Introduction

Semiconductors as photocatalysts are expected to play an increasingly important role in many applications for the utilization of solar energy, for example, the conversion of water to hydrogen gas by photocatalytic water splitting [1], the creation of self-cleaning surfaces [2], disinfection of water [3], degradation of organic contaminants [4], as well as photoreduction of CO2 to gaseous hydrocarbons [5, 6]. As a traditional and most popular photocatalyst, TiO2 has been extensively used and investigated. However, the visible light photocatalytic activity of TiO2 is limited because of the wide band gap of 3.2 eV [7, 8]. Although many methods such as impurity doping [9], metallization [10], and sensitization [11] have been applied to TiO2 to extend the photoresponsive range towards the visible light, high synthesis cost and processing complexity limited enhancement in photocatalytic efficiency make them less appealing for widespread commercial application. Thus, it is highly desirable to develop a simple method to prepare the catalysts with high stability, high activity, low-cost and strong visible light response.

In the past decades, nanoscaled noble metals (such as silver and gold) have been extensively studied due to their unique surface plasmon resonance (SPR) properties in the visible region and a relative low Fermi level, which could decrease the recombination of photogenerated electrons and holes. Therefore, noble metal nanoparticles (NPs) could be utilized as a kind of visible light harvesting and converting centers to develop new plasmon enhanced photocatalysts. For example, the Au (or Ag)-loaded TiO2 [12], WO3 [13], ZnO [14], AgX (X=Cl, Br, I) [15], and other large-bandgap semiconductors [16] exhibit high photocatalytic performance in the visible light region. Recently, Ag@AgCl plasmonic photocatalysts, consisting of silver salt particles decorated with in situ grown Ag NPs, have been developed into promising photocatalysts for environmental purification applications under sunlight or visible light irradiation [17, 18]. It is well known that the AgCl NPs exhibit strong absorption only in the ultraviolet spectral region and weak absorption in the visible region due to the large bandgaps, direct bandgap of 5.15 eV (241 nm) and an indirect bandgap of 3.25 eV (382 nm) [17]. Nevertheless, (as for Ag0) the electric fields around silver NPs can be enhanced when the incident lights are in the resonance with the localized surface plasmon of silver NPs [19–21]. Hence it is an effective way to improve the photocatalytic activity of AgCl by constructing Ag@AgCl. Huang et al. [22] first synthesized Ag@AgCl as plasmonic photocatalyst by an ion exchange reaction between aqueous solutions of Ag2MoO4 and HCl followed by UV irradiation. Since then, Ag@AgCl as a promising plasmonic photocatalyst was further studied and developed. For example, multistep thermal polyol process [23], microwave-assisted [24], and sonochemical polyol techniques [25] were developed for the synthesis of Ag@AgCl particles with good photocatalytic activity. However, most of those methods mentioned above are time consuming, cost-intensive, and/or they have to be carried out under elevated temperatures, which do not meet the synthetic requirement of green chemistry in modern science. Consequently, new and simple approach for rapid synthesis of plasmonic Ag@AgCl photocatalyst is needed to be developed.

Besides synthetic macromolecular compounds such as sodium dodecylbenzene sulfonate (SDBS), polyvinylpyrrolidone (PVP), and cetyltrimethylammonium bromide (CTAB), some biomolecules employed as eco-friendly and inexpensive directing agents and/or capping agents for the synthesis of photocatalytic materials have attracted much attention in the past few years. Amino acids are promising and interesting biomolecular surfactants due to their multiple functional groups and adjustable polarity. For example, L-cysteine as a surfactant was widely used in the fabrication of photocatalysts such as ZnO nanorod arrays, flower-like In2S3, pagoda-like PbS with hierarchical architectures, NiS microcrystals, and CdS nanoshperes [26–30]. To the best of our knowledge, however, there is no report on the synthesis of Ag@AgCl plasmonic materials by using amino acids as surfactant. Herein, various amino acids were exploited for the first time for the one-pot synthesis of the Ag@AgCl plasmonic photocatalysts under ultrasonic conditions. The readily fabricated Ag@AgCl presents excellent visible light activity for photocatalytic degradation of Rhodamine B (RhB) and Formic acid (FA).

