Structural, morphological and optical investigations of θ-Al2O3 ultrafine powder

Journal of Alloys and Compounds 718 (2017) 1e6

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Structural, morphological and optical investigations of q-Al2O3 ultrafine powder Ahmed S. Jbara a, b, c, *, Zulkafli Othaman b, c, M.A. Saeed c, d a

Physics Department, Science College, Al-Muthanna University, Samawah, 66001, Iraq Center for Sustainable Nanomaterials, Universiti Teknologi Malaysia, Skudai, 81310, Johor Bahru, Malaysia c Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, Skudai, 81310, Johor Bahru, Malaysia d Division of Science and Technology, University of Education, Township, Lahore, 54770, Pakistan b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 January 2017 Received in revised form 1 May 2017 Accepted 7 May 2017 Available online 10 May 2017

Single-phase q-Al2O3 nanopowder has been synthesized by co-precipitation technique. The synthesized powders were sintered at a temperature ranging from 900 to 1200
C. A stable monoclinic phase is observed for the whole sintering temperature range. The purity, chemical bonds, morphology and optical properties of the powders were investigated by different characterization techniques. X-ray diffraction and BrunauereEmmetteTeller analysis confirms the existence of ultrafine alumina powders with particle diameter of ~5 nm and surface area of 100 m2/g. The novel optical results such as band gap of 5.8 eV would reveal the viability of observed phase of alumina in advanced semiconductor applications. © 2017 Elsevier B.V. All rights reserved.

Keywords: q-Al2O3 Co-precipitation Ultrafine powder Optical band gap

1. Introduction Currently, solar cell industry is facing great challenges regarding incorporation of nanostructured materials for substantial improvements in the efficiency of solar cells along with generation, conversion and transmission of energy. In recent years, alumina has been emerged as a key material to overcome these challenges. Core-shell structures such as Al2O3-ZnO [1], Ag-Al2O3 [2] and SnO2Al2O3 [3] are being used as a new strategy to improve optical absorption in solar cells. Alumina can also be used as a template for other nanostructured material such as CdS nanoparticles [4], carbon nanotubes [5], Au nanorods [6], Si quantum dots [7] Si thin film [8], shell of CoFeZr core [9,10], as well as for abrasives and polishing materials [11,12]. In addition, metal-oxide-semiconductors are widely used in photonic device technology providing different designs compared to the conventional ones and are suitable for high-performing integrated optoelectronic circuits [13]. Alumina has become an important oxide material in this kind of model and can have various applications such as in transistors [14,15], where it is used to amplify or switch electronic signals, in FLASH memory

* Corresponding author. Physics Department, Science College, Al-Muthanna University, Samawah, 66001, Iraq. E-mail address: [email protected] (A.S. Jbara). http://dx.doi.org/10.1016/j.jallcom.2017.05.085 0925-8388/© 2017 Elsevier B.V. All rights reserved.

cells [16], capacitors [17], to inject or eject charges from the tunneling oxide layer, in light emitting diodes [18] and in solar cells [19]. However, because of their highly disordered nature, existing crystallographic models are insufficient to describe the structure of many important alumina phases [20]. Various properties of alumina can be tuned with its constituent transition phases compared to single-phase material. Arifa, Boukhachem, Askri, Boubaker, Yumak and Raouadi [21] find that the band gap for kAl2O3 is around 4.053 eV, which is smaller than that for a-Al2O3 (8.8 eV) [22]. This indicates semiconducting behavior for some of the alumina phases. Furthermore, it has been observed that qalumina nanowires are suitable for many fast optical sensor applications because of its decay time of ~2.23 ns [23]. In-depth exploration of earlier studies reveals that these materials are usually synthesized by means of conventional mechanical routes and expensive physical methods. However, inhomogeneity and non-controllable particle size of the products remains a problem. Physical routes can be used to synthesize most of the ceramic nanopowders (Nps) and are believed to be better to achieve a high purity of products. However, physical routes have many limitations regarding the synthesis of a wide range of ultra-pure and ultra-fine powders. Alternatively, chemical methods have recently received enormous attention for producing fine, mixed and uniform powders without any contamination.

