Solid State Sciences 8 (2006) 634–639www.elsevier.com/locate/ssscie

Formation of Ni–Ti-layered double hydroxides using homogeneousprecipitation method

Xin Shu a, Wenhui Zhang a, Jing He a,∗, Fanxing Gao a, Yuexiang Zhu b

a State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, PR Chinab State Key Laboratory of Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking University,

Beijing 100871, PR China

Received 10 January 2006; received in revised form 15 February 2006; accepted 24 February 2006

Available online 29 March 2006

Abstract

Ni(II)/Ti(IV) layered double hydroxide (LDH) was synthesized by homogeneous precipitation method utilizing urea hydrolysis. The structureand composition of the LDHs were characterized by PXRD, FT-IR, ICP-AES, SEM, nitrogen adsorption/desorption isotherms, TG-DTA, and insitu high temperature X-ray diffraction techniques. It was found that Ti4+ cation are incorporated in the host layers with cyanate anion as theinterlayer anions. The titanium content incorporated in the LDH slabs increases with prolonged synthesis time. The Ni–Ti-LDH crystallinity de-creases with increasing Ti4+ dosage in the synthesis mixture. No extra-framework titanium species was observed existing in the Ni–Ti-LDH. Boththe surface area (182 m2/g) and pore volume (0.86 cm3/g) for the LDH synthesized using the urea method are higher than that by conventionalmethods. The Ni–Ti-LDH is converted to Ni–Ti-LDO above 473 K.© 2006 Elsevier SAS. All rights reserved.

Keywords: Layered double hydroxide; Homogeneous precipitation; Ti-containing

1. Introduction

There is growing interest in layered double hydroxides(LDHs), a kind of natural and synthetic hydrotalcite, due totheir potential application in numerous domains such as cata-lysts [1–3], precursors for catalysts [1], catalyst supports [4,5],anion exchangers [6], adsorbent [7], electrochemical reac-tions [8], and bioactive nanocomposites [9]. The structure ofLDHs can be described as [MII

1−xMIIIx(OH)2]x+(An−)x/n·

yH2O, where MII = Mg2+, Zn2+, Ni2+, etc., MIII = Al3+,Fe3+, Ga3+, etc., and A = 1/2(CO3

2−), Cl−, OH−, etc. Struc-turally they consist of stacked brucite-like (M2+(OH)2) layersin which M2+ ions are partially substituted by M3+ ions. Thesubstitution of M3+ for M2+ on the layers demands for the in-corporation of interlayer anions to balance the resulting positivecharge [1]. In addition to divalent and trivalent cations, a widerange of cations in monovalence or higher valence such as Li+,Sn4+, Zr4+, Ti4+, etc., [10–14] may also be accommodated in

* Corresponding author. Fax: +86 10 64425385.E-mail address: jinghe@263.net.cn (J. He).

1293-2558/$ – see front matter © 2006 Elsevier SAS. All rights reserved.doi:10.1016/j.solidstatesciences.2006.02.029

the octahedral sites of the layers. Because Ti-containing ma-terials have been found to exhibit good performance in thetransformations of organic molecules in liquid phase reactions[15] and photocatalysis [16–19], the synthesis of Ti-containingLDHs, therefore, has attracted recent attention [12,13].

In this paper, we report the synthesis of Ni–Ti-LDH by ho-mogeneous precipitation from aqueous solutions in the pres-ence of urea. The hydrolysis of urea has been used to promotethe precipitation of metal hydrous oxides with uniform size,by heating homogeneous aqueous solutions containing solublemetal salts [20]. By the urea hydrolysis method, LDHs mate-rials possessing large particle sizes have already been synthe-sized [8,21,22].

2. Experimental

2.1. Preparation

A typical synthetic procedure is as follows: 0.5 ml of TiCl4solution (the solution was prepared by TiCl4 and HCl with thevolume ratio as 1:1, thereinto TiCl4 is 0.002 mol), 0.01 mol of

X. Shu et al. / Solid State Sciences 8 (2006) 634–639 635

Ni(NO3)2·6H2O (the molar ratio of Ni2+/Ti4+ is 5:1) and 6.5 gof urea were dissolved in 100 ml deionized water under vigor-ous stirring. The resulting suspension was stirred for a certaintime at a refluxing temperature (373 K in aqueous solutions),and then filtered. The filter cake was washed twice with deion-ized water and once with anhydrous ethanol, and finally driedovernight at 333 K. The Ni–Ti-LDH with different molar ratioof Ni2+/Ti4+ can also be prepared using the same procedureby varying Ni2+ dosage.

