Scholarly article on topic 'Comparison of catalytic activity of carbon-based AgBr nanocomposites for conversion of CO2 under visible light'

Comparison of catalytic activity of carbon-based AgBr nanocomposites for conversion of CO2 under visible light Academic research paper on "Chemical sciences"

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{AgBr / Nanocomposite / "Photocatalytic activity" / "CO2 reduction" / Conversion / "Visible light"}

Abstract of research paper on Chemical sciences, author of scientific article — Zekun Fang, Shuzhen Li, Yibin Gong, Weichen Liao, Shuanghong Tian, et al.

Abstract The catalytic activities of carbon-based AgBr nanocomposites (AgBr/CNT, AgBr/GP, AgBr/EG, and AgBr/AC) for CO2 reduction to hydrocarbons under visible light were investigated in this study. The carbon-based AgBr nanocomposites were prepared on carbon-based supporting materials (CNT, GP, EG, and AC) by the deposition–precipitation method in the presence of cetyltrimethylammonium bromide (CTAB). The photocatalytic activities of AgBr nanocomposites on different supporting materials (CNT, GP, EG, and AC) were investigated by CO2 reduction yield in the presence of water under visible light (λ > 420 nm). The results of X-ray diffraction (XRD) and transmission electron microscopy (TEM) showed that AgBr nanoparticles were well dispersed on the surface of supporting materials. AgBr/CNT and AgBr/GP had a relatively higher reduction yield under visible light due to the transfer of photoexcited electrons from the conduction band of well-dispersed AgBr to carbon supporting materials. In addition, the carbon-based AgBr nanocomposites were stable in the repeated uses under visible light. The total product yields of carbon-based AgBr nanocomposites after the 5 repeated uses almost remained about 83% of the first run. Therefore, carbon-based AgBr nanocomposite is an effective and stable visible-light-driven photocatalyst for CO2 photoreduction.

Academic research paper on topic "Comparison of catalytic activity of carbon-based AgBr nanocomposites for conversion of CO2 under visible light"

King Saud University Journal of Saudi Chemical Society

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ORIGINAL ARTICLE

Comparison of catalytic activity of carbon-based AgBr nanocomposites for conversion of CO2 under visible light

Zekun Fang a, Shuzhen Li a, Yibin Gong a, Weichen Liao a, Shuanghong Tian a'b, Chun Shan c, Chun He a b *

a School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China b Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Guangzhou 510275, China

c Guangdong Polytechnic Normal University, Guangzhou 510665, China

KEYWORDS

Nanocomposite; Photocatalytic activity; CO2 reduction; Conversion; Visible light

Abstract The catalytic activities of carbon-based AgBr nanocomposites (AgBr/CNT, AgBr/GP, AgBr/EG, and AgBr/AC) for CO2 reduction to hydrocarbons under visible light were investigated in this study. The carbon-based AgBr nanocomposites were prepared on carbon-based supporting materials (CNT, GP, EG, and AC) by the deposition-precipitation method in the presence of cetyltrimethylammonium bromide (CTAB). The photocatalytic activities of AgBr nanocomposites on different supporting materials (CNT, GP, EG, and AC) were investigated by CO2 reduction yield in the presence of water under visible light (k > 420 nm). The results of X-ray diffraction (XRD) and transmission electron microscopy (TEM) showed that AgBr nanoparticles were well dispersed on the surface of supporting materials. AgBr/CNT and AgBr/GP had a relatively higher reduction yield under visible light due to the transfer of photoexcited electrons from the conduction band of well-dispersed AgBr to carbon supporting materials. In addition, the carbon-based AgBr nanocomposites were stable in the repeated uses under visible light. The total product yields of carbon-based AgBr nanocomposites after the 5 repeated uses almost remained about 83% of the first run. Therefore, carbon-based AgBr nanocomposite is an effective and stable visible-light-driven photocatalyst for CO2 photoreduction.

© 2013 Production and hosting by Elsevier B.V. on behalf of King Saud University.

* Corresponding author at: School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China. Tel./ fax: +86 20 39332690.

E-mail address: hechun@mail.sysu.edu.cn (C. He). Peer review under responsibility of King Saud University.

