Applied Mechanics and Materials Vol. 563 (2014) pp 94-101 Online available since 2014/May/28 at www.scientific.net © (2014) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMM.563.94
Behavior of Layered Double Hydroxides Having Different Divalent Transition Metal Groups M. Mamat1,a, T. Tagg1,b, Wan M. Khairul1,c, M.A.A. Abdullah1,d, N. Mohd Tahir2,e, Z. Jubri3,f and R.A. As’ari1,g 1
Advanced Materials Research Group, School of Fundamental Science, Universiti Malaysia Terengganu, 21030 Kuala Terengganu, Malaysia.
2
Environmental Research Group, School of Marine Science and Environment, Universiti Malaysia Terengganu, 21030 Kuala Terengganu, Malaysia.
3
Department of Engineering Sciences & Mathematics, College of Engineering, Universiti Tenaga Nasional, 43000 Kajang, Malaysia. a
[email protected],
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d
Keywords: layered double hydroxide, divalent transition metal, cation radius
Abstract The layered double hydroxides (LDHs) with different divalent transition metal groups and nitrate as a counter anion were investigated. Three d-block divalent metals namely cobalt (Co), nickel (Ni) and copper (Cu) were selected. The cobalt/aluminium (CoAN)-, nickel/aluminium (NiAN)- and copper/aluminium (CuAN)-layered double hydroxides were successfully synthesized via coprecipitation method. All the obtained LDHs were characterized by PXRD, FT-IR, ICP-OES, CHNS and TGA/DTG analysis. Interestingly, behavior of the LDHs was dependent on the size of divalent cations. PXRD showed the basal spacing decrease in the order NiAN (0.88nm)> CuAN (0.87nm) > CoAN (0.74nm), and in a linear correlation with the increasing radii of the divalent cations. Similar trend is observed for the weight loss of LDHs, where NiAN has the highest weight loss (53%), followed by CuAN (43%) and CoAN (34%). Further elemental analysis showed the content of trivalent metal cations, nitrate anions and water molecules in the LDHs decrease with the increasing radii. Introduction Layered double hydroxide (LDH) or so-called hydrotalcite-like compound or anionic clay is one of the layered materials that have attracted much attention due to its potential applications such as catalyst [1], oxyanions remover [2] and as a working electrode for glucose biosensor [3]. The general formula of LDH can be represented by [M2+1-xM3+x(OH)2]x+[An–]x/n.yH2O [4] where M2+ and M3+ are di- and trivalent metal cations respectively, and A is the anion. The structure of LDH resembles the structure of brucite, Mg(OH)2 where Mg is surrounded by six oxygen atoms in the form of hydroxide, forming an octahedral unit. Each octahedral unit is attached to the adjacent unit by edge sharing to form infinite sheets. In the LDH, M2+ will be partially substituted by M3+ thus, giving rise to the positive charge that will be balanced by the anions and water molecules located in the interlayer region [5]. LDH can be synthesized via a wide range of methods and in many possible combinations of M2+/M3+ and metal-anion. The most common method is co-precipitation, or post-synthesis modifications, such as ion exchange [6]. The reaction conditions required in this method can vary depending on the features of the desired materials [7]. Previous work on Ni/Al– and Co/Al–LDHs synthesized by co-precipitation technique showed higher stability for the Ni/Al-LDH compared to that of Co/Al-LDH [8].
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We here report the synthesis and properties of novel LDHs with different first-row transition metal divalent cations. LDH materials exhibit different stabilities in the range of pHs [9] and metal ratios. It is difficult to synthesize LDH materials under similar pH and ratio conditions, therefore the LDHs in this study were prepared at different ratios and pH values based on the previous reports [8,10,11]. Experimental Aluminium nitrate nanohydrate [Al(NO3)3·9H2O, ≥99.0%, Merck], cobalt nitrate hexahydrate [Co(NO3)2.6H2O, ≥99.0%, Merck], nickel nitrate hexahydrate [Ni(NO3)2·6H2O, ≥99.0%, Merck], copper nitrate trihydrate [Cu(NO3)2.3H2O, ≥ 99.5%, Merck] and sodium hydroxide [NaOH, > 99.0%, Essex] were purchased and used without further purifications. All LDHs were prepared via co-precipitation method. The solution of 2M sodium hydroxide (NaOH) was added dropwise into the mixed nitrate solution of M2+/Al3+ (M2+ = Co2+, Ni2+, Cu2+) at preset M2+/Al3+ molar ratio until the mixture reached the constant appointed pH. The preset molar ratio and pH for each M2+/Al3+ system is shown in Table 1. Titration was performed under N2 atmosphere and the solution was stirred constantly. The resulting slurry was aged at 70 ºC with continuous stirring for 18 hours. The precipitate was filtered, washed with distilled water and dried in an oven at 70°C. They were then ground into fine powder products. The products were characterized by powder X-ray diffractometer (PXRD), fourier transform infrared spectrophotometer (FT-IR), inductively coupled plasma optical emission spectrophotometer (ICP-OES), carbon, hydrogen, nitrogen and sulfur analyzer (CHNS) and thermogravimetric/derivative thermogravimetric (TG/DTG) analyzer. Table 1: Preset molar ratio and pH of products LDH system Co/Al/NO3 Ni/Al/NO3 Cu/Al/NO3
Group of M2+ 9 10 11
Ratio of M2+/Al3+ 3 2 2
pH 9 6 8
The synthesized LDHs were analyzed by PXRD using a Rigaku, Miniflex II, desktop X-ray diffractometer under the following conditions: 30 kV, 15 mA, Cu Kα radiation (λ = 0.1541841 nm) and scan rate of 2°/ min in the range from 3°-60°. The FT-IR spectra were obtained using Perkin Elmer Precisely, Spectrum 100 FT-IR spectrophotometer in the range 4000 - 400 cm-1. Elemental compositions of the products were determined by ICP-OES, using a VISTA-PRO CCD Simultaneous ICP-AES under the standard conditions and a CHNS analyzer model CHNS-O (FLASH EA 1112 Series). Thermal degradation was determined using TGA/SDTA 851 instrument at 35ºC -900ºC at a heating rate of 10º/min in N2. Results and Discussion PXRD patterns of the synthesized LDHs are shown in Fig.1. They exhibited symmetrical peaks at low 2θ and weaker, less symmetric peaks at high 2θ values. Poor crystallinity phase which resembles the hydrotalcite-like compound was observed in all LDHs with no other crystalline phases. The basal spacing, d of (003) peak for Co/Al/NO3 (CoAN), Ni/Al/NO3 (NiAN) and Cu/Al/NO3 (CuAN) are shown in Table 2. The d value represents the thickness of a single layer and relates to the size and orientation of the charge balancing interlayer anions [12], in this case, the nitrate anions. The intensity of (003) peak and the d value strongly depends on the radius of divalent metal cation. As the intensity of the (003) peak increase, the d value of this peak for all LDHs decreased in a linear correlation (NiAN > CuAN > CoAN) with the increasing radii of divalent metal cations.
