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Revealing the effects of oxygen defects on the electro-catalytic activity of nickel oxide
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Revealing the effects of oxygen defects on the electro-catalytic activity of nickel oxide Shiguang Jin a,1, Yunmin Zhu a,1, Zuyun He a, Huijun Chen a, Xi Liu a, Fei Li a, Jiang Liu a, Meilin Liu b, Yan Chen a,* a
Guangzhou Key Laboratory for Surface Chemistry of Energy Materials, Guangdong Engineering and Technology and Research Center for Surface Chemistry of Energy Materials, State Key Laboratory of Pulp and Paper Engineering, School of Environment and Energy, South China University of Technology, Guangzhou, 510006, P.R. China b School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, 30332-0245, USA
highlights
graphical abstract
Thin films are used as model systems to reveal the role of oxygen defects. NiO
thin
films
show
strongly
enhanced OER activity after Ar plasma treatments. Oxygen vacancies change the surface adsorption characteristics of NiO thin film. The NiO thin film treated by 50 W Ar plasma shows the highest OER activity.
article info
abstract
Article history:
Low-cost transition metal oxides (TMO) has received extensive attention as new-
Received 24 June 2019
generation oxygen evolution reaction (OER) catalysts. In recent years, introducing oxygen
Received in revised form
defects via various strategies emerged as an efficient way to increase electro-catalytic
5 October 2019
activity of TMO. Despite many successful examples, the physical and chemical origin of
Accepted 18 October 2019
such enhanced OER activity induced by oxygen defects is still not well understood. In this
Available online xxx
work, we systematically investigate the correlation between the concentration of oxygen vacancies and OER activity by using a NiO thin film model system grown by pulsed laser
Keywords:
deposition. Oxygen vacancies are introduced into NiO thin films by Ar plasma treatments
Oxygen defects
with different power (50 W, 100 W, and 150 W). The OER activities of NiO thin films are
Plasma treatment
found to be significantly enhanced after plasma treatment. The improvement in electro-
Oxygen evolution reaction
chemical OER performance is attributed to the creation of oxygen defects, which lead to the
Thin films
strongly modified surface adsorption properties, as confirmed by X-ray photoelectron
* Corresponding author. E-mail address: [email protected] (Y. Chen). 1 Equally contributed. https://doi.org/10.1016/j.ijhydene.2019.10.130 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Jin S et al., Revealing the effects of oxygen defects on the electro-catalytic activity of nickel oxide, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.130
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spectroscopy and contact angle measurements. Our results demonstrate that plasma treatment as a promising method for precisely tuning physical and chemical properties of TMO through oxygen defects engineering to obtain high performance electro-catalysts. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Due to its sluggish reaction kinetics, oxygen evolution reaction (OER) is considered the critical reaction which determines the efficiency of several energy devices such as water electrolysis cells [1e9] and rechargeable metal-air batteries [10e12]. Currently, noble metals and their oxides such as RuO2, IrO2, Pt and PtM (alloys of Pt and transition metals) are most widely used catalysts for OER, hydrogen evolution reaction (HER), and oxygen reduction reaction (ORR). However, the high price of noble metals strongly limits their large scale application. Transition metal oxides (TMO) with low cost, low over-potential and long-term stability are considered alternative OER catalysts [13e17]. The electro-catalytic activity of TMO, however, is still not sufficient for practical application. While modifying electronic structure of TMO through doping of first-row transition metal elements (i.e. Co, Ni, Fe, etc) have been widely used for developing high performance electrocatalysts [18e20]. Tailoring oxygen vacancy in TMO recently emerged as an extra degree of freedom for rational design of novel catalysts [21e30]. The presence of oxygen vacancy critically modifies the electronic structure [31e33], transport properties [31e34], and surface adsorption characteristics [33,35], leading to significant changes in OER activity of many TMO catalysts, such as CoOx [25], MnO2 [26], Fe2O3 [27], CoFe2O4 [28], LaCoO3-d [29], and NiCo2O4 [30]. Several approaches have been developed for tailoring oxygen defects in TMO, such as thermal treatment, solution process, and cation doping [21,36e38]. Annealing the materials in a reducing atmosphere (such as H2 and NH3) at elevated temperatures is widely used as a simple and efficient approach to introducing oxygen vacancies [39,40]. In many case, however, high annealing temperature leads to agglomeration of nanostructures and degradation in materials performance [41]. It is challenging to control the density and distribution of oxygen vacancies in solution process [42e45]. The effects of cation doping depends highly on the materials system and preparation techniques [46e49]. Plasma technologies have been widely used to modify materials surface for energy applications such as proton exchange membrane fuel cells, water electrolysis cells, Li-ion batteries, and supercapacitors [50e52]. Plasma treatment are applicable to different materials system and can wellmaintain the microstructure of the materials. The oxygen defects introduced by plasma can be controlled by the power, pressure, and the treatment time. There have been previous successful attempts to use plasma treatment to enhance the electro-catalytic performance of TMO. For instance, Xu et al. [31] demonstrated that the OER activity of Co3O4 nanosheets treated by Ar plasma was significantly improved. Zhu et al. [27] used air plasma treatment to improve photoelectrochemical OER performance of a-Fe2O3 nanoflakes. Lu et al. [29] observed better
OER activity of LaCoO3-d powder via Ar plasma treatment. All these examples demonstrated the potential application of plasma treatment in oxygen defects engineering for achieving high performance TMO catalysts. Despites many successful examples of enhancing the OER performance of TMO by oxygen defects engineering, the underlying mechanism is still not wellunderstood. For instance, the existence of oxygen vacancies was found to boost the OER activity of CaMnO3-d [34] and LaCoO3d [29], but degrade that of PrBaCo2O6-d [53], La0.7Sr0.3CoO3 [22], and La0.6Ca0.4CoO3 [54]. For rational design of novel OER catalysts using plasma treatment and other approaches, it is necessary to reveal the role of oxygen defects in determining the surface reaction kinetics. In this work, we report our findings in systematic investigation into the effect of oxygen vacancies generated by Ar plasma treatment on the OER activity of nickel oxides. Using pulsed laser deposition (PLD), NiO thin films with well controlled thickness and surface structure can be synthesized. The use of such model system with controlled thickness and surface roughness can isolate the contribution of oxygen defects from factors like microstructure changes. Furthermore, surface sensitive techniques such as X-ray photoelectron spectroscopy (XPS) and contact angle measurement were used to quantify the changes in surface properties induced by oxygen vacancies. The use of inert Ar plasma treatment can avoid complications arisen from anion doping, and we can focus on the impact of oxygen vacancies. It was found that the NiO thin films showed better OER performance after Ar plasma treatment. The current density at 1.7 V vs. reversible hydrogen electrode (RHE) for the NiO films subjected to 50 W, 100 W, and 150 W Ar plasma treatments showed 5.8, 2.8, and 1.6 folds enhancement compared with that for the pristine NiO film, respectively. The enhancement in OER performance is attributed to the change in Ni valence states and improved surface adsorption characteristics of the film, as supported by X-ray photoelectron spectroscopy (XPS) and contacting angle measurements. Our results showed the importance of oxygen vacancy on the electro-catalytic activities and demonstrated plasma treatment as a promising technique for achieving high performance electro-catalysts for energy applications. The mechanistic understanding obtained in this work using the PLD grown model system can be applied to other material system synthesized by other methods for practical application.
