TiO₂ Nanosheet Arrays:Photoelectronic And SERS Properties Modified By Facet

As one of the most important n-type semiconductors,TiO₂ has been widely used in solar cells,environmental purification,self-cleaning,antifogging coatings,and electrochemical energy storage.It is well-known that the properties of a metal oxide semiconductor are not only related to their crystal phase,surface area,defects In the lattice,crystal morphology,crystallinity,but also exposed crystal facets.More specifically,both theoretical and experimental studies have shown that the {001}facets of anatase TiO₂ exhibit more active than the thermodynamically more stable{010} and {101} facets.In this work,we synthesized a set of TiO₂ nanosheet arrays?TNS?films with different percentages of {001'} facets on FTO,using HF as a capping and shape-controlling agent.However,the practical application of TiO₂ is hindered by its wide band gap and high recombination rate of the photogenerated electron-hole pairs.Thus,considerable efforts have been dedicated to fabricate {001}exposed TiO₂ composite materials.The work based on {001}TNS composite films can be described as follows.A set of TNS films with different percentages of {001} facets were synthesized by a hydrothermal method.The effects of the HCl concentration on morphology,crystal structure,and optical property have been investigated by SEM,TEM,XRD,Raman,and UV-Vis spectrophotometer.When the HCl concentration increases from 4.8 to 5.2 mol/L,dense nanosheets tend to grow on the FTO substrate surface,and the percentage of exposed {001} facets increases from 76%to 84%.The intensity of Raman peak E.at 144 cm-1 decreases,and the photocurrent increases with more proportion areas of {001} facets are exposed.The sample prepared at HCl concentration of 4.8 mol/L shows the highest photocatalytic activity.Ag/TNS films with high exposed {001} facets were synthesized by a hydrothermal method and Ag reduction method.The morphology,microstructure,optical,and photoelectrochemical?PEC?properties were investigated and discussed compared with those of bare TNS.The Ag/TNS films showed improved absorbance in the visible range and enhanced PEC properties.With the increase of Ag particle,band gap values of the samples vary from 3.21 to 3.08 eV,the photocurrent increases from 18 to 50 ?A/cm2.The excellent PEC properties can be attributed to in-situ growing TNS with high energy {001} facets on FTO combined with surface plasmon resonance?SPR?of Ag nanoparticles.The deposition of CdS nanoparticles on the {101} facets by controlling F-content on the surface of TNS is developed.For CdS/TiO₂,under light irradiation,the photo-generated electrons flow from {001} to {101} facets,meanwhile,the photo-generated electrons transfer from CdS nanoparticles to the TNS.The deposition of CdS nanoparticles on the {101} facets will shorten the path-ways that electron must travel.Herein,the synergistic effect of selective deposition of CdS nanoparticles on the {101} facets of TNS combined with the different band edge positions of {001}and {101} facets,which would facilitate the spatial separation of electrons and holes on different facets,leading to enhanced photoelectronic performance.A Cu₂O/{101}TiO₂ nanosheet?Cu₂O/{101?TNS?array film was prepared by the electrodeposition of Cu₂O on the {101} facets of anatase nanosheet arrays by varying electrodeposition potential.When the applied potentials are-0.1 V and-0.2 V,octahedral Cu₂O particles with side length 0.6-1.0 ?m exist mainly on the {101}facets of the TNS films.When the applied negative potential increases to-0.3 V,Cu₂0 particles have obvious agglomerate and dendritic branching Cu₂O are formed.At deposition potential-0.4 and-0.5 V,the dendritic branching structures gradually decrease and stacking particle become prominent.XPS spectra show two peaks at 932.4 and 952.2 eV,corresponding to Cu 2p3,2 and Cu 2p1/2 of Cu+ or Cu.In addition,satellite peak of Cu 2p3/2 centered at-942 eV can not be seen,indicating no CuO here.UV-Vis diffuse reflection absorption spectra of Cu₂O/{101}TNS array films show two absorption peaks at around 440 and 520 nm,the first peak at-440 nm is related to the Cu₂O sensitization,the second peak at-520 nm can be assigned to the SPR absorption characteristic of the metallic Cu.For TNS,due to different band structures and band edge positions between {001} and {101} facets,Cu₂O/{101} facets of Tio2 have higher band offset value which supply a larger driving force to increase the transport efficiency of carriers.Besides,owing to the directional flow of photogenerated electrons from {001} to {101} facets,the electrodeposition of Cu₂O on the {101} surface of TNS will shorten the path length that the electrons must travel,thus reduce recombination of photogenerated electron-hole pairs.In addition,as the applied negative potential is high enough,a part of Cu+ is reduced to Cu,which is beneficial for the photoexcited electrons transfer from CB of Cu₂O to that of TiO₂.The enhanced PEC properties of Cu₂O/{101}TNS array films can be attributed to the cocontributions of different band edge positions between {001} and {101} facets,Cu₂O-Cu-TiO₂ ternary components,and vertically aligned single-crystal TiO₂ nanosheet arrays structure.TNS decorated with Ag nanoparticles were synthesized on FTO substrate by a hydrothermal method and a magnetron sputtering method.When the Ag sputtering time is 3 s,Ag nannoparticles are distributed on the TNS and the mean sizes of Ag particles are 15.2 nm.When the sputtering time is 10 s,the mean sizes of Ag particles are 50.0 nm.however,after 20 s of sputtering,the surface of TNS is entirely covered with Ag film.With the sputtering time increasing from 3 to 10 s,the SPR peaks red shift from 560 to 715 nm.Using R6G as the probe molecule,we investigated the sensitivity,reversibility and uniformity of SERS substrates.It is noted that the TNS with 10 s Ag-sputtering achieves the highest Raman signals,its EF is estimated to be about 3.0×105,and the RSD of the SERS intensity at 1360 cm-1 is about 13.4%.the intensity of the SERS spectra on TNS/Ag has a significant enhancement compared with Ag film.The excellent SERS performance is attributed to synergistic effect of EM and CT enhancement mechanism.Furthermore,the band bending of the adjacent region between {001} and {101} facets,provides an additional CT from Ag to TNS,which resulting in strong enhancement of the SERS signal.