Silver Silicate@Carbon Nanotube Nanocomposites for Enhanced

Jan 20, 2017 - The material shows a better visible light degradation effect on organic dyes in comparison to commercial photocatalysts. ... Zero dimen...
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Research Article pubs.acs.org/journal/ascecg

Silver Silicate@Carbon Nanotube Nanocomposites for Enhanced Visible Light Photodegradation Performance Ya-Qiong Jing,† Chen-Xi Gui,† Jin Qu,*,† Shu-Meng Hao,† Qian-Qian Wang,† and Zhong-Zhen Yu*,†,‡ †

Beijing Key Laboratory of Advanced Functional Polymer Composites, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China ‡ Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *

ABSTRACT: The abilities of adsorption and separation of electron hole pairs are two important factors in photodegradation, and a balance between them is required to obtain an excellent photodegradation performance. As a new photocatalyst, silver silicate has a poor conductivity that hinders its photodegradation ability. Herein, silver silicate (AgSiOx)@carbon nanotube (CNT) and AgSiOx@reduced graphene oxide (RGO) nanocomposites are prepared for the first time to improve the photodegradation performance of AgSiOx. The influences of CNT and RGO contents on improving the photodegradation efficiency of AgSiOx are different due to the differences in the concentration of oxygencontaining groups and the electrical conductivity. The photodegradation efficiency of AgSiOx@CNT nanocomposites first increases and then decreases with increasing the concentration of CNTs, while the removal efficiency of pollutants by AgSiOx@RGO nanocomposites increases with the GO concentration owing to the residual oxygen-containing functional groups on RGO. The AgSiOx@CNT nanocomposite with a trace amount of CNTs (0.1 wt %) shows fairly effective photodegradation activity, and its photodegradation process is completed in 10 min with a higher removal efficiency and rate constant than reported. KEYWORDS: Silver silicate, Photodegradation, Visible light, Carbon nanotube, Reduced graphene oxide

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INTRODUCTION

Several approaches have been developed for visible light photocatalysts, such as deposition of noble metals, compounding with metals or semiconductors, ion-doping, and dye sensitization.4 Nanoparticles of noble metals, such as silver and gold, are able to absorb visible light because of their localized surface plasmon resonance effect. When the electric field of an incident light couples with the conduction band electrons of the noble metals, a plasmonic oscillation localizes on the surface of nanoparticles; the localized surface plasmon resonance could extend the optical absorption of noble metals to visible light region and accelerate the separation of electron hole pairs. Therefore, Ag is widely used to couple with other photocatalysts to enhance the visible light absorption ability.5−7 Many visible light induced silver based photocatalysts have been discovered including Ag2O,8 Ag3PO4,9−11 Ag/AgCl,12,13 Ag/AgBr,14,15 and Ag/AgI.16,17 Silver halide has a good

Since the first discovery of photocatalytic splitting of water in 1972, visible light induced photocatalysts have had great potential due to their efficient utilization of solar energy. When the photocatalyst is excited by the light within a certain range of wavelengths that matches the band gap of the material, the electrons in the valence band are excited to the conduction band, and the holes are left in the valence band. The holes and the excited electrons form the electron hole pair. The electrons initiate redox reaction with the water molecules and form hydroxyl radicals, which react with organic pollutants and generate less toxic products.1 TiO2 is the most widely studied photocatalyst because of its high photocatalytic activity, good thermal and chemical stability, nontoxicity, and low cost. However, the main drawback of TiO2 is the narrow band gap that can only be excited by ultraviolet (UV) light and the fast recombination of electron hole pairs that limits the photodegradation efficiency.2 Visible radiation accounts for ca. 45− 50% of the total solar radiation.3 To improve the performance of a photocatalyst, its optical absorption should be extended to the visible light region, and the recombination rate of electron hole pairs should be retarded. © 2017 American Chemical Society

Special Issue: Asia-Pacific Congress on Catalysis: Advances in Catalysis for Sustainable Development Received: November 1, 2016 Revised: January 6, 2017 Published: January 20, 2017 3641

