Conductive Tungsten Oxide Nanosheets for Highly Efficient Hydrogen Evolution

Transcription

1 Supporting Information for Conductive Tungsten Oxide Nanosheets for Highly Efficient Hydrogen Evolution Tingting Zheng, Wei Sang, Zhihai He, Qiushi Wei, Bowen Chen, Hongliang Li, Cong Cao, Ruijie Huang, Xupeng Yan, Bicai Pan, Shiming Zhou* & Jie Zeng* Hefei National Laboratory for Physical Sciences at the Microscale, Key Laboratory of Strongly-Coupled Quantum Matter Physics of Chinese Academy of Sciences, Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui , P. R. China. These authors contributed equally to this work. Correspondence and requests for materials should be addressed to S.Z. ( or to J.Z. ( S1

2 Experimental Section Theoretical calculations. All calculations are performed using Vienna ab initio simulation package (VASP) based on the spin-polarized density functional theory. The interaction between ions and valence electrons was described with using projector augmented wave (PAW) potentials. The exchange-correlation between electrons was described by using the generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerhof (PBE) form. The kinetic energy cut off for the plane-wave basis set was 500 ev. To simulate the WO 3 (010) surface, a seven-layer ( 2 2 ) R45 o slab with a vacuum region of 15 Å was adopted, corresponding to (WO 3 ) 24 (96 atoms). The Brillouin zone was sampled with k-points within the gamma centered Monkhorst-Pack scheme. In our optimization, the atomic coordinates were fully relaxed until the forces on all the atoms were less than 0.02 ev Å -1. More details see the Note S1. Synthesis of WO 3 -r and WO 3 -p NSs. WO 3 -r NSs were synthesized via a liquid exfoliation method. First, tungsten oxide precursors were prepared by a solvothermal method as described in the literature S1. In brief, 2 g WCl 6 (Aladdin) was added into ethanol (100 ml) and reacted at 160 C for 24 h. Then the precursors (1 g) were dissolved in water and ethanol solution (50 ml, V water : V ethanol = 1:1) and ultrasonicated in scientz-iid ultrasonic cell disruptor for 5 h. Finally, WO 3 -r NSs were obtained by centrifuging the resultant dispersion at 2000 rpm for 10 min and collecting the supernatant. WO 3 -p NSs were prepared by annealing the WO 3 -r NSs at 300 C about 10 min in air. Characterizations. TEM images were taken using a Hitachi H-7650 transmission electron microscope at an acceleration voltage of 100 kv. HRTEM images were collected on a JEOL ARM-200F field-emission transmission electron microscope operating at 200 kv accelerating voltage. XPS were carried out on an ESCALAB 250 X-ray photoelectron spectrometer with Al Kα as the excitation source. XRD patterns were performed on a Rigaku TTR-III diffractometer using Cu Kα radiation (λ = Å) at room temperature. Raman spectra were collected on a JYLABRAM-HR spectrometer equipped with an integral microscope. AFM image was performed by using a Veeco DI Nano-scope Multi Mode V system. The temperature dependent resistivities of the pellets of all the samples were measured with a four-probe configuration on a Quantum Design physical property measurement system. Electrochemical measurements. 5 mg of the catalyst was dispersed in water and ethanol solution (1 ml, V water :V ethanol = 1:1) in a 5-mL vial. Afterwards, 40 µl of Nafion solution was S2

3 added into the vial. After ultrasonication of the solution for at least 1 h to form homogeneous ink, 10 µl of the ink was deposited onto rotating disk electrode with a diameter of 5 mm (loading, mg cm -2 ). Finally, the as-prepared catalyst films were dried at room temperature. Electrochemical measurements were carried out with a three-electrode system on an IM6 electrochemical workstation (Zahner, Germany) at room temperature. A graphite rod and saturated Ag/AgCl electrode were used as the counter and reference electrodes, respectively. All potentials were converted to values with reference to a reversible hydrogen electrode (RHE) in the high purity hydrogen saturated 0.5M solution. Before the electrochemical measurement, the electrolyte was degassed by bubbling hydrogen for 30 min. The polarization curves were obtained by sweeping the potential from 0 to -0.5 V (vs RHE) at a scan rate of 2 mv S -1 with the rotating speed of 1600 rpm. The Nyquist plots were conducted with the frequencies ranging from 0.1 MHz to 1 Hz. The amplitude of the applied voltage was 5 mv. The accelerated stability tests were performed by potential cycling from +0.1 to -0.3 V (vs RHE) at a sweep rate of 100 mv S -1 with the rotating speed of 1600 rpm. Hall coefficient (R H ) and carrier concentration (n) measurements. The charge carrier concentration is calculated from the Hall coefficient measured on a flat sample in a magnetic field. The Hall voltage V H is the voltage arising perpendicular to both the magnetic field and current direction. The Hall resistance is R = V H /I and Hall coefficient R H =R d/b, where I is the current, d is the sample thickness and B is the perpendicular magnetic field. The Hall carrier concentration is calculated as n H = 1/(eR H ), where e is the elementary charge. The Hall carrier concentration is related to the true carrier concentration n by n = r H n H, where r H is the Hall factor which is generally only equal or close to 1 in the free electron model and the limit of high doping levels in a single parabolic band. S3

