As one of the most prevalent anionic defects on solid surfaces, the essential role of oxygen vacancies originates not merely from local geometric incompleteness, but from their profound reshaping of surface electronic-state distributions. Extensive theoretical and experimental studies have demonstrated that oxygen vacancies often serve as electron-rich active centers capable of markedly altering water adsorption configurations, enhancing interfacial electron-transfer capability, and thereby reshaping water activation pathways and subsequent reaction routes.

In typical transition-metal oxide systems, oxygen vacancies exhibit dual characteristics as both “electron traps” and “reactive sites.” Wang et al. employed hydrogen reduction of fluorine-doped TiO₂ nanosheets (F-TiO_2-x) to induce and stabilize surface oxygen vacancies, thereby significantly enhancing the photocatalytic degradation efficiency of indoor volatile organic compounds (V_OCs)^[35]. DFT calculations elucidated that the introduction of oxygen vacancies increased the adsorption energy of water molecules from 0.727 eV to 1.257 eV and provided key active sites that facilitate electron transfer to adsorbed water molecules (Δq = -0.25 e) [Figure 3A]. This process substantially lowered the reaction barrier for water dissociation to generate ·OH radicals, thereby establishing a reaction pathway in which surface fluorine and oxygen vacancies cooperatively promote water activation. Similarly, Li et al. combined DFT calculations with X-ray absorption fine structure (XAFS) spectroscopy to demonstrate that oxygen vacancies on the BiOCl (010) facet induce dissociative water adsorption^[39].

However, oxygen vacancies do not universally exhibit strong activation effects across all material systems. For example, on the WO_3-x (001) surface, the introduction of oxygen vacancies triggers pronounced surface reconstruction, causing originally undercoordinated W sites to evolve toward geometric and electronic configurations similar to the W_5c sites on stoichiometric surfaces. This reconstruction-induced “defect healing” effect thermodynamically stabilizes the surface structure but simultaneously weakens the defect’s ability to activate reactants^[40]. Computational results show that the adsorption energy of water molecules on the defective surface (1.05 eV) differs only marginally from that on the pristine surface (0.95 eV), with water molecules favoring molecular adsorption rather than dissociation^[40]. This indicates that defect-induced water activation can be significantly constrained by lattice reconstruction and stabilization mechanisms.

Taken together, these observations indicate that the essence of oxygen-vacancy engineering lies not in a simple increase in defect concentration, but rather in the precise regulation of defect-induced electronic states and local structural evolution. Rationally stabilized oxygen vacancies can effectively reduce the energy barriers associated with water activation and electron transfer, whereas excessive structural relaxation and coordination reconstruction may lead to defect passivation or even functional failure. Moreover, the role of oxygen vacancies extends beyond initial adsorption to include the dynamic modulation of proton supply during subsequent reaction steps.

Recent studies have reshaped the conventional understanding of surface hydroxyls. Rather than being merely passive products of water dissociation or inert adsorption sites, surface hydroxyls are now widely recognized as key active species governing the thermodynamics and kinetics of catalytic reactions at the water-solid interface. Extensive in situ spectroscopic investigations and theoretical calculations have demonstrated that surface hydroxyls can directly participate in proton-coupled electron transfer (PCET) processes and, through the construction of ordered interfacial HBNs, significantly lower the activation barrier associated with the rate-determining step^[36,41].

Chen et al. combined in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) with DFT calculations to elucidate the promoting role of surface hydroxyls in the conversion of dioxymethylene (DOM) to formate^[36]. Mechanistic calculations identified two distinct reaction pathways^[36]: H transfer from DOM to a surface lattice oxygen species involves a high activation barrier of 1.95 eV, whereas H transfer to a surface terminal hydroxyl requires a barrier of only 0.12 eV and proceeds exothermically. Beyond direct participation in reaction pathways, surface hydroxyls and interfacial water structures can also indirectly regulate reaction kinetics through collective hydrogen-bonding effects. Liu et al. combined in situ Raman spectroscopy with AIMD simulations to show that the accumulation of high-density surface holes induces the rearrangement of interfacial water molecules, forming highly ordered HBNs^[41]. In this system, adjacent Fe^v=O intermediates activate interfacial adsorbed water molecules - rather than hydroxide ions - via hydrogen bonding, enabling a multi-electron reaction pathway with an almost negligible activation barrier^[41].

