Vertical Growth of WO3 Nanosheets on TiO2 Nanoribbons as 2D/1D Heterojunction Photocatalysts with Improved Photocatalytic Performance under Visible Light

Abstract

We report the construction of 2D/1D heterojunction photocatalysts through the hydrothermal growth of WO₃ nanosheets on TiO₂ nanoribbons for the first time. Two-dimensional WO₃ nanosheets were vertically arrayed on the surface of TiO₂ nanoribbons, and the growth density could be simply controlled by adjusting the concentration of the precursors. The construction of WO₃/TiO₂ heterojunctions not only decreases the band gap energy of TiO₂ from 3.12 to 2.30 eV and broadens the photoresponse range from the UV region to the visible light region but also significantly reduces electron–hole pair recombination and enhances photo-generated carrier separation. Consequently, WO₃/TiO₂ heterostructures exhibit improved photocatalytic activity compared to pure WO₃ nanosheets and TiO₂ nanoribbons upon visible light irradiation. WO₃/TiO₂-25 possesses the highest photocatalytic activity and can remove 92.8% of RhB pollutants in 120 min. Both further increase and decrease in the growth density of WO₃ nanosheets result in an obvious reduction in photocatalytic activity. The kinetic studies confirmed that the photocatalytic degradation of RhB follows the kinetics of the pseudo-first-order model. The present study demonstrates that the prepared WO₃/TiO₂ 2D/1D heterostructures are promising materials for photocatalytic removal of organic pollutants to produce clean water.

1. Introduction

The demand for fresh water increases exponentially with global population growth and industrial development [1,2,3]. Clean water scarcity is one of the most pressing issues for human beings to confront nowadays, especially in developing countries [4]. According to World Wildlife, two-thirds of the world’s population will face water shortages by 2025 [5]. To address this challenge, many advanced technologies have been developed to remove aqueous pollutants for clean water production [6,7], including adsorption [8,9], membrane filtration [10], and semiconductor photocatalytic technology [11]. Among various techniques, semiconductor photocatalysis offers a green and sustainable water purification technology because harmful pollutants can be completely decomposed to CO₂, H₂O, and other small molecules without the involvement of any chemicals [11,12,13,14]. Mainly, photocatalysts absorb photons as an energy source to (1) generate electron−hole pairs, (2) separate the photoexcited charges, (3) transfer electron and hole to the photocatalyst’s surface, and (4) utilize the active charges on the photocatalyst’s surface for catalytic decomposition of pollutants [12,13,14,15,16,17].

Heterojunction photocatalysts, particularly systems consisting of a low-dimensional 1D and 2D semiconductor, are recognized as prospective building blocks for next-generation advanced photon harvesting and conversion technologies [18,19]. The considerable potential of 1D/2D heterojunctions arises from high charge carrier mobility along the 1D nanostructures and effective charge recombination prevention ability of 2D nanostructures due to the strong electron confinement effect in atomic-thick layers. Among four different configurations based on interfacial contact, heterojunctions with vertically aligned 2D nanosheets on 1D nanostructures are highly desirable for photocatalytic systems [19]. That is because the vertically aligned 2D nanosheets on 1D nanostructures not only allow maximum exposure of the entire surface, thereby maximally providing the edge active sites, but also create better electronic contacts and optimized electron transport pathways, leading to enhanced photo-generated electron–hole separation efficiency [18,19]. This unique 2D array on 1D structures widely exists in nature, such as leaf array on a tree branch and the bony plates along stegosaur backs, and their advancement in light absorption and conversion have been proven in nature for millions of years.

