J Mater Sci: Mater Electron (2017) 28:1869–1876 DOI 10.1007/s10854-016-5738-0

Enhanced photocatalytic activity and photoelectrochemical performance of InOOH nanosheets prepared via a facile solvothermal route Shuang Yang1,2 • Cheng-Yan Xu1,2 • Bao-You Zhang1,2 • Sheng-Peng Hu1,2,3 Jing Yu1,2 • Liang Zhen1,2

•

Received: 18 May 2016 / Accepted: 26 September 2016 / Published online: 1 October 2016 Ó Springer Science+Business Media New York 2016

Abstract Two-dimensional materials, such graphene and transition metal dichalcogenides with layered structures, and other non-layered materials, have found broad applications in optoelectronics, energy storage and photocatalysis, utilizing the unique properties arising from the twodimensional characteristics. In this work, we report the solvothermal synthesis of uniform InOOH nanosheets with average length of c.a. 1.5 lm, width of c.a. 500 nm, and thickness of c.a. 60 nm. The obtained InOOH nanosheets are characterized by X-ray diffraction, scanning electron microscope and transmission electron microscope. The possible growth mechanism of the InOOH nanosheets was discussed. Photocatalytic and photoelectrochemical experiments indicated that the InOOH nanosheets present enhanced photocatalytic activity for the degradation of Rhodamine B under UV light irradiation, which can be ascribed to its high BET surface area as well as enhanced electron–hole separation originated from the two-dimensional morphological characteristics.

Electronic supplementary material The online version of this article (doi:10.1007/s10854-016-5738-0) contains supplementary material, which is available to authorized users. & Cheng-Yan Xu [email protected] & Liang Zhen [email protected] 1

School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China

2

MOE Key Laboratory of Micro-System and Micro-structures Manufacturing, Harbin Institute of Technology, Harbin 150080, China

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School of Materials Science and Engineering, Harbin Institute of Technology at Weihai, Weihai 264209, China

1 Introduction Since the isolation of graphene, two-dimensional (2D) materials have attracted intense interest over the past ten years [1, 2]. Because of their large surface areas and quantum confinement effects within nanometer thick sheets [3, 4], 2D materials have a wide range of applications, including catalysts [5–7], optoelectronics [8, 9], biological sensors [10], supercapacitors [11–13], solar cells [14–16], and lithium ion batteries [17, 18]. Among these applications, photocatalysis is a kind of surface reaction, the performance of which highly depends on the surface structures of photocatalysts such as specific surface area, porosity, defects, type of exposed crystal facets, and surface atomic structure [19, 20]. Therefore, 2D nanomaterials become attractive photocatalysts, which usually expose their high-reactivity facets. Han and co-workers [21] report a facile hydrothermal route for synthesizing sheet-like anatase TiO2 with the highly reactive (001) facets, which exhibit excellent activity in the photocatalytic degradation of methyl orange. Ye and Xi [22] demonstrated that the exposed {001} facets of the m-BiVO4 nanoplates lead to a remarkable enhancement of visible-light photocatalytic activity. Hu’s group [23] prepared the ultrathin SnS2 nanosheets with exposed {001} facets via a solvothermal process with the assistance of L-glutatione. These ultrathin SnS2 nanosheets show the superior photocatalytic activity for degradation of methyl orange under visible light irradiation. Thus researchers have sought a way to improve the performance of photocatalysis by preparing the substances with two-dimensional nanostructures. As an n-type semiconductor with a wide band gap of 3.6 eV, indium oxyhydroxide (InOOH) has recently emerged as attractive photocatalysts for the degradation of

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different organic pollutants under UV irradiation [24–26]. Hydrothermal and solvothermal methods, hydrolysis reaction, and template-assisted approaches have been utilized to synthesize InOOH micro/nanostructures with various morphologies [25–33], including hexagram, nanoparticles, nanofibers, nanowires, nanorods, nanotubes, microspheres, urchin-like structures and hollow nanostructures. To the best of our knowledge, there has been no report related to the synthesis of InOOH nanosheets as attractive photocatalysts. Herein, we report a simple, one-step solvothermal procedure to prepare InOOH nanosheets. By simply adjusting the ratio between ethylene glycol and water, it was found that the products could be systematically tailored, yielding different morphologies like InOOH nanosheets and microflowers. The photocatalytic and photoelectrochemical properties of InOOH nanosheets were evaluated under UV illumination. The superior photocatalytic activity of InOOH nanosheets can be ascribed to the sheet-like nanostructures which have larger specific surface area and could enhance the separation and affect the inhibition of recombination of excited electrons and holes.

