Enhanced Photocatalytic Activity of Nanoparticle-Aggregated Ag–AgX(X = Cl, Br)@TiO2 Microspheres Under Visible Light

Ag–AgX(X = Cl, Br)@TiO2 nanoparticle-aggregated spheres with different mass ratio of R = TiO2/Ag(X) from 35:1 to 5:1 were synthesized by a facile sol–gel technique with post-photoreduction. The photocatalytic activities of both Ag–AgCl@TiO2 and Ag–AgBr@TiO2 under visible light are effectively improved by ~3 times relative to TiO2 NPAS under the simulated sunlight for the decomposition of methyl orange (MO). Ag–AgBr@TiO2 showed 30% improvement and less stable in photocatalytic activity than that of AgCl@TiO2. The role of Ag and AgX nanoparticles on the surface of Ag–AgX(X = Cl, Br)@TiO2 was discussed. Ag on these samples not only can efficiently harvest visible light especially for AgCl, but also efficiently separate excited electrons and holes via the fast electron transfer from AgX(X = Cl, Br) to metal Ag nanoparticles and then to TiO2-aggregated spheres on the surface of heterostructure. On the basis of their efficient and stable photocatalytic activities under visible-light irradiation, these photocatalysts could be widely used for degradation of organic pollutants in aqueous solution.


Introduction
As an important photocatalyst, titanium dioxide (TiO 2 ) has been widely used in decomposition of toxic and hazardous organic pollutants, and in water splitting for hydrogen production [1][2][3]. However, the rapid recombination of photoinduced electrons and holes and its wide band gap greatly lower the TiO 2 quantum efficiency. Therefore, it is of great importance to improve the visible-light harvesting and the separation of photoinduced electron-hole pairs in TiO 2 for its further applications. TiO 2 -based heterostructures or composites formed by semiconductors with narrower band gap (e.g. Ag 2 O/TiO 2 , BiVO 4 /TiO 2 /BiVO 4 , BiOI/TiO 2 , Cu 2 O/TiO 2 , and Bi 2 O 3 /TiO 2 ) have been intensively investigated to develop high activities [4][5][6][7][8]. In addition, noble metal-TiO 2 is used to improve the generation and separation of photoinduced carriers due to the surface plasmon resonance (SPR) of noble metal nanoparticles such as Ag and Pd [9][10][11][12]. The photoinduced electrons in noble metal transfer onto the surface of semiconductor, which results in visiblelight photocatalytic activity, such as Ag@TiO 2 or Ag nanoparticles and graphene-co-loaded TiO 2 . Therefore, noble metal-semiconductor junction has potential application for the visible-light response in environmental purification. Specifically, because noble metal has a strong electron storage property and large work function, the excited electrons transfer from TiO 2 to a noble metal nanoparticle is an ultrafast process [9]. This phenomenon improves photocatalytic activity by effective charge separation. However, there are two opposite electron transfers in the metal-semiconductor structure which is from noble metal to semiconductor and from semiconductor to noble metal, respectively, under an irradiation. Thus, it is important to steer the transfer of the photoinduced carriers.
Herein, we developed a facile low-temperature method to prepare Ag-AgX(X = Cl, Br)@TiO 2 NPAS heterojunction photocatalysts and the influence of weight rate R (R = TiO 2 :AgX) and band structures of heterojunctions on the photocatalytic activity were systematically studied. Ag-AgX(X = Cl, Br)@TiO 2 double heterostructures exhibited much higher photocatalytic activity for the degradation of MO under visible-light irradiation (k [ 400 nm). The possible mechanism and the key role of metallic Ag on the photocatalytic activity of the Ag-AgX(X = Cl, Br)@TiO 2 double heterostructures were proposed.

Preparation of Samples
All chemical reagents used in this experiment are of analytical purity and without further purification.

Preparation of Ag-AgCl@TiO 2 NPAS
TiO 2 NPAS were prepared by diglycol-mediated process with minor modification, which was reported elsewhere [24]. For Ag-AgCl@TiO 2 NPAS preparation, 0.23 g of TiO 2 NPAS was dispersed in 20 mL deionized water with vigorous stirring for 1 h, while 0.15 g cetyltrimethylammonium chloride (CTAC) was added dropwise to the suspension and stirred for 1 h. Then, 800 lL 0.1 mol L -1 AgNO 3 aqueous solution was added and continually stirred for 1 h and then irradiated for 30 min under the simulated sunlight. The precipitation was collected by a washing and centrifugation process, and then the obtained sample was dried at 60°C in a drying oven for 12 h. Finally, the precursors were completely crystallized and converted into Ag-AgCl@TiO 2 NPAS by heating at 300°C for 8 h in nitrogen. Ag-AgCl@TiO 2 with weight ratio (R = TiO 2 :-AgCl) of 1:0, 30:1, 25:1; 20:1; 15:1; 10:1; 5:1 denoted as sample S1-S7 was prepared by changing the amount of TiO 2 NPAS as shown in Table 1.

