Supporting Information: Quantifying Surface Area of Nanosheet Graphene Oxide Colloid Using a Gas-Phase Electrostatic Approach

Transcription

1 Supporting Information: Quantifying Surface Area of Nanosheet Graphene Oxide Colloid Using a Gas-Phase Electrostatic Approach Wei-Chang Chang, 1 Shiuh-Cherng Cheng, 1 Wei-Hung Chiang, 2 Jia-Liang Liao, 2 Rong- Ming Ho, 1 Ta-Chih Hsiao, 3 De-Hao Tsai 1,* 1 Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan, Republic of China. 2 Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan, Republic of China. 3 Graduate Institute of Environmental Engineering, National Central University, Zhoung-Li, Taiwan, Republic of China. * Corresponding Author. dhtsai@mx.nthu.edu.tw; Tel: ; Fax: Table of Contents 1. Synthesis of carbon nanotubes (CNTs) 2. Additional TEM images of graphene oxide nanosheets without size classification Figure S1. TEM images of GOSWCNT without size classification. The scale bars are 100 nm. Figure S2. TEM images of GOMWCNT without size classification. The scale bars are 100 nm. Figure S3. TEM images of GOgraphite without size classification. The scale bars are 100 nm. S-1

2 3. TEM image analysis of drop-casted GOgraphite colloid versus electrosprayed GOs Figure S4. TEM images of GOgraphite colloid collected by the drop-cast method (i.e., without the use of electrospray ionization and electrostatic deposition). The scale bars are 100 nm. 4. Additional TEM images of graphene oxide nanosheets with size classification Figure S5. TEM images of size-classified GOSWCNT. 1: 20 nm; 2: 35 nm; 3: 50 nm. The scale bars are 100 nm. Figure S6. TEM images of size classified GOMWCNT. 1: 35 nm; 2: 50 nm. Figure S7. TEM images of size-classified GOgraphite. 1: 20 nm; 2: 35 nm. The scale bars are 100 nm. 5. Calculation of number concentration of GOs for ES-DMA-CPC/ASAA 6. Calculation of the average mobility diameter of GO colloid 7. Conversion of aerosol current to equivalent surface area 8. Determination of correlation factor (C) Figure S8. Aerosol current (Ip) versus the selected mobility diameter (dp,m) of spherical particles. 9. Technical descriptions of diffusion battery Figure S9. Effect of diffusion battery on the mobility size distributions of GOs. (a) GOSWCNT. (b) GOgraphite. 10. Determination of specific surface area on a mass basis S-2

3 11. Defoliation of carbon nanotubes to form GO Figure S10. Cartoon depiction of the defoliation process for carbon nanotubes. References S-3

4 1. Synthesis of carbon nanotubes (CNTs) The CNTs used in the present study were synthesized using a catalytic chemical vapor deposition (CVD). Details of the CNT growth process have been described in the previous study. 1 Briefly, at 1 atmospheric pressure in a 3-inch quartz tube, iron (Fe) film (i.e., the catalyst for the growth of CNT) and an alumina (Al2O3) support layer (40 nm in thickness) were sputtered onto a polished silicon (Si) substrate with a silicon dioxide (SiO2) layer of 600 nm in thickness. To synthesize single-walled CNT and multi-walled CNT, the thickness of Fe film was controlled to 1.0 nm and 8.2 nm, respectively. The average thickness of as-deposited Fe films was characterized by ex-situ atomic force microprobe (Veeco, Dimension 3100, Plainview, NY, U.S.A.). All other parameters of the CVD process were the same for the growth of different types of CNTs. S-4

5 2. Additional TEM images of graphene oxide nanosheets without size classification S-5

6 Figure S1. TEM images of GOSWCNT without size classification. The scale bars are 100 nm. S-6

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8 Figure S2. TEM images of GOMWCNT without size classification. The scale bars are 100 nm. S-8

9 Figure S3. TEM images of GOgraphite without size classification. The scale bars are 100 nm. S-9

10 3. TEM image analysis of drop-casted GOgraphite colloid versus electrosprayed GOs Figure S4. TEM images of GOgraphite colloid collected by the drop-cast method (i.e., without the use of electrospray ionization and electrostatic deposition). The scale bars are 100 nm. From the TEM images, we identify the flexible GO nanosheets became crumpled both after drop-cast and electrostatic aerosol deposition. The effect of drying on the morphological change of GO has been reported by Wang et. al. (i.e., using an ultrasonic nebulizer to aerosolize micro-sized droplets containing graphene). 2 The results show that graphene oxide was in folded structure, preventing graphene sheet from restacking (i.e., the loss of surface area). In comparison, the electrospray generates the droplets with a much smaller diameter ( 150 nm). 3-5 Therefore, the capillary effect of inducing morphological changes of flexible GO nanosheet could be less significant during the droplet evaporation (i.e., less extent of folding on GO nanosheet). Because the aerosol drying process can possibly change both the surface area and size or GO, 2 we report the measured results (ES-DMA-CPC/ASAA, ES-ASAA/CPC) as mobility diameter and equivalent surface area. The results are comparable to the results of the TEM image analysis and the BET analysis S-10

