The photocatalytic properties of hollow (GaN)1-x(ZnO)x composite nanofibers synthesized by electrospinning

Accepted Manuscript Title: The photocatalytic properties of hollow (GaN)1−x (ZnO)x composite nanofibers synthesized by electrospinning Author: Ding Wang Minglu Zhang Huaijuan Zhuang Xu Chen Xianying Wang Xuejun Zheng Junhe Yang PII: DOI: Reference:

S0169-4332(16)32412-6 http://dx.doi.org/doi:10.1016/j.apsusc.2016.11.053 APSUSC 34357

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APSUSC

Received date: Revised date: Accepted date:

14-9-2016 7-11-2016 7-11-2016

Please cite this article as: Ding Wang, Minglu Zhang, Huaijuan Zhuang, Xu Chen, Xianying Wang, Xuejun Zheng, Junhe Yang, The photocatalytic properties of hollow (GaN)1-x(ZnO)x composite nanofibers synthesized by electrospinning, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.11.053 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 proof before it is published in its final 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.

The photocatalytic properties of hollow (GaN)1-x(ZnO)x composite nanofibers synthesized by electrospinning Ding Wanga, Minglu Zhanga, Huaijuan Zhuanga, Xu Chena, Xianying Wanga, Xuejun Zheng b,c

, Junhe Yanga

a

College of Materials Science and Engineering, University of Shanghai for Science & Technology,

Shanghai, 200093, People’s Republic of China. b

School of Mechanical Engineering, Xiangtan University, Xiangtan, 411105, People’s Republic of

China, E-mail: [email protected] c

Key

Laboratory

of

welding

robot

and

application

technology

of

Hunan

Province, Xiangtan University, Xiangtan, 411105, People’s Republic of China



Corresponding author. Tel: +86-021-55270588, Fax: +86-021-55270588 E-mail address: [email protected] (X.Y. Wang), [email protected] (X.J. Zheng)

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Graphical abstract

Highlights 1: (GaN)1-x(ZnO)x composite nanofibers were obtained by electrospinning method. 2: Phase transition from ZnGa2O4 to (GaN)1-x(ZnO)x under NH3 was observed. 3: Hollow and porous structure was confirmed by SEM and TEM investigation. 4: GaN:ZnO with optimal ratio of 1:2 displayed highest photocatalytic activity.

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Abstract (GaN)1-x(ZnO)x composite nanofibers with hollow structure were prepared by initial electrospinning, and the subsequent calcination and nitridation. The structure and morphology characteristics of samples were investigated by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM). The characterization results showed the phase transition from ZnGa2O4 to (GaN)1-x(ZnO)x solid-solution under ammonia atmosphere. The preparation conditions were explored and the optimum nitridation temperature and holding time are 750 C and 2 h, respectively. The photocatalytic properties of (GaN)1-x(ZnO)x with different Ga:Zn atomic ratios were investigated by degrading Rhodamine B under the visible light irradiation. The photocatalytic activity sequence is (GaN)1−x(ZnO)x (Ga:Zn=1:2)> (GaN)1−x(ZnO)x (Ga:Zn=1:3) >ZnO nanofibers>(GaN)1−x(ZnO)x (Ga:Zn=1:4)> (GaN)1−x(ZnO)x (Ga:Zn=1:1). The photocatalytic mechanism of the (GaN)1−x(ZnO)x hollow nanofibers was further studied by UV-vis diffuse reflectance spectra. The excellent photocatalytic performance of (GaN)1−x(ZnO)x hollow nanofibers was attributed to the narrow band gap and high surface area of porous nanofibers with hollow structure. Key words: Composite nanofibers; (GaN)1-x(ZnO)x; Electrospinning; Hollow and porous structure; Photocatalytic activity.

