Facile hydrothermal synthesis and improved photocatalytic activities of Zn²⁺ doped Bi₂MoO₆ nanosheets

Abstract

A novel sheet-like Zn²⁺ doped Bi₂MoO₆ photocatalyst was fabricated through a facile hydrothermal strategy. The morphologies and chemical compositions of the samples were investigated with the help of powder X-ray diffraction (XRD), X-ray photoelectron spectrum (XPS), energy dispersive X-ray analysis (EDS), high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), Raman and FT-IR spectra. These experimental results indicated that Zn²⁺ ions could be incorporated into the crystal lattices of Bi₂MoO₆ and substituted the Bi³⁺ ions. Photocatalytic experiments revealed that the as-obtained Zn²⁺ doped Bi₂MoO₆ nanosheets possessed excellent UV-visible light induced photocatalytic ability for the decomposition of Rhodamine B (RhB) molecules by comparison with undoped Bi₂MoO₆. The effect of some scavengers on the photocatalytic efficiency was assessed. Moreover, a possible enhancement mechanism of photocatalytic activity over Zn²⁺ doped Bi₂MoO₆ nanosheets was presented based on the systematical characterization results.

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1. Introduction

Since the discovery of the Honda–Fujishima effect, semiconductor-based photocatalysis technology has afforded a potentially promising route to deal with serious energy and environmental problems.^1–5 Without doubt, in order to achieve efficient photocatalytic technology, it is necessary to first consider how to obtain efficient photocatalysts.^6–9 In the past several years, various photocatalysts have been developed and the most famous one is titanium dioxide.^10–12 However, an inherent shortcoming of titanium dioxide, namely the relatively wide band-gap, may seriously restrict its practical application.¹³ Therefore, considering the full use of solar energy, it is imperative to design and develop new photocatalyst with efficient visible-light activity.^14–16

Among many bismuth-based photocatalysts, Bi₂MoO₆ attracts keen research interest because of its fascinating physical and chemical properties, including suitable band gap, excellent chemical inertness and cheapness.^17–19 As a typical Aurivillius oxide, Bi₂MoO₆ owes interesting layered structures in which [Bi₂O₂]²⁺ layers are sandwiched between MoO₄²⁻ slabs, and can be regarded as a new kind of visible-light photocatalyst with two-dimensional layered structure. Latest researches have demonstrated that Bi₂MoO₆ can display photocatalytic ability for environmental treatment and water splitting under the irradiation of visible light irradiation.^20–22 However, it remains a challenge to realize the practical applications of Bi₂MoO₆ photocatalyst owing to some immanent defects, such as relatively low quantum yield and poor charge separation efficiency.^23–25

Doping impurities into a semiconductor can be regarded as one of the most commonly used methods to prolong the lifetime of photogenerated charges and improve the photocatalytic efficiency.^26–28 For example, Ding et al. found that the reactive species regulation over Bi₂MoO₆ under visible light could be realized through a Bi self-doping technology via a simple soft-chemical method, leading to the significant enhancement of photocatalytic property.²⁹ Alemi and coworkers reported the successful preparation of novel Ln(Gd³⁺, Ho³⁺ and Yb³⁺)-doped Bi₂MoO₆ photocatalysts and presented a possible enhancement mechanism of photocatalytic ability.³⁰ Recently, Ren et al. demonstrated that the photocatalytic efficiency of Bi₂WO₆ matrix might be significantly improved by introducing Zn²⁺ to replace Bi³⁺ lattice sites based on a first-principles calculation method.³¹ Due to the structure similarity between Bi₂WO₆ and Bi₂MoO₆, it is reasonable to believe that the above design idea (Zn²⁺ doping technology) should be suitable for the photocatalytic ability improvement of Bi₂MoO₆. However, to the best of our knowledge, report on the Zn²⁺ doped Bi₂MoO₆ photocatalyst is still quite rare up to now. Hence, we report the successful preparation of sheet-like Zn²⁺ doped Bi₂MoO₆ photocatalyst through a facile hydrothermal strategy. The photocatalytic ability of the Zn²⁺ doped Bi₂MoO₆ sample was evaluated by decolorizing RhB under the UV-visible-light irradiation, which revealed that the photocatalytic performance of Zn²⁺ doped Bi₂MoO₆ could be greatly enhanced compared to pure Bi₂MoO₆. Furthermore, a possible enhancement mechanism of photocatalytic activity was proposed in detail.

