XPS study of basic aluminum sulphate and basic aluminium nitrate

Materials Letters 59 (2005) 1932 – 1936 www.elsevier.com/locate/matlet

XPS study of basic aluminum sulphate and basic aluminium nitrate Loc V. Duonga, Barry J. Woodb, J. Theo Kloproggea,T a

Inorganic Materials Research Program, Queensland University of Technology, 2 George Street, GPO Box 2434, Brisbane, Q 4001, Australia b Brisbane Surface Analysis Facility, University of Queensland, Brisbane, Qld 4072, Australia Received 16 September 2004; accepted 5 February 2005 Available online 17 March 2005

Abstract Basic aluminium sulphate and nitrate crystals were prepared by forced hydrolysis of aluminium salt solution followed by precipitation with a sulphate solution or by evaporation for the basic aluminium nitrate. X-ray Photoelectron Spectroscopy (XPS) confirms the chemical composition determined by ICP-AES in earlier work. High resolution XPS scans of the individual elements allow the identification of both the central IVAlO4 group and the 12 aluminium octahedra in the [IVAlO4AlVI(OH)24(H2O)12] building unit by two Al 2p transitions with binding energies of 73.7 and 74.2 eV in both the basic aluminium sulphate and nitrate. Four different types of oxygen atoms were identified in the basic aluminium sulphate associated with the central AlO4, OH, H2O and SO4 groups in the crystal structure with transitions at 529.4, 530.1, 530.7 and 531.8 eV, respectively. D 2005 Elsevier B.V. All rights reserved. Keywords: Al13; Basic aluminium nitrate; Basic aluminium sulphate; Characterisation methods; Keggin structure; X-ray techniques

1. Introduction One of the most important aluminium complexes in solution is the so-called Al13, a Keggin-type cage structure in which a central AlIVO4 is surrounded by 12 AlVI (OH)24(H2O)12. Forced hydrolysis of Al3 + solutions by the addition of a base like sodium carbonate or sodium hydroxide or homogeneous hydrolysis by the decomposition of urea is known to result in the formation of this large aluminium (oxo-)hydroxide complex. The structure of this complex was first studied by X-ray diffraction after precipitation in the form of two different basic aluminium sulphates in which the Al13 structure is retained [1–4]. Johansson and coworkers [1–4] described the precipitation of basic aluminium sulphates containing the Al13 building unit linked by hydrogen bonding to the oxygen atoms of the sulphate groups. The sodium-containing aluminium sulphate crystallised in the cubic system whereas the sodium-free sulphate crystallised in the

T Corresponding author. Tel.: +61 7 3864 2184; fax: +61 7 3864 1804. E-mail address: [email protected] (J.T. Kloprogge). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.02.029

monoclinic system. In earlier work, we [5,6] reported the precipitation of monoclinic basic aluminium sulphate with a small amount of sodium and a trace of nitrate and of basic aluminium nitrate. Based on the chemical analyses by ICP-AES, a chemical composition per unit cell of Na 0.1 [AlO 4 Al 12 (OH) 24 (H 2 O) 12 ](SO 4 ) 3.55 d 9H 2 O was reported for the sulphate. 27Al Solid-state Magic-Angle Spinning Nuclear Magnetic Resonance spectroscopy showed that the Al13 units were still present in the crystal structure. Kloprogge et al. [5,7] and Teagarden et al. [8] have described the infrared spectrum and near infrared spectrum [9] of the monoclinic basic aluminium sulphate. The spectrum of basic aluminium sulphate is dominated by two strong water bands at 3247 and 1640 cm 1, a strong Al–OH stretching band at 3440 cm 1. The infrared spectrum was further dominated by the r1 and r3 bands at 981 and 1051 cm 1 of the sulphate group in the Al13 sulphate structure. Furthermore, the band at 724 cm 1 is assigned to an Al–O mode of the polymerised Al–O–Al bonds in the Al13 Keggin structure [10]. The Raman spectrum showed the r2 and r4 SO42 triplets at 446, 459 and 496 cm 1 and 572, 614 and 630 cm 1. The r1 was

L.V. Duong et al. / Materials Letters 59 (2005) 1932–1936

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observed as a single band at 990 cm 1, partly overlapped by the r3 triplet at 979, 1009 and 1053 cm 1 [11,12]. To date, there are no publications available on the X-ray Photoelectron Spectroscopy (XPS) of the basic aluminium sulphate and nitrate complexes. The objective of this report is to describe in detail the high-resolution XPS spectra of basic aluminium sulphate and compare those with the spectra of basic aluminium nitrate in order to get more insight in the structure of this complex aluminium salt. As such, this paper forms a continuation of our earlier work on infrared, infrared emission and Raman spectroscopy of these basic aluminium salts.

