Using renewable energy sources instead of fossil fuels allows to produce few greenhouse emissions, reducing environmental impact and sustaining the future social and economic social needs [1] . Production and storage are the two important steps in the exploitation of renewable technologies. Storage technologies can be electrical, chemical, electrochemical, mechanical or thermal [2] . Electrochemical techniques related to research and development for clean energy storage gained importance for their strategic value [3] and currently electrochemically generated hydrogen is one of the most encouraging energy storage mediums [4] . Though greatest amount of the hydrogen is existent in molecular forms, it can be yielded exploiting water electrolysis (WE) [5] , where water breaks into H₂ and O₂. The oxygen evolution reaction (OER) occurs at the anode while the hydrogen evolution reaction (HER) occurs at the cathode.

A considerable amount of electrochemical overpotential is required to start HER in normal conditions [6] . Therefore, extremely efficient WE needs the use of an electrocatalyst to reduce the energy barrier associated with HER. Noble metals like Pd or Pt are considered as the best electrocatalysts because of their outstanding activity in acidic environment for HER [7] . Meanwhile, their insufficient availability and high cost caused researchers to look for cost-effective substitutes. Therefore, research concentrated on the look for materials substitute to Pt [8] , which is the most admired catalyst because of its robust stability and high electrocatalytic activity [9] [10] . Currently, alternative cost-effective electrocatalytic materials are unearthed and some of them have been developed to a large extent. Important examples are metal sulphides [11] , phosphides [12] - [17] , selenides [18] , nitrides [19] carbides [19] , some transition metals or alloys [20] [21] and their nanoparticles (NP) [22] [23] .

Nickel phosphide is evaluated as a substitute of Pt-based electrocatalytic materials in the future. Nickel phosphides (Ni₂P) compound showed low HER overpotentials and good stability [12] [24] . Besides highly poisonous phosphine-based chemical reaction, substitute methods to prepare Ni₂P have been established. Specifically, nickel phosphide has been prepared, in the form of nanowires or NPs, via solvothermal [25] [26] or hydrothermal synthesis [27] [28] , by the decomposition of metal-phosphine moieties [24] , and by direct phosphorization of Ni [29] [30] [31] . Other nickel-phosphorus alloys, like Ni₃P or Ni₁₂P₅, are identified by stimulating electrocatalytic properties [12] . Electrodeposition or sputtering can be used to obtain nickel phosphide thin films by direct phosphorization of Ni [32] [33] . However, this synthesis process involves noxious precursors, like phosphine [34] .

Nickel phosphide has a wide range of usages for electrochemical devices [35] [36] [37] . Due to high theoretical specific capacity, much work has been recently done to use nickel phosphide as anode in rechargeable lithium (Li) battery [38] [39] . Recently, the high electrochemical behaviour, great conductivity and thermal stability of transition metal phosphides have made them suitable as supercapacitors [40] [41] . Among different transition metal phosphides, nickel phosphide (NiP) is an important class.

Electroless nickel deposition is a nickel deposition method that takes place in an aqueous solution in the presence of a chemical reducing agent. It involves no external current source. The deposition having a low phosphorus content is better resistant to wear and the hardest, whereas the surface will have a better corrosion resistance if it has a high phosphorus content [42] . Electroless deposition of Ni-P films find extensive application in fields such as medical, aviation, aerospace, automobile, chemical processing, textile, oil and gas industries since they can be grown on substrates of many different materials and with complicated shapes, easily processed, and exhibit excellent resistance to corrosion, good lubricity, and high hardness [43] [44] .

In the present work, nickel phosphide is investigated in the form of thin film. Electroless deposition of Ni-P thin films was obtained using the reducing agent sodium hypophosphite. Then, the resulting alloys were annealed to support P compound formation and interdiffusion. Annealing parameters were optimized to reduce oxidation, to give the best conditions for Ni₂P formation and to reduce the formation of secondary phases. X-ray photoelectron spectroscopy (XPS) is used to identify the existence of oxides on the surface and to determine the chemical state of the elements present in the material [45] [46] [47] . In this research work, XPS is used to investigate the chemical states and surface compositions of as-deposited and annealed Ni-P film. The results would be helpful to enhance surface nature of Ni-P film.

In the research work, the soda-lime glasses (SLG) were used as substrate. They were first cleaned by washing and scrubbing with alconox. Then sonication in acetone and methanol was done for 20 minutes. Afterwards, DI water and isopropanol was used to wash them. Then, N₂ gas was used to dry the substrates. An aqueous solution of 0.2M NiCI₂, 0.3 M NaH₂PO₂ and 0.2 M Na-succinate have been used for electroless deposition. To ensure a good dispersion of precursor materials, the aqueous solution was stirred for the whole duration of the experiment. The substrate temperature was controlled within ± 2˚C of 80˚C by means of a hot plate to which a thermocouple is attached. The duration of deposition was 50 minutes. Hydrochloric acid (HCl) was used to maintain the pH of the solution around 4.

