J Mater Sci: Mater Electron DOI 10.1007/s10854-017-6690-3

Sol–gel deposition and characterization of multilayer 2% Cu doped ­TiO2 nano structured thin films M. I. Khan1 · K. A. Bhatti2 · Rabia Qindeel3 · Fazal‑e‑Aleem1 · Naeem‑ur‑Rehman1 · Norah Alonizan3 

Received: 6 October 2016 / Accepted: 1 March 2017 © Springer Science+Business Media New York 2017

Abstract  Multilayer of 2% Cu doped ­TiO2 thin films have been grown by sol–gel spin coating on glass substrate in the form of three, five and seven layers. X-ray diffractrometer (XRD) confirms the doping of Cu into ­TiO2. Scanning electron microscopy (SEM) confirms the formation of nano particles with average size of 19, 25 and 35  nm for three, five and seven layers of thin films, respectively. The electrical resistivity of these multilayer films is found as 2.19 × 107, 1.20 × 107 and 1.11 × 107 ohm-m respectively. UV–vis shows that the films have 80% transmittance in the visible region which is good for solar spectrum. The optical band gap energy decreases with an increase in the number of layers as 3.778, 3.768 and 3.736  eV respectively for 3, 5 and 7 layered thin films. This work provides an environment friendly and low cost use of an abundant material for optoelectronic devices.

1 Introduction Multilayer thin films are now widely used in the design of optical reflectors, computer disks, anti-reflecting coating and solar cells [1]. Amongst various applications in solar cells, gas sensing, photonic devices etc, T ­ iO2 is widely * M. I. Khan [email protected] 1

Department of Physics, The University of Lahore, Lahore 54000, Pakistan

2

Laser and Optronics Centre, Department of Physics, University of Engineering and Technology, Lahore 54800, Pakistan

3

Department of Physics and Astronomy, College of Science, King Saud University, Riyadh 11451, Saudi Arabia





used in various forms [2–5]. In view of the importance of this compound, we undertake a novel study of multilayer of 2% Cu-doped ­TiO2 nano structured thin films deposited by the sol–gel spin coating method. Structural, morphological, electrical and optical properties are undertaken using XRD, SEM, four point probe and UV–vis techniques. Common phases of ­TiO2 are anatase, brookite and ruttile. Ruttile is most stable while the remaining two are metastable [6]. Stability of these phases has been discussed in several reports [6–9]. It is observed that phase transition is mostly dependent on particle size besides, pH, surface energy, and solution chemistry [10, 11]. It has been reported [12] that “Mostly, anatase stable phase is obtained for crystal sizes below 11  nm, brookite is most stable for crystal sizes between 11 and 35 nm, and ruttile is most stable for crystal sizes exceeding 35 nm”. Reported results show that brookite has high photocatalytic activity compared to the other two phases [13]. Brookite is however difficult to prepare and therefore very little information about optical and electrical properties of this phase is available. In our work, we have prepared brookite phase of stack layered Cu doped T ­ iO2 in a simple manner. It has been found that the optical properties of T ­ iO2 can be enhanced by reducing the band gap through doping of metallic and non-metallic ions, like Zinc [14, 15], Yttrium [16], Niobium [17, 18], Copper [18], Nitrogen [19] etc. Copper usually exists as ­Cu2+ ion in oxide and Ti exists as ­Ti4+ ion in ­TiO2. It is found [20] that doping of Cu into ­TiO2 could donate oxygen vacancies in the lattice, and reduces the band gap of ­TiO2 which enhances the optical and electric properties of ­TiO2. To the best of our knowledge, there is no study on stacked multilayer of Cu doped T ­ iO2 thin films. We have therefore undertaken a fabrication of brookite phase of 2% Cu-doped ­TiO2 nano structured multilayered thin films by

13

Vol.:(0123456789)



