Development and Application of TiO 2 Nanoparticles Coupled with Silver Halide

Titaniumdioxide (TiO 2 ) is proposed to be effective photocatalyst forwastewater treatment, air purification, and self-cleaning ability, because of its strong oxidation and superhydrophilicity. In order to conquer the limits of TiO 2 , a variety of methods have been used.This paper presents a critical review of novel research and achievements in the modification of TiO 2 nanoparticles with silver halide (AgX, X = Cl, Br, I), which aims at enhancing the visible light absorption and photosensitivity. Herein we study the synthesis, physical and chemical properties, and the mechanism of this composite photocatalyst.


Introduction
Water is one of the most essential substances for all life.However, an increasing number of organic pollutants are being discharged into aqueous environment along with wastewater.Various treatment techniques and processes have been used to remove the organic pollutants from wastewater.Among all the approaches proposed, the photocatalysis technology for the treatment of contaminated water has attracted significant attention in the past few decades, because of its high stability, low cost, and nontoxicity toward both humans and the environment [1].Among a variety of photocatalysts, TiO 2 is considered as one of the most ideal semiconductors for photocatalyst.But there are some drawbacks which have greatly limited the application of TiO 2 : one is the inefficient utilization of solar energy, and the other is the rapid recombination rate of photogenerated electron-hole pairs [2].
In response to these deficiencies, several approaches for TiO 2 modification have been developed: doping by metals (noble metals, transition metals, lanthanide metals, alkaline and alkaline earth metals, cadmium sulphide, etc.) and nonmetals (nitrogen, fluorine, sulfur, carbon, etc.) and modification with dye-sensitization and composite semiconductors.
After the phenomenon of surface plasmon resonance (SPR) was discovered, a novel photocatalyst named plasmonic photocatalyst was developed, which was a composite photocatalyst composed of noble-metal nanoparticles and a polar semiconductor, such as silver halide (AgX).In the photographic process, an electron is liberated by photon adsorption and electron-hole pair creation.The liberated electron will combine with an Ag + cation to form an Ag 0 atom.Continuous irradiation will bring limitless photons, which will be absorbed by the AgX particles to form a cluster of Ag 0 atoms, and this phenomenon is the very basis for chemical photography.Considering its instability under sunlight, AgX is scarcely ever used as a photocatalyst.In the study of the photocatalyzed H 2 production from a CH 3 OH aqueous solution in the presence of AgBr dispersed on a silica support, Kakuta et al. observed that Ag 0 atoms take place on the surface of AgBr particles and AgBr particles are not destroyed under UV illumination [3].This research leads to the synthesis of the first visible light plasmonic photocatalyst Ag/AgCl.Similarly, the combination of AgX and TiO 2 can also have this kind of property.The Ag/AgCl ought to have both advantages, which can be excited under visible light irradiation and can be stable when illuminated.

TiO 2 Coupled with Silver Halide
2.1.AgCl/TiO 2 .With a direct band gap of 5.6 eV and an indirect band gap of 3.25 eV, silver chloride (AgCl) is widely acknowledged as photosensitive material and is one of the source materials of photographic films [4].Since 2008, Wang et al. reported the synthesis of a highly efficient and stable plasmonic photocatalyst Ag/AgCl under visible light irradiation through treating Ag 2 MoO 4 with HCl to form AgCl and subsequently reducing some Ag + to Ag 0 nanoparticles on the AgCl surface by illumination.Taisuke et al. prepared Ag(core)-AgCl(shell) composite crystal-loaded TiO 2 by a two-step method involving electrochemical reduction/electrochemical oxidation.They showed that an energy gap of more than 0.7 eV is needed for efficient one-directional interfacial electron transfer from the CB (conducting band) edge of TiO 2 to the redox potential of Ag/AgCl [5].It was reported that TiO 2 nanotubes were first synthesized by hydrothermal method.Since then, they had been widely used in photocatalysis [6].Yu et al. fabricated visible-lightdriven plasmonic photocatalyst Ag/AgCl/TiO 2 nanotubes (NTs) by AgCl nanoparticles doping into the self-organized TiO 2 NTs which were grown by anodization of Ti foils in an aqueous solution.They also revealed the reason why the photoexcited electrons at Ag NPs can inject into the TiO 2 conduction band is that a Schottky barrier is formed at the Ag/TiO 2 interface and the electric field in the space-charge layer was able to promote transport of excited electrons at the Ag surface to the walls of TiO 2 NTs [7].Wen and Ding used AgCl nanoparticles to load onto the TiO 2 nanotubes by a precipitation reaction, and some AgCl nanoparticles were reduced to Ag particles under irradiation.The product Ag@AgCl/TiO 2 nanotubes exhibited high photocatalytic activity due to the surface plasmon resonance (SPR) effect [8].Two-step method consisting of electrochemical anodization process and electrodeposition process was used to prepare the Ag/AgCl/TiO 2 nanotube arrays.It was found that the major reactive species produced in situ are h + , • OH, and O 2− via the degradation of microcystin LR.The results showed that the degradation reaction was highly promoted in NaCl electrolyte, as well as in an acid solution [9].