2.2. Characterization

The Ni–Ti-LDH materials were characterized by X-ray dif-fraction, FT-IR, TG/DTA, ICP, nitrogen adsorption/desorptionisotherms, SEM and TEM techniques. Powder X-ray diffraction(XRD) data were collected on a Shimadzu XRD-6000 diffrac-tometer using Cu Kα source, with a scan step of 0.02◦ and ascan range between 3◦ and 70◦. FT-IR spectra were recorded ona Bruker Vector 22 spectrometer in air at room temperature. Thesample was pressed into a disc with KBr. The spectrum of eachsample was recorded in triplicate by accumulating 20 scansat 2 cm−1 resolution between 400 and 4000 cm−1. Thermalanalyses were carried out on a PCT-1A thermal analysis sys-tem in air. The temperature-programmed rate was 10 ◦C/min,and the measured range from 30 to 600 ◦C. Elemental analysiswas performed with a Shimadzu ICPS-7500 ICP instrument onsolutions prepared by dissolving the samples in dilute HNO3.The low-temperature nitrogen adsorption-desorption experi-ments were carried out using a Quantachrome Autosorb-1 sys-tem. The specific surface area was calculated using the BETmethod based on the absorption isotherm and pore size distri-bution was calculated using the BJH method based on the des-orption isotherm. SEM and TEM micrographs were performedon HITACHI S-4300 and HITACHI H-800, respectively.

3. Results and discussion

3.1. Structure characterization of Ni–Ti-LDH

Fig. 1 illustrates the powder XRD patterns for Ni–Ti-LDHwith the Ni2+/Ti4+ molar ratio of 5:1, 5:2 and 3:2 in the syn-thesis mixture. The XRD patterns give the basal reflectionsof (003), (006), and (110) planes, which are characteristic ofLDHs structure. It can also be noted clearly that only the reflec-tions corresponding to LDHs phase (LDH: JCPDS File No. 38-487) are observed for all samples, although the crystallinitydecreases with increasing Ti4+ content. Assuming a hexago-nal crystal system, the lattice parameters in a direction werecalculated from (110) reflections and shown in Table 1. The de-crease in the lattice parameter a (= 2d110) with increasing Ti4+content clearly demonstrated the effective incorporation of Ti4+in the LDH framework. The isomorphous substitution of Ni2+(r = 72 pm) in octahedral coordination by Ti4+ (r = 68 pm)results in the decrease in parameter a. Moreover, the decreasein a is also attributed to the shorter band length of Ti–O(0.1948 nm) [23] than Ni–O (0.2084 nm) [24]. It is worthwhile

Fig. 1. Powder XRD patterns for Ni–Ti-LDH with different molar ratio ofNi2+/Ti4+ molar ratio (a) 5 : 1, (b) 5 : 2, (c) 3 : 2 (synthesis time = 10 h).

Table 1Preparation conditions and lattice parameters of the Ni–Ti-LDHa

Molar ratioof Ni/Tib

d003 (nm) d110 (nm) Latticeparameter c

(nm)

Latticeparameter a

(nm)

5 : 1 0.7303 0.1550 2.1908 0.31005 : 2 0.7214 0.1543 2.1642 0.30863 : 2 0.7144 0.1511 2.1432 0.3022

a Synthesis time = 10 h. c = 3d003, a = 2d110.b The ratio in the synthesis mixture.

to note that a similar observation was reported on the introduc-tion of Zr4+ in the brucite layer [25]. Generally, it is possibleto obtain pure LDHs when the molar ratio of M2+/M3+ variesin a certain range, such as 2.0–4.0 for Mg2+/Al3+ [1]. In thisstudy, pure Ni–Ti-LDH is observed while the molar ratio ofNi2+/Ti4+ ranges from 5:1 to 3:2. However, the crystallinitydecreases with increasing Ti4+ dosage in the synthesis mixture,and the suggested molar ratio of Ni2+/Ti4+ is 5:1 in the inves-tigated range.

The effect of synthesis time on the incorporation of Ti4+was also investigated. As shown in Fig. 2, the (015) reflectionof Ni–Ti-LDH appears with prolonged synthesis time, indicat-ing a better crystallization is achieved. The related structuralparameters are summarized in Table 2. The lattice parame-ter a gradually decreases with increasing synthesis time on thewhole, indicative of the gradual substitution of Ti4+ for Ni2+on the slabs.

The particle size in a direction, which was calculated by theSherrer’s equation based on XRD patterns, exhibit no obviouschange with extended synthesis time. It could probably be ex-plained by the fact that the excessive urea over metal ions inthe initial solution results in the rapid accomplishment of theformation of the Ni–Ti-LDH nuclei. The following growth ofcrystallite particles is uniformly carried out under vigorous ag-itation.

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Fig. 2. Powder XRD patterns for Ni–Ti-LDH with different reaction time(a) 1.5 h, (b) 2 h, (c) 10 h, (d) 17 h, (e) 24 h, (f) 31 h, (g) 43 h (Ni/Ti = 5).

