Ag-decorated K2Ta2O6 nanocomposite photocatalysts with enhanced visible-light-driven degradation activities of tetracycline (TC)

Author's Accepted Manuscript

Ag-Decorated K2Ta2O6 nanocomposite photocatalysts with enhanced visible-light-driven degradation activities of tetracycline (TC) Dongbo Xu, Weidong Shi, Min Chen, Bifu Luo, Lisong Xiao, Wei Gu

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S0272-8842(14)01901-4 http://dx.doi.org/10.1016/j.ceramint.2014.11.136 CERI9581

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Ceramics International

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9 October 2014 22 October 2014 26 November 2014

Cite this article as: Dongbo Xu, Weidong Shi, Min Chen, Bifu Luo, Lisong Xiao, Wei Gu, Ag-Decorated K2Ta2O6 nanocomposite photocatalysts with enhanced visible-lightdriven degradation activities of tetracycline (TC), Ceramics International, http://dx.doi. org/10.1016/j.ceramint.2014.11.136 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Ag-decorated K2Ta2O6 nanocomposite photocatalysts with enhanced visible-light-driven degradation activities of tetracycline (TC) Dongbo Xu, Weidong Shi*, Min Chen*, Bifu Luo, Lisong Xiao, Wei Gu

School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, P. R. China

* Corresponding author: E-mail address: [email protected] (WD Shi), [email protected] (M Chen),

Abstract In this paper, octahedral K2Ta2O6 nanocrystals have been synthesized by a single-step hydrothermal

method.

Then

the

visible-light-driven

plasmonic

Ag-K2Ta2O6

nanocomposite photocatalysts are prepared by a photochemical reduction process. The product was characterized using various techniques, such as X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), UV-visible diffuse-reflectance spectroscopy (DRS). The results show that the Ag nanoparticles are uniformly loaded on the surface of the K2Ta2O6. The prepared Ag-K2Ta2O6 exhibits an efficient photocatalytic activity for degradation of tetracycline (TC) under visible-light irradiation and shows good repeatability and stability. In particular, the highest photocatalytic activity at the nominal atomic ratio of silver to tantalum as 3%, which is more than 3 times that of pure K2Ta2O6.

1

Keywords Plasmonic; Ag-K2Ta2O6; photocatalysts; tetracycline (TC); 1 Introduction In the past two decades, tantalates semiconductor photocatalysts have attracted extensive attention due to their wide potential applications in environmental procedures such as air purification, water disinfection, hazardous waste remediation, and water purification because tantalates possess conduction bands consisting of a Ta5d orbital located at a more negative position than that of titanates (Ti3d), which have conduction bands at a high potential, so a variety of mixed metal oxides containing Ta5+ transition-metal ions have been studied recently.1-5 For example, Kudo and his group have reported many tantalates, K3Ta3Si2O13,6 alkaki tantalates ATaO3 ( A = Li, Na, and K ).7 Tatsumi Ishihara as well as have reported KTaO3,8 K2Ta2O6.9 Alkaline earth tantalates BTa2O6 ( B = Ca, Sr, and Ba ),10 K3Ta3B2O1211 have been reported by Kato and his group. Masato Machida and other groups have reported lanthanum-doped NaTaO3,12 SnMn2O6, and SnM2O7 ( M = Nb and Ta ),13 lanthanide tantalates LnTaO4 ( Ln = La, Ce, Pr, Nd, and Sm ),14 and so on. These tantalates show high activities for photocatalytic water splitting and dyestuff degradation under UV light irradiation. Very recently, nanoparticles (NPs) of noble metals (Au, Pt, Ag) on the surface of semiconductor strongly absorb visible light due to their surface plasmon resonance (SPR), in which their conducting electrons undergo a collective oscillation induced by the electric field of visible light.15–18 The SPR phenomenon has led to important applications such as colorimetric sensors,19 photochromic devices,20 photocatalysis21 and so on. Naturally, Ag NPs supported on semiconductor such as TiO2 were found to be highly efficient photocatalysts for degrading MB.22 Attempts to develop photocatalysts effective under visible light have led to numerous plasmonic photocatalysts including Ag@AgCl,23 Ag@AgBr,24 and Ag/AgBr/WO3•H2O.25 In these photocatalysts noble-metal NPs are deposited on the surface of polar semiconductors so that visible light is absorbed by the 2

