Optical and Optoelectrical Properties of Ternary Chalcogenide CuInS₂/TiO₂ Nanocomposite Prepared by Mechanochemical Synthesis

Abstract

In this work, a nanocomposite consisting of ternary chalcogenide CuInS₂ and TiO₂ was prepared and its optical and optoelectrical properties were investigated. The CuInS₂/TiO₂ nanocomposite was produced via one-step mechanochemical synthesis and characterized from the crystal structure, microstructural, morphology, surface, optical, and optoelectrical properties viewpoints. X-ray diffraction confirmed the presence of both components, CuInS₂ and TiO₂, in the nanocomposite and revealed a partial transformation of anatase to rutile. The presence of both components in the samples was also proven by Raman spectroscopy. HRTEM confirmed the nanocrystalline character of the samples as crystallites ranging from around 10 nm and up to a few tens of nanometers were found. The presence of the agglomerated nanoparticles into larger grains was proven by SEM. The measured optical properties of CuInS₂, TiO₂, and CuInS₂/TiO₂ nanocomposites demonstrate optical bandgaps of ~1.62 eV for CuInS₂ and 3.26 eV for TiO₂. The measurement of the optoelectrical properties showed that the presence of TiO₂ in the CuInS₂/TiO₂ nanocomposite increased its conductivity and modified the photosensitivity depending on the ratio of the components. This study has demonstrated the possibility of preparing a CuInS₂/TiO₂ nanocomposite material with promising applications in optoelectronics in the visible region in an eco-friendly manner.

1. Introduction

Ternary chalcogenide semiconductors of the I–III–VI group with promising applications in electronics, optics, and catalysis have been intensively studied in recent years [1]. However, the majority of the best-investigated sulfide-based semiconductors contain toxic heavy metals, which seriously limits their potential application.

The ternary chalcogenide CuInS₂ as a semiconductor with direct and indirect bandgap values of 1.60 eV and 1.51 eV [2] enables the absorption of radiation in the region of the solar radiation spectrum and thus is a promising semiconductor for the development of solar cells with low toxicity and good chemical stability. Its combination with other metallic inorganic semiconductors (e.g., TiO₂ and ZnO) with wider bandgaps can lead to better optical properties due to the elimination of surface non-radiative recombination defects. Nanostructured TiO₂ semiconductors are extensively used in solar cells and photocatalysis. CuInS₂/TiO₂ nanocomposite represents a non-toxic alternative to other previously studied nanocomposites, such as CdSe/TiO₂, CdS/TiO₂, and PbS/TiO₂, that contain toxic heavy metals [3,4,5].

Many synthetic techniques have been reported for the synthesis of TiO₂-coated CuInS₂ nanomaterials, including solvothermal synthesis, the ultrasonication-assisted cathodic electrodeposition strategy, the ionic layer adsorption and reaction (SILAR) method, robotic spray pyrolysis, hydrogen plasma treatment, etc. [6,7,8,9,10,11,12,13].

CuInS₂/TiO₂ composites with different depositing amounts were prepared by the deposition of CuInS₂ on TiO₂ with one-step or two-step solvothermal reactions [6]. Enesca et al. [7] prepared a CuInS₂/TiO₂/SnO₂ heterostructure for air decontamination by deposition of layers by spray pyrolysis followed by annealing. A new type of photoelectrode has been successfully realized using an n-type CuInS₂ heterostructure modified with a TiO₂ array of nanotubes (NTs) by cathodic electrodeposition using ultrasound [8]. Shen et al. [9] investigated the properties of p-type CuInS₂ quantum dots synthesized via the solvothermal route and bound to n-type TiO₂ nanoparticles by a heat treatment process. Also, CuInS₂–TiO₂ heterojunction solar cells were prepared by Nanu and co-workers using the atomic layer deposition method [10]. CuInS₂/TiO₂ electrodes formed from nanotube heterojunction arrays were prepared by synthesis using a modified sequential ion layer adsorption and reaction (SILAR) method [11]. Chen et al. conducted an experiment on the preparation of a TiO₂ NR/CuInS₂ solar cell by formation in hydrogen plasma [12]. The spin coating-assisted SILAR method was used for synthesis of CuInS₂ QD-sensitized TiO₂ films, which were further used as promising photoelectrodes for solid-state QDSSCs [13].