11 4. Additional TEM images of graphene oxide nanosheets with size classification S-11

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13 Figure S5. TEM images of size-classified GOSWCNT. 1: 20 nm; 2: 35 nm; 3: 50 nm. The scale bars are 100 nm. S-13

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15 Figure S6. TEM images of size classified GOMWCNT. 1: 35 nm; 2: 50 nm. S-15

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17 Figure S7. TEM images of size-classified GOgraphite. 1: 20 nm; 2: 35 nm. The scale bars are 100 nm. 5. Calculation of number concentration of GOs for ES-DMA- CPC/ASAA The actual number and mass concentrations of GO suspensions (i.e., after the removal of large, non-dispersible aggregates of GO from the suspensions) can be calculated based on the gas-phase number concentration of electrosprayed GO measured by ES-DMA-CPC (for Route 1). By integrating the peak in the size distribution obtained by ES-DMA-CPC (i.e., after correcting for charge distribution and transfer function), Np,g,GO and Nl,GO can be determined by Eq. (1) 4,5. S-17

18 N d p, m,max dn p p, g dd p (1), dd d p p, m,min where dp,m,max and dp,m,min are the maximum and minimum dp,m for a peak, respectively. Nl,GO is then calculated using Eq. (2). N (2). p, g, GO N l, GO N l, AuNP N p, g, AuNP Here, Nl,AuNP can be obtained based on the vendor information (i.e., 30 nm-aunp). Based on the mobility size distributions measured by ES-DMA-CPC, the number concentrations of GO sheet in the suspensions were (4.5 ± 0.4)*10 12 cm -3, (3.0 ± 0.1)*10 11 cm -3, and (6.1 ± 0.7)*10 12 cm -3 for GOSWCNT, GOSWCNT, and GOSWCNT, respectively. In addition, we also consider the possible effect of droplet-evaporation induced aggregation. 3-5 To measure the particle size and surface area using ES-DMA- CPC/ASAA, a necessity is to have no more than one GO sheet present in one aerosol droplet. Due to the relative low particle concentration and small droplet size ( 150 nm), the average number of GO in a droplet was 1 % in our study. Therefore, the effect of multiple GOs per droplet can be negligible. Note that the maximum number concentration of GOs for operation was 1.1*10 14 particles/cm 3 in order to avoid the droplet evaporation-induced aggregation during the measurement of ES-DMA- CPC/ASAA. 6. Calculation of the average mobility diameter of GO colloid The average mobility diameter (dp,m,avg) is calculated using Eq. (3). 4,6 d p, m, avg dn dd g, total, t p, m d p, m dd p, m dn dd g, total, t p, m dd p, m (3), where Ng,total,t is the total concentration of aerosolized GO measured in gas phase. By calculation, the dp,m,avg of GOSWCNT, GOMWCNT and GOgraphite are 23.3 nm, 30.4 nm and 30.8 nm, respectively. S-18

19 7. Conversion of aerosol current to equivalent surface area The equivalent surface area of GOs can be derived based on the measured aerosol current, Ip, which is correlated the lung-deposited surface area concentration: 7 2 ( ndp d p )( DFa ) dd p 0 SA (4). LD Here SALD is the total lung-deposited surface area concentration (unit: μm 2 /cm 3 ). DFa represents the fraction of the deposited particles as a function of particle size. ndp is the particle number concentration for a given particle size, and the term (πdp 2 /ndp) represents the equivalent surface area per particle (SA) determined by this method. Here the SALD is determined based on the aerosol current measured, using Eq. (5), 8 SALD (μm 2 /cm 3 ) = 413 Ip (pa). (5). Combining Eq. (4) and Eq. (5), the correlation of SA versus Ip can be obtained (Eq. 1 of the main text). 8. Determination of correlation factor (C) In the main text, C is a constant of converting the measured Ip to the equivalent surface area derived from the correlation of spherical particles, C SA sphere I sphere DF a sphere (6). Here, SAsphere is the surface area of spherical particles (i.e., equal to πdp,m 2 ). DFa(sphere) is the deposition efficiency of charges on a spherical particle surface as a functional of dp,m, which can be derived based on the fraction of particles depositing in the alveolar region of human lung. 7 The size-dependent DFa(sphere) is calculated based on the model developed by the International Commission on Radiological Protection (ICRP). 7 By calculation, DFa (sphere) are 0.48, 0.46, 0.43, 0.40, 0.37, 0.34, and 0.31 when dp,m S-19

