Effect of Substrate Temperature on the Electrochemical and Supercapacitance Properties of Pulsed Laser-Deposited Titanium Oxynitride Thin Films Free

Due to the depletion of fossil resources and the rise in environmental degradation, producing clean energy has become a top concern for the world. Energy has a significant impact on our socioeconomic development and the quality of our lives [1]. Researchers have been motivated by this issue to create effective, sustainable, and clean energy solutions. The two most popular types of energy storage devices are batteries and capacitors. Both batteries and capacitors perform the same function of storing energy; the difference between the two devices comes from the way they perform this task. Battery stores and distributes energy slowly, while capacitors store and distribute energy instantaneously. During the discharge process in batteries, the chemical energy held is converted to electrical energy and discharged. Capacitors, on the other hand, use electrostatic attraction to store electrical energy, making them suitable for devices that require a high power density [2].

Supercapacitors have drawn a lot of attention from electrochemical energy storage devices as one of the alternative energy sources because of their quick charge–discharge speed, high specific capacitance, extended lifespan, safety, and dependability [3–6]. Based on the charge storage process, supercapacitors are divided into electrical double-layer capacitors (EDLCs) and pseudocapacitors (PCs) [7]. EDLCs store energy through the accumulation of electrostatic charges at the electrolyte/electrode interface due to a non-Faradaic process [8]. However, the energy storage mechanism in PCs incorporates quick redox events brought about by Faradaic reactions [9]. Carbonaceous materials such as activated carbons, graphene, and carbon nanotubes have been exploited as electrode materials for EDLCs due to their high surface area and porosity [4,10,11]. As opposed to that, electrochemical active materials such as nitrides [12–14], sulfides [15,16], transition metal oxides (TMO) [17–19], and oxynitrides [20,21] are employed as electrode materials for pseudocapacitor due to their energy densities and high capacitance than carbon materials. However, poor electrical conductivity and structure degradation of TMO as a consequence of Faradaic redox reactions eventually lead to low cyclic stability. A means to improve supercapacitor technology is made possible by novel electrode materials with distinctive physicochemical features. As an alternative to TMO, metal nitrides such as titanium nitride (TiN) are used as electrode materials due to their chemical stability and high thermal and electrical conductivity [22–24]. Despite these advantages, metal nitrides exhibit poor cycling rate capabilities, low charge storage, and limited long-term durability [25].

Metal-based oxynitrides have recently emerged as promising intermediate compounds between insulating metal oxides and conducting metallic nitrides. These oxynitrides possess a varied range of attractive properties: high electronic conductivity, corrosion resistance, and biocompatibility, which enable them to circumvent the difficulties related to metal oxides and nitrides [26,27]. Wang et al. synthesized titanium oxynitride nano grid film, which exhibits excellent capacitance retention and cyclability [28]. Yu et al. have also reported outstanding cycling stability (∼10⁵ cycles) exhibited by tungsten oxynitride fibers [29].

In this work, we have explored the electrochemical and supercapacitor properties of titanium-based oxynitride (TiNO) thin films synthesized using a pulsed laser deposition (PLD) method. The general formula of the TiNO homolog series is expressed as $Ti(1−x3)3+Ti(vac)x33+N(1−x)3−Ox2−$⁠. The series exists in the whole range of $0≤x≤1$ [30,31]. In this formula, Ti³⁺ (vac) is the number of Ti³⁺ vacancies in each formula unit, which is created to maintain the charge neutrality in the resulting compounds. This molecular formula takes into account the substitution of trivalent N anions by bivalent O anions in an ionic TiN lattice and the maintenance of charge neutrality in the lattice. The formula is also built on the assumption that the valences of Ti, N, and O are maintained at +3, −3, and −2, respectively [31]. The terminal compounds (TiN with x = 0 and TiO with x = 1), as well as all the intermediate compounds with x between 0 and 1, all have rock salt structures, but they possess wide-ranging physicochemical properties [32,33]. One of the terminal compounds of the family, TiN (x = 0), has been known for its well-established properties for decades [19,34–38]. The TiNO material system has several favorable properties and lends itself to a variety of applications, making it a suitable electrode material candidate [39–42].

To investigate the electrochemical performance of the TiNO film for supercapacitor applications, we have carried out three and two electrode measurements. A symmetric cell is fabricated to evaluate the cyclic stability, energy, and power density for its real-time application.

