Recent Advances of Bismuth OxychIoride PhotocataIytic MateriaI:Property,Preparation,and Performance Enhancement

Wen-Wen Liu | Ren-Fu Peng

Abstract—Due to its unique layered structure,bismuth oxychloride (BiOCl) has potential applications as a photocatalytic material in clean energy utilization and environmental purification.In recent years,researchers have carried out a lot of studies and made important progress for the preparation and performance enhancement of BiOCl.In this review,the charge separation property of BiOCl was analyzed based on the crystal structure and electronic structure.Subsequently,the common preparation method of BiOCl was expounded,and the growth mechanism of BiOCl was introduced.Furthermore,the strategies for modulating the photocatalytic performance of BiOCl via doping,vacancy creation,internal electric field tuning,co-catalysts modification,composites construction,and using sensitizers were summarized.Finally,in view of the current research status of the BiOCl photocatalytic material,some problems still need to be addressed including exploring the method of controlled synthesis of BiOCl exposed other high energy surfaces,developing advanced characterization methods to clarify the detailed transfer path of the photogenerated charge,and expanding the photocatalytic application range of BiOCl.

1.Introduction

With the continuous development of the social economy,energy consumption and demand have increased.At present,the energy needed for production and life of human society mainly relies on nonrenewable fossil energy.However,based on the world’s proven fossil energy sources,current consumption,and expected consumption rate,the exhaustion of fossil energy will become an inevitable reality[1],[2].Moreover,various pernicious gases are released,while a large amount of fossil energy is used,which not only causes problems,such as greenhouse effect and acid rain,but also seriously threatens the global ecological balance and directly endangers human health[3]-[5].In addition to the energy crisis,environmental pollution,especially water pollution,is also one of the potential challenges that constrain the sustainable development of human society[6],[7].

Semiconductor photocatalytic technology that simulates natural photosynthesis can use solar energy to split water into hydrogen through photocatalytic materials,reduce carbon dioxide into carbonaceous fuel,and mineralize pollutants into small molecules,which is one of the research hotspots in the current energy and environment field[8]-[12].Due to the huge application potential of semiconductor photocatalytic technology in solving the energy crisis and environmental pollution,it has received extensive attention from scientists[13]-[16].Recently,various layered materials,such as graphene,carbon nitrides,transition metal dichalcogenides,perovskites,and layered double hydroxides have been widely used in photocatalytic technology[17]-[21].Of these layered materials,bismuth oxychloride (BiOCl)has potential application prospects in clean energy utilization and environmental purification due to its unique layered structure,stable chemical and optical properties,easy synthesis,and non-toxicity,and has received extensive attention in the field of photocatalysis in recent years[22]-[24].However,BiOCl has a wide bandgap and can only respond to ultraviolet (UV) light in the solar spectrum.The low light utilization efficiency and easy recombination property of the photogenerated electrons and holes resulted in low photocatalytic activity[25]-[27].To date,many researchers have adjusted the photogenerated charge separation efficiency or light absorption ability of BiOCl by doping,creating vacancies,regulating internal electric field,supporting co-catalysts,constructing composites with semiconductors,and using sensitizers to improve the catalytic performance.

In the rest of this review,we first analyze the charge separation property of BiOCl based on its crystal structure and electronic structure.Next,we focus on its common preparation method and introduce the growth mechanism.In the subsequent section,we summarize various strategies to enhance its photocatalytic performance.Finally,some tentative suggestions on future development of new BiOCl photocatalytic materials are presented.

2.Charge Separation Property

The charge separation property of BiOCl can be analyzed from its crystal structure and electronic structure.As a ternary compound semiconductor composed of V-VI-VII main group elements,BiOCl crystals belong to a typical PbFCl type tetragonal system structure,and the space group is P4/nmm[28].Further,BiOCl is an open layered structure in which [Cl-Bi-O-Bi-Cl] repeating units are alternately stacked along thec-axis by a weak van der Waals force interaction between Cl atoms (Fig.1 (a))[29].In each [Cl-Bi-O-Bi-Cl] layer,a Bi atom coordinates with the surrounding four O atoms and four Cl atoms to form a conical decahedral structure with the opposite directions and upper and lower asymmetry.This unique structure gives BiOCl a large space to polarize atoms and atomic orbitals,which is beneficial for inducing the internal electrostatic field along the [001] direction[30],[31].When BiOCl is excited by light,the internal electrostatic field promotes the transfer of electrons between layers and improves the separation efficiency of electrons and holes.

