English

• 0.   Introduction

  With the rapid growth and development of the world economy, an increasing number of people recognize the importance of the threat of water pollution, which results from industrial and mining activities. Water pollution has increased people concerns about its negative consequences for health and public safety as well as its impact on environ-ment. Over the past decades, as far as control and increasing regulation over contaminants, a wide range of methods including chemical precipitation, biological treatment, adsorption and photocatalysts have been proposed for wastewater treatment^[1-4]. However, several drawbacks are needed to overcome: biological proc-esses are not efficient for degrading complex and bio-recalcitrant contaminants, while chemical precipitation is severely affected by pH and complexing agent. Additionally, expensive regeneration of adsorbents restricts the development of adsorption processes^{[2, 5-7]}. On the contrary, semiconductor photocatalysts have been proposed to degrade and remove contaminants from wastewater due to their high efficiency, nontoxic and feasibility with sunlight^[8-10]. Among various deve-loped semiconductor photocatalysts, TiO₂ is one of the most widely used photocatalyst for water remediation because of nontoxicity, low processing cost and high stability in aqueous medium^[11-14]. However, TiO₂ has several disadvantages such as that it only absorbs UV photons as it has a large band gap about 3.2 eV, and it has a short lifetime of the photo-generated electron-hole pairs^[15-16]. Therefore, to overcome such drawbacks and increase photocatalytic activity, researches have focused on doping TiO₂ surface with metals or nonmetal ions, combination with other semiconductors or development of non-TiO₂ based and novel photocatalytic materials^[17-19].

  Recently, Wang et al. first used metal-free poly-meric graphite-like carbon nitride (g-C₃N₄) as photo-catalyst, which exhibits high photocatalytic perfor-mance for hydrogen generation^[20]. C₃N₄ photocatalyst can be prepared by directly heating N-rich precursors (such as melamine, urea) at different temperature and heating rate^[21-23]. Although g-C₃N₄ has been regarded as attractive photocatalyst due to its unique properties, such as facile preparation, low processing cost, moderate band gap and high chemical stability, the poor absorptivity and low electron mobility rate restrict the development of g-C₃N₄^[24-26]. Therefore, among the various strategies for improving photocatalytic performance of g-C₃N₄, its combination with transition metal oxides is a feasible one^[27-29]. For example, Zhang et al. synthesized g-C₃N₄/TiO₂ (B) nanofibers with exposed (001) plane and the composites exhibited enhanced visible-light activity^[30]. Hu et al. reported Fe³⁺doped g-C₃N₄ and Fe₂O₃/g-C₃N₄ catalysts using melamine and ferric nitrate as precursors^[31]. C₃N₄/CoO was designed and fabricated for the first time and exhibit more-efficient utilization of solar energy than pure g-C₃N₄^[27]. Furthermore, it was found that MoO₃ has a band gap of 2.8~3.2 eV and can be active in both UV and visible light^[32]. Cu_0.33MoO₃ nanorod was synthesized via hydrothermal route and used for degradation of toluidine blue "O" and chlorobenzene^[33]. MoO₃/polymer polyimide was synthesized via an in situ crystal growth approach, the obtained composite exhibits enhanced photocatalytic activity for H₂ evolution^[34]. Li et al. prepared MoO₃-g-C₃N₄ which show high efficiency for the degradation of methylene blue^[32]. He et al. prepared MoO₃-g-C₃N₄ composites exhibiting higher activity than pure g-C₃N₄, which is attributed to the strong and wide adsorption of visible light and high separation of photogenerated electron-hole pairs^[35]. Although MoO₃-C₃N₄ composite catalyst has better catalytic activity under visible light irradiation, there are few reports about the photocatalytic mechanism over MoO₃-C₃N₄ composites prepared by different methods.

  In this work, a series of MoO₃-C₃N₄ photocatalysts were successfully synthesized by impregnation method using (NH₄)₆Mo₇O₂₄·4H₂O and C₃N₄ as precursors. The microstructure and photocatalytic efficiency of MoO₃-C₃N₄ under visible light were investigated. Particularly, 1.60%(w/w) MoO₃-C₃N₄ catalysts exhibited high photo-catalytic activity in the degradation of MO under visible light irradiation. The synergic effect between MoO₃ and C₃N₄ and the possible mechanisms of enhancement of photoactivity were also investigated.

  1.   Experimental

  1.1   Preparation of the catalysts

  Pure C₃N₄ powder was prepared by directly heating melamine at 500 ℃ for 4 h with a heating rate of 2.5 ℃·min^-1 in air.

  The MoO₃-C₃N₄ photocatalysts were synthesized by impregnation method. In a typical synthesis, the desired amount (NH₄)₆Mo₇O₂₄·4H₂O (0.003~0.094 g) was dissolved in 0.7 mL of deionized water. Subse-quently, the solution was added drop-wise to C₃N₄ powder in a porcelain crucible under stirring and then dried at 80 ℃ for 6 h. Finally, the obtained powder was calcined at 500 ℃ for 2 h. The final products were denoted as x% MoO₃-C₃N₄, where x% is the mass fraction of MoO₃.