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1. Introduction As one of the most promising catalysts for the decomposition of organic pollutants, ZnO is of superior photocatalytic performance, nontoxicity, photoelectric chemical stability, and good conductivity [1, 2]. However, the wide band gap of ZnO leads to the low utilization of solar energy [3]. It only can absorb ultraviolet light because of its band gap energy (3.2 eV), accounting for only 4% of the total sunlight [4, 5]. In addition, the high recombination rate of the photogenerated electron-hole pairs limits its catalytic activity [6, 7]. Therefore, improving the charge separation efficiency and enhancing visible light utilization efficiency are urgently needed. GaN is a kind of wide bandgap semiconductors with the bandgap energy 3.4 eV at room temperature [8]. GaN has excellent optical properties, electrical properties, thermal stability and mechanical properties, which was widely used in light-emitting diodes (LEDs), laser diodes and photovoltaic electrode, and especially from over-all splitting of water under the irradiation of UV light [9, 10]. The band gap energy of GaN based materials can be regulated from 1.89 eV to 6.2 eV by formation solid solution with AlN or InN, which can emit light between corresponding wavelengths of 210 nm to 660 nm with continuous variation, and the band almost covers the whole of ultraviolet and visible light area [11]. Both ZnO and GaN are hexagonal wurtzite structure with similar forbidden band width and crystal lattice parameters, which is easy to form solid solution [12]. The (GaN)1-x(ZnO)x solid solution was first prepared by Maeda K, which can achieve over-all photocatalytic splitting of water after loading RhO2 and Cr [13]. Zou and co-workers has researched the photocatalytic degradation

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of four kinds of polycyclic aromatic hydrocarbons (PHE), anthracene (ANT), acenaphthene (ACE) and benzene anthracene (BaA) under visible light, and the experimental results show that (GaN)1-x(ZnO)x solid solution has excellent photocatalytic activity to them with the sequence of degrading efficiency is PHE > BaA > ANT > ACE [14]. The structure of materials is also a key factor affecting their photocatalytic performance. Recent years, various nanostructures have attracted considerable interest in the research of photocatalysts [15, 16]. Electrospinning is a simple and versatile method to fabricate nanofibers with diameters in the range of nanometers to a few micrometers [17]. The high surface-to-volume ratio, high porosity and other outstanding properties make the electrospinning nanofibers being widely used in sensors, catalysts and pollution control [17-20]. In view of smaller band gap, visible light response properties and high porosity, (GaN)1-x(ZnO)x hollow nanofibers have important research significance in photocatalytic degradation pollutants under visible light. Hollow (GaN)1-x(ZnO)x composite nanofibers were first synthesized by first electrospinning, and the subsequent calcination and nitridation. The microstructure, morphology, and composition of photocatalysts were investigated by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM). The photocatalytic activities of the as-prepared samples were estimated by the degradation of Rhodamine B (RhB) in aqueous solution under simulated visible light. The

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photocatalytic mechanism was explored based on UV-vis diffuse reflectance spectra and photoluminescence (PL) spectra. 2. Experimental 2.1. Preparation of (GaN)1−x(ZnO)x composite nanofibers All the chemicals were analytical grade from Aladdin Reagent (Shanghai) Co. Ltd. and used without further purification. In a typical procedure, an appropriate amount of gallium (III) nitrate hydrate (Ga(NO3)3·xH2O) and zinc acetate hydrate (Zn(CH3COO)2·2H2O) with the mole ratio of Ga to Zn=1:1, 1:2 and 1:3 and 1:4 were dissolved in the mixture of deionized water and ethanol. The mass ratio of salts, deionized water and ethanol is 1:4:4. And then, the polyvinylpyrrolidone (PVP, Mw=1,300,000 g·mol−1) was added under vigorous stirring to form a homogeneous and translucent precursor solution. The precursor solution was filled into a plastic syringe with a stainless steel nozzle which is positioned at a fixed distance with a metal cathode as collector, and then it was electrospun into composite nanofibers with a computer-controlled syringe pump and an applied voltage between the electrospinning nozzle and the collector was 18 kV. After electrostatic spinning, the Ga(NO3)3/Zn(CH3COO)2/PVP fibers were covered on the surface of aluminum foil. These fibers were placed into electric dry oven at 60 C for 12 h to gain the dried nanofiber membrane. The PVP template and metal salt were decomposed by annealing at 650 °C for 2 h in air atmosphere with a heating rate of 2 °C min-1. Then, nanofibers were nitrided at the temperature of 750 °C for 2 h with the heating rate of 2 °C min-1 in a flow of 200 sccm NH3, and (GaN)1-x(ZnO)x

