X. LUO et al.: SYNTHESIS OF (NH4)XWO3 NANORODS BY A NOVEL HYDROTHERMAL ROUTE 227–23 1 SYNTHESIS OF (NH 4 ) X WO 3 NANORODS BY A NOVEL HYDROTHERMAL ROUTE SINTEZA (NH 4 ) x WO 3 NANOPAL^K Z NOVIM HIDROTERMALNIM POSTOPKOM Xianguo Luo 1 , Shubin Zhu 1 , Xuhong Su 1 , Jianguo Huang 1 , Zekun Zhou 1 , Qiongyu Zhou 2 , Yufeng Wen 3 , Ping Ou 1* 1 Jiangxi University of Science and Technology, School of Materials Science and Engineering, Hongqi Ave. No. 86, Ganzhou Jiangxi 341000, China 2 Foshan University, School of Materials Science and Energy Engineering, Jiangwan Road No. 18, Foshan Guangdong 528000, China 3 Jinggangshan University, School of Mathematical Sciences and Physics, Xueyuan Road No. 28, Ji’an Jiangxi 343009, China Prejem rokopisa – received: 2019-06-13; sprejem za objavo – accepted for publication: 2019-11-08 doi:10.17222/mit.2019.127 We report on a novel hydrothermal route for synthesizing ammonium tungsten bronze ((NH4) xWO3) nanorods. As-prepared (NH4) xWO3 nanorods have an average diameter of 150 nm and length of 2 μm. These single-crystalline nanorods are hexagonal and grow along the c-axis. The formation process of the (NH4) xWO3 nanorods is also discussed based on time-dependent experimental results. Keywords: hydrothermal, (NH4) xWO3, nanorod Avtorji v pri~ujo~em ~lanku poro~ajo o novem hidrotermalnem postopku za sintezo nanopal~k amonijevega volframata ((NH4) xWO3). Pripravljene nanopal~ke (NH4) xWO3 so imele povpre~ni premer 150 nm in dol`ino 2 μm. Te monokristalini~ne nanopal~ke imajo heksagonalno strukturo in rastejo vzdol` c-osi. Potek procesa tvorbe (NH4) xWO3 nanopal~k avtorji opisujejo na osnovi ~asovno odvisnih preizkusov. Klju~ne besede: hidrotermalni postopek, (NH4) xWO3, nanopal~ke 1 INTRODUCTION Hexagonal tungsten bronzes are a group of non- stoichiometric compounds with the general formula M x WO 3 (M=Cs, K, Na, Rb, NH 4 , etc.) that consists of mixed-valence tungsten ions (W 6+ and W 5+ ). M x WO 3 particles have been demonstrated to exhibit excellent near-infrared (NIR) absorption properties when dis- persed in a one-dimensional form due to their unique surface electronic structures and crystallographic defects. 1–6 The designed growth of (NH 4 ) x WO 3 nanorods is of great significance because they also exhibit mixed-valence tungsten ions and show NIR shielding abilities. Additionally, a substitution of alkali metal tungsten bronzes with (NH 4 ) x WO 3 can avoid the consumption of alkali metals, especially the expensive Cs. Furthermore, owing to the open-tunnel structure and special electronic properties of (NH 4 ) x WO 3 , the mobility of the cations in the channels of the WO 3 framework allows a dramatic modification of their electronic pro- perties due to ion exchange or intercalation, giving the (NH 4 ) x WO 3 species broad application prospects as catalytic, battery, gas sensing and electrochromic ma- terials. 