All The Catalytic Active Sites of MoS 2 for Hydrogen Evolution

Transcription

1 Supporting Information All The Catalytic Active Sites of MoS 2 for Hydrogen Evolution Guoqing Li 1,2, Du Zhang 3, Qiao Qiao 4, Yifei Yu 1, David Peterson 5, Abdullah Zafar 5, Raj Kumar 1, Stefano Curtarolo 6, Frank Hunte 1, Steve Shannon 5 Yimei Zhu 4, Weitao Yang 3, and Linyou Cao 1,7* 1 Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695; 2 College of Textile, North Carolina State University, Raleigh, NC 27695; 3 Department of Chemistry, Duke University, Durham, NC 27708; 4 Department of Condensed Matter Physics and Materials Science, Brookhaven National Laboratory, Upton, NY 11973; 5 Department of Nuclear Engineering, North Carolina State University, Raleigh, NC 27695; 6 Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27708; 7 Department of Physics, North Carolina State University, Raleigh, NC * To whom correspondence should be addressed. lcao2@ncsu.edu This PDF document includes Figure S1-S19 S1. The catalytic inactivity of the sulfur vacancies created by Ar plasma treatment S2. Calculation of the turnover frequencies at single active sites S1

2 Figure S1. Typical EIS measurement results collected from MoS2 films. Figure S2. AFM characterizations of monolayer MoS 2 films and flakes. (a) AFM image of a typical as-grown triangle flake on sapphire substrates. (b) Height profile of the white dash line shown in (a). (c) AFM image of a typical as-grown monolayer MoS2 film on sapphire substrates. (d) Height profile of the white dash line shown in (c). S2

3 Figure S3. Pre-test cycling of monolayer MoS 2 film and flakes. We perform cyclic voltammetry at the film and flakes transferred onto glassy carbon in the range of (vs. RHE) till the catalytic performance appears to be stable. The polarization curves collected from (upper) a typical MoS 2 film and (lower) typical MoS 2 flakes with different cycles as indicated. Figure S4. XPS results of monolayer MoS 2 film and flakes before and after the catalytic reactions. The peaks of Mo and S are labeled as shown S3

4 Figure S5. FFT of a HRTEM image from MoS2 film. The circles in different colors indicate the points used to construct the dark field IFFT image showing grain boundaries given in Fig. 2c in the main text. Figure S6. Raman spectra of as grown monolayer MoS 2 film and flakes. The A 1g peak at 405 cm -1 and the E 1 2g peak at cm -1. The intensity is normalized to the intensity of the Raman peak of sapphire substrates at 420 cm-1. S4

5 . Figure S7. XPS results of monolayer MoS 2 flakes before and after the treatment of sulfur vacancy treatment. Figure S8. Tafel plots of MoS2 films. These Tafel plots are derived from the polarization curves given in Fig. 3c of the main text. S5

6 Figure S9: AFM measurement before and after repair process. AFM images of monolayer MoS 2 films (a) before and (b)a fter the repair of sulfur vacancies. (b) and (d) the height profile for the white dash line in (a) and (c), respectively. The change in thickness is negligible, indicating this is no polymer residue layer on the top of samples after the repair process;, suggesting negligible physical coverage of residual MPS molecules Figure S10. Dependence of exchange current densities on the length of grain boundaries. (a) and (b) False color fast fourier transformation (FFT) image of two typical films showing different grain sizes. (c) Exchange current density as a function of the density of grain boundaries. The exchange current densities are extracted from the polarization curves measured at the defect-repaired films and the flake-merged films. S6

7 Figure S11. Measured XPS results and corresponding fitting for monolayer MoS 2 film with different densities of sulfur vacancies. S7

8 Figure S12. STEM HAADF image showing vacancies in (a) MoS 2 film (left) and (b) triangle flakes. The vacancies are indicated by yellow circles. The STEM HAADF image given in (a) is collected from the film whose density of sulfur vacancies is estimated to be 10% from XPS measurement. We use ten different HAADF images from the film to count its sulfur vacancy percentage (about 4x4 nm 2 each), and find that the vacancy density is 8.9 +/- 1.6 %, in very good agreement with the XPS measurement. By the same token, we find that the vacancy concentration in the triangle flakes is 1.1 +/- 0.6 %, very close to the % estimated from XPS measurement. Figure S13. Raman spectra of monolayer MoS2 films with different densities of sulfur vacancies. The density of sulfur vacancies is labeled as shown. S8

9 a 2 1 b c Figure S14. Mirror boundaries in monolayer MoS 2. (a) STEM ADF image showing mirror boundaries in monolayer MoS 2. The number 1 and 2 indicate and 12-4 structures in the boundary. (b) Structure model (left) and ADF image (right) for structures. (c) Structure model (left) and ADF image (right) for 12-4 structures. S9

