Nearly Barrierless Four-hole Water Oxidation Catalysis on Semiconductor Photoanodes with Highly Accumulated Surface Holes

doi:10.21203/rs.3.rs-4708170/v1

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Nearly Barrierless Four-hole Water Oxidation Catalysis on Semiconductor Photoanodes with Highly Accumulated Surface Holes

https://doi.org/10.21203/rs.3.rs-4708170/v1

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The sluggish water oxidation reaction (WOR) is considered the kinetic bottleneck of artificial photosynthesis, due to the complicated four-electron and four-proton transfer process. Herein, we find that the WOR can be kinetically nearly barrierless on four representative photoanodes (i.e., α-Fe₂O₃, TiO₂, WO₃, and BiVO₄) under the concentrated light irradiation, wherein the rate-limiting O − O bond formation step is driven by highly accumulated surface photo-generated holes that exhibit a superior fourth-order kinetics. The activation energy is quantitatively estimated by combining the population model with Eyring-like equation and is further confirmed by density functional theory (DFT) calculations. The WOR rate under this condition shows more than one order of magnitude enhancement compared with that has 1st, 2nd or 3rd -order kinetics. Focusing on α-Fe₂O₃, the highly accumulated surface holes form adjacent Fe^V=O intermediates that effectively activate surface-adsorbed H₂O molecules via hydrogen bonding effect as revealed by operando Raman measurements and ab initio molecular dynamics (AIMD) simulations. This work discloses a systematic understanding of the internal relations between activation energy and reaction orders of surface holes for future WOR study.

Physical sciences/Chemistry/Photochemistry/Photocatalysis

Physical sciences/Chemistry/Catalysis/Photocatalysis

Physical sciences/Chemistry/Catalysis/Heterogeneous catalysis

Water oxidation

Rate law analysis

Activation energy

Hydrogen bond network

Semiconductor photoanodes

Water oxidation reaction (WOR) is a primary and key anodic half-reaction.^1–5 It is commonly utilized in conjunction with cathodic half-reactions, such as the hydrogen evolution reaction and CO₂ reduction reaction, which are crucial for energy and environmental engineering towards achieving sustainability.^6–10 However, WOR requires the transfer of four electrons and four protons with concomitant formation of O − O bond, leading to high energy barriers and slow kinetics in addition to the unfavorable reaction thermodynamics, thus hindering its further applications.^11–13

One of the most widely studied approaches for accelerating WOR involves the use of photoelectrochemical (PEC) cells,^14–17 wherein a semiconductor photoanode with optical bandgaps in the visible or near-ultraviolet can directly utilize photo-generated holes for driving WOR.^18–20 Understanding the mechanisms of WOR at a molecular level is essential for improving the PEC efficiency.^21–25 Previous studies have focused on exploring the molecular-level mechanisms of PEC WOR by using hematite (α-Fe₂O₃) as a model photoanode.^{21, 26–32} These efforts have investigated the proton transfer mechanisms^{26, 33, 34} and O − O bond formation pathways^{27, 35, 36} for PEC WOR, and further extends these WOR mechanisms to oxygen atom transfer reactions.^{28, 37, 38} Recently, we systematically explored the proton-coupled electron transfer pathways for WOR on a series of semiconductor photoanodes by conducting rate law analysis of H₂O molecules.³⁹ These studies provide critical insights into the fundamental processes governing PEC water oxidation.

Exploring the kinetic characteristics of the photo-generated holes at semiconductor/electrolyte interfaces is crucial for understanding the mechanisms of WOR, as the performance of WOR is limited by the surface hole density.^{40, 41} Currently, a population model has been used to describe WOR kinetics.^{18, 30, 42–45} This model assumes that the kinetics are correlated with the surface hole density by a power law:

$$\:{J}^{ph}={k}_{\text{w}\text{o}}{\left[{h}^{+}\right]}^{\beta\:}$$

where J^ph is the photocurrent density of WOR, k_WO is the rate constant of WOR, [h⁺] is surface hole density, and β is the apparent reaction order for surface holes. WOR is a complex four-hole reaction and the reaction order with respect to surface holes can be first, second, third or fourth-order.¹⁸ Reaction orders greater than one suggest that the accumulation of multiple oxidizing equivalents occur prior to the rate-determining step (RDS), indicating of a cooperative effect between two or more surface active species that results in a multi-hole transfer pathway for WOR.¹⁸ Durrant et al. revealed a transition from a first-order kinetics at low surface hole density to a third-order kinetics at high surface hole density on various semiconductor photoanodes by modulating the illumination intensity of light.⁴⁵ Our group observed a first-order kinetics and a second-order kinetics for surface holes on α-Fe₂O₃ photoanodes in near-neutral and highly alkaline environments, respectively.²⁷ Additionally, altering the wavelength of light can induce variations in reaction kinetics due to different separation processes of surface charges under different wavelength excitations.^{43, 46, 47} Although WOR is inherently a four-hole reaction, very few studies have reported the fourth-order kinetics on semiconductor photoanodes.^48–50 It is crucial to elucidate the WOR pathways driven by high-order kinetics of surface holes and their implications for WOR performances.

Previous research indicated that high order kinetics of surface holes can significantly improve the PEC WOR performance, and this typically required much higher light intensities that were significantly greater than the standard 1 sun used for PEC WOR performance tests.^{30, 44, 45} Notably, in certain regions on Earth, natural sunlight intensity can exceed 1 sun. Recently, Janáky et al. achieved a photocurrent density over 110 mA·cm^− 2 for PEC cells under the concentrated sunlight irradiation (up to 10 sun), whose performance was comparable to industrial electrochemical methods.⁵¹ Inspired by these findings, we believe that achieving the fourth-order kinetics under concentrated light intensity irradiation is both feasible and valuable for significantly enhancing PEC WOR performance, making it a practical and promising approach for real-world applications.

In this work, we conducted the rate law analysis of surface holes for PEC water oxidation on four well-known semiconductor photoanodes (i.e., α-Fe₂O₃, TiO₂, WO₃, and BiVO₄). A superior fourth-order kinetics was observed under the concentrated light irradiation (up to 760 mW·cm^− 2) and the WOR can proceed via a kinetically nearly barrierless pathway, in which the key O − O bond formation step can be driven by highly accumulated surface photo-generated holes. The activation energy (E_a) was estimated using a population model combined with an Eyring-like equation, and this was confirmed by density functional theory (DFT) calculations. We revealed the relationship between E_a and surface hole density, and provided a quantitative understanding on the over-fourth-order kinetics. Under these conditions, the WOR rate was improved by more than one order of magnitude compared to the first, second, or third-order kinetics. Operando Raman spectroscopy, intensity-modulated photocurrent spectroscopy (IMPS) measurements and ab initio molecular dynamics (AIMD) simulations revealed that on α-Fe₂O₃ photoanodes, adjacent Fe^V=O intermediates formed by the highly accumulated surface holes effectively activated surface-adsorbed H₂O molecules through hydrogen bonding at the interface.

2.1. Rate law analysis of surface holes

Physical characterizations and PEC performances of four typical photoanodes were illustrated in Figures S1 − S4 and Figure S5, respectively. The surface hole density was measured by the EIS method according to our previous works (detailed in the Supporting Information), as shown in Figures S6 and S7. The rate law analysis of surface holes was conducted on the four emblematic metal oxide photoanodes: hematite (α-Fe₂O₃), titanium dioxide (TiO₂), tungsten oxide (WO₃), and bismuth vanadate (BiVO₄), as depicted in Fig. 1. The reaction order of surface holes can be determined from the slope of log(J^ph) − log([h⁺]) curve according to Eq. 1. For these four photoanodes with low surface hole densities (under low light intensities), α-Fe₂O₃ (Fig. 1a) and TiO₂ (Fig. 1b) photoanodes exhibited a second-order kinetics, and WO₃ (Fig. 1c) and BiVO₄ (Fig. 1d) photoanodes exhibited a third-order kinetics. As surface hole densities increased, all four photoanodes showed a fourth-order kinetics of surface holes for WOR. The light intensity of the transition from second- or third-order to fourth-order was 260 mW·cm^− 2 for α-Fe₂O₃ and WO₃, 190 mW·cm^− 2 for BiVO₄, and 90 mW·cm^− 2 for TiO₂. The fourth-order kinetics of surface holes implied that the four photoanodes can accumulate four surface holes at the active site before the RDS of WOR, the O − O bond formation step.⁵²

