Nickel and Lithium Doping Face Off in Zinc Oxide Films for Supercapacitors

Zinc oxide is one of the most workhorse materials in modern electronics, a cheap, abundant, transparent semiconductor that shows up everywhere from solar cells to sensors. Now a team of researchers from Pakistan, Saudi Arabia, the United Arab Emirates, Uzbekistan and Tunisia has carried out one of the most complete head-to-head comparisons yet of two dopants vying to upgrade it: nickel and lithium. Their study, published in the Journal of the Saudi Chemical Society, grew three kinds of thin films side by side — pure ZnO, Ni-doped ZnO and Li-doped ZnO — and then subjected all of them to the same battery of structural, optical, wetting, electrical and electrochemical tests. The verdict is nuanced but useful: lithium wins on conductivity and charge storage, while nickel offers its own surprises, including a dramatic light-triggered switch from water-repelling to water-loving behavior.

The films were made with one of the simplest and cheapest techniques in materials science: sol–gel spin coating. The researchers dissolved zinc acetate dihydrate in 2-methoxyethanol with monoethanolamine as a stabilizer, added nickel acetate or lithium acetate at a 5 percent doping level, stirred the solutions at 60 degrees Celsius, and spun them onto glass at 3000 rpm. Ten coating cycles, each followed by a brief bake at 200 degrees, built up films thick enough to study, and a final anneal at 400 degrees for an hour locked in good crystallinity. For the electrochemical measurements, the same recipe was applied to fluorine-doped tin oxide electrodes. The low-temperature, solution-based route matters because it avoids the expensive vacuum equipment of sputtering or spray pyrolysis, keeping the door open for large-area, low-cost manufacturing.

X-ray diffraction confirmed that every film, doped or not, crystallized in the hexagonal wurtzite structure of zinc oxide, with no impurity phases sneaking in. The main diffraction peaks, corresponding to the (100), (002) and (101) crystal planes, shifted slightly to higher angles in the doped samples — a fingerprint that the foreign atoms had actually taken up zinc sites in the lattice rather than clustering into separate compounds. Using Scherrer’s equation on the dominant (002) peak, the team calculated average crystallite diameters of 23.79 nanometers for pure ZnO, 20.82 nanometers for the Ni-doped film and 22.5 nanometers for the Li-doped film. Rietveld refinement of the diffraction data gave a weighted profile factor of 9.35 percent and a goodness-of-fit of 2.56 for the parent compound, indicating a solid structural model.

Under the scanning electron microscope, all three films revealed a wrinkled surface texture studded with nanograins. The wrinkles, the authors suggest, arise from compressive stress that builds up as the wet film dries and the temperature difference between substrate and material drives shrinkage; the dopant atoms add their own tensile stress as they squeeze into zinc sites. Energy-dispersive X-ray spectroscopy verified the presence of zinc, oxygen and, in the nickel case, the dopant itself. Lithium, with an atomic number of just 3, sits below the detection threshold of EDS, which can only resolve elements heavier than atomic number 5, so its incorporation had to be inferred from the X-ray peak shifts instead.

Optically, the undoped film was the star of transparency, transmitting more than 90 percent of visible light. Doping dimmed that performance — below 80 percent for nickel and below 85 percent for lithium — because impurity atoms scatter light and disrupt crystallinity. Yet doping also delivered something valuable: a slight red shift of the absorption edge and a narrowing of the optical band gap, from 3.33 electronvolts in pure ZnO to 3.30 for Ni-ZnO and 3.315 for Li-ZnO, as extracted from Tauc plots. The team attributes the narrowing to higher carrier concentrations introduced by the dopants, with crystallite size, strain and lattice parameters also playing a role. Band gap engineering of this kind is precisely what supercapacitor designers look for, since a smaller gap can improve charge transfer at the electrode surface.

Deeper optical analysis sharpened the contrast between the two dopants. The lithium-doped film kept its refractive index below 2 across the visible spectrum, while the nickel-doped film pushed above 2, and the extinction coefficient — a measure of optical loss — was correspondingly lower for lithium. Packing density, computed from the refractive index, was highest for the Ni-doped film, and the volume and surface energy loss functions showed that energy dissipation occurred mainly inside the films rather than at their surfaces. Optical conductivity rose steadily with photon energy in all samples as photons excited electrons across the gap. Taken together, the optical data mark the lithium-doped film as the better candidate for transparent optoelectronic devices.

