Mechanochemically Synthesized Chalcogenide Cu₃BiS₃ Nanocrystals in an Environmentally Friendly Manner for Solar Cell Applications

Abstract

Ternary wittichenite Cu₃BiS₃ nanocrystals were prepared mechanochemically using a planetary ball mill from elemental copper, bismuth and sulfur in a stoichiometric ratio in only 5 min. The orthorhombic wittichenite Cu₃BiS₃ was nanocrystalline with an approximate crystallite size of 38 nm ± 9 nm, as confirmed by Rietveld refinement. The nanocrystalline character of orthorhombic Cu₃BiS₃ was also proven by transmission electron microscopy. The measured Raman spectrum confirmed the formation of pure wittichenite Cu₃BiS₃. The morphology characterization demonstrated the homogeneity of the sample. The value of the specific surface area for pure mechanochemically prepared Cu₃BiS₃ after 5 min was 2.7 m²g⁻¹. The optical properties were investigated using UV–Vis absorption and micro-photoluminescence spectroscopy. From the absorption UV–Vis spectrum, the value of the bandgap energy was determined to be 1.52 eV, which creates an assumption for the use of wittichenite Cu₃BiS₃ in photovoltaic applications. The optoelectrical properties of the prepared Cu₃BiS₃ nanocrystals were verified by current–voltage measurements in the dark and under white light illumination. The photocurrent increased by 26% compared to the current in the dark at a voltage of 1 V. The achieved results confirmed a very fast and efficient way of synthesizing a ternary wittichenite Cu₃BiS₃, which can be used for applications in solar cells.

1. Introduction

Ternary copper–bismuth–sulfide (Cu-Bi-S) materials have recently gained importance in solar cell investigation as substitutive absorbers because of their low toxicity, low cost and earth-abundant elements [1].

Wittichenite Cu₃BiS₃ has a p-type conductivity, direct bandgap (1.10–1.86 eV) and large absorption coefficient (>10⁵ cm⁻¹). Furthermore, Bi, compared with Te and In, is less toxic, readily accessible and relatively inexpensive [2]. Cu₃BiS₃ has been proposed as an potential absorber material for photovoltaics [3] due to the suitable direct optical bandgap, which is close to the optimum value for efficient solar energy conversion [4]. Cu₃BiS₃ nanomaterials also have potential applications in bio-imaging [5,6].

Wittichenite Cu₃BiS₃, with many potential applications, has been tentatively synthesized using various techniques. Cu₃BiS₃ nanorods with strong absorption in the second near-infrared window were successfully synthesized by a facile organic route using oleylamine as the stabilizing agent [7]. Semiconducting Cu₃BiS₃ thin films produced by the solid-state reaction from chemically deposited CuS and thermally evaporated bismuth thin films were studied in paper [8]. Cu₃BiS₃ nanorods with different aspect ratios were synthesized via an ethanol–thermal pathway between CuCl₂, H₂O, BiCl₃ and thiourea using different solvents in a previous study [9]. Large-grain and highly crystalline Cu₃BiS₃ thin films were synthesized by a one-step dimethyl sulfoxide-based solution coating process [10]. One-dimensional Cu₃BiS₃ nano- and micro-structures prepared by a solvothermal route using the structural directing agent polyethylene glycol as a soft template were investigated in paper [11]. Single tetra-pod-like 3D-architecture Cu₃BiS₃ nanopowders for photodetector applications were prepared using the solvothermal route by Murali et al. [12]. Yan et al. [13] prepared Cu₃BiS₃ nanostructures with various morphology through the solvothermal method. Cu₃BiS₃ dendrites prepared by cyclic microwave radiation using L-cysteine and a complexing agent were investigated by Aup-Ngoen et al. [14]. Zeng et al. [15] prepared flower-like Cu₃BiS₃ hierarchical structures using the facile hydrothermal method. Coral-shaped Cu₃BiS₃ nanostructures were synthesized by a rapid polyol process by Shen et al. [16]. Deng et al. [17] synthesized Cu₃BiS₃ nanosheets and nanoparticles using 1-octadencence and 1-dodecanethiol as capping ligands through a single-source method. Wittichenite Cu₃BiS₃ nanocrystals, which exhibit a clear photoresponse in I-V measurement, were prepared by the hot-injection method by Yan et al. [18]. Cu₃BiS₃ nanoparticles synthesized by hydrothermal and solvothermal methods using L-cysteine biomolecules and a complexing agent were studied by Aup-Ngoen et al. [19]. Wittichenite Cu₃BiS₃ semiconductor thin films deposited on glass slides via a spray pyrolysis approach were investigated by Liu et al. [20]. Zhong et al. [21] synthesized flower-like Cu₃BiS₃ nanorods using a facile biomolecule-assisted solvothermal method employing CuCl₂·2H₂O, Bi(NO)₃·5H₂O and L-cystine as reactants. Cu-Bi-S thin films deposited onto soda lime glass substrates using a one-stage co-evaporation process from Cu₂S and Bi₂S₃ sources were studied by Hussain et al. [22]. Cu₃BiS₃ films for efficient hydrogen evolution from solar-driven photoelectrochemical water splitting prepared using the spray pyrolysis method were investigated in study [23]. Cu₃BiS₃ was also prepared by a simplified route through chemical bath deposition in paper [24]. A comprehensive review of the synthesis, characterizations, processing, and solar photovoltaic applications of Cu₃BiS₃ thin films was reported in paper [2].

