物理化学学报 (Wuli Huaxue Xuebao) Acta Phys. -Chim. Sin. 2014, 30 (11), ,2 杨子祥

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1 November 物理化学学报 (Wuli Huaxue Xuebao) Acta Phys. -Chim. Sin. 2014, 30 (11), [Article] doi: /PKU.WHXB 非晶态镍硼酸盐结构的 EXAFS 研究 1,2 刘红艳 1,* 房春晖 1 戈海文 1,* 房艳 1,2 杨子祥 1 周永全 1,2 唐玉玲 ( 1 中国科学院青海盐湖研究所, 中国科学院盐湖资源与化学重点实验室, 西宁 ; 1 朱发岩 2 中国科学院大学, 北京 ) 摘要 : 由摩尔比分别为 1:2 和 1:8 的 NiCl 2 6H 2O 和 Na 2B 4O 7 10H 2O 作为反应物, 合成两种非晶态镍硼酸盐, 同时通过水热法合成 β-ni(oh) 2. 化学分析和热重 - 微商热重法 (TG-DTG) 分析结果确定两种非晶态镍硼酸盐的分子组成分别为 NiO 0.8B 2O 3 4.5H 2O 和 NiO B 2O 3 3H 2O. 激光拉曼 (Raman) 实验结果表明镍硼酸盐样品中主要存在的硼氧阴离子为 B 3O 3(OH) 5 和 B 2O(OH) 6. 同步辐射扩展 X 射线吸收精细结构 (EXAFS) 方法对样品进行结构解析, 通过数据拟合给出样品中 Ni 原子周围近邻配位原子种类 配位数以及原子间距离. 用不同晶体结构作为标准对两种非晶态镍硼酸盐进行拟合的结果表明, 样品中 Ni 原子周围局域结构与 Ni 3B 2O 6 晶体 (ICSD No ) 中的吻合较好. Ni 原子周围配位原子为 O B 和 Ni, 对于 NiO 0.8B 2O 3 4.5H 2O, 配位数分别为 和 3.8, 配位距离分别为 和 nm; 对于 NiO B 2O 3 3H 2O, 配位数分别为 和 4.0, 配位距离分别为 和 nm. 关键词 : 硼酸镍 ; 氢氧化镍 ; 扩展 X 射线吸收精细结构 ; 局域结构 ; 配位原子 中图分类号 : O641 EXAFS Study of the Structure of Amorphous Nickel Borate LIU Hong-Yan 1,2 FANG Chun-Hui 1,* FANG Yan 1,* ZHOU Yong-Quan 1 ZHU Fa-Yan 1 GE Hai-Wen 1 YANG Zi-Xiang 1,2 TANG Yu-Ling 1,2 ( 1 CAS Key Laboratory of Salt Lake Resources and Chemistry, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining , P. R. China; 2 University of Chinese Academy of Sciences, Beijing , P. R. China) Abstract: In this work, two amorphous hydrated nickel borates were synthesized with nickel chloride hexahydrate and borax as reactants at mole ratios of 1:2 and 1:8, respectively. The chemical compositions of these nickel borates were determined to be NiO 0.8B 2O 3 4.5H 2O and NiO B 2O 3 3H 2O through thermogravimetryderivative thermogravimetry (TG-DTG) and chemical analysis, in which the main anions were determined to be B 3O 3(OH) 5 and B 2O(OH) 6, respectively, by Raman spectroscopy. The local structure of these samples was studied by synchrotron radiation extended X-ray absorption fine structure (EXAFS). Based on the processing and fitting of the EXAFS experimental data, the neighboring coordination atoms type, the interatomic distances, and the atom-pair numbers of Ni were determined. The fitting results of the amorphous nickel borate show that the local structure around the Ni atom has a similar structure to that of the Ni 3B 2O 6 crystal. The neighboring coordination atoms of Ni in these amorphous nickel borates are O, B, and Ni. For NiO 0.8B 2O 3 4.5H 2O, the interatomic distances for, Ni B, and Ni Ni are 0.208, 0.263, and nm, and the atom-pair numbers are 5.7, 3.8, and 3.8, respectively. The interatomic distances of NiO B 2O 3 3H 2O are 0.207, 0.262, and nm, and the atom-pair numbers are 6.0, 4.0, and 4.0, respectively. The first shells of Ni 2+ in NiO 0.8B 2O 3 4.5H 2O and NiO B 2O 3 3H 2O are octahedral with six oxygen atoms. Received: August 6, 2014; Revised: September 25, 2014; Published on Web: September 25, Corresponding authors. FANG Chun-Hui, fangch@isl.ac.cn. FANG Yan, fangy8@isl.ac.cn. The project was supported by the National Natural Science Foundation of China ( ) and Main Direction Program of Knowledge Innovation of Chinese Academy of Sciences, China (KZCXEW-307). 国家自然科学基金项目 ( ) 和中国科学院知识创新工程重要方向项目 (KZCXEW-307) 资助 Editorial office of Acta Physico-Chimica Sinica

