Photocatalytic reduction of Cr(VI) by polyoxometalates/TiO₂ electrospun nanofiber composites

Abstract

Polyoxometalates (Ti_0.75PW₁₂O₄₀, Ti-PTA)/TiO₂ nanofiber composites were fabricated by a simple electrospinning technique and then high temperature calcination. In the structure, Ti-PTA, as an electron relay, can accept the photo-generated electrons from the conduction band of TiO₂, which promote the separation of photo-generated charges in TiO₂. Then the electrons stored on Ti-PTA further transfer to the Cr(VI) in the solution to realize the removal of Cr(VI). The Ti-PTA/TiO₂ nanofiber composites exhibit enhanced photocatalytic performance for photocatalytic reduction of Cr(VI), which might be a potential photocatalyst for Cr(VI) removal in environmental therapy.

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Introduction

Chromium is a common water contaminant because of wide applications in metallurgy, staining glass, anodizing aluminum, organic synthesis, leather tanning and wood preservation industries. Cr(VI) affects human physiology, accumulates in the food chain and causes severe health problems ranging from simple skin irritation to lung carcinoma. Its concentration in water is necessarily restricted to be less than 0.05 ppm by the environmental quality standards for water pollution control.^1,2 Contrarily Cr(III) is low toxic and an essential human nutrient, which does not readily migrate in groundwater since it usually precipitates as hydroxides, oxides, or oxyhydroxides. It is also quite soluble in aqueous phase over almost the entire pH range, thus it is quite mobile in the natural environment. In a word, Cr(VI) is more toxic than Cr(III). Therefore, reduction of Cr(VI) to Cr(III) is beneficial for the environment and is a feasible method for removal of Cr(VI).^2,3

Cr(VI) is a strong oxidant and therefore can be reduced in the presence of electron donors. In the recent years, photocatalytic removal of toxic substance in aqueous suspension of semiconductor has received considerable attention in view of solar energy conversion.^3–8 This photocatalysis was achieved for rapid and efficient destruction of environmental pollutants. Among of all the semiconductors, TiO₂ exhibits excellent photocatalytic activity due to its good chemical and biological stability, low cost and toxicity.^9,10 Thus, photocatalytic reduction of Cr(VI) over TiO₂ catalysts was extensively investigated including morphologies,^5,11 organic sacrificial agent,^12,13 and modified TiO₂.^14,15 However, low photocatalytic efficiency limits TiO₂ photocatalyst's practical application due to high electron–hole recombination rate result in low quantum efficiencies.¹⁶ Many studies have been carried out to promote the charge separation process, such as noble metal,^16–18 oxygen vacancy,^11,19,20 mixed phase junction^21,22 and heterojuction.^23,24

Spatially separation of the photogenerated charge carriers is a feasible approach for enhancing the photocatalytic performance. Employing an electron scavenger to accept the electron from the conduction band of TiO₂ is an effective way to restrain electron–hole recombination. Polyoxometalates (POMs) are good multielectron acceptors and show satisfactory photoactivity in homogeneous systems. However, it is hard to remove from homogeneous system after treatment, which results in a secondary pollution. Loading POMs onto catalyst carrier is a prefer way to solve this problem. For POMs–TiO₂ composites, POMs easily accept photogenerated electron from TiO₂ conduction band and electrons are temporarily stored in the form of reduced POMs and later the collected electron can be transfer to adsorbed substance. The POMs, such as PW₁₂O₄₀³⁻, SiW₁₂O₄₀⁴⁻, facilitate the transfer of photogenerated TiO₂ electron to dioxygen as a means of increasing the efficiency of the photo-degradation of 1,2-dichlorobenzene and dye molecules.^25,26 Although there are a few reports on the high efficiency photo-degradation of organic waste,²⁷ the metal ions waste removal by POMs–TiO₂ composites is rarely reported.²⁸ Generally, loading POMs on the TiO₂ by physical adsorption, resulting that POMs may desorb from the supporting materials. On the other hand, the limited amount POMs can be loaded on the supporting materials such as TiO₂. Electrospinning technique is a robust method to fabricate the nanofibers composites, whose composition can be simply tuned via the feed materials ratio.

Herein, we fabricated POMs–TiO₂ nanofibers composites through simple and facile electrospinning technique. H₃PW₁₂O₄₀ (PTA) was mixed with titania sol–gel precursor and polyvinylpyrrolidone (PVP). The electrospun nanofibers were thermal treated at 400–600 °C to remove the polymer and transfer TiO₂ from amorphous to anatase phase. The results Ti_0.75PW₁₂O₄₀ (Ti-PTA)/TiO₂ nanofiber composites exhibit enhanced photocatalytic activity of Cr(VI) reduction at optimum POMs amount and thermal treatment temperature. Based on the above results, we proposed a possible reaction mechanism – heterojunction of Ti-PTA/TiO₂. POMs accepted the photogenerated electron from TiO₂ conduction band and further reduced the Cr(VI) ion adsorbed on the Ti-PTA/TiO₂.