1. Introduction

The atmospheric concentration of CO2 has been reported to increase each year by about 2 ppm [24]. The significant rise in atmospheric CO2 levels resulting from the combustion of fuels is one of the primary causes of global warming problems. The reduction and fixation of CO2 to organic compounds have been regarded as an important research area for solving the global warming and depletion of fossil fuel problems. Therefore, the techniques for photo-chemically transforming CO2

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into a source of fuel could offer an attractive way to decrease atmospheric concentrations and simultaneously finding new energy sources. Novel developments in CO2 reduction methods are required. Thus, the synthesis of the novel nano-structured catalysts for CO2 reduction methods has opened a new possibility for creating advanced photocatalytic materials for this purpose. One way to accomplish this conversion method is through the light-driven reduction of carbon dioxide to hydrocarbons with electrons and protons derived from water [28]. These nanostructured catalysts acting as semiconductors under light irradiation have been extensively studied for about four decades. In 1972, Fujishimna and Honda discovered the pho-tocatalytic splitting of water on TiO2 electrodes [8]. This event marked the beginning of a new era in heterogeneous photoca-talysis, and this is one of the promising research studies which focuses on the economic photocatalytic reduction of CO2 to generate products of interest carried out using various semiconductor catalysts in a heterogeneous system [21,22].

Recent innovations in photocatalytic technology have realized CO2 conversion to a potentially promising application. In this process, visible light as an energy source for semiconductor electron-hole pairs, was utilized for exciting these electron-hole pairs, then, the photoexcited electrons reduce CO2 with H2O on the catalyst surface to form the energy-bearing products such as carbon monoxide, methane, methanol, formaldehyde, and formic acid [19]. A variety of photocatalysts such as TiO2, CdS, ZrO2, ZnO, and MgO have been studied. Among them, wide band-gap catalysts are considered the most convenient candidates in terms of stability such as TiO2 [4,17,20,27]. Modifications on the catalysts by doping some transition metals on the catalyst surface were required to enhance the reaction rate, to increase visible light utilization, and to control the selectivity of products, with a function as ''charge-carrier traps'' for suppress recombination of photoexcited electron-hole pairs. [19] However, it is still a challenge to explore the highly-efficient photocatalysts with a narrow band gap for the reduction of CO2 under visible light, due to the instability of the transition metal-doped titania.

It is well known that silver bromide (AgBr) is a photosensitive material with an indirect band gap of 2.64 eV (470 nm) [10,11], which can be applied in photocatalytic reactions by coupling some supporting materials [1,30]. In the photosensitive process, the silver bromide absorbs a photon and generates an electron and a positive hole [1,12,23,30,32]. If the photographic process (i.e. interstitial ions combined with electrons to form silver atoms) is inhibited, the generated electron and hole can be used in the photocatalytic process. To effectively suppress the photographic process, the electron-hole pair should be quickly separated and the generated electron must be transferred into some supporting materials [1,31,32]. Our recent work on the photocatalytic reduction of CO2 has demonstrated that AgBr coupled with TiO2 can maintain its catalytic activity by injecting electrons into the conduction band of TiO2 upon excitation with visible light [1]. Therefore, the charge transfer between the nanostructure interfaces is very important to determine the photoreduction efficiency of CO2 and photocatalyst stability [18,29].

Recently, carbon-based nanocomposites have been found to be effective in improving the charge transfer between the nano-structured interfaces and exhibit the high catalytic activity, due to their intrinsic physical and chemical properties of carbon-based materials [18,29,5,9,7]. Shaban et al. firstly

demonstrated the high photoactivity of carbon-doped TiO2 in the photoconversion efficiencies for the splitting water into H2 and O2 [25]. Akhavan et al. prepared a graphite oxide-Tita-nia composite using a sol-gel technique and found the enhancement for the pollutants degradation compared to bare anatase TiO2 [2]. At the same time, Kamat and his research team successfully prepared multifunctional photocatalyst by anchoring Two-dimensional (2D) carbon to oxide-semiconductor (TiO2, ZnO) and metal nanoparticles (Au, Pt) [16]. However, the main interest of the previous studies was focusing on the principle of photocatalytic mechanism and the structure of the photocatalysts for efficient treatment. Nano-size effects on photoreactions of CO2 reduction for semiconductor nano-particles have only been recently studied. Therefore the carbon-based materials which have a large specific surface area, high porous structure and low cost such as graphite powder (GP), expanded graphite (EG), and activated carbon (AC) could be considered as good candidates to improve the charge transfer in the photoreactions of CO2 reduction.