7 Unfortunately, due to the high structural distortion of (NH 4 ) x WO 3 as a result of the insertion of large NH 4+ ions into the WO 6 octahedral framework, it is still fairly difficult to synthesize. Until now, there have been few reports on the synthesis of one-dimensional (NH 4 ) x WO 3 . S. Guo et al. 6 reported a solvothemal method for prepar- ing (NH 4 ) x WO 3 nanorods at 200 °C for 72 h. However, the practical applications of this process are limited by complicated procedures, long reaction times and the use of expensive and environmentally unfriendly organic solvents. Consequently, exploiting simple, mild, low-cost routes for the preparation of (NH 4 ) x WO 3 is quite imper- ative and exceedingly challenging. In this work, we propose a novel, simple hydrother- mal route for synthesizing (NH 4 ) x WO 3 nanorods at 200 °C for 12 h, with sodium tungstate dihydrate (Na 2 WO 4 ·2H 2 O), thiourea (CH 4 N 2 S) and citric acid monohydrate (C 6 H 8 O 7 ·H 2 O) as the starting materials. Single-crystalline (NH 4 ) x WO 3 nanorods were obtained under mild conditions at a low cost. The formation of the (NH 4 ) x WO 3 nanorods is also discussed based on time- dependent experiments. Materiali in tehnologije / Materials and technology 54 (2020) 2, 227–231 227 UDK 620.1:542:620.3 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(2)227(2020) *Corresponding author's e-mail: opyp@163.com (Ping Ou) 2 EXPERIMENTAL PART In a typical procedure, 3.07 g of Na 2 WO 4 ·2H 2 O, 1.78 gofCH 4 N 2 S and 1.92 g of C 6 H 8 O 7 ·H 2 O were dissolved in 30 mL of deionized water under vigorous stirring for 10 min. The obtained mixed solution was transferred into a 50-mL stainless-steel Teflon-lined autoclave, which was filled with deionized water up to 80 % of the total volume. The hydrothermal treatment was performed by placing the sealed autoclave in an oven and maintaining it at 200 °C for different reaction times. Then the auto- clave was taken out and cooled to room temperature in air. The products were filtered and washed with deionized water and ethanol several times in turn, and finally oven dried in air at 60 °C for 12 h. Powder X-ray diffraction (XRD) patterns of the sam- ples were measured on a D8 advance X-ray diffrac- tometer with high-intensity Cu-K ( = 0.15418 nm) radiation. The surface composition and binding energy of the samples were determined with an Esccalab 250Xi X-ray photoelectron spectrometer (XPS). Scanning-elec- tron-microscope (SEM) images were obtained with a Zeiss igma field-emission scanning-electron micro- scope. Transmission-electron-microscope (TEM) and high-resolution TEM (HRTEM) images were obtained using a FEI Tecnai G2 20 transmission electron micro- scope at 200 kV. 3 RESULTS AND DISCUSSION Figure 1a shows the XRD pattern of the as-prepared sample hydrothermally synthesized at 200 °C for 12 h. It was found that all the diffraction peaks could be well indexed to the hexagonal (NH 4 ) x WO 3 with known lattice constants of a = 0.7388 nm and c = 0.7551 nm (ICDD PDF no. 73-1084), and no characteristic peaks for im- purities such as WO 3 or WO 3-x were observed. The che- mical composition and valence state of the as-prepared (NH 4 ) x WO 3 sample was examined with XPS. The fully scanned spectra clearly show that N, W, O and C existed in the sample (Figure 1b). The presence of carbon in the final product may be related to the residual, chemically or physically adsorbed organics originating from CH 4 N 2 SorC 6 H 8 O 7 ·H 2 O. The XPS peak of N1s located at 402.2 eV may be related to the NH 4+ ions in (NH 4 ) x WO 3 . For tungsten, a complex energy distribution of W4f photoelectrons was obtained, as shown in Figure 1c. After deconvolution, the W4f core-level spectrum could be well fitted to two spin-orbit doublets, corres- ponding to W atoms in two different oxidation states. The main peaks, W4f 5/2 at 37.8 eV and W4f 7/2 at 35.7 eV, could be attributed to the W atoms being in the 6+ oxidation state. The second doublet, with lower binding energies of 34.9 eV and 36.3 eV could be assigned to the emission of the W4f 5/2 and W4f 7/2 core levels from the atoms in the oxidation state of 5+. These results for the core level of tungsten ions in tungsten bronze are in good agreement with reported values. 