2 1980 Acta Phys. -Chim. Sin Vol.30 Key Words: Nickel borate; Nickel hydroxide; Extended X-ray absorption fine structure; Local structure; Coordination atom 1 Introduction Metal borates are widely applied in fuel cells, 1 laser applications, and non-linear optical. 2,3 They are also known for their interesting magnetic, catalytic, and phosphorescence properties. 4-6 In recent years, crystalline anhydrous nickel borate was found to be used as cathode material 7 of lithium ion battery. Bediako et al. 8,9 have shown that anhydrous nickel borate can also act as a catalyst for solar-driven water splitting to H 2 and O 2, and which is an attractive method of accomplishing the solar-to-fuels conversion necessary to meet the growing global energy demand by renewable sources. 10,11 At present, although the crystalline nickel borate structure was well developed, the structure of amorphous hydrated nickel borate has not been reported yet. With the wealth of experience acquired in the field of crystal and nanometer nickel borate, 115 in the present work, we decided to study the amorphous hydrated nickel borate using a combination of various techniques. The amorphous nickel borates were characterized by X-ray diffraction (XRD), Raman spectrum, and the thermogravimetry- derivative thermogravimetry (TG- DTG) analysis. To obtain a comprehensive understanding of the local structure of amorphous hydrated nickel borates, we conducted a detailed study using synchrotron radiation (SR) extended X-ray absorption fine structure (EXAFS) method. 2 Experimental 2.1 Materials and sample preparation NiCl 2 6H 2O(AR), Na 2B 4O 7 10H 2O(GR), H 3BO 3(AR), NaOH (AR), and NiSO 4 6H 2O(AR) were commercially available. The amorphous hydrated nickel borates were synthesized with NiCl 2 6H 2O and Na 2B 4O 7 10H 2O as reactants with mole ratio of 1:1 and 1:8, respectively. The reactants were dissolved in distilled water separately at room temperature, mixed together and heated to about 50 C. 16 The H 3BO 3 solution (10% (w)) was used to adjust the ph value of solution to 8.8 and 7.9, respectively. The precipitation of nickel borate products was obtained after reaction for 4 h and static overnight, then the precipitation was washed with the H 3BO 3 solution and distilled water for several times. In order to check the reliability of the SR EXAFS experiment and data processing, the β-ni(oh) 2 was synthesized with NaOH and NiSO 4 6H 2O in aqueous solution by direct precipitation method. 17,18 Element analysis for Ni was determined by ethylene diamine tetraacetic acid (EDTA) complex metric titration with murexide as indicator and B by mannitol titration method Characterization methods Thermal stability of the samples was characterized by a synchronous thermal analyzer (the United States thermoelectric company, SDT Q600) at 2.5 C min -1 from 25 to 1000 C. X-ray diffraction was gotten from an X Pert Pro X-ray diffractometer (PANalytical company, Holland), using Cu K α radiation. A laser confocal Raman spectrometer (LabRAM HR800, Jobin Yvon) was employed to measure Raman spectrum, with excitation wavelength of nm and exposed time of 10 s, the samples were placed on microscope slides and exposed one time. The EXAFS spectra at the Ni K α1-edge ( ev) were recorded in the transmission mode at the Beijing Synchrotron Radiation Facility (BSRF). The storage ring ran at energy of GeV and an average electron current of about 80 ma. The radiation was monochromatized by a Si(111) double-crystal monochromator, and a 50.0% harmonic rejection was achieved by slightly detuning at two crystals from parallel alignment. All data were collected at room temperature and the sample was kept in a cell with Kapton film windows. Data analysis of the EXAFS spectra was performed with the IFEFFIT software, 20,21 which is based on the theoretical calculation of the X-ray absorption