Trans. Nonferrous Met. Soc. China 27(2017) 1127-1133

Photochemical synthesis and enhanced photocatalytic activity of MnO_x/BiPO₄ heterojunction

Hai-bin LI, Guo-you HUANG, Jian ZHANG, Sheng-hao FU, Teng-gan WANG, Hong-wei LIAO

School of Materials Science and Engineering, Changsha University of Science and Technology, Changsha 410114, China

Received 4 April 2016; accepted 18 December 2016

Abstract: Monoclinic BiPO₄ with rod-like shape was prepared via a CTAB-assisted hydrothermal route. MnO_x nanoparticles were loaded on the surfaces of BiPO₄ rods by a photo-deposition process to form MnO_x/BiPO₄ heterojunctions. The as-prepared samples were characterized by XRD, SEM, TEM, XPS, FL, and UV-Vis diffuse reflectance measurements. The results showed that MnO_x nanoparticles were strongly anchored to the surfaces of BiPO₄ rods when the mole ratio of Mn to Bi was controlled at a low level, forming MnO_x/BiPO₄ heterojunctions with effective and sound interfaces. The MnO_x/BiPO₄ heterojunctions exhibited higher photoactivity than pristine BiPO₄ for photodegradation of methyl blue under UV irradiation, which could be attributed to the efficient charge transfer at the heterojunction interfaces. The higher light absorption ability of MnO_x/BiPO₄ in the range of 300-420 nm compared with pristine BiPO₄ was also responsible for the enhanced photocatalytic activities of MnO_x/BiPO₄ heterojunctions.

Key words: BiPO₄; photocatalysis; hydrothermal method; MnO_x; heterojunction; photodeposition

1 Introduction

Semiconductor photocatalysis is expected to be a very promising technology to address problems in environmental remediation and energy utilization [1,2]. Recently, many Bi-based semiconductors, such as  BiVO₄ [3], Bi₂MoO₆ [4], BiWO₆ [5,6], and BiPO₄ [7], have been applied as photocatalysts. Among these photocatalysts, BiPO₄ has attracted more and more attention. BiPO₄ mainly exists in three crystalline phases: monoclinic structure, monoclinic monazite structure, and hexagonal structure. Among them, the monoclinic BiPO₄ shows the best photocatalytic activity due to its high photocatalytic activity, excellent absorption of UV, strong oxidation ability and chemical stability in aqueous solution, while that of the hexagonal BiPO₄ is the worst [7]. It was reported that the photocatalytic activity of monoclinic BiPO₄ is much better than that of Degussa P25 TiO₂ [8]. However, monoclinic BiPO₄ still suffers from its wide bandgap (4.2 eV), poor adsorptive performance, and large size [9], which limits the utilization of visible light and the separation of photoinduced charge carriers, hindering further improvement of photocatalytic activity. Therefore, it is highly necessary to develop an effective method to solve those problems.

Combining with a cocatalyst, such as a noble metal or another semiconductor, to form heterojunction is an effective way to broaden the light absorption range and promote the separation of photoinduced charge carriers of a photocatalyst. Noble metals, such as Ag, Pt, and Au, are deriving-electron type cocatalysts, while some semiconductors, such as MnO_x, PbO_x, and Ag₃PO₄, are deriving-hole-type cocatalysts [9-11]. It was reported that both Ag and Ag₃PO₄ could enhance the separation efficiency of the photoinduced holes and electrons of BiPO₄, improving the photocatalytic activity [11,12]. MnO_x is a photocatalyst with high activity owe to its small particle size, strong adsorptivity, and excellent absorption of visible light [10]. So, anchoring MnO_x on the surface of BiPO₄ to form a stable heterojunction may cover the shortage of BiPO₄. In addition, photo-induced holes are the main active species of BiPO₄ for dye degradation [10]. Therefore, deriving-hole-type MnO_x may effectively improve the separation efficiency of the photoinduced charge carriers of BiPO₄. However, to our best knowledge, there are no reports about deposition of MnO_x on BiPO₄.