6

composite nanofibers were finally synthesized. The GaN nanofiber was prepared by the same method as (GaN)1-x(ZnO)x with the absence of Zn(CH3COO)2·2H2O. ZnO naofiber was prepared by the same electrostatic spinning method with Zn(CH3COO)2·2H2O and PVP as raw material. The ZnO nanofiber was obtained by calcining the Zn(CH3COO)2/PVP nanofibers in air atmosphere. 2.2. Characterization The crystal structure of samples was determined by XRD (Bruker, D8 Advance, Germany) with Cu-Kα (λ= 0.15418 nm) radiation in the range of 20-80° at room temperature. The morphology and elementary composition were investigated via SEM image and EDS pattern obtained by FE-SEM (FEI, Quanta FEG 450, USA), and the porous and hollow microstructure were confirmed by TEM (JEOL, JEM 2100F, Japan). Thermogravimetric analysis (TGA) was performed under air flow from room temperature to 900 °C, with a heating rate of 10 °C/min, on a Pyris 1 TGA Thermogravimetric Analyzer. Absorption spectra and UV–vis diffuse reflectance spectra were measured using a UV/vis absorption spectrophotometer (Shimadzu Corporation, UV-vis-2550, Japan). The room temperature photoluminescence (PL) spectra were carried out by Raman Microscopy (Horiba, LabRAM HR Evolution, France) by using 325nm He-Cd laser as the excitation source. The composition of photocatalyst was investigatedby XPS (TMO, ESCALAB 250 Xi, England), and the position of the C1s peak (284.5 eV) was used to correct the XPS binding energies. The Brunauer−Emmett−Teller (BET) surface area was determined by N2 adsorption method using an automatic gas adsorption system (Quantachrome, Autosorb-1, Yuasa

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Co.). 2.3. Photocatalytic degradation measurement The photocatalytic activities of the as-prepared samples were confirmed by the degradation of RhB in aqueous solution under simulated sunlight. A high pressure mercury lamp was used as simulated sunlight source (CHF-XM35-500W, Beijing ChangtuoCo.) and the ultraviolet light was filtered out by filter. The photocatalysts (10 mg) were placed in a RhB aqueous solution (50 mL, 10 mg/L). This solution was magnetically stirred in the dark for 30 min as a control to measure the adsorption capacity of the sample to RhB. The photodegradation experiments were then performed under simulated visible light for 150 min. The solutions were sampled every 30 mined analyzed by recording the variations in the absorption band at 554 nm using a UV-vis 2550. The other photocatalysts were investigated under the same conditions for comparison. 3. Results and discussion 3.1. Preparation conditions It is necessary to confirm the thermal decomposition temperature of Ga(NO3)3/Zn(CH3COO)2/PVP nanofibers. Fig. S1 is the TGA curve of the sample with the Ga:Zn=1:1. A tiny mass loss was observed before 100 C due to the volatilization of adsorption water. The decomposition temperature of PVP and inorganic salts is between 250 C and 550 C. After 650 C, the TGA curve begins to steady and no mass loss, indicating that the organic compounds have been completely removed and the inorganic salts transform into oxides. Therefore, the precursor fiber

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composite was calcined at 650 C for 2 h to prepare Zn-Ga-O nanofibers. Nitridation temperature and nitridation time are two key parameters in the preparation of (GaN)1-x(ZnO)x composite nanofibers. The XRD patterns of Zn-Ga-O nanofibers (Ga:Zn=1:1) and (GaN)1-x(ZnO)x nitrided at 650 C, 750 C and 850 C are compared in Fig. 1. The diffraction peaks of Zn-Ga-O nanofibers can be indexed as the wurtzite ZnO (JCPDS 36-1451) and cubic phase ZnGa2O4 (JCPDS 38-1240) [21-23]. The phases of ZnO and ZnGa2O4 were remained at 650 C, which indicates that the nitridation temperature needs to increase. The diffraction peaks of the cubic phase ZnGa2O4 are disappeared when the nitridation temperatures are over 750 C. As shown in Fig. 1, the positions of the diffraction peaks are located between those of GaN and ZnO, indicating that the obtained samples were not physical mixtures of GaN and ZnO phases but rather solid solutions. The intensity of diffraction peaks increases with the nitridation temperature. In addition, the diffraction peaks are shift towards large angle with the increase of nitridation temperature, which indicates the decrease of zinc and oxygen in the samples because of the larger ionic radius of Zn2+ (0.74 A ° ) compared to Ga3+ (0.61 A ° ). The similar phenomenon is also reported by

