The synthesis of a Bi2MoO6/Bi4V2O11 heterojunction photocatalyst with enhanced visible-light-driven photocatalytic activity

A novel Bi2MoO6/Bi4V2O11 heterostructured photocatalyst was successfully fabricated using a facile one-pot solvothermal method. This heterojunction consists of homogeneous dispersed Bi4V2O11 nanocrystals anchored on the surface of Bi2MoO6 nanoflakes, endowing the heterojunction with nanosized interfacial contact. Based on the favorable interfacial contact, the band alignment at the heterojunction effectively facilitated photo-generated carrier transfer, which was verified by photoelectrochemical and photoluminescence measurements. Thereby, in contrast with pristine Bi2MoO6 and Bi4V2O11, the as-synthesized heterojunction with nanoscale contact exhibited significantly enhanced photocatalytic activity towards the degradation of MB and the reduction of Cr(vi). In addition, the as-fabricated Bi2MoO6/Bi4V2O11 heterojunction exhibited good cycling stability for MB degradation after 4 cycles. Finally, a plausible photocatalytic mechanism for MB degradation over the Bi2MoO6/Bi4V2O11 heterojunction was discussed in detail. This work not only reports a highly efficient photocatalyst but also sheds light on the design and optimization of a heterojunction.


Introduction
In recent years, water pollution has become a serious problem all around the world. A large number of pollutants, such as organic dyes, heavy metal ions, drugs etc. are discharged into both industrial waste water and domestic sewage. Until now, the conventional water treatment methods such as adsorption, coagulation, microbial degradation, and ultra-ltration were commonly used to treat waste water, however, this approach has the disadvantage of low removal efficiency and difficulty in removing low concentrations of contaminants. [1][2][3] As a kind of green energy technology, semiconductor photocatalytic technology can remove all kinds of pollutants under the irradiation of sunlight and thus, has attracted a large amount of attention. Photocatalysis possesses a number of advantages, such as the room-temperature oxidation of contaminants even at low concentrations, reduced secondary pollution, non-toxicity and low-cost, which is suitable for the degradation of contaminants. 4,5 To date, titanium dioxide (TiO 2 ) is undoubtedly considered to be the most exceptional photocatalyst for solar energy conversion and environmental applications under UV illumination (l < 400 nm). As is well known, among the solar spectrum, ultraviolet-light makes up only less than 5%, while visible-light (760 nm> l > 400 nm) makes up approximately 40%. However, the major drawback of TiO 2 is the wide band gap (3.2 eV), which greatly reduces the efficiency of solar energy utilization and limits its commercial applications. 6,7 Therefore, over the past few decades, a great deal of effort has been made to exploit more efficient visible-light-responding photocatalysts in which Bi-based semiconductors have attracted a substantial amount of attention due to their peculiar electronic structure and the low cost of raw materials. 8 Recently, as an example of a Bi-based semiconductor, Bi 2 MoO 6 has been regarded as a promising photocatalyst because of its narrow band gap (2.5-2.8 eV), high chemical stability and non-toxicity. Unfortunately, the rapid recombination of photo-induced electron-hole pairs and sluggish charge transport in pristine Bi 2 MoO 6 greatly restrict its photocatalytic activity. 9,10 To overcome the obstacles of rapid charge carrier recombination, many strategies have been developed, such as controlling the morphology, 11 doping 12 and constructing a heterojunction. Among them, the method of constructing a heterojunction has been the most popular, since it can boost charge carrier separation and transfer efficiently originating from the as-introduced built-in electric eld at the heterostructured interface. Very recently, Bi 4 V 2 O 11 with a peculiar layered structure that has intrinsic oxygen vacancies in perovskite (VO 3.5 , 0.5 ) 2À (, represents the intrinsic oxygen vacancies) slabs sandwiched between (Bi 2 O 2 ) 2+ layers, has been considered as an excellent photocatalyst for oxygen evolution and water purication. [23][24][25] Because of the narrow band gap ($2.1 eV) and excellent charge carrier transport originating from its unique crystalline structure, Bi 4

Photocatalysts preparation
All chemicals reagents were received from Aladdin Chemical Co. Ltd. and used without further purication.