The high-energy ball milling process affords all the benefits for the expanding field of mechanochemistry [14,15]. This method represents a simple, low-temperature, and environmentally friendly alternative to the traditional preparation methods. Its eco-friendly process means that synthesis takes place in the absence of solvents and toxic organometallic precursors and unwanted side-products. We have already prepared other metal sulfide-based semiconductor–TiO₂ nanocomposites via mechanochemical synthesis in the past [16,17].

According to our best knowledge, the CuInS₂/TiO₂ nanocomposite has not been prepared via mechanochemical synthesis so far. In the present work, the CuInS₂/TiO₂ nanocomposite in two different CuInS₂:TiO₂ molar ratios was prepared via mechanochemical synthesis and its optical and optoelectrical properties were investigated. The novelty of this study is the simple and environmentally friendly mechanochemical synthesis of CuInS₂/TiO₂ nanocomposite in a very short time, at ambient pressure and temperature, as suitable material for solar cell applications.

2. Materials and Methods

2.1. Materials

The CuInS₂/TiO₂ nanocomposite was prepared from CuInS₂, which was synthesized from copper (99.7%, Aldrich, Darmstadt, Germany), indium (99.99%, Aldrich, Darmstadt, Germany), sulfur (99%, Ites, Vranov n/T, Slovakia), and commercially available TiO₂ Degussa P25 (Degussa, The Netherlands) (75% anatase and 25% rutile).

2.2. Mechanochemical Synthesis of CuInS₂/TiO₂ Nanocomposite

The mechanochemical synthesis of CuInS₂ was performed in a planetary mill Pulverisette 6 (Fritsch, Idar-Oberstein, Germany) by milling of 1.31 g of elemental copper, 2.37 g of elemental indium, and 1.32 g of elemental sulfur. A tungsten carbide milling chamber (volume 250 mL) filled with 50 balls (diameter 10 mm) of the same material was used for milling, and the milling was carried out at 550 rpm for 60 min in an argon atmosphere according to Equation (1) and the procedure provided in [18]. Mechanochemically prepared CuInS₂ according to Equation (1) and commercially available TiO₂ were used to synthesize the CuInS₂/TiO₂ nanocomposite (Equation (2)) (in molar ratios of 4:1 and 1:4 chosen based on our results published in papers [16,17] for similar nanocomposites). The synthetic process of the CuInS₂/TiO₂ nanocomposite is displayed in Figure 1 and can be described by the following reactions:

Cu + In + 2S→CuInS₂

(1)

CuInS₂ + TiO₂→CuInS₂/TiO₂

(2)

The mechanochemical synthesis of CuInS₂/TiO₂ nanocomposite was also carried out in a planetary ball mill Pulverisette 6 (Fritsch, Idar-Oberstein, Germany) in argon atmosphere (>99.998%, Linde Gas group, Bratislava, Slovakia). Milling took 30 min without breaking cooling due to the shorter milling time. A 250 mL tungsten carbide chamber with 50 tungsten carbide balls with a diameter of 10 mm was vented with argon for 3 min to ensure an inert atmosphere. The rotational speed of the mill was set at 500 rpm. A ball to powder ratio of 72:1 was used.

2.3. Characterization Techniques

X-ray diffraction (XRD) measurements were performed using a D8 Advance diffractometer (Bruker, Bremen, Germany) equipped with a θ-θ goniometer, in CuKα radiation (40 kV, 40 mA), a secondary graphite monochromator, and a scintillation detector. The XRD pattern was scanned from 15° to 60° with a step of 0.03° and a counting time of 12 s. The XRD pattern was analyzed using Diffracplus Eva software with the ICDD—PDF2 database. Rietveld refinement was carried out using TOPAS Academic software [19,20].

TEM analyses were carried out using a 200 kV JEM 2100 microscope (JEOL, Akishima, Japan) with a LaB6 electron source and equipped with an energy-dispersive X-ray spectrometer (EDS). A small amount of the sample was ultrasonically homogenized in absolute ethanol. A drop of the homogenized suspension was deposited on a carbon-coated copper grid and dried. Prior to TEM analyses, the sample was covered with carbon to prevent charging under the electron beam.

Morphology was studied using a field emission-scanning electron microscope (FE-SEM, Mira 3, Tescan, Czech Republic) coupled with an EDS analyzer (Oxford Instruments, Oxford, UK).

The values of the specific surface area were obtained by using a NOVA 1200e Surface Area & Pore Size Analyzer (Quantachrome Instruments, Boynton Beach, FL, USA).