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spectrum of the samples was analyzed by using UV–vis spectrometer (Shimadzu UV-2550). 2.3 Photocatalytic performance evaluation The photocatalytic activity of InOOH nanosheets and microflowers was evaluated by recording the degradation of RhB in aqueous solution under UV light irradiation. The photocatalytic experiments were conducted as follows: 10 mg of catalyst were immersed in Rhodamine B (RhB) solution (1.0 9 10-5 M, 40 mL) and then magnetically stirred in dark for 30 min to reach adsorption equilibrium and uniform dispersibility. The solution was then exposed to UV light irradiation from a 10 W Hg lamp (UV-TEC, GPH212T5L) for up to 50 min. During the irradiation, the suspension was magnetically stirred. The temperature of the photocatalytic reaction system was maintained at room temperature by a flow of water. At given time intervals, 3 mL of aliquots were sampled and centrifuged immediately to remove the photocatalyst particles. The dye concentration in the supernatant solution was analyzed by measuring the absorption intensity of RhB at 554 nm. 2.4 Photoelectrochemical measurements

2 Experimental section 2.1 Synthesis All the reagents are of analytical grade and are used as received without further purification. In a typical synthesis, 0.30 mmol of In(NO3)34.5H2O was dissolved into the mixture of 10.0 mL of ethylene glycol (EG) and 5.0 mL of distilled water, which was then transferred to a 20 mL Teflon-lined autoclave and heated at 180 °C for 12 h in an oven. After the completion of reaction, the autoclave was allowed to cool to room temperature naturally. The white products were collected by centrifugation, washed with distilled water and absolute ethanol for several times, and dried at 60 °C in ambient atmosphere.

The photoelectrochemical (PEC) performance of InOOH nanomaterials was carried out in a conventional threeelectrode system consisting of a working electrode, a Pt wire counter electrode, and a saturated calomel electrode (SCE) performed using an electrochemical workstation (CHI 660E, Chenhua, China). The work electrode was prepared by depositing 1 mL catalyst solution (3 mg mL-1) on 1 cm 9 1 cm indium tin oxide (ITO)coated glass. Then, the working electrode was dried at 60 °C for 1 h to volatilize the solvent and steady the electrode materials. The electrolyte was comprised of 0.5 M Na2SO4 aqueous solution (pH = 6). A 300 W Xe lamp (CEL-HXF 300, Beijing Au-light, China) with UVREF filter was employed as incident light source to study PEC response of the samples.

2.2 Characterization

3 Results and discussion The phases were examined by means of X-ray diffraction (XRD) with Rigaku D/max 2500 diffractometer using Cu Ka radiation (k = 0.15418 nm). The morphology and microstructure of the products were characterized by a field-emission scanning electron microscope (FE-SEM, FEI Quanta 200F), a transmission electron microscope and a high magnification transmission electron microscope (TEM and HRTEM, JEOL JEM 2100). Nitrogen adsorption–desorption isotherms were performed on a Micrometrics ASAP 2020. The UV–vis diffusion reflectance

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The crystal structure and phase of products were examined by XRD. As shown in Fig. 1, all the diffraction peaks can be indexed to orthorhombic InOOH (JCPDS No. 71-2283; space group P21 nm) with lattice constants of a = 0.526 nm, b = 0.456 nm, and c = 0.327 nm. XRD peaks for other impurities such as In2O3 or In(OH)3 are not detected, indicating the high purity of the solvothermal synthesized products. Compared with the standard diffraction pattern, XRD pattern of InOOH nanosheets