2 mmol of Bi(NO 3 ) 3 $5H 2 O was added into 15 mL of ethylene glycol and magnetically stirred in a water bath at 80 C to form a clear solution. Then, appropriate stoichiometric amounts of (NH 4 ) 6 Mo 7 O 24 $4H 2 O and NH 4 VO 3 were added into the abovementioned bismuth nitrate solution, which was continuously stirred for 20 min at 80 C to obtain a transparent solution. Subsequently, the pH of the above solution was adjusted to about 8 by the slow dropwise addition of 2 M sodium hydroxide solution. Then, the abovementioned solution was poured into a 25 mL Teon-lined stainless autoclave, which was subsequently sealed and maintained at 160 C for 16 h in an oven. Aer the reactor cooled down to room temperature, the asobtained product was centrifuged three times using water and absolute ethanol.
Finally, the photocatalyst was obtained aer drying at 80 C in air for 4 h. According to the abovementioned method, Bi 2 MoO 6 /Bi 4 V 2 O 11 heterojunction photocatalysts with different molar amounts of Mo and V at 0.2 mmol and 0.8 mmol (denoted as BMV-28), 0.4 mmol and 0.6 mmol (denoted as BMV-46), 0.5 mmol and 0.5 mmol (denoted as BMV-55), and 0.6 mmol and 0.4 mmol (denoted as BMV-64), respectively, were prepared. In addition, pristine Bi 2 MoO 6 and Bi 4 V 2 O 11 were also fabricated for the purpose of comparison.

Characterization
The crystalline structures of the as-fabricated samples were analyzed using X-ray diffraction (XRD) on a Rigaku D/max-2000 diffractometer with Cu Ka radiation (l ¼ 1.5406Å) in the range of 2w ¼ 20-90 at a scanning rate of 4 C min À1 with a scan width of 0.02 . The morphology of the samples was observed using eld emission scanning electron microscopy (FE-SEM, HELIOS NanoLab 600i) at an accelerating voltage of 20 kV. The transmission electron microscopy (TEM) and highresolution TEM (HRTEM) analyses were carried out on a JEM-2100 transmission electron microscope at an accelerating voltage of 200 kV. The X-ray photoelectron spectroscopy (XPS) was conducted on a Thermo Scientic ESCALAB 250Xi X-ray photoelectron spectrometer coupled with a pass energy of 20.00 eV and an Al Ka excitation source (1486.6 eV). The UV-vis diffuse reectance spectra (DRS) were obtained on a spectrophotometer (HITACHI UH-4150) using BaSO 4 as the reectance standard. Photoluminescence (PL) analysis was accomplished on a HORIBA FluoroMax-4.

Photocatalytic activity and photoelectrochemical measurements
The photocatalytic performance of the as-fabricated samples was evaluated by the degradation of methylene blue (MB) and reduction of Cr(VI) under visible-light illumination using a 300 W Xe lamp (Trusttech PLS-SXE 300, Beijing) with a UV cutoff lter (l $ 400 nm). In a typical photocatalytic process, 0.05 g of the as-fabricated sample was added into 100 mL of MB solution (10 mg L À1 ) or Cr(VI) solution (10 mg L À1 , which was based on Cr in a dilute K 2 Cr 2 O 7 solution). The photocatalyst was dispersed in the above solution under ultrasonic treatment for 10 min, and the mixed solution was magnetically stirred in the dark for 30 min to achieve an adsorption-desorption equilibrium between the photocatalyst and the pollutants. When the photodegradation experiment began, 4 mL of the solution was collected from the suspension at xed-time intervals, centrifuged at 10 4 rpm for 5 min to remove catalyst powders, and the MB and Cr(VI) concentrations were determined using a HITA-CHI UH-5300 UV-vis spectrometer at 664 and 352 nm, respectively. The photoelectrochemical measurements were carried out on a CHI604C electrochemical workstation using a standard three-compartment cell with the Bi 4 V 2 O 11 , Bi 2 MoO 6 and Bi 2 MoO 6 /Bi 4 V 2 O 11 samples coated on FTO glass used as the working electrode, a piece of Pt sheet as the counter electrode, standard Ag/AgCl in saturated KCl as the reference electrode, and a 0.5 M Na 2 SO 4 aqueous solution as the electrolyte.
