Zhengcui Wu,* Yali Zhang, Xia Wang and Zexian Zou

Anhui Key Laboratory of Molecule-Based Materials, The Key Laboratory of Functional Molecular Solids, Ministry of Education, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, P. R. China.

New Journal of Chemistry

Abstract

Mesoporous nanosheets-assembled flower-like SrTiO3/Ag nanoparticles composites with tunable  Ag content were constructed via room-temperature liquid-phase deposition of Ag nanoparticles on  mesoporous nanosheets-assembled flower-like SrTiO3, which were pre-synthesized by a facile  solvothermal approach and subsequently annealed in air. The as-fabricated nanocomposites were  Ag nanoparticles with ca. size of 2 nm uniformly distributed on the surface of mesoporous  nanosheets-assembled flower-like SrTiO3 with a size about 600 nm in diameter. The Ag content on  flower-like SrTiO3 greatly affected the catalytic performance on the photodegradation of methyl  orange and the catalytic reduction of 4-nitrophenol, and there is a different optimum amount of Ag  nanoparticles for two kinds of catalytic reactions. The optimum Ag@SrTiO3 nanocomposite  exhibited a much enhanced full-arc light photocatalytic activity for the degradation of methyl  orange and excellent activity and stability for the catalytic reduction of 4-nitrophenol. This work  provides a simple method to synthesize Ag@SrTiO3 nanocomposite, developing a new catalyst for  photocatalytic degradation and catalytic reduction of organic pollutants.

1. Introduction

The efficient disposal of environmental pollutants is a major challenge for the sustainable  development of modern society due to the inverse impacts of industrialization. Among various  remediation technologies, the photocatalytic degradation and catalytic conversion of chemically  stable organic pollutants occupy a prominent place,1-12 because the organic pollutants can be  completely mineralized into CO2, H2O and inorganic substances without leaving any harmful  residues by photocatalytic degradation, or be turned into useful chemicals by catalytic conversion  with a catalyst. A perovskite metal oxide of strontium titanate (SrTiO3), with a bandgap energy of  3.2 eV and the conduction band edge 200 mV negative than TiO2, offers favorable energetics for  photocatalysis in water splitting and degradation of organic pollutants under ultraviolet  irradiation,13,14 where its stable crystallographic structure endowed its stable photocatalytic activity.  However, the large band gap of SrTiO3 renders it inactive under visible light and consequently  inefficient in the utilization of solar energy. Intentional modification of SrTiO3 for utilizing visible  light of the solar spectrum in photocatalysis has been carried out by manipulating its electronic  structure, such as noble metal decorating,15-17 oxygen vacancy introducing,18 narrow bandgap  semiconductor coupling19,20 and transition metal ion doping.21,22 The photo-induced charge transfer  behaviors at the interfacial region were critically influenced by these modified approaches, of which  incorporating Au or Ag nanostructure on SrTiO3 surface can enhance the visible photocatalytic  activities of SrTiO3 by the localized surface plasmonic resonance (LSPR) of Au or Ag  nanostructure.15-17 So far, only several Ag@SrTiO3 nanocomposites have been reported for  photocatalysis, such as Ag-SrTiO3 film,23 Ag nanoparticles-modified cubic-like SrTiO3, 24 Ag  nanoparticles on 5×5×0.5 mm3 SrTiO3 (100) facet,17 Ag nanoparticles on SrTiO3 nanotubes,16,25 Ag nanoparticles on SrTiO3 nanoparticles,15,26 where the incorporating Ag nanoparticles were mostly  nonuniform with size from several nanometers to more than ten nanometers. Therefore, it is highly  attractive to develop new Ag@SrTiO3 nanocomposite catalysts with uniform small size of Ag that  can work efficiently for the photocatalytic degradation of organic pollutants under solar energy.  On the other hand, Ag nanoparticles are the first one of metal nanocatalysts to be used for the  reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) by NaBH4, 27 which transformed a toxic  pollutant into a potent intermediate for manufacturing many analgesic and antipyretic drugs.28,29 However, pristine Ag nanoparticles with small size are susceptible to aggregation to minimize their  surface energy during catalytic processes, which resulted in a significant reduction of their catalytic  activity.30 Moreover, the recovery of Ag nanoparticles catalysts is difficult due to their small sizes,  which hinders their wide practical applications in heterogeneous catalysis. An extensive effort has  been devoted to enhance the stability and facilitate the catalyst recovery. A most straightforward  and widely adopted approach is to immobilize Ag nanoparticles onto appropriate solid supports,31-36 where not only the Ag nanoparticles are stable in the reaction, but also the catalytic performance of  composite can be improved. However, there was no report about depositing Ag nanoparticles on  SrTiO3 surface for catalyzing 4-NP reduction. Herein, uniform Ag nanoparticles with ca. size of 2  nm distributed on mesoporous nanosheets-assembled flower-like SrTiO3 was synthesized via  room-temperature liquid-phase deposition of Ag nanoparticles on mesoporous nanosheetsassembled flower-like SrTiO3, pre-prepared by a facile solvothermal approach and subsequently  annealed in air. The Ag@SrTiO3 nanocomposite exhibited not only much enhanced full-arc light  photocatalytic activity for the degradation of MO, but also excellent activity and stability for the  catalytic reduction of 4-NP.  