noble-metal NPs, and the photo-generated electrons and holes are separated by the metal–semiconductor interface. The photocatalytic oxidation on these plasmonic photocatalysts takes place typically on the surface of the polar semiconductors.26 Herein, the octahedral K2Ta2O6 nanocrystals was prepared through hydrothermal synthesis.27 Subsequently, we prepared a visible-light-driven plasmonic Ag-K2Ta2O6 nanocomposite photocatalyst by a photochemical reduction process. The detailed synthesis process, characterization, and photochemical property testing for this composite catalyst were also discussed. In addition, the Ag-K2Ta2O6 nanocomposite was applied for the degradation of tetracycline (TC) to evaluate the plasmonic performance. TC is widely used in aquiculture and live stocking, but the waste TC is usually discharged along with waste water, which causes serious ecological pollution. The prepared Ag-K2Ta2O6 exhibits an efficient photocatalytic activity for degradation of TC under visible-light irradiation. 2 Experimental 2.1 Material synthesis. Octahedral K2Ta2O6 nanocrystals were prepared according to previously published procedures.27 In a typical process of preparing Ag-K2Ta2O6 nanocomposite, the obtained K2Ta2O6 (0.5 mmol) was added into 50 mL of AgNO3 solution by magnetic stirring in the dark. Photoreduction was carried out under a 1000 W Xe lamp for 2 min, during which silver ions were reduced to form silver nanoparticles on the surface of K2Ta2O6. The obtained Ag-K2Ta2O6 was then washed with deionized water, and dried in an oven at 60 °C for 12 h. The molar ratios of Ag to K2Ta2O6 were 0.5%, 1%, 2%, 3%, 4% and 5%. 2.2 Characterization X-Ray diffraction (XRD) patterns were obtained using a Bruker D8 advanced X-ray powder diffractometer with Cu Kα radiation. Scanning electron microscopy (SEM) and Energy Dispersive Spectrometer (EDS) images were obtained with a Hitachi S-4800 microscope. Transmission electron microscopy (TEM) and high resolution transmission 3

electron microscopy (HRTEM) measurements were carried out on a JEM-2100 microscope. The UV−vis diffuse reflectance spectra (DRS) were measured on a Shimadzu UV-3100 UV/Vis spectrophotometer using BaSO4 as a reference. The binding energies were characterized by using X-ray photo-electron spectroscopy (XPS) (VG Micro Tech ESCA 3000 X-ray photoelectron spectroscope using monochromatic Al KR with a photon energy of 1486.6 eV at a pressure of >1*10-9 Torr.) The XPS spectra were charge corrected to the adventitious C 1s peak at 284.6 eV. 2.3 Photocatalysis Tetracycline (TC) was chosen to evaluate the photocatalytic properties of Ag-K2Ta2O6 nanocomposite. Two 500 mL beakers with a water jacket were used as photoreactor. The suspensions were kept at constant temperature by circulating thermostatted water through the jacket. Ag-K2Ta2O6 photocatalyst (0.1 g) was dissolved in 100 mL of TC solution (20 mg/L). Prior to visible-light irradiation, the mixture was stirred magnetically in the dark for 45 min to obtain the equilibrium adsorption state. The light source was a 300 W Xe arc lamp with UV cutoff filter (providing visible light with > 425 nm). After exposure to visible light for 30 min, 5.0 mL of solution was taken out and used for fluorescence spectrum measurements. 2.4. Kinetics of Photocatalytic Degradation of TC. To further understand the reaction kinetics of TC degradation, the apparent pseudo-first-order model28-29 expressed by eqn (1) was applied in the experiments: ln (C0/C) = Kapp · t

(1).

