Development of Cu3N electrocatalyst for hydrogen evolution reaction in alkaline medium

A wide variety of electrocatalysts has been evolved for hydrogen evolution reaction (HER) and it is reasonable to carry out HER with low cost electrocatalyst and a good efficiency. In this study, Cu3N was synthesized by nitridation of Cu2O and further utilized as an electrocatalyst towards HER. The developed Cu3N electrocatalyst was tested and results showed a low overpotential and moderate Tafel slope value (overpotential: 149.18 mV and Tafel slope 63.28 mV/dec at 10 mA/cm2) in alkaline medium with a charge transfer resistance value as calculated from electrochemical impendence spectroscopy being 1.44 Ω. Further from the experimental results, it was observed that the reaction kinetics was governed by Volmer–Heyrovsky mechanism. Moreover, Cu3N has shown an improved rate of electron transfer and enhanced accessible active sites, due to its structural properties and electrical conductivity. Thus the overall results show an excellent electrochemical performance, leading to a new pathway for the synthesis of low cost electrocatalyst for energy conversion and storage.

www.nature.com/scientificreports/ In general, platinum is the ideal catalyst for HER with the desired characteristics such as low onset potential, Tafel slope and high durability but the high cost and scarcity hampered its large scale application in hydrogen production [13][14][15] . Thus, it is crucial to develop abundant and highly efficient electrocatalysts for large scale hydrogen production. Over the past few decades, research is focused on developing first row transition metals as efficient electrocatalyst for HER 16 . Copper is a promising catalyst and similar analogue to Pt metal, but has a limited activity towards HER due to its deficiency in the capture of H atom [17][18][19][20][21] . Numerous efforts have been taken to synthesize copper with transition metal sulphides, carbides, phosphides and dichalcogenides to overcome this issue for improving HER performance [22][23][24][25][26][27][28][29] . Copper Nitride (Cu 3 N) is a metastable semiconductor that has been proposed as efficient cathodic materials for energy conversion and storage applications, because of their unique physiochemical optical, electrical and its thermal properties [30][31][32] . Cu 3 N has drawn attention in other fields like optical device storage, fuel cells, high-speed ICs, metallic microscopic links, CO 2 reduction, energy storage and energy production [30][31][32] . Various routes have been explored for the reduction of particle size and different morphology of Cu 3 N. For instance, Pereira et al. prepared Cu 3 N from CuF 2 at 300 °C in NH 3 atmosphere. XRD measurements revealed dark green power of Cu 3 N without any trace of oxidation or residual CuF and TEM images exhibited nanodomains of Cu 3 N materials. The obtained Cu 3 N were used as negative electrode for lithium battery application 33 . In recent years, Cu 3 N in the form of thin films have been mainly synthesized by molecular beam epitaxy (MBE), radio frequency (RF), active laser deposition (ALD), ion assisted deposition, ultrasonic plasma spray method and magnetron sputter ion plating. Other preparation method for Cu 3 N particles includes solvothermal and ammonolysis method 32,34 . Deshmukh and co-workers reported the synthesis of ultra-small Cu 3 N nanoparticles via one step reaction between copper (II) methoxide and benzylamine. TEM imaged confirmed that Cu 3 N has ultra-small particle morphology with ~ 2 nm thickness. These Cu 3 N nanoparticles provided pathways for the development of efficient cathode materials to enhance lithium ion batteries application 35 . Here in, Cu 3 N nanoparticles have been explored as an efficient electrocatalyst for electrochemical hydrogen evolution reaction. The prepared Cu 3 N material as electrocatalyst possesses intrinsic HER activity, which might be related to their electronic structure and oxidation state of Cu, resulting in Cu + increasing the electrochemically active surface to enhance hydrogen evolution performance. In this work Cu 3 N nanoparticle were synthesized from nitridation of Cu 2 O and to further confirm the formation and morphology, various investigations were done like XRD, FTIR, SEM and BET measurements. Cu 3 N as electrocatalyst exhibited a considerable catalytic performance of HER in alkaline electrolyte, a reasonable current density of 10 mAcm −2 at an overpotential of 149.18 mV. The good HER performance might owe to the large surface area and favourable electrical conductivity of Cu 3 N particles.