Experimental Fabrication of thin film The NiO model thin films were prepared on one side polished quartz substrate by PLD under an oxygen pressure of 1 Pa at 500 C. The laser energy was set at 300 mJ per pulse. The
Please cite this article as: Jin S et al., Revealing the effects of oxygen defects on the electro-catalytic activity of nickel oxide, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.130
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deposition time was 5 min at a pulse frequency of 5 Hz and 30 min at a pulse frequency of 10 Hz. The PLD target was synthesized by pressing NiO (Aladdin, 99.5%) powder into a ceramic pellet and fired at 1200 C in air for 2 h. Before PLD deposition, a gold pattern with a comb shape was deposited onto the quartz substrate using ion sputtering. After PLD deposition, the sputtered gold pattern was buried underneath the NiO thin film and was then used as current collector in the electrochemical tests [Fig. 1(a)].
SU8010) at 5 kV. The surface chemistry analysis of the XPS was performed on a Thermo Fisher K-Alpha instrument. The C 1s peak at 284.6 eV was used as the internal reference peak for calibrating binding energies. For the wettability measurement, a droplet of electrolyte solution with 2 ml volume was placed onto the NiO films and the contact angle of the electrolyte and film surface were tested by optical surface contact angle meter (DSA25, Germany).
Plasma treatment
Results and discussion
The NiO thin films with embedded Au current collector were put into the chamber of low-temperature radio frequency (RF) plasma machine for Ar plasma treatment for 10 min. The pressure of Ar in the chamber was maintained at 20 Pa. Three different RF power were used for the plasma treatment, including 50 W, 100 W, and 150 W. The pristine NiO thin film and the ones subjected to plasma treatment under different power were referred as PeNiO, 50-NiO, 100-NiO, and 150-NiO, respectively. All the samples were taken out of the chamber after plasma treatment and stored in the same ambient condition before further analysis.
Crystal structure and surface morphologies
Characterizations For the electrochemical tests, a silver wire was attached to the Au pattern with silver slurry. All the exposed silver and the back side of quartz substrate were covered by epoxy resins to avoid the contact to the electrolyte during OER tests [Fig. 1(a)]. The electrochemical performance was tested on a CHI660E electrochemical workstation using a three-electrode system (Ag/AgCl as the reference electrode, platinum wire as the counter electrode). The electrolyte (1 M KOH) was purged with oxygen for above 35 min before each test to ensure the O2 saturation. Cyclic voltammograms were recorded in the potential range from 0.2 to 0.8 V (vs. Ag/AgCl) at a scan rate of 10 mV/s. The electrochemical impedance measurements were performed at a cell voltage of 0.66 V (vs. Ag/AgCl) under the influence of an alternating current voltage of 5 mV in the frequency range from 1 MHz to 0.1 Hz. NiO thin films were examined by XRD using an X-ray diffractometer (Bruker D8 Advance, Cu-Ka, l ¼ 1.5418 A). The surface morphology was probed by a Field Emission Scanning Electron Microscope (HITACHI,
The crystal structures of pristine NiO (PeNiO) and the ones subjected to plasma treatment (50-NiO, 100-NiO, and 150-NiO) were characterized by X-ray diffraction (XRD). As shown in Fig. 1(b), all films were polycrystalline NiO without any secondary phases. The diffraction peak of Au current collector can also be seen in the XRD pattern. There was no noticeable difference in the XRD patterns for samples before and after Ar plasma treatment, which is not in accord with the reduction effect of Ar plasma treatment reported by Osonio et al. [55]. The comparison of the NiO (200) XRD peaks of PeNiO, 50-NiO, 100-NiO, and 150-NiO was shown in Fig. 1(c). The shape and height of NiO (200) XRD peaks did not show noticeable changes after plasma treatment. The reason is likely that the changes introduced by our plasma treatment are limited to the surface region. Since XRD probed the bulk crystal structure of the whole thin films, it may not be sensitive to the changes introduced by oxygen vacancy in the surface region. As a result, we did not observe noticeable differences for the pristine film and the films subjected to plasma treatment. The thickness of the NiO thin films, as revealed by the scanning electron microscope (SEM) images, was about 120 nm (Fig. 2). The surface morphology of NiO thin films remained unchanged before and after plasma treatment (supplementary materials, Fig. S1).
Chemical structures As reported in many previous studies, plasma treatment can generate oxygen vacancies on the surface of TMO [27,29,31], but it is difficult to probe the surface oxygen vacancies in these films due to its small volume [22,23,56]. To confirm the
Fig. 1 e (a) Illustration of experimental process for electrochemical measurements; (b) XRD patterns of PeNiO, 50-NiO, 100NiO, and 150-NiO; (c) Comparison of the NiO (200) XRD peaks of PeNiO, 50-NiO, 100-NiO, and 150-NiO. (The standard PDF cards used for reference are PDF#47e1049 and PDF#65e2870 for NiO and Au, respectively). Please cite this article as: Jin S et al., Revealing the effects of oxygen defects on the electro-catalytic activity of nickel oxide, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.130
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Fig. 2 e (a) SEM images of the pristine NiO thin film (PeNiO) and (b) Cross-section SEM image of NiO thin films on silicon substrate.
