Growth evolution and phase transition from chalcocite to digenite in nanocrystalline copper sulfide: Morphological, optical and electrical properties

Summary Copper sulfide is a promising p-type inorganic semiconductor for optoelectronic devices such as solar cells, due its small band gap energy and its electrical properties. In this work nanocrystalline copper sulfide (CuxS), with two stoichiometric ratios (x = 2, 1.8) was obtained by one-pot synthesis at 220, 230, 240 and 260 °C in an organic solvent and amorphous CuxS was obtained in aqueous solution. Nanoparticle-like nucleation centers are formed at lower temperatures (220 °C), mixtures of morphologies (nanorods, nanodisks and nanoprisms) are seen at 230 and 240 °C, in which the nanodisks are predominant, while big hexagonal/prismatic crystals are obtained at 260 °C according to TEM results. A mixture of chalcocite and digenite phases was found at 230 and 240 °C, while a clear transition to a pure digenite phase was seen at 260 °C. The evolution of morphology and transition of phases is consistent to the electrical, optical, and morphological properties of the copper sulfide. In fact, digenite Cu1.8S is less resistive (346 Ω/sq) and has a lower energy band gap (1.6 eV) than chalcocite Cu2S (5.72 × 105 Ω/sq, 1.87 eV). Low resistivity was also obtained in CuxS synthesized in aqueous solution, despite its amorphous structure. All CuxS products could be promising for optoelectronic applications.

and has a lower energy band gap (1.6 eV) than chalcocite Cu 2 S (5.72 × 10 5 Ω/sq, 1.87 eV). Low resistivity was also obtained in Cu x S synthesized in aqueous solution, despite its amorphous structure. All Cu x S products could be promising for optoelectronic applications.

Introduction
Metallic chalcogenides based on cadmium, such as cadmium telluride, CdTe, or cadmium sulfide, CdS, have been widely investigated regarding their application in the optoelectronic field, mainly in photovoltaic devices due to the semiconducting, electronic and optical properties [1][2][3][4][5]. Cadmium is a toxic heavy metal, which limits its applications in the optoelectronic area. In fact, the current trend is to develop environmentfriendly nanometric semiconductors with adequate optoelectronic properties for solar cells. It is well known that all properties (physical, chemical, magnetic) of nanometric materials differ from the bulk semiconductor due to the quantum effects [6]. Among the non-toxic nanomaterials with a small energy band gap that are promising for photovoltaic devices are: iron sulfide (FeS 2 ), tungsten sulfide (WS 2 ) and copper sulfide (Cu 2 S) [7]. The last is a terrestrially abundant and interesting semiconductor due to its stoichiometric variety usually depicted as Cu x S. Copper-rich sulfides (Cu 2 S), Cu x S with x = 0.03, 0.2, 0.25, and CuS are widely reported [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27]. The stoichiometric ratio can be tailored by changing the concentration of copper or sulfide precursors, the reaction parameters and the kind of solvents. The following phases were obtained: djurleite (Cu 1.97 S), digenite (Cu 1.8 S) or analite (Cu 1.75 S) . These crystalline phases are stable p-type compounds, which could be used as absorber materials in solar cells [30][31][32]. However, the exact identification of the crystalline structure is controversial due to the stock of 86 XRD patterns for Cu x S, some of which have reflections with narrowly spaced positions (see Table 1). This proximity makes it difficult to clearly assign diffraction patterns to certain crystalline phases.
On the other hand, the control of size, shape, distribution and stoichiometry of Cu x S is an essential challenge nowadays, because these parameters are dependent on several factors [12,13,15,18,21]. For example, the reaction temperature modified the shape, size and optical properties of monodisperse Cu 2 S obtained from a simple one-pot route [15]. In fact, there exists wide research about the synthesis of copper sulfide nanostructures obtaining different Cu/S ratios [9,11,16,20,[23][24][25][26]. However, the lack of knowledge about the growth evolution and the phase transitions of copper sulfide is the motivation of this work.
In this work, the growth evolution and the phase transition of copper sulfide in the temperature range from 220 to 260 °C in an organic solvent is reported. The full electrical, morpholog-ical and optical properties of these crystalline samples synthesized in the organic solvent were compared with the amorphous Cu x S obtained from aqueous solution.

