A general method to diverse silver/mesoporous–metal–oxide nanocomposites with plasmon-enhanced photocatalytic activity

A r t i c l e i n f o

Article history: Received 11 August 2014 Received in revised form 9 October 2014 Accepted 14 October 2014 Available online 23 October 2014

Tongxuan Liua, Benxia Li a,∗, Yonggan Haoa, Fang Hanb, Linlin Zhangc, Luyang Hua

a School of Materials Science and Engineering, Anhui University of Science and Technology, Huainan, Anhui 232001, PR China

b Anhui Entry-Exit Inspection and Quarantine Bureau, 329 Tunxi Road, Hefei, Anhui 230029, PR China

c School of Chemistry Engineering, Anhui University of Science and Technology, Huainan, Anhui 232001, PR China

A b s t r a c t

Incorporating plasmonic Ag nanoparticles into mesoporous metal–oxide (MMO) semiconductors will achieve collective effect to greatly increase the photocatalysis. This work demonstrated a general twostepmethod to obtain diverseAg/MMO composite photocatalysts with plasmon-enhanced photocatalytic activity. Several typical MMO (TiO2, ZnO, and CeO2) semiconductors were synthesized by integrating evaporation-induced self-assembly and in situ pyrolysis of metal precursors. Different amounts of Ag nanoparticles were then loaded in these MMO semiconductors through a facile photodeposition process. The Ag nanocrystals with sizes of 50–100 nm were embedded in MMO semiconductors, endowing the Ag/MMO composites notable visible light absorption. The photocatalytic activities of the as-prepared diverse photocatalysts were studied systematically. The influencing factors including MMO species, mesoporous structure, and Ag loading amount on the photocatalytic activity were discussed in detail. The Ag/MMO composites exhibited much improved photocatalytic activity than their pure MMO semiconductors. The mesoporous TiO2 with the Ag-loading amount of 5 wt.% exhibited the best photocatalytic performance for photodegrading both methylene blue and phenol under a simulated sunlight. The enhancement in photocatalysis is attributed to the synergistic effect of the mesoporous structures for efficient mass transfer as well as the Ag nanoparticles providing plasmonic enhanced light absorption.

1. Introduction

Mesoporous materials have attracted considerable attention for their broad applications in catalysis [1,2], energy conversion and storage [3–6], sensors [7–9], and so on, due to their large surface areas, tunable pore sizes and shapes, various structures, multitude of compositions, and ease of functionalization [6,10]. Mesoporous metal oxide (MMO) semiconductors are gaining popularity for photocatalytic applications due to their large specific surface area in a continuous structure instead of discrete particles [11–13]. The mesoporous feature is expected to greatly facilitate the light harvesting, photogenerated charge carriers transfer, and carrier-induced surface reaction, resulting in higher photocatalytic activity [14,15]. However, the single-component semiconductor photocatalysts are encountering bottlenecks in improving their solar-driven catalytic efficiency due to their inherent limitations such as the high rate of charge-carrier recombination and limited light absorption. In this regard, developing the hybrid nanocomposites with synergistic effects is an effective approach to make a breakthrough in the photocatalytic performance of semiconductor photocatalysts.