X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos Axis Supra XPS instrument (Manchester, UK) with monochromatic AlKα X-rays (hυ = 1486.69 eV). Scans were performed at an emission current of 15 mA and in hybrid lens mode. Survey XPS and high-resolution (HR) spectra were obtained using scan steps of 1.0 and 0.1 eV, respectively. Prior to data analysis, the C1s spectrum was calibrated to 284.8 eV. Low-resolution broad scan and high-resolution scan were performed for Cu, In, S, Ti, and O. The low-resolution wide survey and high-resolution scans were performed for Cu, In, S, Ti, and O, respectively. The HR spectra were used to analyze the elemental composition and chemical state of the sample. Deconvolution and fitting of the elements of interest were performed using CasaXPS software (version 2.3.17, Casa Software Ltd., Teignmouth, UK) using Spine Shirley background in the HR spectra and Gaussian/Lorentzian line shape to fit the XPS peaks.

UV–Vis spectra of individual CuInS_2, commercially available TiO₂ and mechanochemically synthesized CuInS₂/TiO₂ nanocomposite were measured using a UV–Vis light source connected to an optical fiber and to a stage for measuring reflectance using two optical fibers. The reflectivity of the samples could only be measured on samples prepared in the form of a thin slice formed by pressing the nanocrystalline powder. The spectra were measured by the OceanOptics system and processed after software correction in the range 200–1000 nm.

Micro-Raman and PL spectra measurements at room temperature were performed using a confocal Raman microscope in backscattering geometry (Spectroscopy, Imaging, Warstein, Germany) and using a focused Ar laser beam tuned to excitation wavelength of 514 nm. The calibration of the measuring system before the measurement was carried out using the frequency of the Raman line of crystalline Si at 520 cm⁻¹.

The optoelectrical properties of individual CuInS_2, commercially available TiO₂, and CuInS₂/TiO₂ nanocomposite were realized by measuring the I–V characteristics in the dark and under focused white light with an illumination intensity of ~600 mW/cm² using a halogen lamp and Agilent 4155C semiconductor parameter analyzer. Each sample for measurement was prepared by dropping a solution of nanocrystalline powder in isopropyl alcohol onto a Au interdigital structure with an area of 3 × 3 mm and with finger/gap dimensions of 30/12 µm. The electrical connection to the socket was completed using Au wires glued with silver paste.

3. Results and Discussion

3.1. Structural and Microstructural Characterization

For comparison, the X-ray diffraction (XRD) patterns of previously mechanochemically prepared CuInS₂, commercial TiO₂, and mechanochemically synthesized CuInS₂/TiO₂ 4:1 and 1:4 nanocomposites are shown in Figure 2. The phase composition of the prepared CuInS₂/TiO₂ nanocomposites was carried out by Rietveld refinement (Figure 3). In the XRD pattern of the CuInS₂/TiO₂ 4:1 (Figure 3a), the four most intensive diffraction peaks can be ascribed to the tetragonal structure of roquesite CuInS₂ (JCPDS 00-027-0159). In addition to this, a small peak at 25.1°, which can be indexed to anatase (TiO₂ P25) with a tetragonal structure (JCPDS 01-075-2545), was found. A very small amount of tungsten carbide wear coming from milling balls and a vial was also evidenced. The approximate crystallite size of roquesite nanocrystals in this sample was calculated to be 22 ± 4 nm, which is in accordance with the 18 nm reported in [18]. In both cited papers, the crystallite size is reported for the as-prepared pure CuInS₂. In the case of the CuInS₂/TiO₂ 1:4 sample (Figure 3b), the diffraction lines characteristic of anatase modification (TiO₂ P25) with a tetragonal structure (JCPDS 01-075-2545) are much stronger; however, the diffraction lines of roquesite CuInS₂ (JCPDS 00-027-0159) with a tetragonal structure are present and are still intensive. In addition, a small amount of rutile (another tetragonal crystallographic modification of TiO₂ with JCPDS 01-076-9000) was found. This fact points to partial phase transition of anatase into rutile during milling [16]. A greater amount of tungsten carbide from the wear of the milling balls and the vessel has also been demonstrated. The diffraction peaks of WC are much more intense; namely, the one at 31.3° (overlapping with the weaker reflection of roquesite) is significant. In general, all the diffraction lines are broad, indicating the formation of fine crystallites, as well as structural disorder that is common during the process of milling [21].

