Modulation of electrocatalytic activity by tuning anion electronegativity: case study with copper chalcogenides

Anion-tuning in metallic chalcogenides has been shown to have a significant impact on their electrocatalytic ability for overall water splitting. In this article, copper-based chalcogenides (Cu2 X, X= O, S, Se, and Te) have been systematically studied to examine the effect of decreasing anion electronegativity and increasing covalency on the electrocatalytic performance. Among the copper chalcogenides, Cu2Te has the highest oxygen evolution reaction (OER) activity and can sustain high current density of 10 and 50 mA cm−2 for 12 h. The difference in intrinsic catalytic activity of these chalcogenide surfaces have been also probed through density functional theory calculations, which was used to estimate energy of the catalyst activation step. It was observed that the hydroxyl adsorption on the surface catalytic site is critically important for the onset and progress of OER activity. Consequently, it was also observed that the –OH adsorption energy can be used as a simple but accurate descriptor to explain the catalytic efficiency through volcano-like correlation plot. Such observation will have a significant impact on developing design principle for optimal catalytic surface exhibiting high performance as well as prolonged stability.

Transition metal chalcogenides have become increasingly important in the field of electrocatalysis for their unprecedented high efficiency in oxygen evolution reaction (OER) [1][2][3][4][5][6].This reaction comprises a critical step for several electrochemical energy conversion processes including water splitting, fuel cells, and clean hydrogen generation [7].Transition metal chalcogenides have been found to be particularly effective in promoting electrochemical energy conversion due to their unique electronic structure that facilitates interfacial charge transfer through modulation of electron density around the catalytically active site [8].Transition metal chalcogenides typically containing one or more transition elements such as iron, cobalt, or nickel, combined with a chalcogen atom such as sulfur, selenium, or tellurium, frequently form layered structure [9].Such layered morphology are expected to exhibit improved electrocatalytic efficiency owing to better accessibility to active sites, and facile intermediate adsorption on the surface.Such properties also lead to improved OER activity.Moreover, the electronic structure of metal chalcogenides is highly tunable through anion modulation, which leads to further tuning of their electrocatalytic properties [3,10,11].This is especially useful for optimizing the OER, as the reaction is highly sensitive to the presence of certain metals and chalcogen atoms.Over the past several years, intense research has led to the identification of several transition metal chalcogenides which shows highly efficient OER activity [12][13][14][15].Several transition metal chalcogenides comprising, Ni, Co, and Fe have also been identified from our previous research [8,[16][17][18][19].During the course of these studies, it was further revealed that the anion electronegativity had a critical role to play in the observed OER electrocatalytic efficiency.Specifically, the electronegativity of the coordinated anion affects the ionicity or covalent strength of the M-X bond (M-X, where M refers to transition metal and X = S, Se, Te), respectively.The electrochemical redox of the transition metal site is more impacted by the covalent character of the M-X bonding, which ameliorates with bond strength [20].Because the degree of covalency in transition metal tellurides is higher than that of the corresponding selenides, most transition metal tellurides have the intrinsic property of enhanced metallicity, which can lead to better conductivity and charge transfer rate of the transition metal tellurides [21].Additionally, transition metal chalcogenides are relatively inexpensive compared to precious metal based OER catalysts, which makes them attractive for large-scale applications.Moreover, metal chalcogenides exhibit excellent chemical stability, which is important for maintaining the activity of the electrode over time [16].The chalcogens in the compounds form strong covalent bonds with the metal atoms, which renders the compounds resistant to corrosion or degradation.Improvements in electrocatalyst performance have been proposed through the use of a wide range of control mechanisms, including the development of hierarchical structures, the doping of heteroatoms and the processing of alloys [17,[22][23][24][25]. Few studies have also investigated the effect of anion species on the performance of water splitting electrocatalysts [16,18,[26][27][28].
In our previous study, we have reported the effect of varying anion covalency in Ni-based chalcogenide [18].Specifically, it was observed that electrocatalyst efficiency increased significantly down the chalcogen series resulting in telluride showing the highest OER efficiency.The electrocatalytic activity trend in these chalcogenides could be also comprehended in a manner akin to the Sabatier principle [29] by following the hydroxyl (-OH) adsorption energy on the catalyst surface.The initial -OH adsorption on the surface preceded by local oxidation of the active site comprises the activation step and in our previous studies we have proposed that this -OH adsorption energy can be used as a descriptor and correlated with the observed OER catalytic activity.Nevertheless, a systematic study of the variation of OER catalytic activity solely as a function of anion electronegativity and its correlation with changes in -OH adsorption energy is still missing.However, such insight is critically needed for developing effective catalyst design principles by elucidating how surface anion coordination alters catalyst activation and further charge transfer across the interface.By exploring the direct correlation between anion electronegativity and -OH adsorption energy in various catalytic systems, we can gain deeper insights into the reaction mechanisms, surface intermediates, and reaction kinetics involved in the OER.
In this article, we have investigated the variation of inherent electrocatalytic activity for a series of copper chalcogenides with generic formula, Cu 2 X (X = O, S, Se, and Te).These copper chalcogenides show similar crystal structure.However, the OER catalytic activity shows a significant enhancement with decreasing anion electronegativity with Cu 2 Te showing the best activity.Estimation of the -OH adsorption energy through density functional theory (DFT) calculations revealed its clear correlation with OER activity further confirming the appropriateness of using -OH adsorption energy as a descriptor for the OER efficiency.Furthermore, these copper chalcogenides can be optimized to provide both high catalytic activity and stability.The novelty of our research lies in the systematic study of the variation of OER catalytic activity based solely on the anion electronegativity.To the best of our knowledge, such a comprehensive analysis had been missing from the existing literature, and our work fills this critical gap.By understanding the impact of anion electronegativity on catalytic performance, we aim to unravel the underlying principles governing OER and contribute to the rational design of highly efficient catalyst surfaces.

