Structural characterization

As shown in Figure 1, the Bi2WO6/TiO2-NT nanocomposites were fabricated by depositing Bi2WO6 nanoparticles onto the hierarchical TiO2 tubes via the solvothermal method, where the TiO2 tubes were obtained by the calcination of the TiO2-gel/cellulose composite, which in turn was prepared through the surface sol−gel method. According to the theoretical contents (30, 50, 70, and 90 wt %) of the Bi2WO6 component in the Bi2WO6/TiO2-NT nanocomposites, they were denoted as 30%−Bi2WO6/TiO2-NT, 50%−Bi2WO6/TiO2-NT, 70%−Bi2WO6/TiO2-NT, and 90%−Bi2WO6/TiO2-NT nanocomposites, respectively. The practical contents of the Bi2WO6 component in these Bi2WO6/TiO2-NT nanocomposites were measured by EDX to be 38.4, 54.3, 72.9, and 95.2 wt %, respectively, as displayed in Supporting Information File 1, Figure S1 and Table S2.

As shown in Figure 2a, the XRD patterns of the Bi2WO6/TiO2-NT nanocomposites all exhibit characteristic diffraction peaks at 2θ = 28.3, 32.7, 47.1, 55.9, 58.5, 68.6, 75.7, and 78.2°, which are attributed to the (131), (200), (202), (133), (262), (400), (391), and (460) planes of the russellite phase Bi2WO6 (JCPDS No. 39-0256), respectively [34]. Besides, the peak located at 2θ = 25.3° is also observed in these XRD patterns, which is assigned to the (101) plane of the anatase phase titania (JCPDS No. 21-1272) [34]. The diffraction peaks in the XRD pattern of pure Bi2WO6 powder are all consistent with those of the Bi2WO6/TiO2-NT nanocomposites and assigned to the russellite phase Bi2WO6. The XRD pattern of pure TiO2-NT shows other weak peaks at 2θ = 37.8, 48.0, 53.9, 55.1, 65.7, and 75.0°, which belong to the (004), (200), (105), (211), (204), and (215) planes of the anatase phase titania (JCPDS No. 21-1272), respectively [34]. These weak peaks are only presented in the XRD pattern of the 30%−Bi2WO6/TiO2-NT nanocomposite due to the rather low contents of the TiO2 component in other Bi2WO6/TiO2-NT nanocomposites. Besides, the intensities of the characteristic diffraction peaks in the XRD patterns of the Bi2WO6/TiO2-NT nanocomposites weaken with the increased content of the TiO2 component in the corresponding Bi2WO6/TiO2-NT nanocomposite. The presence of the hierarchical TiO2 nanotubes influences and inhibits the crystallinity of Bi2WO6 in between the interface of the two phases, revealing the strong interaction between TiO2 and Bi2WO6 phases [35,36].

Figure 2: (a) XRD patterns and (b) FTIR spectra of (i) pure TiO2-NT and (ii) pure Bi2WO6 samples, as well as of hierarchical (iii) 30%−Bi2WO6/TiO2-NT, (iv) 50%−Bi2WO6/TiO2-NT, (v) 70%−Bi2WO6/TiO2-NT, and (vi) 90%−Bi2WO6/TiO2-NT nanocomposites.

Figure 2b displays the FTIR spectra of the hierarchical Bi2WO6/TiO2-NT nanocomposites, pure TiO2-NT, and pure Bi2WO6 powder samples, where all present two similar absorption bands at 1625 and 3420 cm−1 which can be indexed to the stretching vibration of adsorbed H2O and –OH group on the sample surface [37]. Apart from the 30%−Bi2WO6/TiO2-NT nanocomposite, the FTIR spectra of the other Bi2WO6/TiO2-NT nanocomposites all exhibit three apparent absorption bands at approx. 555, 736, and 1077 cm−1, which are indexed to the stretching vibrations of Bi−O and W−O covalent bonds and to the bridge stretching vibration of the W−O−W bond in the Bi2WO6 phase, respectively [38]. All the FTIR spectra of the Bi2WO6/TiO2-NT nanocomposites display bands centered at approx. 480 cm−1 which are assigned to the Ti−O stretching vibration in the TiO2 phase except for the 90%−Bi2WO6/TiO2-NT nanocomposite [39]. The FTIR spectrum of the pure Bi2WO6 powder exhibits similar absorption bands to those of the 90%−Bi2WO6/TiO2-NT nanocomposite, which are all attributed to the Bi2WO6 phase. In the spectrum of pure TiO2-NT, apart from the band indexed to the –OH group (3420 cm−1), another wide band centered at 600 cm−1 is detected which is assigned to the Ti−O stretching vibration in the TiO2 phase. As compared to the spectra of pure TiO2-NT and pure Bi2WO6 powder samples, the absorption bands in the Bi2WO6/TiO2-NT nanocomposites all exhibit slight red shifts, demonstrating the close connection, strong interaction, and formation of heterostructures in between Bi2WO6 and TiO2 phases. It is deduced from the XRD and FTIR characterizations that the Bi2WO6/TiO2-NT nanocomposites are only composed of the anatase phase TiO2 and the russellite phase Bi2WO6, while strong mutual effects and well-proportioned heterostructures are organized in between the two phases.

