Hydrothermal synthesized delafossite CuGaO2 as an electrocatalyst for water oxidation

Hydrogen production from water splitting provides an effective method to alleviate the ever-growing global energy crisis. In this work, delafossite CuGaO2 (CGO) crystal was synthesized through hydrothermal routes with Cu(NO3)2·3H2O and Ga(NO3)3·xH2O used as reactants. The addition of cetyltrimethylammonium bromide (CTAB) was found to play an important role in modifying the morphology of CuGaO2 (CGO-CTAB). With the addition of CTAB, the morphology of CGO-CTAB samples changed from irregular flake to typical hexagonal sheet microstructure, with an average size of 1–2 μm and a thickness of around 100 nm. Furthermore, the electrocatalytic activity of CGO-CTAB crystals for oxygen evolution reaction (OER) was also studied and compared with that of CGO crystals. CGO-CTAB samples exhibited better activity than CGO. An overpotential of 391.5 mV was shown to be able to generate a current density of 10 mA/cm2. The as-prepared samples also demonstrate good stability for water oxidation and relatively fast OER kinetics with a Tafel slope of 56.4 mV/dec. This work highlights the significant role of modification of CTAB surfactants in preparing CGO related crystals, and the introduction of CTAB was found to help to improve their electrocatalytic activity for OER. Graphical abstract


Introduction
To alleviate the global energy crisis and environmental pollution, the development of clean energy storage and conversion devices is required [1][2][3]. Water electrolysis is one of the most important energy technologies for producing clean hydrogen fuel [4,5]. The oxygen evolution reaction (OER) in water electrolysis is considered to be an important semi reaction for carbon dioxide electro-reduction, metalair batteries, hydrogen production, and nitrogen electroreduction, because the slow OER significantly affects the overall reaction efficiency of water electrolysis [6][7][8]. Currently, noble metal oxides [9][10][11] (such as RuO 2 and IrO 2 ) are still efficient catalysts for OER. Nevertheless, both the high cost of precious metals and the low catalytic efficiency of the oxides hinder their application. Hence it is necessary to develop efficient and earth abundant non-noble metal OER catalysts.
In recent years, many oxide-based electrocatalysts for water splitting, including transition metal oxides [12,13], hydroxides [14,15], selenides [16,17], and phosphates [18,19] have exhibited certain OER catalytic activities. A new class of transition metal oxides, delafossite materials (ABO 2 ), have also shown great potential for electrocatalytic applications, such as CuFeO 2 [20]. CuCoO 2 [21][22][23], CuMnO 2 [24]. Zhang et al. [25] have successfully customized the surface charge of silver delafossite AgCoO 2 by adjusting the surface charge transfer state and transferring it into an efficient OER catalyst. Compared with pure AgCoO 2 (395 mV), and RuO 2 (369 mV) or IrO 2 (338 mV) catalysts, AgCoO 2 /Ag catalysts need an overpotential of 271 mV to reach a current density of 10 mA/cm 2 . Mao et al. [24] synthesized CuMnO 2 powder by sol-gel method. The CuMnO 2 working electrode showed a current density of 12.3 mA/cm 2 at 1600 r/min and 11.9 mA/cm 2 without rotation for OER. In contrast to lots of work focusing on the application of p-type ABO 2 oxides in water electrolysis, very few studies have paid attention to CuGaO 2 (CGO) oxides. CGO is of great potential in catalytic materials because of its ultra-high carrier mobility, appropriate bandgap (~ 2.0 eV), and excellent long-term stability [26][27][28]. Ahmed and Mao [29] prepared three different morphologies of CGO, namely nanoparticles, submicron hexagons, and micron-sized particles, by a hydrothermal method at 190 °C, a sono-chemical method at 850 °C, and a solid-state reaction at 1150 °C, respectively. The CGO nanocrystals produce a current density of 23 mA/ cm 2 when the applied voltage is 0.6 V in 1 mol/L KOH solution for OER. They also successfully synthesized CGO nanoparticles at 850 °C by the sol-gel method [30]. The current densities of CGO nanoparticles were found to be 15 and 18 mA/cm 2 for H 2 and O 2 generation, respectively, in 0.5 mol/L KOH solution.
