Strengthening of Graphene Aerogels with Tunable Density and High Adsorption Capacity Towards

Supplementary Information

Strengthening of Graphene Aerogels with Tunable Density and High Adsorption Capacity towards Pb2+

Zhuo Han1,2, Zhihong Tang1*, Shuling Shen1, Bin Zhao1, Guangping Zheng1,2 Junhe Yang1*

1School of Materials Science and Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China

2Department of Mechanical Engineering and Shenzhen Research Institute, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China

*Corresponding Author: ；

Preparation of graphene oxide (GO):

GO was chemical exfoliated from natural graphite flakes via a modified Hummer’s method. Typically, graphite flakes (5.0g) (CP, Aladdin Reagent Database Inc., Shanghai, China) and sodium nitrates (2.5g) were put into concentrated sulfuric acid (115ml) at room temperature. The mixture was under ice bath for 25 mins with mild agitation.Potassium permanganate (15.0 g) was added gradually and the temperature of suspension was kept to below 10oC for another 25 mins. Then, the mixed suspension was heated to 35oC and kept for 45 mins until a thick paste was formed. Deionized water (140 ml) was added and the temperature of the solution was kept at 98oC for 45mins.When the brown mixture turned into yellow, the mixture solution was diluted to 700 ml, followed by adding 30 ml H2O2 (30%). The mixture was thenfiltered and washed with 50ml of HCl solution. Finally, the solution was centrifuged for several times at 11000rpm until the pH of the system was about 7. The resulting sample was dried in vacuum at 60oC for 72hrs. 2mg/ml of GO solution was prepared by the sonication of 2.2g graphite oxide in 1L water for about 2 hrs, after which the aggregates were removed by mild centrifugation (3000 rpm for 10mins). The resulting dispersion was ready for the preparation of GH.

Characterization:

The mechanical strength was measured by a compression testing machine (Materials Testing Machine, ZWICK). The GAs were sliced and polished into the cylindrical shape with a height of 8mm and a diameter of 4mm.Two cross sections of the cylindrical sample were abraded until they were parallel. During thecompression tests, the compressive speed was 1mm/min, and the load-displacement relation was recorded until a suddenly change in the applied stress. The apparent density of samples was calculated by the weight divided by its volume:ρ = MGA / VGA. The porosity of samples was calculated by the formulaN = ( 1- (MGH - MGA) / MGH) * VGA / VGH *100%,where N represents the porosity ratio, MGH refers to the mass of GH, MGA refers to the mass of GA, and V represents the volume.

The morphology and structure of GAs after soaking in ammonia solution of different concentrations were observed by field emission scanning electron microscopy (FESEM, FEI Quanta FEG) with an accelerating voltage of 20 kV. Before SEM measurementon their inner structures, the samples were made directly by breaking the sample into two parts with fresh fracture surfaces. Functional groups and chemical bonds of samples were determined by X-ray photoelectron spectroscopy (XPS, PHI 5000C ESCA) at a base pressure of 1×10-9 mbar. Specific surface areas of the samples were measured by nitrogen adsorption-desorption isotherms performed at 77 K on a Micromeritics ASAP-2020 volumetric adsorption system, and the specific surface area (SSA) was calculated by the Brunauer-Emmett-Teller (BET) method.

Organic solvent experiments

The samples were weighed and then put into different kinds of organic solvents for weight gaining tests. After kept in an incubator shaker for 30mins under a rotating speed of 150rpm, the samples were removed from the organic solvents and weighed again after the surface oils had been removed with a filter paper.

The absorption capacity (Q) was calculated using the equation:

Q = (M- M0) / M0 ,

where M0 and M are the weights of the samples before and after absorption, respectively. The weight measurements on the organic solvents with absorbed oil were done quickly to avoid evaporation of the oils.

Electrochemical measurements

Electrochemical measurements were carried out with a three-electrode cell using an electrochemical station (CHI760d, CH Instrument). The working electrode was a glassy carbon (GC) electrode of 3.0 mm in diameter. The samples were triturated by a filter sieve with 250 mashes and then dispersed in nafion/ethanol (1%) solution by ultra-sonication and 20 μL of the resulting dispersion was loaded drop by drop on the surface of the pre-treated GC electrode, thus forming a uniform coating layer on the electrode after it was dried in air at room temperature.

The procedure of lead ion’s detection in ASV method was as follows: the treated GC electrode was first soaked in the mixture of 0.005M hydrochloric acid solution containing 0.001M Pb(NO3)2 under stirring for 60s.Then the GC electrode was rinsed by deionized water and dipped into the measurement cell containing hydrochloric acid (0.01 M) at a constant potential of −1.0 V (vs. Ag/ AgCl) for 60s to electrodeposite Pb(II). The parameters of the differential pulse mode was set as follows: potential scanned from -0.8 to -0.2V, potential step of 5mv, pulse width of 0.2 s, pulse amplitude of 50 mV, and pulse period of 500 ms.

Figure S1 Typical SEM images of GA-RT. Rare porescould be found in GA-RT sample, and the surface was tight and smooth.

Figure S2 N2 adsorption-desorption isotherms of GA (a), a-GA-10% (b), a-GA-14% (c), a-GA-18% (d) and a-GA-22% (e).

Figure S3 Cycle adsorption performance of a-GA-10% towards Pb2+

Figure S4 (a) Histogram plots of weight gains of a-GA-10% in different kinds of organic solvents. (b) Histogram plots of weight gains of a-GA-N in toluene solution.