A reply to the Comment by M. Calatayud et al. on "Imaging of the Hydrogen Subsurface Site in Rutile TiO2" (Physical Review Letters, Volume 102, Issue 13). DOI: 10.1103/physrevlett.102.136103.Peer reviewe

Besenbacher, Flemming

Enevoldsen, Georg H.

Foster, Adam S.

Hammer, Bjørk

Hofer, Werner A.

Jensen, Mona C. R.

Lauritsen, Jeppe V.

Pinto, Henry P.

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This is an electronic reprint of the original article.This reprint may differ from the original in pagination and typographic detail.Author(s): Enevoldsen, Georg H. & Pinto, Henry P. & Foster, Adam S. & Jensen,Mona C. R. & Hofer, Werner A. & Hammer, Bjørk & Lauritsen, JeppeV. & Besenbacher, FlemmingTitle: Reply to the comment on "Imaging of the Hydrogen Subsurface Sitein Rutile TiO2''Year: 2010Version: Final published versionPlease cite the original version:Enevoldsen, Georg H. & Pinto, Henry P. & Foster, Adam S. & Jensen, Mona C. R. &Hofer, Werner A. & Hammer, Bjørk & Lauritsen, Jeppe V. & Besenbacher, Flemming.2010. Reply to the comment on "Imaging of the Hydrogen Subsurface Site in RutileTiO2''. Physical Review Letters. Volume 104, Issue 11. 119604/1. ISSN 0031-9007(printed). DOI: 10.1103/physrevlett.104.119604.Rights: © 2010 American Physical Society (APS). This is the accepted version of the following article: Enevoldsen,Georg H. ; Pinto, Henry P. ; Foster, Adam S. ; Jensen, Mona C. R. ; Hofer, Werner A. ; Hammer, Bjørk ;Lauritsen, Jeppe V. ; Besenbacher, Flemming. 2010. Reply to the comment on "Imaging of the HydrogenSubsurface Site in Rutile TiO2''. Physical Review Letters. Volume 104, Issue 11. 119604/1. ISSN 0031-9007(printed). DOI: 10.1103/physrevlett.104.119604, which has been published in final form athttp://journals.aps.org/prl/pdf/10.1103/PhysRevLett.104.119604.All material supplied via Aaltodoc is protected by copyright and other intellectual property rights, andduplication or sale of all or part of any of the repository collections is not permitted, except that material maybe duplicated by you for your research use or educational purposes in electronic or print form. You mustobtain permission for any other use. Electronic or print copies may not be offered, whether for sale orotherwise to anyone who is not an authorised user.Powered by TCPDF (www.tcpdf.org)Enevoldsen et al. Reply: In Ref. [1], from simultaneouslyrecorded atomic force microscopy and STM images anddensity functional theory (DFT) calculations, we showedthat hydrogen species could be atomically resolved in thesurface and subsurface of rutile TiO2. To explain an ap-parent tip-assisted movement of H atoms between the twosites, we predicted a barrier for hydrogen to diffuse directlyfrom bridging oxygen (Ob) on TiO2 (110) to a subsurfacesite (Osub) of 2.4 eV for a H coverage of 1=8 ML usingDFT. This barrier is in agreement with the 2.6 eV given inrecent calculations for a higher coverage of 1 ML inRef. [2], but is now questioned in the precedingComment, Ref. [3]. A number of studies, Refs. [2–4],suggest that a lower energy path via an intermediate in-plane oxygen site (O3f) exists. For a coverage of 2 ML thebarrier is predicted to be 1.1 eV [4], and for a coverage of1 ML it is further reduced to 0.9 eV [2].To investigate whether the alternative path has the low-est energy for the low coverage case (<0:1 ML) reflectingthe experimental conditions in Ref. [1], we calculated itusing the same