Experimental

Reagents

Silver nitrate (AgNO3), sodium chloride (NaCl), ethylene glycol (EG), L-lysine (Lys), L-valine (Val), L-glutamic (Glu), PVP, ethanol, Rhodamine B, and Formic acid were purchased from Shanghai Jingchun Industry Co., Ltd. All the reagents were used as received without further treatment.

Preparation of photocatalysts

Ag@AgCl photocatalysts were synthesized by a facile and rapid ultrosonic-assisted nonaqueous synthesis route. In a typical procedure, NaCl (300 mg) and the amino acid (225 mg) were added into 100 mL of EG to form a clear solution at room temperature. Then, 8 mL of EG solution containing AgNO3 (510 mg) was added dropwise to the above Cl−-amino acid solution under ultrasonic conditions. A white milky suspension immediately formed after the addition of the Ag+ solution, and the white suspension gradually became pale red in the sonication duration. The temperature of the suspension during sonication was monitored with a thermometer, and the apparent temperature was 70–80 °C. After reacting for 30 min under ultrasonic conditions, the hybrid particles were cooled down to room temperature, collected by centrifugation, washed three times with alcohol and water, and dried at 80 °C for 12 h. The Ag@AgCl photocatalysts synthesized by using various amino acids, such as L-lysine (Lys), L-valine (Val), and L-glutamic acid (Glu) were denoted as Ag@AgCl-Lys, Ag@AgCl-Val, and Ag@AgCl-Glu.

For comparison, the Ag@AgCl-PVP photocatalyst was synthesized under the same reaction conditions by using PVP to substitute amino acid. Moreover, the Ag@AgCl-Lys-H photocatalyst was synthesized by hydrothermal heating method instead of sonication during the reaction process.

Characterization

Powder X-ray diffraction (XRD) measurements were performed on a Bruker AXS-D8 diffractometer with CuKα radiation (λ = 0.15,418 nm). UV/Vis absorption spectra were recorded on a Hitachi U-3900H instrument by using BaSO4 as the reference. The morphology was observed with a Nova Nano SEM450 field-emission scanning electron microscopy (SEM). Surface electronic states were analyzed by X-ray photoelectron spectra (XPS, Perkin-Elmer PHI-5300 spectrometer, Al KR). All of the binding energies were calibrated by using the contaminant carbon (C1s 284.6 eV) as a reference.

Photocatalytic activity

The photocatalytic activities of the samples were evaluated by the photocatalytic degradation of RhB and FA aqueous solution at ambient temperature under a 300 W Xe lamp with UV cut off filter (λ ≥ 420 nm). In a typical procedure, 50 mg Ag@AgCl photocatalyst was suspended in 50 mL RhB aqueous solution (10 mg L−1) in a 100 mL beaker. Prior to irradiation, the suspension was stirred for 30 min in the dark to ensure an adsorption/desorption equilibrium between the photocatalyst and RhB dye. During the photocatalytic degradation processes, 2 mL of suspensions were sampled at certain time intervals and centrifuged to remove the photocatalyst particles. The upper clear liquid was analyzed by recording the characteristic absorption peak of RhB at 553 nm with an UV–Vis spectrophotometer to calculate the concentrations of the dye.

The photocatalytic degradation of FA solution (1 mmol L−1) was carried out by the same procedure like that of RhB. The concentration of FA samples was detected by UV–Vis spectrophotometer at 206 nm.

Photoelectrochemical measurements

Photocurrents were measured using an electrochemical analyzer (CHI660C Instruments) in a standard three-electrode system with the prepared samples as the working electrodes with an active area of catalyst 1.0 cm2, a graphite as the counter electrode, and SCE (saturated calomel electrode) as a reference electrode. A 300 W Xe lamp equipped with an ultraviolet cut off filter to provide visible light with λ ≥ 420 nm served as the visible light source. A 0.5M Na2SO4 aqueous solution was used as the electrolyte.