TiNO thin films were grown on 10 mm × 15 mm × 1 mm titanium plates using a PLD method. The schematic of the PLD method is shown in Fig. 1(a). In our PLD experiments, a high-purity (99.99%) TiN target was used to deposit the TiNO films by controlling the substrate temperature, oxygen pressure, laser energy density, and laser repetition rate. A KrF laser (Coherent Complex Pro, wavelength 248 nm, pulse duration 30 ns, repetition rate 10 Hz) was used. A fixed number of laser pulses (shots) of 10,000 (deposition time = 1000 s) was used. We set the oxygen pressure at 50 mTorr, varied the substrate temperature from 300 °C to 500 °C, and the laser energy density at 2 J/cm². The surface morphology of the TiNO electrode before and after the electrocatalysis measurements was studied using a Hitachi SU8000 scanning electron microscopy (SEM). The electrical resistivity was determined using an Ossila T2001A standard four-probe measurement. The unit lattice models were simulated using the visualization for electronic and structural analysis. The film-substrate orientation was analyzed using an X-ray diffractometer (Bruker D8) with a CuKα source (λ_ave = 0.154 nm). XPS measurements were carried out using a Thermo Escalab Xi+ (XPS). Raman spectroscopy measurements were carried out using a WiTec alpha300R Confocal Raman microscope, specifically at the laser excitation wavelength of 532 nm.

The electrochemical activities of the TiNO films and supercapacitor cells were performed using three-electrode and two-electrode techniques, respectively. The electrolytic solution used for these experiments was an aqueous 1 M KOH solution. For the three-electrode measurement, an Hg/HgO electrode is used as a reference electrode, platinum wire is used as the counter electrode, and TiNO is used as the working electrode. The supercapacitor device was fabricated using two-working electrodes separated by an ion-transporting layer (chromatography paper) (Fig. 1(b)). Before testing, the symmetric cell was soaked in the 1 M KOH electrolyte for 1 h. A symmetric supercapacitor refers to similar supercapacitive electrodes. In the present study, Ti plates coated with TiNO films are used as electrodes on the two sides of the ion-transporting layer [43]. The charge storage capacity of the electrode was studied using cyclic voltammetry (CV) and galvanostatic charge–discharge methods. Electrochemical impedance spectroscopy (EIS) measurement was carried out by applying an AC voltage in a frequency range from 100 kHz to 100 µHz at open circuit potential. Electrochemical measurements were performed on VSP-3e Biologic EC-lab potentiostat. In the two-electrode cell, one electrode acts as a counter electrode, and the other electrode serves as the working electrode. The active electrode surface for both positive and negative electrodes was 0.75 cm². A constant charging–discharging current was applied to record the charge/discharge curve of supercapacitors, and then the change of cell voltage with time was recorded. All the electrochemical data were analyzed after iR correction, using the solution resistance obtained from the EIS measurement at 0 V versus Hg/HgO.

Figure 2 shows the X-ray diffraction patterns of the TiNO thin films grown on a polycrystalline titanium (Ti) plate substrate at three different deposition temperatures. The thin films exhibited the characteristic peaks of the face-centered cubic TiNO phase with the appearance of the (111) plane at 36.7 deg on all three samples. All other peaks, indexed s, belong to the polycrystalline Ti substrate. A careful examination of the X-ray diffraction (XRD) patterns shows a slight shift in the (111) peak position with respect to the (111) plane in pure TiN (111), which is presented as a line pattern using the JCPDS data [44,45]. This shift in the peak position is attributed to the partial oxidation of TiN to TiNO. The lattice constant (a) of the TiNO films was calculated using the XRD d (111) values according to the formula 1/d²_hkl = (h² + k² + l²)/a², where (hkl) is the Miller indices of the plane [46]. The lattice constant values of TiNO films deposited at 300 °C, 400 °C, and 500 °C were found to be 4.21 Å, 4.23 Å, 4.24 Å with a ratio of a₃₀₀: a₄₀₀: a₅₀₀: 1.000, 1.004, 1.007. The increase in the TiNO lattice constant with an increase in the deposition temperature is explained on the basis of reduced oxygen content in the films since the ionic radius of O²⁻ (1.42 Å) is smaller than that of N³⁻ (1.71 Å), the TiNO films with more oxygen will have a smaller lattice constant [47]. The higher oxygen content in TiNO films deposited at lower temperatures (i.e., more oxidation of TiN) is attributed to a reduced out-diffusion process from films at lower deposition temperatures [48]. The gettering effect of Ti due to its high oxygen-affinity also leads to the leaching away of oxygen from the TiNO films and the formation of a TiO₂ layer on the Ti plate at higher temperatures [49,50]. The formation of TiNO takes place via partial oxidation of TiN in the presence of oxygen as the following reaction: $TiN→1/2(O2)TiNO→1/2(O2)TiO2+12N2$⁠. The present method of oxidizing TiN to TiNO system via controlled oxidation during the PLD process has an energy advantage over the conventional method of nitridation of TiO₂ to TiNO. The energy advantage comes from the higher activation energy of the nitridation process (672 KJ/mol) than that of the oxidation process (496 kJ/mol) [51]. The favorable energetics of an oxidation process coupled with the existence of isostructural rock salt phase between TiN (no oxygen substitution) and TiO (N fully substituting O) can result in the realization of a much wider doping range of O into TiN than N into TiO₂ system. However, due to the absence of the isostructural phase between TiN and TiO₂, the substitutional nitrogen doping beyond 5.5 atm% has not been realized when TiO₂ is subjected to nitration. The possibility of oxygen substituting the nitrogen instead of occupying an interstitial site in the TiN lattice is ruled out due to positive incorporation energy for the intestinal site (+1.59 eV) and a negative incorporation energy (−3.54 e V) at the N substitutional site [52].

