Advanced Composites and Hybrid Materials

, Volume 1, Issue 2, pp 269–299 | Cite as

Regulations of silver halide nanostructure and composites on photocatalysis

  • Yingying Fan
  • Dongxue Han
  • Zhongqian Song
  • Zhonghui Sun
  • Xiandui Dong
  • Li Niu


The silver halides and their corresponding composites would constitute remarkable photocatalytic systems not only in energy production but also in environmental remediation. The major advantage in these silver halides system is that plasma metal Ag0 species would be spontaneously generated by silver halides under light irradiation, which play dual roles of expanding light absorption range and charge carriers separation. In this tutorial review, the origin and development of silver halides, synthesis methods, construction of composite structures with other materials, photocatalytic applications, and various and controversial mechanisms are all exhaustively described.

Graphical abstract

The silver halides and their corresponding composites would constitute remarkable photocatalytic systems not only in energy production but also in environmental remediation.


Silver halide Photocatalysis Composites materials 

1 Introduction

From the past to the present, the continued population growth combing with sustainable industry development have bring about beyond controlled environment containments and avaricious power requirements. Among multiple contaminations, the water pollution government is considered as one of the critical events that intimately associate with human’s livelihood [1, 2].

Thus, urgent conductions are demanded to be applied on the treatment approaches of water pollution especially the organic dye component, which is difficult to be decomposed no matter through chemical or biological methods. In the energy demand aspects, most of the energy production is supplied by the depletion of the fossil resources, such as oil, coal and natural gas. It is well known that these fossil sources are stored limited and environment troubles would be yield following the combustion of the traditional fuels, which urgently demand for a developing sustainable energy technologies [3, 4, 5, 6, 7, 8].

Inexhaustible solar energy is the most important sustainable resource, whose annual energy output for Earth is capable for satisfying mankind’s yearly energy consumption with 5% UV, 43% visible, and 52% IR [9, 10]. For today, many applications of solar energy have been proposed, such as photoelectrochemical reaction and various solar cells [5, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27], which are all indirect utilization depending on meticulous devices. Photocatalysis is a direct conversion technique between solar and chemical energy in presence of catalyst and accelerates the photoreaction [16, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37]. The corresponding applications are mainly exhibited in photocatalytic hydrogen evolution from water splitting [38, 39, 40, 41, 42], organic contaminants degradation [43, 44, 45, 46, 47, 48, 49, 50], water disinfection [51, 52, 53, 54], CO2 photoreduction, and surface self-cleaning [55, 56, 57, 58, 59, 60, 61]. Nowadays, photocatalysis energy conversion technology based on the semiconductor with suitable bandgap has emerged as one of the saviors for the traditional power mode, which could utilize the solar energy to degrade the water pollutions and produce clean energy [62, 63, 64, 65, 66, 67]. Alien to the metal possessing a continuous electronic state, semiconductor possesses a bandgap with a void electron region, which extends from the top of filled VB to the bottom of vacant CB. Thus, when the light irradiates on the semiconductor nanoparticles, pairs of electrons and holes would be generated and have sufficient time (nanosecond) for arrival at the surface reaction before recombination owning to the existence of bandgap [68, 69]. For environment cleanup, the electrons and holes would integrate with oxygen molecules (O2) or water molecules (H2O) to form the radicals, which play a vital role in organic dyes degradation. While in clean energy aspect, mainly the electrons are delivered to the precursor reactants (such as H2O and CO2) to generate the hydrogen (H2) and hydrocarbon fuels, which are actively regarded as energy vectors for converting solar to chemical and electrical energy. The requirement for transforming the irradiation energy to hydrogen and hydrocarbon fuels could be generalized into two reasons. First, amplitude diversifications of solar irradiation, according to daily and seasonal changes, are incompatible with the successive energy supplying in life and industry demand. Thus, conversion of the solar energy into hydrogen and hydrocarbon fuels becomes necessary. Second, due to H2O and CO2 as raw materials and combustion product, hydrogen and hydrocarbon fuels are pragmatic for large-scale operations because of its sustainable properties and readily available resource.

1.1 The development origin of AgX

In 1972, Fujishima and Honda discovered that TiO2 was able to photoelectron-splitting water under light, which opened new era for photocatalysis [70]. Since then, large number of metal oxides similar to TiO2 with large bandgap has been focused as photocatalysts. As well known, although large bandgap is enable to maintain semiconductor’s stability during reaction process, low solar energy absorption (only ultraviolet light) creates poor photocatalysis efficiency. Therefore, remedies for enhancing solar energy utilization have been devoted, such as doping various non-metal or transition metal atoms into large-band semiconductor’s crystal [71, 72, 73]. Based on many reports on doping methods, even though certain promotions are detected in both irradiation absorption and catalytic ability, large distance is still required to catch up [74, 75].

The photocatalytic capability could be determined by many factors. Especially, the light utilization characteristics for controlling the charge carrier generation and the corresponding charge carriers separation, both of which are critical important toward the photoinduced chemistry efficiency. In this perspective, plasmon-enhanced photocatalyst constituting by plasmon and semiconductor is able to vividly express its superiority in these two photocatalytic enhancement aspects. First, plasmonic metal nanoparticles can act as an antenna to localize the irradiation energy and then transfers it to the semiconductor, which makes the photocatalyst a high absorption coefficient [76, 77, 78, 79]. Second, irradiation light is likely to be scattered by larger metal particles on the semiconductor surface, thus the path length of light through the semiconductor is expanded [80, 81, 82]. Hereafter, a significant proportion of scattered light tends to be trapped and absorbed by the semiconductor. Third, bulk recombination of electrons and holes in semiconductor is regularly happen due to the fact that the minority carrier distance (L) is shorter than the physical size of semiconductor structure [83, 84, 85]. As the plasmon could localize the optical confining to the near-surface region of a semiconductor, the electrons and holes generated in there only need traveling less than L to engage the reaction, which greatly minimize the recombination ratio. In addition, plasmonic metal nanoparticle is able to sensitize the semiconductor to below-band gap light through transferring its own excited electrons into the CB of semiconductor so that the photocatalysis activity of semiconductor can be apparently improved [86]. Moreover, in plasmonics, metal or even noble metal are the noumenon particles (such as Cu, Au, and Ag), which promises their resistance to decomposition. Based on these two advantages, plasmonic nanoparticles (NPs) are considered as favorable photosensitive materials for enhancing the light absorption coefficient of large bandgap semiconductors and could also maintain their stabilities [87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102]. Great quantities of works on constituting plasmonic NPs and large-bandgap semiconductors have already achieved. For instance, Cu NPs dispersed on TiO2 supports and mesoporous Au/TiO2 nanocomposite have been prepared for H2 production and wastewater treatment reactions, respectively [103, 104, 105]. However, assemble efficiency between plasmonic NPs and semiconductor is not able to get well controlled due to the external precursors introduction, leading to uncertain sensitive efficiency of plasmonic NPs on semiconductor photocatalysts. If the semiconductor photocatalysts act as the precursor to generate plasmonic NPs, then seamlessly integration between semiconductor photocatalysts and plasmonic NPs will be realized and result in high charge transport efficiency among them. Throughout all photocatalysts, only AgX semiconductors can be in situ partially converted to plasmonic Ag NPs under light irradiation constituting Ag/AgX hybrid particles with clean and well-define metal/semiconductor interfaces.

1.2 What is silver halide and its development in nanotechnology

Silver halides (AgX) are related to one kind of semiconductor crystal constituted by positively charged silver ion (Ag+) and negative halogen ions (X = Cl, Br and I) [106, 107]. The key characteristic of AgX is the photosensitivity, as photolytic silver could be formed under light exposure, which make AgX become pivotal source material in photographic film. In photographic aspects, microscopic sized AgX grains are integrated into gelation to form photographic emulsions, in which negative emulsion is built up by AgBr containing a small percentage of AgI while AgCl grains constitute the positive emulsion [108, 109]. Through certain light exposure, pairs of electron and positive hole could be liberated within AgX crystal. Due to the important role of crystal defect, positive holes likely recombining with electrons would be captured by X traps, which create higher merge opportunity for the generation of Ag0 particles by the combination of interstitial Ag+ and photoelectrons. In this way, the term of “latent image” is developed accounting for the photographic emulsion exposure [110]. However, these beneficial features of silver halide in photographic process simultaneously reflect that the silver halides are unstable under irradiation of light, which inhibit their application in the photocatalysis aspects. In 1921, Baur and Rebmann investigated the photochemical properties of AgCl in water splitting and discovered minor O2 evolution under UV irradiation [111]. In order to take insight into the oxygen evolution, Chandrasekaran and Thomas in 1983 opened a mechanism exploration work and elucidated that water oxidation by AgCl/Ag+ system under irradiation was not catalytic though a similarity between this system and photosynthetic energy storage was existed [112]. However, in 1996, one more time Calzaferri et al. carried out a photochemical oxidation of water to O2 with thin AgCl layer on SnO2-coated glass plate in small excess presence of Ag+ ions in aqueous solution under different illumination conditions [113]. This work revealed that the role of Ag+ ion was to repair the damaged AgCl species during the photochemical oxidation of water, which ensured an intact AgCl composition before and after the reaction. Simultaneously, convincing mechanism was proposed explaining the reasons of acidic condition, an excess of Cl, chlorine and oxygen formation during the water decomposition combining with a thoroughly reaction process analysis. And this mechanism indirectly indicated the photochemical oxidation of water on AgCl layers was a photocatalytic process. After that in 1999, Noriyoshi Kakuta et al. used a AgBr/SiO2 composite prepared from Schumann emulsion to generate hydrogen (H2) from CH3OH/H2O photolysis under UV illumination and this reaction was continued for 200 h without destruction of AgBr although Ag0 was observed [114]. Maybe it was the first time to confessedly recognize the silver halide as photocatalyst. In 2006, a visible-light-induced photocatalyst AgI/TiO2 was prepared by Chun Hu et al. and was found to show high efficiency for the degradation of azodyes, which based on their previous work visible-light-driven photocatalyst AgBr/TiO2 [115]. The above-mentioned is primary regarding the origin of silver halides on the photocatalysis application. From then on, tremendous works with respect to the property and morphology modification on silver halides sprang up to advance their photocatalytic performance [114, 115, 116, 117, 118, 119, 120, 121].

