Au nanoparticles embedded on urchin-like TiO2 nanosphere: An efficient catalyst for dyes degradation and 4-nitrophenol reduction

Materials and Design 121 (2017) 167–175

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Au nanoparticles embedded on urchin-like TiO2 nanosphere: An efficient catalyst for dyes degradation and 4-nitrophenol reduction Zheng-Hui Ren a,1, Hai-Tao Li a,1, Qiang Gao a,⁎, Hao Wang a, Bo Han b, Kai-Sheng Xia b, Cheng-Gang Zhou b,⁎ a b

Department of Chemistry, Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China Sustainable Energy Laboratory, Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• AuNPs/uTiO2 was prepared via photoreduction deposition of Au nanoparticles (AuNPs) on urchin-like TiO2 nanosphere (uTiO2). • AuNPs/uTiO2 showed high surface area (147.5 m2 ⋅ g-1), large pore volume (0.52 cm3 ⋅ g-1), and high dispersity of AuNPs. • uTiO2 support could prevent leaching or aggregation of AuNPs, while allowing guest molecules to diffuse in and out easily. • AuNPs/uTiO2 exhibited superior catalytic efficiencies for dyes degradation and 4-nitrophenol reduction. • Catalytic activity of AuNPs/uTiO2 could remain almost unchanged after being recycled for several times.

a r t i c l e

i n f o

Article history: Received 20 November 2016 Received in revised form 19 February 2017 Accepted 20 February 2017 Available online 21 February 2017 Keywords: Environmental remediation Oxidative degradation Reductive conversion Heterogeneous catalyst AuNPs/uTiO2 Superior catalytic properties

a b s t r a c t The development of efficient heterogeneous catalysts for degrading organic pollutants or converting them into harmless and even useful products is of vital significance for environmental remediation. Herein, we reported the facile synthesis of a highly active and stable nanocatalyst (AuNPs/uTiO2) via a simple photoreduction deposition of Au nanoparticles (AuNPs) on urchin-like TiO2 nanosphere (uTiO2), and demonstrated its excellent performances as a catalyst for oxidative degradation of organic dyes and reductive conversion of 4-nitrophenol (4NP). The AuNPs/uTiO2 nanocomposite showed high surface area (147.5 m2·g−1), large pore volume (0.52 cm3·g−1), and high dispersity of AuNPs. In particular, the uTiO2 support could combine AuNPs strongly and serve as a shield to prevent leaching or aggregation of AuNPs, while allowing guest organic molecules to diffuse in and out easily. Benefiting from the excellent characteristics, the AuNPs/uTiO2 exhibited superior catalytic properties for dyes degradation and 4-NP reduction with significantly higher catalytic efficiencies than many previously reported heterogeneous catalysts. Moreover, the catalytic activity of AuNPs/uTiO2 could remain almost unchanged after being recycled for several times, demonstrating its long-term stability. The AuNPs/uTiO2, combining the advantages of high activity, favorable kinetics, and excellent durability for dye degradation and 4-NP reduction, should be very promising for wastewater treatment. © 2017 Elsevier Ltd. All rights reserved.

⁎ Corresponding authors. E-mail addresses: [email protected] (Q. Gao), [email protected] (C.-G. Zhou). 1 The two authors contributed equally to this work.

http://dx.doi.org/10.1016/j.matdes.2017.02.064 0264-1275/© 2017 Elsevier Ltd. All rights reserved.

