Morphology-controllable synthesis of novel Bi₂₅VO₄₀ microcubes: optical properties and catalytic activities for the reduction of aromatic nitro compounds

Abstract

In the present paper, novel Bi₂₅VO₄₀ microcubes with relatively good dispersion have been fabricated via a facile, fast and mild hydrothermal synthesis strategy. The morphology and compositional characteristics of the Bi₂₅VO₄₀ sample were characterized by various testing techniques including powder X-ray diffraction (XRD), inductively coupled plasma-atomic emission spectroscopy (ICP-AES), FT-IR spectroscopy, energy dispersive spectrometry (EDS), X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM). Several experimental parameters including synthesis temperature, time and the amount of additives (e.g., polyacrylamide, sodium citrate and sodium hydroxide) were discovered to play vital roles in the construction of these Bi₂₅VO₄₀ microcrystals. From the UV-vis diffuse reflectance spectrum (DRS) of the Bi₂₅VO₄₀ its band gap was calculated to be 2.42 eV. The photoluminescence (PL) spectrum showed that Bi₂₅VO₄₀ microcubes had two obvious emission peaks centered at 458 and 639 nm. Moreover, catalytic experiments showed that the as-prepared Bi₂₅VO₄₀ microcubes possessed superior catalytic performance for the conversion of 4-nitrophenol (4-NP) and 4-nitroaniline (4-NA) in the presence of excess NaBH₄ solution.

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1. Introduction

During the past decade, the controllable preparation of nano- or micro-scale metal oxides with uniform dimensions and shapes has attracted considerable interest, because the physical and chemical properties of materials depend highly on their sizes and shapes.^1–10 Therefore, morphology-controllable synthesis of novel and well-defined metal oxide micro/nanostructures has been one of the hot areas in the field of chemistry and material research.^11–15 Recently, studies further demonstrate that ternary oxides usually possess better performances than binary counterparts in many cases such as optical, electronic, magnetic, mechanical, and piezoelectricity properties due to their stoichiometric diversity and structural richness.^16–20 However, phase and morphology control of these complex metal oxides with a desired chemical composition is still a big challenge up to now because of their various stoichiometries (e.g., typical chemical formulas of ABO₄, A₄B₂O₁₁, A₂₅BO₄₀, A₄₆B₈O₈₉ and so on) and the associated complex structures.^21–27

Bismuth vanadates are an important class of Bi-based ternary metal oxides. Usually, there are many phases existed in the well-known Bi–V–O family including BiVO₄, Bi₄V₂O₁₁, B₂₅VO₄₀, Bi₄₆V₈O₈₉ and so on.^24–27 In addition, it has showed that these kind complex metal oxides exhibit excellent oxide ion conductivity, photocatalytic, dielectric, ferroelectric and pyroelectric properties.^28–30 Therefore, controllable fabrication of these functional micro/nanomaterials with various morphologies is very significant from the perspective of the fundamental research and technological application. Actually, subsequent studies have proved this point. For example, Xing's research team prepared hierarchically structural BiVO₄ photocatalysts with special morphologies through a facile one-pot method in the presence of soluble starch.³¹ Kuang and coworkers synthesized a new polymorph of bismuth vanadate Bi₄₆V₈O₈₉ with excellent ionic conductivity by a high temperature solid state reaction.²⁵ Chen et al. reported the successful preparation of hierarchical Bi₄V₂O₁₁ hollow microspheres with excellent visible activities by a facile template-free solvothermal route.³² Besides the above three materials, other bismuth vanadates with different atomic ratio of Bi : V : O such as Bi₂₅VO₄₀, has been discovered years ago. Researches show that Bi₂₅VO₄₀ belongs to the sillenite group with a pseudo-body centered cubic unit cell and the noncentrosymmetric space group I23 and it usually exhibits unique photorefractive, photochromic, electrooptic and dielectric properties.^33–38 Therefore, realization of the morphology-controllable synthesis of Bi₂₅VO₄₀ micro/nanomaterials not only expects to obtain some novel nanostructures and enriches the preparation technology of nanomaterials, but also has great significance for the performance modulation and optimization of Bi₂₅VO₄₀ materials. Unfortunately, to the best of our knowledge, report about the fabrication of Bi₂₅VO₄₀ micro/nanostructures still remains rare, especially for Bi₂₅VO₄₀ microcubes.

