Facile Synthesis of Highly Dispersed WO3·H2O and WO3 Nanoplates for Electrocatalytic Hydrogen Evolution

Abstract

The highly dispersed WO3·H2O nanoplates have been synthesized by a facile hydrothermal reaction assisted by citrate acid. WO3 nanoplates have been prepared by the calcination of as-prepared WO3·H2O at 450°C. XRD data show that WO3·H2O and WO3 have good crystal structure and high purity. SEM images show that WO3·H2O and WO3 have the uniform nanoplates morphology with the edge length of about 100-150 nm. The selective absorbance of citrate acid with many OH groups onto [010] facet of tungsten oxide precursors can result in the controlled growth of WO3·H2O, thus leading to the good dispersion and small size of WO3·H2O nanoplates. The electrocatalytic activity of WO3·H2O and WO3 for hydrogen evolution reaction (HER) has been investigated in detail. The good electrocatalytic activity for HER has been obtained, which may be attributed to the good dispersion and small size of nanoplates. And the growth mechanisms of WO3·H2O and WO3 nanoplates have been discussed.

Wen-Hui Hu 1 and Guan-Qun Han 1,2 and Bin Dong 1,2 and Chen-Guang Liu 1

Academic Editor:Chunyi Zhi

1, State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266580, China
2, College of Science, China University of Petroleum (East China), Qingdao 266580, China

Received 9 December 2014; Accepted 17 January 2015; 2 February 2015

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Hydrogen evolution reaction has attracted the growing interest because hydrogen as a promising sustainable energy carrier could accelerate the transition from the hydrocarbon economy to sustainable energy economy [1]. One of the promising fashions of hydrogen production is to adopt electrochemical [2] route. Novel metals such as Pt are the highly active electrocatalysts for HER [3], but the disadvantages of Pt are high cost and limited reserve, preventing the utilization of novel metal electrocatalysts. Therefore, designing the earth-abundant elements as active electrocatalysts represents future development of the electrocatalysts for HER [4].

As important n -type semiconductors, tungsten oxide hydrates (WO₃ ·n H₂ O) and tungsten oxides (WO₃ ) have obtained more and more attention due to their polytypic structures and excellent physical/chemical properties [5]. Many applications have been extensively investigated such as lithium-ion batteries [6], supercapacitors [7], gas sensors [8], photocatalysts [9, 10], solar energy devices [11], and electrocatalyst in electrolysis of water for HER [12]. Furthermore, the monoclinic WO₃ is more stable phase than any other WO₃ structures owing to the structure consisting of a three-dimensional network of WO₆ octahedrons [13, 14].

Up to now, many approaches have been adopted to synthesize WO₃ nanostructures with different morphologies including nanotubes [15], nanowires [16], nanorods [17], nanoplates [18], and hollow spheres [19]. Among the different morphologies, nanoplate-like structure displays excellent gas response and catalytic properties because of their high density of surface sites [20]. WO₃ ·n H₂ O has been usually prepared through the liquid-phase synthesis routes, and WO₃ has been synthesized by annealing WO₃ ·n H₂ O to remove crystal water. The hydrothermal method can be used owing to some advantages in controlling the morphology, size, and homogeneity at the mild temperature for large-scale production [21]. However, developing a facile route for large-scale production of WO₃ with high crystal phase and high purity is still a challenge.

In our work, highly dispersed WO₃ ·H₂ O nanoplates have been synthesized by a facile hydrothermal reaction assisted by citrate acid. WO₃ nanoplates have also been prepared by the calcination of WO₃ ·H₂ O at 450°C for 4 h. SEM images show that as-prepared WO₃ ·H₂ O and WO₃ samples have the uniform nanoplates morphology with the edge length of about 100-150 nm. The electrocatalytic activity of the two samples for HER properties has been investigated in detail. The good electrocatalytic activity for HER has been obtained, which may be attributed to the good dispersion and small size of nanoplates. And the growth mechanisms of highly dispersed WO₃ ·H₂ O and WO₃ nanoplates have been discussed.

2. Experimental

2.1. Preparation of WO₃ ·H₂ O and WO₃ Nanoplates

All chemical reagents were of analytic purity and used directly without further purification. WO₃ ·H₂ O nanoplates have been prepared by dissolving 0.4 g citrate acid and 0.53 g Na₂ WO₄ ·2H₂ O in 40 mL deionized water in order under stirring. The pH value of the solution was adjusted to about 1.0 by the addition of HCl. Then the mixture was transferred into a 50 mL Teflon-lined stainless steel autoclave and sealed, and the autoclave was placed in a preheated oven at 120°C for 3 h. After being naturally cooled down to room temperature, the yellow precipitates were collected by filtration and washed with deionized water and ethanol for several times and dried in oven at 80°C. The as-prepared WO₃ ·H₂ O nanoplates have been calcined at 450°C for 4 h to prepare WO₃ samples. The yellow WO₃ ·H₂ O samples changed to white-yellow WO₃ samples.

