Chemically Modified Graphene Oxide-Wrapped Quasi-Micro Ag

Article pubs.acs.org/JPCC

Chemically Modified Graphene Oxide-Wrapped Quasi-Micro Ag Decorated Silver Trimolybdate Nanowires for Photocatalytic Applications Kan Zhang,† Nansra Heo,‡ Xinjian Shi,‡ and Jong Hyeok Park*,†,‡ †

SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 440-746, Republic of Korea School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea

‡

S Supporting Information *

ABSTRACT: We report a simple and versatile strategy for hybridizing 1-D silver trimolybdate wires (Ag2Mo3O10 Ws) and graphene oxide (GO) by a direct solution process. Because of the photoactive nature of Ag2Mo3O10 Ws, Ag particles were uniformly formed on their surfaces with the assistance of solar light. Enriched surface functional groups of GO from chemical activation induce a chemical coordination with these Ag particles. As a result, a uniform ternary hybrid mixture was successfully formed with metallic Ag nanoparticles as a bridge connecting activated GO and Ag2Mo3O10 Ws. We found that both excited GO and Ag2Mo3O10 could generate electron/hole pairs separated in space by metallic Ag as a solid-state electron mediator. The novel photocatalytic mechanism was confirmed using photocurrents, the electronic-band structure, and photoactivity correlation analysis.

1. INTRODUCTION Silver molybdates have attracted much attention as an important family of conducting glass and ammonia sensing materials due to easy manipulation of their morphologies by solution synthesis.1−6 More recently, one-dimensional, quasimicrostructured Ag2Mo3O10 wires (Ws) have been developed as a plasmonic photocatalyst for degradation of Rh.B and have mechanisms similar to those of previous silver salt (halide or phosphate) photocatalysts.7 These plasmonic silver salt photocatalysts can significantly absorb visible light due to their localized surface plasmon resonance when photoinduced electrons reduce Ag ions to metallic Ag0 particles on the surface of the photocatalyst.8 Similarly, Ag2Mo3O10 Ws can also generate an electron/hole pair to reduce Ag+ ions to metallic Ag0 particles, leading to a cluster of silver atoms on an Ag2Mo3O10 Ws backbone during repeated absorption of photons. Although the large surface-to-volume ratio of linear Ag2Mo3O10 Ws may provide excellent optical, chemical, and physical characteristics, it has attracted less attention due to lower photocatalytic activity compared to silver salt-based photocatalysts. For silver salt-based photocatalysts, the photogenerated holes within the silver halide photocatalysts are able to oxidize halide ions to halide atoms, which have strong oxidation ability for organic molecules.9 However, they have limited practical applications due to several drawbacks, including poor stability in aqueous medium and morphology control difficulties, which reduces their photocatalytic ability. For instance, silver orthophosphate has been demonstrated to produce an extremely high quantum yield of about 80% at wavelengths less than 480 nm for O2 evolution and degradation © 2013 American Chemical Society

of organic contaminants, but it is soluble in aqueous solution, which greatly reduces its structural long-term stability.10 In addition, the synthesis of silver halide nanostructures has not been as well studied. The uncontrollable nanostructure of silver halide leads to the loss of its unique electronic and optoelectronic properties. Most recently, construction of graphene oxide (GO) or reduced graphene oxide (RGO)/silver halide or phosphate hybrids have attracted much attention in order to improve their chemical structure and properties via efficient charge separation and robust interfacial structure. For example, Zhang et al. reported that an RGO-grafted Ag@AgCl hybrid can enhance photocatalytic activity and stability for degradation of rhodamine B.11 Also, Zhu and co-workers reported that GOwrapped Ag@AgX (X = Cl, Br) composites showed enhanced photocatalytic activity and stability for the degradation of methyl orange.12 The advantages of GO or RGO hybridization can be found in most silver halide and phosphate cases,13−18 in which they can act as an electron acceptor to effectively suppress the charge recombination, resulting in more reactive species.19 Notably, for numerous RGO- or GO-based semiconductor nanocomposites in photocatalytic systems,20,21 there seems to be a consensus that the photoactivity enhancement of semiconductors after hybridization with RGO or GO is attributed to a 2D network with an electron sink to accept and shuttle electrons photogenerated in the semiconductors.22 Received: July 16, 2013 Revised: October 8, 2013 Published: October 11, 2013 24023

dx.doi.org/10.1021/jp407038c | J. Phys. Chem. C 2013, 117, 24023−24032

The Journal of Physical Chemistry C

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Figure 1. (a) UV−vis spectra of GO, activated GO, and graphene. Inset is GO (left) and activated GO (right) aqueous solution (b) photoelectrochemical properties of GO and activated GO. C1s XPS spectra of GO (c) and activated GO (d).

