Supporting Information. Ultrathin Lanthanum Tantalate Perovskite Nanosheets Modified. by Nitrogen Doping for Efficient Photocatalytic Water Splitting

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1 Supporting Information Ultrathin Lanthanum Tantalate Perovskite Nanosheets Modified by Nitrogen Doping for Efficient Photocatalytic Water Splitting Meilin Lv a, Xiaoqin Sun a, Shunhang Wei a, Cai Shen b, Yongli Mi a,c and Xiaoxiang Xu a, * a Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, 1239 Siping Road, Shanghai, , China. xxxu@tongji.edu.cn, telephone: b Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences Zhongguan Road, Zhenhai District, Ningbo, Zhejiang, China c The Hong Kong University of Science and Technology, Department of Chemical and Biomolecular Engineering, Kowloon, Hong Kong, China 1. Supplementary experimental 1.1. Open-circuit voltage decay (OCVD) measurements First, photo-electrodes of as-prepared samples were fabricated by an electrophoretic deposition method: 1-2 two pieces of fluorine doped tin oxide (FTO) glass (30 10 mm) were immersed into 50 ml acetone solution containing 50 mg sample powders and 10

2 mg iodine. The two pieces of glass were kept in parallel with a 10 mm separation and the conductive sides facing inward. A constant bias (15 V) was applied between the two pieces of glass for 10 min under potentiostatic control (Keithley 2450 Source Meter). The prepared electrodes were then calcined at 473 K for 10 min to remove absorbed iodine. Open-circuit voltage decay (OCVD) experiments were performed with a three-electrode configuration setup using a Zahner electrochemical workstation. The sample photo-electrode, Pt foil (10x10 mm) and Ag/AgCl electrode were used as the working, counter and reference electrodes, respectively. An aqueous solution of K 3 PO 4 /K 2 HPO 4 (0.1 M, ph = 7.95) was used as an electrolyte and a buffer. A 300 W Xenon lamp (Perfect Light, PLX-SXE300) coupled with a UV cutoff filter (λ 420 nm) was applied as the light source. The incident light was rectified by an electronic timer and shutter (DAHENG, GCI-73). Experiments started when the open-circuit voltage (V oc ) of sample photo-electrode is stable in the dark. Light illumination was introduced onto sample photo-electrode by opening the shutter for 100 s and was terminated by closing the shutter. V oc of photo-electrode was then allowed to decay in the dark until its value restored to the initial one. 2. Supplementary figures

3 Fig. S1. Observed and calculated XRD patterns for RbLaTa 2 O 7, the refinement converged with good R-factors (R p = 4.19%, R wp = 5.60%, χ 2 = 1.934). O 1s O 2- RbLaTa 2 O 6.77 N 0.15 RbLaTa 2 O 7 Intensity (a.u.) OH Binding Energy (ev)

4 Fig. S2. X-ray photoelectron spectra (XPS) of O 1s state for RbLaTa 2 O 7 before and after nitrogen doping. The peaks centred around 529 ev and 531 ev are assignable to lattice O 2- species and surface hydroxyl groups 3-4 Fig. S3. Calculated band structures, total density of states (DOS) and partial density of states (PDOS) of constituent elements of RbLaTa 2 O 6.77 N 0.15 Fig. S4. Field emission scanning electron microscopy images of RbLaTa 2 O 7 (a) and RbLaTa 2 O 6.77 N 0.15 (b)

5 100.3 RbLaTa 2 O 6.77 N Mass % m = % Temperature ( C) Fig. S5. Thermogravimetric analysis (TGA) of RbLaTa 2 O 7-x N x in air with a heating rate 20 K/min from 200 C to 950 C, a weight increase ~ 0.24% was noted which corresponds to the replacements of N with O in RbLaTa 2 O 7-x N x. The N content in the sample can then be determined to be 2.2% and the chemical formula of N doped sample can be written as RbLaTa 2 O 6.77 N 0.15.

6 Fig. S6. Schematic representation of ion exchange and ultrasonically exfoliating RbLaTa 2 O 6.77 N 0.15 into LaTa 2 O 6.77 N nanosheets: Rb + cations in the interlayer is sequentially replaced by hydrated protons (I), protonated ethylamine (II) and tetrabutylammonium cations (III) to lower the interlayer coulombic interactions. The compound is finally exfoliated into nanosheets by sonication. A digital image of the sonicated nanosheet suspension is displayed.

7 Fig. S7. XRD patterns of RbLaTa 2 O 7, RbLaTa 2 O 6.77 N 0.15, protonated sample (HLaTa 2 O 6.77 N 0.15 ) and ethylamine intercalated sample (EA-LaTa 2 O 6.77 N 0.15 ) and exfoliated nanosheet. (001) peak around 9 is gradually depressed.