2 Preparation of silver halide

As AgX is composited by Ag+ and Cl, common features for synthesis are definitely existed among them, such as a precipitation reaction of Ag+ and X and ion-exchange between silver salt and HX [122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135]. The architecture and morphology of AgX, especially with high-surface-energy crystalline facets, are able to manage and define their unique photocatalytic properties [136, 137, 138, 139, 140, 141, 142, 143, 144]. And some photocatalytic performance discussed in Section 3.1 is exactly in accordance with the well-designed architectures of AgX. At present, various types of synthesis modus have been created for fabricating AgX crystals with different architectures and shapes. Table 1 listed an overview of many respective AgX architectures that prepared in various synthesis methods, including polyvinylpyrrolidone-assisted precipitation, ion-exchange, and template-direction. [1, 145, 146, 147]. Here, several synthetic approaches have been partitioned to systematically analyze the reaction process and mechanisms.
Table 1

Summary of the shapes, morphology illustrations, photocatalyst kinds, methods for synthesis and photocatalytic applications of AgX nanostructures discussed in this review

2.1 Polyvinylpyrrolidone-assisted precipitation

The most convenient way to synthesize AgX crystal is the chemical precipitation between Ag+ and X [148, 149, 150]. However, violent reaction rate of direct precipitation likely to cause particles aggregation into bulk, which wrecks the desired nano-size properties. Thus, restraint tactics should be exerted in precipitation process to obtain uniform nano or micro level particles [151]. As an excellent capping agent, polyvinylpyrrolidone (PVP) has been widely applied in morphology control of many inorganic crystals, such as metal single crystal, metallic salts and inorganic semiconductors [152, 153, 154, 155, 156, 157, 158]. For examples, tetrahedral Pt nanoparticles (NPs) stabilized by PVP has been synthesized employing H2 reduction method [159]. Applying a facile solvothermal technology in presence of PVP, controllable CdS colloidal spheres with monodisperse submicrometer have been prepared [160]. Especially in silver composition aspect, PVP has been taking as dominant morphology agent to form large-scale silver crystals with uniform distribution [1, 161]. This selectivity can be ascribed to the preferred binding tendency between PVP and Ag+, which forms PVP-Ag+ complex and plays a vital role in Ag+ stabilization. Within unite structure from PVP molecule chain, electron clouds are inclined to show partiality for polar groups (C = O) and ligands -N, which property enable interaction between Ag+ and C = O as well as N atom in the pyrrole ring. As shown in Fourier transform-infrared spectroscopy (FT-IR) spectra of PVP-Ag+ and pure PVP in Fig. 1a, the interaction between Ag+ and pyrrole N was reflected at 1388 cm−1, and the strong band generated at 2436 cm−1 was attributed to the formation of Ag+…-O = C, respecting the PVP-Ag+ configuration [162]. Due to steric effect of PVP, the chemical reaction of Ag+ and X become sluggish and controllable. With reaction occurrence, crystal diameter gradually increases and steric effect is strengthened. Finally, the PVP molecule cover layer generated on the surface of AgX crystal succeeds in prohibiting growth and agglomeration of crystal particles. More significant, each crystal face is terminated with different ion states. Some surfaces are mainly constituted with Ag+ that mass PVP molecules would attach, thus suppressed growth on these planes would happen. While other planes dominated by X would gain less influence from PVP and wanton growth is extended. Finally, a morphology management is achieved based on PVP adjustment as shown in Fig. 1b, c [162, 163].
Fig. 1

a FT-IR spectrum of the freeze-dried PVP–Ag+ hybrid precursor with RPVP/Ag = 0.05. Reproduced with permission from ref. [162]. Copyright 2014 Elsevier. b, c SEM images of Ag/AgCl crystal particles synthesized in the presence of PVP. b Reproduced with permission from ref. [164]. Copyright 2011 Royal Society of Chemistry. c Reproduced with permission from ref. [163]. Copyright 2014 Royal Society of Chemistry

Monodispersed cubic AgCl NPs were synthesized by Yugang Sun group through a typical precipitation reaction between Ag+ ions and Cl ions, in which PVP was added to assist the morphology control [6]. This approach points a vital role of PVP in AgX architecture construction and is received as popular approach to design various AgX architectures. In a similar cube-like AgCl crystal synthesis (Fig. 1b), Shaojun Dong’s group has investigated the crystal morphology changes with increasing amounts of PVP founding reduced crystal size and more structured architecture [164]. Besides the regular cube shapes, cube-tetrapod-like AgCl constructions assembled by four cubes were also synthesized through higher reaction temperatures (Fig. 1c) [61, 163, 164]. In AgBr system, hexagonal nanoplates exposed with high energy facets were fabricated using PVP as capping agent as well, first revealing facet effect on AgBr photocatalytic properties [61, 142]. On the other hand, polar-faceted AgI microplates with [143] surfaces were prepared in a facile PVP-assisted precipitation, whose archetypes demonstrated preferred diametric migration of charge carriers [143]. In these typical synthesis, Ag+ cations and X anions are dissolved into viscous solvents with addition of PVP (such as ethylene glycol, glycerol or deionized water with P-tolyl sulfonic acid), respectively, before the stages of reaction. In this process, PVP mainly serves as stabilizer of both ions, and may also complex with Ag+. Then, both solutions are injected into each other under vigorous stirring generating milky dispersions, in which several or tens nanometers sized particles would be formed. Alternatively, such processes are proceeded in certain temperatures, larger crystals under micron level would be synthesized with uniform morphologies.

2.2 Template-directed formation

Since appropriate templates are able to defined both architectures and sizes of target productions, template-directed methods have been regarded as a versatile method in designing various desired nanostructures [165, 166, 167]. Similar to other general NPs composites, template-based AgX synthesis can also be divided into two types of hard template and soft template.

2.2.1 Hard templates

The morphology of AgX nanostructures based on hard template is only affected by original template morphology and their dimensions can also be tight controlled with rigid growth along templates [168, 169]. Commonly used hard templates are anodic aluminum oxide (AAO) and water-soluble sacrificial salt-crystal-template, the latter serves as both reactants and templates during synthesis process. Anodic aluminum oxide (AAO) containing uniform and tunable pore distribution can be used to efficiently guide the synthesis of AgCl nanoparticle nanowires and various AgI nanowires [170, 171, 172, 173, 174], in which electrochemical deposition and electroless precipitation approaches play major roles in embedding AgX into the pores of AAO templates. In Fig. 2a, representative AgI nanowire synthesis is listed based on a highly ordered AAO membrane [173]. Initially, a layer of metal (such as Ag, Au or Pt) is sputtered onto one side of membrane to enable AAO conductive and serve as working electrode. After that, high-orientated Ag nanowire array along template channel direction is electrodeposited within nonporous AAO membrane and subsequent iodination is carried out in an electrochemical cell to obtain the final AgI product. For AgCl nanowire synthesis, a direct precipitation of Ag+ and Cl also can offer uniform deposition for filling AAO membrane pores in high yield [170]. Outstanding advantages of this AAO-template direct synthesis are that desired AgX nanowires or nanorods can homogeneously and periodically aligned among AAO matrix with diameter and length changes in accordance with template pore size. However, with AAO in product, extra template elimination step must be added after synthesis which increases the experiment’s complexity. Water-soluble sacrificial salt-crystal-template-directed synthesis is capable of generating AgX crystal structures with hollow interior morphologies and achieved successful cases including AgCl nanoframe, Ag/AgCl and Ag/AgBr cubic cage [175, 176, 177]. In these materials, salt-crystal-templates (such as KBr, NaCl) are first prepared through injecting their saturated aqueous solution into low solubility alcohol solution leading to salt crystal precipitation in Fig. 2b. Subsequently, AgNO3 solution is added along with ion exchange diffusion reaction between X and Ag+ to form AgX crystal on the surface of salt crystal. When the AgX crystal growth is completed, the obtained salt crystal/AgX samples are washed with water to remove and dissolve the inside salt crystal corn; thus, AgX hollow interior architectures are acquired.
Fig. 2

