One pot microwave-assisted synthesis of Ag decorated yolk@shell structured TiO₂ microspheres

Abstract

A facile and economical one-pot microwave-assisted approach for the synthesis of Ag decorated yolk@shell structured TiO₂ microspheres (Ag-TS) is reported. The rapid and uniform microwave heating could reduce the reaction time to 30 min, an order of magnitude shorter than that of conventional methods. The characterization data confirmed that the resultant mesoporous structured Ag-TS were highly uniform in size with an average diameter of ∼0.5 μm, which was constructed by small anatase TiO₂ nanoparticles, along with Ag nanoparticles ranging from 10 to 50 nm homogeneously dispersed on the microspheres. Nitrogen adsorption–desorption measurement revealed that all the Ag-TS samples had high specific surface areas (>100 m² g⁻¹) and abundant mesoporous structures. The growth model of Ag-TS was proposed based on a series of contrast experiments, the unique selective heating of reaction solvent (deionized water and ethanol) by the microwave method was found to be critical. At the initial stage, amorphous solid microspheres were formed by heating of ethanol molecules through absorbing microwave energy due to the better microwave absorbing performance. Then water molecules were heated by the microwave irradiation, the crystallization of anatase TiO₂ on the surface of the solid microspheres started, followed by the Ti species diffusing spontaneously towards the outer surface of the solid microspheres and leading to the formation of the outer shell due to the Ostwald ripening process. Finally, water continuously diffused through the outer shell and guided the subsequent crystallization of anatase TiO₂, resulting in the formation of the core. Besides, the application of Ag-TS for the removal of water contaminants including toxic heavy metal hexavalent chromium (Cr(VI)) ions and organic dye methylene blue (MB) were also evaluated.

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Introduction

As one of the most important transition metal oxides, TiO₂ with a wide bandgap has been widely investigated and used in the field of photocatalysis.^1–5 A large number of TiO₂ photocatalysts with various morphologies and structures (such as nanoparticles, nanorods, nanosheets, nanotubes) have been synthesized,^6–9 among them, yolk@shell structured TiO₂ have attracted special interest due to their unique properties, such as low density, high surface area, good surface permeability and great light-harvesting capacity.^10–13 It has been reported that yolk@shell structured mesoporous TiO₂ microspheres exhibited considerably high activity in the photocatalytic degradation of phenol,¹⁰ due to the large accessible surface area and excellent light harvesting properties. Over the past few years, huge efforts have been made to fabricate yolk@shell structured TiO₂.^10–15 For example, Cui et al. have reported the fabrication of yolk@shell structured TiO₂ microspheres using linear polymer polyethylene glycol as a template.¹⁴ At the present time, the template route is the most widely used method for materials chemists to synthesize the yolk@shell structured TiO₂, which need additional steps to remove the templates.^14,15 There were also some reports about the “template-free” methods for the synthesis of yolk@shell structured TiO₂,¹² but most involved the long duration of reaction, and required the use of expensive and toxic precursors such as TiCl₄, greatly limiting the widespread applications of such materials. Therefore, finding a facile, fast and economical method for the synthesis of yolk@shell structured TiO₂ is still a great challenge in materials science.

Besides the controlling of morphologies and structures, surface decoration with noble metal Ag has been proved to be a further means to enhance the photocatalytic efficiency of TiO₂ photocatalysts.^16,17 It was reported that the sponge-like TiO₂ nanostructure showed much better activity towards photo-degradation of Rhodamine B and salicylic acid after photo-deposition of Ag nanoparticles.¹⁸ The enhanced activity may attribute to the rapid transfer of photoelectrons and enhanced photo absorption capability introduced by Ag. A variety of strategies have been used to decorate TiO₂ with Ag, but most of those methods involved the preparation of TiO₂ matrix first, and then the decoration of Ag by photo-deposition,^19,20 chemical deposition,^16,18 and impregnation method,¹⁷ these multi-step methods complicated the preparation process. There were also few reports about the in situ growth of Ag in the preparation process of TiO₂,^21,22 but the long reaction time of those methods also limited the preparation efficiency. Thus, an in situ and time-saving route for the decoration of Ag on TiO₂ matrix is still required to meet economic and industrial needs.