Herein we are encouraged to compare the employment of AgBr nanocomposites on different carbon-based supporting materials, i.e. multi-walled carbon nanotubes (CNT), graphite powder (GP), expanded graphite (EG) and activated carbon (AC), to investigate AgBr potential for improving the CO2 reduction activity under visible light. It is expected that carbon-based AgBr nanocomposites could be good visible-light photocatalyst candidates for the reduction of CO2 in the presence of carbon-based supporting materials to maintain their stability. This study here offers a comprehensive critical investigation on the feasibility, stability and efficiency of carbon-based AgBr nanocomposite (AgBr/CNT, AgBr/GP, AgBr/ EG, and AgBr/AC) as an effective photocatalysts for the reduction of CO2 under the visible light (k > 420 nm).

2. Materials and methods

2.1. Materials

Multi-walled carbon nanotubes (CNT-1020 with 5-15 im length) were purchased from Shenzhen Nanotech. Port Co., Ltd.. Graphite intercalated compounds were purchased from Hebei Laiyin Company. Graphite powder (GP) was purchased from Tianjin Fuchen Reagent Co., Ltd.. Expanded graphite (EG) was prepared from graphite intercalated compounds before applying the previous steps by a reported process in the literature [20]. Activated carbon (AC) was purchased from Tianjin Fuchen Reagent Co., Ltd. with 80 mesh of AC particle. CO2 gas (99.99%) was obtained from Foshan Ruike Gas Co., Ltd. in China. Cetyltrimethylammonium bromide (CTAB, C16H33N+ (CH3)3Br~; purity:>98%) was purchased from Tianjing Fuchen Chemical Reagent Co., Ltd. in China. AgNO3 with analytical grade was purchased from Guangdong Guanhua Chemical Co., Ltd. in China. Other chemicals with analytical grade were obtained as reagents and used without further purification. Deionized water was used throughout this study.

2.2. Preparation of catalysts

Carbon-based AgBr nanocomposites were prepared by the deposition-precipitation method in the presence of cation

surfactant of CTAB [6]. An appropriate amount of the carbon-based materials was oxidized by HNO3 solution (HNO3:-H2O = 1:4, v/v) for 1 h boiling then washed with distilled water until the filtrate was neutral. The sample was added to 100 mL of CTAB aqueous solution with a concentration of 0.01 M (10 times above CMC of CTAB 9.8 x 10-4 M), and the suspension was sonicated for 30 min and then stirred magnetically for 30 min. Then 0.0012 mol of AgNO3 in 2.3 mL of ammonia hydroxide (25 wt.% NH3) was quickly added to the suspension. AgBr content in carbon-based AgBr nanocompos-ites (AgBr/CNT, AgBr/GP, AgBr/EG, and AgBr/AC) was 20.0%. In this process, cationic surfactant CTAB was adsorbed onto the surface of carbon-based supporting materials at alkaline condition to limit the number of nucleation sites for the formation of AgBr aggregates, leading to well-dispersed AgBr on carbon-based materials. In addition, the amount of bromide ion from CTAB is more than sufficient to precipitate Ag+ from the added AgNO3 in aqueous solution. The resulting mixture was stirred at room temperature for 12 h. The product was filtered, washed with deionized water, dried at 75 0C, and subsequently sintered in a muffle furnace at 500 0C in the presence of N2 for 3 h.

2.3. Characterization of carbon-based AgBr nanocomposites

The prepared carbon-based AgBr nanocomposite powders were first analyzed by X-ray diffraction (XRD) method using a diffractometer (Bruker AXS, Germany) with radiation of Cu target (Ka, k = 1.54059 A). Transmission electron microscopy (TEM) images were obtained with a transmission electron microscope (TEM, JEM-2010HR). The content of AgBr in the catalyst was determined by SEM-EDS-EBSD thermal field emission scanning electron microscope (JEOL, Japan). AgBr/CBM powders were then examined by a transmission electron microscope (TEM, JEM-2010HR). The EIS measurements were carried out using a potentiostat/galvanostat (Autolab, PGSTAT302N) coupled to a lock-in amplifier (PGSTAT 302N). The impedance data were collected as a function of frequency scanned from the highest (1000 Hz) to the lowest (0.001 Hz) using the "single-sine" (lock-in amplifier) method. The measurements were conducted in a conventional H-type electrochemical Pyrex cell with a standard three-electrode assembly. This assembly had a AgBr nanocomposite working electrode, a Pt wire as a counter electrode (area, 0.5 x 1.0 cm2), and a saturated calomel electrode (SCE) as a reference electrode.

