Controllable Syntheses of Hierarchical WO3 Films Consisting of

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Controllable Syntheses of Hierarchical WO3 Films Consisting of Orientation-Ordered Nanorod Bundles and Their Photocatalytic Properties Shuai Zhang, Shufang Wu, Jinming Wang, Jingpeng Jin, and Tianyou Peng Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01254 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 23, 2017

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Crystal Growth & Design

Controllable

Syntheses

of

Hierarchical

WO3

Films

Consisting of Orientation-Ordered Nanorod Bundles and Their Photocatalytic Properties Shuai Zhang, Shufang Wu, Jinming Wang, Jingpeng Jin, Tianyou Peng* College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, P. R. China

Abstract: A series of hierarchical hexagonal WO3 (h-WO3) films consisting of orientationordered nanorod bundles paralleling to the fluorine-doped tin oxide (FTO) glass substrate were synthesized through a hydrothermal reaction of Na2WO4 solution (pH2.30). By varying the Na2WO4 concentration, the aspect ratios of the h-WO3 nanorods and the shapes of the orderly stacked nanorod bundles can be conveniently adjusted, which then influence the morphology of the resultant hierarchical h-WO3 films on the FTO substrate. With enhancing the Na2WO4 concentration, the nanorod bundles are changed from flat shape into fusiform one to form h-WO3 film with relatively smaller porosity and smoother surface. Compared with the h-WO3 film with fusiform-shaped nanorod bundles, the h-WO3 film with flat-shaped ones exhibits better photocatalytic O2 generation activity as the direct consequences of its larger specific surface area, higher roughness, and better light absorption property. ■ INTRODUCTION Nowadays, hierarchical structures of semiconductor have drawn a great attention not only for their peculiar properties because of the well-organized morphology but also for the fundamental

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scientific significance.1,2 Among various hierarchical structures developed, one-dimensional (1D) nanomaterials, such as nanorods, nanowires and nanotubes, are the most used building blocks due to their specific physical properties and potential applications.3-5 Usually, the size, density, orientation and crystalline structure of 1D building blocks have a great influence on the material performance.6,7 Therefore, in-depth understandings of 1D building block’s crystal growth and its hierarchical structure formation process are necessary for controllable synthesis of hierarchically structured materials.8-10 Among numerous metal oxides, tungsten trioxide (WO3) is a promising candidate for wideranging applications in the fields of photocatalysis, electrochromic/photoelectrochemical devices, Li-ion battery and gas sensors due to its unique physicochemical properties.11-17 In the field of photocatalytic water splitting, WO3 is regarded as an attractive photocatalyst for O2 generation since it exhibits an absorption of ∼12% of the solar spectrum, good hole diffusion property and moderate electron Hall mobility.18 Compared with the bulk WO3, 1D WO3 nanostructures have advantages of smoother electron movement and larger specific surface area, and thus many efforts have been devoted to the syntheses of 1D WO3 nanomaterials and their hierarchical structures.19-22 For instance, Zheng and co-workers20 have synthesized WO3/BiVO4 core/shell nanowire photoanode, in which the WO3 nanowire acted as electron conductor, and the obtained photoanode exhibited a considerable photoelectrochemical performance. Among various fabrication approaches, hydrothermal process has mostly been applied to the syntheses of 1D WO3 nanomaterials or their hierarchical structures by using inorganic salt or low-molecular-weight organics as directing agent.23-27 For example, hexagonal WO3 (h-WO3) nanowire networks have been hydrothermally deposited on a fluorine-doped tin oxide (FTO) glass substrate by using thiourea as directing agent, and the resultant h-WO3 nanowire film


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showed excellent photochromism performance.24 Similarly, Li and co-workers25 have hydrothermally synthesized hierarchical nanostructures self-assembled with h-WO3 nanorods by using (NH4)2SO4 as directing agent, and this hierarchical nanostructure exhibited mixed protonic-electronic conductivity and high redox capacity. Although directing agents with functional groups can effectively direct the formation of 1D building blocks and their special morphologies,23-26 there is almost no research on the controllable synthesis of hierarchically structured h-WO3 film made up of 1D building blocks in the absence of directing agent till now. Also, in-depth investigations on the formation mechanism of those hierarchically structured hWO3 films are inadequate to the best of our knowledge. In this study, a series of hierarchical h-WO3 films consisting of orientation-ordered nanorod bundles paralleling to the FTO substrate were synthesized through a hydrothermal reaction of Na2WO4 solution (pH2.30) without additional directing agent. The growth processes of the hWO3 films were investigated, and it was found that the growth rate of nanorods, the shape of the component units and the morphology of h-WO3 films can be controlled by changing the hydrothermal conditions (such as Na2WO4 concentration, pH value, reaction temperature, reaction time, and WO3 seed layer). The optical and photocatalytic O2 generation properties of the resultant hierarchically structured h-WO3 films consisting of different orderly stacked nanorod bundles were also investigated in detail. ■ EXPERIMENTAL SECTION Hydrothermal Synthesis of WO3 Films. All reagents used in the present experiments were obtained from commercial sources as analytical reagent grade without further purification. The commercial FTO glass was cleaned by water, ethanol, acetone, and isopropanol sequentially in an ultrasonic bath and dried for the further use. In the first step, WO3 seeds were prepared and