fine spectral signal and a subsequent refinement of the structure parameters. The EXAFS oscillations equation, χ(k), as the following expression: χ(k)=σ(n j/kr j2 )S 02 (k)f j(k)exp[-2(r j-δ)/λ j-2σ j2 k 2 ]sin[2kr j+φ j(k)] This equation is known as the EXAFS universal equation, which represents the superposition of the contribution of different coordination layers. The neighboring atoms to the central atom Ni are divided into j shells, with all atoms with the same coordinator number and distance from the central atom grouped into a single shell. Where k is the wave vector, F j is the ab initio amplitude function for shell j, R j is the average coordination distance from the central atom Ni and neighbor atoms in the jth shell. N j is the average coordination number in the jth shell, σ j2 is the EXAFS Debye-Waller factor, representing the mean-square variation in R j due to both static and thermal disorder. S 02 is an amplitude reduction factor due to shake-up/shake-off processes at the central atom Ni and Φ j(k) is the phase shift suffered by the photoelectron in the outgoing and scattering processes. λ is the mean free path of a photoelectron, and Δ is a correction factor (Δ=R 1) in the mean free path concept. 224 The EXAFS equation was used to fit the experimental data with N, R, and σ j2 as variable parameters. The χ(k) data of Ni were generally weighted by k 2 using a Hanning window with dk=0.10 nm. The fitting was applied to both the real and imaginary parts of χ(r) in the region of 0.11 nm<r<0.32 nm. During the fitting process, structural parameters have been varied, taking into account the standard and criteria in XAFS. We have chosen a k- range from ~26 to ~105 nm -1 that allows us to have, following the Nyquist criterion, a maximum number of relevant independent points (p) of thirteen. The Nyquist theorem was used to determine the number of p for fitting the data, the formula is usually given by 25 p=(2δrδk/π)+2 where ΔR is the R region in R phase over which the fit is made and Δk is the k region of the XAFS spectrum analyzed in k space. In our fittings, we have determined upto twelve independent

3 No.11 LIU Hong-Yan et al.: EXAFS Study of the Structure of Amorphous Nickel Borate 1981 parameters for three shells (Ni-O, Ni-H/B, and Ni-Ni/B). 3 Results and discussion 3.1 Chemical analysis Chemical analysis determined the molar ratio of NiO and B 2O 3 in the samples. Element analysis for Ni was determined by EDTA complex metric titration with murexide as indicator and B by mannitol titration method. 19 The results of the chemical analysis as shown in Table 1 indicated that the molar ratios of NiO and B 2O 3 for the amorphous hydrated nickel borates are 1:0.8 and 1:1, respectively, and therefore the composition of the molecular formula of samples can be expressed as NiO 0.8B 2O 3 xh 2O and NiO B 2O 3 yh 2O, respectively. 3.2 TG-DTG analysis Fig.1 depicts the typical TG-DTG curves of NiO 0.8B 2O 3 xh 2O, NiO B 2O 3 yh 2O, and β-ni(oh) 2. As shown in Fig.1(a, b), TG curves only have one process of weightlessness, the start temperatures are 66.7 and 67.2 C and the end temperatures are 802 and 539 C, and their weightlessness are 33.42% and 28.22% (w), corresponding to the number of water molecules are 4.5 and 3.0, respectively. Meantime, the DTG curves have a broad peak. Combined with the results of chemical analysis, the molecular formula of the amorphous nickel borates are determined to be NiO 0.8B 2O 3 4.5H 2O and NiO B 2O 3 3H 2O, respectively. Fig.1(c) depicts the TG-DTG curves of β-ni(oh) 2, the