Eff ects of Oxygen Vacancy on the Adsorption of Form aldehyde on Rutile TiO2(110)Surface

Li-m ing Liu,Jin Zhao

HefeiNational Laboratory for Physical Sciences at M icroscale,and Key Laboratory ofStrongly-Coupled Quantum Matter Physics,Chinese Academ y of Sciences,and Department of Physics,and Synergetic Innovation Center ofQuantum Information&Quantum Physics,University ofScience and Technology of China,Hefei230026,China

Eff ects of Oxygen Vacancy on the Adsorption of Form aldehyde on Rutile TiO2(110)Surface

Li-m ing Liu,Jin Zhao?

HefeiNational Laboratory for Physical Sciences at M icroscale,and Key Laboratory ofStrongly-Coupled Quantum Matter Physics,Chinese Academ y of Sciences,and Department of Physics,and Synergetic Innovation Center ofQuantum Information&Quantum Physics,University ofScience and Technology of China,Hefei230026,China

Oxygen vacancy(Ov)has significant influence on physical and chem ical p roperties of TiO2system s,especially on surface catalytic processes.In this work,we investigate the effects of Ovon the adsorption of formaldehyde(HCHO)on TiO2(110)surfaces through fi rstprincip les calculations.W ith the existence of Ov,we find the spatial distribution of surface excess charge can change the relative stability of various adsorption configurations.In this case,the bidentate adsorption at five-coordinated Ti(Ti5c)can be less stable than the monodentate adsorption.And HCHO adsorbed in Ovbecomes themost stable structure. These results are in good agreement w ith experim ental observations,which reconcile the long-standing deviation between the theoretical prediction and experim ental resu lts.This work brings insights into how the excess charge affects the molecule adsorption on metal oxide surface.

TiO2,Formaldehyde,Oxygen vacancy,Excess electrons

I.INTRODUCTION

Titanium dioxide(TiO2)is a versatilematerial for a w ide range ofapplications,to namea few,as UV blocking pigments and sunscreen in industry,m ixed conductor and synthetic single crystals sem iconductor in electronic devices,most im portantly,asabundant and toxic free photocatalyst which have been focused since the water splitting work done by Fujishima and Honda in 1972[1].

Form aldehyde(HCHO)on TiO2is of particular interest because in num erous organic catalytic reactions HCHO isa key species(reactant,intermediate,or product)such as resins synthesis[2],CO2reduction[3],and hydrogen production[4].Besides,HCHO is a common interior pollutantwhich causesseverehealth issues, TiO2-based HCHO decom posing devices play a crucial role in the control of air pollution.

The adsorption of HCHO on TiO2(110)surface has attracted attentions from both theoretical and experimental researchers for the last two decades[5?9].Despite numerous innovationalmaterials based on TiO2were synthesized to catalyze decom posing HCHO,the theoretical predictions and experimental observation of adsorption configurations of HCHO on TiO2were not in accordance,even on themostw idely investigated ru-tile TiO2(110)surface.In 2001 for the fi rst time,Idrisset al.theoretically addressed the adsorption configurations and energies of HCHO on TiO2(110)surface using a cluster m odel based on density functional theory (DFT)calculations[10].A lthough only onemonodentate adsorption configuration was taken into consideration and the cluster model m ay not be an accurate description of extended surface,nevertheless,thiswork pioneered the ambition of using accurate DFT calculations to reveal them icro picture of aldehydes such as HCHO adsorption on m etal oxides.In 2011,Haubrichet al.reported a systematic investigation of the effectsof different surfaceand subsurface point defectson theadsorption ofHCHO on TiO2(110)surfaces through DFT calcu lations[11].They declared a bidentate adsorption configuration which has almost tw ice the adsorption energy of other monodentate adsorption configurationson five-coordinated Ti(Ti5c).From then on, theoreticalstudies continued confi rm ing that the bidentate adsorption configuration is themost stable structure.However,the bidentate adsorption configuration was hard ly confi rm ed from experim ental observations. For instance,in 2013,Yuanet al.reported the photocatalytic oxidation ofmethanolon TiO2(110)surface by means of thermal desorption spectroscopy(TPD) and X-ray photoelectron spectroscopy(XPS)[7].They found the form ation of HCHO and the shift of C 1s level indicated that no bidentate configuration was detected. In early 2016,Yuet al.reported,at low coverage(submonolayer)and low tem peratures(45 K to 65 K),themost abundant species of HCHO on TiO2(110)surface isa chem isorbedmonomer bounded to Ti5csites(Lew is acid)in a tilted m onodentate configuration[12].To understand the deviation between theoretical p redictions and experimental observations is urgent.

