Polyvinyl pyrrolidone-assisted synthesis of size-tunable polymer spheres at elevated temperature and their conversion to nitrogen-containing carbon spheres

Journal of Colloid and Interface Science 549 (2019) 162–170

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Polyvinyl pyrrolidone-assisted synthesis of size-tunable polymer spheres at elevated temperature and their conversion to nitrogencontaining carbon spheres Pramila P. Ghimire, Arosha C. Dassanayake, Nilantha P. Wickramaratne, Mietek Jaroniec ⇑ Department of Chemistry and Biochemistry, Kent State University, Kent, OH 44242, USA

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 21 February 2019 Revised 15 April 2019 Accepted 17 April 2019 Available online 19 April 2019 Keywords: Polyvinyl pyrrolidone Size-tunable carbon spheres Adsorption Nitrogen-doped carbons

⇑ Corresponding author. E-mail address: [email protected] (M. Jaroniec). https://doi.org/10.1016/j.jcis.2019.04.059 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

a b s t r a c t Polyvinyl pyrrolidone (PVP)-stabilized polymer spheres synthesized at elevated temperature by Stӧberlike method are carbonized to obtain carbon spheres. Their size is tunable in the range from 875 nm to 60 nm by adjusting the amount of PVP during synthesis at elevated temperature. These spheres are obtained by using resorcinol and formaldehyde as carbon precursors, ethylene diamine (EDA) as nitrogen source and basic catalyst, and PVP as stabilizer and an additional source of nitrogen dopant. A synergistic effect is shown between (i) elevated temperature that facilitates the formation of a large number of small polymeric nuclei and (ii) varying amount of PVP that controls the growth of formed nuclei by hindering polymerization of resorcinol and formaldehyde. The elevated temperature synthesis not only produces monodispersed carbon spheres but also eliminates the need for hydrothermal treatment that generally requires high-pressure autoclave vessel. The nitrogen content increases from 3.0 wt% to 5.0% with increasing PVP amount. Besides nitrogen originating from ethylene diamine, an additional amount of doped nitrogen in carbon spheres is supplied by PVP during its decomposition. The resulting carbon spheres were subjected to the post-synthesis CO2 activation in order to improve their structural properties. The surface area of activated carbon spheres increased almost twice from (516 m2/g
581 m2/g) to (1010 m2/g
1060 m2/g). A significant enhancement of microporosity as well as presence of nitrogen species in activated carbon spheres resulted in high CO2 uptake at ambient temperatures. The tunable size, high microporosity, nitrogen doping and cost-effective synthesis make these carbon spheres attractive for a variety of uses ranging from adsorption, catalysis, electrode materials to biomedical applications. Ó 2019 Elsevier Inc. All rights reserved.

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1. Introduction Over the past several years, porous carbon spheres have attracted a significant attention as they integrate unique features of porous carbon materials and spherical morphology [1–3]. Some of the remarkable properties such as regular geometry, good liquidity, large surface area, tunable porosity and high chemical inertness are exhibited by the porous carbon spheres that make these materials very popular in many different fields including adsorption, catalysis, energy storage, and separations [1,4–9]. Recently, numerous studies report potential applications of carbon spheres even in some sophisticated applications such as drug delivery, biodiagnostics, etc. provided that their size is reduced below 200 nm to avoid their expulsion by phagocytes [10]. Even though carbon nanotubes, nanorods etc. have been demonstrated for these applications [11–13], carbon spheres are more preferable because of the absence of sharp edges and also lower diffusion distances [10,14]. The synthesis of carbon spheres with the well-defined size, high monodispersity and tunable surface properties is also highly desirable for energy-related applications like supercapacitor electrodes, lithium ion batteries, and so on [15,16]. For instance, in the case of small-sized carbon spheres, the diffusion distance needed for transport of electrolytes to micropores is significantly lowered that results in higher capacitance and better rate performance of supercapacitors [16,17]. On the other hand, monodispersity is regarded as a crucial factor to be considered in the aforementioned application as it offers high packing density, and creates uniform pores between the particles that are easily accessible for electrolytes and consequently lower the charge-transport resistance [18,19]. Up-to date, there are several studies aimed to obtain monodispersed carbon nanotubes and carbon nanoparticles using template-based, surfactant-assisted and template-free synthesis strategies [20]. Although, template-free strategies like direct pyrolysis of polymer spheres [21], thermal treatment of hydrocarbons offer facile ways for the synthesis of carbon spheres [20], the limited number of thermally stable carbon precursors and formation of large particles are current disadvantages of this thermal treatment route. Nevertheless, emulsion polymerization leads to the synthesis of sub-micrometer-sized particles; serious agglomeration and low carbon yield are some of the major drawbacks associated with this technique [22]. Stӧber-like synthesis is the most widely adopted method based on sol–gel technique that involves the formation of primary polymer colloids, which are then converted to polymer spheres and finally to uniform, monodispersed colloidal carbon spheres upon thermal treatment [23]. However, the particle size obtained by Stӧber-like method is around 500 nm or slightly higher [23]. Different strategies have been employed to reduce the size of carbon spheres such as change in the reaction temperature, reagent concentration, reaction time, and so on [14,23–27]. For example, Lu et al. obtained polydopamine-derived carbon spheres with size ranging from 780 nm to 120 nm by adjusting the NH3/dopamine molar ratio [14]. Friedal and co-workers utilized the reaction between melamine and formaldehyde in a basic solution to synthesize polymer spheres, which upon subsequent carbonization at different temperatures varying from 500 °C to 800 °C produced carbon spheres of sizes between 860 nm and 400 nm [24]. Likewise, Qian et al. discovered ‘‘seeded” strategy, where colloidal seeds in the range of 10 to 20 nm were firstly obtained by controlled polymerization of phloroglucinol and terephthalaldehyde, and next enlarged by subsequent addition of resorcinol and formaldehyde to produce polymer nanospheres. These spheres upon carbonization were transformed into carbon particles with tunable size between 30 and 90 nm [25,27]. Dong et al. reported fabrication of polymer and carbon spheres with size ranging from

