NiO Surface Research Articles

Effect of α-FeOOH in KOH Electrolytes on the Activity of NiO Electrodes in Alkaline Water Electrolysis for the Oxygen Evolution Reaction

Iron cation impurities reportedly enhance the oxygen evolution reaction (OER) activity of Ni-based catalysts, and the enhancement of OER activity by Fe cations has been extensively studied. Meanwhile, Fe salts, such as iron hydroxide and iron oxyhydroxide, in the electrolyte improve the OER performance, but the distinct roles of Fe cations and Fe salts have not been fully clarified or differentiated. In this study, NiO electrodes were synthesized, and their OER performance was evaluated in KOH electrolytes containing goethite (α-FeOOH). Unlike Fe cations, which enhance the performance via incorporation into the NiO structure, α-FeOOH boosts OER activity by adsorbing onto the electrode surface. Surface analysis revealed trace amounts of α-FeOOH on the NiO surface, indicating that physical contact alone enables α-FeOOH to adsorb onto NiO. Moreover, interactions between α-FeOOH and NiO were observed, suggesting their potential role in OER activity enhancement. These findings suggest that Fe salts in the electrolyte influence OER performance and should be considered in the development of OER electrodes.

Dopant effects on the environment-dependent chemical properties of NiO(100) surfaces

NiO is a promising material for applications such as electronic devices and catalysts due to its low cost, environmental friendliness, and optimal chemical and electronic properties. Inclusion of a dopant element into the NiO near surface structure can further tune material properties. Despite extensive studies on doped NiO (M−NiO) materials, there remains a lack of knowledge regarding the connection between dopant, environment-dependent dominant near surface structures, and overall chemical properties. Here, the dopant effects on the near surface structure and electronic properties on M−NiO(100) are systematically examined using a combination of density functional theory (DFT) and ab initio phase diagrams. The simultaneous factors tested include dopant element (Al, Mo, Nb, Sn, Ti, V, W, or Zr), dopant placement (surface or subsurface), Ni/O vacancies (surface or subsurface), and oxygen species (O* or O2*) adsorption. The results reveal the dominant structures and compositions of M−NiO(100) under a wide range of environmental conditions (i.e. varying temperature and pressure). Subsequent electronic analyses of the dominant M−NiO(100) structures show non-uniform changes in charge redistribution, band gap, work function, and d-band center with dopant and near surface structure, emphasizing the role of dopants in tuning M−NiO(100) both geometric and electronic properties. Overall, these multiscale modeling results enable a rapid, effective, and a priori prediction of dominant M−NiO(100) structures with distinct chemical properties, crucial for guiding NiO-based materials design with enhanced performance for advanced electronics, energy storage, and catalytic systems.

Regulating Local Coordination Sphere of Ir Single Atoms at the Atomic Interface for Efficient Oxygen Evolution Reaction.

Single-atom catalysts dispersed on an oxide support are essential for overcoming the sluggishness of the oxygen evolution reaction (OER). However, the durability of most metal single-atoms is compromised under harsh OER conditions due to their low coordination (weak metal-support interactions) and excessive disruption of metal-Olattice bonds to enable lattice oxygen participation, leading to metal dissolution and hindering their practical applicability. Herein, we systematically regulate the local coordination of Irsingle-atoms at the atomic level to enhance the performance of the OER by precisely modulating their steric localization on the NiO surface. Compared to conventional Irsingle-atoms adsorbed on NiO surface, the atomic Ir atoms partially embedded within the NiO surface (Iremb-NiO) exhibit a 2-fold increase in Ir-Ni second-shell interaction revealed by X-ray absorption spectroscopy (XAS), suggesting stronger metal-support interactions. Remarkably, Iremb-NiO with tailored coordination sphere exhibits excellent alkaline OER mass activity and long-term durability (degradation rate: ∼1 mV/h), outperforming commercial IrO2 (∼26 mV/h) and conventional Irsingle-atoms on NiO (∼7 mV/h). Comprehensive operando X-ray absorption and Raman spectroscopies, along with pH-dependence activity tests, identified high-valence atomic Ir sites embedded on the NiOOH surface during the OER followed the lattice oxygen mechanism, thereby circumventing the traditional linear scaling relationships. Moreover, the enhanced Ir-Ni second-shell interaction in Iremb-NiO plays a crucial role in imparting structural rigidity to Ir single-atoms, thereby mitigating Ir-dissolution and ensuring superior OER kinetics alongside sustained durability.