DOI: 10.1021/acssuschemeng.6b02650 ACS Sustainable Chem. Eng. 2017, 5, 3641−3649

Research Article

ACS Sustainable Chemistry & Engineering sensitivity to light and is widely used in photographic films. When the incident light generates an electron hole pair on silver halide, silver ions react with the electrons and form silver atoms and thus extend the light absorption region of the material to visible light region. However, the methods to prepare these materials are usually complicated and timeconsuming. It is desirable to develop other visible light active materials. Silicate nanomaterials have been widely used in adsorption, catalyst, energy storage, and drug delivery because of their simple preparation, rich source, unique porous structure, and low cost.18−21 Previously, we reported the synthesis and application of hierarchical zinc silicate, 22 sandwich-like magnesium silicate,23 and other metal silicate nanotubes24 for the adsorption of water pollutants and explored lithium storage properties of the layered nickel silicate hybridized with carbon nanotubes (CNTs)25 or reduced graphene oxide (RGO) sheets.26 Recently, Lou et al. developed a novel silver silicate material for visible light absorption.27 There are two structure features that benefit the charge separation: internal polar electric field and nonequivalent metal oxygen polyhedral bridged by metal oxygen bonds. For silver silicate, the transition metal−SiO4 tetrahedral could easily be distorted and forms an internal polar electric field, and the coordination conditions benefit the flow of charge carriers from one tetrahedral to another with an extended light absorption region. The material shows a better visible light degradation effect on organic dyes in comparison to commercial photocatalysts. However, a major disadvantage of silicate materials is their low electrical conductivity, which is not beneficial for the separation of electron hole pairs. It is therefore imperative to combine an electrically conductive second phase with silicates to enhance the photodegradation performances. The inhibitation of electron hole pair recombination is usually achieved by coupling with semiconductors with a narrow band gap or using electron donors or acceptors. Zero dimensional fullerenes, one-dimensional CNTs, and twodimensional graphene or graphene oxide (GO) have been used to enhance the performance of various photocatalysts.28−34 It is commonly agreed that the multiple graphitic layers of multiwalled CNTs could facilitate the migration of electrons and extend the light absorption wavelength,35−37 and RGO sheets with a large specific surface area could serve as the supporting substrate for various photocatalysts and provide a platform for the free flow of electrons.38,39 Enrichment of the organics is another contribution of RGO as its large specific surface area and residual oxygen-containing groups could enhance the adsorption capacity of its nanocomposites and bring the organics to the photocatalysts.40 However, there is no agreement yet on the comparison of the enhancement of carbon additives in terms of their types and concentrations. Some believe that graphene or RGO shows a better photocatalytic performance than do CNTs, while others argue that graphene is essentially the same as other carbon additives including activated carbon and CNTs.41 Herein, both AgSiOx@CNT and AgSiOx@RGO nanocomposites are synthesized in situ with a one-step approach, and their visible light photodegradation enhancements are explored. The influence of carbon phase content is studied, and the removal efficiencies of these two nanocomposites are compared. Thanks to the better conductivity of CNT than RGO, a trace amount of CNTs (0.1 wt %) is sufficient in improving the photodegradation activity than that of AgSiOx@

RGO nanocomposites, while further increase in CNT concentrations leads to a reduction in the photodegradation efficiency due to the decreased synergistic effect of the nanocomposites by improving the electron transfer effect. Differently, the residual oxygen-containing groups of RGO still endow the AgSiOx@RGO nanocomposites with good adsorption of dyes rather than photodegradation efficiency.