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loaded on the FTO substrate. Typically, 1.0 g of Na2WO4 was dissolved in 15.0 mL of water, followed by dropping 10.0 mL of HCl solution (3.0 M) to obtain a yellow precipitate, which was washed three times with water and separated through centrifugation. 1.0 mL of H2O2 (30wt%) and 5.0 mL of isopropanol were added into the yellow precipitate in sequence under stirring to obtain WO3 seed solution. WO3 seed-loaded FTO substrate was prepared by spin-coating the seed solution on the FTO substrate followed by annealing at 500°C for 30 min in air. Hierarchical h-WO3 films were synthesized through a hydrothermal process. Typically, Na2WO4 was dissolved in 30.0 mL of water under stirring, and then the pH value was adjusted to 2.30 using HCl solution (3.0 M). After stirring for 30 min, the resultant mixture was transferred into a Teflon-lined autoclave (50 mL), and then a piece of the WO3 seed-loaded FTO substrate was immersed and leaned against the wall of the autoclave with the seed layer facing down, which was kept at 160°C for 4 h hydrothermal treatment. After cooling down to room temperature, the FTO substrate was taken out and rinsed with water, followed by drying at room temperature. With varying the Na2WO4 addition amount (0.5, 1.0, 2.0, and 3.0 mmol) in the hydrothermal solution (pH2.30), a series of h-WO3 films were fabricated and referred as HW0.5, HW1.0, HW2.0, and HW3.0, respectively. Material Characterization. X-ray powder diffraction (XRD) pattern was performed on a MiniFlex 600 X-ray diffractometer with Cu Kα irradiation (λ = 0.154 nm) at 40 kV and 15 mA. A scan rate of 4° min-1 was applied to record the XRD patterns in the range of 10° ≤ 2θ≥ 50° with a step of 0.02°. The morphology of the obtained film was studied with Zeiss-Sigma field emission scanning electron microscope (FESEM). The high-resolution transmission electron microscopy (HRTEM) observation was conducted on a LaB6 JEM-2100 (HR) electron microscope (JEOL Ltd.) working at 200 kV. UV-vis diffuse reflectance absorption spectra


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(DRS) were obtained by a Shimadzu UV-3600 spectrophotometer equipped with an integrating sphere with BaSO4 as the reference. Liquid N2 adsorption-desorption measurement was performed on a Micrometrics ASAP2020 at 77 K after sample was degassed at 120°C. Photocatalytic Activity Measurement. The photocatalytic O2 evolution activities of the obtained h-WO3 films were measured in a close gas-circulation system under a 300 W Xe-lamp irradiation. The photochemical reaction was performed in a reaction cell (Pyrex glass) containing h-WO3 film (4.0 cm2) and 0.05 M Fe(NO3)3 solution (10 mL). Prior to the irradiation, the above solution containing h-WO3 film was thoroughly degassed to remove air completely, and the reactor was irradiated from the top. The amount of evolved O2 was analyzed using a gas chromatograph (GC, SP-6890, thermal conductivity detector, 5 Å molecular sieve columns, and Ar carrier). ■ RESULTS AND DISCUSSION Crystal Phase Analyses. Figure 1 depicts the XRD patterns of the obtained h-WO3 films (HW0.5, HW1.0, HW2.0 and HW3.0) on the FTO substrate, which are derived from 160oC hydrothermal treatment 4 h of the reaction solution (pH2.30) containing 0.5, 1.0, 2.0 and 3.0 mmol Na2WO4, respectively. As can be seen, all films can be ascribable to the hexagonal WO3 (h-WO3) (JCPDS No. 33-1387) since the diffraction peaks at 2θ = 13.9°, 24.3° and 28.2° correspond well to the reflections of (100), (110) and (200) planes,27,28 respectively. Nevertheless, the XRD patterns of those h-WO3 films show no (001) and (101) diffraction peak of h-WO3 when compared with our previous work.27 It implies that the present h-WO3 films have a feature of ordered arrangement or preferred growth related to the [001] crystal direction (caxis).29 The very similar XRD patterns of those h-WO3 films also indicate that the Na2WO4 concentration has little effect on the crystal phase formation and the possible preferred growth