Converting between the measured potential (vs. Ag/AgCl) and NHE was achieved using eqn (1). 28 The light source employed was a 300 W Xe lamp. The Mottschottky measurements were carried out at a frequency of 100 Hz and an amplitude of 10 mV.  Fig. 3. The TEM image of the BMV-55 composite at low magnication ( Fig. 3a) illustrates that the BMV-55 composite is comprised of nanoakes with a thickness of about 10 nm and length of about 100-200 nm, which is consistent with the SEM results described above. Interestingly, as observed from the magnied TEM image (Fig. 3b), nanocrystals with a size of ca. 10 nm are   homogeneously dispersed and tightly adhered to the surface of the nanoakes. By observing the HRTEM image of the BMV-55 composite (Fig. 3c), it can be further conrmed that the nano-akes are Bi 2 MoO 6 and the as-anchored nanocrystals are Bi 4 V 2 O 11 because the observed interplanar spacings of 0.811 nm and 0.312 nm correspond well to the (020) plane of the orthorhombic Bi 2 MoO 6 and the (113) plane of the orthorhombic Bi 4 V 2 O 11 , respectively. These results suggest that the Bi 4 V 2 O 11 nanocrystals are embedded on the Bi 2 MoO 6 nanoakes in the BMV-55 composite, forming a Bi 2 MoO 6 /Bi 4 V 2 O 11 heterojunction with a nanosized interfacial contact. With the aim of further verifying the coexistence of Bi 2 MoO 6 and Bi 4 V 2 O 11 , surface electronic states analysis of the BMV-55 composite have been performed using the X-ray photoelectron spectroscopy (XPS). Fig. 4 shows the high-resolution XPS spectra of the asfabricated BMV-55 composite photocatalyst. The full survey spectra and C 1s spectra of BMV-55 are shown in Fig. S3 in the ESI. †

Results and discussion
The peak positions in all of the XPS spectra were calibrated using the C 1s peak (284.6 eV) as the reference. First, the two characteristic peaks located at 164.5 and 159.1 eV in the Bi 4f spectra (Fig. 4a) were assigned to Bi 4f 5/2 and Bi 4f 7/2 , respectively, which corresponded to Bi 3+ . 29 Then, in the Mo 3d XPS spectra (Fig. 4b), the binding energies for Mo 3d 3/2 and Mo 3d 5/2 located at 235.5 and 232.4 eV, respectively were assigned to Mo 6+ in Bi 2 MoO 6 . 9,29 Furthermore, for the V 2p peaks (Fig. 4c), the two peaks located at 524.1 and 516.7 eV were assigned to V 2p 1/2 and V 2p 3/2 , respectively and belonged to V 5+ in Bi 4 V 2 O 11 . 27,30 Moreover, as shown in Fig. 4d, the O 1s spectra could be divided into three peaks, and the binding energies located at 529.8, 530.3 and 531.2 eV could be assigned to the lattice oxygen in Bi-O, 13 lattice oxygen in Mo-O, 18 and intrinsic oxygen vacancy (V o ) in Bi 4 V 2 O 11 , 31 respectively. The XPS results further indicated the simultaneous existence of Bi 2 MoO 6 and Bi 4 V 2 O 11 species in the BMV-55 composite photocatalyst. In a bid to investigate the optical absorption capacity of the asprepared photocatalysts, UV-vis diffuse reection spectra (DRS) analysis was carried out and is displayed in Fig. 5. As can be seen from Fig. 5a, the absorption edges of pristine Bi 2 MoO 6 , Bi 4 V 2 O 11 and the BMV-55 composite were observed at about 475, 624 and 600 nm, respectively. Obviously, the absorption edge of the BMV-55 composite exhibited a red shi compared with that of pristine Bi 2 MoO 6 , which illustrated that the introduction of Bi 4 V 2 O 11 was conducive to enhancing the light absorption capacity of the BMV-55 composite. In addition, the band gaps of pristine Bi 2 MoO 6 and Bi 4 V 2 O 11 were calculated using the Kubelka-Munk function (eqn (2)). 32 where a, h, n, A, and E g represent the absorption coefficient, Planck constant, light frequency, a constant, and band gap, respectively. In this function, n is determined by the type of optical transition in the semiconductor. Generally, n ¼    Fig. S4. † Obviously, the photocatalytic activity of BMV-55 in the absence of H 2 O 2 is much lower than that found upon adding H 2 O 2 into the system of which the degradation rate is only 75% aer 90 min, indicating that H 2 O 2 plays a key role in our photocatalytic reactions. To further study the photocatalytic application of the as-fabricated photocatalysts, we have also used a toxic heavy metal ion, Cr(VI), as a model pollutant, and we have conducted the photocatalytic reduction of Cr(VI) over the pristine Bi 2 MoO 6 and Bi 4 V 2 O 11 samples, and the BMV-55 composite samples with the results