  Selective deposition of noble metal Pt was performed. 50 mg of 1.60% MoO₃-C₃N₄ catalyst was dispersed in 80 mL of 15% (V/V) lactic acid solution and ultrasonically dispersed for 5 min. Then, 300 μL of 0.02 g·mL^-1 H₂PtCl₆ aqueous solution was added to the suspension and ultrasonically dispersed for 3 min. Finally, a 300 W xenon lamp was used as light source and the mixture was irradiated for 0.5 h under stirring.

  1.2   Characterization methods

  XRD measurements were recorded on a RigakuD/MAXRB diffractometer with Cu Kα radiation (λ=0.154 18 nm) within the range of 2θ=10°~70° at a voltage of 40 kV and a current of 300 mA. HR-TEM images were obtained on a JEM-2100 PLUS trans-mission electron microscope at an accelerating voltage of 200 kV. Fourier transform infrared (FT-IR) spectra of the catalysts were recorded on a Nicolet Nexus-670 spectrometer. The specific surface area and pore size of the samples were measured by a SSA-4200 pore size specific surface area analyzer (Beijing builder electronic technology). Ultraviolet visible (UV-Vis) diffuse reflection spectra were measured using a UV-Vis spectrophotometer (Shimadzu UV 3600, Japan) within the range of 200~800 nm. X-ray photoemission spectroscopy (XPS) was measured in a PHI5300 ESCA system. The beam voltage was 3.0 eV, and the energy of Ar ion beam was 1.0 keV. The binding energies were normalized to the signal for adventitious carbon at 284.8 eV.

  1.3   Evaluation of the photocatalytic activity

  The photocatalytic activity of MoO₃-C₃N₄ nano-composites was evaluated by degradation of MO (10 mg·L^-1) aqueous solution in a quartz glass reactor under visible light irradiation. Before irradiation, 40 mg of MoO₃-C₃N₄ samples were ultrasonically dispersed in 80 mL MO solution for 10 min and the suspension was stirred in the dark for 30 min to establish an adsorption-desorption equilibrium. The visible light used for photocatalytic degradation was generated with a CEL-HXUV 300 Xe lamp coupled with UV cutoff filter (λ > 420 nm, Zhongjiao Jinyuan Technology). During the photocatalytic reaction, the reaction system was magnetically stirred and 5 mL liquids were collected at specific reaction time. Then, the collected samples were centrifuged. Finally, the concentration of catalyst-free MO solution was analyzed by recording variations of the absorbance maximum (λ=463 nm) using a 754PC UV/Visible spectrophotometer (Shanghai Jinghua Group Corporation). As a comparison, photo-catalytic degradations of MO in the presence of MoO₃ or C₃N₄ were also performed under visible light irradiation.

  Hole and free radical trapping experiment was similar to the photocatalytic degradation experiment. 1, 4-benzoquinone (BQ, ·O₂^- scavenger), tert-butyl alcohol (TBA, ·OH scavenger) or ammonium oxalate (AO, h⁺ scavenger) were added to MO (10 mg·L^-1) aqueous solution with 40 mg of 1.60% MoO₃-C₃N₄ photocatalyst before ultrasound^[36].

  2.   Results and discussion

  2.1   Structure of the photocatalysts

  The XRD patterns of MoO₃, C₃N₄ and x% MoO₃-C₃N₄ composites are shown in Fig. 1. It can be seen that all the diffraction peaks of pure MoO₃ can be indexed as the orthorhombic structure (PDF No.99-0080). The main peaks at 12.77°, 23.33°, 25.65°, 27.33° and 39.04° correspond to the (020), (110), (040), (021) and (060) planes, respectively^[37]. Pure g-C₃N₄ exhibited two diffraction peaks at 13.11° and 27.43°, corresponding to the in-plane repeated units of (100) and the inter-layer structural packing of (002) planes, respectively^[38]. g-C₃N₄ can be detected in all MoO₃-C₃N₄ composites. However, diffraction peaks of MoO₃ were not observed in samples of 0.16%~0.80% MoO₃-C₃N₄ composites, which should be due to the low content of MoO₃ and homogeneously dispersion in composites. Further increasing the contents of MoO₃ to 1.6% and 5.0%, the MoO₃ phase can be detected by XRD. The diffraction peaks of g-C₃N₄ phase were gradually weakened with the introduction of MoO₃, may be attributed to the reduced crystallinity of g-C₃N₄. Interestingly, the diffraction peaks of the (002) plane of g-C₃N₄ in MoO₃-C₃N₄ composites shifted to a higher 2θ value by about 0.45° compared to pure g-C₃N₄, indicating the decreased interlayer distance of C₃N₄^[38].

  Figure 1

  

  Figure 1.  XRD patterns of MoO₃, C₃N₄ and x% MoO₃-C₃N₄

  下载: 全尺寸图片 幻灯片

  The morphology and structure of pure g-C₃N₄ and 1.60% MoO₃-C₃N₄ were also characterized by TEM. As shown in Fig. 2a, pure g-C₃N₄ exhibits typical two-dimensional lamellar structure. The TEM image of 1.60% MoO₃-C₃N₄ (Fig. 2b) displays that the surfaces of MoO₃ nanoparticles were coated with thin amorphous layers of C₃N₄, leading to the formation of MoO₃-C₃N₄ heterostructure. This hybrid structure will decrease the recombination rate of photogenerated electron-hole pairs as well as prolong lifetime of charge carriers during the photocatalytic reaction. Moreover, EDS (Fig. 2c) exhibited that the selected areas of the sample are composed with Mo, O, C and N elements. More detailed structural characteristics for 1.60% MoO₃-C₃N₄ were studied by HR-TEM as shown in Fig. 2d. The fringes with an interplanar spacing of about 0.350 nm correspond to (040) crystal plane of MoO₃, which is in consistent with the XRD patterns. In addition, another phase without fringes can be assigned to C₃N₄.