Facile synthesis of highly basic, nanocrystalline Fe2O3-NiO composites as promising and durable catalysts for selective dehydrogenation of 2-butanol.

Nanocrystalline Fe2O3-NiO composite catalysts were prepared using a sonication-assisted green preparation method. The prepared catalysts were characterized using different techniques, including thermal analyses (TGA/DTA), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, surface area measurements (SBET), and scanning electron microscopy (SEM). The surface basicity of the prepared catalysts was measured using the temperature-programmed desorption of CO2 (CO2-TPD) as a highly acidic probe molecule. The catalytic activity of all the prepared catalysts was tested at a temperature range of 250-325 °C towards the dehydrogenation of 2-butanol to methyl-ethyl ketone (MEK), which is considered a promising fossil fuel alternative and has several industrial applications. The composite catalysts showed better catalytic activity compared to the pure oxides (i.e., Fe2O3 and NiO) due to the strong synergetic effect between the two oxides. Fe2O3 prevented the coke formation over the surface of NiO by the oxygen-scavenging effect of Fe, which promotes the oxidation of the carbonaceous species and increases the catalyst's resistance to deactivation. The effect of weight hourly space velocity (WHSV) on the catalytic activity was tested over a selected catalyst. In addition, the stability and durability of the catalyst were tested across four successive reaction cycles, demonstrating remarkable performance throughout all the reaction cycles.

Nickel nanocatalyst on lanthanum aluminate: Optimizing preparation and evaluating performance in glycerol dry reforming for syngas production

In this study, mesoporous nanocrystalline Ni/La-Al catalysts with varying nickel (Ni) loadings (5, 10, 15, and 20 wt.%) were synthesized using a solid-state method for the glycerol dry reforming (GDR) reaction. The effects of different Ni loadings and the addition of steam on the structural and catalytic performance of the catalysts were investigated. Characterization techniques, including Brunauer-Emmett-Teller (BET) surface area analysis, temperature-programmed reduction (TPR), temperature-programmed oxidation (TPO), and X-ray diffraction (XRD) were employed to analyze the catalysts' properties. Results showed that increasing Ni loading to 10 wt.% improves the catalytic properties. However, by increasing the Ni loading, a decrease in catalytic properties due to increasing particle size, creating NiO bulk phase and surface pore blockage is obvious. A series of experiments were performed to investigate the effect of steam usage on catalytic properties. TPO analysis revealed that steam addition reduces the carbon deposition and shifts oxidation peaks to lower temperatures; consequently, the activity and stability of the catalyst will be enhanced. Although the 5 - 7.5 vol.% of steam in the feed composition significantly stabilizes the catalyst, it is possible to produce H2-rich synthesis gas, and conversely, increasing the temperature reduces the H2/CO ratio. Various examinations were conducted under different conditions, including varying gas hourly space velocity (GHSV), carbon-to-glycerol ratio (CGR), and temperature, to obtain more comprehensive data on the catalysts.

Role of NiO in wide-bandgap perovskite solar cells based on self-assembled monolayers

NiO/self-assembled monolayer (SAM) double hole transport layers (HTLs) has become the mainstream choice in high-efficiency single-junction and tandem perovskite solar cells (PSCs). However, the underlying role of NiO in double HTLs from the microscale is not systematically revealed currently. Herein, we reveal that NiO plays an important role in inducing a more uniform and thinner SAM from three aspects. Firstly, compared with indium tin oxide (ITO), the NiO surface exposes less crystal facet orientations and a higher metal atom density, suggesting that sufficient chemical binding sites for SAMs are provided to facilitate the close-packed assembly of SAMs. Secondly, NiO modified ITO exhibits smaller surficial roughness and strongly bonded −OH, which is beneficial to the uniform distribution of SAMs and strong adhesion to the NiO surface. Utilizing the well-constructed SAM, we fabricate wide-bandgap (>1.75 eV) PSCs using vacuum-assisted technology and achieve an impressive photoelectric conversion efficiency of 19.55 %. These findings not only deepen the understanding of SAM assembly on substrates but also provide essential guidance for the future industrialization of PSCs.

