Correlation between crystallite size and photocatalytic performance of micrometer-sized monoclinic WO₃ particles

2.1 Micrometer-sized WO₃ catalyst preparation

Micrometer-sized WO₃ particles were prepared by heating APT-typed ammonium tungstate (ammonium tungstate pentahydrate powder (ATP); >99%; Kanto Chemical Co., Inc., Japan). In a typical experimental procedure, 2 g of ATP was heated using an electrical furnace in the atmospheric condition under a fixed condition: a heating rate of 50 °C/min and a holding time at a specific temperature for 30 min. To obtain the control of particle crystallinity, heating temperatures were varied from 150 to 800 °C. To gain monoclinic phase, the heated material was cooled to room temperature with a cooling rate of 50 °C/min. In addition, ATP was purchased and used without further purification.

2.2 WO₃ particles characterization

Several analyses were employed to characterize the physicochemical properties of the heated ATP. Thermal Gravimetry (TG) and Differential Thermal Analysis (DTA) (DTG-60A, Shimadzu Corp., Japan; operated at a heating rate of 5 °C/min with a flow of air (200 mL/min)) was used to analyze the mass transformation profile of ATP into WO₃ as a function of temperature. Fourier Transform Infrared Spectroscopy (FTIR; FTIR-8400, Shimadzu Corp., Japan) and X-ray diffraction (XRD; XRD; PANalytical X’Pert PRO; Philips Corp., The Netherland) were utilized to analyze the chemical composition and the crystal phase/pattern respectively. Scanning Electron Microscope (SEM; SEM, JSM-6360LA; JEOL Ltd., Japan) was used to characterize the particle outer diameter and morphology. The nitrogen sorption analysis (BET Nova 4200e; Quantachrome Instruments Corp., US; operated at 77 K) was also conducted to analyze the surface area and porous structure of the sample.

2.3 Photocatalytic performance analysis

For analyzing the effect of crystallite size on the photocatalytic performance, the produced WO₃ powder with a specific crystallite size was used for degrading 100 ppm of curcumin solution. The curcumin solution itself was obtained from extraction process of turmeric (Curcuma longa L; collected from Bandung, Indonesia), in which the extraction process is described in detail in elsewhere (Nandiyanto et al., 2017a, 2017b).

The mixed catalyst-curcumin suspension was put in the photocatalytic process. The photocatalytic process was carried out in a 400-mL capacity of borosilicate photochemical batch glass reactor having dimension of 10 and 8 cm for height and diameter, respectively. 36 W of Neon Lamp was used as the light source. The experiments were performed at ambient temperature between 25 °C and 30 °C at atmospheric condition. All experiments were stirred using a magnetic stirrer with a mixing rate of 600 rpm. To keep the concentration of dissolved oxygen in the suspension constant during experiments, 1 L/min of air was bubbled into the reactor.

Prior to each experiment, the catalyst-curcumin suspension was allowed to equilibrate for 5 min without any illumination. Then, the photocatalytic process was conducted for 6 h. The progress of the process was monitored by withdrawing definite quantity of aliquot and measured using a portable spectrophotometer (Vis-spectro, Indonesia; see reference (Nandiyanto et al., 2016b)) and UV–Vis spectrophotometer (Shimadzu UV–VIS-Spectrophotometer, UV-3150) for decoloration.

3.1 Preparation of WO₃ particles with controllable crystallite sizes

To analyze the impact of temperature for controlling the formation of WO₃, TG-DTA analysis was conducted (Fig. 1). The TG result showed the detection of loss in mass as a function of temperature. Decreases in mass were found gradually from the beginning of the heating process. However, after passing 500 °C, the mass was relatively stable. The decreases in mass are believed to have a correlation with the existence of reaction, strengthened by the DTA analysis for the detection of either an exothermic or an endothermic process at temperatures of 120, 190, 280, and 420 °C. The loss in mass in TG analysis was in a good agreement with the theoretical stoichiometrical approximation (see attached Table in Fig. 1).

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Fig. 1

TG-DTA analysis result of ATP. Attached table is a prediction of mass calculated using stoichiometry.