Kazunari Domen.[23] The morphologies of Zn-Ga-O nanofibers (Ga:Zn=1:1) and (GaN)1-x(ZnO)x nitrided at 650 C, 750 C and 850 C were imaged by FE-SEM. As shown in the Fig. 2a, the abundant and homogeneous nanofibers with the smooth surface were observed for Zn-Ga-O composite nanofibers. After nitriding at 650 C, the morphology of sample is almost no change in Fig. 2b, which is consistent with the XRD characterization in Fig. 1. The increased roughness and decreased diameters

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after further increasing nitridation temperature were observed in the Fig. 2c and Fig. 2d, which attribute to H2O produced in the reaction from oxides to nitrides [24]. In addition, the ZnO can react with H2 coming from the thermal decomposition of NH3 used as a nitrogen source. [23-25] The metallic Zn obtained by reduction of ZnO will evaporate from the fibers and lead to the dropping of Zn content in the product (Fig. 1). The nanofiber was destroyed when the nitridation temperature is 850 C, which is too high to maintain the fiber structure. Therefore, the optimum nitridation temperature is 750 C. The nitridation time is also an important influencing factor. XRD patterns of samples after nitriding at 750 C for 2 h and 4 h were shown in Fig. 3. Both of XRD patterns indicate the formation of solid solution. The intensity of the diffraction peak increases with the extension of nitridation time. Furthermore, the diffraction peaks shift towards large angle, which indicates the reducing of the Zn content in the product. The morphologies of samples with different nitridation time were imaged by FE-SEM. As shown in Fig. 4, both samples are of the continuous nanofiber composed of nanoparticles. From the former, the fiber morphology keeps well and there is no obvious commissure between nanofibers. From the latter, the surface roughness of nanofibers is increased and part of the nanofibers is fractured, which is due to the reduction of ZnO in the nitridation and the evaporation of the produced metallic Zn [26]. Above all, in order to maintain the good nanofiber morphology with high crystallinity, 750 C for 2 h was chosen as the nitridation condition. XRD patterns of (GaN)1-x(ZnO)x with different Ga:Zn atomic ratios are

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compared in Fig. 5. All samples take on the characteristic peak of wurtzite structure due to the formation of solid solution. The low diffraction peak intensity of (GaN)1-x(ZnO)x (Ga:Zn=1:1) indicates the small crystallite size and low crystallinity. With the increase of Zn ratio, the diffraction peak intensity was initially increase and then decrease, which may be due to the nature of chemical bond, the distribution of the constituent elements and so on [23]. The diffraction peak of ZnO nanofiber is consistent with the hexagonal wurtize structure (JCPDS 36-1451), and the high and sharp diffraction peaks indicate the high purity and good crystallinity. According to the Debye-Scherrer formula, the average crystallite sizes of (GaN)1-x(ZnO)x solid solution first increase and then decrease which indicates that the excessive Zn inhibits the grain growth and leads to smaller grain size [22] FE-SEM images of (GaN)1-x(ZnO)x samples with different Ga:Zn atomic ratios were further performed, as depicted in Fig. 6. The diameters of (GaN)1-x(ZnO)x (Ga:Zn=1:1) nanofibers were between 160 ~ 250 nm. According to the Debye-Scherrer formula, the average crystallite size of the sample is about 18 nm. The diameter of nanofiber is almost no changes with the Ga:Zn atomic ratios. However, the average crystallite size of sample is affected by the ratio of the Ga to Zn (Table 1). The FE-SEM image of ZnO shows the smooth surface and uniform nanofiber. Fig. 6f shows the TEM image of (GaN)1-x(ZnO)x (Ga:Zn=1:1) nanofiber. A bright contrast (dark/bright) between the boundary and the center of the nanofiber confirms their hollow nature. The outer diameter of nanofiber is about 200 nm and the wall thickness is about 30 nm. In addition, the wall of hollow nanofiber is of porous