10 23 tilt boundary Figure S15. Tilt boundaries in monolayer MoS 2. (Upper) STEM ADF image showing 23 tilt boundaries in monolayer MoS 2. (Lower) Structure model (left) and magnified ADF image (right) for the 5-7 structures that are involved in tilt boundaries. Figure S16. Dependence of the catalytic activity of sulfur vacancies on crystalline quality. (a) Raman spectra, (b) PL spectra, (c) XPS results, and (d) polarization curves of two monolayer MoS 2 films. The two films show reasonably comparable densitiesof sulfur vacancies as indicated by the XPS results, but have different amount of grain boundaries as evidenced by the Raman and PL measurement. S10

11 S1. The catalytic inactivity of the sulfur vacancies created by Ar plasma treatment Our experimental results indicate that the sulfur vacancies created by Ar plasma treatment are not catalytic active. This is in stark contrast with what reported previously by a recent work. The previous study reports an increase in the catalytic activity of MoS 2 films after being treated by Ar plasma and ascribes the increase to the creation of sulfur vacancies. However, we find that the increase in catalytic activity resulting from Ar plasma treatment is mainly due to other two effects of the treatment: cleaning the surface and creation of cracks that may have edge sites. The sulfur vacancies created by Ar plasma treatment actually contribute, if any, little to the catalytic activity. For the Ar plasma treatment of MoS 2, MoS 2 samples were bombarded with argon ions using a radio-frequency, inductively-coupled plasma source in cylindrical chamber that is 4in in diameter and 6in in length with a quartz housing. Using a pressure control system coupled to the chamber, all experiments were conducted at 20 mtorr. The sample was suspended in the middle of the chamber attached to a ceramic rod. The plasma is produced inside the chamber using a 3- turn copper coil wrapped around the quartz housing, driven by a pulsing generator at 13.56MHz. The generator was pulsed at 50 W for 20% of a 1 khz duty cycle. This power delivery scheme allows for a lower particle flux to the substrate compared to constant power source operation. Ions produced in this discharge are accelerated to approximately 15 V before impacting the samples, with the ion flux up to 11.0 A/m 2 (6.9E19 particles/m 2 /s) for each second of sample exposure time. We have investigated the catalytic activity of MoS 2 films treated by Ar plasma in a way similar to what reported by the previous work. The film is grown using the same process for the synthesis of MoS 2 flakes, similar to what used in the previous study. The film is then transferred onto glassy carbon substrates for catalytic characterization using the surface-energy-assisted transfer technique we developed. We perform Ar plasma treatment at the film using two different ways. In one way, we first perform cyclic voltammetry (CV) at the film in the range of V (vs. RHE) for thousands times till the catalytic activity appears to be stable, and then treat the film with Ar plasma. In the other way, we treat the transferred film with no pretreatment cycling using Ar plasma. The treatment conditions at both ways are kept to be comparable. During the Ar plasma treatment, MoS 2 samples were bombarded with argon ions using a radiofrequency, inductively-coupled plasma source in a cylindrical chamber that is 4in in diameter and 6in in length with a quartz housing. Using a pressure control system coupled to the chamber, all experiments were conducted at 20 mtorr. The sample was suspended in the middle of the chamber attached to a ceramic rod. The plasma is produced inside the chamber using a 3-turn copper coil wrapped around the quartz housing, driven by a pulsing generator at 13.56MHz. The generator was pulsed at 50 W for 20% of a 1 khz duty cycle. This power delivery scheme allows for a lower particle flux to the substrate compared to constant power source operation. Ions produced in this discharge are accelerated to approximately 15 V before impacting the samples, with the ion flux up to 11.0 A/m 2 (6.9E19 particles/m 2 /s) for each second of sample exposure time. S11