Hence, we provide a facile hydrothermal method to the synthesis of novel Bi₂₅VO₄₀ microcubes. Some experimental parameters affecting the phase and morphology of the final sample are studied in detail. In addition, the optical and catalytic performances of Bi₂₅VO₄₀ sample are investigated. Several distinguishing features are presented below: (1) novel Bi₂₅VO₄₀ microcubes are obtained; (2) the preparation strategy is relatively facile, mild and fast; (3) no post-treatment is required; (4) the Bi₂₅VO₄₀ sample possesses superior catalytic reactivity towards the reduction of 4-nitrophenol and 4-nitroaniline in excess NaBH₄ aqueous system.

2. Materials and methods

For the preparation of Bi₂₅VO₄₀ microcubes, Bi(NO₃)₃·5H₂O, NH₄VO₃, Na₃C₆H₅O₇·2H₂O, NaOH and polyacrylamide (PAM, molecular weight = 3 million) were employed as Bi source, V source, coordination agent, mineralizer and surfactant, respectively. In a typical synthesis process, 0.0012 mol of Bi(NO₃)₃·5H₂O was added into 15 mL of deionized water followed by addition of 0.0024 mol of Na₃C₆H₅O₇·2H₂O. Then, 15 mL of aqueous solution containing NH₄VO₃ (0.0001 mol), NaOH (0.3 g) and polyacrylamide (0.3 g) was slowly dropped into this reaction system under vigorous stir for 40 min. Finally, the as-obtained suspension was poured into a Teflon-lined oxidation-resisting steel autoclave, and kept at 150 °C for 2 h, then cooling to atmospheric temperature naturally. The precipitant was recovered by centrifugation, followed by washing several times to remove excess surfactants and residual ions, and drying at 80 °C for 8 h.

XRD measurement was recorded on a Shimadzu XRD-6000 powder X-ray diffractometer in the 2θ range from 10° to 70°, with Cu Kα radiation (λ = 1.5418 Å). SEM image of the sample was obtained using a field emission scanning electron microananlyser (S-4800 from Hitachi Corp.), operated at an acceleration voltage of 5 kV. EDS, being attached to the SEM, was employed to measure the composition of the product. HRTEM was carried out on a JEM-2100 high resolution transmission microscope, employing an accelerating voltage of 200 kV. XPS data was collected on a thermo ESCALAB 250XI X-ray photoelectron spectrometer. FT-IR spectrum was obtained for KBr-diluted sample on a Nicolet Magna 750 IR spectrometer in the range of 400–1000 cm⁻¹. The UV-vis DRS of the product was collected with a UV-vis spectrophotometer (UV-4100 from Hitachi Corp.). PL spectrum was recorded on an Edinburgh FLSP 920 fluorescence spectrophotometer at room temperature. The Bi/V atomic proportion of the sample was determined by ICP-AES (Profile Spec from Leeman Corp.).

To assess the catalytic ability of Bi₂₅VO₄₀ microcubes for the transformation of 4-NP to 4-aminophenol (4-AP), some solutions that were utilized in the reaction need to be prepared freshly. The catalytic experiments were actualized as follows: 1.0 × 10⁻⁴ mol L⁻¹ of 4-NP and appropriate amount of Bi₂₅VO₄₀ catalysts were first mixed; a certain volume of NaBH₄ solution was then added into the above reaction system to give total 4 mL of the solution. The reduction process was monitored by UV-vis spectrophotometer (UV-9000S, metash, China). For the reduction reaction of 4-NA, 1.0 × 10⁻⁴ mol L⁻¹ of 4-NP was replaced by 2.0 × 10⁻⁴ mol L⁻¹ of 4-NA, while keeping other experimental conditions constant.