2 Methodology

3.1 XRD results XRD pattern of 2% Cu doped T ­ iO2 three, five and seven layered thin films deposited by sol–gel spin coating technique is shown in Fig. 1. Diffraction peaks in XRD pattern confirm the doping of Cu atoms in ­TiO2 multilayer thin films. In three layers of 2% Cu doped T ­ iO2 thin film, three peaks corresponding to (111), (212) and (022) planes of T ­ iO2 brookite phase are observed (PDF #65-2448). In case of five layer Cu doped ­TiO2 thin film, the formation of brookite phase is confirmed by matching five peaks corresponding to (111), (220), (212), (022) and (123) planes which match with data card (PDF #65-2448). For the third sample with seven Cu doped ­TiO2 layers, pattern similar to five layers is observed that confirms the formation of same phase. These XRD patterns confirmed the formation of brookite phase of T ­ iO2 with no detectable trace of crystalline ruttile or anatase phases. Similarly, no evidence of Cu segregation, clustering or formation of oxide phase has been observed which demonstrates the complete dissolution of ­Cu2+ cations to form Cu–O–Ti bonds. This fact is in accordance with the literature [25]. Also, all the diffraction peaks of doped Cu were shifted to the left, confirming that Cu was incorporated into the T ­ iO2 lattice.

2% Cu doped TiO2

240 220 180 160 140 120 80 60 40

(022)

(212)

100

5 Layers

(212)

(022)

(220)

200

7 Layers

(123)

260

(123)

280

(212) (022)

300

(220)

(111)

(111)

13

3 Results and discussion

(111)

Thin films can be deposited by using different techniques, including chemical vapor deposition [21], thermal evaporation [22], sol–gel process [6] and spray pyrolysis [23]. Among these, sol–gel method for depositing thin film has many unmatched advantages as compared to other methods, like simple equipment (cost effective), deposition of thin films at room temperature and efficient control in microstructure and composition of thin film [24]. We have therefore used sol–gel spin coating method for preparing Cu doped ­TiO2 thin films. In our study, 0.4 g of ­TiO2 nano-powder was dissolved in 5  ml of ethanol while 5  ml of Diethylene glycol was added as a stabilizing agent and 0.02  g of Cu was added in this solution. The mixture was subjected to stirring for a period of 3 days at room temperature to get the solution of Cu doped ­TiO2. This solution was then aged for 24  h that resulted in a clear homogeneous solution of Cu doped ­TiO2. The solution was then used for fabrication of multilayer thin films. Substrates were cleaned prior to deposition by acetone and ethanol followed by rinsing with distilled water. Initially, one layer of Cu doped T ­ iO2 was coated by adding drops of solution to the glass substrate spinning on the coater followed by subsequent heating at 120 °C for 1 min. Rotation speed of the spin coater was kept at 2000 rpm for 20 s. Instead of annealing at high temperature (500–600 °C) for the preparation of Cu doped ­TiO2 films as employed by some researchers [20], we used an easier method of stirring for a long duration. This resulted in Cu cations being seated in the ­Ti4+ position. In our method, the excess time stirring resulted in the incorporation of ­Cu2+ ions at the T ­ i4+ positions. This fact is endorsed by the EDX analysis which indicates the presence of Cu in ­TiO2 films (discussed in the later part). XRD results (described in the later part) also showed the diffraction peaks related to the brookite phase of T ­ iO2. These results did not indicate the presence of any extra phase related to Cu. The following layers were also deposited in the same way to get 3, 5 and 7 layered thin films. The structure was analyzed using XRD (PANalytical X’Pert PRO) with CuKα radiations. The scanning range was 10–70 degrees. Morphology of the films was studied by using scanning electron microscopy (SEM) (Quanta 250 fei). Four-point probe technique (KIETHLEY Instrument) having 2 nA to 105 mA, 6220 DC and 1–120 nV, was used to study the electrical properties. HITACHI

U-2800 UV–vis spectrometer was used to study the optical properties of films.

Intensity (a.u)

‘sol–gel spin coating method’ in a user friendly and economical method. Structural, morphological, electrical and optical properties are measured by XRD, SEM, four-point probe and UV–vis respectively.