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The photograph images of samples produced with various temperatures are shown in Fig. 2. Physical appearances of WO₃ changed from white, orange, to yellowish green along with the changes in temperatures. Meanwhile, the initial ATP was white (Fig. 2a), and the color¹ did not change when ATP was heated with temperatures lower than 250 °C (Fig. 2b and c). Then, heating temperatures of about 380 °C gave the outcome for the production of material with orange color (Fig. 2d and e). Finally, the color changed into yellowish green gradually when using temperatures of more than 380 °C (Fig. 2f–h). This result confirmed the impact of the change in chemical composition on the material physical properties.

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Fig. 2

Photograph images of ATP heated with various temperatures. Figure (a) is the initial ATP powder. Figures (b), (c), (d), (e), (f), (g), and (h) are samples heated at 150, 210, 380, 480, 600, 700, and 800 °C respectively.

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The FTIR analysis results of ATP heated with various temperatures are shown in Fig. 3. The results revealed the changes in the chemical composition, as confirmed by the changes in the FTIR peaks and pattern. The FTIR of the initial ATP detected some peaks, corresponding to the tungstate-related specific bond (Nandiyanto et al., 2016a; Madarász et al., 2004) (in the range below 1000 cm⁻¹), OH molecules (Nandiyanto et al., 2009b) (at the band of about 1600 and 3600 cm⁻¹), and N-H bond (Nandiyanto et al., 2009b) (at the band of about 1400 and 3200 cm⁻¹). Then, the peak intensities relating OH and N-H bond decreased with the increasing temperature. Further, the peaks disappeared after 300 °C. This result verified that the chemical composition in the product was strongly dependent on the temperature used for the heating process, specifically relating to OH and NH-related molecule removal.

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Fig. 3

FTIR analysis results of ATP heated with various temperatures.

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Fig. 4 presents the XRD patterns of ATP heated with various temperatures. Changes in the patterns along with the selection of temperature process were detected. The spectra of initial sample were in a good agreement with that of pure ATP (Nandiyanto et al., 2016a; Madarász et al., 2004). By heating the sample at 150 °C, the disappearances of some peaks in the range of 20, 30–40, and higher than 50° were detected. The use of temperature of 280 °C resulted in the formation of materials with amorphous phase. Then, starting from 300 °C, all peaks were re-assigned. The detection of diffraction peaks corresponding to monoclinic WO₃ based on Joint Committee on Powder Diffraction Standards (JCPDS) No. 72-1465 was observed. No impurity peaks and no transition to other phases were detected, indicating that the products are pure in both chemistry and crystalline phase. The peak intensities become more prominent with the increasing temperature from 380 °C, indicating the formation of material with higher degree of crystallinity and larger crystallite size. Using the similar calculation method referred in Tahara et al. (2014), we found that in the degree of crystallinity case, all samples prepared at temperatures from 380 to 800 °C were completely crystalline. According to the Scherrer equation, the crystallite sizes of the sample increased from 19 to 42 nm with the increasing temperatures from 380 to 800 °C. Based on these results, the heating temperature plays an important role for regulating the degree of crystallinity and crystallite sizes in the product.

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Fig. 4

XRD analysis results of ATP heated with various temperatures.

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Fig. 5 depicts the SEM images of the samples produced with various temperatures. The results showed that agglomerated-free particles with polyhedra shapes were observed for all temperature variations. A change in the outer diameter was observed depending on the heating temperature (see Fig. 5 h). The mean outer diameters of initial ATP particles were about 120 μm (Fig. 5a), and the sizes decreased along with the increase of the temperatures (Fig. 5b–g). Increasing the level of temperature has promoted to the existence of smaller particles, in which these particles were probably from the breakage of initial ATP particles. However, in the case of process using temperature higher than 380 °C (See Fig. 5d–g), the constant particle size with the mean outer diameters of about 60 μm were obtained.

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Fig. 5

SEM images of ATP heated with various temperatures. Figure (a) is the initial ATP powder. Figures (b), (c), (d), (e), (f), and (g) are samples heated at 150, 210, 380, 480, 600, and 800 °C respectively. Figure (h) is the outer diameters of the particle versus heating temperatures.

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High-magnified SEM images are depicted in Fig. 6. The modification of surface morphology in the particle was found along with the change in temperature. A smooth surface was found for initial ATP particles (Fig. 6a). Adding heat into the ATP, the surface with smooth shape changed into that with some cracks (Fig. 6b). The cracks enlarged with increasing temperature (Fig. 6c and d). In addition to the cracks, the rougher surface was also observed when the temperature increased (Fig. 6d).