Where C is the concentration of solute remaining in the solution at irradiation time of t and C0 is the initial concentration at t = 0. Kapp denotes the degradation rate constant. The apparent rate constant (Kapp) has been chosen as the basic kinetic parameter for the different photocatalysts since it enables one to determine a photocatalytic activity independent of the previous adsorption period in the dark and the concentration of solute remaining in the solution.30 4

3 Results and discussion 3.1 Crystal structure, XRD, SEM, TEM, EDS and HRTEM analysis Fig. 1 shows the XRD patterns of the pure octahedral K2Ta2O6 nanocrystalline powders, and the different molar ratios of Ag to K2Ta2O6 from 0.5% to 5% of Ag–K2Ta2O6 nanocomposite. All peaks can be indexed to the standard PDF card of metatantalate K2Ta2O6 (JCPDS 35-1464), and no traces of other phases are examined.31 Moreover, due to the fact that the Ag nanoparticles are dispersed and trace to load on the surface of the octahedral K2Ta2O6 nanocrystals, no signal about silver can be detected for the Ag element. The SEM image shown in Fig. 2A indicates the octahedral morphology of the pure K2Ta2O6 sample with an average size of around 100 nm and the surface of K2Ta2O6 is smooth. But the surface of Ag–K2Ta2O6 nanocomposite is plicate as showen in Fig. 2B. Fig. 3 shows TEM images of Ag-K2Ta2O6 with different Ag contents, respectively. Fig. 3a–f show the TEM images of the Ag-K2Ta2O6 composites with AgNO3 to K2Ta2O6 theoretical molar ratios 0.5%, 1%, 2%, 3%, 4%, 5%, respectively. As shown in Fig. 3a, b, with the addition of AgNO3 at low concentration, it can be seen that there are microcrystalline Ag particles on the surface of K2Ta2O6. For the 2% and 3% composites, the Ag particles can be clearly observed (Fig. 3c, d). When higher concentration of AgNO3 is added, more and more Ag nanoparticles are deposited and the particles of Ag are bigger than previous, as shown in Fig. 3e, f. The Ag NPs seen as bright dots in the images with diameter of 2–10 nm are deposited homogeneously on K2Ta2O6-crystalline surface. The EDS analysis of Ag-K2Ta2O6 nanocomposite was taken during the SEM measurements confirmed the presence of Ag NPs (Fig. 4a). The representative HRTEM image (Fig. 4b) of an individual Ag-K2Ta2O6 nanocomposite shows clear lattice fringes on the edge of the K2Ta2O6, and the Ag interplanar spacing is 0.22 nm on the surface of K2Ta2O6 which interplanar spacing is 0.26 nm. Furthermore, the K2Ta2O6 interplanar spacing corresponds to the plane of the isometric phase, which is in good agreement with 5

the XRD results.

Fig. 1 The XRD patterns of pure K2Ta2O6 (a) and the Ag–K2Ta2O6 nanocomposite samples prepared with varying molar ratios 0.5% (b), 1% (c), 2% (d), 3% (e), 4% (f), 5% (g), respectively.

6

Fig. 2 SEM images of as-prepared photocatalysts: (a) pure K2Ta2O6; (b) 3% Ag–K2Ta2O6.

Fig. 3 TEM images of as-prepared Ag–K2Ta2O6: (a) 0.5% Ag–K2Ta2O6; (b) 1% Ag–K2Ta2O6; (c) 2% Ag–K2Ta2O6; (d) 3% Ag–K2Ta2O6; (e) 4% Ag–K2Ta2O6; (f) 5% Ag–K2Ta2O6.