Silver nanoparticles, an accessible noble metal nanomaterial, have recently proven to be promising in harvesting photon energy for chemical reactions due to their extraordinary localized surface plasmon resonance (LSPR) properties in the visible light region [16–18]. The energetic electron-driven photocatalysis of silver nanostructures have been demonstrated in the catalytic oxidation reactions [19] and water-splitting [20] under visible light irradiation. The researches in recent years have demonstrated that the incorporation of plasmonic Ag nanocrystals in semiconductor nanostructures can gain greatly improved photocatalytic performance due to the LSPR-induced light focusing in the vicinity of Ag nanocrystals and/or plasmon-sensitized photocatalysis of semiconductors [21–23]. Moreover, the photogenerated charge carriers can be efficiently separated at the interface between Ag and semiconductor, which allows the efficient formation of reactive oxygen species for the degradation of organic compounds [24,25]. Therefore, coupling metal oxide semiconductors with silver nanoporticles presents a promising strategy for designing advanced photocatalysts and elicits widespread research interest [26,27]. Awazu et al. deposited TiO2 film on nanoparticles comprising an Ag core covered with a silica (SiO2) shell to prevent oxidation of Ag by direct contact with TiO2 [28]. The most appropriate diameter for Ag nanoparticles and thickness for the SiO2 shell giving rise to LSPR in the near UV were estimated from Mie scattering theory. The photodegradation rate of methylene blue (MB) on such a film was increased by a factor of 7 under near-UV illumination. Kumar et al. investigated the effect of the silica layer thickness on the overall photocatalytic performance of TiO2 (15 nm)/SiO2/Ag nanoparticle architectures by use of atomic layer deposition to achieve precise film thickness of SiO2 and TiO2 layers [29]. The composite photocatalyst with a 2 nm SiO2 interlayer exhibited the best photocatalytic performance, suggesting thatthe plasmonic near-field enhancement effect plays a dominant role. Besides, the plasmonic photocatalysts composed of Ag/AgX (X= Cl, Br, I) loaded on metal–oxide supports were prepared by the simple ion-exchange and the light-induced chemical reduction methods, in which the supports with hierarchical nanostructures could endow higher surface-to-volume ratio and more reactive sites to the photocatalysts [30,31].

In particular, the MMO nanomaterials are fascinating to act as functional supports for fabricating plasmonic composite photocatalysts. It has been demonstrated that the Ag nanoparticles synthesized within the mesopore structure of SBA-15 by microwave-assisted alcohol reduction displayed controllable sizes and color, having enhanced catalytic activity under visible light irradiation owing to LSPR, compared to Ag NPs obtained by pure thermal processes [32]. The integration of plasmonic Ag nanoparticles and MMO semiconductors would obtain a further efficient utilization of Ag nanoparticles and enhance photocatalytic activity in a most effective way [33–35]. The synthesis of various Ag/MMO composite photocatalysts via a general route is of great significance for facilitating the studies of the plasmon-enhanced photocatalysis and thereby guiding the design and creation of advanced photocatalysts, but a challenge as well. Based on these considerations,the present work demonstrates a facile and generaltwo-stepmethod to obtain diverse Ag/MMO composite photocatalysts with plasmonenhanced photocatalytic activity. The photocatalytic activities of the Ag/MMO nanocomposites were evaluated by photodegrading methylene blue (MB) and phenol in aqueous solution under a simulated sunlight. The Ag/MMO composites exhibited much enhanced photocatalytic activity than their corresponding pure MMO semiconductors. Moreover, the mesoporous TiO2 with the Ag-loading amount of 5 wt.% exhibited the best photocatalytic activity. This work not only provides a general method for preparing Ag/MMO composite photocatalysts but also sheds some light on the plasmon enhancement in photocatalysis by the synergistic effect between plasmonic metal nanoparticles and MMO semiconductors.

2. Experimental

2.1. Materials

All the chemicals are analytical reagent and used as received without further purification. Triblock copolymer Pluronic F127 (EO106PO70EO106; EO: ethylene oxide; PO: propylene oxide) was purchased from Sigma–Aldrich. Coumarin (C9H6O2) was purchased from Aladdin Reagents (Shanghai) Co., Ltd. Zinc nitrate hexahydrate [Zn(NO3)2·6H2O], cerous nitrate [Ce(NO3)3·6H2O], tetrabutyl orthotitanate [Ti(OBu)4], silver nitrate (AgNO3), concentrated hydrochloric acid (HCl, 37 wt.%), and absolute alcohol were purchased from Shanghai Sinopharm Chemical Reagent Co., Ltd. Deionized water was used throughout this work.