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Fig. 1 XRD pattern of as-synthesized products. The lower panel shows the standard diffraction pattern of orthorhombic InOOH (JCPDS No. 71-2283)

show no distinct crystallographic orientation, which is probably due to the multi-crystal nature of nanosheets (see below TEM results). The morphology of as-synthesized InOOH nanosheets was first examined by FE-SEM. Figure 2a shows a typical low-magnification SEM image of the products, demonstrating typical sheet-like characteristics of InOOH with average length of c.a. 1.5 lm, and width of c.a. 500 nm. High-magnification SEM image shown in the inset of Fig. 2a presents the side-view of a nanosheet with thickness of about c.a. 60 nm. In addition, it could find that

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these nanosheets prefer to lie on the substrate due to 2D characteristics, which would be beneficial for enhanced photoelectrochemical performance. Figure 2b shows a typical TEM image of InOOH nanosheets, further demonstrating that the obtained products are typical sheetlike structure. The corresponding selected-area electron diffraction (SAED) pattern is presented in Fig. 2c, showing discrete diffraction rings. These diffraction rings can be indexed to the (010), (101), (001), (020), (120), and (211) planes of orthorhombic InOOH. This suggests that the single sheet is comprised by multi crystals. HRTEM image (Fig. 2d) of the nanosheet (see the black rectangle in Fig. 2b) shows three sets of clear lattice fringes with interplanar distances of 0.265, 0.277 and 0.456 nm, corresponding to (011), (010), and (101) planes of InOOH, respectively. These clear lattice fringes in the HRTEM image confirm the good crystallinity of InOOH nanosheets. In this work, the effect of EG/H2O volume ratio on the formation of InOOH nanosheets was systematically investigated with other reaction conditions kept the same as typical experiment. Figure 3 presented SEM images of the obtained products with different volume ratios of EG/H2O. When only H2O was used as solvent, the irregular cubes with size from several tens nanometers to micrometers were obtained (Fig. 3a). As seen from XRD pattern in Fig. 4, the irregular cubes were pure In(OH)3, which was formed by the hydrolysis of In3? in aqueous solution [33].

Fig. 2 Morphology and structure characterization of InOOH nanosheets: a SEM image; b TEM image; c SAED pattern; d HRTEM image. Inset in (a) is the side-view of an indivual InOOH nanosheet

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Fig. 3 SEM images of the samples prepared at 180 °C for 12 h with different volume ratios of EG/H2O: a pure water; b 1:4; c 1:2; d 1:1

Fig. 4 XRD patterns of the samples prepared at 180 °C for 12 h with different volume ratios of EG/H2O

When less amount of EG was used as solvent (e.g., EG/ H2O of 1:4), the morphology and phase of the products remain unchanged (Figs. 3b, 4). As higher EG:H2O volume ratio of 1:2, the products consists of cubes and microflowers (Fig. 3c). XRD pattern in Fig. 4 showed that the products were mainly In(OH)3, and the diffraction peaks from InOOH phase can be also observed. Based on the previous reports, the cubes was likely to be In(OH)3 [34]. When the EG/H2O was 1:1, the obtained products were microflowers with average diameter of about 1.5 lm, which are composed of nanosheets (Fig. 3d). The phase of

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products was InOOH as evidenced by XRD analysis (Fig. 4). When the EG/H2O was 2:1, the InOOH nanosheets with average length of c.a. 1.5 lm, width of c.a. 500 nm, and thickness of c.a. 60 nm were synthesized (Fig. 2a, b). Based on the above experimental results, it is evident that ethylene glycol is a crucial factor in the control of phase and morphology of InOOH nanosheets. The effects of ethylene glycol on the products will be discussed in following section. In the solvothermal reaction, In(NO3)3 reacted with H2O to form In(OH)3, and then In(OH)3 decomposed to form InOOH in water scarcity conditions or at relatively higher reaction temperatures [33]. Large amount of EG could change the viscosity of solution due to the coordination habit, which will decrease the reaction rate, and then yield products with uniform size [35]. In contrast, the reaction with pure H2O as solvent is rather fast, which will increase the growth rate of products [36]. It is hard to obtain the uniform products in water adequate conditions. As previously reported, ethylene glycol underwent several steps of intermediate reactions to form longer chains at relatively higher reaction temperature (170–195 °C) [37]. In the present work, In3? could combine with EG, leading to the formation of longer chains [37]. The long chains could further self-assembled into nanosheets through van der Waals interactions [38]. Based on the above discussion,

