Preparation of a Ti0.7W0.3O2/TiO2 nanocomposite interfacial photocatalyst and its photocatalytic degradation of phenol pollutants in wastewater

A Ti0.7W0.3O2/TiO2 nanocomposite interfacial photocatalyst was designed and prepared for the photocatalytic degradation of phenol pollutants in wastewater. The detailed properties of the Ti0.7W0.3O2/TiO2 nanocomposite interface (NCI) were analyzed by XRD, SEM, EDX, DRS, UPS and XPS technologies, showing that anatase TiO2 nanospheres (NSs) were uniformly dispersed on the surface of rutile Ti0.7W0.3O2 nanoparticles (NPs) and formed the nanocomposite interface. The DRS and UPS results of 5 wt% Ti0.7W0.3O2/TiO2 NCI indicated a greatly broadened light response range with a wavelength shorter than 527 nm and a shorter band gap energy of 2.37 eV. The conduction band of TiO2 NSs, Ti0.7W0.3O2 NPs and 5 wt% Ti0.7W0.3O2/TiO2 NCI were measured based on the results of the valence band and band gap energy obtained via XPS and DRS, and then the energy level diagram of Ti0.7W0.3O2/TiO2 NCI was proposed. The photocatalytic degradation of phenol at Ti0.7W0.3O2/TiO2 NCI with different loading ratios of Ti0.7W0.3O2 NPs was investigated under optimum conditions (i.e., pH of 4.5, catalyst dosage of 0.45 g L−1 and phenol initial concentration of 95 ppm) under the illumination of ultraviolet visible light. Also, 5 wt% Ti0.7W0.3O2/TiO2 NCI exhibited the highest photocatalytic activity, with the initial rate constant (k) calculated as 0.09111 min−1. After recycling six times, Ti0.7W0.3O2/TiO2 NCI showed good stability and recyclability. The involvement of superoxide radicals in the initial reaction at Ti0.7W0.3O2/TiO2 NCI was evidenced by the use of a terephthalic acid (TA) fluorescent probe. Besides, UV-Vis spectroscopy, UHPLC-MS and GC-MS technologies were used to analyze the main intermediates in the photocatalytic degradation of phenol. The probable photocatalytic degradation mechanism of phenol at Ti0.7W0.3O2/TiO2 NCI was also proposed.


Introduction
Environment pollution has become a worldwide problem.2][3] Over the last several years, the utilization of direct solar light has become a much greener approach for energy generation as well as for environmental clean-up.Therefore, the development and design of a UV-Vis active photocatalyst for direct sunlight harvesting has drawn broad interdisciplinary attention and much research fascination.The photocatalytic decomposition of organic contaminants is one of the most promising techniques for wastewater treatment and purication.
However, the total quantum efficiency of TiO 2 is very low, [26][27][28] which has limited the potential value of actual production and application of TiO 2 nanomaterials.A lot of studies have been done to address the drawbacks mentioned above, with noble metal deposition considered as one of the most effective and promising solutions.Pt is one such representative noble metal, which has been widely used to improve the performance of TiO 2 nanomaterials in wastewater treatment and air purication with a superior performance.Wang 29 successfully synthesised a Pt/TiO 2 NW photocatalyst.The recombination rate of electrons and holes was reduced greatly for Pt NPs, resulting in good conductivity.Pt NPs are superior electron acceptors on the photocatalyst surface and enable the timely transfer of electrons.Emilio et al. 30 observed an increase in the lifetime of electrons by Pt modication on the TiO 2 surface due to the better separation of charge carriers caused by the Schottky barrier between Pt and TiO 2 .2,25,[31][32][33] However, noble metals are scarce and particularly expensive, which may limit their large-scale application.Thus, novel relatively economical photocatalysts are highly desirable.
In the present paper, a Ti 0.7 W 0.3 O 2 /TiO 2 NCI was synthesized via a sol-gel and combustion technique, and was shown to possess several positive aspects, such as good stability, good visible light response range and effectively decreased recombination of charge carriers by a fast photogenerated electron transfer.The photocatalytic activity of the Ti 0.7 W 0.3 O 2 /TiO 2 NCI was investigated for the degradation of phenol under simulated solar light illumination, and it showed higher photocatalytic activity.Furthermore, the main intermediates and mechanism for the photocatalytic degradation of phenol at the Ti 0.7 W 0.3 O 2 / TiO 2 NCI were also analyzed and discussed.This type of a photocatalyst may nd application in low concentration organic wastewater clean-up.