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1. Introduction The contamination of water supplies by organic compounds has become an issue of serious international concern due to the extensive industrialization and consequently the massive discharge of various types of organic toxicants, including chlorinated solvents, fertilizers, dyes, phenols, and nitro aromatic compounds etc. [1]. Among them, the organic dyes and nitro aromatic compounds are biologically and chemically stable, which makes them difficult to eliminate by natural degradation processes [2]. Such contaminants once released can adversely affect aquatic organisms and pose serious threats to human health [3,4]. Thus, removing these organic contaminants or converting them into harmless and even useful products before discharge is of utmost practical importance [2]. Organic dyes can be removed by various techniques including biodegradation, chemical oxidation, and adsorption etc. [5,6]. Among these techniques, Fenton oxidation degradation, based on the catalytic decomposition of H2O2 to yield highly reactive •OH, is very promising for dyes removal [7]. By contrast, the preferred way of detoxificating nitro aromatic compounds is to convert them into industrially valuable aromatic amines via catalytic reduction in the presence of NaBH4 as the reducing agent [8–10]. In general, a crucial issue for both Fenton oxidation degradation of organic dyes and catalytic reduction of nitro aromatic compounds is the design and fabrication of catalysts [11]. During the past two decades, a great deal of research on Fenton oxidation degradation of dyes has been conducted over various heterogeneous catalytic systems with the metal oxides or hydroxides (e.g., Fe2O3, Fe3O4, Co3O4, FeOOH, MnOOH, and Mn3O4) as catalysts [12]. Practically speaking, however, since the catalytic processes take place only at the solid–liquid boundary, the heterogeneous catalysts commonly show much lower efficiencies as compared to the homogeneous analogues [13]. Introduction of external energies such as ultrasound, UV light, microwave, and electricity, can largely promote heterogeneous Fenton oxidation process, but it also results in the need for specific equipment [14]. To investigate the possibility of facilitating the heterogeneous Fenton reaction without external energy input, some studies have recently been done on the nanoscaled noble metals [12,15]. Fortunately, it has been discovered that Au nanoparticles (AuNPs), dispersed on a suitable solid support, have a powerful potential in catalytically decomposing organic pollutants [16–20]. In essential, AuNPs can act as excellent electron-donor or acceptor, so they can significantly promote the decomposition of H2O2 to •OH via a redox cycle process without aid of external energy [19]. Moreover, AuNPs have a great advantage in catalytic durability over conventional heterogeneous Fenton catalysts because Au is rather “inert” and highly resistant to chemical attacks under various reaction conditions [19]. But it should be also noted that the support of AuNPs can affect the surface area, pore size, pore volume, exposed active sites, stabilization of AuNPs, which all have significant impacts on the overall catalytic performance [21]. To date, only a limited number of solid materials such as hydroxyapatite, carbon, diamond, titania, and hematite have been used as the supports of AuNPs to construct AuNPs-based Fenton catalysis systems for organic pollutants degradation [16–20]. Even though these supports can disperse homogeneously AuNPs on their surfaces, they either have low surface areas or lack of abilities to prevent AuNPs from migrating and even leaching. On the other hand, AuNPs have attracted great attention as one of the most promising catalysts for reduction of nitro aromatic compounds in recent years [22]. In these applications, AuNPs are generally immobilized on the supporting materials, such as polymer, carbon, silica, and metal oxides, to prevent particle aggregation [23]. Among them, the immobilization of AuNPs on TiO2 is of particular interest because the TiO2 support is chemically/mechanically stable and highly effective for the dispersion of AuNPs in a catalytically active form [24–27]. However, in the majority of cases, the used TiO2 supports have low porosities and cannot provide sufficient surfaces for adsorbing and activating reagents, and therefore, high activities are often hard to achieve [28].

Encapsulation of AuNPs in porous TiO2, though relatively scarcely reported, may be an effective way to alleviate the above problem [23,29, 30]. For example, a mesoporous AuNPs/TiO2 nanocomposite was synthesized recently by Ismail et al., exhibiting a high catalytic activity for 4-NP reduction [29]. More recently, a macro-/meso-porous AuNPs/ TiO2 composite was developed by Li et al. as the catalyst of 4-NP, which also showed a favorable catalytic ability [30]. Nonetheless, these porous composites usually possess randomly aligned mesopore channels and a large part of AuNPs stay deeply inside the mesopores, so the guest molecules would be difficult to diffuse through the pores to react with the active sites on the internal surface, resulting in a relatively slow reaction kinetics as compared to exposed catalysts [31]. In addition, the immobilization of AuNPs on TiO2 usually requires multiple processes (e.g., impregnation followed by high-temperature reduction), which is somewhat tedious, time-consuming, and probably limits the wide applications of these catalysts. Therefore, a facile and efficient route to fabrication of porous AuNPs/TiO2 catalyst with highly accessible active sites and further superior catalytic properties is still a challenge and of particular significance. In the present work, we reported a facile synthesis of AuNPs embedded on urchin-like TiO2 spheres (AuNPs/uTiO2) and demonstrated its superior catalytic performances for Fenton degradation of organic dyes and reduction of 4-NP (Fig. 1). The uTiO2 support with highly opened structure was facilely fabricated by a combination of surfaceprotected etching and calcination with the use of TiO2 solid spheres as precursor [32]. Then, the AuNPs were successfully deposited on the uTiO2 by a simple one-step photoreduction process under room temperature over a short period of time (only 1 min). It was confirmed that the resulting AuNPs/uTiO2 showed a high surface area (up to 147.5 m2·g− 1), large pore volume (up to 0.52 cm3·g− 1), and high dispersity of AuNPs. In particular, the uTiO2 support could combine AuNPs strongly and serve as a shield to prevent leaching or aggregation of active AuNPs, while allowing guest molecules to diffuse in and out easily. As a consequence, this AuNPs/uTiO2 nanocomposite was demonstrated to be a highly efficient catalyst for both dyes degradation and 4NP reduction, simultaneously fulfilling the important requirements of high activity, favorable kinetics, and excellent durability in the two kinds of reactions. 2. Experimental 2.1. Chemicals and reagents Hydrogen tetrachloroaurate (III) hydrate (HAuCl4·4H2O), polyvinylpyrrolidone (PVP, 10 k), ammonium hydroxide (NH3·H2O, 28 wt%), ethanol, acetonitrile, hydrogen peroxide (H2O2, 30 wt%), methylene blue (MB), auramine O (AO), basic red 5 (BR), and basic blue 17 (BB), sodium tetraborohydrate (NaBH4), 4-nitrophenol (4-NP) were all purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Titanium isopropoxide (TTIP, Ti(OC3H7)4, 98%) was obtained from J&K Scientific Ltd. (Beijing). All chemicals and regents used in this study were commercially available and used without further purification. Distilled water was used throughout our experiments. 2.2. Preparation of AuNPs/uTiO2 The uTiO2 support was synthesized by surface-protected etching and sequential calcination processes, according to a reported method with a minor modification [32]. First, amorphous TiO2 nanosphere precursor was synthesized through a controlled hydrolysis of TTIP. In detail, 0.38 g of NH3·H2O (28 wt%) and 0.91 g of water were added into a mixed solution containing 150 mL ethanol and 100 mL acetonitrile. Afterwards, TTIP (5 mL) was promptly injected into the above solution under vigorous stirring. A milky suspension was formed immediately. After further stirring for 7 h, the product was centrifuged and washed with ethanol for several times. Second, the TiO2 precursor powders

