Effect of Li doping on passivation of trap states and improvement in charge transport in TiO$_2$ thin films

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Effect of Li doping on passivation of trap states and improvement in charge transport in TiO

₂

thin films

MOHINI PANDAY, GAURAV K UPADHYAY and L P PUROHIT^∗

Semiconductor Research Lab, Department of Physics, Gurukul Kangri University, Haridwar 249 404, India

∗Corresponding author. E-mail: proflppurohitphys@gmail.com

MS received 24 December 2020; revised 5 April 2021; accepted 9 April 2021

Abstract. In the framework of this study, pure and Li-doped TiO2 (0.5, 1.5, 2.5 at.%) thin film samples were synthesised via sol–gel spin coating method. The structural, morphological and optical properties were examined by using X-ray diffraction, scanning electron microscopy (SEM) and UV–Vis spectroscopy respectively. XRD pattern reveals the polycrystalline nature of Li-doped TiO2 thin films with anatase crystal phase. The average crystallite sizes were found to be in the range of 19–23 nm. The energy-dispersive X-ray spectroscopy (EDS) demonstrates the oxidation states of Ti, O and Li in the deposited thin films. The optical band gap of TiO2thin films was varied from 3.15 to 3.26 eV on increasing Li doping. For the study of light emission properties of Li-doped TiO2thin films, the PL spectra were recorded in the wavelength range of 350–700 nm. The doping of Li on TiO2

films show improved charge transport properties which can be used in optoelectronics and energy storage devices.

Keywords. Li-doped TiO2; sol–gel; thin films; energy-dispersive X-ray spectroscopy; photoluminescence.

PACS Nos 78.20.–e; 61.10.–i; 81.15.–z; 81.20.–n; 68.37.–Hk; 73.61.–r

1. Introduction

TiO2has attracted great attention because of its promi- nent and extensive range of properties such as non- toxicity, cost effectiveness, long term stability, trans- parency and high refractive index [1–3]. To a great extent, TiO2 has been broadly investigated due to its features and numerous applications in chemical sen- sors, photocatalysis, dye-sensitised solar cells (DSSC), medicines, cosmetics, Li-based batteries [4–7] etc.

Moreover, TiO2 has three crystallographic phases: (i) anatase phase, (ii) rutile phase and (iii) brookite phase.

Among these, anatase phase of TiO2 is the most stable phase with respect to rutile and brookite phases and the formation of phases is restricted by synthesising temperature [8]. In recent years, great interest has been developed for titanium oxide (TiO₂) thin films because of its excellent transmittance in the visible region [9,10].

Inclusion of metal ions into TiO2 matrix enhance its electrical, chemical and optical properties. TiO₂doped with metal ions (Cu, CO, Ni, Li, Ag) [11] and non- metal elements (N, F, S, Cl and Br) have been examined for various applications. Bessekhouad et al have also reported alkaline (Li, Na, K)-doped TiO2nanoparticles

synthesised by sol–gel method [12,13]. Among them, lithium is found to be one of the most promising material as a dopant [14]. Li-doped TiO2thin films have found applications in various fields like nanosensors, lithium- ion batteries and passivation layer in perovskite solar cells [15–17]. Moreover, Li is responsible for ejecting electrons and transport to TiO2. Besides, lithium ions have high mobility by which it offers better conductiv- ity when doped with TiO2and provides better electron transfer with passivation of oxygen vacancies [18].

The synthesis process of nanosized Li-TiO2provides the synergistic effect to induce oxygen and trap states.

This is happened due to the development of Ti³⁺state and confirmed by the functional density theory along with the wave function approaches. Moreover, sound arrangement of Ti atoms on the surface with a preferable coordination further creates Ti³⁺ surface ions which affect the crystallographic, morphological and optical properties of Li-doped TiO2[19,20]. Beside this, ionic radii of Ti⁴⁺ion (0.605 Å) and Li⁺ion (0.76 Å) are much different, leading to the modification in the surface, crystallographic properties and shorter cation oxygen bonds which provide higher oscillation with a good crystal field resulting in clear emission [21].