dispersed into strong basic conditions (pH > 10). Enough amount of chloroacetic acid was added and stirred for 10 h in order to activate epoxide and ester groups and convert hydroxyl groups to carboxylic acid (COOH) moieties.28 (Note: Because of excellent dispersity of GO in basic conditions, the samples were washed and collected by centrifugation of high speed (15 000 rpm) at low temperature (4 °C). 2.2. Synthesis of Ag2Mo3O10 Nanowire. The synthesis of ultralong Ag2Mo3O10 NWs was based on the procedure reported previously with slight modification.7 Briefly, 0.1062 g of AgNO3 were dissolved in 20 mL of distilled water as solution A, and 0.2194 g of (NH4)6Mo7O24·4H2O were dissolved in 20 mL of distilled water as solution B, respectively. Then, solutions A and B were mixed, and pH was adjusted to 2 by adding HNO3. The precursor solution was transferred into a 50 mL Teflon-lined stainless autoclave and maintained at 140 °C for 6 h. The yellowish precipitates were washed with water and dispersed into deionized water with 5 mg/mL concentration. The XRD pattern of silver trimolybdate NWs can be indexed as dominated Ag2Mo3O10 phase, as shown in Figure S1a, Supporting Information. 2.3. Synthesis of Ag@Ag2Mo3O10/GO or RGO Hydrides. GO wrapped Ag@Ag2Mo3O10 composite was obtained by a photoassisted self-assembled process in water−ethanol solution. The XRD pattern of Ag@Ag2Mo3O10 is shown in Figure S1b, Supporting Information. As-prepared activated GO was added to 20 mL of Ag2Mo3O10 suspension, and 30 mL of EtOH as hole scavenger was then added. The mixture was stirred for 1 h under solar light irradiation. The resulting precipitates were collected and dried at 50 °C. Three kinds of Ag@Ag2Mo3O10/GO composites were obtained with different graphene contents named as 10%, 20%, and 30% Ag@ Ag2Mo3O10/GO, respectively. The Ag@Ag2Mo3O10/RGO composite was obtained by reducing GO using 35% hydrazine.

With this in mind, it is expected that hybridizing GO or RGO with Ag2Mo3O10 will enhance the aforementioned poor photocatalytic activity of Ag2Mo3O10. However, hybridizing GO or RGO with nanowire-shaped materials to well-tailored nanostructures is still a large challenge due to the inherent morphological nature of nanowire-shaped materials (for example, curling and twisting). Less successful cases of hybridized GO or RGO for other kinds of nanowire materials have also been reported.23−27 Among them, only two chemical approaches were achieved: one is simply random “hard” hybridization;24,25 another is anchoring of chemical bonds via carboxyl groups of GO.23,26,27 In this study, well-tailored GO nanosheet-wrapped, Agdecorated Ag2Mo3O10 Ws were synthesized via a strong chemical interaction between carboxyl groups on a GO surface and Ag nanoparticles on Ag2Mo3O10 Ws. To maximize the interaction between the GO and Ag nanoparticles, a novel experiment was carefully designed, during which GO was first activated to have more carboxyl groups using a chloroacetic acid treatment under strong basic conditions,28,29 followed by hybridization with Ag2Mo3O10 Ws with the assistance of solar light. Because of the strong attraction between the carboxyl group on the GO and Ag particles on Ag2Mo3O10 Ws, the GO nanosheets were well hybridized with Ag2Mo3O10 Ws, which resulted in greatly improved photocatalytic activity and stability of Ag2Mo3O10 Ws.