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650 nm to 30 nm by varying the catalyst/water and resorcinol/ water molar ratios during polymerization of resorcinol and formaldehyde in presence of Lysine as a basic catalyst. However, the resulting polymer and carbon spheres were polydispersed and aggregated [26]. Surfactants such as Pluronic F127, in addition to their role in controlling the size of metallic nanoparticles, they are also useful for tailoring the size of polymer and carbon spheres [28]. For instance, Choma and co-workers illustrated the size dependence of polymer spheres with the addition of various amount of Pluronic F127 block polymer. When a small amount of F127 (0.01 g) is used, the resulting polymer spheres were big and irregular. As amount of F127 increased (0.4 and 1 g), the size of polymer spheres was reduced and finally reached 30–40 nm [28]. Very recently, Fang and co-workers have successfully developed low-concentration (10-7 M F127 block copolymer) hydrothermal route to form highly ordered body-centered cubic mesoporous structure of carbon spheres with uniform size. The size was highly controlled from 20 nm to 140 nm by changing the reagent concentration while keeping all other parameters constant [29]. Later, the study carried out by Zhang’s group demonstrated that the modulation of carbon chain in the non-ionic surfactant can be used for the efficient size control of carbon spheres [30]. Likewise, Qiao et al. extended the well-known Stӧber method to synthesize nitrogen-doped mesoporous carbon spheres by adopting a strictly controlled hydrothermal treatment using dual surfactants as a soft-template in aqueous-alcoholic solutions. The size and morphology were highly tunable by changing different experimental parameters like ethanol–water ratio, concentration of dual surfactant, etc. [31]. It is noteworthy that most of these syntheses involve an additional hydrothermal treatment (HT) that is generally performed to cure polymer and to obtain high quality carbon spheres without cracks [32,33]. However, hydrothermal treatment requires high temperature and high-pressure autoclave vessel that limit its scalable production. Similar to Pluronic F127, polyvinyl pyrrolidone (PVP) also plays a significant role in controlling the size of metal nanoparticles. PVP adheres to the metal surface due to the bonding formed between metal ion and lone pair of electrons of nitrogen and oxygen in PVP. The steric hinderance from long vinyl polymeric chain prevents the agglomeration of the synthesized nanoparticles [34]. Similar phenomenon was demonstrated in the case of nickel and palladium nanoparticles [35]. PVP has carbonyl group and nitrogen atoms with lone pair of electrons [34,36], thus can form hydrogen bond with the hydroxyl group of methylated polymer spheres. Therefore, the size control of metal nanoparticles by capping action of PVP was anticipated to be true in the case of polymer spheres too. Moreover, PVP is a source nitrogen that is doped in the carbon spheres during thermal treatment that could be highly beneficial for the improvement of surface polarity and electron donor properties of the carbon matrix [37,38]. Another set of reports indicates the significant role of reaction temperature in controlling the size of polymer and carbon spheres [39]. For example, low initial reaction temperature would slow the reaction rate initiating fewer nuclei in the reaction solution. As a result, larger polymer spheres are formed. On the contrary, higher temperature minimizes the energy barrier and leads toward faster reaction rate resulting in numerous smaller primary nuclei in the solution [40,41]. Besides, the high temperature ensures production of uniform polymer and carbon spheres. Till-date, the number of reports on the development of synthetic strategies at elevated temperatures to improve monodispersity of carbon spheres is very small [19,39]. For instance, the temperature-dependence of nucleation and growth was clearly evidenced by Wang et al., who reported size-tunable polymer and carbon spheres by using benzoxazine chemistry [39]. By precise control of the initial reaction