Experimental

Materials

Tetrabutyltitanate (TBT), phosphotungstic acid (H₃PTA), potassium dichromate were purchased from Aladdin Reagent Inc. Polyvinylpyrrolidone (PVP, M_w 1 300 000) was obtained from Alfa Aesar Inc. All chemicals were used without any further purification.

Solution preparation

0.7 g of PVP was dissolved into 15 mL isopropanol (IPA) till to form clear solution. Then 0.2 mL acetic acid was added into PVP solution. After stirring for 1 hour, about 0.5 mL TBT was added into PVP solution and stir over 1 hour for hydrolysis of TBT. Finally, designed amount PTA (5–30 mol% relative to TBT) was added into above solution and stir till to completely dissolved.

Electrospinning conditions

The above solution was loaded into 10 mL syringe connected with a 22 gauges blunt needle. PVP/TiO₂/PTA fibers were collected on the alumina foil. The syringe was set up with a tip-to-collector distance of 15 cm. The solution feed rate was set 0.5 mL h⁻¹ and 15 kV was applied to the needle and collector. The collected fibers were thermal treated at 400–600 °C for 5 hours to remove PVP.

Characterization

The morphology of the fibers was observed with a scanning electron microscopy on a JEOL JSM 4800F and transmission electron microscopy (TEM, FEI Tecnai G2 operated at 200 kV). The crystalline structure was recorded by using an X-ray diffractometer (XRD) (Bruker AXS D8 Focus), using Cu Kα radiation (λ = 1.54056 Å). The UV-vis absorption spectra were measured on a Shimadzu UV 2600 UV-vis spectrophotometer. X-ray photoelectron spectrum (XPS) analyses were performed on an ESCALABMKII spectrometer with an Al-Kα (1486.6 eV) achromatic X-ray source.

Photocatalytic reduction of Cr(VI)

The photocatalytic activity of the samples was evaluated through the reduction of K₂Cr₂O₇ in aqueous solution and isopropanol under a 300 W high voltage UV light mercury lamp at a 20 cm distance. The experiments were performed at 25 °C as follows: 20 mg of Ti-PTA/TiO₂ nanofiber composites was added into 20 mL of K₂Cr₂O₇ solution (160 ppm) and 20 mL isopropanol. Before irradiation, photocatalysts were not submitted to any previous treatment and then were dispersed in K₂Cr₂O₇ solution under magnetic stirring for 30 min and covered from any source of light to assure that the adsorption–desorption equilibrium between the TiO₂ catalyst and K₂Cr₂O₇ was reached. Then the solution was stirred and exposed to the UV light irradiation. After irradiation for a designated time, aliquots were taken from the irradiated reaction flask and submitted to centrifugation to separate Ti-PTA/TiO₂ composites. The K₂Cr₂O₇ concentration was monitored by the absorbance value at the maximum peak (365 nm) using a Shimadzu UV-2600 spectrometer.

Results and discussion

Electrospinning is a simple and facile technique for fabrication of nanofibers, which are about 200 nm in diameter. Generally, TiO₂ sol–gel precursor was added into PVP solution to form TiO₂–PVP composites, which is electrospun into fibers. The TiO₂ also assumes the shape of nanofibers is maintained after removing PVP via high temperature sintering process. To obtain uniform Ti-PTA/TiO₂ nanofiber composites, H₃PTA was added PVP and TiO₂ sol–gel precursor mixed solution to form a homogeneous solution. Fig. 1A and B display the morphology of as-spun PVP/PTA/TiO₂ nanofibers at the optimum electrospinning conditions. The fibers show quite good uniformity with 188 ± 36 nm in diameter. The Ti-PTA/TiO₂ nanofiber composites have lower fiber diameters (87 ± 15 nm) than as-spun PVP/PTA/TiO₂ nanofibers because of the degradation of PVP during the sintering process (Fig. 1C and D). Transmission electron microscopy (TEM) image of a single PTA-TiO₂ nanofiber is shown in Fig. 1E. It discloses that the nanofiber is actually made of small nanoparticles with ∼15 nm in diameter. Some dark nanoparticles are observed on the fiber, which may relate to Ti-PTA in the fiber due to tungsten existence. Fig. 1F shows high resolution TEM (HR TEM) and fast Fourier transform (FFT) images corresponding to the different area. The HR TEM indicates the anatase phase of TiO₂ and the lattice fringes correspond to 0.35 nm. In the dash rectangle region, the FFT pattern shows two symmetry dots which is consistence with the TEM image. The solid rectangle region shows different lattice fringe and FFT pattern, which is believed to correspond to the Ti-PTA nanoparticles.

