Host-guest interaction creates hydrogen-evolution electrocatalytic

Subscriber access provided by UNIV OF DURHAM

Article

Host-guest interaction creates hydrogen-evolution electrocatalytic active sites in 3d transition metal-intercalated titanates Ruiqin Gao, Guangtao Yu, Wei Chen, Guo-Dong Li, Shuang Gao, Zengsong Zhang, Xiaopeng Shen, Xuri Huang, and Xiaoxin Zou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15617 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Host-guest interaction creates hydrogen-evolution electrocatalytic active sites in 3d transition metal-intercalated titanates

Ruiqin Gao, ‡

a

Guangtao Yu, ‡b Wei Chen,*b Guo-Dong Li,a Shuang Gao, a Zengsong Zhang,b

Xiaopeng Shen,b Xuri Huangb and Xiaoxin Zou*a a

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of

Chemistry, Jilin University, Changchun 130012, P. R. China b

Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry,

Jilin University, Changchun 130023, P. R. China ‡ Ruiqin Gao and Guangtao Yu contributed equally to this work. KEYWORDS: electrocatalysis; water splitting; host-gest interaction; hydrogen evolution reaction; two-dimensional material

ABSTRACT The hydrogen evolution reaction (HER) is involved in energy-intensive water- and chlor-alkali electrolyzers, and thus, highly active and stable HER electrocatalysts in alkaline media are needed. Titanates, a family of representative two-dimensional materials with negatively charged main layers, are chemically and structurally stable under strongly basic conditions, but they have never been shown to have electrocatalytic activity for HER. Herein, we report that intercalating 3d metal cations, including Fe3+, Co2+, Ni2+ and Cu2+ ions, into the interlayer regions of titanates yields efficient and robust electrocatalysts for the alkaline HER. The intercalation of 3d metal cations in titanates is achieved by rapid cation-exchange reaction

1 Environment ACS Paragon Plus

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

between Na+-containing titanates and 3d metal cations at room temperature. Among the 3d metal-intercalated titanates we synthesize, the Co2+-containing material is found to show the best electrocatalytic activity. Experimental and theoretical results reveal that the strong electronic interaction between 3d metal cations and negatively charged main [TiO6]∞ layers renders good catalytic activity to the outermost oxygen atoms in the [TiO6]∞ layer, further making 3d metalintercalated titanate an efficient electrocatalyst for the HER.

1. INTRODUCTION Two-dimensional layer-structured materials have recently become a boom area of research since the discovery of graphene for many of them have exhibited excellent properties in electronics, optoelectronics, catalysis, photochemistry and electrochemistry.1-3 There are an enormous variety of layer-structured materials which can be subdivided into three major categories according to the charge of the main layers. (1) Charge-neutral layer materials4-6: their layers are joined together by van der Waals force, such as graphene and transition metal dichalcogenides; (2) Positively charged layer materials7,8: they consist of positively charged layers and charge balancing anions between the layers. The most common family is layered double hydroxide (LDH); (3) Negatively charged layer materials (e.g., titanate)9,10: they comprise negative-charged layers and intercalated cations. The unique structural features of layer-structured materials, i.e. the strong covalent bond in the main layers and weak interaction between the layers, endow them with open interlayer space to be utilized.11-13 This, in turn, provides new possibilities to enhance their properties by the introduction of suitable guest species in the interlayer regions, and to investigate the host-guest interaction between main layers and intercalated species.


Page 2 of 21

Page 3 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Of particular interests are the catalytic properties of layer-structured materials, which have been shown to be significantly improved/tuned by the introduction of guest species in some cases. For example, MoO42- has been immobilized in the interlayer of hydrotalcite, generating a composite material with efficient catalytic activity for oxidizing various substituted olefins into hydroperoxides.14 In another report, Pt nanoclusters have been deposited in the interlayer space of layered KCa2Nb3O10 through electrostatic attraction, and the strong interactions between them results in significantly enhanced photocatalytic activity for water splitting.15 Besides the above examples in which the activity of guest species are improved by host layers, a few recent works show that the catalytic activity of main layers can also be tailored by guest species. For instance, Dong et al. report that introducing K+ ions into the interlayer space of g-C3N4 could extend the π conjugated system of g-C3N4 and thereby improve the photocatalytic activity of the latter.16 Electrocatalytic HER in alkaline medium is one of the key steps involved in energy-intensive water- and chlor-alkali electrolyzers.17-21 Hence, many researchers have been actively searching for efficient HER catalysts operating in alkaline solution, especially those based on non-precious metals. However, the development of such HER catalysts is relatively slow, mainly due to the intrinsic lower HER rate in alkaline media than in acidic media,23,24 as well as the poor corrosion resistance of many materials in alkaline media.23,24 Up to now, there are only a few noble-metal free catalysts that are active for HER in alkaline solution, including metal alloys25-29, metal carbides,30-32 metal phosphides33,34 and metal sulphides.35,36 In addition, the catalytic activities of several layer materials in alkaline solution have also been studied. For example, Ni-Fe layered double hydroxide has recently been shown to electrocatalyze HER in alkaline electrolyte, but its catalytic stability is still inferior to commercially used metallic Ni catalyst.37 Therefore, it is