2. Experimental

2.1 Preparation of pure Bi₂MoO₆ sample

The pure Bi₂MoO₆ sample was firstly prepared based on previous report.³² 0.001 mol of Bi(NO₃)₃·5H₂O and 0.0005 mol of Na₂MoO₄·2H₂O were added in 15 mL of ethylene glycol. After agitating for 2 h, the mixture was transferred into stainless-steel autoclave with a Teflon liner of 40 mL capability, followed by heating treatment at 160 °C for 6 h. When the temperature was cooled to ordinary temperature, the product (denoted as S1) was washed five times repeatedly, and then dried at 60 °C for 7 h.

2.2 Preparation of Zn²⁺ doped Bi₂MoO₆ photocatalyst

Firstly, 0.5 g of the as-obtained pure Bi₂MoO₆ sample (S1) was introduced into a Teflon liner. Then, 30 mL of aqueous solution containing 0.0055 g of zinc acetate and 0.01 g of NaOH was added. After stirring for 0.5 h, the Teflon liner was sealed and heated at 180 °C for 24 h. The subsequent washing and drying procedures of the sample (denoted as S3) are consistent with that of pure Bi₂MoO₆ sample (Section 2.1). For the preparation of the other two samples, the amount of zinc acetate was controlled at 0.0033 (denoted as S2) and 0.0077 g (denoted as S4), respectively, keeping other experimental parameters unchanged.

2.3 Characterization

The crystal structures of the products were characterized by XRD technology (XRD-6000, Shimadzu, Japan; Cu Kα radiation, λ = 1.5418 Å). SEM and EDS images of the samples were recorded on a field emission scanning electron microanalyser (S-4800, Hitachi, Japan). HRTEM was carried out on a high resolution transmission microscope, employing an accelerating voltage of 200 kV (FEI Tecnai G20, America). FT-IR spectra were obtained on an IR spectrometer (Magna 750, Nicolet, America). Raman spectra were taken at room temperature in the spectral range of 200–1000 cm⁻¹ using a Raman spectromicroscope (LabRAM HR800, Horiba Jobin Yvon, France). The UV-vis diffusion reflectance spectra of the samples were analyzed with a UV-vis spectrophotometer (UV-3600, Shimadzu, Japan). XPS spectrum was recorded on a multifunctional imaging electron spectrometer (Thermo ESCALAB 250XI, America). The surface photovoltage spectra of the products were obtained by surface photovoltage measurement system, which was comprised of a monochromator and a lock-in amplifier (model SR830-DSP) with an optical chopper. The Brunauer–Emmett–Teller (BET) surface area was measured using a micropore & chemisorption analyzer (ASAP 2010, Micromeritics, America). Photoluminescence spectra were recorded employing a fluorescence spectrophotometer (Edinburgh FLSP 920, England). The doped amounts of Zn²⁺ ions in the samples were analyzed by inductively coupled plasma-atomic emission spectroscopy (J-A1100 from Jarrell-Ash Corp).

2.4 Activity measurement

A 240 W Xe lamp (BL-GHX-Xe-300, Shanghai BILON Co., Ltd., China) was employed as a light source. Firstly, 40 mg of Zn²⁺ doped Bi₂MoO₆ was introduced into 10 mg L⁻¹ of RhB aqueous solution (40 mL). Before turning on the light, the above mixed solution was agitated in the dark room for 1 h. Next, ∼2 mL of the reaction solution was got out at suitable intervals. Centrifugation technique was used to dislodge the solid powder. The concentration of RhB solution was calculated by UV-visible spectrophotometry (UV-3600, Shimadzu, Japan).