3. Results and discussion

The phase and purity of the Bi₂₅VO₄₀ microcubes was first checked by XRD analysis. Fig. 1a shows the experimental XRD pattern of the sample synthesized at 150 °C for 2 h. Every diffraction peak can be readily assigned to the standard Bi₂₅VO₄₀ material with a cubic phase, which is in accordance with the reported data (JCPDS file Card no. 46-0419). The intensity and shape of the peaks reveal that the Bi₂₅VO₄₀ sample is well crystallized. No other diffraction peaks for impurity phases is found from the XRD pattern. The vibration spectrum of Bi₂₅VO₄₀ sample was further analyzed through FT-IR spectrum (Fig. 1b). The two peaks at 475 and 763 cm⁻¹ are contributed by the bending and stretching vibration of VO₄³⁻, respectively.^39,40 The peak centered at 525 cm⁻¹ corresponds to the stretching vibration of Bi–O–V, and a weak peak at 600 cm⁻¹ is attributed to the vibration of Bi–O bond.⁴¹ Therefore, it is reasonable to believe that the pure Bi₂₅VO₄₀ product has been successfully prepared via the hydrothermal synthesis strategy.

Fig. 1 XRD pattern (a) and FT-IR spectrum (b) of the as-obtained Bi₂₅VO₄₀ microcubes prepared at 150 °C for 2 h.

More detailed surface compositions and chemical states of the elements in Bi₂₅VO₄₀ sample were revealed by XPS (Fig. 2). Besides the Bi, V and O signals deriving from Bi₂₅VO₄₀ sample, the C 1s peak centered at 284.8 eV, which is considered to be added in the sample preparation process, can also be found and employed as a binding energy reference, as shown in Fig. 2a. The narrow spectrum of Bi 4f is shown in Fig. 2b, which possesses two Bi 4f signals at about 158.8 eV (Bi 4f_7/2) and 164.1 eV (Bi 4f_5/2), indicating that Bi is in the Bi³⁺ oxidation state.³⁰ As for the V 2p shown in Fig. 2c, the peaks at 523.8 and 516.3 eV can be accordingly attributed to the V 2p_1/2 and V 2p_3/2, which is the characteristic of the V species with a +5 valence in Bi₂₅VO₄₀.⁴² In Fig. 2d, the O 1s peak can be separated into three signals at 529.6, 531.0 and 532.6 eV, which should be ascribed to the binding energies of O in the V–O bond, Bi–O bond and hydroxyl (or crystal water) adsorbed on the surface, respectively.⁴³ Moreover, ICP-AES technology was employed to analyze the atomic proportion of Bi/V in the final sample. Experiment indicates that this value is about 24.9 : 1, which is highly consistent with the stoichiometric ratio of Bi₂₅VO₄₀, indicating that the final sample is comprised of pure Bi₂₅VO₄₀ sample.

Fig. 2 XPS spectra of the as-obtained Bi₂₅VO₄₀ microcubes prepared at 150 °C for 2 h.

The morphology and size of the Bi₂₅VO₄₀ sample was investigated by SEM. As displayed in Fig. 3a, it is easily found that the final product is comprised of many Bi₂₅VO₄₀ microcubes with uniform size and shape. High magnification SEM image displayed in Fig. 3b indicates that the whole microcubes have smooth faces and the edge-length is around 1–2 μm. EDS elemental mapping of Bi₂₅VO₄₀ sample also describes the space distribution of Bi, V and O in an individual microcube (Fig. 3c–f), which shows that three elements are evenly distributed in the cube-like structure.

