Visible Light-Responsive Photocatalytic Activity of Boron Nitride Incorporated Composites

Photocatalysts are essential to promote the highly efficient applications of solar energy in water splitting and/or the degradation of organic contaminations. Especially, the visible light-responsive photocatalysts could benefit with the cost-effective splitting or degradation due to the unlimited sunlight and the absence of expensive light emitter. In the photocatalysts, the charge transfer rates as well as the hole-electron recombination rate are two critical factors that determine the photocatalytic activity, which could also be affected by the dimension, defects, doping and morphologies controlled by the synthesis methods. Boron nitride (BN) is an ultrawide-bandgap semiconductor, and the combination of BN with the visible light-responsive photocatalysts has been found to be effective in enhancing the photocatalytic activities. Therefore, it should be meaningful to understand the BN incorporated photocatalytic composites in depth, including the synthetic approaches, the activity improving mechanisms and the versatile applications. In this review, we mainly focused on the assembly method of BN incorporated photocatalysts; the activity enhancing mechanism by introducing the BN in the photocatalytic composites as well as the properties and the applications. In the end, we gave a conclusion and an outlook for the BN incorporated photocatalytic composites.


INTRODUCTION
Photocatalysts are the semiconductor materials that could promote the water splitting and the degradation of organic contaminations via converting the light energy of irradiation to the chemical energy (redox reaction) of hole-electron pairs, which has attracted intense interests for decades (Molinari et al., 2010). The most impressing advantage of photocatalysts is the utilization of unlimited sunlight on the earth, ranging from the ultraviolet to visible light (Zhu and Wang, 2017;Di et al., 2018;Guo et al., 2018;Jiang et al., 2018;Ji M. et al., 2018;Liang et al., 2018;Nasr et al., 2018;Sivaprakash et al., 2018;Xue et al., 2018), which benefits the human beings with an alternative approach to get the H 2 /O 2 and self-cleaning in an environment friendly way.
The activities of the photocatalysts were mainly determined by three factors: (a) the light absorption, (b) the mobility of light-generated carriers, and (c) the recombination rate of the hole-electron carriers. The light absorption was mainly dependent on the bandgap of the photocatalysts. For example, TiO 2 (E g = 3.0-3.2 eV)/SnO 2 (E g = 3.8 eV)/ZnS (E g = 3.7 eV)/ZrO 2 (E g = 5.0 eV) could be used as the ultraviolet lightresponsive photocatalysts, while the WO 3 (E g = 2.8 eV)/CdS (E g = 2.5 eV)/GaP (E g = 2.3 eV)/V 2 O 5 (E g = 2.0 eV)/CdSe (E g = 1.7 eV)/GaAs (E g = 1.4 eV) could be used as the visible light-responsive photocatalysts (Molinari et al., 2010). Besides, the surface plasmon resonance (Zheng et al., 2011;Rayalu et al., 2013) as well as the heterostructure design Xu et al., 2017a,b) could also improve the light absorption. The mobility of the hole/electron carriers in the photocatalysts was relevant to the conductivity of the photocatalysts (Tang and Ye, 2005;Xu et al., 2012;Crossland et al., 2013) and the effective mass of the carriers . On the other hand, the higher mobility could facilitate the well-separation of the hole/electron pairs , and thus improve the photocatalytic activities.
Hexagonal boron nitride (BN) is an analog of graphite, characterized with a layered structure. BN has an ultra-wide bandgap (5-6 eV), which benefits the material with high electrical insulation, high thermal and chemical stability (Tsao et al., 2018). The as-synthesized BN nanostructures were always negatively charged intrinsically (Peng et al., 2013), which makes them the good h + carrier acceptor, and could be used to improve the h + /e − carrier separation in photocatalysis. As reported in literatures (Chen et al., 2014;Wang J. et al., 2015;Štengl et al., 2016;Liu et al., 2017;Nasr et al., 2018), the combination of BN in the photocatalysts could effectively enhance the photocatalytic activities via improving the visible light absorption and reducing the recombination rate of hole/electron carriers. Therefore, the BN incorporated nanocomposite could be an alternative photocatalyst with enhanced visible light-responsive photocatalytic activities.
This work aims to review the recent progress of visiblelight responsive BN incorporated photocatalysts, including the methodology for heterostructure assembly, characterization, and the activity enhancing mechanisms. Finally, several approaches will be proposed for further improving the photocatalytic activities of the BN incorporated nanocomposites.