Density functional theory calculations shown in Fig.1 (b) reveal that the fundamental bandgap across BiOCl is indirect,and the fundamental indirect bandgap of BiOCl is 3.37 eV[29],a typical UV-excitable wide bandgap p-type semiconductor.Its valence band maximum is mainly contributed by Cl 3p and O 2p electronic states and the conduction band minimum is contributed by Bi 6p electronic states[32].When an equal photon excites an electron from the Cl 3p state to the Bi 6p state in BiOCl,a pair of electrons and holes appear.For indirect bandgap semiconductors,photogenerated electrons must pass through a certainkspace distance to return to the valence band and then recombine with holes,which also reduces the possibility of electron-hole recombination to some extent[33].Therefore,the open layered crystal structure and indirect bandgap characteristics of BiOCl are favorable for the hole-electron separation and charge transfer,which is beneficial to the photocatalytic reaction.

3.Preparation and Growth Mechanism

Fig.1.Crystal and band structure of BiOCl:(a) crystal structure of BiOCl with [Cl-Bi-O-Bi-Cl] bilayers stacked along the c-axis and (b) scalar relativistic (dashed lines) and fully relativistic (black lines) electronic band structure of BiOCl[29](Reproduced with Permission[29].Copyright 2016,American Chemical Society).

In addition to its relatively good charge separation property,facile synthesis and low cost of raw materials are also important factors for BiOCl applications in the field of photocatalysis[34],[35].As we know,the preparation method can affect the morphology,size,and surface area of the photocatalyst,which affect the photocatalytic performance[36],[37].This section mainly describes several common liquid preparation methods for the preparation of BiOCl,including the hydrolysis,water (solvent) thermal method,template method,and reverse microemulsion method,and introduces the relevant growth mechanism.

3.1.HydroIysis Method

The hydrolysis by using a compound containing bismuth,such as BiCl3,Bi(NO3)3,and Bi2O3,is a method which is employed earlier in the preparation of BiOCl.The reaction conditions of this method are mild,but the dimensional uniformity of the product is poor.For instance,Armelaoet al.used BiCl3as both the Bi source and Cl source,then hydrolyzed BiCl3at 65 °C for 6 h,and thus successfully obtained BiOCl nanoparticles.In order to stabilize the BiOCl nanoparticles,acetylacetone was used as an auxiliary solvent in the synthesis,and an acidic environment was provided by adding HCl[38].Songet al.used Bi(NO3)3and HCl as raw materials,adjusted the pH of the reaction system to about 2 by Na2CO3,then hydrolyzed it at room temperature for 30 min,and finally synthesized irregular BiOCl nanosheets with a thickness of about 21 nm to 85 nm[39].

3.2.Water (SoIvent) ThermaI Method

The water (solvent) thermal method is the most common method for preparing various nanostructured BiOCl.During the thermal synthesis of water (solvent),the spontaneous pressure generated during heating helps to enhance the solubility and reactivity of the precursor,thereby triggering chemical reactions that are difficult to occur under atmospheric conditions[40].More importantly,the properties of BiOCl,such as size,shape,crystal phase,and exposed surface,can be conveniently controlled by adjusting thermodynamic and kinetic parameters.For instance,after dissolving Bi(NO3)3and NaCl in ethylene glycol,the reaction was carried out at 160 °C for 12 h,Gaoet al.successfully obtained BiOCl microspheres.Moreover,different sizes of BiOCl microspheres could be obtained by adjusting the precursor concentration[41].By dissolving Bi(NO3)3into the mannitol aqueous solution at room temperature,slowly dropping the NaCl aqueous solution into this mixture to form suspension and then reacting at 160 °C for 3 h,Liuet al.successfully prepared BiOCl nanoplates exposed (001) crystal facets on their top and bottom surfaces while (110) crystal facets at their four side surfaces[42].

