Efficient oxidation of water to O₂ is key to the production of H₂ fuel and for the reduction of CO₂ by electrolysis, photocatalysis, photoelectrocatalysis, and other approaches. Therefore, considerable attention has been paid to developing viable water oxidation catalysts (WOCs) [1-6]. Polyoxometalates have been selected as carbon-free inorganic ligands for the construction of these catalysts because of their high stability towards oxidative degradation and capacity to transfer electrons and protons [7]. As a class of homogenous molecular WOCs, a series of ruthenium-[8-14] and cobalt-[15-21] polyoxometalate complexes, including [{Ru₄O₄(OH)₂(H₂O)₄} (γ-SiW₁₀O₃₆)₂]^10- and [Co₄(H₂O)₂(α-B-PW₉O₃₄)₂]^10-, have been intensively studied. These investigations have demonstrated the promise of polyoxometalates for multi-electron-transfer catalysis.

Recently, immobilization of molecular polyoxometalate-based WOCs for the development of electrodes and photoelectrodes has drawn interest. We found that the high reactivity of molecular WOCs was retained when supported on various materials [22-24]. Bonchio et al. [25] deposited [Ru₄(H₂O)₄(μ-O)₄(μ-OH)₂(γ-SiW₁₀O₃₆)₂]^10-@multi-walled carbon nanotubes (MWCNTs) on an ITO substrate to obtain an oxygen-evolving electrode that produced an over-potential (η) as low as 0.35 V and TOFs approaching those of the cluster in homogeneous solution (306 h^-1 at η = 0.60 V). Hill et al. [26] immobilized [Ru₄^ⅣO₅(OH)(H₂O)₄(γ-PW₁₀O₃₆)₂]^9- and [{Ru₄^Ⅳ(OH)₂(H₂O)₄} (γ-SiW₁₀O₃₄)₂]^10- on TiO₂/FTO electrodes via the a silanization-cationization process, which resulted in a continuously enhanced photocurrent or catalytic water oxidation activity.

In 2015, we reported a new WOC, Ag⁺-based polyoxometalate complex, [H₃Ag^Ⅰ(H₂O)PW₁₁O₃₉]^3-, and proposed its mechanism of chemical water oxidation in the presence of S₂O₈^2- [27]. Herein, this Ag⁺-polyoxometalate complex was further immobilized on a nanocrystalline TiO₂ electrode owing to its sufficient stability and notable catalytic ability when combined with photosensitizers. Moreover, the electrocatalytic and photoelectrocatalytic effects of [Ag(H₂O)(H₃PW₁₁O₃₉)]^3- on a TiO₂ anode for water oxidation were investigated and the electrocatalytic mechanism is proposed.

All chemicals were commercially available and used without further purification. K₃[Ag(H₃PW₁₁O₃₉)]·12H₂O (AgPW₁₁) and K₄[H₃PW₁₁O₃₉]·14H₂O (PW₁₁) were synthesized according to reported methods [27, 28]. The TiO₂ powder was commercial P25. The TiO₂ paste was prepared according to a previous report [29]. X-ray powder diffraction (XRD) patterns were recorded on a Bruker AXS D8 Advance diffractometer with the use of Cu K_α radiation (λ = 1.5418 Å) in the 2θ range of 5°–60° with a step size of 0.02°. Scanning electron microscope (SEM) images and energy dispersive X-ray (EDX) analytical data were obtained on a scanning electron microscope (ZXM6360-LV) with an EDX detector.

TiO₂/ITO electrodes: a piece of ITO conductive glass was successively ultrasonically cleaned with detergent, isopropanol, ethanol and deionized water for 20 min each, and finally dried in air. The clean ITO substrate was then coated with TiO₂ pasted by a screen-printing [30] technique to obtain a film with an area of 0.36 cm². The screen-printing process was repeated three times. Finally, a TiO₂/ITO electrode was obtained by annealing the TiO₂-coated ITO at 450 ℃ for 1 h.

AgPW₁₁-TiO₂/ITO electrode: The TiO₂/ITO electrode was dipped into 20 mL of K₃[H₃AgPW₁₁O₃₉]·12H₂O solution (2.00 mmol L^-1) overnight, followed by washing with 5 mL deionized water three times followed by drying in air.

AgNO₃-TiO₂/ITO and PW₁₁-TiO₂/ITO electrodes were fabricated by the same process except that AgNO₃ or PW₁₁ replaced the AgPW₁₁.

