Facile Synthesis of Highly Dispersed WO 3 ⋅ H 2 O and WO 3 Nanoplates for Electrocatalytic Hydrogen Evolution

The highly dispersed WO 3 ⋅H 2 O nanoplates have been synthesized by a facile hydrothermal reaction assisted by citrate acid. WO 3 nanoplates have been prepared by the calcination of as-preparedWO 3 ⋅H 2 Oat 450C. XRDdata show thatWO 3 ⋅H 2 OandWO 3 have good crystal structure and high purity. SEM images show that WO 3 ⋅H 2 O andWO 3 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 WO 3 ⋅H 2 O, thus leading to the good dispersion and small size of WO 3 ⋅H 2 O nanoplates. The electrocatalytic activity of WO 3 ⋅H 2 O and WO 3 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 WO 3 ⋅H 2 O andWO 3 nanoplates have been discussed.


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 earthabundant elements as active electrocatalysts represents future development of the electrocatalysts for HER [4].
As important n-type semiconductors, tungsten oxide hydrates (WO 3 ⋅nH 2 O) and tungsten oxides (WO 3 ) 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 WO 3 is more stable phase than any other WO 3 structures owing to the structure consisting of a threedimensional network of WO 6 octahedrons [13,14].
Up to now, many approaches have been adopted to synthesize WO 3 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].WO 3 ⋅nH 2 O has been usually prepared through the liquid-phase synthesis routes, and WO 3 has been synthesized by annealing WO 3 ⋅nH 2 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 largescale production of WO 3 with high crystal phase and high purity is still a challenge.
In our work, highly dispersed WO  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 WO 3 ⋅H 2 O and WO 3 nanoplates have been discussed.

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 2 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).

Electrochemical Measurement.
The glassy carbon electrode (geometric surface area of glassy carbon = 0.1256 cm 2 ), Pt plate, and Ag/AgCl electrode were used as working, counter, and reference electrodes, respectively.0.5 M H 2 SO 4 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.To investigate the morphology and size of WO 3 ⋅H 2 O, the as-prepared samples have been measured by SEM (in Figure 2).As shown in Figure 2 3.It can be seen that all the diffraction peaks of WO 3 samples are consistent with the monoclinic WO 3 phase (JCPDS number 43-1035).No other peaks from XRD patterns were detected, indicating that WO 3 ⋅H 2 O samples completely transform to the pure WO 3 phase with three distinct diffraction peaks of (002), (020), and (200), respectively.

Results and Discussion
The SEM images of the calcined WO 3 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 WO 3 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 WO 3 nanoplates appears with the slight increasing of thickness of WO 3 nanoplates due to the effect of the calcination process at 450 ∘ C.
The transformation process from WO 3 ⋅H 2 O nanoplates to WO 3 nanoplates is illustrated in Figure 5. Firstly, H 2 WO 4 phase formed when adding HCl into Na 2 WO 4 solution (1) as follows: Secondly, H 2 WO 4 precursor changed to [WO(OH) 4 (OH 2 )] (2), with the sixfold coordinated W 6+ 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 (OH 2 )] converted to the octahedral [WO 5 (OH 2 )] layer by oxolation, which is the structural unit of WO 3 ⋅H 2 O with more OH on the [010] facets [9].There are stable hydrogen bonds force-derived from the terminal oxygen atoms and coordinated water molecules between the neighbouring layers of WO 3 ⋅H 2 O, thus leading to the formation of WO 3 ⋅H 2 O with layers structure (Figure 5(b)).When being calcined at 450 ∘ C, the water molecules between the neighbouring layers of WO 3 ⋅H 2 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 WO 3 is obtained in Figure 5(c).During the hydrothermal process, the growth of WO 3 ⋅H 2 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 WO 3 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 [010] facet because there are many OH groups on the surface of [WO 5 (OH 2 )] 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   for HER.However, in our work, WO 3 nanoplates have a higher current density but a higher Tafel slope than that of WO 3 ⋅H 2 O.According to the previous report [24], which electrocatalyst is better depends on the targeted current  The electrochemical impedance spectra of the two different samples can be shown in Figure 7. WO 3 sample shows the smaller charge-transfer resistance than WO 3 ⋅H 2 O, which implies that WO 3 has much faster electron transfer and improved efficiency for HER than WO 3 ⋅H 2 O.The enhancement of conductivity also benefits from the compact structure of WO 3 after calcination at high temperature.The poor HER activity of WO 3 ⋅H 2 O may be attributed to the existence of crystal H 2 O. Firstly, the crystal H 2 O may occupy some active sites for HER, resulting in poor catalytic activity.Secondly, the poor conductivity of WO 3 ⋅H 2 O impedes the transportation of the electrons between the active sites and the electrode.So WO 3 ⋅H 2 O has the worse HER activity than WO 3 .