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temperature at 15 °C, 24 °C and 28 °C, they successfully synthesized uniform and monodispersed carbon spheres of 150 nm, 105 nm and 95 nm, respectively. Similarly, Zhao and co-workers adopted Stӧber-like synthesis to polymerize 3-aminophenol and formaldehyde at various initial reaction temperatures that ultimately led to size tuning of carbon spheres [19]. Yet, these studies inevitably require post-synthesis hydrothermal treatment that stands as a drawback. To the best of our knowledge, this work reports for the first time the effect of combined role of elevated temperature (120 °C) and varying amount of PVP to synthesize highly uniform, monodispersed, size-tunable nitrogen-containing polymer and carbon spheres by slightly modified Stӧber-like synthesis. A synergistic effect originates from both higher temperature and PVP; higher temperature produces many smaller polymeric nuclei, which later grow into polymeric spheres of controlled size depending upon amount of polyvinyl pyrrolidone used as a stabilizer. Moreover, this method reduces the use of hydrothermal treatment that requires high-pressure autoclave vessel making this synthetic approach industrially scalable. The elevated temperature or PVP alone reduces the size almost by half, which is further reduced when synthesis is carried out at higher temperature in the presence of PVP due to a synergistic effect. Moreover, this strategy also produces highly porous carbon spheres with effective nitrogen doping amount (even up to 5.0%), which is beneficial for various uses such as adsorption as illustrated for CO2, catalysis and electrochemical applications. 2. Experimental Materials. Resorcinol (C6H4(OH)2, 98%), formaldehyde (HCHO, 37 wt%), ethylenediamine were purchased from Acros Organics (Geel, New Jersey, USA). Polyvinyl pyrrolidone (PVP) was obtained from Fisher Scientific. Technical grade ethanol and deionized water were used in all experiments. 2.1. Synthesis of PVP-stabilized polymer spheres and carbon spheres A series of PVP-stabilized polymer spheres was synthesized by slightly modified Stӧber-like method [23,38]. In a typical synthesis, a mixture of 60 mL of DI water and 24 mL of ethanol (95% v/v) were added to a round bottom flask fitted with Leibeg’s condenser and placed in oil bath at 60 °C. To this solution, 0.45 mL of ethylene diamine were added under stirring and then the temperature was adjusted to 100 °C. Various amount of PVP (0.1 g, 0.2 g, 0.3 g, 0.4 g) was added to the reaction mixture and stirred for additional 20 min. Thereafter, 0.6 g of resorcinol was added and stirred till its complete dissolution. Later, 0.9 mL of HCHO was delivered to the solution and temperature was raised immediately to 120 °C. The resulting solution was stirred continuously for additional 48 h. The resulting brown colloidal solution was centrifuged and colloidal particles were separated, washed several times with DI water and dried overnight under hood. The samples were labelled as ETPS-x PVP (where ‘‘ET” refers to the synthesis at elevated temperature, ‘‘PS” represents polymer spheres and x = 0.1, 0.2, 0.3 and 0.4 is the amount of PVP added in grams). The brown dried powder was grinded and heated thermally at 1 °C/min up to 350 °C under flowing N2 and dwelled for 2 h. The temperature was further increased to 600 °C with the same ramping rate and was kept at this temperature for additional 2 h. The samples were labelled as ET-CSx PVP (where ‘‘ET” refers to the synthesis of PSs prepared at elevated temperature that were used to obtain carbon spheres ‘‘CS” and x = 0.1, 0.2, 0.3 and 0.4 denotes the amount of PVP added in grams). Analogous sample of carbon spheres was also synthesized using the same recipe as that for ET-CS-0.2-PVP except the order