Since 2015, our team has been working on the hydrothermal synthesis of oxides, and we have successfully synthesized CuAlO 2 [31], CuCrO 2 [32,33], CuFeO 2 [34,35], CuMnO 2 [36,37], CuScO 2 [38], CuCoO 2 [21-23, 39, 40]. Du et al. [21] synthesized Ca doped CuCoO 2 via a simple polyvinylpyrrolidone-assisted hydrothermal process, which required an overpotential of 470 mV to achieve a current density of 10 mA/cm 2 and a small Tafel slope of 96.5 mV/ dec in alkaline solution for OER. Deng et al. [38] synthesized CuScO 2 by a hydrothermal method. The CuScO 2 particle-loaded foam nickel electrode requires an overpotential of 490 mV to provide a current density of 10 mA/cm 2 for OER. In this work, we intend to study the influence of the water oxidation performance of CGO with cetyltrimethylammonium bromide (CTAB) used as a surfactant in hydrothermal reaction. In detail, the effect of mineralizer on the synthesis of CGO was studied by changing the amount of NaOH at first, and then by adding a surfactant of CTAB to modify the morphology and structure of CGO crystals. Finally, CGO electrocatalyst with an excellent OER activity was obtained as expected.

Materials synthesis
All the chemicals in the experiment were of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd without further treatment. CGO is synthesized by a one-step hydrothermal method, modified from our previous works [32,33]. Usually, 10 mmol Cu(NO 3 ) 2 ·3H 2 O and 10 mmol Ga(NO 3 ) 3 ·xH 2 O were dissolved in 70 mL deionized solution and 5 mL ethylene glycol. Different amounts of NaOH (1.0, 2.0, 3.0 g NaOH), and surfactant (0, 1.8 g CTAB) were added to the above solution and stirred evenly. Then the solution was put into a 100 mL Teflon-lined autoclave and reacted in an oven at 190 °C for 24 h. The obtained precipitate was washed several times with ammonia, deionized water, and absolute ethanol, and then dried at 70 °C for 4 h for further characterization.

Structural characterization
The crystal structure of the prepared CGO samples was analyzed by X-ray diffraction (XRD, D8 Advance, Bruker). The morphology, microstructure, and chemical composition of CGO powders were observed by using field-emission scanning electron microscopy (FESEM, S4800, Hitachi) coupled with energy-dispersive X-ray spectroscopy (EDS). The surface chemical states of CGO powders were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Escalab 250Xi), and the C 1s line (284.80 eV) corresponding to the surface adventitious carbon (C-C line bond) was used as the reference binding energy. The BET (Brunauer-Emmett-Teller) specific surface areas and porosity parameters of these CGO samples were measured by N 2 adsorption-desorption isothermetry (Micromeritics TriStar II 3020 3.02).

Electrochemical measurements
The three electrode system was used for the electrochemical performance test. The saturated calomel electrode and platinum wire were used as the reference electrode and counter electrode, respectively, and 1 mol/L KOH solution was used as the electrolyte. The working electrode was prepared as follows. Typically, 15 mg CGO powder was ultrasonically dispersed in 500 μL water, 480 μL isopropanol, and 20 μL Nafion solution to prepare CGO suspension. The 20 μL CGO suspension was dripped on nickel foam (1 cm 2 ) with a pipette gun and then dried to obtain the working electrode, the loading mass of CGO electrocatalyst was 0.30 mg/cm 2 . The same loading mass of RuO 2 electrode, supported by nickel foam, was fabricated as a reference sample. The electrocatalytic performance of OER was evaluated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and chronopotentiometry at room temperature (~ 25 °C) using a CS2350H electrochemical workstation (Wuhan Corrtest Instruments Corp., China). All current density values were normalized relative to the geometrical surface area of the working electrode. All CV curves presented in this work were IR-corrected. The electrochemical data processing methods were based on previous works [21][22][23].

