An ecient Turing-type Ag2Se-CoSe2 multi-interfacial oxygen-evolving electrocatalyst

Although the Turing structures, or stationary reaction-diffusion patterns, have received increasing attention in biology and chemistry, making such unusual patterns on inorganic solids is fundamentally challenging. We report a simple cation exchange approach to produce Turing-type Ag2Se on CoSe2 nanobelts relied on diffusion-driven instability. The resultant Turing-type Ag2Se-CoSe2 material is highly effective to catalyze the oxygen evolution reaction (OER) in alkaline electrolytes with an 84.5% anodic energy eciency. Electrochemical measurements show that the intrinsic OER activity correlates linearly with the length of Ag2Se-CoSe2 interfaces, determining that such Turing-type interfaces are more active sites for OER. Combing X-ray absorption and computational simulations, we ascribe the excellent OER performance to the optimized adsorption energies for critical oxygen-containing intermediates at the unconventional interfaces. Our work offers opportunities for creating Turing structures in other inorganic nanomaterials with unexplored catalytic abilities. Atomic Resolution Analytical Microscope an acceleration voltage of kV. Raman spectra were taken on a Raman microscope (Renishaw®) excited with a 514 excitation laser. ICP-AES data were investigated by an Optima 7300 DV instrument. Ultraviolet-photoelectron spectroscopy was obtained at the BL11U beamline Synchrotron The X-ray absorption spectra of Co L-edges were taken on the BL10B The X-ray absorption spectra of and Se K-edges out XPS X-ray photoelectron


Introduction
Almost seven decades ago, Alan Turing predicted the chemical reaction-diffusion model, in which a pair of activator and inhibitor can interact and self-regulate to form spatiotemporal stationary patterns 1 . This reaction-diffusion model has become a classic mechanism for morphogenesis in biological 2,3 (e.g., skin patterns of the puffer sh; Fig. 1a) and chemical systems 4 . In experiments on chemical systems, previous research has led to stationary Turing patterns occurred in the chlorite-iodide-malonic acid [5][6][7] (CIMA) and the Belousov-Zhabotinsky 8,9 (BZ) reactions. Later, a number of two-and three-dimensional Turing structures was investigated in chemical 10,11 and living systems 12,13 . Very recently, Tan and co-workers reported the preparation of a Turing-type polyamide membrane, which shows markedly enhanced watersalt separation performance compared to the conventional desalination membranes 14 .
Creating Turing structures in chemical systems currently remains a huge challenge. The di culty comes from the necessary requirement that the reaction-diffusion process should evolve far from thermodynamic equilibrium 1,15,16 , in which the inhibitors have a higher diffusion coe cient than the activators, leading to short-range activation and long-range inhibition 17 (Fig. 1b). Nevertheless, in homogeneous media, most chemical reactions involve small molecules with similar or inappropriately differing diffusion coe cients 18,19 . Such di culty can, in principle, be overcome by introducing an unreactive reagent that reversibly binds the activator species, thus causing suitable differences in the diffusion coe cients 6,7,14 . This has been demonstrated via coupling starch or polyvinyl alcohol with triiodide (activator) in the CIMA reaction 7 . Although considerable progress, there is no observation of stationary Turing patterns in inorganic solid nanomaterials has been reported thus far.
Here, we report the rst experimental demonstration of Turing-type silver selenide (Ag 2 Se) nanostructures that patterned on cobalt diselenide (CoSe 2 ) nanobelts by reacting Ag + ions with previously synthesized CoSe 2 (ref. 20) in a diethylenetriamine (DETA)-deionized water (DIW) binary solution at room temperature.
The conversion from CoSe 2 to Ag 2 Se is a thermodynamic driving process 21,22 , in which the low-mobility CoSe 2 is the activator and the fast-diffusing Ag + ions are the inhibitor 23 , thus resulting in a diffusiondriven instability and forming Turing structures. The resultant new material possesses rich Turing-type Ag 2 Se-CoSe 2 interfaces, which manifest excellent activity for electrocatalytic oxygen evolution reaction (OER). Moreover, we show the high robustness of the Turing-type Ag 2 Se nanostructures under harsh OER process. The observed OER performances can be attributed to the large number of Turing interfaces, which are experimentally and computationally con rmed as more favorable sites for the adsorption of oxygen-containing OER intermediates.

