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Dense and uniform NiO thin films fabricated by one-step electrostatic spray deposition
Materials Letters 153 (2015) 24–28
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Dense and uniform NiO thin films fabricated by one-step electrostatic spray deposition Bussarin Ksapabutr a,b,n, Pathompong Nimnuan a, Manop Panapoy a,b,n a Department of Materials Science and Engineering, Faculty of Engineering and Industrial Technology, Silpakorn University, Muang, Nakhon Pathom 73000, Thailand b High Performance and Smart Materials, Center of Excellence for Petrochemical and Materials Technology, Chulalongkorn University, Bangkok 10330, Thailand
art ic l e i nf o
a b s t r a c t
Article history: Received 6 January 2015 Accepted 31 March 2015 Available online 8 April 2015
NiO thin films were prepared using one-step electrostatic spray deposition without additives. Dense and crack-free NiO films with (1 1 1) preferred orientation could easily be obtained by tailoring the solvent system in a precursor solution and the annealing temperature. To reduce the evaporation during the deposition and to allow a better spreading of the droplets on the substrate, a mixture of ethanol and butyl carbitol was used as a solvent. The films deposited using the mixed solvent system showed a much smoother and denser surface compared with those deposited using ethanol. The structural parameters, oxygen/nitrogen atomic ratio, electrical conductivity and optical transmittance were improved as the annealing temperature increased in the range of 400–800 1C. Furthermore, the resistance of NiO film remained constant over time at each measurement temperature for 5 h for stability test by heating and cooling process between 200 and 300 1C. & 2015 Elsevier B.V. All rights reserved.
Keywords: NiO film Electrostatic spray deposition Electrical properties Optical properties
1. Introduction Nickel oxide (NiO) is an important p-type semiconductor with a wide band gap of 3.6–4.0 eV and has many practical applications in gas sensors [1], pseudocapacitor electrodes [2], electrochromic devices [3], catalysts [4], dye sensitized solar cells, transparent conducting oxides (TCOs) [5], and thermistors [6–7]. Although pure stoichiometric NiO is an insulator with a room-temperature resistivity of the order of 1013 Ω cm [8], non-stoichiometric NiO shows ptype conduction due to the presence of Ni3 þ ions, resulting from Ni2 þ vacancies and/or interstitial oxygen in NiO crystallites. Controlling the crystallographic orientation and deposition of the films is very important in using NiO films. Chen and Yang [9] fabricated NiO films on glass substrate with (1 1 1) preferred orientation by radiofrequency (RF) magnetron sputtering and reported that their compositions were non-stoichiometric ratio. The NiO films with (2 0 0) orientation are formed near stoichiometric ratio. The (2 0 0) plane of ionic rock salt materials is considered as non-polar cleavage plane and is thermodynamically stable, and the most stable NiO termination has a surface energy of 1.74 J/m2. Meanwhile, the (1 1 1) plane is polar and unstable because of its non-stoichiometric ratio and higher
n Corresponding authors at: Department of Materials Science and Engineering, Faculty of Engineering and Industrial Technology, Silpakorn University, Muang, Nakhon Pathom 73000, Thailand. Tel./fax: þ 66 34219363. E-mail addresses: [email protected] (B. Ksapabutr), [email protected] (M. Panapoy).
http://dx.doi.org/10.1016/j.matlet.2015.03.151 0167-577X/& 2015 Elsevier B.V. All rights reserved.
surface energy (4.5 J/m2), respectively [9–11]. By increasing the temperature Ni2 þ has more kinetic energy migrating to the equilibrium position which leads to a decrease of Ni vacancy defects and results in an increase of the (2 0 0) plane intensity that is attributed to the densely packed plane of Ni2 þ . Jang et al. [10] also prepared NiO films on glass substrate by RF magnetron sputtering and concluded that the preferred orientation changed from (1 1 1) to (2 0 0) with increasing substrate temperature and the resistance of NiO films increased with aging time, showing the instability of (1 1 1) surface. Echresh et al. [12] synthesized NiO films on Si substrate by thermal evaporation method and showed that texture coefficient (TC) of (1 1 1) plane of NiO films decreased, whereas TC of (2 0 0) plane increased as the annealing temperature increased from 600 to 1000 1C. Similar reports presented by Gandhi et al. [13] showed that the ratio of integrated intensity of (1 1 1) and (2 0 0) of NiO nanowalls prepared on Ni grid substrate decreased with annealing temperature. Previous studies of LaNiO3 thin films revealed that the resistivity and band gap energy of dense and crack-free films decreased even with increasing annealing temperature [14]. Different techniques have been developed for preparing dense NiO thin films, such as RF magnetron sputtering [15], ultrasonic spray pyrolysis [16], supersonic atmospheric plasma spraying [7], pulsed laser deposition [17]. However, some techniques require expensive vacuum chambers yielding restrictive operating conditions. Therefore, developing a simple approach for low cost and convenient pathway is a great challenge. Electrostatic spray deposition (ESD) [18–20] is a spray pyrolysis technique which
B. Ksapabutr et al. / Materials Letters 153 (2015) 24–28
involves the atomization of a precursor solution when a high voltage is applied between a nozzle containing the precursor solution and a heated substrate. The spray is transported by the electrical field and impacts the substrate to form the thin film. This method offers several advantages, such as a simple setup, nonvacuum conditions, inexpensive precursors, and a well-controlled structure and composition [20–22]. However, it is difficult to simultaneously achieve long-term electrical stability at high temperature, (1 1 1) preferred orientation and high electrical conductivity, which are required for various applications. Moreover, the electrical conductivity of sputtered NiO film is unstable and decreases with time [10]. From the viewpoint of the application, efforts to stabilize high electrical conductivity of NiO films are essential. Therefore, the present work is focused on the fabrication of dense and uniform NiO thin films using ESD technique and investigates the effect of solvent mixtures and annealing temperature on their surface morphology, electrical conductivity and longterm electrical stability at high temperature.