Fig. 3 SEM (a and b) and EDS mapping (c–f) of the Bi₂₅VO₄₀ prepared at 150 °C for 2 h.

The morphology of the sample was revealed to be seriously influenced by the synthesis temperature. If the synthesis temperature was set at 120 °C, keeping other experimental conditions unchanged, micro-scale cubes were readily obtained (Fig. 4a). However, high magnification SEM, TEM images and the size distribution histogram of the sample displayed in Fig. 4b–d shows that many polydisperse nanoparticles with wide size distribution (32–42 nm) attach to the surface of cube-like microstructures. When the temperature was enhanced to 150 (Fig. 3) or 180 °C (Fig. 4e), the final product was comprised of uniform microcubes. Furthermore, it should be noted that all the XRD patterns of these three samples can be indexed to pure Bi₂₅VO₄₀ with cubic phase (Fig. 1 and 5). Therefore, it is reasonable to believe that Bi₂₅VO₄₀ nanoparticles may first form during the synthesis process. At higher reaction temperature, such particle-like precursors gradually dissolve and more Bi₂₅VO₄₀ microcubes with larger size are obtained. Actually, this phenomenon can be supported by well-known Ostwald ripening mechanism.⁴⁴

Fig. 4 SEM, TEM images and the size distribution histogram of the samples prepared at different reaction temperature: (a–d) 120 and (e) 180 °C.

Fig. 5 XRD patterns of the samples prepared at different reaction temperature.

It has demonstrated that the amount of sodium hydroxide in the reaction system can significantly influences the shape and phase of the sample. When the amount of sodium hydroxide was controlled at 0.15 g, amorphous agglomerate particles were produced (Fig. 6a and b and 7, down). While increasing the amount of sodium hydroxide to 0.3 (Fig. 1 and 3) or 0.45 g (Fig. 6c and 7, middle), monodisperse Bi₂₅VO₄₀ microcubes were obtained. Further increasing the amount of sodium hydroxide to 0.6 g, microrods and microcubes survived together in the sample (Fig. 6d and e) and the corresponding XRD pattern displayed in Fig. 7 (upper) revealed that the sample was comprised of two phase, which could be assigned to cubic Bi₂₅VO₄₀ (no. 46-0419) and orthorhombic BiVO₄ (no. 12-0293), respectively. As a conventional mineral reagent, the presence of appropriate amount of sodium hydroxide in the hydrothermal or solvothermal process can remarkably speed up the reaction rate, which is in favor of the nucleation and growth of target product.⁴⁵ In our experiment, the lower amount of sodium hydroxide means the slower formation rate of Bi₂₅VO₄₀, which may take responsibility for the appearance of amorphous agglomerate particles. However, excess sodium hydroxide in the reaction medium usually leads to the generation of some impurities, which has been adequately demonstrated in the preparation of other complex metal oxides nanomaterials such as BiFeO₃, Cd₂Ge₂O₆, Bi₄Ge₃O₁₂ and so on.^46–48 Therefore, in order to prepare pure Bi₂₅VO₄₀ sample, the optimal amount of sodium hydroxide should be controlled at 0.3 g.

Fig. 6 SEM and TEM images of the samples with different amount of sodium hydroxide: (a and b) 0.15, (c), 0.45 and (d and e) 0.6 g.

Fig. 7 XRD patterns of the samples with different amount of sodium hydroxide.

During the current hydrothermal process, we found that the organic macromolecule PAM also has a crucial effect on the morphology and dispersity of Bi₂₅VO₄₀ sample. In the absence of PAM, the as-prepared sample was composed of agglomerate cube-like Bi₂₅VO₄₀ microstructures (Fig. 8a and b), while other conditions keeping the same with the typical synthesis. If the amount of PAM was set at 0.3 (Fig. 1 and 3) or 0.6 g (Fig. 8c and d), uniform Bi₂₅VO₄₀ microcubes were the exclusive product. On one hand, the viscosity of the solution becomes greater due to the presence of PAM, which can effectively slow down the reaction rate. On the other hand, the amide groups of PAM molecule can coordinate with metal ions to generate stable coordination compound, and further adjust the reaction rate. Therefore, it is reasonable to believe that the improvement of agglomerating phenomenon can be attributed to the effective control of reaction rate resulting from the above double effect of PAM molecule.⁴⁹

Fig. 8 SEM images and XRD patterns of the samples with different amount of PAM: (a and b) 0 and (c and d) 0.6 g.

