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Aug 4, 2016 - virgin and fluorinated SiCDCs. We demonstrate that with increasing fluorination level the hydrophobic character of the low and medium ...
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Fluorination-Induced Changes in Hydrophobicity of Silicon CarbideDerived Nanoporous Carbon Ali Shahtalebi,† Maimonatou Mar,‡ Katia Guérin,‡ and Suresh K. Bhatia*,† †

School of Chemical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia Institut de Chimie de Clermont-Ferrand, Université Blaise Pascal, F-63000 Clermont Ferrand, France

‡

ABSTRACT: We report the effect of fluorine doping on hydrophobicity of microporous silicon carbide-derived carbon (SiCDC), exploring water vapor adsorption in virgin and fluorinated samples experimentally and using semiempirical isotherm models. The surface chemistry of the virgin carbon and samples fluorinated to three different levels is characterized by X-ray photoelectron spectroscopy and 19F nuclear magnetic resonance analysis techniques. Besides having different textural features such as surface area and pore size distribution, the virgin and fluorinated SiCDCs show different surface chemistries in terms of the nature and quantity of the C−F bonds. The comparison of the characterization results of the samples and model parameters is used as the methodology to understand the water adsorption mechanism in virgin and fluorinated SiCDCs. We demonstrate that with increasing fluorination level the hydrophobic character of the low and medium fluorinated SiCDC samples remains almost constant while the highly fluorine-doped SiCDC is less hydrophobic. We identify a pore accessibility problem for argon in the fluorine-doped SiCDCs, demonstrating that fluorine functionalization blocks the carbon pores for the nonpolar argon while allowing polar H2O molecules to form clusters that enter the internal volume of the micropores. These findings provide important new understanding of the mechanism of water adsorption in microporous carbons and their fluorinated counterparts. eventually merge, leading to the bulk filling of the carbon pores.9,11,12 Most of the experimental studies on water adsorption on microporous carbons have shown that the role of surface functional groups is prominent,8,13−16 and some of these studies have investigated the role of surface groups by exposing carbon materials to controlled oxidative treatments.16,17 Computer simulations on H2O adsorption have also shown the formation of clusters on primary adsorption centers, indicating the strong enhancement of H2O adsorption by primary surface groups.1,18−22 The effect of several types of polar oxygen-containing sites considering different densities and local distributions on the carbon surface has been investigated in a number of studies.19,23,24 The presence and structure of water adsorbed in carbon micropores at ambient temperature have been investigated by different techniques such as X-ray diffraction,25 differential scanning calorimetry,26 and dielectric relaxation spectroscopy.26,27 Iiyama et al.25 have investigated water confined in carbonaceous micropores using in situ X-ray diffraction, showing solidlike structure for water molecular assembly at 303 K based on electron radial distribution function analysis.

1. INTRODUCTION The synthesis and design of adsorbents to increase hydrophobicity is crucial for industrial applications because H2O is ubiquitous in most environments and its presence poses significant challenges in many different areas of technology, such as gas separation. While extensive work on gas adsorption in carbon pores has been reported, our understanding of H2O adsorption in carbon materials is far from complete.1−4 The presence of H2O can lead to complex adsorption behavior and affect the efficiency of CO2 capture by adsorption2,5 because it inhibits CO2 adsorption. Water adsorption isotherms on microporous carbons usually show sharp increase in uptake at medium or high relative pressures with clear hysteresis. McBain et al.6 related water uptake to capillary condensation, while in another approach, first proposed by Pierce and Smith7 and extended by Dubinin and Serpinsky (DS),8,9 it is assumed that H2O adsorption in carbonaceous pores involves clustering around primary adsorption centers. In this phenomenological model, the adsorption of H2O is believed to be due to the presence of significant numbers of adsorption centers (for example oxygencontaining centers) which have high affinity for H2O,1,4,10 although the bare carbon surface is hydrophobic. The initial H2O molecules adsorbed on these centers act as nuclei for the formation of H2O clusters,4,10 leading to the growth of larger hydrogen-bonded clusters. The hydrogen-bonded clusters © 2016 American Chemical Society

Received: May 9, 2016 Revised: June 24, 2016 Published: August 4, 2016 18595

DOI: 10.1021/acs.jpcc.6b04655 J. Phys. Chem. C 2016, 120, 18595−18606

Article

The Journal of Physical Chemistry C

SiCDCs. The structural properties of the virgin and fluorinated samples were evaluated by Ar and CO2 adsorption, and the surface chemistry (i.e., C−F bond nature and its quantity) is determined by 19F nuclear magnetic resonance (NMR) and Xray photoelectron spectroscopy (XPS) analysis. In order to study the interactions of H2O with fluorine doped atoms contributing as functional groups, we apply a number of semiempirical isotherm models whose application to microporous CDCs has not previously been reported.

