Structural and Optical Characterization of Ni and Al Co-Doped ZnO Nanopowders Synthesized via the Sol-Gel Process

Abstract

We have successfully synthesized (Ni,Al) co-doped ZnO nanostructured powders via the sol-gel technique at low temperature. The elemental analysis confirms the incorporation of the Ni and Al ions into the ZnO matrix. The structural study revealed that the nanopowder samples are assembled in flower-shaped Zn_0.9-xNi_0.1Al_xO nanostructures with average crystallite sizes ranging from 39 to 53 nm. The XRD patterns show that the Zn_0.9-xNi_0.1Al_xO nanopowders have a hexagonal wurtzite polycrystalline structure. Weak diffraction peaks related mainly to nickel oxides are also detected in the samples. The highest crystallite size, lowest lattice parameters and unit cell volume are obtained for the nanopowder samples that contain 1.5 at.% of aluminum. The decomposition process of the dried gel system is investigated by thermogravimetric analysis (TGA). Raman scattering and FT-IR measurements confirm the wurtzite structure of the synthesized Zn_0.9-xNi_0.1Al_xO nanopowders. The energy band gap of the synthesized nanopowders (~3.32 eV) was estimated by using the Brus equation and the crystallite sizes obtained from XRD data, for comparison. The strain in the nanopowder samples (~2.7 × 10^–3) was also calculated according to the Stokes-Wilson equation.

1. Introduction

Recently, semiconductor nanoparticles have attracted much attention due to different physical and chemical properties compared to their bulk materials¹⁾. When the size of a nanostructure approaches the Bohr radius of exciton, optical, electronic and structural properties are greatly affected by the quantum confinement effect, meaning that nanosized materials are very different from their bulks^2–5). Therefore, semiconductor particles are viewed as promising candidates for many important technological future applications¹⁾. Among various semiconductor nanoparticles, nanosized zinc oxide (ZnO) particles are the most frequently studied due to their frequent applications in industrial areas. ZnO nanoparticles which have great potential for use as efficient UV light absorbers were recently applied to improve the finishing of textile products⁶⁾ and are being utilized for improved cosmetic products⁷⁾. To synthesize uniform nanosized ZnO particles and control their sizes and morphology, a variety of techniques has been used such as thermal decomposition⁸⁾, chemical vapor deposition⁹⁾, sol-gel^10–13), spray pyrolysis¹⁴⁾, and precipitation¹⁵⁾.

It is well known that adding impurities into a wide gap semiconductor such as ZnO induces great changes in the optical, electrical and magnetic properties¹⁶⁾. Thus doping certain elements into ZnO has become an important route to control and optimize its optical, electrical and magnetic performance. It was reported that ZnO has been doped with elements of the groups IA such as Li, IIIA such as Al, VA such as N and VIII such as Ni, Co, etc.^17,18). Doping of ZnO with magnetic ions such as Ni introduces magnetic properties ultimately forming dilute magnetic semiconductors, which are interesting materials for application in the spintronic domain. In addition, nanosized nickel oxide has demonstrated other excellent properties such as catalytic¹⁹⁾, electrochromic²⁰⁾, optical^16,21) and electrochemical properties²²⁾. There is considerable interest in the development of zinc-oxide-based (ZnO: transition metal) diluted magnetic semiconductors because of their high Curie temperature which is essential for spintronic devices³⁾. Recently, co-doped ZnO nanomaterials with Al and transition metals have been investigated for scientific and practical interests. It was found that for the Ni and Al co-doped ZnO (Zn(Ni,Al)O) film, the ferromagnetic behavior was enhanced by increasing the Ni content^23–25). However, the co-doping of ZnO with metallic ions such as nickel (Ni²⁺) and aluminum (Al³⁺) is not widely investigated. The Zn(Ni,Al)O semiconductor is an interesting material due mainly to its ferromagnetic behavior with a high Curie temperature.