3.3.TempIate Method

The template method usually uses a substance with a specific nanostructure as a template,which mediates the growth of the material.The method can obtain BiOCl structures with different sizes and shapes according to the template structure,so the template method is another effective method for synthesizing different nanostructures of BiOCl.For instance,Cuiet al.used Bi(NO3)3and HCl as raw materials and carbonaceous microspheres as a sacrificial template,then calcined the template adsorbed Bi3+and Cl-at 400 °C under atmospheric conditions,and thus successfully prepared BiOCl hollow microspheres.The size of the microspheres was uniform,the diameter was about 200 nm,and the average shell thickness was about 40 nm[43].Recently,Yanet al.successfully prepared three-dimensional hierarchical BiOCl by using butterfly wings as a biological template (Figs.2 (a) and (b)).The results show that after the amination of butterfly wings and subsequent BiOCl seeds desition,two-dimensional BiOCl nanosheets can grow uniformly on the surface of the template and replicate the original fine and complex structure of butterfly wings (Fig.2 (c))[44].

Fig.2.SEM images of (a) Euploea mulciber butterfly forewing scales and (b) BiOCl-E.And (c) schematic diagram of the synthesis process,including amination treatment,BiOCl seeds deposition,and BiOCl nanosheets growth[44] (Reproduced with Permission[44].Copyright 2019,Royal Society of Chemistry).

3.4.Other Preparation Methods

In addition to the above several preparation methods,some other methods have also been used to prepare BiOCl photocatalytic materials.For instance,Henleet al.synthesized BiOCl nanoparticles using a reverse phase microemulsion containing heptane,a nonionic surfactant,and a saline solution.Reversed micelles are used as nanoscale templates for the ion precipitation process,and the micelle can be used to adjust the size of the nanoparticles from 3 nm to 22 nm[45].Yanget al.reported that BiOCl can be prepared by the ionic liquid-assisted ultrasonic method.The synthesis process is simple and the reaction conditions are quite mild.Bi(NO3)3and the ionic liquid providing the Cl source are first dispersed in a solvent,stirred at room temperature for 20 min,and ultrasonically irradiated for 10 min,then BiOCl with different morphology can be obtained by changing the reaction solvent[46].Liuet al.prepared zero-dimensional BiOCl nanoparticles with about 5 nm to 10 nm via in-situ chemical conversion by using Bi4Ti3O12nanosheets as substrates and selected HCl as the Cl source.The reaction conditions are simple and only need to be stirred at room temperature for a certain period of time[47].

3.5.Growth Mechanism

During the liquid phase preparation,the formation of semiconductor nanocrystals typically involves the nucleation,growth,and assembly process[48].Understanding the nucleation,growth,and assembly process of nanocrystals is a prerequisite for the controlled synthesis of BiOCl photocatalytic materials with the uniform size,morphology,and unique structure.When BiOCl is prepared,the Bi source and Cl source are first dissolved in an aqueous solution to produce Bi3+and Cl-,and then Bi3+is hydrolyzed to form an intermediate substance (Bi2O2)2+.Subsequently,the positively charged (Bi2O2)2+and the negatively charged Cl-form a large number of tiny Cl-Bi-O-Bi-Cl nuclei under the action of the Coulomb force,wherein Bi and O are connected by a covalent bond.Since the nucleus spontaneously reduces the surface energy,Cl-Bi-O-Bi-Cl grows in a direction perpendicular to thec-axis,forming a [Cl-Bi-O-Bi-Cl] layer by a weak van der Waals force.Further extending the reaction time,the[Cl-Bi-O-Bi-Cl] layers are stacked on each other to produce various two-dimensional structures.Finally,these twodimensional structures are assembled into various nanostructures by different assembly processes,such as the oriented attachment,Kirkendall diffusion,and Ostwald ripening[49].

For instance,Zhanget al.successfully prepared three-dimensional hierarchical BiOCl microspheres by controlling the rate of nucleation formation using ethylene glycol as the reaction solvent.During the growth process,Bi(NO3)3is firstly combined with ethylene glycol to obtain alcoholate.This complex can effectively slow the release of Bi3+in the solution and reduce the growth rate of the [Cl-Bi-O-Bi-Cl] nucleus.This facilitates the formation of twodimensional ultra-thin structures mediated by the Ostwald ripening,and then assembles into microspheres composed of two-dimensional nanostructures under the large viscosity of ethylene glycol and the Kirkendall diffusion[50].In addition,Zhanget al.synthesized a BiOCl microstructure with an adjustable morphology from nanosheets to hollow microspheres via hydrolyzing the BiCl3precursor by adding citric acid and polyvinylpyrrolidone (PVP) to a solution of water and ethanol (Fig.3).The [C6O7H5]3-produced by citric acid hydrolysis chelates with Bi3+to form [C6O7H5]3-Bi3+,which also controls the growth rate of nucleation.As a structuredirecting agent,PVP plays an important role in the assembly of BiOCl nanosheets into the hierarchical nested structures and hollow structures[51].