All electrochemical experiments were performed on a CHI600B electrochemical workstation (Shanghai Chenhua Instrument Corp., China) with a three-electrode system. A Pt wire and Ag/AgCl (3.00 mol L^-1 KCl) were used as the counter and reference electrodes, respectively. The working electrodes included a glassy carbon electrode, a TiO₂/ITO electrode and modified TiO₂/ITO electrodes. The glassy carbon electrode was polished for 60 s with 0.05 μm alumina particles and sonicated twice for 30 s in reagent grade water prior to use. Cyclic voltammograms (CVs) were collected in 0.10 mol L^-1 NaH₂PO₄-Na₂HPO₄ electrolyte, having a pH in the range of 5.3–6.7, at different scan rates in the range of 100–900 mV s^-1 and at 1.0 or 0.1 mA V^-1 sensitivity. Other electrochemical measurements were performed in 0.10 mol L^-1 Na₂SO₄ electrolyte. Electrochemical impedance spectra (EIS) were measured at a bias voltage of -0.3 V with an alternating current (ac) bias signal of 5 mV in the frequency range of 0.01–1×10⁵ Hz. The photo-electrochemistry was measured under simulated AM 1.5 G illumination (1 sun, 100 mW cm^-2) from a 300 W Xe arc lamp without a filter.

The TiO₂ powder used to fabricate the electrodes was commercial P25. XRD patterns of the electrodes are shown in Fig. 1(a). The ITO conductive glass showed strong diffraction peaks at 2θ = 26.5°, 33.7°, 37.9°, 51.7°, 61.8°, and 65.9°. Both TiO₂/ITO and AgPW₁₁-TiO₂/ITO electrodes showed characteristic peaks from anatase at 2θ of 25.2°, 48.0°, and 54.4° and characteristic peaks from rutile at 2θ of 27.3°, 36.0°. No diffraction peaks were observed for AgPW₁₁, likely because of the small amount present on the TiO₂ surface [31].

SEM imaging was conducted to provide detailed information about the surface morphology and homogeneity of the TiO₂ and AgPW₁₁-TiO₂ films on the ITO substrate. As shown in Fig. 2, both the TiO₂ and AgPW₁₁-TiO₂ films showed typical granular patterns and no cracks. The film thickness was estimated to be approximately 7 μm (Fig. 1(b)). The pure TiO₂ film consisted of particles with size in the range of 10–40 nm (Fig. 2(a)); however, the average particle size was slightly larger (15–60 nm) for the AgPW₁₁-decorated TiO₂ film (Fig. 2(b)), which could be attributed to the introduction of AgPW₁₁. The EDX spectra indicated the existence of Ag, P, and W on the AgPW₁₁-TiO₂/ITO electrode (Fig. S1). This result confirms that AgPW₁₁ was present on the TiO₂ surface.

The behaviors of the TiO₂/ITO and AgPW₁₁-TiO₂/ITO electrodes in the electrocatalytic oxidation of water were studied in Na₂SO₄ solution (0.1 mol L^-1). Fig. 3 shows the results of linear sweep voltammetry of the TiO₂/ITO electrode and AgPW₁₁-TiO₂/ITO electrode. When the applied voltage was less than 1.3 V vs. Ag/AgCl, the anode currents of both the TiO₂/ITO electrode and AgPW₁₁-TiO₂/ITO electrode were small and showed no obviously differences. However, when the applied voltage was more than 1.3 V vs. Ag/AgCl the anode currents of the TiO₂/ITO electrode and AgPW₁₁-TiO₂/ITO electrode increased gradually as the voltage increased. Moreover, the increase of the range of the latter was clearly greater than that of the former. In other words, a larger applied voltage led to a greater difference in their catalytic behaviors. At an applied voltage of 1.5 V vs. Ag/AgCl, the anode current density of the TiO₂/ITO electrode was less than 0.3 mA cm^-2; however, that of the AgPW₁₁-TiO₂/ITO electrode reached 1.5 mA cm^-2. Thus, AgPW₁₁ has a pronounced electrocatalytic effect on water oxidation at a TiO₂ anode, particularly at a higher applied voltage. To further verify the catalytic effect, the behaviors of TiO₂/ITO, PW₁₁-TiO₂/ITO, AgNO₃-TiO₂/ITO, and AgPW₁₁-TiO₂/ITO electrodes at bias voltages of 1.5 V vs. Ag/AgCl were studied by chronoamperometry. As shown in Fig. 4, their anode currents were stable in the range of 300 s, and their current densities were -0.1, -0.2, -0.4, and-1.0 mA cm^-2, respectively. The current density of the AgPW₁₁-TiO₂/ITO electrode was 10 times as high as that of the TiO₂/ITO electrode and 2.5 times as high as that of AgNO₃-TiO₂/ITO electrode.