2. EXPERIMENTAL SECTION Materials. Graphite flakes were supplied by Huadong Graphite Factory (China) with an average diameter of 13 μm. Multiwalled CNTs were purchased from Shenzhen Nanotech Port Co. (China) with an average diameter of 40−60 nm. Concentrated nitric acid (HNO3, 65−68%), sulfuric acid (H2SO4, 98%), hydrochloric acid (HCl, 37%), and hydrogen peroxide (H2O2) were bought from Beijing Chemical Factory (China). Silver nitrate (AgNO3), sodium silicate (Na2SiO3·9H2O), methylene blue (MB), ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), and tert-butanol were supplied by the Sinopharm Chemical Reagent Co. (China). All chemicals are analytical grade and were used without further purification. Synthesis of AgSiOx. A total of 510 mg of AgNO3 was dissolved in 50 mL of deionized water as solution A, and 284 mg of Na2SiO3· 9H2O was dissolved in 50 mL of deionized water as solution B. After stirring solution A for 30 min, solution B was added dropwise with a constant pressure funnel. The mixture was further stirred for 2 h, and the resultant product was collected by centrifuging, washed with deionized water and ethanol three times, and dried at −55 °C in a FD1C-50 freeze dryer (China). Syntheses of AgSiOx@CNT and AgSiOx@RGO Nanocomposites. AgSiOx@CNT nanocomposites were prepared with different CNT concentrations of 0.05, 0.1, 0.2, 0.5, 1, 5, and 10 wt %, which are based on the initial amount of CNTs. In a typical procedure, CNTs were dispersed in 50 mL of deionized water by ultrasonication for 1 h, and then 510 mg of AgNO3 was added and stirred for another 30 min to get a CNT/AgNO3 mixture. Na2SiO3·9H2O was dissolved in 50 mL of deionized water, and the resultant solution was added dropwise using a constant pressure funnel to the CNT/AgNO3 mixture, which was further stirred for 2 h. The resulting product was collected by centrifuging, washed with deionized water and ethanol three times, and freeze-dried using the freeze dryer. By a similar approach, AgSiOx@RGO nanocomposites were also prepared with different GO concentrations of 0.05, 0.1, 0.2, 0.5, 1, 5, and 10 wt %. GO was prepared using a modified Hummers method.42,43 Characterization. Microstructures of the nanocomposites were observed with a Zeiss Supra 55 field-emission scanning electron microscope (SEM) and a JEOL JEM-3010 transmission electron microscope (TEM). X-ray diffraction (XRD) measurements were carried out using a Rigaku D/Max 2500 diffractometer with Cu Kα radiation (λ = 1.54 Å) at a generator voltage of 40 kV and a generator current of 40 mA. AgSiOx, AgSiOx@CNT, and AgSiOx@RGO were characterized with a Thermo VG RSCAKAB 250X high-resolution Xray photoelectron spectroscope (XPS) and a Nicolet Nexus 670 Fourier-transform infrared spectroscope (FT-IR). A conventional three-electrode system was used for electrochemical impedance spectroscopy (EIS) measurements. A 5 mg sample was ultrasonicated in 1 mL of ethylene glycol, and 20 uL of the resultant slurry was dropped on the indium tin oxide (ITO) glass (1 × 0.5 cm2) and dried at ambient temperature overnight. The working electrode was immersed in an electrolyte consisting of 0.1 M KCl solution with 5 mM K3[Fe(CN)6]/K4[Fe(CN)6] for 1 day. The counter electrode was platinum wire, and the reference electrode was Ag/AgCl in saturated KCl solution. The EIS Nyquist plots were recorded on a Metrohm Autolab PGSTAT 302N electrochemical workstation with an AC modulation amplitude of 5 mV and in the frequency range 50 kHz to 0.1 Hz. Photoluminescence (PL) spectra were measured on a Hitachi F-5400 fluorescence spectrophotometer with an excitation wavelength of 270 nm. The scanning speed, photomultiplier voltage, and excitation and emission slits were 1200 nm/min, 900 V, 10 and 10 nm, respectively. 3642