Schematic illustration for the synthesis process of AgI/Ag hetero-nanowire structure using AAO membrane with electrochemical method (a), and the water-soluble sacrificial salt-crystal-template route for the formation of Ag/AgCl cubic cages (b). a Reproduced with permission from ref. [174]. Copyright 2015 American Chemical Society. b Reproduced with permission from ref. [252]. Copyright 2013 Wiley

2.2.2 Soft templates

Generally, microemulsions composited with various micelles and reverse micelles are the most common soft templates in nanoparticles synthesis of inorganic crystals, noble metals and metal oxides [116, 178, 179, 180, 181, 182, 183, 184, 185]. The professional term of “microemulsion” was proposed by J. H. Schulman, who introduced the “micro emulsion” systems describing a stable transition between oil-rich and water-rich mixture [186, 187]. The surfactant molecules, containing amphiphilic groups, aggregate to form direct and reversed micelles in oil/water (O/W) and water/oil (W/O) microemulsions with hydrophilic groups oriented to water while hydrophobic groups associated with organic solvent [188]. Since then, numbers of nanoparticles practical synthesis on this topic have carried out and attained significance in basic research area. So far, quantities of nanoparticles with variable architectures have been successfully fabricated including nanorods [189, 190, 191], nanocubes [192], nanowires [193, 194], nanoplates [195, 196, 197], and feather or dendrite-like crystal structures [198]. Initially, micelle-based synthesis is capable of preparing AgX nanoparticles through mixing two identical reverse micelles each containing silver ions or halide ions [188, 199, 200, 201, 202, 203, 204]. Two kinds of reverse micelles after mixing rapidly repeat collide with association and dissociation processes companying reaction ions exchanges.

Thus, Ag+ and X combine together and nucleate with AgX nanoparticles formation within the water pools of reverse micelles. In these reverse micelles reaction for AgX, the surfactant molecules have a strong effect on the morphologies of AgX nanostructures through acting as soft templates as well as affecting reaction parameters, such as precursor ions reaction trace and reaction rate [199]. In addition, taking advantage of Br and Cl that existed in molecules of cetyltrimethylammonium (CTAB) and cetyltrimethylammonium chloride (CTAC), AgBr and AgCl can be facilely fabricated without addition of external X resource [147, 202, 205, 206, 207]. Liu group used an oil-in-water mixture microemulsion of CTAC chloroform solution and AgNO3 aqueous solution to one-pot synthesize two controllable morphology Ag/AgCl nanostructures (sphere and cube) through the changes of CTAC concentration in chloroform solution [147]. When CTAC chloroform solution was injected dropwise into a bulk aqueous solution of AgNO3, microemulsion of oil-in-water could be form with the hydrophobic tail of CTAC toward micro-oil core while hydrophilic point directing bulk water phase. In this direct micelles, solution concentration of CTAC was shown to induce a change to the Ag/AgCl morphology (Fig. 3a), forming cube-like (Fig. 3b) or sphere-like (Fig. 3c) nanospecies by adding diluted or highly concentrated CTAC chloroform solution, respectively. The regulation effect of CTAC surfactant was the reasonable illustration for such reaction phenomenon proving that high surfactant concentration facilitated cube-like architecture manufacture and low concentration favored sphere structure formation. In addition, different surface content of metallic Ag0 was observed comparing this oil-in-water synthesis process with previous water-in-oil Ag/AgCl preparation [205]. This is attributed to reaction discrepancy of direct micelles and reverse micelles for the Ag/AgCl preparation: Nanostructured AgCl crystals were generated owning to the collision of Ag+ and Cl ions both in oil-in-water and water-in-oil system and the surface metallic Ag0 grains were induced by ambient light irradiation. However, in oil-in-water case, the collision reaction was mainly occurred at the outer interface of micro-oil phase, in which reactant Ag+ for AgCl and extra Ag+ reduced to produce Ag0 could both constantly provided from the bulk water phase. As a result, comparatively more enriched Ag species could prone to be generated in oil-in-water than water-in-oil microemulsion system. What more important is that ternary alloyed AgClxBr1-x nanocrystals (NCs) with tunable bandgaps can be obtained with CTAC and CTAB selected as both halogen sources and surfactants [208]. In Fig. 3d, non-polar solvent chloroform solution of CTAC/CTAB was injected dropwise to polar aqueous solution of AgNO3 leading to the formation of microemulsion [208]. Surfactant CTAC/CTAB molecules with amphiphilic groups tended to agglomerate at the interface with hydrophilic halogen ion end toward water phase while hydrophobic alkyl chains contacting with chloroform solution. Ag+ dissolved in water phase would like to collide with X within the same phase immediately generating AgX species, whose aging process was controlled by diffusion states of CTAC and CTAB from non-polar phase. Consequently, well-defined ternary alloys of AgClxBr1-x nanocrystals, such as AgCl0.75Br0.25 (Fig. 3e), AgCl0.5Br0.5 (Fig. 3f), and AgCl0.25Br0.75 (Fig. 3g), were obtained by competitive reaction between CTAC and CTAB with Ag+. However, apart from these advantages in the soft-template technique, many challenges remain: Comparing with hard-template technology, the nanostructures produced via soft-template method are still difficult to achieve uniform distributions no matter in size or shapes; The surfactants served as adjuvants or used as precursors during synthesis are not easy to be removed especially in light- and heat-sensitive materials, which ultimately turn into impurities.
Fig. 3

a Possible explanation of the one-pot controllable synthesis of Ag/AgCl-based nanospheres and quasi-nanocubes via an oil-in-water medium. bc SEM images of the bare Ag/AgCl quasi-nanocubes (b) and bare Ag/AgCl nanospheres (c). d Schematic illustration of the synthesis procedure of AgClxBr1-x NCs. eg Typical SEM images of AgClxBr1-x samples. ac Reproduced with permission from ref. [147]. Copyright 2011 Royal Society of Chemistry. dg Reproduced with permission from ref. [208]. Copyright 2013 Royal Society of Chemistry

2.3 In situ ion exchange

Similar with hard template directed synthesis, in situ ion exchange of nanofabrication offer precious control on the placement of silver halide nanostructures over the preformed anion or cation salts, which acts as both templates and silver ion or halide ion sources [209]. Conventional in situ anion exchange synthesis method utilizes silver-based compounds such as Ag3PO4, Ag2CO3 and Ag2Mo2O7 as templates as well as silver source for fabricating heterostructured silver-based compound/silver halide structure [1, 44, 210, 211]. In general, as shown in Fig. 4a, large number of silver-based compounds (Ag-based compounds) acte as templates are first prepared or purchased to simplify experimental process and save time. After adding halide ions (X), ion-exchange reaction between anion from Ag-based compound and X occur resulting AgX crystals formation and depositing on the surface of substrate precursors. During this anion exchange process, AgX generation on Ag-based compounds surface in hybrid system can be tuned by varying X ion concentration with high resolution. In addition, benefiting from the heterostructure of Ag-based compounds/AgX material system, photocorrosion obstacle occurring in practical application can be eliminated. Shown in Fig. 4b–d, Ag3PO4/AgBr cubic crystals (Fig. 4b), Ag2CO3/AgBr.
Fig. 4

a Schematic diagram of the proposed basic anion exchange mechanism of Ag-based compound into Ag-based compound/AgX composite. Exposing Ag-based compound with halide ion (X) led to ion exchange between anion ions from Ag-based compound and X, which was then grown into Ag-based compound/AgX composite. Using anion exchange reaction, cubic Ag3PO4/AgBr/Ag heterostructures (b), Ag2CO3/Ag/AgBr ternary nanorods (c) and Ag2CO3/AgBr/Ag nanorods (the inset was the corresponding photographs) (d) have all been successfully synthesized. e Schematic illustration of the conversion processes from Ag2CO3 nanorods to AgnX nanotubes via an acid-etching anion exchange reaction. And the corresponding prepared Ag2S, AgCl, Ag3PO4, and Ag2C2O4 nanotubes (f). b Reproduced with permission from ref. [1]. Copyright 2013 Royal Society of Chemistry. c Reproduced with permission from ref. [44]. Copyright 2015 Royal Society of Chemistry. d Reproduced with permission from ref. [210]. Copyright 2015 Royal Society of Chemistry. (e, f) Reproduced with permission from ref. [214]. Copyright 2014 Royal Society of Chemistry

nanorods (Fig. 4c) and Ag2Mo2O7/AgBr rods (Fig. 4d) were all prepared through anionic ion exchange synthesis method between PO4 3−, CO3 2− and Mo2O7 with Br, and showed higher photocatalytic stability than pristine Ag3PO4, Ag2CO3 and Ag2Mo2O7 [1, 7, 8, 44, 210, 212]. However, this point is also regarded as its major drawback, as single silver halide component is difficult to be formed. Another acid-etching anion exchange reaction can also be used to fabricate series of AgX nanostructures [213]. While differ from conventional anion exchange reaction comparing Fig. 4a, typical silver halides rather than mixtures of Ag-based compounds/silver halides can be created. In this section as shown in Fig. 4e [214], pregrown Ag2CO3 nanorods were taken as templates, hydrogen ion (H+) from various acid was injected to react with Ag2CO3 releasing Ag+ ions around vicinity of templates. The released Ag+ ions surrounding Ag2CO3 templates were again reacted with the acid radical ions (Xn-) to generate a thin layer AgnX coating over the substrates, which resulted in intermediate formation of Ag2CO3-AgnX yolk-shell nanostructures. Furthermore, the resist made by local shell of AgnX could change the reaction progress by altering diffusion condition. A common ionic radius of Xn- is much larger than H+; thus, most of Xn- ions could be delayed out of shell while H+ smoothly went across reacting with Ag2CO3 core to generate large amount of Ag+. Due to concentration difference between internal and external, Ag+ ion tended to diffuse from core region to outside and react with Xn- with more AgnX directly precipitated above the tubular surface. After removal of the core region of Ag2CO3 with acid etching, an ordered hollow interior structure was finally patterned. Acid-etching anion exchange reaction allows for precise control over the structure and placement of silver halides, including external architecture preservation, size and pure phase acquisition. It can generate a wide range of AgnX, such as Ag2S, AgCl, Ag3PO4 and Ag2C2O4 (Fig. 4f). In addition, as a major advantage, anion ion-exchange reaction can produce double anion silver halide, such as Ag(Br, Cl) and Ag(Br, I), just taking the formed AgX as Ag-based compound to anion exchange with another halide ion [213, 215, 216].