Since the discovery of the Honda–Fujishima effect in 1972 [20], TiO₂ has been considered to be the most promising photocatalyst because of its high photosensitivity, excellent stability, nontoxic nature, and low cost [13,21,22]. In particular, 1D TiO₂ nanoribbons have received much more attention for advanced photocatalyst design because of the following features [22,23,24]. First, the well-defined 1D geometry facilitates fast and long-distance electron transport, leading to long-time photocatalytic stability. Second, 1D nanoribbons possess larger specific surface areas than corresponding bulk materials, providing more active catalytic sites. Third, the large length-to-diameter ratio of 1D nanoribbons increases light absorption and scattering, enhancing light use efficiency [19,20]. Beside TiO₂, WO₃ is another extensively investigated photocatalyst driven by its narrow band gap and visible light absorption ability [25,26,27]. Among various morphologies, 2D WO₃ nanosheets possess an extremely high percentage of exposed surfaces and a strong quantum confinement effect in the thinnest dimension, leading to abundant active sites and enhanced light conversion efficiency, which are highly desirable for high-performance photocatalysts [25,26,27,28,29,30]. Inspired by the booming progress in 1D TiO₂ nanoribbon and 2D WO₃ nanosheet photocatalysts, the rational design of 2D/1D WO₃/TiO₂ multidimensional heterojunction photocatalysts is expected to integrate the merits of both 1D and 2D nanogeometry and lead to enhanced photocatalytic performance. Particularly, a 2D WO₃ array on a 1D TiO₂ heterojunction is of special interest for highly efficient photocatalyst design. Coupling narrow band gap WO₃ (2.4–2.8 eV) can not only broaden the absorption wavelength of TiO₂ from UV to visible light regions, resulting in the formation of visible light-responded heterojunction photocatalysts, but also effectively suppresses electron−hole pair recombination, thus improving the photocatalytic activity [31]. To date, substantial efforts have been devoted to construct WO₃/TiO₂ heterojunction systems, including WO₃ nanorod/TiO₂ nanofiber composite [32], WO₃ nanoparticle/TiO₂ nanoparticle composite [33,34], heterostructured TiO₂/WO₃ porous microspheres [35,36], a WO₃ nanoparticle/TiO₂ nanotubes system [37,38,39], heterostructured WO₃/TiO₂ nanosheets [40,41], and WO₃ nanosheet/TiO₂ nanoparticle composite [42]. However, to the best of our knowledge, 2D/1D heterojunction photocatalysts composed of TiO₂ nanoribbons and WO₃ nanosheets have not been reported. Demonstrating the vertical growth of WO₃ nanosheets on TiO₂ nanoribbons for 2D/1D heterojunction photocatalysts is especially challenging.

Herein, we report the vertical growth of WO₃ nanosheets on TiO₂ nanoribbons as 1D/2D heterojunction photocatalysts with improved photocatalytic performance under visible light. The WO₃/TiO₂ heterojunctions were constructed through the introduction of TiO₂ nanoribbons into the hydrothermal growth system of WO₃ nanosheets. The as-obtained WO₃/TiO₂ sample was characterized by using XRD, SEM, TEM, XPS, UV–vis, and PL analysis. WO₃ nanosheets vertically arrayed on the surface of TiO₂ nanoribbons lead to the formation of 1D/2D heterojunctions with maximum exposure of the entire surface, which not only broadens the light adsorption spectrum into the visible region but also inhibits the recombination of photoinduced carriers, resulting in enhanced photocatalytic degradation of aqueous pollutants.

2. Results and Discussion

2.1. XRD Analysis

The hydrothermal growth process is accompanied by an obvious color change in TiO₂ from white to yellow-green (Figure 1a), indicating the successful and uniform growth of WO₃ on TiO₂. In order to confirm WO₃ growth, XRD patterns of the samples before and after hydrothermal reaction were recorded (Figure 1b). Before growth of WO₃ nanosheets, all the diffraction peaks located at 25.3°, 37.8°, 48.0°, 53.9°, 55.0°, 62.1°, 62.7°, and 68.8° can be indexed to anatase TiO₂ (JCPDS file no. 21-1272), which is in good agreement with previous reports [43,44]. After the growth of the WO₃ nanosheets, except for the peaks originating from TiO₂ nanoribbons (marked with red boxes), new peaks (marked with blue circles) centered at 16.5°, 25.6°, 30.5°, 33.4°, 34.1°, 34.8°, 38.9°, 44.1°, 45.9°, 46.3°, 49.6°, 52.6°, 56.2°, 57.2°, 58.4°, 61.2°, 64.3°, and 66.0° were observed, which match well with WO₃•H₂O (JCPDS file no. 43-0679) [45], indicating the formation of WO₃/TiO₂-25 composites [32,33,34].