TG curve has two weight losses with 5.15% and 19.02% (w), respectively. The former corresponds to the evaporation of adsorbed water molecules from 60.3 to C, the latter is associated with the loss of water produced by dehydroxylation of the hydroxide layers between and C with 19.02% (w) of weight losses, almost in agreement with the weight losses of theory calculated with 19.43% (w) according to Ni(OH) 2=NiO+ H 2O, which can be confirmed that the weightless in higher temperature is attributed to the Ni(OH) 2 decomposed and generated NiO, accordingly, in the DTG curve having a shaper peak. 26, X-ray diffraction Fig.2 shows the XRD patterns of NiO 0.8B 2O 3 4.5H 2O, NiO B 2O 3 3H 2O, and β-ni(oh) 2. No obvious Bragg diffraction peaks were found in the NiO 0.8B 2O 3 4.5H 2O and NiO B 2O 3 3H 2O, which indicated that the nickel borates are amorphous. Fig.2(c) indicated that synthesized Ni(OH) 2 was in the β-ni(oh) 2 form No peaks of α-ni(oh) 2 or nickel oxide were detected, the pattern matches well with that of reported β-ni(oh) 2 (PDF No ). 3.4 Raman spectroscopy Fig.1 TG-DTG curves of NiO 0.8B2O3 xh2o (a), NiO B2O3 yh2o (b), and β-ni(oh)2 (c) Raman spectra of NiO 0.8B 2O 3 4.5H 2O, NiO B 2O 3 3H 2O, and β-ni(oh) 2 were recorded and displayed in Fig.3. The range of cm -1 is the most favorable zone for the investigation of borate, which is considered as the characteristic absorption bands of polyborates. 31,32 As Fig.3(a, b) shown, the peaks in the range of cm - 1 in Raman spectra are assigned to asymmetric stretching vibration of B O tetrahedron (BO 4). The peaks in the range of cm - 1 are assigned to symmetric stretching vibration of B O planar triangle (BO 3). 33 The peaks produced by symmetric stretching vibration of BO 4 locate at 818 and 863 cm -1 in Fig.3(a) and 833 cm -1 in Fig.3(b). In Fig.3(a), the characteristic absorption peaks of borate anions at 468, 610, and 640 cm -1 are assigned to B 3O 3(OH) 5, 34,35 while there is a weak peak at 471 cm -1 in Fig.3(b). 34 For the weak peaks caused by characteristic vibration Table 1 Optimal fitting results of β-ni(oh)2 as shown in Fig.6 Structural parameter Sample Shell N 5.8 (0.6) * R/nm (0.001) 10 4 Δσ 2 /nm (0.2) ΔE0/eV (0.25) R-factor Ni(OH)2 Ni H Ni Ni 5.8 (0.6) 5.8 (0.6) (0.001) (0.001) 1.2 (0.2) 1.2 (0.2) (0.25) (0.25) * data in brackets: error bar; σ 2 : Debye-Waller factor; ΔE0: energy zero offset Fig.2 XRD patterns of NiO 0.8B2O3 4.5H2O (a), NiO B2O3 3H2O (b), and β-ni(oh)2 (c)

4 1982 Acta Phys. -Chim. Sin Vol.30 Fig.3 Raman spectra of NiO 0.8B2O3 4.5H2O (a), NiO B2O3 3H2O (b), and β-ni(oh)2 (c) of B 5O 6(OH) 4- at 553, 526, and 551 cm -1. The band near 588 cm -1 is the characteristic peak of B 4O 5(OH) 4 in Fig.3(a) and the strong peak 603 cm -1 is assigned to B 3O 3(OH) 4- in Fig.3(b). 36 The strongest peaks near 755 cm -1 attribute to the B 2O(OH) 6 in Fig.3(a, b). Raman spectrum of β-ni(oh) 2 was given in Fig.3(c). A small peak was located at 477 cm -1, together with a strong sharp peak at 3581 cm -1 emerged, which was consistent with the literature Raman spectrum of crystalline β-ni(oh) 2. 32,37 The absence of the peak centered around 510 cm -1 indicated the nice crystallization of the product with few proton vacancies and stacking fault. 