Effect of pH in the hydrothermal preparation of monoclinic tungsten oxide of Solid State Chemistry

This paper presents the preparation of monoclinic WO 3 by a one-step hydrothermal method. The effect of very acidic pH (0.1) and the signi ﬁ cance of various additives (CH 3 COOH, NaClO 4 , Na 2 SO 4 ) were investigated. To clarify the role of pH on the obtained crystal structure and morphology, every synthesis using pH 1 were repeated, and the effect of temperature, using 180 and 200 (cid:1) C, was also studied. All samples prepared at pH 0.1 were pure, well crystallized monoclinic WO 3 independently from the temperature, the presence and the quality of the ad- ditives. At 180 and 200 (cid:1) C, applying CH 3 COOH and NaClO 4 resulted nanosheets similar in size. With Na 2 SO 4 additive at 180 (cid:1) C sheets, at 200 (cid:1) C sheets and also rods formed indicating that SO 42 (cid:3) was a capping agent only at 200 (cid:1) C. For comparison, at pH 1at both temperatures the crystalline phases and the morphologies varied depending on the type of the additive.

In most cases the one-step hydrothermal treatment results hexagonal (h-) or orthorhombic (o-) WO 3 phase, and for the preparation of monoclinic (m-) WO 3 there is always need for a post calcination step [21,[42][43][44][45][46][47]. The implementation for one-step hydrothermal synthesis of m-WO 3 without any post calcination would be huge improvement, since it is the most examined phase of WO 3 . So far, however, only some papers have reported about its one-step hydrothermal synthesis [48][49][50][51][52]. Although, in these works the authors studied the effect of various parameters such as temperature, time, pH in the acidic range or concentration of the acid, the role of pH is still not clear. Since these preparations were carried out always in the presence of some additive, it is not clarified, whether the additive itself, the additive together with the adjusted pH, or only the pH is responsible for the formation of m-WO 3 .
In this report, we prepared m-WO 3 by one-step hydrothermal reaction without any calcination using 0.1 pH. Moreover, we investigated the effects of different additives such as CH 3 COOH in the volume of 1.25/ 2.5/5 and 7.5 mL, as well as Na 2 SO 4 and NaClO 4 . To get a definite answer to the effect of pH we carried out syntheses also at pH 1 and also without any additive. For revealing the influence of temperature, we used 180 and 200 C as well. We investigated the obtained crystalline phases and morphology of the samples by XRD and SEM, then the pure m-WO 3 products were further examined by FT-IR, Raman, UV-Vis spectroscopy, TEM and EDX. Finally, their specific surface area and band gaps were also determined.

Hydrothermal synthesis
Every synthesis was carried out as the following: first, 4.1 g Na 2 WO 4 ⋅H 2 O was dissolved in 100 mL ion exchanged water. Under continuous stirring 3 M HCl acid solution was used to set the required pH. Next, 3.2 g (COOH) 2 ⋅2H 2 O was added and the solution was completed up to 250 mL. In our former work we studied the role of oxalic acid and found, it improved the crystallinity of the products without changing the crystalline phases [53]. Based on this result, we used it in every preparation. In the following step, 30 mL solution was taken out and mixed with certain additive. Then, it was poured into a 45 mL autoclave (Parr Instruments) and put into a furnace at 180 or 200 C for 24 h. Finally, the product was filtered, washed with ion exchanged water followed by ethanol and dried at 60 C for 2 h.
The performed reactions are listed in Table 1. The obtained minor crystal phases are written in italics.

Characterization
For X-ray powder diffraction (XRD) measurements a PANanalytical X'Pert Pro MPD diffractometer with Cu Kα radiation (λ ¼ 0.15418 nm), while for investigating the morphology of the samples a LEO 1540 XB electron microscope was used. Energy-dispersive X-ray spectroscopy (EDX) analyses were carried out by a JEOL JSM 5500-LV instrument. FT-IR spectra were taken by a PerkinElmer 2000 FT-IR spectrometer between 450 and 4000 cm À1 applying KBr pellets (1 mg sample/300 mg KBr). Raman spectra were recorded by a Jobin Yvon LabRam spectrometer equipped with an Olympus BX41 optical microscope using a frequency doubled Nd-YAG laser (532 nm), while diffuse reflectance UV-Vis spectra by a Jasco V-570 UV/VIS/NIR spectrometer. Transmission electron microscopy (TEM) images were taken by a FEI Tecnai G2 20 X-TWIN instrument operated at 200 keV. N 2 adsorption was measured at -196 C with a Nova2000e (Quantachrome) computercontrolled apparatus. The apparent surface area (S BET ) was calculated from the Brunauer-Emmett-Teller (BET) model [55].