  Figure 2

  

  Figure 2.  TEM images of C₃N₄ (a) and 1.60% MoO₃-C₃N₄ (b); EDS spectrum of 1.60% MoO₃-C₃N₄ (c) and HRTEM image of 1.60% MoO₃-C₃N₄ photocatalyst (d)

  下载: 全尺寸图片 幻灯片

  Fig. 3 shows the FT-IR spectra of MoO₃, C₃N₄ and MoO₃-C₃N₄ heterojunctions with different MoO₃ contents. For the pure C₃N₄, the peaks at 1 242~1 636 cm^-1 are assigned to typical CN heterocycle stretching vibration modes, while the sharp peak located at 810 cm^-1 is related to the characteristic vibrational mode of triazine units^[38]. The peaks observed at 993 cm^-1 corresponds to Mo=O stretching mode. The peaks at 562 and 867 cm^-1 are ascribed to the stretching mode of oxygen linked with three metal atoms in the Mo-O-Mo units, respectively^[39]. The main characteristic peaks of C₃N₄ appeared in all MoO₃-C₃N₄ photocatalysts. The infrared-active modes of MoO₃ were not detected in 0.16%~1.6% MoO₃-C₃N₄ composites due to the low concentration of MoO₃, which is in consistent with XRD patterns.

  Figure 3

  

  Figure 3.  FT-IR spectra of synthesized MoO₃, C₃N₄ and x% MoO₃-C₃N₄

  下载: 全尺寸图片 幻灯片

  XPS measurements were performed to determine the valence states of various species. Fig. 4 shows the XPS spectra of MoO₃, g-C₃N₄ and 1.6% MoO₃-C₃N₄. C1s high resolution XPS spectra are shown in Fig. 4A and it was found that pure g-C₃N₄ has two C1s peaks centered at 284.8 and 288.3 eV which could be attributed to adventitious carbon on the surface and N-C-N coordination in graphitic carbon nitride, respectively^[40-41]. The N1s binding energy of pure g-C₃N₄ was observed at 398.6 eV, which can be assigned to the presence of sp²-bonded graphitic carbon nitride^[42]. The observed slight shift of N1s peak (398.4 eV) in 1.6% Mo-C₃N₄ composite can be attributed to the interaction between Mo and N atoms. MoO₃ has a strong O1s peak at 531.2 eV which could be attributed to the O^2- species in molybdenum oxide, while pure g-C₃N₄ has a low intensity O1s peak produced by adsorbed H₂O. The O1s peak of 1.6% Mo-C₃N₄ composite can be thought to be the overlap of the two oxygen species. The Mo3d_5/2 (233.2 eV) and the Mo3d_3/2 peaks of pure MoO₃ showed that only Mo⁶⁺ species are detected; while, the Mo3d peaks of 1.6% Mo-C₃N₄ composite shift to 232.3 and 235.5 eV, which could be attributed to the existence of both Mo⁵⁺ and Mo⁶⁺ species on the surface^[43].

  Figure 4

  

  Figure 4.  XPS spectra of MoO₃, C₃N₄ and 1.60% MoO₃-C₃N₄ composite

  (A) C1s; (B) N1s; (C) O1s; (D) Mo3d

  下载: 全尺寸图片 幻灯片

  The UV-Vis DRS spectra of MoO₃, C₃N₄, 1.60% MoO₃-C₃N₄ and 5.00% MoO₃-C₃N₄ composites are shown in Fig. 5. As can be seen in Fig. 5A, MoO₃ and g-C₃N₄ had absorption edge at approximate 440 and 470 nm, respectively. The absorption edge of MoO₃-C₃N₄ composite is weaker than that of pure g-C₃N₄ while the absorbance of composite is obviously higher than pure g-C₃N₄. Optical band gaps (E_g) were obtained via Tauc-plots, a method invented by the physicist^[44]. The band gap energy of a semiconductor can be estimated by the following: αhν=A(hν-E_g)^n/2, where α, h, ν, A and E_g are the adsorption coefficient, Planck constant, light frequency, a proportionality constant and band gap energy, respectively. Considering that MoO₃ and C₃N₄ are direct transition and indirect transition semiconductors, respectively, for MoO₃ and C₃N₄, the values of n are 1 and 4, respectively^{[39, 45]}. The E_g can be obtained from the plot of (αhν)^2/n versus hν, by extrapolation of the linear part near the onset of the absorption edge to intersect the energy axis. As shown in Fig. 5B, the E_g of MoO₃ was 2.86 eV according to a plot of (αhν)² versus energy (hν) and the E_g of C₃N₄ was estimated to be 2.60 eV according to a plot of (αhν)^1/2 versus energy (hν). The valence bands (VB) of MoO₃ and C₃N₄ are shown in Fig. 6. The position of the VB edges of MoO₃ and C₃N₄ were 3.28 and 1.58 eV, respectively.