Characterization
X-ray photoelectron spectroscopy (XPS) patterns of Ag-AgX@TiO 2 NPAS were determined by photoelectron spectrometer (ESCALAB 250Xi). The morphological features of the samples were characterized by a scanning electron microscope (SEM, TESCAN VEGA 3 SBH SEM), a field emission scanning electron microscopy (FE-SEM, Nova 400 Nano SEM) and a high-resolution transmission electron microscopy (HRTEM, JEOL-4000EX). The elemental composition of the samples was recorded by energy-dispersive X-ray spectroscopy (EDS, OXFORD). UV-Vis absorption spectrum was measured by the UV-Vis spectrophotometer (UV-3600, Shimadzu) with an integrating sphere attachment. Photoluminescence (PL) measurement was obtained by an Edinburgh FLS920 fluorescence spectrometer with the excitation source of steady-state Xe900 450 W xenon lamp. All the measurement was taken at room temperature.

Photocatalytic Degradation
Twenty milligrams of samples was added into 50 mL aqueous suspension of MO (14 mg L -1 ) in a 100 mL beaker. The suspension was magnetically stirred in dark for 30 min to establish adsorption/desorption equilibrium before illumination. An simulated sunlight instrument (CHF-XM-500 W) with a power of 100 mW cm -2 was used as an illumination source, and the 420 nm cutoff filter was placed on the end of light transmission tube to ensure the irradiation only in visible-light wavelength. The catalysis after degradation was collected by a washing and centrifugation process and then dried at 60°C. In order to keep the mass of catalysis constant, some catalyst (about 2 mg) were added.

Crystal Structure, Morphology, and Formation
Mechanism of Ag-AgX(X 5 Cl, Br)@TiO 2 NPAS Figure 1a shows the XRD patterns of the Ag-AgCl@TiO 2 samples S1-S7. The sample S1 with R = 1:0 exhibits pure anatase phase  [29]. Figure 1b shows the XRD patterns of the Ag-AgBr@TiO 2 samples S8-S14. Similar to Ag-AgCl@TiO 2 , To further obtain the structure and morphology information of Ag-AgX(X = Cl, Br)@TiO 2 samples, FE-SEM, TEM, and HRTEM characterizations have been conducted as shown in Figs. 2 and 3. Figure 2a shows the FE-SEM and enlarged image in the inset of pristine TiO 2 (S1) which indicates nanoparticles-aggregated spheres with the diameter of 370-450 nm. The Ag-AgCl@TiO 2 with R from 30:1 to 5:1 (S1-S7) is shown in Fig. 2b-g, from which we can see that the diameter is about 500-600 nm with Ag-AgCl nanoparticles coated on the surface. The morphology of Ag-AgCl@TiO 2 is similar to that of the TiO 2 -aggregated spheres, which indicates that the precipitation of Ag-AgCl on the surface of TiO 2 does not affect the structure of 20 30 Intensity (a.u.)