Synthesis of the TiO 2 NSs
The pure anatase TiO 2 NSs was prepared via a hydrothermal method. 34First, 6 mM NaOH particles were added to 40 mL absolute ethyl alcohol and stirred for 10 min.Then, 2 mL titanium trichloride solution was added to the above NaOH solution drop-wise under vigorous stirring.Aer 10 min, the mixed solution was transferred to a 50 mL Teon-lined autoclave and heated at 150 C for 18 h.Aer cooling, the precipitate collected through centrifugation was rinsed with distilled water and pure ethanol several times until there were no residual ions le.Then, the products were calcined at 400 C for 2 h aer being dried at 80 C.

Synthesis of the Ti
The Ti 0.7 W 0.3 O 2 NPs were prepared by a modied sol-gel technique.First, 4 mM of WCl 6 powder was added to 10 mL of absolute ethyl alcohol with stirring for 10 min.Then, 1 mL of titanium tetrachloride solution was added to 5 mL of distilled water drop-wise under vigorous stirring.The above solutions were mixed together under air-free conditions.Then, the above mixed solution was stirred by mechanical stirring under 40 C constant temperature water for about 24 h to get a baby blue gel.The product was poured into a drying oven at 100 C for 12 h.The dry power was added to a 50 mL Teon-lined autoclave with a moderate amount of alcohol and heated at 180 C for 8.5 h, and then the precipitate was dried and ground.Finally, the obtained powder was reduced at 1300 C in a H 2 atmosphere for 4 h to obtain a light tan powder.At this point, the W 6+ was fully reduced to W 4+ . 35,364 Synthesis of the Ti 0.7 W 0.3 O 2 /TiO 2 NCI The Ti 0.7 W 0.3 O 2 /TiO 2 NCI was prepared via a simple method.First, 6 mM NaOH were added to 40 mL absolute ethyl alcohol and stirred for 10 min.Then, 2 mL titanium trichloride solution was added to the above NaOH solution drop-wise under vigorous stirring accompanied with a certain quality of pure rutile Ti 0.7 W 0.3 O 2 NPs.34 Aer 30 min, the mixed solution was transferred to a 50 mL Teon-lined autoclave and heated at 180 C for 8.5 h, and then the precipitate obtained was dried and ground.Then, the obtained powders were calcined at 400 C in a N 2 atmosphere for 2 h.All referenced Ti 0.7 W 0.3 O 2 / TiO 2 NCI samples were also prepared by the method described above and used for the photoactivity tests.

Synthesis of the Pt/TiO 2 nanocomposites
The synthesis of the Pt/TiO 2 nanocomposites and the characterization results are described in the ESI (Section 1, Fig. S1-S4 †).
Meanwhile, the detailed information on the characterization, photocatalytic test and analysis of the intermediates in the photocatalytic degradation of phenol are shown in ESI (Section 2 †).

Characterization
PXRD analysis was carried out to investigate the impact of Ti 0.7 W 0.3 O 2 modication on the phase structure and on the chemical composition of the TiO 2 NSs, as these have a great inuence on the photocatalytic activity.The PXRD patterns of the prepared samples are depicted in Fig. 1(A).][39] All diffraction peaks of the samples (a and i) could be indexed to the International Centre for Diffraction data of pure anatase TiO 2 (JCPDS no.21-1272) and rutile TiO 2 (JCPDS no.21-1276), respectively.The crystallite sizes were calculated by Scherrer's formula given in eqn (1).
where l is the wavelength of the Cu-Ka used, b is the full width at half-maximum of the diffraction peak, K is a shape factor (0.94) and q is the angle of diffraction.The average crystalline size calculated from the major diffraction peak (101) of anatase TiO 2 NSs was about 149.1 nm. 34The average crystalline size calculated from the major diffraction peak (110) of rutile Ti 0.7 W 0.3 O 2 NPs was about 1077 nm. 36he average particle size and distribution of the anatase TiO 2 NSs and Ti 0.7 W 0.3 O 2 NPs were obtained using a laser particle size analyzer and are shown in ESI (Section 3, Fig. S6 †).The average particle size and distribution of anatase TiO 2 NSs were determined to be about 287 nm, which was larger than the average crystalline size calculated from the major diffraction peak (101) in the XRD analysis.The average particle size and distribution of Ti 0.7 W 0.3 O 2 NPs were determined to be about 1189 nm, which was consistent with the result calculated from the major diffraction peak (110) in the XRD analysis of the rutile Ti 0.7 W 0.3 O 2 NPs.The possible reasons for the deviation were as follows.On the one hand, some TiO 2 NSs reunite aer high temperature calcination.On the other hand, the principles of the two kinds of detection methods were different, whereby the results of the XRD analysis were estimated using an empirical formula, whereas the laser particle size analyzer detection needed the samples to be dispersed in water, and the dispersion of the TiO 2 NSs was not very good and they were prone to reunion.This might lead to an increase in the error of the result.
The PXRD patterns of Ti 0.7 W 0.3 O 2 /TiO 2 NCI loaded with 1 wt%, 2 wt%, 5 wt%, 10 wt%, 20 wt% and 50 wt% Ti 0.7 W 0.3 O 2 NPs are shown in Fig. 1(A)(b-g).All the samples were identical with the pure anatase and rutile phase aer calcination at 400 C , respectively.None of the diffraction peaks were changed signicantly aer deposition, which indicated that the Ti 0.7 W 0.3 O 2 NPs did not affect the phase structure and chemical composition of the TiO 2 NSs.However, further observation showed that the diffraction peaks corresponding to TiO 2 NSs exhibited relatively weaker peak intensities and broader diffraction peak widths.It could be inferred from this that the average crystallite size was slightly decreased by Ti 0.7 W 0. The detailed morphological features of the Ti 0.7 W 0.3 O 2 /TiO 2 NCI were characterized by SEM technology and are shown in Fig. 2(A-D).The pure anatase TiO 2 was present as nanospheres, with a uniform size distribution as shown in Fig. 2(A).Fig. 2(B) shows the morphology of the pure rutile Ti 0.7 W 0.3 O 2 NPs, which were irregular and schistose particles with a smooth surface and highly dense quality.The elemental composition of the synthesis of the samples was conrmed by EDX spectra.][49] Furthermore, the atomic% of Ti/the atomic% of O of anatase TiO 2 was measured as 0.48, which was close to the mol ratio of Ti/O (0.50) in TiO 2 .The atomic% of Ti/the atomic% of W of