The amounts of sodium citrate can also sharply affect the morphology and phase of the sample. Several controllable experiments were carried out to check the effect of the sodium citrate in the construction of Bi₂₅VO₄₀ microcubes. For example, polydisperse Bi₂₅VO₄₀ microcubes and some irregular particles were obtained in the absence of sodium citrate (Fig. 9a). The corresponding XRD pattern displayed in Fig. 9b demonstrates that the sample is composed of two phases with cubic Bi₂₅VO₄₀ (no. 46-0419) and Bi₂O₃ (no. 27-0052). Raising the amount of sodium citrate to 0.0024 mol, monodisperse Bi₂₅VO₄₀ microcubes were obtained (Fig. 1 and 3). While the amount of sodium citrate was set at 0.0042 mol, the morphology of the sample hardly changed (Fig. 9c). However, the XRD pattern of this sample reveals that the BiVO₄ impurity also exists in the system (Fig. 9d). As is well known, sodium citrate is a common coordination agents, which can reacts with Bi³⁺ to form a stable metal complex.⁵⁰ Due to the coordination and dissolution equilibrium, the appropriate amounts of sodium citrate may sharply affect the concentration of free metal ions in the reaction system and further adjust the reaction and diffusion rate of the reagents, which may be responsible for the formation of uniform Bi₂₅VO₄₀ microcubes.

Fig. 9 SEM images and XRD patterns of the samples with different amount of Na₃C₆H₅O₇·2H₂O: 0 (a and b) and 0.0042 mol (c and d).

It has been demonstrated that the morphology of the obtained precursors can be easily adjusted by the reaction time. Fig. 10 represents the SEM images and XRD patterns of the samples synthesized at various reaction periods of 1, 2 and 12 h, respectively. As displayed in Fig. 10a and b, one can find that the final sample was composed of Bi₂₅VO₄₀ microcubes and nanoparticles when the reaction time was controlled at 1 h. Extending the reaction time to 2 h, nanoparticles gradually disappeared and more microcubes were obtained (Fig. 1 and 3). If the synthesis time was set at 12 h, the shape and phase of the sample remained unchanged (Fig. 10c and d). Therefore, the possible formation mechanism of the Bi₂₅VO₄₀ microcubes may be attributed to the distinguished Ostwald ripening process.⁴⁴

Fig. 10 SEM images and XRD patterns of the samples obtained at 150 °C with different reaction time: 1 (a and b) and 12 h (c and d).

The optical absorption of the as-obtained Bi₂₅VO₄₀ microcubes was investigated. Fig. 11 depicts the typical UV-vis DRS of the Bi₂₅VO₄₀ sample. For a crystalline semiconductor, the optical absorption near the band edge follows the equation αhν = A(hν − E_g)^n/2, where α, h, ν, A and E_g are the absorption coefficient, Planck constant, light frequency, a constant and band gap, respectively. Also, n can be a value of 1 for the direct band gap and 4 for the indirect band gap.³⁵ Therefore, from the long wavelength extrapolation of the band edge (Fig. 11b), the band gap of the Bi₂₅VO₄₀ micorcubes can be calculated to be 2.42 eV.

Fig. 11 DRS spectrum (a) and the plots of (αhν)² vs. photo energy for the estimation of E_g of Bi₂₅VO₄₀ microcubes prepared at 150 °C for 2 h (b).