Electrochemical impedance spectra of TiO₂/ITO, AgNO₃-TiO₂/ITO and AgPW₁₁-TiO₂/ITO electrodes were measured at a bias voltage of -0.3 V vs. Ag/AgCl (Fig. 5). The Nyquist curves of the three electrodes represent the charge transfer resistance (R_ct) at the electrode-electrolyte interface. A variety of impedance parameters can be obtained by fitting the Nyquist curve [32]. The R_ct values were 4774, 1257 and 840 Ω, respectively. The AgPW₁₁-TiO₂/ITO electrode showed the lowest R_ct value, which suggested that AgPW₁₁ had the capacity to transfer electrons at the electrode-electrolyte interface. This result explains the catalytic effect of the AgPW₁₁-TiO₂/ITO electrode for water oxidation. The R_ct value of the AgPW₁₁-TiO₂/ITO electrode was lower than that of AgNO₃-TiO₂/ITO, which suggests that thePW₁₁ ligand plays an important role in transferring electrons and proton in the water oxidation process.

Photo-electrocatalysis with the TiO₂/ITO and AgPW₁₁-TiO₂/ITO electrodes was performed in 0.1 mol L^-1 Na₂SO₄ solution under simulated AM 1.5 illumination (100 mW cm^-2). The Ⅰ-Ⅴ curves of AgPW₁₁-TiO₂/ITO and TiO₂/ITO electrodes under light irradiation are shown in Fig. S2. The current densities of both electrodes increased upon irradiation. Thus, light has a certain role in the process. Furthermore, AgPW₁₁-TiO₂/ITO showed a higher current density than that of TiO₂/ITO under illumination. Fig. 6 shows current density-time curves measured at different bias voltages for the TiO₂/ITO electrode and the AgPW₁₁-TiO₂/ITO electrode. As shown in Fig. 6(a), the TiO₂/ITO electrode began to give out an anode current under no illumination when the applied voltage was more than 1.4 V vs. Ag/AgCl; however, under illumination it exhibited an anode current only when the applied voltage was greater than 1.2 V vs. Ag/AgCl, and the anode current density was approximately 0.3 mA cm^-2 and the photocurrent density was approximately 0.2 mA cm^-2 at the applied voltage of 1.5 V vs. Ag/AgCl. For the AgPW₁₁-TiO₂/ITO electrode (Fig. 6(b)), the anode current density reached 1.4 mA cm^-2, i.e., 4.6 times as high as that of the TiO₂/ITO electrode at 1.5 V vs. Ag/AgCl under illumination. The photocurrent density of the AgPW₁₁-TiO₂/ITO electrode was approximately 0.3 mA cm^-2. Because the oxidation of water is a slow reaction, the illumination could not improve the photocurrent greatly and we observed tailed photocurrent peaks [33].

To determine the stability of AgPW₁₁-TiO₂/ITO electrode, we examined reuse of the electrode. Fig. 7 shows the anode current density of the AgPW₁₁-TiO₂/ITO electrode after 15 reuses. The anode current was more than three times as high as that of the TiO₂/ITO electrode although the values decreased slightly as the number of uses increased. Thus, the AgPW₁₁-TiO₂/ITO electrode was relatively stable. For reuse of the AgPW₁₁-TiO₂/ITO electrode, EDX analysis showed the relative element composition. As shown in Table S1, the content of P, W and Ag remained identical for the reused electrodes. Furthermore, AgPW₁₁ was retained on the electrode after multiple catalysis reactions. To determine the amount of Ag adsorbed from AgPW₁₁ and AgNO₃ onto the TiO₂ anode, ICP measurements were performed, as listed in Table S2. The amounts of Ag adsorbed for AgPW₁₁ and AgNO₃ on AgPW₁₁-TiO₂/ITO and AgNO₃-TiO₂/ITO electrodes were similar at 1.2756 and 1.3259 ppm, respectively. Hence, the difference in the catalytic activity could be attributed to the polyoxometalate ligand.

In the phosphate buffer solution (pH = 6.0, 0.1 mol L^-1), with the use of a glassy carbon electrode as the working electrode, the cyclic voltammetry of AgPW₁₁ at concentrations of 0, 0.05, 0.10, 0.15 and 0.20 mmol L^-1 were studied under the conditions of 1.0–1.6 V vs. Ag/AgCl at a scanning speed of 100 mV s^-1. As shown in Fig. 8, the anode currents were considerably enhanced in the solution containing AgPW₁₁, and the larger concentration of AgPW₁₁ resulted in greater enhancement of the anode currents when the applied voltage was more than 1.25 V vs. Ag / AgCl. In the range of 1.1–1.6 V vs. Ag/AgCl, a pair of redox peaks A₁ and C₁ (E_pa≈1.31V, E_pc≈1.23V) appeared with a ΔE_p value of more than 58 mV, indicating that the electrochemical reaction is quasi-reversible at the electrode surface.