highly desirable to search highly active HER catalysts that comprise earth-abundant metals and possess good chemical and catalytic stability in strongly alkaline media. Herein, we report, for the first time, that high-performance alkaline HER is achieved by 3d transition metal-intercalated titanates, in which intercalated cations can be Fe3+, Co2+, Ni2+ and Cu2+ ions with Co2+ as the best one. The intercalation of 3d metal cations in titanates is obtained by rapid cation-exchange reaction between Na+-containing titanates and 3d metal cations at room temperature. Experimental results and theoretical calculations suggest that the strong electronic interaction between 3d metal cations and negatively charged main [TiO6]∞ layers activates the outermost oxygen atoms in the [TiO6]∞ layer, creating efficient electrocatalytic active sites for HER. 2. EXPERIMENTAL SECTION 2.1. Reagents and materials. Ti foil (99.8 %, 0.15 mm in thickness) was purchased from Baoji Yingdatai Metal Material Co., Ltd. Nickel foam (thickness: 1.5 mm, bulk density: 0.23 g/cm3, porosity: 97 %, p.p.i.: 100, surface density: 350 g/cm2) was purchased from Changsha Lyrun Material Co., Ltd. Sodium hydroxide (NaOH) was purchased from Tianjin Fuchen Chemical Factory. Potassium hydroxide (KOH), CuCl2·4H2O, ZnCl2 were purchased from Beijing Chemical Factory. CrCl3·6H2O, MnCl2·4H2O and FeCl2·4H2O were purchased from Tianjin Guangfu Fine Chemical Factory. Co(NO3)2·6H2O, Ni(NO3)2·6H2O were purchased from Sinopharm Chemical Reagent Co., Ltd. 20 wt.% Pt/C was purchased from Sigma-Aldrich. Highly purified water (>18 MΩ cm) was provided by a PALL PURELAB Plus system. 2.2. Characterizations and Instruments. Powder XRD was recorded on Rigaku D/Max 2550 X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). The morphology of the obtained sample was examined JEOL JSM 6700F at an accelerating voltage of 5 kV. Transmission


Page 4 of 21

Page 5 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electron microscopy (TEM) images were recorded with a Philips-FEI Tecnai G2S-Twin microscope operated at 200 kV. The The X-ray photoelectron spectroscopy (XPS) was performed on on an ESCALAB 250 X-ray photoelectron spectrometer with a monochromatic Xray source (Al Kα hυ = 1486.6 eV). The Raman spectra were obtained with a Renishaw Raman system model 1000 spectrometer with a 20 mW air-cooled argon ion laser (514.5 nm) as the exciting source. 2.3. Preparation of 3d transition metal-intercalated titanates To prepared 3d transition metal-intercalated titanate, Na-titanate was synthesized firstly. Ti foil was treated by ultrasonic treatment sequentially with acetone, water and ethanol for 20 min. Four pieces of cleaned Ti foil (0.5 cm × 5 cm) were placed against the wall of a 45 mL Teflonline stainless steel autoclave filled with 27 mL of 1 mol/L NaOH, and then were kept at 220 oC for 5h. The resulting material (denoted as Na-titanate) is the sodium-containing titanate nanowire film grown on Ti foil. Na-titanate was immersed in an aqueous solution with 3d metal salt (0.25 mol/L) at room temperature for 1 h. The product was washed with deionized water and ethanol several times, resulting in the 3d transition metal-intercalated titanates. The resulting material was labeled as M-titanate (M = Cr, Mn, Fe, Co, Ni, Cu or Zn). 2.4. Electrocatalytic measurement. The electrochemical tests of all samples were implemented with a CHI Instrument (Model 630E) in a typical three-electrode system configuration with an Hg/HgO reference electrode and a carbon rod counter electrode at room temperature. The alkaline electrolyte solution was prepared by dissolving KOH in deionized water at a concentration of 1 mol/L. The electrode potential of Hg/HgO reference electrode (versus the normal hydrogen evolution (NHE)) was calibrated to be 0.099 V. The



measured potential vs Hg/HgO electrode without iR correction was converted to revisable hydrogen electrode (RHE) potential according to following equation: E vs RHE = E vs Hg/HgO + 0.099 V + 0.059 V*pH. The scan speed of linear sweep voltammograms (LSV) measurement was set to be 0.1mV/s. The stability was tested using chronoamperometric measurement at an overpotential of 276 mV. Electrochemical impendence spectroscopy (EIS) was measured at a potential of -1.3 V vs. RHE with a sinusoidal voltage of 5 mV and a frequency ranging from 106 Hz to 1Hz. Faradaic efficiency of Co-titanate was measured according to previous report.38 The geometric area of working electrode exposed to electrolyte was defined to 0.25 cm2. Except for the working area, the titanate was scratched form Ti foil before pasted with epoxy resin. The Ni foam working electrode was also pasted with epoxy resin except for the working area of 0.25 cm2. The Pt/C loaded working electrode was prepared as following: (1) a homogeneous ink contained 2 mg of 20% Pt/C and 100 µL isopropanol was obtained by ultrasonic dispersion; (2) 6 µL of the ink was drop-casted on Ti foil and dried in air (the loading of 20 % Pt/C catalyst was 0.48 mg/cm2, which is the same as titanate). (3) 6 µL of 0.3% Nafion solution was drop-casted on top of the electrode and dried in air. 2.5. Theoretical calculations.

