The oxygen evolution reaction (OER) is a bottleneck to water splitting to evolve H₂ and to comprehensive utilization of CO₂ at industrial scale [1]. It is mechanistically complicated, involving the transfer of four electrons and four protons coupled with the formation of an O−O bond [2, 3]. Photosystem II in nature exhibits very high catalytic efficiency with TOFs in the range of 100-400 s^-1 [4], which provides an example to motivate researchers to develop efficient, robust, and economical oxygen evolution catalysts (OECs) [5]. In a steady stream, exciting research results have been reported in past decades [6-16]. To date, Ru and Ir-based oxides and carbides have been widely acknowledged as excellent electrocatalysts. At a current density of 10 mA cm^-2, their overpotentials are 0.28-0.36 V in 1 M NaOH and 0.34-0.44 V in 1 M H₂SO₄ [17, 18]. However, the scarcity and high cost of such noble metals, as well as their stability, hampers their widespread usage [17-21]. Therefore, inexpensive and earth-abundant transition-metal (TM = Fe, Co, Ni, Mn, Mo, V, W, etc.) alternatives such as oxides [22-25], hydroxides [26-29], sulfides [30-32], selenides [33], phosphides [34, 35], nitrides [36], and borides [37] have been explored in connection with promising advances in recent years.

The economy and abundance of Ag-based catalysts have attracted deep interest because both Ag and Ru are in the fifth period so that they have similar properties. Moreover, both have higher electron density and more adjustable coordination numbers than those of the 3d transition metals. In addition, Ag can exist in three oxidation states (I, II, and III). This makes it advantageous to employ catalysts based on Ag as OECs. In fact, it has been reported that both simple Ag⁺ ions and the coordination compound [H₃Ag^I(H₂O)PW₁₁O₃₉]^3-, as homogeneous OECs, exhibit high catalytic activity in acidic aqueous solutions [38-41]. Li's group [42-45] has fabricated Ag₂O-AgO/ITO electrodes by in situ electrochemical deposition in K₂B₄O₇ and NaHCO₃ electrolytes, and the electrodes have higher overpotentials than the corresponding Co and Ni-based electrodes do at room temperature. However, the overpotential decrease to 132 mV at 1 mA cm^-2 (when the OER takes place) occurs at 70 ℃.

In recent years, crystal facet engineering has emerged as an important strategy in designing efficient electrocatalysts for water splitting [46]. A semiconductor with selective exposure of facets exhibits different electrochemistry of charge transfer, adsorption-to-reaction of reactants, and catalytic performance due to differences in atomic coordination and configuration of different facets. For example, the different preferentially exposed crystal facets of Co₃O₄ exhibit different activities for electrochemical water oxidation, of which the order is {111} > {112} > {110} > {001} [47, 48]. It is reported that Ag₂O can be fabricated to have many morphologies, including cubes, great rhombicuboctahedra, cuboctahedra, corner-truncated octahedra, octahedra, nanocubes, rhombic dodecahedra, edge and corner-truncated cubes, small rhombicuboctahedra, and edge-truncated octahedral [49]. Different morphologies mean differences in the exposed crystal facets, which exhibit distinctive properties. For instance, the cubic Ag₂O particles enclosed by the {100} facets adsorb bacteria more easily [50]. Ag₂O octahedra and hexapods bounded by positively charged {111} facets respond repulsively when dispersed in a solution of positively charged methylene blue solution, but can be suspended in a solution containing negatively charged methyl orange [51]. The cubic Ag@Ag₂O photocatalyst with exposed {100} facets show the greatest activity for the degradation of methyl orange (MO) under visible light irradiation [52]. For the exposed crystal facets, the order of stability against chemical etching or dissolution in NH₃-NaOH solution is {111} > {110} > {100} [53].