25
morphology, thickness and composition were examined by a scanning electron microscope (SEM; Model S3400N, Hitachi) coupled with energy dispersive X-ray (EDX) analyzer. The EDX detector was calibrated using a high purity Ni-standard (99.99%). To avoid the interference from the oxygen in a glass substrate, the O/Ni atomic percent in the NiO films was determined by subtracting oxygen in all metal oxides from the glass substrate. The electrical conductivity of the samples was measured by dc fourpoint probe technique (200–300 1C) using a Keithley 2420. The resistance was determined using the two-probe method, connected directly to the silver interdigitated electrodes (Hioki 3801 Hitester) for stability test by heating and cooling the sample in one cycle between 200 and 300 1C. The resistance measurement was performed at each temperature for 5 h. The optical properties were performed using a UV–vis spectrophotometer (Shimadzu UV-1800).
3. Results and discussion 2. Experimental Dense NiO thin films were deposited on glass substrate (Sail Brand, China) through a vertical ESD setup used in our previous works [18]. A precursor solution was prepared by dissolving nickel nitrate hexahydrate (99%, Carlo Erba) in a mixed solvent of ethanol (99.8%, boiling point of 78 1C) and butyl carbitol (99 þ %, boiling point of 230 1C) in a ratio of 80:20 (Et/Bu). The butyl carbitol was used because its high boiling point is suitable for controlling film deposition at lower evaporation rate. The concentration of precursor solution was fixed at 0.1 M. The precursor solution was characterized for its surface tension by a ring method using a surface tension analyzer (DST-60, SEO) and for its conductivity using a CyberScan PC510 conductivity meter. The obtained solution was pumped through the metal nozzle at a flow rate of 1 ml/h. The applied voltage, deposition time and deposition distance were set at 15 kV, 1 h and 7 cm, respectively. Suitable deposition temperature was determined by thermogravimetric analysis (Perkin Elmer, TGA7). The TGA result showed major weight losses below 300 1C, corresponding to the evaporation of solvent and water and the decomposition of organic components and nitrate in the precursor solution. The spraying temperature was thus controlled at 400 1C and measured during deposition using a digital surface probe thermometer (CT-1000 Type K). The assprayed film at 400 1C using the mixed solvent was denoted as AS-Et/Bu400. The effect of annealing temperature on properties of NiO films was investigated. Therefore, after deposition AS-Et/ Bu400 was annealed at different temperatures for 2 h. For comparison, another as-sprayed film was prepared using ethanol (Et) as solvent in the precursor solution of the same deposition condition. Sample abbreviations are given in Table 1. The crystal structure of NiO films were characterized by X-ray diffraction (XRD) with an automated (Rigaku D/Max 2000HV) diffractometer equipped with CuKα radiation source. The film
Table 1 Sample abbreviations for the NiO thin films.
Fig. 1(a)–(d) show the surface morphologies and cross-sectional structures of as-sprayed films (AS-Et/Bu400 and AS-Et400) and annealed films at different temperatures (F-Et/Bu600, F-Et/Bu800). Dense film with crack-free surface was obtained for Et/Bu solvent even at low deposition temperature of 400 1C. Meanwhile, the film prepared using Et has a more porous structure as compared with those using the mixed solvent. For the ESD process, the effect of conductivity, surface tension, and density of precursor solution on droplet size is described by the Kelvin relation [23]. The conductivity, surface tension and density of precursor solutions using Et and Et/Bu were 2.49 mS/cm, 18.89 mN/m, and 0.85 g/cm3 and 1.99 mS/cm, 18.10 mN/m, and 0.87 g/cm3, respectively. The calculated droplet size of precursor solution using Et/Bu was only 1.15 times larger than that using Et. The charged droplets atomized under electric field on the heated substrate cause film formation due to solvent evaporation and decomposition of the precursor solution. With higher boiling point solvent, the impacting droplets can easily spread on the substrate, resulting in dense and crackfree film. Similar results have been obtained in previous studies for fabricating dense GDC and ScSZ [21–22]. After annealing, the thin films obtained from Et/Bu are still highly dense and crack-free. The thickness of AS-400Et/Bu decreased from 295 7 5 nm to 1847 6 nm and 1247 12 nm after annealing at 600 and 800 1C, respectively. Meanwhile, AS-400Et has an average thickness of 688 7179 nm. XRD analysis of as-sprayed and annealed NiO films (Fig. 2(a)) indicated that the patterns show (1 1 1), (2 0 0) and (2 2 0) crystal planes of bulk NiO. For dense films obtained from Et/Bu, all peaks exhibit only cubic NiO structure from JCPDS 04-0835 with (1 1 1) preferential orientation. In contrast, for porous films prepared using Et, all peaks can be indexed to cubic NiO structure (JCPDS 01-078-0643) with (2 0 0) preferential orientation. The reflection intensities for each peak contain information related to the preferential or random growth of polycrystalline thin films which is studied by calculating the texture coefficient (TC(h k l)) for all planes using the equation [24]: TCðh k lÞ ¼ N 100
Sample
Solvent ratio (ethanol:butyl carbitol)
Condition
As-Et400 F-Et600 As-Et/Bu400 F-Et/Bu600 F-Et/Bu700 F-Et/Bu800
100:0 100:0 80:20 80:20 80:20 80:20
As-sprayed at 400 1C Annealed at 600 1C As-sprayed at 400 1C Annealed at 600 1C Annealed at 700 1C Annealed at 800 1C
Iðh k lÞ X Iðh k lÞ = I o ðh k lÞ I o ðh k lÞ
ð1Þ
where N is the number of reflections observed in the XRD pattern, I(h k l) is the measured intensity of X-ray reflection, I0(h k l) is the corresponding standard intensity from the JCPDS file No. 04-0835 and 01-078-0643 for Et/Bu and Et systems, respectively. Furthermore, XRD profiles can also be used to calculate crystallite size, dislocation density and micro strain which play an important role in several properties, such as thermal stability, electrical
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B. Ksapabutr et al. / Materials Letters 153 (2015) 24–28
Fig. 1. SEM images of (a) As-Et400, (b) As-Et/Bu400, (c) F-Et/Bu600 and (d) F-Et/Bu800 (the insets show the corresponding cross-sectional SEM images).
