In-situ synthesis of amorphous silver silicate/carbonate composites for selective visible-light photocatalytic decomposition

Coupling two different semiconductors to form composite photocatalysts is an extremely significant technique for environmental remediation. Here, a one-step in-situ precipitation method has been developed to prepare amorphous silver silicate/carbonate (AgSiO/Ag2CO3) nanoparticles (NPs) composites, which are well dispersed sphere-like particles with the sizes of around ~50–100 nm. The high-efficiency photocatalytic activities under visible light (VL) have been carefully evaluated, and the AgSiO/Ag2CO3 NPs composites exhibit selective photocatalytic degradations on Methylene Blue (MB) and Rhodamine B (RhB). The maximum degradation rate for MB can reach ~99.1% within ~40 min under VL irradiation, much higher than that of RhB (~12%) in the same condition, which can be ascribed to (I) the smaller molecule size of MB than that of RhB, (II) the fast charge separation between AgSiO NPs and Ag2CO3 NPs, abundant heterojunction interfaces as well as fully exposed reactive sites. These composites are proposed to be an example for the preparation of other silicate composite photocatalysts for practical applications in environmental remediation.


Photocatalytic tests.
Photocatalytic degradation experiments were conducted in a photocatalytic reactor equipped with a 420 nm cut-off filter and a 500 W Xe lamp as the light source. For the photocatalytic reaction, the photocatalysts (20 mg) were mixed into a MB and RhB solution (50 mL, 10 mg/L). To guarantee the adsorption-desorption counterpoise, the mixed solution was stirred for 60 min in the darkness. Then, the solution was illuminated by VL and a small portion of the suspension (~3.0 mL) was sampled at 10 min illumination spacing. The catalytic efficiency was tested by using a UV-vis spectrophotometer (UV-2500, Shimadzu) for the degradation of RhB and MB.

Results and Discussion
A facile in-situ precipitation method was designed to synthesize AgSiO/Ag 2 CO 3 nanoparticles as illustrated in Fig. 1. The chemical reaction process can be described as follows: By adding AgNO 3 to the NaHCO 3 solution, the CO 3 2− present in the solution will react with the added Ag + by electrostatic interaction to produce Ag 2 CO 3 . Then, the fixed Ag + cations will further react with SiO 3 2− to generate AgSiO with the addition of Na 2 SiO 3 solution, resulting in the formation of AgSiO/Ag 2 CO 3 [5][6][7] .
The XRD patterns of pure AgSiO, Ag 2 CO 3 and AgSiO/Ag 2 CO 3 composites are shown in Fig. 2. The broad peak at about 34° could be indexed to the (−124) and (−115) planes for pure AgSiO in Fig. 2A, suggesting its amorphous structure 6 . For the pure Ag 2 CO 3 NPs (Fig. 2B), the positions and intensities of the diffraction peaks are well matched with the standard JCPDS card 2,3,11 . The XRD pattern of AgSiO/Ag 2 CO 3 composites is shown in Fig. 2A. Obviously, the diffraction peaks of AgSiO and Ag 2 CO 3 NPs can be clearly found in curve and without any other peaks. The peaks of Ag 2 CO 3 decreased with increasing AgSiO contents, and the characteristic peak intensity of AgSiO increases with the increase of the AgSiO contents, indicating that AgSiO/Ag 2 CO 3 composites with high purity have been successfully synthesized by an in-situ precipitation 1,8 .
