photoluminescence technique using terephthalic acid as a probe molecule, to be produced over the irradiated Bi 2 WO 6 particles. Based on the experimental results, photoexcited hole (h) and superoxide (∙O 2 ) are suggested to be the two main active species responsible for the dye degradation, while ∙OH plays a negligible role in the photocatalysis.


Experimental
All raw materials and reagents used were of analytical grade without further purification.0.33 g of Na 2 WO 4 ⋅2H 2 O was dissolved in 20 mL distilled water to form solution A, and 0.97 g of Bi(NO 3 ) 3 ⋅5H 2 O was dissolved in 20 mL acetic acid solution (2.5 mol⋅L −1 ) to form solution B. The above process was accompanied by a constant magnetic stirring to make the additives dissolve fully.Then solution A was slowly added to solution B drop by drop under constant magnetic stirring, and immediately a milk-white suspension solution was formed.After being further stirred for 30 min, the suspension solution was adjusted to a pH of 10 by adding NaOH solution (2.5 mol⋅L −1 ) and then filled up to 70 mL by adding distilled water.The resultant solution was transferred and sealed in a stainless steel autoclave with a Teflon liner of 100 mL capacity and submitted to hydrothermal treatment at a certain temperature.After a certain time of reaction, the autoclave was naturally cooled down to room temperature.The resultant yellowish precipitate was collected and washed several times with distilled water and absolute ethanol and then dried in a thermostat drying oven at 60 ∘ C for 8 h to obtain final Bi 2 WO 6 particles.
The phase purity of the as-prepared Bi 2 WO 6 particles was examined by means of X-ray powder diffraction (XRD) with Cu K radiation.The particle morphology and microstructure were investigated by field-emission scanning electron microscope (SEM) and field-emission transmission electron microscope (TEM).The ultraviolet-(UV-) visible diffuse reflectance spectrum was measured using a UV-visible spectrophotometer equipped with an integrating sphere attachment.
The photocatalytic activity of Bi 2 WO 6 particles was evaluated by the degradation of RhB under simulated-sunlight irradiation from a 200 W xenon lamp at room temperature.RhB was dissolved in distilled water to make a 2 mg⋅L −1 RhB solution.The photocatalyst loading was 0.1 g in 100 mL of RhB solution.Before illumination, the mixed solution was mildly stirred by a magnetic bar for 1 h in the dark to reach the adsorption-desorption equilibrium of RhB on the photocatalyst particles.During the photocatalysis experiment, the water-jacketed reactor was cooled with water-cooling system to keep the solution at room temperature.At given irradiation time intervals, a small amount of the reaction solution was sampled for examining the RhB concentration, which was determined by measuring the absorbance of the solution at a fixed wavelength of 554 nm using a UV-visible spectrophotometer.Before the absorbance measurements, the reaction solution was centrifuged at 3000 r⋅min −1 for 10 min to remove the photocatalyst.The effects of ethanol, KI, and benzoquinone (BQ) on the photocatalytic efficiency were investigated, aiming at revealing the dominant species responsible for the dye degradation.A photoluminescence (PL) technique was used to examine the hydroxyl (•OH) radicals formed over the irradiated Bi 2 WO 6 as well as P25 photocatalysts by using terephthalic acid (TPA) as a probe molecule.TPA tends to react with •OH radicals to produce 2-hydroxyterephthalic acid (TAOH) that is a highly fluorescent compound.The PL intensity of TAOH at around 429 nm is in proportion to the amount of produced •OH radicals.TPA was dissolved in sodium hydroxide solution (1.0 mmol⋅L −1 ) to make a 0.25 mmol⋅L −1 TPA solution.0.1 g of the photocatalyst was added to 100 mL of the TPA solution.After being magnetically stirred for several minutes in the dark, the mixed solution was irradiated by a 200 W xenon lamp (simulated sunlight).The reaction solution was centrifuged at 3000 r⋅min −1 for 10 min to remove the photocatalyst.The upper clear solution in the centrifuge tube was used for the PL measurements at a fluorescence spectrophotometer with the excitation wavelength of 315 nm.