creation of oxygen vacancies on the surface of NiO after plasma treatment, we probed the Ni 2p and O 1s XPS spectra of all the NiO films, as shown in Fig. 3(a) and (b). The Ni 2p3/2 peak shifted slightly towards lower binding energy after Ar plasma treatment [Fig. 3(a)], indicating the reduction of Ni valence states and the creation of oxygen vacancies on the surface of plasma treated NiO [22,56,57]. After plasma treatment, more apparent changes were observed in the O 1s spectra. As shown in Fig. 3 (b), the O 1s spectra can be fitted into four peaks, which located around 529.5 eV (OI), 530.7 eV (OII), 531.4 eV (OIII) and 532.6 eV (OIV), corresponding to lattice oxygen, defective O, hydroxyl groups or surface-adsorbed oxygen and absorbed water molecule, respectively [41,58]. The height of defective oxygen peak (OII) represents the amount of oxygen defects within the films. As shown in Fig. 3(b), the
intensity of the defective O peak (OII) increased after plasma treatments compared to the pristine film, indicating the creation of oxygen vacancies. The presence of oxygen defects after plasma treatment can be seen more clearly from Fig. 3(c), which shows the decrease in the area ratio of lattice oxygen peak OI to Ni 2p and the increase in the area ratio of the defective O peak. The 50-NiO showed the lowest OI (lattice oxygen) to Ni 2p ratio and the highest OII intensity (defective O peak), suggesting a decrease of oxygen vacancy density as the plasma power further increase. The 50-NiO showed the highest defective O peak (OII) and lowest lattice oxygen to Ni 2p ratio, suggesting a decrease of oxygen vacancy density as the plasma power further increase. It is also need to note that the peaks corresponding to adsorbed oxygen species (OIII and OIV) in O 1s spectra increased after plasma treatment
Fig. 3 e (a) Ni 2p and (b) O 1s X-ray photoelectron spectra of PeNiO, 50-NiO, 100-NiO, and 150-NiO; The O 1s peak were fitting by four peaks, which are attributed to lattice oxygen (OI), defective O (OII), hydroxyl groups or surface-adsorbed oxygen (OIII) and absorbed water molecule (OIV), respectively. (c) The area ratio of lattice O peak (OI) to the Ni 2p peak (left) and the area ratio of defective oxygen peak OII to the summary of the OI, OII, and OIII peaks (right). Please cite this article as: Jin S et al., Revealing the effects of oxygen defects on the electro-catalytic activity of nickel oxide, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.130
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[Fig. 3(b)], suggesting a change in the surface adsorption characteristics introduced by oxygen defects [41,59]. As mentioned above, no noticeable difference in XRD patterns were observed for all NiO films. In contrast, XPS showed the evidence of the formation of oxygen vacancies. The XPS and XRD results suggest that the oxygen vacancies created by plasma located mainly on the surface region of the film. As a result, only the surface sensitive XPS can probe the changes introduced by plasma treatment.
Electrochemical activities Confirming the formation of oxygen vacancies after plasma treatment, the influence of these oxygen vacancies on the OER performance were evaluated by electrochemical tests. The cyclic voltammetry (CV) spectra for all NiO films are plotted in Fig. 4(a) for comparison. The plasma treated NiO thin film showed noticeable enhancement in OER activity, with much lower onset potential and higher current density for the plasma treated samples than that for the pristine NiO. The current density at 1.7 V (vs. RHE) for PeNiO, 50-NiO, 100-NiO, and 150-NiO are 1.68 mA/cm2, 9.76 mA/cm2, 4.75 mA/cm2, and 2.68 mA/cm2, respectively. Consistent with the CV test results, the electrochemical impedance spectra (EIS) and the Tafel plots also suggested a significant enhancement in OER activity for the NiO films subjected to plasma treatment [Fig. 4(b) and (c)]. The EIS spectra exhibited a characteristic semicircle in the Nyquist plots, which can be fitted with a charge transfer resistance (Rct), a constant phase element, and an ohmic resistance (Rs) from the solution and all ohmic contact [41,53]. The ohmic resistance (Rs) did not show noticeable difference after plasma treatment, suggesting the conductivity of the
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thin film sample stay unchanged after plasma treatment. The charge transfer resistance (Rct) of 50-NiO, 100-NiO, and 150NiO were 60U, 168U, and 243U, which was reduced to about 1/6, 1/2, and 2/3 of the Rct for the pristine NiO (365U) [Fig. 4(b)]. Such decrease in Rct suggested a strongly facilitated OER reaction on the defective NiO surface [60e63]. The Tafel slope for NiO films decreased after Ar plasma treatment and the 50-NiO exhibited the lowest Tafel slope [Fig. 4(c)]. Both the CV and EIS tests indicate that the performance of the pristine and the plasma treated samples follow the order of 50-NiO > 100-NiO > 150-NiO > PeNiO [Fig. 4(d)]. Such trend is in consistent with the amount of oxygen vacancy in the pristine NiO film and the ones subjected to plasma treatment probed by XPS [Fig. 3 (b) and (c)]. The 50-NiO showed highest oxygen vacancies density on the surface and exhibited the highest OER activity, indicating the positive influence of oxygen vacancies on the OER performance of NiO. We further found that the enhanced OER performance after plasma treatment can be maintained after long-term stability measurements (Fig. S2). Both the XRD patterns (Fig. S3) and SEM images (Fig. S4) of the pristine and the plasma treated NiO thin films showed no noticeable changes after OER electrochemical measurements, indicating the NiO thin films remained stable during OER process. Transition metal atoms are normally regarded as the active catalytic sites in TMO, such as NiO [64], Co3O4 [31], and Fe2O3 [65]. To further evaluate the activity of NiO thin film, the turnover frequency (TOF) of nickel sites were quantified based on the electrochemical surface area (ECSA). The detailed information about ECSA measurement and TOF quantification can be found in Supporting information Section S2 and S3. The TOF of all NiO films calculated using the current density at 1.6 V vs. RHE were compared with other transition metal and
Fig. 4 e (a) CV polarization curves; (b) Electrochemical impedance spectra and (c) Tafel plots for PeNiO, 50-NiO, 100-NiO, and 150-NiO (d) Dependence of current density at 1.7 V (vs. RHE); and charge-transfer resistance of NiO thin film on the plasma power. Please cite this article as: Jin S et al., Revealing the effects of oxygen defects on the electro-catalytic activity of nickel oxide, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.130
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Table 1 e Comparison of TOF for different catalysts. Catalysts PeNiO 50-NiO 100-NiO 150-NiO Fe in iron fluoride-oxide NiO RuO2
TOF at 1.6 V vs. RHE 0.14556 0.61354 0.38559 0.22871 0.2141 0.07 0.01724
ref This This This This [65] [66] [67]
work work work work
noble metal based electrocatalysts as presented in Table 1. The TOF for pristine NiO thin film was comparable to NiO powder reported in literature [66]. After Ar plasma treatment, the TOF showed considerable increase, suggesting enhanced OER activity. The NiO film treated by 50 W Ar plasma showed higher TOF than iron from iron oxides [65], RuO2 [67], and NiO [66] in the previous literatures.