Materials and method
The reagents were analytical grade and used as received.Copper (II) chloride dihydrate (CuCl

Synthesis of Cu 2 Se:
On the basis of the synthesis approach described in our earlier paper [10], copper selenide was synthesized by an enhanced hydrothermal process.In a typical synthesis procedure, powdered copper oxide (Cu 2 O) and selenium (Se) were mixed in an equimolar ratio.The original molar ratio of Cu 2 O to Se was chosen to be 1:1.First, 8 ml of deionized water was used to dissolve Cu 2 O (0.001 M) in a Teflon-lined 23 ml acid digestion bomb.Se (0.001 M) was added to the mixture after mixing vigorously for 20 min.Finally, 3 ml of N 2 H 4. H 2 O were added to the mixture, followed by 20 min of continuous stirring.The resulting solution was sealed in a stainless-steel autoclave lined with Teflon and heated at 185 • C for 24 h.After that, the autoclave was allowed to cool to room temperature naturally.The final product was then centrifuged and repeatedly washed with an ethanol and deionized water solution.Finally, a vacuum oven at 60 • C was used to dry the final product.
Synthesis of Cu 2 Te: Cu 2 Te were also synthesized following a similar protocol as above.In a typical synthesis procedure, powdered copper chloride (CuCl 2 .6H 2 O) and tellurium (Te) were mixed in a molar ratio of 2:1.Both Cu and Te precursors were added in 6 M KOH solution.Finally, 3 ml of N 2 H 4 .H 2 O were added to the mixture, followed by 20 min of continuous stirring.The final mixture was sealed and heated at 175 • C for 24 h in a Teflon-lined stainless-steel autoclave.The resultant products were recovered using centrifugation, and the final product was dried by heating at 60 • C .