Recently, Zhao et al. [54] reported that AgO crystals with mainly {111} exposed crystal facets yield superior catalytic activity for the OER over those facets with mainly {202} exposed crystal facets. It (an exposed {111} facet) has a modest overpotential of 418 mV at 1 mA cm⁻² and a Faradaic efficiency of 93%. It is of interest to study the relationship of crystal facets in relation to the activity of Ag₂O. Therefore, in this work a series of Ag_2-xO electrodes was fabricated by galvanostatic electrocrystallization in 0.1 M K₂B₄O₇ electrolyte. It was found that the Ag_2-xO crystals with preferentially exposed {111} facets show higher electrochemical water oxidation activity. In addition, the mechanism of electrocatalytic water oxidation on the Ag_2-xO electrode was also studied.

All reagents including AgNO₃ (99.8%), K₂B₄O₇ (99.5%), and H₂O₂ (30.0%) employed were commercially available and used as supplied without further purification. Ultrapure water with a resistivity of ∼18 MΩ cm⁻¹ was used to prepare all solutions. F-doped SnO₂-coated (FTO) glass slides (15 Ω γ^-1, Sheet Glass) were purchased from Dalian Heptachroma Solar Tech Co., Ltd. The FTO glass slides were washed ultrasonically in acetone and then rinsed with ultrapure water before being used.

Anodic electrolysis deposition was performed by galvanostatic electrocrystallization according to reported methods [42, 55], using a LANHE CT2001A electrochemical workstation at current density 0.50, 1.00, 2.00, 3.00, 4.00, and 7.00 mA cm^-2 at room temperature in an H-type divided cell separated by a G2 glass filter. As shown in Fig. S1, the anode compartment of the H-type cell contained an aqueous solution of 1.00 mM AgNO₃-0.10 M K₂B₄O₇ and the cathode compartment contained an aqueous 0.10 M K₂B₄O₇ solution. An FTO glass slide was used as a positive electrode and the electrodeposition area was controlled to remain at 1.00 cm². The reference electrode was a saturated Hg/Hg₂Cl₂ electrode (SCE) and the counter electrode was a platinum sheet. Electrochemical deposition was carried out until the charge reached 5.0 C. After deposition, the Ag_2-xO/FTO-i electrode was rinsed with ultrapure water, dried in the air, and then stored for subsequent experiments. Based on the current density in the electrodeposition process, they were marked as Ag_2-xO/FTO-0.5, Ag_2-xO/FTO-1, Ag_2-xO/FTO-2, Ag_2-xO/FTO-3, Ag_2-xO/FTO-4, and Ag_2-xO/FTO-7, respectively.

X-ray powder diffraction (XRD) patterns were recorded on an Advance diffractometer (Empyrean) with Cu K_α radiation (λ = 1.5406 Å) in the 2θrange of 20~ to 70~ with operating potential of 30 kV. SEM images were obtained using a scanning electron microscope (ZXM636-LV). HRTEM images were recorded on a JEM 2100F field emission microscope operating at 200 kV. Raman spectra were collected using a 532 nm diode-pumped solid-state laser, commercial Raman spectrometer (Invia). XPS patterns were recorded on a spectrograph (VG ESCALABMKII) with Al K_α radiation (1253.6 eV). A vacuum pressure of 6.2 × 10^-6 Pa was maintained during the testing process.

All electrochemical experiments were performed on a CHI660E electrochemical workstation (Shanghai Chenhua Instrument Corp., China) with a three-electrode system and collected in 0.10 M K₂B₄O₇ electrolyte at pH 9.2 with a scan rate of 100 mV s^-1. A platinum sheet and an SCE electrode were used as the counter and reference electrodes, respectively. The working electrodes were of Ag_2-xO/FTO-i. Electrode potentials were converted relative to a reversible hydrogen electrode (RHE) scale, using the formula E_RHE = E_SCE + 0.244 + 0.059 × pH. CVs were obtained in the potential range of 1.4-2.0 and 1.4-1.7 V vs. RHE. Linear sweep voltammetry (LSV) was performed in the potential range of 0.8-2.4 V vs. RHE. EIS was measured at a bias potential of −0.5 V in the frequency range of 10^-2 to 10⁵ Hz. The I-t curve was recorded at 1.20 V.