methodology [1,5]. Specifically, DFT cal-culations were performed using the plane wave VASP code[6] with the local spin density approximation plus HubbardU [ðLSDAÞ þU] method, which was shown to give betteragreement with experiments [5,7]. In order to see if thefunctional plays a role in the size of the barrier, we alsocalculated it using the standard generalized gradient ap-proximation (GGA) method.Our results (see Fig. 1) show that the barrier for Ob toO3f is around 1.0 eV for the GGA functional, but there isno local minimum at the O3f site with LSDAþU.However, in both cases the rate determining barrier isfound when passing through the Ti-O plane, and this isabout 1.6 eV for GGA and 1.8 eV for LSDAþU. This islower than the original prediction of Ref. [1], which waslikely overestimated due to constraints on the migrationpath. The barriers remain significantly higher than thepredictions of Refs. [2,4], and assuming the barrier forthe total path was also considered, the difference must bedue to the higher coverages. However, we also speculatethat Calatayud et al. [3] might have neglected to includethe reaction energy from Ob to O3f to the energy barrierfrom O3f to Osub. In any case, the fact that the barrier issmaller than originally considered actually strengthens theconclusion of our Letter [1] that H is manipulated to asubsurface site.We disagree with the interpretation of room temperaturebehavior in the Comment [3]. Regardless of the H cover-age, calculations predict a diffusion barrier of at least 1 eV.This value is too high for thermally activated processes tobe active at room temperature, and spontaneous diffusionof H should be an extremely rare process. Indeed, thedynamics of surface H was studied experimentally inhigh statistical detail in recent fast STM studies (e.g.,[8]). Here, the removal or displacement of H adatoms,other than through a surface reaction, was never observed.Georg H. Enevoldsen,1 Henry P. Pinto,2Adam S. Foster,2,3,* Mona C. R. Jensen,1Werner A. Hofer,4 Bjørk Hammer,1 Jeppe V. Lauritsen,1and Flemming Besenbacher1,†1Interdisciplinary Nanoscience Center (iNANO) andDepartment of Physics and AstronomyAarhus UniversityAarhus, Denmark2Department of Applied PhysicsAalto UniversityP.O. Box 1100, FI-02015 TKK Helsinki, Finland3Department of PhysicsTampere University of TechnologyP.O. Box 692, 33101 Tampere, Finland4Surface Science Research CentreThe University of LiverpoolLiverpool, United KingdomReceived 30 November 2009; published 19 March 2010DOI: 10.1103/PhysRevLett.104.119604PACS numbers: 68.47.Gh, 61.72.jj, 68.37.Ef, 68.37.Ps*adam.foster@tut.fi†fbe@inano.dk[1] G. H. Enevoldsen et al., Phys. Rev. Lett. 102, 136103(2009).[2] P.M. Kowalski, B. Meyer, and D. Marx, Phys. Rev. B 79,115410 (2009).[3] M. Calatayud et al., preceding Comment, Phys. Rev. Lett.104, 119603 (2010).[4] X.-L. Yin et al., Chem. Phys. Chem. 9, 253 (2008).[5] G. H. Enevoldsen et al., Phys. Rev. B 78, 045416 (2008).[6] G. Kresse and J. Furthmu¨ller, Comput. Mater. Sci. 6, 15(1996).[7] H. P. Pinto et al., Nanotechnology 20, 264020 (2009).[8] J. Matthiesen et al., ACS Nano 3, 517 (2009).FIG. 1 (color online). Calculated diffusion barriers for hydro-gen migration from surface to subsurface site using (a) GGA and(b) LSDAþU.PRL 104, 119604 (2010) P HY S I CA L R EV I EW LE T T E R Sweek ending19 MARCH 20100031-9007=10=104(11)=119604(1) 119604-1  2010 The American Physical Society