2.4. Photocatalytic CO2 reduction experiments

Photocatalytic reduction was carried out in a stainless steel vessel with valves for evacuation and gas feeding as shown in Fig. 1, in which an O-ring sealed glass window was placed at the top for admitting light irradiation. A 150 W Xe lamp (Shanghai Aojia Lighting Appliance Co., Ltd.) with UV cutoff filter (providing visible light with k > 420 nm) was used for irradiation. In a typical batch, 0.5 g of prepared carbon-based AgBr nanocomposites was suspended in 100 mL of 0.2 M KHCO3 solution in the vessel. Prior to the irradiation, pure CO2 (99.99%) was passed via a flow controller through the solution for 30 min to remove the oxygen, and then closed maintaining a pressure of 7.5 MPa inside the reactor. During

-t^J-dls —S]—IZZI 9

Figure 1 Schematic diagram of photocatalytic reduction system: (1) CO2 gas cylinder; (2) flow meter; (3) pressure digital meter; (4) stainless steel vessel; (5) O-ring glass window; (6) Xe lamp; (7) cooling system; (8) gas sampling; (9) liquid sampling; (10) magnetic stirrer; V1, V2, V3, and V4 are valves.

the reaction, the powders were continuously agitated to prevent the sedimentation of the catalyst by a magnetic stirrer, and the photocatalytic reaction conducted lasted for 5 h at room temperature. After illumination, small aliquots of the suspension were withdrawn by syringe, filtered through Milli-pore membranes, and then analyzed. Gas samples were performed with a gas-tight syringe through a septum. Reaction products in liquid phase were analyzed using a gas chromatog-raphy (Agilent HP6890N) equipped with a flame ionization detector (FID) and a HP-5 capillary column (30 m x 320 im x 0.50 mm). The product in gas phase was analyzed by GC/MS Hewlett-Packard (HP) 6890 gas chroma-tography with a HP 5973 mass detector. A 60 m length x 0.32 mm I.D. 1.8 m film thickness HP-VOC column (Agilent Scientific, USA) was used. The oven was programed at following rates, the initial temperature of the column was 40 0C hold for 2 min, followed by a ramp of 5 0C min-1 to 120 0C (1 min hold), injection model splitless, for 1 iL sample.

3. Results and discussion

3.1. Characterization of carbon-based AgBr photocatalyst

The prepared fresh carbon-based AgBr nanocomposites were first examined by XRD as shown in Fig. 2. The XRD results

20 30 40 50 60 70 29 (degree)

Figure 2 XRD pattern of (a) AgBr/GP, (b) AgBr/EG, and (c) AgBr/AC.

i^i—îk—

showed that CNT, GP, EG and AC had the hexagonal structure and their identified peaks are sharp at around 2h = 26.4°, corresponding to 00 2 with an interlayer spacing of 0.34 nm. In the meantime, the crystalline AgBr was found in the fresh carbon-based AgBr nanocomposites at around 2h = 26.7°, 30.9°, 44.3°, and 55.0°, indicating a hexagonal structure for the synthesized AgBr according to the report by Rodrigues and Mar-tyanov et al. [23]. In our synthesis process, a large quantity of the silver species can be precipitated and deposited by CTAB, since the amount of bromide species from CTAB is more than sufficient than that of silver species. In addition, XRD pattern of the used carbon-based AgBr nanocomposites after 5 h visible light irradiation with a 150 W Xe lamp was almost the same as that of fresh carbon-based AgBr nanocomposites. The diffraction peaks which are assigned to metallic Ag (38.2°, 44.4°, 64.4°, and 77.4°) were not detected in both of fresh and used carbon-based AgBr nanocomposites. Earlier studies reported that the irradiation led to the appearance of metallic Ag, for example, 1.2wt.% Ag was formed after 14 h irradiation in AgBr/TiO2 [6], however there are still some reports related to photostability without any formation of Ag for AgI/TiO2 [13] and AgCl/TiO2 [14] photocatalyst as well.

