GO composite: A photocatalyst of extraordinary photoactivity and stability

Accepted Manuscript One-step synthesis of Ag/AgCl/GO composite: a photocatalyst of extraordinary photoactivity and stability Lin Liu, Jiatao Deng, Tongjun Niu, Gang Zheng, Pei Zhang, Yong Jin, Zhifeng J, Xiaosong Sun PII: DOI: Reference:

S0021-9797(16)30916-X http://dx.doi.org/10.1016/j.jcis.2016.11.039 YJCIS 21767

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

16 August 2016 10 November 2016 11 November 2016

Please cite this article as: L. Liu, J. Deng, T. Niu, G. Zheng, P. Zhang, Y. Jin, Z. J, X. Sun, One-step synthesis of Ag/AgCl/GO composite: a photocatalyst of extraordinary photoactivity and stability, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/10.1016/j.jcis.2016.11.039

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One-step

synthesis

of

Ag/AgCl/GO

composite:

a

photocatalyst of extraordinary photoactivity and stability Lin LIU, Jiatao DENG, Tongjun NIU, Gang ZHENG, Pei ZHANG, Yong JIN, Zhifeng JIAO and Xiaosong SUN* School of Materials Science and Engineering, Sichuan University No. 29 Wangjiang Road, Chengdu 610064, Sichuan, P. R. China

Abstract Recently, the photocatalytic applications of silver chloride have been paid closed attention for the excellent ability to photodegrade organic pollutants. Comparing with other catalysts, the silver chloride presents outstanding photocatalytic activity. However, it also suffers from the poor photocatalytic stability. This very paper is focusing on the one-step wet chemical process of preparing Ag/AgCl/GO photocatalyst with high photocatalytic activity and stability. The detailed characterizations were particularly carried out in order to investigate the photo-catalytic activity and stability. Meanwhile the morphology, chemical composition as well as crystalline structure were investigated. It is found that the as-prepared Ag/AgCl/GO composite exhibited an ultrahigh photocatalytic activity and stability in the process of photodegrading RhB. The unique catalytic activity has been discussed based on the SPR effect in Ag nanoparticles on AgCl surface and the separation of photo-generated electron-hole pairs, the primary benefit of the stability owes a great deal to GO which can capture the photo-generated electrons in case they reduce Ag+ ion or recombine the excited holes.

Keywords: photocatalysis, Ag/AgCl/GO, photocatalyst, photo-degradation, RhB, SPR, * Author to whom correspondence should be addressed: [email protected]

1. Introduction The application of photocatalysis has attracted tremendous interest because of the energy crisis, environment pollution and climate changes. People need to find sustainable ways of utilizing resources effectively to deal with the crises aforementioned. The exploitation of solar energy is, so far, the fruitful strategy. Aside from directly converting solar energy into electrical energy with photovoltaic cell, the photo-generated electrons and holes in the catalyst could get involved in the procedure of splitting water to get hydrogen or degrading the pollutant for water purification and disinfection. The research on the application of titanium dioxide (TiO2) has made great progress[1-6]. However, due to the wide band-gap of 3.0~3.2 eV, TiO2 could only be excited by UV light [1, 7], thus, the most part of the solar energy (ca. 95%) cannot be used. To solve this problem, people have prepared the hybrid of TiO2, such as metal/TiO2 [8, 9], compound/TiO2 [10, 11] and as well as organic-polymer-TiO2 [12, 13]. Recently some other wide bandgap materials such SnO2 [14, 15], BiOCl [16], and AgX (X=Cl, Br, I) [17] have also been extensively studied. These promising candidates, by forming the hybrid with Ag or Au nanoparticles, give excellent photocatalytic performances under visible light irradiation [18]. Among all these materials, Silver chloride, AgCl, is highly photo sensitive, which makes it an important photocatalyst [19-21]. However, AgCl still suffers from the serious

reduction of Ag+ with photo-generated electrons in the

photocatalytic process that limits the reuse of the catalyst [22]. To overcome this shortcoming, the photo-generated electron ought to be removed or exhausted as fast as possible. Therefore, the noble metal/AgCl hybrid structure [23] or noble metal/AgCl/semiconductor hybrid structure[24-29] is rationally proposed and investigated, as the noble metals will collect the photo-generated electrons, and, the surface plasmon resonance (SPR) effect will enhance the absorption of visible light [25-29]. In addition, Ag/AgCl/carbon materials such as reduced graphene oxide