12 Figure S17. Structural and compositional characterization of monolayer MoS 2 films. (a) Raman spectra of the film before pre-testing cycling (black), after pre-testing cycling (red), and after an treatment of Ar plasma for 2s (blue). The arrows are associated with defects created by the Ar plasma treatment. (b) XPS measurement of the film before pre-testing cycling (black), after pre-testing cycling (dashed red), and after an treatment of Ar plasma for 2s (blue). The Ar treatment may indeed induce defects, as indicated by the defect peaks in Raman spectra (Fig. S15a). This defect peaks are similar to what reported in the previous work, suggesting the formation of comparable defects. Our XPS measurement also shows an decrease in the stoichiometric ratio of S:Mo after the treatment, indicating the formation of sulfur vacancies as reported by the previous study. From the XPS measurement, we can quantitatively estimate that around 10% sulfur vacancies are created by the treatment. Additionally, the Raman and XPS measurements have confirmed that the pre-treatment cycling may not change the composition and structures of the film at all. We monitor the catalytic performance of the films through the process of the treatment, and plot the results in Fig. S16. The catalytic performance of the film shows dramatic improvement during the pre-treatment cycling process and usually tends to be stable after thousands of cycles. This cycling process just serves to clean the surface to expose catalytic active sites. A couple of recent works and our own studies have demonstrated that air-borne carbonaceous contaminants may adsorb to the surface of the film in ambient environment. However, the Ar treatment shows negligible effects on the catalytic performance of these pre-cycled films. This indicates that the sulfur vacancies created by the Ar plasma are not catalytically active. To further confirm the catalytic inactivity of the plasma-generated sulfur vacancies, we have repaired the sulfur vacancies at the Ar-treated films using the process reported in the main text, and find negligible changes in the catalytic performance after the repair. On the other hand, we do observe an increase in catalytic activity at the non-pre-cycled film after the Ar treatment. But we believe that this increase is just due to the cleaning effect of Ar plasma treatment, which may remove the carbonaceous contaminants at the surface of the film as the cycling process. The catalytic performance of the treated non-pre-cycled film may be further improved by post-treatment cycling to be similar to that of the thoroughly pre-cycled film with no Ar treatment. S12

13 Figure S18. Effect of Ar plasma treatment on the catalytic activites. (a) Polarization curves of the monolayer MoS2 films at diiferent stage, initial, after being cycled (in the range of V vs. RHE) for 5000 times, treated by Ar plasma for 2 s, and after trated by the process of repairing sulfur vacancies. (a) Polarization curves of the monolayer MoS2 films at different stage, initial, treated by Ar plasma for 2 s, and after cycling (in the range of for 5000 times) for 5000 cycles. We have also examined the catalytic performance of the films treated by Ar plasma for different durations. The films are cycled till stable catalytic performance prior to the Ar plasma treatment. Figure S17a shows the Raman of the films treated by Ar plasma for 2, 10, and 15 seconds. The Raman indeed shows more defects at the films treated longer time, and the defect-associated peaks are again similar to what reported in the previous work, indicating the formation of comparable defects. The catalytic characterization results for these treated films are plotted in Fig. S17b. The results of the films prior to the treatment are also given as a reference. There are negligible changes in catalytic performance at the films treated for 2s and 10s, while obvious improvement in catalytic activity at the film treated for 15 s. However, we believe that this increase results from the formation of many cracks in the films due to the treatment, instead of sulfur vacancies. SEM images clearly indicate the presence of holes (dark areas) with missing materials in the film whose sizes and densities increases with the treatment duration. The cracks may provide edge sites to be catalytic active sites for the HER. To further confirm that the increase of catalytic activity not resulting from the formation of sulfur vacancies, we perform the repair of sulfur vacancies at the films treated for 15 s, and found negligible change afterward. S13

14 Figure S19. Effect of the treatment time on the catalytic activities. (a) Raman spectra of the films treated by Ar plasma for 2s (black), 10s (blue), and 15s (red). (b) Polarization curves of the monolayer MoS2 films before and after the Ar plasma treatment. We also performed the repair of sulfur vacancies at the film treated for 15s. And the polarization curve after the sulfur vacancy repair is also given. (c) SEM images of the films treated by Ar plasma for 2s (top), 10s (middle), and 15s (bottom). The black areas are holes with MoS 2 materials missing. S2. Calculation of the turnover frequencies at single active sites The TOF is estimated by using the exchange current density at 0V vs. RHE. We have provided detailed calculation of reaction rate for S vacancy is provided in S2. The turnover frequency is calculated as following TOF = R H2 /N active where R H2 is the number of hydrogen molecules produced per unit time and unit area at 0V vs. RHE and N active is the number of catalytic active sites per unit area. The hydrogen molecules produced per unit time and unit area can be calculated from the exchange current density at 0V vs. RHE J 0 as R H2 = J 0 /2e. For sulfur vacancies, the number of active sites per unit area is calculated from the measured density of sulfur vacancy a% using the following equation S14

15 a% N sulfur = ( ) 10 8 For simplicity, we assume the sulfur vacancies evenly distributed in the top and bottom layers of sulfur atoms. We only consider the sulfur vacancies at the top layer of sulfur atoms to be catalytically active, because the sulfur vacancies in the bottom layer of sulfur atoms is embedded underneath and may not be accessible for reaction. For the edge sites, we consider each edge site or grain boundary occupy a length of 0.32 nm, which is the lattice constant of MoS 2. N edge = (measured length/unit area)/0.32 nm The number of grain boundary can be calculated using the same method as the calculation for the edge sites. S15