and mechanical properties of materials. The average crystallite size was calculated from the strongest peak (1 1 1) and (2 0 0) planes for dense and porous films, respectively, based on Debye–Scherrer formula [24]. The dislocation density (δ) defined as length of dislocation lines per unit volume and the microstrain (ε) of NiO films were estimated from the relations [24]:
δ ¼ 1=D2
ð2Þ
ε ¼ β=4 tan ðθÞ
ð3Þ
where D is the average crystallite size, β is the full width at half maximum and θ is the Bragg angle. These calculated values are reported in Table 2. As-Et/Bu400 exhibits the (1 1 1) reflection as the strongest peak in the diffractogram. The ratio of the intensities of (1 1 1) and (2 0 0) diffraction peaks (I(1 1 1)/I(2 0 0)) and TC(1 1 1) increased with increasing annealing temperature, and vice versa for Et system. In comparison with the intensity of XRD reference lines of NiO powder (I(1 1 1)/I(2 0 0) ¼0.91), all the films deposited from Et/Bu showed preferential growth of the crystallites along the (1 1 1) plane parallel to the substrate. In contrast, the films deposited from Et solution revealed preferential growth of the crystallites along the (2 0 0) plane. This might be because of more Ni2 þ vacancies for NiO films from the Et/Bu solution. These results can be confirmed by EDX measurement, showing the O/Ni atomic percent was 52.03:47.97, 52.70:47.30 and 54.21:45.79 for As-Et/Bu400, F-Et/ Bu600, and F-Et/Bu800, respectively. The atomic percents of oxygen are greater than 50%, and these values increased with increasing annealing temperature. Therefore, the increase in substrate temperature increased the non-stoichiometry of the NiO
film. Excess of oxygen in non-stoichiometric NiO films will create vacancies at Ni2 þ sites [25], leading to an increase in nickel vacancy with annealing temperature, which might be due to a decrease in film thickness. The (1 1 1) plane is the most densely packed plane of O2 for the NiO crystal structure [9]. The O/Ni atomic percent was 50.87:49.13 and 50.37:49.63 for As-Et400 and F-Et600 respectively. Moreover, the dislocation density and microstrain decreased with increasing crystallite size indicating a lower number of lattice imperfections. This might be due to formation of dense and crack-free films without any voids, resulting in an enhanced crystal growth with increasing temperature [26]. Fig. 2(b) shows the temperature dependence of electrical conductivity at 200–300 1C for the as-sprayed and annealed films. F-Et/Bu800 had the highest conductivity compared with F-Et/ Bu600, As-Et/Bu400, and As-Et400, respectively, due to more Ni2 þ vacancies (shown in EDX results). The activation energy (Ea) was 0.39, 0.41, 0.48 and 0.60 eV for As-Et400, As-Et/Bu400, F-Et/Bu600, and F-Et/Bu800, respectively. The value of Ea increased with annealing temperature because of the increase in crystallite size and decrease in defects [27]. In addition, the resistivity at room temperature of As-Et/Bu400, F-Et/Bu600, and F-Et/Bu800 was 0.98, 4.29, and 14.78 Ω cm, respectively. To investigate electrical stability of NiO films, the aging and dynamic tests were performed. As shown in inset of Fig. 2(b), the resistance remains constant over time at each temperature, indicating that F-Et/ Bu800 has high electrical stability at high temperature. The transmittance spectra of NiO films as a function of wavelength (200–1000 nm) are shown in Fig. 3(a). The optical transmittance of NiO films increased with annealing temperature due to the increase in crystallite size and decrease in defects. At a wavelength of 550 nm, the transmittance of As-Et400, As-Et/
B. Ksapabutr et al. / Materials Letters 153 (2015) 24–28
Fig. 2. (a) XRD patterns of NiO thin films and (b) electrical conductivity of NiO thin films as a function of temperature (the inset shows electrical stability and dynamic test of F-Et/Bu800).
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Fig.3. (a) Spectral transmittance of NiO thin films and (b) the typical variation of (αhν)2 vs. photon energy (hν) (the inset shows a photograph of NiO (F-Et/Bu800) coated on glass substrate compared with the uncoated glass substrate).
Table 2 Crystallite size, I(1 1 1)/I(2 0 0), TC(1 1 1), TC(2 0 0), TC(2 2 0), dislocation density and microstrain of NiO thin films. NiO films
Crystallite size (D) (nm)
I ð1
As-Et400 F-Et600 As-Et/Bu400 F-Et/Bu600 F-Et/Bu700 F-Et/Bu800
5.09 9.78 13.71 23.56 31.31 38.76
0.69 0.67 1.46 1.60 1.84 2.12
1 1Þ =I ð2 0 0Þ
TC (1 1 1)
TC (2 0 0)
TC (2 2 0)
Dislocation density (δ 1015 lines/m2)
Microstrain ε 10 3
1.67 1.53 2.55 2.72 2.83 2.97
1.57 1.54 1.21 1.11 0.94 0.82
1.27 1.43 1.01 1.02 1.14 1.22
38.59 10.46 5.32 1.81 1.02 0.67
18.94 9.78 8.15 4.73 3.55 2.87
Bu400, F-Et/Bu600 and F-Et/Bu800 was 28.73, 74.92, 77.44, and 78.55%, respectively. The band gap energy (Eg) of these films was obtained by extrapolating the linear portion of the curve in Fig. 3 (b). The Eg value of As-Et/Bu400 was reduced from 3.76 to 3.56 eV with increasing temperature, whereas that of As-Et400 was 3.96 eV. The decrease in Eg is probably because of an increase in
crystallite size (the structural parameters in Table 2). These results are consistent with those reported by Zayed et al. [14]. From these results, the obtained NiO thin films can be a promising candidate not only for TCO application due to their high transmittance and low electrical resistivity but also for thermistor application due to their high Ea and long-term electrical stability at high temperature.