Experimental
All the chemicals and reagents used were of analytical grade and used without any further purification. Copper (II) sulphate pentahydrate (CuSO 4 ·5H 2 O), Sodium hydroxide (NaOH), l-Ascorbic acid (C 6 H 8 O 6 ) were purchased from Sigma-Aldrich Chemicals Pvt. Ltd. and double distilled water was used in the synthesis by using Milli-Q water.
Preparation of Cu 2 O nanoparticles. The synthesis procedure of cuprous oxide (Cu 2 O) was adopted from the previous literature report 36 with slight modifications. Typically, 2 mmol of copper sulphate solution was dissolved in 50 ml of DI water and simultaneously 20 mmol of NaOH was added drop wise into the mixture. Then the mixture was continuously stirred at ambient temperature. Later, a capping agent of 4 mmol ascorbic acid was added into the above solution. Finally the reaction mixture was stirred continuously, stirred for 30 min at ambient temperature. The resultant product turns the solution to brick red colour as given in Fig. 1, which indicated the formation of cuprous oxide (Cu 2 O) nanoparticles. Further the obtained Cu 2 O nanoparticles were washed with DI water and ethanol for several times and dried at 60 °C for 12 h in vacuum oven.

Preparation of Cu 3 N nanoparticles.
The Cu 3 N nanoparticles were prepared via nitridation process of Cu 2 O 37 . Briefly, Cu 2 O nanoparticles was kept in an alumina tube and placed inside a furnace, which was subsequently heated under purified argon at 30 min. The tubular furnace was heated at a temperature of 250 °C for 21 h under ammonia atmosphere. The flow rate of ammonia gas was 60 ml/min for 1.5 h and the product was isolated by centrifugation (7500 rpm for 10 min). The resultant product was transferred into a petri dish, dried at 80 °C for 12 h. Further, Cu 2 O nanoparticles were heated with NH 3 gas of different concentrations at different temperature, which is labelled as Cu 3 N-300/120 ml/min, Cu 3 N-300/160 ml/min and Cu 3 N-250/60 ml/min.

Mechanism of Cu 3 N formation.
Copper sulphate (CuSO 4 ) reacts with NaOH solution in the reaction to form copper hydroxide Cu(OH) 2 . Then ascorbic acid as surfactant was added into copper hydroxide solution leading to the formation of copper oxides (Cu 2 O) 38 . In the last step, Cu 2 O powder was heated in NH 3 atmosphere, which reacts with Cu 2 O to form Cu 3 N. The reaction mechanism for the formation of Cu 2 O and Cu 3 N is given below: Preparation of electrodes. The glassy carbon electrode having a geometrical surface area of 0.07 cm 2 was first polished with alumina slurry of 0.05 micron, followed by rinsing it with DI water, ethanol and acetone. The working electrode was prepared from 5 mg of Cu 3 N catalyst dissolved in 250 µl of ethanol. Later, 5 µl of the catalyst/5 µl of Nafion was pipetted with micro syringe and coated on cleaned glassy carbon electrode (GCE) surface using drop casting method. The coated electrode was then dried at room temperature for 12 h. Electrochemical testing was carried out by CHI 660C electrochemical workstation. Cyclic voltammetry, linear sweep voltammetry, Tafel plot and electrochemical impedance spectroscopy techniques were done to evaluate HER performance.