Mechanochemistry is a branch of chemistry used for the extensive diversity of synthesis of different organic and inorganic materials [25,26,27]. Currently, its application potential is already exhaustive and new applications regularly appear [28]. By applying the tools of high-energy ball milling, it offers a satisfactory option to prepare nanocrystalline materials in a one-step, solvent-free, environmentally friendly manner without the need to apply external heating or pressure, in contrast to the classical methods [29]. Mechanochemistry is a scalable and reproducible method. Moreover, the mechanochemically synthesized compounds are composed of morphologically inhomogeneous, weakly crystallized and structurally disordered nanoparticles. The mechanochemical solid-state approach has also been successfully applied for ternary chalcogenide synthesis by our group [30,31,32,33,34].

To our best knowledge, the wittichenite Cu₃BiS₃ preparation by a facile one-step environmentally friendly mechanochemical synthesis is reported for the first time in this study. The main focus is on its optical and optoelectrical properties.

2. Materials and Methods

Ternary Cu₃BiS₃ nanocrystals were prepared in the planetary ball mill Pulverisette 6 (Fritsch, Idar-Oberstein, Germany) from 1.92 g of copper (99.7%, Merck, Darmstadt, Germany), 2.11 g of bismuth (99.5%, Aldrich, Taufkirchen, Germany) and 0.97 g of sulfur (99%, Ites, Vranov n/T, Slovakia) in a Cu:Bi:S stoichiometric ratio of 3:1:3, according to the reaction (Equation (1)). The particle size distribution of the elemental precursors used for the synthesis is shown in the Supplementary Materials (SM) in Figure S1.

3Cu + Bi + 3S→Cu₃BiS₃

(1)

The mechanochemical synthesis was performed using a tungsten carbide milling chamber (250 mL in volume) filled with 50 tungsten carbide milling balls (10 mm in diameter) at 550 rpm, in an argon atmosphere (>99.998%, Linde Gas group, Bratislava, Slovakia), for 0.5–5 min without break cooling due to the very short milling times. The milling chamber was not filled with Ar using a glow box; instead, Ar gas was purged for about 3 min into the chamber while air was being pushed out of it via the other ventil. A ball-to-powder ratio of 73:1 was used. The yield of mechanochemically synthesized Cu₃BiS₃ was 96%.

XRD characterization was carried out using a D8 Advance diffractometer (Brucker, Bremen, Germany) with the CuKα radiation in the Bragg–Brentano configuration. The generator was adjusted to 40 kV and 40 mA. The divergence and receiving slits were 0.3° and 0.1 mm, respectively. The XRD patterns were observed in the range of 10–70° 2θ with a step of 0.03°. Rietveld refinements from the XRD data of the synthesized samples were carried out using the TOPAS Academic program [35,36]. For the purpose of obtaining a proper geometry setup and to eliminate instrumental broadening, the instrumental resolution function was identified by the refinement of LaB6 standard specimen. The phase identification was made using the JCPDS PDF2 database [37].