2.2. Characterization of Morphology and Structure

Crystallographic structure of all as-prepared samples was investigated with X-ray powder diffraction (XRD, X'Pert PRO MPD, Cu KR) at a scanning rate of 8°C min^-1 . XRD data were collected in the 2θ ranges from 10 to 60°. The morphology of the samples was examined with field-emission scanning electron microscopy (SEM, Hitachi, S-4800). Selected area electron diffraction (SAED) was used to examine samples' crystallinity (TEM, JEM-2100UHR with an accelerating voltage of 200 kV).

2.3. Electrochemical Measurement

The glassy carbon electrode (geometric surface area of glassy carbon = 0.1256 cm² ), Pt plate, and Ag/AgCl electrode were used as working, counter, and reference electrodes, respectively. 0.5 M H₂ SO₄ solution was used as electrolyte for linear sweep voltammetry and electrochemical impedance spectra on the Gamry reference 600 electrochemical workstation. The working electrodes were prepared by catalytic ink-dispersing 20 mg catalyst in 1 mL ethanol and 0.01 mL of 5 wt% Nafion under 30 min ultrasonic radiation.

3. Results and Discussion

Figure 1 displays XRD patterns and SAED patterns of WO₃ ·H₂ O synthesized by hydrothermal route. As shown in Figure 1(a), all the peaks clearly demonstrate that the samples are the orthorhombic WO₃ ·H₂ O crystal, corresponding to the (020), (111), and (002) diffraction at 2θ of 16.5°, 25.7°, and 35.1° (JCPDS number 43-0679 with lattice constants of a = 0.5238 nm, b = 0.1070 nm, and c = 0.5120 nm). The high intensity and narrow peaks of (020) and (111) mean good crystallinity of WO₃ ·H₂ O. No other impurity peaks can be observed from the patterns, indicating high purity of the sample. It can be concluded that the hydrothermal process assisted by citrate acid could produce the pure WO₃ ·H₂ O with the orthorhombic phase. Figure 1(b) shows the corresponding SAED patterns of WO₃ ·H₂ O, indicating that these nanoplates have a polycrystalline orthorhombic WO₃ ·H₂ O phase.

(a) XRD patterns of WO₃ ·H₂ O as-prepared by hydrothermal method. (b) Selected area electron diffraction (SAED) patterns of WO₃ ·H₂ O.

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

To investigate the morphology and size of WO₃ ·H₂ O, the as-prepared samples have been measured by SEM (in Figure 2). As shown in Figure 2(a), SEM images of WO₃ ·H₂ O assisted by citrate acid show the uniform nanoplates morphology with the edge length of about 100-150 nm. With higher magnification, it can be seen from Figure 2(b) that the thickness of each WO₃ ·H₂ O nanoplate is about 30 nm. The coarse surface and monodispersed structure of WO₃ ·H₂ O nanoplates can also be clearly observed, which maybe indicate the lager specific surface area and more active sites for HER [12]. From Table 1, it can be seen that WO₃ ·H₂ O nanoplates have much higher Brunauer-Emmett-Teller (BET) specific surface area of 23.28 m² ·g^-1 than WO₃ nanoplates (11.99 m² ·g^-1 ), which implies that the BET area of WO₃ nanoplates is decreased after the calcination.

Table 1: BET area of WO₃ ·H₂ O and WO₃ nanoplates.

Materials	WO3 ·H2 O	WO3
BET area (m2 ·g-1 )	23.28	11.99

SEM images of WO₃ ·H₂ O nanoplates.

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

XRD patterns of WO₃ after calcination at 450°C are shown in Figure 3. It can be seen that all the diffraction peaks of WO₃ samples are consistent with the monoclinic WO₃ phase (JCPDS number 43-1035). No other peaks from XRD patterns were detected, indicating that WO₃ ·H₂ O samples completely transform to the pure WO₃ phase with three distinct diffraction peaks of (002), (020), and (200), respectively.

Figure 3: XRD patterns of WO₃ after being calcined in 450°C for 4 h.

[figure omitted; refer to PDF]

The SEM images of the calcined WO₃ samples are shown in Figure 4. It can be found in Figure 4(a) that the similar nanoplates morphology has been maintained during the process of calcination at 450°C. However, the surface of WO₃ nanoplates obviously becomes smooth, which may be attributed to the elimination of water molecules between the tungstite layers. Figure 4(b) shows that some aggregation growth of WO₃ nanoplates appears with the slight increasing of thickness of WO₃ nanoplates due to the effect of the calcination process at 450°C.

SEM images of WO₃ nanoplates.

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

The transformation process from WO₃ ·H₂ O nanoplates to WO₃ nanoplates is illustrated in Figure 5. Firstly, H₂ WO₄ phase formed when adding HCl into Na₂ WO₄ solution (1) as follows: [figure omitted; refer to PDF]

Figure 5: Schematic illustration of the formation mechanism of WO₃ nanoplates.