4.Strategies for Performance Enhancement

For a semiconductor photocatalyst,the photogenerated charge separation efficiency and light absorption capacity are two main aspects which determine the photocatalytic performance[52],[53].Among all the bismuth oxyhalide materials,BiOCl exhibits the best photocatalytic activity under UV light,even higher than commercial P25 for photocatalytic degradation of dyes[54].Although BiOCl has a relatively good charge separation property,the ubiquitous recombination of electrons and holes leads to a very small amount of electrons or holes that can participate in a catalytic reaction.Moreover,BiOCl has a wide bandgap and can only respond to UV light,resulting in low light utilization efficiency.Therefore,the photocatalytic performance of BiOCl is far from meeting the needs of practical applications.In order to enhance the photocatalytic performance of BiOCl,researchers have developed a variety of methods to adjust the photogenerated charge separation efficiency and light absorption capacity.As shown in Table 1,these strategies include doping,creating vacancies,regulating the internal electric field,supporting co-catalysts,constructing composites with semiconductors,and using sensitizers.The details are shown as the following.

Fig.3.Formation of different BiOCl nano/microstructures under various concentrations of citric acid[51] (Reproduced with Permission[51].Copyright 2012,American Chemical Society).

Table 1:Summary of modulating photoactivity strategies,preparation methods,and photocatalytic activity of BiOCl

4.1.Doping

Doping is one of the most widely used strategies for enhancing the photocatalytic activity of semiconductor materials due to its role in regulating the optical properties and charge dynamics of materials.It has been reported that doping or co-doping of elements,such as Co,Fe,La,Zn,C,or S,forms impurity levels in BiOCl,causing the absorption edge to red shift[55]-[60].Compared with pure BiOCl,the doped BiOCl will have a more broader light absorption range,thereby exhibiting the enhanced visible light photocatalytic activity.However,the high symmetry characteristics of the BiOCl layer structure may cause heteroatomic doping to enhance its thermal instability,increase charge trapping points,and reduce the redox ability[61].

Compared with heteroatom doping,since BiOCl,BiOBr,and BiOI have the similar layer structure and atomic arrangement,the same group of halogen atoms is more favorable for entering the lattice of BiOCl and replacing the Cl atoms therein.Therefore,the researchers synthesized a number of different structures of Br doped BiOCl and I doped BiOCl[62]-[67].For instance,Liuet al.synthesized BiOCl nanoplates with different I doping concentrations by the hydrothermal method and investigated the degradation performance of RhB and tetracycline hydrochloride under visible light.The results show that I doping not only broadens the light absorption range of BiOCl nanoplates,but also enhances the separation ability of the photogenerated charge (Figs.4 (a) and (b)).Compared with pure BiOCl,the doped BiOCl significantly improved the photocatalytic degradation effici ency of RhB and tetracycline hydrochloride (Figs.4 (c) and (d)).The rate constant of the optimal doped BiOCl for the RhB degradation can reach 5.3 times than that of pure BiOCl[42].On the basis of results,the enhanced photocatalytic degradation efficiency can be attributed to the improved the spatial carrier separation ability (Fig.4 (e)).

Fig.4.Charge dynamics and degradation efficiency of doped BiOCl:(a) transient photocurrent responses of BOC,BOC-1,BOC-2,and BOC-3 normalized by light absorption and (b) calculated average lifetime of BOC,BOC-1,BOC-2,and BOC-3.The degradation efficiency of (c) RhB and (d) tetracycline hydrochloride over BOC,BOC-1,BOC-2,and BOC-3 under visible light irradiation.And (e) schematic illustration of enhanced spatial carrier separation[42] (Reproduced with Permission[42].Copyright 2017,Royal Society of Chemistry).