2. EXPERIMENTAL SECTION 2.1. Synthesis of GO and Activated GO. Graphite powder (purity 99.9995%) was obtained from Alpha Aesar. Other chemical reagents were purchased from Aldrich without further purification. GO was prepared by a Hummers method.30 Activation of GO was based on the procedure reported previously;29 briefly, the as-prepared GO was 24024



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Figure 2. SEM images of Ag2Mo3O10 (a,b) and Ag@Ag2Mo3O10/activated GO (c,d), full image element mapping of Ag@Ag2Mo3O10/activated GO (e,f), and EDS line analysis of Ag@Ag2Mo3O10/activated GO (g). TEM image (h) and HR-TEM image (i) of Ag@Ag2Mo3O10/GO.

2.4. Analysis Instruments. Scanning electron microscopy (SEM) images of the product were taken on a field emission scanning electron microscope (FESEM, JSM-7000F, Japan). XRD patterns were obtained with a diffractometer D500/5000 in Bragg−Bretano geometry under Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) was conducted on an AESXPS instrument (ESCA2000, VG Microtech, England) equipped with an aluminum anode (Al Kα = 1486.6 eV). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) observations were performed on a JEOL, JEM-2100F (Japan) electron microscope. Raman spectra were acquired at room temperature using an excitation energy of 2.41 eV (514 nm, Ar+ ion laser)

(Renishaw, RM-1000 Invia). Fourier transform infrared (FTIR) spectra were recorded in KBr pellets with Bruker FTIR (Bruker IFS-66/S). UV−vis diffuse reflectance spectra were obtained using a UV−vis spectrophotometer (Neosys-2000, Scinco Co. Ltd., Korea) using BaSO4 as a reference at room temperature. GO or activated GO electrodes were prepared by electrodeposition of GO onto an FTO glass substrate for measuring the photoelectrochemical properties. In the electrodeposition, two FTO electrodes were held apart at a constant distance of 2 cm in the GO or activated GO dispersion of water/acetonitrile, while a potential of 10 V was applied for 20 min. The photoelectrochemical properties were measured with a three electrode system consisting of working (GO or 24025



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potential curves of their electrodes under chopped solar light irradiation (AM1.5G, 100 mW/cm2). Both electrodes present clear anodic/cathodic photoresponses, and the photocurrent sign switched at ∼−0.4 V vs Ag/AgCl, which is similar to a previous report.32 The results indicated photoactive GO or activated GO semiconductor because the FTO substrate generated only weak photocurrent (a few microamperes) and did not generate a cathodic photocurrent since FTO is an ntype semiconductor. The work function of pristine graphene has been evaluated to be 4.6 eV,33 which allows electron transfer from the graphene to FTO (4.9 eV). However, no standard literature reported a work function of GO, but it is believed that the oxidative graphene would change its work function.34,35 Therefore, when GO or activated GO was irradiated, the excited electrons might be transferred to FTO at a ranging of applied bias, leading to anodic photocurrent. On the contrary, the excited electrons could be also transferred to electrolyte at an applied bias, such as more negative potential, leading to cathodic photocurrent. The higher photocurrent density of the activated GO indicates better photocatalytic activity than pure GO. Figure 1c,d shows the full-scale XPS spectra of GO and activated GO, respectively. The deconvolution of the C1s peak in the GO demonstrates four types of carbon bonds: C−C (284.6 eV), C−O (epoxy and hydroxyl, 286.5 eV), CO (carbonyl, 288.2 eV), and O(H)− CO (carbonyl, 289.6).36 Compared to GO, the deconvolution of the C1s peak in the activated GO shows an additional π−π* shakeup satellite (291 eV), indicating a π electron transition from occupied to unoccupied valence orbitals (Figure 1d).37 These results also demonstrate that the activated GO is a better macromolecular photosensitizer for an efficient photocatalytic process and thus displays a higher photocurrent than GO. In addition, the ratio between C−C and C−O in GO is slightly increased after chemical activation, suggesting successfully activating GO’s epoxide groups and converting hydroxyl groups. The morphologies of Ag2Mo3O10 Ws and Ag@Ag2Mo3O10/ activated GO composites were observed in FE-SEM images. Figure 2a,b shows uniform Ag2Mo3O10 Ws with lengths of at least several hundred micrometers and diameters of several hundred nanometers, suggesting quasi-microscale wires. It can be seen from Figure 2c,d that the Ag@Ag2Mo3O10 Ws are distinctly wrapped with gauze-like GO nanosheets. Both element mapping and EDS line analysis of the Ag@ Ag2Mo3O10/activated GO display core/shell-like structures with homogeneous distributions of C, O, Ag, and Mo (Figure 2e−g). Moreover, the surface morphologies of Ag@Ag2Mo3O10 Ws became rough after photoirradiation, suggesting photochemical reduction of Ag+ to Ag0 nanoparticles on the surfaces of the Ag2Mo3O10 Ws (a TEM image of the Ag2Mo3O10 Ws is shown in Figure S2, Supporting Information). Compared with Ag2Mo3O10/GO (Figure S3, Supporting Information), the activated GO with COOH moieties could readily coordinate with Ag nanoparticles formed on the backbone of the Ag2Mo3O10 Ws.38 The TEM and HR-TEM images in Figures 2h,I further confirm that the Ag2Mo3O10 Ws are wrapped with microscale GO sheets and have Ag nanoparticles with diameters of about 10 nm. The 0.24 nm lattice spacing of the Ag nanoparticles is in agreement with the value of the lattice spacing of the (111) plane of cubic Ag, and 0.783 nm corresponds to layer distance of activated GO (inset in Figure 2i).