3. Results and discussion

Fig. 1a depicts the XRD patterns of pure Bi₂MoO₆ (S1) and Zn²⁺ doped Bi₂MoO₆ with different zinc concentrations (S2, S3 and S4). It is observed that all the diffraction patterns corresponding to the above four samples belong to the orthorhombic phase of Bi₂MoO₆ (JCPDS card no. 76-2388). The diffraction peaks attributed to some impurities fail to be found in Fig. 1a, implying that the existence of Zn²⁺ ions has no significant impact on the micro-scale structures of Bi₂MoO₆. The partial enlarged detail of the four XRD patterns in the range of 25°–32° is presented in Fig. 1b. This clearly shows that the (131) diffraction peak slightly shifts toward the higher angles with the increasing of the zinc concentrations. For the rest diffraction peaks, similar phenomenon can also be observed. As is well known, the differences in ionic radius will affect the lattice parameters.³³ Usually, the ionic radius of Zn²⁺ and Bi³⁺ ions are 0.074 and 0.103 nm, respectively. If some Bi³⁺ lattice sites in the Bi₂MoO₆ matrix are replaced by the smaller Zn²⁺ ions, the lattice parameters of Bi₂MoO₆ will decrease and lead to the slight shift of diffraction peaks toward higher angles, which is consistent with the XRD observation.³⁴ The calculated cell parameters of various samples (S1, S2, S3 and S4) are shown in Table S1 (ESI†).

Fig. 1 XRD patterns of pure Bi₂MoO₆ (S1) and doped Bi₂MoO₆ samples with different Zn²⁺ content (S2, S3 and S4).

Raman spectroscopy is a very sensitive test technology to characterize the local structure of micro/nanomaterials. By means of it, some changes of bonding states in the coordination polyhedron can be observed.¹⁷ As for pure Bi₂MoO₆ (S1) and Zn²⁺ doped sample (S3), many characteristic bands for Bi₂MoO₆ sample can be easily found: 282, 324, 351, 399, 714, 798 and 844 cm⁻¹ (Fig. 2a). The A_1g peak at 798 cm⁻¹ is originated from the symmetric stretch of a MoO₆ octahedron, while the A_1g modes at 714 and 844 cm⁻¹ show the character of orthorhombic distortions of the MoO₆ octahedron in Bi₂MoO₆. The presence of modes below 400 cm⁻¹ can be attributed to both Bi–O stretches and lattice modes. This result is consistent with previous report.²⁵ Although no obvious shift of Raman peaks can be observed, the relative intensity of Raman peak has changed. As shown in Fig. 2a, the intensity of band at 351 cm⁻¹ remains the same all the time, while the intensity of 282 and 324 cm⁻¹ increase when the Zn²⁺ ions is introduced into the reaction system. The reason for these differences is unclear up to now. However, it is reasonable to believe that such change may be attributed to the tiny changes in the crystal structure of Bi₂MoO₆, even if both two samples possess the same orthorhombic phase. Actually, similar experimental phenomenon in Bi₂MoO₆ system has been demonstrated by Zhu's research team.³⁵

Fig. 2 Raman (a) and FT-IR spectra (b) of pure Bi₂MoO₆ (S1) and doped Bi₂MoO₆ samples (S3).

Fig. 2b shows the FTIR spectra of undoped (S1) and doped (S3) Bi₂MoO₆ samples. For S1, the bands located at about 841 and 796 cm⁻¹ can be attributed to the asymmetric and symmetric stretching mode of MoO₆ involving vibrations of the apical oxygen atoms, respectively.³ The absorption bands located at 555 and 733 cm⁻¹ are attributed to the bending vibration and asymmetric stretching modes of MoO₆, respectively. The band at 438 cm⁻¹ corresponds to the Bi–O stretching mode. In addition, similar FT-IR spectrum can be easily found in S3. However, it should be noted that the intensity of FT-IR spectrum over S3 significantly reduce and some peaks located at 438 and 555 cm⁻¹ shift slightly, which may be ascribed to the interactions between Zn²⁺ and Bi₂MoO₆ in the Zn²⁺ doped Bi₂MoO₆ system.³⁶