Results and Discussion
Structural properties from X-ray diffraction The structural properties of the copper sulfide samples (Cu x S) depend on the synthesis and the reaction temperature ( Figure 1). A fully amorphous product is obtained from aqueous solution according to the X-ray diffraction pattern ( Figure S1 in Supporting Information File 1). However, the crystallinity of organic products is dependent on the temperature reaction. At 220 °C, Cu x S presents three peaks with low intensity at 2θ = 38, 46.5 and 49° corresponding to the chalcocite structure (JCPDS 31-0482) (Figure 1a). Above a temperature of 230 °C, the Cu x S product is more crystalline. There are four peaks with broadening and better intensity at 2θ = 37.84, 46.5, 48.82, and 54.94°, which match both to the chalcocite (JCPDS 31-0482) phase and djurleite phase (JCPDS 20-0365). At 240 °C (Figure 1b) Three shapes of unit cells of Cu 2 S chalcocite phase can be presented: monoclinic (low chalcocite), hexagonal (high chalcocite), and cubic (cubic chalcocite) [30]. It is well known that the transformation from monoclinic (α,γ-Cu 2 S) to hexagonal (β-Cu 2 S) occurs at 103.5 °C and 101.8 °C for bulk and nanostructure chalcocite, respectively [33]. According to Machani et al. [34] the monoclinic phase changes to djurleite in ambient air and the real phase obtained is djurleite instead of chalcocite, even though, the chalcocite phase is usually reported [8,[12][13][14][15]. In fact, the djurleite phase is obtained in ambient air [18], while chalcocite is obtained under argon atmosphere [14]. So, the products reported here obtained at 220 °C and 230 °C really are the chalcocite phase despite some peaks which match with djurleite. In fact, the Cu x S products maintained the crystalline phases after we stored them for one year at room temperature, which is indicative of a good stability of the Cu 2 S chalcocite and Cu 1.8 S digenite phases (results not shown here).     The grain size and stress of the crystalline copper sulfide samples from organic synthesis at 230-260 °C were obtained from the full widths at a half maximum (FWHM) of the diffraction peaks and the linear combination of the following equation [35]: where β is the FWHM measured in radians, θ the Bragg angle of the peaks, λ the XRD wavelength, in our case in nanometers (λ = 0.154 nm), D is the effective crystallite size, and ε is the effective strain. A plot of β cos(θ)/λ versus sin(θ)/λ for all the samples gives the grain size and the strain, as shown in Figure  S2 in Supporting Information File 1. The intercept is the inverse of the grain size and the slope is the strain, respectively. The grain size increases as the temperature increases (24.5 to 28.3 nm), the effective strain decreases in the samples shown that the least stress was at 260 °C (−8.26 × 10 −5 ) and the highest was at 230 °C (−2.73 × 10 −3 ).

Morphology from TEM and HRTEM
TEM images revel that amorphous Cu x S from aqueous solution is constituted of nanometric particles with undefined shape that are agglomerated into clusters (See Figure S3 in Supporting Information File 1), which is in concordance with Cu x S obtained in similar aqueous systems [21].
The morphology of Cu x S samples from organic solution depends on the reaction temperature, for example irregular particles below 10 nm can be observed for Cu In order to verify the full transition of the digenite phase an HRTEM analysis of the crystals was made. The distance between the lines in the HRTEM image ( Figure 3) is approximately 0.32 nm. This corresponds to the (0015) plane spacing of the digenite phase, which matches the peak of 46% of intensity in the XRD pattern shown in Figure 1b. The diffraction pattern of electrons obtained by the Fourier transformation (inset of Figure 3) shows an interplanar distance of about 0.197 nm, close to the value 0.19644 nm for the (110) spacing of the digenite phase (the peak for 100% intensity in the XRD pattern).
From TEM images, it can be observed that the phase transformation occurs from 220 to 260 °C and involves three stages: the

Cu/S ratio from EDS
The EDS patterns shows two peaks at 0.9 and 8.0 keV attributed to Cu Kα and Cu Lα emission, while a third peak at 2.3 keV is due to the S Kα emission.