In summary, interfacial water molecules play a dual functional role: dissociated water generates surface hydroxyls that provide low-barrier proton-transfer channels, while intact water molecules assemble into ordered HBNs that cooperatively promote efficient substrate activation.

At the atomic scale, the electronic structure of metal active centers exhibits high tunability regarding the local coordination environment. Evolving from isolated single atoms to sub-nanometer metal clusters, parameters such as coordination number, ligand electronegativity, and bond length serve as important descriptors for regulating interfacial electronic reconstruction^[44]. SACs are firmly anchored through the formation of strong covalent or ionic bonds with substrate heteroatoms (such as O, N, S). Due to the lack of homonuclear metal-metal bonds, they exhibit significant charge transfer and electron-deficient (cationic) characteristics^[45]. The evolution of electronic structures under these different coordination configurations is crucial for revealing the flipping of water molecule adsorption configurations at the solid-liquid interface and the mechanism by which the d-band center regulates the dissociation barrier of the O-H bond.

Huang et al. developed an entropy-driven strategy for single-atom photocatalyst design by confining Ni single atoms within an interlayer structure via a hydrothermal method^[37], which can induce and strengthen an internal macroscopic polarization electric field (PEF). Driven by the enhanced PEF, photogenerated charge carriers overcome Coulombic attraction and are efficiently separated and directionally transported to surface Ni-(s-triazine)₄ active sites through channels formed by Ni-N bonds^[37]. Su et al. employed an aqueous-phase reforming strategy to construct a hydrophilic hydroxylated Ru single-atom interface (HO-Ru/TiN) on a metallic TiN substrate^[46]. Surface-bonded hydroxyls were shown to participate in the formation of unique HO-RuN₅-Ti Lewis acid-base pairs, which not only improved interfacial wettability but also acted as active centers for efficient water capture and dissociation. Detection of OH and OOH intermediates by in situ spectroscopy confirmed that water molecules are oxidized by photogenerated holes at the interface to release O₂ and supply the protons and electrons required for CO₂ reduction, thereby establishing an interfacial hydroxyl-mediated water-solid synergistic catalytic mechanism.

Overall, regulation of the single-atom coordination environment provides an effective pathway for optimizing water-solid interfacial interactions. By constructing specific coordination configurations, such as confined architectures or surface hydroxylation, catalyst surface polarization and wettability can be enhanced, promoting water chemisorption and activation^[46]. This strengthened interfacial coupling facilitates charge injection to the surface and effectively lowers the O-H bond dissociation barrier, thereby improving reaction kinetics.

Isolated metal surfaces often exhibit limited capability for water activation due to weak metal-water interactions, which hinder the simultaneous stabilization of protonic and hydroxyl intermediates, resulting in kinetically constrained water dissociation^[47]. In contrast, when metal nanoparticles are supported on oxide substrates, the interfacial regions formed between the two components, commonly referred to as perimeter sites, frequently display interfacial chemical properties distinct from those of either component alone^[48]. These interfaces not only reconstruct local electronic structures and electric-field distributions but, more importantly, introduce a dual-site synergistic adsorption mode: oxide sites preferentially stabilize hydroxyl species (OH*), whereas metal sites favor the adsorption of protons or hydrogen atoms (H*). Such spatial and electronic cooperation lowers O-H bond cleavage barriers, rendering water dissociation both thermodynamically favorable and kinetically accessible^[48].