S3
With the increase of Ag-AgBr in Ag-AgBr@TiO 2 , there are more and more small particles on the surface of samples. The TEM and HRTEM images of sample S11 are presented in Fig. 3g The surface chemical states of the Ag-AgX(X = Cl, Br)@TiO 2 with further investigation by XPS are shown in Fig. 4. The peak positions in all the XPS spectra were calibrated with C 1s at 286.4 eV. The XPS spectrum of the Ag-AgX(X = Cl, Br)@TiO 2 indicates that the product consists of Ag, Ti, O, and X (X = Cl, Br) elements (Fig. 4). The peak of N 1s results from the surface adsorption of nitrogen during preparation of Ag-AgX(X = Cl, Br)@TiO 2 at 300°C for 8 h under the protection of nitrogen. Two strong peaks at 464.3 and 458.6 eV are attributed to Ti 2p of TiO 2 (Fig. 4c), which is slightly shifted toward lower binding energies compared with that of pristine TiO 2 due to the direct contact between Ag nanoparticles and TiO 2 [21]. The peak of Cl can be divided into two peaks Cl 2p 3 and Cl 2p 1 peaks (Fig. 4d) [19], which correspond to 200.5 and 197.6 eV, respectively. Br 3d 5/2 and Br 3d 3/2 binding energies in XPS spectra correspond to 67.8 and 68.8 eV, respectively, which could be assigned to Brin AgBr (Fig. 4e) [18]. There are two strong peaks for Ag 3d at about 374 and 367.8 eV as shown in Fig. 4f-  Br)@TiO 2 [17]. On the basis of XPS results, it can be confirmed that the coexistence of Ag(0) and AgX on the surface of TiO 2 . EDS results are consistent with those of XPS, demonstrating distribution of Ag, Ti, O, and X (Cl, Br) on the samples as shown in the inset in Fig. 4a, b. The element of Cl and Br comes from C 16 H 33 (CH 3 ) 3 NCl (CTAC) and C 16 H 33 (CH 3 ) 3 NBr (CTAB), respectively. Their long molecular chain, about 2.3-2.5 nm, is favorable to connect with TiO 2 , but is not in tangle. On the basis of the above experimental results, a possible two-step growth mechanism for the formation of Ag-AgX(X = Cl, Br)@TiO 2 NPAS is proposed. In the first step, homogenously distributed AgX(X = Cl, Br) nanoparticles on the surface of TiO 2 -aggregated sphere are formed via an interaction between CTAC or CTAB and AgNO 3 , which induces nanoparticles to assemble into the final AgX(X = Cl, Br)@TiO 2 microspheres. The microspheres of Ag-AgX(X = Cl, Br)@TiO 2 are formed in the second step when they are exposed to the simulated sunlight. Unbound metallic Ag (Ag 0 ) appears due to the photo-induced reduction reaction of AgX nanoparticles. Free metallic Ag(Ag 0 ) attaches each other on the surfaces of AgX(X = Cl, Br)@TiO 2 and forms the final Ag-AgX(X = Cl, Br)@TiO 2 NPAS. The reduced Ag 0 atoms may also react with other silver species (Ag ? ions, Ag 0 atoms, or silver clusters). The formation process is shown by the following equations [30]: AgBrÀ! hv Ag 0 + Br 0 ð2Þ Ag 0 þ Ag qþ r ! Ag qþ To understand the formation of Ag clusters and its role in photoactivity in Ag-AgX@TiO 2 NPAS, we prepared the AgX@TiO 2 NPAS in the same way without irradiation. Ag-AgBr@TiO 2 , AgBr@TiO 2 , and AgCl@TiO 2 . It is well known that the band gap of anatase TiO 2 is about 3.2 eV. However, the absorption edge of the four samples is around 410 nm (about 3.1 eV), which is smaller than the band gap of anatase TiO 2 . It might result from surface defect of nanoparticle aggregation spheres with large specific surface and imprecise measurement, which contribute to the absorption edge starting at a little larger wavelength [31]. In addition, AgCl and AgBr on the surface of TiO 2 have little influence on the absorption edge of Ag-AgX@TiO 2 NPAS. AgCl@TiO 2 has no absorption in visible light because of the large band gap of AgCl (about 3.3 eV), while AgBr@TiO 2 has weak absorption in the visible light due to its smaller band gap (about 2.6 eV). Obviously, Ag-AgCl@TiO 2 has weak visible-light absorption. Thus, the photoactivity of Ag-AgCl@TiO 2 extends from the UV into the visible-light region. The absorption of Ag-AgBr@TiO 2 in the range of 400-800 nm is stronger than that of AgBr@TiO 2 because of the emergence of Ag. Therefore, as the accumulated excited electrons on the surface of AgX combine with Ag ? to form Ag 0 clusters under the simulated sunlight irradiation, Ag-AgX@TiO 2 is formed. Silver clusters have empty energy levels below the conduction band of AgCl, which enables a new electronic transition from the valence band of AgCl to the empty Ag cluster energy levels [30]. This is the well-known self-photosensitive property of AgX semiconductor, and it extends the light absorption from the UV to the visible region because of the silver clusters adsorbed on AgCl@TiO 2 surface [30]. The self-sensitization is different from the spectral sensitization effect of Ag clusters that makes an electron injection from Ag clusters into the AgCl conduction band [19]. Thus, the absorption in the visible light comes from the combined action of the surface plasmon resonance of metallic Ag and the self-sensitization effect of Ag clusters.
Silver clusters have contributed not only to light absorption but also the separation of the excited carriers.  The photoluminescence (PL) emission spectroscopy provides the separation efficiency of the electron-hole pairs and carrier lifetime in semiconductors [32]. The PL spectra of Ag-AgCl@TiO 2 , Ag-AgBr@TiO 2 , AgBr@TiO 2 , and AgCl@TiO 2 (Fig. 5b) show two characteristic bands with peaks at 451 and 523 nm caused by the e-h recombination with 350 nm excitation, which correspond to the emission of TiO 2 [33]. The PL spectra are much similar for the four samples except for the strength. The PL intensity of Ag-AgX@TiO 2 shows a notable decrease compared with that of AgX@TiO 2 . Thus, the decrease in the PL intensity suggests that Ag-AgX@TiO 2 NPAS possess a longer carrier lifetime, which demonstrates the enhancement of the photoinduced electron/hole pair separation. It is responsible for the enhanced photocatalytic activity for the pollutant degradation.