2.2. FTIR Analysis

In order to further confirm the formation of WO₃/TiO₂ composites, FTIR spectra of all samples have been collected and shown in Figure 1c. For the spectrum of pure TiO₂ nanoribbons (green line), the broad adsorption band centered at 3430 cm⁻¹ could be ascribed to the stretching vibrations of the surface hydroxyl groups and molecularly chemisorbed water [46]. The peak at 1630 cm⁻¹ results from -OH bending of molecularly physisorbed water [46]. The large absorption in the range 492 cm⁻¹ can be attributed to the strong stretching vibrations of the Ti-O bond in TiO₂ nanoribbons [46]. For the spectrum of pure WO₃ nanosheets (red line), the broad adsorption band center at 3410 cm⁻¹ is related to the stretch region of the surface hydroxyl groups with hydrogen bonds and crystal water of WO₃•H₂O, while the peak centered at 1633 cm⁻¹ is from O–H bending of physisorbed water. The weak band observed around 600–750 cm⁻¹ is attributed to the O–W–O stretching modes of WO₃, and the peak located at 941 cm⁻¹ is associated with W=O stretching [46]. For WO₃/TiO₂ composites with different ratios, the broad peak around 3412–3420 cm⁻¹ of each sample can be ascribed to the stretching vibrations of -OH groups [46]. These characteristic absorption peaks of WO₃ and TiO₂ located in the range of 400–1000 cm⁻¹ are all observed in the spectra of WO₃/TiO₂ composites, indicating the successful combination of WO₃ and TiO₂. From WO₃/TiO₂-5 to WO₃/TiO₂-75, the intensity of peaks from TiO₂ nanoribbons decreases, and the intensity of peaks ascribed to WO₃ nanosheets increases, indicating the increasing growth density of WO₃ nanosheets on TiO₂ nanoribbons, which is in good agreement with the raw material proportioning.

2.3. SEM and TEM Analysis

In order to reveal the configuration of WO₃/TiO₂ heterojunctions, SEM and TEM were applied to observe the pure TiO₂ nanoribbons and the WO₃/TiO₂-25 heterostructure. Before WO₃ growth, pure TiO₂ nanoribbons exhibit typical 1D morphology with a length from several tens to several hundreds of micrometers (Figure 2a). TEM images (Figure 2b–d) of an individual nanoribbon show that the width of the nanoribbons is around 100 nm and the surface of the nanoribbons are flat and clean, which provides comfortable platforms for the nucleation and growth of WO₃ nanosheets. After WO₃ growth, the 1D typical morphology is still maintained, while the surface of the nanoribbons is vertically arrayed with tetragonal nanosheets at a length of 100 nm to 400 nm and thickness of 40 nm (Figure 2e). A similar structure has also been reported in BiOBr/TiO₂ systems [43]. A TEM image (Figure 2f) of a single heterostructured nanoribbon demonstrates that 2D nanosheets were grown on both sides of the nanoribbons, leading to the formation of vertically aligned 2D/1D heterostructures with maximum exposure of junctions and the entire surface. High-magnification TEM images (Figure 2g,h) show very clear crystalline planes with a d-spacing of 0.347 nm, which closely matched the inter-planar spacing of the (111) facets of WO₃ [45], further confirming the successful formation of WO₃/TiO₂ heterojunctions. Figure 2i–l show HAADF-STEM-EDS mapping images of a typical nanoribbon arrayed with sheet structures. Ti and O elements can be found in the nanoribbons, while W and O elements existed in the nanosheets, which directly proves that the nanoribbons were TiO₂ and the nanosheets arrayed on nanoribbons were WO₃. These results visually verified that the flat TiO₂ nanoribbons served as the backbones and the WO₃ nanosheets with a tetragonal profile were arrayed on them, leading to the formation of typical vertically aligned 2D on 1D heterostructures. This unique 2D array on 1D structures widely exist in nature and their advancement in light absorption and conversion have been proved in nature for millions of years. For example, trees adopt a leaf array on branch structures to obtain enough access to maximum light. Stegosaurs possessed bony plates along their backs to absorb more heat from the sun to warm their blood on cool days. Therefore, 2D WO₃/1D TiO₂ heterostructures are anticipated to be highly desired for the design of advanced photocatalytic systems.

The growth density of WO₃ nanosheets can be regulated simply via control over the precursor concentration. When the concentration of the precursors was decreased to 5 mM, few nanosheets could be observed on the surfaces of the nanoribbons (Figure S1a). At 10 mM precursor concentration, only a small amount of nanoribbons was decorated with WO₃ nanosheets (Figure S1b). On increasing the precursor concentration to 50 mM, a much higher density of nanosheets could be grown on the nanoribbon backbones, and some isolate nanosheets (Figure S1c) were found between nanoribbons. Further increasing the precursor concentration to 75 mM would lead to a large amount of free nanosheets (Figure S1d), indicating the formation of the mixture of WO₃/TiO₂ heterojunctions and WO₃ nanosheets (Figure S2).