  Figure 5

  

  Figure 5.  UV-Vis spectra of MoO₃, C₃N₄, 1.60% MoO₃-C₃N₄ and 5.00% MoO₃-C₃N₄ composites (A) and the estimated band gaps of MoO₃ and C₃N₄ (B)

  下载: 全尺寸图片 幻灯片

  Figure 6

  

  Figure 6.  Valence band XPS of MoO₃ and C₃N₄

  下载: 全尺寸图片 幻灯片

  Generally, the ability of adsorption, desorption and diffusion of reactants and products are mainly determined by the S_BET and pore volume of the catalyst. Therefore, a catalyst with high specific surface area (S_BET) and big pore volume is significant to the enhancement of catalytic performance. The surface area and pore volume of the prepared samples were obtained from N₂ adsorption-desorption isotherms. As shown in Fig. 7, all samples except MoO₃ exhibit a type Ⅳ isotherm with H3 hysteresis loop, suggesting the presence of mesopores. The high-pressure hysteresis loop (0.9 < P/P₀ < 1) is related to the larger pores formed between secondary particles, suggesting the presence of lamellar pore structures. MoO₃ shows a type Ⅲ isotherm without hysteresis loop, which means MoO₃ sample do not have pore structure. As listed in Table 1, pure MoO₃ and g-C₃N₄ show relatively low specific surface area (7 and 8 m²·g^-1, respectively). Compared with pure phases, all MoO₃-C₃N₄ composites exhibit high BET surface areas, which might be attributed to thermal exfoliation of g-C₃N₄ into thinner nanosheets during compositing process. With increase of the MoO₃ concentration, BET surface areas of samples increased from 8 to 131 m²·g^-1 until the content reached 1.60%. While further increasing of MoO₃ content to 5% will induce dramatic reduction of BET surface area, which may be due to that MoO₃ can promote the combustion of g-C₃N₄. The pore volume of samples showed a similar change tendency and 1.60% MoO₃-C₃N₄ catalyst has the highest pore volume which was 0.893 mL·g^-1. 1.60% MoO₃-C₃N₄ catalyst shows the highest specific surface area and pore volume.

  Figure 7

  

  Figure 7.  Nitrogen adsorption-desorption isotherms of the samples

  下载: 全尺寸图片 幻灯片

  Table 1

  

  Table 1.  Specific surface areas and pore volumes of the samples

  下载: 导出CSV

  Sample	Surface area/(m2·g-1)	Pore volume/(cm3·g-1)
  C3N4	8	0.06
  0.16% MoO3-C3N4	89	0.67
  0.40% MoO3-C3N4	101	0.50
  0.80% MoO3-C3N4	104	0.66
  1.60% MoO3-C3N4	131	0.90
  5.00% MoO3-C3N4	48	0.37
  MoO3	7	0.02

  2.2   Photocatalytic test

  For evaluation of photocatalytic performances of MoO₃, pure C₃N₄ and MoO₃-C₃N₄ composites, as-prepared materials were used for degradation of MO under visible-light irradiation. As illustrated in Fig. 8A, under visible light irradiation, the self-degradation of MO is negligible in the absence of catalyst, indicating that the photolysis could be ignored. In the presence of pure MoO₃ or C₃N₄ catalysts, 5% or 17% of MO was degraded under visible-light irradiation in 120 min, respectively. However, obvious degradation of MO was observed in the presence of the composition catalysts. When the proportion of MoO₃ reached 1.60%, the as-prepared photocatalyst exhibited highest photocatalytic activity, reaching 100% MO conversion. The photocatalytic activity began to decrease when the content of MoO₃ was higher than 1.60%. So, it can be concluded that the effective degradation of MO occurs when MoO₃-C₃N₄ composites take part in the photocatalytic reaction and the 1.6% MoO₃-C₃N₄ sample displayed the highest photocatalytic activity of degradation of MO under visible-light. It has been discussed that 1.6% MoO₃-C₃N₄ photocatalyst has the highest BET surface area and pore volume, so the surface area and pore volume must be one of the reasons why 1.6% MoO₃-C₃N₄ photocatalyst showed the highest photocatalytic activity. High surface area will increase the number of surface active sites and diffusion of the MO toward the active sites. Additi-onally, this increased photocatalytic performance should be ascribed to the effective separation of electrons holes pairs and transfer of photogenerated charges^{[35, 39]}.

  Figure 8

  

  Figure 8.  Photocatalytic degradation of MO over MoO₃, C₃N₄ and MoO₃-C₃N₄ under visible light irradiation (A) and the corresponding kinetic studies (B)

  下载: 全尺寸图片 幻灯片

  It can be found that with the presence of photocatalysts the degradation data of MO showed exponential relationship with irradiation time, suggesting the reaction follows a first order kinetics. Thus the rate was based on performance equation which could be written as follow^[46]:

  $ \ln \left( {{C_0}/{C_t}} \right) = kt $

  Where C₀ is the concentration of MO after adsorption-desorption equilibrium, C_t represents the concentration of remaining MO after photocatalytic degradation at irradiation time (t) and k is the corresponding rate constant. So, the degradation rate constant of MoO₃, C₃N₄ and a series of MoO₃-C₃N₄ photocatalysts (Fig. 8B) can be obtained according to linear fit of plot of ln(C₀/C_t) vs t, respectively. The k of pure g-C₃N₄ is 0.001 9 min^-1. While, the rate constant of 1.60% MoO₃-C₃N₄ photocatalyst is up to 50 times higher than that of pure g-C₃N₄. In other words, the prepared MoO₃-C₃N₄ heterostructure is much more effective than g-C₃N₄ and there is an optimum content of MoO₃ to obtain an active heterostructure for degradation of methyl orange under visible light.