Results and discussion
3.1. Crystal structure and morphology 3.1.1. Samples obtained at 180 C, pH 0.1 Based on XRD patterns, samples 1-7 are identified as pure m-WO 3 (ICDD 04-005-4272) independently on the used additive (Fig. 1). The samples are well crystallized indicated by the sharp and narrow peaks without any other phases or impurities. In the case of using CH 3 COOH a slight difference in crystallinity can be observed, i.e. the smaller acid volume improves it more, which is demonstrated by the much sharper reflections of samples 1 and 2.
Based on the SEM images, not only the crystalline phase is the same, but also the morphology of samples 1-7 is very similar. Uniformly, the morphology consists of mostly sheets with cuboid-like shapes. In the case of using CH 3 COOH additive, the sheets are generally 20-100 nm thick and 200-300 nm wide and long, and the ratio of the cuboids is getting more significant due to the larger volume of the acid (samples 1-4, Fig. 2). The sample prepared with NaClO 4 has similar morphology, containing bit thicker, 50-200 nm sheets (samples 5-6, Fig. 2), while using Na 2 SO 4 additive results much more robust appearance of sheets, namely 100-200 nm thickness, 200-300 nm width and 350-800 nm length. Intriguingly, without additive, the morphology has the similar characteristics as sample 1 (sample 7, Fig. 2).

Samples obtained at 180 C, pH 1
All XRD reflections are assigned to orthorhombic WO 3 •0.33H 2 O (ICDD 01-087-1203) in the case of using CH 3 COOH additive (samples 8-11, Fig. 3). The sharp, well distinguished peaks prove high degree of crystallinity of every sample independently from the volume of the acid.
On the SEM images homogenous morphology can be seen containing strongly agglomerated forms (Fig. 4). The WO 3 ⋅0.33H 2 O crystals formed in angular shapes with 100-200 nm thickness and width and 200-300 nm length in the case of 1.25-5 mL CH 3 COOH (samples 8-10, Fig. 4). These become longer when 7.5 mL acid was used, in general, more than 300 nm (sample 11, Fig. 4).

Samples obtained at 200 C, pH 0.1
Every sample is identified as pure, single phase m-WO 3 with high crystallinity (Fig. 5). The lower volume of CH 3 COOH is more beneficial as it gives narrower and sharper XRD peaks. Similar was observed in the case of 180 C. The asterisk at the samples prepared at pH 1 with the usage of NaClO 4 , Na 2 SO 4 or without additive refers to samples which we prepared in one of our former studies [53,54]. Thus, these results are not here reported in detail along with the last sample in Table 1 (200 C, pH 1, without additive). Fig. 6 shows the homogenous, well defined morphology of samples 12-18. Nanosheets are the characteristic forms when CH 3 COOH or NaClO 4 and even when no additive were used, however, elongated, anisotropic forms appear when Na 2 SO 4 was applied. The sheets are 20-100 nm thick and 200-300 nm wide in the case of CH 3 COOH, similarly to those observed at 180 C, but became much longer from 300 to 800 nm due to the higher temperature. Beside the thin sheets, cubes and wider angular forms also appear in some places (samples 12-15, Fig. 6). When NaClO 4 was used, 100-200 nm thick and 200-300 nm width sheets formed with more than 400 nm length (sample 16, Fig. 6). When no additive was used, the obtained morphology was similar to samples 12-16 (sample 18, Fig. 6). In the case of Na 2 SO 4 additive, however, the morphology is mainly consisted of rods along with the sheets and consisted of mainly rods along with sheets. These rods are 200-400 nm thick and wide and can be even more than 1 μm long.     The effect of Na 2 SO 4 additive on the crystalline phase and morphology in the hydrothermal reaction of Na 2 WO 4 and HCl is well known. It results hexagonal (h-)WO 3 with rod-like morphology at around pH 1 [53,56,57]. Its role as a capping agent in this work, however, seemed to be varying. Based on the XRD and SEM results neither h-WO 3 , nor rods formed at pH 0.1, 180 C (Figs. 1-2). At 200 C, however, the effect of Na 2 SO 4 additive on the morphology can be observed and the product contained rod-like figures as well beside nanosheets.

Samples obtained at 200 C, pH 1
Samples 19-22 were identified as crystalline WO 3 ⋅0.33H 2 O with small amount of m-WO 3 (Fig. 7). The ratio of m-WO 3 phase decreases along with the decreasing volume of CH 3 COOH.
Samples 19-22 have similar morphology consisting of objects with shorter and longer angular shapes (Fig. 8). They are, in general, at least 200 nm thick, 200-300 nm wide and 200-800 nm long. Comparing them to samples 8-11, prepared at 180 C, we can conclude, that higher temperature is favourable for the growth of larger crystals. Due to that much greater but fewer crystal formed.
From the above detailed results we concluded, that neither the quality of the additive, nor the temperature had considerable influence on the obtained crystalline phase at pH 0.1. The formation of the m-WO 3 phase can be attributed only to the highly acidic pH. Therefore, we further investigated only the as-prepared new m-WO 3 phase found in samples 1-7.