2.2. Synthesis of MMO semiconductors (MO = ZnO, TiO2 and CeO2)

All the MMO samples were synthesized using a general process integrating evaporation-assisted self-assembly and in situ pyrolysis of metal precursors. In a typical synthesis, 0.5 g of Pluronic F127 were dissolved in 15 mL of absolute alcohol (EtOH), and then 5 mmol of the respective metal precursor (Table 1) was added into the solution with vigorous stirring. For the synthesis of mesoporous TiO2, 3 mL of concentrated hydrochloric acid was used to restrain the hydrolyzation of Ti(OBu)4. The resulting solution was keptin an oven at 50 ◦C for 24 h, and then dried at 100 ◦C for 6 h. The as-made xerogels were then calcined at 400 ◦C for 4 h to remove the block copolymer surfactant species, with a heating rate of 1 ◦C/min from room temperature. Finally, the MMO samples were obtained.

2.3. Preparation of Ag/MMO (MO = ZnO, TiO2 and CeO2) nanocomposites

The Ag/MMO (MO = ZnO, TiO2 and CeO2) nanocomposites were prepared through a facile and eco-friendly photodeposition process. Before photodeposition, a certain amount (∼0.24 g) of MMO powder was dispersed in deionized water (15 mL) to form a suspension. An AgNO3 aqueous solution with concentration of 3.936 mg/mL (the equivalent concentration of Ag is 2.5 mg/mL) was prepared, which was used for all of the Ag loading experiments. For the preparation of Ag/MMO nanocomposites, the AgNO3 solution with different amounts was injected into the MMO suspensions under magnetic stirring. The dosage of AgNO3 solution was calculated according to that the theoretical Ag loading mass percent in the targeted composites is 1 wt.%, 2 wt.% and 5 wt.%, respectively. Then,the reaction mixture was irradiated by UV–vis lightfrom a Xe lamp for 30 min under continuous stirring. Finally, the dispersion was centrifuged and washed by deionized water for several times, and then dried at 60 ◦C.

The as-prepared Ag/MMO samples with different Ag loading amounts were denominated as Ag(x)/MMO, where x represents the theoretical Ag loading mass percent calculated from the experimental dosage of AgNO3, as shown in Table 2.

2.4. Characterizations

The X-ray diffraction (XRD) patterns were recorded on a Japan Shimadzu XRD-6000 equipped with graphite monochromatized high-intensity Cu K radiation ( = 0.1542 nm), and operated at 40 kV voltage and 30 mA current. The field emission scanning electron microscopy (FESEM) images were performed on JEOL JSM-6700F (Japan) at an accelerating voltage of 20 kV. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images associated with the energydispersive X-ray (EDX) spectra were obtained on TEM JEOL-2010. X-ray photoelectron spectroscopy (XPS) measurements were performed on a VGESCALAB MKII X-ray photoelectron spectrometer with an exciting source of Mg-Ka. Brunauer–Emmett–Teller (BET) nitrogen adsorption/desorption was measured using a Micromeritics ASAP 2020 V4.01 (V4.01 E) analyzer (USA). The UV–vis diffuse reflection spectra (DRS) of the samples were recorded on a spectrophotometer of Perkin Elmer Lambda 950. The pH values of the solutions forphotodepositionofAg before andafter lightirradiation were determined on a Colloidal Dynamics Zeta Probe (USA). The actual mass percent of loaded Ag in each Ag/MMO nanocomposites was measured by Inductively Coupled Plasma Optical Emission Spectra (ICP-OES) on a Perkin Elmer Optima 8300 optical emission 

spectrometer (USA), in which the analyte is the Ag emission spectrum at 328.068 nm.