Under the frame of Vienna ab initio simulation package

(VASP),39,40 the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof exchange-correlation functional41 are employed to perform all the density functional theory (DFT) computations, where a semi-empirical van der Waals (vdW) correction is included to account for the dispersion interactions.42,43 The projector-augmented plane wave (PAW) is used to describe the electron-ion interactions,44,45 and a 400-eV cutoff for the plane-wave basis set. In this work, 2×5×5 Monkhorst-Pack grid k-points are employed for geometric optimization of the


Page 6 of 21

Page 7 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


bulk H2Ti2O5·H2O andNa2Ti2O5·H2O, 1×3×5 and 1×3×3 Monkhorst-Pack grid k-points are used for the doped bulk Na2Ti2O5·H2O by the transition metals (TM) with the 1×2×1 and 1×2×2 supercell sizes, respectively. Additionally, 5×5×1 Monkhorst-Pack grid k-points are adopted to optimize the slab structures of pristine and TM-doped Na2Ti2O5·H2O. In this work, the convergence threshold is set as 10-4 eV in energy. For all the calculations of slab models, the symmetrization is switched off and the dipolar correction is included. 3. RESULTS AND DISCUSSION We synthesized sodium titanate nanowire film on Ti foil via hydrothermally treating Ti foil in NaOH aqueous solution at elevated temperatures, according to the previously reported procedures.46 This material is denoted as Na-titanate hereafter. We also synthesized several materials with different interlayer cations by fast cation exchange from Na-titanate. Aqueous solutions for cation exchange contained 0.25 mol/L Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+ or Zn2+. Correspondingly, the resulting material is donated as M-titanate (M = Cr, Mn, Fe, Co, Ni, Cu or Zn). The loading of the M-titanate was found to be about 0.48 mg/cm2. The loading content (0.098 mg/cm2) of transition metal on the surface of M-titanate nanowire is obtained from XPS measurement, which indicates the atom ratio of transition metal to titanium is varied from 1:4 to 1:7. Among the Na-titanate and M-titanate materials we synthesized, Co-titanate showed the highest electrocatalytic activity for HER, and thus this particular material was characterized and discussed in a more detailed way. The characterizations of other M-titanate are presented in Supporting Information (Figures S1-S6). The powder X-ray diffraction (XRD) pattern of Na-titanate suggests that Na-titanate is composed of body-cantered orthorhombic Na2Ti2O5·H2O (Figure 1A). Further comparison of the XRD patterns of Na-titanate and Co-titanate reveals that Co-titanate has almost the same XRD



pattern as Na-titanate, indicating that Na-titanate and Co-titanate should have similar crystal structure. In addition, compared with Na-titanate, Co-titanate exhibits a slight shift (~0.06o, 2theta) of the diffraction peaks to smaller angles (Figure 1B). This result suggests that Co-titanate possesses a little larger interlayer spacing than Na-titanate. The typical scanning electron

Figure 1 (A) XRD patterns of Na-titanate and Co-titanate; (B) the corresponding (200) diffraction peaks shown in an expanded way; (C) SEM image of Co-titanate (inset is the cross section of Co-titanate nanowire film to show its thickness); D) HRTEM image of an individual nanowire in Co-titanate. microscope (SEM) images of Co-titanate (Figure 1C) reveal that the film grown on Ti foil comprises of randomly oriented and entangled nanowires with a diameter of 50 nm and the thickness of the nanowire film is about 10 µm. High-resolution transmission electron microscopy (HRTEM) image of an individual nanowire in Co-titanate (Figure 1D) shows a wide lattice fringe with d = 0.78 nm that is associated with the (200) crystallographic plane of titanate.


Page 8 of 21

Page 9 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The lemental composition and surface bonding structure of Co-titanate and Na-titanate were studied by X-ray photoelectron spectroscopy. The high resolution profile of the Co 2p for Cotitanate in Figure 2A shows the binding energies of Co 2p3/2 and Co 2p1/2 at 782.1 and 797.8 eV with two satellite peaks at 787.2 and 803.5 eV, respectively. This suggests that Co species are high spin Co2+ ions in Co-titanate.47 The Ti 2p core levels of Co-titanate (Figure 2B) locate at 458.5 and 464.3 eV, indicating the existence of Ti(Ⅳ) in the material.48 The peaks of O 1s (Figure 2C) at 529.7, 530.7, 533.7 eV are associated to the oxygen of Ti-O, OH- and H2O, respectively.49 Furthermore, comparison of the Ti 2p and O 1s spectra of Co-titanate and Natitanate reveals that the Ti 2p and O 1s peaks of Co-titanate shift to higher binding energy. This result reveals that the [TiO6]∞ main layers of Co-titanate should possess relatively lower electron density than those of Na-titanate. This result further indicates that strong electronic interaction

Figure 2 (A) Co 2p, (B) Ti 2p and (C) O 1s high resolution XPS spectra of Co-titanate and Natitanate; (D) Raman spectra of Co-titanate and Na-titanate.



should occur between interlayer Co2+ ions and the [TiO6]∞ main layers, resulting in a shift of electron-density from the [TiO6]∞ main layers to interlayer cations. In order to further reveal the interaction between the [TiO6]∞ main layers and interlayer cations and its effects, the Raman spectra of Co-titanate and Na-titanate are compared, as shown in Figure 2D. In the Raman spectrum of Na-titanate, the Raman bands at 276, 442, 701 cm-1 can be assigned to the three Ag symmetric modes of Ti-O-Ti in the main layers, and the band at 662 cm1

is originated from the stretching mode.50 The Raman bands at around 809 and 906 cm-1 are

ascribed to the vibration of short Ti-O-Na moieties.51 Comparison of the Raman spectra of Cotitanate and Na-titanate can reveal that (1) Co-titanate generally has similar Raman bands with Na-titanate due to their similar structure of the main layers; (2) Co-titanate exhibits two very weak Raman bands at 809 and 906 cm-1, which are associated with Ti-O-Na vibration, because there is a lower content of Na+ in Co-titanate than that in Na-titanate; (3) the Raman bands for Co-titanate, which are associated with Ti-O-Ti stretching model, shifts to lower frequency in comparison with those for Na-titanate, indicating the relatively weaker strength of Ti-O bond in Co-titanate;51 and (4) compared with that of Na-titanate, the Raman spectrum of Co-titanate exhibits more broadened Raman bands, indicating the increased structural distortion of main layers in Co-titanate. All of these phenomena should be originated from the stronger electronic interaction between the [TiO6]∞ main layers and interlayer cations. To further access the structure of Co-titanate, detail theoretical studies were performed on Co-titanate. Although some transition metal cations exchanged titanates have reported to have interesting properties, such as magnetism, their structure is still not clear. To obtain the structure of Co-titanate, we firstly constructed the structure of Na2Ti2O5·H2O based on H2Ti2O5·H2O in view of the fact that the Na2Ti2O5·H2O has a similar structure to