Micro-Raman and micro-PL spectra were recorded by a UV–Vis–NIR confocal Raman microscope (Spectroscopy & Imaging, Warstein, Germany). An Ar laser with a wavelength of 514 nm was used to excite the measured sample.

The microstructural characterization was investigated using a TEM-200 kV JEOL-2100-PLUS microscope (JEOL, Akishima, Japan) equipped with a LaB6 filament (point resolution of 0.25 nm) and an energy-dispersive X-ray detector (EDX, Oxford Instruments, Nanolab Technologies Inc. Milpitas, CA, USA) detector. High-resolution TEM (HRTEM) images, lattice spacing, selected area electron diffraction (SAED) patterns, fast Fourier-transform (FFT) and phase interpretation were analyzed with the Gatan Digital Micrograph software (Gatan Inc., Pleasanton, CA, USA) and the Java version of the electron microscope software (JEMS-SWISS, Jongny, Switzerland). The elemental analysis quantification was made with the INCA software. For the SEM/TEM measurement, a small quantity of the powder sample was dispersed in ethanol and deposited on carbon-coated nickel grids (to avoid the interference between the Cu grid and the Cu from the sample in the EDX analysis).

The morphology was studied using scanning electron microscopy (SEM). SEM images were recorded on an S-4800 field emission microscope (Hitachi, Japan) in secondary electron mode at an acceleration voltage of 2 kV. Energy-dispersive X-ray spectrometry (EDX) was realized by means of a Bruker detector coupled to the SEM microscope to enable elemental mapping using an acceleration voltage of 20 kV.

EDX analysis was conducted in several areas of the sample in both TEM and SEM microscopes and the presented values are the averages in each case. The detectors coupled to the microscopes have internal standards that allow a semi-quantitative analysis, showing the elemental ratios in each analyzed area.

The chemical composition of the sample was as well analyzed by the inductively coupled plasma–optical emission spectrometry (ICP-OES) method on a dual-view iCAP 7200 (Thermo Fisher Scientific, Waltham, Massachusetts). A sample mass of 10 mg was mixed with 4 mL of HCl (37% Fisher Chemical, Hampton, New Hampshire) and 2 mL of HNO₃ (58% AnalR Normapur, AVANTOR, VWR, Radnor, Pennsylvania). Then, the mixture was introduced into a reactor vessel and digested using an ETHOS EASY (Milestone) microwave digestion system, using 1800 W at 250 °C/15 min. The digested sample was diluted to 50 mL using ultrapure water and analyzed in the ICP-OES equipment using three injections.

The specific surface area was determined by the low-temperature nitrogen adsorption method using a NOVA 1200e Surface Area and Pore Size Analyzer (Quantachrome Instruments, Boynton Beach, FL, USA). The values were calculated using BET theory.

A Mastersizer Scirocco 2000M (MALVERN Instruments, Malvern, Great Britain) working in laser diffraction mode was employed to measure the particle size distribution in the range 0.1–200 μm.

The absorption ultraviolet–visible (UV–Vis) spectrum was acquired by the UV–Vis spectrophotometer Helios Gamma (Thermo Electron Corporation, Warwickshire, UK). The sample was dispersed in absolute ethanol and placed in quartz cuvettes for the spectral analysis. The energy bandgap (Eg) can be determined by the utilization of the Tauc equation (Equation (2)):

$$\mathsf{\alpha }=\frac{\mathrm{A}{(\mathrm{h}\mathsf{\nu }-{\mathrm{E}}_{\mathrm{g}})}^{\mathrm{n}}}{\mathrm{h}\mathsf{\nu }}$$

(2)

where α—absorption coefficient, h—Plank’s constant, A—constant, ν—frequency and n is a constant associated with different kinds of electronic transitions. The optical bandgap, Eg, was estimated by plotting (αhν)² as a function of the photon energy hν. The optical bandgap energy was found by the extrapolation of the straight-line portion of the Tauc plot for the zero absorption coefficient (α = 0).

The optoelectrical properties of the prepared Cu₃BiS₃ nanocrystals were verified by current–voltage measurements in the dark and under focused halogen white light with an illumination intensity ~600 mW/cm². The sample for measurement was prepared by dropping a solution of nanocrystalline powder in isopropyl alcohol onto a structure with interdigital Au contacts. The interdigital structure used has an area of 3 × 3 mm with finger/gap dimensions of 30/12 μm. The electrical connection of the interdigital structure to the socket was realized using Au wires glued with silver paste.