Results
Synthesis and characterization of Turing-type Ag 2 Se-CoSe 2 Stationary Turing patterns have been widely observed in living systems 12,13,15 , such as diodon holocanthus (commonly known as the puffer sh; Fig. 1a). The striped patterns in the puffer sh inspired that a high density of interfaces are likely created when Turing structure can be made in inorganic materials. Chemical transformation of one inorganic solid to another by atom exchange in solution is generally considered as a reaction-diffusion process 24 . This transformation, also known as ion-exchange reaction, has previously been used to pattern solids on nanoscale, leading to new materials with higher compositional and structural complexity 25 . We hypothesize here that cation exchange may offer the ability to generate a Turing pattern once the diffusion coe cients of two cations are appropriately different.
We want to react CoSe 2 nanobelts with Ag + ions to produce Ag 2 Se, considering that the conversion to Ag 2 Se is thermodynamically favored with a driving force of around -1000 kJ mol -1 in the bulk 26 . The previously developed CoSe 2 nanobelts 20 ( Supplementary Fig. 1) are the material of choice because they can provide adequate surface regions for the development of Turing structures. In a typical synthesis, the as-synthesized CoSe 2 nanobelts were dispersed in a mixture of DETA and DIW (2/1, v/v) under vigorous stirring. After completely dispersing, AgNO 3 was added and the reaction solution was continuously stirred at room temperature for 4 hours. In this system, CoSe 2 is the activator, and AgNO 3 is the inhibitor (Fig.   1b). The reaction starts when the fast-diffusing Ag + comes to the CoSe 2 surface and replaces Co 2+ ions in the solid phase. Because the markedly lower diffusion of Co 2+ , this activator initially replaced by the locally available Ag + inhibitor. As the reaction proceeds, more Co 2+ activator diffuses to exchange with nearby inhibitor, and eventually a Turing-type Ag 2 Se pattern forms across the surface of the CoSe 2 nanobelts (Fig. 1c). We note that the diffusion coe cient of Ag + ions is ~10 -5 cm 2 s -1 (ref. 23), orders of magnitude higher than that of Co 2+ ions in CoSe 2 . Such differences in the diffusion coe cients of the two species thus meet the requirement of activator/inhibiotr-mediated patterning, which result in the unusual Turing structures (Fig. 1c).
We examined the surface morphology of the as-synthesized sample by scanning electron microscopy (SEM), which shows regular striped patterns over the entire CoSe 2 support (Fig. 2a, Supplementary Fig.   2). Transmission electron microscopy (TEM; Fig. 2b) and scanning TEM (STEM; Fig. 2c) studies reveal that very re ned and interconnected Ag 2 Se networks grew on CoSe 2 , forming the nanoscale Turing-type structures. To visualize the Ag 2 Se-CoSe 2 interfacial structure, we performed high-angle annular dark-eld STEM (HAADF-STEM). Figure 2d validates the formation of distinct solid-state interface with the contrast mainly from the difference in atomic number between Ag 2 Se and CoSe 2 ( Supplementary Fig. 3). The dashed circles in Figure 2d highlight different lattice con gurations, and their corresponding fast Fourier transform (FFT) patterns feature cubic CoSe 2 (Fig. 2e) and orthorhombic Ag 2 Se (Fig. 2f), respectively.
Energy-dispersive X-ray spectrum (EDX) elemental mappings further evidence that Ag 2 Se developed on the CoSe 2 support, where Ag appears only in the Turing-patterned regions (Supplementary Figs. 4,5).
We carried out a series of control experiments to explore the formation of Turing-type Ag 2 Se-CoSe 2 structure. Despite cation exchange is commonly prohibited at room temperature in the bulk phase, results show that our reaction happened at room temperature with a fast rate. This phenomenon could be ascribed to the reduced reaction barrier in nanosized materials 27 , as well as the favorable thermodynamic driving force for forming Ag 2 Se (ref. 26). We tracked the evolution of the Turing structures as a function of reaction time (Supplementary Figs. 7,8). The diffusion-driven instability allowed this Turing structures to develop well at mere 1 h; markedly prolonging the reaction time to 12 h resulted in a hollowed-out structure. Moreover, the concentration of AgNO 3 appears to be also critical (Supplementary Figs.9-11). At low Ag + concentration, the transformation reaction initiated only at the edges of CoSe 2 nanobelts where the Co 2+ diffusion is easier because of the energy minimization. The addition of excess Ag + would cause the formation of over-carved material. Our experiments thus reveal that reacting 7.84 mmol L -1 AgNO 3 with CoSe 2 at room temperature for 4 h yields the optimal Turing-type Ag 2 Se-CoSe 2 structure.
We investigated the physicochemical properties of the novel Turing-type Ag 2 Se-CoSe 2 material by multiple characterization techniques. Differential scanning calorimetry (DSC) in Figure 2g exhibits a pair of endothermic (located at 417 K) and exothermal (located at 360 K) peaks for the Ag 2 Se-CoSe 2 , which could be the result of a phase transition from low-temperature cubic to high-temperature superionic phase of Ag 2 Se (ref. 28) (Insets in Fig. 2g). This superionic conducting phase transition of Ag 2 Se-CoSe 2 thus hints at potentially high cation mobility of the new structure 29 . Electrical conductivity tests as a function of temperature reveal consistently higher conductivity of the Ag 2 Se-CoSe 2 than that of metallic CoSe 2 (Fig. 2h). Such conductivity enhancement suggests that the Turing-type Ag 2 Se-CoSe 2 interfaces facilitate electron transfer. Moreover, we performed the work function measurement of Ag 2 Se, CoSe 2 , and Ag 2 Se-CoSe 2 by ultraviolet photoelectron spectroscopy (UPS; Fig. 2i). The results show a lower work function of 3.73 eV for Ag 2 Se-CoSe 2 , versus 3.93 eV for CoSe 2 and 4.24 eV for Ag 2 Se, indicating a modulated electronic structure of Ag 2 Se-CoSe 2 that permit an easier electron transfer, in line with conductivity results 30 .
