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PABA-assisted hydrothermal fabrication of W18O49 nanowire networks and its transition to WO3 for photocatalytic degradation of methylene blue
Advanced Powder Technology xxx (2018) xxx–xxx
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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
PABA-assisted hydrothermal fabrication of W18O49 nanowire networks and its transition to WO3 for photocatalytic degradation of methylene blue Hang Chen a, Wanlin Cai b, Xiaoqing Gao a, Jianmin Luo a, Xintai Su a,⇑ a b
Ministry Key Laboratory of Oil and Gas Fine Chemicals, College of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, China College of Science, Xinjiang Institute of Education, Urumqi 830000, China
a r t i c l e
i n f o
Article history: Received 12 April 2017 Received in revised form 30 January 2018 Accepted 16 February 2018 Available online xxxx Keywords: Tungsten oxide p-Aminobenzoic acid Hydrothermal method Photocatalytic properties Hydrophilic property
a b s t r a c t W18O49 nanowire networks have been fabricated by a facile hydrothermal method. In this method, p-aminobenzoic acid (PABA) was used as an assistant agent to control the morphology transformation. W18O49 and its products annealed at different temperature were characterized by XRD, SEM, TEM, UV– vis absorption spectroscopy, XPS, TGA, and FTIR. Formation mechanism and thermal stability of W18O49 nanowire networks were studied in detail. The experiment data showed that PABA played an important role in the induced crystal growth of W18O49 nanowires along [0 1 0] axis. In transformation, the structure of samples was controlled: from irregular particles to nanowire networks. W18O49 nanowire networks were annealed at different temperature. The nanowire networks collapsed at 450 °C, while WO3 nanocrystals were obtained. The W18O49 nanowire networks annealed at 400 °C have a superior photocatalytic performance to degrade methylene blue and its specific surface area was up to 147 m2 g 1. Ó 2018 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Nowadays, nanomaterials have achieved a widespread attention due to their enhanced physical and chemical properties [1–5]. Among all kind of nanomaterials, one dimension nanostructure has attracted extensive interests due to the unique electronic and optical properties [6–8]. Tungsten oxide nanowires and nanorods, as one of the crucial transition metal oxide semiconductors, have a wide applications in electrochromic devices, gas sensors, optical devices, hydrogen reduction, and photocatalysts [9–15]. Specially, tungsten oxide is well-known for its nonstoichiometric properties, as the lattice can withstand a considerable amount of oxygen vacancy [12,16,17]. In particular, monoclinic W18O49 is of special interest for its unique defect structure and distinct physical and chemical properties in the nanometer regime [4,18–20]. Hitherto, W18O49 based materials have exhibited excellent performance in various fields, such as catalysis [21], gas sensing property [22], optical property [23], magnetic and near-infrared adsorption properties [24,25]. Recent research shows that the reductivity of W18O49 can realize the direct growth of metal particles on metal oxides in situ [26]. Furthermore, the acidic surface of ⇑ Corresponding author. E-mail address: [email protected] (X. Su).
the W18O49 porous nanomaterials enhances their adsorption on basic dyes [27,28]. Guangcheng Xi and co-workers have reported the preparation of ultrathin W18O49 nanowires with diameters below 1 nm that are efficient in the photochemical reduction of carbon dioxide by visible light [4]. Y.M. Zhao and Y.Q. Zhu have researched the room temperature ammonia sensing properties of ultra-thin W18O49 nanowires with diameter less than 5 nm, and the nanowire was prepared by a solvothermal technique [18]. Chongshen Guo et al. have investigated the morphologycontrolled synthesis and near-infrared (NIR) absorption properties of W18O49 which applied to innovative energy-saving windows [25]. The general method to fabricate W18O49 nanomaterials is alcoholysis of WCl6, W(CH3CO)6, or W(CO)6 [18,26,29]. However, the reagents are expensive and unstable, which are disadvantageous for practical applications. Therefore, it is highly desirable and challenging to develop a facile and economical method to produce W18O49 with porous nanostructures for their large scale applications in water treatment and others. Once, we have reported a communication about the hydrothermal fabrication of W18O49 nanowire networks with Na2WO42H2O as tungsten resource [30]. In that work, tungsten acid was used as precursor while p-amino-benzoic acid (PABA) was utilized as a reductive and structure-directing agent, respectively. Herein, the
https://doi.org/10.1016/j.apt.2018.02.020 0921-8831/Ó 2018 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
Please cite this article in press as: H. Chen et al., PABA-assisted hydrothermal fabrication of W18O49 nanowire networks and its transition to WO3 for photocatalytic degradation of methylene blue, Advanced Powder Technology (2018), https://doi.org/10.1016/j.apt.2018.02.020
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formation mechanism of the W18O49 nanostructure has been studied in detail by discussing the amount of PABA, the reaction time. Furthermore, the surface feature of the samples has been characterized by FT-IR spectrum and XPS spectrum. The thermal stability research shows that the product has a high stability and the W18O49 nanowire networks can transform to monoclinic WO3 nanocrystals at 450 °C and is of an excellent hydrophilicity. The W18O49 nanowire networks have an excellent visible-light photocatalytic activity in present work. 2. Experimental In this experiment, all chemical reagents were of analytical grade and used without further purification.
SDT Q600 instrument and measured from 20 to 800 °C with a heating rate of 10 °C min 1 in air. Fourier transform infrared spectra (FTIR) were recorded with a Bruker Equinox 55 in the range of 400–4000 cm 1. Before the FTIR spectra test, the pretreatment process of samples was as follows: about 2 mg of each sample was grounded into fine particles and mixed with 100 mg potassium bromide (KBr), then the mixed powder was dried and pressed into tablets for test. The Brunauer-Emmett-Teller (BET) surface area and pore volume were measured by the nitrogen gas adsorptiondesorption method using a Micromeritics ASAP (accelerated surface area and porosimetry) 2020 system. The samples were degassed under vacuum at 180 °C for 6 h, and the N2 adsorption/ desorption isotherms were measured at 77 K. 2.5. Contact angle measurement
2.1. Synthesis of tungsten acid precursor The basic synthetic process is listed as follows: 10 g of Na2WO42H2O was dissolved in 1 mol L 1 HNO3 to form tungsten acid (H2WO4nH2O) precipitate, and precipitate was dried with a vacuum freeze dryer. 2.2. The fabrication of W18O49 nanowire networks Four group experiments were conducted to investigate the role of PABA in synthesizing W18O49 nanowire networks by using different amount of PABA. 1 g of H2WO4nH2O and PABA (0/0.5/1/2/4 g) were mixed into 60 mL of deionized water with continuous stirring for 2 h, respectively. Then the mixture was transferred into a 90 mL Teflon-lined stainless steel autoclave. Hydrothermal treatment of mixture was carried out at 180 °C for 24 h, and then the autoclave was cooled down naturally. The final products were washed with deionized water and ethanol several times, and dried in air at 60 °C. The samples prepared at different amount of PABA (0/0.5/1/2/4 g) were noted as S-0, S-0.5, S-1, S-2, S-4. S-2 was selected for researching the formation mechanisms of W18O49 nanowire networks by controlling the hydrothermal time (0/4/8/12/24 h).