The Ag_2-xO/FTO-i electrodes were formed at the current density of 0.50, 1.00, 2.00, 3.00, 4.00, and 7.00 mA cm^-2 in 0.10 M K₂B₄O₇ electrolyte by galvanostatic electrocrystallization. When the amount of electric charge reached 5.0 C cm^-2, the electrolysis was stopped. The variation of the electrolysis potential with the quantity of electric charge is shown in Fig. S2. It was found that the potential fluctuated slightly when the charge was < 1.0 C cm^-2 (when the potential remained basically unchanged), but increased with increase in the applied current density. The stable potentials were about 1.00, 1.10, 1.15, 1.20, 1.28, and 1.32 V vs. SCE when the current density was 0.50, 1.00, 2.00, 3.00, 4.00, and 7.00 mA cm^-2. A uniform, dark, smooth Ag_2-xO film was formed on an FTO glass slide. No significant shedding was observed in the experiments.

Fig. 1 shows the normalized XRD patterns of the Ag_2-xO/FTO-i (i = 0.5, 1, 2, 3, 4, and 7) electrodes. Except for the diffraction peaks of the FTO substrate, the diffraction peaks at 26.6º, 38.0º, and 65.4º correspond to {110}, {200}, and {311} crystal facets of Ag₂O (JCPDS no. 41-1104). This demonstrates that the electrode films are composed of Ag₂O. However, it was noted that the diffraction peak at 32.8º for the {111} crystal facets of standard Ag₂O, obviously shifts to 33.4º with a Ag_2-xO/FTO-0.5 electrode and to 33.9º for other Ag_2-xO/ FTO-i (i = 1, 2, 3, 4, and 7) electrodes. The shift of the diffraction peak of the {111} crystal facet indicates that Ag⁺ ions on the {111} crystal facet are partially oxidized into Ag²⁺ ions during the electrodeposition process. Therefore, it was determined that a more accurate description of the component of the electrode films should be Ag_2-xO. This result is similar to that of an earlier reference [45], but the shifts are larger because more Ag²⁺ ions were generated in our work. Furthermore, by close inspection of the XRD patterns of the Ag_2-xO/FTO samples, it was noted that the diffraction peak intensity ratios of {111} and {200} crystal facets (Table S1) are approximately 1.59 for the Ag_2-xO/FTO-1 electrode and 0.4, 0.48, 0.39, 0.32, and 0.36 for the other Ag_2-xO/FTO-i electrodes (i = 0.5, 2, 3, 4, and 7, respectively). This result indicates that a Ag_2-xO/FTO-1 electrode exhibits a larger exposed {111} facet, and more noteworthy, that some of the Ag⁺ ions on the {111} facet are oxidized to Ag²⁺ ions. This could affect the OER.

Fig. 2a shows the XPS survey spectrum of the Ag_2-xO/FTO-1 electrode. The C 1s hydrocarbon peak was fixed at the binding energy of 284.6 eV to subtract the surface charging effect. This shows that the electrode film consists of O and Ag elements and that no other elements exist. Fig. 2b is the Ag 3d XPS spectrum of the Ag_2-xO/FTO-1 electrode. The two broad peaks appearing at 368.02 and 374.03 eV were assigned to the binding energy of Ag 3d_5/2 and 3d_3/2, respectively. The peak at ~377.5 eV was assigned to a weak satellite signal. According to the fitting results, the two peaks at 368.21 and 374.10 eV could be attributed to Ag⁺ ions [56], while the other two peaks at 367.81 and 373.89 eV were derived from Ag²⁺ ions. The ratio of the peak areas belonging to Ag⁺ and Ag²⁺ was approximately 1.46. This indicates that some Ag²⁺ ions exist on the Ag_2-xO/FTO-1 electrode, which is in good accord with the XRD results. The higher valence state Ag²⁺ ions may be advantageous for the OER [57-63].

