3.1 Structural characterization
Table 1 pore parameters of LTCs
Figure 2 showed the N2 adsorption-desorption isotherms and the pore size distribution obtained by the BJH algorithm for all samples LTC-T-X-Y tested at 77k temperature. It could be seen that the adsorption isotherm of LCZ belonged to type IV line type in the IUPAC classification[20]. For all samples, the relative pressure was smaller for N2 adsorption in the low-pressure, indicated that there was less microporous filling. In addition, the hysteresis of the desorption curve in the medium-pressure and the low adsorption at a P/P0 of 0.99 in the high-pressure indicated the presence of fewer macropores and a large number of mesopores in LCZ. When the carbonization time and QAL addition were constant, the adsorption capacity of the high-pressure increased gradually with the increase of the regulation temperature, and the desorption curves were also more hysteresis. That was because more mesopores and macropores appeared after the carbonization temperature increased, and the specific surface area and pore volume also increased. As shown in Table 1, the specific surface area reached a maximum value of 310.249 m2·g− 1 at a carbonization temperature of 600°C, indicating that more mesopores were produced and the specific surface area increased at a carbonization temperature of 600°C. The pore volume reached a maximum value of 0.3976 mL·g− 1 at a carbonation temperature of 600°C. However, more macropores were produced after the carbonation temperature over 600°C, which increased the pore volume, but too many macropores reduced the specific surface area.
When the carbonization temperature and QAL addition were constant, the regulated carbonization time gradually increases. It could be found that there was little change in the high-pressure, while the adsorption amount in the medium-pressure was increased and the hysteresis of the desorption curve is more severe, indicated that more mesopores were generated. As shown in Table 1, the specific surface area reached a maximum value of 310.249 m2·g− 1 at 2h, while the pore volume also reached a maximum value of 0.3976 mL·g− 1.
The amount of regulated QAL addition was continuously increased when the carbonation temperature and carbonation time were constant. It could be found that the maximum specific surface area of 317.765 m2·g− 1 and the maximum pore volume of 0.4344 mL·g− 1 can be achieved at the addition of 4 g of QAL. It was showed that QAL addition affects the number of pore creation, but relatively too much QAL may reduce pore generation by self-encapsulation of the carbon layer and collapse of the pore channel. As shown in the pore size distribution diagram, the pore size of LCZ was mainly concentrated between 1 ~ 30 nm, a large number of mesopores were distributed between 2 ~ 10 nm, and a small number of micropores and macropores existed. In summary, LTC-600-2-4 with excellent pore structure was obtained when carbonized at 600°C for 2h and the addition of QAL was 4g.
The structures of pure ZnO and LTC-600-2-4 were tested by XRD diffraction. As shown in Fig. 3, the characteristic peaks of ZnO appeared at 2θ of 31.8°, 34.3°, 36.2°, 47.5°, 56.6°, 62.9°, 67.9°, 69.2°, which correspond to the ZnO crystals at (100), (002), (101), (102), (110), (103), (200), (112), (201), respectively. The characteristic peaks and positions of LTC-600-2-4 and ZnO are consistent, indicating that lignin did not destroy the crystalline structure of ZnO. This was because the intensity of the amorphous characteristic peaks of lignin carbon at 20 ~ 30°was much weaker than that of the ZnO. Moreover, the amount of lignin carbon in the composites was relatively small, which made it difficult to observe the characteristic peaks of lignin carbon in the XRD spectra. This was similar to most reported XRD results of graphene/ZnO composites[13–14].
To further investigate the structure of LTC-600-2-4, Raman spectroscopy was performed on the composites. As shown in Fig. 4, the characteristic peaks D peak (1340 cm− 1) and G peak (1590 cm− 1) of lignin carbon could be observed. The D-band reflected the degree of defects in the carbon structure, and the G-band was due to the vibration of sp2-hybridized carbon atoms in the two-dimensional direction of the hexagonal lattice. The obtained LTC-600-2-4 was similar to C/ZnO reported by other researchers using other carbon materials[12–14]. The strength ratio of G-band to D-band (ID/IG) was used to express the degree of graphitization of carbon in the composite. The smaller the ID/IG carbon value, the higher the degree of graphitization of the material. The low degree of graphitization of the prepared LTC-600-2-4 could be clearly seen from the figure.