Mitigation mechanisms of ash deposition and adhesion by different coatings on heated surfaces at macro and molecular scales

Research on the mitigation of ash deposition on coals rich in alkali and alkaline earth metals using coatings is limited. Existing research methods, mainly focusing on ash composition and melting characteristics, cannot fully elucidate the key mechanisms underlying ash adhesion properties on various coated surfaces. Therefore, this study explored the mitigation mechanisms of nickel and chromium coatings on ash deposition using a drop tube furnace, custom-made ash adhesion strength tester, and molecular dynamics. The results revealed that nickel and chromium coatings effectively mitigated ash deposition, with a more significant effect at temperature less than 1000 °C. Above 1000 °C, the ash melting led to a significant increase in ash adhesion strength, diminishing the inhibitory effect of coatings. The key minerals, Na2SO4, Ca2MgSi2O7, and Ca2Al2SiO7, mainly physically adsorbed onto the NiO(001) and Cr2O3(001) surfaces. The binding energy of these minerals with NiO(001) and Cr2O3(001) was significantly lower than that on α-Fe2O3(110) surfaces. The α-Fe2O3(110) system was more stable, and minerals readily adhered to the α-Fe2O3(110) surface, whereas none of these minerals readily bound to NiO(001) and Cr2O3(001), maintaining their adhesion state. Consequently, nickel and chromium coatings demonstrated excellent resistance to ash adhesion and deposition, with nickel coatings being more effective.

NiO surface reduction by Nd overlayers

Metal/metal oxide interfaces have significant impact on modern technologies like microelectronics, solar cells, magnetic storage, and sensors due to their distinct electronic and structural properties. Such interfaces sometimes deviate from ideal bulk-termination which influences the system's overall performance. Here, we present the study of the interfacial electronic structure formed by the deposition of Nd layers onto NiO thin films utilizing X-ray photoelectron spectroscopy (XPS) and Ultraviolet photoemission spectroscopy (UPS). Our study indicates that the interface between the rare earth metal (Nd) and NiO differs from the ideal sharp interface. The portion of NiO surface reduced to metallic Ni while the deposited Nd metal is oxidized, indicating the interfacial oxidation/reduction process. The thickness of this interfacial Ni layer increases with increase in Nd metal deposited and reaches maximum depth of 3.0 Å when effective Nd overlayer thickness reaches 7.9 Å. Such an interface reaction could be utilized in heterostructures where an intentional reduction of the oxides is desired.

Regulating Lattice Oxygen on the Surfaces of Porous Single-Crystalline NiO for Stabilized and Enhanced CO Oxidation

The long-range ordered lattice structure and interconnected porous microstructure of porous single crystals (PSCs) provide structural regularity and connectivity in remote electron movement to stabilize oxygen vacancies and activate lattice oxygen linked to surface active sites. In this work, we prepare NiO powder, single-crystal (SC) NiO, and PSC NiO. NiO contains a significant amount of oxygen vacancies. We find that the structure of porous NiO can create more oxygen vacancies. We load Pt onto these NiO crystals by atomic layer deposition (ALD) to activate lattice oxygen on definite NiO surfaces. The results show that Pt-loaded NiO effectively exhibits CO oxidation performance, in which Pt-loaded PSC NiO completely oxidizes CO at 65 °C. With 1% CO fully adsorbed, the density of activate lattice oxygen becomes an essential factor affecting performance. PSC NiO with deposited Pt clusters exhibited stable CO oxidation catalysis when run in air at ~65 °C for 300 h.

Production of synthesis gas from dry reforming of propane with carbon dioxide over ceria-promoted nickel foam catalysts

A series of nickel foam supported cerai catalysts (ceria/NiF) for dry reforming of propane with carbon dioxide were prepared and their performance were evaluated at temperatures in the range of 520-600°C. Among the catalysts examined the 4 wt.% CeO2/NiF exhibited better low-temperature activity for the DRP, which is attributed to the formation of well- isolated nanosize CeO2 particles and highly reducible metal oxide species over the NiF support. The XPS and temperature programmed reduction (TPR) data also supported the incorporation of ceria into the NiO surface layer formed over NiF support, which was substantially higher in 4% CeO2/NF catalyst. Due to these favorable properties, the 4 wt.% CeO2/NiF could be considered as an excellent catalyst for DRP.

On the VOC loss in NiO-based inverted metal halide perovskite solar cells

This study assesses the effect of key NiO properties, especially resistivity and surface energy, on the device's performance.