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Fig. 1. Schematic illustration of the synthesis of AuNPs/uTiO2 and its application in both dyes degradation and 4-NP reduction.

were dispersed into water to form a suspension with a concentration of 20 mg·mL−1. 23.1 mg of NaF were then added into the suspension as the etching agent. After stirring for 2 h, 33.3 mg of PVP were added. The suspension was continuously stirred for 2 h before transferring to a Teflon-line autoclave. A hydrothermal reaction was carried out at 110 °C for 4 h to conduct the crystallization and surface etching processes. The obtained white solids were collected via centrifugation. After being washed with diluted NaOH (1 mmol·L−1) and water, the uTiO2 was obtained by calcining at 350 °C for 2 h with a heating rate of 1 °C·min−1. Then, the AuNPs/uTiO2 was synthesized by a photoreduction deposition of Au nanoparticles (AuNPs) on uTiO2. Typically, 40 mg of uTiO2 were dispersed in ethanol (10 mL), and then 2.04 μmoL of HAuCl4·4H2O were added. Before the irradiation process, gaseous N2 was bubbled to ensure that the solution had no dissolved O2. Then, the suspension was submitted to the UV light irradiation (wavelength of 210 nm) for 1 min using a 500 W high-pressure mercury lamp under stirring. After the UV-irradiation, the color of the sample changed from white to blue-purple, indicating the formation of AuNPs on the surface of uTiO2 by the reduction reaction between Au(III) and the surface photoexcited electron [33]. The solid product (i.e., AuNPs/uTiO2) was centrifuged, washed repeatedly with water and ethanol, and dried at 60 °C. 2.3. Characterization of materials The morphologies of as-prepared samples were observed by a fieldemission scanning electron microscope (SEM, SU8010, Hitachi, Japan) and transmission electron microscopy (TEM, Philips CM12). Wideangle X-ray diffraction (XRD) measurements were carried out on an X-ray diffractometer (D8-FOCUS, Bruker, Germany). X-ray photoelectron spectroscopy (XPS) analysis was performed in a VG Scientific ESCALAB Mark II spectrometer equipped with two ultra-high vacuum (UHV) chambers to analyze the chemical states of constituent elements. N2 adsorption/desorption measurement was performed on a Micromeritics ASAP2020 surface area analyzer at 77 K. 2.4. Fenton degradation of dyes The catalytic oxidation activity of AuNPs/uTiO2 was evaluated by catalyzing degradation of typical organic dyes (MB, AO, BR, and BB) in the