Figure 3 presents the morphologies and microstructures of cellulose-derived Bi2WO6/TiO2-NT nanocomposites. As exhibited in the FE-SEM images (the first two columns in Figure 3), all Bi2WO6/TiO2-NT nanocomposites are assembled by composite microtubes, which are composed of cross-linked nanotubes, revealing the hierarchical network structures replicated from the initial cellulose template. Besides, it is apparent that the uniform Bi2WO6 nanoparticles are compactly coated on TiO2 nanotubes. The TEM images (the last two columns in Figure 3) of individual composite nanotubes isolated from the Bi2WO6/TiO2-NT nanocomposites show similar nanotube diameters of approx. 100 nm, and the uniform Bi2WO6 nanoparticles (sizes: 10−20 nm) are compactly coating the TiO2 nanotubes which are composed of tiny TiO2 nanocrystallites. With an increased Bi2WO6 content in the Bi2WO6/TiO2-NT nanocomposites, the thicknesses of the isolated nanotubes increase in the order of 15, 25, 40 and 60 nm, and the densities of uniform Bi2WO6 nanoparticles on the TiO2 nanotubes also increase.

Figure 3: Electron micrographs of the hierarchical (a1−a4) 30%−Bi2WO6/TiO2-NT, (b1−b4) 50%−Bi2WO6/TiO2-NT, (c1−c4) 70%−Bi2WO6/TiO2-NT, and (d1−d4) 90%−Bi2WO6/TiO2-NT nanocomposites. The first two columns represent FE-SEM images of the nanocomposites showing the three-dimensional network structures. The last two columns exhibit TEM images of an individual composite nanotube which was separated from the corresponding nanocomposite.

As a comparison, the pure TiO2-NT sample retains the three-dimensional network structure of natural cellulose, and the TiO2 nanotube has a tube diameter of 100 nm and a thickness of 5 nm (Supporting Information File 1, Figure S2a), proving that the deposition of Bi2WO6 nanoparticles does not affect the hierarchically cross-linked structures but thickens the composite nanotubes of the Bi2WO6/TiO2-NT nanocomposites. The pure Bi2WO6 powder sample is formed by aggregated Bi2WO6 particles with sizes of 10−20 nm (Figure S2b). The Bi2WO6/TiO2 sample that was prepared without the cellulose template is composed of several aggregated Bi2WO6 nanoparticles on the bulk TiO2, while the distribution of Bi2WO6 nanoparticles is rather uneven with very different sizes (Figure S2c). It is hence concluded that the cellulose-derived three-dimensional network structures of the Bi2WO6/TiO2-NT nanocomposites promote the uniform dispersion of the Bi2WO6 nanoparticles on the TiO2 nanotubes, which is beneficial to the formation of active sites and well-proportioned heterostructures for the photocatalytic reactions. As illustrated in Figure 1, when the hierarchical TiO2 nanotubes were mixed with the precursor solutions of Bi2WO6, the Bi3+ ions were uniformly dispersed on the negatively charged tube surfaces due to its unique morphology, which resulted in more uniform formation of the Bi2WO6 nanoparticles in the Bi2WO6/TiO2-NT nanocomposites. The detailed mechanism was revealed in the hierarchical Ag2O-nanoparticle/TiO2-nanotube composite reported by our group [31].