J Mater Sci: Mater Electron

3 Layers

20 10

20

30

40

50

60

70

Angle 2θ (degree)

Fig. 1  XRD pattern of 2% Cu doped T ­ iO2 three layers, five layers and seven layers thin films

J Mater Sci: Mater Electron

3.2 SEM results Morphological properties are important structural characteristics like crystal structure and crystallite size. SEM micrographs of 2% Cu doped ­TiO2 three, five and seven layer thin films show that all the three films consist of nano particles (Fig. 2a–c). The average size of nano particles show an increasing trend with rising number of layers and found to be 19, 25 and 35 nm for three, five and seven layers, respectively. The size of particles, growth orientation, their shapes and distribution play a significant role in the performance and reliability of such type of polycrystalline films. The distribution of particle size is highly sensitive to the deposition temperature and impurities/defects that usually segregate at the grain boundaries [26]. Correlation between the sample morphology and consequent optical and electrical properties is somewhat difficult. Researchers usually correlate them to the grain size, grain boundaries and their orientation [27]. In our work we can interpret that nanostructure multilayer thin films are attributed to the change

in particle shape that in turn improves the crystallinity related properties of the material [26]. When an additional Cu doped ­TiO2 layer was deposited on the previously deposited layer, the underlying nanoparticles provided the base for the growth of upper layer. The little difference in the XRD patterns confirms the nonexistence of the interfacial layers that usually are formed at the interface of two deposited layers. This absence of the interface may basically provide a chance for the particles to grow further and get larger particle sizes when the number of layers was increased. EDX of 2% Cu doped ­TiO2 three, five and seven layered thin films are shown in Fig.  3a–c. The EDX analysis confirms the presence of Cu, Ti and oxygen atoms in the thin films. The extra peaks present in the EDX spectra appear due to the elements present in the underlying glass substrates. The gold (Au) peak appears when EDX has been done then for making film conducting, a layer of Au is deposited on the film. The weight and atomic percent of Ti, Cu and O is listed in Table 1. It can be seen that the percentage of oxygen is increased with an increase in the

Fig. 2  SEM micrographs of 2% Cu doped ­TiO2 a three layers b five layers and c seven layers thin films

13



J Mater Sci: Mater Electron

Fig. 3  EDX micrographs of 2% Cu doped ­TiO2 a three layers b five layers and c seven layers thin films

3.2.1 Four‑point probe results Table 1  Weight percentage of materials present in 2% Cu doped multilayer thin films Elements

Wt% of three layers

Wt% of five layers

Wt% of seven layers

O Ti Cu

41.23 38.93 1.86

39.72 2.85 0.75

37.85 3.63 1.64

number of layers. This is due to the reason that between two layers of thin films, a thin layer of oxygen is deposited. Consequently, as the layers increase, titanium quantity is decreased. This is observed in EDX studies given in Table 1.

13

Electrical properties are studied using four-point probe results. The graph of average resistivity against number of Cu doped ­TiO2 layers is plotted in Fig. 3. Following relationship was used to find the resistivity of semiconductors thin films [6]

𝜋 V . (1) ln(2) I where ρ is average resistivity, V the voltage and I is the current. Calculated average resistivity of three, five and seven layer thin films is 2.19 × 107, 1.20 × 107 and 1.11 × 107 ohm-m, respectively. Figure  4 shows a decrease in average resistivity with the increase in number of Cu doped ­TiO2 layers. That in turn shows an increase in conductivity and improved electrical behavior of Cu doped T ­ iO2 films

𝜌=

J Mater Sci: Mater Electron Three layers of 2.0% Cu doped TiO2

22000000

Five layers of 2.0% Cu doped TiO2

Seven layers of 2.0% Cu doped TiO2

20000000

18000000

Abs, (a. u.).

