Facile Electron Transfer in Atomically Coupled Heterointerface for Accelerated Oxygen Evolution

An efficient and cost-effective approach for the development of advanced catalysts has been regarded as a sustainable way for green energy utilization. The general guideline to design active and efficient catalysts for oxygen evolution reaction (OER) is to achieve high intrinsic activity and the exposure of more density of the interfacial active sites. The heterointerface is one of the most attractive ways that plays a key role in electrochemical water oxidation. Herein, atomically cluster-based heterointerface catalysts with strong metal support interaction (SMSI) between WMn2 O4 and TiO2 are designed. In this case, the WMn2 O4 nanoflakes are uniformly decorated by TiO2 particles to create electronic effect on WMn2 O4 nanoflakes as confirmed by X-ray absorption near edge fine structure. As a result, the engineered heterointerface requires an OER onset overpotential as low as 200 mV versus reversible hydrogen electrode, which is stable for up to 30 h of test. The outstanding performance and long-term durability are due to SMSI, the exposure of interfacial active sites, and accelerated reaction kinetics. To confirm the synergistic interaction between WMn2 O4 and TiO2 , and the modification of the electronic structure, high-resolution transmission electron microscopy (HR-TEM), X-ray photoemission spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) are used.


Introduction
Oxygen electrochemistry plays a major role in energy conversion and storage devices, particularly in the field of fuel cells, metal-air batteries, and water electrolyzers. [1] However, this technology has many particular bottlenecks. The major challenge in catalysts is greatly dependent on the surface charge state of the designed materials. [16,17] Thus far, innumerable efforts have been devoted to construct electrocatalysts with a suitable surface electronic structure for enhancing the catalytic activity, like interface engineering to create strong metal support interaction, [18][19][20][21] alloying, [22] heteroatom doping, [23] and phase control. [24] Compared with singlecomponent catalysts, heterointerface catalysts have several advantages including the electronic interactions between different components, and the synergistic effects. Apart from this, the general guideline to design active and efficient OER catalysts is to achieve higher intrinsic activity and a high density of interfacial active sites. In this regard, engineering heterointerface is one of the most attractive ways that plays a key role in electrochemical water oxidation.
Inspired by the motivations mentioned above, we designed heterointerfaces between WMn 2 O 4 and TiO 2 , where WMn 2 O 4 nanosheets are uniformly and densely decorated by TiO 2 nanoparticles to maximize the density of the interfacial active sites. WMn 2 O 4 exhibits high intrinsic activity, but limited durability and electronic conductivity. TiO 2 nanoparticles can act as a good potential support material for catalysts due to its environmental friendliness, high stability, moderate cost, ability to alter the electronic properties of oxide catalysts, and commercial availability. [3] Thus, the hybridization of WMn 2 O 4 with TiO 2 would provide an effective charge transfer, long-term stability, and enhanced adsorption of targeted reactants. In this regard, the resulting heterostructure material would possess a hierarchical structure, which enables a strong electronic coupling and convenient charge transfer at the interconnected interface, thus promoting electron transport. Herein, we introduce a strong metal support interaction (SMSI) effect on WMn 2 O 4 -TiO 2 heterostructure, hypothesizing that the interface could bring about more active environment to trigger the adsorption and desorption of oxide species to eventually produce O 2 gas via water-splitting. We also demonstrate the change in Mn atomic environment near the metal atoms before the formation of the heterointerface via W atom doping (Scheme S1, Supporting Information); and hence correlate their effect in enhancing the charge transfer phenomena during OER catalysis. The resulting electro-catalyst exhibits an onset overpotential of 200 mV versus reversible hydrogen electrode (RHE), a small Tafel slope (67 mV dec −1 ), and stable Chronopotentiometric measurement in 0.1 M KOH solution at various current densities (10, 20, and 30 mA cm −2 ) for a total of 30 h (10 at each current density)

Synthesis of WMn 2 O 4 Nanoflakes and TiO 2 Nanoparticles
In a typical synthesis of WMn 2 O 4 nanoflakes, 698 mg of (Mn(ac)) 2 and 344 mg of WCl 6 were dissolved in 50 mL of deionized (DI) water under gentle magnetic stirring to afford a homogeneous solution. Subsequently, 288 mg of NaOH was dissolved in 5 mL of DI water and added dropwise into the reaction mixture under vigorous stirring. The color of the solution first turned green and gradually evolved into greenish-brown, brown, and finally dark brown with the addition of NaOH. Then, the reaction mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave and a hydrothermal reaction was carried out at 120 °C for 6 h. [9] The resulting precipitate was collected by centrifugation, was washed thoroughly with anhydrous ethanol and dried in a vacuum at 60 °C for 12 h.