DOI: 10.1021/acssuschemeng.6b02650 ACS Sustainable Chem. Eng. 2017, 5, 3641−3649

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Figure 1. TEM images of (a) AgSiOx, (c) AgSiOx@CNT (1%), and (d) AgSiOx@RGO (1%) nanocomposites. (b) XRD patterns of AgSiOx, AgSiOx@CNT (1%), and AgSiOx@RGO (1%) nanocomposites. Photocatalytic Measurements. A total of 50 mg of AgSiOx, AgSiOx@CNT, or AgSiOx@RGO was mixed with 50 mL of MB solution at a content of 50 ppm, and the mixture was stirred in the dark for 30 min to achieve an adsorption equilibrium. The photocatalytic experiments were carried out in a photocatalytic reaction chamber under a CEL-HXUV300 xenon lamp with visible light wavelength between 400 and 780 nm. A total of 2.5 mL of the mixture was taken out every 2 min until the photocatalytic process was completed. The mixture was centrifuged at 10 000 rpm, and its supernatant was taken for UV−vis analysis using a Thermal Scientic Evolution 200 UV−visible (UV−vis) spectrophotometer (USA). The total amount of the radical scavengers used was 0.1 mmol.

image of the AgSiOx@CNT nanocomposite (Figure 1c) clearly shows the spherical nanoparticles of AgSiOx that are uniformly attached along the nanotubes like a nanosized necklace, indicating that there should be physical or chemical interactions between them to improve the electron and ion transport properties at the interfaces. GO exhibits a typical sheet structure (Figure S1b); after its decoration with AgSiOx, a large number of nanoparticles are observed on the surface of RGO sheets (Figure 1d). No isolated AgSiOx nanoparticles are observed even after ultrasonic treatment of the specimen, suggesting that the AgSiOx nanoparticles are well dispersed and tightly attached on the RGO sheets. The concentration of CNTs plays an important role in the microstructure of AgSiOx@CNT nanocomposites. Figure 2a−c show SEM images of AgSiOx@CNT nanocomposites with different CNT concentrations. At the low CNT concentration of 0.1%, AgSiOx nanoparticles are attached on CNTs like strings and covered all CNT surfaces, few isolated AgSiOx nanoparticles are observed (Figure 2a). However, at the higher CNT concentration of 1%, some part of the CNT surface is exposed in the air, which may reduce the utilization efficiency of CNTs (Figure 2b). A further increase of the CNT concentration to 10% leads to an increased exposure of the CNT surface, and fewer AgSiOx nanoparticles are attached on CNTs (Figure 2c). Compared to the one-dimensional AgSiOx@CNT nanocomposite, AgSiOx nanoparticles are uniformly deposited on both sides of RGO sheets (Figure 2d). The assembly of AgSiOx onto one-dimensional CNTs and two-dimensional RGO forms nanocomposites with different morphology and dimensionality, which could influence the

3. RESULTS AND DISCUSSION Figure 1 shows TEM images and XRD curves of AgSiOx, AgSiOx@CNT, and AgSiOx@RGO nanocomposites. It is seen that AgSiOx nanoparticles are spherical with sizes of around 50 nm (Figure 1a). Although its XRD pattern exhibits the main features of silver silicate (Ag6Si2O7, JCPDS: 85−281), the peaks are not very sharp, indicating that the silver silicate is not fully crystallized. Actually, poor crystallization could lead to more defects on the silver silicate surface and thus provide better chemical activity. The compounding with carbon components does not change the crystallographic structure of AgSiOx, which is pretty critical as the purpose of the incorporated carbon phase is to improve the electron transfer efficiency and adsorption capacity and serve as the supporting matrix for a satisfactory dispersion of AgSiOx nanoparticles.44 Since the main visible light photodegradation component is still AgSiOx, whose original structure should be preserved (Figure 1b). The multiwalled CNTs exhibit diameters around 20−40 nm and wall thickness of 5−10 nm (Figures 1c and S1a). The TEM 3643

DOI: 10.1021/acssuschemeng.6b02650 ACS Sustainable Chem. Eng. 2017, 5, 3641−3649

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Figure 2. SEM images of AgSiOx@CNT nanocomposites with the CNT concentrations of (a) 0.1%, (b) 1%, and (c) 10%. (d) SEM image of the AgSiOx@RGO nanocomposite (1%).