Average Resistivity (ohm-m)

2.0

16000000

1.5

1.0

14000000

0.5

12000000

10000000

3

4

5

6

7

0.0 300

Number of 2% Cu doped TiO2 Layers

3.3 Optical properties The optical properties of Cu doped T ­ iO2 thin films were determined using UV–vis spectrometer in the wavelength range of 300–800 nm. Figure 4, shows the optical absorption spectra of three, five and seven layered thin films. Absorption edge of the ­TiO2 films can be seen in the region between 300 and 380 nm of wavelength. The absorption of photons decreases slightly with an increase in the number of layers that indicate the highest transparency of 7 layers of Cu doped T ­ iO2 film as compared to the 3 and 5 layers of films. The band gap absorption edges of all films appear around 360  nm approximately. The absorption edges show the transition from absorption to transmittance region. These films show more than 80% transmittance in the visible region. The optical band gap energy of three, five and seven layers of Cu doped T ­ iO2 thin films is calculated by plotting a graph between (𝛼h𝜐)2 and photon energy, where 𝛼 is absorption coefficient [32] as shown in Fig. 5. Tauc’s relation was used to find out optical band gap energies [33] (Fig. 6).

450

500

550

600

650

700

750

800

Fig. 5  2% Cu doped T ­ iO2 thin films having three, five and seven layers

some inherent defects and oxygen vacancies in ­TiO2 thin film. This decrease of band gap is also due to the exchange interaction of sp and d orbitals of “band electrons of ­TiO2 and localized d-electrons of ­Cu+2 ions”, respectively [34]. “The ­Cu2+ introduces new energy levels in band gap of host that are located below the conduction band. Consequently, the band gap energy of doped samples is decreased i.e., the optical response of doped materials is shifted toward visible region” [35]. Sahu et al. [25] have nicely deliberated on this point as to how the optical band gap is reduced with an increase of multilayer. They deliberate that “copper doped into the ­TiO2 lattice resulted in the rearrangement of neighbor atoms to compensate for charge deficiencies, thereby shifting the band gap”. 3.00E+015

Three layers of 2.0% Cu doped TiO2, Eg=3.778 eV. Five layers of 2.0% Cu doped TiO2, Eg=3.768 eV.

Seven layers of 2.0% Cu doped TiO2, Eg=3.736 eV. 2.50E+015

2.00E+015

2

[28]. Furthermore, decrease in resistivity is attributed to the increase in grain particle size. The mobility of electrons is therefore improved with the reduction in resistivity [29–31].

400

Wavelength, l (nm).

( h ) , (a.u.).

Fig. 4  2% Cu doped T ­ iO2 thin films having three, five and seven layers

350

1.50E+015

1.00E+015

5.00E+014

(𝛼h𝜐) = A(h𝜐 − Eg )n

(2) where, “A” is constant and exponent “n” is 1/2 and 2 for the direct and indirect band-gap semiconductors [34] respectively. The calculated band gap energies of three, five and seven layers are 3.778, 3.768 and 3.736  eV respectively. The decrease in E ­ g values is credited to the occurrence of

0.00E+000 3.55

3.60

3.65

3.70

3.75

3.80

3.85

3.90

Photon energy, h , (eV).

Fig. 6  Band gap of 20% Cu doped ­TiO2 three, five and seven layers, thin films

13



After having discussed electronic and optical properties, we will now deliberate on optoelectronic applications of these films. In the process of forming Cu doped T ­ iO2 multilayer, we observe that optical band gap and electrical resistivity are reduced. Our work indicates that these multilayered films have more than 80% transmittance in the visible region of spectrum. This makes it a good candidate for various optoelectronic applications including solar cells and optical filters etc. In solar cells, these films can be used as a transparent conducting oxide layer due to its low resistivity and high transmittance in the visible region of spectrum. At the same time, these thin films have very high absorption in the UV region of spectrum. Consequently, optical filter of this material can be made which protect the optoelectronic devices from the UV radiations. The devices being cost effective and user friendly makes it useful for optoelectronic applications.