illustrated in Fig. 7. As can be seen from the gure, the BMV-55 composite shows a distinctly enhanced photocatalytic activity for the reduction of Cr(VI) compared with pristine Bi 2 MoO 6 and Bi 4 V 2 O 11 , demonstrating that our heterojunction photocatalysts have a good application in wastewater purication. It is speculated that the highest activity of the BMV-55 composite could be associated with the higher separation and transport efficiency of the photo-induced carriers originating from the construction of the heterojunction between the Bi 2 MoO 6 nanoakes and Bi 4 V 2 O 11 nanocrystals. Aiming at conrming this viewpoint, two important approaches, including photoelectrochemical and photoluminescence (PL) analyses, have been used to evaluate the separation and transport efficiency of photo-induced carriers. Fig. 8 illustrates the results of the electrochemical measurements consisting of the photocurrent and electrochemical impedance spectroscopy (EIS) over the as-prepared pristine Bi 2 MoO 6 and Bi 4 V 2 O 11 samples, and the BMV-55 composite, under visible irradiation. As demonstrated in Fig. 8a, the BMV-55 composite exhibits a remarkably enhanced photocurrent response in striking contrast with the pristine Bi 2 MoO 6 and Bi 4 V 2 O 11 samples. In detail, the photocurrent intensity of the BMV-55 composite is approximately 8-and 18-fold larger than that of pristine Bi 2 MoO 6 and Bi 4 V 2 O 11 , implying that the separation and transport of the photoexcited charge carriers can be effectively promoted by the construction of the heterogeneous structure by anchoring the Bi 4 V 2 O 11 nanocrystals on the Bi 2 MoO 6 nanoakes. Fig. 8b shows the EIS Nyquist plots. Apparently, the BMV-55 composite exhibits a smaller arc radius relative to that of the pristine Bi 2 MoO 6 and Bi 4 V 2 O 11 samples, indicating the signicantly enhanced interfacial charge carrier transport ability of the BMV-55 composite. These results can furnish the valid evidence that the BMV-55 composite possesses facilitated charge carrier separation and transport properties compared with pristine Bi 2 MoO 6 and Bi 4 V 2 O 11 . To shed light on the recombination rate and uorescence lifetime of the photoinduced charge carriers, PL analysis has been performed, as shown in Fig. 9. As depicted in Fig. 9a, the BMV-55 composite exhibits a lower PL emission intensity compared with pristine Bi 2 MoO 6 and Bi 4 V 2 O 11 samples, manifesting that the recombination of the photo-induced charge carriers is effectively suppressed in the Bi 2 MoO 6 /Bi 4 V 2 O 11 heterojunction. Moreover, Fig. 9b presents the time-resolved uorescence decay spectra monitored at 460 nm under an excitation wavelength of 345 nm and the corresponding tting curve of pristine Bi 2 MoO 6 and Bi 4 V 2 O 11 , and the BMV-55 composite. Obviously, the decay kinetics of the BMV-55 composite are slower when compared to those of pristine Bi 2 MoO 6 and Bi 4 V 2 O 11 . Consequently, the radiative lifetimes are extracted by means of a reconvolution t using a 3-exponential model (eqn (3)). 34 where t is the decay time aer the absorption, I (t) is the uorescence intensity at time t, I 0 is the intensity at time t ¼ 0, s i is the lifetime and B i is the pre-exponential factor. In addition, the averaged-lifetime sis calculated using eqn (4). 34 The best tting curve and kinetic parameters observed for pristine Bi 2 MoO 6 and Bi 4 V 2 O 11 , and the BMV-55 composite are displayed in Fig. S5 in the ESI. † In detail, the average-lifetime of carriers in pristine Bi 2 MoO 6 and Bi 4 V 2 O 11 were shortened to ca. 1.983 and 0.487 ns, respectively. In contrast, the averagelifetime of the BMV-55 composite was distinctly prolonged to 7.059 ns, indicating that the formation of the Bi 2 MoO 6 /Bi 4 V 2 O 11 heterojunction is tremendously benecial to prolong the radiative lifetimes of the photo-induced charge carriers, and the greatly prolonged lifetime of the charge carriers could be closely related to the high separation efficiency of the charge carriers. The results of the PL spectra and time-resolved uorescence decay spectra analysis further reveal that the BMV-55 composite exhibits a signicantly enhanced separation and transport efficiency for the photo-induced charge carriers relative to those of pristine Bi 2 MoO 6 and Bi 4 V 2 O 11 , which is very consistent with the results of the photoelectrochemical analysis.