Fig. 3 showed TEM images of carbon-based AgBr nano-composites. It can be seen that AgBr nanoparticles with a size of 5-10 nm are deposited on the surface of carbon-based materials, indicating that AgBr nanoparticles were dispersed well on the surface of carbon-based materials (small black dots overlapped with parts of carbon-based supporting materials). It is believed that this deposition of AgBr with carbon-based supporting materials was beneficial to the charge transfer between the interface of carbon and AgBr nanoparticles.

To investigate the optical properties, AgBr and carbon-based AgBr nanocomposites were analyzed by UV-Vis absorption spectra in the wavelength range of 250-800 nm and the absorption spectra of AgBr and carbon-based AgBr nanocomposites are shown in Fig. 4. It can be seen that AgBr shows an absorption edge at 470 nm (an indirect band gap of 2.64 eV), which can be applied in photocatalytic reaction by coupling some supporting materials. Thus, AgBr/TiO2 catalysts had a substantial red shifting of the absorption edge due to dispersed AgBr as well as a better optical absorption generally in the visible region of 400-700 nm, compared to TiO2 catalysts. With respect to carbon-based AgBr nanocomposites, it is difficult to observe the shift of band gap due to the strong absorption of the black supporting materials. However, it should be noted that all carbon-based AgBr nanocomposites had better optical absorption generally in the visible light range. Thus, it is believed that the better optical absorption of carbon-based AgBr composites in the visible light range is an essential condition to obtain the high photocatalytic performance under visible light.

3.2. Photocatalytic reduction activity of carbon-based AgBr nanocomposites

Fig. 5a illustrated the dependence of CO2 reduction yield on the different supporting materials. It can be seen that the pho-tocatalytic products, i.e. methane, methanol, CO and ethanol, were formed in the photocatalysis using different carbon-based AgBr nanocomposites. Indeed, the conduction band of AgBr is located at —1.04 eV versus NHE, which is more cathodic than the conduction band of TiO2 (—0.5 eV) [3]. The potential for the reducing CO2-CH3OH, CH4, and CO was —0.38, —0.24,

§ 1.0

300 400 500 600 700 800 Wavelength (nm)

Figure 4 UV-vis diffuse reflectance spectra of photocatalysts absorbance vs. light-absorbed wavelength.

§ 120

Л 100

о 60 ■о

о 40 Q-

"o 60 ■o

o 40 0.

Figure 5 Product yields for different carbon-based AgBr nanocomposites.

and —0.53 V versus NHE, respectively. Therefore, the conduction band of the semiconductor was more negative than the reduction potential of CO2, leading to the reduction of CO2 to hydrocarbons. Moreover, the product yield under visible light for 5 h increased in the order of AgBr/GP«AgBr/ CNT > AgBr/EG > AgBr/AC, in which AgBr/GP produces a maximum yield of methane, methanol, ethanol, and CO with 156.11, 94.44, 16.13, and 39.02 imol g—1 respectively. The different activity is probably related to the electron transfer from the conduction band of AgBr to different carbon supporting materials. Experimental results demonstrate that AgBr/GP

and AgBr/CNT are the effective catalysts compared with AgBr/EG and AgBr/AC possibly due to the excellent conductivity of GP.

Pioneer research groups investigated the zeolite [31] and titanium dioxide [6,13] on the efficiency for AgBr. Therefore, we also compared the catalytic activity of carbon-based AgBr nanocomposites for photocatalytic reduction of CO2 with AgBr/TiO2 and AgBr/zeolite. The results shown in Fig. 5b illustrated a significant yield of photoreduction products for AgBr/GP compared with that of AgBr/TiO2 and AgBr/zeolite. This result may help in providing optimal catalytic structure which reduces carbon dioxide under visible light with a relatively higher reduction yield.

Since the photoreduction process of CO2 involved H. radicals and carbon dioxide anion radicals formed by electron transfer from the conduction band [27,29,33], the pH value was crucial in the photoreduction of CO2. It can be seen from Fig. 6 that the yield of photocatalytic products for CO2 reduction using carbon-based AgBr nanocomposites is dependent on the pH value. Firstly, the product yield of CO2 reduction for four carbon-based AgBr nanocomposites increased with the pH value up to 8.5, and then decreased upon the further increase in the pH value. Obviously, the neutral and weak alkaline pH value is beneficial to the photoreduction of CO2. This can be explained by the following reasons: (1) The OH~ ions in aqueous solution could act as strong hole-scavengers and reduce the recombination of hole-electron pairs [15], beneficial to facilitate the reduction of CO2. In contrast, H+ in aqueous solution could be involved in the electron competition