presence of H2O2 as the oxidizing agent. Typically, 10 mL of MB solution (50 mg·L−1) containing 4.5 wt% H2O2 was added into a vial that contained 5 mg of catalysts, and the mixture was shaken in the dark with a speed of 200 rpm at 35 °C. At predetermined time intervals, the catalyst was separated immediately from the solution with a filter. The concentrations of MB in the course of degradation were measured at the maximum absorption wavelength (665 nm) by using the UV– vis spectrophotometer. With the use of a process similar to that described above, the oxidative degradations of AO, BR, and BB were also investigated. The absorption wavelengths for determining AO, BR, and BB were 431, 525, and 631 nm, respectively. 2.5. Reductive conversion of 4-NP The catalytic reduction activity of AuNPs/uTiO2 was evaluated by catalyzing conversion of 4-NP to 4-amino-phenol (4-AP) in the presence of NaBH4 as the reducing agent. In a typical experiment, 0.2 mL of 4-NP solution (5 mmol·L− 1), 3 mL of NaBH4 solution (0.2 mol·L−1), and 6.8 mL of water were added to a vial. After addition of the catalyst particles (5 mg), the yellow solution was gradually faded as the reaction proceeded. The reaction temperature was maintained at 30 °C and UV–vis spectra of the solution were recorded at predetermined time intervals to monitor the progress of the reduction reaction. 3. Results and discussion 3.1. Characterization of materials The synthesis procedure for AuNPs/uTiO2 is illustrated in Fig. 1. The uTiO2 is synthesized by surface-protected etching and sequential calcination processes with the use of TiO2 nanospheres as precursor [32]. Once uTiO2 powders are dispersed in Au(III)-contained ethanol and are subsequently irradiated by UV light, the photo-generated holes will be scavenged by ethanol while the photo-excited electrons are trapped by the Au(III) present in the interfacial region [33]. Consequently, the AuNPs are supported on the surface of uTiO2 to form our target product AuNPs/uTiO2. To verify whether the physicochemical characteristics of AuNPs/uTiO2 were well developed, different characterization

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Fig. 2. SEM images of TiO2 precursor (a), uTiO2 (b), and AuNPs/uTiO2 (c); TEM images of AuNPs/uTiO2 with low (d) and high (e) magnification, where the inset of (e) shows the particle size distribution of AuNPs in AuNPs/uTiO2; EDS spectrum from SEM measurement (f).

techniques such as SEM, TEM, XRD, XPS, and N2 adsorption/desorption measurement were employed. SEM images were collected for TiO2 precursor, uTiO2, and AuNPs/ uTiO2 to distinguish their morphological difference. As seen from Fig. 2a, the TiO2 precursor displays, basically, a spherical morphology with relatively smooth surface. The diameter of TiO2 nanospheres is mainly around 450–500 nm. During the hydrothermal treatment, these TiO2 nanospheres were readily coated by PVP through the hydrogen bonding between the carbonyl groups of PVP and the hydroxyl groups on TiO2 surface [32]. The inner TiO2 would be etched by the F− ions, eventually forming an urchin-like sphere with numerous radially arranged nanothorns [32]. Such an urchin-like morphology of uTiO2 was confirmed by SEM image, as shown in Fig. 2b. Thanks to the surface protection by PVP, the uTiO2 nanospheres still keep a good monodispersity, and their diameter is not changed visibly as compared with TiO2 precursor. After decoration with AuNPs, the urchin-like morphology is not obviously influenced (Fig. 2c), but a lot of AuNPs have been embedded on uTiO2 according to the TEM images (Fig. 2d and e).

Noticeably, these AuNPs are well separated by nanothorns of uTiO2, and their size has a relatively narrow distribution mainly centered at 5.6–14.9 nm (Fig. 2e). Compared with the common loading of AuNPs on relatively smooth surfaces of various supports [34], such embedding is likely to be highly advantageous for preventing leaching or aggregation of active AuNPs. Furthermore, the EDS analysis of AuNPs/uTiO2 at various different regions, taken during the SEM measurements, confirmed the presence of AuNPs (Fig. 2f). The molar ratio of Au/Ti in the AuNPs/uTiO2 nanostructures is 0.73:100. Phase structures of uTiO2 and AuNPs/uTiO2 were determined by XRD, and the results are shown in Fig. 3a. In pattern of uTiO2 sample, the peaks can be assigned to the diffractions of (101), (004), (200), (105), (211), and (204) crystalline planes of anatase (JCPDS card No. 04-0477) [35]. However, the XRD profile of AuNPs/uTiO2 shows a single diffraction pattern of anatase, but does not show that of AuNPs, which should be due to the low content of AuNPs [36]. It is worthy to point out that the ratio of AuNPs on TiO2 seems to be much higher from EDS (Fig. 2f) than the XRD result (Fig. 3a). This is mainly attributed to the

Fig. 3. XRD patterns of uTiO2 and AuNPs/uTiO2 (a); XPS spectrum of AuNPs/uTiO2, where the insets are the narrow region XPS for Ti 2p (top) and Au 4f (bottom).

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Fig. 4. N2 adsorption/desorption isotherms of AuNPs/uTiO2, where the inset is its pore size distribution.