3. Results

3.1. Structural Characterization

3.1.1. X-ray Diffraction

The kinetics of the mechanochemical synthesis of wittichenite Cu₃BiS₃ was investigated by X-ray diffraction (XRD). The XRD patterns were observed at different milling times (0.5, 2, 3.5 and 5 min), as shown in Figure 1. The desired phase for wittichenite Cu₃BiS₃ (PDF 00-043-1479) was identified at the shortest milling time (0.5 min) and unreacted bismuth Bi, syn (PDF 01-085-1329), is also present. After 2 min of milling, the unreacted bismuth Bi is still present. However, after 3.5 min of milling, only Cu₃BiS₃ as the final product is present, the unreacted Bi disappears, and the mechanochemical reaction is almost complete. A further 1.5 min of treatment did not bring about any significant changes in the XRD pattern. A substantial peak broadening reveals the obvious nanocrystalline character of the sample, and this was also established by calculations from the XRD. The estimated phase compositions obtained by Rietveld refinement match well with the analysis of the XRD patterns described above. Specifically, the content of unreacted elemental Bi for the reaction mixtures milled for 0.5 min and 2 min was 8.6 ± 0.6 and 7.3 ± 0.6%, respectively. Afterwards, a Bi⁰ phase was not present.

By means of Rietveld refinement, the crystallite size of the produced Cu₃BiS₃ was also estimated (Figure 1b). During its rapid formation after 5 min, the crystals are slightly less than 350 nm in size and the size drops well below 100 nm for all other cases. Interestingly, the crystallite size of the non-reacted bismuth is 67 ± 5 nm and 61 ± 5 nm in the mixtures after 0.5 and 3.5 min, respectively. The crystallite size for the Cu₃BiS₃ phase obtained after 3.5 and 5 min is practically identical (38 ± 9 nm), meaning that the supply of more energy does not lead to a larger reaction. In the case of these two samples, microstrain also comes into play, being 0.72 ± 0.22 and 0.65 ± 0.17 for the product obtained after 3.5 and 5 min, respectively. This means that the crystal lattice becomes deformed as the reaction completion approaches. Hussain et al. [22] prepared Cu₃BiS₃ thin films with the crystallite size from 38 to 44 nm via one-stage thermal evaporation with increasing deposition temperatures up to 400 °C. Santhanapriya et al. [38] also synthesized the pure single-phase orthorhombic structure of Cu₃BiS₃ with an average crystallite size of 38 nm. These reported results are in good agreement with our results.

The added value of this research strategy for preparing Cu₃BiS₃ nanoparticles by mechanochemical synthesis is, namely, its economic affordability: its very rapid synthesis, solvent-free character, ambient pressure and room temperature conditions. The approach is reproducible, ensuring high yield, is simple and is easy to operate. The final pure sample prepared after only 5 min of milling was selected for further characterization of the structural and microstructural properties as well being investigated from optical and optoelectrical points of view.

3.1.2. Raman Spectroscopy

The results of Raman spectroscopy recorded between 80 and 800 cm⁻¹ confirming the crystallinity and structural phase analysis of the prepared Cu₃BiS₃ sample are shown in Figure 2. The representative micro-Raman spectrum obtained using laser excitation at 514 nm shows modes of 120, 150, 207, 250, 288, 430 and 515 cm⁻¹. The observed modes are close to those of the Cu₃BiS₃ phase reported in the literature [18,22,39,40,41] and RRUFF database. The measured spectrum from the micro-Raman measurements confirmed that the synthesized Cu₃BiS₃ is phase pure and the results are in agreement with the measurements obtained from the XRD analysis (Figure 1).