4.2.Creating Oxygen Vacancies

Photocatalytic reactions usually occur on the surface of materials,so the photocatalytic performance is closely related to the surface properties of materials[68]-[70].Creating oxygen vacancies is considered to be an important method to modify the surface properties of materials,and there are two main characteristics in regulating the photocatalytic performance of BiOCl[71]-[73].One is that oxygen vacancies can modulate the electronic properties of BiOCl,thereby enhancing light absorption and charge transfer.For instance,Guanet al.synthesized a BiOCl ultrathin nanosheet with the thickness of 2.7 nm by the solvothermal method and found that the main defect of this ultrathin nanosheet is triple vacancy associates (Fig.5 (a)).The experimental results show that the triple vacancy associates with BiOCl ultrathin nanosheets can not only reduce the bandgap,enhance the light absorption range,but also effectively reduce the recombination of electron-hole pairs.Therefore,the ultrathin BiOCl nanosheet showed the obvious superior photodecomposition performance over RhB (Fig.5 (b))[74].Cuiet al.synthesized a BiOCl nanosheet with a large number of oxygen vacancies by the solvothermal method,and studied the relationship between oxygen vacancies and photo-oxidation properties under visible light irradiation.Theoretical and experimental results show that after introducing oxygen vacancies,a new defect energy level appears in the energy band,which can extend the light absorption range from UV to visible light,so that BiOCl nanosheets with oxygen vacancies exhibit visible-light-driven photocatalytic activity towards oxygen evolution (Fig.5 (c)).In addition,BiOCl with abundant oxygen vacancies exhibits a higher visible light photocurrent response and more efficient photogenerated charge separation and transfer than BiOCl with a small number of oxygen vacancies(Figs.5 (d) and (e))[75].

Fig.5.Schematic representation of vacancies in BiOCl and degradation efficiency,electronic properties,and charge dynamics of BiOCl with oxygen vacancies.Schematic representations of trapped positrons of (a) defect andvacancy associates,respectively,(b) comparison of photodecomposition of RhB with ultrathin BiOCl nanosheets and BiOCl nanoplates under the simulated solar irradiation[74] (Reproduced with Permission[74].Copyright 2013,American Chemical Society),(c) schematic illustration of the band structure of OV-rich/poor BOC and pure BOC,(d) surface photovoltage spectrum of OV rich/poor BOC and pure BOC (current density transients of OV-rich/poor BOC and pure BOC under visible-light irradiation are shown in the inset),and (e) the Nyquist impedance plots of OV-rich/poor BOC and pure BOC[75] (Reproduced with Permission[75].Copyright 2018,Royal Society of Chemistry).

The second role of oxygen vacancies is to alter the adsorption and activation of molecules on the surface of BiOCl.For instance,Zhaoet al.created oxygen vacancies on the (001) and (010) facets of BiOCl by using UV radiation and studied the effect of the surface structure on activated molecular oxygen.The results show that the created oxygen vacancies enhance the adsorption of O2,in which the (001) facet tends to reduce O2toby one-electron transfer and the (010) facet tends to formby two-electron transfer[76].Using the high oxygen density characteristics of the BiOCl(001) facet,Liet al.created the oxygen vacancies on the (001) facet by microwave irradiation,resulting in the selective nucleation and growth of Ag on BiOCl nanosheets.Selective deposition of Ag by oxygen vacancies has tighter contact than random free deposition,so Ag selectively deposited BiOCl nanosheets have better performance for the reduction of Cr(VI) and the oxidation of sodium pentachlorophenolate[77].

4.3.ReguIating InternaI EIectric FieId

According to the charge separation property of BiOCl,its unique layer structure is favorable for inducing the internal electrostatic field along the [001] direction,which is beneficial to enhance the photocatalytic performance.Taking advantage of the internal electric field,Jianget al.optimized the photoreactivity of BiOCl.The degradation results showed that the BiOCl nanosheets exposed to the (001) surface had higher activity for photocatalytic degradation of salicylic acid under UV light than the exposed (010) surface BiOCl nanosheets because the photogenerated charge has more efficient separation and transfer in the [001] direction than the [010] direction under the action of the internal electric field.Photocurrent and photoluminescence spectroscopy results show that the exposed (001) surface BiOCl nanosheets can more effectively utilize the advantages of internal electric fields[78].Later,Liet al.found that the intensity of the internal electric field is related to the exposure ratio of the (001) facet by measuring the surface voltage and charge density.The larger the exposure ratio of the (001) facet,the stronger the internal electric field and the higher the efficiency of charge separation and transfer.Regulating the internal electric field of BiOCl controlled by the crystal facet design provides a new way to improve the photocatalytic performance[79].