[figure omitted; refer to PDF]

Secondly, H₂ WO₄ precursor changed to [WO(OH)₄ (OH₂ )] (2), with the sixfold coordinated W⁶⁺ including one oxygen atom and water molecule along b -axis, two OH groups along a -axis, and two OH groups along c -axis, respectively (as shown in Figure 5(a)). Thirdly, during hydrothermal process, [WO(OH)₄ (OH₂ )] converted to the octahedral [WO₅ (OH₂ )] layer by oxolation, which is the structural unit of WO₃ ·H₂ O with more OH on the [figure omitted; refer to PDF] facets [9]. There are stable hydrogen bonds force-derived from the terminal oxygen atoms and coordinated water molecules between the neighbouring layers of WO₃ ·H₂ O, thus leading to the formation of WO₃ ·H₂ O with layers structure (Figure 5(b)). When being calcined at 450°C, the water molecules between the neighbouring layers of WO₃ ·H₂ O have been released with keeping the layer structure. The dehydration process has been proved to be a topotactic process [22]. Finally, the structure of monoclinic WO₃ is obtained in Figure 5(c). During the hydrothermal process, the growth of WO₃ ·H₂ O nanoplates has largely been affected by the citrate acid as dispersant and control agent. Similar research has been reported that certain crystal faces of WO₃ can be impeded by using that preferentially adsorbed to specific crystal faces with the assistance of malic acid [23]. In our work, citrate acid including many OH groups can be selectively absorbed onto [figure omitted; refer to PDF] facet because there are many OH groups on the surface of [WO₅ (OH₂ )] layer. The selective absorbance may be attributed to the strong interaction between the OH groups (Figure 5(d)). Therefore, the stacking growth along (010) facet has been controlled by the adding of citrate acid. And the good dispersion and small size of WO₃ ·H₂ O nanoplates could be obtained.

The electrocatalytic activity for HER of WO₃ ·H₂ O and WO₃ nanoplates has been studied in 1 M H₂ SO₄ solution over the potential range of -0.60 to +0.8 V at a scan rate of 50 mVs^-1 by the linear sweep voltammetry (LSV), Tafel plots, and electrochemical impedance spectra. Figure 6 shows the LSV curves and Tafel plots of WO₃ ·H₂ O and WO₃ nanoplates. It can be seen from Figure 6(a) that WO₃ ·H₂ O exhibits the electrocatalytic activity for HER with onset potential of -0.20 V (versus RHE) and the exchange current density of -4.5 mA cm^-2 at overpotential of 300 mV. The onset potential for the HER of WO₃ nanoplates is about -0.09 V. The exchange current density of WO₃ sample is about -7.5 mA cm^-2 at overpotential of 300 mV.

(a) Linear sweep voltammogram curves of WO₃ ·H₂ O and WO₃ nanoplates. (b) Tafel plots of WO₃ ·H₂ O and WO₃ nanoplates.

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

Figure 6(b) shows Tafel slopes of 97 and 101 mV dec^-1 for WO₃ ·H₂ O and WO₃ nanoplates, respectively. The ideal electrocatalysts should have low Tafel slopes and high cathodic current densities. For example, Pt has a high current density in the order of 10^-3 A cm^-2 and a Tafel slope of 30 mV dec^-1 for HER. However, in our work, WO₃ nanoplates have a higher current density but a higher Tafel slope than that of WO₃ ·H₂ O. According to the previous report [24], which electrocatalyst is better depends on the targeted current density. For example, to obtain the targeted current density of 10 mA cm^-2 , WO₃ ·H₂ O sample requires -390 mV of overpotential, while WO₃ sample requires -340 mV. Therefore, WO₃ is the better electrocatalyst for HER.

The electrochemical impedance spectra of the two different samples can be shown in Figure 7. WO₃ sample shows the smaller charge-transfer resistance than WO₃ ·H₂ O, which implies that WO₃ has much faster electron transfer and improved efficiency for HER than WO₃ ·H₂ O. The enhancement of conductivity also benefits from the compact structure of WO₃ after calcination at high temperature. The poor HER activity of WO₃ ·H₂ O may be attributed to the existence of crystal H₂ O. Firstly, the crystal H₂ O may occupy some active sites for HER, resulting in poor catalytic activity. Secondly, the poor conductivity of WO₃ ·H₂ O impedes the transportation of the electrons between the active sites and the electrode. So WO₃ ·H₂ O has the worse HER activity than WO₃ .

Figure 7: Electrochemical impedance spectra of WO₃ ·H₂ O and WO₃ nanoplates.

[figure omitted; refer to PDF]

4. Conclusions

The highly dispersed WO₃ ·H₂ O and WO₃ nanoplates have been synthesized by a facile process assisted by citrate acid. The as-prepared WO₃ ·H₂ O and WO₃ have the uniform nanoplates morphology with good crystal structure and high purity, which may be attributed to the adding of citrate acid. The selective absorbance of citrate acid onto [figure omitted; refer to PDF] facet of [WO₅ (OH₂ )] layer may result in the good dispersion and small size of WO₃ ·H₂ O nanoplates. The electrocatalytic activity of WO₃ ·H₂ O and WO₃ for HER has been studied. The poor electrocatalytic activity of WO₃ ·H₂ O compared to WO₃ may be attributed to the existence of crystal H₂ O.

Acknowledgment

This work is financially supported by the National Natural Science Foundation of China (no. U1162203).

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

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