Results and discussion
In our previous reports [21,37,38,41], the concentration of NaOH in the precursor solution was the key factor affecting the crystal growth of ABO 2 oxides during hydrothermal synthesis. Using Cu(NO 3 ) 2 ·3H 2 O and Ga(NO 3 ) 3 ·xH 2 O as reactants, the amount of mineralizer NaOH (samples No. 1, No. 2, No. 3 in Table 1) was adjusted to prepare CGO crystals. As shown in Fig. 1a, when the amount of NaOH was 2 and 3 g, the majority of the XRD diffraction peaks of the as-obtained product  (Fig. 1c) shows that the size of CGO crystals is not uniform, ranging from 200 to 600 nm. The thickness of CGO particles is also uneven, up to 120 nm. It is known that the activity of the electrocatalyst is greatly influenced by its morphology and active sites [42]. The smaller the CGO particles are, the larger the specific surface area is, and the more active sites are exposed. Generally, the surfactant used in the hydrothermal reaction can help to control the particle size and the morphology of the product. By binding on the particle surface, surfactant molecules create a barrier for crystals proximity and induce strong spatial repulsion [43,44]. To optimize the structure and morphology of CGO, 5 mmol CTAB was added to improve the reaction kinetics and reduce the agglomeration of CGO particles (No. 4 in Table 1). XRD analysis in Fig. 1b confirms the formation of the CGO phase when 5 mmol CTAB (denoted as CGO-CTAB) was added in hydrothermal precursors. The CGO-CTAB samples (Fig. 1d) exhibit a typical hexagonal morphology with a thickness of about 100 nm and a transverse size around 1-2 μm. After adding the CTAB surfactant, the morphology and size of CGO particles become more uniform.
In addition, the chemical composition of CGO-CTAB samples was also studied. The SEM-EDS spectrum in Fig. 2a shows that there are four elements (Cu, Ga, O, Pt) in CGO-CTAB powders, of which Pt comes from platinum powder sprayed to enhance the conductivity of the sample before the SEM test. The atomic ratio of Cu/Ga is 19.06:20.08 (1:1.05) for the CGO-CTAB samples, which is   Fig. 2 a Elemental analysis report, b SEM image, c-e EDS elemental mappings, f N 2 adsorption-desorption, and g corresponding pore size distribution plots of CGO samples very consistent with the stoichiometric ratio. Figure 2c-e suggests a uniform distribution of the Cu, Ga, and O elements in the CGO-CTAB nanoplates. The nitrogen adsorption-desorption isotherms of as-prepared CGO powders are presented in Fig. 2f-g. The absorption isotherms of CGO and CGO-CTAB samples showed typical type IV adsorption isotherms and hysteresis rings, indicating the existence of typical mesoporous structures [13,45,46]. CGO-CTAB samples showed a high specific surface area, 24.69 m 2 /g, which is larger than that of CGO (9.50 m 2 /g) in this work and CuFeO 2 (11.38 m 2 /g) in Ref. [47]. The high specific surface area of CGO-CTAB provides a large electrode-electrolyte interface for the reactants and mesoporous morphology can provide developed transportation channels for water electrocatalytic reactants [46]. Besides, the pore volume of CGO-CTAB (0.091 cm 3 /g) is larger than that of CGO (0.023 cm 3 /g). The high porosity of catalysts is conducive to the separation of reactants and products during the OER process [48]. The pore size distribution of CGO-CTAB is centered around 2 nm in the mesoporous range according to the BJH models ( Fig. 2 g), which is in favor of the escape of newly generated oxygen gas molecules. These results show that CGO-CTAB with a large number of mesopores results in a large specific surface area and can provide more catalytic active sites, thereby helping to improve their electrocatalytic performance.