Annealing treatment was performed with the help of an MTI Corporation GSL-1100X furnace. After depositing Ni-P thin films on soda-lime glass (SLG) substrates, they were placed in a quartz boat which is centered in the quartz tube chamber inside the furnace. The system was sealed after placing the quartz tube inside the furnace. High-purity argon gas was used to vacuum purge the chamber for three cycles. The annealing process was done at 350˚C on the substrates for a duration of 60 minutes in argon atmosphere. The annealed Ni-P thin films showed a change in morphology.

XPS was done to investigate the composition of NiP thin films. Monochromatic Al Kα radiation (1486.6 eV) through a Kratos AXIS Ultra DLD XPS system was used to obtain XPS spectra. An electronic neutralization gun was used to remove the charge effect on the sample surface. The base pressure of the system was 5 × 10⁻¹⁰ Torr. The sample was pressed to a disc of size 1 × 13 mm at first and then fixed to the sample-holder. Afterwards, it was degassed in the load lock chamber overnight. After that, XPS study was done on it by removing it to the test chamber. The value of contaminant carbon (C 1s 284.6 eV) was used as a reference to calibrate all binding energy values.

The composition and chemical purity of Ni-P thin films were analyzed by XPS. The typical XPS survey spectrum of as-deposited Ni-P film is showed in Figure 1(a). The peaks arising from Ni 2p, 3s, 3p, Ni Auger, O 1s, C 1s, P 2s and P 2p are clearly seen in the spectrum. No other impurities are observed in the spectrum. The nickel, Ni_LMM, Auger peak is found at 700 eV. High resolution spectra of Ni 2p core level, P 2p core level and O 1s core level are shown in Figure 1(b), 1(c) and 1(d) respectively. The two peaks at 856.4 eV and 873.7 eV can be assigned to the binding energy of Ni 2p_3/2 and 2p_1/2. The separation of Ni 2p doublet is by 17.3 eV. These binding energy values of Ni 2p are characteristic of nickel phosphide in the oxidation state (Ni_xP_yOz; where x, y and z vary) [48] . The P peak at binding energy of 133 eV is characteristic of nickel phosphide in the oxidation state (Ni_xP_yOz; where x, y and z vary) [48] . The resolved peaks of O centered at binding energies of 531 eV and 532.5 eV (Figure 1(d)) corresponds to Ni(OH)₂ and silicon dioxide (SiO₂) respectively [49] . The peaks corresponding to silicon dioxide arise due to glass slides that were used for deposition. However, the peak of the O 1s core level is in the general XPS spectra (Figure 1(a)) suggest that the film was oxidized.

The XPS survey spectrum of annealed Ni-P thin film is showed in Figure 2(a). The peaks arising from Ni 2p, 3s, 3p, Ni Auger, O 1s, C 1s, P 2s and P 2p are clearly seen in the spectrum. No other impurities are observed in the spectrum. The nickel, Ni_LMM, Auger peak is found at 700 eV. High resolution spectra of Ni 2p core level, P 2p core level and O 1s core level are shown in Figure 2(b), Figure 2(c) and Figure 2(d) respectively. The two peaks at 856.5 eV and 873.8 eV can be assigned to the binding energy of Ni 2p_3/2 and 2p_1/2. The separation of Ni 2p doublet is by 17.3 eV. These binding energy values of Ni 2p are characteristic of nickel phosphide in the oxidation state (Ni_xP_yOz; where x, y and z vary) [48] . The peak has shifted about 0.1 eV to higher BE side compared with the peak before annealing. The P peak at binding energy of 133.5 eV is characteristic of nickel phosphide in the oxidation state (Ni_xP_yOz; where x, y and z vary) [48] . The oxidized NiP peak has shifted about 0.5 eV to higher BE side compared with the peak before heat treatment. The resolved peaks of O centered at binding energies of 531 eV and 532.5 eV (Figure 2(d)) corresponds to Ni(OH)₂ and silicon dioxide (SiO₂) respectively [49] . The peaks corresponding to silicon dioxide arise due to glass slides that were used for deposition. However, the peak of the O 1s core level in the general XPS spectra (Figure 2(a)) suggests that the film was oxidized. But the intensity is lower compared to as-deposited film.

In this present research work, the electroless as-deposited and annealed Ni-P film has been analyzed using XPS study. The XPS analysis presents that film contains the elements nickel, phosphorus, oxygen, sodium and carbon. The phosphorus content is increased in the Ni-P film after annealing as observed from the survey spectrum. The P 2p binding energy corresponding to oxidized Ni-P is increased by 0.5 eV after heat treatment. These facts suggest that oxygen is transferred to phosphorus from nickel to form phosphorus oxides after annealing [45] . A chemical shift is observed for Ni (2p3/2) spectra.

The work was supported by the Advanced Support Program for Innovative Research Excellence-(ASPIRE-I), grant number 15530-E404 and Support to Promote Advancement of Research and Creativity (SPARC), grant number 15530-E413 of the University of South Carolina, Columbia, USA.

The author declares no conflicts of interest regarding the publication of this paper.

Chowdhury, T.A. (2024) XPS Studies on Electroless As-Deposited and Annealed Ni-P Films. Engineering, 16, 123-133. https://doi.org/10.4236/eng.2024.165010