Laboratory experiments on a variety of hydrophobic carbons such as activated carbon fibers (ACFs),28 some activated carbons,29 and carbide-derived carbons (CDCs)30,31 have shown that H2O isotherms on these carbons are of type V based on the IUPAC classification, and strong H2O adsorption occurs at moderate relative pressures. Based on the DS approach,8,9 a number of semiempirical water adsorption models have been presented in the literature providing fundamental information about the behavior of H2O in porous carbons. 32 Barton et al. 11,33 have developed equations incorporating empirical models to fit isotherm shapes and describe the experimental water adsorption on carbon. Gauden34 has investigated the mechanism of water adsorption on carbons having high-energy centers using the DS approach and the equations formulated by Barton et al.11,33 explaining that these models provide adequate descriptions of water adsorption isotherms only for carbons having a small density of primary adsorption sites. On the basis of the adsorption of H2O molecules on the active sites, Talu and Muenier35 have also developed a model assuming that at low loading the system behavior is solely controlled by adsorbate−solid interactions, and at intermediate loading in which the type V isotherm inflection occurs, the adsorbed molecules form large hydrogenbonded clusters. Do and Do (DD)36 have developed a twostage water adsorption isotherm model in which, in stage 1, H2O molecules strongly bond to primary sites and form Hbonded clusters. In their work they have assumed pentamers or clusters of five H2O molecules to create sufficient dispersive force for water to adsorb and fill the micropores compared to a single H2O molecule. Several modifications to the DD isotherm model36 have been presented in the literature to describe water adsorption on carbon.4,10,17,37 Although the model proposed by Do et al.4 can describe a variety of isotherm shapes for water adsorption in activated carbon, several physicochemical parameters such as the size of water clusters and the equilibrium constant of water vapor in the micropores of activated carbon cannot be measured. Yao et al.38 have developed a new model to calculate these parameters. The doping of carbonaceous materials with heteroelements is known to be an effective method to induce modifications of their hydrophilicity and hydrophobicity and water adsorption behavior,39−43 for example, fluorine-doped carbonaceous materials potentially form a new class of materials exhibiting promising characteristics and properties that make them attractive not only for gas adsorption processes but also for other applications such as in supercapacitors and batteries.44−47 In a recent simulation study from this group, the effect of fluorine doping on the hydrophobic−hydrophilic characteristics of SiCDCs has been reported.41 In this simulation study it is shown that more hydrophilic surfaces are obtained upon fluorination, but these hydrophilic carbon surfaces also display increased hydrophobic character due to the enhanced energy barriers arising from the stronger adsorption. However, the effect of fluorination on the hydrophobicity and hydrophilicity of CDCs has not been investigated experimentally, although studies with activated carbons have been reported.48,49 In our previous experimental work50 we have studied the effect of fluorine-doping of silicon carbide-derived carbon (SiCDC) on its structure as well as CO2 adsorption. The present work contributes to the understanding of the effect of fluorination of microporous SiCDC on experimental water adsorption, to enable tuning of the hydrophobic−hydrophilic characteristics of

