Fabrication And Photoelectrochemical Properties Of Ⅱ-Ⅵ Compound / TiO₂ Nanorod Heterostructures

With many advantages, heterosturctures have been widely used in the area of solar cells. One of them is window effect which means that incident light passed through the large band material（e.g. TiO₂） without any loss and was absorbed by the inlayer materials（e.g. Cd S, Cd Se） with the photon electron-holes excited. Meanwhile, with the property of the bands of herterostructures, the photon electron-holes would be quickly separated with the inhence of the transformation efficiency of solar cells. Three different heterostructures, containing TiO₂/Cd S, TiO₂/Cd Se and TiO₂/Cd S/Cd Se, have been widely studied and various solar cells have been fabricated by different method. Electrochemical methods have many advantages, such as: laboratory equipment is easy to operate, the reactions can be performed at room temperature, the deposition can be accurately adjusted experimental parameters, the experiments cost less time, the depositions could be taken place over a variety of conductive substrates, and it is easy to achieve co-deposition and fabricate large area devices. Therefore, in this paper, the above three heterostructures were fabricated by electrochemical methods, including the potentiostatic method and the constant current method. During the experiment, we studied the growth dynamics of TiO₂/Cd S heterostructures with different electrochemical methods for the different interfaces potentials, and TiO₂/Cd S heterostructure nanorods with whole Cd S shells covered, TiO₂/Cd S QDs and TiO₂/Cd S caps were fabricated. During the fabrication of TiO₂/Cd Se, the p H of the electrolytes which would influence the morphologies of the samples were studied and core-shell TiO₂/Cd Se heterostructure nanorods with Cd Se shells whole covered were successful fabricated. Moreover, core-shell TiO₂/Cd S/Cd Se heterostructure nanorods which had two different shells were fabricated by the potentiostatic method with Cd Se shells covered over the as-parepred TiO₂/Cd Se nanorods, and the step-wise bands were formed. Then, photoelectrochemical properties of all the samples were tested. At last, some valuable results were obtained.（1）Through the potentiostatic method, we got the core-shell nanorods with the Cd S shells deposited over the original TiO₂ nanords. From the SEM and TEM images, it is clear to see the growth processes of these depositions with different experiment time. With the discussion with the potential at the interfaces, we studied the growth dynamics of the core-shell TiO₂/Cd S heterostructure nanorods. We got that the deposition started on the top at first, and then Cd S covered over the whole surfaces of the nanorods. Finally, core-shell TiO₂/Cd S heterostructure nanorods were formed with thicker Cd S shells. Meanwhile, Cd S was also deposited over the original TiO₂ nanords by the constant current method, and Cd S “caps” were formed on the tops for during this process the potentials located at the interfaces had no changes, which was different to the potentiostastic process.（2）After treated with TiCl₄ aqueous solution, there were many TiO₂ particles lay over the surfaces which provided large numbers of new lattice defect centers. At the same conditions, Cd S was also deposited by the constant current method, and we got Cd S QDs formed over the nanorods surfaces. It is because that Cd S had been continuously deposited at these lattice defect centers. Similarly, over the treated TiO₂ nanorods, Cd S was also deposited by the potentiostatic method, and from the SEM images we could see that the fabricated nanorods were also cylinders which were the same to the samples over the original TiO₂ nanorods.（3）Through XRD pattern, it is easy to find that the lattice parameters were the same between the original TiO₂ nanorods and the TiO₂ particles which were formed by the TiCl₄ aqueous solution, but the relative intersities of the diffraction peaks were different for the different growth orientations of the two TiO₂. Meanwhile, comparing the diffraction peaks of all prepared Cd S, there was no difference, and we could get that all Cd S were the same crystalline form when they were fabricated by electrochemical methods. From the UN-visible spectra, it is clear to see that the absorb peaks were redshift when Cd S were deposited long time, and we can analyze the thicknesses of Cd S during the experiment processes through their absorption properties. Under the three electrodes system, the photoelectrochemical properties of all samples were tested, with the result that the photocurrent densities and the fill factors of each series of the samples increased at first and then decreased, and we got the largest photocurrent density when Cd S were deposited over the treated TiO₂ nanorods by the constant current method.（4）In the experiment of deposited Cd Se, the activity of Na₂ Se SO₃ lain in the electrolyte with lower p H would be high, so that the chemical reaction rate was high, in another word, the value of current in the whole current circuit was lager. However, with a higher p H, the activity and the reaction rate would be lower. Thus, utilizing the electrolyte with lower p H, Cd Se would be deposited all areas without order, and TiO₂/Cd Se heterostructures were not well fabricated with bare TiO₂ nanorods. Through the potentiostatic method, core-shell TiO₂/Cd Se heterostructures nanorods were fabricated with the high p H electrolyte. Meanwhile, TiO₂/Cd Se heterostructures whose morphologies were similar to the above TiO₂/Cd S nanorods were also fabricated by the constant current method with the high p H electrolyte. Through the XRD pattern, it is proved that Cd Se deposited by electrochemical methods is blende structure. With the three electrodes system, photoelectrochemical properties of core-shell TiO₂/Cd Se heterostructures nanorods were tested. With the result, we can see that the photocurrent and FF were increased with the deposition and the decreaed, and we get the best photocurrent when Cd Se were deposited 20 min.（5）Over the as-prepared TiO₂/Cd S core-shell heterostructure nanorods, Cd Se shells were deposited by the potentiostatic method. From the SEM images, it is clear to see that Cd S were perfectly coated by Cd Se shells, and the morphologies have significantly differences that the diameters were larger after deposited Cd Se shells and the roughnesses of the surfaces were different. Through the UN-visible spectra, we got that the peaks had a redshift from 480 nm to 650 nm which had proved that the existence of Cd Se shells had broden the absorption area which was lagerly improved the absorption of light and increased the transformation efficiency. From the HRTEM image, we can clearly see the core-shell TiO₂/Cd S/Cd Se heterostructure nanorods and the interfaces among them. Comparing the photoelectrochemical properties results, we can see that the output potentials and the fill factors had no obvious change, however, the photocurrent density of the TiO₂/Cd S/Cd Se core-shell nanorods had lagerly improved from 2.17 m A/cm2 to 6.57 m A/cm2. The result demonstrated that the nanorods covered with Cd Se had broadened absorption area and enhanced the ustilze of solar spectrum energy. Meanwhile, the existence of Cd Se shells promoted the photo electrons transformed follow the stepwise energy levels, and reduced the recombination of the electrons and holes.