Figure 4 shows the Raman spectra of the synthesized CuInS_2, commercially available TiO₂, and mechanochemically synthesized CuInS₂/TiO₂ 4:1 and 1:4 nanocomposites. The measured spectra confirmed the appearance of characteristic peaks that correspond to the formation of crystalline nanoparticles. It is clear from the figure that the dominant feature of the measured spectrum of the CuInS₂/TiO₂ nanocomposite includes TiO₂ peaks, which indicate the formation of the compound in different crystallographic forms—polymorphism [22]. The intense peak at 143 cm⁻¹ and lower peaks near 397 and 514 cm⁻¹ correspond to anatase-form TiO₂ with symmetries E_g, B_1g, and A_1g, respectively. The broad peak around 406 cm⁻¹ includes two mixed peaks from 398 to 437 cm⁻¹, which may be related to the formation of anatase and rutile forms of TiO₂ with A_1g and E_g symmetry, respectively [22]. The lower-intensity peaks at 245, 296, and 338 cm⁻¹ correspond to CuInS₂ phase with symmetries A₁, E, and B_2, respectively [18,23,24]. The results of Raman measurement confirm the results obtained by means of XRD as well as the presence of WC peaks at 808, 1420, and 1600 cm⁻¹ for the CuInS₂/TiO₂ 1:4 sample, as shown in the inset of Figure 4, corresponding to published data [25].

Microstructural characterization of CuInS₂/TiO₂ nanocomposite in 4:1 and 1:4 ratios was performed by transmission electron microscopy (TEM). Both the CuInS₂/TiO₂ nanocomposites consist of agglomerated nanoparticles, which was confirmed by a low-magnification TEM image (Figure 5a,b).

The crystallinity, crystallite size, and chemical composition of the samples were inspected by selected-area electron diffraction (SAED), high-resolution TEM (HRTEM), and energy-dispersive X-ray spectroscopy (EDXS). The results of the analyses for the 4:1 and 1:4 nanocomposites are shown in Figure 5c,d, respectively. The SAED patterns of both samples are diffraction rings, indicating that the products are composed of randomly oriented nanoparticles. Based on the measured d-values of the rings, we confirmed that the main phases in both nanocomposites are roquesite CuInS₂ and anatase TiO₂. In the 4:1 sample, roquesite is the predominant phase, whereas, in the 1:4 sample, the situation is the other way around. HRTEM analyses of both samples additionally disclose that the size of the crystallites in both samples is in the nanometer range, ranging from around 10 nm up to a few tens of nanometers. The presence of CuInS₂ and TiO₂ in different ratios in both nanocomposites was confirmed by EDXS. The analyses recorded from larger areas of the sample clearly show the prevalence of Cu, In, and S in the 4:1 sample and a significantly higher fraction of Ti in the 1:4 sample. For evaluation of CuInS₂ and TiO₂ distribution in the samples, EDXS mapping at the nanoscale would be required.

3.2. Surface and Morphological Characterization

Mechanically activated or mechanochemically synthesized samples are characterized by a variety of methods, and one of the most important characteristics is the measurement of the specific surface area (S_A) value [21]. The S_A of pure CuInS₂, from which the studied nanocomposite was synthesized, was 4 m²g⁻¹, as was reported in paper [18]. Upon the introduction of TiO₂, the S_A value considerably increased (values of 6.3 m²g⁻¹ and 9.7 m²g⁻¹ have been evidenced for the CuInS₂/TiO₂ 4:1 and 1:4 samples, respectively). The S_A value of TiO₂ P25 is 28.7 m²g⁻¹. The S_A values of the mechanochemically synthesized CuInS₂/TiO₂ reported here are lower compared with the samples prepared by other methods [26].

The morphology of the prepared products was investigated by scanning electron microscopy. The SEM micrographs of the synthesized CuInS₂/TiO₂ nanocomposite are displayed in Figure 6.

The SEM images of the CuInS₂/TiO₂ 4:1 and 1:4 nanocomposites show that the prepared sample consists of fine nanoparticles that form densely arranged irregular aggregates with irregular size and shape. In addition to larger micrograins, a large number of nanoparticles (100–300 nm in size) can also be found. Smaller nanoparticles can be found on the surface of larger particles and are also dispersed among larger particles.

For the surface elemental analysis, EDS mapping was performed, and the results are shown in Figure 7. The layered EDS images containing all the elements are shown in Figure 7A,B for the CuInS₂/TiO₂ 4:1 and 1:4 nanocomposites, respectively, and in Figure 7b–f (for sample A) and Figure 7b–f (for sample B). The individual energy-dispersive X-ray spectroscopy (EDS) mapping for Cu, In, S, Ti, and O can be viewed. The distribution of all the elements in the nanocomposite appears to be homogeneous. The signal of titanium is stronger in nanocomposite CuInS₂/TiO₂ 1:4.