3.2. Microstructural Characterization

The wittichenite Cu₃BiS₃ sample was further described by TEM and related techniques. A low-magnification image of the sample is presented in Figure 3a, showing the typical appearance of mechanochemically synthesized samples, where small crystallites are agglomerated, forming large particles. The medium particle size is between 0.5 and 2 µm. A HRTEM study indicates that the nanocrystals can be indexed in an orthorhombic symmetry consistent with the Cu₃BiS₃ structure (space group P2₁2₁2₁ with lattice parameters a = 7.723 Å, b = 10.395 Å and c = 6.716 Å, α = β = γ = 90°), in good agreement with the XRD results. A representative micrograph directed along the [2 1 5]_Cu3BiS3 zone axis is shown in Figure 3b and the appropriate FFT and simulated EDP are inset. However, the Cu₃BiS₃ sample was unstable under the beam irradiation. It was found that small nanocrystallites of Bi₂S₃ are ejected from the as-received Cu₃BiS₃ during TEM analysis. This was confirmed by EDX analysis and the results are displayed and described in Figure 4.

An X-ray spectroscopy (EDX) study was performed to interpret the instability of the sample under the beam irradiation. EDX allowed us to determine the chemical composition of the particles before and after being irradiated, as well as that of the crystallites that were ejected out of the Cu₃BiS₃ particles. The composition before irradiation (EDX spectrum of a particle, Figure 4a) shows an atomic ratio in accordance with Cu₃BiS₃ stoichiometry (42 at % of Cu, 18 at % of Bi and 40 at % of S). However, after some minutes of irradiation, the composition changes to 51 at % of Cu, 13 at % of Bi and 36 at % of S (Figure 4b), meaning that the content of Bi and S has decreased, whereas that of Cu has increased. This composition change is due to the ejection of some small Bi₂S₃ crystallites out of the irradiated particle (see inset in Figure 4b,c). The HRTEM study shows that such crystallites have a structure according to the orthorhombic Bi₂S₃ (unit cell parameters, a = 11.15 Å, b = 11.30 Å and c = 3.98 Å, α = β = γ = 90° and space group Pbnm). A Bi₂S₃ nanocrystal oriented along [1 1 1]_Bi2S3 is presented in Figure 4c and the corresponding FFT and the calculated EDP are inset.

3.3. Morphological Characterization

The morphology and chemical composition of the mechanochemically synthesized Cu₃BiS₃ after 5 min of milling are documented by SEM study and elemental mapping analysis (Figure 5). The representative SEM image of mechanochemically synthesized Cu₃BiS₃ showing several grains at lower magnification is displayed in the Supplementary Materials (SM) in Figure S2. The chemical composition was also analyzed using ICP. The Cu, Bi and S elemental maps show that the three elements are homogeneously distributed thought the sample, with an elemental ratio according to a monophasic Cu₃BiS₃. These data are in good agreement with the ICP results, where 10 mg of Cu₃BiS₃ sample in 50 mL of solution gave 75.75 ppm of Cu, 85.36 ppm of Bi and 39.10 ppm of S. It must be taken into account that in the mechanosynthesis method, all of the elements that are introduced as precursors will be present in the final product of the reaction, regardless of whether or not the reaction has been completed. This was confirmed via the ICP results, which indicated the amounts of each element that were introduced into the reaction and were present in the dissolved sample.

3.4. Specific Surface Area Measurement

The nanosized crystallites usually possess a high specific surface area; however, they have a tendency to form agglomerates because of their small size. The surface properties were investigated by the low-temperature nitrogen adsorption method. The specific surface area values increased with the increase in the milling time from 0.7 m²g⁻¹ to 2.7 m²g⁻¹. The highest value 2.7 m²g⁻¹ was measured for the pure Cu₃BiS₃ sample milled after 5 min. The measured values are in accordance with those reported for other mechanochemically prepared ternary sulfides [33,42]. Contrary to our mechanochemical synthesis, it is possible to obtain Cu₃BiS₃ with a higher specific surface area around 17 m²g⁻¹ when applying the biomolecule-assisted hydrothermal and solvothermal method [19].

3.5. Optical Properties

3.5.1. UV–Vis Spectroscopy

The absorption properties of mechanochemically synthesized wittichenite Cu₃BiS₃ prepared after 5 min of milling were investigated using UV–Vis absorption microscopy. The measured spectrum in the range from 200 nm to 800 nm, displayed in Figure 6, shows a broad absorption peak with increasing absorbance from 250 nm and a maximum at approximately 670 nm.