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Fig. 6

High magnification of SEM images of ATP heated with various temperatures. Figure (a) is the initial ATP powder. Figures (b), (c), and (d) respectively are samples heated at 210, 600, and 800 °C.

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Fig. 7 shows the nitrogen adsorption/desorption isotherms of samples prepared with various temperatures. Although there is a change in the maximum volume adsorbed values, the values are relatively small (less than 30 cc/g in STP condition).

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Fig. 7

Isotherm adsorption/desorption results of samples prepared with various temperatures.

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To confirm detailed information gained from nitrogen sorption analysis, Table 1 shows the characteristics of the samples (i.e., surface area, pore volume, micropore volume, and average pore radius). Results showed that the surface area increased from about 2–13 m²/g. Then, samples heated with temperatures higher than 380 °C had similar surface area of about 13 m²/g. These surface areas are relatively low compared to porous materials (Nandiyanto et al., 2009b, 2013). The increases in the surface area confirmed the changes of particle size as shown in Fig. 5. In short, increasing the processing temperature has a direct correlation to the production of smaller particles. Indeed, the smaller particles have a consequence to the higher surface area. Then, when the particle outer diameters are nearly the same (such as samples heated with temperatures of higher than 380 °C), the surface area will be the same.

The pore radiuses for all samples were less than 7 nm. This pore size range is relatively not active for photocatalytic process due to the limitations in the oxygen and light penetration into the deepest position in the pores (Nandiyanto et al., 2009a). Thus, the present study can assume that the photocatalytic process will be done on the outer surface of the catalyst only.

3.2 Photocatalytic performance of WO₃ particles with controllable crystallite sizes

The absorbance at all wavelengths decreased over time due to the degradation of curcumin. Using Beer’s Law, the absorbance can be re-expressed as a unit of concentration (Nandiyanto et al., 2009a).

The normalized curcumin concentrations (with respect to the initial concentration) over time during photocatalysis experiments are shown in Fig. 8. Photodegradation was measured for 6 h of the process, which was an ample amount of time to observe the differences in the photocatalytic performances of WO₃ with various crystallite sizes. WO₃ samples with a constant particle outer diameter were used to confirm that the photodegradation rate is from the effect of the crystallite size. As a comparison, we also investigated the catalytic performance of ATP and amorphous WO₃ particles. The evaluation of ATP and amorphous particles is to show the importance of crystallinity on the photocatalytic performance.

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Fig. 8

Photodecomposition profile of curcumin catalyzed by WO₃ particles with various crystallite sizes. The samples were conducted using 100 ppm of curcumin with 3.30 g/L of WO₃ catalyst in the ambient condition.

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Curcumin decomposing rates relied on the type of WO₃ catalysts used. The concentration of irradiated curcumin without additional WO₃ displayed insignificant discoloration (final concentration of 98%). Next, the slight change in the concentration was found when using ATP and amorphous WO₃. When WO₃ particles with crystallite sizes of 19, 23, 26, 30, and 42 nm were used as the catalyst, the final concentrations of curcumin were 80, 61, 55, 40, and 33%, respectively. Since the fastest degradation of curcumin was obtained when using WO₃ with larger crystallite, the result confirmed the important roles of the degree of crystallinity and crystal sizes on the photocatalytic performance.

To ensure the influence of crystallity parameter on the photocatalytic performance empirically, the Langmuir–Hinshelwood kinetic model for solid–solute reactions was used (Nandiyanto et al., 2009a). In short, the equation is expressed by

Fig. 9 shows the calculated photocatalytic rate versus crystallite size. To confirm the effect of crystallite size, the photodecomposition process of curcumin were tested in different catalyst loadings. The result showed that the more additional catalyst had correlation to the increases in the photocatalytic rate. But, too high concentration of catalyst added has a negative impact to the lowering photocatalytic performance. However, the main idea gained from this figure is to get the tendency in the crystallinity parameter compared to above analysis in Fig. 8. In general, all catalyst loading parameters agreed the increases in the crystallite size have a direct impact to the increases in the photocatalytic rate. For example, for process with catalyst loading of 3.30 g/L, the photocatalytic rate of WO₃ with crystallite sizes of 19, 23, 26, 30, and 42 nm were 0.049; 0.105; 0.175; 0.211; and 0.272 h⁻¹, respectively, whereas that with WO₃ with amorphous phase was 0.018 h⁻¹.