It is a significant task to prepare Zn(Ni,Al)O nanomaterials with better electrical, optical and ferromagnetic behavior for potential applications in magnetoelectronic and optoelectronic devices. The aim of the present contribution is the elaboration as well as the structural and optical characterization of Zn(Ni,Al)O nanoparticles synthesized by the sol-gel process where many environmental parameters such as temperature, pressure and doping concentration can be varied to control the growth of the crystal²⁶⁾. This study is a prerequisite to investigate the ferromagnetic behavior in these powdered samples. The prepared Zn_0.9-xNi_0.1Al_xO samples have an Ni composition fixed at 10 at.% and various Al contents ranging from 0.0 to 2.5 at.%. The composition and structure of the Zn(Ni,Al)O nanopowders are studied using energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The optical property and thermal analysis characteristics of the nanopowder samples are measured by using an FT-IR spectrophotometer and a thermogravimetric analyzer. The vibrational properties are studied by Raman scattering at room temperature in back-scattering configuration.

2. Experimental details

Zn(Ni,Al)O nanocrystals were prepared by the sol-gel method using 16 g of zinc acetate dehydrate as a precursor in 112 ml of methanol. After 10 min magnetic stirring at room temperature, 2.8 g of nickel chloride hexahydrate corresponding to [Ni]/[Zn] = 10 at.% and an adequate quantity of aluminum nitrate-9-hydrate corresponding to [Al]/[Zn] ratios of 0.0, 1.5 and 2.5 at.% were added. After an additional 15 min magnetic stirring, the solution was placed in an autoclave and dried under supercritical conditions of ethyl alcohol (EtOH).

The phase identification of nanopowders was analyzed by X-ray diffraction using a Philips X’pert diffractometer equipped with copper X-ray tube (λ_kα1 = 1.5406 Å), nickel filter, graphite crystal monochromator, proportional counter detector, divergence slit 1° and 0.1 mm receiving slit. The working conditions were 40 kV and 30 mA for the X-ray tube, scan speed 0.05° and 2 s measuring time per step. The morphology of the Zn(Ni,Al)O nanoparticles was observed by scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDX). FT-IR spectroscopy (Thermo Scientific Nicolet iS10 FT-IR Spectrometer) was used at room temperature in the range of 500–4000 cm⁻¹ to study the optical properties of the synthesized Zn_0.9-xNi_0.1Al_xO nanopowders. Thermal gravimetric analysis (TGA) curves of the (Ni,Al) co-doped ZnO nanopowders was performed using a TGA apparatus (Perkin Elmer). Approximately 2.9 mg of a sample was placed in a platinum crucible on the pan of the microbalance and heated from 35 to 950°C at a rate of 10°C/min⁻¹. The Raman spectra were performed at room temperature with a Labram system equipped with a microscope in back-scattering configuration. The excitation line was at 514.5 nm from an Ar⁺ laser.

3. Results and discussion

3.1 Composition and structure

Fig. 1 shows typical EDX spectra of the (Ni,Al) co-doped ZnO nanopowders. The spectra in Fig. 1 show the existence of Zn, Ni, Al and O in the powdered samples, indicating the incorporation of Ni and Al ions into the ZnO matrix. It is clear from the elemental analysis that the percentage of Ni atoms is almost constant (around 11 at.%) for all the doped samples (Table 1). The intensity of the characteristic Al peak increases with increased dopant level. The average Al concentrations in the doped nanopowders are 0.0, 2.04 and 3.38 at.% for 0, 1.5 and 2.5 at.% Al doped Zn(Ni)O nanocrystals, respectively (Table 1).

Fig. 1

Energy dispersive X-ray analyses of the Zn_0.9-xNi_0.1Al_xO nanoparticles synthesized by the sol-gel process. (a) x = 0 at.%, (b) x = 1.5 at.% and (c) x = 2.5 at.%.

Table 1 The atomic content of the different elements of the three Zn_0.9-xNi_0.1Al_xO nanopowders obtained from the EDX measurements

Nominal Al concentration (at.%)	EDX Results
	Oxygen (at.%)	Nickel (at.%)	Aluminum (at.%)	Zinc (at.%)
0	47.39	9.26	—	43.35
1.5	51.68	13.19	2.04	33.09
2.5	51.49	12.64	3.83	32.05