Recently,the disagreement between the computational and experimental results started decreasing.In 2015,Zhanget al.reported that the spatial distribution of the extra charge near a Ovsite strongly aff ected the binding of the Ti-bound formaldehyde,especially decreased the stability of bidentate adsorption con figurations[9].In late 2016,Fenget al.reported confi rmation of the bidentate configurations of HCHO on Ti5cthrough STM observations[13].

From the view ofexperimentalendeavor,stoichiom etric TiO2surfaces are very challenging to grow.They are easily being reduced or reconstructed w ith various defects and reconstruction.Among various point defectswhich m ake TiO2a typicaln-type sem iconductor, oxygen vacancy(Ov)is very common.Removing one oxygen atom from the surface would break two Ti?O bonds and release two electrons back to surface,those unpaired electrons are defined as excess charge.It is believed to have significant influence on molecular adsorption and surface catalytic reactions.Therefore,a system atical study on how Ovaff ect the adsorption of HCHO is essential.

In this work,we reported the adsorption of HCHO on both stoichiometric and reduced TiO2(110)surfaces. The interaction between m olecular adsorption and excess charge was discussed in full details.The relative stability of variousadsorption configurations in the presence of excess electrons induced by Ovwas investigated through the com parison of geometries,energies and electronic structures.We found that the excess charge induced by Ovaffects the adsorption of HCHO significantly.W ith the existence of Ov,the bidentate adsorption at Ti5c,can be less stab le than the monodentate adsorption.And HCHO adsorbed in Ovbecomes themost stable structure.These results are in good agreement w ith experimental observations,which interpret the long-standing deviation between the theoretical prediction and experimental results.

II.CALCULATION M ETHODS

Periodic DFT calculations were performed by using the viennaab initiosimulation package(VASP)[14,15]. The generalized gradient approximation(GGA)functional was adopted w ith the Perdew-Burke-Ernzerhof (PBE)exchange-correlation description[16,17].The energy cutoff of 400 eV for p lane-wave basis sets was used to expand the valence electronic wave function w ith the configurationsofC(2s22p2),H(1s1),O(2s22p4) and Ti(4s23d2).The projector augmented wave(PAW) m ethod was used to describe the electron-ion interaction.Dipole correctionswere adopted to cancel the interactions between the excess charge and its periodic im ages for all calculations.A 5×2 slab model containing 3 Ti-O-Ti tri-layer was chosen to simulate the TiO2(110)surfaces.The atom s in the bottom Ti-O-Ti tri-layer were fixed to the positions w ithin bulk TiO2during the structure optim ization.To screen the effects of un-paired electrons from bottom Tiand O,the pseudo hydrogen saturationmethod wasadopted.Since a Ti atom in TiO2bulk off ers four electrons to bind to six O atom s and each O atom in bulk bonded to three Ti,pseudo hydrogens w ith the valence of 2/3(H0.66) and 4/3(H1.33)electron charge were added to bottom O and Ti correspondingly as shown in FIG.1.The H0.66?O and H1.33?Ti bond lengths were determ ined to be 1.02 and 1.88?A by the geometry optim ization procedure which kept all Ti and O atom s fixed in bulk position then allowed the pseudo hydrogen atom s to relax.Then by fixing the bottom layer,the slab was optim ized until the force on each atom is smaller than 0.02 eV/?A.To avoid the interlayer interaction,a vacuum layer of 15?A was added between slabs.To correct the self-interaction error in DFT,we app lyU=4.5 eV on Ti3d orbitals.

