Accessible Graphene Aerogel for Efficient Harvesting. Solar Energy

Transcription

1 Supporting Information Accessible Graphene Aerogel for Efficient Harvesting Solar Energy Yang Fu, Gang Wang, Tao Mei, Jinhua Li, Jianying Wang, and Xianbao Wang Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry-of-Education Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei Key Laboratory of Polymer Materials (Hubei University), School of Materials Science and Engineering, Hubei University. Wuhan , PR China. Corresponding author. Tel.: ; fax: Figure S1. Schematics of experimental setup for solar steam generation. S1

2 Figure S2. Nitrogen absorption and desorption measurements of GOA, GOAM and GA with the BJH pore size distribution. Those surface areas of GOA, GOAM and GA are m 2 /g, m 2 /g, and m 2 /g, respectively. Average pore sizes are nm for GOA, nm for GOAM, and nm for GA. BJH desorption cumulative volume of pores between and nm width are cm 3 /g for GOA, cm 3 /g for GOAM, and cm 3 /g for GA. Note S1. Concentrations of GO for fabricating GA In order to select the proper GO concentration to fabricate GA with efficient solar energy conversion, we design a series of GO concentrations from 1 mg ml -1 to 6 mg ml -1 (5 ml in general except special notes). Mass changes of water for GA from GO with different concentrations under light irradiation is recorded in Figure S3. It can be seen that the mass loss increases with the decreasing of GO concentrations. Because the greater the GA weight is, the greater the density is under the same volume and structure. The larger density GA could sink into the water-air interface comparing with the low density GA floating on the interface, which indicates that more light is reflected though the interface and heat transfers to the bulk water more swimmingly. So the lowest weight of GA owns the largest mass loss in same period of time. However, it is hard to form a complete GA film when the GO concentration was reduced to 1 mg ml -1 (Figure S4). And the better mechanical and stable properties can be obtained in 3 mg ml -1 GO dispersion compared to 2 mg ml -1. So we chose the S2

3 proper concentration of 3 mg ml -1 GO dispersion in this experiment. Figure S3. The mass changes of water for GA from GO with different concentrations under light irradiation (3 kw m -2 ). Figure S4. Optical photos of GOAM of different weight. Note S2. Concentrations of GO dispersion and rgo dispersion In the optical test, 8 mg GO and rgo were used to dispersed in 100 ml water to compare with floating GA (8 mg), respectively, so the concentration of GO and rgo dispersion is mg ml -1. Note S3. GOAM is partially reduced to GA though irradiation When GOAM was exposed under the intensive light (light intensity > 10 suns) within 5 s, a gray gas appeared on the surface of GOAM and brown GOAM was rapidly S3

4 changed to the black state like graphene. It indicated that GOAM was reduced to GA. Four methods were used to certificate this reduction process as follows (Figure S5). FT-IR spectra (Figure S5a) shows the following functional groups of samples: O H stretching vibrations (ca cm -1 ), C=O stretching vibration ( cm -1 ), C=C from unoxidized sp 2 C C bonds ( cm -1 ), and C O vibrations (ca cm -1 ). 1-2 The peak intensity of the hydroxyl groups for GA weakens, and the absorption bands of other oxygen-containing groups are not pronounced, compared with that of the corresponding GOAM. XPS is another effective method for analysis of surface elements of material, which can be used to determine the relative content of elements of materials. XPS analysis shows that the intensity of O peak for GA has decreased drastically in comparison with that of GOAM (Figure S5b). The O/C atomic ratio in GA is 1 : 2.0 compared with 1 : 4.7 in GOAM, which proves that some oxygen-containing groups have been removed by sunlight irradation. XPS data were also used to demonstrate the uniform reduction of the thick GOAM and changeless composition of GA before and after use as solar steam generator (Figure S6). Thick GOAM, surface and cross-section of the same sample after photo-reduction were labeled as TGOAM, GA-EX and GA-IN, respectively. GA before and after use as solar steam generator were labeled as GA-before and GA-after, respectively. XPS analysis shows that those percentages of O atom are 29.4% for TGOAM, 20.07% for GA-IN, 18.49% for GA-IN/GA-before and 18.7% for GA-after, respectively, which could provide the evidence that both inside and outside of TGOAM are reduced during photo-reduction process. In addition, the little change of those contents of O, N and C in Figure S6 demonstrates that the composition of GA remains stable after photothermal test. Crystalline structures of GA and GOAM are identified by XRD, and the result is shown in Figure S5c. GOAM shows an obvious diffraction peak at 8.4, which is corresponding to a (001) reflection. 3 According to the Bragg equation, the derived layer-to-layer distance (d-spacing) is approximately Å, which is much larger than that of pristine graphite (~3.4 Å). The peak at 2θ = 10.6 almost disappears in GA, which is replaced by a weaker and broader (002) peak at ca. 2θ = 25.0 indicative S4

5 of a graphene layer with a d-spacing of 0.36 nm, signifying the reduction of GO to graphene. Raman fingerprints are one of the most unambiguous and useful techniques to identify the degree of order and defect. As shown in Figure S5d, two distinct peaks are easily observed, which are attributed to the D band at ca cm -1 and G band at ca.1584 cm -1. The D band is related to the structural disorder at defect sites, whereas the G band is associated with the in-plane bond-stretching motion of pairs of sp 2 C atoms. 4 The D/G intensity ratio ( I D / I G ) are usually taken as a measure of the defect density in graphene, and the ratio approaches zero for a highly ordered pyrolitic graphite. 5 It is obvious that the I D / I G increases to 0.96 in GA compared with that of GOAM (0.88), showing a decrease in the average size of the graphitic domains. It is caused by the small re-graphitized sp 2 domains, indicating that more defects exited in GA and GOAM was reduced in some degree 6. Those provide clear evidences for the partial reduction of GO. Figure S5. (a) FT-IR spectra, (b) XPS survey, (c) XRD data and (d) Raman spectra of GOAM and GA. S5