Results and discussion

Characterization of Ag@AgCl photocatalysts

The XRD patterns of the as-prepared Ag@AgCl photocatalysts are shown in Fig. 1. All patterns matched very well with the JCPDS standard data of AgCl (JCPDS No. 31-1238), and no impurity phase is found. The diffraction peaks at 2θ values of 27.8, 32.2, 46.2, 54.8, 57.5, 67.5, 74.5 and 76.7o are assigned to the (111), (200), (220), (311), (222), (400), (331) and (420) planes of AgCl, in order. It is noted that there is no reflection assigned to Ag metal in the XRD patterns, possibly because that the low Ag content [31] was below the detection limit and/or the Ag particles on the surface of AgCl are poorly crystallized [32, 33]. Furthermore, the intensity ratio of I200/I111 for the Ag@AgCl samples is about 2.2, 2.06, 2.16, which is synthesized by using different amino acids (Lys, Val, Gul). These results are higher than the literature data (JCPDS No.31-1238) of I200/I111 = 2.0. It was reported that the oxygen atoms of PVP preferentially bind to (100) crystallographic planes of FCC silver, which can inhibit AgCl (100) faces and reduce the area of (111) faces during the growth of AgCl cubic crystals [34]. It is envisioned that the oxygen atoms of amino acid could play the similar role like that of PVP. Therefore, the AgCl cuboids were obtained by joining amino acids as directing and assembling agent [35].

Fig. 1
figure 1

XRD patterns of a Ag@AgCl-Lys, b Ag@Ag-Val, and c Ag@AgCl-Glu

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The morphologies of the as-synthesized Ag@AgCl photocatalysts were observed by SEM. As shown in Fig. 2, all the Ag@AgCl particles synthesized by using various amino acids have a cuboid shape and uniform sizes, which is consistent with the results of XRD.

Fig. 2
figure 2

SEM images of a Ag@AgCl-Lys, b Ag@AgCl-Val, and c Ag@AgCl-Glu

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The elemental composition, chemical status and silver (Ag0) content of the Ag@AgCl sample are further analyzed by means of XPS. The survey spectra (Fig. S1) of the as-synthesized Ag@AgCl samples are similar and confirmed that the Ag and Cl are the main components. In addition, small amounts of O, N, and C are detected. The N 1 s, C 1 s, and O 1 s peaks are attributed to the trace amounts of amino acid molecules adsorbed on the surfaces of Ag@AgCl particles. Ag 3d spectrum (Fig. 3) consists of two peaks at 367.7 and 373.7 eV, which correspond to the binding energies of Ag 3d3/2 and Ag 3d5/2, respectively. The two peaks can be further deconvoluted into two different peaks at 367.5 and 368.8 eV, as well as 373.5 and 374.6 eV. The bands at 367.5 and 373.5 eV are attributed to the peaks of Ag+ (AgCl), and the bands at 368.8 and 374.6 eV could be ascribed to the peaks of metallic Ag0 [36]. According to the XPS results, the surface Ag0 contents of the Ag@AgCl-Lys, Ag@AgCl-Val and Ag@AgCl-Glu samples are calculated to be 8.4, 7.9, and 7.3 mol%, respectively. Moreover, two peaks at about 199.6 and 197.9 eV were observed in the Cl 2p spectra (Fig. S2), corresponding to the binding of Cl 2p1/2 and Cl 2p3/2 respectively. This illustrates the existence of Cl–.

Fig. 3
figure 3

XPS spectra of Ag 3d in the as-prepared Ag@AgCl samples

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From the above results, we can see that the Ag0 content of the as-synthesized Ag@AgCl samples is in the order of Ag@AgCl-Lys > Ag@AgCl-Val > Ag@AgCl- Glu. Xu et al. [37] considered that PVP not only acted as a growth controlling agent but also as a reducing agent to reduce partial Ag+ to metallic Ag in the synthesis process of Ag@AgBr samples. Herein, we speculate that the amino acids also play similar roles, acting as both a shape-directing agent for the synthesis of AgCl cuboids and a reducing reagent for the formation of Ag0 in the synthesis process of Ag@AgCl photocatalysts. Meanwhile, the amino acid molecules with more amino groups could exhibit stronger reducing ability. Accordingly, the reducing ability of the amino acids is in the order of L-lysine > L-valine > L-glutamic acid, which is in consistent with the Ag0 content trend in corresponding Ag@AgCl samples.