of PVP addition; in the former sample PVP was added after addition of resorcinol. These spheres are denoted as ET-CS-0.2-PVP*. The post-synthesis activation of the synthesized carbon spheres was carried out by heating carbon spheres until 850 °C ramping at 10 °C/min under CO2 flow and then holding at the same temperature for 4 h. The samples were labelled as ET-CS-x-PVP-act (where x = 0.1, 0.2, 0.3 and 0.4 is the amount of PVP in grams added during the synthesis). ET-CS-0.2-PVP*-act was obtained after activation of ET-CS-0.2-PVP*. To investigate the effect of temperature and PVP separately, polymer spheres without PVP and with 0.2 g of PVP were synthesized at room temperature (RT) followed by 24 h hydrothermal treatment, centrifugation and drying. The polymer spheres were labelled as RT-PS-0 PVP and RT-PS-0.2 PVP respectively. The carbonization and activation were carried out in a similar way as mentioned above and the samples were labelled as RT-CS-0 PVP, RT-CS-0.2 PVP (after carbonization) and RT-CS-0 PVP-act and RTCS-0.2 PVP-act (after activation). Here ‘‘RT” refers to the synthesis performed at room temperature followed by 24 h-hydrothermal treatment at 100 °C. 2.2. Characterization Nitrogen adsorption isotherms were measured at
196 °C on ASAP 2010/ASAP 2020 volumetric analyzers (Micromeritics, Inc., Norcross, GA). All samples were degassed under vacuum at 200 °C for 2 h prior to the adsorption measurements. Thermogravimetric (TG) and differential thermogravimetric (DTG) profiles were obtained on Hi-Res 2950 and Q-500 thermogravimetric analyzers (TA Instruments, Inc., New Castle, DE) from 25 to 800 °C under flowing nitrogen (polymer spheres) and under air (ET-CS-x-PVPact) at a heating rate of 10 °C min
1. CO2 measurements were performed for carbons at both 0 °C and 25 °C using an ASAP 2020 volumetric analyzer (Micromeritics, Inc., Norcross, GA). Morphology was examined on an environmental scanning electron microscope (ESEM) using an FEI model Quanta 450 FEG (FEI. Hillsboro, OR). A Bruker Vector 22 FTIR (Fourier-transform infrared) spectrometer was used to collect FTIR spectra in the frequency range of 4000–500 cm
1. 2.3. Calculations The Brunauer–Emmett–Teller specific surface areas (SBET) of the samples were calculated from N2 adsorption–desorption isotherms in the relative pressure range of 0.05–0.2 [42]. The specific total pore volumes (Vt) of these materials were obtained by using the amount of nitrogen adsorbed at relative pressure (P/Po) = 0.99 [43]. The 2D-nonlocal density functional theory (2D-NLDFT) method for heterogeneous surfaces was used to obtain the pore size distribution (PSD) with SAIEUS software provided by Micromeritics [44]. The ultramicropore volume (for pores below 0.7 nm, V0.7 nm) and micropore volume (for pores below 2 nm, V2 nm) for the samples studied were obtained from cumulative pore volume calculated using the 2D-NLDFT method. 3. Results and discussion The high-resolution scanning electron microscopy (HRSEM) images of RT-CS-0 PVP, RT-CS-0.2 PVP, ET-CS-x PVP (x = 0, 0.1, 0.2, 0.3 and 0.4 denotes the mass of PVP in grams), ET-CS-0 PVPact and ET-CS-0.2 PVP-act are displayed in Fig. 1 showing their spherical morphology. Since in the current study the carbon spheres were synthesized at elevated temperature using different amounts of PVP, the effect of temperature and PVP on the particle size needed to be studied separately in order to fully understand

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RT-CS-0 PVP

750

800

850 900 950 Particle size (nm)

300

500 nm ET-CS-0.1 PVP

0

100 200 Particle size (nm)

350

400 450 Particle size (nm)

150

500 nm

ET-CS-0.2 PVP

ET-CS-0.3 PVP

0

50 100 150 Particle size (nm)

200

0

50 100 150 Particle size (nm)

ET-CS-0 PVP-Act

150

200

0

50 100 150 Particle size (nm)

350

200

500 nm

500 nm

ET-CS-0.4 PVP

200 250 300 Particle size (nm)

500 nm

300

500 nm

500 nm

ET-CS-0 PVP

RT-CS-0.2 PVP

200

500 nm

ET-CS-0.2 PVP-Act

250 300 350 400 Particle size (nm)

0

50 100 150 Particle size (nm)

200

500 nm

Fig. 1. HRSEM images of RT-CS-0 PVP, RT-CS-0.2 PVP, ET-CS-0 PVP, ET-CS-0.1 PVP, ET-CS-0.2 PVP, ET-CS-0.3 PVP, ET-CS-0.4 PVP, ET-CS-0 PVP-act and ET-CS-0.2 PVP-act. The insets show the particle size distribution of the carbon spheres obtained by size measurement of 100–120 spheres using HRSEM images.

the mechanism of formation of these spheres. As seen in Fig. 1, the carbon spheres synthesized at room temperature without PVP (RTCS-0-PVP) are larger with size centered at 875 nm. The size of the ET-CS-0 PVP spheres was drastically lowered to 325 nm with increasing reaction temperature to 120 °C. This pronounced reduction in size with an increase in the synthesis temperature might be due to lowering the energy barrier for nucleation that results in the formation of smaller polymeric nuclei [40,41]. In contrary, the reduction in size to 411 nm was observed for carbon spheres, RTCS-0.2 PVP, fabricated in the presence of PVP alone that could be due to the adherence of PVP on emulsion droplets hindering the polymerization of resorcinol and formaldehyde. Excitingly, when these two effects are combined, the synergistic effect is observed for the spheres prepared in the presence of PVP and at higher temperature. For instance, the spheres with particle size of 325 nm observed for ET-CS-0 PVP were decreased to 223 nm with the addition of 0.1 g of PVP. It is interesting to note the presence of low fraction of small-sized particles of 70 nm and 50 nm in this sample too. However, this polydispersity might have occurred due to smaller amount of PVP, which was insufficient to cover the surface of all polymer spheres uniformly. A further increase in the PVP amount to 0.2, 0.3 and 0.4 g caused a significant decrease in the particle size to 93 nm, 87 nm and 63 nm, respectively. As mentioned earlier, the decrease in the size of particles could be due to the presence of PVP around emulsion droplets, which hindered further polymerization of resorcinol and formaldehyde. Another possibil-