activated GO/FTO), counter (Pt), and reference (Ag/AgCl) electrodes. A 1 M Na2SO4 solution in a quartz cell was used as the electrolyte. 2.5. Photocatalytic Test. In order to analyze the photocatalytic effect, the decolorization or degradation reaction of Rh.B and 4-chlorophenol (4-CP) in water was tested. Powdered samples of 30 mg were dispersed in 50 mL of 3 × 10 −5 M Rh.B or 1 × 10 −5 M 4-CP solution under ultrasonication for 1 min. For the irradiation system, solar simulator irradiation (Ls-150-Xe lamp, USA) was used at the distance of 100 mm from the solution. The suspension was irradiated with light source as a function of irradiation time. Samples were then withdrawn regularly from the reactor, and removal of dispersed powders was carried out through centrifugation. The clean transparent solution was analyzed by UV−vis spectroscopy (Optizen 2120, Mecasys Co. Ltd., Korea). The concentration of Rh.B or 4-CP solution was determined as a function of irradiation time from the absorbance region at a functional wavelength.

3. RESULTS AND DISCUSSION Natural exfoliated GO sheets have great potential applications for hybridizing with different kinds of materials via a low-cost

Figure 3. (a) Raman spectra of GO, activated GO, and Ag@ Ag2Mo3O10/activated GO composite: (1) GO, (2) activated GO, (3) 10% Ag@Ag2Mo3O10/activated GO, (4) 20% Ag@Ag2Mo3O10/ activated GO, (5) 30% Ag@Ag2Mo3O10/activated GO, and (6) Ag2Mo3O10/activated GO.

solution processing method due to surface functional groups (e.g., epoxides, hydroxyls, and carboxylic acids) being able to render themselves suspendable in polar solvents. As described in the experimental section, activation of GO was achieved by chemical modification using chloroacetic acid in a strongly basic solution. The color of the GO solution changed from light brown (left inset of Figure 1a) to dark brown (right inset of Figure 1a). However, high water solubility was maintained due to chloroacetic acid activating GO’s epoxide and ester groups and converting hydroxyl groups to carboxylic acid (COOH) moieties.29 Figure 1a shows the optical absorption spectra of GO and activated GO. The maximum absorption peak of activated GO is smaller than that of GO, indicating that the oxygen functionalities reduced the symmetry of the π−π* system to open the gap and eventually causing the top energy level of the valence band to be counterchanged to the O 2p orbital.31 In order to confirm the GO photoresponse, the photoelectrochemical properties of GO and activated GO were measured and compared. Figure 1b shows the current− 24026



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Figure 4. Full XPS data of GO and Ag@Ag2Mo3O10/activated GO composite (b), and C1s (c), Mo3d (d), and Ag 3d (3) deconvolution spectra of Ag@Ag2Mo3O10/activated GO composite.

