Diff usion of Form aldehyde on Rutile TiO2(110)Assisted by Surface Hydroxyl Groups

Da-weiGuanRui-min WangXian-chi JinDong-xu DaiZhi-o MaHong-jun FanXue-m ing Yangad

a.Shanghai Advanced Research Institute,Chinese Academy of Sciences,Shanghai201210,China

b.State Key Laboratory ofMolecularReaction Dynam ics,Dalian Institute ofChem icalPhysics,Chinese Academ y of Science,Dalian 116023,China

c.University ofChinese Academy of Science,Beijing 100049,China

d.School of Physical Science and Technology,ShanghaiTech University,Shanghai201210,China

Diff usion of Form aldehyde on Rutile TiO2(110)Assisted by Surface Hydroxyl Groups

Da-weiGuana,b,c,d?,Rui-min Wangb,c?,Xian-chi Jinb,c?,Dong-xu Daib,Zhi-bo Mab?,Hong-jun Fanb?,Xue-m ing Yanga,b,d?

a.Shanghai Advanced Research Institute,Chinese Academy of Sciences,Shanghai201210,China

b.State Key Laboratory ofMolecularReaction Dynam ics,Dalian Institute ofChem icalPhysics,Chinese Academ y of Science,Dalian 116023,China

c.University ofChinese Academy of Science,Beijing 100049,China

d.School of Physical Science and Technology,ShanghaiTech University,Shanghai201210,China

As the photo-dissociation product ofmethanol on the TiO2(110)surface,the diff usion and desorption processes of formaldehyde(HCHO)were investigated by using scanning tunneling m icroscope(STM)and density functional theory(DFT).Themolecular-level images revealed the HCHO molecules could diffuse and desorb on the surface at 80 K under UV laser irradiation.The diff usion was found to bem ediated by hydrogen adatom s nearby,which were produced from photodissociation ofmethanol.Diff usion ofHCHO wassignificantly decreased when there was only one H adatom near the HCHO molecule.Furthermore,single HCHO molecule adsorbed on the bare TiO2(110)surface was quite stable,little photo-desorption was observed during laser irradiation.Them echanism of hydroxyl groups assisted diff usion of formaldehyde was also investigated using theoretical calculations.

Diff usion,Desorption,Formaldehyde,Scanning tunneling m icroscope

I.INTRODUCTION

The understanding of diff usion process on surfaces is im portant in surface physical and chem ical processes, especially in heterogeneous catalysis.Becausem ost of chem ical reactions occur on active sites such as interface,steps,surface point defects[1?3].Reactants and interm ediates need to be close to those active sites by diff usion p rocess,and the products also need to be carried away from the active positions to keep reacting.So the diffusion processshould bea key step in actual reaction[4?5].Sometim es reactivity could be related w ith diff usivity of reactants and intermediates.Therefore, the research of diffusion process on surface in fundamental level plays an im portant guidance to optim ize catalyst.

Since water splitting in a photo-electrochem ical cell was reported in 1972,titanium dioxide(TiO2)has received extensive and increasing attention because of its potentialapplications in clean hydrogen production[6]. It has been reported that pure TiO2hasweak catalytic activity for water sp litting,and adding somem ethanol could increase the productivity of H2obviously[7].As the most im portant intermediate product,formalde-hyde(HCHO)acted a key role in this catalytic process. The research of HCHO diffusion on TiO2surface could contribute to the understanding of the effect of HCHO in O?H bond cleavage ofwatermolecules.

The rutile TiO2(110)has becom e a well-studied model catalyst surface partly due to its stability [6,8?12].The surface consists of protruding twocoordinated bridging oxygen(Obr)rows and fivecoordinated Ti(Ti5c)rows running in the[001]direction.As a major surface point defect,Obrvacancies can be easily introduced by ultrahigh vacuum(UHV) annealing.In order to investigate the m icrostructure and fundamental chem ical property of TiO2(110),highresolution scanning tunnelingm icroscope(STM)isperformed to provide detailed information at submolecular level.STM studies have shown the surface structure [13,14],m olecular adsorption[15?20],m etal-doping [21?25]and chem ical reactions[26?28].Furthermore, time-lapsed STM studies by Besenbacheret al.could record the fast changing of STM im ages as a STM movie,and they show a dram atic diff usion m ovie about ethanol diffusion along and across the Ti rows by assistant of hydrogen adsorbed on bridging bonded oxygen (BBO-H)[29].