Elemental composition (EDX)
A typical EDX spectrum shows the main components of samples 1-7, namely W and O (Fig. 9A). There are no other elements present referring to any impurity.

FT-IR, Raman spectroscopy
In the FT-IR spectra of samples 1-7, the region of 1000 and 500 cm À1 refers to the characteristic lattice vibrations of WO 3 (Fig. 9B). There are three well distinct peaks at around 750, 815 and 950 cm À1 , which belong to ν(W-O), ν(W-O-W) and ν(W-O, W--O), respectively. The broad peak at 3400-3600 cm À1 along with the band at 1600 cm À1 and in the case of some samples at 1400 cm À1 are ascribed to the vibration of ν sym (OH) of hydroxyl group as well as δ(OH) and ν(OH) of W-OH interaction, respectively (framed in Fig. 9B) [11,[58][59][60][61]. The appearance of the latter is more considerable when the volume of the acetic acid additive increases (samples 3-4) and in the case of NaClO 4 additive (sample 5). Fig. 9C shows the Raman spectra of samples 1-7 which are the same without any difference in contrast with the FT-IR spectra. The main peaks at 810 and 710 cm À1 are ascribed to the stretching vibrations of m-WO 3 . Bands appearing at 320 and 270 cm À1 are assigned to the bending modes, while the less intensive ones below 200 cm À1 belong to the lattice vibrations of the monoclinic phase [15,23,[62][63][64][65].

UV-Vis spectroscopy, band gap
According to the diffuse reflectance UV-Vis spectra of samples 1-7, significant absorption develops only in UV and the UV-near region of the   visible light (Fig. 9D). Their absorption edge is located at 450-460 nm in good agreement to the light yellow colour of m-WO 3 phase. Based on the spectra, the band gaps were also calculated ( Table 2).
Calculation was carried out using αhυ ¼ A ðhυ À E g Þ n equation, where α is molar absorption coefficient, hυ is the photon energy, A is a constant, E g is the band gap energy and n is depending on the direct or indirect allowed or forbidden type of the electron transition of the material, but is 2 for WO 3 . With plotting αhυ against hυ (Tauc-plot), drawing a tangent line onto the linear range and extrapolating, the value at hυ ¼ 0 gave the band gap energy (eV). For approximating A, Kubelka-Munk function was used [51,[66][67][68]. Table 2 shows the band gap energy of each sample derived from diffuse reflectance spectra which are in good accordance with the reported range (2.5-3.0) [17,21,22,51,[69][70][71]. The calculated values are technically equal which means, that neither the quality, nor the quantity of the used additives change the optical properties of m-WO 3 phase.

TEM
The TEM images show, that the 150-250 nm wide sheets are single crystalline and have smooth surface (Fig. 10). Their strict, straight edges can be clearly seen confirming the angular shapes appeared on the SEM images. Their thinness agrees with the SEM images and is obvious due to the visibility of other sheets ordered under each other. This up and down ordering is confirmed by the step-like edges at samples 5 and 6.

Specific surface area
The apparent surface area of the samples is listed in Table 3. S BET of samples 1 and 5 are comparable, considering the similar nanosheet morphology, however, 6 has much smaller area which can be attributed to the more robust appearance of sheets, which were considerably thicker than in the case of samples 1 and 5, as discussed in 3.1.1.

Conclusion
In this study, we successfully prepared m-WO 3 using pH in the very acidic range (pH 0.1) during a one-step hydrothermal method without any post-calcination. Besides, we investigated the effect of various additives such as CH 3 COOH in different volumes (1.25/2.5/5.0/7.5 mL), as well as NaClO 4 , Na 2 SO 4 and changed the temperature from 180 to 200 C. As reference we carried out reactions using no additive, as well. To find out the role of pH in the formation of m-WO 3 we repeated every synthesis at pH 1. The samples prepared at pH 0.1 at 180 C and 200 C were pure m-WO 3 in all cases, independently on the type or on the presence of the additive. The samples had similar, nanosheet-like morphology in the case of CH 3 COOH, NaClO 4 and even when no additives were used, at 180 and also at 200 C. When Na 2 SO 4 was used, however, elongated sheets were obtained at 180 C, but the morphology consisted of mainly rods with more than 1 μm length at 200 C. The appearance of the elongated sheets and rods can be attributed to the structure directing role of Na 2 SO 4 . Applying pH 1, the usage of CH 3 COOH in every volume, NaClO 4 and the absence of any additives resulted WO 3 ⋅0.33H 2 O, but Na 2 SO 4 gave h-WO 3 . The morphology was affected by the type of the additives. The m-WO 3 samples prepared at pH 0.1 were further studied by EDX, FT-IR, Raman and UV-Vis spectroscopies, TEM and their band gap (2.57 eV) and specific surface area were  For further investigating the nanosheet morphology of m-WO 3 , TEM images were also taken of samples 1, 5 and 6, and their specific surface area were also determined.   Table 3 Specific surface area of samples 1, 5 and 6.