Fig. 3 shows the SEM and HRTEM images of Ag_2-xO/FTO-i electrodes. The SEM images show that Ag_2-xO/FTO-i (i = 0.5, 1, 4, and 7) electrodes display relatively dense Ag_2-xO film while Ag_2-xO/FTO-i (i = 2 and 3) electrodes are composed of sparse Ag_2-xO particles with poor density. The morphologies of Ag_2-xO particles can be observed at magnification of 10000 times in the insets of Fig. 3a-3f. It is shown that Ag_2-xO/FTO-0.5 and Ag_2-xO/FTO-1 exhibit a triangular slice of the vertical matrix and parallel matrix, respectively, whereas Ag_2-xO/FTO-i (i = 2, 3, 4, and 7) electrodes show wheat-ear shapes with a parallel matrix. The triangular surface morphology of the Ag_2-xO/FTO-1 electrode corresponds to the preferred growth orientation of {111} crystal facets of Ag_2-xO [53]. The HRTEM image of Ag_2-xO was examined (Fig. 3g) and showed that Ag_2-xO has a crystalline structure with lattice spacing of 0.267 nm, which was attributed to the d-spacing for the {111} plane of Ag_2-xO [54]. It is known that many more terminal Ag ions are exposed on the {111} crystal facet (than on other facets), which might lead to higher activity for electrocatalytic water oxidation there.

The electrocatalytic water oxidation performance of the Ag_2-xO/FTO-i electrodes was studied by linear sweep voltammetry (LSV) in 0.10 M K₂B₄O₇ solution at pH 9.2. As shown in Fig. 4, the anode currents of Ag_2-xO/FTO-i electrodes start and continue until the applied potential increases to ∼1.76 V vs. RHE without compensation for an iR drop. Additionally, it is noted that Ag_2-xO/FTO-1 electrodes show obviously higher oxidative current density than the other Ag_2-xO/FTO-i (i = 0.5, 2, 3, 4, and 7) electrodes do. For instance, at 2.39 V vs. RHE, the current density was 10.00 mA cm^-2 for the Ag_2-xO/FTO-1 electrode, but was 5.18, 4.22, 4.43, 5.86, and 5.44 mA cm^-2 for Ag_2-xO/FTO-i (i = 0.5, 2, 3, 4, and 7, respectively), and only 0.55 mA cm^-2 for a bare FTO electrode. That is, the Ag_2-xO/FTO-1 electrode has oxidative current density nearly 2 times higher than that of the other Ag_2-xO/FTO-i (i = 0.5, 2, 3, 4, and 7) electrodes and 18 times higher than that of the FTO electrode. The Tafel plots (Fig. S3) and Tafel equations can be obtained by analyzing the data in Fig. 4. As listed in Table 1, the onset potentials of the Ag_2-xO/FTO-i electrodes are all similar (1.74-1.77 V vs. RHE). The overpotentials (η = V_RHE - iR - 1.23 V, where V_RHE is the applied potential vs. RHE) are 433, 417, 439, 438, 428, and 431 mV at 10.00 mA cm^-2 and the Tafel slopes are 63, 47, 59, 88, 68, and 61 mV dec^-1 for Ag_2-xO/FTO-i (i = 0.5, 1, 2, 3, 4, and 7, respectively). As a consequence, Ag_2-xO/FTO-1 electrodes have the lowest overpotential and the smallest Tafel slope, which are ascribed to their larger exposed {111} crystal facets and to the existence there of high valence Ag²⁺ ions. The above results prove clearly that Ag_2-xO O is an efficient OEC under alkaline conditions and that the {111} crystal facets are more active. Compared to a AgO/FTO electrode [54], a Ag_2-xO/FTO-1 electrode shows lower overpotential. The OER catalytic performances of partial Ag-based catalysts and of Ir/C, IrO₂ or RuO₂ catalysts, are listed in Table S2 for comparison.