ity could be due to the increase in viscosity of reaction solution with increasing PVP amount that lowers the polymerization of resorcinol and formaldehyde [34,45]. The observed decrease in the yield of spheres with increasing PVP amount also supports the above explanation. A further increase in the amount of PVP to 0.5 and 0.6 g produced highly polydispersed particles including significant population of larger particles in addition to smaller ones that might be due to the formation of smaller polymer spheres that tend to agglomerate during high thermal treatment (Fig. S1). Ideally, the PVP polymer instantaneously is uniformly distributed in the solution after its addition and adsorbs on the surface of particles. At higher concentration of PVP, the viscosity of the solution increases that may hinder the free movement of PVP polymer to cover the entire surface of spheres. This may lead to an extensive local adsorption of PVP at the point of initial contact between PVP and colloidal polymer particles, whereas some portion of the sphere remains exposed [46]. Also, the excess amount of PVP polymer increases the number of the extended loops and chains of the adsorbed PVP that become available to the exposed surface of another particle to link or bridge them together, which may possibly result in an aggregation during thermal treatment [47,48]. Thus, an optimum amount of PVP is required to decrease the size of carbon spheres and maintain their proper quality. Since the size reduction can partly occur during carbonization, we performed the HRSEM analysis of the as-synthesized polymer spheres (see Fig. S2 in supporting information) in order to under-

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stand the contribution of the thermal treatment on the size reduction. The size of ET-PS-0 PVP, ET-PS-0.1 PVP, ET-PS-0.2 PVP and ETPS-0.3 PVP polymer spheres (as shown in Fig. S2) is centered around 550 nm, 245 nm, 114 nm and 110 nm, respectively. High degree of polydispersity is observed for ET-PS-0.1 PVP with additional peaks around 298 nm, 268 nm, 103 nm and 74 nm. This result is in a good agreement with high polydispersity observed for ET-CS-0.1 PVP and ET-CS-0.1 PVP-act. The trend of decreasing the sphere size with increasing amount of PVP even in polymer spheres (before carbonization) clearly indicates that the ability of particle size tuning is associated with the PVP amount and the polymerization reaction itself carried out at elevated temperature (120 °C). However, further slight reduction of carbon particles occurs during activation because of partial oxidation or burning off the outer carbon layer in flowing CO2 gas (Fig. S3 in supporting information). The synthesis was also carried out for PVP added after resorcinol; the resulting polymer spheres are labelled as ET-PS-0.2 PVP* (amounts of all the reactants were similar to those used for synthesis of ET-PS-0.2 PVP). The resulting carbon spheres, ET-CS-0.2 PVP* (shown in Fig. S1) are comparatively more agglomerated and polydispersed than ET-CS-0.2 PVP. In addition, the structural parameters obtained for ET-CS-0.2 PVP are better than those determined for ET-CS-0.2 PVP* (see discussion later). Since, the present study aims to make spherical materials with controlled size while ensuring their monodispersity, thus the option of adding PVP after resorcinol was not explored further. The observed difference in the quality of spheres caused by changing the order of addition of reagents could be as follows; dissolving PVP after adding resorcinol can disturb the formation of continuous polymeric PVP framework, which can cause the formation of shorter polymeric PVP chains, reducing its stabilization property [49,50]. The size reducing ability of PVP commonly used during the synthesis of metal nanoparticles was successfully extended to the sol– gel synthesis of polymer spheres and carbon spheres. Based on the mechanism of size control of metal nanoparticles with PVP that is due to strong bond formation of metal ions with nitrogen and oxygen lone pair of electrons of PVP [34], their role in tuning the size of