Here, Ag decorated yolk@shell structured TiO₂ microspheres was synthesized with a high yield and excellent uniformity, based on a one-pot and extremely fast (30 min) microwave-assisted route in a special hydro-alcohol system. The growth model was proposed for the formation of Ag-TS. The application of Ag-TS for the removal of Cr(VI) and MB in wastewater was evaluated and discussed in detail.

Experimental section

Sample preparation

All reagents were commercial and used without further purification. Titanium oxysulfate (Aladdin Chemistry Co. Ltd, AR), urea (≥99.0%, Guoyao Chemical Reagent Co. Ltd, AR) and silver nitrate (≥99.8%, Tian Jing Guang Fu Science and Technology Development Co. Ltd, AR) were chosen as raw materials. In a typical preparation procedure, titanium oxysulfate (2 mmol) and urea (4 mmol) were added to 40 mL mixed solvent of deionized water and ethanol (the volume ratio of deionized water to ethanol was 1 : 1), and then different amounts of silver nitrate (0, 0.044, 0.074, and 0.22 mmol) were added. After continuous stirring for 3 h, the mixed suspension was transferred into a 100 mL Teflon-lined autoclave, which was heated at 120 °C for 30 min during microwave irradiation (heating rate was about 20 °C min⁻¹) (QWAVE 4000, 2450 MHz, up to 1200 W, Questron Technologies Corp) and cooled to room temperature. The precipitates were collected, washed with deionized water followed by a rinsing in ethanol, and afterwards dried in an oven at 70 °C for 5 h. Four samples obtained by using 0, 0.044, 0.074, and 0.222 mmol silver nitrate were labeled as Ag-TS0, Ag-TS1, Ag-TS2 and Ag-TS3, respectively.

Characterization

The phases of the products were identified by X-ray diffraction analysis (XRD, Philips X'pert PRO) using Ni-filtered monochromatic CuKα radiation at 40 keV and 40 mA. The morphology and structure of the product was characterized by field emission scanning electron microscope (FESEM, Sirion 200 FEI) using an accelerating voltage of 5.00 kV, and transmission electron microscopy (TEM, JEOL-2010, 200 kV) with an energy dispersive X-ray spectrometer (EDX, Oxford, Link ISIS). The powders were ultrasonically-dispersed in ethanol. Then the suspensions were dropped onto the SEM stub and holey-carbon grid for FESEM and TEM examination respectively. X-ray photo electron spectroscopic (XPS) analyses were conducted on a Thermo Scientific ESCALAB 250Xi system, equipped with a hemispherical energy analyser, an Al Kα X-ray source (hν = 1486.6 eV) was operated with a base pressure of 3 × 10⁻⁸ Pa. The photoluminescence (PL) measurement was performed on a LabRam confocal Raman microscope made by JY Company, excited by the 325 nm line of a continuous He–Cd laser at room temperature. The content of Ag decorated on the different samples was measured by an inductive coupled plasma optical emission spectrometer (ICP, ICP 6300, Thermo Fisher Scientific).

Photocatalytic activity evaluation

The evaluation of photocatalytic activity of the photocatalysts for the removal of Cr(VI) in aqueous solution was performed at ambient temperature. K₂Cr₂O₇ was used as the sources for Cr(VI). Typically, 40 mg photocatalysts were added into 80 mL of 10 mg L⁻¹ Cr(VI) solution. The pH value of the reaction suspension was adjusted to 4 using dilute hydrochloric acid. Before irradiation, the suspension was stirred for 30 min in the dark to reach adsorption–desorption equilibrium between the photocatalysts and Cr(VI). After the confirmation of no further decrease of Cr(VI) concentration caused by adsorption on the catalysts, the photoreaction vessel was exposed to the UV light irradiation. A 300 W high pressure mercury lamp with maximum emission at 365 nm was used as light source. After different irradiation intervals, 3 mL solution analytical samples were taken out from the reaction suspensions and centrifuged to remove the photocatalyst powders. Changes in Cr(VI) concentration were followed by the spectrophotometric method of the diphenylcarbazide at 540 nm using a spectrophotometer (CARY-5E). ICP emission spectrometer (ICP 6300) was used to measure the total Cr ions concentrations. The evaluation of photocatalytic degradation of 10 mg L⁻¹ MB in water was performed with the similar procedures. The concentrations of MB were determined by the absorption peaks in UV-vis absorption spectra at 664 nm.