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the reported XPS data [34]. The peak separation between the 2p1/2 and 2p3/2 lines is 5.7 eV, which is also consistent with the +4 oxidation state [38]. The inset of Fig. 3b shows that the binding energies of Au 4f7/2 and Au 4f5/2 are 86.7 and 83.2 eV, respectively, and the peak separation value of 3.5 eV indicates the metallic nature of AuNPs [36,39]. A 0.8 eV negative shift of Au0 4f (from 84.0 to 83.2 eV) suggests strong interactions between the AuNPs and the uTiO2 support [39,40]. N2 adsorption/desorption measurement was conducted to determine the textural parameters of AuNPs/uTiO2. As shown in Fig. 4a, the AuNPs/uTiO2 exhibits a type VI isotherm with an obvious H1 hysteresis loop, indicating its mesoporous characteristic [41]. Pore size distribution (PSD) of AuNPs/uTiO2 obtained by Barrett–Joyner–Halenda (BJH) method displays a narrow distribution of mesopores centered at about 10.1 nm (Fig. 4b). The detailed texture parameters are calculated and it is found that the AuNPs/uTiO2 has a surface area of 147.5 m2·g−1 and a pore volume of 0.52 cm3·g− 1. Generally, the catalytic performance of a heterogeneous catalyst is positively correlated with its surface area. The large surface area of this urchin-like composite, in conjugation with its highly open structure, distinct crystallinity, and high monodispersity of AuNPs, should be considerably beneficial to its catalytic performance. 3.2. Catalytic oxidation activity

fact that most of AuNPs are located on the tip of nanothorns of uTiO2 nanostructures (Fig. 2e). The survey XPS spectrum (Fig. 3b) indicates that the AuNPs/uTiO2 is mainly composed of Au, Ti, and O, where the C emission peak should be due to the ex situ preparation process or the transfer process of the sample into the UHV chamber [37]. The sample exhibits Ti 2p1/2 (463.9 eV) and Ti 2p3/2 (458.2 eV) peaks as shown in Fig. 3b, which can be assigned to the Ti4+ oxidation state according to

Catalytic oxidation activity of AuNPs/uTiO2 was firstly evaluated via the degradation reaction of MB in the presence of H2O2. As shown in Fig. 5a, the AuNPs/uTiO2 shows a considerably high catalytic efficiency. Specifically, the degradation degree of MB reaches 84.7% in 30 min, 96.2 in 60 min, and nearly 100% in 90 min. Many previous investigations showed that the time required to achieve a more or less complete

Fig. 5. Degradation kinetic curve of MB catalyzed by AuNPs/uTiO2 along with the results from control experiments (a); under identical conditions, degradation kinetic curves of AO (b), BR (c), and BB (d). Each inset shows the UV–vis absorption spectra of the corresponding dye during degradation process.

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degradation of MB (concentrations of MB ≤ 50 mg·L− 1) for different heterogeneous Fenton systems was in the range of 90–600 min, e.g., 90 min for FePt [42], 120 min for ferrocene [43], 120 min for βMnO2 [44], 120 min for Fe/Cu bimetallic system [45], 240 min for Fe3O4/FeMnOx [46], and 600 min for Multi-nFe2O3@Meso-SiO2 nanoreactor [47]. The time value in this work falls to the bottom of the range, implying the superior catalytic activity of AuNPs/uTiO2. Such a rapid kinetics in catalytic degradation of MB might be attributed to the high dispersity of AuNPs on the high-surface-area uTiO2, ensuring the maximum exposure of active sites for MB degradation. Moreover, the highly open structure of uTiO2 support can also allow organic molecules to diffuse in and out easily. Control experiments were performed to compare the removal efficiencies towards MB, and all these experiments were conducted in the dark to avoid the influence of light. It is observed that use of H2O2 alone only gives rise to a slight removal of MB within 90 min, which is negligible compared with the rapid degradation of MB by heterogeneous Fenton-like reaction based on AuNPs/uTiO2 nanocomposite (Fig. 5a). When AuNPs/uTiO2 is used in the absence of H2O2, the concentration of MB only decreases by 8.5% after 90 min (Fig. 5a), indicating the AuNPs/uTiO2 has a low efficiency for MB adsorption. Taken together, these results do indeed suggest that the oxidative degradation of MB is induced by H2O2, which can be significantly promoted by the AuNPs/ uTiO2 catalyst. In order to gain insight into the universality of AuNPs/uTiO2, its catalytic oxidation activity for the Fenton degradation of several other typical organic dyes including AO, BR, and BB was also investigated. All the degradation reactions for these dyes were carried out under the same conditions as that for MB. As shown in Fig. 5a–d, the catalytic degradation rates of the dyes follow the order: BR N BB N MB N AO. The degradation rates involved in the removal of these four organic dyes differ from each other, which is probably attributed to the difference in molecular structure and different degradation mechanism [48]. Interestingly, our results reveal that all these organic dyes can be efficiently degraded in our reaction systems (Fig. 5), indicating the AuNPs/uTiO2 will be a promising catalyst for oxidative degradation of a variety of organic dyes. Degradation kinetics is one of the most important characteristics that represent the catalytic efficiency of the Fenton catalyst and, therefore, largely determines the potential applications of catalyst [49]. In this study, the kinetic experiments of MB degradation with the use of AuNPs/uTiO2 as catalyst were conducted by investigating the degradation rate versus the degradation time at different temperatures (35, 45, 55, and 65 °C), and the results are shown in Fig. 6a. It is evident that the degradation rate of MB increases with the increase of temperature. Moreover, it can be observed from the plots of ln(C0/Ct) against time that the degradation reaction of MB obeys the pseudo-first-order kinetic model (Fig. 6b) [49]. The reaction rate constants (k) of MB degradation were found to be 0.05986 min− 1 (R2 = 0.9947) at 35 °C, 0.10001 min−1 (R2 = 0.9978) at 45 °C, 0.15976 min−1 (R2 = 0.9938) at 55 °C, and 0.24746 min−1 (R2 = 0.9769) at 65 °C, respectively. Furthermore, the activation energy (Ea) can be calculated from the plots of lnk against 1/T according to the Arrhenius equation (Fig. 6c) [49]. The Ea value is estimated to be 40.96 kJ·mol−1 for AuNPs/uTiO2 system. Generally, the Ea values of ordinary chemical reactions are usually between 60 and 250 kJ·mol−1 [50]. The results presented in this study imply that the catalytic oxidation of MB in aqueous solution by our heterogeneous catalytic system requires a lower activation energy and can be easily achieved.