In the following section, we specifically focused on α-Fe₂O₃ photoanodes to further investigate the detailed reaction mechanism. The fourth-order kinetics of surface holes were also observed under white LED illumination (Figure S8) or in near neutral conditions (pH = 8 ~ 10) (Figure S9a) on the α-Fe₂O₃ photoanode. More importantly, it can be seen from Fig. 1e that the reaction orders of surface holes could be further divided into three distinct regions with the surface hole density ranging from 0.4 to 1.8 h⁺·nm^− 2. In addition to the second-order kinetics at surface hole densities below 1 h⁺·nm^− 2 and fourth-order kinetics at surface hole densities between 1 h⁺·nm^− 2 and 1.7 h⁺·nm^− 2, an unexpected eighth-order kinetics of surface holes appeared with surface hole densities exceeding 1.7 h⁺·nm^− 2. Accordingly, the J^ph increased from 0.02 ~ 0.09 mA·cm^− 2 to 0.12 ~ 0.74 mA·cm^− 2 and was further enhanced to 1.10 mA·cm^− 2 for the third region, which was nearly two orders of magnitude enhancement. Besides, an ultra-high reaction order (~ seventh-order) was also observed in 1 M NaOH solution at high surface hole densities on the α-Fe₂O₃ photoanode (Figure S9b). Similar cases of reaction orders exceeding fourth-order have been documented in previous studies and were explicitly mentioned by Cowan et al.,^48–50 but no further understanding for this ultra-high reaction order of surface holes was provided.

The theoretical maximum reaction order of surface holes for WOR should be four, so we believe that the ultra-high reaction order is an illusion derided from the over-simplified application of Eq. 1. It is important to recognize that the apparent reaction order obtained from Eq. 1 may be influenced by other factors. For instance, k_wo may be not constant and impact the apparent reaction order. Recently, Teschner et al. discovered that the Ea of WOR on metal oxide electrocatalysts was not constant but decreased linearly with the increase of surface hole density.⁵³ The relationship between k_wo and E_a is described by:

$$\:{k}_{wo}={k}_{0}\text{exp}\left(-\frac{{E}_{a}}{{k}_{B}T}\right)$$

where k₀ is the pre-exponential factor and independent of surface holes. k_B and T are the Boltzmann constant and temperature, respectively. Combining Eq. 1 and Eq. 2 with Eyring theory, a new exponential linear formula can be derived (for detailed discussions, see Supporting Information Note 1), as shown in Eq. 3:

$$\:ln\left({J}^{ph}\right)=\beta\:\text{l}\text{n}\left(\right[{h}^{+}\left]\right)-\frac{\xi\:\left[{h}^{+}\right]}{{k}_{B}T}+{\text{l}\text{n}(k}_{0})-\frac{{E}_{0}}{{k}_{B}T}$$

where E_a = ζ[h⁺] + E₀ and ζ is the constant determining the change of E_a with surface hole densities. E₀ is the constant to determine the E_a at surface hole density of 0 h⁺·nm^− 2.^{53, 54}