2. METHODOLOGY 2.1. SiCDC Synthesis and Fluorine Doping. We prepared SiCDCs in our laboratory by chlorination of commercial micrometer-sized SiC powders provided by Alfa Aesar as described previously in detail in our previous work.51 The temperature that was used for the SiCDC synthesis was 1073 K. Three different fluorine-doped SiCDCs denoted LF (low fluorinated), MF (medium fluorinated), and HF (high fluorinated) were prepared by solid−gas reaction of CDC with diluted fluorine gas for different times (10, 20, and 60 min, respectively). Before fluorination the samples were initially outgassed overnight in the oven under primary vacuum at 393 K to eliminate adsorbed molecules. A passivated nickel reactor covered with NiF2 was used for the fluorination of SiCDC. After outgassing, the oven temperature was cooled to room temperature and the samples were subjected to a mixed gaseous atmosphere having various F2/N2 ratios for the above duration. We used gaseous molecular fluorine purchased from Solvay Fluor (purity 98−99% V/V with maximum 0.5% V/V HF and other impurities, primarily O2/N2, at approximately 0.5% V/V) in the fluorination. Upon the completion of fluorination, we applied a nitrogen purge to flush the residual molecular fluorine. A detailed study of the effect of fluorine-doping of SiCDC on its structure, morphologies, and also CO2 adsorption has been presented in our recent publication.50 Different techniques including high-resolution transmission electron microscopy (TEM), Fourier transform infrared (FTIR) and Raman spectroscopies, and thermogravimetric analysis (TGA) techniques were used for the structural characterization of virgin and fluorinated samples in our previous work.50 In the previous work we presented the Ar adsorption isotherms at 87 K and CO2 adsorption at 273 K. The surface area and pore volume values as well as pore size distributions of each sample were obtained from the experimental Ar and CO2 isotherms by applying appropriate density functional theory (DFT) algorithms and Dubinin−Radushkevich (DR) equations. Table 1 provides key sample characterization data. 2.2. X-ray Photoelectron Spectroscopy. We have conducted X-ray photoelectron spectroscopy (XPS) analysis with a VG Scientific Escalab 220iXL X-ray photoelectron spectrometer equipped with a monochromatized Al Kα (1253.6 Table 1. Textural Characteristics of Virgin and Fluorinated Samples Obtained Using DFT50 argon

18596

CO2

sample

pore volume (cm3/g)

surface area (m2/g)

pore volume (cm3/g)

surface area (m2/g)

virgin LF MF HF

0.519 0.367 0.241 0.164

1527.3 1118.5 642.4 401.9

0.236 0.205 0.172 0.091

1955 1754 1402 854

DOI: 10.1021/acs.jpcc.6b04655 J. Phys. Chem. C 2016, 120, 18595−18606

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The Journal of Physical Chemistry C eV) source. Some details on the XPS have been presented in our recent work.50 The survey spectra were used to determine the atomic content of carbon, oxygen, and fluorine on the surface of the material. 2.3. NMR Experiments. NMR experiments were performed at ambient temperature using a Bruker Avance spectrometer with working frequency of 282.2 MHz for 19F. A magic-angle spinning (MAS) probe (Bruker) with a 4 mm rotor was used. MAS spectra were obtained using a simple sequence performed with a single π/2 pulse length of 4.0 μs. 19 F chemical shifts were referenced to CF3COOH. 2.4. Water Adsorption Experiments. To obtain the water adsorption−desorption isotherms at 303 K, the samples were first degassed at 473 K using our volumetric Micromeritics ASAP2020 adsorption analyzer. Then the samples were cooled to ambient temperature, and the isotherm measurements were conducted using the analysis port of the Micromeritics ASAP2020 instrument. 2.5. Water Adsorption Isotherm Models. Here, we report the application of the Dubinin−Serpinsky (DS) equation,9 the equation proposed by Barton et al.,11,33 and the model proposed by Talu and Meunier35 to the water adsorption isotherms measured for virgin and fluorinated SiCDC samples. Here we briefly discuss these models and equations. The DS equation is still widely applied to describe the water adsorption mechanism on carbons because of its simplicity32 while considering the key mechanism of water clustering around primary surface groups. The original DS model, which covers only the relative pressures until the upswing of the isotherm, is presented as DS‐1:

a ch cμ = 0 1 − ch

which appears to perform better than the DS-2 model. The BEM equation is obtained by the introduction of the term (1 − exp[−k2(cμ − ac)2]) and a fourth parameter, ac. Barton et al.11,33 have suggested that the parameters k and ac initiate the reduction in adsorption and k controls the rate of this reduction as adsorption proceeds. Barton et al.33 have shown that ac values are always larger than those of saturation adsorption, as, at P/P0 = 1, while not assigning any physical meaning to ac.32,33 Gauden34 explains that this parameter (ac) is an indication of the total hypothetical number of adsorbed water molecules, equal to or larger than as, needed to saturate all possible adsorption centers when the water molecules are ideally arranged. In this work we have also applied the proposed model by Talu and Meunier35 to the water adsorption isotherms measured for virgin and fluorine-doped SiCDC samples to investigate the water adsorption mechanism. Talu and Meunier35 have suggested a semiempirical model of water adsorption in micropores using association theory, which considers water molecule cluster creation on the adsorbent surface. This model is expressed by the equations Talu−Meunier:

h=

cμ (2)

where k (g/mmol) is a constant involved in decreasing active site concentration. This parameter represents the loss of the secondary adsorption sites. Parameters c, a0, and k are fitting parameters, with c and a0 being similar to those in DS-1. In the derivation of the above equation, the term (1 − kcμ) represents the decrease in adsorption sites as adsorption proceeds. However, it has been shown that the DS-2 equation does not satisfactorily predict the entire water adsorption isotherm, particularly their final section, for largely microporous carbons.11,33 Subsequently, Barton et al.33 proposed the empirical isotherm BEM:

h=

ψ=

−1 + (1 + 4Kξ)1/2 2K

(5)

ξ=

NmN Nm − N

(6)

3. RESULTS AND DISCUSSION 3.1. Surface Chemistry and Physicochemical Characterization. Understanding the hydrophobicity and hydrophilicity of fluorine-doped carbons requires fundamental knowledge of the nature and the amount of surface functional

cμ ca0 + ccμ(1 − exp[ −k 2(cμ − ac)2 ])

(4)

where H0 is the inverse of the Henry’s law constant, K a parameter related to the equilibrium constant for the cluster formation in micropores, Nm the saturation capacity of the pore, and N the amount of adsorbed water (mmol/g) at pressure P.35 Talu and Meunier35 state that although a single equation of state (Volmer EOS) is used for the derivation of their model, their association theory is not strictly a homogeneous surface model. However, they explain that in their model the vertical interactions appear in the Henry’s law constant as a single value, as in homogeneous models. This is different from the heterogeneous models in which the adsorption potential is a canonical average. In this association theory there is an infinite number of different energy sites for the molecules due to clustering, in which the energy of adsorption of each molecule equals to the summation of the ratio of the single vertical interaction value and cluster size and the energy of reaction. Clusters with different size can occupy a single site, and the adsorption potential can take any value from zero to the Henry’s law value.35 They have used water adsorption on activated carbon to test this theory and have reported good fits. McCallum et al.19 have successfully used this theory for the interpretation of water adsorption isotherms predicted by simulation for various pore widths. Mowla et al.52 have also used this model to describe experimental data for different types of activated carbons.

(1)

c(a0 + cμ)(1 − kcμ)

⎛ ψ ⎞ H0ψ exp⎜ ⎟ 1 + Kψ ⎝ Nm ⎠

where

where a0 (mmol/g) is the concentration of the surface functional groups; h is the relative pressure (P/P0); P and P0 are the equilibrium and saturation pressure, respectively; cμ (mmol/g) is the amount of water adsorbed; and c (unit-less) represents the equilibrium constant. Assuming that water clusters are created at larger relative pressures caused by further adsorption on the carbonaceous material along with decrease of secondary adsorption sites, and also with similar initial stages of water adsorption mechanism as DS-1, the implicit DS-2 model11,32 is obtained as DS‐2:

P=

(3) 18597

DOI: 10.1021/acs.jpcc.6b04655 J. Phys. Chem. C 2016, 120, 18595−18606

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fluorination. The XPS F 1s spectra of the fluorinated samples confirmed that the weakened covalent CF bonds were more dominant when the fluorination level increased. MAS NMR spectroscopy on 19F nucleus was also used to identify the various fluorinated groups and the nature of C−F bonds for the LF, MF, and HF samples. Deconvolution of the NMR spectra, as shown in Figure 2, led to identification of the different covalence of carbon−fluorine bonds and relative contents in the bulk material. The CF3 and CF2 groups correspond to the −5 ppm line and −45 ppm broad line, respectively. These shifts are likely to appear in the case of decomposition. The content of CF3 groups were at the limit of detection ( 0 for all samples and is estimated to be equal to 6.13, 5.81, 6.32, and 5.11 for the virgin, SiCDC-LF, SiCDC-MF, and SiCDC-HF, respectively. This behavior indicates thatthe hydrophobic character of the virgin, low, and medium fluorinated SiCDC samples does not change significantly and remains almost constant after fluorination, while the decrease in the value of [d2Cμ/dh2]/a0 at Cμ = 0 for SiCDC-HF is a clear indication of decrease in the hydrophobicity of the high fluorine-doped SiCDC sample. This behavior is probably due to the negligible effect of fluorine at low (6.2%) and medium (8.7%) levels of fluorination, while at high levels of fluorination (30.7%), the effect of fluorine is more prominent which reduces the hydrophobicity. The decreased hydrophobic character of HF sample is also consistent with the XPS and NMR analysis observation in which strong C−F covalent bonds decrease in the HF sample while weakened covalent C−F bonds significantly increase. Parmentier et al.49 have argued that the weakened covalent C−F bonds react with water during their contact with water and are converted into C−OH groups. However, Li et al.,48 who have reported fundamentally different isotherms for the fluorinated microporous carbon fibers, suggest fluorination can create perfect hydrophobicity in the carbonaceous microporous system in which fluorinated carbon micropores repel strongly water without formation of water clusters. While the nature of the C− F bonding in their experiments is not discussed by Li et al.,48 we may speculate that the increased hydrophobicity in their experiments is due to an increase in strong C−F bonds. We have also plotted the pore volume and surface areanormalized isotherms of H2O at 303 K, based on the structural properties obtained from argon adsorption, to investigate the effect of fluorine−water and fluid−fluid interactions in the fluorine-doped and virgin SiCDC samples. Figure 7a,b shows that the normalized isotherms differ from each other, with the fluorine-doped isotherms shifting to higher adsorbed values