Chen et al. engineered highly controllable Pt-PdO perimeter sites on carbon nanotube supports via atomic-scale interface design, achieving simultaneous enhancement of catalytic activity and stability^[38]. MD simulations combined with DFT calculations revealed that Pd atoms preferentially segregate to nanoparticle surfaces and undergo partial oxidation under thermodynamic driving forces, ultimately forming a Pt-rich core-PdO-rich shell heterostructure. At the Pt-PdO interface, a pronounced bifunctional synergistic effect was observed [Figure 3F]: PdO sites substantially reduced the activation barrier for water dissociation from 0.83 eV to 0.11 eV, thereby accelerating the rate-determining step. In a classical model system, Fujitani et al. investigated Au/TiO₂(110) and directly observed water dissociation at the junction between gold nanoparticles and the TiO₂ support, yielding surface H and OH species^[49]. This experimental evidence conclusively confirms that metal-oxide perimeter sites, rather than isolated metal or oxide surfaces, serve as the key active centers for water activation and subsequent surface reactions.

In other words, the perimeter effect introduced by metal-oxide interfaces fundamentally reshapes water adsorption and dissociation pathways on solid surfaces. By breaking the adsorption-dissociation equilibrium constraints inherent to single-component surfaces through interfacial synergy, these interfaces continuously generate highly reactive hydroxyl or hydrogen species.

Proton transport at the water-solid interface predominantly follows the Grotthuss mechanism, in which charge migration proceeds through the concerted breaking and formation of O-H bonds within a HBN, rather than via the physical diffusion of hydrated protons^[53]. The efficiency of this mechanism under interfacial conditions critically depends on whether the HBN maintains long-range connectivity and orientational coherence^[54]. At water-solid interfaces, however, the solid surface potential can disrupt the cooperative orientation of interfacial water molecules, leading to fragmentation or localization of HBNs and raising questions regarding the viability of efficient proton shuttling^[55]. Recent in situ characterizations and AIMD simulations demonstrate that, as long as HB channels spanning multiple water molecules are preserved, the Grotthuss mechanism not only persists at interfaces but can even be significantly amplified under specific conditions^[56].

Cai et al. computationally compared two proton-transfer pathways on the ZrO₂(111) surface in aqueous environments^[57]. They showed that direct proton hopping between surface oxygen sites (O_2c to O_3c) encounters an energy barrier as high as 128 kJ mol^-1, rendering this pathway kinetically unfavorable [Figure 4A]^[57]. In contrast, the presence of liquid water activates a Grotthuss-type proton relay mediated by interfacial water molecules, reducing the energy barrier to approximately 47 kJ mol^-1 [Figure 4B]^[57]. This finding highlights the essential role of liquid water: rather than serving merely as a passive solvent, it constructs a low-barrier HBN that dramatically facilitates dynamic proton migration and active-site reconstruction on catalyst surfaces. Cheng et al. further employed random phase approximation (RPA) theory combined with surface-charging methods to analyze the electrochemical environment of water-solid interfaces^[58]. Despite perturbation in water adsorption orientations induced by electrode potentials, surface-adsorbed water was shown to maintain cooperative proton transfer via Grotthuss-type HBNs, thereby enabling efficient *CO reduction^[58].

In summary, interactions at the water-solid interface are crucial for overcoming proton-transfer energy barriers. Through chemisorption to surface atoms and HB bridging, interfacial water molecules effectively alleviate spatial constraints imposed by solid lattices^[50]. This intimate interfacial coupling preserves long-range Grotthuss channels even under complex electrochemical conditions and establishes the importance of water-assisted mechanisms in lowering activation energies and promoting surface proton flux.

The wetting layer at the water-solid interface, characterized by structures distinct from bulk water, acts as a critical barrier to charge and mass transport^[59]. Driven by surface physicochemical properties as well as HB interactions, interfacial water molecules undergo reconstruction to form a “Janus-type” interface with specific dipole orientations^[50]. Substrate wettability governs the orientation of water dipoles and induces the assembly of ordered HBNs. Such interfacial ordering not only alters system entropy but also critically determines interfacial charge-transfer efficiency and reaction selectivity.