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B. Ksapabutr et al. / Materials Letters 153 (2015) 24–28
4. Conclusions A facile approach to fabricate dense and uniform NiO thin film was presented using ESD process with the mixed solvent of ethanol and butyl carbitol. Compared with using only ethanol, higher boiling point solvent caused more spreading of charged droplets impinging on the substrate, resulting in dense and uniform films. After annealing, thin films are still highly dense and crack-free. However, the crystallite size increased with annealing temperature but the dislocation density and microstrain decreased, indicating higher quality films. The NiO films annealed at 800 1C provided the highest values of TC, I(1 1 1)/I(2 0 0), O/Ni atomic ratio, crystallite size, and %transmittance (78.55%) and showed the lowest electrical resistivity and Eg. The resulting films also exhibited high Ea (0.6 eV) and long-term electrical stability at elevated temperature. These results suggest that the obtained NiO thin films are expected to be used as both TCO and temperaturesensing materials. Acknowledgments This work was supported by the Department of Materials Science and Engineering, Silpakorn University and the Center of Excellence for Petrochemical and Materials Technology, Chulalongkorn University. References [1] Fasaki I, Kandyla M, Kompitsas M. Properties of pulsed laser deposited nanocomposite NiO:Au thin films for gas sensing applications. Appl Phys A— Mater 2012;107:899–904. [2] Vijayakumar S, Nagamuthu S, Muralidharan G. Supercapacitor studies on NiO nanoflakes synthesized through a microwave route. ACS Appl Mater Interfaces 2013;5:2188–96. [3] Sawaby A, Selim MS, Marzouk SY, Mostafa MA, Hosny A. Structure, optical and electrochromic properties of NiO thin films. Physica B 2010;405:3412–20. [4] Jana S, Samai S, Mitra BC, Bera P, Mondal A. Nickel oxide thin film from electrodeposited nickel sulfide thin film: peroxide sensing and photodecomposition of phenol. Dalton Trans 2014;43:13096–104. [5] Joseph DP, Saravanan M, Muthuraaman B, Renugambal P, Sambasivam S, Raja SP, et al. Spray deposition and characterization of nanostructured Li doped NiO thin films for application in dye-sensitized solar cells. Nanotechnology 2008;19:485707–17. [6] Huang CC, Kao ZK, Liao YC. Flexible miniaturized nickel oxide thermistor arrays via inkjet printing technology. ACS Appl Mater Interfaces 2013;5:12954–9. [7] Liang S, Zhang X, Bai Y, Han ZH, Yang J. Study on the preparation and electrical properties of NTC thick film thermistor deposited by supersonic atmospheric plasma spraying. Appl Surf Sci 2011;257:9825–9.
[8] Silva MR, Scalvi LVA, Antonia LHD, Santos DI. Deposition and photo-induced electrical resistivity of dip-coated NiO thin films from a precipitation process. J Mater Sci: Mater Electron 2013;24:1823–31. [9] Chen HL, Yang YS. Effect of crystallographic orientations on electrical properties of sputter-deposited nickel oxide thin films. Thin Solid Films 2008;516:5590–6. [10] Jang WL, Lu YM, Hwang WS, Hsiung TL, Wang HP. Effect of substrate temperature on the electrically conductive stability of sputtered NiO films. Surf Coat Technol 2008;202:5444–7. [11] Freundy HJ, Kuhlenbecky H, Staemmler V. Oxide surfaces. Rep Prog Phys 1996;59:283–347. [12] Echresh A, Abbasi MA, Shoushtari MZ, Farbod M, Nur O, Willander M. Optimization and characterization of NiO thin film and the influence of thickness on the electrical properties of n-ZnO nanorods/p-NiO heterojunction. Semicond Sci Technol 2014;29:115009–15. [13] Gandhi AC, Huang CY, Yang CC, Chan TS, Cheng C, Ma YR. Growth mechanism and magnon excitation in NiO nanowalls. Nano Res Lett 2011;6:485–99. [14] Zayed MK, Solieman A, Ebaid M. Discrepancies in the morphology and physical properties of amorphous and crystalline sprayed lanthanum nickel oxide films. Acta Mater 2013;61:5674–84. [15] Ryu HW, Choi GP, Lee WS, Park JS. Preferred orientations of NiO thin films prepared by RF magnetron sputtering. J Mater Sci 2004;39:4375–7. [16] Denayer J, Bister G, Simonis P, Colson P, Maho A, Aubry P, et al. Surfactantassisted ultrasonic spray pyrolysis of nickel oxide andlithium-doped nickel oxide thin films, toward electrochromic applications. Appl Surf Sci 2014;321:61–9. [17] Cao L, Wang D, Wang R. NiO thin films grown directly on Cu foils by pulsed laser deposition as anode materials for lithium ion batteries. Mater Lett 2014;132:357–60. [18] Ksapabutr B, Panapoy M, Choncharoen K, Wongkasemjit S, Traversa E. Investigation of nozzle shape effect on Sm0.1Ce0.9O1.95 thin film prepared by electrostatic spray deposition. Thin Solid Films 2008;516:5618–24. [19] Hwang BH, Chang CL, Hsu CS, Fu CY. Electrostatic spray deposition of NiO/CGO films. J Phys D: Appl Phys 2007;40:3448–55. [20] Fu CY, Chang CL, Hsu CS, Hwang BH. Electrostatic spray deposition of La0.8Sr0.2Co0.2Fe0.8O3 films. Mater Chem Phys 2005;91:28–35. [21] Ksapabutr B, Chalermkiti T, Wongkasemjit S, Panapoy M. Controllable deposition of gadolinium doped ceria electrolyte films by magnetic-field-assisted electrostatic spray deposition. Thin Solid Films 2013;546:423–30. [22] Ksapabutr B, Chalermkiti T, Wongkasemjit S, Panapoy M. Fabrication of scandium stabilized zirconia thin film by electrostatic spray deposition technique for solid oxide fuel cell electrolyte. Thin Solid Films 2010;518:6518–21. [23] Ksapabutr B, Poungchun G, Panapoy M. Architectural control of nanostructured ZrO2 thin films via the electrostatic spray deposition technique using zirconatrane as a precursor. Phys Scr 2010;T139(014055):1–5. [24] Larbi T, Ouni B, Boukhachem A, Boubaker K, Amlouk M. Investigation of structural, optical, electrical and dielectric properties of catalytic sprayed hausmannite thin film. Mater Res Bull 2014;60:457–66. [25] Chen SC, Wen CK, Kuo TY, Peng WC, Lin HC. Characterization and properties of NiO films produced by rf magnetron sputtering with oxygen ion source assistance. Thin Solid Films 2014;572:51–5. [26] Sharma V, Kumar P, Singh N, Upadhyay S, Satsangi VR. Dass Sahab et al. Photoelectrochemical water splitting with nanocrystalline Zn1 xRuxO thin films. Int J Hydrogen Energy 2012;37:12138–49. [27] Laukaitis G, Virbukas D. The structural and electrical properties of GDC10 thin films formed by e-beam technique. Solid State Ionics 2013;247–248:41–7.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Dense and uniform NiO thin films fabricated by one-step electrostatic spray deposition Bussarin Ksapabutr a,b,n, Pathompong Nimnuan a, Manop Panapoy a,b,n a Department of Materials Science and Engineering, Faculty of Engineering and Industrial Technology, Silpakorn University, Muang, Nakhon Pathom 73000, Thailand b High Performance and Smart Materials, Center of Excellence for Petrochemical and Materials Technology, Chulalongkorn University, Bangkok 10330, Thailand
art ic l e i nf o
a b s t r a c t
Article history: Received 6 January 2015 Accepted 31 March 2015 Available online 8 April 2015
NiO thin films were prepared using one-step electrostatic spray deposition without additives. Dense and crack-free NiO films with (1 1 1) preferred orientation could easily be obtained by tailoring the solvent system in a precursor solution and the annealing temperature. To reduce the evaporation during the deposition and to allow a better spreading of the droplets on the substrate, a mixture of ethanol and butyl carbitol was used as a solvent. The films deposited using the mixed solvent system showed a much smoother and denser surface compared with those deposited using ethanol. The structural parameters, oxygen/nitrogen atomic ratio, electrical conductivity and optical transmittance were improved as the annealing temperature increased in the range of 400–800 1C. Furthermore, the resistance of NiO film remained constant over time at each measurement temperature for 5 h for stability test by heating and cooling process between 200 and 300 1C. & 2015 Elsevier B.V. All rights reserved.