4.4.Supporting Co-CataIyst

Supporting a co-catalyst on a semiconductor photocatalyst serves three main functions in the photocatalytic performance improvement,including enhancing the charge separation efficiency,enhancing light absorption,and providing reactive sites[80].For instance,Yuet al.prepared a series of noble metal (Rh,Pd,Pt)/BiOX(Cl,Br,I)composite photocatalysts and discussed the effect of noble metal loading on the photocatalytic performance of BiOXin degrading acidic orange II under visible light and UV light.The results show that the enhanced photocatalytic performance is mainly attributed to the enhancement of the visible light absorption intensity of BiOXand the separation rate of photogenerated electron-hole pairs after loading the noble metal[81].Recently,Liet al.studied the selective catalytic oxidation of aromatic alcohols after loading a small amount of Pd nanoparticles and Au nanoparticles on BiOCl ultrathin nanosheets by photodeposition.The results show that the oxidation of aromatic alcohol to aromatic aldehyde is the most efficient after BiOCl is loaded with Pd nanoparticles (Fig.6 (a)).After a series of experimental explorations and theoretical calculations,it is confirmed that the electron coupling between the Pd and BiOCl(001) surfaces is beneficial to promote the separation and transfer of the photogenerated charge,and the surface of Pd nanoparticles can promote the adsorption and activation of alcohol molecules (Figs.6 (b)and (c))[82].

Fig.6.Functional specifications of the co-catalyst for the BiOCl photocatalyst:(a) results for the selective oxidation of benzyl alcohol to benzaldehyde in the presence of different photocatalysts,(b) DFT-calculated adsorption energy of O2 and benzyl-alcohol molecules respectively on the sites of Pd,Au,and Vo,(c) band diagram and charge transfer in the Pd-BiOCl photocatalyst[82] (Reproduced with Permission[82].Copyright 2018,Royal Society of Chemistry),(d) schematic illustration of Ag-(001)BiOCl(001)-PdOx,Ag-(110)BiOCl(110)-PdOx,Ag-(001)BiOCl(110)-PdOx,and Ag-(110)BiOCl(001)-PdOx hybrid structures,(e) average O2 evolution rates with BiOCl and Ag-BiOCl-PdOx as photocatalysts under visible light(400 nm＜λ＜780 nm) irradiation,and schematic illustrating the facet-dependent interfacial hole transfer ability resulted from the different thicknesses of contact barrier layer on (f) Ag-BiOCl and (g) BiOCl-PdOx interfaces[83] (Reproduced with Permission[83].Copyright 2017,John Wiley & Sons,Inc.).

It is worth noting that the load position is also an important parameter affecting the catalytic performance when the co-catalyst is supported on the semiconductor photocatalyst.Baiet al.selectively supported Ag and PbOxcocatalysts on the surfaces of BiOCl(001) and BiOCl(110) to construct four different contact interface composites,including Ag-(001)BiOCl(001)-PdOx, Ag-(001)BiOCl(110)-PdOx, Ag-(110)BiOCl(001)-PdOx, and Ag-(110)BiOCl(110)-PdOxcomposite materials,and compared their photocatalytic oxygen evolution performance(Fig.6 (d)).The catalytic results show that the average oxygen evolution rates of Ag-(110)BiOCl(110)-PdOxare 5.9 times,1.9 times,and 1.6 times than that of Ag-(001)BiOCl(001)-PdOx,Ag-(001)BiOCl(110)-PdOx,and Ag-(110)BiOCl(001)-PdOx,respectively (Fig.6 (e)).The charge kinetic analysis indicates that the ability of Ag-(110)BiOCl and BiOCl(110)-PdOxto transfer holes at the interface is better than that of Ag-(001)BiOCl and BiOCl(001)-PdOx.The first possible reason is that the thinner contact barrier layer based on the BiOCl(110)interface leads to the stronger electron coupling ability,and the second is that the distance from the hole to the reaction site is shorter after loading Ag and PdOxon the BiOCl(110) plane (Figs.6 (f) and (g))[83].

