SEM images of the B-PTA-1 was measured, and 3D porous structure can be clearly observed as shown in Fig. 2a and 2b. The B-PTA-1 was composed of BC and BDCF as shown in Fig. 2c, and BC was disordered distribution on BDCF, which was entangled randomly to form an interconnected 3D porous structure. Compared with the B-PTA-1, the surface of BDCF in B-PTA-0 is smooth as shown in Fig. S3. The element mapping and energy dispersive X-ray (EDX) spectrum of the B-PTA-1 represented the major elements of C (73.79%), O(22.97%) and Ca (3.24 %) as shown in Fig. 2d and Fig S4. The XRD patterns of B-PTA-0, B-PTA-1 and BC were presented in Fig. 2e. In the diagram of B-PTA-0, 2-theta degrees were located at 16° and 22° corresponding to the (1 1 0) and (2 0 0) diffraction planes of cellulose, respectively (Mohiuddin et al. 2015; Rodriguez-Restrepo et al. 2020). The broad peak at 22.8° and weak peak at 44° of the BC were ascribed to (0 0 2) and (1 0 0) plane, respectively, suggested the formation of amorphous carbon (Sayed et al. 2020). The characteristic diffraction peaks at 16°, 22.6° and 44.5° in the B-PTA-1 indicated the existence of cellulose and amorphous carbon. The XRD patterns of B-PTA-0.5 and B-PTA-1.5 were shown in Fig. S5. In the Raman spectra of B-PTA-1 (Fig. 2f), the peaks at 1340 cm− 1 and 1580 cm− 1 attributed to the disorder D-band of amorphous carbon and the in-plane sp2 vibration of graphite crystal G-band, respectively (Zhu et al. 2019).
The surface chemical compositions and functional groups of the B-PTAs were researched through FT-IR and XPS. The FT-IR characteristic absorption peaks located at 3458, 2900, 1428, 1030 cm− 1 can be assigned to the OH, CH2, CH symmetrical deformation, and COC stretching vibration of cellulose, respectively as shown in Fig. 3a (Gao et al. 2020). A peak was observed at 1625 cm− 1 that was related to C = O functional groups as a result of the natural aging of cellulose (Salama 2020). XPS analysis of B-PTA-1 was researched as shown in Fig. 3b and 3c. The major elements of the B-PTA-1 were carbon and oxygen. The high-resolution spectrum of the C 1s peak can be assigned into three components including aliphatic C-C, hydroxyl carbon C-O and carboxy carbon O-C = O, which were consistent with the FT-IR, and further indicated the hydrophilic property of the B-PTAs.
The hydrophilicity, water transport and storage capacity of B-PTAs are extremely important in the solar steam generation device. Hydrophilicity was further demonstrated through water contact angle measurement as shown in Fig. 3d. A water droplet was wetted immediately when was dropped onto the surface of B-PTA, which indicated that the B-PTAs were super-hydrophilicity. White B-PTA-0 was researched to clearly observe the water transport ability. Once the B-PTA-0 is touched with the water of the cotton core, the wetting process starts immediately. The B-PDA-0 was completely wetted within 30 seconds as shown in Fig. 3f, which demonstrated that the B-PTA-0 can effectively transport water in the interconnected hierarchical porous networks. Moreover, the water hold ability of B-PTA-1 was researched. 0.232 g B-PTA-1 (height: 1 cm, diameter: 3 cm) can hold a weight of approximately 5.305 g water, which was 23 times weight without any obvious deformation as shown in Fig. S6. The results manifested that the B-PTAs have super-hydrophilicity, water transport and capacity storage.
The light absorption abilities of B-PTAs were investigated through UV–Vis–NIR spectrophotometer and weighted by the standard air mass 1.5 global (AM 1.5 G) solar spectrum as shown in Fig. 4a. The average integrated absorption of B-PTA-0, B-PTA-0.5, B-PTA-1 and B-PTA-1.5 in the wavelength range from 300 to 2500 nm was 51%, 93%, 95% and 96%, respectively. In the whole spectrum, B-PTA-0.5 exhibited stronger light absorption than B-PTA-0, which indicated that the CB can effectively enhance the light absorption ability of the B-PTAs. In addition, the absorption ability of B-PTA-1 and B-PTA-1.5 was similar and slightly stronger than B-PTA-0.5, which implied that B-PTA-1 is the best candidate for solar steam generation. In consideration of the excellent light absorption abilities, the light to heat capacities of the B-PTAs were researched. The temperature change of the B-PTAs under 1-sun illumination was traced through IR camera. The top surface temperatures of B-PTA-0 increased from 29.1°C to 42.0°C in the first 1 minute, and kept up near 48.7°C continuing to light for 8 minutes as shown in Fig. 4b. Relatively, the top surface temperature of B-PTA-0.5, B-PTA-1 and B-PTA-1.5 sharply increased from 27.8°C, 28.8°C and 27.8°C to 62.4°C, 71.3°C and 71.3°C in the first 1 minute, respectively, and maintained near 81.2°C, 85.3°C and 84.9°C with the extension of illumination time to 8 minutes, which indicated that the light to heat ability was B-PTA-1.5 ≈ B-PTA-1 > B-PTA-0.5 > B-PTA-0. IR images of B-PTA-0 and B-PTA-1 was presented in Fig. 4c and 4d. When the light was off 1 minute, the top surface temperature of B-PTA-0 decreased to 33.6°C, and B-PTA-0.5, B-PTA-1, B-PTA-1.5 dropped sharply to 41.4°C, 39.1°C, 40.1°C, respectively. IR images of B-PTA-0.5 and B-PTA-1.5 were shown in Fig. S7. Furthermore, the rate of increase and decrease in temperature of B-PTA-1, B-PTA-1.5 was close and faster than B-PTA-0.5, which was further proved that the light to heat capacities was B-PTA-1.5 ≈ B-PTA-1 > B-PTA-0.5 > B-PTA-0. These results were consistent with the UV–Vis–NIR research.