Ma et al. combined in situ ultraviolet spectroscopy, X-ray photoelectron spectroscopy (XPS), and DFT calculations to investigate water adsorption on anatase TiO₂(001)-(1 × 4) surfaces^[60]. At sub-monolayer coverage, water molecules exhibit weak interactions with surface 5-fold coordinated Titanium (Ti_5c) sites, characterized by physisorption and high lateral mobility. Scanning tunneling microscopy (STM) images and height profiles reveal that as coverage approaches and exceeds one monolayer, water molecules self-assemble into ordered interfacial structures. Additional water molecules shorten O-O distances and construct a complex interfacial HBN, increasing the average adsorption energy to 0.71 eV and stabilizing the wetting layer [Figure 4C-E]. More importantly, this network breaks the spatial isolation of terrace sites and opens cascaded proton and hole transfer pathways, enabling photogenerated holes to efficiently oxidize and dissociate water molecules. Wang et al. revealed that hexagonal boron nitride (h-BN) surfaces exhibit Janus-like behavior driven by a dynamic equilibrium between chemisorbed and physisorbed hydroxide ions^[61]. This unique interfacial chemistry induces spontaneous surface charging and markedly strengthens interfacial water structuring, providing a molecular-level explanation for the higher water friction of h-BN compared to graphene^[61].

Collectively, these findings demonstrate that the interfacial wetting layer is not a simple extension of bulk water but a structured water phase formed under surface potential constraints. The orientation and HB connectivity of the first water layer thus govern charge distribution, proton transport pathways, and intermediate stability^[62,63].

In nanoporous materials such as zeolites and metal-organic frameworks, pore sizes are often comparable to the dimensions of water clusters (< 2 nm). Such extreme confinement prevents water molecules from sustaining the tetrahedral HBN characteristic of bulk water, forcing them to reorganize into one-dimensional chains, small clusters, or quasi-gaseous monomers with distinct thermodynamic and kinetic properties^[69-71].

Zhang et al. constructed a spatially graded hydrophilic-hydrophobic solid-water interface by introducing hydrophobic silica nanoparticles into a porous hydrogel framework^[72]. They demonstrated that this engineered solid-water interfacial network can significantly reduce surface water content, regulate the hydrogen-bonding states of water molecules, and effectively decrease the enthalpy of water evaporation^[72]. At the solid-water interface, abundant hydrophilic functional groups within the porous hydrogel framework restructure water molecules into three distinct states through interfacial interactions: bound water (BW), free water (FW), and intermediate water (IW) [Figure 4F]. Raman spectroscopic analysis of O-H stretching vibrations [Figure 4G] further confirms that this spatially engineered solid interface effectively weakens the dense hydrogen-bonding network among water molecules and significantly increases the proportion of IW, which exhibits a lower evaporation energy barrier. Consequently, the overall enthalpy of water evaporation is reduced at the microscopic molecular scale, providing a fundamental thermodynamic basis for efficient liquid-vapor phase transitions. Treps et al. used DFT calculations to investigate hydrated ZSM-5 zeolites with different surface orientations^[73]. They found that water speciation strongly depends on local topology: at nanopore mouths, water tends to dissociate into cavity-stabilized bridging hydroxyls with strong Brønsted acidity, whereas on outer surfaces it remains molecularly adsorbed, acting as mild Brønsted acid sites whose stability depends on HBNs formed by surface silanol groups^[73].

Overall, nanoscale spatial confinement induces water restructuring, providing additional degrees of freedom for regulating reaction processes and transforming water from a background solvent into a key factor influencing reaction pathways and energy barrier distributions^[74,75]. Thus, understanding the structural response of water in confined environments is a prerequisite for explaining anomalously enhanced reaction behaviors in water-solid systems.