2. Experimental 2.1. Basic aluminium sulphate and nitrate The synthesis and characterisation of the monoclinic basic aluminium sulphate used in this study has been extensively described by Kloprogge et al. [5–7,10,13]. The tridecameric aluminium polymer was obtained by forced hydrolysis of a 0.5 M aluminium nitrate solution with a 0.5 M sodium or potassium hydroxide solution until an OH/Al molar ratio of 2.2 was reached. Next, the basic aluminium sulphate was precipitated by the addition of the appropriate amount of 0.5 M sodium sulphate and aged for 42 days before removal from the solution. Crystals collected from the wall of the container were shown by XRD and SEM to be phase pure (Fig. 1). The basic aluminium nitrate was prepared from the same hydrolysed aluminium solution followed by very slow evaporation of the excess water at room temperature. This sample was shown to have an impurity in the form of KOH. 2.2. XPS analysis The basic aluminium nitrate and sulphate samples were analyzed in freshly powdered form in order to prevent surface oxidation changes. Prior to the analysis, the samples were out gassed under vacuum for 72 h. The XPS analyses were performed on a Kratos AXIS Ultra with a monochromatic Al X-ray source at 150 W. Each analysis started with a survey scan from 0 to 1200 eV with a dwell time of 100 ms, pass energy of 160 eV at steps of 1 eV with 1 sweep. For the high resolution analysis, the number of sweeps was increased, the pass energy was lowered to 20 eV at steps of 100 meV and the dwell time was changed to 250 ms. Band component analysis was undertaken using the Jandel dPeakfitT software package, which enabled the type of fitting function to be selected and allows specific parameters to be fixed or varied accordingly. Band fitting was done using a Lorentz–Gauss cross-product function with the minimum number of component bands used for the fitting process. The Gaussian–Lorentzian ratio was main-

Fig. 1. SEM images of a basic aluminium sulphate crystal at room temperature and after calcination at 400 8C.

tained at values greater than 0.7 and fitting was undertaken until reproducible results were obtained with squared correlations of r 2 greater than 0.995. Band width and shape of the O and Al transitions were set prior to the band component analysis, based on the analysis of standard aluminium compounds corundum, a-Al2O3 and boehmite, AlOOH.

3. Results and discussion The basic aluminium sulphate crystals exhibit a clear tetrahedral morphology. Some surface cracks are visible probably due to partial dehydration (Fig. 1). Calcination of the basic aluminium sulphate does not significantly alter the morphology, although some minor powder formation can be

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L.V. Duong et al. / Materials Letters 59 (2005) 1932–1936 5000

O 1s

4500 4000

Intensity (CPS)

3500 3000 2500 2000 1500

C 1s Na 1s

S 2p N 1s

Al 2p

1000 500 0 1200

1000

800

600

400

200

0

Binding energy (eV) Fig. 2. XPS survey scan of basic aluminum sulphate.

observed on the surface. Energy dispersive X-ray analyses showed no change in the overall composition. Fig. 2 shows the XPS survey scan of the basic aluminium sulphate, clearly showing the presence of sodium, aluminium, oxygen and sulphur. In addition, there is a minor amount of N present as well. The chemical composition based on the survey scans of the basic aluminium sulphates at room temperature and after calcination at 200 and 400 8C does not show any significant differences (Table 1). The composition is very close to the composition reported earlier based on ICP-AES analysis of this compound, although the sodium content is somewhat higher [5]. This can probably be explained by the fact that ICP-AES of the redissolved basic aluminium sulphate crystals is less sensitive for sodium than XPS of the solid crystals. The basic aluminium nitrate shows a similar composition to that of the basic aluminium sulphate but is characterised by slightly lower oxygen content, due to the fact that the SO4 groups have been replaced by NO3 groups in the crystal lattice and the presence of the KOH impurity. The high resolution scans of the different elements present in the basic aluminium sulphate crystals at room temperature and after calcination at 200 and 400 8C are similar in both intensities and binding energies within the experimental error of the instrument (Table 2), confirming the general observations discussed above. For sodium a single 1s transition is observed with a binding energy of 1073 eV, indicating a single position in the basic aluminium Table 1 Chemical composition (in at.%) from the XPS analyses of the basic aluminium sulphate at room temperature and after calcination at 200 and 400 8C and basic aluminium nitrate O 1s Al 2p S 2p N 1s Na 1s