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structure, which can enhance the ability of pollutant molecules adsorption and helps to improve their photocatalytic properties. EDS patterns were used to confirm the elemental composition of the (GaN)1-x(ZnO)x with different Ga:Zn atomic ratios and they are given in Fig. S2. There are Ga, Zn, N and O elements in the EDS analysis, and it indicates that the (GaN)1-x(ZnO)x composite nanofibers were successfully prepared. Furthermore, the proportion of Zn after nitriding is lower than ratios of design, which confirmed volatilization losses of metallic Zn in the nitridation. XPS was used to study the surface composition and the surface chemical bonding of the photocatalysts. The XPS spectra of the Ga 2p2/3, Zn 2p2/3, O 1s and N 1s states for Zn-Ga-O and (GaN1−x)(ZnO) x are shown in Fig. 7. for comparison. The binding energy of Ga 2p3/2 after nitriding (1117.7 eV) is lowerer than the Zn-Ga-O values (1118.9 eV) attributable to the loss of oxygen species on the surface as seen in the O1s spectra. The Zn 2p3/2 peaks for the two samples appear at almost the same positions. The range of the binding energy covers 389.0-400.4 eV for the N 1s state. The abundant peaks of N1s indicate the chemical bond between N to Ga and Zn is complex. 3.2. Photocatalytic performance RhB, as an organic dye, is often used to evaluate the photocatalytic properties of catalysts. The photocatalytic activities of the (GaN)1−x(ZnO)x with different Ga:Zn atomic ratios and the pure ZnO nanofibers were evaluated by the photocatalytic degradation of RhB under simulated visible light. The time-dependent UV-vis spectra

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are respectively described as Figs. S3. Degradation E

efficiency

can

A –A C0 – C  100%  0  100% C0 A0

be

expressed

as

follows

[26]

(1)

where C0 and C are the concentration of RhB at reaction time 0 and t minutes, while A0 and A are the corresponding absorbance values. The relationship between degradation efficiency and degradation time can be calculated from Eq. (1), and the degradation efficiency vs degradation time curves are given in Fig. 8a. When the degradation reaction lasts 150 min, its degradation efficiency reaches 54.5%、97.8%、 84.9% and 78.4%. The composites with mass ratio of GaN:ZnO=1:2 show the highest photocatalytic activity. The photocatalytic activities of pure ZnO nanofinbers were also researched. When the degradation reaction lasts 150 min, the degradation efficiency of ZnO nanofiber reaches 83.1%. As shown in Fig. 8b, the characteristic absorption peaks at λ=554 nm decrease rapidly with the increasing irradiation time in the presence of photocatalysts and they almost disappeared completely after being irradiated for 150 min, indicating that most of RhB was degraded. The degradation kinetics of organic pollutants in water generally followed a Langmuir-Hinshelwood mode [27, 28] r

dC kKC  dt 1  KC

(2)

where K and k are the adsorption constant of RhB and the true rate constant which includes various parameters such as the mass of catalyst, the flux of efficient photons, the coverage in oxygen, etc. Because the initial concentration of pollutants is low (the

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term KC→0), the Eq. (2) can be reduced to the pseudo first-order kinetics equation [29, 30]

ln  C0 / C   kt

(3)

The ln (C0 /C) can be calculated by using the data plotted in Fig. 8a, so the photocatalytic reaction kinetic curves are given in Fig. 8b for the degradation of RhB in the presence of (GaN)1−x(ZnO)x composites photocatalyst. Here, the fitted line slopes

representing the photodegradation rate constants k of RhB are respectively

0.0053 min-1, 0.0259 min-1, 0.0125 min-1 and 0.0096 min-1 for (GaN)1−x(ZnO)x nanofibers with mass ratios of 1:1, 1:2, 1:3 and 1:4. It can be seen that the activity of (GaN)1−x(ZnO)x nanofibers first increased and then decreased with the increase of ZnO amount in the solid solution. The sequence of photodegradation rate constants are