2. Experimental

2.1 Synthesis of mesoporous nanosheets-assembled flower-like SrTiO3

All chemicals were of analytical purity and used without further purification. In a typical synthesis, 1 mmol SrCl2·6H2O was dissolved in 20 mL of glycol, while 1 mL of tetrabutyltitanate was  dissolved in 20 mL of glycol. Then, the above two solution was mixed and added 3 mL of 1,  6-hexanediamine under stirring for 10 min. The as-obtained mixture was loaded into a 50 mL  Teflon-lined stainless steel autoclave and heated at 160 °C for 18 h, then allowed to cool to room  temperature naturally. The resulting white precipitate was collected by centrifugation and washed  with deionized water and ethanol three times, then dried at 60 °C for 6 h. Afterward, the precursor  was placed in a crucible and carefully heated from room temperature to 550 °C at a rate of  10 °C·min-1 and then maintained at 550 °C for 6 h in air. Finally, the product was collected.  

2.2 Synthesis of Ag@SrTiO3 nanocomposites

1 mmol SrTiO3 was ultrasonically dispersed into 30 mL of deionized water. Then, a certain amount  of AgNO3 (0.0085, 0.017 or 0.034 g) was added under stirring. Next, a certain volume of glycol (1.5,  3, or 6 mL) was added. The above mixture was stirred at room temperature for 60 min. The  resulting product was collected by centrifugation and washed with deionized water and ethanol  three times, then dried at 60 °C for 6 h. For convenience, the products were individually denoted as  2%, 5% and 10% Ag@SrTiO3 according to the atomic ratio of Ag to SrTiO3.

2.3 Materials Characterization

Power X-ray diffraction patterns (XRD) were recorded on a D8 advance diffraction system with  high-intensity Cu Kα radiation. The field-emission scanning electron microscope (FESEM) images  were obtained on a Hitachi S-4800 field-emission scanning electron microscope operated at an accelerating voltage of 5.0 kV. The transmission electron microscopy (TEM) images were taken on  a JEOL 2010 at an acceleration voltage of 200 kV. The scanning transmission electron microscopy  (STEM) images were obtained on a JEOL JEM-ARF200F. The Fourier transform infrared (FTIR)  spectroscopic study was carried out with a MAGNA-IR 750 (Nicolet Instrument Co.) FTIR  spectrometer. UV-visible diffuse reflectance spectra were carried out on a Shimadzu UV-2450  spectrophotometer using magnesium oxide as a standard. X-ray photoelectron spectroscopy (XPS)  spectra were performed on an ESCALAB MK II X-ray photoelectron spectrometer. The metallic  contents were determined by using Optima 5300DV inductively coupled plasma spectrometry. The  pH value was measured on a PHSJ-4F pH meter (Leici, China). Brunauer–Emmett–Teller (BET)  nitrogen adsorption–desorption was tested by a Micromeritics ASAP 2460 accelerated surface area  and porosimetry system.  

2.4 Photocatalytic and Photo-electrochemical Measurements

Photocatalytic activities of as-prepared pristine SrTiO3 and Ag@SrTiO3 nanocomposites were  evaluated by measuring the photodegradation of Methyl Orange (MO) under irradiation of a 300 W  full-arc Xe lamp (CEL-HXF300, Beijing, China). The power density of light source used in the  measurement is 50 mW cm−2, and the wavelength range is from 320 nm to 760 nm. Typically, 15  mg catalyst was dispersed into 100 mL of 10−5 M MO aqueous solution by constant stirring in the  dark for 30 min to reach an adsorption/desorption equilibrium of MO on the catalyst surface. At a  given time interval, about 3 mL of the suspension was withdrawn under irradiation and centrifuged  to remove the photocatalyst for UV-vis absorption spectrum measurement (Shimadzu UV-2550,  Japan). The concentration of MO was determined through checking the characteristic absorption.