Unlike confinement-dominated water restructuring in nanopores, solvation reconstruction at water-solid interfaces in electrochemical systems primarily reflects interactions between water molecules and solid surfaces^[76,77]. Near interfaces with high surface energy or strong interactions, water molecules couple with surface functional groups, defect sites, and localized charges to form solvation layers with specific orientations and stabilities^[78]. These interfacial solvation structures govern not only water adsorption and dissociation but also the stability and migration of protons and reaction intermediates^[79].

Wang et al. employed melamine as a multifunctional electrolyte additive to regulate Zn²⁺ solvation and interfacial interactions, thereby significantly enhancing zinc anode stability^[80]. The strongly electronegative groups in melamine exhibit higher binding energies with Zn²⁺ than water molecules, partially stripping water from the Zn²⁺ solvation sheath and reconstructing the HB network of the electrolyte, which reduces the HOMO-LUMO gap to accelerate charge transfer and promotes preferential melamine adsorption at the water/zinc interface. Additionally, Joos et al. systematically investigated transport properties across the full concentration range of Li-SCN-H₂O electrolytes, revealing that trace water molecules can act as dopants by substituting anions and forming H₂O-SCN defects^[81]. This mechanism simultaneously increases lithium vacancy concentration and mobility, enhancing ionic conductivity by nearly three orders of magnitude^[81].

In conclusion, electrolyte solvation reconstruction arises from the dynamic evolution of chemical environments and intermolecular interactions at water-solid interfaces. Through functional additives or solvent composition tuning, water solvation structures can be precisely engineered at the molecular scale - either by stripping solvation shells to suppress side reactions or by inducing defects to enhance ionic conductivity^[80,81]. These insights underscore that water at solid-liquid interfaces is not an inert medium but a highly tunable functional component that directly governs charge transfer and material transformation processes^[79].

The spontaneously formed built-in electric field at the solid-water interface constitutes a central physical factor driving various electrochemical processes. In energy storage systems such as batteries and supercapacitors, this type of field - originating from intrinsic material and electrolyte properties - governs interfacial water structure, ion transport, and interfacial reactions, thereby determining overall device performance. For example, in the field of piezoelectric catalysis, BON materials with asymmetric unit-cell-layer structures and abundant surface hydroxyl groups can eliminate interlayer electric-field shielding and achieve in situ self-polarization, thereby significantly enhancing the material’s piezoelectricity and built-in electric field. This leads to improved mechanical energy conversion efficiency and higher yields in piezocatalytic water splitting^[82].

In supercapacitors, Shi et al. constructed a MnO/MnS heterojunction combined with a BCN substrate and utilized the synergistic effect between the heterojunction’s built-in electric field and the BCN matrix to effectively widen the voltage window of an aqueous supercapacitor^[83]. This built-in electric field works in concert with the proton-attracting effect of N sites in BCN, markedly enhancing the adsorption and migration of Li⁺ on the electrode surface. As a result, the electrode surface becomes positively charged during discharge, electrostatically repelling H⁺ in the electrolyte, thereby suppressing the hydrogen evolution reaction and raising its overpotential. Simultaneously, a reverse electric field forms during charging, accelerating Li⁺ desorption and further inhibiting the oxygen evolution reaction^[83]. Similarly, in zinc-based battery systems, introducing amino acid-based additives into the electrolyte can regulate the solvation structure of Zn²⁺ and promote the formation of a stable solid-electrolyte interphase layer, thereby suppressing hydrogen evolution and dendrite growth triggered by interfacial water, and improving the corrosion resistance and cycling stability of the zinc anode^[84].

Furthermore, introducing a 7,7,8,8-tetracyanoquinodimethane (TCNQ) charge-transfer complex can form a Zn(TCNQ)₂ semiconductor interface on the zinc surface. The work-function difference generates a built-in electric field, which homogenizes the electric-field distribution, guides rapid Zn²⁺ transport, and suppresses water molecule activity, thus alleviating dendrite growth and side reactions on the zinc anode^[85]. In summary, by modulating interfacial water structure, ion solvation, and transport behavior, builtin electric fields can effectively suppress side reactions, enhance deposition uniformity, and improve interfacial stability. This provides a crucial physical foundation for addressing performance bottlenecks in a variety of electrochemical devices.