The synthesis procedure for TiO 2 nanoparticles can be found in the Supporting Information.

Synthesis of WMn 2 O 4 -TiO 2 Heterostructure
WMn 2 O 4 -TiO 2 heterostructures were prepared by a modified calcination method. In a typical synthesis, 0.1 g of TiO 2 nanoparticles (1:1 molar ratio of WMn 2 O 4 to TiO 2 ) and WMn 2 O 4 nanoflakes were dispersed in 20 mL deionized water. Then, the obtained solution was stirred and heated at a temperature of 80 °C in oil bath. The temperature was slowly increased to 200 °C and kept for 2 h to obtain a complete evaporation and drying of the solvent. Then, the powder was grinded to obtain a uniform size of the heterostructures and then annealed to create strong metal support interaction (SMSI) at 600 °C with a rate of temperature increase equal to 5 °C min −1 with 40 sccm an Ar flow with a chamber pressure pumped down to ≈0.1 Pa. The sample was kept at this temperature for 4 h to obtain WMn 2 O 4 -TiO 2 , as schematically represented in Figure 1.

Sample Characterization
The crystalline structure of the samples was investigated through powder X-ray diffraction (XRD). The morphology of the samples was investigated through scanning electron microscopy (SEM) combined with energy dispersive X-ray (EDS) spectroscopy for the elemental analysis. Transition Electron Microscopy (TEM) images were recorded with high-resolution transmission electron microscope. To study the change in electronic environment, X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) measurements were performed with a standard X-ray source and at the VUV-Photoemission beamline of the synchrotron Elettra (Trieste, Italy). Details of the characterization techniques were found in the supporting information.

Oxygen Generation from Water Splitting
Electrochemical activity measurements were performed using a computer-controlled potentiostat (PGSTAT302N, Metrohm Autolab) assembled with a rotational system (Pine Research Instrumentation, Durham, NC, USA) with a standard threeelectrode glass cell (graphite rod as a counter electrode, Ag/ AgCl/sat. KCl reference electrode, carbon fiber with catalysts as the working electrode) in 0.1 M KOH electrolyte at room temperature. The working electrode was prepared by ultrasonically dispersing 5 mg of catalysts in 1 mL of isopropanol and DI water (3:1) and Nafion solution (70 µL, 0.05 wt.% in alcohol), and then drop casting the ink on the surface of pretreated carbon fiber. EIS measurements were carried out in 0.1 M KOH electrolyte solution at 25 °C. The detailed procedure is described in the Supporting Information.

Morphological and Structural Characterizations
The scheme for the preparation of the WMn 2 O 4 -TiO 2 heterostructure is reported in Figure 1. As can be seen in Figure S1a, Supporting Information, a typical SEM image of pristine TiO 2 , which possesses well-dispersed nanoparticles. Similarly, Figure S1b, Supporting Information, shows the typical SEM image the WMn 2 O 4 nanoflakes. These nanoflakes have a minimum selfaggregation and an open structure, which is advantageous for the electrocatalytic reactions thanks to its high specific surface area and exposed sites. The WMn 2 O 4 -TiO 2 nanoflakes (Figure S1c, Supporting Information) exhibit an entangled network, in which the TiO 2 nanoparticles are uniformly dispersed on the surface of WMn 2 O 4 , without agglomeration. Such structure may enhance ion diffusion and electronic motion, and may extend the durability of the electrode. The corresponding EDS elemental mapping ( Figure S1d, Supporting Information) reveals the presence of evenly distributed W, Mn, Ti, and O, as expected, with no contaminants, within the detection limit of the technique.