(GaN)1−x(ZnO)x

(Ga:Zn=1:2)>

(GaN)1−x(ZnO)x

(Ga:Zn=1:3)>ZnO>(GaN)1−x(ZnO)x (Ga:Zn=1:4)> (GaN)1−x(ZnO)x (Ga:Zn=1:1). Obviously, the k value of (GaN)1−x(ZnO)x composite with the mass ratio of GaN:ZnO=1:2 is the highest among all the samples. In Fig. 8c, the recyclability can be proved by the recycling experiment with the used (GaN)1-x(ZnO)x hollow nanofibers as photocatalyst and fresh RhB solution as target pollutant. The slight decrease of activity after several runs might be due to the loss of catalysts during the transfer process. 3.3 Photocatalytic mechanism

The photocatalytic mechanism of (GaN)1−x(ZnO)x solid solution may be due to the following reasons:

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(i) Photoabsorption is one of the key factors affecting the photocatalytic performance. Photoabsorption properties of the photocatalysts were investigated by the UV–vis diffuse reflectance spectra. As shown in Fig. 9a, the ZnO nanofiber has almost no absorption at >400 nm, whereas the (GaN)1−x(ZnO)x composite nanofibers show an obvious absorption in the visible light region ranging from 400 to 600 nm, and the light absorption of (GaN)1−x(ZnO)x (Ga:Zn=1:2) nanofiber in the visible region is the strongest, which can help to absorb the full sunlight spectrum. When the content of Zn is lower, the band gap of (GaN)1−x(ZnO)x solid solution is larger, which needs more energy to generate electron-hole pair. When the Zn content in solid solution is larger, the excess Zn will produce more lattice defects, which become recombination center of electronic-hole pairs and decrease the photocatalytic activity. (ii) The photo-luminescence (PL) spectroscopy of pure ZnO nanofibers, Zn-Ga-O and (GaN) 1−x (ZnO) x were studied in comparison to reveal the migration of photo-generated electron–hole pairs. As shown in Fig. 9b, the pure ZnO nanofibers exhibit a strong emission peak at around ∼383 nm, which can be attributed to the band-band PL phenomenon. However, the intensity of Zn-Ga-O and (GaN) 1−x (ZnO) x emission peaks dropped significantly, suggesting that the recombination of photo induced charge carrier is inhibited greatly, which lead to a lower recombination rate of photo-generated electron–hole pairs. (iii) In the preparation of (GaN)1−x(ZnO)x solid solution, part of the ZnO was removed by the high nitridation temperature, which leads to fiber roughness increase. The nanofiber composes of a lot of aggregate pore and is formed by the smaller grain

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size of particles bonding between the particles. The porous makes the surface of nanofiber adsorbing more reactant and increasing the contact area to photocatalysts. Moreover, the smaller grain size can effectively reduce the recombination of electronic-hole

pair,

and

improves

the

photocatalytic

activity

[31].

N2

adsorption-desorption isotherms and pore size distribution of ZnO nanofibers, Zn-Ga-O and (GaN)1−x(ZnO)x were measured to investigate the specific surface area and porous structure, and the results are presented in Figs. 9c and 9d. The N2 adsorption−desorption isotherms of all samples belonged to type IV adsorption isotherms with clear hysteresis loops at high relative pressures, indicating the presence of mesopores. The ZnO nanofibers showed a low N2 uptake at the medium relative pressure (P/P0) yet a dramatically predominant adsorption at higher P/P0, which is usually associated with multilayer adsorption on external surfaces and in larger mesopore or macropore. The hysteresis loop at the medium relative pressure range (P/P0 = 0.4−0.8) was associated with capillary condensation in small mesopores (3−4 nm), which was consistent with the pore size distribution in Fig. 9b. As shown in Fig. 9b, the porous of ZnO nanofibers is not obvious. However, the narrow and strong distribution at 4.2 nm was observed clearly for Zn-Ga-O. There are two types of pores for (GaN)1−x(ZnO) x. The larger porous in the 19 nm is due to the loss of metallic Zn in the nitridation. The specific surface areas of ZnO nanofibers, Zn-Ga-O and (GaN)1− x(ZnO)x are