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films were grown on soda-lime glass substrate and properties of these prepared thin films were investigated by using UV–Vis–IR spectroscopy, PL, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and field emission scanning electron microscopy (FE-SEM).

2. Experimental

2.1 Sample preparation

The materials and reagents used in the experiment were titanium tetra isopropoxide (C12H28O4Ti; purity 98.5%), 2-methoxyethanol (C3H8O2; purity 99.0%) and lithium hydroxide (LiOH; purity 99.5%). All the chemicals and reagents utilised in the experiment were purchased from Alfa Asear and used without further purification.

The undoped and Li-doped TiO2thin films were synthesised on soda lime glass substrates by using sol–gel spin coating technique. Chemical reagents used for the synthesis of undoped and Li-doped TiO2 were titanium tetra isopropoxide (TTIP) as the precursor of TiO2, lithium hydroxide (LiOH) as the precursor of lithium, 2-methoxyethanol (2-MEA) as the solvent and hydrochloric acid (HCl) as the stabiliser.

Primarily, the precursor solution of TiO2 (0.5 mol/l concentration) was prepared by dissolving TTIP in 2- MEA at room temperature (30^◦C). Then, appropriate amount of HCl was added drop by drop to the precursor solution and stirred for 1 h to obtain a transparent solution. The resultant solution was aged for 24 h before the film deposition. Further, the same procedure was repeated with the addition of LiOH in TiO₂ solution and Li-doped TiO2 solutions (0.5, 1.5, 2.5 at.%) were obtained. Finally, the prepared undoped and Li-doped TiO₂ solutions were deposited on soda lime glass substrate by a spin coater at 2500 rpm for 30 s. After every deposition, the coated samples were dried at 200^◦C in

Probe III) was used for the analysis of the element and bonding strength. Scanning electron microscopy (Carl Zeiss Ultra Plus) was used for the analysis of surface morphology. Optical properties were analysed using a UV–Vis–IR spectrophotometer (Shimadzu UV- 3600, Japan) and the photoluminescence (PL) data were recorded using HORIBA JOBIN YVON (Model- FLOROCUBE).

3. Results and discussion

3.1 Structural analysis

The diffraction patterns of the undoped and Li-doped TiO₂ thin films are shown in figure 1. The observed diffraction patterns of all the samples were matched with the JCPDS card #21-1272 to show polycrystalline anatase crystal phase. The highest intense peak centred at 25.28^◦ for all thin films corresponds to (101) plane and other smaller peaks were found at 37.78^◦, 48.02^◦, 53.85^◦and 55.01^◦corresponding to (004), (200), (105) and (211) planes, respectively. A slight variation was observed in the intensity and width of all peaks with Li doping in TiO2. Thus, the changes in texture and crystallite size were observed for all the samples. In addition, no other impurity peaks were available in the diffraction pattern after the Li doping in TiO2. Apart from the anatase phase, a small peak was also appeared for 2.5 at.% Li doping in TiO2 which was centred at 32.80^◦. This might be due to the Li interaction directly with TiO₂and a new phase of Li₂Ti₃O₇was developed. Fur- ther, 2.5% of Li doping in TiO2might be responsible for generating this new phase by the fact of mixing inter- stitial and substitutional sites [30]. Ravishankar et al have also reported similar XRD patterns corresponding to very low Li doping concentration in which Li atoms were replaced by Ti atoms in lattice sites [31].

The crystallite sizes corresponding to (101) plane were estimated for all the samples and analysed by the

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Figure 1. (a)XRD pattern of the undoped and Li-doped TiO2thin films for different doping concentrations and (b) Li2Ti3O7

peak at 2θ=32.80^◦.

Scherrer’s formula [31,32], D = Kλ

βcosθ, (1)

whereDis the crystallite size, Kis a constant (taken as 0.94),λis the wavelength of X-ray radiation (λ= 0.154 nm for Cu Kα X-rays) andβ is the broadening factor of the diffraction line measured at half of its maximum intensity (FWHM) for Bragg’s angle of diffraction(2θ) [33,34]. The crystallite size is inversely proportional to the FWHM and a variation in crystallite size from 19 to 23 nm was observed for all the samples with respect to the (101) plane as shown in table1.