Enhanced electrochemical performance of NiO surfaces via selective Li+ doping.

The effects of selective Li+ doping on NiO films' structural, optical and electrochemical properties were investigated in this study. Our results highlighted a strong correlation among bandgap modulation, conductivity and selective Li-ion doping on the surfaces of NiO films. Electrochemical impedance spectroscopy (EIS) demonstrated the impact of subtle Li+ doping on the electrochemical behavior of the NiO thin films due to an increase in the diffusion resistance (Rd) up to 49.32 Ω for 0.5% Li-doped NiO, suggesting faster and more efficient hole transport from the electrode to electrolyte unlike NiO:Li4.0% electrodes. A linear and symmetric forward curve demonstrating ohmic behaviour was observed only for 0.5% Li-doped NiO thin films during current-voltage (I-V) analysis. Interestingly, the 0.5% Li-doped NiO films, which exhibited the largest anodic peak area in the CV curve under illumination, outperformed the 4.0% Li-doped NiO films in electrochemical activity, emphasizing the effectiveness of minimal doping.

(Invited) Limitation of Molecular Twisting: Upgrading a Donor-Acceptor Dye to Drive H2 Evolution

Dye-sensitized photoelectrochemical cells (DSPECs) have attracted wide attention to convert solar energy into high-energy chemicals. A DSPEC consists of a photoanode performing the oxidation half reaction and aphotocathode performing the reduction half reaction. In this presentation I will demonstrate how the performance of a dye-sensitized photocathode can be manipulated to enhance performance and even produce hydrogen! We have used a state- of-the-art dye (D-A P1 dye) on the surface of optimized NiO, and co-adsorbed myristic acid (Tetradecanoic Acid), which has a carboxyl anchoring group and a long apolar alkyl chain. Time-resolved photoluminescence and Density Functional Theory studies show that twisting lowers the energy levels of the photoexcited D-A dye, while twisting is inhibited in case myristic acid is co-adsorbed on the NiO surface. The presence of myristic acid also favors light-induced charge separation, as apparent from femtosecond transient absorption, and increases the apparent photocurrent. Very interestingly, only in the presence of myristic acid, light-induced H2 evolution is observed in aqueous media, despite the absence of a H2 evolution catalyst. We assign the H2 generation to a synergetic effect of inhibited twisting of the D-A dye radical anion increasing its electrochemical potential, combined with charge transfer and conversion of H+ on the hydroxylated NiO surface. Our work illustrates the importance of understanding effects of photoinduced intramolecular twisting and demonstrates that control thereof offers a simple design approach for efficient solar fuel devices.

Theoretical Study of the Reactive Mechanisms of Li-Doped Ni-Based Oxygen Carrier during Chemical Looping Combustion.

Ni-based oxygen carriers (OCs) are considered promising materials in the chemical looping combustion (CLC) process. However, the reactivity of Ni-based OCs still offers the potential for further enhancement. In this work, the Li doping method has been employed for the modification of Ni-based OCs. The reactivity and microreaction mechanisms of different concentrations of Li-doped Ni-based OCs with CO in CLC are clarified using density functional theory (DFT) simulation. The structures, energy, and density of states are obtained through computational investigation of the reaction path in elementary reactions. The results show that (1) the adsorption energies of CO molecules on NiO surfaces with 4, 8, and 12% Li doping concentrations are -0.53, -0.48, and -0.54 eV, respectively, demonstrating an enhanced reactivity compared to that of pure NiO (-0.41 eV); (2) the calculation of the transition state indicates that the most favorable pathway for CO oxidation takes place on the surface of NiO with an 8% Li doping concentration, exhibiting the lowest energy barrier of 0.51 eV; and (3) the oxygen vacancy formation energies on the surface of NiO are 3.05, 2.30, and 2.10 eV for 4, 8, and 12% doping concentrations, respectively. Additionally, the decrease in oxygen vacancy formation energies exhibits a gradual decline with an increasing Li doping concentration. By comprehensive analysis, 8% is considered to be the optimal doping concentration of NiO for chemical looping combustion.

Rotaxane-Functionalized Dyes for Charge-Rectification in p-Type Photoelectrochemical Devices.