with CO2 reduction (i.e. H+ + e--> H-), leading to lesser

yields of hydrocarbons [27]. (2) More CO2 can be dissolved in basic solution to form HCO3- ions (i.e. CO2 + OH~ fi HCO3~) than pure water or acidic solution [26], resulting in higher yields of hydrocarbons in the photore-duction process. (3) The neutral and weak alkaline pH value is beneficial to the adsorption of CO2 on the catalyst from the zeta potential result of carbon-based AgBr nanocomposites (Fig. 7). It can be seen that the electrostatic force existed between carbonic ions and carbon-based AgBr nanocomposites due to positively charged carbon-based AgBr nanocomposites at pH < 6.0, while the electrostatic repulsive force existed between CO2 and carbon-based AgBr nanocomposites due to negatively charged carbon-based AgBr nanocomposites at pH > 6.0. A relatively higher photocatalytic reduction activity was achieved in the neutral and weak alkaline pH range, representing the overall combined effect of the higher concentration of OH~ ions but lower electrostatic repulsive force.

3.3. Stability of carbon-based AgBr nanocomposites

The practical application of such a photocatalyst is the major scale which determines the photocatalyst efficiency, at the same time not only the photocatalytic activity of a catalyst is important, but its stability is also critical, since AgBr as a component of carbon-based AgBr nanocomposites is a photosensitive material. Alternatively, both GP-based AgBr nano-composites and AgBr were tested for their repeated uses and stability for CO2 photocatalytic reduction in aqueous solution.

It can be seen from Fig. 8a that GP-based AgBr nanocom-posites kept the photocatalyst stability during 5 repeated uses. AgBr/GP retained its yield of methane, methanol, ethanol, and

AgBr AgBr/CNT AgBr/GP AgBr/GO AgBr/AC

, 150 120 90 60 30 0

160 140 120 100 80 60 40 20

2.0 4.0 6.0 7.0 8.5 10.0 12.0 pH value

7.0 8.5

pH value

pH value

Figure 7 Zeta potential for suspension of carbon-based AgBr nanocomposites in 0.02 M KCl solution.

CO in 5 repeated uses at 147.18 ± 7.04, 86.59 ± 6.07, 12.71 ± 2.51, and 33.74 ± 4.95 imol g—1, respectively. The total yields of the photocatalytic products on AgBr/GP after the 5 repeated uses remained about 83% of the first run. The results indicated that AgBr/GP is an effective and chemical-stable catalyst. However, AgBr demonstrated a significant decline of product yield after 3 repeated uses from 36.73 to 0 imol g—1

= 140 о

■g 100 ф

Ü 60 TD

2 40 CL

-К 120 100

о n TD О

7.0 8.5

pH value

7.0 8.5

pH value

Figure 6 Effect of pH value on product yields using (a) AgBr/ CNT, (b) AgBr/GP, (c) AgBr/EG, and (d) AgBr/AC under visible light irradiation.

2 30 о

Number of batch run (N)

Number of batch run (N)

Figure 8 Product yields for (a) AgBr/GP nanocomposite, and (b) AgBr on number of batch runs.

О 140

for methane, 32.16 to 3.71 imolg-1 for methanol, 6.49 to 1.46 imol g-1 for ethanol, and 9.18 to 0 imolg-1 for CO, respectively (Fig. 8b). The total yields of the photocatalytic products on AgBr after the 3 repeated uses decreased to about 6% of the first run.

Furthermore, the used AgBr and AgBr/GP after repeated uses were examined by XRD measurement. Fig. 9 showed the XRD patterns of the fresh and used AgBr. It can be seen clearly that the crystalline Ag can be found in the used AgBr at around 2h = 38.20 (Fig. 9b) compared to that of fresh AgBr (Fig. 9a), indicating the formation of metallic Ag through the photographic process under visible light irradiation. This result indicated that the decreased activity of pure AgBr is ascribed to the decomposition of the catalyst. However, with respect to AgBr/GP, the XRD results show that AgBr/GP after repeated use appeared to display no changes as before, indicating that AgBr nanoparticles well-dispersed on GP can maintain their stability and photocatalytic activity. In this photocatalytic process, the transfer of photoexcited electrons from the conduction band of well-dispersed AgBr to carbon-based materials is beneficial for the stability of significant advantage for CO2 photocatalytic reduction. Therefore, in practice, the stability of carbon-based AgBr nanocomposites would be a significant advantage for CO2 photocatalytic reduction under visible light.