Page 10 of 21

Page 11 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


H2Ti2O5·H2O.52 Our computed results reveal that all three optimized lattice parameters (a=20.118, b=3.647 and c=3.191 Å) for the bulk H2Ti2O5·H2O (Figure 3A) are close to the corresponding those from the experimental results.53 Based on the bulk structure of H2Ti2O5·H2O, we explore the bulk structure for Na2Ti2O5·H2O by using Na atoms to replace the correlative H atoms, where all three possible configurations are considered, as illustrated in Figure S7. It is found that the structure in figure 3B is the most energetically stable configuration for Na2Ti2O5·H2O, in which

Figure 3 (A) The bulk structure of H2Ti2O5·H2O; (B) the structures of bulk Na2Ti2O5·H2O; (C) the structure of Co-doped Na2Ti2O5·H2O in a 1×2×2 supercell. all four Na atoms in supercell are located between the two Ti-O layers, and the interlayer distance is a little larger than that of H2Ti2O5·H2O, which is consistent with the previously reported results.52 This can be explained by the larger ionic radius of Na+ compared to H+. Subsequently, we explored energetically stable bulk structure for Co-titanate following the steps depicted Supporting Information. All the possible configurations are illustrated in figure S8. Our computed results reveal that the most energetically stable configuration for the bulk Co-titanate is the structure in figure 3C with a staggered arrangement between any two neighboring Co



atoms. Moreover, the computed results also show that the substitution of Na+ with Co2+ can further increase the interlayer distance as well as the local structural distortion of the main layers due to the strong chemical bonding between interlayer Co2+ ions and the oxygen atoms with the main layers. Those theoretical results are also well consistent with our above experimental observations. Next, the electrocatalytic activity of M-titanate towards HER was accessed in 1 mol/L KOH solution with a three-electrode system at room temperature (see details in Experimental Section). As shown in Figure 4A and Figure S9, Na-titanate，Cr-titanate, Mn-titanate and Zn-titanate demonstrate ignorable catalytic current density in the voltage window applied, while the Fe-

Figure 4 (A) Polarization curves of Co-, Ni-, Fe-, Cu- and Na-titanate; (B) comparison of current densithy of M-titanate and Ni foam at an overpotential of 300 mV; (C) polarazation curve of Cotianate, Ni foam and 20% Pt/C; (D) i-t curves of Co-titanate, Ni-titanate and Ni foam at an overpotential of 274 mV.


Page 12 of 21

Page 13 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


titanate, Co-titanate, Ni-titanate and Cu-titanate show significant activity towards HER, with their activity decreasing in the order of Co-titanate > Ni-titanate > Cu-titanate > Fe-titanate. To compare the activity of M-titanate more precisely, their electrocatalytic activity at an overpotential of 300 mV are presented in Figure 4B. The current densities of Co-titanate, Nititanate, Cu-titanate and Fe-titanate at this overpotential are found to be 64.0, 25.5, 13.7 and 5.7 mA/cm2, respectively. The electrochemical impedance spectroscopy (EIS) measurement (Figure S10) indicate the value of charge transfer resistance decrease in the following sequence Natitanate (∞) >> Fe-titanate (5.1 Ω) > Cu-titanate (3.5 Ω) > Ni-titanate (3 Ω) > Co-titanate (1.2 Ω). This result suggests Co-titanate has a fast charge transfer process in HER among them, and correspondingly exhibits the best catalytic activity. To further evaluate the activity of the two most active M-titanate (M = Co and Ni), their activities were compared with those of 20 wt% Pt/C (a benchmark noble-metal-based electrocatalyst for HER) and Ni foam (metallic Ni as inexpensive catalyst is used in industrial water splitting). As shown in figure 4C, Pt/C exhibits the highest activity among them, whereas Co-titanate and Ni-titanate exhibit better catalytic activity than Ni foam. Because of the low content of nickel or cobalt in Ni (or Co)-titanate (c.a. 0.098 mg/cm2), the higher earth-abundance of titanium compared with Ni and Co (e.g., abundance ratio of nickel to titanium in crust is 1/63), as well as high corrosion resistance of titanium, the Ni (or Co)-titanate may have potential practical applications in alkaline water splitting device. The stability of the M-titanate (M = Fe, Co, Ni or Cu) catalysts was evaluated at an overpotential of 274 mV for 20 h. Co-titanate, Ni-titanate (Figure 4D) generate a current density of about 45mA/cm2 and 17 mA/cm2, respectively; Cu-titanate and Fe-titanate (Figure S11) generate a current density of 9 mA/cm2 and 3 mA/cm2, respectively; and there is no noticeable