A supramolecular photovoltaic strategy is applied to enhance power conversion efficiencies (PCE) of photoelectrochemical devices by suppressing electron-hole recombination after photoinduced electron transfer (PET). Here, the author exploit supramolecular localization of the redox mediator-in close proximity to the dye-through a rotaxane topology, reducing electron-hole recombination in p-type dye-sensitized solar cells (p-DSSCs). Dye PRotaxane features 1,5-dioxynaphthalene recognition sites (DNP-arms) with a mechanically-interlocked macrocyclic redox mediator naphthalene diimide macrocycle (3-NDI-ring), stoppering synthetically via click chemistry. The control molecule PStopper has stoppered DNP-arms, preventing rotaxane formation with the 3-NDI-ring. Transient absorption and time-resolved fluorescence spectroscopy studies show ultrafast (211 ± 7 fs and 2.92 ± 0.05ps) PET from the dye-moiety of PRotaxane to its mechanically interlocked 3-NDI-ring-acceptor, slowing down the electron-hole recombination on NiO surfaces compared to the analogue . p-DSSCs employing PRotaxane (PCE = 0.07%) demonstrate a 30% PCE increase compared to PStopper (PCE = 0.05%) devices, combining enhancements in both open-circuit voltages (VOC = 0.43 vs 0.36V) and short-circuit photocurrent density (JSC = -0.39vs -0.34mA cm-2 ). Electrochemical impedance spectroscopy shows that PRotaxane devices exhibit hole lifetimes (τh ) approaching 1 s, a 16-fold improvement compared to traditional I- /I3 - -based systems (τh = 50ms), demonstrating the benefits obtained upon nanoengineering of interfacial dye-regeneration at the photocathode.

Ab-initio electronic, magnetic, and optical properties of Fe-phthalocyanine on NiO(001)

The adsorption of organic molecules on antiferromagnetic surfaces forms interfaces with potential applications in organic spintronics devices. Molecules modify the dispersion of spin excitations in the substrate through charge transfer and crystal deformations, and offer the possibility to couple them to light excitations. Here, we follow an ab initio approach to the study of the interface between the magnetic organo-metallic molecule Fe-phthalocyanine and the (001) surface of NiO. By applying Hubbard-corrected density-functional theory (DFT+U) calculations we determine the most stable adsorption configuration as that with the molecule lying flat with Fe above a surface O atom. We find a strong hybridization between Fe orbitals and surface ones, with the Fe spin coupled antiferromagnetically to subsurface Ni through the O atom. Moderate changes to the magnetic structure of the components and charge displacements are shown. Optical spectra show reduced absorption onsets and are investigated for the possibility of a coupling with the system spin properties.

Influence of Transition Metal Elements on Ni Migration in Solid Oxide Cell Fuel Electrodes

In the present study, the electrochemical performance and microstructure evolutions of Ni-M (M = Fe, Cu, Mn) bimetallic fuel electrodes were investigated under SOFC and SOEC operations. Ni-Fe, Ni-Mn, Ni-Cu and pure Ni patterned fuel electrodes were sputtered on YSZ pellets. During the SOFC tests, the electrochemical performance of all Ni-M bimetallic fuel electrodes were lower than pure Ni electrode, while the degradation rates of Ni-Fe and Ni-Mn electrodes were smaller than the others. The spreading of Ni film on YSZ surface was observed for all samples, and such Ni migration was suppressed by Fe and Mn addition, whereas it was enhanced by Cu addition. During the SOEC tests, the cell performance degraded with Cu and Fe addition, but improved with Mn addition. The adhesion between Ni film and YSZ substrate was enhanced by doping Fe and Mn, which correlated well with the inhibited degradation in both fuel cell and electrolysis operations. The Ni migration phenomenon is discussed by the strength of Ni-O bond, surface tension and melting point, which are influenced by the addition of transition metal elements.

Electrochemical Activation of Atomic-Layer-Deposited Nickel Oxide for Water Oxidation.