Results and discussion
Structure and morphology. The nature of crystallinity and phase structure of the synthesized cuprous oxide (Cu 2 O) nanoparticles were confirmed from XRD measurements as given Fig. 2a (210) respectively. In trial-1, Cu 3 N was not found with a trace mount of CuO and residual of Cu in the material. With an enhanced temperature and flow rate of NH 3 gas in trial-2, Cu 2 O was preheated at 300 °C in NH 3 atm (flow rate 120 ml/min), wherein Cu 3 N was not obtained as shown in Fig. 3a,b. Finally, Cu 2 O was preheated at 300 °C in NH 3 atm (flow rate 160 ml/min) and given in Fig. 2b. The diffraction peaks observed at 23°, 33°, 41°, 48°, 54°, 59°, 69° and 74° corresponds to the crystal plane (100), (110), (111), (200), (210), (211), (220) and (300) respectively, which confirm the formation of Cu 3 N nanocrystal as per the JCPDS card No. 47-1088 with a crystalline size of Cu 3 N being 12 nm. The morphology and structural features of the Cu 3 N (300 °C/160 ml/min) were analysed by scanning electron microscopy as given in Fig. 4a,b. Cu 3 N materials are nanoclustered flower like morphology with nanoflowered structure. The www.nature.com/scientificreports/ average particle size was calculated to be 18.8 nm and particles distribution ranged from 30 to 40 nm respectively as given in the inset of Fig. 4b and the corresponding morphology of the Cu 2 O nanoparticles is given in Fig. 4c,d. Figure 5 shows the TEM image of Cu 3 N and from the result the lattice was found to be cubic crystal and further from the SAED pattern, it could be seen that apart from the Cu 3 N pattern, a trace amount of impurities could be seen that might be due to the presence of minor amount of unreacted Cu 2 O but the proportion was very less as observed from XPS. To further investigate the functionality and molecular structure, Fourier transform infrared spectroscopy (FTIR) analysis was carried out for Cu 3 N catalyst. As shown in Fig. 6a, FTIR spectrum of Cu 3 N nanoflower exhibited prominent peaks at 652 cm −1 , which is ascribed to the intrinsic lattice mode vibration of Cu-N. The sharp peaks at 819 cm −1 is assigned to the surface of Cu-N 3 bond. The peak at 2049 cm −1 corresponds to the stretching vibration of N 3 azide confirming the formation of Cu 3 N. Further Raman spectrum was conducted to examine the Cu 3 N electrocatalyst and as given in Fig. 6b, two distinct peaks at 625 cm −1 and 1570 cm −1 correspond to the stretching and bending of Cu-N bond and the peak at 218 cm −1 is assigned to the vibrational mode of Cu. The porosity of electrocatalyst was investigated by nitrogen adsorption-desorption isotherm to understand the accessible surface properties, as shown in Fig. 7. The Brunauer-Emmett-Teller (BET) surface area was calculated to be 70.731 m 2 /g for Cu 3 N obtained at 300 °C/160 ml/min. It shows type II adsorption isotherm and hysteresis loop has been observed, which shows mesoporous pore size structure. The cumulative pore volume was calculated to be 5.448 × 10 -2 cc/g with a diameter pore size of 1.92 nm. This high surface area and micropores can offer efficient active sites and also promote diffusion of ions in the electrolyte to accelerate the electrochemical process of HER. Further TGA analysis was done to understand the thermal stability of the synthesized samples. The thermogravimetric analysis of the material synthesized at various temperatures under N 2 atmosphere was done and given in Fig. 8. As observed from the figure, the TGA curves could be identified into three different weight loss regions. During the first stage, a minor weight loss occurred at a temperature ranging  www.nature.com/scientificreports/ from 0 to 150 °C, which is related to the loss of trapped water molecules. The second stage weight loss occurring at 250 °C is associated to the removal of organic solvents present on the surface of the particle. The third stage weight loss at 400 to 550 °C is due to the thermal decomposition of Cu and N 2 . Moreover, thermogram of Cu 3 N exhibited three weight losses, which is in agreement with the previous reported Cu 3 N materials [32][33][34][35]40,41 . DSC is a very effective characterization tool for analysing the thermal properties and heat capacity of the material and the synthesized Cu 3 N material has an exothermic peak at 520 °C. To further analyse the material, XPS was taken for Cu 2 O and Cu 3 N samples (Fig. 9a,b) and from the figure, it could be observed that Cu-related peaks exhibit a symmetric shape with no satellite peak around 943 eV, ruling out the presence of Cu 2+ . In the deconvoluted XPS spectrum of Cu 3 N, Cu 2p peak at binding energy of 932.4 eV was found with a shoulder around 934 eV. The first peak around 932 eV is attributed to Cu 3 N; two other peaks around 933 eV and 934 eV are attributed to Cu 2p3/2 and Cu 2+ respectively. The former energy is close to the reported value of Cu 3 N from the energy of Cu metal (932.1 eV; not shown), and this slight difference between Cu and Cu 3 N agrees with close binding energies of Cu 0 and Cu 1+ as shown in Fig. 10.