polymer spheres is anticipated as shown in Scheme 1. As previously reported, the reaction of formaldehyde and EDA in basic medium forms intermediate (structure 1), which upon reaction with excess of EDA, formaldehyde and resorcinol gives polymer framework with AOH, ANHA and ether groups (structure 2) [6,38]. The PVP in contact with the polymer network forms Hbond between above-mentioned functionalities and AC@O group and ANHA groups of PVP (structure 3). More PVP surrounds the polymer spheres with the increment of PVP that consequently hinders the polymerization and thus control the size of polymer spheres. The polymer spheres upon subsequent carbonization produce nitrogen-containing microporous carbon spheres of varying sizes. The participation of EDA itself in the polymerization helps to create N-containing polymer matrix [38]. Additionally, PVP is also the source of nitrogen that can be doped in the carbon framework during carbonization [51]. Namely, the observed significant increase in N % from 3.1% (ET-CS-0 PVP) to 4.1% (ET-CS-0.2 PVP) and 5.0% (ET-CS-0.4 PVP) can be ascribed to the PVP amount increase, which has high nitrogen content [51]. Since the carbon spheres were further activated with CO2 gas at higher temperature to increase the amount of ultramicropores [52], some fraction of nitrogen might be removed during this activation process, which resulted in lower N% values, namely 3.0%, 3.1% and 3.8% obtained for ET-CS-0 PVP-act, ET-CS-0.2 PVP-act and ET-CS-0.4 PVP-act, respectively. It is noteworthy to mention that the size of activated carbon spheres was slightly reduced because of oxidative behavior of CO2 at higher temperature that peels off carbon when it encounters carbon spheres (Fig. 1 and Fig. S3). For example, the size of final activated carbons ET-CS-0 PVP-act (310 nm) and ET-CS-0.2 PVPact (78 nm) are slightly lower than carbonized ET-CS-0 PVP and ET-CS-0.2 PVP samples (Fig. 1). Namely, CO2 diffuse into the pores, react with pore wall of carbon widening the pores by peeling the outer layer of carbon spheres. Similar observation was reported in several studies while post synthesis activation of carbon materials [38,53,54]. The effect of PVP on the stabilization of polymer resin spheres is further supported by the investigation of chemical structure

Scheme 1. Possible mechanism for the formation of size-tunable carbon spheres.

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around polymer spheres using FTIR. The FTIR spectra of pure PVP polymer, ET-PS-0 PVP (pure polymer spheres) and PVPincorporated polymer spheres (ET-PS-0.1 PVP, ET-PS-0.2 PVP and ET-PS-0.3 PVP) were obtained in the range of wavenumbers between 4000 and 500 cm
1 and shown in Fig. S4 (see supporting information). As can be seen in Fig. S4, a broad peak between 3400 cm
1 and 3100 cm
1 is visible for almost all the samples due to OAH stretching vibration. For pure PVP polymer, the signature peaks are observed at wavenumber 1655, 1426, 1287 cm
1 that correspond to the C@O stretching vibration, CH2 bending vibration and CAN vibrations respectively [55]. On the other hand, the spectra of RF polymer resins have peaks at 1613 cm
1, 1453 cm
1, 1233 cm
1 and 1100 cm
1 that can be attributed to the C@C stretching vibration and CH2 deformation. A peak at 1230 cm
1 refers to aromatic CAO and OAH stretching vibrations. All these peaks are the signature peaks for polymer resin as reported elsewhere [56]. For PVP-incorporated polymer spheres, in addition to the peaks observed for RF resin, there is a new peak around 1653 cm
1, the intensity of which increases with increasing PVP amount. Conversely, the intensity of peak at 1613 cm
1 assigned to C@C stretching vibration in the RF polymer resin network is lowered with increasing PVP amount showing the wrapping/ stabilizing behavior of polyvinyl pyrrolidone. Nitrogen adsorption–desorption isotherms measured for the carbon spheres at
196 °C are depicted in Fig. 2; panels A and B present data for non-activated and activated samples of varying PVP amount, respectively. The pore size distributions obtained from these isotherms are shown in Figs. S5 and S6 (see supporting information) respectively. The specific surface area and pore structural parameters are listed in Table 1. All adsorption isotherms except for ET-CS-0 PVP and ET-CS-0 PVP-act are type IV, with high N2 uptake at lower pressure region and distinct hysteresis loops at high pressure region clearly indicating presence of both micropores and mesopores; however, mesopores created might be due to the voids between the particles [57]. The adsorption isotherms for ET-CS-0 PVP and ET-CS-0 PVP-act are type I with high N2 adsorption plateau and absence of hysteresis loop indicating their solely microporous nature. Combining these results together with the HRSEM images, one can conclude that the space between the particles below 325 nm lies in mesopore range and for larger particles, the voids are in the macropore range for which adsorption hysteresis is not observed. This is further supported by the similar nitrogen adsorption–desorption isotherms and pore size distribution curves for RT-CS-0 PVP, RT-CS-0.2 PVP and their respective activated products RT-CS-0 PVP-act and RT-CS-0.2 PVP-act (see Fig. S7).