OER performance of Turing-type Ag 2 Se-CoSe 2 catalyst The boosted electron transfer property of Ag 2 Se-CoSe 2 prompted us to explore this new structure for negotiating the sluggish OER catalysis, considering that decent OER activities have been observed previously on CoSe 2 -based catalysts [31][32][33][34] . We compared the OER activity of our Turing-type Ag 2 Se-CoSe 2 catalyst with that of four references CoSe 2 , Ag 2 Se, NiFe LDH, and 20 wt% Ir/C catalysts. Rotating disk electrode (RDE) measurements in O 2 -saturated 0.1 M KOH exhibit that Ag 2 Se-CoSe 2 needs an overpotential of mere 221 mV at 10 mA cm -2 , which increased to 399 mV for CoSe 2 , 350 mV for NiFe LDH, and 393 mV for Ir/C (Fig. 3a). By sharp contrast, Ag 2 Se itself affects negligible OER activity. Tafel analysis (Fig. 3b) yields a slope of 52, 175, 66 and 101 mV decade -1 for Ag 2 Se-CoSe 2 , CoSe 2 , NiFe LDH and Ir/C catalyst, respectively. A considerably smaller slope achieved in Ag 2 Se-CoSe 2 implies markedly enhanced kinetics for OER. We also conducted electrochemical impedance spectroscopy (EIS) at 340 mV overpotential to probe the charge transfer resistance (R ct ) for these catalysts. Our measurements ( Supplementary Fig. 12 property, so the rich Turing interfaces aforementioned could be responsible for the superior OER activity. We also underscore that the performance gained from the RDE testing here ranks among the superb for the OER electrocatalysts reported previously ( Supplementary Fig. 17 Fig. 19).
To verify the hypothesis that Turing-type Ag 2 Se-CoSe 2 interfaces are more OER active sites, we plotted the exchange current density (j 0 ; the most inherent measure of OER activity 36 respectively. We nd that the j 0 increases linearly as the interface length is increased (Fig. 3c). When plotting versus the Ag 2 Se covered area, a volcano-shaped dependence of the j 0 was obtained (Fig. 3d), further revealing that the reaction rate is directly proportional to the amount of interface sites. These results thus elucidate that the OER reaction takes place more energetically at the Ag 2 Se-CoSe 2 interfaces.
We used gas chromatography (GC) to detect and quantify the O 2 product evolved from the Ag 2 Se-CoSe 2docerated carbon paper electrode at 10 mA cm -2 . The measured O 2 gas perfectly matches with the theoretical value, corresponding to a Faradaic e ciency of ~100% (Fig. 3e). As a result, we achieved a high anodic energy e ciency up to 84.5%, which far exceeds that of CoSe 2 (73.7%) and also compares favorably to 76.2% for NiFe LDH and 74.4% for Ir/C catalyst (Fig. 3f), respectively.
Spectroscopic studies of the Turing-type Ag 2 Se-CoSe 2 catalyst We now study the impact of Ag 2 Se-CoSe 2 interfaces on the electronic structure and catalytic character by using diverse spectroscopic techniques. Prior reports have demonstrated that orthorhombic Ag 2 Se contains Ag (1) occurring in tetrahedral coordination and Ag (2) in triangular coordination 38 ( Supplementary Fig. 23). Our valence band spectrum of Ag 2 Se reveals two intensive features at 5.31 and 6.15 eV (Fig. 4a), originating from Ag (1) 5d and Ag (2) 5d states 39 , respectively. When forming interfaces with CoSe 2 , a large negative shift (~190 mV) of the Ag (1) 5d state is observed, whereas the Ag (2) 5d feature is undisturbed. This result suggests that Ag + coordinates tetrahedrally with Se 2at the novel Turing-type interfaces ( Supplementary Fig. 24). Figure 4b presents the Co K-edge X-ray absorption near-edge structure (XANES), which shows that the absorption edge of Ag 2 Se-CoSe 2 is shifted to a lower energy versus CoSe 2 because of charge transfer from Ag 2 Se to Co (Supplementary Fig. 25) 40 . Extended X-ray absorption ne structure (EXAFS) spectra ( Fig. 4c)  of CoSe 2 , is still observed after OER (Fig. 4c). By contrast, single CoSe 2 undergoes a complete surface self-reconstruction to form CoOOH after OER (Supplementary Fig. 26). The EXAFS wavelet transform (Fig.  4d) analyses a technique that can discriminate the backscattering atoms further verify that Co-Se bond (~ 7.82 Å -1 in k space) remains in Ag 2 Se-CoSe 2 after OER.
Unexpectedly, our Ag 3d X-ray photoelectron spectroscopy (XPS) analysis (Fig. 4e) reveals that the Ag valence state is unaltered after we performed OER on Ag 2 Se-CoSe 2 catalyst at a 221 mV overpotential (10 mA cm -2 ) for 12 h. Moreover, the signal from Ag-Se bond (at 54.2 eV) 42 in Se 3d (Fig. 4f, Supplementary  Fig. 27) further supports that Ag 2 Se phase survives after OER, agreeing with Se K-edge XANES results ( Fig. 4g, Supplementary Fig. 28). By contrast, only oxidized Se species 43 were detected from single CoSe 2 catalyst after OER (Fig. 4f), suggesting a complete surface self-reconstruction to CoOOH.
On the basis of above results, we become clear about the nature at the Turing-type Ag 2 Se-CoSe 2 interfaces. In the reaction-diffusion system, Ag + ions replace the Co 2+ ions in CoSe 2 nanobelts and tetrahedrally coordinates with Se 2-, creating Turing-type Ag 2 Se-CoSe 2 interfaces where the e g lling of adjacent Co cations is increased. This hence causes a near-unity e g occupancy of surface Co cations, leading to enhanced OER activity 40,44 . Moreover, such interfaces also show extreme structural robustness against harsh OER corrosion (Fig. 4h).