Herein, monoclinic BiPO₄ rods were prepared via a hydrothermal approach, MnO_x nanoparticles were then loaded on the surface of BiPO₄ rods by a photo-deposition process to form a novel MnO_x/BiPO₄ heterojunctions. The photoactivities of the as-prepared samples were evaluated by degradation of methyl blue (MB) under UV irradiation.

2 Experimental

2.1 Preparation of BiPO₄

All reagents were of AR grades and used without further purification. Deionized water was used in all experiments. BiPO₄ was synthesized as follows: 1.15 g of Na₃PO₄·12H₂O and 0.25 g of CTAB were dissolved into 15 mL of deionized water, while 1.46 g of Bi(NO₃)₃·5H₂O was dissolved into 15 mL of HNO₃ solution (1 mol/L). The two solutions were then mixed up. NaOH solution (2 mol/L) was dropwise added to tailor the pH of the mixture under magnetic stirring. The obtained suspension was fixed to 40 mL by adding deionized water. After being stirred for 30 min, the suspension was transferred into a 50 mL Teflon-lined autoclave stainless steel autoclave. The autoclave was heated at 140 °C for 15 h, and then cooled to room temperature naturally. The product was collected, washed with deionized water and absolute alcohol three times, respectively, and finally dried at 80 °C for 20 h to obtain BiPO₄ powders.

2.2 Preparation of MnO_x/BiPO₄ heterojunctions

MnO_x/BiPO₄ heterojunctions were synthesized by photo-deposition. 1 g of BiPO₄ was ultrasonically dispersed into 100 mL deionized water, then a certain amount of MnSO₄·H₂O was dissolved into the suspension (the mole ratios of Mn to Bi were controlled at 0.05, 0.08, and 0.15, respectively). The mixture was magnetically stirred for 24 h in the dark, and then irradiated for 3 h under UV light. The product was collected and thoroughly washed with deionized water and ethanol respectively, and finally dried at 80 °C for 20 h to obtain MnO_x/BiPO₄ heterojunctions. The as- prepared samples were characterized and confirmed for the Mn to Bi mole ratio via atomic absorption spectroscopy (AAS) using Chem. Tech Analytical 2000 spectrophotometer.

2.3 Characterization

The crystalline structure of the sample was analyzed by a Rigaku D/Max 2500 powder diffractometer (XRD) with Cu K_α radiation (λ=1.5406  ). The morphology of the as-prepared samples was investigated by transmission electron microscopy (TEM, Philips Tecnai20G2S-TWIN) and environmental scanning electron microscope (ESEM, FEI QUANTA 250). X-ray photoelectron spectroscopy (XPS) data of the samples were determined with a K-Alpha 1063 electron spectrometer from Thermo Fisher Scientific using 72 W Al K_α radiation. UV-Vis diffuse reflectance spectra were measured with a Specord 200 UV spectrophotometer. Photoluminescence spectroscopy analysis (PL) of the samples was carried out on a Hitachi F-4500 fluorescence spectrophotometer.

2.4 Photocatalytic test

The photocatalytic properties of MnO_x/BiPO₄ heterojunctions were assessed by photodegradation of MB aqueous solution under ultraviolet irradiation with a 45 W ultraviolet lamp. 0.2 g of photocatalyst was mixed with 100 mL of 10 mg/L methyl orange aqueous solution. The mixture was ultrasonically dispersed for 10 min and then magnetically stirred in dark for 30 min before commencing the photocatalytic reactions to allow the system to reach an adsorption/desorption equilibrium. All photocatalytic reactions were carried out in a laboratory constructed photoreactor. 3 mL of sample solution was taken at given time intervals and separated through centrifugation. The concentrations of MB solution were evaluated by an UNICO UV-2100 spectrophotometer at 660 nm.

