For directly evaluating the light-to-thermal conversion performance of the TP based evaporators, an IR camera were employed to follow the tracks of temperature of the evaporator surface under the dry and wet condition respectively at the light intensity of 1000 W m− 2. Under the dry and wet condition, the Na-CFTP exhibit excellent light-to-thermal performance. As shown in Fig. 5, the dry surface temperature of Na-CFTP rapidly increased from 17.5 ℃ to 89.6 ℃ only using 30 s, and further ramped up to 105.3 ℃ for 1 min, and then reached 130.4 ℃ in the next 30 min; howbeit, other samples temperature did not increase so fast and high like Na-CFTP, for instance, the final temperature of Na-CFTP is higher than those of CTP and CFTP for 13.3 ℃ and 9.8 ℃ respectively under the dry condition. Additionally, under the wet condition, the surface temperatures of Na-CFTP rapidly ramped to 44.2 ℃ during 3 min, and slowly grown to 53.9 ℃ in the next 57 minutes under one sun, which temperatures rose more speedy and higher than that of other samples as exhibited in Fig. 6. As discussed above, it is displays that Na-CFTP showed best light-to-thermal conversion performance under the dry and wet condition. Excellent light-to-thermal performance of the Na-CFTP can be attributed to its unique porous structure and high surface area that can capture the light in an efficient way and further change to heat.
Solar steam generation performance
Given the typical porous structure and good light capture and light-to-thermal performance of the porous Na-CFTP, we developed different height (1 cm, 5 cm, 10 cm) Na-CFTP based 3D evaporators to investigate their water steam generation performance, where the height of 1, 5 and 10 cm evaporator are named Na-CFTP1, Na-CFTP5 and Na-CFTP10 respectively. We firstly systematically explored the water conveying performance of the different height of the Na-CFTP evaporators. Commercial cotton rods with a length of 15 cm and a diameter of 8 cm are used as standard water absorbent, and a thermal imaging camera is used for recording the water transmission process. As shown in Fig. 7and Figure S7, with the help of capillary effect, water can be quickly transported to the middle and top of the cotton rod for Na-CFTP1 evaporator during the 60 s and 180 s respectively. Moreover, for sample Na-CFTP10, water also can be transported about 2.5 cm within 3 min, and few water moisture slowly reach to the top of the cotton rod for 30 min. Obviously, With the increase of the sample height, the water transport speed in evaporator slows down. The water transmission speed is in the cotton on the Na-CFTP10 is slower than that of Na-CFTP1 and Na-CFTP5. The transmission speed of the water in cotton rod on Na-CFTP10 is slower than in other two samples, but its water transmission performance is still meet the requirements of the evaporation. Such good water transmission performance is consistent with the results of the water contact angle as shown in Fig. 4e and Figure S6.
To get the detail evaporation datum of the TP based evaporator, a home-made equipment was used to survey performance of water evaporation, where the environment humidity and temperature were kept about 40% and 25°C respectively. Here, a computer controlled electronic balance was used to keep an account of mass changes of water in container every 5 s for 1 h. As exhibited in Fig. 8a, under one sun illumination (1000 W m2), the amount of water volatilization for per hour were 1.16, 1.29, 1.49, 1.65, 1.67, 1.72, 1.89 kg m− 2 for CTP, CFTP, Zn-CFTP, P-CFTP, K-CFTP, Cu-CFTP, Na-CFTP evaporator, respectively. As nicely shown in Fig. 8a and b, the evaporation performance of the Na-CFTP is significantly higher than that of other chemical agents treated TP based evaporators with corresponding steady evaporation speed is 2.26 kg m− 2 h− 1 after one hour, which is 1.65, 1.55, 1.36, 1.11, 1.22, 1.15 times of the CTP, CFTP, Zn-CFTP, P-CFTP, K-CFTP, Cu-CFTP.
Considering the geometry of toilet paper and for better using the typical structure, different heights of TP were treated by NaOH and carbonized in an atmosphere furnace in nitrogen for 2h. As exhibited in Fig. 8c and d, With the increase of evaporation height, the evaporation rate of the TP based evaporator activated by NaOH is largely improved. Na-CFTP10 evaporator exhibits a much higher evaporation performance than the Na-CFTP1 and Na-CFTP5. As shown in Fig. 8c and d, when the height of the TP based evaporator increase from 1 cm to 5cm and 10 cm, the water weight loss in the cup increases from 1.89 to 2.54 and further to 3.10 kg for per m2, with the corresponding evaporation rate enhance from 2.26 to 2.86, and further increase to 3.37 kg m− 2 h− 1.
It should be noted that evaporation and apparent energy utilization rates of Na-CFTP10 evaporator are far beyond 1.592 kg m− 2 h− 1 (theoretical value) and 100%[38]. Na-CFTP10 has achieved such good results, on the one hand because of its rich microporous (Fig. 3c), well vertical layer-by-layer structure (Fig. 2c) and good water transportation and steam escape capacity (Fig. 4e and Fig. 7b) which can catch more lights and, own good light-to-heat conversion efficiency and provide enough water for evaporating; on the other hand because expand the water evaporation area by additional side area like the previous literature[36].
Above results demonstrated that TP based evaporator exhibits excellent solar assisted water evaporation performance. In this manuscript, we triumphantly utilize the traditional carbonized and activated technology to change the traditional, easy scale-up and cheap industrial products (Toilet paper) as high as an efficient evaporator. This would open an easy way to utilize the industrial products for solar energy utilization and distillation and desalination in future.