Fig. 2 shows the X-ray diffraction spectra (vertically offset for clarity) of the prepared Zn(Ni,Al)O nanopowder samples. The crystal structure of Zn_0.9-xNi_0.1Al_xO (x = 0.00, 0.015 and 0.025) was wurtzite with preferred orientation along [101], [100] and [002] directions for all the samples. The reflection peak positions of the products coincide well with JCPDS data for the wurtzite structure of bulk ZnO^27,28). With Ni and Al incorporation, the dominant peaks of ZnO are retained but are accompanied by extra peaks marked by asterisks. The intensity of these extra peaks is seen to decrease with Al concentration due to incorporation of Al ions in the ZnO lattice. The occurrence of extra peaks and the intensity dependence on the Al concentration signifies that these peaks are due to the presence of additional phases in wurtzite ZnO mainly related to metallic Ni and NiO-like crystalline structures. It is known that Ni is very unsTable in the ZnO matrix and has the tendency to form clusters of metallic Ni or NiO²⁹⁾. The dominant phase is, however, the wurtzite because of smaller concentrations of Ni (10%) and Al (0–2.5%) atoms in the prepared samples. The results show that the addition of Ni and Al atoms as dopants did not affect the lattice patterns of ZnO nanocrystals.

Fig. 2

XRD patterns of the three Zn_0.9-xNi_0.1Al_xO nanopowder samples synthesized by the sol-gel method: (a) x = 0 at.%, (b) x = 1.5 at.% and (c) x = 2.5 at.%.

The Zn_0.9-xNi_0.1Al_xO crystallite sizes (D), calculated from the widths of the major diffraction peaks observed in Fig. 2 using the Scherrer formula³⁰⁾, are given in Table 2. The average crystallite sizes range from 39 to 53 nm. The slight changes in the lattice parameters (Table 3) are expected due to the ionic radii mismatch. Both ionic radii of Al³⁺ (0.57 Å) and Ni²⁺ (0.69 Å) are smaller than the ionic radius of Zn²⁺ (0.74 Å)²⁴⁾. Fig. 3 illustrates the shift occurring in the (101) peak of ZnO with increase in Al concentration. It is clear from Fig. 3 that the (101) peak of ZnO shifts to higher angles for x = 0.015 and 0.025 compared to x = 0. However, for the higher doping concentration (x = 0.025), a smaller shift is detected. The lattice parameters (a and c) and the unit cell volume decrease (Table 3) with the introduction of Ni and Al ions; this can be attributed to the larger ion radius of Zn²⁺ compared with that of Ni²⁺ and Al^{3+ 31)}. Therefore, Al atoms are more incorporated into the ZnO lattice at x = 0.015. The lattice strain of the Zn_0.9-xNi_0.1Al_xO nanopowder samples is also calculated according to the Stokes-Wilson equation. The different values of the strain are 2.7 × 10⁻³, 2.3 × 10⁻³ and 2.9 × 10⁻³ for x = 0, 0.015 and 0.025, respectively.

Table 2 Peak position, full width at half maximum (FWHM) and the estimated crystallite sizes from XRD data along the major reflection planes of the three Zn_0.9-xNi_0.1Al_xO nanopowders

Nominal Al concentration (at.%)	(101)		(100)		(002)
	Peak position and (FWHM) (degree)	Crystallite size D(nm)	Peak position and (FWHM) (degree)	Crystallite size D(nm)	Peak position and (FWHM) (degree)	Crystallite size D(nm)
0	36.34 (0.199)	41	31.86 (0.181)	45	34.51 (0.198)	42
1.5	36.43 (0.173)	48	31.96 (0.154)	53	34.59 (0.169)	49
2.5	36.38 (0.204)	40	31.89 (0.209)	39	34.54 (0.197)	42

Table 3 Lattice parameters, unit cell volume and energy band gap of the Zn_0.9-xNi_0.1Al_xO nanoparticles

Nominal Al concentration (at.%)	Energy gap (eV)	Unit cell volume (Å3)	Lattice parameters (Å)
			a	c
0	3.322	47.2	3.241	5.194
1.5	3.325	46.8	3.231	5.181
2.5	3.321	47.1	3.237	5.189

Fig. 3

Enlarged view of the diffraction angle ranges 2θ = 35.6°–37.2° for Zn_0.9-xNi_0.1Al_xO nanopowders: (a) x = 0.00; (b) x = 0.015 and (c) x = 0.025.