Results and Discussion
Figure 1 shows the XRD patterns of Bi 2 WO 6 particles prepared at different hydrothermal reaction temperatures and times (the samples are separately denoted as 200 ∘ C-24 h, 140 ∘ C-24 h, and 200 ∘ C-8 h).It is seen that all the diffraction peaks of Bi 2 WO 6 particles can be indexed in terms of the orthorhombic Bi 2 WO 6 phase (PDF card number 73-2020), and no traces of other impurity phases are detected in the XRD patterns.
Figures 2(a)-2(c) show the SEM images of 200 ∘ C-24 h, 140 ∘ C-24 h, and 200 ∘ C-8 h samples, respectively, revealing that the particles are regularly shaped like spheres without any adhesive behavior.For all the samples, the size of the particles ranges from 60 to 120 nm and the average particle size is ∼85 nm.This indicates that the hydrothermal reaction temperature and time have almost no effect on the particle size.
Figure 3(a) shows the TEM image of 200 ∘ C-24 h sample, revealing a spherical morphology with an average particle size of ∼85 nm, which is well in agreement with the SEM  observation.The selected area electron diffraction (SAED) pattern of a single particle is inserted in Figure 3(a), which presents clearly polycrystalline diffraction rings.This indicates that the individual particle is not a single crystal but is composed of several grains or subgrains.Figure 3(b) gives the high-resolution TEM image obtained from a particle, showing that the particle is made up of subgrains with size of 5-10 nm.
Figure 4 shows the UV-visible diffuse reflectance spectra of Bi 2 WO 6 samples.The insert in Figure 4 shows the corresponding first derivative of the reflectance () with respect to wavelength  (i.e., /).The absorption edge ascribable to the electron transition from valence band to conduction band can be determined from the peak wavelength in the first derivative spectra.All the samples have a similar absorption edge located at 423 nm, from which the bandgap energy   of the samples is obtained to be 2.93 eV.
Figure 5 shows the photocatalytic degradation of RhB over Bi 2 WO 6 samples as a function of irradiation time (), along with the blank experiment results. 0 is the initial concentration of RhB and   is the RhB concentration after irradiation for time .In the absence of the photocatalyst, RhB appears to be stable under simulated-sunlight irradiation and its degradation percentage is only ∼6.1% after 5 h of exposure.

Adsorption Degradation
Figure 6: Effects of ethanol (5% by volume), KI (1.0 mmol⋅L −1 ), and BQ (1.0 mmol⋅L −1 ) on the photocatalytic degradation rate of RhB at a testing of 5 h (where 200 ∘ C-24 h sample is used as the photocatalyst), along with the corresponding adsorption rates after 1 h in the dark.
In the absence of simulated-sunlight irradiation, Bi 2 WO 6 samples show a moderate adsorption toward RhB (about 5.2-6.3% after 1 h of adsorption).On irradiation with simulated sunlight in the presence of Bi 2 WO 6 samples, the degradation of RhB increases substantially with increasing irradiation time and reaches ∼76.4,∼72, and ∼74.9% after 5 h of irradiation for 200 ∘ C-24 h, 140 ∘ C-24 h, and 200 ∘ C-8 h samples, respectively.These values are much larger than the blank degradation as well as the adsorption value, implying a pronounced photocatalytic activity of as-prepared Bi 2 WO 6 particles toward the dye degradation.
It is known that ethanol and BQ can be, respectively, used as the scavenger of •OH and superoxide (•O 2 − ), whereas KI is an efficient scavenger to both •OH and photoexcited h + [31,32].By investigating the effects of ethanol, KI, and BQ on the photocatalytic efficiency of Bi 2 WO 6 particles toward the RhB degradation, we can clarify the role of •OH, h + , and •O 2 − in the photocatalysis.Figure 6 shows the effects of ethanol (5% by volume), KI (1.0 mmol⋅L −1 ), and BQ (1.0 mmol⋅L −1 ) on the photocatalytic degradation rate of RhB at testing of 5 h, where 200 ∘ C-24 h sample is used as the photocatalyst.The corresponding adsorption rate after 1 h in the dark is also given in Figure 6, which demonstrates a minor change when adding the scavengers.The photocatalytic results show that ethanol has almost no effect on the photocatalytic degradation of RhB.This indicates a negligible role of •OH in the dye degradation.However, KI and BQ exhibit a substantial suppression of RhB degradation, which implies that h + and •O 2 − are the dominant active species responsible for the dye degradation over Bi 2 WO 6 particles.
Figure 7 shows the PL spectra of the TPA solution after reaction for 3 h over the simulated-sunlight irradiated Bi 2 WO 6 (200 ∘ C-24 h sample) and P25 photocatalysts.It is well known that •OH radicals can be readily generated over the irradiated commercial Degussa P25 (a mixed-phase TiO 2 photocatalyst) in water solution [33].As a result, when P25 is used as a photocatalyst, the TPA reaction solution shows obvious PL signal at around 429 nm.However, on irradiation in the presence of Bi 2 WO 6 photocatalyst, the TPA reaction solution shows no PL signal at around 429 nm, indicating no •OH radicals produced over the irradiated Bi 2 WO 6 photocatalyst.This result further reveals that •OH radicals play a minor or negligible role in the dye degradation.Generally, the conduction band (CB) and valence band (VB) edge potentials of a semiconductor play a key role in the photocatalysis process.In Bi 2 WO 6 , the hybridized O 2p and Bi 6s states form the top of valence band while the W 5d states form the bottom of conduction band, where the energy difference between the valence and conduction bands is known as the forbidden energy gap (  ) [34].The conduction band and valence band potentials of Bi 2 WO 6 can be calculated using the following relation [35]: where  is the absolute electronegativity of Bi 2 WO 6 (defined as the arithmetic mean of the electron affinity and the first ionization of the constituent atoms) and is estimated to be 6.2 eV according to the data reported in the literature [36],  e is the energy of free electrons on the hydrogen scale (4.5 eV), and   is the bandgap energy of Bi 2 WO 6 (2.93 eV).The conduction band and valence band potentials of Bi 2 WO 6 particles versus normal hydrogen electrode (NHE) are therefore calculated to be +0.24 and +3.17 V, respectively.When Bi 2 WO 6 is irradiated with light of energy greater than its   , electrons are excited from the valence band to the conduction band, thus generating e − /h + pairs.The photoexcited electrons and holes migrate to the Bi 2 WO 6 particle surface and participate in a series of redox reactions to produce a number of active species.Generally, •O 2 − that is suggested to be a dominant active species in the photocatalysis is derived from the reaction of the photoexcited e − and O 2 .From a thermodynamic point of view, the conduction band potential of a semiconductor is required to be negative to the redox potential of O 2 /•O 2 − (−0.13 V versus NHE [37]) so that the reaction between e − and O 2 can proceed.Although the conduction band potential of Bi 2 WO 6 particles calculated from (1) is positive to the redox potential of O 2 /•O 2 − , their actual value in the solution could be changed.It is noted that the conduction band potential of a semiconductor has an increasing trend with the solution pH [38].In our photocatalytic experiments, the pH value of the reaction solution is measured to be 6.5, which is much higher than (pH = 0) when calculating the conduction band potential.This indicates that the conduction band potential of Bi 2 WO 6 particles in the solution is expected to undergo a negative shift compared to their calculated value, consequently making the generation of •O 2 − from the reaction between e − and O 2 thermodynamically possible.
The redox potentials of OH − /•OH and H 2 O/•OH are +1.89 and +2.72 V versus NHE [39], respectively, both of which are negative to the valence band potential of Bi 2 WO 6 (+3.17V).From this point of view, it seems that the photoexcited h + can readily react with OH − and H 2 O to produce •OH radicals.However, no •OH is found to be produced over the irradiated Bi 2 WO 6 particles.A possible reason is that the photoexcited h + forms as Bi 5+ oxidation state, and the redox potential of Bi 5+ /Bi 3+ , being +1.59 V versus NHE [40], is negative to those of OH − /•OH and H 2 O/•OH.This makes it reasonable that the photoexcited h + cannot react with OH − /H 2 O to generate •OH.
The reusability of Bi 2 WO 6 particles was examined by the recycling photocatalytic experiment.After the first cycle of the photocatalysis was completed, the catalyst was collected by centrifugation, washed with water, and dried.The recovered catalyst was introduced to the fresh RhB solution for the next cycle of the photocatalytic experiment under the same conditions.This process was repeated four times.Figure 8 shows the degradation of RhB after simulatedsunlight irradiation for 5 h over Bi 2 WO 6 particles (200 ∘ C-24 h sample) during the five photocatalytic cycles.It reveals that the degradation percentage of RhB maintains 75% to 76% for five consecutive recycles.This indicates an excellent photocatalytic reusability of Bi 2 WO 6 particles.