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However, the atomic% of Ti/the atomic% of O of anatase TiO 2 and the atomic% of Ti/the atomic% of W of rutile Ti 0.7 W 0.3 O 2 NPs in Ti 0.7 W 0.3 O 2 /TiO 2 NCI were measured to be 0.54 and 2.02, respectively.The error of the results had thus increased, which might be due to the interaction between the two nanomaterials.
A UV-Vis spectrometer was used to record diffuse reectance spectra in the range 200-800 nm.Fig. 3 (A ) were generated and quickly spread to the valence band due to the shorter band gap energy, which might make the semiconductor have higher conductivity. 53The volume resistivity and conductivity of pure anatase TiO 2 , 0.3 wt% Pt/TiO 2 and 5 wt% Ti 0.7 W 0.3 O 2 /TiO 2 are shown in Fig. 3(F).The volume resistivity of pure anatase TiO 2 was over 10 times that of 5 wt% Ti 0.7 W 0.3 O 2 /TiO 2 .The volume resistivity of 0.3 wt% Pt/TiO 2 was over 2 times that of 5 wt% Ti 0.7 W 0.3 O 2 /TiO 2 .It was thus indicated that the conductivity of pure anatase TiO 2 could be improved greatly by modifying the Ti 0.7 W 0.3 O 2 NPs, and that the performance of Ti 0.7 W 0.3 O 2 NPs was superior to that of Pt NPs, which could fully prove the above conjecture.
Meanwhile, the 5 wt% Ti 0.7 W 0.3 O 2 /TiO 2 NCI was extended to the visible absorbance region with a wavelength shorter than 527 nm and had a shorter band gap energy of 2.37 eV.The above results were fully proved by ultraviolet photoemission spectroscopy (UPS) and the results are shown in ESI (Section 4, Fig. S7 †).The band gap energies of the pure TiO 2 NSs and 5 wt% Ti 0.7 W 0.3 O 2 /TiO 2 NCI were 3.38 and 2.43 eV, respectively, which were slightly larger than that from the DRS.The reasons for the deviation may be due, on the one hand, to the detection depth of UPS technology, which was 10 atoms, while on the other hand, the carbon pollution signal would be higher for the solid powder sample.
In short, Ti 0.7 W 0. The rutile Ti 0.7 W 0.3 O 2 was further characterized by XPS, as seen in Fig. 4(A-D), to illustrate the structural features and composition.The XPS survey scan spectrum of rutile Ti 0.7 W 0.3 O 2 is shown in Fig. 4(A).The relative concentrations of Ti and W of Ti 0.7 W 0.3 O 2 were determined by the respective XPS peak areas and atomic sensitivity factors and n Ti /n W was measured as 2.13, which is close to the mol ratio of Ti/W (2.33) in Ti 0.7 W 0.3 O 2 54 and consistent with the EDX result.The XPS narrow scan spectra of Ti 2p, W 4f and O 1s of the rutile Ti 0.7 W 0.3 O 2 are shown in Fig. 4(B-D).The Ti 2p 1/2 and Ti 2p 3/2 peaks of Ti 0.7 W 0.3 O 2 were located at 464.6 and 458.8 eV and assigned to TiO 2 , respectively.The W 4f 5/2 and W 4f 7/2 peaks were located at 34.2 and 33.1 eV, and assigned to WO 2 , proving that the W 6+ was fully reduced to W 4+ in Ti 0.7 W 0.3 O 2 .
3.2 Photocatalysis 3.2.1 Optimal photocatalytic conditions (pH, initial phenol concentration, photocatalyst dosage).In order to nd the optimal initial pH value of the solution, the photocatalytic degradation of phenol was carried out at a pH of 3.5-10.0,catalyst dosage of 0.45 g L À1 , irradiation time of 360 min and phenol concentration of 95 ppm and the results are shown in Fig. 5(A).Obviously, the Ti 0.7 W 0.3 O 2 /TiO 2 NCI showed the highest photocatalytic activity under pH 4.5, indicating that phenol photodegradation in the acidic solution was higher than that in natural solution, neutral solution and alkaline solution, which was consistent with the research conclusions of Khataee 55 and Kim. 56They believed that the oxidation ability of the hydroxyl radical (cOH À ) under acidic conditions was higher than in alkaline solution.The formation of HCO 3 À and CO 3 2À in alkaline solution would interfere in the reaction between pollutants and cOH À , resulting in reducing its oxidation potential and leading to a lower photocatalytic activity.Meanwhile, with the appearance of HCO 3 À and CO 3 2À , the low adsorption of negatively charged system components resulted in a lower production of superoxide radical anions (cO 2 À ) and hence a lower oxidation ability.However, the phenol photodegradation at pH 3.5 was also lower than that at pH 4.5.This may be due to the change in the Ti 0.7 W 0.3 O 2 /TiO 2 NCI structure under a too acidic environment.Herein, the results revealed that the optimal initial pH value was 4.5.
The most appropriate initial phenol concentration was investigated with the initial concentration ranging from 50-125 ppm and the results are shown in Fig. 5(B).Obviously, the complete photodegradation time of phenol increased with the increase in the initial concentration from 50-95 ppm, and the photodegradation of phenol at 95 ppm could be just nished with 360 min irradiation.The photocatalytic efficiencies of 110 ppm and 125 ppm phenol were 94.3% and 86.3% aer 360 min irradiation, respectively.Further increases decreased the photocatalytic efficiency, indicating that there was an optimum value.The reasonable explanations for this are as follows: rst, too many phenol molecules and its intermediates would also absorb a part of the irradiation and limit the light absorption capability of the photocatalysts.Second, excessive amounts of phenol molecules and its intermediates also deactivate more active sites and reduce the light penetration to active sites situated on the surfaces of Ti 0.7 W 0.3 O 2 /TiO 2 NCI.The above two disadvantages also result in a lower production of superoxide radical anions (cO 2 À )/cOH À and ultimately a lower oxidation ability. 57e effect of the Ti 0.7 W 0.3 O 2 /TiO 2 NCI dosage was investigated by varying the dosage from 0.15 g L À1 to 0.90 g L À1 and the results are shown in Fig. 5(C).When raising the Ti 0.7 W 0.3 O 2 /TiO 2 NCI dosage from 0.15 g L À1 to 0.60 g L À1 , the phenol photocatalytic efficiency increased from 58.7% to 100% as more active sites were available, increasing the response surface area and leading to a greater production of cO 2 À /cOH À .However, further increasing, the dosage to 0.90 g L À1 decreased the photocatalytic efficiency.According to the literature, [58][59][60][61][62] the reasons for this might be due to the following aspects: on the one hand, an excessive dosage of photocatalysts would result in lower solution transparency, light scattering and interception and the prevention of the light induction of some catalysts particles.On the other hand, too many photocatalysts particles would prevent the effective collisions between phenol molecules and a variety of free radicals.Moreover, the pore volume and available surface area of the photocatalysts would also be diminished with excessive dosage, resulting in a lower photocatalytic activity.
The stability and recyclability of all heterogeneous photocatalysts are critically important for application in wastewater treatment plants.The stability and recyclability of Ti 0.7 W 0.3 O 2 / TiO 2 NCI were investigated in a batch reactor under pH 4.5, a catalyst dosage of 0.45 g L À1 and phenol concentration of 95 ppm.Aer each experiment, the used photocatalyst was collected from the suspension turbid solution and washed with 50% ethanol solution to remove residue phenol and other photodegradation products on the photocatalysts surface.Then, the wet photocatalyst was dried at 105 C for 4 h.This sequence was repeated six times and the phenol photodegradation efficiency of each cycle recorded and the results are shown in Fig. 5(D).Aer six recycles, the photocatalytic degradation efficiency of Ti 0.7 W 0.3 O 2 /TiO 2 NCI was reduced from 100% to 94.5%, indicating that the Ti 0.7 W 0.3 O 2 /TiO 2 NCI showed high photocatalytic activity with good stability and recyclability.The reduction could be explained by a loss of photocatalyst during the washing process, which was consistent with the PXRD results.Phenol photocatalytic degradation rate (%)