Fig. 4 shows SEM images of the Zn(Ni,Al)O nanopowders prepared with different nominal concentrations of aluminum. The nanoparticles have flower-like shapes consisting of a large number of primary nanocrystallites which are detected by XRD. No appreciable change either in shape or in size of the ZnNiO particles could be observed on Al-doping (Fig. 4(a–c)). From SEM micrographs, a non-uniform distribution of particles is observed. It consists of either several single particles or a cluster of particles. The magnified image shows that flower-shaped structures are constituted by the accumulation of several hundred sharp sheets of Zn(Ni,Al)O nanoparticles. The typical diameters of these individual nanosheets are in the range of 0.5–1 μm with a thickness of 50–100 nm. We note that similar morphologies of the nanostructured materials, consisting of nanometric homogeneous subunits aggregated into larger particles, have been reported previously for materials prepared by controlled precipitation³²⁾ or thermal decomposition of metalorganic precursors³³⁾.

Fig. 4

SEM images showing the morphology of nanocrystalline Zn_0.9-xNi_0.1Al_xO nanopowders at different magnifications: (a) x = 0 at.%, (b) x = 1.5 at.% and (c) x = 2.5 at.%.

In order to compare the energy band gap (E_g) of the different Zn_0.9-xNi_0.1Al_xO samples, we used the Brus equation derived from the effective mass model³⁴⁾ and the crystallite sizes obtained from XRD data. The parameters of ZnO bulk used in the calculations were taken from Ref. 35. The different values of E_g are given in Table 3. We note that E_g is almost constant (∼2.2 eV) when the Al content increases from 0 to 2.5 at.%.

3.2 Thermal properties

The thermal behavior of dried Zn(Ni,Al)O gel has been investigated by thermogravimetric analysis (TGA). Fig. 5 depicts the results of the TGA measurements on the nanosized Zn_0.9-xNi_0.1Al_xO particles. It can be seen that the sample weight decreases continuously with increase in temperature in the interval 25–450°C. However, for temperatures up to 450°C, the weight loss decreases albeit at a small rate (Fig. 5), which corresponds to the release of entrapped gases formed during the decomposition of acetate ions³⁶⁾. The TGA curves show four main regions. The first weight loss is from room temperature up to 150°C due to the dehydration of Zn_0.9-xNi_0.1Al_xO nanopowders. The second weight loss is from 150 to 220°C, which is attributed to the decomposition of chemically bound groups. The third step from 220 to 350°C is related to decomposition of the organic groups. The last weight loss from 350 to 450°C is attributed to decomposition of the impurity ions from the ZnO lattice and the formation of ZnO pure phases. The TGA study also shows the stability of the Zn_0.9-xNi_0.1Al_xO nanopowders. We note that for x = 0, the weight loss is more important compared to samples with x > 0; this can be related to the addition of Al atoms.

Fig. 5

TGA curves of nanosized Zn_0.9-xNi_0.1Al_xO powders at a heating rate of 10°C/min. (a) x = 0.00, (b) x = 0.015 and (c) x = 0.025.

3.3 Raman scattering

Room temperature Raman spectra of Zn(Ni,Al)O nanopowders, synthesized by the sol-gel process, are reported in Fig. 6. The intense Raman peak located at about 502 cm⁻¹ could be assigned to multiphonon scattering of the transverse optical (TO) E₁ and E₂ (low) modes. The peak at 708 cm⁻¹ can be assigned to combinations of longitudinal acoustic (LA) and TO modes at the M point. In the low-frequency region, the peaks at 190, 330, 384 and 409 cm⁻¹ are attributed to 2E₂ (low), 2E₂ (M), A₁ (TO) and E₁ (TO) modes³⁷⁾, respectively. Phonons in a nanocrystal are confined in space and all types of phonons over the entire Brillouin zone will contribute to the Raman spectrum. The intense peak at ∼434 cm⁻¹ is the E₂(high) mode, characteristic of the ZnO crystallinity³⁸⁾. Observation of the later modes indicates that the Zn(Ni,Al)O product has the wurtzite structure as confirmed by XRD analysis. The broad Raman peak at around 573 cm⁻¹ may be due to the A₁(LO) mode³⁹⁾. Recent reports related the appearance of this mode to lattice defects, namely either oxygen vacancies or zinc interstitials or their combination^40,41).

Fig. 6

Room temperature Raman backscattering spectra of the nanocrystalline Zn_0.9-xNi_0.1Al_xO nanopowders. (a) x = 0.00, (b) x = 0.015 and (c) x = 0.025.