Adsorption properties One critical factor that determines the OER activity of TMO is the ability for reactant and reaction intermediates adsorbing on and desorbing from the surface of catalysts [68,69]. To further understand the change of the surface properties and OER activity for NiO films induced by oxygen vacancies, we evaluate the wettability of the NiO films to the electrolyte (1 M KOH) by measuring the contact angle of the pristine and the plasma treated NiO samples. The contact angles of the electrolyte solution on PeNiO, 50-NiO, 100-NiO, and 150-NiO were 32.1 , 26.7 , 28.9 , and 31.8 , respectively (Fig. 5). All the plasma treated samples showed smaller contact angles than the pristine one, and the 50-NiO surface showed the best wettability to the electrolyte. Similar decrease in contact angle after plasma treatment were observed formerly in other transition metal oxides [70]. The oxygen-to-metal ratio on the surface of metal oxides is crucial for the wettability [71]. Harju et al. [72] observed that the oxygen vacancies in plasma sprayed TiO2 and Cr2O3 films can enhance water adsorption capacity by using isothermal gas adsorption studies. Through density functional theory calculation, Zhu et al. [59] reported that the plasma treated Sr2Fe1.3Ni0.2Mo0.5O6-d showed lower adsorption energy of water due to the oxygen vacancies created by plasma treatment in 5% H2/Ar, and thus exhibited higher OER activity. Chen et al. [73] reported that the Zeolitic Imidazolate Framework-67 template with oxygen vacancies created by O2eAr RF plasma can lower the adsorption energy of water for
nickel cobalt layered double hydroxides on the template, and further increase the OER activity. OER activity can be hindered by insufficient adsorption of water molecules and hydroxyl groups, which are generally the rate-determining step in the multistep OER reaction process [74e76]. For catalysts with high OER activity the surface needs to present with suitable hydrophilicity to facilitate the adsorption of intermediate species (i.e. hydroxyl) onto its surface, while not hinder the release of oxygen gas produced from the surface [33]. The oxygen vacancies created by Ar plasma treatments on the surface of NiO films facilitated the adsorption of water molecules and hydroxyl groups, as evidenced by the enhanced wettability to electrolyte in the contact angle measurements (Fig. 5) and increased intensity of surface adsorbed oxygen species in O 1s XPS spectra [Fig. 3 (b)]. With adequate water molecules and hydroxyl groups adsorbed onto the surface of oxygen deficient NiO film, the OER catalytic process can proceed smoothly and exhibited higher current density. Our observation suggested that the oxygen vacancy generated by plasma treatment can promote the adsorption of reactant and reaction intermediates on the catalyst surface, thus enhance the OER performance. As shown in the electrochemical test (Fig. 4), the 50-NiO film exhibited highest OER activity compared to pristine NiO, 100-NiO and 150 NiO films. The reason why the high plasma power leads poor OER performance is likely due to the lower surface oxygen vacancies density when the plasma power becomes too high. Plasma treatment is a complicated physical and chemical process involving energetic electrons, excited species, free radicals, and ions. It is normally used as an efficient strategy to introduce defects into variety of materials [51]. However, it was also reported that plasma treatment can also eliminate the existed defects in materials [77e80]. For instance, Yang et al. [78] reported decreased oxygen vacancies of In2O3 films with the increase of Ar plasma power during the deposition procedure. Xu et al. [31] demonstrated that the OER activity of Co3O4 nanosheets treated by Ar plasma for 120s was better than 180s and 240s. Similarly, it was reported that Ar plasma treatment with medium time length lead to highest OER activity for cobalt oxides [81] and NiFe layered double hydroxides [82]. Lim et al. [77] observed that the oxygenvacancies-related peak decreased in SnO2 nanowire after N2 plasma through photoluminescence spectra. Similar elimination effects on the oxygen vacancies were also observed in O2 plasma treatment [79,80]. For our case, plasma treatment with 50 W power was found to generate highest oxygen vacancy density and best wettability to the electrolytes. Further
Fig. 5 e (aed) Contact angle measurement results of 1 M KOH solution on (a) PeNiO, (b) 50-NiO, (c) 100-NiO, and (d)150-NiO; (e) Contact angle of 1 M KOH on NiO thin films as a function of plasma treatment power. Please cite this article as: Jin S et al., Revealing the effects of oxygen defects on the electro-catalytic activity of nickel oxide, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.130
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increase of plasma power to 100 W or 150 W leads to a decrease in oxygen vacancy concentrations and degraded OER performance. Our results indicate the existence of optimal plasma treatment condition for tuning the electro-catalytic activity of TMO by oxygen defect engineering.
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Conclusion [4]
In conclusion, we investigated the change of the OER activity for nickel oxides influenced by oxygen vacancies generated by plasma treatment by using a thin film model system prepared by PLD. Ar plasma treatments with different power were applied to introduce oxygen vacancies into the surface of NiO thin films. XRD and SEM tests showed no difference in the crystal structure and morphology between the pristine and the Ar plasma treated NiO. The OER activity for NiO films were found to be significantly enhanced after plasma treatment. The current density (1.7 V vs. RHE) for the NiO films subjected to 50 W, 100 W, and 150 W Ar plasma treatments showed 5.8, 2.8, and 1.6 folds enhancement compared with that of the pristine NiO film, respectively. Such enhancements are attributed to plasma induced surface oxygen vacancies which improved surface adsorption characteristics, as confirmed by surface contacting angle measurement. The 50-NiO exhibited the best OER performance compared with the PeNiO, 100-NiO, and 150NiO. The reason is likely due to the elimination of pre-existing oxygen defects from the surface during treatment as the plasma power become higher. Our results showed that oxygen defects can influence the intrinsic OER activity of TMO. Plasma treatment represents a promising approach for the optimization of OER activity for TMO electro-catalysts through the degree of freedom of oxygen defect engineering. The method and knowledge gain in this work can be applied to other transition metal oxides and sulfides for energy applications.
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Acknowledgements [13]
This work was supported by the National Natural Science Foundation of China (11605063, 11975102 and 91745203), Guangzhou Science and Technology Program General Projects (201707010146), the Fundamental Research Funds for the Central Universities (2018MS40), State Key Laboratory of Pulp and Paper Engineering (2018TS08), Guangdong Pearl River Talent Program (2017GC010281), Guangdong Innovative and Entrepreneurial Research Team Program (2014ZT05N200).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.130.