activity loss during the whole potentialstatic HER process. In comparison, Ni foam exhibit inferior stability than above M-titanate. Moreover, for example, after 20 h long-term stability measurement, the nanowire morphology and layer structure of Co-titanate remain unchanged, as observed from its SEM and TEM images in Figure S12. To determine the Faradaic efficiency of HER, the H2 produced during electrolysis at η = 200 mV was collect for 100 min. The Faradaic efficiency obtained from the comparison of hydrogen experimentally quantified and theoretically calculated suggests that the electocatalytic HER efficiency is close to 100% (Figure S13). To gain further insight into the origin of electrocatalytic activity, we performed density functional theory (DFT) computations on the representative system, Co-titanate. It is well known that the hydrogen evolution reaction (HER) activity can be closely correlative with the adsorption energy of a single H atom. Consequently, the free energy of H* (∆GH*) can serve as an effective descriptor to evaluate HER activity, where the smaller ∆GH* absolute value, the better HER activity in general.54 Considering that the target atom Na is located between two Ti-O layers in the pristine Na2Ti2O5·H2O, we can reasonably speculate that the replacement of Co could effectively activate the Ti-O layer, resulting in the evident improvement of HER activity. To further confirm this viewpoint, we have performed the correlative DFT computations by constructing the theoretical model as depicted in SI to simulate the Ti-O surface of Co-doped system. The possible adsorption sites are depicted in figure 5A-B, including the Ti1~Ti4 and O1~O4 atoms, as well as the representative Ti-O bridge bond and ring sites. Our computed results reveal that for the pristine Na2Ti2O5·H2O system, the ∆GH* values of only four O-sties can be obtained on the surface of Ti-O layer, which are -1.413, -0.989, -1.397 and -1.003 eV for the O1, O2, O3 and O4 sites, respectively. Such a considerably negative ∆GH* value indicates a poor HER activity for the pristine Na2Ti2O5·H2O system, well consistent with our experimental result.


Page 14 of 21

Page 15 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Comparatively, for the Co-doped Na2Ti2O5·H2O system, we also merely obtain the ∆GH* values of four O-sties on the surface of Ti-O layer, which are 0.466, 0.437, 0.265 and 0.199 eV for the O1, O2, O3 and O4 sites, respectively. Obviously, the exchange of Na with transition metal Co can significantly decrease the ∆GH* absolute value of O-stie and effectively enhance the HER activity, which is in good agreement with our experimental result.

Figure 5 The possible adsorption sites of H* for (A) Na2Ti2O5·H2O and (B) Co-doped Na2Ti2O5·H2O (side and top views); (C) the schematic diagram of electron transfer process in Co-doped Na2Ti2O5·H2O system; (D) The combination of H 1s orbital and O 2p orbital forms a fully occupied bonding orbital and a partially occupied anti-bonding orbital for the adsorption of H* at the O site on the surface of Ti-O layer for the Co-doped Na2Ti2O5·H2O. Note that a, b and c mean three directions for the supercell; O1~O4 and Ti1~Ti4 represent the top site of the corresponding O and Ti atoms, as well as B and R mean the sampled Ti-O bond and ring sites, respectively. 15 Environment ACS Paragon Plus


Next, we performed the following mechanism analysis to predict the activity of Co-titanate from the standpoint of the molecular orbital theory. Specifically, we can understand that there are two Ti atoms around each O atom in the Ti-O layer and thus two valence electrons for O atom are employed to participate in the formation of two Ti-O ionic bonds. As a result, when the H* is adsorbed at the O-site, it can interact with either of two remained lone pairs on O atom to form one fully filled bonding orbital (σO-H) and one partially filled anti-bonding orbital (σ*O-H), as shown in Figure 5C. According to molecular orbital theory, the strength of H-O bond can be determined by the occupancy of the partially filled anti-bonding orbital, that is, the higher σ*O-H occupancy, the weaker bonding strength. Our computed Bader charge results show that the exchange of Na with the transition metal Co can induce the electron transfer towards the O-sites on the surface of Ti-O layer (Figure 5D), resulting in more filled p-states of O atom. Consequently, σ*O-H occupancy can increase when the adsorption of H* at the O-site. Meanwhile, the extra electron that the surface O-site gets can also result in the less electron transfer from H to O. As a result, the interaction between H and O can be effectively weakened and the HER catalytic activity can be effectively enhanced. 4. CONCLUSION In summary, 3d metal cations-intercalated titanates as a novel class of hydrogen evolution electrocatalysts that can operate stably and efficiently in alkaline media are presented. The intercalation of 3d metal cations in titanates is easily achieved by rapid cation-exchange reaction between Na+-containing titanates and 3d metal cations at room temperature. Intercalating 3d metal cations, e.g., Co2+, into the interlayer regions of titanates results in the strong electronic interaction between intercalated cations and negatively charged main [TiO6]∞ layers, rendering good catalytic activity to the outermost oxygen atoms in the [TiO6]∞ layer. Our results not only 16 Environment ACS Paragon Plus

Page 16 of 21

Page 17 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


show our ability in tuning layered materials from catalytically inert into highly active by simple host-gest chemistry, but also provide a novel way to create high-performance water splitting catalysts based on earth abundant elements. Supporting Information The process to obtain the bulk structures of the Pristine and Co-doped Na2Ti2O5·H2O. Possible bulk structures of the Pristine and Co-doped Na2Ti2O5·H2O. The Computations of Free-Energy of H* for the Pristine and Co-doped Na2Ti2O5·H2O. Characterization of the other M-titanate (M = Cr, Mn, Fe, Ni, Cu, Zn). Electrocatalytic performance of Cr-titanate, Mn-titanate and Zntitanate. Nyquist plots of M-titanate. Stability of Cu-titanate, Ni-titanate and Ni foam. Faradaic efficiency of Co-titanate. SEM and TEM images of Co-titanate after HER measurement. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work finacially surported by NSFC No. 21401066, 21771079, 21673094, 21673093, 21373099 and 21573090. X. Zou thanks the support of National Key R&D Program of China, Grant No. 2017YFA0207800, Young Elite Scientist Sponsorship Program by CAST, Jilin Province Science and Technology Development Plan 20150520003JH and 20170101141JC, and Science and Technology Research Program of Education Department of Jilin Province [2016]



