Transmittance spectra above pure-PVA sample at different heights from sample surface under 520 nm LED
Description
Transmittance spectra above pure-PVA sample at different heights from sample surface under 520 nm LED
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To measure the transmission spectrum of the vapor at different heights during light-driven hydrogel evaporation operation, a special sample stage was made with water-cooled LED so that the sample stage could fit into the sample chamber of the UV-VIS-NIR spectrometer. The probe beam of the spectrometer is limited with an IRIS of the size 1mm (vertical) × 3 mm (horizontal). The height of the sample can be controlled with the help of a movable stage. Since the probe beam is fixed, this configuration enables us to scan the direct transmittance of the moist air at different heights over the working samples by lowering the sample. The LEDs are fixed in the UV-VIS sample chamber. We ensured that the sample at different heights could receive the same input energy (1 sun) by adjusting the LED input voltage. The sample chamber was kept closed during testing to reduce the noise, but vapor can leak out since the sample chamber is not hermetically sealed. We carefully checked all these measurements and did not find condensation inside the sample chamber. For all the tests of PVA-ppy sample and pure PVA sample, the incident light intensity is the same at one sun (1000 W/m2). To heat DI water under solar radiation, we put a black absorber at the bottom of the sample holder and adjusted the incident light intensity to obtain the same surface temperature as the PVA-ppy sample at one sun. We also carefully measure the direct transmission of dry air, which is obtained after first purging both the sample chamber and the internal chamber with pure nitrogen gas and then turning the purging off. It is a common baseline to process the collected data at different heights for various samples. We collect the signals five times at each sampling height and take their average. Under LED lamp radiation, the detector of the UV-VIS spectrometer quickly gets saturated due to scattered light. To avoid this situation, we put two filters with cut-off wavelengths at 530 nm and 610nm in the front of the detector window to cut off the lights shorter than these wavelengths. However, the beam shape of the UV-VIS changes along the path length. Although long the sample diameter, the change is small. Due to this change, the beam position should be understood as nominal.