Cation exchange reaction is similar to anion exchange in that halide-based compound is used as template for the formation of silver halide with silver nitrate. Copper nitrate (CuBr) microspheres were used in cation exchange reaction with silver nitrate and employed as chemical template, similar to Ag-based compound in anion exchange reaction, which could provide Br source and then be applied as a matrix for the deposition of AgBr [217]. The drawback of cation exchange reaction is also semblance with anion exchange reaction: single silver halide component is difficult to be obtained as original core materials cannot be removed entirely [218].

3 Engineering silver halide for photocatalysis applications

The ability to engineer the physical parameters of AgX, including morphology control and energy band regulation, or composite with other materials provide great promise for performance improvement in photocatalysis [219, 220]. This section will concentrate on the connection between the photocatalytic properties of AgX composite with their configuration organizations. Physical parameters will also be linked with their engineering method in Section 2. In addition, we will also discuss the photocatalysis enhancement of well-regulated AgX composite with corresponding reaction mechanism in specific application, which is another important section to illustrate AgX in Section 4.

3.1 Modification of morphology

The typical approach to engineer the photocatalytic properties of a single AgX component is to manipulate its morphology [221]. The morphology modification, including controls of the size, shape, and surface structures, determines the chemical reactions efficiency and selectivity in heterogeneous photocatalysis through changing some physicochemical properties, such as face-specific reactant molecular adsorption and interfacial charge transfer [222, 223, 224, 225, 226, 227, 228, 229]. Therefore, morphology modification is an important variable that should be deliberately designed to achieve prompted photocatalytic properties.

Highly reactive crystal plane is difficult to be formed due to the surface energy minimizing trends in crystal growth process. However, Lu group reported a successful synthesis of uniform anatase TiO2 single crystals with a large percentage of more active facets exposure rather than thermodynamically stable {101} facets [230]. Since then, nano- or micro-sized photocatalyst crystals with high specific facets exposure have been devoted significant efforts demonstrating superior activity no matter in organic compounds degradation, hydrogen evolution or CO2 reduction [231, 232]. When polyhedral nanostructure is fabrication, more active catalytic sites are emerged including specific facets, edges, corners and steps, where more reactive intensity can be gained. Figure 5 shows the photocatalytic activities of AgX nanostructures (AgCl, AgBr, and AgI) with a set of controlled geometries (near-spherical, cube, concave cube, hierarchically superstructure, and nanoplates), which have been calculated using organic dye degradation and oxygen evolution from water oxidation under light irradiation [233]. From methyl orange (MO) degradation effect (Fig. 5 a, c, d) and O2 evolution comparisons (Fig. 5b), a general theme can be observed, that the photocatalytic performance improves as the crystal surface complexity increases. Thus, it can be inferred that the configuration and quality of crystal surface play a pivotal role in determining their performances in photocatalytic applications.
Fig. 5

a Photodecomposition of MO 20 mg L−1 dye in solution over cubic Ag@AgCl, near-spherical Ag@AgCl and N-doped TiO2 under visible light (≥ 400 nm) irradiation. Reproduced with permission from ref. [146]. Copyright 2011 Royal Society of Chemistry. b Amount of oxygen evolved during photocatalytic oxidation of water with 3D AgCl hierarchical superstructures, concave cubes, and AgCl cubes as catalysts under irradiation by visible light provided by a 300 W Xe arc lamp. Reproduced with permission from ref. [392]. Copyright 2012 Wiley. c Over concave and spherical AgI particles. The result clearly shows that the concave particles exhibit superior performance to the spherical counterparts. Reproduced with permission from ref. [154]. Copyright 2014 Elsevier. d Photodegradation of MO dyes over the AgBr nanoplates, irregular AgBr particles and Ag3PO4. Reproduced with permission from ref. [142]. Copyright 2012 Royal Society of Chemistry

In morphology control aspect, adequate understanding of crystal surface properties is regarded as the necessary conditions [234, 235, 236]. In AgBr crystal, the surface energy of {111} facets has been calculated to be higher than ordinary {110} and {100} facets [61, 142, 237]. Thus, AgBr nanoplates with {111} facets exposure has been designed using PVP as capping agent. And the photocatalytic performance investigation of AgBr with {111} facets in organic dye degradation was manifested to be 4 times higher than the irregular AgBr microspheres which were generally constituted by stable {110} and {100} facets. Besides, high-purity wurtzite-type β-AgI microplates with polar {0001} facets demonstrated preferential diametric migration of photogenerated charge carriers (electrons, holes) along the c axis indicating an effective vectorial electron–hole separation [143]. Thus, dramatically enhanced photocatalytic ability was achieved in organic dye degradation aspect [238, 239, 240, 241]. Concave structures enclosed by high-index facets regularly draw much more interest than general flat faces attributing to the large number of active sites. Based on the overgrowth preference of corners and edges along <111> and <110> directions, a AgCl concave nanocube structure has been formed from it cubic seeds, and showed much higher photocatalytic performance in O2 evolution comparing with original cube structure [242].

3.2 Regulation of energy level

For certain photocatalytic reaction, the band position of semiconductor should fulfill the corresponding reaction potential requirements [243]. That is, the energy of conduction band (CB) satisfies the required reduction potential while valence band (VB) should be positive than the oxidation potential. Thus, in order to refine the redox potentials, many strategies have been developed to engineer the energy band of semiconductors. Three main tactics have always been explored; first, substituting the cations with alkali metal or alkaline earth metal elements to low the levels of the CB; [244, 245] Second, doping with cations with d10 or d10s2 configurations, 3d-transition elements, and nonmetal elements for lifting VB position; [74, 246, 247, 248] Third, fabrication of a solid solution, which can tune the CB and VB simultaneously [249, 250]. However, as superior photo-sensitive materials with the ability to produce plasmonic metal Ag0, the introduction of other elements will destroy the original performance; thus, the first two methods have been rarely applied in AgX. Solid solution of semiconductor nanomaterials constituted by parent AgX materials can not only maintain their original high photocatalytic ability but also construct new composite energy band [251].

Ternary alloyed photocatalytic silver halide AgX (AgClxBr1-x, 0 ≤ x ≤ 1) nanocrystals with uniform cubic morphology could engineer their energy bandgap from 2.5 to 3.0 eV via tuning the ratio of Cl and Br ions (Fig. 6a), which was calculated from the UV-Visible absorption spectra in Fig. 6b [208]. In AgClxBr1-x alloys, the energy bands (VB and CB) are both constituted by Cl3p, Br 4d, and Ag 4d states, while the ratio changes of halogen ions (Cl, Br) mainly influence the CB rather than VB levels [208]. As shown in Fig. 6c, the proportion increase of Cl rather than Br ion gives rise to a negative shift for the CB minimum, subsequently leading to the bandgap narrowing. In the light of theoretical and experimental results, the absorption efficiency of incident light can be augmented by a smaller bandgap value. Consequently, the photocatalytic performance on dye degradation and CO2 reduction were both found to depend on the chemical compositions of AgClxBr1-x, and AgCl0.75Br0.25 sample exhibited the highest activity. On the other hand, the higher VB and CB dispersion of AgClxBr1-x comparing with pristine AgCl and AgBr indicated more charge carriers’ transmission convenience [252], which was another guarantee for photocatalytic activity promotion. Furthermore, plasmonic Ag@AgBr/AgCl heterostructured nanocashews and Ag@AgCl-AgI photocatalysts with highly efficient visible light activity has also been prepared through ion-exchange process [215, 253].
Fig. 6

a The experimental lattice constants and indirect bandgap values of AgClxBr1-x NCs depending on the composition changes. b UV-Visible absorption spectra of AgClxBr1-x nanocrystals. c Electronic structures of the ternary alloyed AgClxBr1-x NCs (x = 1, 0.75, 0.5, 0.25, 0). Reproduced with permission from ref. [208].2018 Copyright 2013 Royal Society of Chemistry