The surface composition and oxidation states of various elements present in the CuInS₂/TiO₂ nanocomposites were investigated by X-ray photoelectron spectroscopy (XPS) (Figure 8). Survey XPS spectra revealed the presence of different elements, such as copper (Cu), indium (In), sulfur (S), titanium (Ti), oxygen (O), and carbon (C). In the case of the CuInS₂/TiO₂ 4:1 nanocomposite (Figure 8A), the peaks of Cu, O, In, and S appear on the wide spectra of the sample. The HR low-intensity spectrum of Cu2p has a doublet of Cu2p_3/2 and Cu2p_1/2. However, the intensity was not high enough to analyze their chemical states. Moreover, the intensity of the S2p spectrum was not sufficiently high to clearly deconvolute the spectrum; however, it can be presumed that the spectrum is represented by the doublet S2p_3/2 and S2p_1/2 at 162.44 eV and 163.60 eV, respectively. The more intensive peaks at 170.30 and 169.16 eV may correspond to metal sulfate (SO₄)²⁻ or a S–O bond. In the case of the mechanochemically synthesized samples, the presence of some metal sulfates due to mechanochemical surface oxidation can be expected [27]. The intensity of the Ti HR XPS was not high enough to analyze their chemical states. However, deconvolution of the O1s spectrum revealed the presence of three components. The peaks at 532.25 eV and at 533.0 eV are related to the capping agents and the contamination as a result of the sample’s exposure to the atmosphere. The presence of titanium dioxide (TiO₂) is confirmed by a slight signal at 530.07 eV. The core-level region of In3d has well-separated spin-orbit components at 445.99 eV (In3d_5/2) and at 453.52 eV (In3d_3/2), with a charge separation of 7.53 eV, which matches well with that of In³⁺ in CuInS₂ [28]. The peaks’ shapes and positions indicate the potential presence of indium oxides.

In the case of nanocomposite CuInS₂/TiO₂ 1:4 (Figure 8B), the presence of Cu, In, S, Ti, and O elements was discovered in the survey spectrum. The intensities of the Cu HR XPS were not high enough to analyze their chemical states. Similar to the CuInS₂/TiO₂ 4:1 nanocomposite, the deconvolution of the core-level O1s spectrum demonstrates three components, with a much stronger peak due to a higher amount of TiO₂, as was expected. The Ti2p region develops typical Ti2p_3/2—Ti2p_1/2 splitting, with the components appearing at 458.71 eV (Ti2p_3/2) and 464.47 eV (Ti2p_1/2). Similar to the O1s spectrum, the components’ positions confirm the presence of TiO₂. Moreover, a charge separation ΔE of 5.76 eV between the two components also indicates metal oxide presence. Typical In3d_5/2—In3d_3/2 splitting is found again in the In3d core-level region. The components appear to be at 445.81 eV (In3d_5/2) and 453.36 eV (In3d_3/2), with a charge separation of 7.55 eV. The results are in line with those of the In3d region in the sample CuInS₂–TiO₂ (4:1) (A).

3.3. Optical Properties

Mechanochemically synthesized CuInS_2, CuInS₂/TiO₂ 4:1, and 1:4 nanocomposites and commercially available TiO₂ were examined using UV–Vis reflectivity spectroscopy to investigate their optical properties (Figure 9a). Even with relatively low reflectance of the samples, it was possible to determine the change in reflectance and recalculate the relative absorption of individual samples. The diffuse reflectance data (shown in Figure 9a) were transformed to pseudoabsorption data using the Kubelka−Munk function (F(R)). This expresses the absorbance as a function of reflectance: F(R) = α/S = (1 − R)²/(2R), where R is the reflectance, α is the absorption coefficient, and S is the scattering coefficient [29]. The dependence of (F(R) hν)² for a direct semiconductor and (F(R) hν)^1/2 for an indirect semiconductor as a function of photon energy hν was used to determine the optical bandgap energy (Eg) by extrapolating the straight part to the zero line as shown in Figure 9b. As is clear from the measured reflectivity spectra of CuInS₂, TiO₂, and CuInS₂/TiO₂ with different ratios, the measured spectra are composed of the absorption properties of the individual components. Thus, from the measurement of the reflectance CuInS₂/TiO₂ with a molar ratio of 1:4, it was possible to determine both bandgaps of the individual components, which were for CuInS₂ (1.62 eV) as a direct semiconductor (Figure 9b-inset) and for TiO₂ (3.26 eV) as an indirect semiconductor. The determination of the Eg of the chalcogenide CuInS₂ is in good agreement with the published data for thin films [2] and nanoparticles [30]. Similarly, the determination of Eg for TiO₂ as an indirect semiconductor is in good agreement with the published data for P25 nanopowder [31].