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Several studies also indicate that co-precipitation is a method which has potential to produce nanoparticles having size less than 100 nm. Co-precipitation is a simple and cost effective method and is able to produce high-quality nanoparticles [24,25]. Herein, singlephase q-Al2O3 nanopowders synthesized via co-precipitation method and sintered at different temperatures are presented for the first time in order to address the issues mentioned above. Structure, chemical bonds, morphology and optical properties of the prepared powders are investigated, keeping in view the applications of synthesized material in semiconductor devices. 2. Preparation and characterization methods

q-Al2O3 -Nps have been synthesized by co-precipitation method using tri (dimethylamino) aluminum (III) Al(N(CH3)2)3 and ammonia solution (NH3, 25 wt %) purchased from Sigma-Aldrich as starting materials. All chemicals of analytical grades were used without further purification. The experimental details of coprecipitation route are as described previously by Jbara, Othaman, Ati and Saeed [25] with a slight change in the range of heat treatment. The crystalline phase and purity for q-Al2O3 were observed by X-ray diffractometer (XRD, D8 Advanced) with Cu Ka radiation source (l ¼ 0.154178 nm) at 40 kV and 10 mA. The particle size was estimated using Scherrer's formula. Single point BrunauereEmmetteTeller (BET) surface area (SBET) measurement of the powders was performed by ASAP 2000 equipment, using 30% N2 and 70% He as adsorption/desorption gas. Fourier Transform Infrared Spectroscopy (FTIR) (Perkin-Elmer 5DX) and Raman spectroscopy (HORIBA Scientific - XploRA PLUS) were used to investigate the functional groups and the possible bond characteristics of q-Al2O3 -Nps at different sintering temperatures. X-ray Photoelectron Spectroscopy (XPS) analysis was carried out by using SHIMADZU-AXIS ULTRA DLD model. q-Al2O3 morphology and nanoparticles size distribution were examined by using FieldEmission Scanning Electron Microscopy (FESEM) (JEOL JSM6701F), operating at 120 kV. Optical characterization was done using PerkinElmer LAMBDA 35 UVeVis spectrometer with wavelength range of 190e1100 nm. 3. Results and discussion The XRD spectra of all synthesized q-Al2O3 -Nps with different sintering temperature (900, 1000, 1100, and 1200
C) are shown in Fig. 1. XRD patterns confirm the formation of q-Al2O3 single phased monoclinic structure (JCPDS No. 23-1009). All samples of the synthesized powder show several peaks corresponding to the crystalline planes (401), (002), (111), (111), (401), (202), (112), (600), (313), (113), (020), and (403) of monoclinic q-Al2O3 crystal

structures as shown in Fig. 1. Decent agreement of all observed diffraction peaks with the data of JCPDS card and absence of impurity peaks confirm the high purity of powders. The lattice parameters a, b, and c of q-Al2O3 with monoclinic structure are calculated by using the relation between the inter planar distance dhkl, and Miller indices (hkl) of reflecting planes and the angle b between a and c as given below;

dhkl ¼ SinðbÞ



1=2 h2 k2 Sin2 ðbÞ l2 2 h l CosðbÞ þ þ  a c b2 a2 c2

Table 1 shows q-Al2O3 lattice parameters related to the intensity peaks at (002), (111), (401), and (403) for samples sintered at various temperatures, which are in close agreement with the previous results. These observations substantiate the formation of single-phase alumina for each sample. Moreover, the q-Al2O3 appears when the sintering temperature is in the range of 900e1200
C. The structural stability of q-Al2O3 has so far been successfully achieved for the first time by co-precipitation method. The observed structural parameters are comparable with the findings of Vieira Coelho, Rocha, Souza Santos, Souza Santos and Kiyohara [26] for q-Al2O3 synthesized by sol-gel method and sintered at various temperature range of 930e1050
C. They observed a stable q-Al2O3 for samples sintered at temperatures between 900 and 1200
C. The particle diameter (DXRD) of the samples is estimated from the XRD spectra using DebyeeScherrer's equation [27] as:

DXRD ¼

K l bhkl cosðqÞ

(2)

where K is known as the shape factor or Scherrer's constant that varies in the range 0.89 < K < 1. Usually K ¼ 0.9 is used by assuming the particles are spherical in shape. l is the X-ray wavelength Table 1 Structural properties of q-Al2O3 -Nps sintered at various temperatures. Present work Angles

Lattice constant (Å)

Sintering temperature (
C)

a ¼ 90
b ¼ 103.83
g ¼ 90
a ¼ 90
b ¼ 104.43
g ¼ 90
a ¼ 90
b ¼ 104.36
g ¼ 90
a ¼ 90
b ¼ 104.05
g ¼ 90


a ¼ 11.768 b ¼ 2.901 c ¼ 5.615 a ¼ 11.875 b ¼ 2.915 c ¼ 5.648 a ¼ 11.858 b ¼ 2.913 c ¼ 5.635 a ¼ 11.770 b ¼ 2.910 c ¼ 5.618

900

1000

1100

1200

Other work

Fig. 1. XRD patterns of q-Al2O3 -Nps sintered at different temperatures.