20 are equal to 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, and 50 nm, respectively. Here DFa of GO is assumed to be equal to DFa(sphere) under the same dp,m. Figure S8 shows the normalized aerosol current (Ip) versus the selected mobility diameter of three types of spherical particle measured by ES-DMA-CPC/ASAA. The measured Ip increases with selected dp,m for all three types of spherical particle following a correlation of Ip = 10-7 dp,m 1.1. From the Eq. 3 of the main text and Figure S8, we can obtain the correlation factor (C) as pa nm -2. Figure S8. Aerosol current (Ip) versus the selected mobility diameter (dp,m) of spherical particles. 9. Technical descriptions of diffusion battery The diffusion battery used in this study is a customized device made of a stainlesssteel tubing with 0.2 cm of inner diameter and 310 cm in length. In principle, the particles with a smaller dp,m can be preferably removed via diffusion-induced deposition. 9 Therefore, the diffusion battery is to preferably remove the ultrafine nonvolatile particles especially when dp,m < 7 nm. Based on the theory of diffusion, the penetration fraction of particles versus the deposition parameter, u p, for a diffusion battery (i.e., circular tube) can be obtained using Eq. (7), 9 S-20

21 u p D particlel tube Q aerosol (7), where Ltube is the length of the diffusion battery, and Qaerosol is the volumetric flow rate of aerosol flow. Dparticle is the diffusion coefficient of the particles calculated using Eq.(8), 9 D particle ktc c 3d p,m (8). where μ is the viscosity of air (= 1.8*10-5 N*s/m 2 ). Cc is the slip correction factor of particle. The penetration rate (P) can be determined using the following equations: 9 P = u p 2/ u p when u p < (9). P = exp(-11.5 u p ) exp(-70.1 u p ) when u p (10). Based on the layout of the diffusion battery, P was estimated < 70 % for the nonvolatile NP (i.e., not GO; dp,m 7 nm). In comparison, > 92 % of GOs (i.e., dp,m> 20 nm) should remain in the aerosol flow. The results indicate that we can use a diffusion battery to preferably remove the ultrafine non-volatile residue particles (i.e., the background signals). The efficiency of diffusion battery was also measured by ES-DMA-CPC. As shown in Figure S9, the peaks representing the non-volatile residue particles (dp,m 7 nm) disappeared for both GOSWCNT and GOGraphite samples. Simultaneously, the number density of GOs decreased by 54 % for GOgraphite and 66 % for GOSWCNT. The experimental results confirm the model-predicted efficacy of diffusion battery semiquantitiavely, showing a higher removal rate towards the ultrafine non-volatile NPs ( 100 % of removal). S-21

22 Figure S9. Effect of diffusion battery on the mobility size distributions of GOs. (a) GOSWCNT. (b) GOgraphite. 10. Determination of specific surface area on a mass basis In the main text, Equation 4 describes a conversion from SA to the specific surface area on a mass basis, SAmass: SA SA mass N GOgraphite (11). Here NGO graphite is the number concentration of GOgraphite per gram of GO in the colloidal sample, which is obtained by dividing the number concentration of GO in the liquid phase with the concentration on a basis of unit mass. N GO graphite N C l, GO l, GO graphite graphite (12). Here, Nl,GO graphite is the number concentration of GOgraphite in the sample (unit: #/cm 3 ). Cl,GO graphite is the mass concentration of GOgraphite in the sample (unit: g/cm 3 ). Based on the results of ES-CPC, the Nl,GO graphite can be calculated using Eq. (13), S-22

23 N n GOgraphite l, GO N graphite l, AuNP naunp (13). Here, ngo graphite and naunp are the number concentrations of GOgraphite and AuNP measured by CPC (i.e., using 30 nm-aunp). Nl,AuNP is the number concentration of AuNP in the liquid phase. Knowing that the ratio of ngographite to naunp equals to 6.6 and Nl,AuNP equals to #/cm 3, Nl,GO graphite is calculated as #/cm 3. Combining Eq. (12) and Eq. (13), we obtain NGO graphite = particles/g. Using Eq. 4 of the main text and SA = (2395 ± 79) nm 2, SAmass of GOgraphite is calculated as (202 ± 8) m 2 g -1 by ES-ASAA/CPC. 11. Defoliation of carbon nanotubes to form GO Figure S10 shows a cartoon depiction of the defoliation of carbon nanotubes. Due to the confinement in the diameter of carbon source, the lateral aspect ratio of GOSWCNT is shown to be significantly higher ( ) than that of GOMWCNT ( ). Figure S10. Cartoon depiction of the defoliation process for carbon nanotubes. S-23

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