2.4. XPS Analysis

The element composition and valence of WO₃/TiO₂-25 composite were analyzed by X-ray photoelectron spectroscopy (XPS). Figure 3a shows the Ti 2p spectra of pure TiO₂ nanoribbons and WO₃/TiO₂-25 heterojunctions. The pure TiO₂ nanoribbons exhibit two XPS peaks at 458.6 eV and 464.3 eV, which correspond to Ti 2p_1/2 and Ti 2p_3/2, respectively. The energy gap between Ti 2p_1/2 and Ti 2p_3/2 peaks is 5.7 eV, revealing the oxidation state of Ti in TiO₂ nanoribbons is +4 [36,47], which is also consistent with the XRD result. After the growth of WO₃ nanosheets on TiO₂, both the Ti 2p_1/2 and Ti 2p_3/2 peaks exhibit obvious red shifts from 458.6 to 459.1 eV and from 464.3 to 464.8 eV, respectively. This result suggests the conversion of Ti-O-Ti bonds to Ti-O-W bonds, which confirms the strong interaction between WO₃ and TiO₂ in the WO₃/TiO₂-25 heterojunctions [34,36]. Furthermore, the +4 oxidation state of Ti is unchanged after the formation of WO₃/TiO₂-25 heterojunctions as the energy gap between the peaks at 459.1 and 464.8 eV remains 5.7 eV. The W 4f XPS spectra of pure WO₃ nanosheets and WO₃/TiO₂-25 heterojunctions are shown in Figure 3b. The two XPS peaks of pure WO₃ nanosheets at 35.7 eV and 37.8 eV are attributed to W 4f_7/2 and W 4f_5/2, respectively. The energy gap between W 4f_7/2 and W 4f_5/2 peaks is 2.1 eV, which suggests that the W element existed in the form of W⁶⁺ in WO₃ nanosheets [34,36]. For WO₃/TiO₂-25 heterojunctions, the binding energies of W 4f_7/2 and W 4f_5/2 peaks were both shifted to low energy by 0.1 eV, while the energy gap between these two peaks is unchanged, indicating the W⁶⁺ oxidation state in WO₃/TiO₂-25 heterojunctions [34,36,48], which is in good agreement with the XRD data. The high-resolution O 1s XPS spectrum of the samples is shown in Figure 3c,d. It can be seen that the Ti-O bond peak in TiO₂ nanoribbons is located at 530.0 eV (Figure 3c), while the W-O bond peak in WO₃ nanosheets is centered at 530.5 eV (Figure 3d). For WO₃/TiO₂-25 heterojunctions, the binding energy at 530.4 eV corresponded to the lattice oxygen of Ti⁴⁺-O or W⁶⁺-O, indicating that W-O and Ti-O shared the orbital O 1s in the W-O-Ti bond [32,43,44]. The peak exhibited a 0.4 eV up-shift from Ti-O bonds (from 530.0 to 530.4 eV) and 0.1 eV down-shift from W-O bonds (from 530.5 to 530.4 eV). The above XPS results show that the binding energy of Ti 2p shifted to high energy, while the binding energy of W 4f and O 1s shifted to low energy after the formation of WO₃/TiO₂-25 heterojunctions, confirming the electron density decrease on TiO₂ and electron density increase on WO₃, respectively [36,49,50]. The electron density change indicates that the photo-generated carriers have been successfully transferred between WO₃ and TiO₂ in the composite, which further proved the formation of WO₃/TiO₂-25 heterojunction structures [34,36].

2.5. Optical Analysis

The UV–vis diffuse reflection spectra of TiO₂, WO₃, and WO₃/TiO₂ heterojunctions were recorded to understand the light absorption property and are shown in Figure 4a. The TiO₂ nanoribbons show strong absorbance in the ultraviolet region, and the optical absorption edge was found to be around 400 nm owing to the large band gap of anatase TiO₂ [21,22]. After the combination of WO₃, the optical absorption edge exhibits a 130 nm red shift compared to pure TiO₂ nanoribbons. Thus, the hybrid WO₃/TiO₂ heterostructures can utilize a larger fraction of the solar spectrum for photocatalytic reactions. In order to quantitatively determine their band gaps, (αhν)^1/2 vs. photon energy (hν) are generated from the diffuse reflectance spectra (Figure 4b). Therein, α is the Kubelka–Munk function of the diffuse reflectance spectra (α = (1 − R)²/2R, and R is the reflectance). The apparent band gaps of pristine TiO₂ and WO₃ were calculated to be 3.12 eV and 2.26 eV, respectively, while the estimated band gap of WO₃/TiO₂ heterostructures was calculated to be 2.30 eV. The band gap reduction in TiO₂ nanoribbons after WO₃ sheet growth suggests the strong interaction between TiO₂ and WO₃ in WO₃/TiO₂ heterostructures, which can effectively hinder the recombination of e-h+ through the WO₃/TiO₂ heterojunction [27,29,35].