9.07 m2/g, 27.26m2/g and 31.52m2/g, respectively. The high surface area

and large porous diameter is beneficial for enhancing the photocatalytic performance. (iv) The energy level diagram of (GaN)1−x(ZnO)x solid solution was illustrated

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in Fig. 10. The conduction band of GaN is composed of Ga 4s and Ga 4p orbitals, and the valence band of GaN is N 2p orbital. After the formation of solid solution, the atoms Zn and O doping in the crystal lattice of GaN. The orbitals of Zn 3d and O 2p take part in the formation of the valence band of solid solution, and the Zn 3d band lies close to lower part of valence band consisting of hybridized N 2p and O 2p bands [32]. The strong p-d repulsion was formation between Zn 3d, O 2p orbitals and N 2p orbital, which introduces an additional band gap and decrease the band gap. The narrow band gap improves the utilization of sun light and the generation rate of photon-generated carrier, which can be proved by the Fig. 9. When the photocatalysts are illuminated by visible light with photon energy higher than the band gaps of (GaN)1−x(ZnO)x solid solution, electrons in the VB are excited to the CB with simultaneous generation of the same amount of holes in the VB. The photogenerated electrons and holes are free to participate in the chemical reaction, and they can be expressed as follows [33, 34]: O2 + e- →•O2-

(4)

H2O + h+ → H+ + •OH

(5)

•O2- + H+ → •HO2

(6)

•HO2 + e- + H+→H2O2

(7)

H2O2 + e- → •OH + OH-

(8)

H2O2 + •O2- → •OH + OH- + O2

(9)

•OH + RhB → intermediates → CO2 + H2O

(10)

The electronic acceptors such as adsorbed O2 can easily trap the photogenerated

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e- to produce superoxide anion radical (•O2-), and the holes can be captured by adsorbed surface H2O to form hydroxyl radicals (•OH). Then the formed •O2- reacted with e- and H+ to produce H2O2, which would further provide hydroxyl radical (•OH) by reaction with e- and •O2- (Eqs. (4) - (9)) [35, 36]. The hydroxyl radical (•OH) presents extremely strong oxidizing properties for the partial or complete mineralization of organic chemicals (Eq. (10)). The above charge transfer process suggests that the photogenerated electrons and holes are efficiently separated in the composites, thus increases the lifetime of the charge carriers [36] and decrease the recombination rate of hole-pairs in the composites to realize high photocatalytic activity. 4. Conclusion In summary, the (GaN)1−x(ZnO)x nanofibers with hollow structure were prepared by electrospinning, and the subsequent was calcined and ammoniatied. The grain size, the crosslinked feature, the hollow structure and the composition information of samples are confirmed by XRD, FE-SEM, TEM and EDS. The characterization results showed the phase transition from ZnGa2O4 to (GaN)1-x(ZnO)x solid-solution under ammonia atmosphere. The optimum operating nitridation temperature and nitridation time are 750 C and 2 h, respectively. The (GaN)1−x(ZnO)x composite nanofiber with the mass ratio of GaN:ZnO=1:2 is the highest photodegradation rate constants k = 0.0259 min-1 under the irradiation of visible light. The improved photocatalytic performance of (GaN)1−x(ZnO)x solid solution was attributed to the narrow band gap and high surface area of hollow nanofibers.

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Acknowledgments This work was supported by Changjiang Scholar Incentive Program ([2009]17), PCSIRT (IRT_14R48), Shanghai Municipal Science and Technology Commission (16060502300), NNSF of China (51272158, 51402193, 51572173) and the State Key Laboratory of Heavy Oil Processing (SKLHOP201503).