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Fig. 9

Correlation of crystallite size and photodecomposition rate of 100-ppm curcumin under various initial catalyst loadings under ambient temperature.

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In addition, the present research was focused on the evaluation of the effect of crystalline size on the photocatalytic performance. Thus, the present data is reproducible when using high purity of WO₃. No additional chemical, additive, surfactant, or doping was added during the photocatalytic process since the existence of other chemicals may trigger or inhibit photochemical reactions. Study on additional chemical is interesting, but it will be done in our future work.

4.1 WO₃ particle formation

Fig. 10 shows a schematic illustration of the particle formation phenomena during the thermal decomposition process of inorganic material. In this model, thermal decomposition of ATP was used. Based on the DTA analysis (see dashed arrows in Fig. 1), five steps of processes occurred. The stoichiometry analysis was done to ensure the steps, as presented in Table 2. The total reaction of the WO₃ formation from decomposition of ATP can be written as $${({\text{NH}}_{\text{4}})}_{10}[{\text{W}}_{12}{\text{O}}_{41}]·5{\text{H}}_2\text{O}\to 12{\text{WO}}_3+10{\text{H}}_2\text{O}+10{\text{NH}}_3$$

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Fig. 10

Illustration image of the conversion of ATP into WO₃ particles.

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Then, detailed steps are described in the following:

(i) The first step: temperature < 100 °C

The first step is an endothermic process, occurring at less than 100 °C (see step 1 in Fig. 10). The result was identified by the slight decreases in mass (less than 1 wt%) in TG analysis and the small endothermic peak in DTA analysis in Fig. 1. Since there is no pattern change in the FTIR analysis result, the change in TG mass is due to the release of physically adsorbed water in the sample (Nandiyanto et al., 2016a). Indeed, since the release of water involved physical removal only, less energy interfered.

(ii) The second step: temperature = 100–150 °C

The second step occurs in temperature of between 100 and 150 °C (see step 2 in Fig. 10). The mass loss of 2 wt% in TG analysis and strong signal for the endothermic peak at temperature of 135 °C in DTA curve were detected (See Fig. 1). FTIR analysis in Fig. 3 detected no changes in pattern, indicating no serious chemical structure deformation occurs. However, the XRD analysis in Fig. 4 showed the disappearance of some peaks in the position of after 30°. These analyses suggested the phenomena in the release of water crystal. Based on the stoichiometrical calculation (see Table 2), the reaction possibly releases almost 4 mol of water crystals. The reaction can be written as $${({\text{NH}}_4)}_{10}[{\text{W}}_{12}{\text{O}}_{41}]·5{\text{H}}_2\text{O}\to {({\text{NH}}_4)}_{10}[{\text{W}}_{12}{\text{O}}_{41}]·1.41{\text{H}}_2\text{O}+3.59{\text{H}}_2\text{O}$$

(iii) The third step: temperature = 150–200 °C

The third step happens in temperature of between 150 and 200 °C with a DTA endothermic peak at temperature of 188 °C (see step 3 in Fig. 10). The FTIR analysis in Fig. 3 identified a change in the wavenumber of less than 1000 cm⁻¹, in which this is due to the release of the rest of H₂O and some ammonium component (NH₄). The formation of WO₃ component is possibly started in the material. Thus, the reaction can be written as $${({\text{NH}}_4)}_{10}[{\text{W}}_{12}{\text{O}}_{41}]·1.41{\text{H}}_2\text{O}\to 0.946{({\text{NH}}_4)}_{10}[{\text{W}}_{12}{\text{O}}_{41}]+0.054{\text{WO}}_3+1.68{\text{H}}_2\text{O}+0.54{\text{NH}}_3$$ Based on the stoichiometrical analysis, the percentage of possible released NH₃ is about 0.54 mol. This is confirmed by the appearance of NH₃ gas smell when heating ATP at the temperature of more than 150 °C. The number of WO₃ can be produced is about 0.054 mol. Since the amount of WO₃ is too small, the analysis of XRD and FTIR cannot identify its existence. Thus, to clarify the release of NH₃, further analysis must be done.