It can also be found from Tables 2 and 1 that d003 rangesaround 0.73 nm and exhibits no obvious change with synthe-sis time as well as Ni/Ti ratio. The basal spacing distance,smaller than 0.765 nm which was reported for natural hydro-talcite with carbonate as interlayer anion [1], suggests that theinterlayer anions might not be carbonates for Ni–Ti-LDH syn-thesized in this work. The hydrolysis of urea generally pro-ceeds in two steps (Eqs. (1) and (2)), the formation of am-monium cyanate (NH4CNO) being the rate determining step,with subsequent fast hydrolysis of the cyanate to ammoniumcarbonate, resulting in a pH value of 9. But in the synthesisof Ni–Ti-LDH in this work, the final pH for the suspensionwas found up to 8. The pH lower than 9 is supposed to in-hibit the formation of carbonate, giving rise to the dominanceof cyanate in the LDH interlayers. The pH of 8–9 is supposedto be suitable for the complete co-precipitation of Ni(II) andTi(IV) cations because the pH value appropriate for the pre-cipitation of Ni(II) is about 8, and the precipitation of Ti(IV)occurs even in an acidic or neutral aqueous medium. Never-theless, the pH optimization for the synthesis of Ni–Ti-LDH isinteresting to be paid attention on in the following investiga-

Fig. 3. IR spectra for Ni–Ti-LDH with different reaction time (a) 1.5 h, (b) 2 h,(c) 10 h, (d) 17 h, (e) 24 h, (f) 31 h, (g) 43 h (Ni/Ti = 5).

tion, especially for the synthesis of a LDH unreported previ-ously.

CO(NH2)2 → NH4CNO, (1)

NH4CNO + 2H2O → 2NH4+ + CO3

2−. (2)

FT-IR spectra and elemental analysis result confirm the pres-ence of cyanate as interlayer anions. As shown in Fig. 3, the twosharp bands at 2220 cm−1 and 2249 cm−1 indicates the pres-ence of cyanate (CNO−) anions. Another important observationfor the presence of cyanate anion in the interlayer space is thechange of νOH (OH stretching vibration at around 3500 cm−1).It is observed that the broad OH vibration band is clearly re-solved into two bands, which has been attributed to the effect ofthe CNO group near hydroxyl group. The peak of the hydroxylgroup affected by the cyanate group shifted to a low wavenum-ber (i.e., 3448 cm−1) due to the lowering of the O–H bondelectron density, while the unaffected hydroxyl group appearedat high wavenumber (i.e., 3643 cm−1). A similar phenomenonwas reported by Tagaya et al. [14]. A weak band ascribed tocarbonate anion (CO3

2−) is observed around 1385 cm−1. Theshift to high frequency in comparison with interlayer carbonate(1376 cm−1) suggests that the IR absorption comes from thecarbonates interacting weakly with LDH slabs or just adsorbed

Table 2Structural and lattice parameters of the synthesized Ni–Ti-LDHa

Reaction time (h) d003 (nm) d110 (nm) Lattice parameter a (nm) Lattice parameter c (nm) Particle sizeb (nm)

1.5 0.7348 0.1552 0.3104 2.2044 5.462 0.7369 0.1553 0.3106 2.2108 5.51

10 0.7303 0.1550 0.3100 2.1908 5.5617 0.7299 0.1548 0.3096 2.1896 5.6424 0.7378 0.1546 0.3092 2.2135 5.8531 0.7329 0.1546 0.3092 2.1986 5.8643 0.7395 0.1547 0.3094 2.2185 5.92

a Ni/Ti = 5. c = 3d003, a = 2d110.b The particle size was calculated by the Sherrer’s equation based on d110.

X. Shu et al. / Solid State Sciences 8 (2006) 634–639 637

on the slab surface. The bands observed around 400–900 cm−1

is interpreted as the lattice vibration modes of M–O and M–OH [26]. Based on the elemental analysis results, the formulafor Ni–Ti-LDH with a synthesis ratio of Ni/Ti = 5 can be de-scribed as Ni0.85Ti0.15(OH)1.49(CNO)0.50(CO3)0.15·0.47H2O,consistent with the above observation that cyanates are dom-inant interlayer anions. The observed Ni/Ti ratio (0.85 : 0.15)is slightly different from that in the starting solution. The lackof coincidence between the initial ratio of cations in the solutionand the ratio in the final solid is, however, rather common [27]and may be ascribed to a preferential precipitation of one oranother cation as hydroxide or error during measurement.