The contact angle measurements were conducted using a JJ200B2 (provided by Shanghai Zhongchen Digital Technology Apparatus Co., Ltd) in a water environment. Before examination, the powder of samples was pressed into a hard plate and then were put into contact angle meter. Subsequently, 2 lL of ultrapure water was directly released onto the surfaces carefully. The final contact angle values were attained by averaging at least five points at different positions on the same surface. 2.6. Photocatalytic degradation experiments The photocatalytic activities of the samples were evaluated by degradation of methylene blue (MB) in an aqueous solution under visible light from 800 W Xe lamp. 50 mg of photocatalysts (W18O49-400/W18O49-450) was well-dispersed into 50 mL of MB aqueous solution (10 mg L 1) with a constant stirring in a quartz tube at room temperature. Before light was turned on, the solution was continuously stirred for 60 min in dark to ensure the establishment of an adsorption-desorption equilibrium. Then, when light was turned on, samples were drawn from solution at predetermined time intervals, and centrifuged to measure the MB removal via a UV–vis spectrometer (Shimadzu UV-2500 PC). 3. Results and discussion
2.3. Thermal stability analysis S-2 was chosen for investigating the thermal stability of W18O49 nanowire networks. A certain amount of S-2 was heated at 400 °C, 425 °C, and 450 °C for 30 min with the heating rate of 2 °C min 1. The products were denoted as W18O49-400, W18O49-425, and W18O49-450, respectively. Moreover, S-2 was further annealed at 400 °C for 60/90/150/210/300 min, respectively, with the heating rate of 2 °C min 1.
Recently, we have reported W18O49 nanowire networks fabricated by the PABA-assisted method. The product exhibited an excellent water treatment performance. Here, we performed a further research to understand the formation mechanism and thermal stability of the W18O49 nanowire networks. The influencing factors, such as PABA amount, hydrothermal time and annealing temperature have been systematically studied in present work. 3.1. Structure and morphology characterization
2.4. Characterization The crystal phase of the obtained samples was studied with powder X-ray diffraction (XRD) analysis (Bruker, D8-Advance Xray Diffractometer, Cu Ka, k = 1.5418 Å). Scanning electron microscopy (SEM) images were recorded with a field emission scanning electron microscopy (S-4800, Hitachi, Japan). Transmission electron microscope (TEM) images were obtained on a Hitachi H-600 with an accelerating voltage of 100 kV. HRTEM characterizations were performed with a JEOL JEM-2100 (JEOL, Japan) transmission electron microscope. UV/Vis absorption spectra were recorded with a Shimadzu UV-4802S. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Fisher Scientific XPS ESCALAB 250Xi instrument with an Al Ka (1486.8 eV) X-ray source. Thermogravimetric analysis (TGA) was performed on a
In the reported communication we put forward a thought that PABA may play an important role on the formation of W18O49 nanowire networks. Here, the influence of PABA amount has been studied by changing the PABA amount from 0 g to 4 g. Fig. 1 showed the XRD patterns of the products derived from different PABA amount. It was obvious that S-0 was monoclinic WO3 (JCPDS card No. 72-0677). S-0.5 was a mixture of WO3 and WO30.33H2O (JCPDS card No. 43-1035 and 35-0270). With the increase of PABA to 1 g, WO3 (S-1) was obtained (JCPDS card No. 72-0677). When the amount of PABA reached to 2 g or more than it (4 g), oxygendeficient monoclinic W18O49 phase was obtained (JCPDS card No. 36-101). So, it was evident that the amount of PABA played an important role on the fabrication of non-stoichiometric tungsten oxide.
Please cite this article in press as: H. Chen et al., PABA-assisted hydrothermal fabrication of W18O49 nanowire networks and its transition to WO3 for photocatalytic degradation of methylene blue, Advanced Powder Technology (2018), https://doi.org/10.1016/j.apt.2018.02.020
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3.2. Chemical analysis
Fig. 1. XRD patterns of samples: (a) S-0, (b) S-0.5, (c) S-1, (d) S-2, (e) S-4.
Fig. 2 showed the SEM and TEM images of samples obtained with different amount of PABA. Without PABA, S-0 was plateshaped squares with about 500 nm in length and about 100 nm in width. When the amount of PABA reached to 0.5 g, the flower like S-0.5 assembled with nanorods were obtained. The mean diameter of the nanorods was about 50 nm. S-1 was a well-disperse nanorod. When the amount of PABA reached to 2 g, the nanowire networks were obtained. When the amount of PABA increased to 4 g, the nanowire networks still appeared in TEM. The mean diameter of the nanowires was below 5 nm confirmed by HRTEM image (inset of Fig. 2e). The same result also was also reported by our group [30]. It was obviously that the amount of PABA played a key role on the fabrication of W18O49 nanowire networks. Further, the S-2 was chose to perform a systematical research of the W18O49 nanostructures, in the following study.
S-2 was characterized by an ultraviolet/visible (UV/Vis) absorption spectroscopy (Fig. 3a), which exhibited an unusual photophysical property, and it was similar with literature reported [4]. An absorption tail presenting in the visible and near infrared (NIR) regions of absorption spectrum demonstrated that the nanostructure had a certain number of oxygen vacancies [31]. This result was different from those one-dimensional W18O49 nanostructures with larger diameters [32,33]. The full range of XPS spectra of S-2 (Fig. 3b) revealed that S-2 was solely constituted of W and O. The energy distribution of W 4f photoelectrons was shown in Fig. 3c. The peaks with W 4f5/2 at 38.0 eV and W 4f7/2 at 35.9 eV, were attributable to the W atoms being in a 6+ oxidation state, which demonstrated that there was only W6+ existed on the surface of S-2. Further, the energy distribution of O 2s photoelectrons confirmed that the clear surface with the peak at 530.9 eV corresponded to O1s-levels of oxygen atoms O2 in the lattice of W18O49. The surface chemical station of the S-2 was different with the reported ones fabricated with the alcoholysis of WCl6 which contained W5+ [4,25]. 3.3. Formation mechanisms The formation of W18O49 nanowire networks was studied by control of hydrothermal time from 0 h to 24 h, and the amount of PABA was 2 g according to the above results. The morphology of tungsten acid precursor (0 h) was studied by SEM and TEM. Fig. 4a and b showed that the tungsten acid precursor had some irregular particles. From Fig. 4c–f, it can be obviously observed the formation process of W18O49 nanowire networks. Product obtained with hydrothermal time of 4 h which almost kept the initial morphology of tungsten acid and had the tendency of transforming to nanowire. When the reaction time reached to 8 h, part
Fig. 2. SEM and TEM images of samples: (a) S-0 (inset bar is 250 nm), (b–c) S-0.5, (d) S-1, (e) S-2, (f) S-4.
