Large-Scale Synthesis of Single Crystalline NiHPO3·H2O Nanoneedle

CRYSTAL GROWTH & DESIGN

Large-Scale Synthesis of Single Crystalline NiHPO3 · H2O Nanoneedle Bundles Based on the Dismutation of NaH2PO2 Lei Zhang, Yonghong Ni,* Kaiming Liao, and Xianwen Wei College of Chemistry and Materials Science, Anhui Key Laboratory of Functional Molecular Solids, Anhui Normal UniVersity, Wuhu, 241000, P. R. China

2008 VOL. 8, NO. 10 3636–3640

ReceiVed February 20, 2008; ReVised Manuscript ReceiVed May 20, 2008

ABSTRACT: In this paper, we report the successful synthesis of well-aligned single crystalline NiHPO3 · H2O nanoneedle bundles on a large scale via a facile hydrothermal route, employing NiSO4 · 6H2O and NaH2PO2 as the reactants in the presence of NH3 · H2O, hexamethylenetetramine (HMT), and cetyltrimethylammonium bromide (CTAB). The reaction was carried out at 150 °C for 24 h. Generally, in basic solution, H2PO2- is often used as a reductant, but it can also decompose into HPO32- and P3- ions due to dismutation. Namely, the redox and dismutation reactions are competitive. Since the formation of Ni-HMT-NH3 complex reduced the potential of the Ni2+/Ni pair in our work, the dismutation reaction of H2PO2- ions was predominant. This led to the production of NiHPO3 · H2O due to the smaller solubility. The as-obtained product was characterized by X-ray powder diffraction (XRD), high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and field emission scanning electron microscopy (SEM). Some factors influencing the morphology of the NiHPO3 · H2O nanoneedles were systematically investigated.

1. Introduction In the past few decades, metal phosphates with openframework structures have been a subject of extensive research due to their rich structural chemistry and wide potential applications in ion-exchange, adsorption, separation, and catalysis.1-5 Recently, the pseudopyramidal phosphite HPO32group has been investigated as a possible replacement for the traditional phosphate tetrahedron with great success. The data available in the literature3,6-9 show that their stoichiometric diversity and structural richness are comparable to those among the better known phosphate compounds. Besides the basic theoretical interest, these new types of solids have wide potential applications in many fields including as host matrices for intercalation chemistry and possible ionic conductivity or exchange.10-17 Traditionally, phosphites are prepared by the direct reaction between H3PO3 or HPO32- and metals or metal salts.18-20 For example, Marcos et al.18 prepared crystalline microporous phosphites of transition metal ions, M11(HPO3)8(OH)6 (M ) Zn, Co, Ni), employing K2HPO3 and metal salts under hydrothermal conditions at 180 °C for 3 days. At the same time, they successfully prepared NiHPO3 · H2O at the same temperature and time, employing NiCl2 · 6H2O and H3PO3 as the reactants.19 In 2005, Rojo and co-workers20 synthesized novel manganese(II) phosphite with the formula of MnHPO3 via the hydrothermal route at 170 °C for 5 days, using H3PO3 and MnCl2 · 4H2O as the reactants. Recently, Gu et al.21 selected NiCl2 · 6H2O and KH2PO4 as the reactants to prepared Ni11(HPO3)8(OH)6 nanocrystals with a hierarchical structure under hydrothermal conditions at 180 °C for 1-24 h. Some complicated compounds containing a HPO32- group, such as (C2H10N2)[Co3(HPO3)4],22 (C4H8N2H4)[Zn(HPO3)2],5 and ZnHPO3 · N4C2H4,23 were also synthesized under hydrothermal conditions (at above 140 °C for 3 days at least). However, to the best of our knowledge, no report on the preparation of well-aligned single-crystalline NiHPO3 · H2O nanoneedle bundles is found in literature to date, via employing NaH2PO2 as a phosphite source. * Corresponding author: E-mail: [email protected]. Fax: (86)5533869302.