4 Figure S1. (a) Top view and (b) side view of optimized supercell for WO 3 (010) slab. The blue and red spheres represent W and O atoms, respectively. S4

5 Figure S2. Schematic adsorption sites for H on atomic models of (a) W 24 O 72, (b) W 24 O 71, and (c) W 24 O 67, respectively. The blue and red spheres represent W and O atoms, respectively. S5

6 Figure S3. Schematic illustration of (a) conventional semiconductor and (b) n-type of degenerate semiconductor. S6

7 Table S1. Adsorption energy, zero-point energy (ZPE) and free energy of adsorbed H on different adsorption sites in Figure S2. Adsorption sites E H /ev E ZPE /ev G H /ev W 24 O 72 W W 24 O 71 W W W W 24 O 67 W W W S7

8 Figure S4. TEM image of WO3-p NSs. S8

9 Figure S5. (a) AFM image of WO 3 -r NSs and (b) the height of the WO 3 -r NSs according to the line from (a). S9

10 Figure S6. O 1s XPS spectrum of WO 3 -r NSs, WO 3-p NSs and WO 3 bulk. S10

11 Figure S7. Magnetic field dependence of Hall resistivity of WO 3 -r NSs (a), WO 3 -p NSs (b), and WO 3 bulk (c). S11

12 Figure S8. (a) W 4f and (b) O 1s XPS spectrum of WO 3 -r NSs after cycling 1,000 times, respectively. S12

13 Figure S9. Nyquist plots at η = -100 mv for WO 3 -r NSs after 1,000 CV cycles. S13

14 Figure S10. Catalyst durability measured by chronoamperometry (E = -100 mv vs RHE) for 10 hours. S14

15 Figure S11. CVs of (a) WO 3 bulk, (b) WO 3 -r NSs, and (c) WO 3 -p NSs measured in solution at scan rates from 20 to 100 mv S -1, respectively. (d) Charging current density as a function of the scan rate for the different electrodes. S15

16 Table S2. Comparison of the reported HER electrocatalysts in acidic aqueous media. Catalyst (loading mass/mg cm -2 ) WO 3 -r NSs (0.285) 3D urchin-like Mo-W 18 O 49 nanostructure (0.16) WO 2 -C mesoporous nanowires (0.35) WO 2.9 (0.285) P-modified WN/rGO (0.337) RuO 2 Nanowires -g-carbon Nitride (0.171) WS 2 /WO x /C hybrid nanostructure (-) W 2 C-MWNTs (0.56) interconnected network of MoP NPs (0.36) porous MoC x nano-octahedrons (0.8) metallic phase MoS 2 nanosheets (0.043) mesoporous MoO 3-x / carbon cloth (-) WO 3 x Nanoplates-C nanofibers (0.21) WS 2 nanosheets ( ) Electrolyte 0.1 M η@j = -10 ma cm -2 (mv) Tafel slope (mv dec -1 ) Ref This work Ref. S Ref. S Ref. S Ref. S Ref. S Ref. S Ref. S Ref. S Ref. S Ref. S Ref. S Ref. S Ref. S14 S16

17 Note S1. The hydrogen adsorption energy on WO 3 (010) surface is defined by E +, H = Eslab H Eslab 1/2EH 2 where E slab+ H and E slab are the total energies of the WO 3 (010) slab with and without an H adatom, respectively, and EH2 is the energy of a H 2 molecule in the gas phase. The corresponding adsorption free energy is calculated as G = E + E T S, H H ZPE H where EZPE is the difference in zero point energy of hydrogen vibration between the adsorbed state and gas phase state, and SH is the entropy difference of hydrogen between the adsorbed state and the gas phase. The contribution from the configurational entropy in the adsorbed state is small and can be neglected, so we can take the entropy of hydrogen adsorption as SH = 1/2S H2, where S H is the entropy of H 2 2 in the gas phase at standard conditions S14,S15. S17

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