No. 410 for the financial assistance. G. Li acknowledges the financial support from the NSFC 21371070 and Jilin Province Science and Technology Development Plan 20160101291JC. W. Chen and G. Yu thank Science and Technology Research Program of Education Department of Jilin Province (JJKH20170780KJ), and Jilin Province Science and Technology Development Plan (20170101175JC and 20150101005JC) for the financial assistance. References (1) Wang, H.; Yuan, H.; Hong, S. S.; Li, Y.; Cui, Y. Physical and Chemical Tuning of Two-dimensional Transition Metal Dichalcogenides. Chem. Soc. Rev., 2015, 44, 2664-2680. (2) Yu, J.; Wang, Q.; O’Hare, D.; Sun, L. Preparation of Two Dimensional Layered Double Hydroxide Nanosheets and Their Applications. Chem. Soc. Rev., 2017, 46, 5950-5974. (3) Deng, D.; Novoselov, K. S.; Fu, Q.; Zheng, N.; Tian, Z.; Bao, X. Catalysis with Two-dimensional Materials and Their Heterostructures. Nat. nanotech., 2016, 11, 218-230. (4) Tan, C.; Lai, Z.; Zhang, H.; Ultrathin Two-Dimensional Multinary Layered Metal Chalcogenide Nanomaterials. Adv. Mater., 2017, 29, 1701392 -1701416. (5) Naguib, M.; Mochalin, V. N.; Barsoum, M. W.; Gogotsi, Y. 25th Anniversary Article: MXenes: A New Family of Two-Dimensional Materials, Adv. Mater., 2014, 26, 992-1005. (6) Zhang, W.; Li, Y.; Peng, S. Facile Synthesis of Graphene Sponge from Graphene Oxide for Efficient DyeSensitized H2 Evolution. ACS Appl. Mater. Interfaces., 2016, 8, 15187-15195.

(7) Zhao, Y.; Jia, X.; Waterhouse, G. I.N.; Wu, L.-Z.; Tung, C.-H.; O’Hare, D.; Zhang, T. Layered Double Hydroxide Nanostructured Photocatalysts for Renewable Energy Production. Adv. Energy Mater. 2016, 6, 1501974-1501993. (8) Peng, L.; Xiong P.; Ma, L.; Yuan, Y.; Zhu, Y.; Chen, D.; Luo, X.; Lu, J.; Amine, K.; Yu, G. Holey TwoDimensional Transition Metal Oxide Nanosheets for Efficient Energy Storage. Nat. Comm., 2017, 8, 15139-15148. (9) Tang,Y.; Zhang,Y.; Rui, X.; Qi, D.; Luo, Y.; Leow, W. R.; Chen, S.; Guo, J.; Wei, J.; Li, W.; Deng, J.; Lai, Y.; Ma, B.; Chen, X. Conductive Inks Based on a Lithium Titanate Nanotube Gel for High-Rate Lithium-Ion Batteries with Customized Configuration, Adv. Mater. 2016, 28, 1567-1576. (10) Zhu, J.; Liu, X.; Geier, M. L.; McMorrow, J. J.; Jariwala, D.; Beck, M. E.; Huang, W.; T. J. Marks, Hersam, M. C. Layer-by-Layer Assembled 2D Montmorillonite Dielectrics for Solution-Processed Electronics, Adv. Mater. 2016, 28, 63-68. (11) Peng, L.; Zhu, Y.; Peng, X.; Fang, Z.; Chu, W.; Wang, Y.; Xie, Y.; Li, Y.; Cha, J. J.; Yu, G. Effective Interlayer Engineering of Two-Dimensional VOPO4 Nanosheets via Controlled Organic Intercalation for Improving Alkali Ion Storage. Nano Lett., 2017, 17, 6273-6279. (12) Ma, L.; Wang, Q.; Islam, S. M.; Liu, Y.; Ma, S.; Kanatzidis, M. G. Highly Selective and Efficient Removal of Heavy Metals by Layered Double Hydroxide Intercalated with the MoS42- ion. J. Am. Chem. Soc., 2016, 138, 2858-2866. (13) Chen, Z.; Leng, K.; Zhao, X.; Malkhandi, S.; Tang, W.; Tian, B.; Dong, Lei; Zheng, L.; Lin, M.; Yeo, B. S.; Loh, K. P. Interface Confined Hydrogen Evolution Reaction in Zero Valent Metal NanoparticlesIntercalated Molybdenum Disulfide. Nat. Comm., 2017, 8, 14548-14556. (14) Sels, B. F.; Vos, D. E. D.; Jacobs, P. A. Kinetics of the Oxygenation of Unsaturated Organics with Singlet Oxygen Generated from H2O2 by a Heterogeneous Molybdenum Catalyst. J. Am. Chem. Soc., 2007, 129, 6916-6926. (15) Oshima, T.; Lu, D.; Ishitani, O.; Maeda, K. Intercalation of Highly Dispersed Metal Nanoclusters into a Layered Metal Oxide for Photocatalytic Overall Water Splitting. Angew. Chem. Int. Ed., 2015, 54, 26982702.


