High-performance Nanocatalysts And Gas Sensors Design With Atomic Layer Deposition And Interface Tailoring

With the development of society,there are increasing opportunities for green production and sustainable development,the development of chemical industry and environmental monitoring and protection and other industries because of pursuit of a better life.The preparation of high efficiency catalyst to improve the activity selectivity in chemical production and reduce the emission of by-products or pollutants has always been a research focus.Environmental monitoring is a useful method for monitoring and controlling the pollutants in chemical production.Using interfacial active sites to improve the catalytic performance of catalysts and the sensing performance of sensitive materials is the most common and effective method at present.Interfacial sites have been confirmed as the active sites in chemical reactions,however,it is hard to accurately control the decoration sites and the atomic configurations of interfacial sites by traditional methods.More attentions have been paid to understand the mechanism with band theory on heterostructures.It is interesting to understand the sensing mechanism in interfacial active sites.It is theoretical value and practical significance to design and develop more efficient gas sensitive materials by studying the mechanism of interface active sites in efficient catalysts.In consider the problems:accurate regulation of the interface active sites,confirmation of atomic configurations for active interfacial sites,agglomeration of nano-sensitive materials at high temperature,low sensitivity and poor selectivity for the sensing material,we employed the atomic layer deposition,which is of accurate regulation at atomic level,to increase the accuracy for regulation of interfacial sites and to increase the sensing performance.In this work,we prepared the MO-M(metal oxide-metal)interfacial catalysts and MO-MO(metal oxide-metal oxide)interfacial gas sensitive materials via interface strategy with ALD.Several contents are involved in this work,including(ⅰ)the university of precise decoration or facet-selectivity for sub-2 nm Pt NPs with different metal oxide(Al2O3、TiO2、NiO、CoOx and FeOx);(ⅱ)the atomic configuration of interfacial active sites CoOx-Pt with Pt-Co-O-(Co)x is reported for the first time;(ⅲ)the in-situ fabrication of ZnO and CoOx-ZnO film on MEMS as highly sensitive H2S sensors;(ⅳ)directly confirmation of confinement effects with NiO confined in SnO2 nanocoils and application of confinement effects improving gas sensing performance.The main content are as follows:1.The university of precise decoration or facet-selectivity for sub-2 nm Pt NPs with different metal oxide.Al2O3,NiO,FeOx,CoOx and TiO2 with different cycle numbers were used to decorate the surface of Pt NPs by ALD to obtain a series of ultra thin oxides modified Pt NPs(Mo-M structure)catalysts.Taking FeOx-Pt catalyst as an example,transmission electron microscopy(TEM),CO in situ infrared(CO-DRIFTs)and density functional theory(DFT)calculations confirmed that the oxide was decorated on low-coordinated sites on Pt surface.Furthermore,the catalysts with interfacial sites between low-coordination sites and oxide was obtained,and the coordination environment between oxides and Pt NPs was precisely regulated.The maximum proportion of interfacial sites was realized by adjusting ALD cycle number.The selectivity of Pt30Fe30 catalyst in selective hydrogenation of cinnamaldehyde was the highest(85%),nearly twice that of Pt30 catalyst.The XPS and XAFS confirmed that the electrons transferred from FeOx to Pt,and the unique local electron structure promoted the catalytic hydrogenation of C=O.Furthermore,the catalysts with precise decoration of different oxides(Al2O3,NiO,CoOx and TiO2)were prepared.The universality of ALD surface precise modification for interfacial sites coordination environment was further verified by theoretical calculation.DFT theoretical calculation also proved the universality of ALD in precisely modifying low coordination site and interface site coordination environment.2.The confirmation of atomic configuration Pt-Co-O-(Co)x structure for interfacial active sites CoOx-Pt.We reported the precise decoration of Pt NPs lowcoordinated sites by different cycles of CoOx(10,20,30,40,50,70 and 100).The obtained PtCox catalysts were employed for the selective hydrogenation of cinnamaldehyde.The activity and selectivity of PtCox catalysts with interfacial sites increased first and then decreased with increase of CoOx cycles.PtCo40 catalyst displayed the highest