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where c t¼0 is the initial concentration of phenol and c t is the concentration of phenol obtained aer various intervals of time (t).From the experimental study, it was observed that 5 wt% Ti 0.7 W 0.3 O 2 /TiO 2 NCI showed higher photocatalytical activity.The phenol photocatalytic degradation trend was: 5 wt% > 10 wt% > 2 wt% > 1 wt% > 20 wt% > 50 wt% > pure TiO 2 > pure Ti 0.7 W 0.3 O 2 .Here, 98.7% of the phenol was photodegraded by 5 wt% Ti 0.7 W 0.3 O 2 /TiO 2 NCI aer 50 min irradiation, indicating a higher photocatalytic activity.However, only about 47.23% and 90.79% of the phenol was photodegraded for P-25 and 0.3 wt% Pt/TiO 2 aer 360 min irradiation.It was indicated that the Ti 0.7 W 0.3 O 2 NPs may be much superior to Pt NPs for modifying the photocatalytic performance of TiO 2 nanomaterial.
3.2.3Kinetic study of the phenol photocatalytic degradation.Additionally, kinetic analysis of phenol degradation was performed for a better comparison of the photocatalytic efficiency of the different photocatalysts.The dependence of ln(c 0 / c) on the irradiation time (t) in P-25, Pt/TiO 2 and Ti 0.7 W 0.3 O 2 / TiO 2 is shown in Fig. 7.It was indicated that the initial photodegradation of phenol followed a quasi-rst-order-type kinetics, as evidenced by the linear relationship between ln(c 0 /c) and the time (t).Actually, c 0 is the initial concentration of phenol, and c is the concentration of phenol aer irradiation for time (t).
The initial rate constant (k) for phenol photocatalytic degradation in P-25 was calculated as 0.00346 min À1 , while the initial rate constant (k) for phenol photocatalytic degradation in 5 wt% Ti 0.7 W 0.3 O 2 /TiO 2 and 0.3 wt% Pt/TiO 2 was about 26 and 6.8 times that of P-25, respectively.This indicated that the photocatalytic activity of TiO 2 was enormously improved with the proper amount of loading Ti 0.7 W 0.3 O 2 and Pt NPs.Furthermore, the initial rate constant (k) for 5 wt% Ti 0.7 W 0.3 O 2 / TiO 2 was over 3.9 times that in 0.3 wt% Pt/TiO 2 , illustrating that the as-prepared Ti 0.7 W 0.3 O 2 NPs may be much superior to Pt NPs in embellishing the photocatalytic properties of TiO 2 nanomaterials and could even replace them.
Herein, to better assess the photocatalytic activity of the synthesized Ti 0.7 W 0.3 O 2 /TiO 2 NCI, we compared our results with the photodegradation of phenol reported in previous studies, as shown in Table 1.The Ti 0.7 W 0.3 O 2 /TiO 2 NCI showed several advantages in the photocatalytic performance, photocatalytically degrading the most amount of phenol with the least irradiation time and catalyst dosage.The initial rate constant (k) of the Ti 0.7 W 0.3 O 2 /TiO 2 NCI was over 2.6 times that of SnS 2 /TiO 2 nanocomposite catalyst, which showed a higher photocatalytic activity than the other catalysts.Therefore, it was concluded that the Ti 0.7 W 0.3 O 2 /TiO 2 NCI was one of the most efficient catalysts for the photocatalytic degradation of phenol under the selected experimental parameters.
3.2.4Analysis of the intermediates in the photocatalytic degradation of phenol.It is well known that phenol decomposition could follow different complicated multistage pathways, following various by-products.0][81][82][83][84] Ilisz et al. 85 found that phenol was degraded into various by-products (such as hydroquinone, catechol and other ring-opened compounds) with different oxidizing agents under UV-Vis irradiation in the presence of different electron scavengers.Catechol, hydroquinone and 2,4-hexadiendioic acid were found during phenol degradation at mesoporous TiO 2Àx B x by Xiong et al. 73 The UV-Vis absorption spectra from the photodegradation of phenol over P-25, Pt/TiO 2 and Ti 0.7 W 0.3 O 2 /TiO 2 are compared in ESI (Section 6, Fig. S10 †).At P-25, besides the characteristic absorption bands at 270 nm of phenol, a new absorption band at 289 nm appeared, which might be attributed to the ringretaining compounds. 29,59,86,87However, besides the two absorption bands at 270 and 289 nm, there were two new absorption bands at 247 and 257 nm for Pt/TiO 2 , and another two new absorption bands at 333 and 363 nm for Ti 0.7 W 0.3 O 2 / TiO 2 , which might be attributed to ring-opened produciits. 87,88It can be concluded that the phenol photodegradation pathway over Ti 0.7 W 0.3 O 2 /TiO 2 NCI was partially different from that over P-25 and Pt/TiO 2 .