FTIR spectra of AgSiO, Ag 2 CO 3 and AgSiO/Ag 2 CO 3 composites were recorded as shown in Fig. 3. In comparison to pure AgSiO, the absorption bands of AgSiO/Ag 2 CO 3 composites obtained at around ~705 cm −1 , ~893 cm −1 , ~1382 cm −1 , and ~1451 cm −1 are attributed to CO 3 2− in Ag 2 CO 3 1,10 . Compared with pure Ag 2 CO 3 , strong absorption band at around ~1382 cm −1 is found for AgSiO/Ag 2 CO 3 composites, which belongs to the Si-O-Si stretching vibrations 8 . Moreover, there is also another shoulder peak indexed to the Si-O-Si bonds at ~1630 cm −1 , which confirms the successful introduction of AgSiO in the composites. Nevertheless, the characteristic peak located at ~1382 cm −1 shifts to higher wavenumbers (Fig. 3). The observed blue shift in the composites after introducing AgSiO (Fig. 3) indicates the weakened bond strengths of Si-O-Si owing to the conjugation between Ag 2 CO 3 and AgSiO 8,12 . This result demonstrates the strong interfacial coupling effect in the AgSiO/ Ag 2 CO 3 composites. Compared with the aggregated or large-size nanoparticles, the nature of AgSiO nanoparticles on Ag 2 CO 3 acted as nano-islands can facilitate the formation of the heterojunction interfaces and guarantee the higher contact areas 13,14 . Both characters are essentials to promote photocatalytic activity and to enhance the separation efficiency of photogenerated charges.
The UV-vis diffuse reflectance spectra of AgSiO/Ag 2 CO 3 composites together with pure AgSiO and Ag 2 CO 3 are shown in Fig. 3. The light absorption edge of Ag 2 CO 3 is measured to be ~480 nm, and mainly absorptions are ultraviolet light 14 . In addition, the peak at 520 nm can be indexed to the Ag nanocrystals. Therefore, we can see that a broad absorption ranging from ~480 to ~750 nm is detected, which is due to the generation of Ag nanocrystals 15,16 . The AgSiO/Ag 2 CO 3 composites exhibit the stronger absorption than that of the Ag 2 CO 3 NPs in both the visible and ultraviolet light region 17 . Obviously, the introduction of AgSiO can significantly enhance the absorption in the visible-light region and can even extend to near-infrared region, which is attributed to the SPR of Ag nanoparticles. It is inferred that the heterojunction of AgSiO/Ag 2 CO 3 composites results in significantly  decreased interfacial contact barrier and strengthened electronic coupling of the semiconductors to generate more photogenerated electrons/holes with improved photocatalytic performance 18,19 .
The surface chemical compositions of AgSiO, Ag 2 CO 3 and AgSiO/Ag 2 CO 3 -5:1 composite were investigated by XPS (Fig. 4). The full-scan XPS spectra of pure AgSiO, Ag 2 CO 3 and AgSiO/Ag 2 CO 3 -5:1 composite ( Figure S1) indicate the presence of Ag, Si, O in AgSiO, Ag, C, O in Ag 2 CO 3 and Ag, Si, O, C in AgSiO/Ag 2 CO 3 -5:1 composite, respectively 15 . Figure 4A depictes the Si 2p peak of AgSiO/Ag 2 CO 3 -5:1 composite. The divided peaks located at ~96.9, ~102.8 and ~101 eV can be indexed into Ag 4 s, Si 2p 1/2 and Si 2p 3/2 , respectively 6 . Figure 4B shows two XPS peaks located at ~368.1 eV and ~374.1, which can be indexed to Ag 3d 5/2 and Ag 3d 3/2 of pure AgSiO, Ag 2 CO 3 and the AgSiO/Ag 2 CO 3 -5:1 composite 16,17 . These two peaks can be further divided into four peaks, ~368.1 and ~374.1 eV for Ag + 3d 5/2 and 3d 3/2 , and ~368.8 and ~374.7 eV for Ag 0 3d 5/2 and 3d 3/2 , respectively. The peaks at ~368.8 and ~374.7 eV confirm the existence of metallic Ag 0 in our AgSiO/Ag 2 CO 3 -5:1 composite. The carbon element in AgSiO is mostly ascribed to the adventitious hydrocarbon from XPS itself. Therefore the strength of C 1 s obeys the decreasing order of Ag 2 CO 3 > AgSiO/Ag 2 CO 3 > AgSiO 20 . Figure 4D demonstrates that O 1 s peak in AgSiO/Ag 2 CO 3 contains two distinguishable shoulders in the spectrum, demonstrating that two chemical states of oxygen are present on the surface 21 Figure 5B clearly shows that the AgSiO samples are sphere-like particles with the sizes of ~50-100 nm 22,23 . Figure 5C-F indicate that the morphologies of AgSiO/Ag 2 CO 3 with different ratios are similar to AgSiO 24 . The presence of SiO 2− in the sources has great effects on the final morphology of composite. The particle sizes of the composite decrease with the increase addition of SiO 2− . Furthermore, most particles are non-agglomerated and the sizes are mostly less than 100 nm, demonstrating that the samples are really nanosized cluster compounds 24 . The intimate contact between AgSiO and Ag 2 CO 3 will strengthen the photogenerated charge separation and transfer 25 . Compared with traditional aggregated or large contact structures, this AgSiO/Ag 2 CO 3 composites can not only provide more surface active sites for sequential photocatalytic reactions, but also shorten the migration distance of photogenerated charges 10 .