Density functional theory calculations
To better understand the catalytic nature of the Ag 2 Se-CoSe 2 interfaces, we carried out density functional theory (DFT) calculations. On the basis of experimental characterizations above, we created the Ag 2 Se-  Fig. 32), which explains the weaker *OOH adsorption on Ag 2 Se-CoSe 2 interface relative to CoSe 2 .
A plot of the electron density difference (Fig. 5b) displays that the donation of electrons from the Ag 2 Se to nearby Co sites occurs at the interface, which permits e g -orbital occupancy of Co closer to unity and thus enhanced OER activity 40,44 , matching well with our XANES measurements. Moreover, the calculated projected density of states (PDOS; Fig. 5c

Conclusion
Chemical Turing patterns in inorganic nanomaterials have not yet been reported. Here we demonstrate that Turing-driven morphogenesis occurs in a cation exchange process, which was originated from the appropriate differences between the diffusion coe cients of Ag + and Co 2+ (diffused from CoSe 2 ), creating stationary Ag 2 Se Turing patterns on CoSe 2 nanobelts. The obtained new material comprises abundant Turing-structured Ag 2 Se-CoSe 2 interfaces, which enable excellent OER electrocatalytic activity and stability in alkaline electrolyte. Our work opens the possibility of producing stationary reactiondiffusion patterns in inorganic solids that would not otherwise have such structures. Furthermore, the unusual engineered interfaces may see application in a wider spectrum of electrocatalytic processes.