would help improve the photodegradation performance due to the extended optical absorption region and accelerate the separation of electron hole pairs.45 However, much more Ag causes a lesser amount of AgSiOx formed in the nanocomposite, thus resulting in a lower photodegradation performance. The optical absorption property of a photocatalyst is a key factor in determining its photocatalytic performance. Figure 3d shows UV−vis spectra of AgSiOx, AgSiOx@CNT, and AgSiOx@RGO nanocomposites. All specimens exhibit broad absorption in the visible light region, which is favorable for the visible light photodegradation reaction. Although the AgSiOx@ CNT nanocomposite has a slightly lower absorption performance in the range of 400−575 nm than AgSiOx, it has a much higher absorption performance in the range of 575−800 nm. Such an enhanced absorption property would result in better visible light photodegradation activity than that of AgSiOx. Compared to AgSiOx@CNT, the area of enhanced absorption range of 600−800 nm is nearly the same as that of the reduced absorption range of 400−600 nm; therefore, the compounding with RGO may not improve the visible light photodegradation activity of AgSiOx as well as compounding with CNTs. On the basis of the spectra (Figure 3d), the band gaps of the specimens are estimated as follows: AgSiOx (1.58 eV), AgSiOx@RGO (1.48 eV), and AgSiOx@CNT (1.35 eV). The narrowing band gaps indicate that compounding with carbon components effectively enhances the visible light absorption performance, and CNTs perform better than RGO in improving the photocatalytic property of AgSiOx.

synergistic enhancement in photodegradation performances of the nanocomposites. XPS surveys of AgSiOx, AgSiOx@CNT, and AgSiOx@RGO nanocomposites in Figure S2 confirm the existence of Ag and Si elements in AgSiOx. C 1s curve fitting results indicate that the oxygen contents are different in these samples (Figure 3a). The C−C bond is dominant in CNTs, while the C−O bond is introduced after compounding with AgSiOx. Compared to the AgSiOx@CNT nanocomposite, AgSiOx@RGO contains oxygen-containing groups due to the residual oxygen-containing groups of the RGO component, enhancing the adsorption ability of organic pollutants. A moderate adsorption ability of RGO sheets effectively enriches the pollutants around the AgSiOx catalyst, and thus increases the chance for pollutants to be degraded. It is noted that, however, too strong of an adsorption ability would lead to a tight interaction between the adsorbent and adsorbate, making desorption of the adsorbate from the adsorbent difficult, and therefore hindering further improvement in photodegradation efficiency. For the Si 2p peak, 101.5 eV of binding energy is observed in AgSiOx, AgSiOx@CNT, and AgSiOx@RGO nanocomposites, indicating the formation of the Si−O−Ag bonds (Figure 3b). Silver ions are easily reduced to Ag when forming the nanocomposites with other nanomaterials. In the current study, Ag is observed in both AgSiOx@CNT and AgSiOx@RGO nanocomposites (Figure 3c), although it does not exist in AgSiOx. More silver ions are reduced to Ag in the AgSiOx@ RGO nanocomposite than the AgSiOx@CNT nanocomposite (Figure 3c). Similar to the adsorption ability of the RGO supporter, an appropriate amount of Ag in the nanocomposite 3644

DOI: 10.1021/acssuschemeng.6b02650 ACS Sustainable Chem. Eng. 2017, 5, 3641−3649

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Figure 3. (a) C 1s, (b) Si 2p, and (c) Ag 3d curves of CNTs, GO, AgSiOx@CNT (0.1%), and AgSiOx@RGO (0.1%) nanocomposites. (d) UV−vis spectra of AgSiOx, AgSiOx@CNT (0.1%), and AgSiOx@RGO (0.1%) nanocomposites measured in solution with the same concentration of 1 mg/ mL.