2.6. Photoluminescence Analysis

Photoluminescence measurements were further undertaken to unveil the charge recombination behavior of TiO₂ and WO₃/TiO₂-25 heterostructures. It is widely known that fluorescence emission signals are derived mainly from the recombination of photo-generated electron–hole pairs, and a lower fluorescence intensity signifies a higher photocatalytic efficiency [12,31,43]. Figure 5a shows the fluorescence spectra of TiO₂ and WO₃/TiO₂-25 heterostructures in the wavelength range of 475–725 nm. It can be seen clearly that the emission peaks of TiO₂ and WO₃/TiO₂-25 heterostructures are in similar shapes, and the fluorescence emission intensity of TiO₂ shows an obvious reduction after modification with WO₃ nanosheets, indicating that growth of WO₃ nanosheets on TiO₂ nanoribbons can effectively suppress recombination of electron–hole pairs and increase the lifetime of the charge carriers, which provides solid basis for photocatalytic activity improvement. In order to further confirm the enhanced photo-generated electron–hole separation efficiency, photocurrent responses of all samples have been recorded and shown in Figure S4. The photocurrent of all the WO₃/TiO₂ composite samples is significantly higher than that of the pure TiO₂ and WO₃, further confirming the higher charge separation induced by WO₃ nanosheet growth, which is consistent with photoluminescence spectra. It is worth noting that WO₃/TiO₂-25 exhibits the highest photocurrent density of 7.9 μA·cm⁻², which is roughly 7.1, 6.6 times higher than that of pure TiO₂ nanowires and pure WO₃ nanosheets. The best photocurrent response manifests that the WO₃/TiO₂-25 heterostructure composite possesses the highest photoelectrons–holes transfer efficiency and thus should be a promising candidate as a high-performance photocatalyst.

2.7. Photocatalytic Performance

UV–vis diffuse reflection, photoluminescence, and photocurrent measurements demonstrate that construction of 2D/1D heterojunctions through vertical growth WO₃ nanosheets on TiO₂ nanoribbons not only broaden the light utilization region but also effectively suppresses electron–hole pair recombination; thus, theoretically, WO₃/TiO₂ heterostructures should possess higher photocatalytic activity than pure TiO₂ and WO₃ [11,12,31]. Figure 5b shows the photocatalytic degradation behavior of RhB over various photocatalysts under visible light irradiation. In the dark condition, all the samples exhibit similar RhB removal efficiency, owing to the similar BET surface area (Figure S3). All five kinds of WO₃/TiO₂ heterostructures prepared under different precursor concentrations show higher photocatalytic activity than pure WO₃ nanosheets and TiO₂ nanoribbons. Of the five heterojunctions, the WO₃/TiO₂-25 system shows the highest photocatalytic activity, which can remove 92.8% of RhB pollutants in 120 min. The photocatalytic degradation ration of RhB over WO₃/TiO₂-50 and WO₃/TiO₂-10 heterojunctions is 86.3% and 71.7%, respectively, indicating that too much higher and lower growth density of WO₃ nanosheets causes photocatalytic activity decrease. The reason is that over-dense WO₃ nanosheets result in lower exposure of junctions, while sparse growth density leads to few junctions. Figure 5c shows the first-order reaction kinetic curves for the RhB degradation reaction over various photocatalysts. It can be seen that the slope of WO₃/TiO₂ heterostructures is bigger than that of pure WO₃ nanosheets and TiO₂ nanoribbons, suggesting the enhanced photocatalytic rate constant after growth of WO₃ nanosheets on TiO₂ nanoribbons. The enhanced photocatalytic activity of the WO₃/TiO₂ heterostructures is attributed to the synergetic effects of the improved visible light utilization and enhanced electron–hole separation, which have been proven by UV–vis diffuse reflection and photoluminescence spectra. Over five consecutive cycles, there was no notable change for the apparent photocatalytic degradation ratio, indicating the excellent durability of WO₃/TiO₂ heterostructures (Figure 5d).