The XRD patterns for the WMn 2 O 4 and TiO 2 systems, and the resulting WMn 2 O 4 -TiO 2 hetero-structure are shown in Figure 2a.
The typical diffraction peaks from TiO 2 sample (reported for clarity purposes also in Figure S2a Figure S3a,b, Supporting Information. The two lattice fringes indicate d 101 = 0.346 nm and d 220 = 0.296 nm, corresponding to TiO 2 and WMn 2 O 4 , respectively. This suggests the coupling interface of TiO 2 and WMn 2 O 4 at the atomic cluster level to form heterostructure. The STEM characterization and energy dispersive spectroscopy (EDS) elemental mapping ( Figure 2d) shows that W, Mn, and Ti exist in the WMn 2 O 4 -TiO 2 samples, and that the Ti is uniformly and homogeneously distributed, (see also the EDS imaging from SEM reported in Figure S1d). This result indicates the successful decoration of TiO 2 on WMn 2 O 4 flakes, that would deliver a conducive interface for enhanced OER activity. The formation of strong metal support interaction between WMn 2 O 4 and TiO 2 is another fascinating phenomenon that probes the electronic effect behind the enhanced catalytic activity.
Given that catalysis is a surface phenomenon, the surface chemical states, and valence state of Mn, W and Ti were assessed by X-ray photoelectron spectroscopy (XPS) in WMn 2 O 4 , TiO 2, and WMn 2 O 4 -TiO 2 . The Mn 2p3/2 peak (Figure 3a) in both WMn 2 O 4 and WMn 2 O 4 -TiO 2 located at 642.4 eV can be associated with contributions of Mn 3+ oxidation states. [5] The W 4f spectrum in WMn 2 O 4 -TiO 2 (Figure 3b  doubles corresponding to the two different oxidation states of W. The doublet with the W 4f7/2 peaks at 36.18 eV corresponds to W +6 [25] and the doublet with the W 4f7/2 peak at 34.58 eV to W +5 . [26] The doublet with the W 4f7/2 peaks corresponding to W +6 and, W +5 for the pristine WMn 2 O 4 ( Figure S4a, Supporting Information) also appeared in the respective binding energy. However, the low penetration depth profile of XPS spectra limit to see the change in electronic structure on W 4f and Mn 2p spectra in WMn 2 O 4 and WMn 2 O 4 -TiO 2 samples. Therefore, we use XAS characterization for best penetration depth profile and sensitive toward even smaller changes in electronic structures.
The Ti 2p3/2 spectra in WMn 2 O 4 -TiO 2 ( Figure 3c) centered at 458.8 shift to lower BE compared to the Ti 2p3/2 spectra centered at 460.3 eV in TiO 2 . This negative shift in Ti 2p spectra in the heterostructure catalysts is due to the decrease in oxidation number during thermal annealing to different oxidation states of Ti. During thermal annealing, TiO 2 commonly reduces to different Magnéli phase oxides as described with the general formula Ti n O 2n-1 ,3< n < 10. [20,27] In the WMn 2 O 4 sample, the O1s XPS spectrum ( Figure S4b, Supporting Information) shows three deconvoluted peaks at binding energies of 529.7 eV and 531.2 eV, assigned to lattice oxygen, and at 533.02 eV, assigned to surface defects and/or adsorbed surface hydroxyls. The lattice oxygen peaks can be associated to the nature of oxides as in Mn(OH) 2 (529.7 eV) and MnO 2 (531.2 eV). Generally, the change in the oxidation state in the redox couples of Mn 3+ and W 5+ /W 6+ (i.e., W 6+ + Mn 3+ ↔ W 5+ + Mn 4+ ) is induced during annealing to construct surface heterointerface material that improves the electron transfer (Figure 3d).