8 Fig. S8. Field emission scanning electron microscopy images of RbLaTa2O7 (a), RbLaTa2O6.77N0.15 (b), protonated sample (HLaTa2O6.77N0.15) (c), ethylamine intercalated sample (EA-LaTa2O6.77N0.15) (d), LaTa2O6.77N0.15- nanosheets (e) and (f)

9 Fig. S9. Dependence of average photocatalytic hydrogen production rate on the amounts of loaded Pt cocatalysts under full range (λ 250 nm) and visible light illumination (λ 400 nm)

10 100 H 2 Gas evolved / µmol g O Time / h Fig. S10. Temporal photocatalytic water splitting reactions of LaTa 2 O 6.77 N nanosheets in pure water under full range illumination, 2 wt% Pt was loaded onto nanosheets as a cocatalyst. (a) Light on Bulk Nanosheet (b) Bulk Nanosheet V oc (V vs. NHE) τ n (s) Light off Time (s) V (vs NHE) Fig. S11. (a) V oc time profile of bulk sample RbLaTa 2 O 6.77 N 0.15 and LaTa 2 O 6.77 N nanosheets in Ar atmosphere, illumination (λ 420 nm) started after a steady V oc was

11 achieved in the dark and was terminated after 100 s, (b) electron lifetime derived from Equation S1 (see information below) The dissipation of photo-generated electrons and their lifetime can be evaluated by open-circuit voltage decay (OCVD) experiments (see supplementary experimental section for more details). These experiments record the change of open-circuit voltage (V oc ) of semiconductor electrode which is directly linked to accumulation/consumption of electrons in the sample upon light illumination/termination. V oc of photoelctrodes represent the voltage difference between Fermi level of semiconductor photo-electrode and counter electrode. For n-type semiconductors, illuminating the sample electrode under open-circuit condition accumulates photo-generated electrons inside semiconductors as holes migrate to the surface and are consumed during photo-oxidation reactions. 5 This effectively shifts Fermi level of semiconductors negatively, so does V oc. Steady state V oc will be reached as long as electron consumption competes with electron accumulation. Instantaneously terminating illumination of photo-electrodes results in decay of V oc which is controlled by various electron dissipation pathways (e.g. recombined with trapped holes, etc.). This provides a direct evaluation of charge generation and separation situations inside semiconductors. The lifetime of these accumulated electrons can be quantitatively approximated using the following equation: 6-7 k T e dv dt B oc 1 τ n = ( ) (S1)

12 where τ n is potential dependent lifetime, k B is Bolzmann s constant, T is the temperature in K and e is the elementary charge. The LaTa 2 O 6.77 N nanosheets show a much slower V oc decay profile and a much longer electron lifetime compared to its bulk counterpart, highlighting the usefulness of exfoliating RbLaTa 2 O 6.77 N 0.15 into LaTa 2 O 6.77 N nanosheets for extending charge lifetime. 3. Supplementary tables Table S1 Unit cell parameters, BET surface area and band gap values for as-prepared RbLaTa 2 O 7 and nitrogen doped RbLaTa 2 O 7 (RbLaTa 2 O 6.77 N 0.15 ), standard deviation is specified in the parenthesis. Sample Space group a / Å c / Å 3 V / Å 3 Band gap (ev) RbLaTa 2 O 7 P 4/mmm (3) (7) (3) 4.26(2) RbLaTa 2 O 6.77 N 0.15 P 4/mmm (1) (5) (2) 2.09(3) Table S2 Zeta potential of nanosheets before and after nitrogen doping at ph = 8 Sample Zeta potential (mv) LaTa 2 O - 7 nanosheets LaTa 2 O 6.77 N nanosheets References 1. Xie, Y. H.; Wang, Y. W.; Chen, Z. F.; Xu, X. X. Role of Oxygen Defects on the Photocatalytic Properties of Mg-Doped Mesoporous Ta 3 N 5. ChemSusChem 2016, 9, Wang, Y. W.; Zhu, D. Z.; Xu, X. X. Zr-Doped Mesoporous Ta 3 N 5 Microspheres for Efficient Photocatalytic Water Oxidation. ACS Appl. Mater. Interfaces 2016, 8, Sun, X. Q.; Wang, S. W.; Shen, C.; Xu, X. X. Efficient Photocatalytic Hydrogen Production over Rh-Doped Inverse Spinel Zn 2 TiO 4. ChemCatChem 2016, 8,

13 Wu, F. F.; Liu, G.; Xu, X. X. Efficient Photocatalytic Oxygen Production over Ca-Modified LaTiO 2 N. J. Catal. 2017, 346, Turner, J. A. Energetics of the Semiconductor-electrolyte Interface. J. Chem. Educ. 1983, 60, Zaban, A.; Greenshtein, M.; Bisquert, J. Determination of the Electron Lifetime in Nanocrystalline Dye Solar Cells by Open-Circuit Voltage Decay Measurements. ChemPhysChem 2003, 4, Meekins, B. H.; Kamat, P. V. Got TiO 2 Nanotubes? Lithium Ion Intercalation Can Boost Their Photoelectrochemical Performance. ACS Nano 2009, 3,