The experimental study on temperature-regulated hydrogen production photocatalytic activity of Zn0.5Cd0.5S in absence of cocatalyst

Semiconductor-based photocatalytic water splitting is one of the efficient hydrogen preparation methods, but the ability of photocatalytic hydrogen evolution (PHE) is mainly decided by the properties of semiconductors and cocatalysts. In this study, we improved the PHE ability by changing the temperature without the modified materials or co-catalysts. With Zn0.5Cd0.5S as catalyst and the L-lactic acid as sacrificial, the temperature (< 100 degrees C) can regulate and significantly enhance the activity of Zn0.5Cd0.5S in the absence of cocatalyst. With the temperature rise, the PHE activity of Zn0.5Cd0.5S first increased, maximized at 40 degrees C and then decreased. This prediction was verified by a series of experiments. The temperature can greatly enhance the PHE activity of Zn0.5Cd0.5S and the extreme value appeared at 40 degrees C. The hydrogen production rate of Zn0.5Cd0.5S increased from 0.9 mu mol/mg/h at room temperature to 47.7 mu mol/mg/h at 40 degrees C, and the quantum efficiency was enhanced from 0.66% to 37.19% (lambda = 450 nm) and to 71.8% (lambda = 420 nm). The significant increment of quantum efficiency suggested the large improvement of photon conversion and use efficiency due to the temperature promotion. The regulating- temperature assisted the semiconductor to enhance optical absorption ability during reactions, accelerated the migration of photoelectrons and raised the use efficiency and life of photoelectrons. Moreover, this temperature accelerated the water dissociation into more H+, increased the H+ concentration and number of active sites adsorbed onto the material surfaces, reduced the water decomposition voltage and inhibited the reserve reactions of water splitting. The above multiple factors synergically acted to largely enhance the PHE performance of Zn0.5Cd0.5S. Although the temperature favored the hydrogen production performance of the materials in the above many aspects, it can also accelerate the recombination probability of photo-generated electrons and holes, which led to the "first increase and then decrease" of PHE activity with the occurrence of the extreme value.