(1)

Angles

Lattice constant (Å)

Ref.

a ¼ 90
b ¼ 104.2
g ¼ 90
a ¼ 90
b ¼ 103.79
g ¼ 90
a ¼ 90
b ¼ 103.83
g ¼ 90
a ¼ 90
b ¼ 103.42
g ¼ 90
a ¼ 90
b ¼ 103.97
g ¼ 90


a ¼ 11.823 b ¼ 2.908 c ¼ 5.630 a ¼ 11.790 b ¼ 2.906 c ¼ 5.620 a ¼ 11.854 b ¼ 2.904 c ¼ 5.622 a ¼ 11.860 b ¼ 2.950 c ¼ 5.630 a ¼ 11.795 b ¼ 2.908 c ¼ 5.618

[46]

[47]

[48]

[49]

[20]

A.S. Jbara et al. / Journal of Alloys and Compounds 718 (2017) 1e6

3

(1.54178 Å), bhkl is full width at half maximum (FWHM) of the diffraction peak and q is the angle of diffraction. The diameter of the nanoparticles calculated from XRD data is observed to vary from 5 to 31 nm as given in Table 2, which confirms the ultrafine nature of synthesized powders. The size of nanoparticles is found to be much smaller compared to the one observed previously [28] for samples prepared by sol-gel technique. Surface area measurements (SBET) of all samples show that Nps have a high surface area at low sintering temperature [26], which is 100 m2/g for powders sintered at 900
C. To endorse the grain diameter results obtained from XRD data, BET method is used to calculate nanoparticles diameter (DBET) using the equation reported earlier [29,30] as:

DBET ¼

6 SBET r

(3)

where r is the theoretical density (3.9 g/cm3). The grain diameter observed from this relation is about 15 nm and is better than that  pez and Go  mez reported by Wang, Bokhimi, Morales, Novaro, Lo [28], as listed in Table 2. Here, it is worth mentioning that the sintering temperature is a major factor used to control nanoparticles size in ceramics materials, as well as the incubation time [31]. It is also established that increase in sintering temperature ultimately enhances the grain size of alumina powders. The diameters of nanoparticle are also estimated from FESEM images of the samples and are given in Table 2. It was observed that q-Al2O3 powders have grains approximately below 40 nm confirming the synthesis of ultrafine powders. FESEM observations are correlated with XRD and BET results. The functional groups and the possible bonds characteristics of q-Al2O3 -Nps were investigated using FTIR. The pellets alumina samples were prepared with alumina and potassium bromide (KBr) ratio of 1:100. FTIR spectra are collected in the range of 4000e400 cm1 (Fig. 2). Two strong peaks exhibited at approximately 562 and 832 cm1, correspond to the stretching vibration of the AleO bonds [32]. These peaks become broader at sintering temperatures of 900 and 1000
C, which may be ascribed to the hydrate transformation of an oxide that confirms the conversion from AlO(OH) to Al2O3. At higher sintering temperatures, an improvement regarding the arrangement of the atoms in the unit cell is observed, which leads to the stretching of Al-O bond. This observation is in good agreement with that reported by Gangwar, Gupta, Kumar, Tripathi and Srivastava [23]. Two other sharp peaks at 1637 and 3460 cm1 indicate the presence of bending and stretching vibration, respectively, of OH in hydration water due to the existence of moisture from KBr [32]. The intensity of hydroxyls bands shrinks with increasing sintering temperature resulted from the condensation of water molecules. It never disappears and is observed until at 1200
C, which refers to the ability of alumina phases to retain OH bands in their structures. The FTIR spectra are very similar to those of q-Al2O3 reported by Boumaza, Favaro, dion, Sattonnay, Brubach, Berthet, Huntz, Roy and Te tot [33]. In Le addition, these spectra reveal weak transmittance bands at approximately 1412, 2855, and 2924 cm1. These bands correspond