In contrast to built-in electric fields, externally applied electric fields provide an effective means for actively and precisely manipulating solid-water interfaces. By applying an electric field through external electrodes, phenomena such as electrowetting, electroosmotic flow, and electric-field-induced phase transitions can be induced^[86]. Further studies have shown that tuning the local electric field on Pd/Cu₂O catalyst surfaces can guide interfacial water molecules to reorient into an “H-down” configuration, shortening the M-H bond length and promoting water dissociation and active hydrogen generation, thereby enhancing the efficiency and selectivity of the electrochemical nitric oxide reduction reaction for ammonia synthesis^[87]. In porous carbon electrode systems, combined model analysis and experimental verification can clarify the relationship between the desolvation state of potassium ions and the critical pore size, while identifying an optimal concentration window for oxygen-containing functional groups. This synergy optimizes both thermodynamic ion adsorption and kinetic diffusion, leading to simultaneous improvements in capacitive performance and rate capability^[88].

Furthermore, anchoring quaternary ammonium cations onto hydrophobic covalent organic frameworks (COFs) and modifying copper electrode surfaces can significantly enhance the local electric field at the electrode-electrolyte interface [Figure 5A]. This strengthens CO intermediate adsorption and promotes CO-CO coupling, thereby improving the selectivity and stability of CO₂ electroreduction to ethylene. The hydrophobicity and microporous structure of the COFs help regulate the transport of CO₂ and H₂O, while the immobilized cations modulate K⁺ migration via the Donnan effect, further intensifying the interfacial electric field^[89]. On highly curved RuIr nanocatalyst surfaces, an enhanced local electric field can drive interfacial water into an ordered “O-down” orientation and strengthen the hydrogen-bond network, thereby accelerating the kinetics of the alkaline hydrogen oxidation reaction (HOR) and improving fuel-cell performance^[90].

The cases discussed above demonstrate that applied electric fields can exert precise control over water molecular orientation, ion transport, and reaction pathways by modulating the intensity and distribution of local electric fields. This offers an effective strategy for addressing key challenges related to efficiency, selectivity, and stability in electrochemical devices.

In photothermal catalytic processes at solid-water interfaces, the photothermal effect stems from the conversion of light energy into heat through non-radiative relaxation, generating a non-uniform temperature field at the interface^[72]. This, in turn, drives mass transfer and reaction processes by altering physicochemical parameters such as interfacial tension, fluid density, and reactant activation energy. This effect constitutes the physical basis for technologies including solar thermal utilization and photothermal catalysis.

For example, in PtW/TiO₂ bimetallic catalysts under photothermal synergistic conditions, acidic hydroxyl groups on W sites can anchor water molecules via hydrogen bonds, transforming the competitive adsorption of O₂ and H₂O into cooperative activation. This promotes the generation of reactive oxygen species, thereby alleviating the poisoning of volatile-organic-compound oxidation catalysts caused by water molecules in high-humidity, low-temperature environments^[91]. Notably, as demonstrated by Chen et al. [Figure 5B], the simultaneous loading of Co single atoms and Ni clusters onto oxygen-vacancy-rich ZrO₂ enables strong photothermal coupling under concentrated solar irradiation: the hybridization between the d-orbitals of Co single atoms and the molecular orbitals of H₂O forms intermediate impurity states, promoting hole generation and transfer under visible-light excitation and accelerating H₂O dissociation^[92]. Meanwhile, Ni clusters convert light energy into hot electrons and localized thermal fields via localized surface plasmon resonance, effectively activating CO₂ and facilitating C=O bond cleavage. This synergistic mechanism involving spatially separated dual active sites significantly enhances the kinetics and efficiency of solar full-spectrum-driven CO₂ reduction with water to CO^[92].