To conduct the photo-electrochemical measurements, the products were fabricated into films on  FTO glasses. The detailed fabrication process of the films was carried out as follows: 5 mg catalyst  was initially ultrasonically dispersed in 1 mL of deionized water, then 0.01 mL dispersion was  uniformly dropped on the 0.28 cm2 FTO-coated glass, afterwards, the FTO-coated glass was heated  at 80 °C for 30 min to volatilize the solvent. The photo-electrochemical test systems were  composed of a standard three-electrode configuration with the catalyst deposited on FTO-coated  glass as the working electrode, the Pt wire electrode and the Ag/AgCl electrode individually as the  counter and the reference, which were immersed in 0.2 mol·L-1 Na2SO4 solution. The  photo-electrochemical properties were measured on a CHI 660B electrochemical workstation  (Chenhua Corp., Shanghai, China) in ambient conditions under a 300 W full-arc Xe lamp  illumination (CEL-HXF300, Beijing, China). The potential was swept from − 0.1 to + 0.8 V (versus  Ag/AgCl) at a sweep rate of 50 mV·s-1 .

2.5 Catalytic reduction of 4-nitrophenol

1 mL of 9×10-4 mol·L-1 4-nitrophenol (4-NP) solution and 1 mL of 6×10-2 mol·L-1 NaBH4 were first  mixed, then 1 mL of 0.6 g·L-1 catalyst aqueous dispersion was added. The time-dependent  absorption spectra were recorded in the UV-vis spectrophotometer at 293K.  

3. Results and discussion

The FESEM image revealed the morphology of SrTiO3 precursor was nanosheets-assembled  flower-like structure (Fig. S1a), while the XRD pattern cannot be indexed with any known standard  substance (Fig. S1b). Further FTIR characterization showed organic groups bonded on the precursor  (Fig. S2), suggesting it was an organic-inorganic hybrid. Fig. 1 shows XRD patterns of the pristine  SrTiO3 and Ag@SrTiO3 nanocomposites at various Ag loading amount. The peaks located at 22.8°, 32.5°, 40.0°, 46.6°, 52.5°, 57.9°, 68.0° and 77.4° can be well-indexed to the characteristic  diffraction peaks of (100), (110), (111), (200), (210), (211), (220) and (310) planes of cubic SrTiO3 (JCPDS No. 84-0443), suggesting that pure SrTiO3 phase was formed and well crystallized after  calcination of the SrTiO3 precursor. Compared with pristine SrTiO3, the diffraction peaks belonging  to SrTiO3 were maintained for all Ag@SrTiO3 nanocomposites, suggesting that the phase structure  of SrTiO3 were well preserved in all nanocomposites. Careful observation of the dominant  diffraction peak positions revealed no shift after Ag loading, excluding the possible doping of Ag+ into SrTiO3 crystal lattice. However, no diffraction peaks belonging to Ag were found, which may  be due to the low loading amount of Ag beyond the detection limit of the diffractometer.  

To understand the morphological and structural characteristics of the pristine SrTiO3 and  Ag@SrTiO3 products, FESEM and TEM characterizations were further performed. Fig. 2a shows  the typical FESEM image of pristine SrTiO3, in which nanosheets-assembled flower-like structure  with average size of 600 nm can be seen. Careful observation can find there were many mesopores  on the nanosheets. A further TEM examination on the product in Fig. 2b clearly shows the  mesoporous structure on the nanosheets, consistent with the FESEM result. Fig. 2c displays the  FESEM image of 5% Ag@SrTiO3 nanocomposite, where the mesoporous nanosheets-assembled  flower-like structure was maintained. The compositional information from the EDX spectrum in Fig.  2d reveals Ag element was successfully coupled on SrTiO3 with atomic ratio of Ag to SrTiO3 of  0.05:1. The TEM image in Fig. 2e presents many small Ag nanoparticles are attached on the  mesoporous SrTiO3 nanosheets. A closer observation in Fig. 2f reveals uniform Ag nanoparticles  with average diameter of 2 nm are distributed on the SrTiO3 nanosheet. The HRTEM image on the  nanocomposite shown in Fig. 2g displays two types of clear lattice fringes. One set of the fringes  spacing was 0.28 nm, which corresponded to the (110) plane of cubic SrTiO3. Another set of the  fringes spacing measured 0.23 nm, corresponding to the (111) lattice spacing of face-centered-cubic  Ag (JCPDS No. 04-0783). Further EDX characterization on 2% and 10% Ag@SrTiO3 nanocomposites showed the atomic ratio of Ag to SrTiO3 were 0.02:1 and 0.10:1, respectively (Fig.  S3), and corresponding TEM images showed Ag nanoparticles have similar size about 2 nm  distributed on mesoporous SrTiO3 nanosheets (Fig. 2h and 2i).