Methanol photocatalysis has been studied on TiO2single crystal surface aswell as on supported nanoparticles using various techniques[30?34]because adding methanol could dram atically enhance the water sp litting effi ciency which wementioned before[7].HCHOis always considered to be the key factor to accelerate the water splitting[35?37].Meanwhile,HCHO could further react w ith another methoxy radical to form m ethyl formate[38,39].And chem ical reaction of HCHO on TiO2(110)has been extensively studied [40?43].Recently,we reported a real-space imaging of a com p lete photocatalytic process for a singlemethanol on TiO2(110)surface by direct high resolution STM imaging[44].Photo-dissociation process can be followed step by step,and HCHO was resolved as a product by experim ental imaging and theoreticalsimulation. Actually theevolution of imageswasdue to thediffusion and desorption of HCHO.A fter the cleavage of both O?H and C?H bond on methanolm olecular,HCHO was adsorbed on the original position.Then it tended to diff use to theadjacent Tisite,and desorbed from the surface finally.This phenomenon reveals the diffusion of HCHO on the TiO2(110)exists which is consistent w ith our p revious conjecture.

In this work,we report all the transfer possibility including diffusion and desorption of HCHO on the surface under UV light irradiation by low-tem perature STM and DFT calculation.The results indicated HCHO could diffuse both paralleland perpendicular to the rowsofObr.In contrastw ith other environmentsof HCHO’sadsorption sites,HCHO presented diff erent activity including diff usion and desorption.Possible reasons are discussed based on the energetic calculation, and the decrease of the diffused barrier is the key point of the whole com p lex phenom ena.

II.EXPERIM ENTS AND COM PUTATIONAL M ETHODS

The experimentswere performed in a UHV chamber equipped w ith a low-tem perature scanning tunneling m icroscope(LT-STM)(M atrix,Om icron).The vacuum in the STM chamberwasmaintained in UHV condition (<4×10?11Torr).CH3OH and HCHO were dosed on TiO2(110)at 80 K,and the realspace STM im ageswere recorded at the same tem perature.In this experiment, wealwaysmadea STM scan on the clean and the dosed CH3OH and HCHO surface before each light irradiation period,and the STM tip is then pu lled back by about 20μm from the surface during CH3OH and HCHO dosing and laser irradiation.A fter a laser irradiation period was com p leted,we then engaged the STM tip to the surface again and found the sam e surface area to trace the change of each individualmolecule.The UV laser irradiation was accom plished by using a 355 nm ns-laser(HIPO,Spectra-Physics)in our photoreaction. The pulse duration is 12 ns,and the high frequency of 50 kHz could ensure that the surfacewas not damaged by this UV laser irradiation.

A ll the calculations were carried out w ith the Viennaab initiosimulation package(VASP)code[45, 46].The generalized gradient approximation(GGA) w ith thespin-polarized Perdew-Burke-Ernzerhof(PBE) functional[47]and p lane augmented waves(PAW)potential[48]were used for characterize op tim ized m olecular structures of TiO2(110).The wave function was expanded by p lane wave w ith kinetic cutoff of 400 eV and density cutoff of650 eV.Our surfacemodelwas cut out of a six-layer slab TiO2crystal to expose the(110) surface[31,49].M onkhorst-Pack grid[50]of(2×1×2)k-points was used for the 4×2 surface unit cell.One HCHO wasadsorbed on the top layer.A ll Ti5csites on the bottom layer were saturated w ith water molecules to maintain the bulk coordination environment[32]. Transition stateswere located by constrainedm inim ization and climbing-im age nudged elastic band m ethods [51,52].

III.RESULTS AND DISCUSSION

A.HCHO ad jacent to various num bers of H atom on TiO2(110)surface at 80 K

FIG.1(a)is a photo-reacted surface which was covered bymethanol.Themethanolwasdosed at 80K and irradiated by UV laser.The bright rows are due to the in-p lane Ti5catom rows,and the dark rowsare two-fold coordinated Obratom rows.Obrvacancies show up as bright spots on the Ti5crows.Most ofmethanol adsorbed on the Ti5csites and two of them reacted after irradiation by 355 nm 50 mW ns-laser which is shown in FIG.1(a).The O?H and one C?H bond on m ethyl cracked and the hydrogen atom s dropped on the adjacent Obrsites.The species of the big spot are one formaldehydem olecule on original adsorption site and two hydrogen atom s adsorbed on ad jacent BBO sites. We reported that three distinct products formed in previous work,which represented reacted m ethanol and the follow ing diff usion and desorption form,respectively [44].Except the diffusion parallel to the Ti5crows we reported,another kind of diff usion was observed,which is shown in FIG.1(e)and(f).The HCHO crossed the Obrrow and adsorbed on the neighbored Ti row which looks like“pole vault”w ith one hydrogen adatom,even theexistenceof BBO-H increased the height of jum ping on topological structure.It is worth noting that these two types of diff usion correspond to the two dim ension on the surface,so it means the formaldehyde can diffuse on the surface easily.We observed both the diffusions happened around the BBO-H and no one cou ld diff use further,which allows us to deduce the hydrogen atom sacted asa im portant role in the diffusion process. Double-irradiation experiment was also performed to measure the diff usion and desorption proportions.A fter the fi rst irradiation,the elliptical spotswhich represent HCHO localized in the m idd le of two BBO-H atom s are m arked.Then the system is illum inated again by 50 mW 355 nm laser for 10 m in.Depending on thein situphoto reaction observation,the statistical resultscould be counted clearly which are shown in Table I. The proportions of diffusion and desorption are 16.1% and 28.5%,respectively.