As shown in Figure 4a, the HR-TEM image of the 70%−Bi2WO6/TiO2-NT nanocomposite shows two kinds of lattice fringes with interplanar spacings of 0.315 and 0.352 nm, which are attributable to the (131) plane of the russellite phase Bi2WO6 and to the (101) plane of the anatase phase TiO2, respectively [40,41]. The SAED pattern of the 70%−Bi2WO6/TiO2-NT nanocomposite (Figure 4b) displays seven discernible diffraction rings which were denoted as 1−7. The rings 2, 4, 6, and 7 are attributed to the (004), (105), (204), and (215) planes of the anatase phase TiO2, while the rings 3 and 5 are assigned to the (202) and (262) planes of the russellite phase Bi2WO6, respectively [42,43]. Owing to the similar interplanar spacings of the (131) plane of the russellite phase Bi2WO6 and the (101) plane of the anatase phase TiO2, the thick ring 1 is ascribed to these two planes.

Figure 4: (a) HR-TEM image, (b) SAED pattern, and (c−h) EDX element mapping images of Bi, W, Ti, and O elements of the composite nanotube surface in the hierarchical 70%−Bi2WO6/TiO2-NT nanocomposite.

Figure 4c−h exhibit the EDX element mapping images of Bi, W, Ti, and O elements of the composite nanotube surface in the hierarchical 70%−Bi2WO6/TiO2-NT nanocomposite. The signals of Bi and W overlap to a great extent and uniformly distribute along the nanotube without aggregation. These signals are a little broader than those of Ti, revealing that Bi2WO6 nanoparticles spread along the TiO2 nanotube surface. Besides, the signals of Bi, W, and Ti interlaced among the composite nanotube, demonstrating the effective formation of heterostructures in between Bi2WO6 and TiO2 phases in the Bi2WO6/TiO2-NT nanocomposite.

As shown in Supporting Information File 1, Figure S3, the XPS survey spectrum of the 70%−Bi2WO6/TiO2-NT nanocomposite only displays the signals of Ti, O, Bi, and W. The high-resolution XPS spectrum of the Bi 4f region (Figure 5a) shows two peaks at 164.1 and 158.8 eV, which are indexed to the binding energies of Bi 4f5/2 and Bi 4f7/2, respectively, proving the existence of Bi(III) in Bi2WO6[44]. The high-resolution XPS spectrum of the W 4f region (Figure 5b) represents the spin–orbit split lines of W 4f5/2 and W 4f7/2 at 36.7 and 34.9 eV, respectively, indicating the existence of W(VI) in Bi2WO6[45]. As displayed in Figure 5c, there are two peaks at 464.1 and 458.3 eV that are attributed to the binding energies of Ti 2p1/2 and Ti 2p3/2 in the high-resolution XPS spectrum of the Ti 2p region [46], which show a distance of 5.8 eV. This demonstrates the Ti(IV) state in TiO2 in the 70%−Bi2WO6/TiO2-NT nanocomposite [47]. The high-resolution XPS spectrum of the O 1s region (Figure 5d) displays two peaks at 529.6 and 530.8 eV, which are ascribed to the lattice oxygen of the TiO2 and Bi2WO6 phases, as well as to the H2O molecules and –OH groups adsorbed on the sample surface [48,49]. As revealed by XPS, cellulose-derived Bi2WO6/TiO2-NT nanocomposites consist of Bi2WO6 and TiO2 phases, and this agrees well with the results of the aforementioned XRD, FTIR, HR-TEM, and SAED characterizations.

Figure 5: High-resolution XPS spectra of the (a) Bi 4f, (b) W 4f, (c) Ti 2p, and (d) O 1s regions of the hierarchical 70%−Bi2WO6/TiO2-NT nanocomposite.

As shown in Figure 6a, based on the definition of IUPAC, the N2 adsorption−desorption isotherms of the 70%−Bi2WO6/TiO2-NT nanocomposite represent the type IV adsorption isotherm and the H3 type hysteresis in the range of 0.6 to 1.0 of the relative pressure (P/P0), demonstrating the mesoporous structure of the nanocomposite [50]. The specific surface area determined by the BET model (SBET) of the 70%−Bi2WO6/TiO2-NT nanocomposite is 26.3 m2·g−1, suggesting approximately the same value as that of pure TiO2-NT (26.4 m2·g−1) [51] but much higher than that of the pure Bi2WO6 powder (16.0 m2·g−1) [52]. This is mainly benefited from the uniform and compact dispersion of Bi2WO6 nanoparticles on the hierarchical TiO2 nanotubes without aggregation. The corresponding pore size distribution pattern analyzed by the BJH model exhibits a sharp peak at approx. 3 nm and a wide peak at approx. 10 nm, which are assigned to the mesopores of the TiO2 nanocrystallites in the TiO2 nanotubes and the gaps between the Bi2WO6 nanoparticles.