7

Fig. 4 (a) EDS image and (b) the representative HRTEM image of an individual 3% Ag–K2Ta2O6 nanocomposite crystalline powders. 3.2 UV−Vis Diffuse Reflectance Spectra The UV−vis diffuse reflectance spectra (DRS) of the Ag-K2Ta2O6 samples were compared to that of pure K2Ta2O6, as shown in Figure 5. According to the spectra, pure K2Ta2O6 sample presents the photoresponse property under the UV light, which is due to the intrinsic band gap transition. The absorption spectra of Ag-K2Ta2O6 are obviously different from the K2Ta2O6. The Ag-K2Ta2O6 samples show a shallow peak with higher intensities observed from 800 to 500 nm under visible light, which is consistent with the formation of more Ag NPs. The prominent absorption in the visible light region could be attributed to the surface plasmon resonance (SPR) effect of Ag nanoparticles. The SPR effect of Ag nanoparticles may have a partial contribution to the photocatalytic activity of Ag-K2Ta2O6.

8

Fig. 5 UV/Vis diffuse-reflectance spectra of naked K2Ta2O6 nanocrystals and AgK2Ta2O6 nanocrystals composites with different Ag NPs loading amount. 3.3 Comparison of photocatalytic activities of Ag-K2Ta2O6 samples To probe the potential application of Ag-K2Ta2O6 nanocomposite in photocatalytic degradation of TC, we evaluated the photocatalytic degradation of TC by K2Ta2O6 crystalline in relation to Ag-K2Ta2O6 nanocomposite under visible light. The inset in Fig. 6 shows that the maximum absorbance decreases greatly after visible-light irradiation within 200 min over the 3% Ag-K2Ta2O6 composite, revealing the degradation of TC (inset in Fig. 6). As shown in Figure 6, this TC solution (see Experimental for details) was kept over the catalyst in the dark for 45 min to obtain the equilibrium adsorption state, under dark conditions only ~4% of TC was adsorbed over the catalyst, the C/C0 of TC without any catalyst, with pure K2Ta2O6 crystalline powder, 0.5% Ag-K2Ta2O6, 1% Ag-K2Ta2O6, 2% Ag-K2Ta2O6, 3% Ag-K2Ta2O6, 4% Ag-K2Ta2O6 and 5% Ag-K2Ta2O6 nanocomposite for 285 min are 2.28%, 14.81%, 14.89%, 24.82%, 30.18%, 49.59%, 38.11% and 17.89%, respectively (Figure 7). Compared with pure K2Ta2O6, the 9

photodegradation significantly increased by further loading Ag. Moreover, the photodegradation in the presence of 3% Ag-K2Ta2O6 nanocomposite was conversely higher as compared to others. Therefore, the effects of Ag as cocatalyst can be clearly drawn, for the DR of 3% Ag-K2Ta2O6 nanocomposite is over 3 times than that of pure K2Ta2O6, which is reasonable to expect the obvious Ag-dependent activity. However, further increasing the Ag content decreases the absorbance (Figure 6, 7); TEM (Figure 4e, f) observations show that a silver molar ratios loading K2Ta2O6 higher than 3% led to aggregation of the Ag NPs and the Ag NPs have a big nanoparticle size. The highest photocatalytic activity for the composite with 3% Ag loading may be attributed to the enhanced visible-light absorption as well as the strongest SPR effect. As far as we know, this work is the first reported visible-light-driven photocatalyst for TC degradation by Ag-K2Ta2O6 until now, so the Ag-K2Ta2O6 as a visible light photocatalyst for antibiotic water treatment is efficient and promising.