3.4 FT-IR measurements

Fig. 7 illustrates the FT-IR spectra of nanosized Zn_0.9-xNi_0.1Al_xO powders in the range 500–4000 cm⁻¹. FT-IR spectra of the synthesized products show a significant spectroscopic band at around 500 cm⁻¹, which is the characteristic band of ZnO⁴²⁾. The broad OH band (3574, 3447) cm⁻¹ appears in all the FT-IR spectra⁴³⁾. Theoretical calculations predict O-H vibrations in ZnO ranging from 3216 to 3644 cm⁻¹, depending on the configuration and number of hydrogen atoms in the complex⁴⁴⁾. The transmission bands at ∼1576 and 1412 cm⁻¹ in all the samples are due to the carbonyl groups of the carboxylate ions which might remain adsorbed on the surface of nanoparticles⁴⁵⁾. The peak at 887 cm⁻¹ is probably due to the nitrate (NO₃⁻) group which is not completely removed during the sol-gel process⁴⁶⁾. Inclusion of Ni and Al ions in the ZnO lattice is confirmed by the emergence of the bands at 1038 and 582 cm^{−1 45)}.

Fig. 7

FT-IR spectra of nanosized Zn_0.9-xNi_0.1Al_xO powdered samples: (a) x = 0.00, (b) x = 0.015 and (c) x = 0.025. The spectra are recorded at room temperature and vertically offset for clarity.

4. Conclusions

In summary, Zn_0.9-xNi_0.1Al_xO nanopowders with crystallite sizes ranging from 39 to 53 nm were successfully synthesized by the sol-gel process and investigated by a variety of techniques for changes in microstructure and optical properties. The XRD patterns indicate high crystallinity in the hexagonal lattice of Zn_0.9-xNi_0.1Al_xO nanoparticles; such a result is also confirmed by Raman measurements. The lowest lattice parameters and unit cell volume are obtained for the nanopowder samples that contain 1.5 at.% of aluminum, which indicates that Al is better incorporated into the ZnO lattice at this Al content. The main weight losses showed by TGA curves occurring in the range between room temperature and 450°C are associated with water emission and the decomposition of organic groups and impurity ions. The TG trace shows the formation of ZnO pure phases up to 450°C. The estimated band gap (∼3.322 eV) from the Brus equation and the average crystallite sizes deduced from XRD data is not strongly dependent on the Al composition of the Zn_0.9-xNi_0.1Al_xO nanopowders in our range of Al content (x = 0–0.025). The synthesized materials have potential applications in photocatalytic, optoelectronic and magnetoelectronic devices.

Author’s short biography

Amor Sayari

Amor Sayari received his bachelor’s degree in physics in 1989 and his Master of Science and PhD graduate degree in solid-state physics in 1991 and 1998, respectively, from Al Manar University, Tunisia. He obtained the habilitation qualification in physics (HDR) at Al Manar University in July 2012, and became an associate professor in 2013. After obtaining his PhD on “Study of folding, confinement and interface effects in GaAs/AlAs superlattices and quantum wells by Raman spectroscopy”, Dr. Sayari combined teaching activities with experimental research in the field of solid-state physics at Al Manar University. He is a member of the Raman Group, Faculty of Science Tunis.

His research interest is vibrational, optical and structural properties, especially for III/V and II/VI semiconductors heterostructures.

In 2006, he joined the King Abdulaziz University, Saudi Arabia, where he became interested in ZnO nanoparticles and their applications in nanotechnology.

Lassaad El Mir

Lassaad El Mir completed his PhD in 1995 at the College of Sciences in Tunis in collaboration with the University of Paris VI in the field of electronic transport in semiconductors and his “HDR” (accreditation to supervise research) in the field of nanotechnology in 2007 from the College of Sciences in Sfax, Tunisia. From 1992 to 2010 he spent several periods of research at the University of Paris VI and in the Institute of NanoSciences in Paris (France). Since 2012 he has been a full professor of physics. From 1997 to 2002 he was head of the Physics Department of the College of Sciences in Gabes University. His main research interest covers the synthesis and characterization of nanoparticles, thin films and nanocomposites for a variety of applications such as solar cells, transparent electrodes, advanced catalyst supports, water treatment, energy storage and gas sensors. Since 2012 he has been a guest professor at the College of Sciences at Al-Imam Muhammad Ibn Saud Islamic University, Saudi Arabia.