In this paper, we successfully prepared novel single-crystalline NiHPO3 · H2O nanoneedle bundles ∼3 µm in length and ∼60 nm in diameter via a facile hydrothermal route, employing NiSO4 as the nickel source and NaH2PO2 as a phosphite source at 150 °C for 24 h. Usually, NaH2PO2 is used as a reductant, but it can also decompose into HPO32- and P3- ions due to dismutation. Namely, the redox and dismutation reactions are competitive. When the redox reaction is suppressed, the dismutation reaction of H2PO2- ions will become predominant. This leads to the production of NiHPO3 · H2O nanostructures due to the smaller solubility. Compared with the above literature, the present approach has three main characteristics: (1) the reaction time is shorter (24 h); (2) NaH2PO2, an often-used reductant, is selected as the phosphite source; (3) abundant well-aligned single-crystalline NiHPO3 · H2O nanoneedle bundles can be obtained.

2. Experimental Section All reagents were purchased from Shanghai Chemical Company and used without further purification. In a typical preparation procedure, NiSO4 · 6H2O (0.005 mol) was dissolved in NH3 · H2O (25% mass) of 5 mL to form a homogeneous solution. Then, hexamethylenetetramine (0.005 mol) and NaH2PO2 (0.005 mol) were added. Finally, 20 mL of aqueous solution contained 0.4 g of CTAB was slowly dropped into the above solution. The as-obtained solutions were transferred into a Teflon-lined steel-stainless autoclave. The reaction was carried out at 150 °C for 24 h. After that, the autoclave was cooled to room temperature naturally. The green products were collected, washed with distilled water several times, and dried in air at 50 °C for 6 h. X-ray powder diffraction patterns of the product were carried out on a Shimadzu XRD-6000 X-ray diffractometer equipped with Cu KR radiation (λ ) 0.154060 nm), employing a scanning rate of 0.02 ° s-1 and 2θ ranges from 10° to 70°. TEM, HRTEM and SAED were carried out on a JEOL-2010 high resolution transmission microscope, employing an accelerating voltage of 200 kV. SEM images and EDS of the products were obtained on Hitachi S-4800 field emission scanning electron microscope, employing the accelerating voltage of 5 kV. Thermogravimetric analysis (TGA) was conducted on TG-209 F 1 thermal analyzer from 100 to 600 °C with a heating speed of 10 °C/ min in Ar atmosphere. The Ni/P molar ratio of the as-synthesized product was analyzed by inductively coupled plasma atomic emission spectroscopy (Optima 5300DV-ICP, Perkin-Elmer). NiCl2 and H3PO4 were used as nickel and phosphor source, respectively. A series of standard solutions containing different concentration of nickel and

10.1021/cg800193j CCC: $40.75  2008 American Chemical Society Published on Web 08/27/2008

Synthesis of NiHPO3 · H2O Nanoneedle Bundles

Crystal Growth & Design, Vol. 8, No. 10, 2008 3637

Figure 2. The thermogravimetrimetric curve of as-prepared NiHPO3 · H2O.

Figure 1. (a) The XRD pattern of the product obtained. (b) Standard XRD pattern of the orthorhombic NiHPO3 · H2O (JCPDS No. 82-0075). (c) EDS spectrum of the as-prepared product. phosphor were prepared. Meanwhile, a certain amount of the product was dissolved in thick HCl solution and then diluted to the appropriate concentration.