Visible-Light Photocatalytic Activity
MO is a kind of organic dye that is often used as a model pollutant to study the catalytic performance of photocatalysts. In this study, the photocatalytic activity of 20 mg Ag-AgX(X = Cl, Br)@TiO 2 is assessed by observing the degradation of MO (14 mg L -1 ) versus time under visiblelight illumination (Figs. 6, 7). Before irradiation, the MO solution with the catalyst was kept in the dark for 30 min to reach the adsorption equilibrium. The concentration of the MO solution slightly decreases in the dark. Comparisons of photocatalytic activities among the Ag-AgCl@TiO 2 samples with different R under 60-min irradiation are shown in Fig. 6a, b. The degradation activity of sample S1 (TiO 2 NPAS) is hardly observed due to the larger band gap (E g = 3.2 eV). But for the samples S2, S4, and S7, the corresponding degradation rates are 82%, 98%, and 85% under visible-light irradiation, which is much higher than that of S0 (TiO 2 NPAS under simulated sunlight, only 34.79%). It indicates that greatly improved photocatalytic activity is achieved by the heterostructure of Ag-AgCl@TiO 2 , and the S4 with R = 20:1 exhibits highest activity for MO degradation as shown in Fig. 6a. The temporal evolution of the absorption spectra of MO aqueous solution in presence of S4 under visible-light irradiation is shown in Fig. 6b. The characteristic peaks centered at 464 nm do not change in position in degradation process. It indicates complete photocatalytic degradation of MO aqueous solution by the Ag-AgCl@TiO 2 NPAS during the reaction, and the catalyzed degradation intermediates are almost not produced. Histogram of photocatalytic activities of Ag-AgCl@TiO 2 NPAS with different R under visiblelight irradiation for 60 min is shown in Fig. 6c. The samples with a wide range R from 10:1 to 25:1 exhibit higher activity for MO degradation with a degradation rate about 98%, which indicates a wide R range for application. It is important to be stable under repeated applications for an economical photocatalyst. Figure 6d shows the cycling performance of sample S4 for five cycles. The photocatalytic activity remains approximately 98%, revealing the excellent long-term photocatalytic stability of Ag-AgCl@TiO 2 NPAS. Figure 7 exhibits the activity of Ag-AgBr@TiO 2 NPAS for MO degradation. It can be seen the efficient removal ratio of 98.78% (for sample S11) in 40 min, which is also far superior to those of Ag-AgCl@TiO 2 NPAS. Compared with Ag-AgCl@TiO 2 NPAS, Ag-AgBr@TiO 2 NPAS have an almost complete photocatalytic degradation of MO and a wider R range from 25:1 to 5:1. Ever after five cycles, there is only about 7.25% of photocatalytic activity decrease for S11, indicating that Ag-AgBr@TiO 2 NPAS have excellent cycling photocatalytic activity.