This relationship provides a more accurate description of the surface holes involved in the WOR. It can be seen that the appearent reaction order of surface holes is determined not only by β, but also by the variation of E_a that is related to the surface hole density. When β was fixed at 2 for the second-order kinetics and at 4 for the fourth-order kinetics, the values of ζ were calculated to be 1.8×10^− 3 and − 3.8×10^− 3, respectively (Figure S10). This suggests that the term ζ[h⁺]/k_BT (0.07 and − 0.15) can be neglected under these conditions, thus making the E_a almost independent on surface hole density for both second- and fourth-order pathways. Hence, Eq. 1 can be directly employed to estimate the reaction order of surface holes under these conditions. Piccinin et al. proposed that the E_a for WOR was weakly dependent on the surface hole density and thus lead to the observed power law on the α-Fe₂O₃ photoanode.⁴⁴ However, when the reaction order was over fourth-order and the value of β was fixed at 4, the the term ζ[h⁺]/k_BT (− 9.34) can not be neglected and the value of ζ was calculated to be − 0.24 (Figure S10). By this theory, we could infer that the over fourth-order kinetics was still essentially fourth-order kinetics, but the E_a exhibited an obvious negative dependency on the surface hole density, resulted in that the slope of log(J^ph) − log([h⁺]) curve can exceed 4 when the surface hole density was greater than 1.7 h⁺·nm^− 2 (Figure S10).

2.2 Activation energy of WOR on α-Fe₂O₃

To obtain the apparent E_a for WOR with fourth-order kinetics, we initially examined the relationship of WOR rates with temperatures and light intensities, as shown in Fig. 2a. The WOR rate under low light intensities was more significantly affected by temperature compared to that under high light intensities. For example, as the temperature increased from 20°C to 48°C, the WOR rate increased by 40% under 100 mW·cm^− 2 illumination. Comparatively, it exhibited an almost temperature-independent reaction rate (around 0.21 mA·cm^− 2) for WOR under 700 mW·cm⁻² illumination. Moreover, surface hole densities obtained by EIS and TPD methods at different temperatures remained almost unchanged under high light intensities (Figure S11). Based on the above results, we consider the temperature dependences of the second- and fourth-order kinetics of surface holes to obtain the apparent E_a for WOR according to Eq. 2, as shown in the rate law analysis plotted in Fig. 2b. The rate constants of WOR at different temperatures could be obtained from the intercepts of log(J^ph) − log([h⁺]) curves with the y-axis. Figure 2c shows the Arrhenius plots for the second- and fourth-order rate constants. The apparent E_a for WOR with second-order kinetics was 0.16 ± 0.02 eV. More importantly, a much lower E_a of only 0.03 ± 0.01 eV was observed for WOR with fourth-order kinetics and the same results were obtained by the TPD method (Figure S11), indicating that the α-Fe₂O₃ photoanode was able to access a fourth-order kinetics reaction pathway to achieve a nearly barrierless WOR. According to the analysis in Supporting Information Note 1, it should be noted that the variations of E_a with surface hole density (i.e., ζ in Eq. 3) was negligible for the fourth-order kinetics pathway. Therefore, the apparent E_a obtained in Fig. 2c corresponds to the E₀ in Eq. 3. Furthermore, the relationship between activation energy and surface hole density can be obtained. As shown in Fig. 2d, the slope of E_a−[h⁺] curve was only − 3.8×10^− 3 with surface hole densities ranging from 1.0 h⁺·nm^− 2 to 1.7 h⁺·nm^− 2, indicating that the E_a of the fourth-order kinetics was almost independent of surface hole densities. In contrast, the slope of E_a−[h⁺] curve was − 0.24 at surface hole densities exceeding 1.7 h⁺·nm^− 2, implying that the E_a of WOR could be further reduced by increasing surface hole densities. Therefore, α-Fe₂O₃ photoanodes can exhibit superior WOR performance at high surface hole densities. The reason for this transition under high-intensity irradiations was further investigated by kinetic isotope effect (KIE) experiments, operando spectroscopies and theoretical simulations.