Synthesis of Cu 2 S:
In a typical synthesis procedure, CuSO 4 .5H 2 O (0.02 mol) was added to 8 ml of DMSO while the mixture was continuously stirred for 20 min to result in a clear, green solution.This mixture was poured in a Teflon-sealed autoclave and kept at 175 • C for 6 h.The resultant products were recovered using centrifugation, cleaned with ethanol and finally a vacuum oven at 60 • C was used to dry the final product.

Synthesis of Cu 2 O:
Typically, 1 mmol Cu(CH 3 COO) 2 and 0.5 mmol glucose are dissolved in 8 ml of distilled water and stirred vigorously to form a blue dispersion.The resultant liquid was placed to a Teflon-lined stainless steel autoclave (23 ml).The autoclave was sealed and placed to 100 • C oven for 12 h before naturally cooling to ambient temperature.The resultant red solid products were recovered using centrifugation, cleaned with ethanol and finally a vacuum oven at 60 • C was used to dry the final product.
Electrode preparation: A homogenous catalyst ink was made by mixing 300 µl of ethanol with 0.8 ml of Nafion (5% by weight) and 2 mg of catalyst powder in order to study the OER activity of the hydrothermally synthesized catalyst.This mixture was homogenized into an ink using an ultrasonicator for about an hour.100 µl of the ink was drop cast onto CC (geometric area of 0.283 cm −2 ).

Characterization technique
The product was analyzed using powder x-ray diffraction (PXRD) with a Philips X-Pert and CuKα (1.5418 Å) radiation without any further purification.By employing a scanning electron microscope (SEM, Helios Hydra) with an acceleration voltage of 15 kV and a working distance of 10 mm, the morphology of the samples was examined.X-ray photoelectron spectroscopy (XPS) was used to determine the valence states of the elements and the chemical composition of the catalystsboth before and after the OER stability studies.X-ray photoelectron spectroscopy (XPS) analyses were conducted with a KRATOS AXIS 165 instrument, utilizing a monochromatic Al X-ray source.All XPS investigations were performed on the pristine catalyst surface without sputtering.An IviumStat potentiostat was used for all electrochemical studies.A graphite rod functioned as the counter electrode, saturated calomel electrode (SCE) as the reference electrode, and the copper-based chalcogenides loaded on CC substrate functioned as the working electrode during the electrochemical studies.Linear sweep voltammetry (LSV) curves were acquired at a scan rate of 5 mV s −1 and the electrochemical workstation automatically corrected the LSV via iR compensation.Electrochemical impedance spectroscopy (EIS) was used to examine the charge transfer resistance from 0.01 Hz to 100 kHz.

Tafel slope
The Tafel slope, which is determined by fitting polarization data to the Tafel equation, is a crucial measure for evaluating OER activity.The dependence of the overpotential on the current density j is represented by the Tafel equation, as shown in equation ( 1): where n is the number of electrons involved in the reaction, α is the transfer coefficient, and F is the Faraday constant.The Tafel slope is given by 2.3RT/αnF.The formula for converting potentials measured with a SCE to reversible hydrogen electrode (RHE) potentials is: where, E(RHE) is the potential in RHE scale.E(SCE) is the potential measured with the SCE.E • (SCE) is the standard potential of the SCE, which is typically considered to be 0.241 V at 25 • C. 0.059 is the conversion factor representing the Faraday constant (F) divided by 1000 (F/1000) to convert from volts to millivolts.
Here, pH represents the pH value of the solution under consideration.

Computational details
The first-principles computations were carried out using the DFT as implemented in the Vienna ab initio simulation package [30].The generalized gradient approximation using the Perdew-Burke-Ernzerhof function ((PAW)) was used to define the exchange correlation approximation [31,32].For the anticipated augmented wave approach, the plane-wave cut-off energy was 520 eV.To prevent erroneous interactions between periodic images, a supercell of 3 × 3 × 3 matrix was built with the vacuum layer of 10 Å.The Brillouin zone integrations were performed using a 3 × 3 × 1 Monkhorst-Pack grid for (001) surfaces.A conjugate gradient algorithm was used to iteratively relax all the ions, and the atomic forces of all systems were reduced to less than 0.01 eV.During the structure relaxation process, a Methfessel-Paxton smearing with a value of σ = 0.05 eV is applied to the orbital occupation.On the other hand, an accurate computation of the electronic structure was performed using a tetrahedron approach with Blöch corrections.