The valence states and compositions of the samples were investigated by XPS. Fig. 3a depicts the survey spectrum of Zn²⁺ doped Bi₂MoO₆ (S3). It can be easily concluded that S3 possesses four elements, including Bi, Mo, Zn and O. The Bi 4f peaks located at 158.98 (Bi 4f_7/2) and 164.28 eV (Bi 4f_5/2) indicate that the valence state of Bi element in the product is 3+ (Fig. 3b).³⁷ In the Mo 3d XPS spectrum (Fig. 3c), the two peaks at 232.26 (Mo 3d_5/2) and 235.42 eV (Mo 3d_3/2) indicate that Mo is in its [MoO₄]²⁻ (Mo⁶⁺) state.³² Two weak peaks centered at 1021.61 eV (Zn 2p_3/2) and 1044.68 eV (Zn 2p_1/2) can be attributed to the Zn 2p binding energy (Fig. 3d), which implies that the presence of Zn element is in the form of Zn²⁺ oxidation state.³⁸ As shown in Fig. 3e, a strong oxygen 1s peak centered at 529.9 eV corresponds to the oxygen in its O²⁻ oxidation state.³² Moreover, the molar percentage of Zn²⁺/Bi³⁺ in the three Zn²⁺ doped Bi₂MoO₆ samples can be calculated to be 0.9, 1.5 and 2.0% by inductively coupled plasma-atomic emission spectroscopy, which corresponds to the S2, S3 and S4, respectively. Therefore, it is reasonable to believe that Zn²⁺ ions have been successfully entered into the Bi₂MoO₆ matrix and substituted the Bi³⁺ ions, leading to the formation of Zn²⁺ doped Bi₂MoO₆ based on above experimental results.

Fig. 3 XPS spectra of doped Bi₂MoO₆ sample (S3): (a) survey spectrum; (b) Bi 4f; (c) Mo 3d, (d) Zn 2p and (e) O 1s.

To characterize the structural and morphological properties, SEM and TEM analysis technologies were performed. As shown in Fig. 4, it can be seen that the as-obtained S3 are composed of many irregular nanosheets. For the other two doped samples such as S2 and S4, similar sheet-like microstructures can be easily found (Fig. S1, ESI†). However, it should be noted that undoped product (S1) is comprised of nanoflakes-assembled Bi₂MoO₆ hierarchical microspheres (Fig. S2, ESI†). This transformation from hierarchical microspheres to nanosheets may be ascribed to the following Zn²⁺ ions doping treatment.

Fig. 4 SEM (a–c) and TEM images (d) of doped Bi₂MoO₆ sample (S3).

Fig. 5 depicts the HRTEM images of pure Bi₂MoO₆ (S1) and doped Bi₂MoO₆ samples with different Zn²⁺ content (S2, S3 and S4). The clear lattice boundaries in the HRTEM images illustrate the high crystallinity of the Bi₂MoO₆ nanosheets. The observed lattice fringes of four Bi₂MoO₆ samples are determined to be 3.156 (S1), 3.148 (S2), 3.138 (S3) and 3.127 Å (S4), respectively, which correspond to the (131) plane of Bi₂MoO₆ crystal. However, it should be noted that a slight decrease in the lattice spacing of (131) crystal plane can be easily found, which may be ascribed to the lattice contraction due to the substitution of Bi³⁺ by Zn²⁺ cations. These experimental facts are in good agreement with the XRD results. Moreover, the EDS spectra of pure Bi₂MoO₆ (S1) and doped Bi₂MoO₆ samples (S3) are shown in Fig. S3 (ESI†), which further prove the successful preparation of Zn²⁺ doped Bi₂MoO₆ nanosheets.

Fig. 5 HRTEM images of pure Bi₂MoO₆ (S1, a) and doped Bi₂MoO₆ samples with different Zn²⁺ content (S2: b; S3: c; S4: d).

The UV-vis diffusion reflectance spectra were employed to detect the photo-responsive performance of the samples. The bandgap value of a semiconductor-based photocatalyst can be determined using the following formula:

αhν = C(hν − E_g)ⁿ (1)

where α, h, ν, C and E_g are the absorption coefficient, Planck constant, light frequency, a constant and band gap, respectively.³⁹ Among them, the exponent n rests with the type of the transition in a semiconductor. For example, n can be assigned a value of 1/2 and 2 for direct inter-band transition and indirect inter-band transition, respectively.⁴⁰ Therefore, the E_g of the resulting photocatalysts can be calculated to be 2.60, 2.71, 2.73 and 2.82 eV for the above four samples (S1, S2, S3, and S4) based on the intercept of the line on abscissa [(αhν)² = 0], respectively (Fig. 6).