Figure 6: (a) N2 adsorption−desorption isotherms and (b) the pore size distribution pattern analyzed from the adsorption isotherm of the hierarchical 70%−Bi2WO6/TiO2-NT nanocomposite.

Photocatalytic performance

Cr(VI) and RhB pollutants are selected as the model pollutants for the evaluation of the photocatalytic performance of the hierarchical Bi2WO6/TiO2-NT nanocomposites. It is reported that the pH value of the Cr(VI) pollutant solution has an important influence in the photocatalytic reduction of Cr(VI) [53]. In order to investigate the photocatalytic reduction activities of the samples toward Cr(VI), the 70%−Bi2WO6/TiO2-NT nanocomposite was set as the representative photocatalyst for the exploration of optimal pH values of the Cr(VI) pollutant solution under visible-light irradiation. It was demonstrated that the reduction efficiency of Cr(VI) under the alkaline condition was poor because the newly formed Cr(OH)3 precipitates covered the active sites of the photocatalysts [54]. Hence, the photocatalytic reduction reactions of Cr(VI) were conducted under a series of acidic conditions. As revealed in Figure 7a and Figure 7b, the order of Kapp values of the photocatalytic reactions under different pH conditions is pH 4 (0.52 h−1) > pH 3 (0.35 h−1) > pH 5 (0.30 h−1) > pH 2 (0.13 h−1), suggesting that the optimal pH value is pH 4. It has been proven that the pH condition of the Cr(VI) pollutant solution has great effects on the surface potentials of photocatalysts, and Cr(VI) ions exist in the form of HCrO4− and Cr2O72− in the reactions shown in Equation 3 and Equation 4 [55].

(3) (4)

Figure 7: (a) Visible-light-induced (λ > 420 nm) photocatalytic reduction profiles toward the Cr(VI) pollutant solution (10 mg·L−1) under various pH conditions by the hierarchical 70%−Bi2WO6/TiO2-NT nanocomposite. (c) Visible-light-induced (λ > 420 nm) photocatalytic reduction profiles toward the Cr(VI) pollutant solution (10 mg·L−1, pH 4) by different samples, as well as (b,d) the corresponding linear fitting curves based on the pseudo-first-order kinetic model.

Under the pH 2 condition, the mass transfer efficiency of Cr(VI) is low due to the poor adsorption capacity of the photocatalyst toward HCrO4− and Cr2O72−. Although the concentration of H+ is high in the pollutant solution, the excessive adsorbed HCrO4− and Cr2O72− block the active sites of the photocatalyst at pH 3, resulting in rather poor photocatalytic properties. In comparison with the pH 4 and pH 5 conditions, although the adsorption capacities toward HCrO4− and Cr2O72− of the photocatalyst are similar, the photocatalytic reduction reaction of Cr(VI) is easier to take place at pH 4 owing to the higher concentration of H+ in the pollutant solution, leading to optimal photocatalytic performances. Similar results have been reported in the literature [54].

As shown in Figure 7c and Figure 7d, the photocatalytic performances of the samples toward the reduction of Cr(VI) were assessed under pH 4 and visible light (λ > 420 nm) conditions. In the dark adsorption stage, pure Bi2WO6 powder exhibits high adsorption capacity, while pure TiO2-NT presents low adsorption capacity toward the Cr(VI) pollutant, which is mainly due to the surface potentials of the related photocatalysts [56]. Hence, the adsorption capacities of the hierarchical Bi2WO6/TiO2-NT nanocomposites enhanced with the increase in Bi2WO6 contents in the corresponding nanocomposites.