The recycle ability of Ag-TS2 was also tested. After the first cycle, all the photocatalysts, including the taken-out samples, were collected and soaked into NH₃·H₂O (AR, purchased from Alfa Aesar) for 24 h. After rinsing with deionized water for several times, the residual photocatalysts were transferred into 100 mL deionized water. Subsequently, ascorbic acid (AR, purchased from Alfa Aesar) solution with a concentration of 10 g L⁻¹ was slowly added into the suspension. The obtained product was washed thoroughly with deionized water and dried in the oven at 70 °C for 10 h. The method of testing the capability of regenerated samples was the same as the photocatalytic activity measurement experiments above.

Photoelectrochemical measurement

The Ag-TS films were fabricated by a spin-coating method. The suspensions of Ag-TS were first prepared by dispersing 45 mg Ag-TS powders in 2 mL ethanol. The ITO conducting glass slides were used as the film substrate and were successively ultrasonically cleaned with water, acetone, water, ethanol, and water for 10 min, respectively. After cleaning treatment, the ITO slide was spin-coated to form Ag-TS films using the suspensions of Ag-TS. The Ag-TS films were used as photoanodes for photoelectrochemical measurements. A three-electrode system in the photoelectrochemical cell with a quartz window for passing light was employed in measuring photocurrent. The three-electrode system using the 0.1 M NaNO₃ aqueous solution as the electrolyte, the prepared Ag-TS film with ITO substrate as the working electrode, a Pt mesh as the counter electrode, and Ag/AgCl (saturated KCl) as the reference electrode was controlled and measured by an electrochemical workstation (CHI760E Instruments). The photoanode was sealed into a portable holder with ca. 0.785 cm² circular area exposed to the electrolyte for light illumination and photoelectrochemical reaction. The irradiating light was 365 nm parallel ultraviolet from a 350 W Xe arc lamp light source fitted with focusing lenses, UV band pass filter (λ < 400 nm), and 365 band pass filter (CHF-XM-350W, Beijing Trusttech Co. Ltd, China). Electrochemical impedance spectroscopy (EIS) was carried out in the frequency range of 0.01 Hz to 100 kHz with an ac amplitude of 10 mV at an applied bias voltage of 0.4 V (vs. Ag/AgCl) under Xe arc lamp light illumination. Photocurrent measurements were carried out using the same electrochemical workstation and the same illumination source. All measurements were carried out under ambient conditions at room temperature.

Results and discussion

Morphology and structure

To investigate the morphologies of the as-prepared samples, representative field emission scanning electron microscope (FESEM) images and transmission electron microscopy (TEM) images of Ag-TS2 are shown in Fig. 1. The as-prepared Ag-TS2 with typical size of about 500 nm were spherical aggregates assembled with primary nanocrystallites, partial broken microspheres displayed hollow interior with yolk@shell structure (Fig. 1a and b), which could be clearly seen from the typical TEM image (Fig. 1c). Ag nanoparticles were homogeneously dispersed onto the surface with the average size around 10–50 nm (Fig. 1c), the high-resolution TEM (HRTEM) image showed the attaching interface of Ag nanoparticle and TiO₂ nanocrystallite (Fig. 1d). The corresponding elemental mapping images indicated the presence of Ti, O and Ag elements in a single microsphere.

Fig. 1 (a) FESEM image, inset in (a) shows the low-magnification image, (b) high-magnification FESEM image, (c) TEM image and (d) HRTEM image of the as-prepared Ag-TS2, (e–g) elemental mapping images of O, Ti and Ag, respectively.

The morphologies of the as-prepared Ag-TS0, Ag-TS1 and Ag-TS3 are also shown in Fig. 2. It was found that all the samples showed the hollow yolk@shell structures similar to that of Ag-TS2, the only difference might lie in the content of decorated Ag, the Ag content of Ag-TS3 increased obviously compared to that of Ag-TS2 from the element mapping images (Fig. 1g and 2g). From the further TEM images (see Fig. S1 in ESI†), the yolk@shell structures of the products can be clearly seen.

