3.1. Samples’ Morphology
The morphology of the carbon spheres was studied using Scanning Electron Microscopy (SEM). The SEM images of the samples carbonized in 700 °C are shown in Figure 1
. For the material without modification (Figure 1
a), small, monodisperse spheres were obtained. The average diameter of the carbon spheres was determined by SEM to be about 400 nm. For the material prepared by the addition of the activator (potassium oxalate, Figure 1
b), two classes of spheres were observed: smaller spheres, about 500 nm in diameter, and larger, about 2–3 µm. The large difference in the diameter of the spheres was the result of the addition of potassium oxalate. The resorcinol–formaldehyde spheres were influenced by the oxalate moieties, and thus larger spheres were formed. However, there was a fraction of the smaller spheres, where oxalate moieties were likely less present. The higher concentration of potassium oxalate (Figure 1
c) resulted in higher saturation of the solution. The spheres containing more potassium oxalate were larger. Nonetheless, there was a large amount of small spheres, which did not contain oxalate moieties. Thus, a large amount of carbon material was not modified.
For the material modified with EDA only (Figure 1
d), the monodispersity of the spheres was kept, however larger spheres (diameter ca. 800 nm) were formed. The larger diameter and the monodispersity of the spheres are evidence that EDA was well dispersed in the whole volume, and so, all carbon spheres contained EDA. In the case of the material modified with both EDA and potassium oxalate (Figure 1
e), the influence of both modifiers can be noticed. The average diameter of the spheres was larger and the spheres were more uniform. EDA provided better dispersion of potassium oxalate in the reaction volume, thus potassium ions were present in a higher amount of the spheres. In the end, a much bigger specific surface area value was reached.
The size distribution of the produced particles was evaluated from the SEM images using the ImageJ software tool and is illustrated in Figure 2
a–e. The quantity of spheres taken into account was 50 for every kind of the sample.
The results for the reference sample are given in Figure 2
a. RF 700 exhibited the highest monodispersity among all the samples. The diameter of the spheres was about 600 nm.
The modification of carbon materials with the lower content of potassium oxalate resulted in higher variation in the spheres’ size distribution. A considerable amount of produced spheres had a diameter from 300 to 1000 nm, as shown in Figure 2
b. With the increase of the carbonization temperature, the formation of larger spheres (about 2000 nm) was observed. For the material carbonized at 600 °C, the majority of spheres were about 500 nm, whereas on the other hand for the sample carbonized at 800 °C, this value shifted to about 700 nm.
Comparing the samples with different amounts of activator, the strong influence of the activator concentration on the spheres’ size was noticed. Higher activator content in the samples resulted in the widest size distribution (Figure 2
c). Moreover, the large spheres of diameter over 2000 nm were formed.
The size distribution of the samples modified with EDA only is presented in Figure 2
d. Compared to the samples modified with potassium oxalate, the highest monodispersity of the spheres was gained. Nonetheless, increasing the carbonization temperature caused the distribution to be broader.
As can be seen in Figure 2
e, the application of both the modificators limited the production of the large spheres (over 2000 nm). Unlike previous distributions, by increasing the carbonization temperature, the shift of distribution towards smaller spheres was noticed.
3.2. Surface Chemistry
The surface composition of materials was analyzed by X-ray Photoelectron Spectroscopy (XPS). The survey spectra acquired for all analyzed samples are shown in Figure 3
. The evaluation of the elemental composition of the surface of all samples is presented in Table 2
. In all samples, carbon and oxygen was present, and potassium was observed in samples prepared with potassium oxalate.
The highest carbon content was observed for the pure carbon material (RF_700); oxygen constitutes only 3% of the surface atoms. The surface of the samples prepared with EDA only also contained a relatively small number of oxygen atoms (approximately 4%), however those surfaces also contained about 4% of nitrogen atoms. The presence of potassium in the internal structure of the material is associated with an increased concentration of oxygen atoms. The more potassium observed in the material, the higher the concentration of oxygen observed, as residual potassium atoms were bound with oxygen. In general, when the carbonization temperature was increased to 800 °C, this resulted in a lower oxygen concentration than that observed for samples carbonized at 600 °C. There is noticeably more potassium retained on the surface of the samples modified by both potassium oxalate and EDA in comparison to materials modified by potassium oxalate only. A possible explanation for this is that a reaction of potassium with amine groups occurred.