8 The chemical compo- sition determined from the deconvolution of the XPS spectrum is (NH 4 ) 0.25 WO 3 . Figure 2a presents the SEM images of the as-pre- pared (NH 4 ) x WO 3 sample hydrothermally synthesized at 200 °C for 12 h. The sample consisted of numerous nanorods, which were straight and had smooth surfaces, and the average diameter and length of the nanorods were 150 nm and 2 μm, respectively. The obtained (NH 4 ) x WO 3 nanorods were further characterized with TEM to determine their structural features and under- stand their crystal-growth behavior. A low-magnification TEM image of a randomly chosen single (NH 4 ) x WO 3 nanorod is shown in Fig- ure 2b. The selected-area-electron-diffraction (SAED) pattern (inset in Figure 2b) of the single nanorod can be indexed to the [010] zone axis of the hexagonal (NH 4 ) x WO 3 , suggesting that the as-prepared (NH 4 ) x WO 3 nanorods are single crystalline in nature. An HRTEM X. LUO et al.: SYNTHESIS OF (NH4)XWO3 NANORODS BY A NOVEL HYDROTHERMAL ROUTE 228 Materiali in tehnologije / Materials and technology 54 (2020) 2, 227–231 Figure 1: a) XRD pattern, b) full-range XPS spectra and c) W4f core-level XPS spectra of the as-prepared (NH 4 ) x WO 3 sample X. LUO et al.: SYNTHESIS OF (NH4)XWO3 NANORODS BY A NOVEL HYDROTHERMAL ROUTE Materiali in tehnologije / Materials and technology 54 (2020) 2, 227–231 229 Figure 3: XRD pattern and SEM image of the products after reaction times of: 2 h (a, b) and 6 h (c, d), respectively Figure 2: a) SEM image, b) TEM image and c) HRTEM image of the as-prepared (NH 4 ) x WO 3 sample. The inset shows the corresponding SAED pattern (b) image of the (NH 4 ) x WO 3 nanorod at a greater magnifi- cation (Figure 2c) shows clear and ordered lattice fringes, confirming that the nanorods are single crystals. In addition, the crystalline-lattice constant in the direc- tion parallel to the nanorod was measured as 0.382 nm, which agrees well with the interplanar spacing of (0 0 2), indicating the preferential growth along the c-axis of the hexagonal bronze structure. To understand the formation process of the single- crystal (NH 4 ) x WO 3 nanorods, time-dependent experi- ments involving changing the hydrothermal reaction time were performed. Initially, the reaction products obtained after2hoftheh ydrothermal treatment were identified as the 5(NH 4 ) 2 O·12WO 3 ·5H 2 O (ICDD PDF no. 18-0128), 5(NH 4 ) 2 O·12WO 3 ·7H 2 O (ICDD PDF no. 18-0126), 5(NH 4 ) 2 O·12WO 3 ·11H 2 O (ICDD PDF no. 18-0127) and (NH 4 ) 10 (H 2 W 12 O 42 )·4H 2 O (ICDD PDF no. 40-1470) phases from their XRD patterns (Figure 3a). The chemical formula of these phases can be represented as 5(NH 4 ) 2 O·12WO 3 ·nH 2 O( n = 5, 7, 11). The obtained 5(NH 4 ) 2 O·12WO 3 ·nH 2 O products display a sheet-like appearance with a lateral size of tens of micrometers (Figure 3b). The 5(NH 4 ) 2 O·12WO 3 ·nH 2 O products were gradually converted into hexagonal (NH 4 ) x WO 3 