The formation energy(Eformation)of oxygen vacancy is defined as:

Eperfectis the total energy of stoichiometric surface,EOvis the total energy of the same surface w ith one oxygen vacancy andEO2is the energy of single oxygen molecule.To com pare the stability of various adsorp-tion configurations,we defined the adsorption energy (Ead)as:

TABLE I Characteristic bonds(as notated in FIG.1)and adsorption energies ofmonodentate and bidentate adsorption con figurations at T i5con reduced T iO2(110)surface,total distortion(TD)is the sum of absolute bond-length change of corresponding bonds com paring to sam e adsorption con figuration on stoichiom etric surface(η1?stoi andη2?stoi).

whereETiO2+HCHOis the total energy of HCHO adsorbed on TiO2(110)surface,EHCHOandETiO2are total energies of single HCHO m olecule and clean TiO2(110)surface correspondingly.

III.RESULTS AND DISCUSSION

A.The adsorption of HCHO on stoichiom etric TiO2(110) surface

In agreem ent w ith previous reports[9,11,13,18, 19],the calculated most stable adsorption configuration of HCHO on pseudo hydrogen saturated stoichiometric TiO2(110)surface is thebidentateadsorption configuration(η2)at Ti5c.FIG.1 shows the top and side viewsof twomonodentate(η1)and one bidentateη2adsorption structures on Ti5c.Forη2(FIG.1(c)),two C?O bonds form sp3hybridization other than sp2hybridization as HCHO m olecule in vacuum orm onodentate adsorption configurations(η1-Ob,η1-Op).Sinceη1-Obandη1-Opare almost identicaleither in geometry(FIG.1(a)and (b)),adsorption energy or electronic structure[20],theη1-Obconfiguration was chosen as the representative for the two monodentate structures of HCHO at Ti5cand remarked asη1.The adsorption energy difference between bidentate and monodentate adsorp tion con figurations is 0.7 eV as shown in Tab le I.

B.Adsorption of HCHO on reduced T iO2(110)surface w ith Ov

1.BBOvas a common defect on TiO2(110)surface

For TiO2(110)surface,bridge bonded oxygen vacancy(BBOv)is known as the m ost stable Ovstructure.It ismore stable than in-plane oxygen vacancy (IPOv)and Ovin sub-surface by 0.36 eV in our calculations.These results are in accordance w ith previous calculations[21].Each Ovinduces two excess electrons to the system.The two excess electrons prefer occupying sub-surface Tiwhich is in agreement w ith previous calculations[22,23].For reduced TiO2(110)surface w ith BBOv,there are two sites favorable for HCHO adsorption.One is at surface Ti5c,the other is in BBOv.

2.HCHO adsorption on Ti5c

For the adsorption of HCHO at Ti5con reduced TiO2(110)surface w ith BBOv,although adsorption configurations are preserved from those on stoichiom etric surface,theadsorption energiesand relative stability among these adsorption configurations are significantly affected by the presence of BBOv.

The fi rst distinguishing feature is that the adsorption energies decrease about 0.4 eV for monodentate adsorption configuration(η1)on reduced TiO2(110) surface than on stoichiometric surface.We labeled diff erent Ti5csites according to their relative position w ith BBOvas online?n,online?nn,and offl ine as shown in FIG.2.As shown in Table I,for monodentate adsorption configurations,the adsorption energies forη1?offl ine,η1?online?nn,η1?online?n are?1.24,?1.19,and?1.17 eV.Com paring to?0.83 eV ofHCHO on stoichiometric surface inmonodentate configurations,molecular adsorption on reduced surface is stabilized.The reason for the energy shift is the electrostatic interaction between excess charge induced by BBOvand dipole m om ent from adsorbed HCHO molecules.

FIG.2 The top view(along[110]direction)of adsorption con figurationsof HCHO on TiO2(110)surfacew ith Ov.Different configurations are labeled as“offl ine,on line?nn and online?n”by the relative distance between the adsorption sites and Ov.