The analysis of high-resolution XPS data brings a more detailed view of the chemistry of the surface of the studied materials. In Figure 4
, the spectral region of binding energy between 280 eV and 300 eV is displayed for two samples of carbon spheres obtained with the weight ratio potassium: carbon of 9:1 (RF_9_1_600 and RF_9_1_800). This region contains the spectrum components originating from C 1s and K 2p electrons. The peak maximum of K 2p3/2
is located at 293 eV and it is accompanied with a K 2p1/2
spin-orbit component at 295 eV. The peak maximum of XPS C 1s spectrum is located at 284.4 eV. This energy is characteristic for highly graphitized carbon materials. However, a distinctive shoulder at about 288 eV is present in the spectrum for both samples, though more prominent on the sample carbonized at 600 °C. This position is usually ascribed to the general group of carbon moieties containing O–C=O bindings. The intensity of the spectra is normalized in respect to the intensity of the main peak of carbon. It can be pointed out that the relative intensity of lines corresponding with potassium atoms as well as carbon atoms in O–C=O bindings decreases in comparison to C–C bonds, reflected by XPS C 1s peak at 284.4 eV. This shows that increased carbonization temperature results in a partial depletion of potassium atoms from the surface as well as a decomposition of a part of C–O bonds. This corresponds well with the quantitative analysis described above. Similar observations are also valid for samples with a lower potassium:carbon ratio.
Slightly different behaviour of the surface species is observed for samples prepared with EDA. In Figure 5
, the spectral region of the binding energy between 280 eV and 300 eV is displayed for two samples of carbon spheres obtained with the weight ratio potassium:carbon of 7:1 with the addition of EDA (RF_7_1_EDA600 and RF_7_1_EDA800). The position of the K 2p peaks is identical to samples without EDA admixture, indicating that the chemical state of potassium atoms is not changed by EDA presence during the preparation stage. However, the peak maximum of the C 1s line for sample RF_7_1_EDA600 is located at 284.6 eV, which is characteristic for C–C bonding in aliphatic sp3
bonds or non-graphitic amorphous carbon. For the sample carbonized at 800 °C, the respective peak maximum of C 1s line is shifted to 284.4 eV. Similar to the samples prepared without EDA, this peak position is assigned to C–C bonds in graphitized carbon material. It is worth noting that the relative intensity of K 2p lines for 600 °C and 800 °C of carbonization is only slightly changed.
3.4. Adsorption Studies
According to the low-temperature nitrogen adsorption–desorption studies, for samples modified with oxalate only, the increase of carbonization temperature resulted in a higher volume of adsorbed nitrogen. The opposite effect was observed for samples modified with EDA only. The addition of both modificators gave a similar result to the use of oxalate only.
Some examples of low-temperature nitrogen adsorption isotherms are shown in Figure 7
The isotherms are of type Ia [35
], characteristic for microporous materials, however a slight increase of adsorbed nitrogen volume at the highest P/P0
can be attributed to the presence of macropores (type II). Spheres, modified with EDA only, adsorbed the lowest nitrogen volume. Modification with potassium oxalate resulted in higher nitrogen adsorption, slightly increasing with dopant concentration. The highest amount of nitrogen was adsorbed in the sample modified with both potassium oxalate and EDA.
Physico-chemical properties of the samples were measured, and the results are shown in Table 4
. In almost all cases, except samples modified with EDA only, an increase in carbonization temperature resulted in an increase of the samples’ density, specific surface area, total pore volume, and CO2
adsorption. An unusual increase in density, simultaneously with an increase in surface area and porosity can be explained by the decomposition of modificators and removal of gaseous decomposition products. The same phenomenon was reported for activated carbon produced from palm shell and modified with potassium carbonate [36
] or phosphoric acid [37
An extremely high increase in specific surface area and CO2 adsorption was observed for the samples modified with both potassium oxalate and EDA. In contrast, samples without potassium oxalate carbonized at higher temperatures did not exhibit larger surface area, and a lower amount of CO2 was adsorbed (because of a lower microporosity). However, a higher concentration of the activator did not improve the specific surface area. Due to the higher saturation of the mixture, bigger spheres were formed, but oxalate moieties were not well dispersed within the volume of the sample (as shown before in SEM images).