When visible light is shone, the pure TiO2-NT displays little photocatalytic reduction activity toward Cr(VI), and the Kapp values of the photocatalytic reactions of the other samples decrease in the following sequence: 70%−Bi2WO6/TiO2-NT (0.52 h−1) > 90%−Bi2WO6/TiO2-NT (0.47 h−1) > 50%−Bi2WO6/TiO2-NT (0.38 h−1) > pure Bi2WO6 powder (0.34 h−1) > 30%−Bi2WO6/TiO2-NT (0.33 h−1). It was concluded that all Bi2WO6/TiO2-NT nanocomposites present better photocatalytic performances toward the reduction of Cr(VI) except for the 30%−Bi2WO6/TiO2-NT nanocomposite. This is attributable to the cellulose-derived hierarchically interwoven structures as well as to the uniform and compact heterostructures formed in between the TiO2 and Bi2WO6 phases of the Bi2WO6/TiO2-NT nanocomposites, leading to higher separation and transfer efficiencies of the photoinduced electron−hole pairs. The optimal 70%−Bi2WO6/TiO2-NT photocatalyst achieves a removal percentage of 96.9% toward the reduction of Cr(VI) upon visible light irradiation for 5 h with a Kapp value of 0.52 h−1, which is 1.5 folds higher than that of pure Bi2WO6 powder. Although the 70%−Bi2WO6/TiO2-NT nanocomposite does not have a superior adsorption capacity toward Cr(VI), the moderate density and uniform dispersion of Bi2WO6 nanoparticles on the hierarchical TiO2 nanotubes result in optimum migration efficiencies of photogenerated electrons and holes, which leads to the optimal photocatalytic reduction activity.

Similarly, as shown in Supporting Information File 1, Figure S4a, all Bi2WO6/TiO2-NT nanocomposites have larger adsorption capacities toward RhB than those of pure TiO2-NT and Bi2WO6 powder samples, except for the 90%−Bi2WO6/TiO2-NT nanocomposite, which is ascribed to the higher specific surface area as well as to close contact and uniform formation of the heterostructures in between the TiO2 and Bi2WO6 phases of the Bi2WO6/TiO2-NT nanocomposites. When visible light (λ > 420 nm) is irradiated, as shown in Supporting Information File 1, Figure S4, the Kapp values of all samples decrease as follows: 70%−Bi2WO6/TiO2-NT (0.65 h−1) > 50%−Bi2WO6/TiO2-NT (0.45 h−1) > 30%−Bi2WO6/TiO2-NT (0.36 h−1) > 90%−Bi2WO6/TiO2- NT (0.14 h−1) > pure Bi2WO6 powder (0.09 h−1) > pure TiO2-NT (0.05 h−1). The optimal 70%−Bi2WO6/TiO2-NT nanocomposite reveals a degradation percentage of 98.2% under visible-light irradiation for 6 h with a Kapp value of 0.65 h−1, which is 13.0 and 7.2 times as high as those of pure TiO2-NT and Bi2WO6 powder samples, respectively. As a comparison, all hierarchical Bi2WO6/TiO2-NT nanocomposites present better photocatalytic properties than those of pure TiO2-NT and Bi2WO6 powder samples, which is attributed to the three-dimensional porous network structures inherited from the original cellulose configuration and the formation of compact heterostructures in between the TiO2 and Bi2WO6 phases of the Bi2WO6/TiO2-NT nanocomposites. This ultimately leads to enhanced adsorption capacities toward RhB as well as to rapid transfer and separation of the photogenerated electron−hole pairs.

As shown in Supporting Information File 1, Figure S5a and S5c, self-reduction of Cr(VI) and self-degradation of RhB during the photocatalytic processes are negligible, and the adsorption of the optimal 70%−Bi2WO6/TiO2-NT nanocomposite toward Cr(VI) and RhB stopped after the achievement of adsorption−desorption equilibrium, suggesting that the photocatalytic reactions of the hierarchical Bi2WO6/TiO2-NT nanocomposites are generated from intrinsic reactions. As a comparison, the Kapp values of the 70%−Bi2WO6/TiO2-NT nanocomposite (0.52 h−1) toward the reduction of Cr(VI) are, respectively, 1.9 and 2.4 times higher than those of the Bi2WO6/TiO2 sample prepared without cellulose template (0.28 h−1) and the Bi2WO6-TiO2 sample prepared by physical blending (0.22 h−1) ( Supporting Information File 1, Figure S5a,b). The Kapp values of the 70%−Bi2WO6/TiO2-NT nanocomposite (0.65 h−1) toward the reduction of Cr(VI) (Supporting Information File 1, Figure S5c and d) are, respectively, 3.6 and 8.1 times higher than those of the Bi2WO6/TiO2 (0.18 h−1) and Bi2WO6-TiO2 (0.08 h−1) samples. This result reveals that the cellulose-derived three-dimensional network structure promotes the homogeneous dispersion of Bi2WO6 nanoparticles on TiO2 nanotubes, as well as an intense interaction and uniform formation of heterostructures in between the TiO2 and Bi2WO6 phases of the Bi2WO6/TiO2-NT nanocomposites, resulting in the superior photocatalytic performances.