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As shown in Table 1, the specific surface area for non-activated (ET-CS-x-PVP) and activated (ET-CS-x-PVP-act) carbon spheres varies from 516 to 581 m2/g and 973–1060 m2/g, respectively. Unsurprisingly, the activated CS possessed higher surface area, which is mainly due to the creation of high volume of ultramicropores formed by etching some portion of carbon during high temperature CO2 activation process [7]. Namely, the volumes of ultramicropores lie in the range of 0.20–0.23 cm3/g and 0.36– 0.42 cm3/g for non-activated and activated samples, respectively. The micropore volumes for all the samples are almost similar, whereas the total pore volumes (combination of micro and mesoporous pore volumes) increase with increasing PVP until ET-CS-0.2 PVP and then decrease. The maximum total pore volume obtained for ET-CS-0.2 PVP is the result of optimal size of the spheres with maximum mesovoids between these particles. Interestingly, for the series of activated samples, the structural parameters are higher for ET-CS-0.1 PVP-act, which can be due to the combination of a small size reduction (because of activation) and the creation of larger voids due to insertion of small carbon spheres between larger ones as evidenced by HRSEM images shown in Fig. S3. The pore volume for ET-CS-0.2 PVP-act is reduced, which can be attributed to the reduction of the void size because of further reduction of spheres size during activation. The nitrogen adsorption-desorption isotherms measured for ETCS-0.2 PVP* and ET-CS-0.2 PVP*-act are shown in Supporting Fig. S8. The structural parameters like the BET surface area, micropore volume and total pore volume reported in Supporting Table S1 are slightly smaller for ET-CS-0.2 PVP* and ET-CS-0.2 PVP*-act as compared to those for ET-CS-0.2 PVP and ET-CS-0.2 PVP-act, respectively. Based on the existing literature, nitrogen-containing porous carbon spheres are potential candidates for CO2 adsorption [38]. Therefore, to validate high nitrogen doping and well-developed microporosity in carbonized and activated carbon spheres, the CO2 adsorption isotherms were measured at 0 °C and 25 °C and the results are displayed in Fig. 3. The CO2 adsorption uptakes at 0 °C and 1 bar for ET-CS-0-PVP, ET-CS-0.1 PVP, ET-CS-0.2-PVP ETCS-0.3 PVP and ET-CS-0.4 PVP are found to be 3.87, 3.43, 3.46, 3.50 and 3.54 mmol/g respectively. The highest adsorption value is for ET-CS-0-PVP due to the high volume of ultramicropores [7,9]. Even though the nitrogen content increases with increasing PVP amount, but at the ambient temperature, the volume of ultramicropores plays the major role in CO2 adsorption. It is well known that the size of ultramicropores is almost two times larger than the kinetic diameter of CO2 molecule [9]. Thus, the adsorption potential experienced by CO2 molecule from opposite walls in these

Fig. 2. Nitrogen adsorption–desorption isotherms for A) non-activated and B) activated carbon spheres studied.

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Table 1 Structural parameters of the carbon spheres studied. Sample

SBET m2/g

Vt cm3/g

V0.7nm cm3/g

V2nm cm3/g

Vmeso cm3/g

nCO2 mmol/g (0 °C)

nCO2 mmol/g (25 °C)

D ± r (nm)

N (%)

ET-CS-0 PVP ET-CS-0.1 PVP ET-CS-0.2 PVP ET-CS-0.3 PVP ET-CS-0.4 PVP ET-CS-0 PVP-act ET-CS-0.1PVP-act ET-CS-0.2PVP-act ET-CS-0.3PVP-act ET-CS-0.4PVP-act

516 543 581 538 521 1013 1083 1060 973 1055

0.28 0.53 0.58 0.46 0.49 0.52 0.82 0.78 0.72 0.76

0.23 0.21 0.22 0.22 0.20 0.38 0.41 0.36 0.40 0.42

0.24 0.23 0.23 0.23 0.22 0.45 0.47 0.45 0.42 0.46

0.03 0.17 0.33 0.22 0.21 0.05 0.35 0.30 0.28 0.29

3.87 3.43 3.46 3.50 3.54 5.67 5.90 5.53 5.75 5.77

2.87 2.52 2.49 2.53 2.72 3.77 3.85 3.59 3.72 3.83

324 ± 13 223 ± 20 93 ± 14 87 ± 13 63 ± 14 310 ± 30 213 ± 2 78 ± 8 66 ± 10 62 ± 12

3.1 4.1 4.1 4.2 5.0 3.0 3.3 3.1 3.3 3.8

Notation: BET specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method in the relative pressure range of 0.05–0.2; Vt is the total single-point pore volume obtained by conversion of the volume adsorbed at relative pressure = 0.99. V0.7nm and V2nm are the volumes of pores below 0.7 nm and 2 nm, respectively, obtained from cumulative pore volume calculated by the 2D-NLDFT method for carbons with slit-shaped pores. Vmeso is the mesopore volume obtained by subtraction of the micropore volume (V2nm) from the total single-point pore volume (Vt). nCO2 is the amount of CO2 adsorbed at 1 bar and 0 °C or 25 °C using adsorption analyzer ASAP 2020 manufactured by Micromeritics. D (nm) represents the average particle size of the carbon spheres obtained from the particle size distributions acquired by analysis of HR-SEM images for 100–120 spheres. r represents the standard deviation. N% denotes the nitrogen percentage obtained by elemental analysis.