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only when larger volume proportion of ethylene glycol is used as solvent, InOOH nanosheets can be obtained. To examine the physico-chemical adsorption properties of InOOH nanosheets, the Brunauer–Emmett–Teller (BET) surface area of as-prepared InOOH nanosheets was determined using N2 adsorption–desorption method. The specific surface area of InOOH nanosheets was determined to be 46.99 m2 g-1. (Fig. 5) It is generally accepted the BET surface area of catalyst has significant influence on the photocatalytic process. In order to compare the photocatalytic performance of InOOH nanosheets, we also examined the BET surface area of the other photocatalysts. As shown in Fig. 5, the specific surface areas of the hexagram shaped InOOH nanostructures, InOOH microflowers and P25-TiO2 were measured to be 40.34, 35.98 and 58.01 m2 g-1, respectively. (In order to compare the photocatalytic activities, the hexagram shaped InOOH nanostructures were synthesized according to the literature [25]. (Fig. S1)). Before photocatalytic evaluation, the optical absorptions of InOOH nanosheets and microflowers were measured by UV–vis diffuse reflectance spectrometer. As shown in Fig. 6a, the InOOH nanosheets and microflowers both show strong absorption in the ultraviolet regions. The optical absorption near the band edge follows the equation [39]:
ðahvÞn ¼ A hv  Eg where hv is the photo energy, a is the absorption coefficient, A is a constant, and n is 1/2 for an indirect semiconductor and 2 for a direct semiconductor. The optical indirect band gap in absorption can be calculated by the functions of (ahv)1/2-hv. As shown in Fig. 6b, by the plots of absorbance versus energy for InOOH nanosheets and microflowers, the band gap both were determined to be 3.57 eV, which show no difference in band gap due to morphology change. The band gap values of InOOH

Fig. 5 N2 adsorption–desorption curves of different photocatalysts: a InOOH microflowers; b InOOH nanosheets; c hexagram shaped InOOH nanostructures; d P25-TiO2

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nanosheets and microflowers are in consistent with that of bulk materials. As a widely used organic dye, RhB was selected as a model pollutant to evaluate the photocatalytic efficiency of InOOH nanosheets and microflowers. Figure 7a shows the temporal UV–vis spectra of RhB solutions irradiated in the presence of InOOH nanosheets by UV light irradiation for different time. The major absorbance of RhB (k = 554 nm) decreased significantly with prolonged irradiation time, which shows excellent photocatalytic properties on RhB. The digital photographs (inset of Fig. 7a) show the color change of RhB solution during the dark experiment and photodegradation process. During the dark experiment, the color of RhB solution hardly changed. While during photodegradation process, the pink color of RhB solution gradually fades with increasingly longer irradiation time. These results indicated that the dyes obviously have been degraded into small molecular in the presence of InOOH nanosheets under UV light irradiation. Figure 7b displays the change of RhB concentration (C/ C0) as a function of time over the process of photocatalytic degradation. As can be seen in Fig. 7b, for dark experiment, RhB degradation is relatively slow in 30 min. Blank tests exhibit little degradation under UV light irradiation, which could be almost ignored. When InOOH nanosheets were used as the photocatalysts, the concentration of RhB decreased to zero after irradiated for 50 min, which is very close to the photocatalytic performance of commercial TiO2 (P25) (Fig. 7b). In the presence of the hexagram shaped InOOH nanostructures and InOOH microflowers under the same experimental conditions, 29.9 and 90.3 % of RhB were degraded within 50 min under UV light irradiation. The photocatalytic activity of hexagram shaped InOOH nanostructures was the worst in three kinds of InOOH catalysts because hexagram shaped InOOH nanostructures were prepared under the strong acid condition. Hydrogen ions adsorbs on the surface of catalysts, which might reduce active hydroxyl radicals to participate in the photocatalytic reaction. Moreover, the photocatalytic reactivity of InOOH nanosheets was higher than that of InOOH microflowers with low specific surface (see Fig. 5). Since photocatalysis is a kind of surface reaction, the surface structures of photocatalysts such as surface area, defects, porosity, the type of exposed planes, and the surface atomic structure will influence the RhB photodegradation efficiency [19, 20]. The fittings of ln(C0/C) plot versus time over the different photocatalysts are shown in Fig. 8a. The photodegradation of RhB catalyzed by four kinds of photocatalysts fits pseudo first-order reaction, i.e. ln(C0/ C) = kt, where C0 and C are the initial and actual concentration of RhB, respectively, k is the apparent rate constant of the degradation [25]. In our experiment, the