selectivity(94%)among these catalysts.Detailed analyses revealed that PtCo40 catalyst has the maximum interfacial active sites.The directly observed interfacial atomic structure was confirmed as Pt-Co-O-(Co)x by AC-HAADFSTEM and EELS.Extended X-ray absorption fine structure(EXAFS)demonstrated demonstrate the existence of Pt-Co bond and Pt-Co-O-(Co)x was further confirmed.XPS,XANES and DFT differential charges results revealed the unique local electronic structure of Co electron transfer to Pt.DFT calculation shows that the unique local electronic structure limits the adsorption of C=C bond at the interface site.The Pt-CoO-(Co)x as active interfacial sites were further confirmed by CAL-DRIFTS.3.In-situ fabrication of ZnO and CoOx-ZnO film on MEMS as highly sensitive H2S sensors.The ultra-thin ZnO film and CoOx/ZnO heterogeneous film with active interfacial sites was in-situ deposited on Micro-Electro-Mechanical Systems(MEMS)as H2S sensor.ALD was employed to controllably fabricate the uniform ZnO film with different thickness on MEMS chips for constructing gas sensors.The sensing response of H2S increased firstly and then decreased with the increase of CoOx cycles.250 ALD ZnO based MEMS sensors displayed excellent sensing performance,including very high sensitivity and selectivity to H2S with a limit of detection(LOD)on ppb level,good stability and quick response and recover dynamics under 70 mW of power dissipation.The sensing film has higher to 1300 times in mechanical strength than the traditional film from drop-coating.ALD CoOx was deposited on ZnO surface to obtain CoOx/ZnO heterojunction and active interfacial sites.The ultra-thin film(20 nm)with 50 ALD CoOx decorated on 250 ALD ZnO displays excellent sensing performance,including very high response(4.45@200 ppb)and selectivity to H2S with a limit of detection(LOD)of 0.38 ppb,long-term sensing stability,high response/recovery performance(7.5 s/15.7 s)and mechanical strength at 230℃.A series of characterizations and DFT calculation uncovered that the heterojunction film thickness with Debye length,the oxygen vacancies and the synergistic effect of active interfacial sites are the main reasons for the high sensing performance.The strategy by fabrication of CoOx/ZnO heterogeneous film within Debye length and employing synergistic effect of active interfacial sites offers a promising route for the design of environmentally friendly gas sensors of CoOx/ZnO.Furthermore,the ALD technique provides a facile in-situ strategy and highthroughput fabrication of MEMS gas sensors.4.Directly confirmation of confinement effects and application in improving gas sensing performance.We reported the fabrication of a novel structure that NiO nanoparticles was confined in SnO2 nanocoils(I-NiSnNCs)via atomic layer deposition(ALD).TEM and EDS confirmed the successful fabrication of NiSnNCs samples.The I-NiSnNCs displayed higher sensing performance to hydrogen than NiO-SnO2 film and NiO outside of-NiSnNCs and NiO both outside and inside of NiSnNCs sensors.These give a directly confirmation of confinement effects in nanoscale confined space.XPS,PL,Raman,and UV-vis characterized the oxygen vacancies in SnO2.The DFT calculation further reveals differences of adsorption energy in oxygen vacancies.Moreover,great effort has been done to demonstrated the gas concentration within the spaced-confined nanocoils.The interaction between NiO and SnO2 and the spillover of dissociated H on NiO to interfacial active sites lead the synergistic effect for NiO and SnO2,leading to funneling effect for I-NiSnNCs,which results in high sensing performance to hydrogen.Funneling effect caused by enrichment and electronic interaction for I-NiSnNCs sensor give a directly proof for confinement effects.This work offers a novel strategy to design sensitive gas sensors without noble metal.Furthermore,the space confined sensors offer a directly and unique insight to understand the confinement effects in design and application of catalysts.The I-NiSnNCs displayed higher sensing performance to hydrogen than NiO-SnO2 film and NiO outside of-NiSnNCs and NiO both outside and inside of NiSnNCs sensors.The synergistic effect of interfacial sites is attributed to the reaction between adsorbed H on NiO and adsorbed O at interfacial active sites."Funneling effect" formed because the concentration of hydrogen on NiO surface is higher than that of the gas state as the adsorbed H quickly react at interfacial active sites.The gas sensors offer a new way to directly confirm the confinement effect.And the confinement effects in return give a novel way to design sensing materials.