Solar steam generation of the B-PTA-0, B-PTA-0.5, B-PTA-1 and B-PTA-1.5 were investigated and the top surface temperature of the wet B-PTAs was recorded by IR camera under 1-sun irradiation. The top surface temperature of the B-PTA-0 increased from 22.4°C to 27.8°C after 3 min irradiation, and continuously risen and kept at average 33.8°C after 15 min illumination as shown in Fig. 5a. An increase in the content of BC of the B-PTAs resulted in the top surface temperature rising under solar steam generation. Compared with the B-PTA-0, the top surface temperature of the B-PTA-0.5 was higher under the same time of illumination, and stabilized at average 37.0°C after 15 min irradiation. Similarly, the top surface temperature of the B-PTA-1 was higher than B-PTA-0.5, and kept at average 39.5°C after 15 min irradiation. For the B-PTA-1.5, the top surface temperature was similar to B-PTA-1, and stabilized around at 39.5°C after 15 min illumination even if the BC content was higher than B-PTA-1. These results were consistent with the UV–Vis–NIR and light to heat research, which implied that the B-PTA-1 has the optimal BC content for solar steam generation.
To systematically evaluate the photothermal ability, the evaporation rates and efficiencies of B-PTAs were accurately recorded through an electronic balance under 1-sun irradiation. The time-dependent water mass change plots of B-PTAs were presented in Fig. 5b. The mass change of water of B-PTA-0, B-PTA-0.5, B-PTA-1 and B-PTA-1.5 were 0.57 g, 0.85 g, 0.96 g and 0.96 g, respectively within 1 hour. The diameter of the B-PTAs is 3 cm. The calculated evaporation rates under 1-sun irradiation was 0.81, 1.20, 1.36 and 1.36 kg m− 2 h− 1, respectively. The dark field evaporation rates of B-PTA-0, B-PTA-0.5, B-PTA-1 and B-PTA-1.5 was 0.25, 0.23, 0.24 and 0,24 kg m− 2 h− 1, respectively. The solar energy conversion efficiency (η) was calculated through the following equations (Li et al. 2018a; Li et al. 2018b):
η = m(HLV + Q)/I
H LV = 1.91846 × 106[T1/(T1-33.91)]2
Q = c(T1-T0)
Where m stands for the net water evaporation rate (kg m− 2 h− 1). HLV represents the liquid-vapor phase change enthalpy. Q is the sensible heat (J kg− 1). I represents the power density of solar illumination. T1 is the temperature of evaporation (K), T0 is the initial temperature of the water. c is the specific heat capacity of bulk water (4.2 J g− 1 K− 1). The net evaporation rates of the B-PTA-0, B-PTA-0.5, B-PTA-1 and B-PTA-1.5 were calculated to be 0.56, 0.97, 1.12 and 1.12 kg m− 2 h− 1, corresponding to energy conversion efficiencies of 38.38%, 66.81%, 77.34% and 77.47%, respectively as shown in Fig. 6a, demonstrated that the evaporation efficiency of the B-PTAs increase with the increase of BC content, and the B-PTA-1 is the best candidate for solar steam generation. The energy conversion efficiency of the B-PTA-1 is higher than the graphene and rice-straw-fiber-based 3D photothermal aerogel (high: 1 cm, diameter: 3 cm), which is 73.60% . Therefore, using cheap bio-waste bagasse to replace expensive graphene to manufacture solar steam generation material is possible. In addition, the evaporation performance of the B-PTA-1 kept stable for 20 cycles with each cycle maintained for 1 hour as shown in Fig. 6b, and represented excellent stability for solar steam generation performance.
To prove the practical application of the B-PTA-1 in desalination, an apparatus for collecting evaporator water was designed to detect ion concentration of evaporation water under the natural sunlight. The diameter of B-PTA-1 in outdoor experiments is about 11 cm (Fig. S8), which can generate about 13 ml water under 1-sun irradiation for 1 hour in theory. During the water evaporation progress, the generated steam condensed into water drops and adhered to the surface of the spherical glass container as shown in Fig. S9, and about 80 mL evaporated water was collected from 9:00 to 18:00 on May 24th, 2021. The ion (Na+, K+, Ca2+, Mg2+) concentrations of evaporated water were tested through ICP-OES method as shown in Fig. 6c, which were significantly decreased to several orders of magnitude, which were satisfied the drinking water standard defined by WHO (Raymond-Whish et al. 2007). The desalinated water can be regarded as clean and safe. To evaluate the application of the B-PTA-1 in purification dye wastewater, methyl blue (20 mg/L) was employed as raw water for the evaporation testing. About 30 ml evaporated water was collected from 11:00 to 3:00 on May 25th, 2021 as shown in Fig. S10. The purified water becomes colorless, the characteristic absorption peaks of methyl blue are removed and the absorbance is close to zero, indicating extremely low concentration of the dyes as shown in Fig. 6d. The results manifested that the B-PTA-1 can effectively purify seawater wastewater and dye wastewater.