Visible-Light Photocatalytic Mechanism of Ag-AgCl@TiO 2
To fully discuss the effects of the heterostructure on the photocatalytic activities of Ag-AgX(X = Cl, Br)@TiO 2 NPAS, the energy band diagram of Ag, AgX, and TiO 2 is presented in Fig. 8. AgCl has a direct band gap of 5.6 eV and an indirect band gap of 3.25 eV [21], and the band gap of AgBr is about 2.6 eV. Meanwhile, Ag-AgCl@TiO 2 NPAS can be excited by visible light as shown in Fig. 5a and it has self-photosensitive effect because of their point ionic defects and electron traps according to the Eqs. (1-5) [13,34], which might help to photochemical decomposition. It is just similar to the research by Hu group that two electrons in one oxygen vacancy in CeO 2 are, respectively, excited to two Ce atoms neighboring the vacancy [35]. S1 S8 S9 S10 S11 S12 S13 S14 However, some researchers proposed that plasma resonance absorption of metallic Ag (or Pd) on the surface of semiconductor contributes to the effective visible absorption and segregation of electron/hole pairs, which results in the higher photocatalytic activity [9,11,12]. Liu et al. found that the double heterostructure Ag/TiO 2 nanoparticles/TiO 2 nanobelts have perfect photocatalytic properties under UV light irradiation with the dominant wavelength of 355 nm and Ag nanoparticles on the surface of the TiO 2 act as a sink for electrons contributing to the interfacial charge-transfer separation [36]. However, the composite photocatalyst with a wide weight ratio of TiO 2 /Ag 2 O exhibits a higher photocatalytic activity, but a bad cyclic stability under UV light irradiation because Ag ? is reduced to Ag [37]. Therefore, the role of Ag on the photocatalytic performance in Ag-AgCl@TiO 2 NPAS is not determined. Thus, it is fascinating to consider how Ag/AgCl particles become an efficient and stable photocatalyst under visible light. To fully understand the photocatalysis process and the role of Ag nanoparticles during photocatalytic degradation MO, the XPS spectrum of the Ag-AgCl@TiO 2 after five cycles is presented in Fig. 4h. The two strong peaks at 373.4 and 367.4 eV corresponding to Ag3d 3/2 and Ag 3d 5/2 of AgCl in Ag/AgCl @TiO 2 become stronger, while the peaks at 374.2 and 368.4 eV corresponding to the metallic Ag of Ag 3d 3/2 and Ag 3d 5/2 become weaker. It indicates that some metallic Ag are oxidized to Ag ? and the selfsensitized effect of AgCl is suppressed during the photocatalysis reaction. Meanwhile, the XPS analysis and the photocatalysis under visible light prove that AgCl is stable at the photocatalytic degradation MO.
Therefore, the photocatalytic process of Ag-AgCl/TiO 2 under the visible light is as following. Firstly, under visible light irradiation, electrons on valence band of AgCl can be excited to empty silver cluster energy levels below the conduction band of AgCl and form photogenerated electron-hole pairs due to the energy of light lower than the band gap of AgCl [30]. Subsequently, because the CB bottom (0.11 eV vs. NHE) and VB top (3.04 eV vs. NHE) of AgCl are below that of TiO 2 (E CB = -0.41 eV, E VB = 2.37 eV), respectively [17], most of excited electrons coming from the surface plasmonic resonance of Ag cluster transfer to the conduction band AgCl and some excited electrons reach to the CB of TiO 2 . The excited electrons react with O 2 adsorbed on the surface of the Ag-AgCl@TiO 2 NPAS to produce ÁO 2 and H 2 O 2 that successively decompose the MO to the final products. Holes transferring to the AgCl surface oxidize Clions to ÁCl radicals, which recombine very fast to form Cl 2 , and then Cl 2 is reduced to chloride ions by reacting with H 2 O under the applied conditions [13,30]. Silver cations act as a catalyst for the decomposition of hypochloric acid into molecular oxygen, protons, and chloride ions. Hypochloric acid can also degrade the MO into micromolecule. Meanwhile, some holes also move to the valence band of TiO 2 . The holes on the surface of TiO 2 can also oxidize OHto yield Á OH. Therefore, the degradation of MO could be attributed to the reaction with Á O À 2 , Á OH, and H 2 O 2 species during the photocatalysis. The excellent photocatalytic performance of Ag-AgCl@TiO 2 NPAS could be due largely to the improvement of electron/hole pairs separation after loading Ag-AgCl on the TiO 2 . The reaction can be written as follows [17,30], Ag À s + O 2 ! Á O 2 + Ag ð9Þ Therefore, chlorine and hypochloric acid are inevitable during the photocatalytic reaction. Combining Eqs. (1) and (5)(6)(7)(8)(9)(10)(11)(12)(13), it indicates the excellent stability of Ag-AgCl@TiO 2 NPAS photocatalyst in the photocatalytic reaction under visible-light irradiation. Furthermore, it is well known that the Ag nanoparticles are likely to be slowly oxidized in the moist environment [13,30]. Therefore, the XPS peaks of Ag ? become stronger after photocatalytic reaction.
In summary, as a promising candidate, Ag-AgCl@TiO 2 photocatalyst shows highly efficient and stable photocatalytic activities under visible light. Ag clusters play a significant role in the light harvesting, and the double heterostructure also plays a role in the prevention of the recombination of photogenerated electron-hole pairs. Figure 8b shows the band structure of Ag, AgBr [16,18,19,38], and TiO 2 , which indicates the photocatalytic mechanism of the Ag-AgBr@TiO 2 photocatalyst. Under visible-light irradiation, electrons in AgBr (E CB = -0.67 and E VB = 1.93 eV) are excited from VB to CB for the narrow band gap of 2.6 eV and the photogenerated electrons easily transfer into metal Ag nanoparticles because the CB potential of AgBr is more negative than that Fermi level of the loaded metal Ag. Then some of the electrons in Ag from the AgBr transfer to the conduction band of TiO 2 (E CB = -0.41 eV, E VB = 2.37 eV) with the help of the Ag plasma resonance. The charge transfer process of AgBr ? Ag ? TiO 2 is similar to the efficient photosynthesis with charge flow steering [38][39][40]. The Ag clusters in the double heterojunction system act as a charge mediator for transferring electrons to the conduction band of TiO 2 and keeping holes in the valence band of AgBr. The stronger photoresponse in visible region and efficient carrier separation of Ag-AgBr@TiO 2 finally enhance the photocatalytic activity more than that of Ag-AgCl@TiO 2 .