3.2 X-ray photoelectron spectroscopy (XPS)

The studies on surface properties of pure TiO₂ and 2.5% Li-doped TiO2 thin film samples were carried out using XPS spectra. Figure 2a shows the full scan XPS spectra as a function of binding energy in the range 0–1100 eV. The spectra of both TiO2 samples show similar characteristic peaks (Ti, O and C), while the Li-doped sample shows one extra peak of Li centred at 54 eV. In addition, the high resolution XPS spectra of Ti shows three peaks, i.e. 2p, 3s and 3p centred at 457, 61 and 36 eV respectively. The main Ti 2p peak was also found in doublet state ²p₃_/₂ and

2p₁_/₂ representing the +4 oxidation state of Ti, as shown in figure2b. The peak position for Ti 2p in the 2.5% Li-doped TiO2 thin film sample shifts to a binding energy band higher than the undoped TiO2 thin film sample. The obtained results confirm that there is low electron density for the Ti atoms in the 2.5%

Li-doped TiO2 compared to the undoped sample. The

HR-XPS spectra of O 1s in pure TiO2 shows two sub- peaks at 529 and 531 eV, respectively (see figure2c).

However, a slight shoulder can be noticed near 531.6 eV for 2.5% Li-doped TiO2 sample, which may be due to the interaction of the oxygen with the lithium atoms. The doping of Li in TiO2can be responsible for reducing Ti⁴⁺ to Ti³⁺ and hence, passivation trap states were observed in TiO2 thin films, resulting in the improvement in charge transport properties of the lithium-doped TiO2[35].

3.3 Microstructural and elemental analysis

The studies on surface microstructure and chemical composition of the fabricated nanomaterials were carried out using field emission scanning electron microscopy (FE-SEM) analysis in 100 nm magnified scale and displayed in figures 3a–3d. All the samples show regular, spherical and uniform distribution of nanograins [36]. The SEM micrographs of undoped TiO2sample show very small size grains, whereas doped sample shows large size grains. The grain size of the doped samples was found to be dependent on doping concentration. In addition, grain size was investigated for undoped TiO2and Li-doped TiO2by ‘Image J’ soft- ware. The grain size was found to be 18, 20, 23 and 24 nm with±0.15 error (standard deviation) for TiO2

and 0.5, 1.5, 2.5 at.% Li-doped TiO2, respectively. The largest grain was observed for the Li-doped TiO₂ thin film sample with 1.5% concentration. Eventually, the variation in surface properties, i.e. texture and crystallite size, were found to support the XRD results. In addition to this, figures 3e–3h show the chemical composition of undoped and Li-doped TiO2samples, as obtained by

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Figure 2. X-ray photoelectron spectra (XPS) of TiO2and Li-doped TiO2(for 2.5% doping).

energy-dispersive X-ray spectroscopy. From the spectra, it is confirmed that Ti and O elements were present in the samples, indicating the proper stoichiometry of the prepared coatings (as shown in the inset of figures 3e–3h. EDX analysis did not show any other element except Ti and O, confirming that the deposited samples were not contaminated. However, the presence of Li was not detected due to the limitation of EDX spectroscopy.

3.4 Optical analysis

For optical study, transmittance spectra of all Li-doped TiO2 thin film samples were recorded in the wavelength range 300–800 nm with respect to air. It is clearly

observed that doping of Li (at.%) significantly influenced the transmittance of TiO2thin films in the visible range while a symmetric variation was not observed with Li concentrations as shown in figure4a. The pure and 1.5% Li-doped TiO2 thin film samples showed the maximum transmittance of 55% whereas 2.5% Li-doped TiO2thin film showed the lowest transmittance of nearly 40% in the visible region. The absorption band edges were found between 300 and 400 nm and the transmittance was found to sharply decrease and become zero for UV light up to 200 nm. The morphology of thin film samples influenced the transmittance of these thin films.