Keywords: NiO film Electrostatic spray deposition Electrical properties Optical properties
1. Introduction Nickel oxide (NiO) is an important p-type semiconductor with a wide band gap of 3.6–4.0 eV and has many practical applications in gas sensors [1], pseudocapacitor electrodes [2], electrochromic devices [3], catalysts [4], dye sensitized solar cells, transparent conducting oxides (TCOs) [5], and thermistors [6–7]. Although pure stoichiometric NiO is an insulator with a room-temperature resistivity of the order of 1013 Ω cm [8], non-stoichiometric NiO shows ptype conduction due to the presence of Ni3 þ ions, resulting from Ni2 þ vacancies and/or interstitial oxygen in NiO crystallites. Controlling the crystallographic orientation and deposition of the films is very important in using NiO films. Chen and Yang [9] fabricated NiO films on glass substrate with (1 1 1) preferred orientation by radiofrequency (RF) magnetron sputtering and reported that their compositions were non-stoichiometric ratio. The NiO films with (2 0 0) orientation are formed near stoichiometric ratio. The (2 0 0) plane of ionic rock salt materials is considered as non-polar cleavage plane and is thermodynamically stable, and the most stable NiO termination has a surface energy of 1.74 J/m2. Meanwhile, the (1 1 1) plane is polar and unstable because of its non-stoichiometric ratio and higher
n Corresponding authors at: Department of Materials Science and Engineering, Faculty of Engineering and Industrial Technology, Silpakorn University, Muang, Nakhon Pathom 73000, Thailand. Tel./fax: þ 66 34219363. E-mail addresses: [email protected] (B. Ksapabutr), [email protected] (M. Panapoy).
http://dx.doi.org/10.1016/j.matlet.2015.03.151 0167-577X/& 2015 Elsevier B.V. All rights reserved.
surface energy (4.5 J/m2), respectively [9–11]. By increasing the temperature Ni2 þ has more kinetic energy migrating to the equilibrium position which leads to a decrease of Ni vacancy defects and results in an increase of the (2 0 0) plane intensity that is attributed to the densely packed plane of Ni2 þ . Jang et al. [10] also prepared NiO films on glass substrate by RF magnetron sputtering and concluded that the preferred orientation changed from (1 1 1) to (2 0 0) with increasing substrate temperature and the resistance of NiO films increased with aging time, showing the instability of (1 1 1) surface. Echresh et al. [12] synthesized NiO films on Si substrate by thermal evaporation method and showed that texture coefficient (TC) of (1 1 1) plane of NiO films decreased, whereas TC of (2 0 0) plane increased as the annealing temperature increased from 600 to 1000 1C. Similar reports presented by Gandhi et al. [13] showed that the ratio of integrated intensity of (1 1 1) and (2 0 0) of NiO nanowalls prepared on Ni grid substrate decreased with annealing temperature. Previous studies of LaNiO3 thin films revealed that the resistivity and band gap energy of dense and crack-free films decreased even with increasing annealing temperature [14]. Different techniques have been developed for preparing dense NiO thin films, such as RF magnetron sputtering [15], ultrasonic spray pyrolysis [16], supersonic atmospheric plasma spraying [7], pulsed laser deposition [17]. However, some techniques require expensive vacuum chambers yielding restrictive operating conditions. Therefore, developing a simple approach for low cost and convenient pathway is a great challenge. Electrostatic spray deposition (ESD) [18–20] is a spray pyrolysis technique which
B. Ksapabutr et al. / Materials Letters 153 (2015) 24–28
involves the atomization of a precursor solution when a high voltage is applied between a nozzle containing the precursor solution and a heated substrate. The spray is transported by the electrical field and impacts the substrate to form the thin film. This method offers several advantages, such as a simple setup, nonvacuum conditions, inexpensive precursors, and a well-controlled structure and composition [20–22]. However, it is difficult to simultaneously achieve long-term electrical stability at high temperature, (1 1 1) preferred orientation and high electrical conductivity, which are required for various applications. Moreover, the electrical conductivity of sputtered NiO film is unstable and decreases with time [10]. From the viewpoint of the application, efforts to stabilize high electrical conductivity of NiO films are essential. Therefore, the present work is focused on the fabrication of dense and uniform NiO thin films using ESD technique and investigates the effect of solvent mixtures and annealing temperature on their surface morphology, electrical conductivity and longterm electrical stability at high temperature.