ASSEMBLY OF BN INCORPORATED NANOCOMPOSITES FOR PHOTOCATALYSIS
The assembly of BN incorporated photocatalysts could be accomplished by hydrothermal, ball-milling, calcined, electrospinning, in-situ precipitation, impregnation, microwave-assisted, and water bath methods.

Hydrothermal Synthesis
Hydrothermal method is a facile and time-saving way for constructing nanostructures. Starting from hBNNS (hexagonal boron nitride nanosheets), InCl 3 , and TAA (thiacetamide), Meng et al. synthesized the BN/In 2 S 3 photocatalysts by a one-pot hydrothermal method (Meng et al., 2016) as shown in Figure 1. The hydrothermal assembled BN/In 2 S 3 composite showed a good interfacial contact between the hBNNS and the sphericallike In 2 S 3 nanofragments. The incorporation of BN was found to be effective to enhance the separation and transfer efficiencies of the photogenerated electron-hole charge carriers, and therefore improve the photocatalytic activities, where the highest activity was reached at the 7% addition level of BN in the composite. Besides the normal hydrothermal approach, the ionic liquid assisted solvothermal synthesis was also applied for the assembly of the photocatalytic composites BN/Bi 4 O 5 I 2 (Ji M. et al., 2018) and BN/BiPO 4 . In the solvothermal synthesis of BN/Bi 4 O 5 I 2 , the ionic liquid 1-hexyl-3-methylimidazolium iodide ([Hmim]I) served not only as the starting materials, but also as the dispersant to optimize the contact at the interface between the BN and the Bi 4 O 5 I 2 . In another case for BN/BiPO 4 , the ionic liquid 1-decyl-3-methylimidazolium dihydrogen phosphate ([Omim]H 2 PO 4 ) was used to assist the solvothermal assembly of the composite with an increased charge transfer efficiency, suppressed hole-electron recombination as well as the enriched active species.

Ball-Milling Synthesis
Ball-milling is a high energy method to introduce numerous defects into the BNNS, which was effective to improve the photocatalytic activities of BN based nanocomposites. Recently, Zhang et al. used the ball-milling method to prepare the Pt/BN/CdS nano-composite photocatalyst (Zhang R. et al., 2015), where the heterogeneous BN/CdS was firstly prepared by a ball milling process, and then the Pt was loaded onto the BN/CdS surface. It was found that the photocatalyst Pt/BN/CdS could exhibit the highly improved photocatalytic activities, and the highest H 2 evolution rate could reach 17.56 mmol/g/h. The enhanced activity could be ascribed to the heterogeneous design of the photocatalyst, where the BN acted as the hole transfer, while Pt acted as the electron acceptor, both of which may resulted in the effective separation of hole/electron carriers. Most importantly, the ball-milling could enhance the tight contact between the CdS and the BN, facilitating the effective charge transfer. In other BN based nano-composite photocatalysts, Fu et al. synthesized the BN/TiO 2 nanocomposite via ball-milling method (Fu et al., 2013a), where an optimized ball milling condition (0.5 wt% BN, milling for 30 min) was applied for the synthesis of BN/TiO 2 nano-composite photocatalyst with 15 and 8 times improvement of efficiency in the degradation of Rhodamine B (RhB) and methylene blue (MB), respectively. Again, the increase of the photocatalytic activities could also be owing to the hole carrier transfer promoting effect of BN (Figures 2a-h). With respect to the BN/ZnO photocatalyst, Fu et al. presented an effective ball milling method (1 wt% BN, milling for 20 min) to assemble the BN/ZnO nanocomposite for the degradation of cationic dyes (Figure 2i), where a degradation efficiency upto ∼80% could be achieved for RhB (Fu et al., 2013b). The BN in the BN/ZnO nanocomposite could not only improve the hole carrier transfer, but also provide the active sites for the absorption of the charged dyes via electrostatic forces.