The wettability experiments produced perhaps the most striking result. A plain water droplet beads up on the Ni-doped film at a contact angle of 105 degrees — comfortably hydrophobic — while the Li-doped film sits at 85 degrees and pure ZnO at 68 degrees, both hydrophilic. The rougher nickel surface traps air under the droplet, propping it up. But when the films were bathed in 254-nanometer ultraviolet light, whose photons carry more energy than the band gap, all three surfaces flipped toward superhydrophilicity within 5 to 15 minutes as photoexcited electrons trapped water molecules and spread the droplet. The Ni-doped film transformed fastest, its contact angle falling to just 0.07 of its initial value after 15 minutes. Left in the dark, the surfaces recovered their original wettability over 21 to 24 hours, a reversible switch with obvious appeal for self-cleaning coatings on windows and photovoltaic panels.

Electrically, lithium pulled decisively ahead. Four-point probe measurements gave sheet resistances of 32.24, 29.37 and 27.1 megaohms per square for the undoped, Ni-doped and Li-doped films respectively, translating into conductivities of 0.134, 0.147 and 0.159 siemens per meter. Doping populates the material with interstitial atoms and oxygen vacancies that ionize easily and release charge carriers, and the lithium film’s better crystalline quality appears to have let those carriers move more freely. For anyone designing transparent electrodes or electronic devices on top of these films, that difference in conductivity is not a rounding error — it is a meaningful edge.

The electrochemical tests, run in a three-electrode cell with 1 molar potassium hydroxide, showed that doping converts ZnO from a resistor-like electrode into a genuine capacitor. Cyclic voltammetry at scan rates from 10 to 100 millivolts per second produced nearly ideal rectangular curves centered on the zero-current axis for both doped films — the classic signature of an electric double-layer capacitor, where charge is stored by fast, reversible ion adsorption rather than by chemical redox reactions. The curves stayed symmetrical even at the highest scan rates, and their enclosed area grew with scan rate as more ions became available to build the double layer. The Li-doped film enclosed the largest area, and galvanostatic charge–discharge measurements quantified the advantage: specific capacitances of 50.16 farads per gram for Li-ZnO versus 33.54 farads per gram for Ni-ZnO at a current density of 0.2 milliamperes per gram, dropping to 35.84 and 24.2 farads per gram at 0.4 milliamperes per gram. Both figures beat a recently reported value of 31.94 farads per gram for an undoped ZnO film, and the team links the capacitance gain to the same band gap narrowing seen optically, with dopant-induced defects extending charge and discharge times through improved ionic mobility.

The broader lesson is that neither dopant is simply better; each earns its place depending on the job. Lithium-doped ZnO is the pick for thin-film supercapacitors and conductive transparent layers, thanks to its superior conductivity, optical clarity and charge storage. Nickel-doped ZnO, with its hydrophobic-to-superhydrophilic switch and stronger band gap reduction, suits self-cleaning surfaces and applications where tailored absorption matters. And because lithium is relatively scarce in nature, the authors note that nickel could serve as a viable substitute where its particular strengths suffice. For a material as cheap and versatile as zinc oxide, a drop of the right impurity — just 5 percent by weight, added in a beaker at 60 degrees — turns out to be enough to reshape its electronic, optical and surface chemistry in ways that matter for the next generation of energy storage devices.

Subject of Research: Comparative effects of nickel and lithium doping on the structural, optical, electrical and electrochemical properties of sol–gel zinc oxide thin films for energy storage

Article Title: Comparative analysis of Ni and Li doped ZnO with enhanced structural, optical, and electrochemical performance in energy storage applications

Article References: Baig, F., Zaheer, Z., Khan, R., Althubeiti, K., Al Otaibi, S., Abdullaev, S., Elboughdiri, N., & Khan, A. (2026). Comparative analysis of Ni and Li doped ZnO with enhanced structural, optical, and electrochemical performance in energy storage applications. Journal of Saudi Chemical Society, 30(2), Article 20. https://doi.org/10.1007/s44442-026-00056-y

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00056-y

Keywords: zinc oxide, thin films, doping, nickel, lithium, supercapacitors, sol-gel spin coating, band gap, wettability, electric double-layer capacitor, optical properties, energy storage