3 Results and discussion

3.1 Structure and morphology of BiPO₄

Figure 1 shows the XRD patterns of the products prepared at different pH values. It can be found that all the diffraction peaks of the samples prepared at pH 1 and 2 are readily indexed to a pure phase of monoclinic BiPO₄ (JCPDS No. 15-0767), while those of the sample prepared at pH 4 are indexed to a pure phase of hexagonal BiPO₄ (JCPDS No. 45-1370). The strong and sharp diffraction peaks indicate that these three samples are highly crystalline. When the pH is tailored to 6, the characteristic diffraction peaks corresponding to both monoclinic BiPO₄ and hexagonal BiPO₄ are found in the XRD pattern, indicating the coexistence of monoclinic BiPO₄ and hexagonal BiPO₄. Moreover, the diffraction peak intensity decreases obviously compared with that of the other samples. It can be deduced that pH plays a key role in the crystal phases of the products in the present case, and strong acid condition favors the growth of monoclinic BiPO₄, which was reported to show the highest photocatalytic activity among the three crystalline phases of BiPO₄ [7].

Figures 2(a) and (b) show the SEM images of the monoclinic BiPO₄ prepared at pH 1 and 2, respectively. It can be clearly seen that both samples consist of rod-like particles with lengths of 1-2 μm and diameters of 100-400 nm. The smooth surfaces indicate the high crystallinity of the products, which is consistent with the XRD results.

Fig. 1 XRD patterns of samples prepared at different pH

Fig. 2 SEM images of monoclinic BiPO₄prepared at pH=1 (a) and pH=2 (b)

3.2 Structure and morphology of MnO_x/BiPO₄ heterojunctions

MnO_x/BiPO₄ heterojunctions were obtained by photodeposition of MnO_x on the surface of monoclinic BiPO₄ rods prepared at pH 2. Figure 3 shows the XRD patterns of MnO_x/BiPO₄ heterojunctions with different mole ratios of Mn to Bi. It can be seen that for all the samples, only characteristic peaks corresponding to monoclinic BiPO₄ (JCPDS No. 15-0767) are found, no characteristic peaks resulted from manganese oxide can be seen even though the Mn to Bi mole ratio of the heaviest MnO_x-loaded BiPO₄ is 0.15:1. Since MnO_x was prepared by photodeposition of Mn²⁺ from MnSO₄ aqueous solution without calcination, it is most likely that the obtained MnO_x is amorphous, resulting in the disappearance of the characteristic diffraction peaks of MnO_x.

Fig. 3 XRD patterns of MnO_x/BiPO₄ with different mole ratios of Mn to Bi

The microstructures of MnO_x/BiPO₄ heterojunctions were investigated by TEM and HRTEM. As shown in Fig. 4(a), the sample with a Mn to Bi mole ratio of 0.08:1 consists of rod-like BiPO₄ with plenty of MnO_x nanoparticles strongly anchored on the surfaces. The insert shows the HRTEM image corresponding to the marked area. The lattice structure of BiPO₄ is very orderly and different from that of MnO_x nanoparticles. The lattice spacing of 0.327 nm is assigned to the interplanar spacing of monoclinic BiPO₄ (200) plane. The lattice structure of MnO_x is rather indistinct, which may be attributed to its amorphism. The obvious interfaces between BiPO₄ rod and MnO_x nanoparticles shown in HRTEM image suggest the formation of well-defined heterojunction structures of MnO_x/BiPO₄ composites, which is beneficial to the charge carrier separation at the interfaces, improving the quantum yield of the photocatalyst. However, MnO_x particles are not found on the surfaces of BiPO₄ rods when the Mn to Bi mole ratio is increased to 0.15:1, but scatter around the BiPO₄ rods, indicating that MnO_x/BiPO₄ heterojunctions are not formed under this condition. We consider the formation of MnO_x/BiPO₄ heterojunctions is mainly due to the heterogeneous nucleation of MnO_x on the surfaces of rod-like BiPO₄ when the concentration of Mn²⁺ in the suspension is very low. The similar process for constructing Ag/TiO₂ nanotube heterojunctions has been demonstrated in our previous work [13]. As the concentration of Mn²⁺ increased to a relatively high level (Mn to Bi mole ratio of 0.15:1), homogeneous nucleation of MnO_x in the suspension dominates, forming scattered MnO_x particles around rod-like BiPO₄.