groups of 4-NP molecules present in the interfacial region, inducing their reduction to amino groups [51]. In our experiments, the addition of AuNPs/uTiO2 into 4-NP/NaBH4 aqueous solution could indeed cause the fading and ultimate bleaching of the yellow color of the solution. The process was monitored by the UV–vis spectrophotometry (Fig. 7a). Intuitively, the absorption of 4-NP at 400 nm decreases quickly, with a concomitant increase of a new peak at 315 nm that can be

3.3. Catalytic reduction activity Catalytic reduction activity of AuNPs/uTiO2 was evaluated via the reduction of 4-NP in the presence of excess NaBH4. As well known, BH− 4 ions can be catalyzed by AuNPs and transfer active hydrogen species to the particle surface to form metal hydride complexes [51]. Subsequently, the adsorbed hydrogen species would transfer to the nitro

Fig. 6. Removal efficiencies of MB under different reaction temperatures (a); the corresponding fitting curves of kinetic data (b); the plot of lnk vs. 1/T (c).

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Fig. 7. UV–vis absorption spectra of 4-NP during reduction reaction (a); reduction kinetic curve of 4-NP catalyzed by AuNPs/uTiO2 along with the results from control experiments (b); the reduction kinetic data at different temperatures and the corresponding fitting curves (c); the plot of lnk vs. 1/T (d).

assigned to 4-AP (Fig. 7a) [52]. Moreover, the UV–vis spectra show an isosbestic point between the two absorption bands, indicating that the 4-NP has been gradually converted to 4-AP without the observation of any side reaction [52]. As shown in Fig. 7b, the conversion rate can reach 87.9% within 2 min and nearly 100% within 6 min in the presence of AuNPs/uTiO2 as the catalyst. Compared with the reported results [53, 54], this reduction efficiency is considerably satisfactory (as will be made clear in the following section). To gain more insight into this reaction and rationally distinguish the contributors to the reductive conversion of 4-NP, the control experiments without AuNPs/uTiO2 or NaBH4 were conducted. As shown in

Fig. 7b, the concentration of 4-NP remains nearly unchanged in the absence of AuNPs/uTiO2, demonstrating that NaBH4 alone cannot reduce 4-NP under our experimental conditions. This phenomenon is understandable, because it is well known that although the reaction between 4-NP and NaBH4 is a thermodynamically favorable process involving E0 for 4-NP/4-AP = −0.76 V and H3BO3/BH− 4 = −1.33 V versus NHE, it is kinetically restricted (does not occur even in 2 days' time) in the absence of catalyst [55]. When AuNPs/uTiO2 is used in the absence of NaBH4, the concentration change of 4-NP is basically negligible over the investigated period (Fig. 7b), indicating an unfavorable adsorption of 4-NP on AuNPs/uTiO2. To sum up, we can deduce that the conversion

Fig. 8. Effect of recycling times on catalytic oxidation of MB (a) and catalytic reduction of 4-NP (b).