2.5. Tests for the photocatalytic activity of Ag/MMO nanocomposites

The photocatalytic properties ofthe as-prepared materials were evaluated by photodegrading the representative organic pollutants of MB and phenol in water under a simulated sunlight. A Xe lamp (CEL-HXF300, Beijing China Education Au-light Co., Ltd) equipped with a long-pass reflector was utilized to provide the broadband light irradiation ( = 350–780 nm) which is well coincident with the dominating light wavelength region of natural sunlight. In each test, 50 mg of photocatalyst was suspended in 100 mL of MB aqueous solution (20 mg/L) or phenol aqueous solution (20 mg/L). The suspension was exposed to the light irradiation under magnetic stirring, after stirred in dark for 2 h to achieve adsorption-desorption equilibrium between photocatalyst and organic pollutant. At given time intervals, about 3 mL of solution was sampled for analyzing the pollutant concentration. The photocatalytic degradation process was monitored using a UV–vis spectrophotometer (Shimadzu UV-2600) to record the characteristic absorption of MB (665 nm) and phenol (269 nm).

2.6. Assays for the produced hydroxyl radicals during photocatalytic process

The photogeneration of hydroxyl radicals (•OH) in the presence of severaltypical photocatalysts of MMO and Ag(5)/MMO was measured by a fluorescence probe method with coumarin (C9H6O2) which can turn into fluorescent umbelliferone upon reaction with •OH radicals. The optimum excitation wavelength (ex) and the maximum emission peak wavelength (em) of umbelliferone are determined to be 372 nm and 478 nm, respectively. For a typical process, 100 mg of a photocatalyst (MMO or Ag(5)/MMO) was dispersed in 100 mL of coumarin solution (1 × 10−3 M) with constant stirring. The suspension was irradiated by the same light source used in the photocatalytic degradation process. At a given time interval (30 min), about 2 mL of the solution was sampled and monitored its maximum photoluminescence emission intensity at 478 nm under the optimum excitation at 372 nm on a fluorescence spectrophotometer (F-380, Tianjin Gangdong Sci. & Tech. Development. Co., Ltd, China).

3. Results and discussions

The MMO samples including ZnO, TiO2 and CeO2 were synthesized via the general route integrating evaporation-induced self-assembly and in situ pyrolysis of metal precursors [36,37]. A typical formation process of the MMO samples is illustrated in Scheme 1. The respective metal precursor,triblock copolymer F127 and absolute ethanol were mixed together to form a transparent solution at room temperature. This solution was kept in an oven at 50 ◦C for 24 h, to achieve the adequate coordination between metal ions and oxygen containing groups of F127 molecules. After that, the solution was then maintained at 100 ◦C for 6 h in air. During this process, anhydrous ethanol was evaporated and the xerogel of the metal-F127 hybrids formed. Finally, the organic substances in the xerogel were removed by combusting and the metal precursors were pyrolyzed in situ to form mesoporous metal oxides.

3.1. Phases and microstructures of pure MMO semiconductors

The phase compositions and crystal structures of the mesoporous metal oxides were characterized by powder X-ray diffraction (XRD). The XRD patterns in Fig. 1 identify these products as the wurtzite phase of ZnO (JCPDS No. 75-1526), anatase phase of TiO2 (JCPDS No. 89-4921), and cubic phase of CeO2 (JCPDS No. 81-0792), respectively. The XRD patterns indicate that these samples have high crystallinity and pure crystal phase. SEM and TEM images in Fig. 2 clearly demonstrate the mesoporous structures of the as-obtained MMO samples. These samples appear to be 3D porous cotton-like bulk materials with rough surface, as shown in Fig. 2a–c. ZnO and TiO2 present similar morphology, while CeO2 has many large pores with sizes in 100–500 nm on its surface. TEM images (Fig. 2d–f) further reveal that the interior of these cotton-like bulk materials presents mesoporous structures forming by the random attachment of the nanoparticles which have sizes of 10–50 nm for ZnO (Fig. 2d), 5–10 nm for TiO2 (Fig. 2e) and CeO2 (Fig. 2f), respectively.