The (111) textured growth of TiNO films on a polycrystalline substrate is understood using the concept of domain epitaxy [53] and lattice arrangement along (111) plane of TiNO and (200) planes. Figure 3 schematically highlights the lattice arrangement at the film-substrate interface. The lattice constants of two-dimensional TiN (111) and Ti (200) planes are 5.19 Å and 4.11 Å, respectively, which results in a 20.8% compressive misfit strain in the TiN lattice. However, this mismatch reduces to 1.06% via the matching of four unit cells of TiN and five unit cells of Ti based on the paradigm of domain matching concept for epitaxial thin film [53,54].

The XPS analysis was performed after 60 cycles of sputtering to remove absorbed carbon, oxygen, and water vapor at the surface using a 500 eV ion gun. The general survey scan is shown in Fig. 4(a). The core levels of titanium (2s, 2p, 3s, 3p) and oxygen (1s) are shown in Figs. 4(b) and 4(c), respectively. As shown in Fig. 4(b), Ti 2p core-level peaks were observed between 454.2 eV and 467.2 eV as a result of spin-orbital coupling, the Ti 2p doublet of Ti 2p_1/2 and Ti 2p_3/2 are also observed. Previous publications [22,31,47] have shown that the instability of TiN in oxygen ambient (and even vacuum conditions that contain residual oxygen) leads to the formation of TiNO and TiO₂. This was further confirmed by the presence of three peaks in Fig. 4(b) (A₁, A_2, and A₃₎, which correspond to the Ti 2p_3/2 peak. For example, in the sample deposited at 500 °C, the peak A₁, observed at 454.88 eV, is connected with the formation of titanium-nitrogen bonding in TiN [47]; peak A₂ at 456.83 eV is related to the titanium-oxygen-nitrogen bonding. The peak A₃ at 458.70 eV is associated with the titanium-oxygen bonding. Thus, the Ti 2p_3/2 peak was a deconvolution of these species. Similarly, the Ti 2p_1/2 peaks for the same sample (500 °C), observed at 464.47 eV, 462.63 eV, and 460.89 eV, are characteristic features of TiO₂ (anatase), TiNO, and TiN, respectively [55]. The molecular fractions of TiO₂, TiNO, and TiN, calculated using the area of these peaks, are plotted in Fig. 4(d) as a function of film deposition temperature. As shown in this figure, the molecular fraction of TiNO is observed to decrease by ∼26.45% while the molecular fractions of TiO₂ and the TiN increase by ∼15.47% and ∼30.1% with a standard deviation less than ±3% determined on two different samples. The difference in binding energies between Ti 2p_3/2 and Ti 2p_1/2 of TiN, TiNO, and TiO₂ remained fairly similar, with an average of 5.93, 5.67, and 5.74 eV for samples synthesized under the three different temperatures. The chemical shifts of the deconvolved peaks (to higher binding energies) are evident with the change in deposition temperatures used for making the TiNO films. For example, the TiN 2p_3/2, TiNO 2p_3/2, and TiO₂ 2p_3/2 peaks demonstrated approximately 0.05%, 0.10%, and 0.07% increase in their binding energy, respectively, signaling higher oxidation state of the TiNO samples deposited at lower temperatures. To avoid errors in XPS data and their analysis, the following care was taken: XPS spectra were recorded with low noise and high peak-to-background ratio, selecting proper model function, reducing the number of free parameters by means of existing information on well-known doublet splitting and intensity ratios, running the fit procedure with Shirley background subtraction and checking the values of the residual standard deviation fittings [56]. For example, the residual standard deviation for our XPS fittings is ∼1.5 units (CPS), which is quite good.