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Please cite this article as: Jin S et al., Revealing the effects of oxygen defects on the electro-catalytic activity of nickel oxide, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.130
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Revealing the effects of oxygen defects on the electro-catalytic activity of nickel oxide Shiguang Jin a,1, Yunmin Zhu a,1, Zuyun He a, Huijun Chen a, Xi Liu a, Fei Li a, Jiang Liu a, Meilin Liu b, Yan Chen a,* a
Guangzhou Key Laboratory for Surface Chemistry of Energy Materials, Guangdong Engineering and Technology and Research Center for Surface Chemistry of Energy Materials, State Key Laboratory of Pulp and Paper Engineering, School of Environment and Energy, South China University of Technology, Guangzhou, 510006, P.R. China b School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, 30332-0245, USA
highlights
graphical abstract
Thin films are used as model systems to reveal the role of oxygen defects. NiO
thin
films
show
strongly
enhanced OER activity after Ar plasma treatments. Oxygen vacancies change the surface adsorption characteristics of NiO thin film. The NiO thin film treated by 50 W Ar plasma shows the highest OER activity.
article info
abstract
Article history:
Low-cost transition metal oxides (TMO) has received extensive attention as new-
Received 24 June 2019
generation oxygen evolution reaction (OER) catalysts. In recent years, introducing oxygen
Received in revised form
defects via various strategies emerged as an efficient way to increase electro-catalytic
5 October 2019
activity of TMO. Despite many successful examples, the physical and chemical origin of
Accepted 18 October 2019
such enhanced OER activity induced by oxygen defects is still not well understood. In this
Available online xxx
work, we systematically investigate the correlation between the concentration of oxygen vacancies and OER activity by using a NiO thin film model system grown by pulsed laser
Keywords:
deposition. Oxygen vacancies are introduced into NiO thin films by Ar plasma treatments
Oxygen defects
with different power (50 W, 100 W, and 150 W). The OER activities of NiO thin films are
Plasma treatment
found to be significantly enhanced after plasma treatment. The improvement in electro-
Oxygen evolution reaction
chemical OER performance is attributed to the creation of oxygen defects, which lead to the
Thin films
strongly modified surface adsorption properties, as confirmed by X-ray photoelectron
* Corresponding author. E-mail address: [email protected] (Y. Chen). 1 Equally contributed. https://doi.org/10.1016/j.ijhydene.2019.10.130 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Jin S et al., Revealing the effects of oxygen defects on the electro-catalytic activity of nickel oxide, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.130
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spectroscopy and contact angle measurements. Our results demonstrate that plasma treatment as a promising method for precisely tuning physical and chemical properties of TMO through oxygen defects engineering to obtain high performance electro-catalysts. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Due to its sluggish reaction kinetics, oxygen evolution reaction (OER) is considered the critical reaction which determines the efficiency of several energy devices such as water electrolysis cells [1e9] and rechargeable metal-air batteries [10e12]. Currently, noble metals and their oxides such as RuO2, IrO2, Pt and PtM (alloys of Pt and transition metals) are most widely used catalysts for OER, hydrogen evolution reaction (HER), and oxygen reduction reaction (ORR). However, the high price of noble metals strongly limits their large scale application. Transition metal oxides (TMO) with low cost, low over-potential and long-term stability are considered alternative OER catalysts [13e17]. The electro-catalytic activity of TMO, however, is still not sufficient for practical application. While modifying electronic structure of TMO through doping of first-row transition metal elements (i.e. Co, Ni, Fe, etc) have been widely used for developing high performance electrocatalysts [18e20]. Tailoring oxygen vacancy in TMO recently emerged as an extra degree of freedom for rational design of novel catalysts [21e30]. The presence of oxygen vacancy critically modifies the electronic structure [31e33], transport properties [31e34], and surface adsorption characteristics [33,35], leading to significant changes in OER activity of many TMO catalysts, such as CoOx [25], MnO2 [26], Fe2O3 [27], CoFe2O4 [28], LaCoO3-d [29], and NiCo2O4 [30]. Several approaches have been developed for tailoring oxygen defects in TMO, such as thermal treatment, solution process, and cation doping [21,36e38]. Annealing the materials in a reducing atmosphere (such as H2 and NH3) at elevated temperatures is widely used as a simple and efficient approach to introducing oxygen vacancies [39,40]. In many case, however, high annealing temperature leads to agglomeration of nanostructures and degradation in materials performance [41]. It is challenging to control the density and distribution of oxygen vacancies in solution process [42e45]. The effects of cation doping depends highly on the materials system and preparation techniques [46e49]. Plasma technologies have been widely used to modify materials surface for energy applications such as proton exchange membrane fuel cells, water electrolysis cells, Li-ion batteries, and supercapacitors [50e52]. Plasma treatment are applicable to different materials system and can wellmaintain the microstructure of the materials. The oxygen defects introduced by plasma can be controlled by the power, pressure, and the treatment time. There have been previous successful attempts to use plasma treatment to enhance the electro-catalytic performance of TMO. For instance, Xu et al. [31] demonstrated that the OER activity of Co3O4 nanosheets treated by Ar plasma was significantly improved. Zhu et al. [27] used air plasma treatment to improve photoelectrochemical OER performance of a-Fe2O3 nanoflakes. Lu et al. [29] observed better
OER activity of LaCoO3-d powder via Ar plasma treatment. All these examples demonstrated the potential application of plasma treatment in oxygen defects engineering for achieving high performance TMO catalysts. Despites many successful examples of enhancing the OER performance of TMO by oxygen defects engineering, the underlying mechanism is still not wellunderstood. For instance, the existence of oxygen vacancies was found to boost the OER activity of CaMnO3-d [34] and LaCoO3d [29], but degrade that of PrBaCo2O6-d [53], La0.7Sr0.3CoO3 [22], and La0.6Ca0.4CoO3 [54]. For rational design of novel OER catalysts using plasma treatment and other approaches, it is necessary to reveal the role of oxygen defects in determining the surface reaction kinetics. In this work, we report our findings in systematic investigation into the effect of oxygen vacancies generated by Ar plasma treatment on the OER activity of nickel oxides. Using pulsed laser deposition (PLD), NiO thin films with well controlled thickness and surface structure can be synthesized. The use of such model system with controlled thickness and surface roughness can isolate the contribution of oxygen defects from factors like microstructure changes. Furthermore, surface sensitive techniques such as X-ray photoelectron spectroscopy (XPS) and contact angle measurement were used to quantify the changes in surface properties induced by oxygen vacancies. The use of inert Ar plasma treatment can avoid complications arisen from anion doping, and we can focus on the impact of oxygen vacancies. It was found that the NiO thin films showed better OER performance after Ar plasma treatment. The current density at 1.7 V vs. reversible hydrogen electrode (RHE) for the NiO films subjected to 50 W, 100 W, and 150 W Ar plasma treatments showed 5.8, 2.8, and 1.6 folds enhancement compared with that for the pristine NiO film, respectively. The enhancement in OER performance is attributed to the change in Ni valence states and improved surface adsorption characteristics of the film, as supported by X-ray photoelectron spectroscopy (XPS) and contacting angle measurements. Our results showed the importance of oxygen vacancy on the electro-catalytic activities and demonstrated plasma treatment as a promising technique for achieving high performance electro-catalysts for energy applications. The mechanistic understanding obtained in this work using the PLD grown model system can be applied to other material system synthesized by other methods for practical application.