25
morphology, thickness and composition were examined by a scanning electron microscope (SEM; Model S3400N, Hitachi) coupled with energy dispersive X-ray (EDX) analyzer. The EDX detector was calibrated using a high purity Ni-standard (99.99%). To avoid the interference from the oxygen in a glass substrate, the O/Ni atomic percent in the NiO films was determined by subtracting oxygen in all metal oxides from the glass substrate. The electrical conductivity of the samples was measured by dc fourpoint probe technique (200–300 1C) using a Keithley 2420. The resistance was determined using the two-probe method, connected directly to the silver interdigitated electrodes (Hioki 3801 Hitester) for stability test by heating and cooling the sample in one cycle between 200 and 300 1C. The resistance measurement was performed at each temperature for 5 h. The optical properties were performed using a UV–vis spectrophotometer (Shimadzu UV-1800).
3. Results and discussion 2. Experimental Dense NiO thin films were deposited on glass substrate (Sail Brand, China) through a vertical ESD setup used in our previous works [18]. A precursor solution was prepared by dissolving nickel nitrate hexahydrate (99%, Carlo Erba) in a mixed solvent of ethanol (99.8%, boiling point of 78 1C) and butyl carbitol (99 þ %, boiling point of 230 1C) in a ratio of 80:20 (Et/Bu). The butyl carbitol was used because its high boiling point is suitable for controlling film deposition at lower evaporation rate. The concentration of precursor solution was fixed at 0.1 M. The precursor solution was characterized for its surface tension by a ring method using a surface tension analyzer (DST-60, SEO) and for its conductivity using a CyberScan PC510 conductivity meter. The obtained solution was pumped through the metal nozzle at a flow rate of 1 ml/h. The applied voltage, deposition time and deposition distance were set at 15 kV, 1 h and 7 cm, respectively. Suitable deposition temperature was determined by thermogravimetric analysis (Perkin Elmer, TGA7). The TGA result showed major weight losses below 300 1C, corresponding to the evaporation of solvent and water and the decomposition of organic components and nitrate in the precursor solution. The spraying temperature was thus controlled at 400 1C and measured during deposition using a digital surface probe thermometer (CT-1000 Type K). The assprayed film at 400 1C using the mixed solvent was denoted as AS-Et/Bu400. The effect of annealing temperature on properties of NiO films was investigated. Therefore, after deposition AS-Et/ Bu400 was annealed at different temperatures for 2 h. For comparison, another as-sprayed film was prepared using ethanol (Et) as solvent in the precursor solution of the same deposition condition. Sample abbreviations are given in Table 1. The crystal structure of NiO films were characterized by X-ray diffraction (XRD) with an automated (Rigaku D/Max 2000HV) diffractometer equipped with CuKα radiation source. The film
Table 1 Sample abbreviations for the NiO thin films.
Fig. 1(a)–(d) show the surface morphologies and cross-sectional structures of as-sprayed films (AS-Et/Bu400 and AS-Et400) and annealed films at different temperatures (F-Et/Bu600, F-Et/Bu800). Dense film with crack-free surface was obtained for Et/Bu solvent even at low deposition temperature of 400 1C. Meanwhile, the film prepared using Et has a more porous structure as compared with those using the mixed solvent. For the ESD process, the effect of conductivity, surface tension, and density of precursor solution on droplet size is described by the Kelvin relation [23]. The conductivity, surface tension and density of precursor solutions using Et and Et/Bu were 2.49 mS/cm, 18.89 mN/m, and 0.85 g/cm3 and 1.99 mS/cm, 18.10 mN/m, and 0.87 g/cm3, respectively. The calculated droplet size of precursor solution using Et/Bu was only 1.15 times larger than that using Et. The charged droplets atomized under electric field on the heated substrate cause film formation due to solvent evaporation and decomposition of the precursor solution. With higher boiling point solvent, the impacting droplets can easily spread on the substrate, resulting in dense and crackfree film. Similar results have been obtained in previous studies for fabricating dense GDC and ScSZ [21–22]. After annealing, the thin films obtained from Et/Bu are still highly dense and crack-free. The thickness of AS-400Et/Bu decreased from 295 7 5 nm to 1847 6 nm and 1247 12 nm after annealing at 600 and 800 1C, respectively. Meanwhile, AS-400Et has an average thickness of 688 7179 nm. XRD analysis of as-sprayed and annealed NiO films (Fig. 2(a)) indicated that the patterns show (1 1 1), (2 0 0) and (2 2 0) crystal planes of bulk NiO. For dense films obtained from Et/Bu, all peaks exhibit only cubic NiO structure from JCPDS 04-0835 with (1 1 1) preferential orientation. In contrast, for porous films prepared using Et, all peaks can be indexed to cubic NiO structure (JCPDS 01-078-0643) with (2 0 0) preferential orientation. The reflection intensities for each peak contain information related to the preferential or random growth of polycrystalline thin films which is studied by calculating the texture coefficient (TC(h k l)) for all planes using the equation [24]: TCðh k lÞ ¼ N 100
Sample
Solvent ratio (ethanol:butyl carbitol)
Condition
As-Et400 F-Et600 As-Et/Bu400 F-Et/Bu600 F-Et/Bu700 F-Et/Bu800
100:0 100:0 80:20 80:20 80:20 80:20
As-sprayed at 400 1C Annealed at 600 1C As-sprayed at 400 1C Annealed at 600 1C Annealed at 700 1C Annealed at 800 1C
Iðh k lÞ X Iðh k lÞ = I o ðh k lÞ I o ðh k lÞ
ð1Þ
where N is the number of reflections observed in the XRD pattern, I(h k l) is the measured intensity of X-ray reflection, I0(h k l) is the corresponding standard intensity from the JCPDS file No. 04-0835 and 01-078-0643 for Et/Bu and Et systems, respectively. Furthermore, XRD profiles can also be used to calculate crystallite size, dislocation density and micro strain which play an important role in several properties, such as thermal stability, electrical
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Fig. 1. SEM images of (a) As-Et400, (b) As-Et/Bu400, (c) F-Et/Bu600 and (d) F-Et/Bu800 (the insets show the corresponding cross-sectional SEM images).