Fig. 1 Scanning electron microscopy (SEM) images of PVP/PTA/TiO₂ (A and B) and Ti-PTA/TiO₂ (C and D). Transmission electron microscopy (TEM) photographs of single Ti-PTA/TiO₂ nanofiber (E). F is high resolution TEM images of Ti-PTA/TiO₂ fibers. The fast Fourier transform images of different region are inserted at right upper corner. The solid line rectangle indicates the Ti-PTA, and dash line rectangle is TiO₂.

To further verify the PTA existence in the fiber, energy dispersive analysis of X-rays (EDAX) and X-ray photoelectron spectroscopy (XPS) were employed. Fig. 2A clearly exhibits signal of Ti, W O and P elements in EDAX pattern of TiO₂-PTA (10 mol% for W) nanofibers. The intensity of W signal increases with the increasing of adding amount of H₃PTA in the original solution (see Fig. S1 in ESI†). The W 4f, W 4d, C 1s, W 4p, Ti 2p and O 1s signal are clearly shown in the full scan survey XPS spectrum of Ti-PTA/TiO₂ (20%) nanofibers. These results are consistence with the EDAX results. Both EDAX and XPS results confirm that Ti-PTA/TiO₂ nanofiber composites form through this simple electrospinning and sintering process.

Fig. 2 Energy dispersive X-ray analysis (EDAX, A) and full scan X-ray photoelectron spectroscopy (XPS, B) of Ti-PTA/TiO₂ nanofiber composites.

The photocatalytic activity strongly depends on the crystalline phase and the molar ratio of TiO₂ and H₃PTA. Firstly, the effect of sintering temperature on the crystalline phase was investigated. The Fig. 3A displays XRD pattern of PTA-TiO₂ (20 mol% W/Ti) after sintering at different temperature from 450–600 °C. XRD patterns matched with the crystalline anatase phase of TiO₂ (JCPDS no. 21-1272). The TiO₂ nanoparticles size was calculated to be about 11.7 nm according to the Scherrer equation. The XRD peak at 25.3 turns more and more sharp, indicating the TiO₂ nanoparticle size grows up with the increase of sintering temperature. TiO₂ nanoparticles size increase from 6.0 nm to 6.4 nm at sintering temperature of 550 °C. When the sample was sintered at 600 °C, a series of new diffraction peaks are observed. These new peaks match with monoclinic phase WO₃ (JCPDS no. 43-1035). These results indicate that the PTA transfer into WO₃ after high temperature treatment.

Fig. 3 Powder X-ray diffraction (XRD) of PVP/PTA-TiO₂ nanofibers composites calcined at different temperature (A) and different molar ratio of PTA to TBT (B).

Thus the PTA amount will affect the photocatalytic activity of PTA-TiO₂ nanofibers. The XRD pattern of different the molar ratio of TBT and PTA is shown in Fig. 3B. After sintering at 550 °C, the (101) diffraction peak of pure TiO₂ nanofibers shows narrower and sharper than Ti-PTA/TiO₂ nanofibers. The TiO₂ nanoparticle sizes were calculated to be 11.7 nm, 7.3 nm, 6.4 nm and 5.9 nm for pure TiO₂, 10, 20 and 30 mol% PTA-TiO₂ nanofibers, respectively. These indicate that the addition of PTA depress the TiO₂ crystal growth to some degree. In addition, WO₃ diffraction peaks are also observed in the PTA-TiO₂ 30 mol% sample, indicating that PTA may be transferred WO₃ at high temperature treatment when the PTA amount is too high.

Fig. 4 Photocatalytic reduction of Cr(VI) by Ti-PTA/TiO₂ nanofiber composites prepared at (A) different temperature and (B) different feed ratio of PTA to TBT.