Experimental Fabrication of thin film The NiO model thin films were prepared on one side polished quartz substrate by PLD under an oxygen pressure of 1 Pa at 500 C. The laser energy was set at 300 mJ per pulse. The
Please cite this article as: Jin S et al., Revealing the effects of oxygen defects on the electro-catalytic activity of nickel oxide, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.130
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deposition time was 5 min at a pulse frequency of 5 Hz and 30 min at a pulse frequency of 10 Hz. The PLD target was synthesized by pressing NiO (Aladdin, 99.5%) powder into a ceramic pellet and fired at 1200 C in air for 2 h. Before PLD deposition, a gold pattern with a comb shape was deposited onto the quartz substrate using ion sputtering. After PLD deposition, the sputtered gold pattern was buried underneath the NiO thin film and was then used as current collector in the electrochemical tests [Fig. 1(a)].
SU8010) at 5 kV. The surface chemistry analysis of the XPS was performed on a Thermo Fisher K-Alpha instrument. The C 1s peak at 284.6 eV was used as the internal reference peak for calibrating binding energies. For the wettability measurement, a droplet of electrolyte solution with 2 ml volume was placed onto the NiO films and the contact angle of the electrolyte and film surface were tested by optical surface contact angle meter (DSA25, Germany).
Plasma treatment
Results and discussion
The NiO thin films with embedded Au current collector were put into the chamber of low-temperature radio frequency (RF) plasma machine for Ar plasma treatment for 10 min. The pressure of Ar in the chamber was maintained at 20 Pa. Three different RF power were used for the plasma treatment, including 50 W, 100 W, and 150 W. The pristine NiO thin film and the ones subjected to plasma treatment under different power were referred as PeNiO, 50-NiO, 100-NiO, and 150-NiO, respectively. All the samples were taken out of the chamber after plasma treatment and stored in the same ambient condition before further analysis.
Crystal structure and surface morphologies
Characterizations For the electrochemical tests, a silver wire was attached to the Au pattern with silver slurry. All the exposed silver and the back side of quartz substrate were covered by epoxy resins to avoid the contact to the electrolyte during OER tests [Fig. 1(a)]. The electrochemical performance was tested on a CHI660E electrochemical workstation using a three-electrode system (Ag/AgCl as the reference electrode, platinum wire as the counter electrode). The electrolyte (1 M KOH) was purged with oxygen for above 35 min before each test to ensure the O2 saturation. Cyclic voltammograms were recorded in the potential range from 0.2 to 0.8 V (vs. Ag/AgCl) at a scan rate of 10 mV/s. The electrochemical impedance measurements were performed at a cell voltage of 0.66 V (vs. Ag/AgCl) under the influence of an alternating current voltage of 5 mV in the frequency range from 1 MHz to 0.1 Hz. NiO thin films were examined by XRD using an X-ray diffractometer (Bruker D8 Advance, Cu-Ka, l ¼ 1.5418 A). The surface morphology was probed by a Field Emission Scanning Electron Microscope (HITACHI,
The crystal structures of pristine NiO (PeNiO) and the ones subjected to plasma treatment (50-NiO, 100-NiO, and 150-NiO) were characterized by X-ray diffraction (XRD). As shown in Fig. 1(b), all films were polycrystalline NiO without any secondary phases. The diffraction peak of Au current collector can also be seen in the XRD pattern. There was no noticeable difference in the XRD patterns for samples before and after Ar plasma treatment, which is not in accord with the reduction effect of Ar plasma treatment reported by Osonio et al. [55]. The comparison of the NiO (200) XRD peaks of PeNiO, 50-NiO, 100-NiO, and 150-NiO was shown in Fig. 1(c). The shape and height of NiO (200) XRD peaks did not show noticeable changes after plasma treatment. The reason is likely that the changes introduced by our plasma treatment are limited to the surface region. Since XRD probed the bulk crystal structure of the whole thin films, it may not be sensitive to the changes introduced by oxygen vacancy in the surface region. As a result, we did not observe noticeable differences for the pristine film and the films subjected to plasma treatment. The thickness of the NiO thin films, as revealed by the scanning electron microscope (SEM) images, was about 120 nm (Fig. 2). The surface morphology of NiO thin films remained unchanged before and after plasma treatment (supplementary materials, Fig. S1).
Chemical structures As reported in many previous studies, plasma treatment can generate oxygen vacancies on the surface of TMO [27,29,31], but it is difficult to probe the surface oxygen vacancies in these films due to its small volume [22,23,56]. To confirm the
Fig. 1 e (a) Illustration of experimental process for electrochemical measurements; (b) XRD patterns of PeNiO, 50-NiO, 100NiO, and 150-NiO; (c) Comparison of the NiO (200) XRD peaks of PeNiO, 50-NiO, 100-NiO, and 150-NiO. (The standard PDF cards used for reference are PDF#47e1049 and PDF#65e2870 for NiO and Au, respectively). Please cite this article as: Jin S et al., Revealing the effects of oxygen defects on the electro-catalytic activity of nickel oxide, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.130
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Fig. 2 e (a) SEM images of the pristine NiO thin film (PeNiO) and (b) Cross-section SEM image of NiO thin films on silicon substrate.