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process. In addition, all XRD patterns show strong and sharp diffraction peaks of h-WO3 with no peak ascribable to the other WO3 crystal phase. It indicates that the present h-WO3 films have high phase purity and crystallinity,27 which will be propitious to the photocatalytic reactions since fewer defects are beneficial for retarding the recombination of photogenerated charge.

Figure 1. XRD patterns of HW0.5, HW1.0, HW2.0 and HW3.0 films derived from 160oC hydrothermal treatment 4 h of Na2WO4 solution (pH2.30) with different concentrations.

Morphology and Microstructure Analyses. The FESEM images (Figure 2) indicate that all h-WO3 films (HW0.5, HW1.0, HW2.0, and HW3.0) have uniform hierarchical structures but with different component units, surface roughnesses and porosities in light of the Na2WO4 concentration. Nonetheless, each film is made up of very similar component unit paralleling to the FTO substrate, and the component units are orderly stacked nanorod bundles as can be seen from the insets in Figure 2. The shape and size of component units change with varying the Na2WO4 concentration, and then influence the morphology of the resultant hierarchical h-WO3 films. For instance, the HW0.5 film derived from 0.5 mmol Na2WO4 solution is made up of flatshaped bundles that are orderly stacked by 1D nanorods with an average length of ∼200 nm


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(Figure 2a), and the nanorod bundles arrange in parallel on the FTO substrate to form numerous pores in the HW0.5 film. Also, the HW1.0 film is formed by flat-shaped bundles but with longer nanorods (400-500 nm) (Figure 2b), which leads to the HW1.0 film having larger pore sizes and rougher surface compared with the HW0.5 film. Once the Na2WO4 addition amount was enhanced to 2.0 or 3.0 mmol, the component units are changed into fusiform shapes (Figure 2c and d) consisting of inner longer nanorods (∼1.5 µm) surrounded with short ones, which then leads to the HW2.0 and HW3.0 films having larger densities, smaller pore sizes and smoother surfaces.

Figure 2. Typical FESEM images of HW0.5 (a), HW1.0 (b), HW2.0 (c) and HW3.0 (d) films. The insets are the corresponding high-magnification FESEM images.

The FESEM images (Figure S1 in the Supporting Information) of HW1.2 and HW1.5 films derived from 1.2 and 1.5 mmol Na2WO4 solution can demonstrate the component units’


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changing trend from flat-shaped bundles to fusiform-shaped ones. Therefore, it can be concluded that the Na2WO4 concentration has an important influence on the morphologies of the component units and the corresponding hierarchical structures of the h-WO3 films. Namely, a higher Na2WO4 concentration can promote the growth of the h-WO3 nanorods and the flat-tofusiform shape changing trend of nanorod bundles to form the hierarchically structured films with relatively smaller porosity and smoother surface.

Figure 3. Typical TEM and HRTEM images of nanorod bundles belonging to HW1.0 (a, b) and HW2.0 (c, d) films. The insets in b and d are the corresponding FFT patterns.

To further insight into the microstructures of those h-WO3 films, TEM images of the nanorod bundles belonging to HW1.0 and HW2.0 films are shown in Figure 3. As can be seen from Figure 3a, the flat-shaped bundles in HW1.0 film are composed of nanorods with ∼10 nm in diameter, and its HRTEM image (Figure 3b) shows clear and regular lattice fringes, revealing the