6 Figure S6. XPS survey of (a) TGOAM, (b) GA-IN, (c) GA-EX / GA-before and (d) GA-after. Figure S7. The mass loss (left axis) and water evaporation rate (right axis) of GOAMs floating on the water- air interface with different irradiation times under light irradiation (3 kw m -2 ). S6

7 Figure S8. XRD data of GA with different ph. Figure S9. Optical photos (1-3) and SEM images (4-9) of GA formed by GO dispersion with different ph. S7

8 Note S4. GA could sustain its wetted surface during the interface water evaporation process In the moment of contact with the interface liquid, the fluid flow to the GA is mainly due to the inertia force and capillary pressure. With the increasing of the viscous resistance, the effect of the capillary pressure is more and more strong, and the net driving force is smaller and smaller. Therefore, the inertia of the liquid flow is also getting smaller and smaller. During the interface water evaporation process, we consider that the fluid in the structure depends on viscous force and the inertia effect and the gravity of the fluid itself are neglected. So, the Washburn s equation can be used to calculated the penetration of water into the GA flowing. 7-8 ll = rr cccccc θθ γγ 11 ηη 22 tt (1) Where l denotes the water penetrate distance, r the average dimension of pores, γ surface tension, θ contact angle, η the dynamic viscosity of water, and t the water penetrate time. The distance is determined through the volume of the droplet divided by the wetted area. The contact angle of light-reduced graphene oxide is 40 o (Fig. S10). The surface tension-to-viscosity ratio of water [γ /η] 1/2 represents the speed of penetration into the GA. We neglect the changes in viscosity and surface tension due to the temperature changes. And the velocity is about 10.7 cm/s. If we suppose that all the flow water was evaporated in one second, the required light intensity could be calculated by equation 2. Q s = ρvh vvvvvv (2) Where Q s denotes the light intensity, ρ density of water, v velocity of water (calculated above), and h vap the total enthalpy of liquid-vapor phase change (sensible heat and potential heat). By calculation, the surface of GA-air would be not dried until the light intensity is up to kw m -2. Thus GA could sustain its wetted surface under the highest light intensity of 10 kw m -2 used in this work. S8

9 Figure S10. The contact angle of light-reduced graphene oxide. Note S5. Thermal conductivity measurement by IR camera The thermal conductivity was measured by sandwiching the material between two quartz glass slides each 3-mm thick (reference materials). The GA with a thickness of 10 mm in air was sandwiched between the two glass slides. The glass slides were spaced from each other by using double-stick tape. In order to maintain at a fixed temperature, this sandwich was placed between a heater (Constant temperature heating table) and a cooler (Semiconductor cooler). IR camera was used to record the temperature gradient of sandwich, through which the heat flux could be obtained. Since the thermal conductivity of glass is known, the thermal conductivity, then, can be calculated by the Fourier equation. Figure S11. The thermal conductivity of GA in air was measured by an IR camera. The inset in the figure is the representative picture by an IR camera. S9

10 Figure S12. The mass loss (left axis) and water evaporation rate (right axis) of GAs floating on the water- air interface with different thicknesses under light irradiation (3 kw m -2 ). Table S1. Quality comparison table about GOAM and GA GOAM is fabricated in different weight of GO, and GA is obtained though lighting-inducing the corresponding GOAM. m (GOAM) / mg m (GA) / mg Mass defect ratio / % Table S2. Density information of GOA, GOAM and GA Sample Apparent density (mg/cm 3 ) True Density (g/cm 3 ) GOA GOAM GA S10

11 Supplementary References. (1) Tamás Szabó; Ottó Berkesi; Péter Forgó; Katalin Josepovits; Yiannis Sanakis; Dimitris Petridis, A.; Imre Dékány, Evolution of Surface Functional Groups in a Series of Progressively Oxidized Graphite Oxides. Chem. Mater. 2006, 18 (11), (2) Chen, Y.; Li, J.; Mei, T.; Hu, X. G.; Liu, D.; Wang, J.; Hao, M.; Li, J.; Wang, J.; Wang, X., Low-temperature and one-pot synthesis of sulfurized graphene nanosheets via in situ doping and their superior electrocatalytic activity for oxygen reduction reaction. J. Mater. Chem. A 2014, 2 (48), (3) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. B. T.; Ruoff, R. S., Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon 2007, 45 (7), (4) Tuinstra, F.; Koenig, J. L., Raman Spectrum of Graphite. J. Chem. Phys. 1970, 53 (3), (5) Ferrari, A. C., Raman spectroscopy of graphene and graphite: Disorder, electron phonon coupling, doping and nonadiabatic effects. Solid State Commun. 2007, 143 (1 2), (6) Graf, D.; Molitor, F.; Ensslin, K.; Stampfer, C.; Jungen, A.; Hierold, C.; Wirtz, L., Spatially Resolved Raman Spectroscopy of Single- and Few-Layer Graphene. Nano Lett. 2007, 7 (2), (7) Oliver J F, Wetting and penetration of paper surfaces. Colloids and Surfaces in Reprographic Technology; M Hair and M D Croucher, Eds.; American Chemical Society: Washington DC, USA, 1992 (8) Sajadi, S. M.; Farokhnia, N.; Irajizad, P.; Hasnain, M.; Ghasemi, H., Flexible artificially-networked structure for ambient/high pressure solar steam generation. J. Mater. Chem. A 2016, 4 (13), S11