Material synthesis
All chemicals are of analytical grade and were used as received without further puri cation.

Synthesis of CoSe 2 /DETA nanobelts
The CoSe 2 /DETA nanobelts were synthesized by a hydrothermal method using our previously developed method 20 . Brie y, 0.249 g Co(AC) 2 ⋅H 2 O and 0.173 g Na 2 SeO 3 were added into a mixed solution (40 ml) with a volume ratio of V DETA /V DIW = 2:1 (DIW = deionzed water). After stirring for 30 min, the obtained wine solution was transferred into a Te on-lined autoclave, which was sealed and maintained at 180 o C for 17 h. The resulting CoSe 2 nanobelts were carefully washed and dried before use.

Synthesis of Turing-type Ag 2 Se-CoSe 2 catalyst
The Turing-type Ag 2 Se-CoSe 2 was prepared through an ion-exchange reaction. Brie y, 20 mg freshly made CoSe 2 nanobelts was added into a mixed solution (15 mL) with a volume ratio of V DETA /V DIW = 2:1.
After completely dispersing, 20 mg AgNO 3 was added and drastically stirring at room temperature for 4 h, the obtained Turing-type Ag 2 Se-CoSe 2 powder was carefully washed and dried before use.
Material characterizations X-ray powder diffraction (XRD) was obtained from a Philips X'Pert Pro Super X-ray diffractometer with Cu Kα radiation (λ = 1.54178 Å). The morphology of the samples was achieved by SEM (Zersss Supra 40) and TEM (Hitachi H7650). The STEM and HAADF images, SAED, and EDX elemental mappings were measured on JEMARM 200F Atomic Resolution Analytical Microscope with an acceleration voltage of 200 kV. Raman spectra were taken on a Raman microscope (Renishaw®) excited with a 514 nm excitation laser. ICP-AES data were investigated by an Optima 7300 DV instrument. Ultravioletphotoelectron spectroscopy was obtained at the BL11U beamline of National Synchrotron Radiation Laboratory in Hefei, China. The X-ray absorption spectra of Co L-edges were taken on the BL10B beamline of National Synchrotron Radiation Laboratory in Hefei (China). The X-ray absorption spectra of Co and Se K-edges were carried out at the beamline 14W1 of Shanghai synchrotron Radiation Laboratory (China). XPS was performed on an X-ray photoelectron spectrometer (ESCALab MKII) with an X-ray source (Mg Kα hυ=1253.6 eV). The O 2 -temperature-programmed desorption analysis (TPD) measurements were taken on AutoChem II 2920. The electrical conductivity measurements were measured by using the standard four probe transport measurement on commercial apparatus of Physical Property Measurement System (Quantum Design, PPMS). Differential scanning calorimetry (DSC) cycling curves were carried out by the NETZSCH DSC Q2000 with a heating/cooling rate of 5 K min −1 between 273 and 473 K.