  As shown in Fig. 9, the photocatalytic degradation efficiency over the prepared MoO₃-C₃N₄ samples depended on the MoO₃ content under visible-light irradiation of 40 min. Increasing the content of MoO₃ within the range of 0 to 1.60%, the degradation efficiency of MO on MoO₃-C₃N₄ composites increased rapidly, while the degradation efficiency began to decrease when the content of MoO₃ was higher than 1.6%. The possible reason might be excessive MoO₃ can cause aggregation of C₃N₄, so the BET surface area started to decrease when content of MoO₃ is higher than 1.60%. Therefore, 1.60% is a suitable content of MoO₃ in the MO photocatalytic degradation.

  Figure 9

  

  Figure 9.  Degradation efficiency changes of MO with the content of MoO₃ in the presence of MoO₃-C₃N₄ composites at the moment of visible-light irradiation of 40 min

  下载: 全尺寸图片 幻灯片

  The influence of the initial MO concentration for degradation of MO over 1.60% MoO₃-C₃N₄ under visible-light irradiation was also studied as shown in Fig. 10a. The degradation efficiency decreased with the increase of initial MO concentration, which might be ascribed to the light screening effect in high concen-tration of MO solution. The influence of different dyes was also investigated and the results are shown in Fig. 10b. Actually, the MoO₃-C₃N₄ photocatalysts prepared by impregnation method exhibited high photoactivity for degradation of MO, rhodamine B(RhB) and methylene blue (MB) dyes, indicating the synthesized MoO₃-C₃N₄ composite is a universal photocatalyst for degradation of organic dyes.

  Figure 10

  

  Figure 10.  Influence of initial MO concentration (a) and different dyes for degradation (b)

  Catalyst: 1.60% MoO₃-C₃N₄, Dosage: 40 mg

  下载: 全尺寸图片 幻灯片

  To investigate the catalytic stability of MoO₃-C₃N₄ heterojunctions, the photocatalytic performance of 1.60% MoO₃-C₃N₄ was investigated in three cycles. As shown in Fig. 11, the degradation efficiency of 1.60% MoO₃-C₃N₄ photocatalyst decreased gradually with the increase of cyclic times, which might be due to the solubility of MoO₃ during water purification.

  Figure 11

  

  Figure 11.  Catalytic stability test of 1.6% MoO₃-C₃N₄ composite

  下载: 全尺寸图片 幻灯片

  2.3   Possible mechanism of MoO₃-C₃N₄ composite

  Fig. 12 shows the kinetic constants of MoO₃-C₃N₄ photocatalyst in the presence of different quenchers. After adding TBA to the reaction system, the constant rate was decreased a little, indicating that ·OH was not the dominant active species. In contrast, the degradation rate declined sharply after addition of BQ or AO, indicating that both ·O₂^- and h⁺ are mainly responsible in the hole and free radical trapping experiment, as BQ and AQ are ·O₂^- and h⁺ scavenger, respectively.

  Figure 12

  

  Figure 12.  Kinetic constants of 1.60% MoO₃-C₃N₄ photocatalyst with different quenchers

  下载: 全尺寸图片 幻灯片

  The bottom energy of the conduction band (CB) positions of C₃N₄ and MoO₃ were calculated by the following empirical equation^[47]: E_CB=E_VB-E_g.

  According to the above equation, the energy of the conduction band (CB) of C₃N₄ and MoO₃ were determined to be -1.02 and 0.42 eV, respectively. Under light irradiation, both MoO₃ and C₃N₄ could absorb photons of energy to provide the driving force for excited electrons (e^-) in the VB to CB and leave holes (h⁺) in the VB. The reduction potential of CB in C₃N₄ was more negative than the standard reduction potential of O₂/·O₂^-, which could reduce O₂ to form ·O₂^-. In addition, the CB in MoO₃ was more positive than O₂/·O₂^-, the electron would only be transferred from the CB of MoO₃ to the VB of C₃N₄. TEM image (Fig. 13a) displayed that the Pt⁴⁺ easily combines with the photogenerated e^- in C₃N₄. The Pt nanoparticles deposited on 1.60% MoO₃-C₃N₄ was further identified (Fig. 13b). The lattice spacing of 0.225 nm was corresponding to the Pt (111) crystal^[48]. The result of selective deposition of noble metal Pt experiments was in consistent with above analysis.