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of 4-NP to 4-AP is reduced by NaBH4 and becomes kinetically favorable with assistance of the AuNPs/uTiO2 catalyst. Considering the concentration of NaBH4 used in our experiments largely exceeds that of 4-NP, the reaction rate can be assumed to be independent of NaBH4 concentration [52]. So, a pseudo-first-order rate kinetics with respect to 4-NP can be applied to evaluate the catalytic rate [52]. Fig. 7c shows the linear relationships between ln(Ct/C0) versus reaction time in the reaction catalyzed by AuNPs/uTiO2 under different temperatures (25, 30, 35, and 40 °C), and the plots well matched the first-order reaction kinetics. According to the linear relationship, we calculated the reaction rate constant k from the slope of the straight lines. The rate constant k at 25 °C is 0.6326 min−1. As the temperature increases to 30 °C, 35 °C, and further to 40 °C, the k values consequently increase (0.8769 min− 1 at 30 °C; 1.0526 min− 1 at 35 °C; and 1.4454 min−1 at 40 °C), clearly demonstrating that the catalytic efficiency of AuNPs/uTiO2 is positively correlated with the reaction temperature. This observation is understandable because the reducibility of NaBH4 is enhanced as the temperature increases [56]. Interestingly, the k values (0.6326–1.4454 min−1) reported in this work are significantly higher than those for many recently reported nanocatalysts such as Pt nanoflowers (0.042 min−1) [57], Pt\\Au pNDs/RGOs [51], silver/iron oxide nanocomposite (0.228 min− 1) (0.133–0.351 min− 1) [58], and hyperbranched polymer-stabilized AuNPs (0.196 min−1) [59]. As mentioned in Section 3.2, the activation energy Ea is a very important parameter of a catalyst, which can be applied to evaluate its catalytic performance. Based on the above calculated rate constant data, the Ea of 4-NP reduction in the presence of AuNPs/uTiO 2 as the catalyst is estimated to be 42.48 kJ·mol − 1 according to the Arrhenius equation (Fig. 7d) [49]. This value is lower than those of several previously reported AuNPs-based catalysts, such as magnetically recoverable Au nanocatalyst (51.2 kJ·mol − 1) [8], Au-based nanoboxes (55.44 ± 3.15 kJ·mol − 1) [60], and Au/PDDA/NCC (69.2 kJ·mol − 1) [61]. These results of both activation energy and rate constants indicate the excellent catalytic reduction activity of AuNPs/uTiO2, which should be related closely to the unique features of this catalyst, such as highly open structure, high surface area, large pore volume, and high dispersity of AuNPs. 3.4. Durability in catalytic oxidation and reduction One of the great advantages of heterogeneous catalysts, besides easy separation from the reaction medium, is the possibility to be recovered and reused in consecutive runs. In the present case, we investigated the reusability of AuNPs/uTiO2 in the catalytic oxidation and catalytic reduction processes. After each use, the catalyst was recovered by simple centrifugation, followed by washing with water for next cycle of catalysis. As seen from Fig. 8a and b, the AuNPs/uTiO2 nanocatalyst can be recycled and reused five times without obvious decline in both degradation and conversion processes. Taking the results in 4-NP reduction as an example (Fig. 8b) to compare with recently reported catalysts such as Cu2O@h-BN (50% activity loss after 4 cycles) [62], PVPh-Ni3Co1 (55% activity loss after 7 cycles) [63], and Ni nanoparticles in hydrogel network (25% activity loss after 5 cycles) [64], our catalyst shows a significantly better durability. This is probably because the uTiO2 support in AuNPs/uTiO2 could serve as an effective shield to prevent leaching or aggregation of active AuNPs, which results in a good durability. 4. Conclusions In summary, a highly active and stable AuNPs-based catalyst (AuNPs/uTiO2) was developed for both dyes degradation and 4-NP reduction. The AuNPs/uTiO2 catalyst, being characteristic of Au nanoparticles (AuNPs) embedded on urchin-like TiO2 sphere (uTiO2), was facilely fabricated by a combination of surface-protected etching, calcination, and photoreduction deposition processes. SEM, TEM, XRD, XPS, and N2