Electrochemical impedance spectra (EIS) of Ag_2-xO/FTO-i electrodes were measured at a bias potential of −0.50 V vs. SCE, and the Nyquist curves are shown in Fig. 5. To interpret these plots, an equivalent circuit (inset) is presented that show electrochemical pseudocapacitance (CPE1) and charge transfer resistance (R_ct) [64, 65]. A variety of impedance parameters can be obtained by fitting the Nyquist curve. The R_ct values are 2.63 × 10⁴, 2.02 × 10³, 3.45 × 10⁴, 2.81 × 10⁴, 1.71 × 10⁴, and 2.21 × 10⁴ Ω for Ag_2-xO/FTO-i (i = 0.5, 1, 2, 3, 4, and 7) electrodes, respectively. The Ag_2-xO/FTO-1 electrode shows the lowest R_ct value, which suggests that Ag_2-xO/FTO-1 electrodes have the preferred capacity to transfer electrons at the electrode and electrolyte interface. This result explains why the Ag_2-xO/FTO-1 electrode has better catalytic activity for the OER.

Moreover, the electrocatalytic durability of catalysts is a vital indicator for their commercial viability. Therefore, we determined the OER stability of a Ag_2-xO/FTO-1 electrode using an I-t test at 1.2 V vs SCE in 0.1 M K₂B₄O₇ electrolyte at pH 9.2. As shown in Fig. 6, electrolysis begins with electrochemical pseudocapacitance, which is ascribed to the oxidation of Ag⁺ to Ag²⁺. After 1 h, the current density gradually rises to 2.84 mA cm^-2 and then decreases slightly to 2.72 mA cm^-2 in 10 h. In this case, the OER current became predominant. Even more encouraging, the Ag_2-xO/FTO-1 electrode remained stable during 10 h of electrochemical testing.

Fig. 7 shows the CV curves of Ag_2-xO/FTO-i electrodes with potential in the range 0.2-2.0 V vs. RHE with a scanning rate of 100 mV s^-1. It is clear that the Ag_2-xO/FTO-1 electrode has a better electrochemical property. It shows oxidative current density 3 times higher than that of other Ag_2-xO/FTO-i (i = 0.5, 2, 3, 4, and 7) electrodes and 16 times higher than that of a bare FTO electrode at 2.0 V vs. RHE. This further proves that the Ag_2-xO/FTO-1 electrode, which has larger {111} well-exposed crystal facets, has superior catalytic activity for the electrochemical OER. Furthermore, the CV curve of Ag_2-xO/FTO-1 clearly shows a strong oxidation peak (I) at 1.4 V vs. RHE and two reduction peaks (II and III) at 0.82 and 1.33 V vs. RHE. This implies that it may offer a convenient opportunity to study the electrochemical and electrocatalytic performance of heterogeneous Ag⁺-based catalysts.

In earlier work, the oxidation peak (I) and reduction peak (II) were assigned to the redox of Ag⁰ and Ag⁺ [66]. The reduction peak (III) may be due to the reduction of Ag²⁺ or O₂^2-, which ions were produced in the water oxidation process at high potential. Accordingly, the CV curves b and c (Fig. 8) were recorded carefully by design. From Fig. 8b, it can be seen that the reduction peak (III) disappears when the scanning potential is from 0.2 to 1.7 V vs RHE; meanwhile, Fig. 8c shows that the reduction peak (III) appears as 0.5 mM H₂O₂ is added to the electrolyte at 0.2 V and 1.7 V vs RHE. This phenomenon indicates that the reduction peak (III) does not originate from the reduction of Ag²⁺ on the Ag_2-xO/FTO-1 electrode, but instead from the reduction of residual O₂^2- generated on the surface of Ag_2-xO at 0.2-2.0 V vs. RHE.