The HRTEM images in Figure S2(A,B) show that the size of AgSiO/Ag 2 CO 3 NPs is of 10 to 20 nm. The lattices of AgSiO/Ag 2 CO 3 NPs are clearly visible in the HRTEM images. The close interface between the AgSiO and Ag 2 CO 3 nanoparticles reveals the formation of nano-heterojunction 26 . From the figure, we can see that Ag 2 CO 3 is uniformly packaged by an amorphous AgSiO, limiting the growth of Ag 2 CO 3 27 . Figure S2(B) shows the lattice spacing of uniformly dispersed Ag 2 CO 3 nanoparticles. The lattice fringes of 0.275 nm is in agreement with the spacings of the (−101) plane of Ag 2 CO 3 and consistent with JCPDS Card No. 26-0339 28 . No lattice fringes of AgSiO can be observed. This kind of heterojunction, which is favorable for the transport of photoexcited carriers, is formed between AgSiO and Ag 2 CO 3 29 . Mainwhile, the composites can be further characterized by element mapping and EDS images (Fig. 6), where the Ag, Si, O and C elements are homogeneously distributed over the  whole profile [30][31][32] . From the above results, we can expect that the strong interfacial coupling effect between AgSiO and Ag 2 CO 3 will promote photogenerated electron-hole pairs separation and transfer, and thus further enhance the photocatalytic performance of AgSiO/Ag 2 CO 3 composites 33 .
Surface area, pore size and pore volume parameters for pure AgSiO, pure Ag 2 CO 3 and AgSiO/Ag 2 CO 3 composites were also investigated. The porosity of the AgSiO/Ag 2 CO 3 composites sample is clearly enhanced. According to Figure S3, all the materials except for Ag 2 CO 3 show a narrow pore size distribution with the average diameter of d > 2 nm 13,34 . With further increase of the AgSiO content, the adsorbed volume drops obviously indicating that the enhancement in adsorption volume is not driven solely by the small particles 6,13,35 . The surface areas of the different AgSiO/Ag 2 CO 3 composites and pure AgSiO and Ag 2 CO 3 are also calculated as shown in Fig. 7B for a better understanding of the composite nanostructure. It can be seen that the surface area of Ag 2 CO 3 is ~11.4 m 2 ·g −1 . With further increasing of the amount of the AgSiO, the surface area values of the AgSiO/Ag 2 CO 3 composites increase obviously. The surface area of AgSiO/Ag 2 CO 3 -6:1 composite can be achieved ~107.9 m 2 ·g −1 . Although the AgSiO/Ag 2 CO 3 -5:1 composite specific surface area isn't the largest, the degradation is best, which can be attributed to its heterogeneous structure.