Fig. 4 shows the UV/Vis absorption spectra of AgCl and Ag@AgCl samples. For pure AgCl, only a peak in the range of 350–400 nm can be observed, which is consistent with the previous report [38]. However, Ag@AgCl samples have strong absorption both in the ultraviolet and visible light regions. The absorption at 350–400 nm can be ascribed to the characteristic absorption of AgCl, and the strong absorption in the region of 400–800 nm can be attributed to the typical SPR of metallic Ag NPs [39]. Compared with Ag@AgCl-Val and Ag@AgCl-Glu, the band of Ag@AgCl-Lys in the visible light region shows a red shift and broadening. Generally, the resonance wavelength strongly depends on the size and shape of the NPs, the interparticle distance, and the dielectric property of the surrounding medium [24]. Here, the increase of the Ag to AgCl ratio with the ordinal use of L-lysine, L-valine, and L-glutamic acid may be responsible for this unique phenomenon.

Fig. 4
figure 4

UV/vis absorption spectra of AgCl and the as-synthesized Ag@AgCl samples

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Photocatalytic activities of Ag@AgCl photocatalysts

In order to investigate the photocatalytic activities of the as-synthesized Ag@AgCl samples and the role of SPR effect during the photocatalysis process, the degradation of RhB aqueous solution (10 mg L−1) under visible light irradiation (≥420 nm) was evaluated. Fig. 5 shows the photodegradation curves of RhB over different photocatalysts. A blank experiment in the absence of the photocatalyst but under visible light irradiation shows that no RhB has been decomposed. Another blank experiment using Ag@AgCl-Lys as photocatalyst without being irradiated demonstrated that the concentration of RhB remained unchanged (not shown in Fig. 5). Those results indicated that the RhB is very stable, and the possible catalytic degradation of RhB over Ag@AgCl-Lys in the dark is excluded. Further control experiments showed that RhB was not photocatalytically degraded on bare Ag NPs. This confirms that the local dielectric environment for Ag NPs is crucial for plasmonic photocatalysis and rules out a surface plasmon induced localized heating effect of Ag NPs for the degradation of RhB as well [40]. Furthermore, a bare AgCl showed very poor visible light driven photocatalytic activity, due to its large indirect band gap (3.25 eV). However, the loading of Ag NPs on the AgCl surface remarkably enhanced the RhB degradation efficiency over the Ag@AgCl samples. It can be seen that the RhB was completely decomposed over Ag@AgCl-Lys within 6 min. Moreover, Ag@AgCl-Val and Ag@AgCl-Glu all exhibited much higher photocatalytic activity than the bare Ag NPs and AgCl. Therefore, the excellent visible light photocatalytic performance of Ag@AgCl may be ascribed to the interfacial interaction between Ag and AgCl. Interestingly, the trend in the photocatalytic activities of the as-synthesized samples follows the order of Ag@AgCl-Lys > Ag@AgCl-Val > Ag@AgCl-Glu, which is in consistent with the order of the Ag0 contents of those samples. This phenomenon can be interpreted as the higher metallic Ag content which increase the density of Ag NPs on the surface of AgCl particles and improve their interface area. This favors the electron–hole separation and interfacial charge transfer [22], which finally enhances the photocatalytic performance of the Ag@AgCl samples. The results suggest that the photocatalytic activities of Ag@AgCl can be rationally tailored by controlling the species of amino acid in the synthesis process. In order to rule out the photosensitization process of the Ag@AgCl photocatalysts, FA was chosen as a colorless model organic pollutant. As shown in Fig. S3, it can be seen that when the parent bare Ag NPs and AgCl are used as photocatalysts, no more than 5 % and 10 % FA, respectively, are decomposed within 40 min under visible light irradiation. While the as-synthesized Ag@AgCl are employed as photocatalysts, they show significantly enhanced photocatalytic activity than that of bare Ag NPs and AgCl. Furthermore, it is found that the photocatalytic activity of the Ag@AgCl samples are enhanced in the order of Ag@AgCl-Lys > Ag@AgCl-Val > Ag@AgCl-Glu, which is consistent with the results of RhB photodegradation. From the results, it can see that the Ag@AgCl as plasmonic photocatalysts exhibit good photocatalytic activity for the degradation of both dyes and colorless organic pollutants.