3. Results and Discussion 3.1. Structures and Morphology Characterization. Figure 1a depicts an XRD pattern of the product at 150 °C for 24 h. All of the reflection peaks could be indexed to orthorhombic NiHPO3 · H2O with calculated lattice constants a ) 8.859 Å, b ) 7.886 Å, and c ) 10.039 Å, which is in good agreement with the literature values (Figure 1b: JCPDS No. 82-0075: a ) 8.898 Å, b ) 7.862 Å and c ) 10.03 Å). A wide and strong peak ranged from 28.5° to 30.6° should be ascribed to the overlapping of two strong peaks centered at 29.1° and 30.3°, respectively. No other impurity peaks are detected in the XRD pattern. Further evidence of the formation of NiHPO3 · H2O came from energy dispersion X-ray analysis. Figure 1c is the energy dispersion X-ray spectrum (EDS) of the as-prepared product. The peaks of Ni, O and P can be easily found. The C peak was attributed to the substrate. ICP-AES was also used to determine the atomic ratio of Ni/P in the product. The experimental result showed that the Ni/P molar ratio of NiHPO3 · H2O powders was 1.03, which was very close to the theoretic value of 1:1. In order to ascertain the proportion of H2O molecule, the thermogravimetrimetric analysis of the product was investigated (Figure 2). The TGA plot reveals 10.1% weight loss for the complex between 100 and

Figure 3. (a, b) SEM images, (c) TEM image, (d) SAED pattern, and (e) HRTEM image of the as-prepared NiHPO3 · H2O with CTAB as surfactant at 150 °C for 24 h.

405 °C. According to calculations, only one water molecule existed in the product. The second weight loss between 405 and 450 °C was very rapid, which can be ascribed the destruction of the framework. Furthermore, the yield of NiHPO3 · H2O nanoneedles produced by the present method is higher than 90%. The morphology of the as-obtained NiHPO3 · H2O was characterized by field emission scanning electron microscopy (SEM) and (high resolution) transmission electron microscopy (TEM/HRTEM). SEM observations show that the final product is composed of a great deal of uniform needle-like NiHPO3 · H2O nanostructures with lengths of ∼3 µm (Figure 3a). These nanoneedles well align into bundles. A high magnification SEM image shown in Figure 3b reveals that the average diameter of the nanoneedles is about 60 nm. TEM observations further confirmed the result of SEM. Figure 3c shows a typical TEM

3638 Crystal Growth & Design, Vol. 8, No. 10, 2008

Zhang et al.

Figure 4. SEM images of the as-prepared NiHPO3 · H2O at different reaction temperatures: (a) 120 °C and (b) 180 °C.

Figure 6. SEM images of the NiHPO3 · H2O nanoneedles prepared from the system with different amounts of CTAB: (a) 0.0 g, (b) 0.1 g, (c) 0.2 g and (d) 0.8 g.

Figure 5. SEM images of the as-prepared NiHPO3 · H2O at 150 °C for the different reaction times: (a) 6 h, (b) 12 h and (c) 36 h.

image of the product. Some well-aligned nanoneedle bundles ∼60 nm in diameter can be easily seen. The SAED pattern of the product shown in Figure 3d proves the single-crystal nature of NiHPO3 · H2O nanoneedles. Figure 3e depicts the HRTEM image of the nanoneedles. The clear interlaced stripes confirm the single-crystal nature of NiHPO3 · H2O nanoneedles. According to measurements, the distances between the neighboring planes are 0.6013 and 0.8034 nm, corresponding to (010) and (220) planes of NiHPO3 · H2O, respectively. 3.2. The Influencing Factors. 3.2.1. The Effect of Temperature. Temperature is an important factor affecting a chemical reaction. In this work, when the reaction was carried out at 120 °C for 24 h, the final product was a mixture comprised of nanoneedles and nanoparticles (Figure 4a). Also, the yield was quite low. When the temperature was increased to 180 °C, larger needle-like structures were obtained (Figure 4b). Importantly, no well-aligned nanoneedle bundles could be obtained at the above two temperatures, which indicates that the temperature plays an important role in the formation of wellaligned NiHPO3 · H2O nanoneedle bundles, and 150 °C should be the optimum temperature to prepare NiHPO3 · H2O nanoneedle bundles. 3.2.2. The Effect of Reaction Time. It has been found that the reaction time can influence the morphology of the final product, too. Figure 5 shows the SEM images of the products obtained from the same system at 150 °C for 6, 12 and 36 h, respectively. After the reaction was completed for 6 h, some nanoparticles and irregular nanorods of various sizes were formed (Figure 5a). With an extension of the reaction time to 12 h, relatively uniform nanoneedle bundles appeared (Figure 5b). After further prolonging the time to 24 h, the final product