The aqueous solutions of phenol degradation over P25, Pt/ TiO 2 and Ti 0.7 W 0.3 O 2 /TiO 2 were detected by UV-Vis spectrometry, UHPLC-MS and GC-MS.Then, the main intermediates were analyzed and inferred by the molecular ions and mass fragment peaks present and from library data.The LC chromatograms, UV-Vis spectrograms and mass spectra from HPLC-MS are shown in ESI (Section 7, Fig. S11a-k †).The GC chromatograms and mass spectra from GC-MS of the intermediates are shown in ESI (Section 8, Fig.S12 and S13 †).The analytical results and possible structures of each intermediate are shown in ESI (Section 9, Table S1 and Fig. S14 †).
In short, ve and six kinds of intermediates were identied in the aqueous suspension of P-25 and Pt/TiO 2 , respectively.Six kinds of intermediates were found in the aqueous suspension of Ti 0.7 W 0.3 O 2 /TiO 2 .The further degradation of all the intermediates might include oxidative hydroxylation and oxidative decarboxylation products, etc. from several reaction pathways operating simultaneously.
3.2.5 Determination of the superoxide radicals using a terephthalic acid (TA) uorescent probe.0][91][92][93] Fig. 8(A) shows the uorescence spectra observed for the supernatant solution of the Ti 0.7 W 0.3 O 2 /TiO 2 NCI suspension containing 3 mM TA irradiated for various irradiation times.Since the observed uorescence spectra were identical to that of TAOH, it was concluded that TAOH is generated from TA by the reaction with superoxide radical anion (cO 2À ) and hydroxyl radical (cOH À ), where superoxide radicals are generated in Ti 0.7 W 0.3 O 2 /TiO 2 NCI suspension and involved in the radical reaction mechanism.
Figure 8(B) presents the uorescence intensity as a function of the duration of irradiation.The uorescence intensity increased linearly with the irradiation time, showing that the formation superoxide radical follows the quasi-rst-order-type kinetics, as evidenced by the linear relationship between the concentration of superoxide radical and irradiation time within a certain range.
3.2.6Mechanism of the photocatalytic degradation of phenol.2][83] The radical reaction and holes reaction mechanism of phenol degradation with mesoporous TiO 2Àx B x was proposed by Xiong et al. 73 Liu et al. 94 well studied the mechanisms of phenol degradation over TiO 2 3D microspheres and proposed three kinds of photodegradation pathways: phenol was transformed into dihydroxybenzene, benzoquinone and 4,4 0 -dihydroxybiphenyl rst, and then transformed into maleic anhydride, which was further photodegraded to CO 2 and H 2 O, nally.][97] Therefore, based on the present experimental data and the referenced studies, 29,59,91,[94][95][96][97] the different intermediates of P-25, Pt/TiO 2 and Ti 0.7 W 0.3 O 2 /TiO 2 indicate the different phenol degradation processes, as clearly illustrated in Fig. 9.
In addition, we believe that the photocatalytic degradation of phenol over P-25 follows a radical reaction mechanism.The photocatalytic degradation of phenol over Pt/TiO 2 and Ti 0.7 W 0.3 O 2 /TiO 2 follows both a radical reaction mechanism and holes reaction mechanism, which proceed in parallel.
The phenol photocatalytic degradation mechanism involves initial reactions at the Ti 0.7 W 0.3 O 2 /TiO 2 NCI, as shown in  ).Acidic conditions could generate a higher affinity towards unpaired e cb À of NCI, leading to the formation of more hydroxyl radicals (cHO 2 À ). 98,99The h vb + could also oxidize pollutants directly.These might account for the extraordinary photocatalytic activity of Ti 0.7 W 0.3 O 2 /TiO 2 NCI.