(iv) The fourth step: temperature = 200–300 °C

The fourth step is the condition when the temperature was between 150 and 200 °C (see step 4 in Fig. 10). Major change in the TG mass was found, followed by the detection of the strongest signal for DTA endothermic peak at temperature of 273 °C (Fig. 1). This result confirmed that the heating process promotes the decomposition of organic-related components (Kim and Kwak, 2007). This hypothesis is also supported by the disappearance of FTIR peaks at about 1400, 1600, 3200, and 3600 cm⁻¹ (Fig. 3), assigned as the removal of OH and NH-related molecules.

OH and NH-related molecules are removed from their originated positions and lost entirely from specimen. The molecules are released as gases, following with the concomitant effect of forming some cracks and holes in the particle (see SEM images in Fig. 6b). The release of these molecules also leads to the collapse of tungstate elemental structure in ATP, changing the position and orientation of tungsten (W) and oxygen (O) elements. The aggregation of W and O elements happens, followed by the creation of an intermediate amorphous phase (see XRD result in Fig. 4) (Nandiyanto et al., 2016a). The beginning of the amorphous formation is at temperature of 280 °C, implying that the minimum temperature for preparing crystalline WO₃ must be conducted at more than 280 °C.

Based on the stoichiometry in Table 2, the ATP was almost completely changed into WO₃ material. However, some ammonium components still exist in small number. The reaction can be generalized as $${({\text{NH}}_4)}_{10}[{\text{W}}_{12}{\text{O}}_{41}]\to 12{\text{WO}}_3+10{\text{NH}}_3+5{\text{H}}_2\text{O}$$

(v) The five step: temperature > 300 °C

When applying temperature of more than 300 °C, almost no significant change in TG mass (see step 5 in Fig. 10) was detected. Small changes in mass occurred due to the release of the rest of ammonium molecules. This result is also confirmed by the XRD analysis results in Fig. 4, showing the increasing intensities of the diffraction peaks (i.e. increasing sizes of WO₃ crystal).

By applying the temperature that is higher than the minimum crystal growth temperature, all organic components (i.e. OH and NH-related molecules) are supposed to be removed. This phenomenon is followed by the enlargement of cracks in the particle (See Fig. 6c–d) and the break of particle into smaller parts (See Fig. 5d–g). The existence of cracks was also confirmed by nitrogen sorption analysis in Table 1, showing the increases in surface area, pore size, and pore volume. W and O elements further connect each other and do further atomic orientations, resulting in the formation of crystalline structure (Sharma et al., 2014). Further, the nucleation and crystal growth of W and O elements occur (Sharma et al., 2014), confirmed by the prominence of XRD peak intensities when heated at temperature of higher than 380 °C (see Fig. 4) (Kim and Kwak, 2007). These phenomena result in the heat excitation that was detected as an exothermic DTA peak. The phenomena in the heat excitation are in a good agreement with that discussed in reference (Fait et al., 2008).

In this range of temperature, no further loss of elements from the particle happens, verified by almost unchanged mass in the TG analysis (Fig. 1). Thus, the process is only re-structurizing elements and the crystal growth. As a consequence, identical mean final particle outer diameters were found for all samples heated with temperatures of more than 380 °C (Fig. 5 h). Although some cases show slight decreases in the outer diameter, the deviation is not significant and this is due to a sintering effect. In addition, the crystal growth phenomena due to the impact of temperature is confirmed by the changes in particle surface morphology (see high-magnified SEM images in Fig. 6d) as well as physical appearances from white, orange, to yellowish green (see Fig. 2).

4.2 Formation of monoclinic structure with controllable crystallite size

The present method is effective to create monoclinic-structured WO₃ material, while other reports reported that the pure monoclinic phase is hardly to obtain (Salje, 1977; Szilágyi et al., 2008). The main reason for the possibility control of crystalline phase is due to the control of heating and cooling rate in the process. These heating and cooling rates must be also optimized with the particle outer diameter (Sharma et al., 2014).