photodegradation rate than that of AgSiOx, and the AgSiOx@ CNT (10%) nanocomposite has the lowest photodegradation efficiency. This is probably because a small amount of CNTs could initiate the synergistic effect of the nanocomposite by improving the electron transfer effect and prolonging the lifetime of electron−hole pairs, thus enhancing the photodegradation efficiency. However, as shown in Figure 2a−c, high CNT concentrations cause more exposure of the CNT surface with no cover of AgSiOx, decreasing the utilization efficiency of CNTs. Additionally, higher CNT concentration means a less active component of AgSiOx, which is not beneficial for improving the photodegradation performance. As for AgSiOx@RGO nanocomposites, their adsorption effect is dominant, evidenced by the significantly decreased MB concentration after 30 min stirring in darkness (Figure 4b). The initial adsorption percentages of AgSiOx@RGO nanocomposites show an obvious increasing trend in the range of 44−85% with adsorption capacities of 22−43 mg/g, which are much higher than that of the AgSiOx@CNT nanocomposite (ca. 15 mg/g). Different from CNTs, RGO contains residual oxygencontaining groups, which improve the adsorption capacity and the enriching effect of organics. It is therefore reasonable that the MB removal rate of AgSiOx@RGO nanocomposites increases with the RGO concentrations. The AgSiOx@RGO (0.05%) nanocomposite has the lowest removal rate, whereas the AgSiOx@RGO (10%) nanocomposite exhibits the highest

To compare the enhancing effect of CNT and RGO on the photodegradation, Figure 4 shows the time-dependent visible light photodegradation curves of AgSiOx@CNT and AgSiOx@ RGO nanocomposites. MB with a concentration of 50 ppm is used as the model organic compound. As a reference, AgSiOx accomplishes the photodegradation process in nearly 20 min (Figure 4a). For AgSiOx@CNT nanocomposites, the low CNT concentration improves the photodegradation ability of AgSiOx. However, a further increase in CNT concentration causes poor photodegradation performance. The AgSiOx@ CNT nanocomposite with 0.1 wt % CNTs exhibits the best photodegradation performance, and its photodegradation process is completed in 10 min with a rate constant of 0.51 min−1, much higher than that of AgSiOx (0.24 min−1, Table 1). Although the rate constant of AgSiOx@CNT (0.1%) is only 10% higher than that reported for AgSiOx (0.46 min−1, Table 1), 1 g of the AgSiOx@CNT (0.1%) nanocomposite could consume as high as 1.25 mg of MB per minute, which is 146% higher than that reported for AgSiOx. Such an excellent photodegradation efficiency is also better than that for previously reported photocatalysts under visible light (Table 1). Photodegradation efficiencies of AgSiOx@CNT (0.2%), AgSiOx@CNT (0.5%), and AgSiOx@CNT (1%) nanocomposites are slightly lower than that of AgSiOx@CNT (0.1%) but still better than those of the AgSiOx@CNT (0.05%) nanocomposite and AgSiOx. At high CNT concentrations, the AgSiOx@CNT (5%) nanocomposite exhibits an even lower 3645

DOI: 10.1021/acssuschemeng.6b02650 ACS Sustainable Chem. Eng. 2017, 5, 3641−3649

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Figure 4. Time-dependent visible light photodegradation of MB (50 mL, 50 ppm) by (a) AgSiOx@CNT and (b) AgSiOx@RGO nanocomposites. (c) Comparison of photodegradation abilities of the AgSiOx@CNT (0.1%) nanocomposite and a mixture of AgSiOx with 0.1 wt % CNTs. (d) Comparison of photodegradation abilities of AgSiOx@CNT (0.1%) nanocomposites under visible light and UV light.

is increased to 10%, the removal time of the AgSiOx@RGO (10%) nanocomposite is only one-third that of AgSiOx, but the photodegradation performance of the AgSiOx@CNT (10%) nanocomposite is even worse than that of AgSiOx. This suggests that CNT and RGO exhibit different enhancement effects at different concentrations. Unlike CNT versus graphene, which is reported to have a similar enhancement effect due to their integrate carbon network, CNT and RGO are quite different in nature. In addition to the oxygencontaining groups of RGO that are beneficial for adsorption, lots of defects on RGO damage the electron conductance and prolong the lifetime of electron−hole pairs, thus improving the photodegradation efficiency. Higher RGO concentration leads to an enhanced adsorption rather than photodegradation. As for the AgSiOx@CNT nanocomposite, although CNT has few oxygen-containing groups and a low adsorption capacity, its high electrical conductivity facilitates the transfer of electrons on its carbon network. On the basis of these results, it is clear that the adsorption capacity and electron transfer effect need to be balanced to achieve the best photodegradation performance. Compounding with CNT is the key to enhancing the photodegradation performance of AgSiOx. To confirm this, a mixture of AgSiOx with 0.1% CNT is fabricated and used for visible light photodegradation of MB with the same procedure as the AgSiOx@CNT (0.1%) nanocomposite. The mixture also accomplishes the photodegradation process in nearly 20 min, which is the same as bare AgSiOx (Figure 4c). It means that a synergistic effect is obtained by the in situ growth of AgSiOx on CNTs. A tight contact between them leads to a better electron transfer effect, thus enhancing the photodegradation efficiency. Encouraged by the excellent property of the AgSiOx@CNT