4 Conclusion In this work, multilayer nano-structured of Cu doped T ­ iO2 thin films have been deposited by sol–gel spin coating. The Cu doping was confirmed by the XRD. The nano-particle size improved with the increase in number of layers which is due to the absence of the interfacial layers among the deposited layers. The larger grain size improves the electrical and optical properties of ­TiO2 films. The decreasing trend in E ­ g is credited to the occurrence of some inherent defects and oxygen vacancies in ­TiO2 thin film. Acknowledgements  This project is partially supported by “King Saud University, Deanship of Scientific Research, College of Science Research Center”, Saudi Arabia where a part of the experiment was carried out. One of us (FA) is greatful to Pakistan Science Foundation for grant under [PSF/NSFC/ENG-P-UOL (02)].

References 1. M. Kitui et al., Optical properties of T ­ iO2 based multilayer thin films: application to optical filters. Int. J. Thin Films Sci. Technol. 4(1), 17–21 (2015) 2. T. Georgakopoulos et  al., On the transient photoconductivity behavior of sol–gel ­ TiO2/ZnO composite thin films. J. NonCryst. Solids 410, 135–141 (2015) 3. J. Wang et  al., Effect of calcinations of T ­ iO2/ZnO composite powder at high temperature on photodegradation of methyl orange. Compos. Part B 45(1), 758–767 (2013) 4. O. Carp, C.L. Huisman, A. Reller, Photoinduced reactivity of titanium dioxide. Prog. Solid State Chem. 32(1–2), 33–177 (2004) 5. S. Chai et al., Novel sieve-like S ­ nO2/TiO2 nanotubes with integrated photoelectrocatalysis: fabrication and application for efficient toxicity elimination of nitrophenol wastewater. J. Phys. Chem. C 115(37), 18261–18269 (2011)

13

J Mater Sci: Mater Electron 6. M. Khan et  al., Investigations of the structural, morphological and electrical properties of multilayer ZnO/TiO2 thin films, deposited by sol–gel technique. Results Phys. 6, 156–160 (2016) 7. M.-H. Tsai, S.-Y. Chen, P. Shen, Laser ablation condensation of ­TiO2 particles: effects of laser energy, oxygen flow rate and phase transformation. J. Aerosol Sci. 36(1), 13–25 (2005) 8. J. Muscat, V. Swamy, N.M. Harrison, First-principles calculations of the phase stability of T ­ iO2. Phys. Rev. B 65(22), 224112 (2002) 9. T. Arlt et al., High-pressure polymorphs of anatase ­TiO2. Phys. Rev. B 61(21), 14414 (2000) 10. M.P. Finnegan, H. Zhang, J.F. Banfield, Anatase coarsening kinetics under hydrothermal conditions as a function of pH and temperature. Chem. Mater. 20(10), 3443–3449 (2008) 11. T. Kandiel, R. Dillert, D. Bahnemann, Titanium dioxide nanoparticles and nanostructures. Curr. Inorg. Chem. 2, 94–114 (2012) 12. H. Zhang, J.F. Banfield, Understanding polymorphic phase transformation behavior during growth of nanocrystalline aggregates: insights from ­TiO2. J. Phys. Chem. B 104(15), 3481–3487 (2000) 13. T.A. Kandiel et al., Brookite versus anatase T ­ iO2 photocatalysts: phase transformations and photocatalytic activities. Photochem. Photobiol. Sci. 12(4), 602–609 (2013) 14. S.K.M. Saad et al., Porous Zn-doped ­TiO2 nanowall photoanode: effect of ­Zn2+ concentration on the dye-sensitized solar cell performance. Appl. Surf. Sci. 353, 835–842 (2015) 15. K.-P. Wang, H. Teng, Zinc-doping in ­TiO2 films to enhance electron transport in dye-sensitized solar cells under low-intensity illumination. Phys. Chem. Chem. Phys. 11(41), 9489–9496 (2009) 16. B. Zhao et al., The influence of yttrium dopant on the properties of anatase nanoparticles and the performance of dye-sensitized solar cells. Phys. Chem. Chem. Phys. 17(22), 14836–14842 (2015) 17. S. Lee et al., Nb-doped ­TiO2: a new compact layer material for ­TiO2 dye-sensitized solar cells. J. Phys. Chem. C 113(16), 6878– 6882 (2009) 18. L. Zhou et  al., Improved performance of dye sensitized solar cells using Cu-doped ­TiO2 as photoanode materials: band edge movement study by spectroelectrochemistry. Chem. Phys. 475, 1–8 (2016) 19. H. Tian et  al., Retarded charge recombination in dye-sensitized nitrogen-doped ­TiO2 solar cells. J. Phys. Chem. C 114(3), 1627– 1632 (2010) 20. S. Wang et  al., Superhydrophilic Cu-doped ­TiO2 thin film for solar-driven photocatalysis. Ceram. Int. 40(4), 5107–5110 (2014) 21. K. Haga et  al., ZnO thin films prepared by remote plasma enhanced CVD method. J. Cryst. Growth 214, 77–80 (2000) 22. Y. Nakanishi et al., Preparation of ZnO thin films for high-resolution field emission display by electron beam evaporation. Appl. Surf. Sci. 142(1), 233–236 (1999) 23. O. Vigil et al., Influence of post-thermal annealing on the properties of sprayed cadmium–zinc oxide thin films. Appl. Surf. Sci. 161(1), 27–34 (2000) 24. L. Li et al., The effects of Cu-doped T ­ iO2 thin films on hyperplasia, inflammation and bacteria infection. Appl. Sci. 5(4), 1016– 1032 (2015) 25. T.-T. Pham et  al., Cu-doped T ­ iO2/reduced graphene oxide thinfilm photocatalysts: effect of Cu content upon methylene blue removal in water. Ceram. Int. 41(9), 11184–11193 (2015) 26. R. Hussin, C. Kwang-Leong, H.H. Xiang, Fabrication of multilayer ZnO/TiO2/ZnO thin films with enhancement of optical properties by atomic layer deposition (ALD). Appl. Mech. Mater. 465(2), 916-921 (2014)