and mechanical properties of materials. The average crystallite size was calculated from the strongest peak (1 1 1) and (2 0 0) planes for dense and porous films, respectively, based on Debye–Scherrer formula [24]. The dislocation density (δ) defined as length of dislocation lines per unit volume and the microstrain (ε) of NiO films were estimated from the relations [24]:
δ ¼ 1=D2
ð2Þ
ε ¼ β=4 tan ðθÞ
ð3Þ
where D is the average crystallite size, β is the full width at half maximum and θ is the Bragg angle. These calculated values are reported in Table 2. As-Et/Bu400 exhibits the (1 1 1) reflection as the strongest peak in the diffractogram. The ratio of the intensities of (1 1 1) and (2 0 0) diffraction peaks (I(1 1 1)/I(2 0 0)) and TC(1 1 1) increased with increasing annealing temperature, and vice versa for Et system. In comparison with the intensity of XRD reference lines of NiO powder (I(1 1 1)/I(2 0 0) ¼0.91), all the films deposited from Et/Bu showed preferential growth of the crystallites along the (1 1 1) plane parallel to the substrate. In contrast, the films deposited from Et solution revealed preferential growth of the crystallites along the (2 0 0) plane. This might be because of more Ni2 þ vacancies for NiO films from the Et/Bu solution. These results can be confirmed by EDX measurement, showing the O/Ni atomic percent was 52.03:47.97, 52.70:47.30 and 54.21:45.79 for As-Et/Bu400, F-Et/ Bu600, and F-Et/Bu800, respectively. The atomic percents of oxygen are greater than 50%, and these values increased with increasing annealing temperature. Therefore, the increase in substrate temperature increased the non-stoichiometry of the NiO
film. Excess of oxygen in non-stoichiometric NiO films will create vacancies at Ni2 þ sites [25], leading to an increase in nickel vacancy with annealing temperature, which might be due to a decrease in film thickness. The (1 1 1) plane is the most densely packed plane of O2 for the NiO crystal structure [9]. The O/Ni atomic percent was 50.87:49.13 and 50.37:49.63 for As-Et400 and F-Et600 respectively. Moreover, the dislocation density and microstrain decreased with increasing crystallite size indicating a lower number of lattice imperfections. This might be due to formation of dense and crack-free films without any voids, resulting in an enhanced crystal growth with increasing temperature [26]. Fig. 2(b) shows the temperature dependence of electrical conductivity at 200–300 1C for the as-sprayed and annealed films. F-Et/Bu800 had the highest conductivity compared with F-Et/ Bu600, As-Et/Bu400, and As-Et400, respectively, due to more Ni2 þ vacancies (shown in EDX results). The activation energy (Ea) was 0.39, 0.41, 0.48 and 0.60 eV for As-Et400, As-Et/Bu400, F-Et/Bu600, and F-Et/Bu800, respectively. The value of Ea increased with annealing temperature because of the increase in crystallite size and decrease in defects [27]. In addition, the resistivity at room temperature of As-Et/Bu400, F-Et/Bu600, and F-Et/Bu800 was 0.98, 4.29, and 14.78 Ω cm, respectively. To investigate electrical stability of NiO films, the aging and dynamic tests were performed. As shown in inset of Fig. 2(b), the resistance remains constant over time at each temperature, indicating that F-Et/ Bu800 has high electrical stability at high temperature. The transmittance spectra of NiO films as a function of wavelength (200–1000 nm) are shown in Fig. 3(a). The optical transmittance of NiO films increased with annealing temperature due to the increase in crystallite size and decrease in defects. At a wavelength of 550 nm, the transmittance of As-Et400, As-Et/
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Fig. 2. (a) XRD patterns of NiO thin films and (b) electrical conductivity of NiO thin films as a function of temperature (the inset shows electrical stability and dynamic test of F-Et/Bu800).
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Fig.3. (a) Spectral transmittance of NiO thin films and (b) the typical variation of (αhν)2 vs. photon energy (hν) (the inset shows a photograph of NiO (F-Et/Bu800) coated on glass substrate compared with the uncoated glass substrate).
Table 2 Crystallite size, I(1 1 1)/I(2 0 0), TC(1 1 1), TC(2 0 0), TC(2 2 0), dislocation density and microstrain of NiO thin films. NiO films
Crystallite size (D) (nm)
I ð1
As-Et400 F-Et600 As-Et/Bu400 F-Et/Bu600 F-Et/Bu700 F-Et/Bu800
5.09 9.78 13.71 23.56 31.31 38.76
0.69 0.67 1.46 1.60 1.84 2.12
1 1Þ =I ð2 0 0Þ
TC (1 1 1)
TC (2 0 0)
TC (2 2 0)
Dislocation density (δ 1015 lines/m2)
Microstrain ε 10 3
1.67 1.53 2.55 2.72 2.83 2.97
1.57 1.54 1.21 1.11 0.94 0.82
1.27 1.43 1.01 1.02 1.14 1.22
38.59 10.46 5.32 1.81 1.02 0.67
18.94 9.78 8.15 4.73 3.55 2.87
Bu400, F-Et/Bu600 and F-Et/Bu800 was 28.73, 74.92, 77.44, and 78.55%, respectively. The band gap energy (Eg) of these films was obtained by extrapolating the linear portion of the curve in Fig. 3 (b). The Eg value of As-Et/Bu400 was reduced from 3.76 to 3.56 eV with increasing temperature, whereas that of As-Et400 was 3.96 eV. The decrease in Eg is probably because of an increase in
crystallite size (the structural parameters in Table 2). These results are consistent with those reported by Zayed et al. [14]. From these results, the obtained NiO thin films can be a promising candidate not only for TCO application due to their high transmittance and low electrical resistivity but also for thermistor application due to their high Ea and long-term electrical stability at high temperature.