creation of oxygen vacancies on the surface of NiO after plasma treatment, we probed the Ni 2p and O 1s XPS spectra of all the NiO films, as shown in Fig. 3(a) and (b). The Ni 2p3/2 peak shifted slightly towards lower binding energy after Ar plasma treatment [Fig. 3(a)], indicating the reduction of Ni valence states and the creation of oxygen vacancies on the surface of plasma treated NiO [22,56,57]. After plasma treatment, more apparent changes were observed in the O 1s spectra. As shown in Fig. 3 (b), the O 1s spectra can be fitted into four peaks, which located around 529.5 eV (OI), 530.7 eV (OII), 531.4 eV (OIII) and 532.6 eV (OIV), corresponding to lattice oxygen, defective O, hydroxyl groups or surface-adsorbed oxygen and absorbed water molecule, respectively [41,58]. The height of defective oxygen peak (OII) represents the amount of oxygen defects within the films. As shown in Fig. 3(b), the
intensity of the defective O peak (OII) increased after plasma treatments compared to the pristine film, indicating the creation of oxygen vacancies. The presence of oxygen defects after plasma treatment can be seen more clearly from Fig. 3(c), which shows the decrease in the area ratio of lattice oxygen peak OI to Ni 2p and the increase in the area ratio of the defective O peak. The 50-NiO showed the lowest OI (lattice oxygen) to Ni 2p ratio and the highest OII intensity (defective O peak), suggesting a decrease of oxygen vacancy density as the plasma power further increase. The 50-NiO showed the highest defective O peak (OII) and lowest lattice oxygen to Ni 2p ratio, suggesting a decrease of oxygen vacancy density as the plasma power further increase. It is also need to note that the peaks corresponding to adsorbed oxygen species (OIII and OIV) in O 1s spectra increased after plasma treatment
Fig. 3 e (a) Ni 2p and (b) O 1s X-ray photoelectron spectra of PeNiO, 50-NiO, 100-NiO, and 150-NiO; The O 1s peak were fitting by four peaks, which are attributed to lattice oxygen (OI), defective O (OII), hydroxyl groups or surface-adsorbed oxygen (OIII) and absorbed water molecule (OIV), respectively. (c) The area ratio of lattice O peak (OI) to the Ni 2p peak (left) and the area ratio of defective oxygen peak OII to the summary of the OI, OII, and OIII peaks (right). Please cite this article as: Jin S et al., Revealing the effects of oxygen defects on the electro-catalytic activity of nickel oxide, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.130
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[Fig. 3(b)], suggesting a change in the surface adsorption characteristics introduced by oxygen defects [41,59]. As mentioned above, no noticeable difference in XRD patterns were observed for all NiO films. In contrast, XPS showed the evidence of the formation of oxygen vacancies. The XPS and XRD results suggest that the oxygen vacancies created by plasma located mainly on the surface region of the film. As a result, only the surface sensitive XPS can probe the changes introduced by plasma treatment.
Electrochemical activities Confirming the formation of oxygen vacancies after plasma treatment, the influence of these oxygen vacancies on the OER performance were evaluated by electrochemical tests. The cyclic voltammetry (CV) spectra for all NiO films are plotted in Fig. 4(a) for comparison. The plasma treated NiO thin film showed noticeable enhancement in OER activity, with much lower onset potential and higher current density for the plasma treated samples than that for the pristine NiO. The current density at 1.7 V (vs. RHE) for PeNiO, 50-NiO, 100-NiO, and 150-NiO are 1.68 mA/cm2, 9.76 mA/cm2, 4.75 mA/cm2, and 2.68 mA/cm2, respectively. Consistent with the CV test results, the electrochemical impedance spectra (EIS) and the Tafel plots also suggested a significant enhancement in OER activity for the NiO films subjected to plasma treatment [Fig. 4(b) and (c)]. The EIS spectra exhibited a characteristic semicircle in the Nyquist plots, which can be fitted with a charge transfer resistance (Rct), a constant phase element, and an ohmic resistance (Rs) from the solution and all ohmic contact [41,53]. The ohmic resistance (Rs) did not show noticeable difference after plasma treatment, suggesting the conductivity of the
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thin film sample stay unchanged after plasma treatment. The charge transfer resistance (Rct) of 50-NiO, 100-NiO, and 150NiO were 60U, 168U, and 243U, which was reduced to about 1/6, 1/2, and 2/3 of the Rct for the pristine NiO (365U) [Fig. 4(b)]. Such decrease in Rct suggested a strongly facilitated OER reaction on the defective NiO surface [60e63]. The Tafel slope for NiO films decreased after Ar plasma treatment and the 50-NiO exhibited the lowest Tafel slope [Fig. 4(c)]. Both the CV and EIS tests indicate that the performance of the pristine and the plasma treated samples follow the order of 50-NiO > 100-NiO > 150-NiO > PeNiO [Fig. 4(d)]. Such trend is in consistent with the amount of oxygen vacancy in the pristine NiO film and the ones subjected to plasma treatment probed by XPS [Fig. 3 (b) and (c)]. The 50-NiO showed highest oxygen vacancies density on the surface and exhibited the highest OER activity, indicating the positive influence of oxygen vacancies on the OER performance of NiO. We further found that the enhanced OER performance after plasma treatment can be maintained after long-term stability measurements (Fig. S2). Both the XRD patterns (Fig. S3) and SEM images (Fig. S4) of the pristine and the plasma treated NiO thin films showed no noticeable changes after OER electrochemical measurements, indicating the NiO thin films remained stable during OER process. Transition metal atoms are normally regarded as the active catalytic sites in TMO, such as NiO [64], Co3O4 [31], and Fe2O3 [65]. To further evaluate the activity of NiO thin film, the turnover frequency (TOF) of nickel sites were quantified based on the electrochemical surface area (ECSA). The detailed information about ECSA measurement and TOF quantification can be found in Supporting information Section S2 and S3. The TOF of all NiO films calculated using the current density at 1.6 V vs. RHE were compared with other transition metal and
Fig. 4 e (a) CV polarization curves; (b) Electrochemical impedance spectra and (c) Tafel plots for PeNiO, 50-NiO, 100-NiO, and 150-NiO (d) Dependence of current density at 1.7 V (vs. RHE); and charge-transfer resistance of NiO thin film on the plasma power. Please cite this article as: Jin S et al., Revealing the effects of oxygen defects on the electro-catalytic activity of nickel oxide, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.130
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Table 1 e Comparison of TOF for different catalysts. Catalysts PeNiO 50-NiO 100-NiO 150-NiO Fe in iron fluoride-oxide NiO RuO2
TOF at 1.6 V vs. RHE 0.14556 0.61354 0.38559 0.22871 0.2141 0.07 0.01724
ref This This This This [65] [66] [67]
work work work work
noble metal based electrocatalysts as presented in Table 1. The TOF for pristine NiO thin film was comparable to NiO powder reported in literature [66]. After Ar plasma treatment, the TOF showed considerable increase, suggesting enhanced OER activity. The NiO film treated by 50 W Ar plasma showed higher TOF than iron from iron oxides [65], RuO2 [67], and NiO [66] in the previous literatures.