Fig. 2. FTIR spectra of q-Al2O3 -Nps sintered at different temperatures.

to the C-H vibrations of the surfactant molecules, which disappear at the higher sintering temperature. Raman spectra for q-Al2O3 -Nps sintered at 900 and 1100
C recorded in the Raman shifts ranging from 200 to 800 cm1 using laser beam of wavelength 785 nm are shown in Fig. 3. As reported by Cava, Tebcherani, Souza, Pianaro, Paskocimas, Longo and Varela [29], various phases of alumina do not exhibit any significant peaks in the Raman spectrum due to the cubic symmetry; however, few Raman modes for cubic spinel materials have been reported earlier [34]. Herein, Raman spectrum for q-Al2O3 is presented for the first time in comparison to the corundum spectrum [35]. Fig. 3 clearly reveals close matching between these spectra, which confirms its alumina phase. The spectra display a rise in the seven Raman peaks at 328, 398, 410, 526, 634, 660, and 686 cm1. The high-intensity bands at 398 and 410 cm1 and another weaker one at 328 cm1 are attributed to Al-O stretching vibrations, while the bands at 526, 634, 686 and at 660 cm1 are ascribed to the hydroxyl stretching [36]. Some

Table 2 Particle diameter for q-Al2O3 -Nps sintered at different temperatures obtained from XRD (DXRD), BET (DBET), and FESEM (DFESEM). DFESEM (nm)

DBET (nm)

DXRD (nm)

Sintering temperature (
C)

26 20 29 40

15.190 20.215 29.714 30.188

5.145e13.855 6.386e14.429 6.745e19.129 27.879e30.545

900 1000 1100 1200

Fig. 3. Raman spectra of q-Al2O3 -Nps sintered at 900 and 1100
C and compared with corundum spectrum [35] with 785 nm laser wavelength.

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In addition, two weak peaks observed at 116 and 978 eV are attributed to the Al 1s and O KLL lines, respectively. These peaks are referred to the valence band and Auger transitions, respectively. Very broad and weak peaks are observed at 96 eV for q-Al2O3 phase sintered at 900 and 1100
C. These occur at the kinetic energy of 1390.69 eV corresponding to Auger transitions of Al KLL. The observed values are in good agreement with the previous works [39]. It is worth mentioning that XPS is a surface analysis and the relevant parameter that can be obtained from XPS data is Auger parameter (AP) [40], which is defined as the sum of the Auger electron kinetic energy (KE) and the core-level binding energy (BE) of the same chemical element. It depends on the change in core electron energy level and the change in intra- and extra-atomic relaxation energies. Auger parameter for two chemical states of Al are determined as follows [41];

AP ¼ KEðAl KLLÞ þ BEðAl 2pÞ Fig. 4. XPS spectra of q-Al2O3 -Nps sintered at 900 and 1100
C.

new Raman peaks are found, which are sharper at 249 cm1 and broader at 447 cm1 that can be assigned to Al-O stretching vibrations. The three moderately weak bands at 561, 739, and 773 cm1 are associated with the hydroxyl deformation [36]. XPS was performed on the surface of the samples sintered at 900 and 1100
C to ensure the composition and phase purity of synthesized powders. Fig. 4 shows XPS survey spectra of q-Al2O3 -Nps showing the presence of Al and O elements. There are three sharp peaks centered at 72, 282 and 529 eV corresponding to the Al 2p, C 1s, and O 1s lines, respectively, which represent the valence band transitions [37], and all the binding and kinetic energies have shifted to a lower energy around 2.4 eV due to charging effects [38].

(4)

The Auger parameter is a one-dimensional quantity used to identify the phase of the material. The result here reveals the value of 1462.69 eV for q-Al2O3, which is close to the earlier result 1462.09 eV by Wagner et al. (1982) [42]. Figs. 5 and 6 represent the FESEM images and grain distribution of q-Al2O3 -Nps, respectively. FESEM images reveal that spherical alumina nanoparticles of small size are generated at a sintering temperature of 900 and 1000
C and are positioned in very close proximity. Fig. 5 (a and b) shows agglomeration of alumina particles where regular hexagonal shapes with high order and homogeneity are observed [43]. However, at higher sintering temperatures i.e. at 1100 and 1200
C, the grain diameter increases to approximately 40 nm. This observation is consistent with the XRD results shown in Table 2. In fact, particles with a size in the nanometer range have a large surface to volume ratio, which leads to the high interfacial

Fig. 5. FESEM micrographs of q-Al2O3 -Nps sintered at a) 900
C, b) 1000
C, c) 1100
C, and d) 1200
C.