These studies illustrate that the photothermal effect, by creating localized thermal fields and modulating interfacial electronic structures, can enhance mass transfer and lower reaction energy barriers, offering an effective strategy for efficiently utilizing solar energy to drive heterogeneous catalytic reactions.

The essence of the photoelectric effect lies in the generation of photogenerated electron-hole pairs and their effective separation and utilization at interfaces. This process enables active control of the electric double-layer structure, carrier injection, and surface reaction kinetics through photovoltage or photocurrent, thereby establishing a coupled “photo-electro-chemical” interplay.

Research has shown that under high-intensity illumination, semiconductor photoanodes (e.g., α-Fe₂O₃, TiO₂, WO₃, BiVO₄) can accumulate a high density of photogenerated holes at their surfaces^[41]. This accumulation drives a shift in the water oxidation reaction mechanism from lower-order kinetics to a nearly barrier-free, fourth-order kinetic pathway [Figure 5C]^[41]. Both theoretical calculations and experimental results indicate that when four holes accumulate at the active site, adjacent high-valent Fe(V)=O intermediates are formed. This configuration lowers the activation energy for O-O bond formation to about 0.03 eV and increases the reaction rate by more than an order of magnitude relative to conventional first-, second-, or third-order kinetics. The process is facilitated by consecutive proton-coupled electron transfers of surface holes and is further enhanced through hydrogen-bond-mediated activation of interfacial water molecules, leading to a significant improvement in the efficiency and kinetics of photoelectrochemical water oxidation^[41].

In terms of heterojunction design, constructing Z-scheme heterojunctions with surface oxygen vacancies (e.g., n-Bi₁₂SiO₂₀/p-Bi₂S₃) can synergistically enhance visible-light absorption, promote charge separation, and generate reactive oxygen species. This approach collectively improves the photocatalytic removal efficiency of low-concentration nitrogen oxides in high-humidity environments, while also suppressing the formation of toxic by-products^[93]. Additionally, introducing N-hydroxymethyl functional groups onto the surface of g-C₃N₄ can enhance the adsorption and activation of O₂, as well as the dehydrogenation kinetics of H₂O, while preserving its intrinsic photoelectric properties^[94]. Constructing a spin-polarized electric field derived from sulfur vacancies in CdS nanorods enables efficient visible-light-driven overall water splitting for hydrogen production without the need for cocatalysts. By promoting charge separation from the bulk to the surface, this strategy mitigates the severe charge-carrier recombination and high reaction barriers commonly encountered in conventional photocatalysis^[95].

In summary, the photoelectric effect - through interface engineering strategies such as defect modulation, heterojunction construction, and surface functionalization - optimizes the generation, separation, and utilization of photogenerated charge carriers, thus offering a key pathway to enhance the efficiency, selectivity, and stability of photocatalytic and photoelectrochemical reactions.

In multicomponent chemical environments, differing physicochemical properties among species often lead to dynamic competition for adsorption sites, reaction pathways, and interfacial energy at the solid-water interface, thereby significantly influencing interfacial composition, charge distribution, and even macroscopic performance.

Lee et al. employed in situ RAXR to probe ion adsorption at charged solid-water interfaces^[96]. Their results show that competition from monovalent cations (Na⁺/Rb⁺) induces nonclassical adsorption behavior of the divalent ion Sr²⁺, driving it from a mixed IS/outer-sphere (OS) configuration toward predominantly OS complexes and significantly weakening its adsorption strength. In particular, Rb⁺ preferentially occupies IS cavity sites on the surface, forcing Sr²⁺ to desorb and exist mainly as fully hydrated OS species. To minimize electrostatic repulsion, these OS-coordinated Sr²⁺ ions further adopt an interleaved spatial arrangement with IS-bound Rb⁺ at the interface [Figure 6A]^[96].