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Biographies Ding Wang received his PhD degree in the field of nano-functional materials in 2013 from Toyama University in Japan. Now, he was a teacher of University of Shanghai for Science & Technology. His research interests focused on sensing materials and catalysis. Minglu Zhang received her BS degree in Materials Physics in 2015 from Xinjiang University. he has been pursuing her MS degree in materials engineering in University of Shanghai for Science & Technology since 2015. Huaijuan Zhuang received her BS degree in environment engineering in 2013 from Anhui Normal University. She has been pursuing her MS degree in environmental science in University of Shanghai for Science & Technology since 2013. Xu Chen received her BS degree in Safety Engineering in 2016 from Liaocheng University. He has been pursuing her MS degree in Materials Engineering in University of Shanghai for Science & Technology since 2016. Xianying Wang received her Ph.D. degree of Materials Science from Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences in 2005. She is now a professor of School of materials science and engineering, University of Shanghai for Science & Technology. Her research interests focused on semiconducting nanomaterials and related nanodevices. Xuejun Zheng received his MS degree in major of structure mechanics in 1989, and PhD degree in the field of fundamental mechanics in 2002 from Xiangtan University. He was appointed a full professor in Faculty of Materials Optoelectronics and Physics,

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Xiangtan University in 2003. Now, he is interested in the field of sensing functional materials, gas sensors and humidity sensors.

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Figure captions: Fig. 1 XRD patterns of Zn-Ga-O and (GaN)1-x(ZnO)x composite fibers (Ga:Zn=1:1) nitridation at 650 C, 750 C and 850 C. Fig. 2 FE-SEM images of of Zn-Ga-O and (GaN)1-x(ZnO)x composite fibers (Ga:Zn=1:1) nitridation at 650 C (b), 750 C (c) and 850 C (d). Fig. 3 XRD patterns of (GaN)1-x(ZnO)x composite fibers (Ga:Zn=1:1) with different nitridation time. Fig. 4 FE-SEM images of (GaN)1-x(ZnO)x composite fibers (Ga:Zn=1:1) with different nitridation time (a) 2h, (b) 4h. Fig. 5 XRD patterns of (GaN)1-x(ZnO)x composite fibers with different Ga:Zn atomic ratios. Fig. 6 FE-SEM images of (GaN)1-x(ZnO)x composite fibers with different Ga:Zn atomic ratios, (a) Ga:Zn=1:1 (b) Ga:Zn=1:2 (c) Ga:Zn=1:3 (d) Ga:Zn=1:4 (e) ZnO nanofibers (f) TEM image of (GaN)1-x(ZnO)x (Ga:Zn=1:1). Fig. 7 The XPS spectra of the Zn-Ga-O and (GaN)1-x(ZnO)x. (a) Ga 2p2/3 XPS spectrum. (b) Zn 2p2/3XPS spectrum. (c) O 1s XPS spectrum. (d) N 1s XPS spectrum. Fig. 8 The photocatalytic performance of photocatalysts (a) the ln(C/C0) vs. time curve of RhB under photocatalysts, (b) the apparent rate constantsof RhB photodegradation in the presence of photocatalysts under visible light, (c) Cycling runs in the photocatalytic degradation of RhB in the presence of (GaN)1−x(ZnO)x under simulated sunlight light irradiation

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Fig. 9 (a) UV-vis diffuse reflectance spectra of ZnO nanofibers and (GaN)1−x(ZnO)x composite fibers with different Ga:Zn atomic ratios, (b) PL spectra, (c) N2 adsorption-desorption isotherms and (d) pore size distribution of ZnO nanofibers, Zn-Ga-O and (GaN)1−x(ZnO)x. Fig. 10 Schematic illustration of the energy level diagram of (GaN)1−x(ZnO)x composite nanofiberss.

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Fig. 1

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Fig. 2

31

Fig. 3

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Fig. 4

33

Fig. 5

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Fig. 6

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Fig. 7

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Fig. 8

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Fig. 9

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Fig. 10

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Table 1. The average crystallite size of (GaN)1 − x(ZnO)x composite fibers with different Ga:Zn atomic ratios and the pure ZnO nanofibers. Ga:Zn

1:1

1:2

1:3

1:4

Pure ZnO

Average crystallite size (nm)

18

21

20

19

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The average crystallite size was calculated with the Scherrer equation.

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