Fig. 12 shows the excitation and emission spectra of the as-obtained Bi₂₅VO₄₀ microcubes at 150 °C for 2 h. In the present work, Bi₂₅VO₄₀ sample exhibits double obvious emission peaks at 458 and 639 nm with excitation wavelength at 360 nm (Fig. 12b). Actually, the origins of the above two PL emission peaks (458 and 639 nm) in Bi₂₅VO₄₀ sample have not been established up to now. However, recent researches have demonstrated that there are many oxygen vacancies existed in such sillenite group complex oxides.^33–35 Therefore, it is reasonable to believe that the peak centered at 458 nm can be ascribed to oxygen vacancies related defects emission, which has been intensively studied in other metal oxides including ZnO and SnO₂.^51,52 The emission peak centered at 632 nm may be attributed to the presence of energy levels located in the bandgap, known as deep donors, which is in good agreement with the previous research report.⁵³

Fig. 12 Room temperature excitation (a) and emission spectra (b) of the as-prepared Bi₂₅VO₄₀ microcubes prepared at 150 °C for 2 h (λ_ex = 360 nm).

In order to assess the catalytic activities of Bi₂₅VO₄₀ microcubes, the reduction reaction of 4-NP to 4-AP in the presence of overabounded NaBH₄ was employed as the model experiment. Generally, 4-NP exhibits a main peak (λ = 317 nm) and a weak peak (λ = 400 nm) in the UV-vis absorption spectrum (Fig. 13a).⁵⁴ The absorption peak of 4-NP undergoes an immediate red-shift from 317 to 400 nm upon the addition of NaBH₄, indicating the formation of 4-nitrophenolate ions.⁵⁵ Researches have demonstrated that such intermediate hardly changes a few days later till catalysts are introduced.⁵⁶ Thus, the changes in absorbance at λ = 400 nm can be chosen as a index to report the transformation from 4-NP to 4-AP. Fig. 13b–d depicts the UV-vis absorption spectra of the solution at various reaction periods in different concentrations of catalyst (10, 15, and 20 mg L⁻¹ Bi₂₅VO₄₀ microcubes). Obviously, the absorbance at λ = 400 nm gradually decreases with extending of the reaction time, and a new peak at about λ = 300 nm emerges that indicates superior catalytic activity of the Bi₂₅VO₄₀ microcubes for the conversion from 4-NP to 4-AP. Fig. 13e displays the correlation between the reaction rate and the concentration of the catalyst. The reaction rate can be quickened with the raising of the concentration of the catalyst from 10 to 15 to 20 mg L⁻¹. The concentration of NaBH₄ is higher than that of 4-NP, the catalytic reaction is in more abidance by the pseudo-first order reaction kinetics. Therefore, such rate constant of the catalytic reactions is obtained from the equation: ln(C₀/C) = kt, where the C₀ and C represent the concentrations of 4-NP solution at time 0 and t, respectively, and k is on behalf of the rate constant.⁵⁷ Fig. 13f is the reaction kinetics of 4-NP solution based on the information recorded in Fig. 13e. The rate constants in different concentrations of Bi₂₅VO₄₀ catalysts are calculated to be 0.033 (10 mg L⁻¹ of Bi₂₅VO₄₀), 0.092 (15 mg L⁻¹ of Bi₂₅VO₄₀) and 0.136 min⁻¹ (20 mg L⁻¹ of Bi₂₅VO₄₀). Fig. 14 depicts the conversion–time curves of 4-NP over different catalysts (Bi₂₅VO₄₀ sample prepared in the absence of PAM, see Fig. 8a; Bi₂₅VO₄₀ sample with 0.6 g of PAM, see Fig. 8c). It can be easily found that the catalytic efficiencies in the presence of the above two catalysts significantly reduced. The serious agglomeration phenomenon of the product may explain the lower catalytic ability of the Bi₂₅VO₄₀ sample prepared without PAM molecules. The high amount of PAM added in the synthesis process may form a protective layer in the outer surface of Bi₂₅VO₄₀ microcubes, which makes against the electron transfer, resulting in the bad catalytic ability (Bi₂₅VO₄₀ sample with 0.6 g of PAM).⁵⁸ Although the catalytic conversion process from 4-NP to 4-AP over the Bi₂₅VO₄₀ catalyst is still unclear, it is reasonable to believe that BH₄⁻ and 4-NP can be firstly adsorbed by the Bi₂₅VO₄₀ catalyst. Then, the electron transfer from BH₄⁻ to 4-NP can be mediated by the catalyst surface, leading to the production of 4-AP.⁵⁹ Further investigations reveal that the Bi₂₅VO₄₀ microcubes also have promising catalysis in the reduction of other aromatic nitro-compounds, such as 4-NA under the same experimental conditions (Fig. 15), indicating huge potential for the applications in industrial production. Moreover, compared with some previous reports (see Table 1), the present Bi₂₅VO₄₀ nanostructures also presented close or better catalytic activity for the reduction of 4-NP. In particular, against Ag and Au catalysts the present Bi₂₅VO₄₀ catalysts have lower cost, which is very important in practical application.