10

Fig. 6 The photodegradation of TC as a function of irradiation time over different catalysts under visible light irradiation. The inset is the UV-visible absorption spectral changes of TC at given time intervals over the 3% Ag-K2Ta2O6 sample;

11

Fig. 7 Photocatalytic degradation ratios of TC with different samples: (a) TC without any catalytic, (b) pure K2Ta2O6, (c) 0.5% Ag-K2Ta2O6 nanocomposite, (d) 1% Ag-K2Ta2O6 nanocomposite, (e) 2% Ag-K2Ta2O6 nanocomposite, (f) 3% Ag-K2Ta2O6 nanocomposite, (g) 4% Ag-K2Ta2O6 nanocomposite, and (h) 5% Ag-K2Ta2O6 nanocomposite under visible light for 285 min. 3.4 The Kinetic Study of Photocatalytic Degradation of TC. In order to further illustrate the photocatalytic reaction, the kinetic behavior is discussed. The photodegradation reaction kinetics of TC can be described by a Langmuir−Hinshelwood model according the report.32 The decomposition of TC approximated the first order kinetic. The variations in ln (C0/C) as a function of irradiation times are given in Figure 8, the highest Kapp which was gained by 3% Ag-K2Ta2O6 can reach 0.162 h-1. The degradation constant of 3% Ag-K2Ta2O6 was found to be about 5.2 times that of K2Ta2O6 (0.0313 h-1), 6.7 times that of 0.5% Ag-K2Ta2O6 12

(0.0242 h-1), 2.7 times that of 1% Ag-K2Ta2O6 (0.0600 h-1), 2.0 times that of 2% Ag-K2Ta2O6 (0.0798 h-1), 1.4 times that of 4% Ag-K2Ta2O6 (0.115 h-1), and 4.0 times that of 5% Ag-K2Ta2O6 (0.0403 h-1).

Fig. 8 TC degradation curves of ln (C0/C) versus time for different catalysts. 3.5 Mechanism of photocatalytic oxidation Furthermore, the PCO process on Ag-K2Ta2O6 photocatalysts can be classified as the ·OH radicals oxidation. In a typical photodegradation procedure, the photogenerated electrons can transfer to the conduction band (CB) from the valence band (VB), leaving the corresponding holes in the valence band, when the semiconductor is irradiated by illumination. Organic compound degradation during the photocatalytic process is resulted from the oxidation reaction of the organic compound with ·OH radicals or holes. A series of reactive species are excited by photo-induced h+ and e-, such as ·OH or ·O2-, which are suspected to be involved in the photocatalytic oxidation process.33 13

Figure 9 shows the results of adding different radical scavengers over the 3% Ag-K2Ta2O6 photocatalyst reaction system under visible-light irradiation. When 10 mL of iso-propanol (IPA)34 for ·OH is added into the reaction system, the removal efficiency of TC is obviously inhibited by IPA and the photodegradation of TC is only 14.68% (entry b in Figure 9), which is largely lower compared to the reaction in the absence of radical scavengers (entry a in Figure 9). A similar and obvious inhibition phenomenon for the photocatalytic reaction is also observed with the triethanolamine (TEA)35−39 scavenger for h+ (entry d in Figure 9). Therefore, it can be concluded that ·OH and h+ are the main active species of Ag-K2Ta2O6 in aqueous solution under visible light irradiation. On the contrary, the photocatalytic degradation of TC obviously increased with the addition of AgNO3 for e− (entry c in Figure 9). The increase suggests that the scavenger of e− has less of an opportunity for electron−hole pairs’ recombination and facilitates the production of more holes. This gives evidence that the degradation of TC is dominated by the ·OH and h+, rather than e− reactions taking place on the surface of the photocatalyst. On the basis of the experimental results, we have proposed a possible mechanistic pathway. As depicted in Scheme 1, when Ag is loaded on the surface of K2Ta2O6, under the polarization field provided by the K2Ta2O6 crystal, the plasmonexcited electrons move and accumulate on the surface of the Ag nanoparticles, the excited electrons can react with O2 producing ·OH radicals.40-41 Subsequently, the presence of ·OH radicals produce from the reaction between the photocatalyst and absorbed water controlled the photodegradation rate. This implies that Ag contributes to the improved photocatalytic activity of K2Ta2O6. Meanwhile, the photogenerated holes from K2Ta2O6 nano-bulk react with water and OH− to produce ·OH radicals. These as-produced ·OH radicals can then degrade the TC molecules into CO2 and H2O. Eventually, it can be estimated that ·OH and h+ are the main active species of Ag-K2Ta2O6 in aqueous solution under visible light irradiation, which would oxidize TC directly.