Fig. 2 FESEM images of the as-prepared (a) Ag-TS0, (b) Ag-TS1 and (c) Ag-TS3, inset in (a) shows the corresponding TEM image, inset in (b) and (c) shows the corresponding enlarged image, (d) TEM image of Ag-TS3, (e–g) the corresponding elemental mapping images of O, Ti and Ag, respectively.

The XRD patterns (Fig. 3a) of all the samples showed the peaks of tetragonal anatase phase TiO₂ (JPCDS no. 21-1272), besides, additional peaks (marked with * in Fig. 3a) which could be assigned to the cubic phase Ag (JCPDS no. 04-0783) were also observed from those Ag decorated samples (Ag-TS1, Ag-TS2 and Ag-TS3), and the intensities of Ag peaks increased with the increased Ag content. The color change of the samples from white to gray and black also indicated the change of Ag content from Ag-TS0 to Ag-TS3. Taking Ag-TS3 as an example, XPS was used to further confirm the valence state of the decorated Ag (Fig. 3b and c), the determined binding energies of Ag 3d_5/2 and Ag 3d_3/2 were 367.7 and 373.7 eV, respectively, the spin energy separation was 6.0 eV, which was the characteristic of metallic silver (Ag⁰).¹⁸

Fig. 3 (a) XRD patterns of the different samples, (b) XPS spectra and (c) Ag 3d high-resolution spectra of the as-prepared Ag-TS3.

In order to quantify the content of decorated Ag, the samples were eluted by nitric acid and determined by the ICP measurement. The calculated Ag weight percentage (wt%) for the different samples are shown in Table 1 (see ESI†), it suggested that the decorated Ag content could indeed be facilely and accurately controlled by the amount of the silver nitrate added without changing the overall morphologies of the yolk@shell structured TiO₂ microspheres. The Ag wt% determined by the EDX spectrometer is also shown for comparison (see Table 1 in ESI†), the result was consistent with the result of ICP. It was worthy mentioned that the Ag decoration percentage (Ag decorated on the TiO₂ matrix/Ag species added in the precursor solutions) by the present in situ method (>60%) was much higher than that of the conventional two-step photo-deposition methods (<10%),^20,23 which greatly saved the waste of expensive Ag source.

To further research into the specific surface area and pore-size distribution of the as-obtained Ag-TS products, nitrogen adsorption–desorption isotherms were measured (Fig. S2a†), all of the samples could be classified as type IV (Brunauer–Deming–Deming–Teller classification), indicating the existence of abundant mesoporous structures in the architectures. Based on the BET equation, the specific surface areas of Ag-TS0, Ag-TS1, Ag-TS2 and Ag-TS3 were evaluated to be about 124, 169, 141 and 126 m² g⁻¹, respectively. The pore size of all the samples centered at 3–5 nm based on the desorption data (Fig. S2b†).

Effects of reaction conditions on the morphology and structure

Typical Ag-TS2 was used as an example to investigate the effect of reaction conditions on the morphology and structure of the product. First, it was found that the reaction solvent was critical for the formation of Ag-TS2 (Fig. 4). After microwave treatment at 120 °C for 30 min in pure deionized water without the addition of ethanol, the white precipitations in the precursor solution were solid spheres with rough surfaces (Fig. 4a), the corresponding XRD pattern (Fig. 4c) matched well with anatase phase TiO₂, no peaks of Ag were detected. When the reaction solvent was pure ethanol, large quantities of microspheres with smooth surfaces could be obtained (Fig. 4b), the corresponding XRD pattern indicated that the product was almost amorphous phase. Only with the mixed solvent of deionized water and ethanol, the Ag-TS2 could be obtained (Fig. 1).

Fig. 4 FESEM images of the products prepared with pure (a) deionized water and (b) ethanol, (c) is the corresponding XRD patterns of the products.