The micro-PL spectra of the CuInS_2, CuInS₂/TiO₂ 4:1, and 1:4 nanocomposites and commercially available TiO₂ when the samples are excited at a wavelength of 514 nm are shown in Figure 10. Broad emission PL spectra are typical in semiconductor nanocrystals based on chalcopyrite I–III–VI2. The size of the CuInS₂ nanoparticles can cause a blue shift in the emission, and the measured PL spectrum thus exhibits size-dependent optical properties, indicating quantum confinement effects [32,33]. As can be observed in Figure 10a, the emission spectra, especially for the CuInS₂/TiO₂ 1:4 nanocomposite, are significantly influenced by the mutual interaction of the individual components by the increased separation and transfer of the photogenerated electrons trapped in TiO₂ compared to pure TiO₂ and CuInS₂ [34]. This is especially evident in the 500–600 nm region, where the intensity of the emission spectrum gradually increases and significantly changes the edge of the TiO₂ emission spectrum. Its expected maximum is in the region of 430 nm [35] and is not shown due to the excitation wavelength of 514 nm.

Figure 10b shows the emission spectrum of the ternary chalcogenite CuInS₂ as a function of energy. To determine the recombination processes in CuInS₂, the measured PL spectrum can be qualitatively decomposed as the sum of two Gaussian functions with maxima of 1.66 and 1.37 eV. The emission spectrum in the region of 1.66 eV is close to the determined bandgap from the absorption measurement and corresponds to interband radiative recombination. The difference from the bulk bandgap of 1.53 eV [36] can be caused by size-dependent optical bandgap properties. The emission peak of 1.37 eV can be assigned to the D–A transition from the donor state of the indium atom, which occupies the copper vacancy (In_Cu) below the edge of the conduction band to the acceptor level of the copper vacancy (V_Cu). This corresponds well to the model of energy levels in undoped CuInS₂ described and published by Ueng [36].

3.4. Optoelectrical Properties

As can be seen in Figure 11, the current–voltage (I–V) characteristics for the measured samples show an almost linear dependence, which confirms the formation of an ohmic contact on the prepared element.

As can be seen from the measured I–V characteristics, the CuInS₂ sample shows the lowest conductivity. By increasing the amount of TiO₂ in the ratios of CuInS₂/TiO₂ 4:1 and 1:4, the conductivity of the samples increases proportionally to the amount of TiO₂. Overall, TiO₂ shows an order of magnitude higher conductivity compared to CuInS₂. Intense illumination causes an increase in the number of generated charge carriers in all the samples, which results in increases in conductivity and photoresponsive current. When comparing the measurement results, the samples show 41%, 61%, 8%, and 28% increases in photoresponsive current under illumination at an applied voltage of 2 V compared to the current in the dark for CuInS₂, the CuInS₂/TiO₂ 4:1 and CuInS₂/TiO₂ 1:4 nanocomposites, and TiO₂, respectively.

4. Conclusions

In this study, CuInS₂/TiO₂ nanocomposites in 4:1 and 1:4 molar ratios were prepared by a facile, low-cost mechanochemical method. X-ray diffraction confirmed the presence of both components in the composites and revealed partial anatase-to-rutile phase transformation and contamination from milling media as a result of the mechanochemical treatment. The micro-Raman measurements confirmed the results from the X-ray diffraction measurements. The nanocrystalline character of the samples was confirmed by TEM as crystallites ranging from around 10 nm up to a few tens of nanometers in size were found. SEM showed that the nanoparticles agglomerated into larger grains. High-resolution XPS analysis confirmed the presence of all the elements with their expected oxidation states and documented the surface oxidation of the produced nanocomposites. Mechanochemically synthesized CuInS₂ and CuInS₂/TiO₂ 1:4 nanocomposite exhibits reflectivity measurements regarding a bandgap near 1.62 eV, while TiO₂ demonstrates a bandgap of 3.26 eV. Measurement of the I–V characteristics showed that the presence of TiO₂ in CuInS₂/TiO₂ nanocomposites increases their conductivity and modifies their sensitivity to light compared to the starting materials, CuInS₂ and TiO₂. The measured optoelectrical properties of the CuInS₂/TiO₂ nanocomposite confirmed its potential for photovoltaic applications.