Calcined Synthesis
Different from the normal assembly methods, calcination is an effective approach to improve the crystallinity as well as the heterogeneous contact in the nanocomposites, which is critical for the photocatalytic activities. As for the BN based nanocomposite, Ide et al. prepared the Au/TiO 2 /BN hybrid photocatalyst though a calcination method, where the BNNS materials were firstly annealed at 1,200 • C for 4 h under N 2 flow to improve the crystallinity, then the heterostructure Au/TiO 2 /BN was prepared by annealing at 400 • C for 2 h under air (Ide et al., 2014). This heterostructure showed high crystallinity and good interface contact ( Figures 3A-E), which benefited the nanocomposite with a high visible light-responsive photocatalytic activity (10.3 µmol/h). Within the hybrid structure, the Au could enhance the visible light absorption, the BN could increase the concentration of reactants, and the charge separation could be improved at the BN/TiO 2 interfaces. Liu et al. prepared the TiO 2 -xN x /BN heterostructure via twostep annealing method, where the TiN/BN composite was firstly obtained after annealing at 1,200 • C in ammonia, and then the TiO 2 -xN x /BN was collected though a calcination at 600 • C in air . As shown in Figure 3F, the nitrogen doped TiO 2 was uniformly grown on the BN fibers, which should be ascribed to the annealing and calcination process, and the good contact between TiO 2−x N x and BN on the interfaces, should lead to the high separation efficiency of the photogenerated hole/electron carriers ( Figure 3G). Recently, Xu et al. reported the calcination assembly of BN/WO 3 hybrid photocatalyst (Xu et al., 2016). In the synthesis, the BNNS and WO 3 nanopowders were firstly mixed and grounded for 30 min, and then the mixture was annealed at 450 • C to get the BN/WO 3 nanocomposite. As shown in Figure 3H, after annealing, the WO 3 nanoparticles could be tightly adhered onto the BNNS (strong ultrasonic treatment could not peel off the WO 3 from the BNNS), which should provide the efficient separation of photogenerated electron/hole carriers on the interfaces (Figure 3I), and the 5 wt% BN/WO 3 showed the best photocatalytic activity 82% for the pollutant RhB, far superior to the single TiO 2 phase.

Electrospinning Synthesis
In the nanostructure assembly, the electrospinning is an intriguing method that permits the controllable synthesis of nanofibers with designed diameters ranging from nano-to micro-meters, depending on the inherent properties of polymers and/or the processing conditions. For the first time, Nasr et al. prepared the BN/TiO 2 nanocomposite via electrospinning method for the photodegradation of methyl orange (Nasr et al., 2017). In the electrospinning synthesis, the BNNS and the TiO 2 precursor were firstly homogenized, and then the mixture was coated onto an aluminum foil using an electrospinning system as the fibers. Finally, the BN/TiO 2 composite nanofibers were collected and calcined at 500 • C in air (Figure 4). As shown in Figures 4A-D, the electrospun nanocomposite photocatalyst showed a morphology with highly interconnected network composed of continuous, randomly oriented nanofibers. The electrospun BN/TiO 2 nanofibers exhibited the enlarged PL emission (Figure 4E), and was seemed to be related with the oxygen vacancies caused by the BN coupled on the TiO 2 surface. In addition, the BN/TiO 2 nanofibers showed higher degradation efficiency (65%) for the methyl orange than that (60%) of the commercialized P25 photocatalyst (Figure 4F). Except for the BN based nanocomposite, the electrospinning method could also been applied for other photocatalysts. For example, the ZnO/TiO 2 /ZnO or TiO 2 /ZnO/TiO 2 heterostructured photocatalysts could be fabricated with an electrospinning and a followed atomic layer deposition (ALD) method (Kayaci et al., 2014); the TiO 2 -polyamide-12 nanocomposites could be electrospun synthesized for clear water (Cossich et al., 2015); the graphene oxide segregated TiO 2 nanofibers could be got by electrospinning for visible light photocatalysis ; the MgO nanofibers could be synthesized by electrospinning for photo induced dye degradation (Mantilaka et al., 2018). Besides, the BiOCl/TiO 2 (Wang K. et al., 2015), Ag/LaFeO 3 , Ag/Ga 2 O 3 (Han et al., 2017), BiOCl x /BiOBr y /BiOI z (Zhang et al., 2016), graphene-ZnO , Ag@AgCl (Zhou et al., 2016), BiOBr (Babu et al., 2016), and mesoporous TiO 2 (Fu et al., 2014) photocatalysts could also been assembled by the electrospinning methods. In all, the electrospinning method is a facile and controllable approach for the assembly of micro/nanofiber structured photocatalysts, and the coreshell or hierarchical structures could also be obtained via the combination of electrospinning and ALD/hydrothermal methods.