Electrochemical measurements
All the electrochemical measurements were performed in a standard three-electrode cell at ambient temperature connected to a VSP-300 potentiostat (BioLogic, France). Ag/AgCl (3.5 M KCl) electrode and graphite rod were used as the reference and counter electrodes, respectively. The potentials reported in this work were normalized versus the RHE through a standard RHE calibration (E = E Ag/AgCl + 0.97 V). A rotating disk electrode (RDE) with glassy carbon (PINE, 5.00 mm diameter, disk area: 0.196 cm 2 ) was used as the working electrode.
To make the working electrodes, 5 mg catalyst powder was dispersed in 1 ml of 1:3 v/v isopropanol/DIW mixture with 20 μL Na on solution (5 wt%), which was ultrasonicated to yield a homogeneous ink. Then, where E 0 is the equilibrium cell potential for water decomposition (E 0 = 1.23 V). FE is the Faradaic e ciency for H 2 O to O 2 conversion, and η ,an is the overpotential at the anode and was measured at 10 mA cm -2 in this work.

DFT calculations
The overall OER process includes four elementary steps that follow: (see Reactions 1-4 in the Supplemental Files) Here, (*) denotes the -OH terminated CoSe 2 or Ag 2 Se-CoSe 2 surface. We used -OH groups to replace the unsaturated Se atoms as models to represent the surface hydroxylation of CoSe 2 or Ag 2 Se-CoSe 2 , because the whole structure is well maintained. These are similar to the -OH groups terminated surface of oxide models 48 . It is more convenient to calculate the thermochemistry of the OER under acidic condition. Following Bajdich et al. 49    h, Temperature-dependent electrical conductivity for CoSe2 and Ag2Se-CoSe2, respectively. i, UPS spectra of the CoSe2, Ag2Se and Ag2Se-CoSe2. surface-reconstruction feature of the Ag2Se-CoSe2 catalyst, showing that the Ag2Se-CoSe2 interface is robust without surface self-reconstruction during OER process. DFT calculations and enhancement mechanism. a, Gibbs free energy change diagrams of the OER process on the CoSe2 (black line) and Ag2Se-CoSe2 (red line) surface models (both terminated with OH groups). b, Three-dimensional electron density difference on the Ag2Se-CoSe2 interface. The cyan and yellow contours represent electron density depressions and accumulations, respectively. c, Calculated density of states (DOS) of CoSe2 and Ag2Se-CoSe2 with the Fermi level aligned at 0 eV. Blue and red dashed line located at the d-band center of CoSe2 and Ag2Se-CoSe2, respectively. d, Temperatureprogrammed O2 desorption analyses for CoSe2 and Ag2Se-CoSe2 catalysts, respectively.

Supplementary Files
This is a list of supplementary les associated with this preprint. Click to download. SITuringtypecatalyst.pdf Formulas.pdf