Photocatalytic Hydrogen Generation Research Based On Photonic-Crystal Heterostructures With Porous Morphologies

ZnO with various nanostructures is widely investigated for high photoelectrochemical(PEC)catalysis performances due to its abundant and inert semiconducting properties with elevated electronic mobility and variable morphologies.Because the solar energy conversion efficiencies could possibly be further enhanced by the introduction of nanophotonic structures with larger surface ratios,ZnO inverse opals(IOs)were achieved by electrochemical deposition,ALD method using H2O and O3 as the oxidant,respectively.And TiO2 protection layers by ALD method using titanium tetrachloride as a precursor enhanced the photochemical stability of ZnO IOs as well as TiO2/ZnO heterogeneous junction in nanometer scale which decrease the chances of recombination of the carriers in the grain.In addition,on the base of ALD ZnO IOs using O3 as the oxidant,the influences of reaction conditions such as ALD reaction temperature,thickness of ZnO,thickness of protective layer of TiO2 and the pore periodicities of the IOs are discussed.Finally,alternate the conformal coating between TiO2 and Al2O3 were studied to improve the light stability further.And the heterostructured photonic crystals with double-layer IO structures with different pore periodicities were also developed for enhancing the PEC performances.The morphology of the films,the elemental analysis,the phase analysis,the crystal quality and the optical absorption properties were determined using a high-resolution scanning electron microscope(SEM),x-ray energy dispersive spectrometer(EDS),the powder X-ray diffractometer(XRD),the photoluminescence(PL)spectra and UV-Vis reflection and absorption spectrum,respectively.The photocatalytic performances were examined in a three-electrode configuration photoelectrochemical cell(PEC)connected to a CHI660E electrochemical workstation.Contents and results of the research are as follows:(1)SEM shows the ZnO IOs of three-dimensional long-range ordered structure can be obtained by electrochemical deposition as well as ALD method using H2O or O3 as the oxidant;Before and after conformal ALD TiO2 samples,the SEM,EDS,XPS analysis,EDS element analysis and the UV-Vis reflection spectrum shows that TiO2 were success coated on the ZnO IOs by ALD method.(2)The photoluminescence spectra,?-? curves and ?-t curves shows that ZnO prepared by ALD using O3 as 0 oxygen source has the relatively strongest intrinsic UV emission peak and the highest photocurrent of 0.8287 mA/cm2,and the ZnO with the worst crystal quality by ALD using H2O as O source share the lowest photocurrent of 0.2213 mA/cm2.(3)When prepared by ALD using O3 as oxygen source,the crystallization quality of ZnO IOs increased as the temperature rising from 160-240?,and the corresponding current 1.0050 mA/cm2 at 240?,30.81%higher than that of 0.7683 mA/cm2 at 160?;As the increase of the ALD circles of ZnO from 170-230 circles,the position of absorption peak has a small red shift,and absorption has been enhanced,but the increase rate has decreased.When deposited 200c,the current of the ZnO IOs is 0.8411 mA/cm2,approximately twice the current of ZnO IOs in 170c and when further increasing to 230c,the photocurrent of ZnO IOs increased by only 10%.(4)When ALD conformal deposition TiO2 in 0c,10c,30c,and 5*(TiO2(5c)+Al2O3(1c)),5*(TiO2(5c)+Al2O3(2c)on the prepared ZnO IOs to further improve the light stability,photocurrent test results show that the light after 20 min,photocurrents and the photocurrent attenuation ratios when compared to 1 min are 0.191(65.41%),0.413(52.58%),0.573(29.80%)and 0.697(10.06%),0.689(9.43%).The increased photocurrent is benefit for the form of TiO2/ZnO heterojunction to improve the transmission efficiency of the carriers;Light stability increased with the increase of TiO2 coating thickness and right amount of Al2O3 can improve the light stability,a little too much will decrease the conductivity of the material due to its insulation.(5)the reflection spectrum shows that the photonic band gaps of ZnO IOs with the diameter of 155 nm,197 nm and 238 nm is respectively at 373.7,439.4 and 495.0 nm.Due to the slow light effects and the light scattering effect of photonic crystals,ZnO IOs has the maximum current 0.9285 mA/cm2 and the minimum current 0.4620 mA/cm2 when with the diameter of 197nm and 155 nm,for its band gap position at 373.7 nm was partly covered spectral region that using for hydrogen production;SEM shows the double laminated ZnO IOs each with the pore diameter of 197 nm to 327 nm.When 197nm ZnO-IOs layer facing to the incident light,the photocurrent reached 1.0529 mA/cm2,higher than that of single layer 197 nm ZnO-IOs of 0.9285 mA/cm2;when 327 nm ZnO-IOs layer facing to the incident light,the photocurrent was only 0.8156 mA/cm2 for the light absorption enhancement area mainly>600 nm which cannot stimulate 197 nm ZnO-IOs layer for the photocatalytic reaction.