References

• 1)   Jagadish  C. and  Pearton  S.J., Zinc Oxide Bulk, Thin Films, and Nanostructures, Elsevier, New York, 2006.
• 2)   Peng  Z.,  Dai  G.,  Chen  P.,  Zhang  Q.,  Wan  Q.,  Zou  B., Synthesis, characterization and optical properties of star-like ZnO nanostructures, Materials Lett., 64 (2010) 898–900.
• 3)   Dietl  T.,  Ohno  H.,  Matsukura  F.,  Cibert  J.,  Ferrand  D., Zener Model Description of Ferromagnetism in Zinc-Blende Magnetic Semiconductors, Science, 287 (2000) 1019–1022.
• 4)   Kadam  P.,  Agashe  C.,  Mahamuni  S., Al-doped ZnO nanocrystals, J. Appl. Phys., 104 (2008) 103501-1–103501-4.
• 5)   Umar  A.,  Al-Hajry  A.,  Al-Heniti  S.,  Hahn  Y.-B., Hierarchical ZnO nanostructures: growth and optical properties, J. of Nanosc. and Nanotech., 8 (2008) 6355–6360.
• 6)   Yadav  A.,  Prasad  V.,  Kathe  A.A.,  Raj  S.,  Yadav  D.,  Sundaramoorthy  C.,  Vigneswaran  Y., Functional Finishing in Cotton Fabrics using Zinc Oxide Nanoparticles, Bull. Mar. Sci., 29 (2006) 641–645.
• 7)   Tani  T.,  Madler  L.,  Pratsinis  S.E., Homogeneous ZnO Nanoparticles by Flame Spray Pyrolysis, J. Nanoparticle Res., 4 (2002) 337–343.
• 8)   Yang  Y.,  Chen  H.,  Zhao  B.,  Bao  X., Size control of ZnO nanoparticles via thermal decomposition of zinc acetate coated on organic additives, J. Cryst. Growth, 263 (2004) 447–453.
• 9)   Purica  M.,  Budianu  E.,  Rusu  E.,  Danila  M.,  Gavrila  R., Optical and structural investigation of ZnO thin films prepared by chemical vapor deposition (CVD), Thin Solid Films, 403–404 (2002) 485–488.
• 10)   El Mir  L.,  Ghribi  F.,  Hajiri  M.,  Ben Ayadi  Z.,  Djessas  K.,  Cubukcu  M.,  von Bardeleben  H.J., Multifunctional ZnO:V thin films deposited by rf-magnetron sputtering from aerogel nanopowder target material, Thin Solid Films, 519 (2011) 5787–5791.
• 11)   El Mir  L.,  Ben Ayadi  Z.,  Saadoun  M.,  von Bardeleben  H.J.,  Djessas  K.,  Zeinert  A., Optical, electrical and magnetic properties of transparent, n-type conductive Zn_0.90–x V_0.10Al_x O thin films elaborated from aerogel nanoparticles, Phys. Stat. Sol. (a), 204 (2007) 3266–3277.
• 12)   El Mir  L.,  Ben Ayadi  Z.,  Saadoun  M.,  Djessas  K.,  von Bardeleben  H.J.,  Alaya  S., Preparation and characterization of n-type conductive (Al, Co) co-doped ZnO thin films deposited by sputtering from aerogel nanopowders, Appl. Surf. Sci., 254 (2007) 570–573.
• 13)   El Mir  L.,  Ben Ayadi  Z.,  Rahmouni  H.,  El Ghoul  J.,  Djessas  K.,  von Bardeleben  H.J., Elaboration and characterization of Co doped, conductive ZnO thin films deposited by radio-frequency magnetron sputtering at room temperature, Thin Solid Films, 517 (2009) 6007–6011.
• 14)   Ayouchi  R.,  Leinen  D.,  Martin  F.,  Gabas  M.,  Dalchiele  E.,  Ramos-Barrado  J.R., Preparation and characterization of transparent ZnO thin films obtained by spray pyrolysis, Thin Solid Films, 426 (2003) 68–77.
• 15)   Dang  Z.M.,  Fan  L.Z.,  Zhao  S.J.,  Nan  C.W., Preparation of nanosized ZnO and dielectric properties of composites filled with nanosized ZnO, Mater. Sci. Eng. B, 99 (2003) 386–389.