To gather further information on the different crystal structures in the samples and on the role of each element, we applied X-ray absorption near edge fine structure (XANES) spectroscopy. Metal K-edges absorption spectra are very sensitive to the interactions with the neighboring atoms and the chemical states of the atoms for the 3d transition metals. The K-edge for an absorption process is due to 1s electrons transition to the empty levels (3d, 4p) over the Fermi level as a final state. Actually, the 1s → 3d transition is forbidden due to a combination of a strong 3d-4p mixing and the overlap of metal 3d orbitals with 2p orbitals of surrounding O atoms. [28] The 3d levels are located just below the Fermi level and can give information about the binding properties of the source atoms. The pre-edge structure may occur just below the main absorption edge and gives information about the forbidden transitions and crystal disorder. On the other side, the post-edge part lies just beyond the main edge and provides information about the interactions with the closest neighboring atoms. The white line part is the large, prominent peak just above the edge. Particularly in L or M edge spectra, it provides information about the d-orbital occupancy. [29] As a result, the Mn K edge spectra were taken before and after OER to see how the change in the Mn valence state is affected. As a result, the pre-edge feature ≈6540.6 eV in the Mn K-edge spectra in both WMn 2 O 4 , and WMn 2 O 4 -TiO 2 is due to quadrupole 1s-3d and/or modifications of the dipole transition probability due to the hybridization between 3d and 4p states. [30] In Mn K pre-edge spectra, No shift was observed in the position, but the intensity of this feature decreased in WMn 2 O 4 -TiO 2 compared to pristine WMn 2 O 4 (Figure 4a inset). This is due to the high degree of disorder in the Mn environment during the surface annealing to form the heterointerface. Nevertheless, this change is pronounced even after OER in the  Similarly, the local structure and the valence state of the W species are determined by the position of the W edge. In this case, the local symmetry is determined by the area of the preedge peak of the W L-edge XANES, which is due to a 2s-5d transition. The 2s-5d transition is a dipole-forbidden transition for regular octahedral symmetry; however, this is partially allowed for a distorted octahedral structure, which gives rise to the absence of an inversion symmetry because the p orbitals are mixed with 5d orbitals. Therefore, a W unit with tetrahedral symmetry exhibits a large pre-edge peak area in W L-edge XANES. Figure 4b  It is worth noting that the pre-edge regime of the Ti K-edge is very sensitive to the local structure geometry and coordination environment. The pre-edge feature of Ti K-edge XANES originates from the 1s−3d transition and appears due to the polarization of p-orbitals and strong 3d−4p orbital mixing. [31] As a result, the pre-edge of Ti k-edge shows three different peaks represented as feature A 1 , A 2, and A 3 . A 1 is related to the Ti 1s-3d (t2g) quadrupole transition and varies upon structural changes in the first coordination shell of Ti. [32,33]   displays the increased intensity of the pre-edge peak with a decrease in coordination number as compared to TiO 2 , (Figure 4c) associated with severe Ti site distortions. The absorption edge shifts towards lower energy for WMn 2 O 4 -TiO 2 in comparison with the TiO 2 sample, probably due to the reduction of TiO 2 to different Magnéli phase oxides as described with the general formula Ti n O 2n-1 ,3< n < 10 during thermal annealing for the heterostructure formation. [20,27] Furthermore, to study the local structure and coordination environment, Fourier transforms (FTs) of the EXAFS data for WMn 2 O 4 , WMn 2 O 4 -TiO 2, and WMn 2 O 4 -TiO 2 after OER is analyzed. As shown in Figure

OER Performance of WMnO 4 -TiO 2 Heterointerface