Other Methods
Apart from the above methods, the scientists also proposed the other approaches for the assembly of BN based photocatalysts, including the in-situ precipitation (Lv et al., 2016), impregnation , the microwave-assisted (Xu et al., 2017a), and the water bath synthesis methods (Chen et al., 2014). As shown in Figure 5, the microwave assisted method was used to synthesize the BN/BiOCl composite (Figures 5A,B) where the BN modified BiOCl microspheres were formed with an enhanced photoreactivity for the reduction of Cr(IV) with visible-light irradiation. The in-situ precipitation was used to prepare the hBN/Ag 3 VO 4 photocatalyst where the Ag 3 VO 4 photocatalyst was stabilized via wrapping with the 3D hBN hierarchical structure during the in-situ precipitation process, which promoted the degradation ability of RhB under the visible light irradiation (Figures 5C,D). The impregnation method has been used to fabricate the BN/Bi 2 WO 6 photocatalyst ( Figures 5E,F), where the BN few-layer structures were loaded onto the Bi 2 WO 6 catalyst by refluxing the mixture in the ethyl alcohol for 11 h, and the 3 wt% BN/Bi 2 WO 6 product exhibited the best visible-light photocatalytic activities for the degradation of RhB. The water bath method could be used to prepare the BN/AgBr photocatalyst (Chen et al., 2014), where a water bath process was used to synthesis the AgBr catalyst on the BN substrates, which gave rise to a high catalytic activity for the degradation of MO under visible light irradiation. In a short summary, the low-energy methods, including the electrospinning, in-situ precipitation, and impregnation are more flexible for the preparation of composite catalysts with various nanostructures, but the weak interface interactions will restrict the performance enhancement, and a followed annealing process may be necessary for the high catalytic activities. In contrast, the high-energy methods, including the hydrothermal, ball milling, calcination, microwave-assisted, and the water bath could result in the strong interface interactions between the BN and the catalysts, thereby leading to the excellent photocatalytic activities, although the flexibility for the nanostructures are restricted.

EFFECT OF BN IN THE PHOTOCATALYSTS
Due to the wide band gap (∼5.5 eV), the hBN material could only take effect in the ultraviolet wavelength for the photocatalysis (Ji H. et al., 2018;Xing et al., 2018;Zhang X. et al., 2018). Despite the wide band gap, the hBN nanostructures were intensively applied for the photocatalytic nanocomposites, owing to its high thermal/chemical stability and the hole promoting ability. In the nanocomposite for photocatalysis, the hBN could play different roles for improving the photocatalytic activities. As summarized in Table 1, within the heterogeneous compound, hBN could be used as carrier promoter, h + transfer channel, and could be used to enhance the visible light/electron absorption, form the B-O-Ti dangling bond, and generate the •OH and h + .

Photogenerated Carrier Transfer Promoter
Reducing the recombination rate of the photogenerated h + /e − carriers is critical for the improvement of photocatalytic activities. Chen et al. reported the modification of bismuth phosphate via the hBN nanosheets . After  the modification, the PL intensity became weaker than the pure bismuth phosphate (Figure 6A), which was ascribed to the reduction of the recombination of the photogenerated h + /e − carrier pairs, and the smaller charge transfer resistance ( Figure 6B) revealed the enhanced photocatalytic efficiency of the hBN modified composites which was then proved by the kinetic analysis of the photodegradation ( Figure 6C). As suggested in Figure 6D, the active species •OH and O • 2 − could be generated on the bismuth phosphate by the redox reaction OH − + h + → •OH (valence band, VB) and O 2 + e − → O • 2 − (conduction band, CB). Due to the efficient carrier (h + /e − ) transfer from bismuth phosphate to the hBN, the recombination of h + -e − pairs was reduced, and more active •OH and O • 2 − could be formed to enhance the photocatalytic efficiencies. Such effect in enhancing the carrier transfer rate and reducing the recombination rate of carrier pairs could also be found in the photocatalyst hBN/Ag 2 CO 3 .
Moreover, in the composite hBN/TiO 2 , it was found that the hBN could behave as an efficient hole carrier transfer promoter via the negatively charged hBNNS (hBN nanosheet) surface after a ball-milling process or though the intrinsic electrostatic potential of hBNNT (hBN nanotube; Tang et al., 2010;Fu et al., 2013a). In the hBN/ZnO photocatalyst, the hBNNS could also serve as the hole transfer promoter via the negatively charged surface cunducted by the ball milling (Fu et al., 2013b).