H/D KIE experiments were conducted to illuminate the impact of the proton-coupled electron transfer (PCET) process on the reaction order of surface holes for WOR. As shown in Fig. 2e, a second-order kinetics of surface holes at low surface hole densities of 0.2 − 0.8 h⁺·nm^− 2 and a third-order kinetics at high surface hole densities of 0.8 − 1.0 h⁺·nm^− 2 were observed for D₂O oxidation on the α-Fe₂O₃ photoanode (Fig. 2e). In addition to the difference in the reaction order, it should be noted that under the same light intensity, the surface hole density for D₂O oxidation was lower than that for H₂O oxidation. These results suggested that it is more difficult to accumulate four holes on the α-Fe₂O₃ surface in D₂O solution compared to that in H₂O solution. Therefore, the surface hole accumulation process has the proton-coupled characterisitcs as show in the below density functional theory (DFT) calculations. As shown in Fig. 2f, the primary KIE values ranging from 2.9 to 3.7 were observed within the entire range of light intensity illumination, indicating that O–H bond cleavage of H₂O molecules was involved in the RDS of WOR. Thus, the E_a measured in this work pertained to the oxidation of H₂O molecules rather than hydroxide (OH^–) ions, consistent with our previous findings.²⁶ The theoretical results of Piccinin et al. suggested that the E_a for reactions involving H₂O molecules (~ 0.40 eV) was substantially higher than for reactions involving OH^– (~ 0.16 eV).⁴⁴ However, our results emphasize that the E_a for oxidizing H₂O molecules could be very low (~ 0.03 eV) at high surface hole densities exceeding 1 h⁺·nm^− 2.

2.3 Theoratical simulations

The WOR pathways on α-Fe₂O₃ with second- and fourth-order kinetics were further compared by DFT calculations. According to our experiments and previous research, the (110) facet of α-Fe₂O₃ saturated by water was constructed as the catalytic surface. The potential energy surfaces of WOR were shown in Fig. 3. For the second-order pathway (Fig. 3a), two Fe^III−OH species (I) undergo two concerted proton-coupled electron transfer (CPET) processes, forming adjacent surface-trapped holes [Fe^IV=O sites (III)] with energy inputs of ~ 1.5 eV per step. Then the O − O bond formed via the nucleophilic attack of an H₂O molecule to the Fe^IV=O sites, a potential-independent chemical step, which is usually deemed the RDS of WOR.^{27, 44, 55} The activation free energy for the O − O bond formation via second-order kinetics was 0.39 eV, which was more favorable than the first-order kinetics (0.54 eV) as previously reported by Durrant’s group.⁴⁵ The formed Fe^III−OOH intermediate (IV) can be further oxidized and finally release O₂ (V). The oxidation of another incoming H₂O molecule on α-Fe₂O₃ subsequently occurs, leading to the regeneration of the Fe^III−OH species (I) and thereby completing one catalytic cycle.

For the fourth-order kinetics (Fig. 3b), there were four surface-trapped holes formed before the O − O bond formation, which can only occur under high-intensity illumination. The produced adjacent Fe^V=O intermediate (E) gains enough energy to enable the following oxidation reactions. In this situation, the activation free energy for O − O bond formation via the nucleophilic attack pathway is 0.31 eV, implying more facile kinetics than second-order kinetics (0.39 eV) and third-order kinetics (0.34 eV). Notably, the O − O bond could also be formed via the coupling of adjacent Fe^V=O intermediate (E) under high surface hole density, which exhibited a faster kinetics with E_a of 0.19 eV than the nucleophilic attack pathway. Both pathways occur under high-intensity illumination.