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single-crystal feature and the high crystallinity of those h-WO3 nanorods. Furthermore, the lattice spacing of ∼0.391 nm between the adjacent lattice fringes corresponds to the d-spacing of (001) planes of h-WO3, indicating that the nanorods grow along the [001] direction.30,31 Similarly, the TEM and HRTEM images (Figure 3c and d) indicate that the width and growth direction of nanorods in the fusiform-shaped bundles of HW2.0 film are identical to the 1D building blocks in the flat-shaped bundles of HW1.0 film. A typical fusiform-shaped nanorod bundle and the hierarchical structure of HW2.0 film can be observed from Figure S2, and the corresponding lattice fringes and fast Fourier transform (FFT) patterns inserted in those HRTEM images also indicate that the h-WO3 nanorods in the fusiform-shaped bundles grow along the [001] direction. Since the above FESEM images (Figure 2) indicate that both flat- and fusiform-shaped bundles are parallel to the FTO substrate, it can be concluded that the (001) facets of nanorods are vertical to the FTO substrate, while the crystal facets (for instance, (100), (200) and (110) facets) paralleling to the c-axis are parallel to the FTO substrate. This conjecture is supported by the above-mentioned XRD patterns (Figure 1), in which the (100), (200) and (110) diffraction peaks have relatively strong intensities, while the (001) and (101) peaks disappear. Besides, the parallelism of lattice fringes in adjacent nanorods illustrates that those nanorods are also aligned in crystallography orientation, and the crystal growth of nanorods is controlled by the oriented attachment growth mechanism.32 Based on the above analyses and discussions, it can be concluded that the formation of h-WO3 nanorod bundles follows the oriented-attachment growth mechanism and the two typical hierarchical bundles have similar growth direction, which is not influenced by the Na2WO4 concentration. Effects of Reaction Conditions on the Morphology of h-WO3 Films. To explore the growth process of the hierarchical h-WO3 films, Figure 4 shows the FESEM images of various films


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derived from different hydrothermal reaction times. After 30 min hydrothermal treatment of 1.0 mmol Na2WO4 solution, only a few irregular deposits are loaded on the FTO substrate (Figure 4a), and very weak (110) and (101) diffraction peaks of h-WO3 can be observed from the corresponding XRD pattern (Figure S3a). It implies that the crystallinity of those deposits is low, and the initial product has certain orientation, which is possibly related to the interaction between the formed WO3 nuclei and the WO3 seeds on the FTO substrate. After prolonging the hydrothermal time to 45 min, some nanorods and nanorod bundles are formed on the FTO substrate (Figure 4b). In those initial hierarchical bundles, shorter nanorods are on the periphery of bundles to form flat-shaped structure, possibly implying that some long nanorods with rapid growth may act as “leader crystals” for the growth of subsequent nanorods in parallel with the leader ones, and thus causing the formation of bundle-like structure.31 In the corresponding XRD pattern (Figure S3a), the emerging (100) and (200) peaks and the relatively intensive (110) peak indicate that the c-axis of nanorods and nanorod bundles are mainly parallel to the FTO substrate. Also, some inclined bundles and irregular particles result in the emergence of (001) peak. With further prolonging the reaction time to 60 min, the FTO substrate is fully covered by hierarchical bundles with flat shapes (Figure 4c), and the corresponding film shows obviously enhanced (100) and (200) diffraction peaks with relatively weak (001) and (101) peaks (Figure S3a). It demonstrates that the basic structure and orientation of the component units have generally formed after 60 min hydrothermal treatment. The difference in the average thicknesses of the flat-shaped bundles in Figure 4c and 2b is ∼300 nm, implying that the thickness of the nanorod bundles grows slowly within the subsequent 3 h hydrothermal treatment. Therefore, it can be concluded that the crystal growth process of 1.0 mmol Na2WO4 solution has gone through


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two basic stages. The first one is heterogeneous nucleation on the FTO substrate, and then fast growth to nanorods and their hierarchical bundles within 60 min. The following stage is ripening process in which the secondary nanorods grow longer slowly without obvious influence on the microstructures of the hierarchical h-WO3 film.

Figure 4. Typical FESEM images of HW1.0 (a-c) and HW2.0 (d-f) films derived from 30 (a, d), 45 (b, e) and 60 (c, f) min hydrothermal treatment.


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Similarly, the crystal growth during the hydrothermal treatment of 2.0 mmol Na2WO4 solution also undergoes the above two stages according to the FESEM images (Figure 4d-f) and the XRD patterns (Figure S3b). On this condition, however, the formation of nanorods and their hierarchical bundles are faster obviously due to the enhanced WO42- concentration, which is crucial for the crystal growth because of the brief chemical reactions occurred as follows:27,33 Na2WO4 + 2HCl + nH2O→ H2WO4·nH2O + 2NaCl

(1)

H2WO4·nH2O → WO3 + (n+1)H2O

(2)