For adsorption of carbon dioxide, the presence of the micropores below 1 nm is considered to be most important, and the pore size distribution in this area was calculated from CO2
adsorption at 0 °C and is shown in Figure 8
In the research paper [21
], doping of the carbon spheres with EDA was described. Increasing the EDA ratio (from 0.2 mL to 0.8 mL for 0.4 g of resorcinol) led to an improvement of specific surface area and CO2
uptake at 25 °C. The work of Sibera et al. [34
] also reported a positive effect of a higher concentration of EDA as a modificator, improving the CO2
uptake. In the present paper, more detailed studies on the influence of carbonization temperature on samples modified with EDA were performed.
The samples modified with EDA showed much lower surface areas and CO2
adsorption than the reference sample RF 700. A higher carbonization temperature resulted in lower surface area (Table 4
) and lower total pore volume, but an increase of the CO2
uptake was observed. This observation can be explained by higher micropore volume, below 0.4 nm for the sample RF EDA 800 (Figure 8
). At elevated temperatures, carbon spheres have a tendency to aggregate, thus the effective surface area decreased. Density measurements proved the increase of density (from 1.59 g/cm3
for RF EDA 600 to 1.72 g/cm3
for RF EDA 800). In contrast, an increase in the carbonization temperature increased the volume of the pores below 0.4 nm (Figure 8
) and consequently the CO2
Significant differences were noticed for the samples modified with potassium oxalate. In the paper [19
], Ludwinowicz and Jaroniec applied three potassium oxalate concentrations, with a K:C ratio of 3:1, 5:1, and 7:1. The growth in surface area (460 m2
/g for pure material and 2130 m2
/g for the highest concentration potassium oxalate) and CO2
adsorption (2.8 mmol/g for pure material and 6.6 mmol/g for the highest concentration potassium oxalate) was observed. In order to investigate the influence of the activator concentration on the physico-chemical properties of the spheres, we employed a higher concentration of potassium oxalate monohydrate (weight ratio K:C = 9:1). The specific surface area values were similar to the values for samples with a lower activator concentration. A significant difference in CO2
uptake at 0 °C and 25 °C was observed for sample RF 9/1 carbonized at 800 °C.
The microporosity of these samples carbonized in 700 °C is given in Figure 9
. The sample RF 700, compared to the samples modified with potassium oxalate, had the lowest specific surface area. This was caused by a lack of energized potassium ions to interact with the carbon matrix and a lack of carbon dioxide released in the result of decomposition of potassium oxalate, creating porosity. For the sample RF 7/1 700, modified with the lower amount of activator, a significant increase of the microporosity in the range of width from 0.3 to 0.7 nm was observed. Application of the higher concentration of the activator in the sample RF 9/1 700 improved the specific surface area, but the lower amount of adsorbed CO2
was noticed, which was in agreement with the lower volume of pores below 0.7 nm, as shown in Figure 9
. The highest values of the specific surface area and CO2
adsorption were obtained for the samples modified with potassium oxalate and EDA simultaneously. Energized potassium ions penetrated the nanocarbon material, but on the other hand, EDA improved the basicity of the material and distribution of the oxalate moieties throughout the nanocarbon sphere. For the sample RF 7/1 + EDA 700, the value of the specific surface area was twice as high as the sample modified with potassium oxalate only, but the CO2
adsorption was only slightly higher. The microporosities of both samples with a diameter of 0.7 nm were comparable. The sample RF 7/1 + EDA 700 had a higher volume of pores from 0.7 to 0.9 nm, however this feature did not improve the CO2
adsorption significantly. Despite the higher value of the specific surface area of the sample carbonized in 800 °C (500 m2
), the CO2
uptake at 0 °C was only slightly better, however at 25 °C, a decrease of the adsorbed value for the sample RF 7/1 + EDA 700 was observed.
The adsorption capacity values of all samples are presented in Figure 10
. The samples carbonized in 600 °C were more resistant to a decrease in CO2
adsorption at the higher adsorption temperature. Mostly, the increase of carbonization temperature led to higher surface area and CO2
adsorption, but a significant decrease of the adsorbed values of CO2
at 25 °C compared to 0 °C was observed.