Further study suggested that the addition of urea in the precursor solution also had a great effect on the morphology and structure of the product (Fig. S3†). When the urea was not added and the other precursors were kept unchanged, microwave treatment at 120 °C for 30 min could only obtain partial hollow yolk@shell structured microspheres along with many random small particles, and the sharp XRD peaks of typical anatase phase TiO₂ were observed, no obvious peaks of Ag were detected, indicating that urea was responsible for the reduction of Ag ions contrast experiment by increasing the microwave reaction temperature to 180 °C with the other conditions kept unchanged was also conducted (Fig. S4a†), mass microspheres with hollow interiors (inset in Fig. S4a†) and rough surfaces similar to Ag-TS2 were obtained, but there were also some random fragments in the product (highlighted by the arrow), which indicated the hollow structures were not stable and would collapse at high temperature of 180 °C. What's more, the microwave heating method was also found to be critical for the formation of Ag-TS2. Fig. S4b† shows the FESEM image of the product prepared at 120 °C for 30 min by the conventional heating method using the electric dry oven, random aggregates of small particles were formed instead of the uniform Ag decorated yolk@shell structured TiO₂ microspheres, indicating the unique advantages of the microwave heating method.

Growth mechanism

On the basis of systematic experiments mentioned above, a plausible growth process of the Ag-TS was proposed, as illustrated in Scheme 1. The formation of the yolk–shell structured microspheres could be interpreted in terms of alcoholysis induced self-assembly, involving the aggregation of TiO₂ building clusters into microspheres and their subsequent reaction, dissolution and re-deposition processes, together with the in situ Ag reduction.²⁵ First, the solid microspheres composed of Ti–O–R and Ag ions were formed immediately at the initial stage of the reaction due to the heating of ethanol molecules by absorbing microwave energy (R represented the organic groups of ethanol molecules). It should be mentioned that ethanol molecules had a much better microwave absorbing performance than the water molecules (the absorbing performance of microwave energy for a specific solvent is determined by the so-called loss tangent tan δ, a larger value of tan δ indicate a better microwave absorbing performance, ethanol molecules have a large tan δ value of 0.941 while compared to 0.123 for water molecules).²⁴ At the later stage, water molecules was heated by the microwave irradiation, the hydrolysis, nucleation and growth of TiO₂ nanocrystals on the surface of the solid microspheres started, followed by the Ti species diffused spontaneously towards the out surface of the solid microspheres and leaded to the formation of the outer shell due to the Ostwald ripening process.^22,25 In fact, the removal of the organic substances (R) resulted in the separation of the outer shell and inner core. Then, water continuously diffused through the outer shell and guided the subsequent hydrolysis, nucleation and growth of TiO₂ nanocrystals, resulting in the formation of the core. At the same time, Ag ions could be reduced to metallic Ag by organic residues in the presence of water molecules and ethanol molecules according to the previous report.²² Above growth model was confirmed by the time-dependent morphological evolution under 120 °C (Fig. S5†). When the reaction was carried out for 1 min, solid microspheres with smooth surface were obtained (Fig. S5a†), the corresponding XRD pattern (Fig. S6a†) indicated that the product was almost amorphous phase, besides, the FTIR spectra (Fig. S6b†) revealed that the solid microspheres displayed strong absorbance peaks around 500 to 1500 cm⁻¹, which was indicative of C–O and/or C=O bonds caused by organic groups of ethanol molecules,²² as the reaction proceeded for 5 min, the surface of the solid microspheres became rough (Fig. S5b†), indicating the formation of TiO₂ nanocrystal. When the reaction time was further increased to 10 min, the solid microspheres became rougher and the yolk@shell structure started forming (Fig. S5c†), the corresponding XRD patterns (Fig. S6a†) indicated that the crystallization of the products increased with the reaction time (the additional peak marked with * could be assigned to the cubic phase metallic Ag), besides, the FTIR spectra (Fig. S6b†) indicated the removal of the organic groups with the reaction proceeded. The matured yolk@shell structured microspheres were obtained with 30 min reaction time (Fig. 1).

Scheme 1 Schematic diagram to illustrate the growth process of the Ag decorated yolk@shell structured TiO₂ microspheres.