Conclusions
In summary, Ti 0.7 W 0.  a higher formation of h vb + and e cb À , and the Ti 0.7 W 0.3 O 2 NPs could quickly transfer e cb À and effectively restrain the recombination of e cb À -h vb + pairs for high conductivity.A large number of superoxide radicals were generated in the suspension in the photocatalytic degradation of phenol by the Ti 0.7 W 0.3 O 2 /TiO 2 NCI, which enhanced the photocatalytic activity.Besides, ve and six kinds of intermediates were identied in the suspension of P-25 and Pt/TiO 2 , respectively, and six kinds of intermediates were found in the suspension of Ti 0.7 W 0.3 O 2 /TiO 2 .The photocatalytic degradation of phenol over P-25 followed a radical reaction mechanism.The photocatalytic degradation of phenol over Pt/ TiO 2 and Ti 0.7 W 0.3 O 2 /TiO 2 followed both a radical reaction mechanism and hole reaction mechanism, which proceeded in parallel.
photocatalytic degradation of phenol pollutants in wastewater.The detailed properties of the Ti 0.7 W 0.3 O 2 /TiO 2 nanocomposite interface (NCI) were analyzed by XRD, SEM, EDX, DRS, UPS and XPS technologies, showing that anatase TiO 2 nanospheres (NSs) were uniformly dispersed on the surface of rutile Ti 0.7 W 0.3 O 2 nanoparticles (NPs) and formed the nanocomposite interface.The DRS and UPS results of 5 wt% Ti 0.7 W 0.3 O 2 /TiO 2 NCI indicated a greatly broadened light response range with a wavelength shorter than 527 nm and a shorter band gap energy of 2.37 eV.The conduction band of TiO 2 NSs, Ti 0.7 W 0.3 O 2 NPs and 5 wt% Ti 0.7 W 0.3 O 2 /TiO 2 NCI were measured based on the results of the valence band and band gap energy obtained via XPS and DRS, and then the energy level diagram of Ti 0.7 W 0.3 O 2 / TiO 2 NCI was proposed.The photocatalytic degradation of phenol at Ti 0.7 W 0.3 O 2 /TiO 2 NCI with different loading ratios of Ti 0.7 W 0.3 O 2 NPs was investigated under optimum conditions (i.e., pH of 4.5, catalyst dosage of 0.45 g L À1 and phenol initial concentration of 95 ppm) under the illumination of ultraviolet visible light.Also, 5 wt% Ti 0.7 W 0.3 O 2 /TiO 2 NCI exhibited the highest photocatalytic activity, with the initial rate constant (k) calculated as 0.09111 min À1 .After recycling six times, Ti 0.7 W 0.3 O 2 /TiO 2 NCI showed good stability and recyclability.The involvement of superoxide radicals in the initial reaction at Ti 0.7 W 0.3 O 2 /TiO 2 NCI was evidenced by the use of a terephthalic acid (TA) fluorescent probe.Besides, UV-Vis spectroscopy, UHPLC-MS and GC-MS technologies were used to analyze the main intermediates in the photocatalytic degradation of phenol.The probable photocatalytic degradation mechanism of phenol at Ti 0.7 W 0.3 O 2 /TiO 2 NCI was also proposed.
3 O 2 NPs modication, indicating that the Ti 0.7 W 0.3 O 2 NPs have a negative effect on the grain growth of TiO 2 NSs.This is because the Ti 0.7 W 0.3 O 2 NPs restrained the crystal growth in the solids by providing dissimilar boundaries and hindered the mass transportation, thus resulting in smaller crystallite sizes. 46Meanwhile, no diffraction peaks of Ti 0.7 W 0.3 O 2 NPs were observed up to 10 wt% (e), indicating that the TiO 2 NSs were uniformly dispersed on the surface of the Ti 0.7 W 0.3 O 2 NPs.The superimposed PXRD patterns for the Ti 0.7 W 0.3 O 2 /TiO 2 NCI before and aer six cycles of irradiation are shown in Fig. 1(B).It is obvious that the two PXRD patterns almost overlap, which indicates that the stability of the Ti 0.7 W 0.3 O 2 / TiO 2 NCI was encouraging, with less decomposition, thus accounting for the higher photocatalytic activity.A feeble and relatively weaker peak intensity was also revealed for the loss of a certain amount of photocatalyst during the experiment.