(a, b) FESEM and TEM images of the pristine SrTiO3, respectively. (c, d) FESEM image and EDX spectrum of 5%  Ag@SrTiO3 nanocomposite, respectively. (e, f) Low- and high-magnification TEM images of 5% Ag@SrTiO3 nanocomposite. (g) The HRTEM image of 5% Ag@SrTiO3 nanocomposite. (h, i) TEM images of 2% and 10%  Ag@SrTiO3, respectively.

Fig. 3 showed the scanning transmission electron microscopy (STEM) image of 5% Ag@ SrTiO3 nanosheet and the corresponding elemental mapping, which reveal that Sr, Ti and O are  homogeneously distributed, while Ag is discretely located on the whole nanosheet.

To identify the elemental and chemical states of Ag@SrTiO3 nanocomposites, XPS spectra were  carried out with the binding energies calibrated using C 1s (284.8 eV). The survey result of 5%  Ag@SrTiO3 in Fig. 4a confirmed the presence of Sr, Ti, O, Ag and C, where the peak for C 1s  ascribed to adventitious carbon during XPS measurement. High-resolution spectra of Sr, Ti, O and  Ag species of the sample are shown in Fig. 4b to 4e, respectively. The two peaks centered at 132.5  and 134.3 eV in Fig. 4b can be individually attributed to Sr 3d5/2 and Sr 3d3/2, confirming Sr exists  in Sr2+ chemical state.16 The binding energies at 463.8 and 457.9 eV in Fig. 4c were Ti 2p1/2 and Ti  2p3/2 peak, respectively, characteristic of dominant Ti4+ . 16 The O 1s spectrum in Fig. 5d exhibited  three binding energies of 529.1, 531.5 and 533.1 eV, the former two peaks correspond to the lattice  O 2-, hydroxyl, and the last peak can be attributed to the chemical adsorbed oxygen.37 The two peaks  centered at 367.4 and 373.5 eV in Fig. 5e correspond to Ag 3d5/2 and Ag 3d3/2, respectively.38 Peak  positions of Ag 3d shift slightly to lower binding energies compared with those of bulk Ag (Ag3d5/2, 368.2 eV; Ag 3d3/2, 374.2 eV). The binding energy shift of Ag is mainly attributed to electron  transfer from metallic Ag to SrTiO3 crystals. The Fermi levels of two components equilibrate when  Ag nanoparticles come into contact with SrTiO3 by transferring some of the electrons from Ag to  SrTiO3 at the interfaces, which results in the higher valence of Ag. The binding energy of  monovalent Ag is lower than that of zerovalent Ag, therefore, the shift to lower binding energies of  Ag 3d5/2 and Ag 3d3/2 further verifies formation of Ag@SrTiO3 nanocomposite.38,39 The 2% and 10%  Ag@SrTiO3 nanocomposites showed similar survey XPS spectra (Fig. S4).  

Fig. 4 XPS spectra of 5% Ag−SrTiO3 nanocomposite. (a) Survey spectrum. (b) Sr 3d. (c) Ti 2p. (d) O 1s. (e) Ag 3d.

The metallic contents in the samples of SrTiO3 and 2%, 5%, 10% Ag@SrTiO3 were determined by  inductively coupled plasma (ICP) spectrometry. The mass contents of Sr and Ti in SrTiO3 are 46.7%  and 25.2%, which are close to the values of 47.8% and 26.1% calculated according to the  stoichiometry. The mass contents of Sr, Ti and Ag in 5% Ag@SrTiO3 are 45.8%, 24.6% and 2.6%， respectively, basically consistent with the atomic ratio by EDX. The ICP results of 2% and 10%  Ag@SrTiO3 (mass content: 46.5% of Sr, 25.1% of Ti and 1.1% of Ag for 2% Ag@SrTiO3; 44.7%  of Sr, 24.1% of Ti and 5.4% of Ag for 10% Ag@SrTiO3) were essentially in agreement with the  EDX results.