Figure 5. (a) Adsorption percentage of Rh.B over Ag@Ag2Mo3O10/activated GO composites. (b) The photocatalytic decolorization of Rh.B over RGO, activated GO, Ag2Mo3O10, Ag@Ag2Mo3O10, Ag@Ag2Mo3O10/activated GO, 20% Ag@Ag2Mo3O10/GO, 20% Ag@Ag2Mo3O10/RGO, and 20% Ag2Mo3O10/activated GO composites under solar light irradiation.

Figure 6. (a) Rh.B decolorization efficiency using 20 wt % Ag@Ag2Mo3O10/activated GO for five cycles. (b) The degradation performance of 4-CP over Ag@Ag2Mo3O10 and 20 wt % Ag@Ag2Mo3O10/activated GO.

spectra are the D band (∼1354 cm−1) and the G band (∼1598 cm−1), which correspond to the first-order scattering of the E2g mode for sp2 carbon domains and local defects and disorders

The Ag@Ag2Mo3O10/activated GO composites were further characterized by Raman and XPS spectra. As shown in Figure 3, the two feature peaks of the carbon materials in the Raman 24027



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As a model experiment to evaluate the photocatalytic properties of Ag@Ag2Mo3O10/activated GO composites, Rh.B was selected as the azo pollutant. The photocatalytic degradation process was monitored by examining the variations in absorbance at 553 nm in the UV−vis spectra. The photocatalytic performances of different photocatalysts were determined by comparing the degradation efficiency of Rh.B under 100 mW/cm2 solar light irradiation (Figure 5). C0 is the absorption intensity of the initial Rh.B aqueous solution at a wavelength of 553 nm, and C is the absorption intensity of Rh.B after physical adsorption combined with photocatalytic degradation as a function of reaction time. It can be noted from Figure 5a that no more than 40% of Rh.B molecules are adsorbed onto any photocatalysts when at equilibrium the adsorption state is under the dark reaction. A blank test (Rh.B alone) under solar light exhibits exceptionally low photodegradation efficiency, which can be almost ignored. It can be also seen from Figure 5b that the photodegradation performance of Rh.B is negligible over graphene sheets (reduction was done using hydrazine), while 20% of the Rh.B dyes can be degraded by the activated GO over the same time duration. This matches well with the previous photocurrent data (Figure 1b), presumably because the activated GO can absorb more light in the visible region. It is clear that a rapid decrease of the Rh.B concentration occurs over all Ag@Ag2Mo3O10/activated GO composites. In particular, the 20 wt % GO sample completely removed the Rh.B dyes with 80 min of irradiation. (Figure S6, Supporting Information, displays the temporal evolution of the spectra during the photodecolorization of Rh.B mediated by 20% Ag@Ag2Mo3O10/activated GO under solar light irradiation. A rapid decrease of absorbance is observed at 553 nm, accompanied by an absorption band shift to shorter wavelengths.) However, the bare Ag@Ag2Mo3O10 only reached 55% removal for the Rh.B dyes, while the Ag@Ag2Mo3O10/GO and Ag@Ag2Mo3O10/RGO in 20 wt % GO or RGO removed 80 and 72% of the Rh.B dyes, respectively. Obviously, the Ag2Mo3O10/activated GO composites have superior photocatalytic activity than either Ag 2 Mo 3 O 10 /GO or the Ag2Mo3O10/RGO composites. In general, the photocatalytic systems with silver halide or phosphate with RGO were better than either photocatalytic system with GO or the photocatalytic system with activated GO because of the efficient photoinduced charge transfer from silver halide or phosphate to RGO.46,47 However, our results show that the photocatalytic activities of Ag@Ag2Mo3O10 with GO or activated GO were much better than those of Ag@Ag2Mo3O10/RGO. SEM images of the Ag@ Ag2Mo3O10/RGO composite show that only a small amount of Ag2Mo3O10 Ws can be anchored on RGO sheets (Figure S7, Supporting Information). Therefore, we propose an additional conclusion that the enhancement of miscibility between Ag2Mo3O10 and activated GO seems to be the more likely the reason for the outstanding photocatalytic performances. The long-term stability of 20 wt % Ag@Ag2Mo3O10/activated GO composites under photocatalytic reaction was investigated as shown in Figure 6a. The photocatalytic activity does not show any significant deactivation over five consecutive recycle experiments. The XPS spectra of the Ag@Ag2Mo3O10/activated GO composite after irradiation show no obvious decrease in O:C atomic ratio (Figure 7). Furthermore, it is well-known that conventional silver halide plasmonic photocatalysts are formed by UV-induced reduction of Ag, after which the Ag formed on the surface of the silver halide has strongly enhanced photocatalytic activity under visible light.48 Herein, the Ag