was composed of well-aligned nanoneedle bundles (Figure 3a). After 36 h, the needle-like structures were still preserved, but the size became larger (Figure 5c). The above observations imply the growth process of NiHPO3 · H2O nanoneedles: In the initial stage, a great deal of NiHPO3 · H2O nanoparticles was produced; then, the crystals continuously grew on these nanoparticles along a certain direction. With a prolonging of the time, NiHPO3 · H2O nanoneedles were finally formed. 3.2.3. The Effect of CTAB. Usually, the presence of surfactants in a system can efficiently control the morphology and size of a product during syntheses of nanomaterials. CTAB is a common ionic surfactant and often used for syntheses of materials. In our work, when CTAB did not exist in the system, only polydisperse and random-aligned nanoneedles were obtained (Figure 6a). While 0.1 g of CTAB was used in the system, the uniform nanoneedles were obtained (Figure 6b). Further increasing CTAB to 0.2 g, some regular nanoneedle bundles started to form (Figure 6c). After 0.4 g of CTAB was used, well-aligned nanoneedle bundles were prepared (Figure 3). After further enhancing the amount of CTAB, for example to 0.8 g, no obvious change was found on well-aligned NiHPO3 · H2O nanoneedle bundles (Figure 6d). This fact indicates that the appropriate amount of CTAB cannot only control the size distribution of the product, but also direct the self-assembly of nanoneedles into bundles. 3.2.4. The Effect of NH3 · H2O. The pH of the system is another important factor affecting the formation of NiHPO3 · H2O nanoneedles. When the reaction was carried out in a pure water system (pH ) 6.8), the final product was comprised of irregular needle-like structures (Figure 7). When 5 mL of NH3 · H2O was added (pH ) 12), well-aligned NiHPO3 · H2O nanoneedle bundles were prepared (Figure 3). If the volume of NH3 · H2O exceeds 5 mL (pH > 12), elemental Ni were obtained. As is well-known, in basic solution, NaH2PO2 is often used as a reductant, but it can also decompose into HPO32- and P3- ions due to dismutation. Namely, the redox and dismutation reactions are competitive. In our experiments, it was found that redox reaction was predominant at strong alkaline medium (pH > 12). While the above system was carried out in neutral or weak alkaline medium (pH < 12), the dismutation reaction of H2PO2ions was predominant. However, in a thick NH3 · H2O system of 25%, neither Ni nor NiHPO3 · H2O was obtained. This should

Synthesis of NiHPO3 · H2O Nanoneedle Bundles

Figure 7. SEM images of the as-prepared NiHPO3 · H2O in pure water.

Figure 8. SEM images of as-prepared NiHPO3 · H2O with different amounts of HMT: (a) 0.003 mol and (b) 0.007 mol.