During the present reaction process, H2WO4 is firstly formed after HCl solution is added into the reaction solution. Under the hydrothermal condition, the formed H2WO4 can be polymerized into polytungstate anions, and then dehydrated into WO3 crystal nuclei on the FTO substrate.33-35 In the initial stage, a higher WO42- concentration can not only cause faster heterogeneous nucleation and make nuclei reach the critical size required for crystal growth quickly,28 but also provide sufficient ion-flow for a rapid crystal growth. For instance, only a few irregular deposits are loaded on the FTO substrate (Figure 4a) after 30 min hydrothermal treatment of 1.0 mmol Na2WO4 solution, while much more nanorods and nanorod bundles (Figure 4d) appear on the FTO substrate when the Na2WO4 addition amount is increased to 2.0 mmol. Meanwhile, similar difference exists between the two films derived from 45 min hydrothermal treatment of 1.0 (Figure 4b) and 2.0 (Figure 4e) mmol Na2WO4 solutions, respectively. Once prolonging the hydrothermal time to 1 h, both of the films are completely covered by hWO3 nanorod bundles, whereas the lengths of 1D building blocks in the flat-shaped bundles (Figure 4c) is in the range of 200-300 nm, much shorter than that (500-600 nm) in the fusiformshaped ones (Figure 4f). Upon further extending the hydrothermal time to 24 h, the lengths of the nanorods derived from 1.0 mmol Na2WO4 solution can also reach 600-700 nm (Figure S4a). It


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implies that more time is needed to form nanorods with certain length in a lower Na2WO4 concentration. Meanwhile, much longer nanorods and larger bundles can be obtained from 24 h hydrothermal treatment of 2.0 mmol Na2WO4 solution (Figure S4a). These observations confirm that the WO42- concentration kinetically controls the nucleation and crystal growth, especially in the initial stage of the hydrothermal reaction. According to our previous work,27 Na+ ions can selectively absorb onto the (200) facets due to its relatively high oxygen-atom density compared with the (001) facets, and such selectively absorbed Na+ ions on the (200) facets could suppress its growth, and then resulting in the h-WO3 crystal preferentially growing along the [001] direction to form 1D nanorods.27 In this work, the Na+ ions of Na2WO4 solution would also influence the formation processes of h-WO3 nanorods and their hierarchical films through the above-mentioned directing effect. When the addition amount of Na2WO4 is 0.5 or 1.0 mmol, the directing effect of Na+ ions will be weak owing to the low Na+ concentrations. Therefore, the limited tungsten source will disperse in both [200] and [001] directions due to the comparable interaction energies with the growth species in a pure hWO3 crystal system.36 As a result, the HW0.5 and HW1.0 films are made up of flat-shaped nanorod bundles with larger size in the [200] direction and smaller size in the [001] direction (Figure 2a and b). When enhancing the addition amount of Na2WO4 to 2.0 or 3.0 mmol, a stronger directing effect of Na+ ions could more efficiently force the tungsten source to interact with the (001) facets of h-WO3 nanorods, which will slow down the stacking of nanorods in the [200] direction and accelerate the growth of nanorods in the [001] direction, and thus resulting in the formation of fusiform-shaped bundles in HW2.0 and HW3.0 films (Figure 2c and d). Especially, the more separate fusiform-shaped bundles in HW3.0 film (Figure 2d) is due to the more efficient


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directing effect because of the larger addition amount of Na2WO4 (3.0 mmol) in the reaction solution. Namely, Na2WO4 concentration has a great influence on the morphology of the component units by controlling the anisotropic growth of the secondary nanorods in the hierarchical bundles. Since the hydrothermal temperature is also a critical parameter for the fabrication of 1D hWO3 nanostructures,23,37 the FESEM images of various HW2.0 films derived from 4 h hydrothermal treatment of 2.0 mmol Na2WO4 solution at different temperatures are shown in Figure 5. The HW2.0 films derived from relatively low temperatures (140oC and 160oC) are composed of nanorod bundles with the inner nanorods longer than the external ones (Figure 5a and b). With enhancing the hydrothermal temperature to 180oC or 200oC, the corresponding films tend to form bundles with similar nanorod lengths (Figure 5c and d). This difference in nanorod bundle morphologies is related to the Ostwald ripening process at different temperatures since a higher temperature can promote a faster dissolution of small particles of h-WO3, which provides sufficient ion-flow for the ripening process. Namely, a higher reaction temperature leads to more sufficient ripening process within the same reaction time, and thus resulting in the similar lengths of secondary nanorods.23 In addition, the hydrothermal temperature also influences the crystal phase of the corresponding films as can be observed from the XRD patterns (Figure S5). The film derived from 140°C hydrothermal treatment shows a h-WO3 XRD pattern with an additional peak at 2θ = 16.5o, which corresponds to (020) diffraction peak of orthorhombic WO3·H2O (o-WO3·H2O) (JCPDS No. 43-0679).37 This diffraction peak cannot be observed from the XRD patterns of the films derived from the hydrothermal temperature higher than 140°C. The existence of oWO3·H2O is attributed to different dehydration rate at different hydrothermal temperatures