Please cite this article in press as: H. Chen et al., PABA-assisted hydrothermal fabrication of W18O49 nanowire networks and its transition to WO3 for photocatalytic degradation of methylene blue, Advanced Powder Technology (2018), https://doi.org/10.1016/j.apt.2018.02.020
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Fig. 3. (a) UV/Vis spectrum of S-2, (b–d) XPS spectra of S-2: (b) full-range XPS spectra, (c) W XPS spectra, (d) O XPS spectra.
Fig. 4. SEM and TEM images of samples prepared at different hydrothermal time: (a, b) 0 h, (c) 4 h, (d) 8 h, (e)12 h, (f) 24 h.
of the product transformed to nanowire (Fig. 4d). When the reaction time reached to 12 h, product was made up of nanowires and some irregular nanoplates (Fig. 4e). Finally, morphology of
product changed into nanowire networks after 24 h hydrothermal reaction at 180 °C (Fig. 4f), whereas the irregular nanoplates disappeared. The morphology transition process demonstrated
Please cite this article in press as: H. Chen et al., PABA-assisted hydrothermal fabrication of W18O49 nanowire networks and its transition to WO3 for photocatalytic degradation of methylene blue, Advanced Powder Technology (2018), https://doi.org/10.1016/j.apt.2018.02.020
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Scheme 1. Schematic illustration of the morphological evolution process of W18O49 nanowire networks: (I) transformation process of tungsten acid precursor to tungsten oxide; (II) formation of W18O49 nanowire networks.
Fig. 5. Thermo-gravimetric analysis of S-2.
tungsten oxide. In our strategy, the amino-group and carboxygroup of PABA with excessive amount could continuously provide a reductive and acid atmosphere. This reductive atmosphere could reduce WO3nH2O into W18O49 and maintain its stability. Also, some research groups had prepared the pure W18O49 using ethanol to provide the reductive environment [4,35]. Secondly, the amorphous tungsten acid precursor was easy to be induced to become the different structures under different agents. Moreover, PABA is a white crystalline powder and easy to dissolve in hot water and crystallize like a needle when temperature decreased. The onedimensional structure characteristics of PABA could serve as a hard template and directly induce amorphous tungsten acid into nanowires structure, so the basic profile of precursor was similar with profile of nanowire networks. Last but the most important, the amino-group of PABA could improve monoclinic W18O49 crystals growth along [0 1 0] axis, and the similar study also demonstrated that growth of ZnO nanorods along longitudinal direction could be promoted in the presence of octylamine [36].
3.4. Thermal stability analysis
Fig. 6. XRD patterns of annealed at different temperatures: (a) 400 °C, (b) 425 °C, (c) 450 °C.
that the nanowire networks were derived from the irregular particles. According to above analysis of the influence of PABA amount and hydrothermal time, the formation process of W18O49 nanowire networks was shown in Scheme 1. The transformation of tungsten acid precursor to W18O49 nanowires involved two steps. In the first step, tungsten acid was transformed to tungsten oxide with assistance of PABA at hydrothermal condition. While in the second step, under the existence of PABA, the nanowires structure was formed. Although the synthesis of W18O49 nanowires was reported by some groups, the formation mechanism was not completely clear [4,30]. According to the description of the two steps, here, we assumed a probable formation mechanism of W18O49 nanowire networks as follows: First, as we are known, there are large amount of tungsten oxides with different stoichiometries, such as W18O49, WO3, WO2.9, W5O14, W3O8, and WO2 [34]. So, it was hard to prepare the pure
The thermal behavior of the W18O49 nanowire networks was displayed in Fig. 5. The TGA curve of S-2 showed a weight loss of around 13.3% from room temperature to 500 °C. The weight loss up to 200 °C might be assigned to desorption of water, including the loss of surface adsorbed water (90 °C) and structural water elimination (200 °C) for S-2 [37]. The weight loss from 200 °C to 500 °C might be the organic adsorbate coming from the heat decomposition of PABA. The thermal stability of the W18O49 nanowire networks was also studied by annealing S-2 at 400 °C, 425 °C and 450 °C (Fig. 6). The XRD pattern of W18O49-400 was similar with S-2 and S-4, which demonstrated that the monoclinic W18O49 nanowire networks had a high thermal stability under 400 °C. From Fig. 6b we can see that when annealing temperature reached to 425 °C, the W18O49-425 was a mixture of monoclinic W18O49 and monoclinic WO3 (JCPDS No. 36-101 and No. 72-0677). When the annealing temperature was 450 °C, S-2 was completely transformed into monoclinic WO3 (JCPDS No. 72-0677). Fig. 7 showed the TEM images of samples obtained by annealing S-2 at 400/425/450 °C, respectively. From Fig. 7a, we could see that W18O49-400 had the similar nanowire networks with S-2, which was consistent with XRD results that W18O49 did not change its physical phase after annealing at 400 °C. Furthermore, the TEM image of W18O49-425 still kept the morphology of nanowire networks. However, with increase of annealing temperature to 450 °C, the nanowire network morphology collapsed and was transformed to nanoparticles with mean diameter of 10 nm. Fig. 8 showed the TEM and XRD results of S-2 annealed at 400 °C and kept at different time. It can be seen clearly the evolution of
Please cite this article in press as: H. Chen et al., PABA-assisted hydrothermal fabrication of W18O49 nanowire networks and its transition to WO3 for photocatalytic degradation of methylene blue, Advanced Powder Technology (2018), https://doi.org/10.1016/j.apt.2018.02.020
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Fig. 7. TEM images of samples annealed at different temperatures: (a) 400 °C, (b) 425 °C, (c) 450 °C.