Page 18 of 21

Page 19 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(16) Xiong,T.; Cen, W.; Zhang, Y.; Dong, F. Bridging the g-C3N4 Interlayers for Enhanced Photocatalysis. ACS Catal., 2016, 6, 2462-2472. (17) Gong, M.; Wang, D.-Y.; Chen, C.-C.; Hwang, B.-J.; Dai, H. A Mini Review on Nickel-Based Electrocatalysts for Alkaline Hydrogen Evolution Reaction. Nano Research, 2016, 9, 28-46. (18) Subbaraman, R.; Tripkovic, D.; Strmcnik, D.; Chang, K.-C.; Uchimura, M.; Paulikas, A. P.; Stamenkovic,V.; Markovic, N. M. Enhancing Hydrogen Evolution Activity in Water Splitting by Tailoring Li+-Ni(OH)2-Pt Interfaces. Science, 2011, 334, 1256-1260. (19) Chen, P.; Xu, K.; Tao, S.; Zhou, T.; Tong, Y.; Ding, H.; Zhang, L.; Chu, W.; Wu, C.; Xie, Y. PhaseTransformation Engineering in Cobalt Diselenide Realizing Enhanced Catalytic Activity for Hydrogen Evolution in an Alkaline Medium. Adv. Mater., 2016, 28, 7527-7532. (20) Jiang, J.; Gao, M.; Sheng, W.; Yan, Y.; Hollow Chevrel-Phase NiMo3S4 for Hydrogen Evolution in Alkaline Electrolytes. Angew. Chem., 2016, 128, 1-6. (21) Zheng,Y.; Jiao, Y.; Zhu, Y.; Li, L. H.; Han, Y.; Chen, Y.; Jaroniec, M.; Qiao, S. Z. High Electrocatalytic Hydrogen Evolution Activity of an Anomalous Ruthenium Catalyst. J. Am. Chem. Soc., 2016, 138, 1617416181. (22) Zhang, W.; Li, Y.; Peng, S.; Template-Free Synthesis of Hollow Ni/reduced Graphene Oxide Composite for Efficient H2 Evolution. J. Mater. Chem. A, 2017, 5, 13072-13078.

(23) Schmidt, T. J.; Ross, P. N.; Temperature Dependent Surface Electrochemistry on Pt Single Crystals in Alkaline Electrolytes: Part 2. The Hydrogen Evolution/Oxidation Reaction. J. Electroanal. Chem., 2002, 524, 252-260. (24) Danilovic, N.; Subbaraman, R.; Strmcnik, D.; Chang, K.-C.; Paulikas, A. P.; Stamenkovic,V. R.; Markovic, N. M. Enhancing the Alkaline Hydrogen Evolution Reaction Activity through the Bifunctionality of Ni(OH)2/Metal Catalysts. Angew. Chem. Int. Ed., 2012, 51, 12495-12498. (25) Solmaz, R.; Döner, A.; Kardas, G. The Stability of Hydrogen Evolution Activity and Corrosion Behavior of NiCu Coatings with Long-Term Electrolysis in Alkaline Solution. Int. J. Hydrogen. Energy, 2009, 34, 2089-2094. (26) Safizadeh, F.; Ghali, E.; Houlachi, G. Electrocatalysis Developments for Hydrogen Evolution Reaction in Alkaline Solutions - A Review. Int. J. Hydrogen Evolution, 2015, 40, 256-274. (27) McEnaney, J. M.; Soucy, T. L.; Hodges, J. M.; Callejas, J. F.; Mondschein, J. S.; Schaak, R. E. Colloidally-Synthesized Cobalt Molybdenum Nanoparticles as Active and Stable Electrocatalysts for the Hydrogen Evolution Reaction under Alkaline Conditions. J. Mater. Chem. A, 2016, 4, 3077-3081. (28) Zhang, Q.; Li, P.; Zhou, D.; Chang, Z.; Kuang, Y.; Sun, X. Superaerophobic Ultrathin Ni–Mo Alloy Nanosheet Array from In Situ Topotactic Reduction for Hydrogen Evolution Reaction. Small, 2017, 13， 1701648-1701654. (29) Lu, Q.; Hutchings, G. S.; Yu, W.; Zhou, Y.; Forest, R. V.; Tao, R.; Rosen, J.; Yonemoto, B. T.; Cao, Z.; Zheng, H.; Xiao, J.Q.; Jiao, F.; Chen, J. G. Highly Porous Non-Precious Bimetallic Electrocatalysts for Efficient Hydrogen Evolution. Nat. Comm., 2015, 6, 6567-6574. (30) Liu, Y.; Yu, G.; Li, G.-D.; Sun, Y.; Asefa, T.; Chen, W.; Zou, X. Coupling Mo2C with Nitrogen-Rich Nanocarbon Leads to Efficient Hydrogen-Evolution Electrocatalytic Sites. Angew. Chem., 2015, 127, 1090210907. (31) Chen,Y.-Y.; Zhang, Y.; Jiang, W.-J.; Zhang, X.; Dai, Z.; Wan, L.-J.; Hu J.-S. Pomegranate-Like N, PDoped Mo2C@C Nanospheres as Highly Active Electrocatalysts for Alkaline Hydrogen Evolution. ACS Nano, 2016, 10, 8851-8860. (32) Lu, C.; Tranca, D.; Zhang, J.; Hernández, F. R.; Su, Y.; Zhuang, X.; Zhang, F.; Seifert, G.; Feng, X.; Molybdenum Carbide-Embedded NitrogenDoped Porous Carbon Nanosheets as Electrocatalysts for Water Splitting in Alkaline Media. ACS Nano, 2017, 11, 3933-3942. (33) Shi, Y.; Zhang, B. Recent Advances in Transition Metal Phosphide Nanomaterials: Synthesis and Applications in Hydrogen Evolution Reaction. Chem. Soc. Rev., 2016, 45, 1529-1541. (34) Callejas, J. F.; Read, C. G.; Roske, C. W.; Lewis, N. S.; Schaak, R. E. Synthesis, Characterization, and Properties of Metal Phosphide Catalysts for the Hydrogen-Evolution Reaction. Chem. Mater., 2016, 28, 6017-6044.