Fig. 5
figure 5

Photocatalytic degradation of RhB over various photocatalysts under visible light irradiation (≥420 nm). Experimental conditions: catalyst dosage = 50 mg, volume of solution = 50 mL, initial concentration = 10 mg L−1

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The photodegradation process of RhB was found to follow pseudo-first order kinetics. As shown in Fig. 6, the rate constant for the Ag@AgCl-Lys was evaluated as 0.5733 min−1. In many reports [23, 25, 36], PVP was usually used for the synthesis of Ag@AgCl or other photocatalysts. In comparison, Ag@AgCl-PVP was synthesized by using PVP, and was evaluated in the degradation of RhB dye. It was found that the rate constant of Ag@AgCl-PVP is 0.3980 min−1, which is much lower than that of Ag@AgCl-Lys. Furthermore, the photocatalysts of Ag@AgCl-Lys-H and Ag@AgCl-PVP-H synthesized by traditional hydrothermal method were also studied, whose rate constants are 0.2250 and 0.1350 min−1, respectively. Obviously, the Ag@AgCl-Lys exhibited outstanding photocatalytic performance than the above photocatalysts. Hence, this study provides a valuable strategy for the synthesis of effective, low-cost, and eco-friendly visible light SPR Ag@AgCl photocatalyst.

Fig. 6
figure 6

Kinetic rate plots of RhB degradation over various photocatalysts synthesized by different methods under visible light irradiation (≥420 nm). Experimental conditions: catalyst dosage = 50 mg, volume of solution = 50 mL, initial concentration = 10 mg L−1

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The stability and recyclability of a photocatalyst is very important for its application. Hence, the stability and recyclability of plasmonic photocatalyst Ag@AgCl-Lys was further investigated by reusing it in the photodegradation of RhB dye. As shown in Fig. S4, RhB is quickly decomposed catalyzed by Ag@AgCl-Lys. The Ag@AgCl-Lys photocatalyst is stable after 5 runs exhibiting no significant loss of activity.

Photocatalytic mechanism

In the photocatalytic oxidation process, electron–hole pairs are directly produced on the photocatalyst surface after illumination. Consequently, a series of photoinduced reactive species including h+, ·OH, and ·O2 − are generated and commonly suspected to be involved in the photocatalytic degradation of organic pllutants. Jiang et al. [31] reported that the direct photohole mainly worked as reactive radical species for the oxidation of organic pollutant molecules over Ag@AgCl photocatalyst. Yu et al. [32] reported that the ·OH were active species and indeed participated in phtotocatalytic reactions over Ag/AgCl/TiO2 composites. Nevertheless, Luo et al. [41] regarded that the active species such as ·OH and ·O2 − all can decompose the adsorbed organic pollution when using Ag/AgCl/rGO as photocatalyst. In order to investigate the photodegradation mechanism in our photocatalytic system, a series of radical- and hole-trapping experiments were designed and carried out over Ag@AgCl-Lys under indoor artificial daylight irradiation. We employed various scavengers to quench these primary active species, i.e., KI as a quencher of h+ and ·OH, isopropanol (IPA) as a quencher of ·OH, benzoquinone (BQ) as a quencher of ·O2 −. The sacrificial agents can capture corresponding radical species, i.e., KI–h+ + ·OH, IPA–·OH, and BQ–·O2 −, which lead to the lower concentration of reactive species to react with the RhB dye in the solution. The depression effects on RhB degradation by adding different quencher are shown in Fig. 7. It can be observed that the addition of KI and IPA almost did not affect the degradation efficiency of RhB throughout the experiments. Meanwhile, the degradation rates of RhB were significantly reduced after the addition of BQ. The results indicate that ·O2 − may be the main reactive species for the degradation of RhB, rather than h+ and ·OH. Because the generation of ·O2 − may occur by direct reaction of the photogenerated electrons with O2 adsorbed on the catalyst surface, an anaerobic experiment was therefore conducted to elucidate the effect of oxygen. It was found that the degradation efficiency of RhB was largely depressed by purging of Ar in the reaction process. This further verifies that ·O2 − maybe act the main reactive species in the photodegradation of RhB over Ag@AgCl-Lys photocatalyst.