3.3 Composited with other materials

3.3.1 Other photoactive semiconductor

Heterogeneous system constituted by photoactive semiconductors is regarded as an effective method to facilitate the photoexcited electron-hole separation with enhanced photocatalytic activity [254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269]. Various metallic semiconductor (such as TiO2, ZnO, BiVO4) and metal-free nitrogen-doped graphene semiconductors have been considered as preferred substances to composite with AgX, shown as Table 2. As photosensitive materials, AgX and Ag/AgX (especially X = Cl and Br) are regarded as suitable semiconductor selections to modify photocatalytic composition system, which can produce efficient coupling photocatalysts with higher activity and stability compared with the single component [270, 271, 272, 273, 274, 275, 276]. Among them, the hybridization between AgX and other metallic semiconductor would significantly improve the light absorption efficiency and facilitate the photoinduced charge carrier separation with similar transmission routes [277, 278, 279, 280, 281, 282, 283, 284, 285, 286]. Such as in TiO2 coupling system, the spectral absorption range tuning of TiO2 is the key factor for its visible light photocatalytic performance modification [287, 288, 289, 290, 291, 292]. The introduction of narrow bandgap AgBr plays a major role in spectral absorption range extension of AgBr/TiO2 system [293], and especially with plasmonic Ag0 generation, further intensive visible light utilization can be obtained [294, 295, 296]. In charge transfer mechanism for photoconversion efficiency enhancement, the photogenerated electrons generated in photo-excited AgBr were all transferred into TiO2 nanoparticles as the CB of AgBr is more negative than TiO2, which promised the effective separation of charge carriers [297, 298].
Table 2

Examples of composite photocatalysts constituted by AgX and other semiconductors

Composite photocatalyst

Synthesized strategy




Deposition and precipitation

Degradation of rhodamine B (RhB) and MO as model dyes



Deposition and photoreduction

Photocatalytic decolorization of MO aqueous solution at ambient temperature

[266, 287]



Visible light degradation of MO and phenol


Ag/AgCl/TiO2/hierarchical porous magnesian


Photocatalytic degradation of gaseous benzene



deposition–precipitation–photoreduction method

Photodegradation of 4-CP



In situ oxidation reaction

degradation of dye RhB



In situ ion exchange approach

Photocatalytic degradation of RhB, MB, MO.




Photocatalytic degradation of RhB



Sol-gel and solvothermal route

Photocatalytic degradation of ibuprofen (IBP) over the as-prepared photocatalysts under white LED irradiation


Ag–AgBr/TiO2 tubes

Deposition and precipitation

Photocatalytic degradation of phenol



Chemical precipitation

Photocatalytic degradation of azo dyes


AgX (X = Br, I)/g-C3N4


Photocatalytic degradation of MO


AgBr supported on g-C3N4-decorated CN ternary nanocomposites


water purification and CO2 reduction




Photocatalytic degradation of MO


NG–AgX@Ag, X = Br, Cl

Deposition and precipitation

Photocatalytic degradation of RhB under visible light


AgI/AgVO3 nanoribbon

In situ ion-exchange approach

Photocatalytic selective oxidation of benzylamine




Photocatalytic degradation of RhB


Bi2SiO5/AgI nanoplate

in situ precipitation

Degradation of ARG aqueous


three-dimensional AgI@TiO2


Photodegradation of organic pollutants


In AgBr-supported metal-free g-C3N4-decorated Nitrogen-doped graphene (CN) ternary nanocomposites (ACNNG-x), Z-scheme heterogeneous structure consisting of AgBr and CN with different energy level has been proved to be an effective rule to photocatalytic degradation of MO and CO2 photoreduction under visible light [299, 300, 301, 302, 303, 304, 305]. Two charge carriers separation mechanisms were involved in this system before and after the generation of metallic Ag0 nanoparticles. Before the metallic Ag0 generation, the photoinduced electrons from the CB of CN would inject into the CB of AgBr while the photoinduced holes remaining in VB would transfer from AgBr to CN. Following this process, high electron-holes separation was obtained and promote the photocatalytic performance. According to the intrinsic photosensitive property of AgBr, the accumulated electrons on the surface of AgBr generated from the repeated absorption of photons will combine with Ag+ to form Ag0 species on the crystal surface. Thus, a AgBr–Ag–CN system with Ag0 species as a conductor was achieved, which formed the known Ohmic contact with low contact resistance and opened a faster charge between AgBr and CN. In this type, the photoinduced electrons generated on the CB of AgBr could directly combine with the holes from CN using metallic Ag0 as reactive sites. Finally, the reserving holes in AgBr and electrons in CN were participated into the forward degradation and reduction reactions.

3.3.2 Other non-photoactive materials

The hybridization of AgX or Ag/AgX with other non-photocatalytic materials (shown in Table 3) specially the carbon-based conductive matters has attracted extensive concerns due to their unique electronic properties and high specific surface area [306, 307, 308, 309, 310, 311, 312, 313, 314, 315]. In hybridization aspect between AgX and photocatalytic substrates, the photocatalysis elevation is based on the Z-scheme heterogeneous structure formation in order to gain high charge separation efficiency [316, 317, 318, 319, 320, 321]. While incorporation of AgX in high specific surface area substrate is another promising strategy in photocatalytic activity promotion, especially in wastewater treatment [322, 323, 324, 325, 326]. With the substrates as dispersing templates, homogeneous and uniform semiconductor crystal morphology could be formed, which in turn can enhance their photocatalytic performance [327, 328, 329, 330, 331, 332, 333, 334]. The graphene sheets with high optical transmittance, large 2D specific surface area, unique electronic properties and locally conjugated aromatic system has regarded as one kind of ideal substrate candidates for photocatalytic deposition. Especially, the 2D planar structure framed by sp 2 hybrid carbon atoms facilitates large amount of dye molecules adsorption through π-π conjugation, which can be a key driving force to promote photocatalytic activity [72]. Among these graphene-based composite photocatalysts, graphene oxide (GO) is generally utilized as a precursor to be reduced as reduced-graphene oxide (rGO), and sometimes it is also applied as a substrate for photocatalysts deposition as well [335, 336]. Furthermore, water-soluble sulfonated graphene (SGE), one kind of graphene sheets with p-phenyl-SO3H groups covalently bonded to their edges, can also be synthesized from GO precursor [72, 337]. Due to the perfect preservation of graphene structure at sheet center and water-soluble groups binding at edges, SGE displays complete combination of great water solubility, aromatic molecules adsorption and photo-induced electron reservoir these features together [338]. Thus, the binding of AgX and SGE together confirmed superior photodegradation ability and high stability.
Table 3

Examples of composite photocatalysts constituted by AgX and other non-photoactive materials

Composite photocatalyst

Synthesized strategy



Ag/AgX-CNTs (X = Cl, Br, I)

An ultrasonic assistant deposition-precipitation method

Photocatalytic degradation of TBP


Ag@AgCl/reduced GO

Deposition-precipitation and photoreduction

Photodegradation of RhB dye




Photodegradation of acridine orange (AO) pollutant



Self-assembled encapsulation

Photodegradation of MB pollutant



Oxidation-chloridization process

Photodegradation of MO pollutant



Sequent double-jet precipitation, hydrothermal, and UV light reduction

Photodegradation of MO pollutant



One-step solution-mixing method

Photodegradation of RhB, MB and MO pollutant


Ag@AgBr/SGE (water soluble sulfonated graphene)

Microemulsion method

Degradation of MO



anion-exchange precipitation method

Degradation of MO


Ag/AgBr/Graphene Oxide

a surfactant-assisted assembly protocol

Degradation of MO


3D AgX/Graphene Aerogels (X = Br, Cl)

In situ growth

Photocatalytic degradation of MO and reduction of CrVI



2D coupling route

Degradation of RhB and H2 evolution



Chemical oxidation method

Photocatalytic decomposition of MO, acid red 18 and MB



Template-free ultrasound-assisted method

Photocatalytic decomposition of RhB




Photocatalytic decomposition ofRhB


3D graphene aerogels (GAs), with cross-linking framework structure and macroscopic block appearance, can be constructed through the graphene sheets overlapping and coalescence [339, 340]. The hierarchically porous structure widespread through the 3D GAs is considered as an ideal support for the deposition of photocatalyst NPs. Low mass density of GAs enable its block structure suspends within the solution without any assistance of external force. Therefore, the photocatalyst uniformly distributed throughout hierarchically porous surface ensures effective contact between the photocatalysts and contaminant molecules. In 3D AgX/GAs (X = Br, Cl) [207], the AgBr and AgCl nanoparticles have been homogeneously in situ grown on the surface of the hierarchical pores of the GAs. The synergistic effect between the large dye molecules adsorption and photocatalytic degradation ability of AgX (X = Br, Cl) make AgX/GAs superior photocatalytic performance of organic dye molecules degradation. On the other hand, macroscopic block appearance of AgX/GAs make it an easily and conveniently recycling operation, which can be treated just using tweezers. It is another advantage of AgX/GAs for photocatalytic applications.