Conclusions
The highly dispersed WO 3 ⋅H 2 O and WO 3 nanoplates have been synthesized by a facile process assisted by citrate acid.The as-prepared WO 3 ⋅H 2 O and WO 3 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 [010] facet of [WO 5 (OH 2 )] layer may result in the good dispersion and small size of WO 3 ⋅H 2 O nanoplates.The electrocatalytic activity of WO 3 ⋅H 2 O and WO 3 for HER has been studied.The poor electrocatalytic activity of WO 3 ⋅H 2 O compared to WO 3 may be attributed to the existence of crystal H 2 O.

Figure 1
Figure 1 displays XRD patterns and SAED patterns of WO 3 ⋅H 2 O synthesized by hydrothermal route.As shown in Figure 1(a), all the peaks clearly demonstrate that the samples are the orthorhombic WO 3 ⋅H 2 O crystal, corresponding to the (020), (111), and (002) diffraction at 2 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 WO 3 ⋅H 2 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 WO 3 ⋅H 2 O with the orthorhombic phase.Figure 1(b) shows the corresponding SAED patterns of WO 3 ⋅H 2 O, indicating that these nanoplates have a polycrystalline orthorhombic WO 3 ⋅H 2 O phase.To investigate the morphology and size of WO 3 ⋅H 2 O, the as-prepared samples have been measured by SEM (in Figure2).As shown in Figure2(a), SEM images of WO 3 ⋅H 2 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 Figure2(b) that the thickness of each WO 3 ⋅H 2 O nanoplate is about 30 nm.The coarse surface and monodispersed structure of Figure 1 displays XRD patterns and SAED patterns of WO 3 ⋅H 2 O synthesized by hydrothermal route.As shown in Figure 1(a), all the peaks clearly demonstrate that the samples are the orthorhombic WO 3 ⋅H 2 O crystal, corresponding to the (020), (111), and (002) diffraction at 2 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 WO 3 ⋅H 2 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 WO 3 ⋅H 2 O with the orthorhombic phase.Figure 1(b) shows the corresponding SAED patterns of WO 3 ⋅H 2 O, indicating that these nanoplates have a polycrystalline orthorhombic WO 3 ⋅H 2 O phase.To investigate the morphology and size of WO 3 ⋅H 2 O, the as-prepared samples have been measured by SEM (in Figure2).As shown in Figure2(a), SEM images of WO 3 ⋅H 2 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 Figure2(b) that the thickness of each WO 3 ⋅H 2 O nanoplate is about 30 nm.The coarse surface and monodispersed structure of
Figure 6 shows the LSV curves and Tafel plots of WO 3 ⋅H 2 O and WO 3 nanoplates.It can be seen from Figure 6(a) that WO 3 ⋅H 2 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 WO 3 nanoplates is about −0.09 V.The exchange current density of WO 3 sample is about −7.5 mA cm −2 at overpotential of 300 mV.

Figure 6 (
b) shows Tafel slopes of 97 and 101 mV dec −1 for WO 3 ⋅H 2 O and WO 3 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
density.For example, to obtain the targeted current density of 10 mA cm −2 , WO 3 ⋅H 2 O sample requires −390 mV of overpotential, while WO 3 sample requires −340 mV.Therefore, WO 3 is the better electrocatalyst for HER.
3 ⋅H 2 O nanoplates have been synthesized by a facile hydrothermal reaction assisted by citrate acid.WO 3 nanoplates have also been prepared by the calcination of WO 3 ⋅H 2 O at 450 ∘ C for 4 h.SEM images show that as-prepared WO 3 ⋅H 2 O and WO 3 samples have the uniform nanoplates morphology with the edge length

Table 1 :
[12]area of WO 3 ⋅H 2 O and WO 3 nanoplates.⋅H 2 O nanoplates can also be clearly observed, which maybe indicate the lager specific surface area and more active sites for HER[12].From Table1, it can be seen that WO 3 ⋅H 2 O nanoplates have much higher Brunauer-Emmett-Teller (BET) specific surface area of 23.28 m 2 ⋅g −1 than WO 3 nanoplates (11.99 m 2 ⋅g −1 ), which implies that the BET area of WO 3 nanoplates is decreased after the calcination.XRD patterns of WO 3 after calcination at 450 ∘ C are shown in Figure