  Figure 13

  

  Figure 13.  HRTEM images of Pt-1.60% MoO₃-C₃N₄ sample

  下载: 全尺寸图片 幻灯片

  On the basis of previous analysis, the possible Z-scheme charge carrier transfer mechanism in photocatalysis was proposed as showed in Fig. 14. Initially, under visible light irradiation, C₃N₄ absorbs photons of energy for generate electrons and holes (Eq.1). At the same time, electrons and holes are generated on MoO₃ as it has suitable band gap to absorb visible light (Eq.2). Subsequently, the photo-generated electrons in the conduction band (CB) of C₃N₄ are scavenged by adsorbed oxygen to form super-oxide radicals (·O₂^-) (Eq.3). The surface adsorbed MO molecules can capture the short-lived superoxide radicals to initiate MO degradation (Eq.4). On the other hand, the electrons in the CB of MoO₃ transfer to the photo excited holes (h⁺) of C₃N₄ (Eq.5) and the h⁺ in the VB of MoO₃ may be able to react with MO molecules (Eq.6). Finally, repeated attacks dye mole-cules by h⁺ in the VB of MoO₃ and ·O₂^- lead to successful degradation of MO.

  $ {{\rm{C}}_3}{{\rm{N}}_4} + h\nu \to {{\rm{C}}_3}{{\rm{N}}_4}\left( {{{\rm{h}}^ + }} \right) + {{\rm{C}}_3}{{\rm{N}}_4}\left( {{{\rm{e}}^ - }} \right) $

  (1)

  $ {\rm{Mo}}{{\rm{O}}_3} + h\nu \to {\rm{Mo}}{{\rm{O}}_3}\left( {{{\rm{h}}^ + }} \right){\rm{ + Mo}}{{\rm{O}}_3}\left( {{{\rm{e}}^ - }} \right) $

  (2)

  $ {{\rm{C}}_3}{{\rm{N}}_4}\left( {{{\rm{e}}^ - }} \right) + {{\rm{O}}_2} \to \cdot {\rm{O}}_2^ - $

  (3)

  $ \cdot {\rm{O}}_2^ - + {\rm{MO}} \to {\rm{degradation\;product}} $

  (4)

  $ {\rm{Mo}}{{\rm{O}}_3}\left( {{{\rm{e}}^ - }} \right) \to {{\rm{C}}_3}{{\rm{N}}_4}\left( {{{\rm{h}}^ + }} \right) $

  (5)

  $ {\rm{Mo}}{{\rm{O}}_3}\left( {{{\rm{h}}^ + }} \right) + {\rm{MO}} \to {\rm{degradation\;product}} $

  (6)

  Figure 14

  

  Figure 14.  Schematic diagram of energy band structure and the possible photocatalytic mechanism of MoO₃-C₃N₄

  下载: 全尺寸图片 幻灯片

  3.   Conclusions

  In summary, MoO₃-C₃N₄ photocatalysts prepared by impregnation method exhibited much higher visible-light catalytic activity than that of pure MoO₃ and C₃N₄. Particularly, when the content of MoO₃ is 1.60%, the MoO₃-C₃N₄ composite has biggest BET surface area and exhibited high catalytic activity in the degradation of MO under visible light irradiation. According to the photocatalytic results, the synergistic mechanism between C₃N₄ and MoO₃ were proposed. A Z-scheme charge carrier transfer mechanism was confirmed for the photocatalytic reaction. The nanostructured layered MoO₃-C₃N₄ heterojunction is low costs, easy preparation and high catalytic activity under visible light radiation, and therefore, ready for application in other related areas, such as solar energy conversion, photovoltaic devices and large scale environmental application.

  Acknowledgements: This work is supported by the National Natural Science Foundation of China (Grant No.21403124) and Natural Science Foundation of Shandong Province (Grant No.ZR2014JL014).

• 1. [1]

     Chong M N, Jin B, Chow C W K, et al. Water Res., 2010, 44(10):2997-3027 doi: 10.1016/j.watres.2010.02.039

  2. [2]

     Fu F L, Wang Q. J. Environ. Manage., 2011, 92(3):407-418 doi: 10.1016/j.jenvman.2010.11.011

  3. [3]

     Fujishima A, Zhang X T, Tryk D A. Surf. Sci. Rep., 2008, 63:515-582 doi: 10.1016/j.surfrep.2008.10.001

  4. [4]

     Crini G. Bioresour. Technol., 2006, 97(16):2173-2181 doi: 10.1016/j.biortech.2005.09.022

  5. [5]

     Gupta V K, Suhas. J. Environ. Manage., 2009, 90(8):2313-2342 doi: 10.1016/j.jenvman.2008.11.017

  6. [6]

     Hermosilla D, Merayo N, Gascó A, et al. Environ. Sci. Pollut. Res., 2015, 22(1):168-191 doi: 10.1007/s11356-014-3516-1

  7. [7]

     Forgacs E, Cserháti T, Oros G. Environ. Int., 2004, 30(7):953-971 doi: 10.1016/j.envint.2004.02.001

  8. [8]

     Abbasi A, Ghanbari D, Salavati-Niasari M, et al. J. Mater. Sci.:Mater. Electron., 2016, 27(5):4800-4809 doi: 10.1007/s10854-016-4361-4

  9. [9]

     Akhavan O. J. Colloid Interface Sci., 2009, 336(1):117-124 doi: 10.1016/j.jcis.2009.03.018

  10. [10]

      Chan S H S, Wu T Y, Juan J C, et al. J. Chem. Technol. Biotechnol., 2011, 86(9):1130-1158 doi: 10.1002/jctb.v86.9