Fig. 9. FT-IR spectra of samples: (a) S-2, (b) W18O49-400, (c) W18O49-450.
Fig. 8. (a–f): TEM images of S-2 annealed at 400 °C with different time: 0 min, 60 min, 90 min, 150 min, 210 min, 300 min; (g) XRD patterns of S-2 annealed at above corresponding conditions.
morphology over annealed time. With the increase of annealed time, the W18O49 nanowire networks structure gradually collapsed into WO3 particles. After 300 min, the S-2 can be totally annealed into WO3. Moreover, the color of powders turned from grey, to brown, and finally changed into light yellow. The surface cleanliness of the S-2, W18O49-400, and W18O49-450 had been evaluated by FTIR spectrum. In the region of 1000–500 cm 1, the bands characteristic for the W-O (950–800 cm 1) units and the stretching vibrations of the bridging oxygen atoms O-WO (600–780 cm 1) appeared in Fig. 9a–c [32]. For S-2, the peak at 3419 cm 1 was attributed to OAH stretching vibration, the peaks
at 2890 cm 1 and 1492 cm 1 may be attributed to stretching vibration and bending vibration of CAH coming from the heat decomposition of PABA, the peaks at 1617 cm 1 may be attributed to the bending vibration of adsorbed water [38]. It was obvious that the surface of W18O49 nanostructure fabricated with PABA was clearer than that with surfactants [14]. For W18O49-400 (Fig. 9b), the peak at 3449 cm 1 was attributed to OAH stretching vibration, the peaks at 1622 cm 1 may be attributed to the bending vibration of adsorbed water. The peaks of CAH were weaker due to the degradation of the organic adsorbate at 400 °C. The FTIR spectrum of W18O49-450 showed that the monoclinic WO3 nanocrystals had a very clean surface. The N2 adsorption-desorption isotherms and pore size distributions of S-2, W18O49-400, and W18O49-450 were displayed in Fig. 10. The isotherms of three samples were of type II (appearance of inflexion point at low P/P0 and absence of a plateau at high P/P0) [39]. Moreover, adsorption isotherms slowly increased with the increase of P/P0 at lower relative pressure while a rapid increase of adsorption isotherm was observed at higher P/P0; a saturation adsorption was not observed even at the saturation vapor pressure. All the phenomena indicated the existence of mesoporous or macropores [40]. The BET specific surface areas of S-2, W18O49400, and W18O49-450 reached to 146, 147, and 47 m2 g 1, respectively (Table 1). It can be concluded that annealing W18O49 at a relative low temperature (400 °C) would not change the pore structure of it. However, when the annealing temperature reached to 450 °C could not only give rise to a collapse of structure, but also a low specific surface area.
Please cite this article in press as: H. Chen et al., PABA-assisted hydrothermal fabrication of W18O49 nanowire networks and its transition to WO3 for photocatalytic degradation of methylene blue, Advanced Powder Technology (2018), https://doi.org/10.1016/j.apt.2018.02.020
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Fig. 12. The photocatalytic degradation of MB over: blank, W18O49-400, and W18O49-450.
Fig. 10. (a) Nitrogen adsorption-desorption isotherms and (b) the corresponding pore size distributions curves for as-prepared samples.
Table 1 Pore structure parameters of the S-2, W18O49-400, and W18O49-450. Samples S-2 W18O49-400 W18O49-450
SBET (m2 g 146 147 47.0
1
)
Average pore size (nm)
Pore volume (cm3 g 1)
11.0 10.0 17.6
0.768 0.702 0.409
nanocrystals. Fig. 10a showed digital photo of the instantaneous state that WO3 nanocrystal powders were dispersing in deionized water. The diffusion of the nanocrystals in water was similar with diffusion of ink. However, the phenomenon couldn’t be observed when S-2 or W18O49-400 was dissolved in water, respectively. Further, Fig. 11b showed that the contact angle of W18O49-450 is just 7.9°. It demonstrated that the nanocrystals have an excellent hydrophilicity. The photocatalytic properties of as-prepared samples were studied by the photodegradation of MB under visible light. Fig. 12 showed photocatalytic degradations of MB with different catalysts: blank (without photocatalyst), W18O49-400, and W18O49-450. Compared with the MB (10 mg L 1) solution without photocatalyst, W18O49-450 exhibited a little photocatalytic property with about 10 % higher than the blank, while the degradation rate of MB solution for W18O49-400 can reach to 40 %. The difference of the photodegradation performance between W18O49-400 and W18O49-450, on the one hand, can be attributed to the morphology and phase structure. On the other hand, the huge specific surface area also meant a high photocatalytic performance. In our previous work, we have discussed the adsorption performance of W18O49 to remove MB. In that work, the adsorption capacity of MB by the as-synthesized W18O49 nanowire networks is about 201 mg g 1, and MB with 10 mg L 1 can be almost completely removed from water with a W18O49 nanowire networks loading amount of 0.1 g L 1 [30]. Thus, we will not further discuss the photocatalytic performance of W18O49 without annealing here.
4. Conclusion
Fig. 11. Digital photos of W18O49-450: (a) aqueous solution, (b) contact angle.
3.5. Hydrophilic and photocatalytic performance The hydrophilic analysis of W18O49-450 powders was shown at Fig. 11, which confirmed the clean surface of monoclinic WO3
In summary, W18O49 nanowire networks have been fabricated by a simple two-pot hydrothermal method with tungsten acid as a precursor and PABA as a reducing and structure-directing agent. The amount of PABA plays an important role on the formation of non-stoichiometric tungsten oxide nanowires. The fabrication mechanism of the W18O49 nanostructure has been proposed by investigating the growth process of these nanowires. The thermal stability research shows that the product has a high stability that it does not decompose under 400 °C until the annealing time reaches to 300 min, and after that it will be transformed into WO3 nanocrystals completely. Additionally, W18O49 nanowire networks can also transform to monoclinic WO3 nanocrystal at 450 °C in 30 min, which exhibits an excellent hydrophilic property. What’s more, the W18O49 nanowire networks annealed at 400 °C show a superior photocatalytic activity to degrade MB aqueous solution under visible-light. Thus, in this article, we provide a more
Please cite this article in press as: H. Chen et al., PABA-assisted hydrothermal fabrication of W18O49 nanowire networks and its transition to WO3 for photocatalytic degradation of methylene blue, Advanced Powder Technology (2018), https://doi.org/10.1016/j.apt.2018.02.020
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specific synthetic and annealing strategy to prepare the tungsten oxides and a basic route to characterize these samples.