3.2 Morphology and microstructure characterization
The micro-morphology and structure analysis of ZnO and LCZ-600-2-4 were investigated by SEM and TEM. As shown in Fig. 5(a), ZnO had a typical nanoparticle structure with diameters ranging from about 50 ~ 180 nm, and pure ZnO nanoparticles exhibited a severe agglomeration behavior. As shown in Fig. 5(c,d), LCZ-600-2-4 was composed of lignin carbon and ZnO nanoparticles. It could also be seen that the ZnO nanoparticles were well dispersed and tightly bound to the lignin carbon nanosheets, and the particle size of ZnO in the composite was much smaller than that of pure ZnO in Fig. 5(b,d,f). The transfer process of photogenerated electrons and holes was closely related to the surface contact between carbon and semiconductor, and the high specific surface area and pore volume of LCZ-600-2-4 provides a rich site for ZnO. This tight interfacial contact was expected to improve the transfer process of photogenerated electrons and holes, thus improving and enhancing their photocatalytic performance.
The elemental composition and content of LCZ-600-2-4 was analyzed by X-ray energy spectrometry (EDS). As shown in Fig. 6(a), LCZ-600-2-4 contained C, O and Zn elements and their atomic percentages are 62.57%, 16.64% and 20.79%, respectively. The O and Zn atomic ratio was close to 1:1, which further indicates that the LCZ-600-2-4 composite was successfully prepared.
3.3 Optical properties characterization
The optical properties of pure ZnO and LCZ-600-2-4 were measured by UV-vis diffuse reflectance spectroscopy. As shown in Fig. 7(a), the sample exhibited a typical strong light absorption in the UV region due to the promotion of electrons from the valence band (VB) to the conduction band (CB) of ZnO. Due to the benzene ring structure and the presence of double bonds, LCZ-600-2-4 exhibited a higher UV absorption than pure ZnO. In addition, LCZ-600-2-4 showed enhanced absorption intensity in visible light due to the absorption of background light by lignin-based carbon compared to pure ZnO.
The photoluminescence (PL) spectra of ZnO and LCZ-600-2-4 were shown in Fig. 7(b). The pure ZnO had a high and broad photoluminescence peak at about 500 nm, which indicated that there were many photogenerated electron/hole pairs in the pure ZnO. However, the photogenerated electrons and holes had a high probability of recombination. However, the luminescence intensity of LCZ-600-2-4 was much lower than that of pure ZnO. The significant decrease in photoluminescence intensity indicates that the recombination probability of photogenerated electrons and holes was greatly reduced and the electron-hole separation efficiency was significantly improved. The photogenerated electrons were effectively transferred to the lignin-based carbon with better conductivity, which greatly improved the photocatalytic activity.
3.4 Evaluation of photocatalytic performance
The photocatalytic performance of the prepared LCZ-600-2-4 composites was evaluated by simulating the photodegradation rate of organic dye pollutants under solar irradiation, and anionic dye MO and cationic dye Rh B were selected as pollutant models. The results are shown in Figs. 8 (a) and (b). The results showed that the LCZ-600-2-4 composite had good photodegradation effect on both MO and Rh b, while the photodegradation effect of pure ZnO solar visible light was very poor. Compared with pure ZnO, LCZ-600-2-4 degraded MO up to 92% and Rh B up to 91% in 2 hours. After compounding with lignin carbon, LCZ-600-2-4 with rich pore structure provideed a large number of adsorption sites and reactive sites for photocatalytic degradationthe.
under solar light irradiation
The photo-degradation mechanism of LC/ZnO composites for MO or Rh B in this experiment can be explained as follows (Fig. 9):
LCZ + hν → ZnO(h+) + LC(e−),
e− + O2 → ·O2−
h+ + H2O/OH- → ·OH
·OH or O2− or h+ + MO(Rh B) → CO2 and H2O
The QAL with positively charged functional groups can combine well with the negatively charged ZnO. The photogenerated electrons and holes generated by ZnO could be rapidly transferred to the lignin-based carbon through a tight interface, inhibiting the effective complexation of holes and electrons. The high specific surface area of LCZ-600-2-4 could facilitate the bulk loading of ZnO, and its porous structure could rapidly adsorb organic dyes onto the composite surface. The degradation of organic dyes in wastewater was further greatly enhanced.
3.5 Evaluation of material stability
It could be seen from Fig. 10 (a) the photodegradation performance showed little difference under different pH conditions. The lignin carbon particles had a certain encapsulation effect, which reduced the excessive contact between ZnO and the degradation solution to a certain extent, indicated that the prepared LCZ-600-2-4 had good pH adaptability.
To further investigate the stability of the samples of LCZ-600-2-4, five successive recycling tests over the sample of LCZ-600-2-4 for photo-degradation of Rh B under solar light have been performed, and relevant experimental results were shown in Fig. 10 (b). It could be seen that the degradation rate of LCZ-600-2-4 could still maintain above 80% after 5 cycles. The slight decrease of degradation rate was due to the incomplete desorption of the adsorbed dyes in the dark reaction stage. The results indicated that the prepared composites had good cycling stability,