J Mater Sci: Mater Electron 27. J. Kar, G. Bose, S. Tuli, Correlation of electrical and morphological properties of sputtered aluminum nitride films with deposition temperature. Curr. Appl. Phys. 6(5), 873–876 (2006) 28. I. Saurdi, M.H. Mamat, M. Rusop, Electrical and structural properties of ZnO/TiO2 nanocomposite thin films by RF magnetron co-sputtering. in Advanced Materials Research (Trans Tech Publications, Zurich, 2013) 29. S. Xue et  al., Effects of annealing on optical properties of Znimplanted ZnO thin films. J. Alloys Compd. 458(1), 569–573 (2008) 30. J. Chang, M.-H. Hon, The effect of deposition temperature on the properties of Al-doped zinc oxide thin films. Thin Solid Films 386(1), 79–86 (2001) 31. S. Jeong et al., Deposition of aluminum-doped zinc oxide films by RF magnetron sputtering and study of their structural, electrical and optical properties. Thin Solid Films 435(1), 78–82 (2003)

32. C. Firdaus et  al., Optical and electrical properties of ZnO and ZnO: ­TiO2 thin films prepared by sol-gel spray-spin coating technique. in Semiconductor Electronics (ICSE), 2012 10th IEEE International Conference on. 2012. IEEE 33. M.F. Khan et  al., 700  keV ­Ni+2 ions induced modification in structural, surface, magneto-optic and optical properties of ZnO thin films. Nucl. Instrum. Methods Phys. Res. Sect. B 368, 45–49 (2016) 34. L. Wang et al., Preparation, characterization, and photocatalytic activity of T ­ iO2/ZnO nanocomposites. J. Nanomater. 2013, 15 (2013) 35. V. Krishnakumar et al., Effect of Cu doping on T ­ iO2 nanoparticles and its photocatalytic activity under visible light. J. Mater. Sci. 27, 7438–7447 (2016)

13