Visible-Light Photocatalytic Mechanism of Ag-AgBr@TiO 2
Since the XPS spectra of Br 3d 5/2 and Br 3d 3/2 in Ag-AgBr@TiO 2 photocatalyst become weak after five cycles of photocatalytic degradation MO as shown in Fig. 4i, AgBr in Ag-AgBr@TiO 2 NPAS is resolved into element Br and element Ag during the reaction of photocatalytic degradation MO according to the Eq. (2), which results in the decrease of Ag-AgBr@TiO 2 NPAS on photocatalytic activity in degradation MO. However, the decomposition of AgBr does not obviously affect the photocatalytic activity, especially at the first three cycles because of the photocatalytic degradation of MO. Ag-AgBr@TiO 2 NPAS are excellent in a wider range ratio (R = TiO 2 /AgBr) from 25:1 to 5:1, and then the stability of photocatalytic activity is gradually worse until R is larger than 25:1. The photocatalytic reaction of Ag-AgBr@TiO 2 agrees with the report of Yue group [38].
Owing to the self-sensitization, AgBr is easily decomposed into Ag and Br under visible-light irradiation. As a strong oxidant, Br photodegrades MO into CO 2 and H 2 O. As a self-sacrificing of AgBr in Ag-AgBr@TiO 2 photocatalyst, the photodegradation of MO with Ag-AgBr@TiO 2 is similar to the Ag 2 O/TiO 2 nanobelts reported by Liu et al. [25]. Therefore, the stability of Ag-AgBr@TiO 2 photocatalyst is slightly worse than that of Ag-AgCl@TiO 2.

Conclusions
In this paper, Ag-AgX(X = Cl, Br)@TiO 2 NPAS synthesized by a facile sol-gel technique and post-photoreduction method. On the basis of their efficient and stable photocatalytic activities, these photocatalysts could be widely used for environmental purification of organic pollutants in aqueous solution. As a self-sensitization material, both AgCl and AgBr have perfect photoresponse in visible light, especially AgBr. Both of them can efficiently separate excited electrons and holes via the fast electron transfer to metal Ag nanoparticles on the surface of heterostructure. The photocatalytic activity of Ag-AgBr@TiO 2 is higher than Ag-AgCl@TiO 2 at the first stage. This work indicates that, as an electronic transmission medium, metallic Ag can enhance photocatalytic activity not only via forming heterostructure with narrow band gap photocatalysts (e.g. E g = 2.6 eV) but also for wide band gap photocatalysts (e.g. E g = 3.2 eV) through the different roles. Ag nanoparticles on the surface of AgCl can effectively help to absorb visible light by self-sensitization and SPR and maintain stability of AgCl, while Ag nanoparticles in Ag-AgBr@TiO 2 system act as a charge mediator for transferring electrons from AgBr to TiO 2 .
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://crea tivecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.