• 16)   Wu  D.W.,  Yang  M.,  Huang  Z.B.,  Yin  G.F.,  Liao  X.M.,  Kang  Y.Q.,  Chen  X.F.,  Wang  H., Preparation and properties of Ni-doped ZnO rod arrays from aqueous solution, J. Colloid Interface Sci., 330 (2009) 380–385.
• 17)   Hong  R.J.,  Jiang  X.,  Szyszka  B.,  Sittinger  V.,  Pflug  A., Studies on ZnO:Al thin films deposited by in-line reactive mid-frequency magnetron sputtering, Appl. Surf. Sci., 207 (2003) 341–350.
• 18)   Ryu  Y.R.,  Zhu  S.,  Look  D.C.,  Wrobel  J.M.,  Jeong  H.M.,  White  H.W., Synthesis of p-type ZnO films, J. Cryst. Growth, 216 (2000) 330–334.
• 19)   Wang  Y.,  Zhu  J.,  Yang  X.,  Lu  L.,  Wang  X., Preparation of NiO nanoparticles and their catalytic activity in the thermal decomposition of ammonium perchlorate, Thermochim. Acta, 437 (2005) 106–109.
• 20)   Lin  S.H.,  Chen  F.R.,  Kai  J.J., Electrochromic properties of nano-composite nickel oxide film, Appl. Surf. Sci., 254 (2008) 3357–3363.
• 21)   Goswami  N.,  Sahai  A., Structural transformation in nickel doped zinc oxide nanostructures, Materials Research Bulletin, 48 (2013) 346–351.
• 22)   Wang  X.,  Song  J.,  Gao  L.,  Jin  J.,  Zheng  H.,  Zhang  Z., Optical and electrochemical properties of nanosized NiO via thermal decomposition of nickel oxalate nanofibres, Nanotechnology, 16 (2005) 37–39.
• 23)   Siddheswaran  R.,  Mangalaraja  R.V.,  Tijerina  Eduardo P.,  Menchaca  J.-Luis,  Meléndrez  M.F.,  Avila  Ricardo E.,  Jeyanthi  C. Esther,  Gomez  M.E., Fabrication and characterization of a diluted magnetic semiconducting TM co-doped Al:ZnO (TM = Co, Ni) thin films by sol–gel spin coating method, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 106 (2013) 118–123.
• 24)   Yu  M.,  Qiu  H.,  Chen  X.,  Liu  H.,  Wang  M., Structural and physical properties of Ni and Al co-doped ZnO films grown on glass by direct current magnetron co-sputtering, Physica B, 404 (2009) 1829–1834.
• 25)   Yu  M.,  Qiu  H.,  Chen  X.,  Liu  H., Magnetic, magnetoresistance and electrical transport properties of Ni and Al co-doped ZnO films grown on glass substrates by direct current magnetron co-sputtering, Mater. Chem. Phys., 120 (2010) 571–575.
• 26)   Lee  J.-H.,  Ko  K.-H.,  Park  B.-O., Electrical and optical properties of ZnO transparent conducting films by the sol–gel method, J. Cryst. Growth, 247 (2003) 119–125.
• 27)   Liu  B.,  Zeng  H.C., Hydrothermal Synthesis of ZnO Nanorods in the Diameter Regime of 50 nm, J. Am. Chem. Soc., 125 (2003) 4430–4431.
• 28)   Shen  G.Z.,  Cho  J.H.,  Yoo  J.K.,  Yi  G.C.,  Lee  C.J., Synthesis and Optical Properties of S-Doped ZnO Nanostructures: Nanonails and Nanowires, J. Phys. Chem. B, 109 (2005) 5491–5496.
• 29)   Mohapatra  J.,  Mishra  D.K.,  Kamilla  S.K.,  Medicherla  V.R.R.,  Phase  D.M.,  Berma  V.,  Singh  S.K., Ni-doped ZnO: Studies on structural and magnetic properties, Phys. Status Solidi B, 248 (2011) 1352–1359.
• 30)   Cullity  B.D.,  Stock  S.R., Elements of X-ray Diffraction, Prentice Hall, New York, 2001.