As shown in Table 1, in comparison with other Bi2WO6/TiO2 composites reported in the literature, cellulose-derived Bi2WO6/TiO2-NT nanocomposites deliver higher Kapp values and larger increments as compared to pure TiO2 under more rigorous conditions of visible-light (λ > 420 nm) irradiation and lower dosage of photocatalyst toward the reduction of Cr(VI) or degradation of RhB. The excellent photocatalytic activities of hierarchical Bi2WO6/TiO2-NT nanocomposites are benefited from the uniform deposition of Bi2WO6 nanoparticles on TiO2 nanotubes and from compact heterostructures built in between the TiO2 and Bi2WO6 phases, which is due to the three-dimensional interwoven structures that duplicated from the natural cellulose template.

Table 1: Comparison of visible-light-induced (λ > 420 nm) photocatalytic performances toward the degradation of RhB or reduction of Cr(VI) with other reported Bi2WO6/TiO2 composites in the literature.

Bi2WO6/TiO2 composites Light source Catalyst dosage Concentration and volume of pollutants Kapp (h−1) Increment compared with pure TiO2 Ref. Bi2WO6/TiO2-NT 350 W Xe, λ > 420 nm 10 mg RhB: 10 mg L−1, 20 mL 0.65 13.0 this work     Cr(VI): 10 mg·L−1, 20 mL 0.52 –   Sb3+ doped Bi2WO6/TiO2 Xe lamp with AM 1.5 filter – RhB: 9.58 mg·L−1 0.55 – [48] Bi2WO6/TiO2/Pt Xe lamp 320 nm < λ < 780 nm 100 mg RhB: 20 mg·L−1, 100 mL 1.26 7.0 [57] Bi2WO6/TiO2 nanotubes 400 W Xe, λ > 420 nm 200 mg RhB: 50 mg·L−1, 220 mL 0.66 8.7 [58] Bi2WO6/mesoporous TiO2 nanotubes 300 W Xe with UV cut-off filter 50 mg Cr(VI): 20 mg·L−1, 100 mL 0.33 – [59]

As shown in Supporting Information File 1, Table S3, in comparison with cellulose-derived Ag2O-nanoparticle/TiO2-nanotube (Ag2O-NP/TiO2-NT) composites [31], g-C3N4/TiO2- nanotube (g-C3N4/TiO2-NT) composites [32], and H3PW12O40/TiO2 nanocomposites [33] reported by our group, the hierarchical Bi2WO6/TiO2-NT composite delivered similar three-dimensional interwoven structures that comprised the composite nanotubes. The Ag2O-NP/TiO2-NT and H3PW12O40/TiO2 composites exhibited excellent photocatalytic performances under UV light irradiation, while the g-C3N4/TiO2-NT and Bi2WO6/TiO2-NT composites had a wider light response to the visible spectral region. Besides, under visible light irradiation, Bi2WO6/TiO2-NT composites show better photocatalytic degradation activities than that of g-C3N4/TiO2-NT composites. Based on these cellulose-derived nanocomposites, structure–activity relationships between photocatalytic activities and structures containing the three-dimensional hierarchical network of the natural cellulose template and the compositions of the composite photocatalysts are revealed.