To understand how the interface affects the OER catalytic performance, the prepared materials were directly evaluated in 0.1 M KOH solution. The electro-catalytic activity of the catalysts supported on carbon fiber was characterized by linear sweep voltammetry (LSV) in O 2 and Ar saturated 0.1 M aqueous KOH solutions (Figure 5), employed to assess the OER kinetics of the catalysts. As can be seen in Figure 5a,b, the heterostructure, benefitting from the active interface, generates a current of 10, 20, and 30 mA cm −2 under an applied potential of 1.5 V, 1.55 V, and 1.6 V versus RHE, respectively, with a significant improvement compared to WMn 2 O 4 (10 mA cm −2 @ 1.6 V, 20 mA cm −2 @1.65 V and 30 mA cm −2 @1.7 V) and RuO 2 (10 mA cm −2 @ 1.56 V, 20 mA cm −2 @ 1.62 V and 30 mA cm −2 @1.67 V). In fact, in the heterointerface catalyst, the electrons can be transferred from one component to another through the boundary surface, that modulates the current density around the active sites. The changes in chemical composition or crystal structure due to strong metal support interaction (SMSI), the setting of atoms at the interface are different from the bulk material, which can induce electronic and geometric effect and new chemical bonding at the boundary surface. [34] To confirm that the interface construction is beneficial to increase the intrinsic activity of the materials, we report the ECSA normalized LSV for WMn 2 O 4 and WMn 2 O 4 -TiO 2 in Figure S5a, Supporting Information. As a result, the intrinsic activity for WMn 2 O 4 -TiO 2 shows an increase after normalization by ECSA, compared to WMn 2 O 4 , thus confirming that the interface construction is beneficial to increase the intrinsic activity. The increased current in WMn 2 O 4 after ECSA normalization might be actually due to thermal annealing affects the surface area for WMn 2 O 4 -TiO 2 . To remove the ambiguity on the origin of the enhanced activity (resulting from thermal annealing or from the  introduction of the heterostructure), we tested the OER activity of WMn 2 O 4 after thermal annealing as can be seen in Figure S5b, Supporting Information. The thermally synthesized WMn 2 O 4 shows worsened OER activity (1.6 V, onset potential) compared to the WMn 2 O 4 sample synthesized via the hydrothermal route. Thus, we can assure that the enhanced OER activity is resulting from the introduction of TiO 2 to form heterostructure. As listed in Table S1, Supporting Information, the optimized WMn 2 O 4 -TiO 2 heterostructure is one of the best among the most active recently reported oxide OER catalysts.
To further understand the kinetics of the OER, Tafel slope were extracted from the polarization curves. The WMn 2 O 4 -TiO 2 hetero-structure exhibited Tafel slope of 67 mV dec −1 , much lower than the WMn 2 O 4 (98 mV dec −1 ) (Figure 5b). The lower Tafel slope suggests the fast reaction kinetics for oxygen evolution and rapid electron transfer in the heterointerface. The significant difference in intrinsic reaction kinetics between WMn 2 O 4 -TiO 2 and WMn 2 O 4 samples also contributes to the different catalytic performance. The formation of heterostructure is associated with the abundant exposure of active sites as a result of the new interface formation between dissimilar materials. Therefore, it is crucial to understand the electrochemical active area of catalysts for OER, which can be estimated by the double layer capacitance (C dl ) around the electrode surface. To unveil this, we calculated the C dl by monitoring the current density in the non-Faradic region with different scan rates. The CVs runs for obtaining these data are presented in Figure S5c,d, Supporting Information. Figure 5c depicts a higher value of C dl in the case of WMn 2 O 4 -TiO 2 (9.8 mF cm −2 ) than WMn 2 O 4 (5.6 mF cm −2 ). The large electrochemical double-layer capacitance indicates the increased density of exposed active sites, which is one of the possible reasons for the enhanced OER activity. To confirm the high catalytic activity of the heterostructured material, electrochemical impedance spectroscopies (EIS) is analyzed in the frequency range from 100 kHz to 0.01 Hz as depicted in Figure 5c. The Nyquist plots  were fitted by using a simple Randles equivalent circuit model, consisting of solution resistance (R s ), and charge transfer resistance (R ct ) that is concerned with OER kinetics, that are related to reaction rate. As can be seen in Figure 5c and Table S2, Supporting Information, WMn 2 O 4 -TiO 2 displayed the R ct (130 Ω), which is lower than that of WMn 2 O 4 (205 Ω) suggesting facile electrode kinetics and faster electronic transport. This highlights that the formation of a heterointerface between TiO 2 and WMn 2 O 4 triggers efficient electron transfer that increase electronic conductivity, thereby yielding accelerated O 2 gas evolution.