It has been known that lower heating rate in the atmosphere condition promotes the formation of WO₃ with monoclinic phase (Salje, 1977; De Wijs and De Groot, 1999). Low temperature rate leads to a relaxation in the interface structure, and the penetration of heat can reach the deepest position inside the particle. However, too low temperature rate can possibly allow the formation of other types of WO₃ phase, instead of monoclinic. With this reason, to achieve homogenous heat transfer in the expected temperature, the present method used small amount of ATP sample, applied relatively rapid heating rate, and held the heating process at a specific temperature. As a consequence, the homogenous heat transfer can be maintained and the final particles contain similar phase and structure (Sharma et al., 2014). In addition to the important of heating rate, the regulation of cooling rate is crucial. Too slow cooling rate has a correlation to the formation of other stable WO₃ crystalline phases (Salje, 1977; Szilágyi et al., 2008). However, to confirm this hypothesis, further analysis must be conducted in the other work.

In addition to the control of heating and cooling rates, the regulation of heating temperature is also crucial for managing the crystallite size (Luis et al., 2011). When the annealing temperature increases, the element inside the material is re-organized, forming crystal cluster. The crystal grain begins to grow, and the growth orientation exploits the vacancy position between the crystal clusters. Since this growth orientation absorbs energy from the heat (see step 5 in DTA analysis in Fig. 1), the more heating temperature permits the more possibility of crystal cluster to exploit vacancy position for growing the crystal (Ohtani et al., 1997). Thus, the crystallite size significantly increases with increasing temperature (Sharma et al., 2014). This hypothesis is in a good agreement with current literatures (Ohtani et al., 1997; Marotti et al., 2006; Sharma et al., 2014; Nandiyanto et al., 2016a; Armaković et al., 2019). Interestingly, we found that in the degree of crystallinity case, samples prepared at temperatures from 380 to 800 °C were completely crystalline. We believe that this phenomenon is due to the control of heating and cooling rate for making good heat transfer to all positions in the particle.

In addition, when heating ATP with temperature of between 380 and 800 °C, the WO₃ materials display different physical colors. The larger crystal results in the more appearance of yellowish green color. The composition of material (i.e. WO₃) and the outer diameter were the same, implying that the different colors are due to the different crystallite sizes. Analysis of the impact of heating temperature on color appearance will be discussed in our future report.

4.3 Influence of crystallite size on photocatalytic performance

The mechanism that constitutes the photocatalytic performance is illustrated in Fig. 11. When semiconductor material absorbs light energy (as photon; see route R1 in Fig. 11) that is equal to or greater than its bandgap energy (about 2.98 eV for WO₃) (Arutanti et al., 2014a), electron (e⁻) is promoted from valence band (VB) to conduction band (CB) (see route R2 in Fig. 11).(Nagaveni et al., 2004). The migrating e⁻ leaves an electron vacancy or hole (h⁺) in the VB. Once the charge separation is kept, e⁻ as the light-generated carrier travels to the catalyst surface (see route R3 in Fig. 11) (De Wijs and De Groot, 1999; Zhang et al., 2007). As a counterpart to stabilize accumulation of the light-generated carriers on the surface, adsorbed species on the outer surface of catalyst such as water (H₂O), hydroxide (OH⁻), and oxygen (O₂) molecules interact and participate in the redox reaction with e⁻ and h⁺ (Nagaveni et al., 2004). Specifically, h⁺ breaks apart the surface-bound H₂O or OH⁻ to form hydroxyl radical (OH^∗) (see route R4 in Fig. 11), whereas e⁻ picks up electron-accepting species (such as O₂) to generate radical anion (e.g. superoxide (O₂^∗)) (see route R5 in Fig. 11). Then, the radical anion endorses the conversion of other H₂O molecule into OH^∗ (see route R6 in Fig. 11) (Vamvasakis et al., 2015). The existence of OH^∗ leads to the conversion of organic molecule into smaller organic component, and further reaction can result in carbon dioxide and water (see route R7 in Fig. 11). This mechanism can be found for the bleaching color in the photocatalytic of dye as reported in elsewhere (Sakthivel et al., 2003; Nandiyanto et al., 2016b, 2017a; Alshammari et al., 2019).

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Fig. 11

Illustration image of the phenomena happen during photocatalysis.

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The activation of the e⁻ and h⁺ depends on the energy band gap between VB and CB in the photocatalyst (Peiro et al., 2001). The band gap decides the acceptable wavelength and transferred energy (De Wijs and De Groot, 1999). To control e⁻ and h⁺ movement, many strategies have been suggested (Abdullah et al., 2016; Alshammari et al., 2019; Arai et al., 2008; Armaković et al., 2019; Arutanti et al., 2014a, 2014b, 2015; Badli et al., 2017; Cheng et al., 2010; Nadarajan et al., 2018; Nagaveni et al., 2004; Nandiyanto et al., 2009a, 2013, 2016b; Radhika et al., 2019; Sakthivel et al., 2003; Vamvasakis et al., 2015; Zhang et al., 2007). However, this study only focuses on the degree of crystallinity and crystallite size, while other parameters will be discussed in other work.