The chemical structures and surface chemical states of the CGO and CGO-CTAB samples were further investigated via XPS. Figure 3a exhibits typical signals of C 1s, O 1s, Cu 2p, and Ga 3d based on the XPS survey spectrum, indicating the presence of Cu, Ga, and O in CGO and CGO-CTAB samples. As shown in Fig. 3b-d, the high resolution spectra of Cu 2p, Ga 3d, and O 1s signals of CGO and CGO-CTAB samples were obtained to study their chemical states. Figure 3b displays the two strong binding energy (BE) peaks at around 932.6 and 952.4 eV which can be attributed to Cu 2p 3/2 and Cu 2p 1/2 , respectively, suggesting the existence of monovalent copper cations (Cu + ) in CGO and CGO-CTAB [49][50][51]. This is consistent with our previous reports on other delafossite oxides CuCoO 2 [21], CuScO 2 [41], and CuMnO 2 [37]. Figure 3c presents a high resolution Ga 3d spectrum for the CGO and CGO-CTAB, which consists of  [54,55]. The high resolution spectra of the O 1s spectrum (Fig. 3d) can be resolved into three peaks. The peaks at 530.0 eV correspond to the O 2− (lattice oxygen) of the delafossite oxides (CGO and CGO-CTAB) [56,57]. The BE at around 531.2 eV supports the -OH (oxygen in a hydroxyl group) [58,59], which is considered as the highest active oxygen, able to improve the generation of active species in the OER process [50]. The fitting peaks at 532.0 eV were assigned to the H 2 O (oxygen of physically absorbed H 2 O molecules) on CGO and CGO-CTAB samples [60,61]. The electrocatalytic activity of CGO for OER was evaluated by CV in 1.0 mol/L KOH. Figure 4a shows the CV curves of the two electrodes loaded with different CGO powder and these two electrodes are expressed as Ni@ CGO, Ni@CGO-CTAB, respectively. For comparison, the OER performances of the bare nickel foam (denoted as bare Ni) and the nickel foam loaded with commercial RuO 2 powder (denoted as Ni@RuO 2 ) were also tested. The corresponding overpotentials of these four electrodes at shows the equivalent circuit model for these CGO based electrodes different current densities were obtained from the reduction branches of the CV curves as shown in Fig. 4b. As seen in Table 2, the OER polarization curves of bare Ni achieved a current density of 10 mA/cm 2 at a low overpotential of 448.6 mV. After loading with CGO powder, Ni@CGO and Ni@CGO-CTAB exhibited overpotentials of 399.6 and 391.5 mV, respectively, while Ni@RuO 2 only required a low overpotential of 312.5 mV. Compared with the bare Ni (η 20 = 481.9 mV) and Ni@CGO (η 20 = 419.9 mV), Ni@ CGO-CTAB (η 20 = 408.9 mV) showed lower overpotentials at 20 mA/cm 2 . To understand the underlying dynamics of OER, Tafel analysis curves (Fig. 4c) were obtained by processing partial reduction curves in CV. The Tafel slopes of Ni@CGO and Ni@CGO-CTAB are 60.8 and 56.4 mV/ dec, respectively, which are much smaller than those of the bare Ni (86.0 mV/dec) and Ni@RuO 2 (79.4 mV/dec). The smaller the Tafel slope, the easier the oxygen evolution. Thus, the reaction kinetics of Ni@CGO-CTAB is the best and the electrocatalytic speed is the fastest. Moreover, the overpotential and Tafel slope of the CGO-CTAB electrode is lower than those of some other non-noble metal oxide catalysts (shown in Table 2), for example, delafossite oxide CuCoO 2 (η 10 = 440 mV, Tafel slope = 92.8 mV/dec) [22], AgCoO 2 (η 10 = 395 mV) [25], CGO nanocrystals (η 23 = 0.6 V vs Ag/AgCl) [29], CGO micro-sized particles nanocrystals (η 5 = 0.6 V vs Ag/AgCl) [29], CuCoO 2 (η 10 = 390 mV, Tafel slope = 70 mV/dec) [23], Ca doped CuCoO 2 (η 10 = 470 mV, Tafel slope = 96.5 mV/dec) [21], and CuScO 2 (η 10 = 470 mV, Tafel slope = 114 mV/dec) [38], or other perovskite electrocatalysts LaNiO 3 (η 10 = 550 mV, Tafel slope = 148 mV/dec) [62], LaNi 0.85 Mg 0.15 O 3 (η 10 = 450 mV, Tafel slope = 95 mV/ dec) [62], LaFeO 3 (η 10 = 420 mV, Tafel slope = 62 mV/dec) [63].