25 8C

200 8C

400 8C

Al13 nitrate

72.2 18.3 4.9 2.3 2.4

71.2 18.9 7.2 – 2.7

68.7 21.9 3.4 – 6.1

64.4 14.3 – 11.2 –

sulphate crystal, which is in accordance with the crystal structure described by Johansson [1–4]. Similarly, the sulphur is only present as a single type of sulphate in the structure as shown by the presence of only one S 2p 1/2 and one S 2p 3/2 transitions (Fig. 3). Much more informative though are the high resolution scans of aluminium and oxygen (Fig. 3). The aluminium high resolution scans show two overlapping bands associated with two different Al 2p transitions with binding energies of 74.2 and 73.7 eV at room temperature (Fig. 3). The ratio of the two types of aluminium is in the order of 11:1 which is very close to the 1:12 ratio observed in the Al13 complex where a central IVAl is surrounded by 12 VIAl. Therefore, the 74.2 eV transition is interpreted as being due to the 12 aluminium octahedra in the Keggin structure, while the 73.7 eV transition is associated with the central AlO4 tetrahedron. Similar values are observed for the Al 2p transitions in the basic aluminium nitrate. The oxygen high resolution scans are rather complex and contain a number of overlapping transitions. Band component analysis indicates the presence of four transitions at 531.6, 530.8, 530.1 and 529.5 eV associated with roughly 8, 40, 36 and 15% of the total amount of oxygen present in the crystal structure. Related work in our laboratory on aluminium (oxo-)hydroxides and oxides such as gibbsite, Table 2 Binding energies (in eV) of the basic sulphate at room temperature and after calcination at 200 and 400 8C Al 2p VIAl Al 2p IVAl S 2p 1/2 S 2p 3/2 O 1s SO4 O 1s H2O O 1s OH O 1s O Na 1s

25 8C

200 8C

400 8C

74.2 73.7 171.0 169.9 531.8 530.7 530.1 529.4 1072.9

74.2 73.5 171.3 170.1 531.7 531.0 530.3 529.5 1073.1

74.6 73.8 171.7 170.5 531.9 531.1 530.4 529.7 1073.3

L.V. Duong et al. / Materials Letters 59 (2005) 1932–1936

a

1935

400 350 300

Intensity (CPS)

VI

Al

250 200 IV

Al

150 100 50 0 78

77

76

75

74

73

72

71

70

Binding energy (eV)

b

3000

2500

Intensity (CPS)

OH H2O

2000

1500

SO4 1000

O

500

0 536

535

534

533

532

531

530

529

Binding energy (eV)

c

350 300

Intensity (CPS)

250

S 2p 3/2 S 2p 1/2

200 150 100 50 0 173

172

171

170

169

168

167

166

Binding energy (eV) Fig 3. (a) Al 2p high resolution spectrum of basic aluminium sulphate. (b) O 1s high resolution spectrum of basic aluminium sulphate. (c) S 2p high resolution spectrum of basic aluminium sulphate.

boehmite and corundum has shown that in general there is a clear distinction between oxygen atoms, hydroxyl groups and water with a shift in the binding energy towards higher values. Oxygen in these minerals is generally observed around 530.7 eV, which is close to the value of 529.4 eV in the basic aluminium sulphate. Similarly, hydroxyl groups in

gibbsite and boehmite are identified by an oxygen 1s transition around 531.8 eV and water around 533 eV. Following an analogues interpretation of the oxygen transitions in the basic aluminium sulphate, the four transitions are interpreted as being due to the central O around the IVAl, hydroxyl group around the VIAl, water in

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both around the VIAl and as crystal water and finally as oxygen in the sulphate groups. This last transition with a transition at 531.6 eV is similar to the value of 532.4 for oxygen in Al2(SO4)3 [14], 532 eV for CaSO4 [15] or 531.7 eV for CdSO4 [16]. Further evidence for this interpretation stems from the relative ratios of these transitions. Based on the chemical composition of the basic aluminium sulphate, one would expect an atom concentration of about 7% for oxygen, 38% for the oxygen atoms in the hydroxyl groups, 34% for oxygen atoms in water molecules and 22% for oxygen atoms in the sulphate groups. These values are close to the observed values. No details can be obtained from the O 1s high resolution spectrum of the basic aluminium nitrate due to interference of the KOH impurity. In summary, this work has shown that XPS is not only a strong tool for obtaining chemical information of materials, but high resolution XPS will also give detailed information on the local environment of the different atoms in the structure. As such, this technique forms an important additional analytical tool to the more common techniques such as X-ray diffraction, Infrared spectroscopy, Raman spectroscopy and solid-state Nuclear Magnetic Resonance Spectroscopy.

Acknowledgements The authors wish to thank the Inorganic Materials Research Program, Queensland University of Technology, Brisbane, for the financial support.

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