The impact of the crystallite sizes on the photocatalytic performance was verified in Figs. 8 and 9. The blank test demonstrated that the degradation of curcumin is extremely slow without WO₃ under light illumination (see Fig. 8 sample “curcumin only”), whereas the curcumin with WO₃ gives responses in the decoloration. This confirms that the main degradation factor for the bleaching curcumin solution is due to the existence of WO₃ catalyst (Zhang et al., 2007).

The decomposition of curcumin with various WO₃ samples is different. The most degraded curcumin was found for samples with crystallite sizes of 42 nm, whereas the lowest was for ATP and amorphous WO₃. Reaction parameters (e.g. outer diameter, and crystal phase of catalyst, as well as the concentration ratio of curcumin/WO₃) were the same and the catalytic particles were classified as non-porous material. This implies that the fundamental reason behind the difference in photocatalytic performance is purely from the degree of crystallinity and the crystallite sizes. In short, the present study demonstrated that the larger crystallite size has a strong correlation to the improvement in the catalytic process, in which this trend was in a good agreement with other reports (Tanaka et al., 1991; Ohtani et al., 1997; Sharma et al., 2014; Armaković et al., 2019).

As mentioned above, the improving photocatalytic performance is due to the correlation of degree of crystallinity, crystallite size, and average diffusion time of the light-generated carriers to reach the surface of catalyst and interact with water molecules (De Wijs and De Groot, 1999; Zhang et al., 2007). The illustration of the movement of light-generated carriers to the surface of catalyst is explained in routes R3 and R4 in Fig. 11. The diffusion time (τ) can be simply described as

1. The more energy can be absorbed (De Wijs and De Groot, 1999; Marotti et al., 2006);

2. The opportunity for the recombination (i.e. re-position of light-generated carriers to its originated state) can be reduced (Tanaka et al., 1991; Kim and Kwak, 2007);

3. The light-generated carriers can easily transfer to the surface of particle (Tanaka et al., 1991; Zhang et al., 2007).

The above advantages can result in the accumulation of the carriers on the surface of the particle (Tanaka et al., 1991). Indeed, this accumulation promotes a more powerful redox reaction for the conversion of H₂O, OH⁻, and O₂ on the surface of catalyst (Kim and Kwak, 2007; Vamvasakis et al., 2015).

The impact of crystallite size was also evaluated by changing the initial amount of WO₃ catalyst (See Fig. 10). Indeed, different rates of photodegradation were found, corresponding to the number of catalyst loading. More catalyst loading results in the more production of OH^∗ species (Sakthivel et al., 2003), allowing the better photocatalytic performance. However, too much catalyst loading has a negative impact on the increases in the turbity that reduce the light transmission through the solution. Thus, the optimum initial catalyst loading must be investigated independently. However, interestingly, although various catalyst loadings were applied, the tendency regarding the crystallite size was consistent. All catalyst loading variations agree that the increases in the crystallite size have a positive correlation to the increases in the photocatalytic rate.

In addition, the above results showed that the photocatalytic rate of all WO₃ samples was relative slower than that of other nanoparticle-typed WO₃ samples (Arutanti et al., 2014b). Solid WO₃ micrometer particles is limited by excessive light penetration on the outer surface of the particle only, preventing the WO₃ activated site inside of the particle for absorbing photons (Nandiyanto et al., 2009a). However, micrometer-sized particle is the best particle for understanding the correlation between crystallite size and photocatalytic performance. Specifically, when employing the micrometer-sized particles, the obstacles that can disturb the measurement (i.e. the particle size and quantum effect (Marotti et al., 2006), as well as contamination from nano-catalyst to the characterized sample (Nandiyanto et al., 2009b)) can be minimized. However, further analysis for the optimization of catalytic particles such as analysis in detail about the surface area is still required in the future work. In addition, we believed that the use of micrometer-sized particles will give more advantages since they are reusable and effective in industrial applications.