[20]
Acknowledgements This work was supported by key project of Xinjiang colleges and universities (XJEDU2013I36) and International Cooperation Project of Xinjiang Science and Technology Bureau (2017E01005, 2017E01016).
[21]
[22]
[23]
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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
PABA-assisted hydrothermal fabrication of W18O49 nanowire networks and its transition to WO3 for photocatalytic degradation of methylene blue Hang Chen a, Wanlin Cai b, Xiaoqing Gao a, Jianmin Luo a, Xintai Su a,⇑ a b
Ministry Key Laboratory of Oil and Gas Fine Chemicals, College of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, China College of Science, Xinjiang Institute of Education, Urumqi 830000, China
a r t i c l e
i n f o
Article history: Received 12 April 2017 Received in revised form 30 January 2018 Accepted 16 February 2018 Available online xxxx Keywords: Tungsten oxide p-Aminobenzoic acid Hydrothermal method Photocatalytic properties Hydrophilic property
a b s t r a c t W18O49 nanowire networks have been fabricated by a facile hydrothermal method. In this method, p-aminobenzoic acid (PABA) was used as an assistant agent to control the morphology transformation. W18O49 and its products annealed at different temperature were characterized by XRD, SEM, TEM, UV– vis absorption spectroscopy, XPS, TGA, and FTIR. Formation mechanism and thermal stability of W18O49 nanowire networks were studied in detail. The experiment data showed that PABA played an important role in the induced crystal growth of W18O49 nanowires along [0 1 0] axis. In transformation, the structure of samples was controlled: from irregular particles to nanowire networks. W18O49 nanowire networks were annealed at different temperature. The nanowire networks collapsed at 450 °C, while WO3 nanocrystals were obtained. The W18O49 nanowire networks annealed at 400 °C have a superior photocatalytic performance to degrade methylene blue and its specific surface area was up to 147 m2 g 1. Ó 2018 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Nowadays, nanomaterials have achieved a widespread attention due to their enhanced physical and chemical properties [1–5]. Among all kind of nanomaterials, one dimension nanostructure has attracted extensive interests due to the unique electronic and optical properties [6–8]. Tungsten oxide nanowires and nanorods, as one of the crucial transition metal oxide semiconductors, have a wide applications in electrochromic devices, gas sensors, optical devices, hydrogen reduction, and photocatalysts [9–15]. Specially, tungsten oxide is well-known for its nonstoichiometric properties, as the lattice can withstand a considerable amount of oxygen vacancy [12,16,17]. In particular, monoclinic W18O49 is of special interest for its unique defect structure and distinct physical and chemical properties in the nanometer regime [4,18–20]. Hitherto, W18O49 based materials have exhibited excellent performance in various fields, such as catalysis [21], gas sensing property [22], optical property [23], magnetic and near-infrared adsorption properties [24,25]. Recent research shows that the reductivity of W18O49 can realize the direct growth of metal particles on metal oxides in situ [26]. Furthermore, the acidic surface of ⇑ Corresponding author. E-mail address: [email protected] (X. Su).
the W18O49 porous nanomaterials enhances their adsorption on basic dyes [27,28]. Guangcheng Xi and co-workers have reported the preparation of ultrathin W18O49 nanowires with diameters below 1 nm that are efficient in the photochemical reduction of carbon dioxide by visible light [4]. Y.M. Zhao and Y.Q. Zhu have researched the room temperature ammonia sensing properties of ultra-thin W18O49 nanowires with diameter less than 5 nm, and the nanowire was prepared by a solvothermal technique [18]. Chongshen Guo et al. have investigated the morphologycontrolled synthesis and near-infrared (NIR) absorption properties of W18O49 which applied to innovative energy-saving windows [25]. The general method to fabricate W18O49 nanomaterials is alcoholysis of WCl6, W(CH3CO)6, or W(CO)6 [18,26,29]. However, the reagents are expensive and unstable, which are disadvantageous for practical applications. Therefore, it is highly desirable and challenging to develop a facile and economical method to produce W18O49 with porous nanostructures for their large scale applications in water treatment and others. Once, we have reported a communication about the hydrothermal fabrication of W18O49 nanowire networks with Na2WO42H2O as tungsten resource [30]. In that work, tungsten acid was used as precursor while p-amino-benzoic acid (PABA) was utilized as a reductive and structure-directing agent, respectively. Herein, the
https://doi.org/10.1016/j.apt.2018.02.020 0921-8831/Ó 2018 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
Please cite this article in press as: H. Chen et al., PABA-assisted hydrothermal fabrication of W18O49 nanowire networks and its transition to WO3 for photocatalytic degradation of methylene blue, Advanced Powder Technology (2018), https://doi.org/10.1016/j.apt.2018.02.020
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formation mechanism of the W18O49 nanostructure has been studied in detail by discussing the amount of PABA, the reaction time. Furthermore, the surface feature of the samples has been characterized by FT-IR spectrum and XPS spectrum. The thermal stability research shows that the product has a high stability and the W18O49 nanowire networks can transform to monoclinic WO3 nanocrystals at 450 °C and is of an excellent hydrophilicity. The W18O49 nanowire networks have an excellent visible-light photocatalytic activity in present work. 2. Experimental In this experiment, all chemical reagents were of analytical grade and used without further purification.
SDT Q600 instrument and measured from 20 to 800 °C with a heating rate of 10 °C min 1 in air. Fourier transform infrared spectra (FTIR) were recorded with a Bruker Equinox 55 in the range of 400–4000 cm 1. Before the FTIR spectra test, the pretreatment process of samples was as follows: about 2 mg of each sample was grounded into fine particles and mixed with 100 mg potassium bromide (KBr), then the mixed powder was dried and pressed into tablets for test. The Brunauer-Emmett-Teller (BET) surface area and pore volume were measured by the nitrogen gas adsorptiondesorption method using a Micromeritics ASAP (accelerated surface area and porosimetry) 2020 system. The samples were degassed under vacuum at 180 °C for 6 h, and the N2 adsorption/ desorption isotherms were measured at 77 K. 2.5. Contact angle measurement
2.1. Synthesis of tungsten acid precursor The basic synthetic process is listed as follows: 10 g of Na2WO42H2O was dissolved in 1 mol L 1 HNO3 to form tungsten acid (H2WO4nH2O) precipitate, and precipitate was dried with a vacuum freeze dryer. 2.2. The fabrication of W18O49 nanowire networks Four group experiments were conducted to investigate the role of PABA in synthesizing W18O49 nanowire networks by using different amount of PABA. 1 g of H2WO4nH2O and PABA (0/0.5/1/2/4 g) were mixed into 60 mL of deionized water with continuous stirring for 2 h, respectively. Then the mixture was transferred into a 90 mL Teflon-lined stainless steel autoclave. Hydrothermal treatment of mixture was carried out at 180 °C for 24 h, and then the autoclave was cooled down naturally. The final products were washed with deionized water and ethanol several times, and dried in air at 60 °C. The samples prepared at different amount of PABA (0/0.5/1/2/4 g) were noted as S-0, S-0.5, S-1, S-2, S-4. S-2 was selected for researching the formation mechanisms of W18O49 nanowire networks by controlling the hydrothermal time (0/4/8/12/24 h).