In the synthesis process, the selection of reaction solvent was critical for the growth of Ag-TS, which has been shown in Fig. 4. The water molecules were responsible for the crystallization of the anatase TiO₂ and the ethanol molecules were responsible for the formation of the initial amorphous solid microspheres, which would evolved into the yolk@shell structured microspheres, they were both very important in the formation of the final product. Besides, it should be mentioned that urea played an important role in the reaction process (see Fig. S3†), the urea would decompose to give OH⁻ under heating and finely control the pH value of the reaction system closed to the isoelectric point pH of TiO₂ (6.8), promoting the self-organization of the TiO₂ nanocrystals into mesoporous microspheres structures,²⁶ besides, the finely controlling the pH value by urea might promote the reduction of Ag ions, as low solubility products AgSO₄ was expected to be formed at a low pH (<4), blocking the further reduction of Ag ions.²⁷ However, the exact role of urea in the reduction of Ag ions was still under investigated.

At last, it should be noted that the unique advantages of volumetric heating of microwave method was critical for the fast formation of the yolk@shell structured microspheres achieved in this work (shown in Fig. S4b†). On one hand, the temperature of the reaction mixture was raised uniformly and quickly throughout the whole liquid volume by direct coupling of microwave electromagnetic radiation energy to the water and ethanol molecules presented in the reaction mixture.²⁴ As demonstrated, the microwave treatment is capable of greatly accelerating the reaction process and reducing the reaction time nearly an order of magnitude in comparison with the conventional heating method using the electric dry oven,²² and the homogeneous reaction conditions created by microwave make sure the uniformity of the products. On the other hand, the microwave method could realize the selective heating of water molecules and ethanol molecules due to their different microwave absorbing performance,²⁴ which was impossible for the conventional heating method using the electric dry oven. In this regard, the appropriate selection of reaction solvent may greatly expand the application range of the microwave-assisted preparation of various nanostructures.

Adsorption and photocatalytic removal of water contaminants

The as-prepared Ag-TS were used for the photocatalytic removal of two typical water contaminants heavy metal ions Cr(VI) and organic dye MB. Dark adsorption was conducted before irradiation. It was found that Cr(VI) concentration decreased considerably after dark adsorption by Ag-TS3 (Fig. 5a), the change of Cr(VI) concentration with the dark adsorption time for the different samples was graphically presented in Fig. 5b, it was observed that the concentration of Cr(VI) rapidly decreased with the dark adsorption time, and reached the equilibrium state in 30 min for all the samples. It was worthy mentioned that the Cr(VI) removal efficiency gradually increased with increasing the Ag content, the removal efficiency of Ag-TS0, Ag-TS1, Ag-TS2 and Ag-TS3 after dark adsorption for 30 min was 21.3%, 28.7%, 34.2% and 60.5%, respectively. The initial pH values of the reaction solution was also found to be critical for the dark adsorption of Cr(VI), the removal efficiency of Ag-TS3 at pH values of 6, 8 and 11 after dark adsorption for 30 min was 49.6%, 30.7% and 0.6%, respectively, indicating that low pH value was beneficial for the absorption of Cr(VI), which was consistent with the previous work.²⁸ The adsorption process of Cr(VI) on Ag-TS at acid condition would follow through the reaction equations:²⁸

TiO₂⁺ + HCrO₄⁻ → TiO₂ − HCrO₄ (1)

3Ag + HCrO₄⁻ + 7H⁺ + 3Cl⁻ → 3AgCl + Cr³⁺ + 4H₂O (2)

where Cl⁻ came from HCl solution which was used for pH modulation. This implied that the decorated Ag nanoparticles could serve as an electron reservoir to effectively reduce the surface-absorbed Cr(VI) to Cr(III) in the adsorption processes, simultaneously, Ag nanoparticles were oxidized to Ag⁺ ions that further interacted with Cl⁻ ions to form AgCl (reaction (2)), above results indicated that the as-prepared Ag-TS showed potential application as an efficient adsorbent for the removal of Cr(VI) with a remarkably fast adsorption rate.

Fig. 5 (a) UV-vis absorption spectra of Cr(VI) solutions before and after dark adsorption by Ag-TS3 for 30 min, (b) the concentration of Cr(VI) with the dark adsorption time for the different samples. C₀ is the starting Cr(VI) concentration (10 ppm), C is concentration of the remaining Cr(VI) at time t.