Results and discussion
The series of copper chalcogenides of generic formula, Cu 2 X (X = S, Se, Te) along with Cu 2 O were synthesized through one-pot solvothermal technique at temperatures between 175  [16].According to the previously reported copper selenide, the Se 3d spectra of the as-synthesized Cu 2 Se (figure 2(f)) exhibit two distinct peaks at 54.4 and 55.4 eV for Se 3d 5/2 and 3d 3/2 , respectively [34].In figure 2(g), the binding energies of S 2-in Cu 2 S were measured to be 161.2 and 162.5 eV, with a 1.2 eV gap between them and an additional peak was detected at 168.6 eV which could be attributed to the partial oxidation of surface sulfur to SO 4 2− species [35].The XPS peaks hence confirmed the formation of the copper chalcogenides containing Cu +2 and the chalcogenide anion.
The electrocatalytic activity for copper chalcogenides for OER was measured in a 1.0 M KOH alkaline solution using the Cu 2 X-modified electrodes.Cu 2 Te demonstrated the highest OER activity compared to the other Cu-chalcogenides, as shown in figure 3(a).Cu 2 Te requires a low overpotential (η) of 255 mV to achieve the current density of 10 mA cm −2 .The other chalcogenides required an overpotential of 297, 334, 379 mV at 10 mA cm −2 for Cu 2 Se, Cu 2 S and Cu 2 O, respectively as shown in figure 3(b).
The blank substrate did not show any OER activity as expected, confirming that the activity observed for the catalyst-loaded electrodes was solely due to the intrinsic catalytic activity of these Cu-chalcogenides.The OER electrocatalytic activity of the Cu-chalcogenides was also compared with that of state-of-the-art RuO 2 catalyst.The comparison of the LSV plots clearly demonstrated that all the copper chalcogenides have higher OER catalytic activity than RuO 2 , while Cu 2 O showed comparable activity.The kinetics of the OER activity was estimated from the Tafel plots by calculating the Tafel slope from the LSV plots near onset of the OER activity.The Tafel plot can also be used to determine the transfer coefficient, which is a measure of the rate of electron transfer between the electrode and the electrolyte.
The transfer coefficient is determined by the intercept of the Tafel plot on the x-axis.In figure 3   figure 4(a), the Cu 2 Te catalyst demonstrated excellent stability over an extended period of time at different applied potentials of 1.49 V and 1.58 V vs RHE with minimum performance degradation.Also, the chronoamperometric measurements of the Cu 2 Te material was measured at higher current densities of 260 mA cm −2 as shown in figure S1(a), to simulate real-world conditions more closely for water electrolysis.Following the extended testing protocol, it was observed that the Cu 2 Te electrocatalyst indeed exhibits commendable stability for OER.After 24 h of continuous OER activity, only a minor deterioration of 4.233% was observed.A multi-step chronoamperometry test was also carried out to investigate stability of the catalysts at various current densities.Figure 4(b) shows the multistep chronoamperometry experiments conducted at range of constant applied potentials.Current density was measured for over two hours at each potential and the applied potentials were initially changed in ascending order from 1.36 V to 1.62 V against RHE, and subsequently in descending order from 1.62 V to 1.48 V vs RHE.As can be seen from figure 4(b), Cu 2 Te showed stable current density at all applied potential in the forward and reverse potential sweep, confirming extensive cycling as well as operational stability.The overpotential change of Cu 2 Te is negligible in the LSV curve obtained after 24 h of multistep chronoamperometry (figure 4(c)), further confirming the exceptional robustness of this catalyst.XPS and of the Cu 2 Te were measured after 24 h of multistep chronoamperometry, in order to examine its compositional stability.XPS measured after long-term chronoamperometry (figure 4(d)) was found to be essentially unchanged when compared to that of the pristine sample, which indicates that the Cu 2 Te catalyst composition was indeed stable for prolonged period of time.The post-OER XPS analysis of Cu 2 Se and Cu 2 S was also carried out and as shown in figure S2, they  The electrochemical surface area (ECSA) is an essential parameter when evaluating the performance of electrocatalysts.It refers to the total active area available for electrocatalysis on the surface of an electrode.A higher ECSA usually implies more active sites available for the reaction, leading to enhanced catalytic activity.The ECSA of an electrode material can be significantly influenced by its composition, crystallinity, morphology, and particle size.In the case of the metal chalcogenides reported here (Cu 2 Te, Cu 2 Se, Cu 2 S, and Cu 2 O), the differences in ECSA can be attributed to their structural differences and material properties.
The ECSA was examined using cyclic voltammetry plots, scanned in the range from 0 to 0.25 V with scan rates between 25 -200 mV.s −1 in N 2 saturated 1 M KOH solution (figures 5(a)-(d).The anodic and cathodic currents were measured at 0.15 V vs Ag/AgCl for Cu 2 Te, Cu 2 Se, Cu 2 S and 0.075 for Cu 2 O as shown in figures 5(e)-(h) which shows the anodic and cathodic current plotted as a function of scan rates.The detailed calculation of ECSA has been described in the supplementary information file.Table S1 shows the ECSA value of each catalyst where Cu 2 Te, has the highest ECSA (24.48), while Cu 2 O, exhibit the lowest ECSA (9.36).Since the ECSA of these chalcogenides varies in the series, the ECSA-normalized current density has also been analyzed to compare the intrinsic catalytic activity across the series.As shown in figure S1(b), Cu 2 Te electrode exhibited the highest current density and OER activity even after normalizing the LSV curves by ECSA.This observation unequivocally points to an enhancement in the intrinsic OER activity of the active sites due to variations in anion electronegativity, with Cu 2 Te demonstrating the most superior activity.