Fig. 13 (a) UV-vis absorption spectra of as-prepared Bi₂₅VO₄₀ microcubes, 4-nitrophenol, and 4-nitrophenol-NaBH₄ solution; (b–d) UV-vis absorption spectra of the systems containing 4-nitrophenol and NaBH₄ in the presence of different amount of Bi₂₅VO₄₀ catalyst; (e) the conversion–time curves of 4-NP and (f) linear relationships between ln(C₀/C) and the reaction time in the presence of different amount of Bi₂₅VO₄₀ catalyst.

Fig. 14 The conversion–time curves of 4-NP over different catalysts.

Fig. 15 (a) UV-vis absorption spectra of the 4-NA and NaBH₄ system in the presence of 10 mg L⁻¹ of Bi₂₅VO₄₀ catalyst for various reaction duration; (b) linear relationships between ln(C₀/C) and the reaction time with 10 mg L⁻¹ of Bi₂₅VO₄₀ catalyst.

Table 1 Comparison of the catalytic capacities of various catalysts reported in the literatures for the reduction of 4-NP to 4-AP by NaBH₄

Catalysts	Rate constants (min^−1)	Reaction time (min)	Amount of catalysts (mg mL^−1)	Reference
Bi25VO40 microcubes	0.136	24	0.02	This work
RGO/Ni-1 nanocomposites	0.015	140	0.06	60
Dendritic Pt	0.045	100	—	61
p(AAm)–CB–Ag	0.037	80	0.02	62
Porous Bi2O3	0.07	40	0.125	63

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4. Conclusions

We have designed and synthesized morphology-controllable Bi₂₅VO₄₀ microcubes that have the edge-length of 1–2 μm through a hydrothermal strategy. General experimental parameters that probably affect the phase and shape of the sample have been investigated in details. By modulation of these experimental parameters, we are capable of creation of the Bi₂₅VO₄₀ microcubes as wanted morphologies that would be more promising for the application in catalysis reaction. Prepared Bi₂₅VO₄₀ microcubes with well-defined structures have the optical absorption and photoluminescence properties that have been measured and analyzed deeply. Importantly, we have demonstrated that the as-prepared Bi₂₅VO₄₀ is able to exhibit superior catalytic activities for the reduction of 4-NP and 4-NA in excess NaBH₄ aqueous system. In comparison to the existing hydrothermal/solvothermal strategy for the formation of similar microcubes, our strategy is more versatile and promising in construction of Bi₂₅VO₄₀ nanoarchitecture, since it is more facile, rapid and mild reaction system.

Acknowledgements

We gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 21501002, 21301005 and 21201006) and the Natural Foundation of Anhui Province (No. 1308085QB34 and 1408085QB31).

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