4 Photocatalytic application of silver halides

Photocatalysis encompasses a fundamental solar light-photocatalyst-matter interaction; as result, many practical and scientific fields have stood to benefit from it. Already, many photocatalytic applications have emerged since Fujishima and Honda achieved the photolysis splitting of water in 1972 [70], which are divided into three categories: (i) remediation of contaminants, (ii) water splitting, and (iii) artificial photosynthesis. The first area reflects the environment application of photocatalysis and has been successfully used for a wide arrange of compounds [341, 342, 343, 344, 345, 346, 347, 348], like phenols, dyes, surfactants, aromatic carboxylic acids, aliphatic alcohols, simple aromatics, pesticides, and alkanes as well as a variety of high oxidative state of heavy metals (such as CrVI, Au3+, Pt4+, and Rh3+) [349, 350, 351, 352]. Among these organism or metal ions degradation cases, complete mineralization and metal reduction have been reported. The second and third exploit the energy-related application of photocatalyst, that is hydrogen evolution from water splitting and reduction of CO2 into carbohydrates [55, 212, 250, 353, 354, 355, 356].

4.1 Organic compound degradation

4.1.1 Advantage analysis of photocatalysis on mechanism aspect

Direct oxidative combust of organic dye molecules by O2 is environmentally friendly and thermodynamically favorable as the entire degradation is exothermic. However, such non-catalytic reaction between triplet ground state O2 (S = 1) and singlet ground state organic dye molecule (S = 0) is kinetically slow in which a high-energy barrier should be conquered during reaction (Fig. 7 Path A). Two main reasons can explain such kinetic resistance [357, 358]: One is spin-forbidden. As mentioned above, the electrons in oxidant O2 molecule is spin-parallel (triple ground state), while organic dye molecule is singlet ground state with spin-paired electrons and some products are usually also spin-paired. Such situation indicates that one of electrons is inevitably spin-reversed during reaction process and this process is spin-forbidden with slow kinetics. Another is unfavorable one-electron reduction potential of O2 molecule. Although O2 molecule has rather high multielectron reduction potential (E(O2/H2O) = +1.229 V and E(O2/H2O) = +0.695 V, NHE, PH = 0) during reaction, its one-electron reduction potential is low (E(O2/O2 -•) = − 0.16 V) meaning the first electron acquirement is difficult during oxidation reaction [359, 360].
Fig. 7

a Three respective reaction paths for the combustion of organic dyes in the presence of triplet ground state O2 (S = 1). Path A is the direct non-catalytic reaction with a high-energy barrier; Path B is the thermal-catalytic degradation process with low energy barrier; Path C is the brief illustration of photocatalytic degradation process with energy supplying. b Corresponding mechanism in photocatalytic reaction. Reproduced with permission from ref. [360]. Copyright 2010 Royal Society of Chemistry

As shown in Fig. 7a, apart from the non-catalytic reaction (path A), other two approaches are also able to achieve reaction target with favorable kinetics that can accelerate overall reaction process between O2 and organic dyes [360]. The first approach is to substitute the noncatalytic reaction with catalytic approach, that is transforming the high-energy barrier into multiple low-energy barrier. The introduction of catalyst can provide some active sites to bound with one of or both of reactants forming intermediates. Then as shown in path B, following the reaction a series of transitional intermediates (T1 and T2) and stationary (S1) are generated with much lower activation energies than the non-catalytic reaction barrier. Therefore, besides thermodynamically favorable, such catalytic reaction is also kinetically satisfied. The second approach is exhibited in Fig. 7 path C. External energy is supplied in the catalytic system constituted by organic compounds, O2 and catalyst, which enable reaction circumvent or overcome the intrinsic barrier during the degradation process. Photocatalytic reaction exactly illustrates this type of process. Under irradiation at ambient circumstance, the semiconductor photocatalyst can absorb the photons with energy matching or exceeding its bandgap, to excite the electrons (ecb ) from VB to CB leaving holes (hvb +) behind as shown in Fig. 7b. The VB holes can capture electrons from the absorbed H2O to generate OH radicals (Eq. (1)) or directly react with organic dye molecules to make them oxidized (Eq. (2)). In open system, dissolved oxygen can scavenge electrons from the CB of semiconductor to produce O2 -• radicals shown as Eq. (3). These activation radicals with oxygen atom are more reactive than the corresponding oxygen molecule O2 and can deliver their energy or electrons into other species or reactants to make chemical changes. Explaining as Eqs. (4)–(6), such reactions always hold low- or non-energy barriers, which guarantee high reaction rate. Overall the entire photocatalytic reaction, only O2 and H2O engage to combust the organic dyes into other degradation products under light irradiation, no photocatalyst component is consumed.
$$ {\mathrm{H}}_2\mathrm{O}+{h}_{vb}^{+}\to {\mathrm{OH}}^{\cdotp }+{\mathrm{H}}^{+} $$
$$ \mathrm{R}-\mathrm{H}+{h}_{vb}^{+}\to {\mathrm{R}\mathrm{H}}^{+\bullet}\rightleftharpoons {\mathrm{R}}^{\bullet }+{\mathrm{H}}^{+}\to \to $$
$$ {\mathrm{O}}_2+{\mathrm{e}}_{\mathrm{cb}}^{-}\to {\mathrm{O}}_2^{-\bullet } $$
$$ \mathrm{R}-\mathrm{H}+{\mathrm{OH}}^{\bullet}\to {\mathrm{R}}^{\bullet }+{\mathrm{H}}_2\mathrm{O} $$
$$ {\mathrm{R}}^{\bullet }+{\mathrm{O}}_2\to {\mathrm{R}\mathrm{OO}}^{\bullet}\overset{{\mathrm{O}}_2/}{\to}\overset{{\mathrm{H}}_2\mathrm{O}}{\to}\mathrm{Degradation} $$
$$ \mathrm{H}-\mathrm{R}+{\mathrm{O}\mathrm{H}}^{\bullet}\to {\mathrm{H}\mathrm{ROH}}^{\bullet}\overset{{\mathrm{O}}_2/}{\to}\overset{{\mathrm{H}}_2\mathrm{O}}{\to }\ \mathrm{Degradation} $$

4.1.2 Performance evaluation modes of silver halide in organic degradation

Some of the first, and most successful, applications of AgX in photocatalysis are the degradation of organic dye molecules [361]. Similar to previously mentioned degradation compounds, general classes of organic substance, though cannot be entirely mineralized have been degraded by AgX. A partial tabulation of various organic compounds that photodegraded by illuminated AgX or AgX-based composites is provided in Table 4. Besides providing these organic matter references, some representative parameters for photocatalysis evaluation have also been summarized and analyzed, such as the apparent rate constant (kapp) and total organic carbon (TOC).
Table 4

Various organic compounds that photodegraded by AgX or AgX-based composites


Photocatalyst composites


Penicillin G (PG)




AgBr/Bi2WO6; Ag–AgBr/TiO2; AgBr/Ag; AgCl

[217, 291, 367, 369]

Methylene blue (MB)

AgBr/Bi2WO6; Ag/AgCl; graphene oxide/Ag@AgCl

[6, 124, 322, 367]

Methyl orange (MO)

AgBr@Ag; AgCl@Ag; AgBr; Ag2Mo2O7@AgBr; Ag/AgCl; CNNs/Ag/AgCl; Ag/AgCl/TiO2; Ag/AgBr/TiO2; AgI

[122, 123, 125, 142, 143, 147, 163, 164, 207, 210, 213, 217, 233, 236, 237, 288, 298, 299, 320, 321, 335, 348, 365, 370]

Rhodamin B (RhB)

AgBr; Ag2Mo2O7@AgBr; Ag/AgCl/PrGO; Ag@AgBr/AgCl; Ag/AgCl; CNNs/Ag/AgCl; Ag@AgBr/AgCl; Ag/AgX/BiOX (X = Cl, Br); MoS2/BiOI/AgI

[124, 210, 215, 238, 267, 269, 321, 336, 364, 366]

Ibuprofen (IBP)



p-nitrophenol (PNP)



17-β-ethinylestradiol (EE2)

Ag/AgCl@helical chiral TiO2;’


the azo dye acid orange 7 (AO7)



2,4-dichlorophenol (2,4-DCP)

AgCl; AgI/BiOI

[241, 369]







acridine orange (AO)