The composite also can be made into thin films.Zhou et al. fabricated the Ag/AgCl/TiO 2 nanocomposite thin films by a sol-gel and photochemical reduction method.The samples show high visible light photocatalytic activity for degradation of 4-chlorophenol aqueous solution and recycling stability [10].
Chemical doping of TiO 2 with nonmetallic elements (C, N, B, S, or F) to extend their visible absorbance is a commonly used method [11,12].A report from Yan et al. chose C and F as codopants, which resulted in a red-shift of the absorption edge to 510 nm from 390 nm.Then AgCl was loaded to these codoped TiO 2 to enhance its photocatalytic property.The brief mechanism is proposed to the transformation of holes from C, F-codoped TiO 2 to the AgCl phase [13].
Compared with the commercial P25 photocatalyst, Sangchaya et al. [14] and their team found that TiO 2 -AgCl powders which were synthesized by a sol-gel method were more efficient under UV and visible light irradiation.In the test of photocatalytic degradation of methyl blue (MB), they found that a photocatalytic reaction rate () can be written as  = 1.00 − where  is a rate constant and  is a treatment time in hour.The result showed that  value of TiO 2 -AgCl powders was 0.47 which was superior to P25's 0.12.This phenomenon indicates the photoinduced electron trapping effect of AgCl, higher concentration of OH radicals on TiO 2 -AgCl surface, and their smaller crystallite size.And the degradation of MB with TiO 2 -AgCl is 50% after 6 h visible light irradiation, while that of P25 is 32%.In some ways, coupling with AgCl can overcome the two major defects of TiO 2 .
Updating the synthesis methods or changing conditions of the degradation progress can not only increase the photocatalytic activity, but also reveal the mechanism, although there are still some details in the reaction that remain unclear.

AgBr/TiO
2 .Previous research showed that silver bromide (AgBr) with a band gap of 2.6 eV is an inorganic photosensitive semiconductor material, which will have high photographic sensitivity under visible light irradiation and can be used to modify TiO 2 to have visible light activity [15].
Hu et al. prepared Ag/AgBr/TiO 2 nanocomposites by the deposition-precipitation method.The catalyst showed high visible light photoexcited efficiency when investigated for the decomposition of azodyes and killing of Escherichia coli [16].Zang and Farnood synthesized the photocatalysts with several levels of AgBr contents on TiO 2 also by the deposition-precipitation method.In the investigation of methyl orange's (MO's) degradation, they fund that at higher levels of AgBr content (>9%) the degradation of MO was dependent on the incident light intensity, while at lower contents the degradation rate increased with the increase of incident light intensity and finally reached a plateau level [17].Similarly, Liu et al. prepared a series of AgBr/TiO 2 visible photocatalysts with different mass ratio of  AgNO 3 / TiO 2 using Ti(OC 4 H 9 ) 4 , KBr, and AgNO 3 as precursors.The result showed that the homogeneously dispersed coupled heterojunction microstructures were formed while  AgNO 3 / TiO 2 = 3.35 which have stronger absorption in the whole visible region and are beneficial to the further improvement of photocatalysis [18].