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Fig. 6 a UV–vis diffuse reflectance spectra of InOOH naosheets and microflowers. b The plots of (ahv)1/2 versus hv for the band gap determination of InOOH naosheets and microflowers

Fig. 7 a UV–vis absorbance spectra depicting the change of RhB concentration as a function of time in the presence of InOOH nanosheets under UV light irradiation. Inset is digital photographs of RhB solution showing the color change during the dark experiment

and photodegradation process. b Photocatalytic degradation curves of RhB solution with blank test and different photocatalysts under UV light irradiation: a blank test; b hexagram shaped InOOH nanostructures; c InOOH microflowers; d InOOH nanosheets; e P25-TiO2

Fig. 8 a The fitting of ln(C0/C) plot vs. time over different photocatalysts: a hexagram shaped InOOH nanostructures; b InOOH microflowers; c InOOH nanosheets; d P25-TiO2. b The

apparent rate constant of the degradation per unit surface area of different photocatalysts: a hexagram shaped InOOH nanostructures; b InOOH microflowers; c InOOH nanosheets; d P25-TiO2

measured k values of the hexagram shaped InOOH nanostructures, InOOH microflowers, InOOH nanosheets and P25-TiO2 were 0.0066, 0.0378, 0.0516 and 0.0931 min-1, respectively (Fig. 8a). In current work, as for the basis for the comparison, the apparent rate constant of the degradation per unit surface area of the three kinds of InOOH catalysts and P25-TiO2 is examined (Fig. 8b). As shown in Fig. 8b, the normalized K value of InOOH

nanosheets is higher than that of the hexagram shaped InOOH nanostructures and InOOH microflowers, which is lower than that of P25-TiO2. The above results further demonstrate that InOOH nanosheets possess the optimal photocatalytic performance of the three kinds of InOOH catalysts. The photoelectrochemical property of InOOH nanosheets and microflowers were measured by monitoring the

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nanosheets and the facile preparation method, it is believed that these InOOH nanosheets may have potential application in the field of photocatalysis. Acknowledgments C.Y. Xu. acknowledges the Fundamental Research Funds for the Central Universities (HIT.BRETIII.201203).

References

Fig. 9 Photoelectrochemical response of InOOH naosheets and microflowers

photocurrent-time (I-t) curve. As shown in Fig. 9, the photocurrent of InOOH nanosheets and microflowers increased under UV light irradiation. The InOOH nanosheets deliver a photocurrent density of 1.12 lA cm-2 under UV light irradiation. More importantly, after repeated on/ off switching, only slight changes have taken place in the photocurrent response. It clearly reveals the good photostability of the as-obtained InOOH nanosheets for up to ten cycles. The photocurrent density of InOOH microflowers was 0.78 lA cm-2, lower than that of InOOH nanosheets. The above experimental results indicate that the different morphologies of samples impacts on their photoelectrochemical properties. As previous reported, photoelectrochemical response behavior could be attributed to the synergistic effect of macroscopic morphological characteristic and microscopic electronic structure [40]. 2D sheetlike morphology possess the higher surface areas, which could enable to harvest signally increased UV light and help electron–hole pairs to transfer faster, and then reduce the recombination rate of photogenerated electrons and holes. Hence, the difference of PEC performance could further explain why InOOH nanosheets possess better photocatalytic performance than InOOH microflowers.

4 Conclusions In summary, InOOH nanosheets have been synthesized via a facile solvothermal route. The results show that the volume ratio of ethylene glycol to distilled water plays important roles in the formation of InOOH nanosheets. The photocatalytic and photoelectrochemical properties of InOOH nanosheets and microflowers were evaluated under UV light irradiation. InOOH nanosheets could complete degrade RhB solution (1 9 10-5 M) within 50 min, which could be ascribed to their unique sheet-like structure and a relative large specific surface area (46.99 m2 g-1). Considering the photocatalytic performance of InOOH

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