Syntheses of MnW12/TiO2 Microflowers and Nanorods for Photocatalytic H2 Evolution


 MnW12/TiO2 microflower has been successfully synthesized by the one-pot method and the microflower could be converted to nanorods by thermal treatment. The hydrogen evolution efficiency of MnW12/TiO2 nanorods is higher than that of MnW12/TiO2 microflowers by means of the photocatalysis test. These results suggest that the photocatalytic property of MnW12/TiO2 could be enhanced by controlling morphology. In addition, massive control experiments have been conducted to explore the effect factors of morphologies. Therefore, the successful preparations in this work not only open up new directions for the structural diversity of isopolyoxometallate based nano/micro materials, but also provide us with an irradiative way to increase the photocatalytic performance of POM/TiO2 nano/micro composite.


Introduction
Polyoxometallates (POMs) as a fascinating species of metal-oxygen clusters have a wide range of potential applications due to their diverse structures and tunable properties in catalysis, materials and photoluminescence, etc [1][2][3][4]. As a unique branch of POM chemistry, POM based nano/micro materials remain less explored to date in comparison with traditional single crystal POM compounds. With the development of nanotechnology, the morphology, component and size of POM based nano/micro materials can be tuned easily through arti cial methods. As a consequence, many researchers have been making signi cant efforts to explore in this eld, and a variety of POM based nano/micro materials have been addressed [5][6][7][8][9][10]. In 2011, Cronin et al. reported a new phenomenon, namely, the growth of hollow mineral tubular architectures, and this may have signi cance in the interpretation of the architectures found in the fossil record that have been to date attributed to biological processes [7]. After this, Mnbased heteropolytungstate microspheres were prepared by Chattopadhyay and coworkers using a unique solvothermal method [11].
To the best of our knowledge, titanium dioxide (TiO 2 ) is an important photocatalyst and has been widely investigated due to its advantages such as relatively low-cost, good chemical stability and higher oxidation potential [12][13][14][15][16][17]. Zou's group fabricates TiO 2 p-n homojunction for photoelectrochemical and photocatalytic hydrogen generation [18]. Yi and coworkers reported W 2 C@C/TiO 2 heterojunction architecture with e cient solar-light-driven hydrogen generation [19].
However, there are relatively few studies on the synthesis and properties of POMs/TiO 2 materials. In 2013, Bansal's group fabricated TiO 2 -POM-bimetal nanocomposites for improved surface enhanced Raman scattering and solar light photocatalysis [20]. Wang et al. synthesized POM/TiO 2 /Ag composite nano bers with enhanced photocatalytic performance under visible light [21]. Lan's group reported POM/TiO 2 Fenton-like photocatalysts with rearranged oxygen vacancies for enhanced synergetic degradation [22]. From these literatures, the morphology is an important factor that has great impacts on the performance of photocatalyst. Because the morphology affects the speci c surface area, separation e ciency and migration rate of photogenerated charges.
From these perspectives, how to enhance the performance of photocatalyst through morphology control has become a goal pursued by our group. In this work, MnW 12 /TiO 2 micro ower has been successfully prepared by a one-pot method through mild conditions. Fortunately, MnW 12 /TiO 2 micro ower could be converted to nanorods by thermal treatment at 550 °C. Under the photocatalysis test, the hydrogen evolution e ciency of MnW 12 /TiO 2 nanorods is higher than that of MnW 12 /TiO 2 micro owers. These results indicate that the performance of photocatalytic could be tuned by controlling morphology. Meanwhile, a large number of control experiments have been carried out to explore the effect factors of morphologies. Therefore, this work not only enrich the diversity of isopolyoxometallate based nano/micro materials, but also nd an enlightening way to improve the photocatalytic e ciency of POM/TiO 2 nano/micro composite.

Materials and methods
TiO 2 (P25) was purchased from Degussa. All chemicals were reagent-grade and used without further puri cation. The materials have been fully characterized by transmission electron microscope (TEM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray powder diffraction (XRD), IR spectroscopy, X-ray photoelectron spectroscopy (XPS), thermogravimetry (TG). XRD (Bruker D8 Advance, Bruker optics Instruments company, Karlsruhe, Germany) was performed using a instrument with Cu Kα radiation (λ = 1.5418 Å). The TEM images were obtained using a transmission electron microscope (JEM-2100F Electronics Co. LTD, Tokyo, Japan). The SEM images and EDX spectrum were obtained using a scanning electron microscope (JSM-7610F Electronics Co. LTD, Tokyo, Japan) with an acceleration voltage of 10 kV. The IR spectra were obtained via an Avatar 360 Fourier transform infrared spectrophotometer (Bruker optics Instruments company, Karlsruhe, Germany) using KBr pellets in the range of 4000−400 cm−1. XPS was conducted using a Thermo ESCALAB 250XI spectrometer (Thermo Fisher Scienti c, Massachusetts, USA). The TG curve was obtained on the STA449F5 thermo-gravimetric analyzer (Netzsch, Selb, Germany).