Another significant effect is the site sensitivity for bidentate adsorption configuration(η2)of HCHO on reduced TiO2(110)surface.It is clearly presented that, forη2configurations,the adsorption energy could vary from?1.99 eV to?1.12 eV at diff erent Ti5csites (FIG.2(a)?(c)).The adsorption energy dependents on the distance between HCHO and BBOv.At offl ine site, HCHO binds to bridge bonded oxygen which has no Ovin the same row.The local environment is sim ilar to that on stoichiometric surface except there is electrostatic interaction between sub-surface excess electron and adsorbed HCHO m olecule.Thus,the adsorption energy decreased from?1.57 eV on stoichiometric surface to?1.99 eV on reduced surface.The 0.4 eV energy shift is sim ilar to the adsorption energies change ofm onodentate adsorption configuration(η1)between stoichiometric and reduced surfaces.For HCHO at Ti5ccloser to BBOv(η2?online?nn andη2?online?n),lattice distortion becom es larger after the m olecular adsorption in Table I.The aggravated distortion would dramatically affect the spatial distribution of excess electrons.The changed distribution of excess electrons alters the electrostatic interaction strength and the adsorption energies forη2?online?nn andη2?online?n become?1.76 and?1.12 eV.

FIG.3 Com parison of adsorption energyvs.adsorp tion site between monodentate and bidentate adsorption con figurations on stoichiom etric and reduced T iO2(110)surfaces.

Thesignificant sitesensitivity ofbidentateadsorption configuration is due to the change of the localization of excess charge induced by BBOv.Forη2?offl ine,sim ilar to the m onodentate adsorption configurations,the excess charge all localized in sub-surface separately as shown in FIG.4(a)?(d)correspondingly.In this case, the overallelectrostatic interaction isattractive leading to stabilizemolecular adsorption.For bidentateadsorption structures(η2?on line?nn andη2?online?n),the aggravated distortion induced by molecular adsorption would drive excess charge to surface localizing closely around BBOvsite then the overall electrostatic interaction is repulsive and them olecular adsorption is destabilized as shown in FIG.4(e)and(f).

It should be noticed that the bidentate adsorption configuration,which is them ost stable structure on stoichiometric surface,could become the least stable one w ith the interaction w ith Ov.Especially when HCHO adsorbing at Ti5cclose to BBOvin FIG.3,the bidentate adsorption configuration(η2?online?n)is less stable than all other monodentate adsorption configurations.

3.HCHO adsorption in BBOv

The adsorption of HCHO in BBOvheals the vacancy in a manner of inserting oxygen end of HCHO into the vacancy.The symm etric adsorption configuration(sym?η1-Ov)w ith m olecular oxygen bonded to two five-folded Ti in BBOvhas the adsorption energy of?1.46 eV(FIG.5(a)?(c)).Beside this symmetric m onodentate adsorption configuration(FIG.5(a)), another asymm etric m onodentate adsorption structure was found w ith an extra hydrogen bond betweenmolecular H and ad jacent bridge oxygen(sym?η1-Ov).Despite an extra hydrogen bond formed(FIG.5(b)),the asymm etric adsorption configuration has theadsorp tion energy of?1.45 eV which is sim ilar to symmetric one because the Ti?Ombonds are stretched on one side and com pressed on the other side.Notice that the adsorption energies ofmonodentate adsorption configurations at BBOvare,not only about 0.6 eV lower than monodentate adsorption con figurations on stoichiom etric surface,but also about 0.2 eV lower than m onodentate adsorption configurations at Ti5cw ith Ov.

FIG.4 Distribution of Ovinduced excess electrons after adsorption of HCHO on reduced TiO2(110)in side(along [001]direction)and top(along[110]direction)view s.The dashed red circles fi lled w ith white represent the position of BBOvwhile the em p ty dashed red circles depict healed BBOvby HCHO adsorption.Spatial distribution(orbitals) of two excess electronswere depicted in yellow.

A bidentate adsorption configuration(η2-Ov)can be formed w ith an extra chem ical bond of C?Ob.The adsorption energy ofη2-Ovis?1.70 eV which is the most stable one among all presented adsorption configurations of HCHO close to BBOv(FIG.5(c)).In other words,if there are BBOvson TiO2(110)surface,HCHO favors adsorbing at Ovsites rather than Ti5csites near BBOv.

FIG.5 Adsorp tion con figurations of HCHO in BBOvon TiO2(110)surface in side and top views(along[001][1-10], and[110]directions for upper,m idd le and lower schem as correspondingly in each box).