To evaluate electrochemical stability, long-term test was conducted using chronopotentiometric measurement at various current densities (10, 20, and 30 mA cm −2 ) for a total of 30 h (10 at each current density). As represented in Figure 5d, WMn 2 O 4 -TiO 2 heterostructure demonstrates very good stability with only minor degradation over long-term testing. It retains 99.8%, 99.6%, and 99.2% activity at 10, 20, and 30 mA cm −2 , respectively. As depicted in Figure 5f, the presence of TiO 2 in the WMn 2 O 4 composite also enables a stronger electronic coupling with the electrocatalyst itself and allows faster charge transfer and increase accessible interfacial contact that accelerate OER. We conclude that the trapped Mn 3+ states are essential for the formation of structurally highly flexible local clusters that could resemble the active sites for water oxidation catalysis. Additionally, we would like to stress that this study again indicates that the stabilization of a fraction of Mn 3+ centers at water oxidation potentials via W and Ti seems to be decisive for catalytic activity.
To get ultrafast OER performance the energy loss between electrodes and active sites, the electronic conductivity, and the interface between catalyst-catalyst support and catalyst-electrolyte are minimized. In order to achieve this, heterostructure catalyst with strong metal support interaction (SMSI) in which one can compensate the drawback of the other as schematically represented in Figure 6 needs to be designed and developed in the field of water splitting catalyst. Furthermore, the heterostructure also exposes more assessable active sites via catalyst surface reconstruction with improved reaction kinetics, giving consequently superior activity. Generally, designing heterointerface 2D material accelerates charge transfer at the electrode/ electrolyte interface and decreases the overall charge transfer resistance by enhancing the electronic conductivity, that induces the improvement of the OER performance.
To examine the structural stability of WMn 2 O 4 -TiO 2 catalyst, post-OER characterization of the catalyst was performed by XRD, and XPS. Interestingly, in the post-OER XRD measurement depicted in Figure 7a (top panel), no observable new phases or peak shift are recorded, which further demonstrates the good structural stability of the as-synthesized catalyst. The broad peak ≈2⊝ = 25 results from the carbon fiber used as a substrate for OER measurement. Figure 7b-d shows the XPS results of Mn 2p, W4f, and Ti 2p for the WMn 2 O 4 -TiO 2 electrode after the OER. Manganese shows two components. The Mn 2p3/2 peak centered at a BE value of 642.4 eV shows a slight positive shift (0.9 eV) compared to the system before OER. the Minor Mn2p3/2 peak centered at BE of 644.6 eV is associated to an oxidation state between 3 and 4. In the W4f spectra, we observe two doublets attributed to W 5+ (blue line) and W 6+ (pink line). The shoulder peak located ≈34 eV corresponds to K3s (from the electrolyte, 0.1 M KOH) at the sample surface. The Ti doublet with Ti 2p3/2 centered at 459.8 eV corresponds to Ti 4+ . A smaller component with the Ti2p3/2 peak centered 458.6 eV is ascribed to Ti 3+ (defect) in WMn 2 O 4 -TiO 2 or Ti 3+ in Ti 2 O 3 . [35] Therefore, we identify the W, Mn, and Ti elements with different oxidation state in the expected binding energy ranges. The existence of viable oxidation states of W, Mn, and Ti benefit for enhanced OER activity.

Conclusion
In conclusion, we have demonstrated cost-effective and ultrafast OER active catalyst via heterointerface engineering. Interface engineering to form SMSI has been recently recognized as one of the most promising strategies to develop efficient water oxidation electrocatalysts. This heterointerface gave rise to the exposure of the active site, which significantly accelerates the OER performance. In the heterointerface catalysts, electrons can be rearranged on heterostructures to modify the exposure of active sites and accelerate the reaction kinetic. As a result, the heterostructure benefitting from the active interface exhibits potent OER catalytic performance with lower overpotential (200 V), with an improvement of 70 mV as compared to a single-component analog of WMn 2 O 4 (270 V). This overpotential shows only negligible change within 30 h of test, suggesting the heterointerface increases the exposure of active species and prevents the collapse of the spinel oxide. Furthermore, the enhanced activity and long-term durability is believed to be originated from the synergetic effect between WMn 2 O 4 and TiO 2 , leading to higher charge carrier density, and enhanced conductivity. To confirm different surfaceinterface and electronic properties, XAS, and HR-TEM characterization tools were used, enabling a full description of the heterointerface from the structural as well as functional point of view. Finally, our approach reveals a novel strategy to manipulate the surface/interface charge states of electrocatalysts for accelerated oxygen evolution, representing a significant advancement in the field of H 2 production through the overall water-splitting process.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.