(r-GO) have shown the interesting potential in catalyst [30-31]. We noticed that, being the monomer of graphite, graphene oxide (GO) possesses peculiar functional groups (hydroxyl and epoxy groups) bound on the basal planes in addition to partial carboxyl groups located at the edges [31, 32]. The surface electrophilic groups would be beneficial to the migration of electron from AgCl to GO in the photocatalytic procedure [33]. For example, Min et al have demonstrated a Z-scheme photocatalytic system using Ag/AgCl encapsulated with GO by a multi-step procedure[34]. To our best knowledge, one-step procedure of fabricating Ag/AgCl/GO composite has been seldom reported yet. Herein, in this paper, we would like to demonstrate the controllable one-step wet chemical procedure to synthesize Ag/AgCl/GO composite. Instead of directly reacting with Cl , Ag+ ions released from the silver-ammonia clathrate would combine with Cl ions on the pre-synthesized GO at slower rate. GO is the electron acceptor as well as the supporter, photo-generated electrons in the Ag/AgCl composite could easily move to GO. Thus, it would obstruct the recombination processes of electron-hole pairs and the reduction of Ag+.

2. Experiment 2.1. Materials All the chemical reagents, silver nitrate (AgNO3), poly (vinyl pyrrolidone) (PVP-K30, molecular weight: 40000), hydrochloric (HCl, 36wt.%), ammonia hydroxide (NH3H2O, 25wt.%), sulfuric acid (H2SO4, 98wt.%), graphite, potassium permanganate (KMnO4), sodium nitrate (NaNO3) and hydrogen peroxide (H2O2, 30wt.%) were commercially purchased. The reagents were analytical pure without further purification. The aqueous solutions throughout the experiments were prepared with deionized water (DI-water, 18.25 M). 2.2. Preparation of silver-ammonia clathrate solution 0.25g AgNO3 was dissolved in 10 ml DI-water under vigorously stirring for

30 min to make a clear solution. Then 0.25 g PVP-K30 and 0.6 ml ammonia (25 wt.%) was added. The mixture solution was vigorously stirred for 20 min to get the silver-ammonia clathrate. 2.3. Preparation of Ag/ AgCl composite 2.5 ml HCl (0.6 M) was added dropwise in 10 ml silver-ammonia clathrate solution under stirring. The mixed solution was irradiated with UV light (=253.7 nm) for 40 min at room temperature. Finally, the resulted precipitate was collected by centrifugation, washed with DI-water and ethanol five times respectively to remove the redundant ionic species or the organic contaminant in the resultant. The final product was dried at 60 ℃ in N2 and denoted as UV-Ag/AgCl. For comparison, a controlled experiment was carried out, in which AgCl sample was prepared in the dark and named as AgCl. 2.4. Preparation of graphene oxide sheet Graphene oxide (GO) was prepared with modified Hummer’s method [35]. In brief, 0.5 g NaNO3 and 1g graphite were dispersed in 23 ml H2SO4 (98 wt.%) in an ice-bath cell (4 ℃) with continuously stirring, then 3 g KMnO4 was slowly added into the solution. After stirring for another 15 min, the mixture was moved into 35℃ oil-bath cell so that the color of the turbid solution turned slowly into dark-brown within 2 h bathing. After that, 46 ml DI-water was added in the vessel. The solution was stewed at 90 ℃ for 15 min. Finally, 140 ml DI-water and 5 ml H2O2 (30wt. %) was sequentially added into the solution. The color of the resultant turned into golden at the end of this process. The product was collected by centrifugation and washed with diluted HCl, DI-water and ethanol five times, respectively, and dried at 60 ℃ overnight in air. 2.5. Prepare Ag/AgCl/GO composite Certain

amount

of

pre-synthesized

GO

was

dispersed

in

silver-ammonia clathrate solution (10 ml), the mixture was stirred for 20 min, then, 2.5 ml HCl (0.6M) was added dropwise. After illumination for 40 min

under UV light, the precipitates were collected with centrifugation and washed with DI-water and ethanol five times, separately. The collects were dried at 60 ℃ in