Based on the analysis above, the WOR kinetics on α-Fe₂O₃ will be accelerated with the surface hole accumulation, following the order: first-order < second-order < third-order < fourth-order (Figure S12). These results are in accordance with the experimental observation. We further dissected the structure of the transition state (TS) during the O − O bond formation via these four kinetics processes. The TS of first-order kinetics showed an H₂O₂-like structure [HO^·---O(H) − Fe^III], while the TS in the other three pathways involved an incompletely dissociated H₂O molecule (Figure S12a). Specifically, the H − O−H angle of the H₂O molecule in the TS of second-order kinetics was 108.9°, which was close to the value of 104.5° for the free H₂O molecule in the ground state but possessed elongated O − H bonds (1.07 Å, versus 0.96 Å in water), indicating of the partly dissociated O − H bonds. The other two TSs (third- and fourth-order kinetics) boasted similar structures of the incoming H₂O molecule. Compared with the TS in second-order kinetics, the H − O−H angle of the H₂O decreased to 92.6° and 94.0° for third- and fourth-order pathways, respectively, illustrating a greater degree of water activation under high surface hole density. The mean length of the O − H bond in both TSs was 1.04 Å, which remained longer than that in free water but shorter than that in the TS of second-order kinetics. This observation indicates that the dissociation of the O − H bond in H₂O tends to occur after the TS for O − O bond formation when the reaction order of surface holes > 2, as the activated H₂O molecule has enough potential energy to break the O − H bond. In other words, the higher density of surface holes on the α-Fe₂O₃ photoanode results in enhanced H₂O activation that subsequently promotes the kinetics of O − O bond formation, thereby accounting for the superior performance of α-Fe₂O₃ under high-intensity illumination.

2.4 Unraveling the Interfacial H₂O Structure

The structures of interfacial H₂O during WOR were probed by operando Raman measurements on α-Fe₂O₃ photoanodes (Fig. 4a). The broad band extending from 3000 to 3700 cm^− 1 was assigned to the O − H stretching mode of H₂O molecules. It can be further deconvoluted into three distinct components of O − H stretching vibration bands via Gaussian fitting, which corresponded to three types of H₂O molecules at the interface.^56–58 Specifically, the peak centered at the highest wavenumbers of ∼3610 cm^− 1 was assigned to Na⁺ ion hydrated water (Na·H₂O) with weak hydrogen-bond interactions. The peak at the wavenumbers of ∼3450 cm^− 1 and the peak at the lowest wavenumbers of ∼3280 cm^− 1 were attributed to 2-coordinated hydrogen-bonded water (2-HB·H₂O) and 4-coordinated hydrogen-bonded water (4-HB·H₂O), respectively. Each of these O − H associated peaks have been validated through a deuterium substitution experiment by using D₂O (Figures S13 and S14). The areas of these three Raman peaks showed a slowly increasing trend in the second-order kinetics region, while they showed a rapidly increasing trend in the fourth-order kinetics regions (Figs. 4a and S15). The increase in peak intensity indicates that highly accumulated surface holes can promotes the adsorption of interfacial H₂O molecules. To better describe the three types of interfacial H₂O molecules on the α-Fe₂O₃ photoanode surface, the proportions of the three O − H stretching Raman bands for different reaction orders were compared. Figure 4b showed that the proportions of 4-HB·H₂O were both more than 50% at the surface hole densities of 0.8 h⁺·nm^− 2 and 1.7 h⁺·nm^− 2, indicating that 4-HB·H₂O dominated the interfacial H₂O structure for WOR with the second-order kinetics and fourth-order kinetics. Furthermore, as the reaction order of surface holes changed from the second-order to the fourth-order, the proportion of 4-HB·H₂O increased from 57–61% and the proportion of 2-HB·H₂O decreased from 36–31%, while the proportions of Na·H₂O remained almost unchanged (Fig. 4b). These results suggested that the dominance of 4-HB·H₂O molecules became more pronounced as the increase of surface hole density, leading to a more ordered interfacial H₂O structure for WOR with fourth-order kinetics. In electrochemical Raman spectroscopy, the vibrational frequency of an adsorbate species varied with electrode potential has been attributed to the vibrational Stark effect.⁵⁹ In this case, it can be seen from Fig. 4c that the vibrational frequencies of these O − H stretching Raman bands showed a red shift with the increase of surface hole density, which could also be attributed to the vibrational Stark effect. At the surface hole density ranging from 0.6 to 1.8 h⁺·nm^− 2, the Stark slopes for Na·H₂O and 2-HB·H₂O were ~ 4.5 and ~ 3.7, respectively. In contrast, for 4-HB·H₂O, the dependency of frequency on surface hole density could be divided into two segments. The Stark slope within the surface hole density of 0.6 − 1.0 h⁺·nm^− 2 was ~ 3.9, and it increased dramatically to ~ 37.3 within the surface hole density of 1.0–1.7 h⁺·nm^− 2. These results showed that the stark shift of O − H stretching in 4-HB-H₂O was more pronounced for fourth-order kinetics, indicating that it was highly susceptible to the impact of surface-trapped holes. This was attributed to the formation of hydrogen bonds between reaction intermediates and interfacial H₂O, bringing the 4-HB·H₂O closest to the photoanode surface.⁶⁰ The more ordered interfacial structure of H₂O molecules for fourth-order kinetics possessed a stronger hydration ability and resulted in the formation of more polarized H₂O molecules. According to previous works, this structure with stronger polarized H₂O molecules enabled the formation of a robust absorbate − H₂O interaction network,⁵⁴ which facilitates the formation of a higher-valent Fe − O species (i.e. adjacent Fe^V=O intermediates) through accumulating four holes on the α-Fe₂O₃ photoanode surface.²⁵