The contact angle measurements were conducted using a JJ200B2 (provided by Shanghai Zhongchen Digital Technology Apparatus Co., Ltd) in a water environment. Before examination, the powder of samples was pressed into a hard plate and then were put into contact angle meter. Subsequently, 2 lL of ultrapure water was directly released onto the surfaces carefully. The final contact angle values were attained by averaging at least five points at different positions on the same surface. 2.6. Photocatalytic degradation experiments The photocatalytic activities of the samples were evaluated by degradation of methylene blue (MB) in an aqueous solution under visible light from 800 W Xe lamp. 50 mg of photocatalysts (W18O49-400/W18O49-450) was well-dispersed into 50 mL of MB aqueous solution (10 mg L 1) with a constant stirring in a quartz tube at room temperature. Before light was turned on, the solution was continuously stirred for 60 min in dark to ensure the establishment of an adsorption-desorption equilibrium. Then, when light was turned on, samples were drawn from solution at predetermined time intervals, and centrifuged to measure the MB removal via a UV–vis spectrometer (Shimadzu UV-2500 PC). 3. Results and discussion
2.3. Thermal stability analysis S-2 was chosen for investigating the thermal stability of W18O49 nanowire networks. A certain amount of S-2 was heated at 400 °C, 425 °C, and 450 °C for 30 min with the heating rate of 2 °C min 1. The products were denoted as W18O49-400, W18O49-425, and W18O49-450, respectively. Moreover, S-2 was further annealed at 400 °C for 60/90/150/210/300 min, respectively, with the heating rate of 2 °C min 1.
Recently, we have reported W18O49 nanowire networks fabricated by the PABA-assisted method. The product exhibited an excellent water treatment performance. Here, we performed a further research to understand the formation mechanism and thermal stability of the W18O49 nanowire networks. The influencing factors, such as PABA amount, hydrothermal time and annealing temperature have been systematically studied in present work. 3.1. Structure and morphology characterization
2.4. Characterization The crystal phase of the obtained samples was studied with powder X-ray diffraction (XRD) analysis (Bruker, D8-Advance Xray Diffractometer, Cu Ka, k = 1.5418 Å). Scanning electron microscopy (SEM) images were recorded with a field emission scanning electron microscopy (S-4800, Hitachi, Japan). Transmission electron microscope (TEM) images were obtained on a Hitachi H-600 with an accelerating voltage of 100 kV. HRTEM characterizations were performed with a JEOL JEM-2100 (JEOL, Japan) transmission electron microscope. UV/Vis absorption spectra were recorded with a Shimadzu UV-4802S. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Fisher Scientific XPS ESCALAB 250Xi instrument with an Al Ka (1486.8 eV) X-ray source. Thermogravimetric analysis (TGA) was performed on a
In the reported communication we put forward a thought that PABA may play an important role on the formation of W18O49 nanowire networks. Here, the influence of PABA amount has been studied by changing the PABA amount from 0 g to 4 g. Fig. 1 showed the XRD patterns of the products derived from different PABA amount. It was obvious that S-0 was monoclinic WO3 (JCPDS card No. 72-0677). S-0.5 was a mixture of WO3 and WO30.33H2O (JCPDS card No. 43-1035 and 35-0270). With the increase of PABA to 1 g, WO3 (S-1) was obtained (JCPDS card No. 72-0677). When the amount of PABA reached to 2 g or more than it (4 g), oxygendeficient monoclinic W18O49 phase was obtained (JCPDS card No. 36-101). So, it was evident that the amount of PABA played an important role on the fabrication of non-stoichiometric tungsten oxide.
Please cite this article in press as: H. Chen et al., PABA-assisted hydrothermal fabrication of W18O49 nanowire networks and its transition to WO3 for photocatalytic degradation of methylene blue, Advanced Powder Technology (2018), https://doi.org/10.1016/j.apt.2018.02.020
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3.2. Chemical analysis
Fig. 1. XRD patterns of samples: (a) S-0, (b) S-0.5, (c) S-1, (d) S-2, (e) S-4.
Fig. 2 showed the SEM and TEM images of samples obtained with different amount of PABA. Without PABA, S-0 was plateshaped squares with about 500 nm in length and about 100 nm in width. When the amount of PABA reached to 0.5 g, the flower like S-0.5 assembled with nanorods were obtained. The mean diameter of the nanorods was about 50 nm. S-1 was a well-disperse nanorod. When the amount of PABA reached to 2 g, the nanowire networks were obtained. When the amount of PABA increased to 4 g, the nanowire networks still appeared in TEM. The mean diameter of the nanowires was below 5 nm confirmed by HRTEM image (inset of Fig. 2e). The same result also was also reported by our group [30]. It was obviously that the amount of PABA played a key role on the fabrication of W18O49 nanowire networks. Further, the S-2 was chose to perform a systematical research of the W18O49 nanostructures, in the following study.
S-2 was characterized by an ultraviolet/visible (UV/Vis) absorption spectroscopy (Fig. 3a), which exhibited an unusual photophysical property, and it was similar with literature reported [4]. An absorption tail presenting in the visible and near infrared (NIR) regions of absorption spectrum demonstrated that the nanostructure had a certain number of oxygen vacancies [31]. This result was different from those one-dimensional W18O49 nanostructures with larger diameters [32,33]. The full range of XPS spectra of S-2 (Fig. 3b) revealed that S-2 was solely constituted of W and O. The energy distribution of W 4f photoelectrons was shown in Fig. 3c. The peaks with W 4f5/2 at 38.0 eV and W 4f7/2 at 35.9 eV, were attributable to the W atoms being in a 6+ oxidation state, which demonstrated that there was only W6+ existed on the surface of S-2. Further, the energy distribution of O 2s photoelectrons confirmed that the clear surface with the peak at 530.9 eV corresponded to O1s-levels of oxygen atoms O2 in the lattice of W18O49. The surface chemical station of the S-2 was different with the reported ones fabricated with the alcoholysis of WCl6 which contained W5+ [4,25]. 3.3. Formation mechanisms The formation of W18O49 nanowire networks was studied by control of hydrothermal time from 0 h to 24 h, and the amount of PABA was 2 g according to the above results. The morphology of tungsten acid precursor (0 h) was studied by SEM and TEM. Fig. 4a and b showed that the tungsten acid precursor had some irregular particles. From Fig. 4c–f, it can be obviously observed the formation process of W18O49 nanowire networks. Product obtained with hydrothermal time of 4 h which almost kept the initial morphology of tungsten acid and had the tendency of transforming to nanowire. When the reaction time reached to 8 h, part
Fig. 2. SEM and TEM images of samples: (a) S-0 (inset bar is 250 nm), (b–c) S-0.5, (d) S-1, (e) S-2, (f) S-4.