adsorption/desorption measurement demonstrated its highly open architecture, high dispersity of AuNPs, distinct crystalline structure, high surface area (147.5 m2·g−1), and large pore volume (0.52 cm3·g−1). Especially, the uTiO2 support could combine AuNPs strongly and serve as a shield to prevent leaching or aggregation of active AuNPs, while allowing guest organic molecules to diffuse in and out easily. Owing to these excellent features, the AuNPs/uTiO2 showed superior catalytic performances for dyes degradation and 4-NP reduction in term of catalytic activity and reaction kinetics. Moreover, it was found that the catalyst was highly stable and could be reused at least five times without significant loss of the catalytic activity. In view of high activity, favorable kinetics, and excellent durability for dye degradation and 4-NP reduction, we believe that the AuNPs/uTiO2 developed in this work will be potentially applicable for treatment of wastewater containing various organic pollutants.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 21303170, 31671820 and 11305145), Natural Science Foundation of Hubei Province (No. 2015CFB187), and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (No. CUGL150414, CUGL140413 and CUG120115). References [1] F. Hernández, M. Ibáñez, T. Portolés, M.I. Cervera, J.V. Sancho, F.J. López, Advancing towards universal screening for organic pollutants in waters, J. Hazard. Mater. 282 (2015) 86–95. [2] C. Singh, A. Goyal, S. Singhal, Nickel-doped cobalt ferrite nanoparticles: efficient catalysts for the reduction of nitroaromatic compounds and photo-oxidative degradation of toxic dyes, Nanoscale 6 (2014) 7959–7970. [3] C. Fernández, M.S. Larrechi, M.P. Callao, An analytical overview of processes for removing organic dyes from wastewater effluents, TrAC Trends Anal. Chem. 29 (2010) 1202–1211. [4] P. Kovacic, R. Somanathan, Nitroaromatic compounds: environmental toxicity, carcinogenicity, mutagenicity, therapy and mechanism, J. Appl. Toxicol. 34 (2014) 810–824. [5] T. Robinson, G. McMullan, R. Marchant, P. Nigam, Remediation of dyes in textile effluent: a critical review on current treatment technologies with a proposed alternative, Bioresour. Technol. 77 (2001) 247–255. [6] Y.W. Wang, Z.L. Mo, P. Zhang, C. Zhang, L.J. Han, R.B. Guo, H. Gou, X.J. Wei, R.R. Hu, Synthesis of flower-like TiO2 microsphere/graphene composite for removal of organic dye from water, Mater. Des. 99 (2016) 378–388. [7] P.V. Nidheesh, R. Gandhimathi, S.T. Ramesh, Degradation of dyes from aqueous solution by Fenton processes: a review, Environ. Sci. Pollut. Res. 20 (2013) 2099–2132. [8] Y.C. Chang, D.H. Chen, Catalytic reduction of 4-nitrophenol by magnetically recoverable Au nanocatalyst, J. Hazard. Mater. 165 (2009) 664–669. [9] P.T. Huong, B.K. Lee, J. Kim, C.H. Lee, Nitrophenols removal from aqueous medium using Fe-nano mesoporous zolite, Mater. Design 101 (2016) 210–217. [10] W. Zhang, G. Wang, Z.Y. He, C.Y. Hou, Q.H. Zhang, H.Z. Wang, Y.G. Li, Ultralong ZnO/ Pt hierarchical structures for continuous-flow catalytic reactions, Mater. Design 109 (2016) 492–502. [11] K. Pirkanniemi, M. Sillanpää, Heterogeneous water phase catalysis as an environmental application: a review, Chemosphere 48 (2002) 1047–1060. [12] A.D. Bokare, W. Choi, Review of iron-free Fenton-like systems for activating H2O2 in advanced oxidation processes, J. Hazard. Mater. 275 (2014) 121–135. [13] H. Lim, J. Lee, S. Jin, J. Kim, J. Yoon, T. Hyeon, Highly active heterogeneous Fenton catalyst using iron oxide nanoparticles immobilized in alumina coated mesoporous silica, Chem. Commun. (2006) 463–465. [14] L.J. Xu, J.L. Wang, Magnetic nanoscaled Fe3O4/CeO2 composite as an efficient Fentonlike heterogeneous catalyst for degradation of 4-chlorophenol, Environ. Sci. Technol. 46 (2012) 10145–10153. [15] A. Dhakshinamoorthy, S. Navalon, M. Alvaro, H. Garcia, Metal nanoparticles as heterogeneous Fenton catalysts, ChemSusChem 5 (2012) 46–64. [16] Y.F. Han, N. Phonthammachai, K. Ramesh, Z. Zhong, T. White, Removing organic compounds from aqueous medium via wet peroxidation by gold catalysts, Environ. Sci. Technol. 42 (2008) 908–912. [17] A. Quintanilla, S. García-Rodríguez, C.M. Doínguez, S. Blasco, J.A. Casas, J.J. Rodriguez, Supported gold nanoparticle catalysts for wet peroxide oxidation, Appl. Catal., B 111–112 (2012) 81–89. [18] S. Navlon, R. Martin, M. Alvaro, H. Garcia, Gold on diamond nanoparticles as a highly efficient Fenton catalyst, Angew. Chem. Int. Ed. 122 (2010) 8581–8585. [19] X.J. Yang, P.F. Tian, C.X. Zhang, Y.Q. Deng, J. Xu, J.L. Gong, Y.F. Han, Au/carbon as Fenton-like catalysts for the oxidative degradation of bisphenol A, Appl. Catal. B 134–135 (2013) 145–152.

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