be ascribed to the absence of free Ni2+ ions in the reaction system due to the presence of abundant NH3 molecules. 3.2.5. The Effect of HMT. To investigate the function of hexamethylenetetramine (HMT) in the formation of well-aligned NiHPO3 · H2O nanoneedle bundles, a series of control experiments was designed. When the system did not contain any HMT, only elemental Ni was produced. This fact indicated that [Ni(NH3)6]2+ complex ions were not stable enough and could be reduced by NaH2PO2 to elemental Ni. When 0.003 mol of HMT was introduced, abundant NiHPO3 · H2O nanoneedles were produced (Figure 8a). After further increasing the amount of HMT to 0.005 mol, well-aligned NiHPO3 · H2O nanoneedle bundles were obtained (Figure 3). However, when the amount of HMT was increased to 0.007 mol, random aligned NiHPO3 · H2O nanoneedles reappeared (Figure 8b). The above facts indicate that the appropriate amount of HMT also plays a crucial role in the formation of well-aligned nanoneedle bundles. 3.3. Possible Growth Mechanism. HMT, a universal and versatile ligand having three fused rings in the chair conformation and four bridgehead nitrogen atoms, is wellknown to form coordination compounds with metal salts. 24 In 1989, Pickardt et al.25 reported that infinite chains of [Zn(HMT)Cl2] could be formed by didentate coordination linking tetrahedral zinc ions. Recently, Gao26 and co-workers reported the preparation of flower-like ZnO nanostructures in the zinc-HMT-en system. They considered that the formation of ZnO nanostructures was related to the above similar complex structure. In fact, HMT can also form stable complexes with Ni2+ ions under the presence of other coordinating reagents. For instance, novel complexes of [Ni(HMT)(NCS)2(H2O)2]n and [M(HMT)(NCO)2(H2O)2]n (where M ) Co and Ni) have been reported by Zhang27 and Roy,28 respectively. Similarly, in our work, a complex structure of Ni-HMT-NH3 should also exist, which had been proven by our experiments. It is well-known that the H2PO2ion is a strong reducing reagent due to its low potential in

Crystal Growth & Design, Vol. 8, No. 10, 2008 3639

an aqueous solution. When the system did not contain HMT, [Ni(NH3)6]2+ ions were formed due to the coordination of Ni2+ and NH3, but elemental Ni could still be produced; while HMT existed in the system, no element Ni was obtained. Contrarily, abundant NiHPO3 · H2O nanoneedles were produced. Distinctly, a more stable complex was formed after HMT was introduced. It led to a great change of the potential of Ni2+/Ni pair due to the decrease of free Ni2+ ions. As a result, elemental Ni could not be produced. Furthermore, H2PO2- ions can also decompose into PH3 and HPO32- in neutral or weak alkaline medium (pH < 12) owing to the dismutation. The produced HPO32- ions reacted with the complex containing Ni (Ni-HMT-NH3) to form NiHPO3 · H2O due to the small solubility of NiHPO3 · H2O. Some possible reactions could be described below:

Ni2++ HMT + NH3 f Ni-HMT-NH3

(1)

23H2PO2 + OH f 2HPO3 + PH3+ H2O

(2)

Ni-HMT-NH3 + HPO23 + H2O f NiHPO3·H2O

(3)

The infinite chains of Ni-HMT-NH3 could serve as an organic template to direct the formation of NiHPO3 · H2O nanoneedles. When the proper amount of CTAB existed in the system, the growth of NiHPO3 · H2O nanoneedles could be controlled, which led to the formation of uniform nanoneedle bundles.

4. Conclusion Well-aligned single-crystalline NiHPO3 · H2O nanoneedle bundles ∼3 µm in length and ∼60 nm in diameter were successfully prepared on a large scale via a facile hydrothermal method. Experiments indicated that the reaction temperature, time, and pH could affect the formation and morphology of the product. The formation of a novel complex containing Ni (NiHMT-NH3) reduced the potential of the Ni2+/Ni pair, so the dismutation reaction of H2PO2- ions became predominant. This led to the production of NiHPO3 · H2O due to the smaller solubility. The infinite chains of Ni-HMT-NH3 complex served as an organic template for the formation of the rod-like structures. The present method is a novel, facile, and reliable route that produces a high yield. It opens a new path to prepare metal phosphite nanostructures. Acknowledgment. The authors thank the National Natural Science Foundation of China (20771005 and 20571002), Science and Technological Fund of Anhui Province for Outstanding Youth (08040106834), the Education Department of Anhui Province (No. 2006KJ006TD).

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