Electroimpedance spectroscopy (EIS) was carried out to investigate the charge transfer rates at the various interfaces of the catalyst-electrode composite, specifically, the charge transfer resistance (R CT ) at the electrode-electrolyte interface.R CT is critically dependent on the material composition and band-edge alignment of the electrocatalyst with the water-oxidation level.Hence, it is expected that R CT will show variation as the catalyst composition changes between different chalcogenides.A lower R CT depicts faster electron transfer at the electrode-electrolyte interface.The EIS spectra for all Cu-chalcogenides along with the equivalent circuit is shown in figure 6 while table 1 lists the estimated values for each component of the equivalent circuit derived from the Nyquist plots.
Comparison of the R CT values showed that the Cu 2 Te electrode exhibited a much lower charge transfer resistance (13.24) than that of Cu 2 Se (24.6), Cu 2 S (34.4), and Cu 2 O (43.8) during the OER process.The trend of R CT corroborates well with the observed OER activity which shows a gradual improvement on going from oxide to telluride.The transition metal tellurides are mostly metallic while their bandgap increases   from telluride to oxide.Hence, copper oxides typically have a larger bandgap compared to copper selenide, while copper telluride shows metallic character.The metallic character in the tellurides enhances electron transfer within the catalyst composite as well as at the electrode-electrolyte interface, thereby, improving the electrocatalytic activity.A comparison with other Cu-based chalcogenides showed that the as-synthesized Cu 2 Te demonstrated highest catalytic activity (table 2).Calculations based on DFT were carried out in order to acquire additional insights of the OER mechanism on the copper chalcogenide surfaces and understand the enhanced activity of the copper telluride.Conventionally OER in basic medium involves four elementary steps on the catalytically active transition metal sites on the surface, generating three different intermediate adsorbates: * OH, * O, and * OOH (the asterisk denotes the adsorption site).Figure 7 shows the calculated free energy profiles of the different OER steps on the Cu site in Cu 2 Te.Apparently, the highest energy barrier is observed for the formation of dioxygen species from hydroperoxide.However, these energy profiles represent adsorption of a single oxygenated species on the catalytically active site and provides a good estimation of the adsorption energies that can be compared between different catalytic surfaces.It should also be noted that in the actual OER mechanism on the catalyst surface, the oxygenated intermediates are expected to adsorb on all exposed In our previous studies we have proposed that the -OH adsorption energy can be used as an appropriate descriptor for OER catalytic efficiency [18,46,47], especially since the OER mechanism involves intermediate adsorption and desorption from only the catalytic site without forming a bridging connection with neighboring lattice sites.We have used DFT calculations to estimate the OH adsorption energy on the Cu sites of (001) lattice planes in Cu 2 O, Cu 2 S, Cu 2 Se, and Cu 2 Te as shown in figure 8.In order to perform the calculation of the OH-adsorption energy, first the surfaces were relaxed to obtain the energy of the clean surfaces.Then OH-ions were placed on top of the active metal site, and the system was relaxed to calculate the total formation energy of the system.The relaxed surface of Cu 2 Te and other copper-based systems after the OH-attachment on the (001) surface is shown in figure S3.The adsorption energy of OH-ions, E ad , was derived as E ad = E sys -E clean -E OH , where E OH is the energy of free OH -.The OH adsorption energy on the (001) surface of Cu 2 O, Cu 2 S, Cu 2 Se, and Cu 2 Te was found to be −1.095,−1.76, −2.075 2.38 eV, respectively, as indicated in figure 8(a).The higher OH adsorption energy on Cu 2 Te surface suggests that catalyst activation and surface coverage by OH is more favorable on Cu 2 Te surface.Typically, the activation of the catalyst comprises attachment of the hydroxide ion to the surface, which initiates OER [48,49].We have further correlated the -OH adsorption energy with the observed overpotential by constructing a volcano-type plot as shown in figure 8(b).Along with Cu-chalcogenides, several other high-efficiency OER electrocatalyst has been compared in the volcano plot as shown in figure 8(b).Interestingly it was observed that the -OH adsorption energy indeed shows a gradual increase as a function of decreasing anion electronegativity corresponding to enhancement of OER activity (reducing overpotential).These observations across many Cu, Co, and Ni-based catalysts confirm that the OH adsorption energy on the catalyst surface can indeed serve as a simple descriptor for OER activity.
Figure S4 shows the partial density of states for the Cu 3d-state before and after OH attachment.Here the occupied spin-up and spin-down Cu states move to the lower negative energy state upon bonding with OH -, as a result of the weaker Cu-Te interactions.In addition, the attachment of the OH ion to Cu 2 Te surfaces results in a redistribution of charges because of electronic hybridization between the orbitals of the adsorbate and the adsorbent.Charge density redistribution frequently occurs in conjunction with adsorption.As a result, we have used charge analysis to investigate the charge density difference in order to interpret the adsorption mechanism.Figures 8(c