Fig. 3. CO2 adsorption isotherms for carbonized samples (ET-CS-x PVP) at 0 °C (A) and 25 °C (C); for activated samples (ET-CS-x PVP-act) at 0 °C (B) and 25 °C (D).

pores is maximal [7]. The increase in the fraction of these micropores leads to the enhanced CO2 uptake. Similar trend of the CO2 uptake is visible for the activated carbon samples. For instance, the volume of ultramicropores decreases in the order of ET-CS0.1 PVP-act > ET-CS-0.3 PVP-act > ET-CS-0 PVP-act > ET-CS-0.2 PVP- act; and correspondingly, the CO2 uptake follows similar order i.e., 5.90, 5.77, 5.75 and 5.53 mmol/g, respectively. Physical adsorption decreases with increasing temperature [9], therefore the CO2 adsorption is higher at 0 °C as compared to that at 25 °C. Similarly, at 25 °C, the CO2 adsorption capacity primarily depends upon the volume of micropores. For the samples with

similar microporosity, the CO2 adsorption capacity increases with increasing nitrogen content. Obviously, the CO2 amount adsorbed by the activated carbons is higher than that on non-activated carbon spheres due to the higher fraction of ultramicropores. Therefore, two effects govern the overall CO2 adsorption on the ET-CS studied: the major one, the large volume of ultramicropores and the minor one, adsorption on basic nitrogen species present in carbon spheres. The TG analysis of pure PVP and PVP-stabilized polymer spheres was performed under nitrogen flow to investigate their thermal stability and the amount of PVP incorporated in the polymers

P.P. Ghimire et al. / Journal of Colloid and Interface Science 549 (2019) 162–170

Acknowledgments

100

Weight (%)

169

80

The HRSEM data were obtained at the lab of the characterization facility of the Advanced Materials and Liquid Crystal Institute.

60

Appendix A. Supplementary material

40

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.04.059.

PVP only ET-PS-0 PVP ET-PS-0.1 PVP ET-PS-0.2 PVP ET-PS-0.3 PVP ET-PS-0.4 PVP

20

References

0 0

200

400 Temperature (o C)

600

800

Fig. 4. Thermogravimetric (TGA) analysis of PVP and PVP-containing polymer spheres.

(Fig. 4). The differential thermogravimetric (DTG) curves correspond to the TG profiles are shown in Fig. S9. As can be seen in Fig. 4, almost all the samples show an initial weight change around 50–70 °C due to the removal of adsorbed moisture, excess solvent, highly volatile oligomers or adsorbed gases. A very sharp decrease of mass at around 400 °C is observed for pure PVP indicating its decomposition at this high temperature. A very small quantity of carbon was yielded which represents that during carbonization the almost entire PVP is decomposed leaving only a trace amount of carbon [58]. Hence, the contribution to the overall carbon yield arising from the PVP-derived carbon is negligible. However, several studies demonstrated a significant nitrogen-doping of the carbon skeleton when the polymeric matrix is thermally treated with PVP [51,59]. For polymer spheres prepared without PVP, such as ET-PS-0 PVP, there are several peaks observed between 200 °C and 300 °C (see Fig. S9) that can be due to the decomposition of phenolic resin. However, no any distinguishable peak related to PVP is observed around 400 °C. In contrast, for ET-PS-x PVP, the peak at 400 °C is observed, the intensity of which increases with increasing PVP amount during the synthesis, suggesting greater incorporation of PVP into polymer with increasing PVP amount. All other peaks remain the same as those observed for polymer spheres prepared without PVP. The TG profiles were also obtained for the activated carbon spheres in air to test their stability and the results are displayed in Fig. S10. All the carbon spheres decompose around 500 °C. This value matches well with those reported elsewhere [6,60].

4. Conclusions The size-tunable nitrogen-containing porous carbon spheres were obtained by a facile one-pot synthesis at elevated temperature using resorcinol and formaldehyde as carbon precursors, EDA as nitrogen source and basic catalyst and polyvinylpyrrolidone as a particle stabilizer. This strategy takes advantage of forming many small-sized polymeric nuclei at elevated temperature and stabilizing effect of PVP to control the size of polymeric spheres by varying the PVP amount. The resulting polymer spheres after subsequent carbonization produced carbon spheres without any cracks. This synthesis strategy eliminates the step of additional hydrothermal treatment that is generally performed in highpressure autoclave vessels to prepare carbon spheres, and reduces the overall cost, which makes it industrially feasible. Moreover, the presence of PVP at high thermal treatment results in nitrogen doping of carbon spheres up to 5%.

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