FIG.1 The diff usion phenom ena of the reacted m ethanol/TiO2surface.Every big protrusion in(a)consists of one HCHO m olecule and two BBOH,and the HCHO m olecule is localized between the two BBOHs.A ll of them are the p roducts of photo-reacted methanol.(b)A fter second time of illum ination,two kinds of diff usion could be observed.(c,e)Detailed im ages of both dissociated processes ofm ethanols which are large versions of(a).(d,f)Detailed im ages of both diff usion processes of HCHO moleculeswhich are large versions of(b).

TABLE I Statistical resu lts of diff usion and desorption probability of HCHO adsorbed ad jacent to 0,1,and 2 BBOH,respectively.

As we know,the dissociation ofmethanol is a typical endotherm ic reaction,the energetics calculation indicated the reaction barrier is 1.6 eV and the energy diff erence between reactant and products is 1.1 eV[31]. This result suggested the energy of photon not only dissociated themethanol but also induced diffusion or desorption of HCHO.Proceeding from this perspective, the HCHO should be very active on Ti5csites.So it is meaningful to investigate binding energy and adsorption situation of HCHO on bare surface.Then the photo-induced diff usion and desorption experiment is performed by the sam e UV light.Surp risingly,the behavior of pure HCHO is very stable under the laser irradiation.The diffusion and desorption proportionsare also listed in Tab le Iwhich showsa significant diff erence from HCHO localized in-between two BBO-Hs.Only four HCHO molecules desorbed from the substrate and no one could diff use to the ad jacent Ti row or neighbored Ti site.Thein situdesorption phenom enon is shown in FIG.2.So we speculated that the two hydrogen adatom s induced the activity of HCHO.

FIG.2 Photo-desorption of HCHO on TiO2(110). (a)HCHO adsorbed on the bare surface.(b)A fter laser irradiation,on ly rare desorption could be observed,which is m arked by a yellow arrow.

FIG.3 The diff usion of HCHO w ith one BBO-H nearby on the hydrate-TiO2(110).(a)Hyd rate-TiO2(110),a target BBO-H ism arked by green×.(b)A fter dosing HCHO molecules,one HCHO molecu lar was localized at the ad jacent T i site.(c)HCHO m olecule diff used to the neighbored T i row after laser irradiation.

To further prove this conjecture,we investigated the movem ent ofHCHO which localized only one BBO-H.If surface hydroxyl could increase the activity of HCHO, the probability of diff usion and desorption should be located in between two statistical results.A special hydrated TiO2(110)surfacewasprepared by dosing water at room temperature the photo-desorption are shown in FIG.3(a)[14,18],water only adsorbed on the vacancies and dissociated spontaneously.The brighter protrusions on the Obrrows are BBO-H atoms.The bigger spots on the surface are im purities to m ark thein situposition.A fter HCHO was dosed on the Ti5csite, we could observe some targetmolecules adsorbed ad jacent to the BBO-H directly.We alsomanipulated som e HCHO molecules which are near the hydrogen atom s move to the adjacent Ti5csite in order to increase the sam ple size.A fter the same laser irradiation,themovem ent of HCHO appeared aswe expected.It could cross the Obrrow and localize just at the symm etrical position.And the probability of diffusion and desorption fall in between situations of no BBO-H and two BBOHs.Considering the assisted eff ect of BBO-H in other system[29],we believe that the hydrogen adatom s on the Obrcould assist the diffusion of HCHO,even reduce the binding energy to make it easy to desorb from the surface.