Conclusion
Spherical Bi 2 WO 6 nanoparticles with an average size of ∼85 nm were synthesized by a hydrothermal route.The individual particle is made up of subgrains with size of 5-10 nm.The bandgap energy of the particles is obtained to be 2.93 eV.The photocatalytic activity of the particles was evaluated by degrading RhB under simulated-sunlight irradiation, and the effects of ethanol, KI, and BQ on the photocatalytic efficiency were investigated.The results reveal that the photocatalytic degradation of RhB has no dependence on ethanol but is substantially suppressed by the addition of KI and BQ.No •OH is found to be produced over the irradiated Bi 2 WO 6 particles.Based on the experimental results, we suggest that h + and •O 2 − are the dominant active species causing the dye degradation, while •OH plays a negligible role in the photocatalysis.In addition, Bi 2 WO 6 particles exhibit good stability in their photocatalytic activity according to recycling photocatalytic experiments.

Figure 1 :
Figure 1: XRD patterns of Bi 2 WO 6 particles prepared at different hydrothermal reaction temperatures and times.

Figure 3 :
Figure 3: (a) TEM image of Bi 2 WO 6 particles (200 ∘ C-24 h sample).The insert shows the SAED pattern of a single particle.(b) High-resolution TEM image obtained from a particle.

Figure 4 :Figure 5 :
Figure 4: UV-visible diffuse reflectance spectra of Bi 2 WO 6 samples.The insert shows the corresponding first derivative of the diffuse reflectance spectra.

Figure 8 :
Figure 8: Degradation of RhB after simulated-sunlight irradiation for 5 h over Bi 2 WO 6 particles during five photocatalytic cycles.