Synthesis of MnW 12 Micro ower
Na 2 WO 4 ·2H 2 O (3.00 g, 9.62 mmol) was dissolved in 30 mL of distilled water, heated to 80 °C with stirring, and boric acid (0.10 g, 1.62 mmol) was added to the solution. Then, after adjusting the pH of the solution to 7.0 with dilute HCl, a small amount of aqueous solution containing MnCl 2 ·4H 2 O (0.40 g, 2.00 mmol) was slowly added dropwise, and after the dropping, it was heated and stirred for the rst 30 minutes.
Then, after adjusting the pH of the solution to 6.0 with dilute HCl, it was heated and stirred for the second 30 minutes, nally cooled at room temperature for 1 hr. The MnW 12 micro owers were collected by centrifugation and washed with water and ethanol to remove excess reagents. MnW 12 micro owers changed into different morphologies after calcination at different temperatures and atmospheres for 2 hrs.

Related Control Experiments
To explore the in uence of different counter cations on the morphology, the synthetic procedure was identical to that of MnW 12 micro owers, but MnCl 2 ·4H 2 O was replaced by NH 4 Cl, KCl, N(C 4 H 9 ) 4 Br, CsCl, respectively.
To explore the in uence of different manganese salts on the morphology of MnW 12 micro owers, the synthetic procedure was identical to that of MnW 12 micro owers, but MnCl 2 ·4H 2 O was replaced by Mn(Ac) 2 , Mn(ClO 4 ) 2 , MnSO 4 , Mn(NO 3 ) 2 , respectively.
To explore the in uence of adding different amounts of PEG4000 on the MnW 12 micro owers, the synthetic procedure was identical to that of MnW 12 micro owers, but different amounts of PEG4000 were added to the solution after boric acid.
To explore the in uence of different stirring times on the appearance of MnW 12 micro owers, the synthetic procedure was identical to that of MnW 12 micro owers, but the rst 30 minutes were changed to 10 min, 1 hr, 2 hrs, respectively.
To explore the in uence of different amount of MnCl 2 on the appearance of MnW 12 micro owers, the synthetic procedure was identical to that of MnW 12 micro owers, but the amount of MnCl 2 were changed to 1 mmol, 4 mmol, respectively.

Synthesis of MnW 12 /TiO 2 Micro owers
Na 2 WO 4 ·2H 2 O (3.00 g, 9.62 mmol) was dissolved in 30 mL of distilled water, heated to 80 °C with stirring, and boric acid (0.10 g, 1.62 mmol) was added to the solution. Then, after a small amount of aqueous solution containing TiO 2 (0.12g, 1.50mmol) was slowly added dropwise, adjusting the pH of the solution to 7.0 with dilute HCl, a small amount of aqueous solution containing MnCl 2 ·4H 2 O (0.40 g, 2.00 mmol) was slowly added dropwise, and after the dropping, it was heated and stirred for the rst 30 minutes. Then, after adjusting the pH of the solution to 6.0 with dilute HCl, it was heated and stirred for the second 30 minutes, and nally cooled at room temperature for 1 hr. MnW 12 /TiO 2 micro owers were collected by centrifugation and washed with water and ethanol to remove excess reagents.

Synthesis of MnW 12 /TiO 2 Nanorods
The MnW 12 /TiO 2 micro owers (3 g) was placed in a porcelain boat and transferred to a tube furnace at room temperature. Then the temperature was ramped to 550 °C at a speed of 5 °C min -1 and maintained for 2 h under air atmosphere. After cooling to room temperature, MnW 12 /TiO 2 nanorods were collected.

Photocatalytic Hydrogen Production
The H 2 evolution experiments were performed in a closed gas-circulating system CEL-PAEM-D8 (China Education Au-light Co., Ltd, Beijing, China) equipped with an external illumination Pyrex reaction vessel (total volume 100 mL) with a magnetic stirrer for vigorous stirring and analyzed by using an automatic H 2 monitoring system. The vessel was lled with a solution containing a sacri cial electron donor CH 3 OH (25 mL), H 2 O (25 mL), H 2 PtCl 6 (0.012 g) and different catalysts. The reaction was tested using a 300 W Xe light (350-780 nm) and the system was evacuated (-0.1MPa) before opening the Xe light. The produced H 2 was analyzed by a gas chromatography (GC7900) with a TCD using Ar as the carrier gas.

Results And Discussion
In the past two decades, many researchers have been working on the morphology and composition of POM based nano/micro materials. However, the progress shows slower in comparison with traditional single-crystal compounds. In addition, morphologies of POM based nano/micro materials are not abundant, compared with other nano/micro materials. Only few morphologies have been characterized, such as polyhedron, tube, wire and spheres. As shown in Scheme 1, the materials based on isopolyoxometallate are rarely reported in recent years. As a unique branch of POM compound, isopolyoxometallate materials lack of su cient concern. Finally, the controllable synthesis and photocatalytic hydrogen production properties of POM/TiO 2 nano/micro composite are rarely studied until now. Therefore, we are making great efforts to explore the design and syntheses of isopolyoxometallate based material and research the photocatalytic hydrogen production. Then the combination of MnW 12 and TiO 2 would create new functional material.