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because the removal of H2O molecules in polytungstate ions is slow and incomplete at a low hydrothermal temperature, which then causes the formation of o-WO3·H2O, while a complete dehydration of polytungstate ions at higher temperature results in the formation of pure h-WO3 (Figure S5).37 Therefore, it can be concluded that the morphology and the crystal phase of the resultant WO3 films are controlled by the hydrothermal temperature due to its effects on the dehydration rate of polytungstate ions and the Ostwald ripening process of h-WO3.

Figure 5. Typical FESEM images of various HW2.0 films derived from 4 h hydrothermal treatment of 2.0 mmol Na2WO4 solution at 140°C (a), 160°C (b), 180°C (c), and 200°C (d).

Similarly, the pH value of reaction solution has an influence on the morphology and crystal phase of the resultant films.28,35 Figure 6 shows the FESEM images of the films derived from the hydrothermal treatment of Na2WO4 solution with different pH values. As can be seen, the resultant film (HW2.0-1.70) derived from 2.0 mmol Na2WO4 solution (pH1.70) is composed of


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nanoplate array perpendicular to the FTO substrate (Figure 6a), and the corresponding XRD pattern (Figure 7a) shows a diffraction peak at 2θ = 18.1o, attributable to the reflection of (111) planes of orthorhombic WO3·0.33H2O (o-WO3·0.33H2O) (JCPDS No. 72-0199).35 It indicates that this film (HW2.0-1.70) is a mixed crystal of h-WO3/o-WO3·0.33H2O. As mentioned above, the existence of hydrous WO3 is due to the incomplete dehydration of polytungstate ions. A higher H+ concentration can lead to a faster reaction, but the faster reaction is not conducive to the removal of water molecules.35 Therefore, the lower pH value (pH1.70) of reaction solution causes the formation of hydrous WO3.

Figure 6. Typical FESEM images of HW2.0 films derived from the reaction solution with pH1.70 (a), pH2.10 (b) and pH2.50 (c), and the film synthesized on the bare FTO substrate under the same hydrothermal condition of HW2.0 film (d).


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Differently, the HW2.0-2.10 and HW2.0-2.50 films derived from the reaction solution with pH value higher than 2.00 are made up of fusiform-shaped bundles (Figure 6b and c) with high crystallinity of single h-WO3 (Figure 7a). It can be due to the slower reaction in a lower H+ concentration, which is beneficial for complete dehydration of the polytungstate ions and the formation of pure h-WO3.35 Moreover, the lengths of secondary nanorods in the bundles tend to be uniform (Figure 6b and c) along with reducing the pH value of reaction solution. This difference in morphologies could be explained as follows: In a higher H+ concentration, the fast reaction, together with more growth species, could lead to relatively sufficient Ostwald ripening process, and then to the secondary nanorods having similar lengths (Figure 6b). With an incomplete ripening under a lower H+ concentration, oppositely, the resultant bundles are composed of nanorods with different lengths (Figure 6c). Therefore, it can be concluded that the pH value of the reaction solution can affect the reaction rate and the growth species amount, which could further control the crystal phase and the morphology of the final films.

Figure 7. (a) XRD patterns of HW2.0-1.70, HW2.0-2.10 and HW2.0-2.50 films derived from the reaction solution with different pH values. (b) XRD patterns of HW2.0 film and its counterpart


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synthesized on the bare FTO substrate under the same hydrothermal condition of HW2.0 film.