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Fig. 3. (a) UV/Vis spectrum of S-2, (b–d) XPS spectra of S-2: (b) full-range XPS spectra, (c) W XPS spectra, (d) O XPS spectra.
Fig. 4. SEM and TEM images of samples prepared at different hydrothermal time: (a, b) 0 h, (c) 4 h, (d) 8 h, (e)12 h, (f) 24 h.
of the product transformed to nanowire (Fig. 4d). When the reaction time reached to 12 h, product was made up of nanowires and some irregular nanoplates (Fig. 4e). Finally, morphology of
product changed into nanowire networks after 24 h hydrothermal reaction at 180 °C (Fig. 4f), whereas the irregular nanoplates disappeared. The morphology transition process demonstrated
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Scheme 1. Schematic illustration of the morphological evolution process of W18O49 nanowire networks: (I) transformation process of tungsten acid precursor to tungsten oxide; (II) formation of W18O49 nanowire networks.
Fig. 5. Thermo-gravimetric analysis of S-2.
tungsten oxide. In our strategy, the amino-group and carboxygroup of PABA with excessive amount could continuously provide a reductive and acid atmosphere. This reductive atmosphere could reduce WO3nH2O into W18O49 and maintain its stability. Also, some research groups had prepared the pure W18O49 using ethanol to provide the reductive environment [4,35]. Secondly, the amorphous tungsten acid precursor was easy to be induced to become the different structures under different agents. Moreover, PABA is a white crystalline powder and easy to dissolve in hot water and crystallize like a needle when temperature decreased. The onedimensional structure characteristics of PABA could serve as a hard template and directly induce amorphous tungsten acid into nanowires structure, so the basic profile of precursor was similar with profile of nanowire networks. Last but the most important, the amino-group of PABA could improve monoclinic W18O49 crystals growth along [0 1 0] axis, and the similar study also demonstrated that growth of ZnO nanorods along longitudinal direction could be promoted in the presence of octylamine [36].
3.4. Thermal stability analysis
Fig. 6. XRD patterns of annealed at different temperatures: (a) 400 °C, (b) 425 °C, (c) 450 °C.
that the nanowire networks were derived from the irregular particles. According to above analysis of the influence of PABA amount and hydrothermal time, the formation process of W18O49 nanowire networks was shown in Scheme 1. The transformation of tungsten acid precursor to W18O49 nanowires involved two steps. In the first step, tungsten acid was transformed to tungsten oxide with assistance of PABA at hydrothermal condition. While in the second step, under the existence of PABA, the nanowires structure was formed. Although the synthesis of W18O49 nanowires was reported by some groups, the formation mechanism was not completely clear [4,30]. According to the description of the two steps, here, we assumed a probable formation mechanism of W18O49 nanowire networks as follows: First, as we are known, there are large amount of tungsten oxides with different stoichiometries, such as W18O49, WO3, WO2.9, W5O14, W3O8, and WO2 [34]. So, it was hard to prepare the pure
The thermal behavior of the W18O49 nanowire networks was displayed in Fig. 5. The TGA curve of S-2 showed a weight loss of around 13.3% from room temperature to 500 °C. The weight loss up to 200 °C might be assigned to desorption of water, including the loss of surface adsorbed water (90 °C) and structural water elimination (200 °C) for S-2 [37]. The weight loss from 200 °C to 500 °C might be the organic adsorbate coming from the heat decomposition of PABA. The thermal stability of the W18O49 nanowire networks was also studied by annealing S-2 at 400 °C, 425 °C and 450 °C (Fig. 6). The XRD pattern of W18O49-400 was similar with S-2 and S-4, which demonstrated that the monoclinic W18O49 nanowire networks had a high thermal stability under 400 °C. From Fig. 6b we can see that when annealing temperature reached to 425 °C, the W18O49-425 was a mixture of monoclinic W18O49 and monoclinic WO3 (JCPDS No. 36-101 and No. 72-0677). When the annealing temperature was 450 °C, S-2 was completely transformed into monoclinic WO3 (JCPDS No. 72-0677). Fig. 7 showed the TEM images of samples obtained by annealing S-2 at 400/425/450 °C, respectively. From Fig. 7a, we could see that W18O49-400 had the similar nanowire networks with S-2, which was consistent with XRD results that W18O49 did not change its physical phase after annealing at 400 °C. Furthermore, the TEM image of W18O49-425 still kept the morphology of nanowire networks. However, with increase of annealing temperature to 450 °C, the nanowire network morphology collapsed and was transformed to nanoparticles with mean diameter of 10 nm. Fig. 8 showed the TEM and XRD results of S-2 annealed at 400 °C and kept at different time. It can be seen clearly the evolution of
Please cite this article in press as: H. Chen et al., PABA-assisted hydrothermal fabrication of W18O49 nanowire networks and its transition to WO3 for photocatalytic degradation of methylene blue, Advanced Powder Technology (2018), https://doi.org/10.1016/j.apt.2018.02.020
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Fig. 7. TEM images of samples annealed at different temperatures: (a) 400 °C, (b) 425 °C, (c) 450 °C.
Fig. 9. FT-IR spectra of samples: (a) S-2, (b) W18O49-400, (c) W18O49-450.