SEM and TEM images
First of all, Scheme S1 shows a schematic of the synthetic processes used to prepare the MnW 12 micro ower. During the procedure, Mn 2+ plays the role as counter ions which would be combined with polyanions. As shown in Figure 1, the MnW 12 micro ower is composed of uniform ower-like morphology. After statistic of 60 particles, the diameter is distributed in the range of 1.5-1.8 μm and the average diameter is around 1.55 μm (Figure 1a, inset).

XRD patterns
The as-prepared MnW 12 /TiO 2 micro owers, MnW 12 /TiO 2 nanorods and their precursors were characterized by XRD. As can be seen from Figure 4a

XPS spectra
In addition, the XPS investigations on MnW 12 /TiO 2 micro owers and MnW 12 /TiO 2 nanorods were recorded to con rm the valence states of Mn, W and Ti. As the valence states of Mn, W and Ti of MnW 12 /TiO 2 micro owers and MnW 12 /TiO 2 nanorods are the same ( Figure S10), take MnW 12 /TiO 2 nanorods as the example for detailed description. As depicted in Figure S10d

TG analysis
The Netzsch STA449F5 thermo-gravimetric analyzer was used for the thermal analysis in nitrogen dynamic atmosphere (50 mL/min) at a heating rate of 10 K/min and 19.2285 mg powder of MnW 12 /TiO 2 micro owers was thermally treated ( Figure 5). The TG curve gives a total weight loss of 6.41% in the range of 23-1000 ℃. The weight loss of 2.68% during the rst step from 23 to 164 ℃ corresponds to the release of adsorbed water molecules. On further heating, the second weight loss of 3.73% between 164-290 ℃ is approximately attributed to the removal of structural water molecules.

Photocatalytic hydrogen generation
As depicted in Figure 6a, the hydrogen production of MnW 12 /TiO 2 micro owers is 21.966 mmol g-1, and the hydrogen production of MnW 12 /TiO 2 nanorods is 28.684 mmol g-1, after 5 hrs of light irradiation. And it is calculated that the hydrogen production of MnW 12 /TiO 2 nanorods is 30.58% higher than that of MnW 12 /TiO 2 micro owers. The higher photocatalytic activity for MnW 12 /TiO 2 nanorods than MnW 12 /TiO 2 micro owers is probably due to the differences in morphologies of MnW 12 /TiO 2 , which will be explained from the following two aspects. On the one hand, the different morphologies of MnW 12 /TiO 2 endow them different speci c surface areas. As shown in Figure 6c, d and Table 1, the speci c surface area of MnW 12 /TiO 2 nanorods is higher than that of MnW 12 /TiO 2 micro owers. A larger speci c surface area can provide more active sites, which contributes to the release of more hydrogen in the photocatalytic process. On the other hand, the morphology affects the separation e ciency and migration rate of photogenerated charges. Due to the quantum size effect, the particle size becomes smaller from micro owers to nanorods. In this case, charges separation effect and migration e ciency become higher, which leads to the improvement of catalytic activity. In short, the morphologies of MnW 12 /TiO 2 are not alike, resulting in different photocatalytic hydrogen production capabilities. In addition, the stability test of MnW 12 /TiO 2 nanorods showed that the total hydrogen production of three cycles were 28.051 mmol/g, 25.381 mmol/g and 25.332 mmol/g, respectively (Figure 6b). After long-term illumination of 15 hours, the MnW 12 /TiO 2 nanorods still maintain the corresponding catalytic activity.
Meanwhile, there is no signi cant change in the XRD pattern of MnW 12 /TiO 2 nanorods before and after three cycles of photocatalytic hydrogen generation ( Figure S11). In all, the above con rm that MnW 12 /TiO 2 nanorods have relatively favorable photocatalytic activity and cycle stability.

Conclusion
In summary, MnW 12 /TiO 2 micro owers were successfully prepared by one-pot method under mild conditions. Moreover, the micro owers turn into nanorods though thermal treatment. The hydrogen production e ciency of MnW 12 /TiO 2 nanorods is better than that of MnW 12 /TiO 2 micro owers through the photocatalytic hydrogen production test. The difference in photocatalytic performance is attributed to the difference in morphology. Meanwhile, a large number of control experiments have also been explored to clarify the reasons for the transformation of morphology. Thus, the successful preparation of MnW 12 /TiO 2 micro owers and nanorods enrichs the structural diversity of isopolyoxometallate based nano/micro materials. And this method reported in this work is proposed to be a general process to improve the photocatalytic e ciency of POM/TiO 2 nano/micro composite.