Adsorption properties One critical factor that determines the OER activity of TMO is the ability for reactant and reaction intermediates adsorbing on and desorbing from the surface of catalysts [68,69]. To further understand the change of the surface properties and OER activity for NiO films induced by oxygen vacancies, we evaluate the wettability of the NiO films to the electrolyte (1 M KOH) by measuring the contact angle of the pristine and the plasma treated NiO samples. The contact angles of the electrolyte solution on PeNiO, 50-NiO, 100-NiO, and 150-NiO were 32.1 , 26.7 , 28.9 , and 31.8 , respectively (Fig. 5). All the plasma treated samples showed smaller contact angles than the pristine one, and the 50-NiO surface showed the best wettability to the electrolyte. Similar decrease in contact angle after plasma treatment were observed formerly in other transition metal oxides [70]. The oxygen-to-metal ratio on the surface of metal oxides is crucial for the wettability [71]. Harju et al. [72] observed that the oxygen vacancies in plasma sprayed TiO2 and Cr2O3 films can enhance water adsorption capacity by using isothermal gas adsorption studies. Through density functional theory calculation, Zhu et al. [59] reported that the plasma treated Sr2Fe1.3Ni0.2Mo0.5O6-d showed lower adsorption energy of water due to the oxygen vacancies created by plasma treatment in 5% H2/Ar, and thus exhibited higher OER activity. Chen et al. [73] reported that the Zeolitic Imidazolate Framework-67 template with oxygen vacancies created by O2eAr RF plasma can lower the adsorption energy of water for
nickel cobalt layered double hydroxides on the template, and further increase the OER activity. OER activity can be hindered by insufficient adsorption of water molecules and hydroxyl groups, which are generally the rate-determining step in the multistep OER reaction process [74e76]. For catalysts with high OER activity the surface needs to present with suitable hydrophilicity to facilitate the adsorption of intermediate species (i.e. hydroxyl) onto its surface, while not hinder the release of oxygen gas produced from the surface [33]. The oxygen vacancies created by Ar plasma treatments on the surface of NiO films facilitated the adsorption of water molecules and hydroxyl groups, as evidenced by the enhanced wettability to electrolyte in the contact angle measurements (Fig. 5) and increased intensity of surface adsorbed oxygen species in O 1s XPS spectra [Fig. 3 (b)]. With adequate water molecules and hydroxyl groups adsorbed onto the surface of oxygen deficient NiO film, the OER catalytic process can proceed smoothly and exhibited higher current density. Our observation suggested that the oxygen vacancy generated by plasma treatment can promote the adsorption of reactant and reaction intermediates on the catalyst surface, thus enhance the OER performance. As shown in the electrochemical test (Fig. 4), the 50-NiO film exhibited highest OER activity compared to pristine NiO, 100-NiO and 150 NiO films. The reason why the high plasma power leads poor OER performance is likely due to the lower surface oxygen vacancies density when the plasma power becomes too high. Plasma treatment is a complicated physical and chemical process involving energetic electrons, excited species, free radicals, and ions. It is normally used as an efficient strategy to introduce defects into variety of materials [51]. However, it was also reported that plasma treatment can also eliminate the existed defects in materials [77e80]. For instance, Yang et al. [78] reported decreased oxygen vacancies of In2O3 films with the increase of Ar plasma power during the deposition procedure. Xu et al. [31] demonstrated that the OER activity of Co3O4 nanosheets treated by Ar plasma for 120s was better than 180s and 240s. Similarly, it was reported that Ar plasma treatment with medium time length lead to highest OER activity for cobalt oxides [81] and NiFe layered double hydroxides [82]. Lim et al. [77] observed that the oxygenvacancies-related peak decreased in SnO2 nanowire after N2 plasma through photoluminescence spectra. Similar elimination effects on the oxygen vacancies were also observed in O2 plasma treatment [79,80]. For our case, plasma treatment with 50 W power was found to generate highest oxygen vacancy density and best wettability to the electrolytes. Further
Fig. 5 e (aed) Contact angle measurement results of 1 M KOH solution on (a) PeNiO, (b) 50-NiO, (c) 100-NiO, and (d)150-NiO; (e) Contact angle of 1 M KOH on NiO thin films as a function of plasma treatment power. Please cite this article as: Jin S et al., Revealing the effects of oxygen defects on the electro-catalytic activity of nickel oxide, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.130
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increase of plasma power to 100 W or 150 W leads to a decrease in oxygen vacancy concentrations and degraded OER performance. Our results indicate the existence of optimal plasma treatment condition for tuning the electro-catalytic activity of TMO by oxygen defect engineering.
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Conclusion [4]
In conclusion, we investigated the change of the OER activity for nickel oxides influenced by oxygen vacancies generated by plasma treatment by using a thin film model system prepared by PLD. Ar plasma treatments with different power were applied to introduce oxygen vacancies into the surface of NiO thin films. XRD and SEM tests showed no difference in the crystal structure and morphology between the pristine and the Ar plasma treated NiO. The OER activity for NiO films were found to be significantly enhanced after plasma treatment. The current density (1.7 V vs. RHE) for the NiO films subjected to 50 W, 100 W, and 150 W Ar plasma treatments showed 5.8, 2.8, and 1.6 folds enhancement compared with that of the pristine NiO film, respectively. Such enhancements are attributed to plasma induced surface oxygen vacancies which improved surface adsorption characteristics, as confirmed by surface contacting angle measurement. The 50-NiO exhibited the best OER performance compared with the PeNiO, 100-NiO, and 150NiO. The reason is likely due to the elimination of pre-existing oxygen defects from the surface during treatment as the plasma power become higher. Our results showed that oxygen defects can influence the intrinsic OER activity of TMO. Plasma treatment represents a promising approach for the optimization of OER activity for TMO electro-catalysts through the degree of freedom of oxygen defect engineering. The method and knowledge gain in this work can be applied to other transition metal oxides and sulfides for energy applications.
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Acknowledgements [13]
This work was supported by the National Natural Science Foundation of China (11605063, 11975102 and 91745203), Guangzhou Science and Technology Program General Projects (201707010146), the Fundamental Research Funds for the Central Universities (2018MS40), State Key Laboratory of Pulp and Paper Engineering (2018TS08), Guangdong Pearl River Talent Program (2017GC010281), Guangdong Innovative and Entrepreneurial Research Team Program (2014ZT05N200).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.130.
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Please cite this article as: Jin S et al., Revealing the effects of oxygen defects on the electro-catalytic activity of nickel oxide, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.130