During the OER process, the transport rate of ions and charge is one of the important factors affecting the kinetics of electrocatalysis. The EIS measurements were performed in the frequency range of 20 mHz-200 kHz under a constant potential of 1.60 V vs RHE. In the Nyquist diagram (Fig. 4d), the intersection of the high-frequency range curve and the real axis is the electrolyte resistance (R s ), and the diameter of the semicircle is the charge transfer resistance (R ct ); the equivalent circuit is shown in the inset of Fig. 4d. The R s values of the bare Ni (R s = 1.3 Ω), Ni@CGO (R s = 1.5 Ω), and Ni@CGO-CTAB (R s = 1.3 Ω) were similar. Furthermore, the bare Ni electrode had the largest charge transfer resistance (R ct = 21.2 Ω), and the Ni@CGO-CTAB (R ct = 3.1 Ω) and Ni@CGO (R ct = 4.7 Ω) are smaller. Therefore, the Ni@CGO-CTAB showed the fastest charge transfer rate in the OER rate control step, which is conducive to the rapid charge transfer between the KOH solution and electrode interface. This result is consistent with the above analysis result of overpotential and Tafel slope.
The electrochemical active surface area (ECSA) is another important factor in the analysis of electrocatalytic activity. ECSA is proportional to the electrochemical double-layer capacitance (C dl ) of the catalyst surface [1,46,64]. As shown in Fig. 4 g-i, at the same scanning speed the CV curve area and current density of Ni@CGO-CTAB are larger. The calculated C dl value (Fig. 4e) of Ni@CGO-CTAB (2.1 mF/cm 2 ) is higher than that of the original value Ni@ CGO (1.8 mF/cm 2 ) and bare Ni (1.5 mF/cm 2 ). The result is in good agreement with BET results, which indicates that CGO-CTAB has a larger effective electrochemical area with more OER active sites.
Stability is another important index to evaluate the catalytic performance of electrocatalyst since the stability of the material can reflect its practical application value. As shown in Fig. 4f, the stability of the material was tested for 18,000 s at a constant current density of 5 mA/cm 2 . The initial potentials of Ni@CGO and Ni@CGO-CTAB electrodes  (Fig. 4a). During the whole electrolysis process, the potential of the bare Ni electrode did not increase obviously (1.66 V vs RHE). However, the required potential of both Ni@CGO and Ni@CGO-CTAB electrodes increased about 35 mV. These two electrodes loaded with CGO powder showed slight degradation, which may be related to the detachment of CGO powder from foam nickel. The XRD pattern in Fig. 5a

Conclusions
In summary, CuGaO 2 (CGO) crystals with a uniform size were obtained by a hydrothermal reaction at 190 °C for 24 h, and were prepared with Cu(NO 3 ) 2 , Ga(NO 3 ) 3 , NaOH, and cetyltrimethylammonium bromide as reactants. The introduction of CTAB not only modified the morphology and surface states through the interaction between surfactant and CGO microstructure but also affected the electrocatalytic activity of OER. Compared with CGO samples without surfactant, CGO-CTAB samples demonstrated a more uniform size (transverse size 1-2 μm, thickness ~ 100 nm), larger specific surface area (24.69 m 2 /g), and higher OER catalytic activity (a lower overpotential of 391.5 mV at 10 mA/cm 2 and a smaller Tafel slope of 56.4 mV/dec). After a long-term stability test for 18,000 s under OER condition, the Ni@ CGO-CTAB electrode still maintained excellent OER catalytic activity, and the potential degradation was only 36 mV. Therefore, the as-prepared delafossite CGO-CTAB has good electrocatalytic activity and durability, which show its broad application prospect for water oxidation.
Han Gao is a master student in the School of Materials Science and Engineering at Wuhan University of Technology, China. She graduated from Anhui University of Architecture, China with a bachelor degree in 2019. Her research interest is hydrothermal synthesis of nanostructured semiconductor and their application in photo/electrocatalytic water splitting.
Miao Yang is a master student in the school of Materials Science and Engineering at Wuhan University of Technology, China. She graduated from South-west University, China with a bachelor degree in 2019. Her current research interests include nanostructured photoelectric functional materials.
Xing Liu is an undergraduate student in the School of Materials Science and Engineering at Wuhan University of technology, China. His research interests include preparation and characterization of inorganic semiconductor hole transport materials for photoelectric devices.
Xianglong Dai is an undergraduate student in the School of Materials Science and Engineering at Wuhan University of technology, China. His research interests include hydrothermal synthesis of nanostructured inorganic semiconductor for photoelectric devices.