  11. [11]

      Roy P, Berger S, Schmuki P. Angew. Chem. Int. Ed., 2011, 50(13):2904-2939 doi: 10.1002/anie.201001374

  12. [12]

      Bavykin D V, Friedrich J M, Walsh F C. Adv. Mater., 2006, 18(5):1124-1129 doi: 10.1021/cm0521875

  13. [13]

      Schneider J, Matsuoka M, Takeuchi M, et al. Chem. Rev., 2014, 114(19):9919-9986 doi: 10.1021/cr5001892

  14. [14]

      Nakata K, Fujishima A. J. Photochem. Photobiol. C, 2012, 13(3):169-189 doi: 10.1016/j.jphotochemrev.2012.06.001

  15. [15]

      Woan K, Pyrgiotakis G, Sigmund W. Adv. Mater., 2009, 21(21):2233-2239 doi: 10.1002/adma.v21:21

  16. [16]

      Tong H, Ouyang S X, Bi Y P, et al. Adv. Mater., 2012, 24(2):229-251 doi: 10.1002/adma.201102752

  17. [17]

      Li Y M, Ji S D, Gao Y F, et al. Sci. Rep., 2013, 5:1370-1382 http://www.ncbi.nlm.nih.gov/pubmed/23546301

  18. [18]

      Asahi R, Morikawa T, Ohwaki T, et al. Science, 2001, 293(5528):269-271 doi: 10.1126/science.1061051

  19. [19]

      Banisharif A, Khodadadi A A, Mortazavi Y, et al. Appl. Catal. B:Environ., 2015, 165(2):209-221 doi: 10.1016/j.apcatb.2014.10.023

  20. [20]

      Wang X C, Maeda K, Thomas A, et al. Nat. Mater., 2009, 8:271-275 doi: 10.1038/nmat2406

  21. [21]

      Yan S C, Li Z S, Zou Z G. Langmuir, 2009, 25(17):10397-10401 doi: 10.1021/la900923z

  22. [22]

      Dong F, Wu L W, Sun Y J, et al. J. Mater. Chem., 2011, 21(39):15171-15174 doi: 10.1039/c1jm12844b

  23. [23]

      Liu J H, Zhang T K, Wang Z C, et al. Mater. Chem., 2011, 21(14):5430-5434 doi: 10.1039/c1jm00049g

  24. [24]

      Wang Y, Wang X C, Antonietti M. Angew. Chem. Int. Ed., 2012, 51(1):68-89 doi: 10.1002/anie.201101182

  25. [25]

      Fang S, Xia Y, Lv K L, et al. Appl. Catal. B:Environ., 2016, 185:225-232 doi: 10.1016/j.apcatb.2015.12.025

  26. [26]

      Ma S L, Zhan S H, Jia Y N, et al. Appl. Catal. B:Environ., 2016, 186:77-87 doi: 10.1016/j.apcatb.2015.12.051

  27. [27]

      Mao Z Y, Chen J J, Yang Y F, et al. ACS Appl. Mater. Interfaces, 2017, 7(9):5320-5330

  28. [28]

      Pan C S, Xu J, Wang Y J, et al. Adv. Funct. Mater., 2012, 22(7):1518-1524 doi: 10.1002/adfm.v22.7

  29. [29]

      Wang Y J, Shi R, Lin J, et al. Energy Environ. Sci., 2011, 4(8):2922-2929 doi: 10.1039/c0ee00825g

  30. [30]

      Zhang L, Jing D W, She X L, et al. J. Mater. Chem. A, 2014, 2(7):2071-2078 doi: 10.1039/C3TA14047D

  31. [31]

      Hu S Z, Jin R R, Lu G, et al. RSC Adv., 2014, 4(47):24863-24869 doi: 10.1039/c4ra03290j

  32. [32]

      Li Y P, Huang L Y, Xu J B, et al. Mater. Res. Bull., 2015, 70:500-505 doi: 10.1016/j.materresbull.2015.05.013

  33. [33]

      Shakir I, Shahid M, Kang D J. Chem. Commun., 2010, 46(24):4324-4326 doi: 10.1039/c000003e

  34. [34]

      Ma C H, Zhou J, Cui Z W, et al. Sol. Energy Mater. Sol. Cells, 2016, 150:102-111 doi: 10.1016/j.solmat.2016.02.010

  35. [35]

      He Y M, Zhang L H, Wang X X, et al. RSC Adv, 2014, 4(110):65163-65172 doi: 10.1039/C4RA11498A

  36. [36]

      Jiang D L, Chen L L, Zhu J J, et al. Dalton Trans., 2013, 42(44):15726-34 doi: 10.1039/c3dt52008k

  37. [37]

      Chen Y P, Lu C L, Xu L, et al. CrystEngComm, 2010, 12(11):3740-3747 doi: 10.1039/c000744g

  38. [38]

      She X J, Wu J J, Xu H, et al. Adv. Energy Mater., 2017, 7:1700025 doi: 10.1002/aenm.201700025

  39. [39]

      Huang L Y, Xu H, Zhang R X, et al. Appl. Surf. Sci., 2013, 283(14):25-32 doi: 10.1016/j.apsusc.2013.05.106