air

and

named

as

UV-Ag/AgCl/GO-5,

UV-Ag/AgCl/GO-10

and

UV-Ag/AgCl/GO-20 referring to the case that 5 mg, 10 mg or 20 mg GO was used, respectively. 2.6. Evaluation of photocatalytic performance. The photocatalytic activity of the prepared samples was tested by degrading RhB under visible light irradiation (>420nm). A 300 W Xe lamp (HSX-F300) equipped with a 420 nm cut-off filter was used. In a typical process, 10 mg sample and 50 ml of RhB solution (10mg/L-1) was mixed in a beaker. Before irradiated with Xe-lamp, the mixture was vigorously stirred in the dark for 30 min to establish the of adsorption-desorption equilibrium. The aqueous solution was extracted every 4 min from the mixture and centrifuged three times (5 min for each) to get rid of the catalyst. The collected supernatant would be transferred to the quartz curette to measure the absorption spectra at 553 nm by a UV-vis spectrophotometer (UNICO UV2102PC) with DI-water as the reference sample. The peak intensity was converted into the C/C0 graph where C0 represented the initial concentration of RhB while C was the concentrations at every 4 min till RhB solution was clear. The cycling experiments were conducted to explore the stability of UV-Ag/AgCl and UV-Ag/AgCl/GO hybrid photocatalyst. Each cycling experiment was conducted the same way above. After each experiment, photocatalyst would be collected and washed with DI-water and ethanol three times, respectively, and dried in the oven. Then 10 mg of the dry powder was taken to carry out the 2nd cycle and so on. 2.7. Characterization The crystalline structure was characterized by X-ray diffraction (XRD, Pert Pro MPD DY129). The surface valence state of elements was identified by X-ray photo electron spectroscopy (XPS, XSAM 800). The scanning electron

microscopy (SEM, JSM-5900LV), transmission electron microscopy and high resolution transmission electron microscopy (TEM, HR-TEM Tecnai G2 F20 S-TWIN) was employed to identify the morphology and the fine structure of the samples. The energy dispersive spectrometer (EDS) equipped on SEM was employed to characterize the composition of the as-prepared samples. The functional group of the GO was identified by infrared absorption spectroscopy (IR, NEXUS 670).

3. Results and discussion 3.1. Detection of XRD

Figure 1. XRD patterns of a) Ag/AgCl, b) UV-Ag/AgCl, c) UV- Ag/AgCl/GO-5

Fig. 1 illustrates the XRD patterns of the samples prepared. Fig. 1a tells the XRD patterns of AgCl prepared in the dark; Fig. 1b shows the XRD patterns of UV-Ag/AgCl and Fig. 1c presents that of UV-Ag/AgCl/GO-5. All the intense diffraction peaks can perfectly fit the PDF card (85-1355) of AgCl, however, the peaks of Ag cannot be found accordingly. The results indicate that the AgCl was successfully produced by the wet chemical procedure, but the Ag nanoparticles could not be identified with XRD. The possible cause is that the quantity of Ag adhered to the AgCl surface was so scarce that the peaks were too weak to be calibrated.

3.2. Characterization of SEM, TEM and EDS Fig. 2 shows the SEM, TEM and EDS characteristics of UV-Ag/AgCl, and UV-Ag/AgCl/GO-5. Fig. 2(a) presents the SEM image of UV-Ag/AgCl, it can be seen that there were lots of cubes which possessed the edge-length of 100~600 nm. Fig. 2(b) shows the SEM image of layered GO, it is found that there were some multi-layered structures, the length of which was 10 μm or more. Fig. 2(c) shows the SEM image of UV-Ag/AgCl/GO-5, it is clearly shown that the composites were consisted of nanocubes and the wrapping layers of GO. Fig. 2(d1) and 2(d2) are the TEM image of UV-Ag/AgCl/GO-5, from which one could clearly see that the cubes were well encapsulated by the thin film. The inset of Fig.2(d1) displays the details of the UV-Ag/AgCl/GO-5, one can recognize the composite structure clearly and tell the AgCl solid cubes apart from the wrapping GO. Besides, the typical HR-TEM gives the information of the Ag/AgCl hybrid structure (Fig. 2(e)). The fringes space of 0.235 nm referred to Ag (111) planes, and that of 0.276 nm strips referred to the (200) planes of AgCl. Fig. 2(f2) shows the element contents of the detected area in Fig. 2(f1) by EDS. Fig. 2(f3)-(f5) gives the elements mapping of C, Ag and Cl in the detected area, respectively. Hence, from the detections above, UV-Ag/AgCl/GO-5 is comprised of Ag NPs, AgCl cubes and GO, specifically, AgCl cubes attached with few of Ag NPs were wrapped with GO layers.

Figure 2. The SEM imagines of (a) UV-Ag/AgCl, (b) GO, (c) Ag/AgCl/GO-5, and (d) TEM imagine of Ag/AgCl/GO-5, (e) HRTEM imagine of Ag/AgCl/GO-5, (f2) gives the EDS spectrum area marked in (f1), (f3)~(f5) indicate the element mapping of C, Ag, and Cl, respectively.