The effect of WO₃/TiO₂-25 catalyst loading on the photocatalytic degradation ratio of RhB is shown in Figure 6a. The photodegradation ratio increased with the increase in catalyst loading in the range of 5–20 mg and reached a maximum when 20 mg of the catalyst was used. Further increase in catalyst loading from 20 mg to 40 mg led to a decline in the photodegradation ratio from 92.8% to 82.6%. The observation can be explained as follows. Increasing catalyst loading from 5 mg to 20 mg would provide more active photodegradation sites, thus leading to an enhancement of the degradation ratio. When the catalyst was increased to 40 mg, the photodegradation reaction system became thick colloid, reducing the light penetration depth, which means only the catalysts in the solution surface layer can be photo activated as degradation sites, while the catalyst in the solution bottom layer make little contribution to RhB photodegradation, thus resulting in a decline in the degradation ratio [51]. Another reason might be due to catalyst aggregation resulting from high catalyst concentration, which would lead to a decrease in total surface area available for degradation, reduction in site density for surface holes and electrons, and an increase in the diffusion path length [51].

In order to reveal the photocatalytic mechanism, several trapping agents, including ascorbic acid (AC), ammonium oxalate (AO), isopropanol (IPA), and hydrogen peroxide (H₂O₂), were applied as scavengers for probing the active radicals in photocatalytic degradation. The scavengers AC, AO, IPA, and H₂O₂ functioned as trapping agents for superoxide anions (O₂⁻), holes (h⁺), hydroxyl radicals (OH), as well as electrons (e⁻), respectively [52]. As shown in Figure 6b, various sacrificial agents have a great impact on the RhB degradation efficiency. After adding AO, AC, and IPA, the RhB photocatalytic degradation efficiency decreased obviously compared with the blank. The photodegradation ratios of RhB corresponding to AC, AO, and IPA were 40.2%, 20.7%, and 32.2%, respectively, indicating that h⁺ and OH constituted the major active species for RhB photodegradation [52]. The photodegradation ratio of RhB reached nearly 100% in the presence of H₂O₂. The addition of H₂O₂ resulted in the consumption of e⁻ in the conduction band (CB) and the enhancement of photoinduced e⁻/h⁺ pair separation. Furthermore, H₂O₂ was reduced by e⁻ to generate OH, as indicated by H₂O₂ + e⁻ → OH + OH⁻ [53]. The generation of extra ·OH further promoted photocatalytic degradation. Therefore, active species detection reveals that photoinduced h⁺ and ·OH were the main active species in the RhB photodegradation process.

The conduction band (CB) and the valence band (VB) edge level potentials of TiO₂ and WO₃ were measured by applying the following formula [54]: E_VB = X − E_c + 0.5E_g, E_CB = E_VB − E_g. Where E_VB is the VB edge level potential and E_CB is the CB edge level potential, and X is the absolute electronegativity of the semiconductor material (electronegativity of TiO₂ and WO₃ is 5.8 eV and 6.59 eV, respectively) [54]. E_c is the energy of free electrons (4.5 eV vs. NHE), and E_g is the band gap of TiO₂ and WO₃. The CB and VB edge level potentials of TiO₂ were determined as −0.26 eV and 2.86 eV, respectively, while the CB and VB edge level potentials of WO₃ were calculated to be 0.96 eV and 3.22 eV, respectively. The photocatalytic degradation mechanism of the WO₃/TiO₂ heterojunctions is schematically demonstrated in Figure 6c. When WO₃/TiO₂ heterojunction photocatalysts are exposed to visible light irradiation, the photo-generated electrons in the WO₃ valence band will be excited to the conduction band, generating holes in the valence band. However, it cannot occur for TiO₂ due to the band gap energy of TiO₂ (3.12 eV) being higher than the visible photon energy. The valence band of WO₃ is higher than that of TiO₂ (3.22 eV vs. 2.86 eV), so the photo-generated holes can be easily transferred to the valence band of TiO₂ through the 2D/1D junction interface [31,32,33,34,35,36]. The photo-generated electrons on the WO₃ conduction band cannot react with O₂ to produce •O₂⁻ radicals because the conduction edge potential of WO₃ was higher than the redox potential of O₂ to •O₂⁻ (0.96 eV vs. −0.33 eV). Thus, the radicals of •O₂⁻ can hardly participate in the photodegradation reaction, which is in good agreement with the scavenger experiment results. Furthermore, photo-generated holes (h⁺) can react with adsorbed H₂O or OH⁻ to generate •OH radicals, which are the main active species for organic pollutant photo-degradation confirmed by the scavenger experiment. The above results showed TiO₂ acted as a photo-generated hole acceptor in the heterojunction, effectively reducing the recombination of photo-generated carriers and enhancing photocatalytic performance, and h⁺ and •OH are responsible for the photodegradation of RhB.