Table 1. Comparison of the Catalytic Performance of the Nanocomposites with the Literaturea mca. (mg)

VMB (mL)

CMB (ppm)

k (min−1)

Qt (mg g−1 min−1)

AgSiOx

50

50

50

0.24

1.00

AgSiOx@ CNT (0.1%) AgSiOx Ag3PO4/ CeO2 branched Ag3PO4 Ag10Si4O13 α-Fe2O3/ Ag6Si2O7 Ag6Si2O7/ WO3

50

50

50

0.51

1.25

100 30

100 80

20 10

0.46 0.063

0.51 0.44

27 46

50

60

10

0.15

0.40

47

50 3

50 10

20 10

0.019 0.227

0.39 0.72

48 49

20

50

9.6

0.116

0.39

50

catalyst

ref this work this work

a

mca., the mass of the catalyst; VMB, the volume of MB; CMB, the initial concentration of MB; k, the rate constant; Qt, the consumption of MB caused by 1 g catalyst per minute.

removal rate due to the excellent adsorption rather than photodegradation. Figure S3 compares the MB removal performances of AgSiOx, AgSiOx@CNT, and AgSiOx@RGO nanocomposites. Only a trace amount of CNT (0.1%) significantly enhances the photodegradation performance of AgSiOx and reduces the removal time to almost half. While RGO at the same concentration shows almost no enhancement. The difference agrees well with the results of Ag 3d XPS (Figure 3c) and UV− vis spectra (Figure 3d). When the carbon phase concentration 3646

DOI: 10.1021/acssuschemeng.6b02650 ACS Sustainable Chem. Eng. 2017, 5, 3641−3649

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Figure 5. (a) EIS Nyquist plots of AgSiOx, AgSiOx@CNT (0.1%), and AgSiOx@RGO (0.1%) nanocomposites in 0.1 M KCl solution with 5 mM K3[Fe(CN)6]/K4[Fe(CN)6]. (b) PL spectra of AgSiOx, AgSiOx@CNT (0.1%), and AgSiOx@RGO (0.1%) nanocomposites. (c) Plots of timedependent visible light photodegradation of MB (50 mL, 50 ppm) by the AgSiOx@CNT (0.1%) nanocomposite with the addition of EDTA-2Na or tert-butanol. (d) Schematic of the degradation of MB on the AgSiOx@CNT nanocomposite.

the electron and hole than AgSiOx and AgSiOx@RGO, which is expected to provide superior photocatalytic activity. To investigate the active species in the photodegradation process, radical scavengers are added into the system. EDTA2Na is the scavenger of photogenerated holes while tert-butanol is the scavenger of hydroxyl radicals. As shown in Figure 5c, the photodegradation of 50 ppm MB by the AgSiOx@CNT nanocomposite is accomplished within 10 min. With the addition of tert-butanol, the photodegradation process is slightly delayed. However, when EDTA-2Na is added into the system, the photodegradation ability is significantly inhibited. These results indicate that the photogenerated holes are the main active species in the photodegradation process of the AgSiOx@CNT nanocomposite. On the basis of the results, a possible mechanism is proposed (Figure 5d). Under visible light irradiation, AgSiOx is excited to generate e−/h+ pairs. The photoinduced electrons transfer quickly from AgSiOx toward CNTs, thus facilitating the separation of photogenerated charge carriers and effectively delaying the recombination of e−/h+ pairs. More conductive CNT is better to enhance the photocatalytic property of AgSiOx. In addition, a proper amount of oxygen-containing groups helps adsorbing organic dyes to the surface of AgSiOx, where photogenerated holes with powerful oxidizing ability directly decompose the dyes to inorganic micromolecules.