Fig. 4 TEM images of MnO_x/BiPO₄ heterojunction with Mn to Bi mole ratio of 0.08:1 (a) and TEM image of MnO_x/BiPO₄ with Mn to Bi mole ratio of 0.15:1 (b) (Insert is HRTEM image of marked area)

Figure 5 shows the high-resolution XPS spectra of the Bi 4f, P 2p, and Mn 2p for MnO_x/BiPO₄ with a Mn to Bi mole ratio of 0.08:1. As shown in Fig. 5(a), the two peaks at 158.9 and 164.2 eV are attributed to the Bi 4f_7/2 and Bi 4f_5/2 of Bi³⁺ in BiPO₄, respectively [10,14]. The P 2p XPS peak can be found at 132.9 eV (Fig. 5(b)), and the peak is ascribed to the P⁵⁺ in BiPO₄ [14]. Figure 5(c) shows the Mn 2p XPS spectrum. The binding energies of Mn 2p_3/2 for the manganese cations in MnO, Mn₂O₃, and MnO₂ were reported at 640.9, 641.8, and 642.4 eV, respectively [15]. Based on the above fact, the Mn 2p_3/2 peak in Fig. 5(c) is fitted by the Gauss-Lorentz method. Three peaks at 640.5, 641.6, and 643.0 eV are obtained, which correspond to Mn²⁺, Mn³⁺ and Mn⁴⁺ in the MnO_x/BiPO₄ heterojunctions, respectively. And the main states of manganese in MnO_x/BiPO₄ are Mn³⁺ and Mn⁴⁺.

Fig. 5 High resolution XPS spectra of Bi 4f (a), P 2p (b), and Mn 2p (c) for MnO_x/BiPO₄ heterojunction with Mn to Bi mole ratio of 0.08:1

3.3 UV-Vis analysis

Figure 6 shows the UV-Vis diffuse reflectance spectra of the pristine BiPO₄ and MnO_x/BiPO₄ heterojunctions with Mn to Bi mole ratios of 0.08:1 and 0.15:1. The pristine BiPO₄ shows strong absorption in the UV range with absorption edge at about 290 nm. The spectrum is steep, indicating that the UV light absorption is resulted from the band-gap transition. The band-gap energy was calculated to be 4.2 eV, according to the formula λ_g=1239.8/E_g, where λ_g is the band-gap wavelength, E_g is the bandgap energy, which is in good agreement with those reported in previous works [16]. For the MnO_x/BiPO₄ heterojunctions, the absorption intensities in the range of 300-420 nm are significantly enhanced compared with that of pristine BiPO₄, which is ascribed to the loading of MnO_x with narrow bandgap on the surface of BiPO₄ rods, indicating that coupling with MnO_x may broaden the light absorption range of the photocatalyst and cover the shortage of BiPO₄.

Fig. 6 UV-Vis diffuse reflectance spectra of pristine BiPO₄ (a) and MnO_x/BiPO₄ heterojunctions with different Mn to Bi mole ratios of 0.08:1 (b) and 0.15:1 (c)