3.3. Characterization of XPS The characteristic of elemental phase and electron status were identified by XPS. Fig. 3 displays the high-resolution XPS spectra of the as-prepared samples, in which Fig. 3a, 3b and 3c illustrate the Ag 3d XPS spectra taken from

AgCl,

UV-Ag/AgCl

and

UV-AgCl/Ag/GO-5,

respectively.

It

is

Figure. 3. XPS spectra of Ag 3d5/2 and 3d3/2 taken from (a) AgCl, (b) UV-Ag/AgCl and (c) UV-AgCl /Ag/GO-5

verified that the peaks at around 367.10 and 367.90 eV in Fig. 3a, 3b and 3c could be ascribed to Ag 3d5/2, and the peaks around 373.09 and 373.72 eV referred to the Ag 3d3/2. The peak of 367.10 eV coupled with that at 373.09 eV corresponded to the peaks of Ag+, while 367.90 and 373.72 eV proved the existence of Ag0 species [36-38] in the samples. Apparently, the content of Ag0 species in AgCl was far less than those in both UV-Ag/AgCl and UV-Ag/AgCl/GO-5. 3.4. Comparison of photocatalytic performance The

photo-degradation

activity

of

as-prepared

samples

was

characterized by degrading RhB solution under visible light (λ≥420nm). Fig. 4 gives the absorption spectra of RhB degraded by AgCl, Fig. 4a, UV-Ag/AgCl, Fig. 4b, and UV-Ag/AgCl/GO-5, Fig. 4c, in which one may find that RhB could be rapidly decomposed by UV-Ag/AgCl and UV-Ag/AgCl/GO-5 within 12 min, Fig. 4b and 4c, while it could not be completely degraded by AgCl, Fig. 4a. Fig. 4d shows the relative concentration ratio (C/C 0) at 553 nm of the RhB absorption spectrum in the presence of prepared photocatalysts (AgCl, UV-Ag/AgCl, UV-Ag/AgCl/GO-5, UV-Ag/AgCl/GO-10, UV-Ag/AgCl/GO-20) as well as that of the controlled experiment without any catalysts. The result of controlled experiment displays that RhB was rather stable under visible light irradiation, there seemed no apparent photo-degradation within 12 min. Besides,

all

the

UV-Ag/AgCl,

UV-Ag/AgCl/GO-5,

UV-Ag/AgCl/GO-10,

UV-Ag/AgCl/GO-20 samples showed an outstanding photo-degradation

capability. The degradation efficiencies were 98.5%, 98.5%, 97.9% and 98.5%,

Figure. 4. The typical UV-vis spectra changes of RhB dye in the presence of (a) AgCl, (b)UV-Ag/AgCl,(c) UV-Ag/AgCl/GO-5 and (d) photocatalytic performance of samples

respectively, except for AgCl which merely gave an efficiency of 86% within the same irradiation time. Thus one can deduce that the UV light irradiation do help to generate Ag0 species in the process of preparing the samples, and the SPR effect worked well, so that the decomposition could be thoroughly carried out within 12 min. However, there seemed no significant effect depending upon the amount of GO. 3.5.

The stability of photo-degradation performance Being a high efficient photocatalyst system, the stability of Ag/AgCl is

always cared about. Herein, Fig. 5 would like to show that, by introducing GO into the Ag/AgCl composite, the stability was significantly improved. Fig. 5 (a) tells the reduction of photo-degradation efficiency of Ag/AgCl within 5 cycling

Figure 5. The cycling performance of (a) UV-Ag/AgCl and (b) UV-Ag/AgCl/GO-5; the SEM images of UV-Ag/AgCl after degradation cycles for: (c) 1st time, (d) 3rd time, (e) 5th time; and the SEM images of UV-Ag/AgCl/GO-5 after degradation cycles for: (f) the 1st time, (g) the 3rd time, (h) the 5th time

experiments. One can see that after 5 cycles, the photo-degradation efficiency of Ag/AgCl decreased about 40%. This can be explained by the reduction of Ag+ to Ag0. Fig. 5(c), 5(d) and 5(e) indicate that Ag/AgCl solid cubes were more rugged after each cycle. On the other hand, UV-Ag/AgCl/GO-5 showed much better stability than that of UV-Ag/AgCl. One can see in Fig. 5 (b) that its catalytic performance was still stable after the 5 th cycling, and from the SEM images shown in Fig. 5(f)-5(h), the surface of the solid cubes was still smooth, and the shape was almost the same as the original ones. 3.6.