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4. Conclusions A facile approach to fabricate dense and uniform NiO thin film was presented using ESD process with the mixed solvent of ethanol and butyl carbitol. Compared with using only ethanol, higher boiling point solvent caused more spreading of charged droplets impinging on the substrate, resulting in dense and uniform films. After annealing, thin films are still highly dense and crack-free. However, the crystallite size increased with annealing temperature but the dislocation density and microstrain decreased, indicating higher quality films. The NiO films annealed at 800 1C provided the highest values of TC, I(1 1 1)/I(2 0 0), O/Ni atomic ratio, crystallite size, and %transmittance (78.55%) and showed the lowest electrical resistivity and Eg. The resulting films also exhibited high Ea (0.6 eV) and long-term electrical stability at elevated temperature. These results suggest that the obtained NiO thin films are expected to be used as both TCO and temperaturesensing materials. Acknowledgments This work was supported by the Department of Materials Science and Engineering, Silpakorn University and the Center of Excellence for Petrochemical and Materials Technology, Chulalongkorn University. References [1] Fasaki I, Kandyla M, Kompitsas M. Properties of pulsed laser deposited nanocomposite NiO:Au thin films for gas sensing applications. Appl Phys A— Mater 2012;107:899–904. [2] Vijayakumar S, Nagamuthu S, Muralidharan G. Supercapacitor studies on NiO nanoflakes synthesized through a microwave route. ACS Appl Mater Interfaces 2013;5:2188–96. [3] Sawaby A, Selim MS, Marzouk SY, Mostafa MA, Hosny A. Structure, optical and electrochromic properties of NiO thin films. Physica B 2010;405:3412–20. [4] Jana S, Samai S, Mitra BC, Bera P, Mondal A. Nickel oxide thin film from electrodeposited nickel sulfide thin film: peroxide sensing and photodecomposition of phenol. Dalton Trans 2014;43:13096–104. [5] Joseph DP, Saravanan M, Muthuraaman B, Renugambal P, Sambasivam S, Raja SP, et al. Spray deposition and characterization of nanostructured Li doped NiO thin films for application in dye-sensitized solar cells. Nanotechnology 2008;19:485707–17. [6] Huang CC, Kao ZK, Liao YC. Flexible miniaturized nickel oxide thermistor arrays via inkjet printing technology. ACS Appl Mater Interfaces 2013;5:12954–9. [7] Liang S, Zhang X, Bai Y, Han ZH, Yang J. Study on the preparation and electrical properties of NTC thick film thermistor deposited by supersonic atmospheric plasma spraying. Appl Surf Sci 2011;257:9825–9.
[8] Silva MR, Scalvi LVA, Antonia LHD, Santos DI. Deposition and photo-induced electrical resistivity of dip-coated NiO thin films from a precipitation process. J Mater Sci: Mater Electron 2013;24:1823–31. [9] Chen HL, Yang YS. Effect of crystallographic orientations on electrical properties of sputter-deposited nickel oxide thin films. Thin Solid Films 2008;516:5590–6. [10] Jang WL, Lu YM, Hwang WS, Hsiung TL, Wang HP. Effect of substrate temperature on the electrically conductive stability of sputtered NiO films. Surf Coat Technol 2008;202:5444–7. [11] Freundy HJ, Kuhlenbecky H, Staemmler V. Oxide surfaces. Rep Prog Phys 1996;59:283–347. [12] Echresh A, Abbasi MA, Shoushtari MZ, Farbod M, Nur O, Willander M. Optimization and characterization of NiO thin film and the influence of thickness on the electrical properties of n-ZnO nanorods/p-NiO heterojunction. Semicond Sci Technol 2014;29:115009–15. [13] Gandhi AC, Huang CY, Yang CC, Chan TS, Cheng C, Ma YR. Growth mechanism and magnon excitation in NiO nanowalls. Nano Res Lett 2011;6:485–99. [14] Zayed MK, Solieman A, Ebaid M. Discrepancies in the morphology and physical properties of amorphous and crystalline sprayed lanthanum nickel oxide films. Acta Mater 2013;61:5674–84. [15] Ryu HW, Choi GP, Lee WS, Park JS. Preferred orientations of NiO thin films prepared by RF magnetron sputtering. J Mater Sci 2004;39:4375–7. [16] Denayer J, Bister G, Simonis P, Colson P, Maho A, Aubry P, et al. Surfactantassisted ultrasonic spray pyrolysis of nickel oxide andlithium-doped nickel oxide thin films, toward electrochromic applications. Appl Surf Sci 2014;321:61–9. [17] Cao L, Wang D, Wang R. NiO thin films grown directly on Cu foils by pulsed laser deposition as anode materials for lithium ion batteries. Mater Lett 2014;132:357–60. [18] Ksapabutr B, Panapoy M, Choncharoen K, Wongkasemjit S, Traversa E. Investigation of nozzle shape effect on Sm0.1Ce0.9O1.95 thin film prepared by electrostatic spray deposition. Thin Solid Films 2008;516:5618–24. [19] Hwang BH, Chang CL, Hsu CS, Fu CY. Electrostatic spray deposition of NiO/CGO films. J Phys D: Appl Phys 2007;40:3448–55. [20] Fu CY, Chang CL, Hsu CS, Hwang BH. Electrostatic spray deposition of La0.8Sr0.2Co0.2Fe0.8O3 films. Mater Chem Phys 2005;91:28–35. [21] Ksapabutr B, Chalermkiti T, Wongkasemjit S, Panapoy M. Controllable deposition of gadolinium doped ceria electrolyte films by magnetic-field-assisted electrostatic spray deposition. Thin Solid Films 2013;546:423–30. [22] Ksapabutr B, Chalermkiti T, Wongkasemjit S, Panapoy M. Fabrication of scandium stabilized zirconia thin film by electrostatic spray deposition technique for solid oxide fuel cell electrolyte. Thin Solid Films 2010;518:6518–21. [23] Ksapabutr B, Poungchun G, Panapoy M. Architectural control of nanostructured ZrO2 thin films via the electrostatic spray deposition technique using zirconatrane as a precursor. Phys Scr 2010;T139(014055):1–5. [24] Larbi T, Ouni B, Boukhachem A, Boubaker K, Amlouk M. Investigation of structural, optical, electrical and dielectric properties of catalytic sprayed hausmannite thin film. Mater Res Bull 2014;60:457–66. [25] Chen SC, Wen CK, Kuo TY, Peng WC, Lin HC. Characterization and properties of NiO films produced by rf magnetron sputtering with oxygen ion source assistance. Thin Solid Films 2014;572:51–5. [26] Sharma V, Kumar P, Singh N, Upadhyay S, Satsangi VR. Dass Sahab et al. Photoelectrochemical water splitting with nanocrystalline Zn1 xRuxO thin films. Int J Hydrogen Energy 2012;37:12138–49. [27] Laukaitis G, Virbukas D. The structural and electrical properties of GDC10 thin films formed by e-beam technique. Solid State Ionics 2013;247–248:41–7.