3.4 Photocatalytic activity of MnO_x/BiPO₄ heterojunction

The effects of MnO_x deposition on the photocatalytic activity of BiPO₄ were evaluated by measuring the degradation of MB in aqueous solution under UV irradiation. Figure 7 shows the photodegradation efficiencies of MB as a function of irradiation time by the pristine BiPO₄ and MnO_x/BiPO₄ heterojunctions with different Mn to Bi mole ratios. After 30 min of UV irradiation, MB removals by the pristine BiPO₄ and MnO_x/BiPO₄ heterojunctions with Mn to Bi mole ratios of 0.05:1, 0.08:1, and 0.15:1 are 49%, 77%, 92%, and 61%, respectively. The time for complete decomposition of MB by the pristine BiPO₄ and MnO_x/BiPO₄ heterojunctions with Mn to Bi mole ratios of 0.05:1, 0.08:1, and 0.15:1 is 90, 50, 40, and 70 min, respectively. These results suggest that coupling BiPO₄ with MnO_x can effectively improve the photocatalytic efficiency of BiPO₄, and there is an optimum value for MnO_x loading. Further increasing the loading amount of MnO_x beyond this level shows detrimental effects on the photodegradation of MB.

Fig. 7 Photocatalytic degradation efficiency of MB by pristine BiPO₄ and MnO_x/BiPO₄ with different Mn to Bi mole ratios

To further understand the the photocatalytic efficiency of the above samples, the plot of ln(C_t /C₀) as a function of time is shown in Fig. 8, where C₀ and C_t are the concentrations of methyl orange in the primary stage of experimental and after UV irradiation [17]. It can be seen that the variation of ln(C_t /C₀) with time follows a linear trend in all the plots. The photocatalytic reaction rate constant k can be calculated according to the following formula: ln(C_t /C₀)=-kt [17]. For the pristine BiPO₄ and MnO_x/BiPO₄ heterojunctions with Mn to Bi mole ratios of 0.05:1, 0.08:1, and 0.15:1, the k values were found to be 0.037, 0.060, 0.079, 0.044 min^-1, respectively.

Fig. 8 Kinetics of degradation of MB by pristine BiPO₄ and MnO_x/BiPO₄ with different Mn to Bi mole ratios

The previous works revealed that the surface area, photoabsorption ability, and the separation and transporting rate of photogenerated electrons and holes are the main factors that affect the catalytic activity of a photocatalyst [18]. Since the pristine BiPO₄ and MnO_x/BiPO₄ heterojunctions possess similar morphology, the photocatalytic activity enhancement of MnO_x/BiPO₄ heterojunctions could be mainly attributed to the efficient charge separation and high transporting rate of photogenerated electrons and holes. Figure 9 presents the PL spectra of the pristine BiPO₄ and MnO_x/BiPO₄ heterojunctions with different Mn to Bi mole ratios of 0.05:1, 0.08:1 and 0.15:1. Photoluminescence is resulted from the recombination of photo-generated electrons and holes. The higher intensity of PL spectrum indicates the higher rate of recombination. Therefore, PL spectrum is usually applied to investigating the efficiency of charge carrier transfer, migration and separation [19,20]. It can be clearly seen that the PL intensity of MnO_x/BiPO₄ heterojunction first decreases with increasing the Mn to Bi mole ratio and reaches the lowest value when Mn to Bi mole ratio is 0.08:1. Further increase of Mn to Bi mole ratio of 0.15:1 leads to a rise in PL intensity. This reveals that coupling a suitable amount of MnO_x with BiPO₄ can effectively enhance the charge carrier separation of BiPO₄. Both BiPO₄ and MnO_x are semiconductor oxides, so a heterojunction will be formed at the interfaces between BiPO₄ and MnO_x when they are closely joined together, and an internal field will emerge due to the potential of band energy difference [9,10]. MnO_x is a deriving-hole-type cocatalyst, the photoinduced holes in the valence band of BiPO₄ may transfer to that of MnO_x due to the internal field at the interfaces. Such electric- field- assisted charge transfer at the heterojunction interfaces promotes the separation efficiency of photo-generated electron–holes and improves the quantum yield, leading to the photocatalytic efficiency enhancement of MnO_x/BiPO₄ composites [11,18,20,21]. The higher photoabsorption ability of MnO_x/BiPO₄ in the range of 300-420 nm compared with pristine BiPO₄, which is revealed by the UV-Vis diffuse reflectance spectra (Fig. 6), is also responsible for the enhanced photocatalytic activity of MnO_x/BiPO₄ heterojunctions. It should be noted that the excess coupled content of MnO_x (Mn to Bi mole ratio of 0.15:1) may result in the scattering of MnO_x particles and the damage of the interfaces between MnO_x and BiPO₄, as observed in Fig. 4(b), which is unfavorable for the charge carrier transfer between MnO_x and BiPO₄, resulting in a low quantum yield and a weak photocatalytic performance compared with that of MnO_x/BiPO₄ with a Mn to Bi mole ratio of 0.08:1.