In addition, the nature of the FTO substrate is another important parameter affecting the morphology of the h-WO3 film. Figure 6d shows the FESEM image of the film derived from hydrothermal treatment of 2.0 mmol Na2WO4 solution using the bare FTO substrate. Compared with HW2.0 film (Figure 2c), much fewer nanorods and hierarchical bundles are loaded on the bare FTO substrate (Figure 6d). It indicates that the WO3 seed layer on the FTO substrate plays an important role as nucleation centers and growth sites for those h-WO3 nanorods and bundles,34 and then leads to the formation of denser film within relatively short hydrothermal time. Moreover, the nanorod bundles on the bare FTO substrate have irregular orientation (Figure 6d), which can also be proved by the corresponding XRD pattern (Figure 7b). The appearance of (001) diffraction peak of h-WO3 demonstrates that the c-axis directions of some individual nanorods or bundles are perpendicular to the FTO substrate. This phenomenon can be ascribed to the bare FTO substrate having rougher surface than the WO3 seed layer (Figure S6). The rougher surface may cause that the angles between the [001] directions of “leader crystals” and the FTO substrate are random, and thus resulting in the disordered arrangement of nanorod bundles. However, the smoother surface of the seed layer would make most of leader crystals paralleling to the FTO substrate, and thus the resultant nanorod bundles having regular orientation. Therefore, it can be concluded that the seeds on the FTO substrate can provide the nucleation sites and smoother substrate for the growth of leader crystal, and thus causing the rapid formation and orderly arrangement of nanorod bundles. Base on the above results and discussions, the possible formation mechanism of the present hWO3 film can be described as Figure 8. The hierarchical h-WO3 film with high crystallinity can


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be derived from the hydrothermal reaction of Na2WO4 solution (pH value higher than ~2.00) at reaction temperature higher than ~140°C. During the initial stage of hydrothermal reaction, fast grown nanorods can act as “leader crystals”, whose c-axes are parallel to the FTO substrate due to relatively smooth seed layer, and then the shorter nanorods attaching with the leader crystals grow along the [001] direction, which is controlled by the oriented-attachment growth mechanism and the directing effect of Na+ ions. Therefore, the nanorod bundles with all secondary nanorods are parallel to the FTO substrate. In spite of the similar structures, the nanorod bundles have various morphologies upon varying the Na2WO4 addition amount in the reaction solution owing to the effects of Na+ ions on the anisotropic growth of secondary nanorods in the hierarchical bundles and the arrangement of the nanorod bundles with different size and shapes, which then causing the different morphologies of the h-WO3 films.

Figure 8. Schematic drawing of the possible formation processes of the h-WO3 films consisting of different orientation-ordered nanorod bundles.

Photocatalytic Activities Analyses. The photocatalytic O2 generation activities of h-WO3 films (HW0.5, HW1.0, HW2.0 and HW3.0) are shown in Figure 9. As can be seen, the O2 generation activity shows an increasing trend with enhancing the Na2WO4 addition amount from


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0.5 to 1.0 mmol, and then goes downhill with further enhancing the Na2WO4 addition amount. Namely, the HW1.0 film exhibits the highest O2 generation activity (54.1 µmol h-1 cm-2) among those films. Moreover, the photocatalytic O2 generation activities of HW0.5 (41.8 µmol h-1 cm-2) and HW1.0 (54.1 µmol h-1 cm-2) films with flat-shaped bundles as component units are higher than that of HW2.0 (33.2 µmol h-1 cm-2) and HW3.0 (30.5 µmol h-1 cm-2) films with fusiformshaped bundles as component units.

Figure 9. Photocatsalytic O2 generation rates of HW0.5, HW1.0, HW2.0 and HW3.0 films under Xe-lamp irradiation.

The difference in activity of h-WO3 films could be ascribed to the following factors. Firstly, the h-WO3 films (HW0.5 and HW1.0) composed of flat-shaped bundles have larger specific surface area (SBET) than the others (HW2.0 and HW3.0) as shown in Table 1. Especially, the HW1.0 film has the largest SBET (1.40 m2 g-1) among those films, which can provide more reactive sites for the O2 evolution reaction, and thus leads to the best photoactivity.2,38 Whereas the HW2.0 and HW3.0 films with fusiform-shaped bundles as component units have relatively smaller SBET values, and thus less reaction chances exist between the photo-induced holes and the


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reactant due to the less reactive sites, which might be the reason for their relatively lower O2 generation activity. Table 1. BET Surface Areas (SBET) of HW0.5, HW1.0, HW2.0 and HW3.0 Films. HW0.5 Film HW1.0 HW2.0 HW3.0 2 -1 SBET (m g ) 0.50 1.40 0.19 0.13