The SEM images showed cracks which were increased for middle range of Li at.% and these films also have

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Figure 3. (a)–(d) SEM images and (e)–(h) EDX spectra of the undoped and Li-doped TiO2thin film samples.

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Figure 4. (a) Transmittance spectra of the undoped and Li-doped TiO2thin films within the wavelength range of 300–800 nm and (b) Optical band gap of thin film samples by Tauc’s plot method.

Figure 5. Photoluminescence (PL) spectra of the undoped and Li-doped TiO2thin films within the wavelength range of 350–700 nm.

low transmittance in the visible region. These cracks decreased the transmittance by light scattering. The optical band-gap energy of the undoped and Li-doped thin film samples were determined from the conventional Tauc’s plot method using eq. (2) [37]

(αhν)² =B(hν−Eg), (2)

wherehis the Planck’s constant,νis the transition fre- quency, Bis a constant (known as band edge constant) andE_gis the optical band-gap energy with respect to a specific transition occurring in the film [38]. The optical band-gap energyEgof thin films evaluated by Tauc’s

plot fitting method as shown in figure4b was in the range of 3.15–3.26 eV and found the lowest for 0.5% Li-doped TiO2thin film and the highest for 1.5% Li-doped TiO2

thin film. Thin film of 1.5% Li doping shows 3.26 eV band gap and shows good agreement with the optical band gap of TiO₂thin film. Further, the variation in the band gap of TiO2 by increasing the doping level of Li was due to the structural modification in TiO2thin films.

The structural deformation in the TiO₂ films could be due to the replacement of either substitutional or inter- stitial TiO2 ions in the TiO2 lattice by Li ions. Similar results were obtained for Cu-doped CdO thin films [39].

PL spectra of undoped TiO2 and Li-doped TiO2 thin films were recorded at an excitation wavelength of 320 nm as shown in figure 5. The emission peaks were observed from 360 to 415 nm and 400 to 500 nm corresponding to the intrinsic composite and composite light of free excitons, respectively [40–42]. This can be attributed to the existing oxygen vacancies and defects on the surface of the films. However, a peak observed at 492 nm corresponds to the exciton recombination, which may be due to small grain and short electron free path. It confirms that the relative intensity of blue emis- sions has been tailored by the introduction of Li ions in the TiO2lattice. Moreover, the 2.5% Li-doped TiO2thin film exhibits low PL intensities compared to the undoped TiO2 thin film due to the reduction of either oxygen vacancies and/or defects [43,44]. Further, in this type of material, electrons and holes are generated by pho- toexcitation and those photoexcited electrons transit to the conduction band and then fill the energy levels corresponding to the excitation energy. The recombination of these carriers takes place and their energy can be emitted as photons. The recombination process can be obtained

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via sublevels present in energy band gap (Eg)or in defect states. However, the increase of defects and oxygen vacancies generate PL spectra with higher intensity [45].

4. Conclusion

The Li-doped TiO2 thin films were fabricated using the sol–gel method. The XRD pattern clearly reveals the polycrystalline anatase phase of Li-doped TiO2thin films. The variation in surface morphology supports the change in texture which was confirmed by FE-SEM. The optical band gap was found in the 3.15–3.26 eV range by varying the Li doping. The crystallite size of TiO2

thin film sample varies from 19 to 23 nm by varying Li doping with respect to the (101) plane. Li doping in TiO2leads to a shift to a higher binding energy band than undoped TiO₂thin films, suggesting lower electron density for the Ti atoms in the Li-doped film. Consequently, this passivation can further improve the charge transport in the Li-treated TiO₂. The Li-doped TiO₂ thin films exhibit low PL intensities compared to the undoped TiO2

thin film due to the reduction of either oxygen vacancies and/or defects. The overall results are encouraging for the application of Li-doped TiO2thin films in optoelec- tronic and energy storage devices such as lithium-ion batteries.

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