3.4. Electron transfer mechanism of carbon-based AgBr nanocomposites

Further electrochemical impedance spectroscopy (EIS) experiments were carried out to explain the reasons of the high photocatalytic activity and stability of the as-prepared carbon-based AgBr nanocomposite photocatalyst. EIS measurements of carbon-based AgBr nanocomposites were

; AgBr (200) A< (2 (a) gBr 20) A2g2B2r AgBr AgBr (|2) (400) (420)

• ' AgBr Ag (200) (2 Ag0 (111) ■ ' Br (b) 20) AgBr AgBr AgBr (222) (400) (420) .......

20 30 40 50 60 70 80

2 9 (degree)

Figure 9 XRD pattern of (a) fresh AgBr, and (b) used AgBr after visible light irradiation.

90 80 70 60

^N 40 30 20 10 0

160 200 240 280 320 360 400

z' (Ohm)

Figure 10 EIS plots of carbon-based AgBr electrodes under visible light.

conducted under visible light to understand the electron transfer process in the photocatalytic conversion of CO2 reduction. The impedance data were collected for nanocomposite electrodes in 0.1 M KCl liquid at pH 5.8 (Fig. 10). It can be seen that only one arc appeared on the Nyquist plot of EIS for all electrodes under visible light conditions, suggesting a simple electrode process under our experimental conditions. The arc radius on EIS Nyquist plot of nanocomposites was of the order of AgBr/GP < AgBr/CNT < AgBr/EG < AgBr/AC. Generally, the smaller the arc radius on the EIS Nyquist plot, the lower the charge transfer resistance. The EIS results demonstrated impedance order as AgBr/GP < AgBr/ CNT < AgBr/EG < AgBr/AC. Thus, the electron accepting and transporting properties of GP and CNT in carbon-based AgBr nanocomposites were much faster than that of EG and AC in carbon-based AgBr nanocomposites, which could be beneficial to the suppression of charge recombination, thereby the highest rate in catalytic reduction of CO2 for Ag@AgBr/ CNT-L would be achieved. In addition, the process of electron transfer is faster than the electron-hole recombination between valence band and conduction band of AgBr, thus plenty of CB-electrons (AgBr) can be stored on the surface of carbon-based supporting materials to be reacted later with CO2. Thus, the timely electron capture on the surface of carbon-based supporting materials plays another important role in the stability of carbon-based AgBr nanocomposites.

4. Conclusions

In this work, the catalytic activities of carbon-based AgBr nanocomposites for CO2 reduction to hydrocarbons under visible light were investigated. The carbon-based AgBr nanocom-posites were prepared by the deposition-precipitation method in the presence of cetyltrimethylammonium bromide (CTAB). The photocatalytic activities of carbon-based AgBr nanocom-posites were evaluated by the reduction yield in the presence of CO2 and water under visible light (k > 420 nm). The experiment results showed that AgBr/GP and AgBr/CNT had a relatively higher reduction yield under visible light compared with AgBr/EG and AgBr/AC. Moreover, it was found that the carbon-based AgBr nanocomposites were stable in the

repeated uses under visible light due to the transfer of photo-excited electrons from the conduction band of well-dispersed AgBr to carbon supporting materials. This investigation in the present work demonstrated that the catalytic activity of carbon-based AgBr nanocomposites can be utilized in the CO2 reduction to hydrocarbons under visible light. Thus, the study, using carbon-based AgBr nanocomposites to reduce CO2 under visible light, will enrich the fundamental theory not only for efficient CO2 conversion and fixation, storage of solar energy, but also for solving environmental problems.

Acknowledgments

The authors wish to acknowledge the following financial supporters of this work: the National Natural Science Foundation of China (Nos. 21273085, 20877025), the Guangdong Province Science and Technology Bureau (No. 2010B090400552), the Natural Science Foundation of Guangdong Province, China (Nos. S2013010012927, S2011010003416, S2011010001836), the Fundamental Research Funds for the Central Universities (No. 13lgjc10), and the Research Fund Program of Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology (No. 2011K0003) for financially supporting this work.

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