Ag-AgBr/TiO 2 composites prepared by a sol-gel method followed by photoreduction showed intrinsic antibacterial activities against Escherichia coli in the dark.Under visible light irradiation, inactivation of E. coli over these Ag-AgBr/TiO 2 composites was attributed to both their photocatalytic disinfection activities and intrinsic antibacterial properties [19].Wang et al. synthesized Ag/AgBr/TiO 2 via sol-gel (Ag/AgBr/TiO 2 ) route and solvothermal route (S-Ag/AgBr/TiO 2 ), respectively.The test of ibuprofen degradation and mineralization revealed that Ag/AgBr/TiO 2 's photocatalysis is a little better than S-Ag/AgBr/TiO 2 , and they both were much superior to the single-component (TiO 2 ) and two-component (Ag/TiO 2 , Ag/AgBr) systems [20].The charge transfer mechanisms in the composites have not been revealed clearly, so Wang et al. developed a simple microemulsion-like chemical precipitation method to construct AgBr/TiO 2 composite.The most difference of the method is the dual role of Br − in the synthetic process, as linkers between cetyltrimethyl ammonium cation surfactants and nanocrystalline anatase TiO 2 in the acidic condition  and as bromine sources to directly produce nanocrystalline AgBr on the surfaces of TiO 2 by chemical precipitation.This work found that the Br − in crystal could effectively capture photogenerated holes, consumedly favoring charge separation, whose factors are responsible for the high activity and excellent stability of the AgBr-TiO 2 [21].Graphene is a single layer which has many outstanding properties, such as high theoretical specific surface area, superior mechanical, chemical stabilities, and so forth.Most recently, Wang et al. synthesized a new photocatalytic nanocomposite, Ag-AgBr/TiO 2 supported on reduced graphene oxide (Ag-AgBr/TiO 2 /RGO).The four-component nanocomposite (Ag-AgBr/TiO 2 /RGO) exhibited a much higher photocatalytic activity for the degradation of penicillin G (PG) under white light-emitting diode (LED-W) irradiation, compared with the single-component (TiO 2 ), two-component (Ag-AgBr, Ag/TiO2, and TiO 2 /RGO), and three-component (Ag-AgBr/RGO, Ag-AgBr/TiO 2 and Ag/TiO 2 /RGO) nanocomposites [22].
To well understand the properties of photogenerated charges and their effects on the photocatalytic performance, a detailed mechanism schematic of transfer and separation of photogenerated charges is presented, as shown in the Figure 1.In the schematic, the energy levels of conduction band (CB) bottom and valence band (VB) top of AgBr and TiO 2 , along with the affinity level of O 2 , are carefully labeled, which are important to reasonably reveal the transfer and separation processes of photogenerated charges.According to [23], the  VB levels for TiO 2 and AgBr stand at 2.86 eV and 1.50 eV via the standard H 2 electrode (SHE), respectively, while their ECB levels are positioned at −0.32 eV and −1.06 eV.
Moreover, the valence band potential of a semiconductor at the point of zero charge also can be calculated by the following empirical equation  VB = X - + 0.5 [24], in which  VB is the VB top potential via the SHE, and the CB bottom potential can be determined by  CB =  VB − .Thus, the  VB levels for TiO 2 and AgBr stand at 2.09 eV and 0.42 eV, respectively, while their ECB levels are positioned at −1.09 eV and −2.14 eV.Based on the levels of  VB and  CB by calculating and obtaining from the references, it is confirmed that the levels of  VB and  CB of AgBr are higher than those of TiO 2 .In general, the O 2 affinity level (O 2 /O 2− ) stands at about 0 eV (−0.046 eV versus SHE) [25].Thus, it is assumed that the photogenerated electrons at the  CB of TiO 2 and AgBr would be effectively captured by the adsorbed O 2 , leading to the SPS occurrence at the DRS edge.
In order to investigate the mechanism of AgBr/TiO 2 , Velmurugan and Swaminathan [26] use the PL emission spectra to analyze the samples.It can be discovered that the positions of the peaks are similar, when comparing AgBr/TiO 2 with pure TiO 2 .However, the PL intensities of TiO 2 are higher than AgBr/TiO 2 .The phenomenon of the reduction emission intensity of AgBr/TiO 2 implies that the recombination of charge carriers is effectively inhibited by AgBr nanoparticles.In addition, it also can be proved by the photocatalytic activity test.From the test, it can be found that in the presence of AgBr/TiO 2 the degradation of RR 120 reached 98% in 40 min under solar light, while 60.1% was observed with TiO 2 .