38 The characteristic peaks of Raman of B 5O 6(OH) 4- and B 4O 5(OH) 4 for NiO 0.8B 2O 3 4.5H 2O are weaker, for the B 3O 3(OH) 5 with three absorption relative weak peaks. The strongest peak is assigned to B 2O(OH) 6, therefore the main anions may be B 3O 3(OH) 5 and B 2O(OH) 6. There are many anions in sample NiO B 2O 3 3H 2O, and one of the strongest peak is assigned to the main anion B 2O(OH) EXAFS analysis At present, the structure of amorphous nickel borate with EXAFS has not been reported. We conducted EXAFS spectra of NiO 0.8B 2O 3 4.5H 2O, NiO B 2O 3 3H 2O, and β- Ni(OH) 2 (as shown in Fig.4), and made a detailed fitting of the experimental data. The detected XAFS spectra at Ni K α1 absorption edge were shown in Fig.4(A) including X-ray absorption near-edge structure (XANES) and EXAFS spectra, and we have to focus on the latter. And then transformed to the k-space spectra in Fig.4(B) and the R-space spectra by Fourier transform in Fig.4(C). The EXAFS data in R-space (R= nm) show two independent peaks so we can use the window function for the inverse Fourier transform. The first peak around 0.20 nm can be assigned to the peak and the second peak at about 0.32 nm to be the Ni Ni peak. As the typical light atoms B and H with extremely weak scattering ability for X-ray, they make none obvious absorption peak in the radial distribution function (RDF). Two obvious dispersion peaks are shown in Fig.4(C), and the overlap or distortion is inevitable. In order to determine the position of the peaks accurately, it is necessary to deconvolve for the spectral peaks to turn into several independent peaks. Hence we make multi- peaks Gaussian fitting for the RDF of NiO 0.8B 2O 3 4.5H 2O, NiO B 2O 3 3H 2O, and β-ni(oh) 2 (displayed in Fig.5). Experimental and fitting peaks are superposed well indicating that the fitting is satisfactory as shown in Fig.5. In combination with the structure parameters of Ni(OH) 2, Ni 3B 2O 6 (ICSD No ), and Ni[B 6O 7(OH) 6] 5H 2O (ICSD No.: 68250) observed that the peaks located at around 0.20 and 0.32 nm can be assigned to the interaction of and Ni Ni (peaks 1 and 3 in Fig.5), respectively; while the little peak located between 0.25 and 0.30 nm can be attributed to the interaction of Ni H or Ni B. In order to clear attribution of each peak, we continue to make EXAFS fitting for samples. Fig.4 Experimental EXAFS results of NiO 0.8B2O3 4.5H2O (a), NiO B2O3 3H2O (b), and β-ni(oh)2 (c) (A) Ni-Kα1 edge absorption spectra; (B) EXAFS k-space spectra; (C) EXAFS R-space spectra

5 No.11 LIU Hong-Yan et al.: EXAFS Study of the Structure of Amorphous Nickel Borate 1983 Fig.5 Multi-peak Gaussian fitting for NiO 0.8B2O3 4.5H2O (a), NiO B2O3 3H2O (b), and β-ni(oh)2 (c) ( ) experimental peaks; ( ) fitting peaks; (- -) theoretical peaks The Ni(OH) 2 crystal (ICSD No ) was used as the standard structure to fit the EXAFS curve of β-ni(oh) 2. The fitted k 2 - weighted χ(r) plots with phase shift correction applied are shown in Fig.6, and the obtained structure parameters and EXAFS parameters from the k 2 -weighted fit in R-space are given in Table 1, k ranges from 24 to 110 nm -1 and R ranges from 0.11 to 0.32 nm. The result of the fitting is satisfactory when k=24-80 nm -1 from Fig.6(A). The radial distribution function for β-ni(oh) 2 are shown in Fig.6(B) and it exhibits two well-resolved peaks. The first peak is attributed to the first shell and the second peak to the first Ni-Ni shell. Multi-peaks Gaussian fitting for β-ni(oh) 2 as presented in Fig.5(c) manifested that a little peak was attributed to Ni H interaction. The EXAFS results for β-ni(oh) 2 indicated the interatomic distances for, Ni H, and Ni Ni are 0.207, 0.278, and nm, and the atom-pair numbers 5.8, 5.8, and 5.8, respectively as shown in Table 1. The experimental and the fitting curves coincident well in Fig.6(B), which determines that the