All AgSiO/Ag 2 CO 3 composites exhibit higher photocatalytic activities than either AgSiO or Ag 2 CO 3 with the order of AgSiO/Ag 2 CO 3 -5:1 > AgSiO/Ag 2 CO 3 -6:1 > AgSiO/Ag 2 CO 3 -4:1 > AgSiO/Ag 2 CO 3 -3:1 > AgSiO/ Ag 2 CO 3 -2:1 > AgSiO > Ag 2 CO 3 , indicating the positive effect of AgSiO contents on enhancing the photocatalytic activities of the composites 8,37 . Full degradation of MB can be observed within ~40 min by VL irradiation in the presence of AgSiO/Ag 2 CO 3 -5:1 composite (Fig. 8A), illustrating the significantly improved photocatalytic activity of the AgSiO/Ag 2 CO 3 composites 36 . However, further increment of proportion (AgSiO/Ag 2 CO 3 -6:1) results in decreased photocatalytic activity, which may be attributed to the recombination of photogenerated electrons and holes, and thenthe photocatalytic efficiency is restrained 38 . Figure 9 shows the variation of the absorption spectra of MB under VL irradiation by AgSiO/Ag 2 CO 3 -5:1 composite. The characteristic peak intensities of MB gradually decrease by prolonging the irradiation time, and the adsorption peaks disappear within ~40 min irradiation 12,39 . The corresponding optical photographs of MB degradation using different photocatalysts under different irradiation times are collected and displayed in Fig. 9B. The color of MB gradually becomes lighter by prolonging the irradiation time, it can be seen that the color of AgSiO/Ag 2 CO 3 -5:1 composite becomes transparent while pure color for AgSiO and Ag 2 CO 3 turns into azury 40 . The variation of the absorption spectra of RhB under VL irradiation by AgSiO/Ag 2 CO 3 -5:1 composite is shown in Fig. 9C. The characteristic peak intensities of RhB gradually decreased by prolonging the irradiation time, and the adsorption peaks became constant within ~40 min irradiation indicating the very slow degradation of RhB 12,38 . These corresponding optical photographs of RhB degradation using different photocatalysts under different irradiation times are in Fig. 9D, showing that the color of RhB almost keeps unchanged.
The selective photocatalytic phenomenon of the AgSiO/Ag 2 CO 3 -5:1 composite was examined by photodegradation of MB and RhB 38 . The absorptions peaks of MB located at ~664 nm almost disappear after irradiation within ~40 min. However, the absorption peaks of RhB located at ~554 nm doesn't change too much (Fig. 9) 39 . As for two cationic dyes MB and RhB, we speculate that the discrepancy in adsorption capacities among them can be ascribed to the molecule size 29,35 . That is, because the size of MB molecules is smaller than those of RhB, MB can be intercalated into the space, while RhB are too large to intercalate into the NPs 37 .
Similarly, we used MB&MO, MO&RhB and MO&CR mixture dyes to further research the selectivity of the material. As shown from the figure S4, When AgSiO/Ag 2 CO 3 composites were added into the binary mixtures, respectively, the characteristic peaks of cationic dyes MB disappeared quickly, while the characteristic peaks of all Photocatalytic mechanism. The electrons and holes produced by photocatalysis have strong reduction and oxidation capacities. The main active species of different photocatalysts may vary due to their different band structure and phase compositions 10,38 . Thus, to explore the mechanism of the high photocatalytic activities and to assess the contribution of the reactive species, trapping experiments of reactive species were conducted using ethylenediaminetetraacetate (EDTA-2Na), iso-propyle alcohol (IPA) and N 2 as h + and OH − and e − scavengers, respectively 10,16 . By adding scavengers into the degradation solutions, the reactive species in the degradation process can be revealed. As shown in Fig. 10, the degradation rate decreases clearly to ~8.8% in the presence of EDTA-2Na (h + scavenger) and the degradation rate is ~98% in the absence of scavengers, which indicates that h + is the major reactive specie for MB degradation (Fig. 10A,B) 8,40 . Introducing IPA displays a significant effect on the K app . It decrease from ~0.102 min −1 to ~0.055 min −1 (Fig. 10C), suggesting that the radical is also a dominant reactive species. And the degradation rate decreases obviously to ~53% and ~66% in the presence of N 2 (O 2− scavenger) and AgNO 3 (e − scavenger), which suggests that O 2− and e − is the partly reactive species for MB degradation (Fig. 10D,E). Through Fig. 10F, we can visually see that the O 2− , e − and h + are reactive species 42 .