The adsorption of HCHO in BBOvhasm inor influence on the distribution of excess charge.For sym?η1-Ovand asym?η1-Ov,a sm all am ount of excess electron (less than 20%)involved in the Ti?Ombond formation,themajority of excess electrons(larger than 80%) still locate at sub-surface(FIG.4(g)and(h)).Forη2-Ovconfiguration,another C?O bond form ing sp3hybridization then no need of excess charge for chem ical bonding thus the excess electrons remain localized in sub-surface(FIG.4(i)).

4.Eff ects of HCHO adsorption on the electronic structure

M olecular adsorption aff ects the electronic structure of reduced TiO2(110)surface in twoways.On onehand, molecular orbitals could emerge in band gap as HOMO or LUMO of the system;on the other hand,the defect states originated from BBOvcould be aff ected by molecular adsorption.

For m onodentate adsorption configurations at Ti5c(η1?offl ine,η1?online?nn,andη1?online?n),the highest occupied molecular orbitalsare about 0.3 eV bellow the valence band maximum(VBM)of TiO2(FIG.6). The gap states originated from BBOvare stabilized by molecular adsorption w ith the downward energy shift of0.5 eV com pared to clear reduced surface.The stabilization of excess charge states is because of the electrostatic attraction between adsorbed HCHO and excess electrons.

FIG.6 Density of states(DOS)for various adsorption configurations of HCHO on reduced T iO2(110)surface.B lack line represents total DOSand red line rep resents the partial DOSof HCHO w ith them agnification of10 tim es for clarity.

For bidentate adsorption configurations at Ti5c(η2?offl ine,η2?online?nn,andη2?online?n),molecular orbital contributes a gap state about 0.2 eV above VBM.As the adsorption of HCHO approaching to BBOv(fromη2?offl ine toη2?online?nn thenη2?on line?n),the m olecular gap state shifts upward a little bit due to the larger local structural distortion in Table I.Gap states induced by BBOvare stabilized forη2?offl ine andη2?online?nn configurationsalso due to the electrostatic attraction between adsorbed HCHO and excess electrons.Forη2?online?n configuration which is close to BBOv,these excess charge gap states are destabilized by shifting up about 0.1 eV because the two excess electrons are closely distributed around BBOvsite due tomolecular adsorption.

As for HCHO adsorbing in BBOvin sym?η1-Ovand asym?η1-Ovconfigurations,an empty state from the hybridization of O 2p and Ti3d emerged about 0.7 eV bellow conduction band m inimum(CBM).The excess charge states have about 0.5 eV energy downward shift sim ilar to the monodentate adsorption at Ti5c(η1?offl ine,η1?online?nn,andη1?online?n).Molecular adsorption is stabilized by excess charge located mainly(larger than 80%)in sub-layer.For HCHO in BBOvinη2-Ovconfiguration,molecular O 2p orbitals contribute a gap state about 0.1 eV above VBM which is sim ilar to HCHO at Ti5cinη2configuration.But the excess charge is still located in sub-surfacewhich is sim ilar to HCHO at Ti5cinη1configuration.

From above analysis,we argued that the bidentate adsorption configurations of HCHO at Ti5chave great potential to be the hole scavenger,while HCHO in BBOvin the sym?η1-Ovand asym?η1-Ovconfigurations have the great potential to be the electron scavenger in photocatalytic process on TiO2(110)surfaces.And molecular adsorption could either stabilize or destabilize the excess charge states by altering the electrostatic interaction between molecu lar adsorption and BBOv.

IV.CONCLUSION

In this work,we investigated the adsorption of HCHO on stoichiometric and reduced TiO2(110)surfaces through fi rst-princip les calcu lations.Com pared to stoichiom etric surface,oxygen vacancy in reduced TiO2would induce excess electrons to the system and affect the relative stability ofmolecular adsorption configurations.For HCHO adsorbing at Ti5c,on onehand,excess electronswould stabilizemolecular adsorption formonodentate adsorption.On the other hand,for bidentate adsorption,theadsorption stability dependsstrongly on the adsorption site.The adsorption is stabilized when theadsorption is far away from BBOv.Yet it can besignificantly destabilized when the adsorption is close to BBOvand becom es less stable than the monodentate adsorption.The adsorption in BBOvis them ost stable configuration near BBOvamong all the configurations we investigated.Our calculations reconcile the longstanding discrepancies between theoretical predictions and experim ental observations of the HCHO/TiO2system and provide valuable insights into the eff ects of Ovon themolecular adsorption on TiO2.