fitted structural parameters are consistent with the structure parameters of Ni(OH) 2 crystal (ICSD No ). Therefore both the experimental and fitting data in the present work for getting the local structure of unknown substance with EXAFS is feasible and reliable. The EXAFS curves of NiO 0.8B 2O 3 4.5H 2O and NiO B 2O 3 3H 2O were analyzed with Ni 3B 2O 6 (ICSD No.: 31387) (fit 1 in Fig.7) and Ni[B 6O 7(OH) 6] 5H 2O (ICSD No ) (fit 2 in Fig.7) as the standard structures. The fitted k 2 -weighted χ(r) plots with phase shift correction applied and the radial distribution function were shown in Fig.7. The obtained structural parameters from the k 2 -weighted fits in the R-space are given in Table 2, k ranges from 24 nm -1 to around 105 nm -1 and R ranges from 0.11 to 0.32 nm. The results of the fitting are satisfactory when k=24-80 nm -1 as shown in Fig.7(A). The radial distribution function for NiO Fig.6 Fitting results of Ni-Kα1 edge EXAFS spectra of β-ni(oh)2 in k-space (A) and in R-space (B) 0.8B 2O 3 4.5H 2O and NiO B 2O 3 3H 2O are shown in Fig.7(B) and also exhibit two well-resolved peaks. Multi-peaks Gaussian fitting for β-ni(oh) 2 as present in Fig.5(a, b) manifested that there also has a small peak that may be assigned to Ni-H or Ni-B interaction. In combination with EXAFS results in Table 2 and fit 1 in Fig.7 indicated the interatomic distances for, Ni B, and Ni Ni shells with the atom-pair numbers 5.7 oxygen atoms, 3.8 boron atoms, and 3.8 nickel atoms at 0.208, 0.263, and nm, respectively, for NiO 0.8B 2O 3 4.5H 2O when fitting with Ni 3B 2O 6 (ICSD No ) (shown as fit 1 in Fig.7) as the standard structure. Meanwhile, the EXAFS results for NiO B 2O 3 3H 2O indicated the local structure around Ni atom with 6.0 oxygen atoms at nm, 4.0 boron atoms at nm, and 4.0 nickel atoms at nm. The results for fitting with the Ni[B 6O 7(OH) 6] 5H 2O (ICSD No.: 68250) (shown as fit 2 in Fig.7) as the standard structure are described as follows. In combination with Table 2 and Fig.5(a, b) indicated that the first peak is attributed to the shell with 4.9 oxygen atoms at nm, the another obvious peak is assigned to the Ni B shell with 1.6 boron atoms at nm and the Ni H shell with 8.1 hydrogen atoms at nm for NiO 0.8B 2O 3 4.5H 2O. In the meantime, the EXAFS fitting results for NiO B 2O 3 3H 2O indicate the, Ni H, and Ni B shell with 4.5 oxygen atoms, 7.5 hydrogen atoms, and 1.5 boron atoms at 0.204, 0.252, and nm, respectively. The experimental curves and the fitting curves with Ni 3(BO 3) 2 (ICSD No ) as standard structure were coincident well as shown in the fit 1 of Fig.7. Compared with the structure param-

6 1984 Acta Phys. -Chim. Sin Vol.30 Fig.7 Fitting results of Ni-Kα1 edge EXAFS spectra of NiO 0.8B2O3 4.5H2O and NiO B2O3 3H2O in k-space (A, A ) and in R-space (B, B ) The solid lines are the experimental data, and the dashed lines are the fitting results. Sample NiO 0.8B2O3 4.5H2O NiO B2O3 3H2O NiO 0.8B2O3 4.5H2O NiO B2O3 3H2O Table 2 Optimal fitting results of NiO 0.8B2O3 4.5H2O and NiO B2O3 3H2O as shown in Fig.7 Structural parameter Shell N R/nm 10 4 Δσ 2 /nm 2 ΔE0/eV fit 1: Ni3B2O6 (ICSD No ) 5.7 (0.9)* (0.002) 1.31 (0.23) (0.31) Ni B 3.8 (0.6) (0.002) 1.31 (0.23) (0.31) Ni Ni 3.8 (0.6) (0.002) 1.31 (0.23) (0.31) 6.0 (0.7) (0.002) 1.08 (0.24) (0.55) Ni B 4.0 (0.5) (0.002) 1.08 (0.24) (0.55) Ni Ni 4.0 (0.5) (0.002) 1.08 (0.24) (0.55) fit 2: Ni[B6O7(OH)6] 5H2O (ICSD No ) 4.9 (0.8) (0.007) 1.15 (0.13) (0.29) Ni H 8.1 (1.3) (0.007) 1.15 (0.13) (0.29) Ni B 1.6 (0.3) (0.007) 1.15 (0.13) (0.29) 4.5 (0.6) (0.004) 