Optical properties
The optical absorbance spectra of the Cu x S are shown in Figure 4. Both, the amorphous sample from aqueous synthesis and the chalcocite Cu x S from organic synthesis at 220 °C, present a weak and broad absorption band at approximately 500 nm. However, crystalline Cu x S samples show a welldefined absorbance band between 490 to 600 nm. In fact, a red shift of about 40 to 60 nm is presented from the chalcocite (Cu 2 S) to the digenite phases (Cu 1.8 S), which is in agreement to the increment of crystal size. This phenomenon is related to the free charges due to the copper deficiency in the samples. For  example, the maximum absorbance band has been reported at 450 nm for Cu 2 S, while it is observed at longer wavelength (950 nm) for CuS [36]. It is clear, that the deficiencies of copper generate a displacement or shift of the optical absorption, which is consistent to the transition of the phases.
The energy band gaps of the samples were computed by the Tauc plot for direct transition ( Figure 5). The indirect plot (inset) did not present a satisfactory straight-line region for all samples. The Cu x S sample prepared in aqueous solution shows an E g about 2.2 eV for the direct and 2.0 eV for the indirect transition, respectively (see inset of Figure 5). This is coherent with the value of 2.3 eV reported for crystalline or amorphous CuS covellite thin films from an aqueous solution [25,37]. On the other hand, the direct E g values of the Cu x S samples prepared in the organic solvent are in the range of 1.57-1.87 eV. These values are adequate for an optical absorption in the visible region, which makes the samples very promising materials for solar cell applications. In Table 2 we observe a clear decrease of E g from 1.87 to 1.60 eV from crystalline chalcocite to the digenite phase, which is in agreement to the increasing crystal size observed with TEM. These values are slightly smaller to those reported for bulk copper sulfide (1.7 and 2.0 eV) [38], so, it is consistent to the size of the nanostructures. On the other hand, an effect was found for chalcocite crystals, namely a shift into the UV region was observed and consequently, large E g values were obtained at high deposition times without modifying the chalcocite phase [13].

Electrical properties
The Cu x S films prepared in aqueous solution are amorphous with undefined morphology. They exhibit a low square electrical resistivity (about 10 3 Ω/sq) as shown in Figure 6. Chalcocite Cu x S from organic solution has a resistance of the order of 10 5 -10 6 Ω/sq, while crystalline Cu x S has a resistivity of about 10 7 Ω/sq at 240 °C and 10 2 Ω/sq at 260 °C, respectively. In fact, the samples obtained at 230 and 240 °C, which consist of a mixture of chalcocite and digenite phases, are more resistive than the digenite phase (sample at 260 °C). This means that the copper deficiency improves the conductivity of the Cu x S, which is consistent to the reports in the literature [20]. Deficient copper structures like analite (Cu 1.75 S) have been grown onto the surface of CuS thin films, which improved their conductivity [28].
The time-photo-current response of Cu x S is reported for the first time (Figure 7). It is clear that the amorphous Cu x S presents a low photosensitivity in contrast to the crystalline Cu x S samples obtained from organic solution, which are  slightly photosensible, suggesting a photo-generation of carrier charges. The current increases gradually as a function of the time exposed to the light, this is attributed to the recombination of charges due to the superficial states in the Cu x S samples.