Figure 8. Experimental adsorption−desorption isotherms of water vapor at 303 K normalized by water-based pore volume.

isotherms are overlapping at high relative pressures as expected, because at high pressures the water density (used as a basis for calculating the water-based pore volume) is approached. On the other hand, at low relative pressures the adsorbed amount of water is higher in fluorine-doped samples compared to the virgin SiCDC sample, which indicates increase of the fluorine− water and fluid−fluid interactions in fluorine-doped SiCDC samples as fluorination level increases, which shifts the water amount adsorbed in the normalized isotherms to higher adsorbed amount of H2O. The increase of the fluorine−water and fluid−fluid interactions is also evident from the desorption branch of the fluorinated SiCDC samples, which displays higher 18603

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H2O compared to simulations for the fluorine-doped SiCDCs, while argon-based values of pore volume and surface area obtained from experiment are considerably lower. This is most likely due to the differences in the SiCDC model constructed by simulation and the experimentally synthesized SiCDC and fluorine-doped samples and the difference in the siting of the fluorine in simulation and experiment. The lower argon-based pore volume in experimental samples compared to those obtained from simulation also suggests an accessibility problem for argon in the experimental virgin and fluorine-doped samples. The larger amount of adsorbed H2O in experiment compared to the simulation is due to the presence of oxygenprimary sites in the experimental samples, leading to the porefilling below the saturation pressure, while in the SiCDC model constructed by simulation, pore-filling does not occur below the saturation pressure.

water uptake than the virgin sample in the low relative pressure regions. The desorption branches show hysteresis due to the strong hydrogen bonds formed between the water molecules. The change of the shape of water adsorption hysteresis reflects the variation of the surface chemistry and composition of the carbon pores as discussed in XPS and NMR analysis. Although all samples yield remarkable hysteresis, as seen in Figures 4 and 8, the desorption equilibria of fluorinated samples exhibit water content higher than that of the virgin sample, and the width of the hysteresis loops increases with increasing level of fluorination. Parmentier et al.49 have also noted similar hysteresis for water adsorption on virgin and fluorine-doped carbons, showing lower water adsorption on fluorinated samples, indicative of increased hydrophobicity, but higher adsorption on the desorption branch, indicating increased hydrophilicity. 3.5. Comparison with Simulation. In our previous work50 it was demonstrated that experimental surface area and pore volume values decrease with increase in fluorination level, in agreement with the trend predicted using fluorinedoped models constructed from the virgin carbon structure derived using hybrid reverse Monte Carlo simulation of SiCDC.41,56 However, the values predicted by simulation were considerably larger than the experimental values, suggesting that the siting of the fluorine in the simulation differs from that attained in the experiment. Here we compare the water-based simulation results with our experimental findings to show the fluorination effect on water vapor adsorption. Farmahini et al.41 have shown that for the virgin sample the pore-filling process does not begin until the pressure approaches the experimental saturation pressure (P/P0 ∼ 1.0), while in our study we observe abrupt increase of water adsorption in the experimental virgin SiCDC. This pattern in our experimental results is similar to grand canonical Monte Carlo (GCMC) simulations for the activated carbons in which it is shown that a pore-filling mechanism gives rise to the amount of water adsorbed at medium relative pressure.28,57 Nguyen and Bhatia1 have also reported the same pattern for their activated carbon fiber (ACF-15) model; however, Farmahini and Bhatia58 have demonstrated that without activated sites pore filling of disordered SiCDC and activated carbon fiber occurs only after the saturation pressure is exceeded. Therefore, the steep water uptake at medium relative pressure in our experimental virgin SiCDC might be due to the existence of significant numbers of oxygen-primary adsorption sites which act as nuclei for the formation of H2O clusters. These oxygen-containing sites also shown in the XPS analysis encourage the growth of the water clusters leading to the carbon pore filling. In simulation works from Brennan et al.24 and Liu and Monson,59 it is also shown that H2O adsorption is greatly enhanced in the presence of activated surface sites. In the Farmahini et al.41 work, their GCMC simulations have also shown that fluorination significantly enhances water uptake and that the amount of H2O adsorbed in the fluorine-doped increases with the level of fluorination, consistent with our observation of enhancement of water uptake for low relative pressures. However, they have observed that for the highly fluorinated SiCDC sample, d2Cμ/dh2 < 0 at low relative pressures, an indication of hydrophilicity, while in our experiments d2Cμ/dh2 > 0 for the virgin and all of the fluorine-doped samples at low relative pressures, and the hydrophobicity is almost constant for the LF and MF samples. Experimental results also show a larger amount of adsorbed