14

15

Fig. 9 Photocatalytic degradation ratios of TC using different radical scavengers over 3% Ag-K2Ta2O6: (a) reaction in the absence of radical scavengers, (b) reaction with iso-propanol for ·OH, (c) reaction with AgNO3 as a scavenger for h+, and (d) reaction with TEA as a scavenger for e− under visible light irradiation for 285 min.

16

Scheme 1. Mechanistic Pathway of Electrons and Holes under Visible Light Illumination on Ag-K2Ta2O6 Photocatalysts

3.6 Recycle of Ag-K2Ta2O6 nanocomposite To investigate the recyclability of the Ag-K2Ta2O6 nanocomposite, sample powders after photocatalytic reactions were collected by natural settling and reused in the photocatalytic reaction for 3 times under the same conditions. Fig. 10 shows the results of three successive TC degradation runs using the reclaimed 3% Ag-K2Ta2O6 catalyst. It can be seen that the catalyst does not exhibit a significant loss of activity in three successive runs. The Ag-K2Ta2O6 nanocomposite catalyst sample displays a good stability and maintains a high photocatalytic performance during three reaction cycles. Because of the mass loss during the sedimentation and transferring processes and the gradually decline in adsorptive capacity of the catalyst, the photocatalytic performance of sample decreased gradually. The results indicate that the Ag-K2Ta2O6 nanocomposite catalyst is sufficient stable during the photodegradation of the TC, which is important for its practical application. 17

Fig. 10 Recycling tests on the 3% Ag-K2Ta2O6 sample for the degradation of TC under visible light irradiation. 3.7 XPS of Ag-KTaO3 nanocomposite Fig. 11 presents the XPS patterns of the 3% Ag-K2Ta2O6 sample before and after three runs under the visible light for the degradation of TC. It can be clearly observed that the peak at 368 eV can be attributed to Ag3d5, and the Ta4f peak for the Ag-K2Ta2O6 sample with the value of about 25.99 eV, confirming that Ta exists mainly in the Ta5+ chemical state on the sample surface. As shown in Fig. 4(b), the HRTEM image of Ag-K2Ta2O6 nanocomposite shows that the Ag NPs are partially embedded in the surface of K2Ta2O6, indicating good contacts between the Ag NPs and K2Ta2O6-nanocrystals. This indicates strong interactions between Ag and the K2Ta2O6 particle,42-43 which is beneficial for the photoexcited electron transfer at the interface. The strong interactions between Ag and K2Ta2O6 also make the loading of Ag NPs stable, and this is confirmed 18

by the stability of photocatalytic experiments.

Fig. 11 The complete XPS spectra of the 3% Ag-K2Ta2O6 nanocomposite. 4 Conclusions In summary, the Ag-K2Ta2O6 nanocomposite with the different molar ratios of Ag to K2Ta2O6 have been successfully synthesized through a photochemical reduction and evaluated the photocatalytic properties of these composites under visible light by monitoring the degradation of TC in aqueous. The probable mechanism for this catalytic reaction involves the visible-light-induced electron transfer from the Ag NPs to the K2Ta2O6 particle, which are ascribed to the synergistic effect of the strong surface plasmonic resonance (SPR), and the degradation of TC by ·OH radicals. Importantly, our work suggest that the idea of noble metal Ag on the surface of K2Ta2O6 can be a plausible strategy to develop efficient photocatalysts with high activity for environmental 19

remediation.

Acknowledgements This work was financially supported by the Zhenjiang Industry Supporting Plan (GY2013023), National Natural Science Foundation of China (21276116, 21301076, 21303074 and 21201085), Program for New Century Excellent Talents in University (NCET-13-0835), Henry Fok Education Foundation (141068) and Six Talents Peak Project in Jiangsu Province (XCL-025).

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