As shown in Fig. 4(c), the core-level O 1s spectrum has been decomposed into three peaks. For example, peak 1 (529.32 eV), peak 2 (529.90 eV), and peak 3 (531.20 eV) in the 500 °C-deposited sample, corresponding to titanium-oxygen-nitrogen bonding in TiNO, titanium-oxygen bonding in TiO₂, and adsorbed oxygen, respectively. The O 1s core-level peaks also undergo a minor shift in peak position as a result of the change in deposition temperature. The evidence of more oxygen content in the TiNO deposited at 300 °C with respect to 400 °C and 500 °C-deposited TiNO films was obtained using X-ray photoelectron spectroscopy (XPS). The XPS fractional oxygen content in TiNO films deposited at 300 °C, 400 °C, and 500 °C were 0.72, 0.71, and 0.69, respectively (O₃₀₀: O₄₀₀: O₅₀₀: 1.00, 0.98, 0.96). The fractional XPS oxygen content ratios in the three films are found to be inversely related to lattice constant ratios, which confirms the correlation between oxygen content in the TiNO films and their lattice parameters.

To further confirm the XPS results about the structural conversion of TiN to TiNO and TiO₂, Raman spectroscopy analysis was carried out using a laser excitation of 532 nm wavelength. The results obtained from the TiNO films deposited at different temperatures are shown in Fig. 5. The presence of the signature peaks for the anatase TiO₂ phase (E_g, B_1g, A_1g, and E_g) in the spectra at 144, 394, 514, 634 cm⁻¹ indicates the formation of anatase TiO₂ [39,40,43,57–59]; the intensity of these peaks increases with an increase in the film deposition temperature. The E_g peak for the sample deposited at 300 °C is clearly present but is very broad; the other peaks are naturally non-existent due to their relatively low intensity. The Raman spectrum (RS) of 300 °C sample is reminiscent of an amorphous TiO₂ phase with low resistivity [60,61]. Thus, it appears that while TiNO films are structurally qualitatively similar with respect to XRD patterns and crystallinity, the TiO₂ phase in the TiN-TiNO-TiO₂ composite mixture is mostly amorphous at 300 °C. Since the amorphous TiO₂ possesses a low resistivity [61], the samples deposited at 300 °C have the lowest sheet resistance in the present study, too (inset of Fig. 5). As the temperature of deposition increases, the resistivity of the composite structure increases due to the fact that TiNO films lose oxygen, making it more resistive, plus the fraction of crystalline and insulating TiO₂ increases. A comparison of these peak positions to the reported Raman peak positions for pure anatase TiO₂ indicates a shift in the peak position [59]. This shift in the peak position with respect to peaks in the pure TiO₂ is attributed to the doping/modification in the band structure of the TiO₂ films due to the hybridization of N (2p) and O (2p) energy levels [47]. As shown in this inset figure, the 300 °C sample has almost five orders of magnitudes lower sheet resistance (3 Ω/square) in comparison to the 400 °C (97,119 Ω/square) and 500 °C (97,371 Ω/square) samples.

The surface morphology of the TiNO electrode was investigated by recording SEM images prior to and after the electrochemical test. The recording of SEM images of the TiNO film surface before and after the electrochemical measurements is used to examine the surface morphology and surface reconstruction during the applied potential window. The images in Fig. 6 on the left column (a), (c), and (e) indicate the surface morphology before electrochemical measurements, and the set of images on the right of Figs. 6(b), 6(d), and 6(f) in the same row are the surface morphologies of the samples after the electrochemical measurements. The creation of dark discolorations (indicated by the red circle) in the aftermath images is observed. This suggests very moderate degradation of the samples, which indicates the stability of the electrodes under the cycling conditions [20,27].

The CVs of the TiNO thin film, deposited at three different temperatures, are presented in Fig. 7(a). All three CVs, recorded at the same scale rate of 5 mV/s, displayed explicit oxidation/reduction peaks. Figure 7(b) shows the variation of the specific capacitance of these films as a function of TiNO film deposition temperatures; the specific capacitance of TiNO films deposited at 300 °C is nearly 20% lower than that of TiNO films deposited at 500. Shown in Fig. 8(a) are the CV curves at various scan rates for the TiNO sample deposited at 300 °C, which has the highest capacitance values. It is clear from Fig. 8(a) that the oxidation peaks shift to more oxidative potentials (moves to the right), and the reduction peaks shift to more reductive potentials (moves to the left). The shift in the oxidation and reduction potential is attributed to irreversible electron transfer reactions, where the electron transfer kinetics are slow [62]. In the case of reversible electron transfer reactions, which have rapid electron transfer kinetics, the oxidation and reduction peak positions are independent of the CV scan rate [62]. Also, the TiNO sample exhibits a non-linear relationship between current density and scan rate. This non-linear CV behavior suggests that the TiNO material system is pseudocapacitive in nature as opposed to the rectangular shape of the electrical double-layer capacitor without the presence of any redox waves [63].