From a practical point of view, the cycle stability of the photocatalysts is a very important factor in evaluating the photocatalytic performance. To evaluate the cycle stability, a cycling experiment for the photodegradation of MB over the BMV-55 composite upon adding 0.1 mL of H 2 O 2 was carried out. Prior to the next cycle experiment, the previously used photocatalyst was subjected to ultrasonic cleaning with distilled water and drying at 60 C. Fig. 10 depicts the results of the recycling experiment aer 4 cycles. It was be clearly seen that the photodegradation rate of MB for 15 minutes remained above 95% aer 4 cycles, which indicated that the as-fabricated BMV-55 composite photocatalyst possessed good cycle stability. For the purpose of unraveling the mechanism of the enhanced photocatalytic activity of the Bi 2 MoO 6 /Bi 4 V 2 O 11 heterojunction, the energy band structures of pristine Bi 2 MoO 6 and Bi 4 V 2 O 11 were investigated by determining their band gaps and at-band potentials. Using the above UV-vis diffuse reection spectra (DRS) analysis (Fig. 5b), the band gaps of pristine Bi 2 MoO 6 and Bi 4 V 2 O 11 were determined to be 2.62 and 2.02 eV, respectively.
In addition, the at-band potentials of pristine Bi 2 MoO 6 and Bi 4 V 2 O 11 are determined via Mott-Schottky analysis, and the results are shown in Fig. 11. It can be seen that both of them show a positive slop, indicating that they are all n-type semiconductors. In addition, the at-band potentials of pristine Bi 2 MoO 6 and Bi 4 V 2 O 11 are ca. À0.39 and 0.26 V, respectively, vs. Ag/AgCl (À0.19 and 0.46 V vs. NHE), which indicates that the at-band potential of Bi 2 MoO 6 is more negative than that of Bi 4 V 2 O 11 by 0.65 V. According to the semiconductor theory, the conduction band potential (E CB ) of n-type semiconductors is very close to (0.1-0.2 V more negative) their at-band potential. 35 Therefore, it can be concluded that the CB position of Bi 2 MoO 6 is more negative than that of Bi 4 V 2 O 11 . Based on above results, possible photo-induced electron-hole pair separation and the photocatalytic degradation of MB mechanism over the Bi 2 MoO 6 /Bi 4 V 2 O 11 heterojunction with the assistance of H 2 O 2 as an electron-trapping agent are proposed, as shown in Scheme 1. On one hand, because of the fact that the CB potential of Bi 2 MoO 6 is more negative than that of Bi 4 V 2 O 11 , the photo-induced electrons can easily migrate from the CB of Bi 2 MoO 6 to the CB of Bi 4 V 2 O 11 driven by the built-in electric eld at the heterostructured interface; it could be then captured   by H 2 O 2 and reacted to generate hydroxide radicals (cOH) with strong oxidizing ability, which will completely degrade MB. On the other hand, since the VB potentials of Bi 2 MoO 6 (2.43 V) and Bi 4 V 2 O 11 (2.48 V) are very close to each other, the migration of the photo-induced holes is very difficult, leading to the holes staying in their valence band positions. 36 In this way, the rapid migration of the photo-induced electrons and the immobilization of the photo-induced holes can lead to the effective separation of the photo-induced electrons and holes over the Bi 2 MoO 6 /Bi 4 V 2 O 11 heterojunction, which ultimately results in the enhanced photocatalytic activity observed during the photodegradation of MB.

Conclusions
In summary, a novel visible-light responsive Bi 2 MoO 6 /Bi 4 V 2 O 11 heterojunction photocatalyst with a nanosized interfacial contact has been successfully fabricated using a facile one-pot solvothermal method. The Bi 2 MoO 6 /Bi 4 V 2 O 11 heterojunctions show excellent photocatalytic MB degradation efficiency under visible-light illumination. In particular, the BMV-55 composite exhibits signicantly enhanced photocatalytic activity during the photodegradation of MB and photoreduction of Cr(VI) compared with pristine Bi 2 MoO 6 and Bi 4 V 2 O 11 . In addition, the Bi 2 MO 6 /Bi 4 V 2 O 11 heterojunctions also display an enhanced transient photocurrent response, lower electrochemical impedance, lower PL intensity and greatly prolonged lifetime, which indicates that the photo-induced charge carriers are more effectively separated and transferred in the Bi 2 MoO 6 / Bi 4 V 2 O 11 heterojunctions. The signicantly enhanced photocatalytic activity and charge carrier separation effect are attributed to the formation of a heterojunction with a nanosized interfacial contact between the Bi 2 MoO 6 nanoakes and Bi 4 V 2 O 11 nanocrystals. Moreover, the Bi 2 MoO 6 /Bi 4 V 2 O 11 heterojunctions exhibit excellent cycle stability aer 4 cycles. This work may be further extended to the research and development of a novel semiconductor heterojunction containing a new kind of Bi 4 V 2 O 11 photocatalyst for water purication and related applications.

Conflicts of interest
There are no conicts to declare.