Photoreduction of Cr(VI) was carried out to evaluate the photocatalytic activity of Ti-PTA/TiO₂ nanofibers (Fig. 4). Because the Ti-PTA/TiO₂ nanofiber are mesoporous materials, adsorption of Cr(VI) on the nanofibers were observed in all case before starting the photocatalytic reduction. Under the UV-vis illumination, Cr(VI) ion concentration has a faint decrease. For pure TiO₂ nanofibers, about 60% Cr(VI) was reduced into Cr(III) in 60 minutes. However, the Ti-PTA/TiO₂ nanofibers thermal treated at 450 °C, the photocatalytic activity is close to the pure TiO₂ nanofibers. With the sintering temperature increase to 500 °C, the photocatalytic performance has an obvious improvement. When the sintering temperature increases to 550 °C, the photocatalytic performance reaches the best point. More than 90% Cr(VI) ion can be removed within 50 minutes. However, the photocatalytic activity significantly drops down if the sintering temperature further increases to 600 °C. The reason is that WO₃ formed under high temperature thermal treatment results in the low photocatalytic performance. Similar results are observed in the different molar ratio of PTA to TBT sample. The photocatalytic performance increase with the relative amount of PTA, then too high amount PTA results in that PTA easily transfer into WO₃ at 550 °C. That leads to that the photocatalytic performance dramatically decreases. Fig. S5† shows the photocatalytic reduction process of Cr₂O₇²⁻ under UV-vis light. There is a characteristic adsorption band of H₃PTA at 260 nm. During photocatalytic reduction process, a shoulder peak at 260 nm was observed at the initial stage, indicating that some H₃PTA adsorbed on the surface of Ti-PTA/TiO₂ nanofibers may be dissolved into the solution. However, the intensity of this peak shows no increase with prolonging the reaction time. That means there is no further PTA release from the Ti-PTA/TiO₂ nanofibers. The formation of Ti-PTA effectively prevents the loss of PTA from the Ti-PTA/TiO₂ nanofibers composites.

Fig. 5 showed the optical images of Ti-PTA/TiO₂ nanofibers composites before and after exposed UV-vis light. Normally, the Ti-PTA/TiO₂ display white color powder and turns blue after exposing UV light. That indicates that the anion PTA was reduced and form reduced PTA in the Ti-PTA/TiO₂. It could gradually recover when it stores in the ambient environment at room temperature. This process clearly demonstrates that the Ti-PTA works as a redox agent in the Ti-PTA/TiO₂ nanofibers composites. As an electron acceptor, it accepts the photo-generated electron from TiO₂ and turns into reduce state, and then the reduced Ti-PTA further can be oxidized by Cr(VI) in the solution and back to Ti-PTA.

Fig. 5 Optical images of Ti-PTA/TiO₂ nanofibers composites before (A) and after (B) exposed UV-vis light.

To decide the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of Ti-PTA, we measured the redox potential of H₃PTA via the cyclic voltammetry measurement. The LUMO potential (0.26 eV vs. NHE) could be calculated from the equation as shown in Fig. S5.† The optical band gap (3.3 eV) is obtained from the UV-vis spectrum of H₃PTA (Fig. S6†). On the basis of these results, we infer the LUMO and HOMO of Ti-PTA are 0.26 and 3.56 eV vs. NHE, respectively. As we know, the conduction band (CB) and valence band (VB) of typical anatase TiO₂ are −0.29 and 2.91 eV vs. NHE and the Fermi level of TiO₂ will locate at ∼0.20 eV below CB.²⁹ Based on the above results and understanding, we believe that the heterojunction between Ti-PTA and TiO₂ forms in Ti-PTA/TiO₂ nanofibers. In the structure, TiO₂ is a matrix, mainly adsorb most of light. Ti-PTA nanoparticles embed into the TiO₂ nanofibers. It works as electron relay, accepting photogenerated electron from TiO₂ and releasing the electron to Cr(VI). Ti-PTA can be easily reduced by the photo-excited TiO₂, thus promote the charge separation of TiO₂. The electron can temporary store in the Ti-PTA prevent from recombination of photo-generated electrons and holes. That is the reason that an enhanced photocatalytic performance was observed for the Ti-PTA/TiO₂ nanofibers composites. While reduced Ti-PTA can also be oxidized by Cr₂O₇²⁻ then recovers to Ti-PTA. The toxic Cr(VI) could be removed from the solution by this process. On the other side, the photo-generated hole can transfer to the sacrificed agent – alcohol. Fig. 6 shows the whole possible reaction mechanism.

Fig. 6 Possible photocatalytic mechanism of PTA-TiO₂ nanofiber composites.

Conclusions

In summary, Ti-PTA/TiO₂ nanofiber composites were fabricated via a simple electrospinning method. The nanofibers morphology can be reserved after removing the polymer by high temperature sintering process. The heterojunction between Ti-PTA and TiO₂ promote the photo-generated electron and hole separation, leading to an improvement of photocatalytic performance. The Cr(VI) removal experiment was designed as to demonstrate the photocatalytic activity of Ti-PTA/TiO₂ nanofibers. This nanofibers shows a great photoreduction capability of Cr(VI) ion under UV-vis light illuministration.

Acknowledgements

We acknowledge the financial support from the National Natural Science Foundation of China (no. 21201159, 61176016, and 21104075), the Science and Technology Department of Jilin Province (no. 20121801), “Hundred Talent Program” CAS and open research fund of Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University).

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Footnote

† Electronic supplementary information (ESI) available: More EDAX and XPS results. See DOI: 10.1039/c4ra08934k

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