Secondly, the better photocatalytic performances of HW0.5 and HW1.0 films are also related to their enhanced light absorption as compared to HW2.0 and HW3.0 films as shown in the UVvis diffuse reflectance spectra (DRS, Figure 10) of h-WO3 films (HW0.5, HW1.0, HW2.0, and HW3.0). As can be seen, all films have approximately same absorption edges at ∼420 nm, corresponding to a bandgap energy (Eg) of ~2.95 eV, which is in accordance with the intrinsic bandgap energy of h-WO3 due to the electron transitions from the valence band (VB) to the conduction band (CB).39 It demonstrates that the effect of Na2WO4 concentration on the intrinsic absorption of h-WO3 can be ignored. Nevertheless, the HW0.5 and HW1.0 films have slightly enhanced absorption intensities compared with the HW2.0 and HW3.0 films, which can be due to their different surface roughnesses and porosities as shown in Figure 2. The rougher surfaces and larger pore sizes of the HW0.5 and HW1.0 films compared with the HW2.0 and HW3.0 films can cause more light scattering and reflection in the pores, which prolong the optical path in the HW0.5 and HW1.0 films, and then resulting in the relatively high absorptions as shown in the DRS spectra (Figure 10).40,41 Therefore, it can be concluded that the difference in photoactivity stems from the different microstructures and morphologies of the resultant h-WO3 films. With appropriate Na2WO4 concentration, rougher surface and larger pores formed among the nanorod bundles could result in larger specific surface area and better light absorption property, which then improve the photocatalytic performance of the corresponding h-WO3 film.


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The present results provide new insights into the significance of hydrothermal reaction conditions on crystallinity, component unit and surface morphology of h-WO3 films, thus paving a new way for the design and synthesis of high-performance, cost-effective h-WO3 film for the application of photocatalytic O2 generation.

Figure 10. UV-vis diffuse reflectance spectra (DRS) of HW0.5, HW1.0, HW2.0 and HW3.0 films.

■ CONCLUSION

In summary, a series of hierarchical h-WO3 films with different component units and morphologies were fabricated on the FTO substrate through a facile hydrothermal treatment of Na2WO4 solution without additional directing agent. The effects of hydrothermal reaction conditions on the microstructures, morphologies and photocatalytic O2 generation activity of the h-WO3 films were investigated. Although each film is uniform and made up of similar component unit (orderly stacked 1D nanorod bundle) paralleling to the FTO substrate, the shape of nanorod bundle, the roughness and porosity of the h-WO3 films can be tuned by varying the hydrothermal reaction conditions. Moreover, the secondary nanorods can stack in parallel to the


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“leader crystals” and grow along the [001] direction to form nanorod bundles because of the directing effect of Na+ ions. With increasing the Na2WO4 concentration, the nanorod bundles are changed from flat shape into fusiform one to form hierarchically structured films with relatively smaller porosity and smoother surface. The h-WO3 films with flat-shaped nanorod bundles as component units exhibit better photocatalytic O2 generation activities than that with fusiformshaped ones, as the direct consequences of the larger specific surface area, higher roughness, and better light absorption property. The present results on the growth mechanism would be helpful for providing valuable information for the controllable synthesis of hierarchically structured hWO3 films, and pave a new way for the design and synthesis of high-performance, cost-effective h-WO3 film for the application of photocatalytic O2 generation. ■ ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxxx/acs.cgd.xxxxxxx. FESEM images of HW1.2/HW1.5 films, HW1.0/HW2.0 films with the hydrothermal treatment time prolonging to 24 h, FTO substrate and WO3 seed layer-loaded substrate; TEM and HRTEM images of the fusiform-shaped nanorod bundle and its larger hierarchical structure in HW2.0 film; XRD patterns of HW1.0/HW2.0 films with different hydrothermal treatment times and HW2.0 films obtained at different temperatures (PDF). ■ AUTHOR INFORMATION

Corresponding Author


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*E-mail address: [email protected]. Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS

This work was supported by the Natural Science Foundation of China (21573166, 21271146, 20973128, 20871096), the Funds for Creative Research Groups of Hubei Province (2014CFA007), and the Natural Science Foundation of Jiangsu Province (BK20151247), China. ■ REFERENCES

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For Table of Contents Use Only

Controllable Syntheses of Hierarchical WO3 Films Consisting of OrientationOrdered Nanorod Bundles and Their Photocatalytic Properties Shuai Zhang, Shufang Wu, Jinming Wang, Jingpeng Jin, Tianyou Peng* Table of Contents Graphic

Synopsis A series of hierarchical WO3 films consisting of orientation-ordered nanorod bundles were synthesized. With enhancing the Na2WO4 concentration of reaction solution, the nanorod bundles are changed from flat shape into fusiform one. The film with flat-shaped nanorod bundles exhibits better photocatalytic O2 generation activity owing to its larger specific surface area, higher roughness and better light absorption property.


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Controllable Syntheses of Hierarchical WO3 Films Consisting of

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