(35) Feng, L.-L.; Yu, G.; Wu, Y.; Li, G.-D.; Li, H.; Sun, Y.; Asefa, T.; Chen, W.; Zou, X. High-Index Faceted Ni3S2 Nanosheet Arrays as Highly Active and Ultrastable Electrocatalysts for Water Splitting. J. Am. Chem. Soc., 2015, 137, 14023-14026. (36) Staszak-Jirkovský, J.; Malliakas, C. D.; Lopes, P. P.; Danilovic, N.; Kota, S. S.; Chang, K.-C.; Genorio, B.; Strmcnik, D. Stamenkovic, V. R.; Kanatzidis, M. G.; Markovic, N. M. Nat.material, 2015, 15, 197-203. (37) Hou, Y.; Lohe, M. R.; Zhang, J.; Liu, S.; Zhuang, X.; Feng, X. Vertically Oriented Cobalt Selenide/Nife Layereddouble-Hydroxide Nanosheets Supported on Exfoliated Graphene Foil: An Efficient 3D Electrode for Overall Water Splitting. Energy Environ. Sci., 2016, 9, 478-483. (38) Zou, X.; Huang, X.; Goswami, A.; Silva, R.; Sathe,B. R.; Mikmekov, E.; Asefa, T. Cobalt-Embedded Nitrogen-Rich Carbon Nanotubes Efficiently Catalyze Hydrogen Evolution Reaction at All pH Values. Angew. Chem. Int. Ed., 2014, 53, 1-6. (39) Kresse, G.; Hafner, J.; Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B, 1993, 47, 558-561. (40) Kresse, G.; Hafner, J.; Ab initio Molecular-Dynamics Simulation of the Liquid-Metal-AmorphousSemiconductor Transition in Germanium. Phys. Rev. B, 1994, 49, 14251-14269. (41) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett., 1996, 77, 3865-3868. (42) Grimme, S.; Semiempirical GGA-Type Density Functional Constructed with A Long-Range Dispersion Correction. J. Comput. Chem., 2006, 27, 1787-1799. (43) Wu, X., Vargas, M. C.; Nayak,S.; Lotrich, V.; Scoles, G.; Towards Extending the Applicability of Density Functional Theory to Weakly Bound Systems. J. Chem. Phys. 2001, 115, 8748-8757. (44) Blochl, P. E.; Projector Augmented-Wave Method. Phys. Rev. B, 1994, 50, 17953-17977. (45)Kresse,G.; Joubert, D.; From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B, 1999, 59, 1758-1774. (46) Shen, L.; Uchaker, E.; Zhang, X.; Cao, G. Hydrogenated Li4Ti5O12 Nanowire Arrays for High Rate Lithium Ion Batteries. Adv. Mater. 2012, 24, 6502-6506. (47) Liu, X.-C.; Shi, E.-W.; Chen, Z.-Z.; Zhang, H.-W.; Chen, B.-Y.; Song, L.-X.; Wei, S.–Q.; He, B.; Xie, Z. Effect of Donor Doping on The Magnetic Properties of Co-Doped ZnO Films. Journal of Crystal Growth, 2007, 307, 14-18. (48) Qian, W.; Greaney, P. A.; Fowler, S.; Chiu, S.-K.; Goforth A. M.; Jiao, J. Low-Temperature Nitrogen Doping in Ammonia Solution for Production of N-Doped TiO2-Hybridized Graphene as A Highly Efficient Photocatalyst for Water Treatment. ACS Sustainable Chem. Eng., 2014, 2, 1802-1810. (49) Seo, H.-K.; Kim, G.-S.; Ansari, S. G.; Kim, Y.-S.; Shin, H.-S.; Shim, K.-H.; Suh, E.-K.; A Study on The Structure/Phase Transformation of Titanate Nanotubes Synthesized at Various Hydrothermal Temperatures. Solar Energy Materials & Solar Cells, 2008, 92, 1533-1539. (50) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K.; Titania Nanotubes Prepared by Chemical Processing. Adv. Mater., 1999, 11, 1307-1311. (51) Kitano, M.; Wada, E.; Nakajima, K.; Hayashi, S.; Miyazaki, S.; Kobayashi, H.; Hara, M.; Protonated Titanate Nanotubes with Lewis and Brønsted Acidity: Relationship Between Nanotube Structure and Catalytic Activity. Chem. Mater., 2013, 25, 385-393. (52) Tsai, C. C.; Teng, H.; Structural Features of Nanotubes Synthesized from NaOH Treatment on TiO2 with Different Post-Treatments. Chem. Mater., 2006, 18, 363-373. (53) Powder Diffraction File of the Joint Committee on Powder Diffraction Standards, Card 47-0124; International Centre for Diffraction Data: Newton Square, PA, 2002. (54) Nørskov, J. K.; Bligaard, T.; Logadottir, A.; Kitchin, J. R.; Chen, J.; Pandelov, S.; Stimming, U. J.; Trends in The Exchange Current for Hydrogen Evolution. Electrochem. Soc., 2005, 152, J23-J26.


Page 20 of 21

Page 21 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents Graphic


Host-guest interaction creates hydrogen-evolution electrocatalytic

Recommend Documents