4.5.Constructing Composites with Semiconductors

Combining other semiconductor materials with BiOCl to form a composite is another common method for enhancing the photocatalytic performance of BiOCl.On one hand,the photogenerated charge can be directed in the direction of the electric field at the interface of the composite to promote the separation of photogenerated electrons and holes.On the other hand,when BiOCl is combined with the narrow bandgap semiconductor,the absorption edge of the composite photocatalyst can be red-shifted,and the utilization efficiency of visible light can be improved.At present,it has been reported that a plurality of semiconductor materials are combined with BiOCl to construct a composite to modulate the photocatalytic performance of BiOCl[84],[85].For instance,Liuet al.first synthesized BiOCl nanoplates by the solvothermal method using Bi(NO3)3as the Bi source,and then synthesized the BiOCl/Bi2S3composite using the synthesized BiOCl nanoplates as the substrate and thioacetamide as the sulfur source by the room temperature ion exchange method.The UV-visible diffuse reflectance spectroscopy results show that the absorption edge of the composite material is red-shifted after the introduction of Bi2S3on BiOCl,and the absorption intensity in the visible region is also enhanced.Photoelectrochemical measurement results show that photogenerated electrons and holes are effectively separated after the formation of the composites.It was used for the degradation of methylene blue,the performance of the composite was obviously enhanced.The degradation rate constant of the optimal composite sample under visible light was 25.8 times than that of pure BiOCl[86].

When BiOCl is combined with other materials,the combination of different crystal faces also affects the charge transfer at the interface and the enhancement of the photocatalytic performance.For instance,Liet al.studied two kinds of g-C3N4/BiOCl(001) and g-C3N4/BiOCl(010) composite photocatalysts with different contact interfaces by loading g-C3N4nanoparticles onto BiOCl nanosheets with different exposed surfaces.The relationship between the orientation of the crystal faces in the nanosheets and the charge separation and movement behavior of the materials,and the visible light catalytic activity of the two composites were evaluated.The g-C3N4/BOC(010)photocatalysts exhibited superior photocatalytic performance to g-C3N4/BOC(001) in MO degradation (Fig.7 (a)).Although the photogenerated electrons could effectively migrate from CB of n-C3N4to CB of BiOCl in both g-C3N4/BiOCl(001) and g-C3N4/BiOCl(010) composites (Fig.7 (b)),the different exposed faces in the g-C3N4/BiOCl composite can induce the migration of photogenerated electrons in different ways.Under the action of the internal electric field,the distance of electrons injected into BiOCl(010) moving to the surface reaction site is shorter than that of electrons injected into BiOCl(001),resulting in the lower loss of electrons before participating in the reduction reaction (Fig.7 (c)).Therefore,the g-C3N4/BiOCl(010) composite has higher photocatalytic performance than the g-C3N4/BiOCl(001) composite[87].By studying the properties of two composite photocatalysts BiOI(001)/BiOCl(001)and BiOI(001)/BiOCl(010) with different interfacial structures and combining theoretical calculation results,Sunet al.also confirmed the critical role of the interfacial structure in composites for enhancing the catalytic performance[88].

Fig.7.Comparison of photocatalytic performance and charge transfer in ng-CN/BOC(001) and ng-CN/BOC(010)photocatalysts:(a) visible-light-induced photocatalytic degradation of MO over samples containing different proportions of ng-CN,(b) band alignments in ng-CN/BOC(001) and ng-CN/BOC(010) heterojunction photocatalysts,and (c) proposed mechanism for photocatalytic reactions occurring on ng-CN/BOC(001) and ng-CN/BOC(010) heterojunction photocatalysts[87] (Reproduced with Permission[87].Copyright 2015,Royal Society of Chemistry).