A.S. Jbara et al. / Journal of Alloys and Compounds 718 (2017) 1e6

600

300 (b)

400

Counts

Counts

( a)

200 0

0

10 20 30 Grain diameter (nm)

100

0

10 20 30 Grain diameter (nm)

40

150 (c)

(d) Counts

150 Counts

200

0

40

200

100 50 0

5

0

10 20 30 Grain diameter (nm)

40

100 50 0

0

10 20 30 Grain diameter (nm)

40

Fig. 6. Grain diameter distribution for q-Al2O3 -Nps sintered at a) 900
C, b) 1000
C, c) 1100
C, and d) 1200
C.

surface tension that might result in agglomeration of nanoparticles. The appearance of quantum dots during the synthesis of powders would be a reason for particle agglomeration [44]. Fig. 6 exhibits the exponential distribution for grain diameter for q-Al2O3 powders.

Optical properties of q-Al2O3 -Nps sintered at 900 and 1100
C were investigated using UVeVis spectroscopy. The absorbance spectra of each sample are shown in Fig. 7 (upper figure). As expected, alumina phase transition is accompanied by a change in the optical properties and thus the band gap values. The band gap calculation for a powder material has been achieved by measuring the light diffused reflection by UVeVis spectroscopy (UVeVis DR). Kubelka-Munk theory [45] can be used to provide the theoretical descriptions of diffused reflectance spectroscopy as explained below:

FðRÞ ¼

a S

¼

ð1  RÞ2 2R

(5)

where F(R) is the Kubelka-Munk function. a, S, and R are the absorption coefficient, the scattering coefficient, and the fractional reflectance, respectively. From the plot of energy (hy) versus [F(R). hy]n, the value of band gap is estimated by extrapolating the straight line of this curve at [F(R). hy]n ¼ 0. Herein, n is the band gap transition dependent exponent, whose value depends on the band gap transitions, either direct or indirect, and whether the transition is allowed or forbidden. The band gap is calculated by plotting [F(R) hy]2 versus energy hy (lower figure of Fig. 7). For direct band gap transition of q-Al2O3, n should be taken as 2 [23]. Optical band gap values for q-Al2O3 -Nps have not been reported yet. The calculated band gap (5.8 eV), however, is in close agreement to that observed for q-Al2O3 nanowires (5.16 eV) [23] and is much smaller to that observed for a-Al2O3 (8.8 eV) [22]. This is probably due to the crystal defects arising from the aggregation of small Al2O3 particles and also attributed to the oxygen vacancies in alumina nanostructures. The observed and small band gap values of q-Al2O3 would help in improving the photocatalytic behavior of this material. It should be noted that similar semiconducting behavior of kAl2O3 with a band gap of 4.053 eV has been reported earlier [21]. 4. Conclusion Fig. 7. UVeVis absorbance (upper) and plot of (F(R). hy)2 (lower) spectra for q-Al2O3 -Nps sintered at 900, and 1100
C.

Pure phase of q-Al2O3 -Nps at the different sintering temperature were prepared by co-precipitation method. X-ray diffraction

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and BET calculations confirm the synthesis of ultrafine powders of alumina comprised of nanoparticles with size up to 5 nm. The FESEM micrographs reveal a significant decrement in agglomerations at higher sintering temperatures. Furthermore, the optical band gap of synthesized alumina phase is found to be 5.8 eV, which is smaller than that of bulk Al2O3. The prepared q-Al2O3 -Nps would have potential applications in advanced optoelectronic devices. Acknowledgments One of the authors (Ahmed S. Jbara) is grateful to the Ministry of Higher Education and Scientific Research (MOHESR) of Iraq for providing research grant. Authors would like to acknowledge Universiti Teknologi Malaysia (UTM) and Ministry of Higher Education of Malaysia for providing the necessary facilities (Vote no. 12-H75/4F736) and Center for Sustainable Nanomaterials and Department of Physics, UTM for technical support.

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