Here Q is the area under the CV curve, $∂v/∂t$ is the scan rate, $ΔV$ is the potential window, and the area is the geometric area of the electrode submerged. The variation of specific capacitance with scan rates at different thin film electrode fabrication temperatures is shown in Fig. 8(b). As shown in this figure, the specific capacitance of the TiNO thin films decreases rapidly with an increase in the scan rates. The lower specific capacitance at the higher scan rate is attributed to insufficient time for the electrolyte to get adsorbed and desorbed on the electrode surface [65]. The decrease in the specific capacitance, however, reaches saturation, which might be attributed to a reduction in the concertation gradient of electroactive species at the electrode surface above a certain scan rate [66], which needs further investigation.

In this equation, $Zc$ is the capacitance of the impedance, $j=−1$⁠, and $ω$ is the angular frequency. The Nyquist plot stems from the electrical circuit contained in the semicircle of the EE, where the diameter represents the charge transfer resistance, R₂. The charge transfer resistance of the interface of the double-layer structure decreases as the diameter decreases, resulting in faster reaction kinetics [19,72,73]. Galvanostatic charge–discharge tests were used to further examine the electrochemical characteristics and charge storage capacity of TiNO thin films in a two-electrode setup. Figure 10(a) shows the galvanostatic charge–discharge characteristics of the TiNO thin film deposited at 300 °C in 1 M KOH electrolyte.

In this equation, I is the discharge current (A), $Δt$ is the discharge time (s), $ΔV$ is the potential window (V), and A is the geometric area of the TiNO film submerged. From the plot (Fig. 10(b)), we can see that the TiNO 300 °C electrode recorded the highest C_sp of 66 mF/cm². This follows our earlier observation from the CV results. The improvement in energy storage could be a result of improved surface conditions as less oxygen dissolves at low deposition temperatures, leading to a high redox activity at the electrode/electrolyte interface.

The Ragone plot of the TiNO supercapacitor cells is shown in Fig. 11, where the power density is presented as a function of energy density. The obtained energy and power densities were comparable among all three samples performed in this study. The highest energy and power density for the device were recorded to be 7.55 Wh/cm² and 952.155 W/cm² for the TiNO_300°C and TiNO_500C cells, respectively. The TiNO_400C had an in-between energy density of 7.37 Wh/cm² and power of 679.29 W/cm². The stability of the supercapacitor has been investigated by monitoring the potential of 300 °C-deposited TiNO symmetric cell versus charge–discharge time. Figure 12 shows the 1st and 1000th galvanostatic cycles of the cell. The supercapacitor made with the TiNO_300°C electrode material showed a negligible loss of capacitance after 1000 galvanostatic cycles with a capacitance retention ability of 90%.

High-performance TiNO thin films were synthesized by a PLD method on polycrystalline titanium metal plates. The highest specific capacitance of 58.7 mF/cm² was observed for the TiNO thin films deposited at 300 °C, which is ∼22% more than those of TiNO films deposited at higher temperatures. The capacitance retention ability of 300 °C-deposited films was ∼90% after 1000 cycles. These films also showed high power and energy densities of 952.155 W/cm² and 7.55 Wh/cm², respectively. The present study has established the thin film deposition temperature as an effective tuning strategy to rationally modulate the morphology and the structure of TiNO films film to achieve a high charge storage performance.

The authors would like to acknowledge the financial support from the NSF Partnership of the Research and Education in Materials (PREM) program via Grant No. DMR-2122067. M.L., S.S., and S.A. acknowledge receiving partial support in the summer from the Energy Frontier Research Center (EFRC) program via Grant No. DE-SC0023415. This work also made use of the Cornell Center for Materials Research Shared Facilities, which are supported through the NSF MRSEC program (DMR-1719875) and of the Joint School of Nanoscience and Nanoengineering (JSNN), a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (Grant No. ECCS-2025462).

There are no conflicts of interest.

The datasets generated and supporting the findings of this article are obtainable from the corresponding author upon reasonable request.