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d = 0.88 nm
intensity/ arbitrary unit
NiAN
d = 0.87 nm CuAN
d = 0.74 nm CoAN
10
20
30
40
50
60
2/ degrees
Fig. 1 PXRD patterns of the LDHs Table 2: Relationship between basal spacing values and divalent metal cations Divalent Group of M2+ metal cation (in periodic table) CoAN Co2+ 9 2+ NiAN Ni 10 CuAN Cu2+ 11 *Cation radius is in octahedral coordination [13]. Sample
Basal spacing, d (nm) 0.74 0.88 0.87
*Radius of M2+ (nm) 0.0745 0.0690 0.0730
The highest d value was obtained for NiAN. Ni2+ has the smallest radius compared to Co2+ and Cu2+. Although periodic trend of the ionic radius follows the order Co2+ > Ni2+ > Cu2+ [14], the crystal field splitting energy (CFSE) of Cu2+ in octahedral coordination is less than that of Ni2+ due to the stabilizing influence of the Jahn-Teller effect. The lower CFSE provides additional stability to the Cu2+ complex, which correlates with ionic radius resulting the order Co2+ < Ni2+ < Cu2+ as shown in the Irving-Williams series [15]. However, the difference between octahedral and tetrahedral crystal field splitting energy or octahedral site preference energy (OSPE) is equally important as a measure of a cation preference for octahedral sites relative to tetrahedral ones. According to Burns [16], Co2+ in octahedral coordination has relatively small OSPE compared to those of Cu2+ and Ni2+. It is suggested that the lower OSPE is a reflection of a weaker binding of a cation to the ligands, i.e, Co2+ with lower OSPE has larger ionic radius than Cu2+ and Ni2+. The trend of ionic radius therefore follows the order Co2+ > Cu2+ > Ni2+, which is in reverse relationship to the d values. Generally, an oxygen atom in a M–O(H) structure is bound preferentially to a metal cation with a small radius, making the hydrogen ion more easily dissociated from the oxygen atom [17]. The same polarizing effect weakens the bonding between the brucite layer and the interlayer anions, causing the expansion of interlayer spacing for small metal cations [9].
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FT-IR spectra for all the LDHs in Fig. 2 are typical for the hydrotalcite-like compounds. Broad bands at around 3400 cm-1 are observed and can be attributed to the OH stretching mode of interhydrogen bonding of hydroxyl groups of the LDHs layers and the interlayer water molecules [18]. The bending mode of water molecules also were found as weak bands at 1640-1620 cm-1 [19–20]. Since the LDHs contain nitrate anions, the NO3- vibration peaks are observed at around 1380 cm-1 [10]. The CO32- anions bands are not found which suggests that all LDHs are free from CO2 contamination [21,22]. The peaks appear below 1000 cm−1 can be attributed to the vibrations of metal-oxygen (M-O, M-O-M, O-M-O) in the layers [11]. Table 3 shows the elemental composition, total weight loss and chemical formula for all LDHs. The observed final molar ratios for NiAN and CuAN differ slightly compared to the initial ratios. This is rather common [23] and may be ascribed to a consequence of the selective redissolution of divalent cations and a preferential precipitation of trivalent ones as a hydroxide due to larger acidity of the trivalent cations and lower solubility of their hydroxides [24].
% transmittance/ arbitrary unit
NiAN
CuAN
CoAN
H2O OH
4000
NO3
3000
2000
1000 -1
wavenumber/ cm
Fig. 2 FTIR spectra of the LDHs
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Table 3: Elemental composition, total weight loss and chemical formula of the LDHs Sample
M2+/Al3+ molar ratio Initial Final
N (%)
Total weight loss (%)
Chemical formula
NiAN
2.0
1.4
3.5
53.1
[Ni2+0.58 Al3+0.42 (OH)2 ] [NO3-]0.42 · 0.81H2O
CuAN
2.0
2.2
3.2
42.8
[Cu2+0.69Al3+0.31 (OH)2 ] [NO3-]0.31 · 0.68H2O
CoAN
3.0
3.0
2.2
34.4
[Co2+0.75 Al3+0.25 (OH)2 ] [NO3-]0.25 · 0.57H2O
All samples do not contain S and C The presence of nitrogen in these composites was determined by CHNS elemental analysis. All LDHs contain nitrogen that is contributed by the NO3- ion. The nitrogen content in all LDHs decreases with the increase of ionic radius. As the radii of M2+ increases (Ni2+