  40. [40]

      Niu P, Liu G, Cheng H M. J. Phys. Chem. C, 2012, 116(20):11013-11018 doi: 10.1021/jp301026y

  41. [41]

      徐梦秋, 柴波, 闫俊涛, 等.无机化学学报, 2017, 33(3):389-395 http://www.wjhxxb.cn/wjhxxbcn/ch/reader/view_abstract.aspx?file_no=20170304&flag=1XU Meng-Qiu, CHAI Bo, YAN Jun-Tao, et al. Chinese J. Inorg. Chem., 2017, 33(3):389-395 http://www.wjhxxb.cn/wjhxxbcn/ch/reader/view_abstract.aspx?file_no=20170304&flag=1

  42. [42]

      Yan H J, Chen Y, Xu S M. Int. J. Hydrogen Energy, 2012, 37(1):125-133 doi: 10.1016/j.ijhydene.2011.09.072

  43. [43]

      Asakura K, Nakatani K, Kubota T, et al. J. Catal., 2000, 194(2):309-317 doi: 10.1006/jcat.2000.2937

  44. [44]

      Soldat J, Marschall R, Wark M. Mater. Res. Bull., 2014, 5:3746-3752 doi: 10.1039/c4sc01127a

  45. [45]

      Wang Y, Di Y, Antonietti M, et al. Chem. Mater., 2010, 22(18):5119-5121 doi: 10.1021/cm1019102

  46. [46]

      Wang Y F, Liu J X, Wang Y W, et al. Mater. Sci. Semicond. Process., 2014, 25:330-336 doi: 10.1016/j.mssp.2014.02.017

  47. [47]

      Dong H J, Chen G, Sun J X, et al. Appl. Catal. B:Environ., 2013, 219(3):86-95

  48. [48]

      Chang Y C, Yan C Y, Wu R J, et al. J. Chin. Chem. Soc., 2014, 61(3):345-349 doi: 10.1002/jccs.v61.3

• 

  Figure 1  XRD patterns of MoO₃, C₃N₄ and x% MoO₃-C₃N₄

  下载: 全尺寸图片 幻灯片
  

  Figure 2  TEM images of C₃N₄ (a) and 1.60% MoO₃-C₃N₄ (b); EDS spectrum of 1.60% MoO₃-C₃N₄ (c) and HRTEM image of 1.60% MoO₃-C₃N₄ photocatalyst (d)

  下载: 全尺寸图片 幻灯片
  

  Figure 3  FT-IR spectra of synthesized MoO₃, C₃N₄ and x% MoO₃-C₃N₄

  下载: 全尺寸图片 幻灯片
  

  Figure 4  XPS spectra of MoO₃, C₃N₄ and 1.60% MoO₃-C₃N₄ composite

  (A) C1s; (B) N1s; (C) O1s; (D) Mo3d

  下载: 全尺寸图片 幻灯片
  

  Figure 5  UV-Vis spectra of MoO₃, C₃N₄, 1.60% MoO₃-C₃N₄ and 5.00% MoO₃-C₃N₄ composites (A) and the estimated band gaps of MoO₃ and C₃N₄ (B)

  下载: 全尺寸图片 幻灯片
  

  Figure 6  Valence band XPS of MoO₃ and C₃N₄

  下载: 全尺寸图片 幻灯片
  

  Figure 7  Nitrogen adsorption-desorption isotherms of the samples

  下载: 全尺寸图片 幻灯片
  

  Figure 8  Photocatalytic degradation of MO over MoO₃, C₃N₄ and MoO₃-C₃N₄ under visible light irradiation (A) and the corresponding kinetic studies (B)

  下载: 全尺寸图片 幻灯片
  

  Figure 9  Degradation efficiency changes of MO with the content of MoO₃ in the presence of MoO₃-C₃N₄ composites at the moment of visible-light irradiation of 40 min

  下载: 全尺寸图片 幻灯片
  

  Figure 10  Influence of initial MO concentration (a) and different dyes for degradation (b)

  Catalyst: 1.60% MoO₃-C₃N₄, Dosage: 40 mg

  下载: 全尺寸图片 幻灯片
  

  Figure 11  Catalytic stability test of 1.6% MoO₃-C₃N₄ composite

  下载: 全尺寸图片 幻灯片
  

  Figure 12  Kinetic constants of 1.60% MoO₃-C₃N₄ photocatalyst with different quenchers

  下载: 全尺寸图片 幻灯片
  

  Figure 13  HRTEM images of Pt-1.60% MoO₃-C₃N₄ sample

  下载: 全尺寸图片 幻灯片
  

  Figure 14  Schematic diagram of energy band structure and the possible photocatalytic mechanism of MoO₃-C₃N₄

  下载: 全尺寸图片 幻灯片

  Table 1.  Specific surface areas and pore volumes of the samples

  Sample	Surface area/(m2·g-1)	Pore volume/(cm3·g-1)
  C3N4	8	0.06
  0.16% MoO3-C3N4	89	0.67
  0.40% MoO3-C3N4	101	0.50
  0.80% MoO3-C3N4	104	0.66
  1.60% MoO3-C3N4	131	0.90
  5.00% MoO3-C3N4	48	0.37
  MoO3	7	0.02

  下载: 导出CSV
•