Revealing the Role of d Orbitals of Transition-Metal-Doped Titanium Oxide on High-Efficient Oxygen Reduction

Open AccessCCS ChemistryRESEARCH ARTICLE1 Nov 2021Revealing the Role of d Orbitals Transition-Metal-Doped Titanium Oxide on High-Efficient Oxygen Reduction Fei Lu†, Weiwei Xie†, Ding Yi, Yan Wang, Fengchu Zhang, Yong Xu, Bo Zhou, Shoujie Liu, Xi Wang and Jiannian Yao Lu† Department Physics, School Science, Beijing Jiaotong University, 100044 Key Laboratory Photochemistry, Institute Chemistry, Chinese Academy Sciences, 100190 †F. Lu W. Xie contributed equally to this work.Google Scholar More articles by author , Xie† Physical Karlsruhe Technology, 76131 Yi Google Zhang Xu Chemistry Chemical Engineering Guangdong Laboratory, Shantou 515031 Zhou Liu *Corresponding author: E-mail Address: [email protected] https://doi.org/10.31635/ccschem.020.202000659 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Precise catalysis is critical for high-quality industry. However, it remains challenging fundamentally understand precise at atomic orbital level. Herein, we propose a new strategy unravel role specific orbitals in catalysis. The oxygen reduction reaction (ORR) catalyzed atomically dispersed Pt/Co-doped Ti1?xO2 nanosheets (Pt1/Co1–Ti1?xO2) used as model z-axis Pt/Co–Ti realms dominate O2 adsorption, thus triggering ORR. In light orbital-resolved analysis, Pt1/Co1–Ti1?xO2 experimentally fabricated, excellent ORR catalytic performance further demonstrated. Further analysis reveals that superior Pt1–Ti1?xO2 Co1–Ti1?xO2 ascribed stronger activation Ti Pt than Co via d–d hybridization. Overall, work provides useful tool underlying mechanisms level opens opportunities catalyst design. Download figure PowerPoint Introduction sluggish kinetics, especially kinetically hindered multielectron transfer process, has long posed one greatest challenges electrocatalysis.1–3 Enormous efforts have been dedicated design class cost-effective high-performance electrocatalysts.4,5 pioneering works provide significant inspiration geometric electronic structure regulation catalysts.6–20 optimizations focus tailoring crystal facets,6,7 refining surface strain,8–11 or downgrading into single-atom scale,12–14 while electronic-structure optimization pursues charge d-band center.15–20 rational d-orbital energies/orientations rarely reported, which requires comprehensive understanding effects each (Figure 1a). To end, three key questions are needed be addressed: (1) How catalysts modulated local environment? (2) What catalysis? (3) do guide toward Figure 1 | (a) Schematic diagrams dyz, dxz, dxy, z 2 x -y within orbitals. (b) diagram nanosheet. (c) pDOS profile dopant Ti. (d) Charge density distribution (0.005 e Bohr?3) corresponding energy window yellow-shaded area (c). (e) ICOHP values Ti–3d Pt–5d orbitals, with inset schematic rotation projection. outline reveal roles individual reactions. method illustrated Pt-doped (Pt1–Ti1?xO2) ( Supporting Information S1a). 1b depicts top view monolayered close proximity, both unsaturated coordination five oxygens,21 may trigger intensive interaction,22,23 confirmed from well-matched levels projected states (pDOS) 1c S1b). clarifies hybridization increases electron unit 1d). Then, two intriguing arise: Which Pt–Ti interaction? play ORR? Experimental Methods Synthesis cation-deficient were initially fabricated. detail, Cs2CO3 (1.63 g) anatase TiO2 (2.14 ground followed calcination 800 °C 20 h twice produce layered titanate Cs0.7Ti1.825?0.175O4 [? represents vacancy (Tivac)]. protonic (H0.7Ti1.825?0.175O4) was obtained acid-exchange procedure mol L?1 HCl times (1 day per period), liquid-to-solid ratio maintained 100 mL g?1. Afterward, products washed distilled water (18.2 M?) several vacuum-dried overnight 80 °C. Finally, (in colloidal state) soft-chemical exfoliation process dispersion 0.017 tetrabutylammonium (TBAOH) solution shaken 10 days 220 rpm, set 250 synthesized an electrostatic adsorption-anchorage employing Pt(NH3)4(OH)2 anchored host precursor, respectively. positively charged Pt2+ could electrostatically adsorbed negatively Tivac site. Specifically, (0.96 mg mL?1) dripped constant stirring. after centrifugation 20,900 rpm freeze-drying 3 days. engineered doping-exfoliation strategy, according previous reports. Initially, K0.8Ti1.7Li0.2Co0.1O4 fabricated grind K2CO3 (0.55 g), Li2CO3 (0.07 CoO (0.07), rutile (1.36 1000 h, second previous-step product h. undergoes