Fig. 2 (
C) and (D) show that the TiO 2 NSs were uniformly dispersed on the surface of the Ti 0.7 W 0.3 O 2 NPs and formed the Ti 0.7 W 0.3 O 2 /TiO 2 NCI.
) shows the DRS of pure TiO 2 NSs, pure rutile Ti 0.7 W 0.3 O 2 and 5 wt% Ti 0.7 W 0.3 O 2 / TiO 2 NCI.The band gap values of the synthesized photocatalysts were calculated by plotting (F(RN)hv) 1/2 versus the photo energy and the plot is shown in Fig. 3 (B).The pure TiO 2 NSs and the pure rutile Ti 0.7 W 0.3 O 2 demonstrated a photoabsorption modi-cation ability for the UV light region with wavelength shorter than 396 and 598 nm, corresponding to band gap energies of 3.21 and 2.05 eV, respectively.The pure rutile Ti 0.7 W 0.3 O 2 had a shorter band gap energy due to W 4+ doped into the lattice of TiO 2 .When the pure rutile Ti 0.7 W 0.3 O 2 was irradiated, conduction band electrons (e cb À

3 O 2
Fig.3(F).The volume resistivity of pure anatase TiO 2 was over 10 times that of 5 wt% Ti 0.7 W 0.3 O 2 /TiO 2 .The volume resistivity of 0.3 wt% Pt/TiO 2 was over 2 times that of 5 wt% Ti 0.7 W 0.3 O 2 /TiO 2 .It was thus indicated that the conductivity of pure anatase TiO 2 could be improved greatly by modifying the Ti 0.7 W 0.3 O 2 NPs, and that the performance of Ti 0.7 W 0.3 O 2 NPs was superior to that of Pt NPs, which could fully prove the above conjecture.Meanwhile, the 5 wt% Ti 0.7 W 0.3 O 2 /TiO 2 NCI was extended to the visible absorbance region with a wavelength shorter than 527 nm and had a shorter band gap energy of 2.37 eV.The above results were fully proved by ultraviolet photoemission spectroscopy (UPS) and the results are shown in ESI (Section 4, Fig.S7 †).The band gap energies of the pure TiO 2 NSs and 5 wt% Ti 0.7 W 0.3 O 2 /TiO 2 NCI were 3.38 and 2.43 eV, respectively, which were slightly larger than that from the DRS.The reasons for the deviation may be due, on the one hand, to the detection depth of UPS technology, which was 10 atoms, while on the other hand, the carbon pollution signal would be higher for the solid powder sample.In short, Ti 0.7 W 0.3 O 2 NPs with higher conductivity can cause fast electron transfer and effectively restrain the recombination of e cb À -h vb + pairs in Ti 0.7 W 0.3 O 2 /TiO 2 NCI, which can diffuse to the surface and react with pollutants and produce more