To evaluate the photocatalytic stability of cellulose-derived Bi2WO6/TiO2-NT nanocomposites, the 70%−Bi2WO6/TiO2-NT nanocomposite was chosen as the representative photocatalyst toward the reduction of Cr(VI) and degradation of RhB. As exhibited in Figure 8a, the percentage removal on the fifth cycle only decreased 2% as compared with the first cycle of the 70%−Bi2WO6/TiO2-NT photocatalyst toward the reduction of Cr(VI) under visible light (λ > 420 nm). This demonstrates the rather high photocatalytic stability of Bi2WO6/TiO2-NT nanocomposites on the reduction of Cr(VI). The XRD patterns of the 70%−Bi2WO6/TiO2-NT nanocomposites before and after the photocatalytic reactions (Figure 8b) exhibit a high degree of consistency, suggesting that the crystal structure of the sample is maintained during the cyclic photocatalysis. As shown in Figure 8c, FE-SEM images of the 70%−Bi2WO6/TiO2-NT nanocomposite after photocatalysis still present the cellulose-derived three-dimensionally porous network structure, and the Bi2WO6 nanoparticles are still uniformly and tightly coated on the TiO2 nanotube surfaces, revealing the morphological and structural stabilities of the Bi2WO6/TiO2-NT nanocomposite. In the high-resolution XPS spectrum of the Cr 2p region of the 70%−Bi2WO6/TiO2-NT nanocomposite after photocatalysis (Figure 8d), there are two strong peaks at 586.8 and 577.2 eV that were attributed to the binding energies of Cr 2p1/2 and Cr 2p3/2, which are assigned to Cr(III) [60]. Besides, the weak peak at 580.2 eV is indexed to the spin–orbit split line of Cr 2p3/2, which corresponds to Cr(VI) [54], suggesting that the 70%−Bi2WO6/TiO2-NT nanocomposite effectively reduced Cr(VI) into Cr(III) in five cycles. Benefiting from the cellulose-derived hierarchical network structures together with the crystal and morphological/structural stabilities of the Bi2WO6/TiO2-NT nanocomposite, the adsorbed Cr species on the nanocomposite have no effect on the cyclic photocatalysis, proving the photocatalytic stability of the Bi2WO6/TiO2-NT nanocomposite.

Figure 8: (a) The visible-light-induced (λ > 420 nm) photocatalytic reduction profiles toward the Cr (VI) pollutant solution (10 mg·L−1, pH 4) for five cycles by the hierarchical 70%−Bi2WO6/TiO2-NT nanocomposite. (b) XRD patterns of the 70%−Bi2WO6/TiO2-NT nanocomposites before and after five-cycle photocatalysis. (c) The FE-SEM image and (d) high-resolution XPS spectrum of the Cr 2p region of the 70%−Bi2WO6/TiO2-NT nanocomposite after five-cycle photocatalysis.

Analogously, as shown in Supporting Information File 1, Figure S6a, the degradation percentage only declines to 13% after five-cycle photocatalytic reactions toward the degradation of RhB, which is mainly due to the loss of the powder photocatalyst during the centrifugation procedure in the cycling processes. The XRD pattern (Supporting Information File 1, Figure S6b) and FE-SEM images (Supporting Information File 1 Figure S6c and Figure S6d) of the 70%−Bi2WO6/TiO2-NT nanocomposite after photocatalysis reveal high stabilities of the crystal and morphological structures. It is concluded that the stabilities of the crystal and morphological structures of the Bi2WO6/TiO2-NT nanocomposite result in the superior photocatalytic stability toward the degradation of RhB.

Photocatalytic mechanism

As presented in Figure 9a, all the UV–vis DRS of the hierarchical Bi2WO6/TiO2-NT nanocomposites show the regions of UV light response and visible light response. The UV–vis DRS of the pure TiO2-NT sample shows an absorption edge at approx. 400 nm that is ascribed to the UV light response without the visible light response, while the UV–vis DRS of the pure Bi2WO6 powder sample displays an absorption edge at approx. 505 nm, which corresponds to both UV and visible light responses. As compared with the pure TiO2-NT sample, the absorption edges in the UV–vis DRS of the Bi2WO6/TiO2-NT nanocomposites extend to approx. 425, 440, 455, and 490 nm, suggesting that the visible light responses of the nanocomposites are remarkably strengthened and the response enhances with the increased Bi2WO6 content in the respective nanocomposite.

Figure 9: (a) UV–vis DRS, (b) bandgaps determined by the intercept on the x-axis of the respective Tauc plots, and (c) PL emission spectra under the excitation at 360 nm of the pure TiO2-NT sample, pure Bi2WO6 powder sample, and the hierarchical Bi2WO6/TiO2-NT nanocomposites.