According to XPS data, the value of Ti2p3/2 in the Ni–Ti-LDH is 456.6 eV, which is different from Ti2p3/2 in the Ti–O–Ti (458.2 eV) coordination condition, indicating the absence ofTiO2 phase in the Ni–Ti-LDH, consistent with XRD observa-tion.

3.2. Morphology and properties of Ni–Ti-LDH

The SEM image, shown in Fig. 4, reveals that the Ni–Ti-LDH is clearly plate-like, but deviates from the hexagonal mor-phology which is typical for LDHs, resulting from the incom-plete crystallization under vigorous agitation. The plate-likeslabs and close stacking of layers can also be clearly observedby TEM image, as shown in Fig. 5. The low temperature nitro-gen adsorption-desorption isotherms of Ni–Ti-LDH, illustratedin Fig. 6, show a type of typically II indicative of interparti-cle mesoporosity. The specific surface area of the Ni–Ti-LDHis determined to be 182 m2/g, higher than that by conventionalprocess, which ranges from 60 to 90 m2/g [28]. The pore-sizedistribution (the inset of Fig. 6) shows that the Ni–Ti-LDH sam-ple has pronounced mesoporosity of a very narrow pore-sizedistribution. The pore diameter at maximum distribution is ca.12.8 nm. The total pore volume is calculated to be 0.86 cm3/g.The larger surface area and higher pore volume than usuallyobserved could be attributed to the aggregation of narrowlysize-distributed fine primary particles, as observed in SEM im-age.

Fig. 4. SEM image of Ni–Ti-LDH (Ni/Ti = 5, synthesis time = 10 h).

Fig. 5. TEM image of Ni–Ti-LDH (Ni/Ti = 5, synthesis time = 10 h).

Fig. 6. Nitrogen adsorption/desorption isotherms and corresponding pore sizedistribution (inset) for the Ni–Ti-LDH (Ni/Ti = 5, synthesis time = 10 h).

The thermal stability of LDHs and phase transformationupon heating was investigated by TG/DTA experiment andin-situ high temperature X-ray diffraction experiment, respec-tively. As shown in Fig. 7, for Ni–Ti-CNO-LDH, one steepweight loss in the TG curve, and correspondingly, an exother-mal peak in the DTA curve were observed around 598 K, whichobviously results from the decomposition and combustion ofcyanate anions. The slow weight loss before 523 K correspondsto the removal of adsorbed water, interlayer water, dehydroxy-lation of the lattice, and CO3

2− adsorbed on the external sur-face of the crystallites which can be released at a rather lowtemperature (423–523 K) [29]. In the in-situ high temperatureXRD patterns shown in Fig. 8, two temperature domains canbe distinguished. In the first domain, from room temperatureto 473 K, the successive XRD patterns exhibit a slight shift to

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Fig. 7. TG/DTA curves for Ni–Ti-LDH (Ni/Ti = 5, synthesis time = 10 h).

Fig. 8. In situ XRD patterns showing the decomposition of Ni–Ti-LDH in the25–500 ◦C temperature range (Ni/Ti = 5, synthesis time = 10 h).

high 2-theta degree in the (003) line position and a progres-sive decrease in the relative intensity of the (006) line to the(003) one. This behavior can be correlated with the extractionof interlamellar water [30], which also results in the gradualdecrease in the interlayer distance, i.e., d003, as represented inFig. 9. When the temperature was increased above 473 K, theLDHs diffractions vanish, including (003) diffraction which in-dicates the layer stacking, and (110) representing the presenceof LDH slabs. Instead, three broad peaks corresponding to theLDO phase [30,31] are observed, meaning that the dehydroxy-lation of the lattice happened around 473 K, consistent with theTG/DTA results. No bulk nickel or titanium oxide is detected.Comparing the in situ XRD and TG/DTA results, it is found that

Fig. 9. Evolution of the interlamellar distance of the decomposition product ofthe Ni–Ti-LDH as a function of the calcinations temperature.

the combustion of interlayer organic anions occurred behind theconversion of LDH to LDO.

4. Conclusions

In summary, Ni–Ti-LDH was synthesized for the first timeby homogeneous precipitation method utilizing urea hydroly-sis. The results indicate that Ti4+ cations are incorporated in thehost layers with cyanate as the interlayer anions. The titaniumcontent in LDH slabs increase with prolonged synthesis time.But the crystallinity decreases with increasing Ti4+ dosage inthe synthesis mixture. Ni–Ti-LDH is converted to Ni–Ti-LDOabove 473 K. Further investigation on the catalytic activity ofNi–Ti-LDH is on-going.

Acknowledgements

The authors are grateful to the financial support from NSFC(20473008 and Key Project: 20531010), Ministry of EducationKey Project (104026), Program for Changjiang Scholars and In-novative Research Team in University (PCSIRT), and Programfor New Century Excellent Talents in University (NCET).

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