3 . 2 . 2
photocatalytic degradation of phenol by P-25, Pt/TiO 2 and Ti 0.7 W 0.3 O 2 /TiO 2 NCI are shown in ESI (Section 5, Fig. S9 †).Fig. 6(A and B) present the phenol photocatalytic degradation by P-25, Pt/TiO 2 and Ti 0.7 W 0.3 O 2 /TiO 2 NCI, showing the differences in the phenol degradation activity with the varying loading rates of Ti 0.7 W 0.3 O 2 /TiO 2 and Pt/TiO 2 .The phenol degradation rate of Ti 0.7 W 0.3 O 2 /TiO 2 NCI increased with the loading value of Ti 0.7 W 0.3 O 2 up to 5 wt%; however, a further increase would decrease the photocatalytic activity, indicating that there was an optimum loading value.The optimum value had a close relationship with the dispersion and particle sizes of Ti 0.7 W 0.3 O 2 NPs.Meanwhile, one could easily nd that the phenol degradation rate of 5 wt% Ti 0.7 W 0.3 O 2 /TiO 2 NCI was always higher than that of 0.3 wt% Pt/TiO 2 at any synchronous irradiation time, revealing its higher photocatalytic activity.Fig. 6(C and D) compare the phenol photocatalytic degradation rates of P-25, Pt/TiO 2 and Ti 0.7 W 0.3 O 2 /TiO 2 NCI aer 360 min irradiation.The phenol photocatalytic degradation rate aer various intervals of time was estimated using the following eqn (2).

Fig. 8 (
Fig. 8 (A) Fluorescence spectral changes observed during the illumination of Ti 0.7 W 0.3 O 2 /TiO 2 NCI in 0.01 M NaOH and 3 mM terephthalic acid solution (under 315 nm excitation); (B) plots showing the induced fluorescence intensities (425 nm) against light illumination time for terephthalic acid.

3 O 2 /
TiO 2 nanocomposite interfacial photocatalysts with loading of different weight ratios of Ti 0.7 W 0.3 O 2 NPs were designed and synthesized for the photocatalytic degradation of phenol in wastewater under the illumination of ultraviolet visible light.The optimum photocatalytic degradation of phenol conditions were pH 4.5, a catalyst dosage of 0.45 g L À1 and phenol initial concentration of 95 ppm.The 5 wt% Ti 0.7 W 0.3 O 2 NPs was the best loading level, and the initial rate constant (k) for 5 wt% Ti 0.7 W 0.3 O 2 /TiO 2 NCI was over 3.9 times that in 0.3 wt% Pt/ TiO 2 .The valence bands of TiO 2 NSs, Ti 0.7 W 0.3 O 2 NPs and 5 wt% Ti 0.7 W 0.3 O 2 /TiO 2 NCI were 3.01, 2.65 and 2.83 eV, the band gap energies were 3.21, 2.05 and 2.37 eV, respectively.Then the conduction bands of the above three materials were measured to be À0.20, 0.60 and 0.46 eV.Ti 0.7 W 0.3 O 2 /TiO 2 NCI showed a greatly broadened light response range, shorter band gap energy and good stability and recyclability.A Schottky barrier was formed at the NCI between TiO 2 NSs and Ti 0.7 W 0.3 O 2 NPs, leading to

Table 1
Comparison of the photocatalytic degradation of phenol in this study with the results reported in the open literature