Photocatalytic performance of the Ag-TS for the degradation of Cr(VI) was demonstrated after dark absorption (Fig. 6). As there was a great difference on the adsorption abilities of Cr(VI) for the different catalysts (Fig. 5b), full adsorption was considered before photo-degradation, the concentration of Cr(VI) solution was adjusted by monitoring the UV-vis absorbance intensity to make sure that the concentration of Cr(VI) solutions was the same after full adsorption for the different catalysts. Without the addition of photocatalyst, the Cr(VI) removal could be ignored under the illumination. The Ag decorated samples (Ag-TS1, Ag-TS2 and Ag-TS3) displayed much higher photocatalytic efficiency than that of the non-decorated sample (Ag-TS0), and the photocatalytic efficiency increased with increasing Ag content. Complete removal of Cr(VI) was observed after 80 min of illumination for Ag-TS3, while 120 min was needed for Ag-TS2, and there was still Cr(VI) remained after 120 min of illumination with Ag-TS0 and Ag-TS1 as catalysts, indicating that Ag decoration successfully gave an enhanced photocatalytic activity and was definitely an indispensable part. It was found that the concentration of total Cr ions measured by ICP (Table 2†) were consistent with the results of UV-vis spectra for all the catalysts, indicating that there was no Cr(III) species desorbed into the photocatalytic reaction solutions. And all the Cr ions species were adsorbed on the surface of the catalysts, which well avoided the secondary pollution.²⁹ The concentration of total Cr ions after 120 min of illumination for Ag-TS2 and Ag-TS3 was found to be about 0.07 and 0.05 ppm, respectively, which was much lower than maximum allowable concentration of Cr⁶⁺ ions in industrial wastewater of 0.25 ppm.²⁹

Fig. 6 Photocatalytic degradation of Cr(VI) by the different photocatalysts with the time evolution. C₀ is the Cr(VI) concentration after the dark adsorption for 30 min (10 ppm), C is concentration of the remaining Cr(VI) at time t.

XPS was used to further study the absorbed Cr species on the surface of Ag-TS3 after photocatalytic reaction. Fig. 7a shows the corresponding high-resolution XPS spectra of the Cr 2p region. The broad peak of Cr 2p^3/2 could be fitted to three main peaks at 576.5, 577.4 and 577.6 eV, consistent with the published XPS spectra for oxides or the hydroxide of characteristics Cr(III) (e.g. Cr(III)_xO_y and Cr(OH)₃), two weak peaks at 578.3 and 579.6 eV corresponded to the characteristics binding energy of Cr(VI) ions,^28,30 indicating that most of the adsorbed Cr(VI) ions on the surface of the Ag-TS3 were reduced to Cr(III). The Ag 3d XPS peak (Fig. 7b) was asymmetric, meaning that Ag existed in at least two valence states.³¹ The XRD pattern (Fig. 7c) of Ag-TS3 after photocatalytic reactions clearly revealed the existence of AgCl (JCPDS no. 01-1013), apart from the peaks related to metallic Ag and anatase TiO₂.

Fig. 7 High-resolution XPS spectrum of (a) Cr 2p, (b) Ag 3d and (c) XRD pattern of the Ag-TS3 powders after photocatalytic reactions.

Based on the above results, the photocatalytic removal process of Cr(VI) by Ag-TS was as follows: before irradiation, the decorated Ag nanoparticles could serve as an electron reservoir to effectively reduce part of Cr(VI) to Cr(III) in the adsorption processes, simultaneously, Ag nanoparticles were oxidized to Ag⁺ ions and further interacted with Cl⁻ ions on the surface to form AgCl. Once the light was on, the TiO₂ matrix would absorb light and induce the photo-excited electrons and holes, and the photo-excited electrons would lead to the further reduction of Cr(VI). On the other hand, the photo-decomposition of AgCl by absorbing photo-excited electrons would produce metallic Ag, which could also reduce part of Cr(VI),^31,32 in this regard, the photocatalytic system facilitated the reactivation of Ag species. Therefore, the Ag-TS could be used as an effective adsorbent and photocatalyst for the removal of heavy metal Cr(VI) in wastewater.