Figure 7. C1s XPS spectra changes of Ag@Ag2Mo3O10/activated GO composites for 1−5 h of irradiation time under solar light.

for sp3 hybridization, respectively.39 The similar intensity and position of the G and D bands for GO and activated GO are clearly observed from their Raman spectra, suggesting that the GO activation strategy does not induce additional defects or disorder in GO. Compared to self-assembled Ag2Mo3O10/GO without Ag nanoparticles, the D and G band intensities of Ag@ Ag2Mo3O10/GO composites are significantly enhanced (approximately 3-fold), which demonstrates a strong chemical interaction between Ag and GO due to the surface-enhanced Raman scattering effect of these Ag particles,40,41 which are shown together with XRD patterns (Figure S4, Supporting Information). Furthermore, the chemical interaction can be confirmed by FT-IR spectra (Figure S5, Supporting Information). The wavenumber blue-shift of carbonyl stretching band of GO is related to the transfer of electron density to form the new Ag−oxygen bond.42,43 Figure 4a shows the full-scale XPS spectra of GO and the Ag@Ag2Mo3O10/activated GO composite. In Figure 4b, the XPS spectrum of the Ag2Mo3O10 /activated GO composite shows additional Mo3d at 232.3 eV, Ag 3d 5/2 at 376.42 eV, and Ag 3d 3/2 at 367.98 eV compared to GO. Figure 4c shows that the deconvolution of the C1s peak in the Ag@Ag2Mo3O10/activated GO has similar peak positions to GO. Figure 4d shows the high-resolution XPS spectrum of the Mo 3d region, which confirms the presence of Mo (IV).44 The high-resolution XPS spectrum of the Ag 3d could be further deconvoluted into four peaks at 367.5, 368.8, 373.5, and 374.6 eV, where the bands at 367.5 and 373.5 eV are attributed to the Ag+ of Ag2Mo3O10, and those at 368.8 and 374.6 eV are ascribed to metallic Ag0, which agrees with other reports (Figure 3e).11,12,45 24028



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Scheme 1. Preparation and Photocatalytic Process of Ag@Ag2Mo3O10/Activated GO Composite through Assistance of Solar Light

Figure 8. VB XPS (a) and UV−vis DRS (b) of the Ag2Mo3O10 NWs. Inset is a plot of (αE)1/2 against the photon energy (E).

degradation rate of 4-CP over Ag@Ag2Mo3O10 was remarkably slower than that over Ag@Ag2Mo3O10/activated GO (UV absorption spectra of 4-CP are shown in Figure S10, Supporting Information). It has been clearly known that OH• radicals (holes trapped) are the main active species for degradation of 4-CP51,52 Therefore, the stable and effective photocatalytic activity means that this is not simply a plasmonic response by those Ag particles; rather, it may be explained as a new method of electron shuttling. Scheme 1 illustrates the preparation steps of the selfassembled Ag2Mo3O10/activated GO composites. First, chemically exfoliated GO was activated into more useful functional groups on a GO surface via chloroacetic acid treatment under strong basic conditions. Photochemically formed Ag0 nanoparticles on Ag2Mo3O10 Ws were readily coupled with activated GO by chemical coordination. The unique 1-D Ag2Mo3O10

content of the Ag2Mo3O10/activated GO composites was monitored by UV absorption spectra, as shown in Figure S8, Supporting Information. The adsorption peak at ∼410 nm clearly indicates formation of Ag nanoparticles, consistent with previous studies.49,50 The characteristic peak intensity is gradually increased with increasing irradiation time to 5 h, suggesting an increase of Ag0 content during the photocatalytic reaction. To quantitatively investigate and compare the change in concentration of the Ag0, Ag 3d XPS spectra are carried out (Figure S9, Supporting Information). The peak area ratios of the Ag0 to the total Ag+ are calculated to 1.2% before being used as photocatalyst, while the relative ratios increase to 2.6% and 8.7% after photocatalytic reaction of 1 and 5 h, respectively. In Figure 6b, the time profiles of the photodegradation of 4-CP over Ag@Ag2Mo3O10 and Ag@Ag2Mo3O10/activated GO under solar light are compared. The results indicate that the 24029