B.Calculated HCHO diff usion assisted by H atom on TiO2(110)surface at 80 K

To understand how BBO-H atom s could assist the diffusion of HCHO,theoretical calculation hasbeen carried out to investigate the role of BBO-H in HCHO diffusion.The optim ized structures of adsorbed HCHO and transition states of diff usion are shown in FIG.4. Three adsorption structures of HCHO and BBO-H atom s are considered:(i)HCHO molecule adsorbs on the Ti5cw ith no BBO-H atom nearby;(ii)HCHO molecule adsorbs on the Ti5cw ith one BBO-H atom nearby;(iii)HCHO molecule adsorbs on the Ti5cw ith two BBO-H atoms nearby,the adsorption energies of HCHO are 0.49,0.51,and 0.45 eV,respectively. The interactions between oxygen atom s of HCHO and Ti5catom s are diff erent because of the existence of BBO-H.W ithout BBO-H nearby,the Ti-OCH2distance is2.28?A.However,adding H atom son theneighbored BBO sites increase the density of Ti5c’s electron cloud,which leads to the change of Ti-OCH2distance. Thus,w ith one and two BBO-H atom s nearby,the Ti-OCH2distance stretches to 2.37?A,whichm eans theadsorption energy should be decreased.However,the adsorption energy ofHCHO w ith one BBO-H atom nearby is bigger than the other two adsorption structures,and the adsorption energies for all the three structures are very sim ilar by considering the eff ect of hydrogen bonding between HCHOmoleculeand BBO-H atoms.W ithout BBO-H atom nearby,only a weak BBO-H-CHO bond is form ed,the length of the weak bond is 2.26?A. W ith one or two BBO-H atom snearby,the length of the weak hydrogen bond increases to 2.40 and 2.52?A,respectively.However,w ith one BBO-H nearby,a strong hydrogen bonding is formed between the BBO-H and O atom of HCHO,the length is 2.36?A.W hen the HCHO molecule adsorbs between two BBO-H atom s, this length decreases to 2.32?A,the bonding energy is further strengthened.Due to a variety of interactions between HCHO and the substrate,the adsorption energy of HCHO does not decreasew ith increasing BBOH atom s nearby.

W hereas,the diff usion barriers are significantly reduced along w ith the increase of BBO-H atom s.The presence of hydrogen bonding could assist HCHO to diff use across the Obrrow.As the number of BBOH atom s increases from 0 to 2,the energy of diff usion barrier decreases from 0.43 eV to 0.29 eV.We calculated the transition-state structures of HCHO cross the Obrrow as shown in FIG.4(a)?(c).W hen there is no BBO-H,only theweak BBO-H-CHO hydrogen bond exist,the distance is 2.64?A.However,after form ing real hydrogen bondingw ith BBO-H,the distance of BBOHOCH2is 1.95 and 1.83?A for one and two BBO-H,respectively.This iswhy diff usion is easier when H atom s are localized on the BBO sites.Therefore,theenergetic results suggested HCHO ismore active for the diffusion crossObrrow when it adsorbed ad jacent to the BBO-H. And the desorption should be related to the diff usion process,because the adsorption energies are not much diff erent.Due to the easy diffusion of HCHO w ith the BBO-H nearby,we speculate that the desorption process could happen in the position of the intermediate state.Direct desorption were stillhard which need several bonds cleaving together,but on the interm ediate state,the Ti-O-CH2already cracked,only bonding energy of BBOH-O-CH2should be overcome.This two-step escape process from the surface should be reliable in energetics.Sim ilarly,the diff usion as shown in FIG. 1(d)could be exp lained based on the sim ilar two-step process.The HCHO in the immediate state did not escape from the surface or cross the BBO row,but fall down on the ad jacent Ti5csite.So we couldn’t say the existence of BBO-H enhanced the probability of both diffusion and desorption generally,actually it reduced the diffi culty of the diff usion and the diff usion is the precondition of desorption.

FIG.4 Side view of HCHO via BBO diff usion reaction for the zero,one and two BBO-H cases are shown in(a)?(c), respectively.Transition states of diff usion are shown in TS(a),TS(b)and TS(c).The chart is the HCHO diff usion barriers w ith zero,one and two BBO-H,and shown by b lack,red and blue lines,respectively.Tiatom s are shown in white,O atom s are shown in red,C atom s are shown in yellow,and H atom s are shown in green.B lue dotted lines indicate hydrogen bonds.

IV.CONCLUSION

In conclusion,we demonstrate a novel use of hydrogen atom sadsorbed on the BBO sites to investigate the strong assistant eff ect of a paralleled and perpendicular diffusion correspondent to BBO row.Both probabilities ofdiff usion and desorption increased continuously along w ith increasing of localized H atom on ad jacent BBO sites.Theoretical calculations also proved this conclusion and further elucidated the relationship between BBO-H and diffusion.Our results indicated BBO-H only increase the diff usion probability and the desorption should be the next step in the interm ediate state of diff usion.This results proved the BBO-H species p lays an im portant role in diffusion even chem ical reactions.

V.ACKNOW LEDGM ENTS

This work was supported by the Chinese Academy of Sciences,the National Natural Science Foundation of China(No.21225315,No.21173210,No.21673224 and No.21210004)and theM inistry of Science and Technology(No.2013CB834605 and No.2013CB834603).

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ceived on March 12,2017;Accepted on March 25,2017)

?These authors contributed equally to this work.

?Authors to whom correspondence shou ld be add ressed.E-m ail: zhbm a@d icp.ac.cn,fanh j@dicp.ac.cn,xm yang@d icp.ac.cn.