Conclusion
To summarize, the effect of anion electronegativity in improving OER catalytic activity was confirmed through a systematic investigation of the copper chalcogenide series.Cu 2 Te showed the lowest overpotential compared to Cu 2 Se, Cu 2 S, and Cu 2 O depicting best OER efficiency.DFT calculations revealed that the enhanced activity could be attributed to the facile -OH adsorption on the catalytic site in Cu 2 Te.It also confirmed that the -OH adsorption energy on single catalyst site can be used an appropriate descriptor for the observed OER catalytic efficiency.Such insight regarding the effect of anion electronegativity and possibility of using the adsorption energy as a descriptor for predicting catalytic efficiency will be extremely useful for optimal catalyst surface design.
(c), Cu 2 O, Cu 2 S, Cu 2 Se and Cu 2 Te exhibited the Tafel slope values of 138.61, 131.13, 126.43 and 74.28 mV dec −1 , respectively.Typically, lower Tafel slopes indicates faster electron transfer and more efficient catalytic activity.Comparison of Tafel slope for all the Cu-chalcogenides revealed that Cu 2 Te shows the lowest value indicating most effective electrocatalytic activity.The stability of these catalysts was tested by carrying our continuous OER for several hours at constant applied potential through chronoamperometry studies.As shown in

Figure 1 .
Figure 1.PXRD patterns for (a) Cu2Te (b) Cu2Se (c) Cu2S, and (d) Cu2O showing good agreement with the standard PXRD patterns as included in the respective plots.