Possible Mechanism AgCl has a direct energy band gap of 5.6 eV and an indirect band gap of

3.25 eV [22]. Thus AgCl cannot be excited by visible-light as usual. But in the case of Ag/AgCl composite, the situation is quite different. The strong absorption at the visible region of Ag/AgCl owes to the SPR effect [18] of Ag nanoparticles attached to the core AgCl. As shown in Fig.6, when the sample is irradiated with visible light, there is a strong absorption because of SPR effect of electrons in Ag nanoparticles. Meanwhile, there are some e-h+ pairs in AgCl due to the self-photosensitivity. Regarding the individual Ag or AgCl,

the Fermi level (EF) of Ag is a bit higher than that of AgCl [39]. Considering to

Figure. 6. The schematic diagram of the photo-degradation mechanism of as-prepared UV-Ag/AgCl and UV-Ag/AgCl/GO samples

the Schottky contact at the interface of this metal-semiconductor structure, the electrons flow from Ag to the AgCl, making the Fermi levels of Ag and AgCl reach the same height to achieve the equilibrium, and forming the Schottky barrier which would suppress the immigration of electrons. Due to the SPR effect, electrons in the Ag would transiently occupy the empty states above Fermi level, which would inject into the AgCl very fast (≤150

fs)[40] for the

strong electronic coupling between the Ag and the AgCl’s conduction bands. There are four destinations these electrons will depart for: (1) transferring directly to the dye; (2) coupling with the O2 or H2O in the aqueous solution to form the radical such as •O2, O22–, O• or •H2O, which is highly active to degrade the dye; (3) recombination with the hole in AgCl; (4) reaction with Ag+

to form the Ag0 species. Meanwhile, the photo-generated holes staying in the Ag would transfer to the interface of Ag/AgCl driven by the electric field of the Schottky contact [38]. The photo-generated holes of AgCl together with the hole generated in Ag will convert surface Cl ion of the AgCl to the Cl0 species, which is highly reactive and can also be further turned into active content to react with the dye pollutants. And compared with other photocatalyst, the Cl0 species could be the feature radical that work in AgCl. The active radical •O2 and the Cl0 species can degrade the RhB effectively. After that, Cl0 turn back to Cl and reform AgCl with the Ag+[21, 39, 41]. However, the Ag/AgCl system in our experiment was not stable, the photocatalytic efficiency was drastically reduced as the experiments going on, which was ascribed to the reduction process of Ag+ to Ag0, as electrons might recombine with Ag+ to form Ag0 and make the loss of AgCl. Fortunately, GO could significantly hold this process back. There are many carboxyl groups on the surface of GO, (Fig.1S and 2S), making it a good electron acceptor, therefore the photo-generated electrons can easily immigrate from Ag/AgCl to the GO to suppress the reduction processes of Ag+ and e--h+ recombination in AgCl, the experimental results have confirmed this assumption.

4. Conclusion In summary, Ag/AgCl/GO composite is a promising material in photocatalysis, which was prepared via the one-step wet chemical procedure under UV light irradiation. The prepared composite exhibited an ultrahigh photocatalytic activity for degradation of RhB aqueous solution, and most interestingly, it possessed the significant photocatalytic stability as well. Regarding the experiments, 50 ml RhB(10 mg/L) could be completely degraded with 10 mg UV-Ag/AgCl/GO in 12 min, and the photocatalytic capability of which was stable after 5 cycles. The high photocatalytic activity is discussed based on the SPR effect of Ag nanoparticles on AgCl surface and the separation of photo-generated electron-hole pairs achieved by GO. The

primary benefit to the stability is that GO can accept the photo-generated electrons so that they cannot reduce Ag + ion or combine holes. To our experiments, GO could drastically stabilize the photocatalytic capability of Ag/AgCl, however, the photocatalytic ability would not be affected by the amount of GO, the optimum and minimum ratio of GO: Ag/AgCl should be investigated carefully. Since the good photocatalytic capacity and moderate cost, the application of Ag/AgCl/GO in chemical catalysis, water disinfection, photovoltaic fuel cells and electrode would make great progresses in the future.

Acknowledgements The authors gratefully acknowledge Shanling Wang, YunfeiTian and Yi He for their contribution to the XRD, XPS, TEM and SEM characterizations.

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