The results indicate that the incorporation of AgSiO with Ag 2 CO 3 photocatalyst successfully improves the VL photocatalytic performance and restrains the photocorrosion in a large level 13,44 . To further comprehend the separation and recombination of electron-hole pairs in pure AgSiO, Ag 2 CO 3 and AgSiO/Ag 2 CO 3 composites, the photocurrent test is carried out under visible light 33 . In this study, electrochemical and photoelectrochemical measurements were performed in 1 M Na 2 SO 4 electrolyte solution in a three-electrode quartz cell. Pt sheet was used as a counter electrode and Hg/Hg 2 Cl 2 /sat. KCl was used as a reference electrode. The pure AgSiO, Ag 2 CO 3 and AgSiO/Ag 2 CO 3 composites on ITO was used as the working electrode for investigation. The photoelectrochemical response was recorded with a CHI 660E electrochemical system 33 . The photocurrent-potential plots of these samples are shown in Fig. 11B. The figure shows the obvious photocurrent intensity of different samples under illumination by 250 lumens LED for 30 second intervals. The best photocurrent intensity (0.7 μA·g −1 ) of AgSiO was obtained when the applied potential is 1.3 V. And the best photocurrent intensity of Ag 2 CO 3 was 1.2 μA·g −1 . However, the photocurrent intensity was improved to 1.8 μA·cm −1 with the same applied potential after a heterojunction formed form AgSiO and Ag 2 CO 3 , which was three times greater than pure Ag 2 CO 3 . This phenomenon reveals that the AgSiO/Ag 2 CO 3 heterostructures possess a larger carrier concentration than the pure AgSiO and Ag 2 CO 3 NPs, and more electron-hole pairs are generated for the charge separation process 45 . These results of the photocurrent tests are in agreement with the results of the photodegradation of MB. The obvious photocurrent demonstrates that the interfacial charge separation between AgSiO and Ag 2 CO 3 NPs exists in this composite 46 .
Based on experimental results, Fig. 12 depicts a diagrammatic sketch for photocatalytic mechanism. Under VL irradiation, AgSiO can absorb VL, leading to the excitation of e − to the conduction band (CB) and whilst keeping h + in the valence bands (VB) 2,7 . For AgSiO/Ag 2 CO 3 heterojunctions, the photogenerated e − on the CB of AgSiO can easily migrate to the CB of Ag 2 CO 3 while the photogenerated h + in the VB of Ag 2 CO 3 migrates to AgSiO 19 . That is to say, the appropriately aligned band edges of AgSiO and Ag 2 CO 3 indicates that the migration of effective photogenerated charges can occur via the heterojunctions with strong interfacial coupling effect in  the composite 47 . The migration of photogenerated charges limit the transmission of photogenerated e − and h + on different sides, which reduces the recombination rate of photogenerated electron-hole pairs and improves the abundance and stability of photogenerated charge in the composite 6,7 . At the same time, the isolated photogenerated charges promote the production of reactive oxidative species, i.e. •O 2− and •OH, which are responsible for degrading MB confirmed by Fig. 12 26,48,49 .

Conclusions
In summary, a facile in-situ precipitation method has been designed and developed to synthesize a series of AgSiO/Ag 2 CO 3 composites with the sizes in the range of 100 nm. The as-synthesized AgSiO/Ag 2 CO 3 -5:1 composite shows superior VL photocatalytic activities, and the degradation of MB reach as ~99.1% under VL irradiation within ~40 min, which can be ascribed to the synergetic effect between AgSiO and Ag 2 CO 3 , including the maximum heterojunction interface with intimate contact, enhanced photogenerated charge separation efficiency, fully exposed reactive sites as well as excellent VL response in the composite. For the selectivity for degradation, we speculate that the discrepancy in degradation capacities among the two anionic dyes can be ascribed to the molecule size. This work will give insights into the importance of rational design of heterojunction systems, and provide a potential method for the construction of efficient heterojunction photocatalysts with controllable sizes and space distributions.