Fig. 7
figure 7

Degradation efficiency of Ag@AgCl-Lys with different quenchers and by Ar purging under visible light irradiation (≥420 nm). Experimental conditions: catalyst dosage = 50 mg, volume of solution = 50 mL, initial concentration = 10 mg L−1

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Based on the above experiments results, an illustration of the Ag@AgCl formation and photocatalytic mechanism is depicted in Fig. S5. Under visible light illumination, Ag@AgCl nanocomposite has a strong absorption in the visible light region due to SPR effect of Ag NPs, generating a certain number of electron–hole pairs. Because of the synergistic effect between the excellent conductivity of Ag NPs and the polarization field provided by AgCl, the photogenerated electrons are transferred to the surface of the Ag NPs far from the Ag@AgCl interface and trapped by oxygen molecules in the solution to form superoxide ions (·O2 −), which will oxidize RhB dye molecules. Meanwhile, the photogenerated holes are transferred to the AgCl surface causing the oxidation of Cl– ions to Cl0 atoms, which may be another active species to oxidize RhB molecules [24, 25, 38] and hence the Cl– will be formed again. On the other hand, the quick migration of photogenerated electrons from the AgCl surface will prevents the photoreduction of AgCl to Ag0. In addition, the recycle of Cl– ions may be the main factors for the high stability of the as-synthesized photocatalyst [39].

Transient photocurrent response

Photoinduced e−–h+ generation, separation, migration and capture by reactive species is considered to be the basic process in semiconductor photocatalyst operation. Photoelectrochemistry tests are powerful tools to monitor these complicated processes. To give further evidence to show the plasmonic enhancements of photocatalytic activity, the photoresponses of the as-synthesized Ag@AgCl and AgCl samples were studied. As shown in Fig. 8, large and rapid anodic photocurrent responses from the light-on to the light-off state can be detected over all Ag@AgCl electrodes, which are mainly ascribed to the quick separation and transportation of photogenerated electrons on the surfaces of working electrodes [42]. The photocurrent response of the samples is in the order of Ag@AgCl-Lys > Ag@AgCl-Val > Ag@AgCl-Glu > AgCl, which is consistent with the order of their photocatalytic activity and Ag0 content in Ag@AgCl samples. It is regarded that the synergy effects between the SPR of the Ag NPs and the polarization field of AgCl matrix make the the exited surface electron transferring from the Ag NPs to the Ag–AgCl interfaces [43]. This interaction facilitates electron–hole separation and interfacial charge transfer, which is beneficial for the generation of photocurrent and the creation of reactive species ·O2 −. In the Ag@AgCl samples, more Ag0 NPs on the surface of AgCl means a stronger interaction between the Ag0 NPs and AgCl matrix. Hence, the as-synthesized SPR photocatalysts of Ag@AgCl show consistent trend with the Ag0 content, photocurrent response as well as photocatalytic activity. Meanwhile, the photocurrents over Ag@AgCl electrodes were maintained at constant levels throughout the entire experimental period, which confirmed that the Ag@AgCl electrodes have photoactivity and steady photoelectrochemical performance under visible light illumination. Moreover, the photocurrent results also provide a direct evidence for the photocatalytic mechanism.

Fig. 8
figure 8

Transient photocurrent response of a Ag@AgCl-Lys, b Ag@Ag-Val, c Ag@AgCl-Glu, and d Ag@AgCl under visible light irradiation (≥420 nm). Experimental conditions: SCE (saturated calomel electrode) as a reference electrode, 0.5M Na2SO4 aqueous solution as the electrolyte

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Conclusion

In summary, we have developed a simple, one-step, effective ultrasonic-assisted approach for the rapid synthesis of Ag@AgCl photocatalysts in an amino acid-EG reaction medium. The composition and SPR adsorption of Ag@AgCl system can be readily modulated by changing the species of amino acid. The synergistic effects between SPR of Ag0 NPs and the polarization fields of AgCl particles facilitate the separation of photoexcited e−–h+ pairs and interfacial charge transfer, thus enhance the photocatalytic activity. Hence, the as-synthesized Ag@AgCl photocatalysts showed excellent photocatalytic performance for the degradation of RhB and FA under visible light irradiation, especially the Ag@AgCl-Lys synthesized by using L-lysine. In addition, Ag@AgCl-Lys showed good stability and recyclability in the photodegradation reaction. Furthermore, the possible degradation mechanism of the plasmonic photocatalytic process was systematically investigated. The active specie ·O2 − is proved to play the most important role in the photodegradation of organic pollutants. It is believed that the obtained Ag@AgCl plasmonic photocatalysts with strong adsorption in the visible light region, the high photocatalytic activity and good photostability have potential applications in the degradation of organic contaminations and environmental cleaning.