Ag@AgX (X = Cl, Br)


acid red G aqueous solution



The real-time concentration changes of organic pollutants during photocatalytic degradation are detected through their UV-visible absorbance. According to Lambert-Beer law, actual concentration changes of colored dye molecules (C/C0) are directly proportional to their corresponding normalized UV-vis absorption values (A/A0). Consequently, through tracing the light absorbance changes, we can successfully monitor photocatalytic degradation states of organic dyes. The pollutant degradation kinetics in heterogeneous photocatalysis from aqueous phase has been widely analyzed in Langmuir-Hinshelwood model (Eq. (7)): [362, 363]
$$ {r}_0=-\frac{\mathrm{d}C}{\mathrm{d}t}=-\frac{kKC}{1+ KC} $$
There, r0 represents the initial reaction rate (mg L−1 min−1), t is the reaction time (min), C is denoted as the real-time concentration of pollutants (mg L−1), K expresses the Langmuir adsorption equilibrium constant (L mg−1), and k signifies the Langmuir-Hinshelwood reaction rate constant (mg L−1 min−1). Under dilute concentration cases (KC < < 1) of water pollutants, a pseudo-first-order kinetics model can be deduced from Eq. (7) through Eq. (8) and (9):
$$ {r}_0=-\frac{\mathrm{d}C}{\mathrm{d}t}=- kKC $$
$$ \ln \left(\frac{C}{C_0}\right)=- kKC=-{k}_{app}t $$
where C 0 represents the original concentration of water pollutants (mg L−1) and k app is the apparent rate constant (min−1). During the overall dye degradation process of AgX, the normalized concentration of dye molecules (ln(C0/C)) show linear relationship with irradiation time indicating that these degradations follow the first-order kinetics (Eq. (9)) [299, 364, 365]. Shown as Fig. 8a, based on the curve state (Ct/C0 ~ t) and time consumption along the entire degradation, an overall degradation process can be obtained. Via pseudo-first-order linear fitting between ln(C0/C) and t, kapp can be determined from linear slop and applied to evaluate the degradation rate among different photocatalyst in the same experiment (Fig. 8b). Besides, one more degradation index, total organic carbon (TOC) is taken to assess the mineralization effect of organic pollutants [366]. The loss value of TOC after the photodegradation, expressing as the percentage of original quantity, suggests the mineralization degree of organic matters as shown Fig. 8c [290, 367].
Fig. 8

a Degradation curve on Ct/C0 against reaction time (t), and the derived linear plots between ln(Ct/C0) and t for determination of kapp. Reproduced with permission from ref. [175], Copyright 2014 Elsevier. b Performance comparison of photocatalytic degradation by the slops (kapp) of ln(C0/C) ~ t. Reproduced with permission from ref. [364], Copyright 2015 Royal Society of Chemistry. c TOC changes during the photocatalytic degradation. Reproduced with permission from ref. [367], Copyright 2014 Royal Society of Chemistry

4.1.3 Photocatalytic mechanism based on different charge transfer modes

Heterogeneous photocatalysis of AgX on organic degradation is a process, similar to general photocatalysis mechanism, where illuminated AgX semiconductors generate photoexcited electrons and holes. Then, H2O and dissolved O2 molecules as well as X ions will react with these charge carriers to produce many radicals (OH, O2 -•) or oxidative X atom, who can oxide the organic contaminants. As mentioned above, photosensitive AgX can absorb photons and excite electrons from its VB to CB leaving holes in VB. The photogenerated electrons migrate to AgX surface and combines with interstitial Ag+ ions producing Ag0 atom. Upton repeated absorption of photons and continue generation of Ag0 on AgX, ultimately silver clusters are able to be produced who can promote their photocatalysis through the localized surface plasmon effect. The localized surface plasmon effect of metallic Ag0 not merely trigger the self-generated electrons and holes engaging in the photocatalytic degradation. On the other hand, the electric field enhancement around Ag/AgX interface can promote incident light absorbance for AgX when Ag0 nanoparticles in plasmon resonance. [368]Ag/AgCl hybrid structure (Fig. 9a), metallic Ag0 nanoparticles deposited on AgCl cube surface, was calculated by 3D finite-difference time-domain simulations (FDTD) to show the electric fields enhancement result under 550 nm excitation. Significant increase of electric field intensity around Ag/AgCl interface has been observed in Fig. 9a right image and Fig. 9b, which can promote the light absorption of AgCl crystal. Thus, photocatalytic promotion effect can also be raised on original AgCl semiconductor due to the metallic Ag0 nanoparticles generation. In accordance with previous studies, two carriers transport modes have been proposed with the introduction of Ag0 in AgX system.
Fig. 9

a Left: 3D FDTD simulation model of a 4 × 4 array of metallic Ag0 nanoparticles deposited on the surface of AgCl substrate under 550 nm excitation. Right: Large magnification on the side view image of (b)

Two of the first, the surface plasmon resonance (SPR) electrons generated in Ag/AgX system remain in the Ag nanoparticles instead of being transferred to AgX, which are mainly reflected in Ag/AgCl and Ag/AgBr composite [6, 237, 348]. The surface of AgX particles are most likely negative charged with X ions termination, and therefore polarize the modified metallic Ag into two opposite charge carriers regions, in which the positive and negative charges is close to or far away from the Ag/AgX interface shown as Fig. 10a [6]. Thus, the SPR electrons generated in metallic Ag0 nanograins are forced to transfer into the negative charged region far away from Ag/AgX interface and react with O2 to from active substance O2 -•. While the remaining holes in metallic Ag0 nanograins migrate into AgX and oxidize X ions and H2O molecules to X atoms and OH, who are also both reactive radical species. It is worth mentioning that through directly oxidizing the dye molecules, X atoms can be again reduced back to X ions, which ensures the stability of AgX [369]. On the other hand, narrow bandgap AgBr also can excite electrons under visible light irradiation (Fig. 10b). Fortunately, due to the excellent conductivity of Ag nanoparticles synergetic with the polarization field of AgBr core, these electrons are able to migrate from AgBr surface to Ag nanoparticles, which reduce the recombination probability of e and Ag+ ions and one more time promise the stability of Ag/AgBr hybrid system [122, 164, 239, 370]. The second mode, contrary to the first explanation above, involves the SPR electrons of metallic Ag0 nanoparticles transferring to the CBs of AgX semiconductors [207, 313, 371]. To elucidate the underlying photocatalysis mechanism in Ag/AgCl composite system, transient absorption (TA) spectroscopy has been taken to investigate the photoelectron migration dynamics between Ag nanoparticles and AgCl semiconductor [252]. Through monitoring the state filling kinetics on the 1s electron level, TA spectroscopy can explore the corresponding electronic transmission status from AgX semiconductor to Ag nanoparticles [372, 373]. Taken Ag/AgCl hybrid structure as analysis object, no photon-induced-electron-plasmon-bleaching (Fig. 11a, iii) and hot-electron-cooling (Fig. 11a, iv) signals can be observed in Ag0 nanoparticles deposited on AgCl substrate comparing single Ag0 nanoparticles (Fig. 11a, i and ii), indicating that a strong electronic coupling exists between metallic Ag0 nanoparticles and AgCl substrate. That is the photon-induced SPR electrons in metallic Ag0 nanoparticles can efficiently inject into the CB of AgCl (Fig. 11a, v) and the injection time is far beyond the TA time resolution, which consist with the previous electron coupling result of other semiconductors [313, 374, 375]. Based on analysis results from TA spectroscopy, the second charge transfer mode from metallic Ag0 nanoparticles to AgCl substrate is proposed under light illumination and shown in Fig. 11b. When the frequency of visible irradiation light matches the intrinsic electron oscillation on metallic Ag0 nanoparticle surface, the corresponding electromagnetic wave intensity will be amplified and confined within certain interfacial hot spots. These produced hot electrons from plasmon-excited Ag nanoparticles can fast inject into the CB of AgCl [320, 376], whose feature is similar to dye sensitization [284]. Afterward, the electrons arrived at the CB of AgCl can be consumed by O2 molecules to produce oxidative radicals or molecules (O2 -•, H2O2) [210, 377]. The positive holes remaining in Ag0 nanoparticles can directly oxide the organic dyes or trapped by H2O to generate OH [124, 378]. These radicals and oxidative molecules (O2 -•, H2O2 and OH) are considered as powerful force to destruct the dye molecules structure [321].
Fig. 10

Schematic illustration of photocatalytic mechanism on Ag/AgCl composite (a) and Ag-AgBr part from Ag-AgBr/γ-Al2O3 nanostructures (b) under illumination. a Reproduced with permission from [122], Copyright 2011 Wiley. b Reproduced with permission from ref. [370], Copyright 2011 Royal Society of Chemistry

Fig. 11

a 470 nm transient absorption kinetics of metallic Ag0 nanoparticles alone (i, ii) and coupled with AgCl substrate (iii, iv) at different delay time after 400 nm excitation. (v) Schematic illustration of electron transfer from metallic Ag0 nanoparticle to AgCl substrate after excitation. b Mechanism illustration of Ag/AgCl hybrid structure for photocatalytic organic dye degradation. a, b Reproduced with permission from [252], Copyright 2013 Wiley. c Energy level distribution on Ag/AgBr interface under dark condition. d With visible light irradiation, energy level changes and charge transfer pathway between metallic Ag0 nanoparticles and AgBr crystal substrates. c, d Reproduced with permission from [380], Copyright 2012 Wiley

Another interpretation for the second mode is from work function perspective of metallic Ag0 nanoparticles and their depositing AgX substrate, taking Ag/AgBr for instance. As shown in Fig. 11c, due to the different work functions between Ag0 nanoparticle (ΦAg = 4.25 eV) and AgBr crystal (ΦAgBr = 5.3 eV) [379, 380], a Schottky barrier can be formed at interface when both sides are attached together. Then electrons from low work function Ag0 nanoparticle will transfer to AgBr crystal with high work function until two Fermi level equilibrium. Under light illumination, the strong SPR enable high electron density generated and enriched on the surface of metallic Ag0 nanoparticle [39, 381], which can raise itself Fermi level and facilitate the electrons injected into the CBof AgBr (Fig. 11d) [382].