We notice that previous reports on electrocatalytic system showed a lower E_a for water dissociation with decreasing degree of hydrogen bonding of H₂O during the operando interfacial H₂O structure evaluation using Raman spectroscopy,^{56, 61} which is in contrast with our results. It is noteworthy that all these electrocatalytic systems follow the Butler–Volmer theory that change the Fermi level of electrocatalysts via applied bias,^{18, 62} while in this work we modulate the surface hole accumulation via irradiation light intensity, which follow the population model.⁴⁵ This operando interfacial H₂O structure evaluation in the PEC system is reported for the first time, thus we further validate the surface-hole-induced H₂O structure variation by using ab initio molecular dynamics (AIMD) simulations.

The model was constructed by using the slab model of α-Fe₂O₃ (110) and incorporating 69 H₂O molecules, and surface holes were introduced by removing hydrogen atoms from the adsorbed H₂O molecules on the surface. The results analysis was conducted based on the AIMD trajectories of 10 ps after the pre-equilibration of 3 ps. As shown in Fig. 4d and e, the presence of the surface hole resulted in the more ordered interfacial H₂O molecules, exhibiting the H-atom-down configuration. The distribution of H atoms along the surface normal direction was shown in Figs. 4f and S16, where the pronounced accumulation of H atoms in the Helmholtz layer (2 − 3.5 Å to the electrode surface) was observed with the increasing of surface hole density. Besides, the average numbers of hydrogen bonds within the interface (1 − 4 Å to the electrode surface) were 6, 7, and 9 for surfaces with surface hole densities of 0, 0.8, and 2.5 h⁺·nm^− 2 (Figure S17), respectively, which agreed with the enhanced hydrogen bonds induced by surface hole accumulation as revealed by operando Raman spectra. Additionally, the AIMD simulations also revealed the polarization of H₂O molecules on the surface exhibiting a high density of surface holes (Figure S18), elucidating decreased average H − O−H angles of 105.1° and 104.5° for interfacial water molecules on surfaces with surface hole densities of 0.8, and 2.5 h⁺·nm^− 2, respectively, compared with that of 107.0° for the H₂O on the hole-free surface. These polarized H₂O molecules on the surface with high hole density exhibit structures that are closer to the transition state for O − O bond formation (Figs. 3 and S12), thereby significantly promoting the WOR kinetics.