The optical properties of the pristine SrTiO3 and Ag@SrTiO3 nanocomposites were studied by  the analyses of the UV-visible diffuse reflectance spectra (Fig. 5). The spectrum of the pristine  SrTiO3 shows a strong UV absorption band characteristic of the wide band gap material of SrTiO3 semiconductor. The Ag@SrTiO3 nanocomposites displayed characteristic localized surface  plasmonic resonance (LSPR) absorption in the visible wavelengths range, which is consistent with  the visible sample color shift from white to gray-brown. Moreover, 5% and 10% Ag@SrTiO3 samples exhibit a surface plasmon absorption peak centered at 475 nm and 433 nm. The result  reveals the higher the Ag content is, the more visible light absorbance the Ag@SrTiO3 nanocomposite will be. The extended absorbance of the Ag@SrTiO3 nanocomposites in the visible  range makes them suitable for efficient utilization of sunlight or visible light to generate more  photoexcited charges in the photocatalytic reactions.  

In this synthetic strategy, nanosheets-assembled flower-like SrTiO3 precursor was firstly  constructed, where glycol was chosen as the solvent to suppress the quick hydrolyzation of  tetrabutyltitanate that is highly reactive toward moisture; while 1, 6-hexanediamine is chosen as a  capping agent, inhibiting the formation of the SrTiO3 precursor skeleton three dimensionally. The  control experiment in absence of 1, 6-hexanediamine clarifies the product was nonuniform  nanoparticles, revealing its crucial role in the formation of nanosheets-assembled flower-like  structure. After calcination at 550 °C in air for 6 h, mesoporous nanosheets-assembled flower-like  SrTiO3 was obtained due to the gradual release of inorganic small molecules such as CO2, N2 and  H2O molecules from the SrTiO3 precursor, while a small quantity of 1, 6-hexanediamine molecules  still bonded on the surface of SrTiO3 nanosheets due to the strong coupling between each other. In  the synthesis of Ag@SrTiO3 nanocomposites, Ag+ was attached on the surface of SrTiO3 nanosheets through bonded with 1, 6-hexanediamine molecules, then reduced to metallic Ag by the  solvent of glycol. Here, the viscosity of glycol could also interfacially stabilize and prevent  aggregation of Ag nanoparticles. The loading amount of Ag could be easily tuned by varying the  concentration of AgNO3 and glycol, such as obtaining 2%, 5% and 10% Ag@SrTiO3 nanocomposites. The formation process of Ag@SrTiO3 nanocomposite was illustrated in Scheme 1.

The photocatalytic activities of the Ag@SrTiO3 nanocomposites were evaluated under full-arc  light irradiation using MO as a probe molecule in aqueous solution, and the relevant data for  pristine SrTiO3 is also given for comparison (Fig. 6a). Prior to the photocatalysis, the catalyst-dye  solution was kept in the dark for 30 min to completely saturate the dye adsorption on the catalyst  surface. The results showed the pristine SrTiO3 and Ag@SrTiO3 nanocomposites all had a small  amount of adsorption for MO molecules, with 1.2% for SrTiO3, while 4.0%, 3.1% and 2.0% for 2%,  5% and 10% Ag@SrTiO3 nanocomposites, respectively. The photocatalytic degradation of the dye  was evaluated after its adsorption/desorption equilibrium on the catalyst surface. The degradation  rate of MO in the presence of SrTiO3 is 6.9% after 1 h. Clearly, Ag@SrTiO3 nanocomposites show  enhanced photocatalytic activities for MO decomposition, far exceeding that of SrTiO3. The 5%  Ag@SrTiO3 nanocomposite presents the highest photocatalytic activity, with which MO  degradation rate reached 92.8% after 1 h. While MO photodegradation remarkably reduced to 52.1%  with 2% Ag@SrTiO3, and further dropped to 35.4% with 10% Ag@SrTiO3. In the absence of  photocatalyst, MO decomposition is 6.4% within the test period, suggesting the photoinduced  self-sensitized photolysis can be induced by photo-absorption of MO itself. The results indicated the  photocatalytic activity of SrTiO3 can be much improved by coupling with Ag nanoparticles, and  there was an optimum amount of Ag nanoparticles. The real pH values of MO without catalyst and  with 5% Ag@SrTiO3 catalyst were 7.4 and 7.6, respectively, demonstrating the addition of catalystbarely change the pH value of MO solution. There was only one report about Ag@SrTiO3 for the  photocatalytic degradation of MO,25 but the catalyst amount was not provided, so we quantitatively  compared the activity of as-prepared 5% Ag@SrTiO3 nanocomposite for MO degradation with  some Ag@TiO2 and Ag@ZnO photocatalysts, as listed in Table 1. Considering the light source and  the amount of MO and catalyst, the activity of our 5% Ag@SrTiO3 nanocomposite was comparable  with or better than these Ag@TiO2 and Ag@ZnO photocatalysts.