Figure 4 .
Figure 4. (a) Chronoamperometry at different applied potentials for Cu2Te.(b) Multistep chronoamperometry at different applied potential steps (ascending and descending).(c) LSV of Cu2Te before and after 24 h of multistep-chronoamperometry.(d) High resolution XPS spectra of Cu 2p, and Te 3d (inset) before and after multistep chronoamperometry.

Figure 6 .
Figure 6.Nyquist impedance plots of as-synthesized Cu based catalysts.Inset represents the equivalent circuit model used for AC impedance fitting.

Figure 7 .
Figure 7.The calculated OER free energy diagram on Cu2Te surface.
)-(f) depict the differential charge densities of adsorbed OH on Cu sites of and Cu 2 Te, Cu 2 Se, Cu 2 S, Cu 2 O and respectively.According to the integrated charge density difference, the Cu-OH layer is surrounded by an apparent electron accumulation at Cu atoms (sky-blue region) and an electron reduction at O atoms (yellow region), leading to new electronic states at the Cu-OH heterointerface.The largest electron cloud, depicted in figure 8(c), indicates a high rate of electron transfer and robust contact between * OH and the Cu 2 Te catalyst surface; this is supported by the value of the adsorption energy, which is negative (−2.38 eV).
[33]d 185 • .The series of copper chalcogenides, Cu 2 Te, Cu 2 Se, Cu 2 S and Cu 2 O were characterized through various structural, microscopy, and surface analysis techniques.Figures1(a)-(d)shows the crystalline PXRD patterns of the hydrothermally synthesized series of copper-based chalcogenides.Each of these phases matched well with their corresponding standard diffraction patterns as shown in the corresponding PXRD plots, which also illustrates phase purity of these samples.The sharp diffraction peaks indicate high degree of crystallinity.Scanning electron microscopy (SEM) was used to study the morphological characteristics of Cu based metal chalcogenides.The SEM images of all four samples, as shown in inset of figures 2(a)-(d) and S1, showed globular-like nanoparticles with particle sizes ranging from 30 to 120 nm.XPS was used to analyze oxidation state of the elements and phase composition for these chalcogenides.In order to investigate the XPS survey spectrum for the Cu 2 X sample, C1s peak (284.6 eV) was used as a reference.The high resolution XPS spectra of Cu in figures 2(a)-(d) shows that there is a spin-orbit splitting of 19.7 eV between the two peaks centered at 933.5 eV and 953.2 eV for all four copper based chalcogenides (Cu 2 Te, Cu 2 Se, Cu 2 S, Cu 2 O, respectively), which are identified as Cu 2p3/2 and Cu 2p1/2, respectively[33].Compositions containing Cu in 2+ oxidation state with a d 9 ground state configuration are distinguished by Cu satellite peaks at 941.5 and 962.3 eV.It has been determined that the peaks at 576.6 eV and 586.8 eV in high resolution spectra of Te 3d correspond to Te 3d5/2 and Te 3d3/2, respectively (figure 2(e))[33].The spin-orbit separation of Te 3d5/2 and 3d3/2 is 10.2 eV.While the remaining two weak peaks at 573.3 eV and 583.7 eV indicate the presence of zero-valent Te

Table 1 .
Equivalent circuit parameters obtained from fitting of EIS experimental data.