• 31)   Tang  G.,  Shi  X.,  Huo  C.,  Wang  Z., Room temperature ferromagnetism in hydrothermally grown Ni and Cu co-doped ZnO nanorods, Ceramics International, 39 (2013) 4825–4829.
• 32)   Pérez-Maqueda  L.A. Matijević  E., Preparation of Uniform Colloidal Particles of Salts of Tungstophosphoric Acid, Chem. Mater., 10 (1998) 1430–1435.
• 33)   Pérez-Maqueda  L.A.,  Diánez  M.J.,  Gotor  F.J.,  Sayagués  M.J.,  Real  C.,  Criado  J.M., Synthesis of needle-like BaTiO₃ particles from the thermal decomposition of a citrate precursor under sample controlled reaction temperature conditions, Journal of Materials Chemistry, 13 (2003) 2234–2241.
• 34)   Brus  L.E., Electron–electron and electronhole interactions in small semiconductor crystallites: The size dependence of the lowest excited electronic state, J. Chem. Phys., 80 (1984) 4403–4409.
• 35)   Studenikin  S.A.,  Golego  N.,  Cocivera  M., Fabrication of green and orange photoluminescent, undoped ZnO films using spray pyrolysis, J. Appl. Phys., 84 (1998) 2287–2294.
• 36)   Barick  K.,  Aslam  M.,  Dravid  V. and  Bahadur  D., Self-Aggregation and Assembly of Size-Tunable Transition Metal Doped ZnO Nanocrystals, J. Phys. Chem. C, 112 (2008) 15163–15170.
• 37)   Calleja  J.M.,  Cardona  M., Resonant Raman scattering in ZnO, Phys. Rev. B, 16 (1977) 3753–3761.
• 38)   Damen  T.C.,  Porto  S.P.S.,  Tell  B., Raman Effect in Zinc Oxide, Phys. Rev., 142 (1966) 570–574.
• 39)   Cusco  R.,  Alarcon-Llado  E.,  Ibanez  J.,  Artus  L.,  Jimenez  J.,  Wang  B.,  Callahan  M.J., Temperature dependence of Raman scattering in ZnO, Phys. Rev. B, 75 (2007) 165202–165213.
• 40)   Chen  Z.Q.,  Kawasuso  A.,  Xu  Y.,  Naramoto  H.,  Yuan  X.L.,  Sekiguchi  T.,  Suzuki  R.,  Ohdaira  T., Microvoid formation in hydrogen-implanted ZnO probed by a slow positron beam, Phys. Rev. B, 71 (2005) 115213–115221.
• 41)   Kaschner  A.,  Haboeck  U.,  Strassburg  M.,  Kaczmarczyk  G.,  Hoffmann  A.,  Thomsen  C.,  Zeuner  A.,  Alves  H.R.,  Hofmann  D.M.,  Meyer  B.K., Nitrogen-related local vibrational modes in ZnO:N, Appl. Phys. Lett., 80 (2002) 1909–1911.
• 42)   Gupta  T.K., Microstructural engineering through donor and acceptor doping in the grain and grain boundary of a polycrystalline semiconducting ceramic, J. Mater. Res., 7 (1992) 3280–3295.
• 43)   Goswami  N.,  Sen  P., Water-induced stabilization of ZnS nanoparticles, Solid State Commun., 132 (2004) 791–794.
• 44)   Lavrov  E.V.,  Weber  J.,  Börrnert  F.,  Vande Walle  C.G.,  Helbig  R., Hydrogen-related defects in ZnO studied by infrared absorption spectroscopy, Phys. Rev. B, 66 (2002) 165205–165212.
• 45)   Saravanana  R.,  Santhia  K.,  Sivakumarb  N.,  Narayananc  V.,  Stephena  A., Synthesis and characterization of ZnO and Ni doped ZnO nanorods by thermal decomposition method for spintronics application, Materials Characterization, 67 (2012) 10–16.
• 46)   Ni  Y.H.,  Wei  X.W.,  Hong  J.M.,  Ye  Y., Hydrothermal preparation and optical properties of ZnO nanorods, Mater. Sci. Eng. B, 121 (2005) 42–47.