V.ACKNOW LEDGM ENTS

Thisworkwassupported by theNationalNaturalScience Foundation of China(No.21373190,No.21421063, No.11620101003)and the National Key Foundation of China,Department of Science&Technology (No.2016YFA0200600 and No.2016YFA0200604),the Fundam ental Research Funds for the Central Universities of China(No.W K 3510000005),the support of National Science Foundation(No.CHE-1213189 and No.CHE-1565704).Calculations were performed at Environmental M olecular Sciences Laboratory at the PNNL,a user facility sponsored by the DOE O ffi ce of Biological and Environmental Research.

[1]A.Fu jishima and K.Honda,Nature 238,37(1972).

[2]Y.H.Lee,C.A.K im,W.H.Jang,H.J.Choi,and M. S.Jhon,Polymer 42,8277(2001).

[3]B.Kaesler and P.Sch¨onheit,Eur.J.Biochem.186,309 (1989).

[4]C.W inner and G.Gottschalk,FEMSM icrobiol.Lett. 65,259(1989).

[5]G.Busca,J.Lam otte,J.C.Lavalley,and V.Lorenzelli, J.Am.Chem.Soc.109,5197(1987).

[6]C.B.Xu,W.S.Yang,Q.Guo,D.X.Dai,T.K.M inton, and X.M.Yang,J.Phys.Chem.Lett.4,2668(2013).

[7]Q.Yuan,Z.F.W u,Y.K.Jin,L.S.Xu,F.Xiong,Y.S. M a,and W.X.Huang,J.Am.Chem.Soc.135,5212 (2013).

[8]Q.Yuan,Z.F.W u,Y.K.Jin,F.X iong,and W.X. Huang,J.Phys.Chem.C 118,20420(2014).

[9]Z.R.Zhang,M.R.Tang,Z.T.Wang,Z.Ke,Y.B. X ia,K.T.Park,I.Lyubinetsky,Z.Dohn′alek,and Q. F.Ge,Top.Catal.58,103(2015).

[10]L.K ieu,P.Boyd,and H.Id riss,J.M ol.Catal.A-Chem. 176,117(2001).

[11]J.Haubrich,E.Kaxiras,and C.M.Friend,Chem istry 17,4496(2011).

[12]X.J.Yu,Z.R.Zhang,C.W.Yang,F.Bebensee,S. Heissler,A.Nefedov,M.R.Tang,Q.F.Ge,L.Chen, B.D.Kay,Z.Dohn′alek,Y.M.Wang,and C.W¨oll Christof,J.Phys.Chem.C 120,12626(2016).

[13]H.Feng,L.M.Liu,S.H.Dong,X.F.Cui,J.Zhao,and B.Wang,J.Phys.Chem.C 120,24287(2016).

[14]G.K resse and J.Hafner,Phys.Rev.B 48,13115(1993).

[15]G.K resse and J.Furthmuller,Phys.Rev.B 54,11169 (1996).

[16]J.P.Perdew,K.Burke,and M.Ernzerhof,Phys.Rev. Lett.77,3865(1996).

[17]J.P.Perdew,M.Em zerhof,and K.Burke,J.Chem. Phys.105,9982(1996).

[18]H.Z.Liu,X.Wang,C.X.Pan,and K.M.Liew,J. Phys.Chem.C 116,8044(2012).

[19]M.R.Tang,Z.R.Zhang,and Q.F.Ge,Catal.Today 274,103(2016).

[20]L.M.Liu and J.Zhao,Surf.Sci.652,156(2016).

[21]M.V.Ganduglia-Pirovano,A.Hofm ann,and J.Sauer, Surf.Sci.Rep.62,219(2007).

[22]P.M.Kowalski,M.F.Cam ellone,N.N.Nair,B.M eyer, and D.Marx,Phys.Rev.Lett.105,146405(2010).

[23]T.Shibuya,K.Yasuoka,S.M irbt,and B.Sanyal,J. Phys.Condens.M atter 24,435504(2012).

ceived on March 22,2017;Accepted on March 31,2017)

?Author to whom correspondence shou ld be addressed.E-m ail: zhao jin@ustc.edu.cn