To determine further the chemical status of the O and Ag elements in Ag_2-xO/FTO-1 during electrocatalysis, XPS spectra were recorded after the LSV measurement and are shown in Fig. 9. As shown in Fig. 9b and 9d, in contrast to the single peak of O 1s at 531.70 eV for the freshly prepared Ag_2-xO/FTO-1, two XPS peaks at 531.70 and 533.10 eV of O 1s emerge for the Ag_2-xO/FTO-1 electrode after the LSV measurement at 0.8-2.4 V vs. RHE. In this case, the peak at 531.70 eV belongs to O^2- ions, and the other at 533.10 eV is assigned to O₂^2- on the surface of Ag_2-xO/FTO-1 [67]. This result further confirms the previous speculation from the CV curves. Fig. 9c shows the Ag 3d XPS spectrum of the Ag_2-xO/FTO-1 electrode before and after the LSV measurement from 0.8 to 2.4 V vs. RHE. Compared to the Ag 3d XPS spectrum of freshly prepared Ag_2-xO/FTO-1, it is found that the binding energies of Ag 3d_5/2 and Ag 3d_3/2 undergo a slight shift to lower energies, which is attributed to increases in Ag²⁺. The peak area ratio of Ag⁺ and Ag²⁺ decreases from 1.46 to 0.65, from which it was inferred that there was a noticeable increase in the content of Ag²⁺ after the LSV at 0.8-2.4 V vs. RHE. It was deduced that the Ag²⁺ and O₂^2- ions coexist in equilibrium.

The visible-Raman spectrum was employed to investigate the O₂^2- species on the surface of Ag_2-xO/FTO-1 electrode. As shown in Fig. 10a and 10b, only two peaks at 430 and 470 cm^-1 appear for both the freshly prepared Ag_2-xO/FTO-1 electrode and that after the CV measurement at 0.2-1.7 V vs. RHE, which are associated with the Ag-O stretching vibration [68]. In contrast, additional peaks at 980, 1053, and 1300 cm^-1 appear for the Ag_2-xO/FTO-1 electrode after the LSV at 0.8-2.4 V vs. RHE, which corresponds to the Ag-O₂^2- species [69].

Based on the above results, a possible mechanism of water oxidation for oxygen evolution is proposed. As shown in Scheme 1, when more Ag⁺ ions are oxidized to Ag²⁺ ions at higher potential, OH^- and H₂O adsorb to the surface of Ag_2-xO, then the electron-transfer from OH^- to Ag²⁺ takes place at the adjacent Ag²⁺-OH species. It can be seen that the active electrons on adjacent oxygen atoms tend to transfer to Ag²⁺ to form Ag⁺, and simultaneously the O-O bond is formed by the departure of two H⁺. Furthermore, O₂ molecules are generated when the Ag⁺ ions connecting the O-O bonds are oxidized to Ag²⁺ or Ag³⁺. Because Ag_2-xO crystals with a larger {111} crystal facet area expose more Ag ions that can preferably combine with OH^- ions and H₂O molecules [53], these facets are remarkably well suited to catalyze water oxidation and generate oxygen evolution on Ag_2-xO/FTO-1 electrodes.

In this work, a series of Ag_2-xO/FTO-i electrodes were fabricated by galvanostatic electrocrystallization. Among them, the Ag_2-xO/FTO-1 electrode exhibits larger {111} well-exposed crystal facets, and its intensity ratio of diffraction peaks of {111} and {200} crystal facets is 3 times higher than the other Ag_2-xO/FTO-i (i = 0.5, 2, 3, 4, and 7) electrodes. Furthermore, it was found that some of the Ag⁺ ions on {111} crystal facets were oxidized to Ag²⁺ ions. The Ag_2-xO/FTO-1 electrode has an overpotential of 417 mV at 10.00 mA cm^-2 and a Tafel slope of 47 mV dec^-1 during electrocatalytic water oxidation in 0.1 M K₂B₄O₇ at pH 9.2. The Ag_2-x O/FTO-1 electrode exhibits obviously better electrocatalytic OER performance than the other Ag_2-xO/FTO-i electrodes, which result is ascribed to the larger exposed {111} crystal facet area and to more Ag⁺/Ag²⁺ ions being exposed in Ag_2-xO/FTO-1. In addition, Ag²⁺ ions and O₂^2- species were fortunately observed in the CV, XPS, and Raman spectra during electrocatalysis, based on which a possible mechanism of electrocatalytic water oxidation on Ag_2-xO/ FTO-1 electrodes is proposed.