Fig. 8. (a–f): TEM images of S-2 annealed at 400 °C with different time: 0 min, 60 min, 90 min, 150 min, 210 min, 300 min; (g) XRD patterns of S-2 annealed at above corresponding conditions.
morphology over annealed time. With the increase of annealed time, the W18O49 nanowire networks structure gradually collapsed into WO3 particles. After 300 min, the S-2 can be totally annealed into WO3. Moreover, the color of powders turned from grey, to brown, and finally changed into light yellow. The surface cleanliness of the S-2, W18O49-400, and W18O49-450 had been evaluated by FTIR spectrum. In the region of 1000–500 cm 1, the bands characteristic for the W-O (950–800 cm 1) units and the stretching vibrations of the bridging oxygen atoms O-WO (600–780 cm 1) appeared in Fig. 9a–c [32]. For S-2, the peak at 3419 cm 1 was attributed to OAH stretching vibration, the peaks
at 2890 cm 1 and 1492 cm 1 may be attributed to stretching vibration and bending vibration of CAH coming from the heat decomposition of PABA, the peaks at 1617 cm 1 may be attributed to the bending vibration of adsorbed water [38]. It was obvious that the surface of W18O49 nanostructure fabricated with PABA was clearer than that with surfactants [14]. For W18O49-400 (Fig. 9b), the peak at 3449 cm 1 was attributed to OAH stretching vibration, the peaks at 1622 cm 1 may be attributed to the bending vibration of adsorbed water. The peaks of CAH were weaker due to the degradation of the organic adsorbate at 400 °C. The FTIR spectrum of W18O49-450 showed that the monoclinic WO3 nanocrystals had a very clean surface. The N2 adsorption-desorption isotherms and pore size distributions of S-2, W18O49-400, and W18O49-450 were displayed in Fig. 10. The isotherms of three samples were of type II (appearance of inflexion point at low P/P0 and absence of a plateau at high P/P0) [39]. Moreover, adsorption isotherms slowly increased with the increase of P/P0 at lower relative pressure while a rapid increase of adsorption isotherm was observed at higher P/P0; a saturation adsorption was not observed even at the saturation vapor pressure. All the phenomena indicated the existence of mesoporous or macropores [40]. The BET specific surface areas of S-2, W18O49400, and W18O49-450 reached to 146, 147, and 47 m2 g 1, respectively (Table 1). It can be concluded that annealing W18O49 at a relative low temperature (400 °C) would not change the pore structure of it. However, when the annealing temperature reached to 450 °C could not only give rise to a collapse of structure, but also a low specific surface area.
Please cite this article in press as: H. Chen et al., PABA-assisted hydrothermal fabrication of W18O49 nanowire networks and its transition to WO3 for photocatalytic degradation of methylene blue, Advanced Powder Technology (2018), https://doi.org/10.1016/j.apt.2018.02.020
H. Chen et al. / Advanced Powder Technology xxx (2018) xxx–xxx
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Fig. 12. The photocatalytic degradation of MB over: blank, W18O49-400, and W18O49-450.
Fig. 10. (a) Nitrogen adsorption-desorption isotherms and (b) the corresponding pore size distributions curves for as-prepared samples.
Table 1 Pore structure parameters of the S-2, W18O49-400, and W18O49-450. Samples S-2 W18O49-400 W18O49-450
SBET (m2 g 146 147 47.0
1
)
Average pore size (nm)
Pore volume (cm3 g 1)
11.0 10.0 17.6
0.768 0.702 0.409
nanocrystals. Fig. 10a showed digital photo of the instantaneous state that WO3 nanocrystal powders were dispersing in deionized water. The diffusion of the nanocrystals in water was similar with diffusion of ink. However, the phenomenon couldn’t be observed when S-2 or W18O49-400 was dissolved in water, respectively. Further, Fig. 11b showed that the contact angle of W18O49-450 is just 7.9°. It demonstrated that the nanocrystals have an excellent hydrophilicity. The photocatalytic properties of as-prepared samples were studied by the photodegradation of MB under visible light. Fig. 12 showed photocatalytic degradations of MB with different catalysts: blank (without photocatalyst), W18O49-400, and W18O49-450. Compared with the MB (10 mg L 1) solution without photocatalyst, W18O49-450 exhibited a little photocatalytic property with about 10 % higher than the blank, while the degradation rate of MB solution for W18O49-400 can reach to 40 %. The difference of the photodegradation performance between W18O49-400 and W18O49-450, on the one hand, can be attributed to the morphology and phase structure. On the other hand, the huge specific surface area also meant a high photocatalytic performance. In our previous work, we have discussed the adsorption performance of W18O49 to remove MB. In that work, the adsorption capacity of MB by the as-synthesized W18O49 nanowire networks is about 201 mg g 1, and MB with 10 mg L 1 can be almost completely removed from water with a W18O49 nanowire networks loading amount of 0.1 g L 1 [30]. Thus, we will not further discuss the photocatalytic performance of W18O49 without annealing here.
4. Conclusion
Fig. 11. Digital photos of W18O49-450: (a) aqueous solution, (b) contact angle.
3.5. Hydrophilic and photocatalytic performance The hydrophilic analysis of W18O49-450 powders was shown at Fig. 11, which confirmed the clean surface of monoclinic WO3
In summary, W18O49 nanowire networks have been fabricated by a simple two-pot hydrothermal method with tungsten acid as a precursor and PABA as a reducing and structure-directing agent. The amount of PABA plays an important role on the formation of non-stoichiometric tungsten oxide nanowires. The fabrication mechanism of the W18O49 nanostructure has been proposed by investigating the growth process of these nanowires. The thermal stability research shows that the product has a high stability that it does not decompose under 400 °C until the annealing time reaches to 300 min, and after that it will be transformed into WO3 nanocrystals completely. Additionally, W18O49 nanowire networks can also transform to monoclinic WO3 nanocrystal at 450 °C in 30 min, which exhibits an excellent hydrophilic property. What’s more, the W18O49 nanowire networks annealed at 400 °C show a superior photocatalytic activity to degrade MB aqueous solution under visible-light. Thus, in this article, we provide a more
Please cite this article in press as: H. Chen et al., PABA-assisted hydrothermal fabrication of W18O49 nanowire networks and its transition to WO3 for photocatalytic degradation of methylene blue, Advanced Powder Technology (2018), https://doi.org/10.1016/j.apt.2018.02.020
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H. Chen et al. / Advanced Powder Technology xxx (2018) xxx–xxx
specific synthetic and annealing strategy to prepare the tungsten oxides and a basic route to characterize these samples.
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Acknowledgements This work was supported by key project of Xinjiang colleges and universities (XJEDU2013I36) and International Cooperation Project of Xinjiang Science and Technology Bureau (2017E01005, 2017E01016).
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Please cite this article in press as: H. Chen et al., PABA-assisted hydrothermal fabrication of W18O49 nanowire networks and its transition to WO3 for photocatalytic degradation of methylene blue, Advanced Powder Technology (2018), https://doi.org/10.1016/j.apt.2018.02.020