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hybridized with 2-D activated GO in an appropriate geometrical space between the wire and sheet allow multiple reflections of electromagnetic waves (including visible light),53 thus allowing more efficient use of the light source and endowing this hierarchical heterostructured hybridization with greatly enhanced properties.54 Previous reports have explained how the band gap energy (Eg) of GO is mainly formed by the antibonding π* orbital as a conduction band with a higher energy level and the O 2p orbital as a valence band.31,55 The energy level of the antibonding π* orbital is much higher than that of H+/H2 as well as dissolved O2, leading to direct electron injection into the solution phase for H2 or O2−• radical generation. Furthermore, Kamat et al. reported that Ag+ can be reduced in graphene/TiO2 photocatalytic systems in which the electrons are trapped by the Ag+ ions.56 The vectoral charge transfer from CB of TiO2 to graphene and graphene to Ag+ shows that the energy level of the antibonding π* orbital is higher than that of metallic Ag. On the basis of the previous literature, it is reasonable to conclude that this is caused by electron transfer from the excited GO to Ag.57 Furthermore, the combination of UV−vis DRS with the XPS valence band spectra for Ag2Mo3O10 was used to determine the electronic band alignments. As shown in Figure 8, the VB maxima of Ag2Mo3O10 NWs are at 1.85 eV. The intrinsic absorption edge of the Ag2Mo3O10 has a visible light response around 468 nm. The derived bandgap from the plots of the transformed Kubelka−Munk function vs light energy is 2.59 eV (inset in Figure 8b). Combined with the above results, the optical CB potential of the Ag2Mo3O10 NWs is at −0.74 eV. A previous report found a CB edge of GO at −0.6 eV vs Ag/AgCl in Na2SO4 electrolyte,58 which can be decreased by about −0.403 eV for NHE. The GO bandgap was also calculated to be about 2.51 eV (Figure S11, Supporting Information), so the VB of GO is approximately 2.107 eV. Therefore, both activated GO and Ag2Mo3O10 Ws as a semiconductor are able to act as an electron donor in the photocatalytic system. The proposed mechanism for the activity of Ag0 is as a solid-state electron mediator that accepts electrons from activated GO and Ag2Mo3O10 Ws.

AUTHOR INFORMATION

Corresponding Author

*(J.-H.P) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NRF of Korea Grant funded by the Ministry of Science, ICT & Future Planning (N R F -2013R1A2A1A09014038, 2011-0006268, 2011-0030254).

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REFERENCES

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4. CONCLUSIONS In summary, we have demonstrated a novel photocatalytic system based on Ag@Ag2Mo3O10/GO composites that show excellent photocatalytic performance under solar light irradiation. The metallic Ag particles on Ag2Mo3O10 serve not only as a bridge by which GO can wrap Ag2Mo3O10 surfaces homogeneously but also can act as a solid state electron mediator to significantly reduce recombination of electron/hole pairs generated in both GO and Ag2Mo3O10 semiconductors. The best photocatalytic activity was found for 20 wt % GOwrapped Ag2Mo3O10 surfaces. This study presents new insights on the coupling of 1-D plasmonic photocatalysts with GO photocatalysts and also opens the way for the use of natural exfoliated GO in the design of new and efficient photocatalytic systems for pollutant removal.

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ASSOCIATED CONTENT

S Supporting Information *

XRD, TEM, SEM, FT-IR, UV−vis, and deconvolution spectra. This material is available free of charge via the Internet at http://pubs.acs.org. 24030



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Chemically Modified Graphene Oxide-Wrapped Quasi-Micro Ag

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