same acid-exchange, wash water, sequence obtain (accurately Ti1.7Co0.1O4) loading ?3.9 wt %. Characterizations microstructure characterized field-emission scanning microscopy (FE-SEM; SU8010; Hitachi, Tokyo, Japan) transmission (TEM; JEM-2010; JEOL, operated accelerating voltage 200 kV, high-angle annular dark-filed TEM (HAADF-STEM; Tecnai G2-F20; FEI, Oregon, USA) 300 kV. thickness estimated force (AFM; Dimension Icon; Bruker, Karlsruhe, Germany). crystallinity X-ray diffraction (XRD) monochromatized Cu K? irradiation (PANalytical Empyrean, Malvern, England). photoelectron spectroscopy (XPS) performed ESCALAB 250Xi (Thermo Fisher Scientific, Massachusetts, USA). UV–vis absorption recorded UV-2600 (Shimadzu, Osaka, Japan). content doped heteroatom weighted inductively coupled plasma optical emission spectrometry (ICP-OES; ICPOES 730; Agilent, California, fine measurements near-edge (XANES) extended (EXAFS) L3-edge K-edge beamline 1W1B Synchrotron Radiation Facility (BSRF) BL14W1 Shanghai (SSRF). foil PtO2 foil, CoO, Co3O4 employed reference samples. XAFS analyses spectra carried out Athena Artemis Demeter software package. quantitative information least-squares curve fitting k2-weighted EXAFS oscillation range 0?6 Å, using ARTEMIS module IFEFFIT (version 1.2.11, IFEFFIT, Copyright 2008, Matthew Newville, University Chicago). Electrochemical as-prepared water. carbon black, weight 1?4 nanosheets, former nanosheet colloid uniform suspension. suspension freeze-dried mixture. mixture (the black 25?75, 6 mg) 600 ?L water/ethanol mixed (50:50 volume ratio) containing 30 5 % Nafion (Dupont) 60-min sonication form homogeneous ink. ink deposited onto glassy rotating disk electrode (RDE; 0.07 cm2, ALS Co., Ltd., serving working electrode. Meanwhile, graphite rod Ag/AgCl serve electrodes three-electrode system (CHI 760E, Chenhua, Shanghai, China), electrolyte test O2-saturated 0.1 M KOH solution. Linear sweep voltammograms (LSVs) varied 400–2500 scan rate mV s?1. electron-transfer number (n) during calculated Koutecky–Levich (K–L) equation: J = L + K B ? / (1)where, JK measured current kinetic-limiting density, rate. 0.62 n F C O D ? ? (2)B determined slope K–L plots aforementioned equation. transferred electrons Faraday (96,485 mol?1), CO (1.2 × 10?6 cm?3) DO (1.9 10?5 cm2 s?1) concentration diffusion coefficients electrolyte, (0.01 kinematic viscosity electrolyte. Computational methods spin-polarized functional theory (DFT) calculations Quantum Espresso Package.24 generalized gradient approximation Perdew–Burke–Ernzerhof-type exchange-correlation (GGA-PBE)25 projector augmented wave (PAW) method26 adopted all kinetic cutoff Ry. A vacuum space 15 Å avoid interaction between periodic units along direction. k-point sampling supercell convergence Ry 10?4 a.u?1, For bonding Hamilton population (COHP) method.27,28 PAW functions basis Wannier90 (v3.1.0).29 It noted original aligned axes global coordinate system. Ti–O Pt–O bond angles ?45° axis correctly display chemical terms rotated angle 45° around evaluated computing reaction-free defined as: ? G H E ZPE T S (3)where ?EZPE ?S differences zero-point entropy, respectively, gas phases. ?E adsorption X species (X O2, OOH, O, OH) DFT sub (4)where Esub+X Esub energies without adsorption. Results Discussion tackle these questions, COHP serves indicator measure covalent strength solids.27,28 Noteworthily, interpretation substantially depends orientation its orbitals.30,31 projection 1e). 1e lists integral Figures S1c S1d) up Fermi overwhelmingly larger integrated (ICOHP) dxz over dxy indicate make dominant contributions interaction. other words, essential ORR.32 Inspired computational findings, route employed.33,34 fairly transparent characteristics fabricated,35,36 titanium (Tivac) clearly identified (Figures 2a 2b). Whereafter, sites Pt1–Ti1?xO2. lamellar spacing ?1.7 nm detected serial 0k0 XRD peaks S2a). (thickness ?1.1 nm) clarified S2b S2c).37 Moreover, impurity introduced donor, narrowing bandgap improving conductivity S2d S2e). 