3. Materials and Methods

3.1. Preparation of WO₃/TiO₂

TiO₂ nanoribbons were synthesized according to our previous report [55,56]. Then, WO₃ nanosheets were vertically grown on TiO₂ nanoribbons using a hydrothermal process. Typically, 0.1 g of TiO₂ nanoribbons were dispersed in 30 mL of distilled water dissolved by 0.25 g of Na₂WO₄•2H₂O (25 mM), 2 mL of 3 mol/L HCl aqueous solution, and 0.3 g of citric acid. Then, the resulting suspension was transferred into a 50 mL Teflon-lined stainless steel autoclave and kept at 120 °C for 24 h. After the hydrothermal reaction was completed, the resulting products were collected by centrifugation, washed several times with deionized water, and dried at 60 °C overnight. For comparison, WO₃/TiO₂ composites with different ratios were prepared in a similar manner. The samples were denoted as WO₃/TiO₂-5, WO₃/TiO₂-10, WO₃/TiO₂-25, and WO₃/TiO₂-50, WO₃/TiO₂-75 in which the number represents the concentration of Na₂WO₄·2H₂O.

3.2. Material Characterization

The morphology and structure of the photocatalysts were examined by a HITACHlS-4800 field emission scanning electron microscope (SEM, Hitachi, Tokyo, Japan). Transmission electron microscope (TEM, Thermo Fisher Scientific, Waltham, MA, USA) and high-resolution TEM (HRTEM, Thermo Fisher Scientific, Waltham, MA, USA) images were taken with an FEI Tecnai G20 electron microscope. For TEM samples, the photocatalyst was dispersed in ethanol using ultrasonic treatment and the TEM samples were prepared by depositing a drop of diluted suspension on a carbon-coated copper grid. UV-vis-near-infrared (NIR, Agilent, Polo Alto, CA, USA) reflection spectra were recorded on an Agilent Australia Carry-5000 spectrophotometer. The UV–Visible absorption spectra were recorded on a Shimadzu UV-2550 spectrophotometer. The X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, Waltham, MA, USA) analysis was carried out on an Escalab 250Xi spectrometer with monochromatic Al Kα radiation (hν = 1486.7 eV). Phase identification of the photocatalysts was measured by a Shimadzu powder difractometer with Cu-Kα radiation. The solid-state fluorescence spectra were characterized using a FLS980 fluorescence spectrophotometer.

3.3. Photodegradation and Photocurrent Test

Rhodamine B (RhB) solution with a concentration of 9 mg/L was used as the model waste water. An amount of 20 mg of samples were dispersed in 100 mL of RhB solution. The solution was kept in the dark for 20 min and then exposed to visible light irradiation. A 500 W halogen lamp was used as the visible light source, and the average light intensity was 60 mW•cm⁻². The distance between the lamp and the solution was 10 cm. During the measurement process, a certain amount of the solution was sampled at certain time intervals and centrifuged (8000 rpm, 3 min) to remove the photocatalyst particles. The supernatant was analyzed to measure the concentration of RhB by using a Shimadzu UV-2550 UV–vis spectrometer (peak center: 554 nm). To analyze the photocatalytic degradation mechanism, ascorbic acid (AC), ammonium oxalate (AO), isopropanol (IPA), and hydrogen peroxide (H₂O₂) reagents were selected as scavengers for anions (O₂⁻), holes (h⁺), hydroxyl radicals (·OH), as well as electrons (e⁻), respectively. Next, 5 mg of AC, 5 mg of AO, 0.2 mL of IPA, and 0.2 mL of H₂O₂ were added into the above photodegradation system while keeping other conditions unchanged.

Photocurrent studies were performed on a CHI 660D electrochemical workstation using a three-electrode configuration where ITO electrodes were deposited with the samples as a working electrode, Pt as a counter electrode, and a saturated calomel electrode as reference. The electrolyte was 0.35 M/0.25 M Na₂S-Na₂SO₃ aqueous solution. For the fabrication of the working electrode, 0.25 g of the sample was grinded with 0.06 g polyethylene glycol (PEG, molecular weight: 20,000) and 0.5 mL ethanol to make a slurry. The slurry was spread onto a 1 cm × 4 cm ITO glass using the doctor blade technique and then allowed to air-dry. A 500 W halogen lamp was used as the visible light source and the average light intensity was 60 mW•cm⁻².