Electrochemical characterization. The electrochemical HER testing was carried out in three electrode cell by using electrochemical workstation (CHI660C instrument) at ambient temperature. Platinum wire, Ag/ AgCl electrode was used as counter and reference electrodes respectively. The catalyst coated glassy carbon electrode was used as working electrode in 1 M NaOH alkaline solution as electrolyte for HER. All the potentials were measured with reference to Ag/AgCl (aq.) electrode and the same was calibrated to the potential versus reversible hydrogen electrode (RHE), in accordance with the equation.
The electrocatalytic activities of Cu 2 O and Cu 3 N towards HER were investigated by cyclic voltammetry at various scan rates (10 mVs −1 to 100 mVs −1 ) in non-faradic current region to evaluate the manifest of electrochemical double layer capacitance (C dl ). HER polarization current was recorded at 2 mVs −1 to determine the onset potential, overpotential, Tafel slope and current density. To improve the electrocatalytic performance of Cu 2 O and Cu 3 N materials in basic medium towards HER, the electron transport and electrochemical surface area are compared (Fig. 11a-d) 38,41 . Thus, ECSA of the catalyst could be directly reflected from the double layer capacitance (cdl) as estimated from the cyclic voltammetry (CV) curves vs. scan rate (10 mVs −1 to 100 mVs −1 ).
EVs.RHE = E vs (Ag/AgCl) + 0.059 pH + 0.199(V ). www.nature.com/scientificreports/ By following McCrory's theory, the capacitance from EDLC was calculated 38,42 and the double layer capacitance value calculated for Cu 2 O and Cu 3 N was calculated to be 0.472 mF cm −2 and 0.803 mF cm −2 in 1 M NaOH. This could be reflected in the higher electrocatalytic activity due to large C dl value. The results indicated that Cu 3 N has higher electrocatalytic activity than Cu 2 O, because Cu 3 N materials have higher electron transfer and conductivity properties. To elucidate the possible kinetic reaction of hydrogen evolution reaction, the involvement of Cu 3 N and Cu 2 O in the reaction is explored using steady state polarization. The linear sweep voltammetry (LSV) curve was recorded at a potential window of − 0.2 V to 0.2 V at a scan rate of 2 mV in 1 M NaOH alkaline medium. In Fig. 12a at the onset potential at 10 mA cm −2 for Cu 3 N and Cu 2 O catalyst, it can be seen that Cu 3 N nanostructure exhibits a remarkable electrocatalytic activity towards HER with onset potential of 0.085 V for      www.nature.com/scientificreports/ during electrochemical process, which might be regarded as active sites for enhancing the electrical conductivity of Cu 3 N nanoflower beneficial for boosting the HER performance. The kinetics reaction of HER was analysed by Tafel plot. The pathway of kinetics for the conversion of (H + to H 2 ) in basic medium in general follows three mechanism viz. Volmer, Heyvosky and Tafel reaction. Volmer is the proton discharge electrosorption (Eq. (8)), electrochemical desorption is the Heyvosky reaction (Eq. (9)) and last step Tafel indicates the recombination of two surface-absorbed H 2 atom (Eq. (10)).
where MH ads represent the absorbed H 2 atom over the surface of the metal and M represents the catalytically active free sites for HER. The Tafel slope was calculated to be 63.28 mVdec −1 and 77.25 mVdec −1 for Cu 3 N and Cu 2 O associated to Volmer-Heyvosky mechanism for the hydrogen evolution. The extrapolation of Tafel plot gives the exchange current density, which was calculated to be 24.2 mA/cm 2 and 11.3 mA/cm 2 respectively. A comparison table of reported Cu 3 N results are discussed in Table 1. Thus, Cu 3 N materials promote electron penetration exposing active sites and mass transfer ability, which suggest the better electrocatalytic activity towards HER. The comparison of HER activity of Cu 2 O and Cu 3 N are given in Table 2.
Electrochemical impedance spectroscopy (EIS) measurements were further done to analyse the interfacial properties of the as obtained electrocatalyst. As given in Fig. 12c,d, the semicircle in the high frequency area of the Nyquist plot was ascribed to the charge transfer resistance (Rct) and higher value of Rct denotes slow reaction rate and lower value of Rct denotes faster reaction rate. www.nature.com/scientificreports/ The cyclic stability test was conducted using linear sweep voltammetry from − 0.2 to 0.2 V and from the result, it was observed that the stability of Cu 3 N was good compared to the Cu 2 O as given in Fig. 13a,b. Overall results show that the synthesized Cu 3 N is an effective catalyst for electrochemical HER (Fig. 14).

Conclusion
In summary, Cu 3 N was synthesized successfully from nitridation of Cu 2 O nanoparticles. The electrochemical hydrogen evolution reaction was carried out using Cu 3 N in alkaline medium in 1 M NaOH. By using Cu 3 N as electrocatalyst, a low Tafel slope of 63.28 mV/decade with a low overpotential of 149.18 mV was observed, which follows Volmer-Heyrovsky reaction mechanism. Thus overall results show that the catalyst has good electrocatalytic activity for HER thus making it a potential candidate for cost effective catalysts in electrochemical hydrogen production.