1.27 ICP-OES. HAADF-STEM image energy-dispersive (EDS) mapping signified mononuclear 2c S2f). Given that, loaded oxide also simulated, locates outside lattice coordinates four relatively oxygens PtO4 configuration S3). As high-resolution shown 2d, exclusive anchorage Ti–vacancy site contrast intensity lattice. images Ti1?xO2. XANES (f) FT-EXAFS references (Pt PtO2). determine refined S4a), decline white-line distinguished, when compared mainly attributed simulated 1d.38 With respect L3-edge, indicates stays 2e S4b–S4d). Fourier transformed (FT-EXAFS) profiles specify prominent peak located ?2.0 exclusively scattering 4.6 2f S4e Table S1). No contribution, Pt–Pt ?2.7 observed first-shell region, confirming Pt1. light-scattering path ?3.3 suggests strong periodicity Pt–O–Ti Thus, designed successfully 2e). activity then RDE S5). cyclic voltammetry (CV) recorded, S6a. ?0.86 V desired activity. polarization curves various rates 400 2500 collected S6b. ?3.88 equation S6c), indicating dominated four-electron pathway.39 Compared commercial Pt/C (20 %), delivers enhanced 3a S6d), more positive onset potential (Eonset) half-wave (E1/2). addition, exhibits comparable JL (limiting current) Pt/C, surpassing factor >19 mass S6e). stability assessed subjecting continuous 10,000 cycles. Notably, superb little change E1/2 determined, shows 20-mV decay E1/2. Pt1 well preserved 10,000-round S7), coalescence Pt/C.40 LSV Pt–d Ti–d interactions. dopant, Ti, initial Pt–Ti. (d e) windows shed Pt1–Ti1?xO2, investigated nature Pt1–Ti1?xO2,41–43 side-on (inset 3b S8). 3b, components Ti–3d, account >99% total Pt/Ti– interacts O2–2s 2px occupied (around ?8 eV S9a S9b), stabilizing Pt/Ti–O bond. While Ti–dxz O–2py character near energy, Pt–dxz features antibonding states, explains dramatically low value Pt–dxz. vivid picture how active interact distributions 3c–3e S9c–S9g). can seen O–2p character, consistent nodal plane perpendicular found, character. general, electron-rich addition intensively platinum-based metals, transition metals,44 Fe,45–48 considered promising candidates Therefore, analogous S10). cobalt atom S11a), any crystalline types indexed. Co1 holds valence approximately +2 S11b), edge ranged against CoO. details Co1-unit 4a S11c S2), CoO5 Co–O ?1.94 confirmed. Co–Ti confirms substitution realms, given similar lay foundation performance. 4 (Co Co3O4). Illustration Pt1–Ti1?xO2–O2 Co1Ti1?xO2–O2. (green bar, increase clear comparison) Pt1/Co1–Ti1?xO2–O2 (red bar), Bader Co1–Ti1?xO2–O2 (charge metal oxygen, blue bar). negligible bare S12a), desirable 4b), JL. 3.85 four-electron-transfer kinetics S12b–S12d). surpasses 10,000-cycles test, implying promising, candidate explain different DOS Co1–Ti1?xO2. analogy Co(Ti) contribution 4c 4d S13), quite proposed atom-realm (AR) effect.17,33,34 order magnitude smaller much weaker This XPS S14), binding shifts lower Co1–Ti1?xO2, namely give explanation inferior When Pt1–Ti1?xO2/Co1–Ti1?xO2 (referred Co1–Ti1?xO2–O2) forming Co/Pt–O bonds, decreases large extent S15 S16), since delocalized transfers absorbed oxygen. Co(Ti)- dominates S3), crucial Pt(Ti)- Pt1–Ti1?xO2–O2. Unexpectedly, strength, TiCo–O Co1–Ti1?xO2–O2, hand, TiPt–O known adsorbate-adsorbent accompanies process. applied estimate 4d).49 show OCo (0.29 |e|) OPt (0.32 |e|), changed OTi significantly (from 0.33 0.45 coinciding analysis. Such interesting findings imply substantial doping Pt. orbital-level insights unraveled demonstrate orbital-level, even spin-level, descriptors (charge, orbital, spin, etc.), introduces AR catalysis, exactly quantum (QC) does, fill gap catalysis.12,50 engineering catalysts, spin effective highly efficient Conclusion We investigate Pt1/Co1–Ti1?xO2. based dopants/Ti dopants dual function, not only center but neighboring Pt1Ti1?xO2 present offers valuable uncover centers reactions so catalysts. available contains supplementary characterization data, including patterns; SEM, AFM, STEM images; spectra; electrochemical CV curves; analyses. Conflict Interest authors declare no conflict interest. Acknowledgments supported Fundamental Research Funds Central Universities (grant nos. 2018JBZ107 2019RC035). financially National Natural Science Foundation China 91961125 21905019), Program International S&T Cooperation Projects Ministry Technology no. 2018YFE0124600), (nos.1932001, 1932004, 1911020, 1911023). appreciate support Excellent One Hundred Project University. W.X. acknowledges research project Virtual Material Design (VirtMat) Program. use beamlines 4B7B BSRF SSRF. References 1. Seh Z. W.; Kibsgaard J.; Dickens C. F.; Chorkendorff I.; Nørskov J. K.; Jaramillo T. F.Combining Theory Experiment Electrocatalysis: Insights Materials Design.Science2017, 355, eaad4998. 2. Shao M.; Chang Q.; Dodelet J.-P.; Chenitz R.Recent Advances Electrocatalysts Reaction.Chem. Rev.2016, 116, 3594–3657. 3. Luo Zhao Z.; Zhan