4. SUMMARY AND CONCLUSIONS To demonstrate the role of fluorine doping on the adsorption behavior and the hydrophilic−hydrophobic character of the synthesized silicon carbide-derived carbon, we successfully fluorinated the synthesized SiCDC samples at three different levels at room temperature. The XPS and NMR analysis confirmed that different C−F bonds were created regardless of the fluorination level, showing that strong covalent C−F bonds decrease on fluorination while C−F bonds with weakened covalence increase. Water adsorption isotherms have been volumetrically measured on fluorinated and virgin SiCDC samples. The hydrophilic−hydrophobic nature of the fluorinated SiCDC has been studied by water adsorption and compared with the virgin SiCDC. It is found that the amount of water adsorbed is correlated to the fluorination level. The weakening of the hydrophobic character of the high fluorinated samples is evidenced by calculating the value of d2Cμ/dh2 at Cμ = 0, which is lower than the values obtained for the LF and MF samples, supporting the presence of the more hydrophilic surface groups and the prominent effect of fluorine in the HF sample. The decreased hydrophobicity in the highly fluorinated sample was attributed to the decreased amount of strong C−F bonds on fluorination along with the increase of C−F bonds with weakened covalence, as observed in the XPS and NMR analysis. It appears that in the pore volume and surface areanormalized isotherms, the fluorine-doped isotherms shift to higher adsorbed value compared to the virgin samples, and the adsorption/desorption branch of the virgin sample is always below the branches of the fluorinated samples. The findings from this research support the recent molecular simulationbased studies of water adsorption in virgin and fluorinated SiCDCs presented from this group.41 We have applied different semiempirical model equations to the water adsorption isotherms of virgin and fluorinated SiCDCs developed for adsorption isotherms of water on porous carbonaceous materials. We conclude that the modified BEM model equation proposed by Barton et al.33 is one of the most suitable equations for the water adsorption on the virgin and fluorinated SiCDCs. It was noticed that for the fluorinated SiCDCs, irrespective of the isotherm model, the water-based pore volumes are larger than the pore volumes obtained based on the argon adsorption. This is likely due to the blockage of fluorine functional groups for nonpolar argon while allowing H2O molecules to form clusters in the pores. Furthermore, it is shown that increasing fluorination level gives rise to a pore accessibility problem of argon at 87 K. The adsorption 18604

DOI: 10.1021/acs.jpcc.6b04655 J. Phys. Chem. C 2016, 120, 18595−18606

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The Journal of Physical Chemistry C isotherms of water on virgin and fluorine-doped SiCDCs show perceptible adsorption hysteresis, in which desorption on fluorinated samples exhibits higher amount of adsorbed water, and the width of the hysteresis loops increases with increasing level of fluorination because of the variation of the surface composition of carbonaceous pores.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +61 7 3365 4263. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research has been supported by a grant (No. DP150101824) from the Australian Research Council under the Discovery Scheme. We thank Dr. Barry Wood for the XPS analysis carried out at the Centre for Microscopy and Microanalysis, the University of Queensland.

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