On the other hand, Zhou et al. combined in situ spectroscopy and theoretical calculations to elucidate that carbonate anions (CO₃^2-) and their derived radicals (CO₃•^-) can form stable hydration layers with water molecules via HBs, promoting the structuring of the interfacial water network^[98]. This ordered water layer not only reduces the energy barrier for proton migration - accelerating the Volmer step of the hydrogen evolution reaction - but also enables CO₃•^⁻ to serve as an additional carbon source that can be reduced to formate or CO. Consequently, a dynamic competitive relationship is established between CO₂ reduction and hydrogen evolution, ultimately leading to the suppression of CO₂ reduction at high cathodic potentials. This finding provides a molecular-level understanding of how anions modulate competing reaction pathways through the structuring of hydration layers^[97,98]. Similarly, on γ-Al₂O₃ surfaces, gas molecules such as H₂O, SO₂, and CO₂ compete with gaseous arsenic pollutants (As₂O₃) for limited surface sites - a mechanism that explains, at the microscopic level, fluctuations in arsenic adsorption efficiency in multicomponent flue gas^[99]. In the low-temperature selective catalytic reduction (SCR) of NO_x with NH₃ over Mn-Cu-Al layered oxide catalysts, water molecules compete with the reactants NH₃ and NO_x for surface active sites and promote the formation of less reactive hydroxylated nitrate intermediates, thereby significantly suppressing low-temperature denitration activity^[100].

In summary, the competitive adsorption and reaction of multiple components at the interface constitute a central factor in determining the interfacial chemical state and, ultimately, the functional output. Managing this dynamic interplay allows for the optimization of the interfacial microenvironment, thereby enabling the fine-tuning of catalytic selectivity, reaction efficiency, and material stability.

In contrast to competition, the coexistence of multiple chemical species at the interface can give rise to synergy through intermolecular interactions, sequential adsorption, or the formation of composite interfacial phases, leading to enhanced functionality.

At the surface of perovskite anodes (e.g., Sr₂Fe_1.5Mo_0.5O_6-δ^[97,101]) in solid oxide fuel cells, water molecules in humid hydrogen atmospheres can synergistically enhance the kinetics of HOR by lowering the surface oxygen vacancy concentration and generating surface hydroxyl species, thereby improving cell performance. Specifically, Qi et al. combined electrochemical relaxation and in situ spectroscopic analysis to reveal that water molecules promote the HOR on Sr₂Fe_1.5Mo_0.5O₆ (SFM)-based perovskites through two synergistic mechanisms [Figure 6B]^[97]. On one hand, the introduction of H₂O increases the surface oxygen chemical potential and reduces the near-surface oxygen nonstoichiometry (δ) and the electron chemical potential, thereby lowering the energy barrier of the rate-limiting step involving “H₂O adsorption and surface oxygen vacancy formation” in the HOR. On the other hand, surface hydroxyl and hydride species formed via water dissociation can further stabilize HOR intermediates by strengthening the interaction between surface hydrogen species and lattice oxygen or oxygen vacancies, thereby accelerating surface reaction kinetics. This work elucidates, from both energetic and structural perspectives, the “co-catalytic” synergistic mechanism of water molecules at perovskite anode surfaces^[98].

Similarly, on the surface of arsenopyrite (FeS₂), the pre-adsorption of water molecules enhances the binding of oxygen and significantly lowers the energy barrier for arsenic oxidation, thereby clarifying the microscopic mechanism by which water vapor synergistically promotes the formation and release of toxic arsenic oxides during combustion^[102]. Additionally, the use of layered rare-earth oxycarbonates (Ln₂O₂CO₃) enables efficient and reversible exchange between surface hydroxyl groups (from H₂O dissociation) and carbonate species (from CO₂ adsorption). This “molecular exchange” mechanism allows for the timely removal of the gaseous product CO₂, thereby preventing its conversion into stable carbonates that would otherwise poison active sites. In this way, it addresses the critical issue of catalyst deactivation due to carbon deposition in the water-gas shift reaction^[103].

These cases demonstrate that through the rational design of interfaces to harness synergy among components, it is possible to construct functional interfaces with high performance and stability, providing a significant paradigm for the design of advanced materials and catalytic systems.