0.81 (0.61) (0.51) Ni H 7.5 (1.0) (0.004) 0.81 (0.61) (0.51) Ni B 1.5 (0.2) (0.004) 0.81 (0.61) (0.51) * data in brackets: error bar R-factor eters in Table 2 indicated that the EXAFS curves fitting with the Ni 3B 2O 6 (ICSD No ) as the standard structure for NiO 0.8B 2O 3 4.5H 2O and NiO B 2O 3 3H 2O is more reasonable, further evidence that the local structures of NiO 0.8B 2O 3 4.5H 2O and NiO B 2O 3 3H 2O are more similar to that of Ni 3B 2O 6 (ICSD No ) crystal. The EXAFS fitting results are in good agreement with the shells of 6 oxygen atoms at nm, 4.0 boron atoms at nm, and 4.0 nickel atoms at nm calculated from the X-ray diffraction results for Ni 3B 2O 6 (ICSD No ) crystal. For the Ni-O shell, the EXAFS analysis gives the interatomic bond distance to be ~0.207 nm, which is slightly lower than the value nm found by X-ray diffraction and reported by McEwen. 39 However, X-ray diffraction gives the coordinates of oxygen atoms (1/3, 2/3, Z) and (2/3, 1/3, Z), where Z values from 0.22 to Taking Z=0.25, the interatomic distance is nm, while taking Z=0.22, the resulting of distance is nm in agreement with the ionic radii of Ni 2 + and O well. The ionic Fig.8 Local structure diagram of amorphous nickel borate

7 No.11 LIU Hong-Yan et al.: EXAFS Study of the Structure of Amorphous Nickel Borate 1985 radius of oxygen is nm 40 and (0.069±0.001) nm for Ni 2+ with 6 neighbor atoms. Taking the ionic radius of O to be nm, the sum of the two radii is about nm. Thus, the EXAFS results are in agreement with the sum of ionic radii. The interpretation further proved that the local structure (as shown in Fig.8) of amorphous nickel borates by synthesis is similar to the structure of the Ni 3B 2O 6 (ICSD No ) crystal. The results of the EXAFS spectra for NiO 0.8B 2O 3 4.5H 2O and NiO B 2O 3 3H 2O revealed that the first shell of Ni atoms is an octahedral structure with six oxygen atoms, the second and third shells are a plane quadrilateral structure with four boron atoms and four nickel atoms, respectively. 4 Conclusions In the present work, two amorphous hydrated nickel borates were synthesized with nickel chloride hexahydrate and borax as reactants at different mole ratios. β-ni(oh) 2 was synthesized and used as a standard substance for checking the reliability of the SR EXAFS experiment and data processing, the result confirmed that the experimental and fitting data in the present work for getting the local structure of unknown substance with EXAFS is feasible and reliable. The samples were characterized and structure analyzed with a variety of experiment methods. The results of characterization revealed that the morphology of the nickel borates is amorphous, combining with the results of the simultaneous thermal analysis determined that the molecular formula of the amorphous hydrated nickel borates can be express as NiO 0.8B 2O 3 4.5H 2O and NiO B 2O 3 3H 2O. The main anions in NiO 0.8B 2O 3 4.5H 2O and NiO B 2O 3 3H 2O are B 3O 3(OH) 5 and B 2O(OH) 6, respectively. The EXAFS data of NiO 0.8B 2O 3 4.5H 2O and NiO B 2O 3 3H 2O were fitted with Ni 3B 2O 6 (ICSD No ) and Ni[B 6O 7(OH) 6] 5H 2O (ICSD No ) as the standard compounds. The results showed that the local structure around Ni atom is consistent with the structure of Ni 3B 2O 6. The neighboring coordination atoms of Ni for the amorphous nickel borates are O, B, and Ni, the interatomic distances for, Ni B, and Ni Ni are ~0.208, ~0.263, and ~0.311 nm, and the atom-pair numbers ~5.7, ~3.8, and ~3.8, respectively. 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