Fig. 6 The UV-vis diffusion reflectance spectra of pure Bi₂MoO₆ (S1, a) and doped Bi₂MoO₆ samples with different Zn²⁺ content (S2: b; S3: c; S4: d).

To estimate the potential application of Zn²⁺ doped Bi₂MoO₆ in the decomposition of some toxic pollutants, four samples including S1, S2, S3 and S4, were adopted as photocatalyst to degrade RhB molecule under the irradiation of UV-visible light. The relevant experimental results can be found in Fig. 7a. Blank experiment implies that RhB molecules are relative stable and the decomposition rate can be negligible. As for undoped Bi₂MoO₆ sample (S1), the degradation value of RhB is 12.9%. When Zn²⁺ doped Bi₂MoO₆ samples are adopted as photocatalysts, all three samples (S2, S3 and S4) possess efficient photocatalytic ability under the irradiation of UV-visible light and S3 possesses the highest photocatalytic degradation efficiency (98.9%). Fig. 7b depicts the linear relation between ln(C₀/C) and t, which implies that the photocatalytic decomposition process reaction of RhB should follows the pseudo-first order model.³ These experimental results obviously reveal that the reaction rates is 0.006, 0.076, 0.128, 0.033 min⁻¹ for S1, S2, S3 and S4 respectively and S3 possesses the optimal photocatalytic performance, which is almost 21.3, 1.7, and 3.9 times higher than those of S1, S2 and S4, respectively.

Fig. 7 (a) Photo-degradation performance of RhB solution as a function of irradiation time under exposure to UV-visible light and (b) kinetics of RhB decolorization in solutions.

To clarify the decomposition process of RhB molecule, many trapping agents including isopropanol, sodium oxalate and benzoquinone were adopted as scavengers of ·OH, h⁺ and ·O₂⁻, respectively.⁴¹ When S3 was employed as photocatalyst, blank experiment revealed that the decomposition efficiency of RhB was 98.9% in the absence of any scavenger (Fig. 8). If benzoquinone, isopropanol or sodium oxalate were adopted as trapping agents, respectively, it could be easily found that sodium oxalate (decomposition efficiency: 1.5%) plays the most key role than isopropanol (decomposition efficiency: 81.1%) and benzoquinone (decomposition efficiency: 30.9%). Hence, h⁺ plays as more crucial roles than ·OH and ·O₂⁻ in the decomposition of RhB.

Fig. 8 Photocatalytic degradation of RhB over the doped Bi₂MoO₆ sample (S3) with or without quenchers.

As is well known, the stronger the surface photovoltage response and the weaker photoluminescence intensity, the higher the photoinduced charge carriers and photocatalytic ability.^42,43 As depicted in Fig. 9, one can find that Zn²⁺ doped Bi₂MoO₆ (S3) exhibits higher surface photovoltage response and lower photoluminescence intensity than undoped Bi₂MoO₆ (S1), which implies that the photo-generated charges can be easily separated in the Zn²⁺ doped Bi₂MoO₆ than pure Bi₂MoO₆.

Fig. 9 The surface photovoltage (a) and photoluminescence spectra (b) of pure Bi₂MoO₆ (S1) and doped Bi₂MoO₆ sample (S3).

In addition, the total density of states of valence band for the pure Bi₂MoO₆ (S1) and doped Bi₂MoO₆ (S3) were measured by valence band X-ray photoelectron spectroscopy. As shown in Fig. 10, one can find that the valence band potential of Zn²⁺ doped Bi₂MoO₆ is estimated to be 0.13 eV more positive than pure Bi₂MoO₆ sample. That is, a ca. 0.13 eV downward in the valence band of Bi₂MoO₆ is obtained upon Zn²⁺ doping, which may imply that the photo-induced holes of Zn²⁺ doped sample possesses stronger oxidizing ability than that of undoped product.⁴⁴

Fig. 10 The valence band XPS spectra of pure Bi₂MoO₆ (S1: a) and doped Bi₂MoO₆ (S3: b).