Further experiments indicated that the Ag decorated samples also showed much higher photocatalytic activity than that of bare TiO₂ in the photo-degradation of organic dye MB (Fig. 8). However, different from the results in photo-degradation of Cr(VI), the increase of Ag content did not always result in the increase of the photocatalytic activity, it was found that Ag-TS2 with an Ag content of ∼3 wt% showed the most superior photocatalytic performance. When the weight percentage of Ag was optimal, it was thermodynamically possible that the Ag acted as an electron well that could effectively separate photo-excited electrons, preventing electron–hole recombination.³³ However, excessive Ag might also cover the surface of TiO₂, reducing the absorption of light by TiO₂.²⁰

Fig. 8 Photocatalytic degradation of MB by the different photocatalysts with the time evolution. C₀ is the MB concentration after the dark absorption for 30 min (10 ppm), C is concentration of the remaining MB at time t.

In addition to the photocatalytic activity, the stability and recycle of the photocatalyst were also important in practical applications. In the present study, the durability of the photocatalytic activity of Ag-TS2 was studied by reusing of the catalysts in fresh Cr(VI) and MB solution. Fig. S7† shows the photocatalytic ability of the Ag-TS2 for three cycles (90 min irradiation for each cycle). It was found that the removal percentage of Cr(VI) and MB after three cycles could still reach 75%, 92%, respectively, which was comparable to that of previous report.²⁸ Indicating the feasibility of regeneration of the as-prepared photocatalysts.

At last, the photoluminescence (PL) emission spectra measurement was used to determine the effect of Ag decoration on the charge recombination behavior and migration efficiency of the TiO₂ samples (Fig. 9a). Typically, the photoluminescence intensity of Ag-TS3 remarkably decreased compared to that of Ag-TS0, indicating that the suppression of the recombination of photo-excited electrons and holes after Ag decoration.^20,23 The photoelectrochemical measurements were used to further investigate the influence of Ag on the charge recombination behavior and migration efficiency of TiO₂.^34–36 We employed electrochemical impedance spectroscopy (EIS) measurement to investigate the properties of electron transfer resistance across the electrolyte interfaces under irradiation (Fig. 9b). It is well known that EIS Nyquist plots can be used to characterize the charge transfer resistance and the separation efficiency of the photo-generated electrons and holes.³⁶ The Ag-TS3 samples exhibited much lower transfer resistance values in comparison with Ag-TS0, indicating a more effective separation of photo-generated electron–hole pairs.³⁶ The transient photocurrent was also measured (Fig. 9c), it could be clearly seen that the photocurrent response of Ag-TS0 was not obvious, while Ag-TS3 produced a pronounced rise in the photocurrent responses under irradiation, due to the effective separation of photo-excited electron–hole pairs at the Ag-TS3/electrolyte interface.^33,36 All the above results suggested that the decoration of Ag could greatly enhanced the separation rate of photo-excited electron–hole pairs, the excellent photoelectrochemical properties of Ag-TS materials are also expected to have promising applications in photoelectrochemical solar cells and other light harvesting devices.

Fig. 9 (a) PL spectra, (b) EIS spectra, and (c) the transient photocurrent response of the different samples.

Conclusions

In summary, we have demonstrated a facile and extremely fast one-pot microwave-assisted method to fabricate Ag decorated yolk@shell structured TiO₂ microspheres. No corrosive and toxic reagents were used in the synthesis process which was environmental friendly. The Ag nanoparticles ranging from 10 to 50 nm were homogeneously dispersed on the yolk@shell structured TiO₂ microspheres, and the content of Ag can be readily controlled by the amount of the precursor of silver nitrate without changing the overall morphology of the yolk@shell structured microspheres. The selection of reaction solvent and the unique selective heating of microwave method were found to be responsible for the growth of Ag-TS. Moreover, it was found that the Ag decoration greatly enhanced the photocatalytic performance, due to the effectively separation of the photo-induced electrons and holes by Ag. The present method is promising for constructing complicated Ag–TiO₂ composites structure in a simple “one-pot” route, which may find application in preparation of other kinds of metal–semiconductor composites.

Acknowledgements

This work was supported by the National Basic Research Program of China (Grant no. 2013CB934302), the Natural Science Foundation of China (Grant no. 51072199, 21177132 and 51272255) and Strategic Priority Research Program of the Chinese Academy of Sciences (Grant no. XDA09030200).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Additional figures including FESEM image, TEM image and so on. See DOI: 10.1039/c4ra14675a

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