For quasi-reversible processes, the transferred electrons can be calculated by the formula: ΔE_p = E_pa–E_pc = 58/n (mV, 25 ℃). As shown in Fig. 8, when the sweep speed was 100 mV s^-1, ΔE_p = E_pa–E_pc = 1.31–1.23 = 0.08 V, from which n was determined to be 0.73. Therefore, the number of electrons transferred in the A₁–C₁ process was considered to be 1. Thus, we deduced that the electrode process was a Ag^Ⅰ/Ag^Ⅱ redox process. The peak at 1.5 V could be assigned to the silver transformation Ag^Ⅱ→Ag^Ⅲ (AgO⁺)[34]. Furthermore, the cyclic voltammogram of AgPW₁₁ was compared with that of AgNO₃ in Fig. S3. We found that the redox peaks of AgNO₃ were similar to those of AgPW₁₁ in the positive potential region, which corresponded to the redox processes of silver. Furthermore, AgPW₁₁ featured a higher peak current and lower peak potential.

Fig. 9(a) is a cyclic voltammetry curve of AgPW₁₁ at different sweep rates in the range of 1.0–1.4 V vs. Ag/AgCl. When the sweep speed was increased from 100 to 900 mV s^-1, the cathode reduction potential (C₁) moved to negative potential and the anodic oxidation potential (A₁) moved to a more positive potential. These shifts resulted in an increase of the peak-to-peak potential difference (ΔE_p) of the oxidation peak A₁ and the reduction peak C₁ increased. These results suggest that the diffusion speed could not keep up with the scanning speed at excessively fast scanning speeds. Furthermore, the A₁–C₁ electrode process was a quasi-reversible process. The A₁, C₁ peak current values are plotted against the square root of sweep rates (ν^1/2), as shown in Fig. 9(b); the R² values of linear fits were 0.99507 and 0.99851, respectively. This good linear relationship indicated that the A₁–C₁ electrode process was mainly controlled by diffusion [34, 35].

To investigate the effect of pH on the AgPW₁₁ catalysis, eight groups of phosphate buffer solutions with different pH were prepared, and an equal amount of 0.2 mmol L^-1 of AgPW₁₁ was added. Fig. 10(a) shows cyclic voltammetry curves measured in solutions of different pH. The peak potentials shifted in a negative direction with increasing pH. As shown in Fig. 10(b), E_pc exhibited a linear relationship with the pH values with a slope of -0.08882. According to the Nernst equation [36] S = 2.303RTm/(αnF), where S is the slope of E_pc, m is the number of transferred protons, T is the temperature, n is the number of transferred electrons, R and F are constants, and α is the transfer coefficient (usually 0.5 for quasi-reversible processes). Hence, the number of transferred protons was deduced to be 1. Therefore, the electrode process of A₁–C₁ was considered to be 1e^- ~ 1H⁺ for [H₃Ag^Ⅰ(H₂O)PW₁₁O₃₉]^3-, expressed below as:

(1)

In the cyclic voltammetry curves, an oxidation process of Ag^Ⅱ/Ag^Ⅲ was not found; however, we noted that the oxidation current of Ag^Ⅰ→Ag^Ⅱ was obviously higher than that of the reduction current of Ag^Ⅱ→Ag^Ⅰ. The disproportionation reaction of Ag^Ⅱ has been reported [37]. Combined with our previous result [27], we deduced that the following disproportionation reaction likely occurred:

(2)

followed by water oxidation:

(3)

where Eq. (3) is the rate-determined reaction.

In the paper, the electrocatalytic and photoelectrocatalytic effects of AgPW₁₁ for water oxidation have been investigated through its immobilization on a TiO₂ electrode. We found that the AgPW₁₁-TiO₂/ITO electrode produced anode currents respectively 10 and 2 times as high as those of the TiO₂/ITO electrode and the AgNO₃-TiO₂/ITO electrode. At higher applied voltages, AgPW₁₁ catalyst exhibited improved catalytic activity. The catalytic effect was attributed to the capacity of the AgPW₁₁-TiO₂/ITO electrode to transfer electrons at the electrode-electrolyte interface. This result proves the critical role of the polyoxometalate ligand in transferring protons and electrons during water oxidation process. However, the photocurrent was not noticeably enhanced under illumination. The AgPW₁₁-TiO₂/ITO electrode was relatively stable. A mechanism for the electrocatalytic water oxidation by AgPW₁₁ is proposed based on our CV studies. The oxidation of AgPW₁₁ is a quasi-reversible process related to one proton and one electron transfer. We deduced that disproportionation of [H₂Ag^Ⅱ(H₂O)PW₁₁O₃₉]^3- might occur, resulting in [H₃Ag^ⅢOPW₁₁O₃₉]^3-, which oxidizes water to O₂.