In order to reveal the reason for activity differences in S1, S2, S3 and S4, the BET surface area of these four photocatalysts were measured (Fig. 11). Compared to pure Bi₂MoO₆ sample (S1: 63.87 m² g⁻¹), doped Bi₂MoO₆ products (S2: 11.21 m² g⁻¹; S3: 8.87 m² g⁻¹ and S4: 15.74 m² g⁻¹) possess a relatively lower specific surface area. Usually, high specific surface area means a strong photocatalytic activity. However, photocatalytic experimental results indicate that S1 owes the worst photocatalytic activity. Therefore, compared to pure Bi₂MoO₆ sample, the higher photocatalytic activity of Zn²⁺ doped Bi₂MoO₆ can be ascribed to below two factors: (1) the recombination of photo-generated charges can be effectively suppressed by Zn²⁺ doping technology. Under the irradiation of UV-visible light, photo-generated electrons are aroused and transferred from valence band to conduction band across the band gap, leading to the formation of photo-generated electron–hole pairs. e⁻ in the conduction band of Zn²⁺ doped Bi₂MoO₆ might be trapped by O₂ to generate ·O₂⁻, which may significantly increase the life of the photo-generated holes.⁴⁵ As a result, more photo-induced carriers will be involved in the photocatalytic reaction, thereby improving the photocatalytic efficiency. (2) When Zn²⁺ ions are incorporated into the crystal lattices of Bi₂MoO₆ and substituted the Bi³⁺ ions, some changes have taken place in the crystal and band structures of Bi₂MoO₆ matrix. Thus, the oxidizability of photo-induced holes can be significantly enhanced based on the valence band XPS analysis results. Therefore, photo-generated h⁺ with higher activity can efficiently oxidize RhB molecule to the final degradation products.

Fig. 11 Nitrogen adsorption–desorption isotherm of pure Bi₂MoO₆ (S1: a) and doped Bi₂MoO₆ samples with different Zn²⁺ content (S2: b; S3: c and S4: d).

Moreover, for three doped Bi₂MoO₆ samples, S3 possesses the lowest specific surface area and the highest photocatalytic activity, which implies that there are other affecting factors on the photocatalytic activities. Recently, many reports have demonstrated that the doping amount can be regarded as a decisive factor associated with the photocatalytic activity in some photocatalytic systems such as Zn²⁺ doped BiOBr,⁴⁶ Zn²⁺ doped BiOCl,⁴⁷ Sr-doped Bi₂WO₆,⁴⁸ and Er³⁺ doped Bi₂MoO₆.⁴⁹ Currently, the generally accepted view is that appropriate amount of doping element can facilitates the rapid migration and separation of photo-generated carriers, leading to the enhancement of photocatalytic ability, which may accounts for the difference of photocatalytic decomposition ability in the above three Zn²⁺ doped Bi₂MoO₆ products (S2, S3 and S4).⁴⁶

4. Conclusion

In summary, Zn²⁺ doped Bi₂MoO₆ nanosheets have been constructed through a facile hydrothermal route. Compared with pure Bi₂MoO₆ product, Zn²⁺ doped Bi₂MoO₆ possessed higher photocatalytic ability and S3 owed the highest photocatalytic performance. The elevated photocatalytic efficiency might be ascribed to the easier transfer and separation of the photogenerated carriers. Investigation of the primary reactive species and their effects over the Zn²⁺ doped Bi₂MoO₆ photocatalysis revealed that the h⁺ served as more crucial roles than ·OH and ·O₂⁻ in this photocatalytic reaction.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21501002 and 21301005), the Natural Foundation of Anhui Province (No. 1408085QB31), the Open Fund of State Key Laboratory of Coordination Chemistry (SKLCC1604), the Young and Middle-Aged Academic Backbone Training Project of Anhui University of Science and Technology.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra06972j

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