4.1.4 Role of dye molecules in organic degradation

When organism degradation mechanism is investigated in photocatalysis, semiconductor photocatalysts are generally regarded as the first objective, that is semiconductor is excited by light and engaged in degradation process. Owning to acting as degradation substance, though dye molecules are found to have sensitizing effect in prompting itself degradation, few works have listed their roles in mechanism illustration. In dye-sensitized perspective, the semiconductor is not acted as photocatalyst to absorb light but as an electron transfer medium from the excited dye molecules pass the CB of semiconductor and then electrons are scavenged by electron acceptors [383, 384, 385]. As shown in Fig. 12a and Eq. (10–12), dye molecules absorb visible light and become excited (path a, Dye*), and then the activated electrons from Dye* are injected into their adjoining CB of semiconductor (ecb , path b), where the ecb can be scavenged by absorbed electron acceptors to produce radicals (path c and d), such as the O2 -•. These radicals, actually the product of dye excitation, will turn over to decompose the dyes (path e), and dyes will also self-decompose due to the loss of electrons (path e). Simultaneously, Fig. 12a clearly
$$ \mathrm{Dye}+\mathrm{hv}\left(\mathrm{visible}\right)\to {\mathrm{Dye}}^{\ast } $$
$$ {\mathrm{Dye}}^{\ast }+\mathrm{Photocatalyst}\to {\mathrm{Dye}}^{+\bullet }+\mathrm{Photocatalyst}\left({\mathrm{e}}_{\mathrm{cb}}^{-}\right) $$
$$ {\mathrm{O}}_2+{\mathrm{e}}_{\mathrm{cb}}^{-}\to {\mathrm{O}}_2^{-\bullet } $$
manifests that the semiconductor just play a medium role for electron transfer. In Fig. 12b, wide and continuously unoccupied electron-acceptor states have distributed among semiconductor CB [386], which ensures adequate space and matched energy level for the electron injection from excited dyes [360]. Besides, the magnitude of density states within CBcan block the back-transfer of electrons from the CBto oxidized dyes, as this backward recombination will lead to an entropy decrease of entire reaction system [387], which is considered as thermodynamically unfavorable. In addition, reduction potential of the oxidized dye is located at forbidden zone of semiconductor, where no energy states exist. Consequently, there is no energy state overlap between the semiconductor and oxidized dye molecules meaning large force intensity should be exerted to drive the back-electron transfer with a low ratio [388, 389].
Fig. 12

a Photodegradation process explained in dye-sensitized perspective. b Electrons injection from vibrational levels of excited dye molecules into the continuously distributed energy states within CB of absorbed semiconductor. Reproduced with permission from ref. [360], Copyright 2010 Royal Society of Chemistry

Although dye sensitization effect enable a photocatalytic activity promotion, such influence will disappear as the dye molecules discolor. As well known, color disappearance just can represent the destruction of chromogenic function group in dye molecules rather than the sufficient degradation. Therefore, photocatalytic activity based on photocatalyst absorbing light is still needed to completely degrade the remaining colorless organic products.

4.2 Other applications of AgX

Large-scale emission of CO2 has created global warming troubles around the world and aroused considerable public concerns. One potential approach to reducing the atmosphere CO2 is photocatalytic reduction of CO2 with H2O to generate hydrocarbon resources (such as methanol and ethanol) utilizing solar energy [390]. Shown as in Fig. 13, the CB edges of AgCl and AgBr nanoparticles are estimated to be − 0.516 and − 0.571 eV (vs NHE), which are more negative than the reduction potential of CO2Θ(CO2/CH3OH) = − 0.38 eV) [61]. These band energy calculation results indicate that the photoexcited electrons in CB of AgBr and AgCl are capable to convert the CO2 molecules into carbohydrate CH3OH [391]. Actually, many works have manifested that the AgX was more suitable for catalytically reducing CO2 into ethanol as φΘ(CO2/C2H5OH) = − 0.085 eV [163, 208, 215]. In alloyed AgClxBr1-x (x = 0, 0.25, 0.5, 0.75 and 1) photocatalysts, the maximum conversion efficiency has been achieved by AgCl0.75Br0.25 sample with methanol and ethanol yields are 181 and 362 mmol g−1, respectively. The corresponding apparent quantum efficiency was calculated to be 3.781% [208].
Fig. 13

Photocatalytic mechanism of Ag/AgX (X = Br, Cl) for visible-light reduction of CO2. Reproduced with permission from ref. [61], Copyright 2012 Royal Society of Chemistry

In addition, under UV/visible light irradiation, the photocatalytic oxidation of H2O to O2 can be realized by AgCl nanoparticles with the presence of a small excess of silver ions in solution [242, 392, 393]. The overall reaction process can be summarized as the following equations: [394].

$$ \left[{nAg}^{+}, mAgCl\right]+\frac{r}{2}{H}_2O\overset{light}{\to }\ \left[\left(n-r\right){Ag}^{+}, mAgCl,{rAg}^0\right]+{rH}^{+}+\frac{r}{4}{O}_2 $$

According to this redox reaction, oxygen can be generated under light illumination by AgCl while the excess of Ag+ is reduced to Ag0, which is also the reason of the necessary existence of Ag+. However, for the same reason, the application of photocatalytic O2 evolution has been expanded.

5 Concluding and perspective

The extensive research based on AgX into photocatalytic applications in recent years has been confirmed as a powerful platform on the utilization of light for environment remediation and energy output. Because of the metallic Ag0 nanoparticles automatic generation under light irradiation, whose plasmonic effect can promote light absorption and charge carriers’ separation, AgX has been the semiconductor of choice for the photocatalytic applications. Due to the progress in both the synthesis and assembly of AgX nanostructures, many advances in performance improvement have been made possible. Through PVP-assisted precipitation, template-directed formation and in situ ion exchange methods et. al, large numbers of nanostructures with various morphologies can be produced. Among them, the uniform nanoparticles with fundamental morphologies (including spheres, near-spheres, cubes, plates and tubes), complex geometries with precisely controlled facets and corners, and hybrid composites constituted by AgX with other substrates have all been investigated and summarized. On the other hand, the growth mechanism during the synthesis has illuminated many critical factors for the various morphology formation, such as surfactant assisting, template directing, and molecules capping [217]. These mechanism insights are benefit for further accurate controlling of AgX geometry in the further researches. The reasons for the large content length on the synthesis of AgX nanostructures is that the photocatalytic responses of single AgX materials are mainly controlled and manipulated by their featured shapes with many active sites. And a number of experimental characterizations and theoretical calculation modes have manifested that much more high-active reaction sites can be exposed in various nanostructure geometry with different synthesis processes. Furthermore, energy band regulation and composition constitution between different AgX species or with other substrates are also considered as critical factors in determining the photocatalytic properties.

Aside from the AgX nanostructure investigations from a fundamental science angle, photocatalytic properties in the final applications are the real purpose, including organic dye degradation, CO2 reduction and O2 evolution. Among them, the most prominent application of AgX at current time is in organic dye degradation. Due to the photosensitivity of dye molecules and spontaneous generation of Ag0 under light irradiation, different reaction mechanisms with various charge carriers transfer pathways are exhaustively analyzed one by one. For instance, the photoinduced electrons from sensitized dye molecules to semiconductors, plasma electrons in metallic Ag0 nanoparticles injecting into adjacent AgX semiconductor or inversely transfer from AgX to metallic Ag0. Some contradictions and conflicts are existed in the field of reaction mechanism under different perspectives, as microscopic reaction process during photocatalytic has not be entirely grasped by scientists. That also a research hotspot in photocatalytic researches. In summary, a rapid development can be observed in the research topic of photocatalysis, and it is promised to be applied to more new scientific and technical fields.

Nevertheless, there is still considerable space for photocatalytic applications of AgX especially in stability aspect. The inevitable integration of Ag+ ions and photogenerated electrons in AgX system would destruct the original crystal construction in a certain extent. Particularly, when special morphology has been exhaustively designed and specific crystal plane should be investigated, the formation of metallic Ag0 substance on AgX surface make experimental program default. Therefore, an urgent probe should be conducted to reduce the combination between Ag+ ions and electrons. Overall, this review is aimed to provide a comprehensive assistance for the further development of AgX and their corresponding composites, so that high efficiency in photocatalytic performance would be constructed.



This work was supported by NSFC, China (21622509, 21527806 and 21475122), Department of Science and Techniques of Jilin Province (20150201001GX and 20150203002YY), Jilin Province Development and Reform Commission (2016C014, 2017C053-1), Science and Technology Bureau of Changchun (15SS05).


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Copyright information

© Springer International Publishing AG 2017

Authors and Affiliations

  • Yingying Fan
    • 1
    • 2
  • Dongxue Han
    • 1
  • Zhongqian Song
    • 1
    • 2
  • Zhonghui Sun
    • 1
    • 2
  • Xiandui Dong
    • 1
  • Li Niu
    • 1
  1. 1.Center for Advanced Research on Analytical Science, School of Chemistry and Chemical EngineeringGuangzhou UniversityGuangzhouPeople’s Republic of China
  2. 2.Engineering Laboratory for Modern Analytical Techniques, Changchun Institute of Applied Chemistry, Chinese Academy of SciencesChangchunPeople’s Republic of China

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