As shown in Figure 9b, the order of the bandgaps of the samples is as follows: pure Bi2WO6 powder (2.40 eV) < 90%−Bi2WO6/TiO2-NT (2.55 eV) < 70%−Bi2WO6/TiO2-NT (2.73 eV) < 50%−Bi2WO6/ TiO2-NT (2.83 eV) < 30%−Bi2WO6/TiO2-NT (2.92 eV) < pure TiO2-NT (3.12 eV). This reveals that the bandgaps of the Bi2WO6/TiO2-NT nanocomposites decrease with the increase in the Bi2WO6 content in the corresponding nanocomposites. As compared with the pure TiO2-NT sample, the enhanced visible light responses and decreased bandgaps of the Bi2WO6/TiO2-NT nanocomposites are attributed to the wider visible-light-responsive region of the Bi2WO6 component and the uniform heterostructures built in between the TiO2 and Bi2WO6 phases in the nanocomposites, leading to the production of more carriers when induced by visible light. It is reported that the change of binding energy due to the atomic bonding or charge transfer transition of the conduction bands in between TiO2 and Bi2WO6 phases in the Bi2WO6/TiO2-NT nanocomposites results in the aforementioned enhancement [39].

The PL emission spectra of the related samples under the excitation at 360 nm are presented in Figure 9c, which are applied to evaluate the separation and transfer efficiencies of the samples. All PL spectra exhibit a strong peak at 456 nm and other three weak peaks at 448, 479, and 490 nm, which are indexed to the recombination of photoinduced electron−hole pairs, freely excited electrons, surface defects, and oxygen vacancies on the band edges, respectively [36,61]. It is apparent that the PL intensities of the Bi2WO6/TiO2-NT nanocomposites at 465 nm are all weaker than those of pure TiO2-NT and Bi2WO6 powder samples, except for the 30%−Bi2WO6/TiO2-NT nanocomposite. This demonstrates that the deposition of Bi2WO6 nanoparticles on the TiO2 nanotubes is effective to inhibit the recombination of photogenerated electron−hole pairs.

In comparison with the Bi2WO6/TiO2-NT nanocomposites with varied contents of the Bi2WO6 component, the weakest PL intensity of the 70%−Bi2WO6/TiO2-NT nanocomposite demonstrates its highest separation and transfer efficiencies of photogenerated electron−hole pairs, which is advantageous to the photocatalytic reduction of Cr(VI) and degradation of RhB. Although the 90%−Bi2WO6/TiO2-NT nanocomposite has the largest amount of heterostructures in between the TiO2 and Bi2WO6 phases owing to its highest content of the Bi2WO6 component, the PL intensity is a little higher than that of the 70%−Bi2WO6/TiO2-NT nanocomposite, which is due to the fact that excessive Bi2WO6 nanoparticles act as the recombination centers and inhibit the transfer of the photoinduced electrons and holes. However, due to the lower contents of the Bi2WO6 component, the heterostructures in the 30%−Bi2WO6/TiO2-NT and 50%−Bi2WO6/TiO2-NT nanocomposites are less than that of the 70%−Bi2WO6/ TiO2-NT nanocomposite, causing the lower separation and transfer efficiencies of the photogenerated electron−hole pairs.

In comparison with the Bi2WO6/TiO2 sample prepared without the cellulose template and the Bi2WO6-TiO2 sample prepared by physical blending (Supporting Information File 1, Figure S7), the cellulose-derived 70%−Bi2WO6/TiO2-NT nanocomposite possesses wider absorption edge in the UV–vis DRS, corresponding narrower bandgap, and weaker PL intensity in the PL spectra, suggesting stronger response to visible light and more efficient separation of the photogenerated electron−hole pairs.

It is reported that the transient photocurrent responses of the samples depend on the amounts of photogenerated charges and the kinetics of charge separation of the corresponding electrodes under irradiation of light [62]. As shown in Figure 10a, under irradiation of visible light (λ > 420 nm) at intervals of 30 s, all the Bi2WO6/TiO2-NT nanocomposites exhibit higher photocurrent responses than that of the pure Bi2WO6 powder sample except for the 30%−Bi2WO6/TiO2-NT nanocomposite, revealing that the recombination of photoinduced electrons and holes is inhibited on the interface of the Bi2WO6/TiO2-NT heterostructures, which promotes the generation of more effective photogenerated charges. Besides, the 70%−Bi2WO6/TiO2-NT and 90%−Bi2WO6/TiO2-NT nanocomposites behave similarly but have higher photocurrent responses than those of the 50%−Bi2WO6/TiO2-NT nanocomposite, which performs analog results as compared to the PL characterization, suggesting the most effective separation and transfer of photogenerated carriers of the 70%−Bi2WO6/TiO2-NT and 90%−Bi2WO6/TiO2-NT nanocomposites.