The determined optical bandgap, 1.52 eV, calculated by the utilization of the Tauc equation (Equation (2)) and displayed by the Tauc plot in the inset of Figure 6 is in good accordance with previous reports [4,18,19,43,44]. Yan et al. [18] and Chakraborty et al. [44] used a hot-injection method to synthesize wittichenite nanocrystals with optical bandgaps of 1.56 eV and 1.5 eV, respectively. Viezbicke et al. [4] prepared Cu₃BiS₃ by a solvothermal method with a direct bandgap at approximately 1.5 eV. Deshmukh et al. [43] prepared Cu₃BiS₃ thin films exhibiting the optical bandgap 1.56 eV using the chemical bath deposition (CBD) technique. The obtained bandgap value of Cu₃BiS₃ is greater than the bandgap of bulk Cu₃BiS₃ (1.40 eV) [3], which can be attributed to the quantum confinement effect in the nanocrystals. This bandgap value corresponds to the optimum for solar cells and confirms that wittichenite is a suitable material for use in solar cells.

3.5.2. Micro-Photoluminescence Spectroscopy

Figure 7 shows the measured micro-PL spectrum obtained upon excitation of the sample with a wavelength of 514 nm. In the spectrum, a broad emission peak is located near 931.6 nm (1.33 eV) with an onset at 809.3 nm (1.53 eV), which agrees with the measured results obtained in a previous report [45]. The values obtained that correspond to the fitted PL Cu₃BiS₃ peaks are in good agreement with the published bandgap data for bulk Cu₃BiS₃ (around 1.40 eV) [3].

3.6. Optoelectrical Properties

The optoelectrical properties of the Cu₃BiS₃ powder nanocrystals dispersed on interdigital contacts from solution were verified by measuring the current–voltage (I–V) characteristics in the dark and under focused halogen light illumination. In doing so, we assumed that in the thin dispersed layer on Au fingers, active bridges from the nanopowder will be created in the gaps for the possibility of measuring electrical and optoelectrical parameters. Our measurements were carried out to confirm the fact that the prepared nanocrystalline powder exhibits suitable electrical conductivity and photosensitivity. This was confirmed by the measurement of the current–voltage characteristics, as shown in Figure 8. The I–V characteristics show only a slightly nonlinear dependence on the voltage, which indicates the formation of an acceptable ohmic contact between the Au fingers of the interdigital structure. Under intense illumination, a thin layer of dispersed Cu₃BiS₃ nanocrystals shows an increase in the current due to the generation of charge carriers, which causes an increase in the photo-responsive current. The generated photo-responsive current shows a 26% increase at 1V compared to the current in the dark for the measured sample.

The I–V characteristics of the synthetized Cu₃BiS₃ nanostructures and their photo-responsive properties have been previously studied in [11] and a similar nanocrystal film prepared by drop-casted Cu₃BiS₃ nano ink on glass is presented in [18], both of which show comparable optoelectrical properties.

4. Conclusions

In this research, the very rapid one-step mechanochemical synthesis of wittichenite Cu₃BiS₃ from copper, bismuth and sulfur precursors using high-energy milling after only 5 min in a planetary mill is reported. The crystallite size of the orthorhombic Cu₃BiS₃ calculated by Rietveld refinement was around 38 nm ± 9 nm. Micro-Raman spectroscopy confirmed the purity of the synthesized Cu₃BiS₃. The nanocrystalline character of the Cu₃BiS₃ was also endorsed by transmission electron microscopy. The morphology characterization demonstrated the homogeneity of the prepared sample. The morphological characterization using SEM and the surface properties investigated by the low-temperature nitrogen adsorption showed that the prepared nanocrystallites are agglomerated into micron-scale particles that exhibit specific surface areas below 3 m²g⁻¹. The medium particle size of wittichenite Cu₃BiS₃ is between 0.5 and 2 µm. The optical bandgap of Cu₃BiS₃ was detected to be 1.52 eV. The Cu₃BiS₃ nanocrystals show ~26% enhancement on the photo-responsive current, which proves that wittichenite Cu₃BiS₃ exhibits acceptable photoelectric properties and provides a prerequisite for use in photovoltaics. The present research shows that mechanochemical synthesis is a feasible way to prepare Cu₃BiS₃ nanocrystals in an environmentally friendly manner that are potentially applicable in solar cells.