4. Conclusions

In conclusion, a novel WO₃/TiO₂ 2D/1D heterojunction photocatalyst was constructed via the introduction of TiO₂ nanoribbons into a WO₃ nanosheet growth system. Two-dimensional WO₃ nanosheets were vertically arrayed on the surface of TiO₂ nanoribbons, and the growth density could be easily controlled by adjusting the concentration of the precursors. The UV–vis diffuse reflection result reveals that vertical growth of WO₃ nanosheets on TiO₂ nanoribbons successfully decreases the band gap energy of TiO₂ from 3.12 to 2.30 eV and broadens the photoresponse range from the UV region to the visible light region. Furthermore, the photoluminescence measurement demonstrates that construction of WO₃/TiO₂ heterojunctions significantly reduces electron–hole pair recombination. Consequently, WO₃/TiO₂ heterostructures with different WO₃ nanosheet growth density show higher photocatalytic activity than pure WO₃ nanosheets and TiO₂ nanoribbons. WO₃/TiO₂-25 possesses the highest photocatalytic activity, which can remove 92.8% of RhB pollutants in 120 min and maintain high removal efficiency after five consecutive cycles. The present study demonstrates that the prepared WO₃/TiO₂ 2D/1D heterostructures are promising materials for photocatalytic removal of organic pollutants to purify water.

Author Contributions

Conceptualization, L.Z.; methodology, L.Z., L.W., K.X., and H.T.; validation, L.W. and H.T.; formal analysis, L.W., K.X., and H.T.; investigation, L.Z.; data curation, L.W., K.X., and L.W.; writing—original draft preparation, L.Z.; writing—review and editing, L.Z. and L.W.; visualization, L.W. and K.X.; supervision, L.Z.; project administration, L.Z.; funding acquisition, L.Z. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The data presented in this study are available on request from the corresponding authors.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures

Figure 1. (a) Photograph of TiO2 before and after hydrothermal growth of WO3. (b) XRD patterns of TiO2, WO3, and WO3/TiO2−25. (c) FTIR spectra of TiO2, WO3, and WO3/TiO2 composites with different ratios.

Figure 2. (a) SEM image of TiO2 nanoribbons. (b–d) TEM images of TiO2 nanoribbons. (e) SEM image of WO3/TiO2-25 heterostructure. (f–h) TEM images of WO3/TiO2-25 heterostructure. (i–l) HAADF-STEM micrograph of WO3/TiO2-25 heterostructure and the corresponding EDS mapping of Ti, W, and O.

Figure 3. XPS spectra: (a) Ti 2p spectra of TiO2 and WO3/TiO2-25 heterojunctions. The orange and blue curves are fitting curves corresponding to the Ti 2p1/2 and Ti 2p3/2, respectively. The black line is the raw data. (b) W 4f spectra of WO3 and WO3/TiO2-25 heterojunctions. The orange, cyan, blue and green curves are fitting curves corresponding to the W 4f7/2 and W 4f5/2, respectively. The black represent the raw data and the red line is the fitting line. (c) O 1s spectra of TiO2 and WO3/TiO2-25 heterojunctions. The magenta, blue and olive curves are fitting curves corresponding to the Ti-O bond. The black represent the raw data and the red line is the fitting line. (d) O 1s spectra of WO3 and WO3/TiO2-25 heterojunctions. The magenta, blue and olive curves are fitting curves corresponding to the W-O bond. The black represent the raw data and the red line is the fitting line.

Figure 4. (a) Diffuse reflectance spectra for the TiO2 nanoribbons, WO3 nanosheets, and WO3/TiO2-25 heterojunction. (b) Plot of (αhν)1/2 as a function of photon energy (hν).

Figure 5. (a) Fluorescence spectrum of TiO2 nanoribbons and WO3/TiO2−25 heterojunction. (b) Photocatalytic degradation of RhB over various photocatalysts under sunlight irradiation, where C0 is the initial concentration of the pollutant, and C is the concentration of the pollutant at irradiation time t. (c) First-order reaction kinetic curves for the RhB degradation reaction over various photocatalysts. (d) Curve of the degradation ratio of RhB versus reuse times of WO3/TiO2-25.

Figure 6. (a) The effect of WO3/TiO2−25 catalyst loading on RhB photodegradation ratio. (b) Trapping test for active species during RhB photodegradation with the WO3/TiO2−25 photocatalyst. (c) Proposed photocatalytic mechanism of WO3/TiO2 photocatalyst.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13030556/s1, Figure S1: SEM images of WO₃/TiO₂ heterostructures prepared under different precursor concentrations: (a) 5 mM, (b) 10 mM, (c) 50 mM, (d) 75 mM. Figure S2: SEM image of pure WO₃ nanosheets. Figure S3: Nitrogen adsorptione-desorption isotherm: (a) TiO₂ nanoribbons; (b) WO₃ nanosheets; (c) WO₃/TiO₂ heterostructures and Figure S4. Photocurrent responses of all samples.

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