Enhancing the Visible Light Absorption
Due to the wide band gap (E g > 2.0 eV), the low absorption of visible light in the photocatalysts (e.g., TiO2, E g = 3.2 eV) restricts the photodegradation applications under the sun light. The hBN incorporation was found to be effective in reducing the band gap to improve the visible light absorption of photocatalysts, which resulted in the enhancement of visible light-responsive photocatalytic activities ( Table 1). As depicted in Table 1, the addition of hBN in the photocatalysts could reduce the Eg of Bi 2 WO 6 , gC 3 N 4 , and TiO 2 from 2.90, 2.56, and 3.13 eV to 2.68, 2.42, and 2.95 eV, respectively, which resulted in the increase of visible light absorption. However, in the hBN/BiPO4 composite, the electronic structure of BiPO4 could not be influenced by the hBN (∼1 wt%; Chen et al., 2017), and in another case for hBN/Bi 2 WO 6 , the band gap was widened by the hBN incorporation . The different behaviors of hBN in tailoring the electronic structure of the photocatalysts should be ascribed to the different strain accumulation on the interface between the hBN and the photocatalyst, since the strain could play a vital role in tuning the band gap of semiconducting materials (Ishikawa et al., 2003;Minot et al., 2003;Biele et al., 2018;Song et al., 2018;Wang et al., 2018;. It should be noticed that the composite prepared with low-temperature methods, e.g., impregnation, always possessed unchanged or widened E g , whilst the high-temperature methods, e.g., hydrothermal, annealing always resulted in narrowed E g . Therefore, it should be careful to tune the interface adhesion via controlling the fabrication methods for successfully reducing the band gap of the hBN incorporated composite, and achieving the high visible light absorption.

Dangling Bond
Introducing the dangling bond in the photocatalysts is an effective approach for improving the visible light absorption and enhancing the photocatalytic activities under the visible light irradiation (Hirsch and Hauke, 2017;Chen H. et al., 2018;Chen T. et al., 2018;Ma et al., 2018). With respect to the hBN incorporated composites, it is possible to form the dangling bond at the edge of the pores of hBNNS. As demonstrated in the porous hBN/TiO 2 composites , the dangling bond B-O-Ti at the open edges of the hBN pores could cause the energy rearrangement of the composite, and thus reduce the band gap from 3.13 to 2.95 eV, and improve the absorption of visible light (Figure 7).
In short, the effect of hBN in the catalytic composites discussed in literatures is mainly focused on the decrease of E g , the hole carrier promoting, and the B-O-Ti dangling bond. However, the E g tuning mechanisms and the molecular dynamics for the hole promoting are rarely reported.

CONCLUSIONS
In summary, the visible light-responsive hBN incorporated photocatalytic composites were fully reviewed based on the recent published works. Firstly, the preparation methods, including the hydrothermal, ball milling, calcined, electrospinning, in-situ precipitation, impregnation, microwaveassisted, and water bath methods, were discussed with respect to the photocatalytic activity, interface adhesion, crystallinity as well as the cost. Among these approaches, the hydrothermal and calcined methods showed the high crystallinity as well as the good interface adhesion, which gave rise to the high photocatalytic activities, while the electrospinning method exhibited the advantages in flexibility and large aspect ratio of the catalysts. The other methods also represented the facile approaches for the fabrication of hBN incorporated photocatalysts. Regarding the activity enhancing mechanism through the incorporation of hBN nanostructures, the possible hypothesis could be the photogenerated carrier (hole/free electron) transfer promoter (e.g., hBN/BiPO 4 and hBN/Ag 2 CO 3 ), the visible light absorption enhancement (e.g., hBN/BiPO 4 and hBN/Bi 2 WO 6 ), and the dangling bond formed at the interface (e.g., hBN/TiO 2 ). Therefore, the optimized hBN incorporated photocatalysts should have the strong adhesion at the interface as well as the high crystallinity, which could result in the reduced recombination rate of the photogenerated carrier pairs, the improved visible light absorption, and thus lead to the enhanced photocatalytic activities under the visible light irradiation.

OUTLOOK
In order to promote the photocatalytic activities of the hBN incorporated composites, the following investigations could be taken into consideration in the future: (a) Band gap engineering for the hBN used in the composites (Ba et al., 2017), e.g., doping, strain engineering. (b) 3D aligned hBN nanostructure (nanotube, nanosheets) for the assembly of the composites with excellent interface adhesion , e.g., ice template (Gao et al., 2014), biomimetic assembly. (c) Doped hBN for the band gap reduction of the composites (Weng et al., 2017), e.g., cations, anions doping. (d) Gradient band gap design at the interface of the composites (Lan et al., 2016;Mitsutaro et al., 2016), e.g., hBN/VO 2 /TiO 2 heterostructures. (e) SPR enhanced activities for the hBN incorporated composites via modification with metal nanoparticles (Wu et al., 2015, e.g., Ag, Au modification on the surface.

AUTHOR CONTRIBUTIONS
NW, GY, HW, RS, and C-PW proposed and wrote the manuscript.