In order to promote the photocatalytic activity or stability, an appropriate synthesis approach and the ration of the different component are all the key aspects.Consequently, optimization of these two matters in the final results.And if there is a selected substrate to support the material, it will be more efficient under irradiation.

AgI/TiO 2 .
As a member of the silver halides, silver iodide (AgI) has three different phases: at room temperature, it is usually a mixture of a hexagonal-AgI phase and a cubic -AgI phase; above 147 ∘ C, -AgI undergoes a first order phase transition into the -AgI phase.When at ambient conditions, the AgI with two distinct phases has a direct band gap of about 2.9 eV [27].Hu et al. prepared photocatalyst Ag/TiO 2 by the deposition-precipitation method that showed high effective in killing Escherichia coli and Staphylococcus aureus.The Ag/TiO 2 also revealed that the photocatalytic degradation of the cell structure caused the cell death by TEM, potassium ion leakage, lipid peroxidation, and FT-IR measurements [28].Li et al. synthesized a core/shell/shell nanostructured AgI/Ag-I 2 /TiO 2 by a feasible approach with AgNO 3 , LiI, and Ti(OBu) 4 .The photocatalytic tests show that the prepared photocatalyst exhibited 4 and 6 times higher than P25 supported AgI, when used to degrade crystal violet and 4-chlorophenol [29].Wang and Liu synthesized Ag/AgI supported TiO 2 acid corrosion nanobelts plasma photocatalyst.The test of degradation of methyl orange (MO) showed the enhancement of the visible light activity, because of the larger area of the photocatalyst after acid corrosion [30].Shila's investigation indicates that modified TiO 2 with AgI also enhanced the adsorption of methyl violet.What is more, they showed the modification of TiO 2 nanoparticles with AgI has no effect on the rate of adsorption but it results in a stable adsorption and higher adsorption capacity.The kinetic test reveals that methyl violet (MV) adsorption onto AgI/TiO 2 followed pseudo-first-order model; however, adsorption of MV onto TiO 2 followed simplified Langmurian model [31].Since direct precipitation of AgI on the supports of TiO 2 would limit the interfacial charge transfer, a three-dimensional nanostructure with irregular shapes and unclear interfaces has been developed.An et al. fabricate a 3D AgI/TiO 2 nanophotocatalyst with an acanthosphere-like morphology composed of nanothorns, which has enhanced performance towards degradation of organic contaminants, in comparison with the conventional nanoparticles and nanotubes [32].
It has been reported that the inner electric field of heterojunction catalyst can decrease the recombination of the electron-hole pairs and prompt the photocatalytic activity of semiconductor photocatalysts [33].And composite particles with different AgX heterojunction with better photographic properties than single AgX have been studied in the field of photographic films [34].
Cao et al. prepared a composite catalyst AgI/AgCl/TiO 2 by ion exchange method.And the result indicates that when the molar percentage of AgI to initial AgCl is 20%, the composite catalyst has the maximal degradation efficiency of MO [35].
Specialized morphology of the particles may lead to specific properties, so the 3D nanoparticles with high specific surface area have an enhanced ability of photocatalysis.And multisemiconductors' common effect also may enhance the initial properties of the single material.

Conclusion and Outlook
AgX is instable when exposed to sunlight alone.And TiO 2 shows relative activity under UV light.However, AgX/TiO 2 system showed high photocatalytic activity and stability under visible light.Great efforts have been made to develop AgX/TiO 2 preparation methods in recent years.In this review, AgX/TiO 2 composite semiconductors with different morphologies, such as Ag/AgX composite, nanotube or other nanostructures, thin film fabrication, and reduced graphene oxide supported AgX/TiO 2 , were prepared to enhance their visible light activity and circulation stability through hydrothermal, ion-exchange methods or precipitation processes.
It was assumed that properties, such as stability, electronhole recombination rate, and visible light photocatalytic activity, of AgX/TiO 2 strongly depended on the preparation method used.However, the properties of AgX/TiO 2 cannot be improved by a single preparation method.Therefore, a major area of future research would be the development of AgX/TiO 2 preparation method, such as preparation of TiO 2 coupled with various types of AgX, ternary composite of AgX/TiO 2 and other semiconductors, and AgX/TiO 2 loaded on supported materials.Such materials preparation together with the development of technically application is crucial for broader scale utilization of photocatalytic systems in commercial application.