In addition,the dimension of the components in the composite also affects the charge transfer at the interface and the photocatalytic performance.At present,many studies have confirmed this by adjusting the size and morphology of the components in the composite[89]-[91].For example,a face-contacted two-dimensional/twodimensional composite has a larger contact area,more charge transport channels,and a faster charge transfer rate than a two-dimensional/zero-dimensional composite with a point contact[92].Using the unique characteristics of the two-dimensional/two-dimensional structure,Liuet al.prepared a BiOCl/C3N4two-dimensional layered composite by a simple calcination method and used it for the photocatalytic degradation of MO under visible light.The results show that the (001) facet of the BiOCl nanoplate and the (002) facet of the C3N4nanosheet are closely attached together in the prepared BiOCl/C3N4two-dimensional layered composite,so that the two-dimensional layered structure has a large contact surface.Moreover,the introduction of C3N4allows the light absorption range of the composite material to extend from the UV region to the visible region.Therefore,the prepared BiOCl/C3N4two-dimensional layered composite has good photocatalytic degradation ability to MO[93].

4.6.Using Sensitizers

BiOCl has a wide bandgap and cannot utilize visible light.A sensitizer can be used to extend the light absorption range from the UV region to the visible region.Under visible radiation,the sensitizer is excited and the excited state electrons are rapidly transferred to the conduction band of BiOCl.Among all the sensitizers,organometallic complex sensitizers,such as copper complexes,ruthenium complexes,and zinc complexes,are most commonly used[94],[95].For instance,Zhanget al.prepared a BiOCl/CuPc composite photocatalyst by using the copper complex copper phthalocyanine (CuPc) to modify BiOCl.BiOCl/CuPc has enhanced the photocurrent density and photocatalytic activity compared with BiOCl.Under simulated sunlight,the photogenerated electrons on the sensitizer CuPc are rapidly transferred to the conduction band of BiOCl.At the same time,RhB in the dye wastewater provides photogenerated holes to participate in the oxygen evolution reaction[96].

In addition to the above-described sensitizer immobilized on the surface of BiOCl by physical adsorption or chemisorption,the sensitizer-modified BiOCl can be directly obtained by hydrolyzing the complex containing a Bi source and a Cl source.For example,Yeet al.used Bin(Tu)xCl3nas a sensitizer and prepared a sensitizer-modified BiOCl nanosheet by hydrolysis.Since the conduction band potential of Bin(Tu)xCl3nis higher than that of BiOCl,the excited electrons of Bin(Tu)xCl3nare injected into the conduction band of BiOCl under visible light irradiation,and react with oxygen to form superoxide radicals.For the degradation of RhB,the photocatalytic activity of sensitizermodified BiOCl was 13 times and 112 times than that of pure BiOCl and P25,respectively[97].

5.ConcIusions and Prospects

BiOCl is a kind of photocatalytic material with good charge separation characteristics.Its preparation method is simple and diverse,and its raw materials are widely available and inexpensive.In recent years,many researchers have carried out a lot of studies on the preparation of BiOCl,the regulation of the photocatalytic performance and related theories,and made important progress.Researchers have improved the light absorption capacity of BiOCl and promoted the separation and transfer of the photogenerated charge by doping,creating vacancies,regulating internal electric fields,supporting co-catalysts,constructing composites with semiconductors,and using sensitizers,thereby the photocatalytic performance of BiOCl is subsequently enhanced.

Although some progress has been made in the research of BiOCl photocatalytic materials,there are still some problems that need to be solved in order to advance its practical application in the energy and environmental fields.First,due to the unique layer structure,the currently synthesized BiOCl is mainly exposed the (001) facet.Although BiOCl exposed to other surfaces is also synthesized,the adsorption and activation of reactants associated with the high energy surface,the desorption of products,and the transfer of the photogenerated charge between the surface and the reactants are also important for enhancing the photocatalytic performance.Therefore,it is necessary to explore a method for the controlled synthesis of BiOCl that exposes other high energy surfaces.Secondly,the existing performance control methods mainly enhance the photocatalytic performance by enhancing the light absorption and charge separation efficiency,but the detailed transfer path of the photogenerated charge is still unclear.Therefore,it is necessary to develop advanced characterization methods to clarify the transfer path of the photogenerated charge in the BiOCl layered structure and the transfer channels in the composite,especially at the contact interface,which is beneficial to guide the design and synthesis of high performance BiOCl photocatalytic materials.Finally,the current photocatalytic application of BiOCl is mainly focused on the degradation of organic pollutants.It is necessary to expand the range of photocatalysis applications,such as splitting water,reducing carbon dioxide,and synthesizing high value-added organic matter.