Mechanism of the formation and phase transition
According to the results presented above, a formation mechanism of the growth and the phase transition from chalcocite to digenite is proposed (Figure 8). It is clear that the nucleation of the crystals begins at 220 °C. It is a key to ensure the growth of nanoparticles at initial stages of the reaction. Above this temperature chains of aligned nanorods are formed and other crystals, nanodisks and prisms, grow. The chains of nanorods are predominant at 230 °C while nanodisks and prisms are the main morphology at 240 °C. A full phase transition from chalcocite to digenite is obtained at 260 °C.
Wang et al. obtained nanodisks of chalcocite Cu 2 S at 220 °C [15]. But, in our case, this temperature is the first stage to the phase transformation from the chalcocite to the digenite phase. According to Wang et al., the growth and rearrangement of the nanodisks are dependent on the concentration of precursors, amount of surfactant, the reaction temperature, and the reaction time. We found that this rearrangement of nanodisks is necessary for the transition of the digenite phase and it is induced only by the temperature.
On the other hand, the amorphous structure of Cu x S prepared from aqueous solution is consistent to its synthesis at low temperatures [37], during which the CuS crystalline covellite phase can be formed above 200 °C [24,25], and the tailoring of the Cu/S stoichiometric ratio and the phase transformation had been reached at temperatures between 230 to 700 °C [21]. Grozdanov and Najdoski found that the electrical sheet resistance decreases as the copper content decreased [25]. This is consistent with our results.

Conclusion
Copper sulfide with 2 and 1.8 of Cu/S ratio were synthesized successfully from chemical synthesis in an organic solvent at 220-260 °C. Amorphous Cu x S was also obtained from aqueous solution at low temperatures with a low electrical resistance, indicative of a high conductivity. The evolution growth, formation of nanostructures, and phase transition were completely described in a scheme based on the TEM images. The full phase transition from chalcocite to digenite is obtained at 260 °C in an organic media. It is clear that the optical and electrical properties are suitable for optoelectronic applications, such as solar cells.

Experimental
Crystalline copper sulfide nanostructures were obtained by onepot synthesis in an organic solvent while raising the reaction temperature from 220 to 260 °C. Amorphous copper sulfide was also synthesized by a chemical reaction in aqueous solution at 40 °C. Films, colloid and powder products were obtained from both reactions.

Synthesis of amorphous copper sulfide from aqueous solution
In this reaction thiourea and copper(II) sulfate pentahydrate (CuSO 4 ·5H 2 O) were the sulfur and copper precursors, respectively, and the TEA ligand was an intermediary in the reaction. The synthesis proceeded as follows: A three-necked reactor containing 440 mL of deionized water was placed on a hot plate with magnetic stirring at 40 °C for 30 min. Clean Corning glass substrates were immersed inside the reactor in order to obtain the films by in situ deposition. Subsequently 1.3389 g of CuSO 4 ·5H 2 O, previously dissolved in 20 mL of deionized water (1.3389 g/20 mL), 0.4354 g/14.5 mL of NaCOOCH 3 and, 5.18 mL/20 mL of TEA. Finally, 0.2 g/31 mL of H 2 NCSNH 2 was added in three aliquots each for 25 min. The substrates were withdrawn from the reactor and rinsed with deionized water. The precipitated products were washed with deionized water three times, immediately they were centrifuged and dried at room temperature. Both films and powder products, received a thermal treatment at 100 °C in air in a stove during 1 h.

Characterization
Powders of two syntheses, aqueous and organic, respectively, of Cu x S were re-dispersed in isopropanol and toluene. One aliquot from these solutions was placed on carbon-coated copper grids for characterization by TEM, in a JEOL JEM-1010 at 80 kV of acceleration potential. Additionally, thin films of aqueous and organic syntheses of Cu x S were characterized by X-ray diffraction (Rigaku, MiniFlex, Cu Kα 1.54 Å and 2θ from 10 to 70°, rate 2°/min each 0.02 s), electrically by the four-points-probe technique, by UV-vis spectroscopy (Thermo Scientific Genesys 10S UV-vis spectrophotometer in the range of 200 to 1100 nm) in order to determine, the structural phase, the electrical resistance and optical absorbance spectra, respectively. The photoresponse measurements were made by applying a potential of 1 V at the sample: 20 s in darkness, 50 s under illumination and another 50 s in darkness. For this, two rectangular metallic contacts (0.5 × 0.2 cm) were painted on the surface of the films with silver paint in a square sample of 0.5 cm 2 .
Energy dispersive X-ray spectroscopy (EDS) was carried out in a JSM-6060LV SEM at 20 keV by using KBr pellets containing granules of Cu x S powder to make the punctual analysis.