Fig. 9 PL spectra of pristine BiPO₄ (a) and MnO_x/BiPO₄ heterojunctions with different Mn to Bi mole ratios of 0.05:1 (b), 0.08:1 (c) and 0.15:1 (d)

Figure 10 presents the results of repeating experiments on photodegradation of MB by MnO_x/BiPO₄ heterojunctions with a Mn to Bi mole ratio of 0.08:1. After each run, the photocatalysts were collected by centrifugation followed by ultrasonic cleaning with distilled water. As shown in Fig. 10, no significant loss is found after five successive cycles, suggesting that the sample is stable and not photocorroded in the photocatalytic reactions.

Fig. 10 Cyclic photodegradation curve for MnO_x/BiPO₄ heterojunctions with Mn to Bi mole ratio of 0.08:1

4 Conclusions

1) The MnO_x/BiPO₄ heterojunctions were successfully synthesized by the photodeposition of MnO_x on the surfaces of hydrothermally prepared monoclinic BiPO₄ rods.

2) The MnO_x/BiPO₄ heterojunctions with a suitable Mn to Bi mole ratio exhibited higher photocatalytic activities than the pristine BiPO₄ for the degradation of MB under UV irradiation.

3) The TEM and PL studies reveal that the effective and sound interfaces of the MnO_x/BiPO₄ heterojunctions play a key role in the efficient separation of electrons and holes for the enhancement of photocatalytic activity. The higher light absorption ability of MnO_x/BiPO₄ in the range of 300-420 nm compared with pristine BiPO₄ is also helpful to enhancing the photocatalytic activities of MnO_x/BiPO₄ heterojunctions.

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MnO_x/BiPO₄异质结光催化剂的光化学合成与增强的光催化活性

李海斌，黄国游，张 健，付圣豪，王腾淦，廖红卫

长沙理工大学 材料科学与工程学院，长沙 400114

摘  要：采用CTAB辅助水热法制备棒状单斜相BiPO₄。通过光沉积在BiPO₄表面负载MnO_x纳米粒子，形成MnO_x/BiPO₄异质结。采用XRD、SEM、TEM、XPS、PL及UV-Vis等手段对样品进行表征。结果表明：当Mn/Bi摩尔比控制在较低水平时，MnO_x纳米粒子牢固附着在BiPO₄表面，形成具有有效界面的MnO_x/BiPO₄异质结。亚甲基蓝光降解实验结果表明，MnO_x/BiPO₄异质结相对BiPO₄具有更高的光催化活性。这是因为异质结的形成，促进了界面电荷迁移，抑制了光生电子-空穴对的复合，从而获得更高的量子效率。而且MnO_x/BiPO₄异质结相对BiPO₄在300~420 nm范围内具有更高的光吸收能力，这也有利于增强光催化活性。

关键词：BiPO₄；光催化；水热法；MnO_x；异质结；光沉积

(Edited by Xiang-qun LI)

Foundation item: Project (51102025) supported by the National Natural Science Foundation of China; Project (14JJ7040) supported by Natural Science Foundation of Hunan Province, China; Project (2014GH561172) supported by China Torch Program

Corresponding author: Hai-bin LI; Tel:+86-731-85258232; Fax: +86-731-82309128; E-mail: coastllee@hotmail.com

DOI: 10.1016/S1003-6326(17)60131-6