Intensity-modulated photocurrent spectroscopy (IMPS) experiments were performed to investigate the charge-transfer kinetics at the interface between α-Fe₂O₃ and electrolyte. The steady-state surface hole density depends on the total holes arriving on the surface, surface hole recombination, and interfacial hole transfer. Therefore, IMPS measurements were used to quantitatively describe the variations in surface hole recombination and interfacial hole transfer with surface hole densities. The IMPS spectra of α-Fe₂O₃ photoanodes under the illumination of different light intensities were displayed in Fig. 5a, from which the surface charge-transfer efficiency (η_ct), the charge transfer rate constant (k_ct), and the surface recombination rate constant (k_rec) were extracted (Figure S19).^{43, 63} The extracted k_ct and k_rec as a function of surface hole density were shown in Fig. 5b. Within surface hole density of 0.4 − 1.0 h⁺·nm^− 2 (i.e., second-order kinetics region), the values of k_ct slowly increased from 0.07 s^− 1 to 0.16 s^− 1 and the values of k_rec also increased from 0.72 s^− 1 to 1.03 s^− 1. This same trend in k_ct and k_rec with the surface hole density caused the very low value of η_ct (< 15%) (Figure S19), resulted in only the accumulation of two holes on the surface to participate in the O − O bond formation step of WOR. Within surface hole density of 1.0 − 1.8 h⁺·nm^− 2 (i.e., fourth-order kinetics region), however, the values of k_ct rapidly increased from 0.16 s^− 1 to 0.51 s^− 1 and the values of k_rec decreased from 1.03 s^− 1 to 0.88 s^− 1. As a result, this opposite trend in k_ct and k_rec caused a significant increase in the η_ct, ranging from 15–36% (Figure S19). This facilitates the accumulation of multiple holes on the surface, leading to a transition from second-order kinetics to fourth-order kinetics of surface holes.

In conclusion, we reveal a superior fourth-order reaction pathway for WOR on four typical photoanodes (i.e., α-Fe₂O₃, TiO₂, WO₃, and BiVO₄) by combining rate law analysis and theocratical simulations. Our findings show that WOR can proceed with nearly barrierless kinetics when the rate-limiting O − O bond formation step is driven by highly accumulated surface holes. Under these conditions, the WOR rate increases by more than an order of magnitude compared to 1st, 2nd, or 3rd -order kinetics. Using a population model modified with an Eyring-like equation, this work elucidates that the over-fourth-order kinetics exhibit a negative dependency of E_a on surface hole density. KIE measurements, operando Raman spectroscopy, IMPS measurements and AIMD simulations further reveal that highly accumulated surface holes form adjacent Fe^V=O intermediates through successive PCET processes on α-Fe₂O₃. These intermediates effectively activate surface-adsorbed H₂O molecules via hydrogen bonding and accelerate charge-transfer kinetics at the interface. This study provides a comprehensive understanding of the intrinsic relationship between activation energy and reaction orders of surface holes, offering valuable insights for future WOR research.

Notes

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the "National Key R&D Program of China" (No. 2022YFA1505000, 2020YFC1808401), NSFC (No. 22072158), the Strategic Priority Research Program of Chinese Academy of Sciences, Grant No. XDB36000000 and CAS Project for Young Scientists in Basic Research YSBR-004.

Author contributions

Y. Zhang directed the project. S. Liu carried out most experiments. S. Liu, Y. Zhang and K. Dang wrote the manuscript, with input from others. K. Dang conducted computational studies. L. Wu helped with the results discussion. All the authors analyzed the results and reviewed the paper.

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Nearly Barrierless Four-hole Water Oxidation Catalysis on Semiconductor Photoanodes with Highly Accumulated Surface Holes

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Version 1

Abstract

Figures

1. Introduction

2. Results and Discussion

2.1. Rate law analysis of surface holes

2.2 Activation energy of WOR on α-Fe₂O₃

2.3 Theoratical simulations

2.4 Unraveling the Interfacial H₂O Structure

3. Conclusions

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1

Nearly Barrierless Four-hole Water Oxidation Catalysis on Semiconductor Photoanodes with Highly Accumulated Surface Holes

Status:

Version 1

Abstract

Figures

1. Introduction

2. Results and Discussion

2.1. Rate law analysis of surface holes

2.2 Activation energy of WOR on α-Fe2O3

2.3 Theoratical simulations

2.4 Unraveling the Interfacial H2O Structure

3. Conclusions

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1

2.2 Activation energy of WOR on α-Fe₂O₃

2.4 Unraveling the Interfacial H₂O Structure