NiO-based electrocatalysts, known for their high activity, stability, and low cost in alkaline media, are recognized as promising candidates for the oxygen evolution reaction (OER). In parallel, atomic layer deposition (ALD) is actively researched for its ability to provide precise control over the synthesis of ultrathin electrocatalytic films, including film thickness, conformality, and chemical composition. This study examines how NiO bulk and surface properties affect the electrocatalytic performance for the OER while focusing on the prolonged electrochemical activation process. Two ALD methods, namely, plasma-assisted and thermal ALD, are employed as tools to deposit NiO films. Cyclic voltammetry analysis of ∼10 nm films in 1.0 M KOH solution reveals a multistep electrochemical activation process accompanied by phase transformation and delamination of activated nanostructures. The plasma-assisted ALD NiO film exhibits three times higher current density at 1.8 V vs RHE than its thermal ALD counterpart due to enhanced β-NiOOH formation during activation, thereby improving the OER activity. Additionally, the rougher surface formed during activation enhanced the overall catalytic activity of the films. The goal is to unravel the relationship between material properties and the performance of the resulting OER, specifically focusing on how the design of the material by ALD can lead to the enhancement of its electrocatalytic performance.

Quench-induced surface engineering of NiO/NiSex hybrid for highly enhanced electrochemical oxygen evolution reaction

To achieve sustainable production of hydrogen fuel through electrochemical water splitting, efficient and low-cost electrocatalysts for the kinetically sluggish oxygen evolution reaction (OER) are urgently desirable. Herein, a calcination-quenching strategy to synthesize core-shell NiO@NiSex nanorods (NiSex is the core and NiO is the shell) with tailored surface compositions and structures is reported. In the calcination step, a NiO@NiSex nanostructure supported on nickel foam (NF) (denoted as NiO@NiSex/NF) was formed via oxidative calcination of highly conductive NiSe in situ grown on NF (denoted as NiSe/NF). Subsequently, the as-obtained NiO@NiSex/NF electrode at the high temperature state was rapidly placed in a cold iron salt solution to achieve the quenching process. The results conclusively demonstrated that the quenching process resulted in iron doping and vacancies generation on the surface of NiO, effectively reconfiguring the desired surface of the catalyst, thus giving rise to notably enhanced electrocatalytic performance for OER in alkaline media. As a result, the quenched-engineered NiO@NiSex/NF electrode requires ultralow overpotentials of 231 and 265 mV to yield current densities of 10 and 100 mA cm−2, respectively, and presents a very low Tafel slope of 28.9 mV dec−1 as well as excellent durability, with the performance superior to most of metal oxide catalysts reported to date. This work extends the use of quenching chemistry in fabrication of high-performance metal oxide catalysts for energy related applications.

Selective Wet and Dry Etching of NiO over β-Ga2O3

In this study, a reaction-limited chemical wet etching process of NiO by an HNO3-based solution, with a complete selectivity over β-Ga2O3 was investigated. The threshold ion energies for inductively coupled plasmas (ICP) dry etching of NiO in BCl3 discharges, its selectivity to β-Ga2O3, and the cleaning effect for chlorine-based residues remaining on the surface after plasma exposure was also examined. One of the major drawbacks of β-Ga2O3 device design is the lack of p-type dopants. An alternative approach is to use a p-type oxide, NiO, to form heterojunctions with n-type Ga2O3. Therefore, a selective wet and dry etching process development is in demand for the optimized pattern transfer process. A solution of 1 HNO3: 4 H2O exhibited measurable etch rates for NiO above 40°C and wet etching activation energy of 172.9 kJ.mol-1 (41.3 kCal.mol-1, 1.8 eV/atom), which is firmly in the reaction-limited regime. The selectivity over β-Ga2O3 was infinite for temperatures up to 55 °C. The strong negative enthalpy for producing the etch product Ga(OH)4 suggests that HNO3-based wet etching of NiO occurs via the formation and dissolution of hydroxides. The BCl3/Ar ICP etching produced maximum etch rates up to 300 Å.min-1 for NiO and 800 Å.min-1 for β-Ga2O3 under moderate plasma power conditions. The selectivity for NiO: Ga2O3 was < 1 under all conditions. The ion energy threshold for the initiation of etching NiO was between 35-60 eV, depending on the plasma power conditions. And its etching mechanism was ion-driven, as determined by the linear dependence between etch rate and the square root of ion energy incident on the surface. By sharp contrast, the etching of Ga2O3 had a stronger chemical component without a well-defined ion energy threshold. After being exposed to BCl3 plasmas, both NiO and Ga2O3 surfaces show chlorine residues, which can both be removed by the standard 1NH4OH: 10H2O or 1HCl: 10H2O solution used for native oxide removal. In addition, according to the location of the Cl 2p3/2 peak in the XPS analysis on the surface, the chlorine particles were ionically bonded. Figure 1