(0.1%) nanocomposite, its UV light photodegradation of MB (50 mL, 50 ppm) is also investigated. The AgSiOx@CNT (0.1%) nanocomposite also has the best photodegradation performance (Figure S4), which is nearly the same as that under visible light (Figure 4d). To explore the charge transfer process, Figure 5a shows EIS Nyquist plots of AgSiOx, AgSiOx@CNT (0.1%), and AgSiOx@ RGO (0.1%) nanocomposites.32 The plots consist of a semicircle at the low and medium frequency ranges and an inclined line at the high frequency range. The diameter of the semicircle corresponds to the electron transfer resistance. Compared to AgSiOx, the addition of RGO leads to a decreased semicircle diameter of the AgSiOx@RGO nanocomposite, implying an improved conductivity in the nanocomposite. As CNT is more electrically conductive than RGO, the AgSiOx@CNT nanocomposite exhibits a better electron transfer effect than the AgSiOx@RGO nanocomposite, which agrees with the results of the XPS C 1s curve (Figure 3a) and MB removal measurement (Figure 4). PL spectra (Figure 5b) confirm the advantage of AgSiOx@ CNT. The strong emission band centered at around 450−480 nm arises from the recombination of photogenerated electrons and holes. The PL intensities decrease in the order AgSiOx > AgSiOx@RGO > AgSiOx@CNT. It is widely accepted that a lower PL intensity corresponds to a lower recombination rate of e−/h+ pairs. These results suggest that the migration of electrons from AgSiOx to CNT prevents the direct recombination of e− and h+, leading to a longer lifetime for

4. CONCLUSIONS AgSiOx@CNT and AgSiOx@RGO nanocomposites are prepared by a one-step method, and the photodegradation 3647

DOI: 10.1021/acssuschemeng.6b02650 ACS Sustainable Chem. Eng. 2017, 5, 3641−3649

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ACS Sustainable Chemistry & Engineering

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enhancements of AgSiOx by CNTs and RGO components are investigated. Adsorption and conductivity properties of AgSiOx@CNT and AgSiOx@RGO nanocomposites are dependent on carbon contents. The electron transfer resistances are reduced after compounding CNTs with AgSiOx. The visible light photodegradation measurements show that only a small amount (as low as 0.1%) of CNTs is sufficient in enhancing the photodegradation efficiency of AgSiOx. Further increasing the concentration of CNTs reduces the performance. As for AgSiOx@RGO nanocomposite, its residual oxygencontaining groups make it an enhanced adsorption performance rather than photodegradation. Therefore, the AgSiOx@CNT nanocomposite has a better photodegradation performance than the AgSiOx@RGO counterpart at a low concentration of carbon components. The removal rate of the AgSiOx@RGO nanocomposite is faster than that of the AgSiOx@CNT nanocomposite at high carbon concentrations. The addition of radical scavenger agents confirms that the main active species are the photogenerated holes. Proper adsorption and conductivity of a photocatalyst are quite important for its excellent photodegradation performance, which could benefit the development of new photocatalysts.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02650. Figures S1−S4 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Fax: +86-10-64428582. E-mail: [email protected] *Fax: +86-10-64428582. E-mail: [email protected]. ORCID

Zhong-Zhen Yu: 0000-0001-8357-3362 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (51402012, 51533001), the Fundamental Research Funds for the Central Universities (YS201402) is gratefully acknowledged.

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REFERENCES

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DOI: 10.1021/acssuschemeng.6b02650 ACS Sustainable Chem. Eng. 2017, 5, 3641−3649

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DOI: 10.1021/acssuschemeng.6b02650 ACS Sustainable Chem. Eng. 2017, 5, 3641−3649