due to a certain amount of hexagonal (NH 4 ) x WO 3 phase, the diffraction peaks of which were detected when the reaction time was extended to6h( Figure 3c). The SEM image also demonstrates that the 5(NH 4 ) 2 O·12WO 3 ·nH 2 O phases dissolved and that spe- cies with a rod-like appearance were formed (Fig- ure 3d). When the reaction time was extended to 12 h, only (NH 4 ) x WO 3 nanorods were found in the sample (Figure 2a), and no sheet-like 5(NH 4 ) 2 O·12WO 3 ·nH 2 O phases remained. In addition, as a comparative experi- ment, when neither CH 4 N 2 S nor C 6 H 8 O 7 ·H 2 O were used in this hydrothermal system, no (NH 4 ) x WO 3 phases were generated even if the other experimental conditions were maintained. Based on the above results, the suggested reaction equations in this hydrothermal system are as follows: 12WO 4 2– + 18H + H 2 W 12 O 40 6– +8H 2 O (1) CH 4 N 2 S+2H 2 O 2NH 3 +H 2 S+CO 2 (2) H 2 W 12 O 40 6– + 10NH 3 +( n+7)H 2 O 5(NH 4 ) 2 O·12WO 3 ·nH2O + 6OH – (3) 5(NH 4 ) 2 O·12WO 3 ·nH 2 O+2 xH 2 S 12(NH 4 ) x WO 3 + + (10-12x)NH 3 +2 xSO 2 +( n+5-4x)H 2 O (4) During the initial stage, WO 4 2– and H + ions were released by Na 2 WO 4 ·2H 2 Oa n dC 6 H 8 O 7 ·H 2 O into the solution, respectively. Then, H 2 W 12 O 40 6– ions were generated due to the reaction of WO 4 2– and H + , 9,10 as shown with Equation (1). The hydrolysis of CH 4 N 2 S could produce massive quantities of NH 3 and H 2 S under hydrothermal conditions, as expressed with Equation (2). As the reaction time increased, the H 2 W 12 O 40 6– ions further reacted with NH 3 and H 2 O, forming sheet-like 5(NH 4 ) 2 O·12WO 3 ·nH 2 O phases according to Equation (3). As the reaction time progressed further, these sheet-like 5(NH 4 ) 2 O·12WO 3 ·nH 2 O phases were gradually transformed into (NH 4 ) x WO 3 nanorods upon an H 2 S reduction through a dissolution-recrystallization mecha- nism. 11,12 The chemical reaction of this process can be expressed with Equation (4). Finally, the pure (NH 4 ) x WO 3 nanorods were obtained at the expense of a complete consumption of the sheet-like 5(NH 4 ) 2 O·12WO 3 ·nH 2 O phases (12 h). The oriented growth of the (NH 4 ) x WO 3 nanorods is associated with their specific crystal structure. In hexagonal tungsten bronzes (M x WO 3 , x 0.33), the structure mainly comprises a rigid tungsten-oxygen framework built of layers containing corner-sharing WO 6 octahedra, which are arranged in six membered rings. The layers are stacked along the c-axis, leading to the formation of one-dimensional open hexagonal channels, which are randomly occupied by cations (Figure 4). 13 Therefore, the growth preferentially occurs along the c-axis of the (NH 4 ) x WO 3 nanorods due to their specific crystal structure. 4 CONCLUSIONS In summary, single-crystalline (NH 4 ) x WO 3 nanorods with hexagonal structures were obtained through a novel hydrothermal route at 200 °C, taking 12 h, with Na 2 WO 4 ·2H 2 O, CH 4 N 2 S and C 6 H 8 O 7 ·H 2 O as the starting materials. The as-prepared (NH 4 ) x WO 3 nanorods had an average diameter of 150 nm and length of 2 μm and they grew along the c-axis. Time-dependent experiments were carried out to further clarify the formation of the (NH 4 ) x WO 3 nanorods. This study provides a simple, mild and economical route for preparing one-dimensional (NH 4 ) x WO 3 nanostructures. X. LUO et al.: SYNTHESIS OF (NH4)XWO3 NANORODS BY A NOVEL HYDROTHERMAL ROUTE 230 Materiali in tehnologije / Materials and technology 54 (2020) 2, 227–231 Figure 4: Crystal structure of the as-prepared (NH 4 ) x WO 3 sample Acknowledgment This work was supported by the Doctoral Scientific Research Starting Foundation of the Jiangxi University of Science and Technology (Grant No. jxxjbs17020) and the Open Foundation of the State Key Laboratory for Advanced Metals and Materials, the University of Science and Technology of Beijing (Grant No. 2018-Z01). 5 REFERENCES 1 C. S. Guo, S. Yin, P. L. Zhang, M. Yan, K. Adachi, T. Chonan, T. Sato, Novel synthesis of homogenous Cs xWO3 nanorods with excellent NIR shielding properties by a water controlled-release solvothermal process, J. Mater. Chem., 20 (2010), 8227–8229, doi:10.1039/c0jm01972k 2 C. S. Guo, S. Yin, M. Yan, T. Sato, Facile synthesis of homogeneous Cs xWO3 nanorods with excellent low-emissivity and NIR shielding property by a water controlled-release process, J. Mater. Chem., 21 (2011), 5099–5105, doi:10.1039/c0jm04379f 3 C. S. Guo, S. Yin, T. Sato, Synthesis of one-dimensional hexagonal sodium tungsten oxide and its near-infrared shielding property, Nanosci. Nanotechnol. Lett., 3 (2011), 413–416, doi:10.1166/nnl. 2011.1182 4 C. S. Guo, S. Yin, L. J. Huang, T. Sato, Synthesis of one-dimensional potassium tungsten bronze with excellent near-infrared absorption property, ACS Appl. Mater. Interfaces, 3 (2011), 2794–2799, doi:10.1021/am200631e 5 C. S. Guo, S. Yin, L. J. Huang, L. Yang, T. Sato, Discovery of an excellent IR absorbent with a broad working waveband: Cs xWO3 nanorods, Chem. Commun., 47 (2011), 8853–8855, doi:10.1039/ c1cc12711j 6 C. S. Guo, S. Yin, Q. Dong, T. Sato, Simple route to (NH4) xWO3 nanorods for near infrared absorption, Nanoscale, 4 (2012), 3394–3398, doi:10.1039/c2nr30612c 7 C. S. Guo, S. Yin, T. Sato, Tungsten oxide-based nanomaterials: morphological-control, properties, and novel applications, Rev. Adv. Sci. Eng., 1 (2012), 235–263, doi:10.1166/rase.2012.1016 8 M. Sun, N. Xu, Y. W. Cao, J. N. Yao, E. G. Wang, Nanocrystalline tungsten oxide thin film: preparation, microstructure, and photochro- mic behavior, J. Mater. Res., 15 (2000), 927–933, doi:10.1557/JMR. 2000.0132 9 J. P. Launay, M. Boyer, F. Chauveau, High resolution PMR of several isopolytungstates and related compounds, J. Inorg. Nucl. Chem., 38 (1976), 243–247, doi:10.1016/0022-1902(76)80402-2 10 J. J. Hastings, O. W. Howarth, A 183 W, 1 H and 17 O nuclear magnetic resonance study of aqueous isopolytungstates, J. Chem. Soc. Dalton Trans., 2 (1992), 209–215, doi:10.1039/dt9920000209 11 L. Q. Jiang, Y. Qiu, Z. G. Yi, Potassium niobate nanostructures: con- trollable morphology, growth mechanism, and photocatalytic activity, J. Mater. Chem. A, 1 (2013), 2878–2885, doi:10.1039/C2TA01056A 12 X. Li, J. L. Zang, Facile hydrothermal synthesis of sodium tantalate (NaTaO3) nanocubes and high photocatalytic properties, J. Phys. Chem. C, 113 (2009), 19411–19418, doi:10.1021/jp907334z 13 A. Hussain, L. Kihlborg, A. Klug, The transformation between hexa- gonal potassium tungsten bronze and polytungstate, J. Solid State Chem., 25 (1978), 189–195, doi:10.1016/0022-4596(78)90102-0 X. LUO et al.: SYNTHESIS OF (NH4)XWO3 NANORODS BY A NOVEL HYDROTHERMAL ROUTE Materiali in tehnologije / Materials and technology 54 (2020) 2, 227–231 231