Reply to the comment on "Imaging of the Hydrogen Subsurface Site in Rutile TiO2''

Aaltodoc

This is an electronic reprint of the original article.
This reprint may differ from the original in pagination and typographic detail.
Author(s): Enevoldsen, Georg H. & Pinto, Henry P. & Foster, Adam S. & Jensen,
Mona C. R. & Hofer, Werner A. & Hammer, Bjørk & Lauritsen, Jeppe
V. & Besenbacher, Flemming
Title: Reply to the comment on "Imaging of the Hydrogen Subsurface Site
in Rutile TiO2''
Year: 2010
Version: Final published version
Please cite the original version:
Enevoldsen, Georg H. & Pinto, Henry P. & Foster, Adam S. & Jensen, Mona C. R. &
Hofer, Werner A. & Hammer, Bjørk & Lauritsen, Jeppe V. & Besenbacher, Flemming.
2010. Reply to the comment on "Imaging of the Hydrogen Subsurface Site in Rutile
TiO2''. Physical Review Letters. Volume 104, Issue 11. 119604/1. ISSN 0031-9007
(printed). DOI: 10.1103/physrevlett.104.119604.
Rights: © 2010 American Physical Society (APS). This is the accepted version of the following article: Enevoldsen,
Georg H. ; Pinto, Henry P. ; Foster, Adam S. ; Jensen, Mona C. R. ; Hofer, Werner A. ; Hammer, Bjørk ;
Lauritsen, Jeppe V. ; Besenbacher, Flemming. 2010. Reply to the comment on "Imaging of the Hydrogen
Subsurface Site in Rutile TiO2''. Physical Review Letters. Volume 104, Issue 11. 119604/1. ISSN 0031-9007
(printed). DOI: 10.1103/physrevlett.104.119604, which has been published in final form at
http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.104.119604.
All material supplied via Aaltodoc is protected by copyright and other intellectual property rights, and
duplication or sale of all or part of any of the repository collections is not permitted, except that material may
be duplicated by you for your research use or educational purposes in electronic or print form. You must
obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or
otherwise to anyone who is not an authorised user.
Powered by TCPDF (www.tcpdf.org)
Enevoldsen et al. Reply: In Ref. [1], from simultaneously
recorded atomic force microscopy and STM images and
density functional theory (DFT) calculations, we showed
that hydrogen species could be atomically resolved in the
surface and subsurface of rutile TiO2. To explain an ap-
parent tip-assisted movement of H atoms between the two
sites, we predicted a barrier for hydrogen to diffuse directly
from bridging oxygen (Ob) on TiO2 (110) to a subsurface
site (Osub) of 2.4 eV for a H coverage of 1=8 ML using
DFT. This barrier is in agreement with the 2.6 eV given in
recent calculations for a higher coverage of 1 ML in
Ref. [2], but is now questioned in the preceding
Comment, Ref. [3]. A number of studies, Refs. [2–4],
suggest that a lower energy path via an intermediate in-
plane oxygen site (O3f) exists. For a coverage of 2 ML the
barrier is predicted to be 1.1 eV [4], and for a coverage of
1 ML it is further reduced to 0.9 eV [2].
To investigate whether the alternative path has the low-
est energy for the low coverage case (<0:1 ML) reflecting
the experimental conditions in Ref. [1], we calculated it
using the same methodology [1,5]. Specifically, DFT cal-
culations were performed using the plane wave VASP code
[6] with the local spin density approximation plus Hubbard
U [ðLSDAÞ þU] method, which was shown to give better
agreement with experiments [5,7]. In order to see if the
functional plays a role in the size of the barrier, we also
calculated it using the standard generalized gradient ap-
proximation (GGA) method.
Our results (see Fig. 1) show that the barrier for Ob to
O3f is around 1.0 eV for the GGA functional, but there is
no local minimum at the O3f site with LSDAþU.
However, in both cases the rate determining barrier is
found when passing through the Ti-O plane, and this is
about 1.6 eV for GGA and 1.8 eV for LSDAþU. This is
lower than the original prediction of Ref. [1], which was
likely overestimated due to constraints on the migration
path. The barriers remain significantly higher than the
predictions of Refs. [2,4], and assuming the barrier for
the total path was also considered, the difference must be
due to the higher coverages. However, we also speculate
that Calatayud et al. [3] might have neglected to include
the reaction energy from Ob to O3f to the energy barrier
from O3f to Osub. In any case, the fact that the barrier is
smaller than originally considered actually strengthens the
conclusion of our Letter [1] that H is manipulated to a
subsurface site.
We disagree with the interpretation of room temperature
behavior in the Comment [3]. Regardless of the H cover-
age, calculations predict a diffusion barrier of at least 1 eV.
This value is too high for thermally activated processes to
be active at room temperature, and spontaneous diffusion
of H should be an extremely rare process. Indeed, the
dynamics of surface H was studied experimentally in
high statistical detail in recent fast STM studies (e.g.,
[8]). Here, the removal or displacement of H adatoms,
other than through a surface reaction, was never observed.
Georg H. Enevoldsen,1 Henry P. Pinto,2
Adam S. Foster,2,3,* Mona C. R. Jensen,1
Werner A. Hofer,4 Bjørk Hammer,1 Jeppe V. Lauritsen,1
and Flemming Besenbacher1,†
1Interdisciplinary Nanoscience Center (iNANO) and
Department of Physics and Astronomy
Aarhus University
Aarhus, Denmark
2Department of Applied Physics
Aalto University
P.O. Box 1100, FI-02015 TKK Helsinki, Finland
3Department of Physics
Tampere University of Technology
P.O. Box 692, 33101 Tampere, Finland
4Surface Science Research Centre
The University of Liverpool
Liverpool, United Kingdom
Received 30 November 2009; published 19 March 2010
DOI: 10.1103/PhysRevLett.104.119604
PACS numbers: 68.47.Gh, 61.72.jj, 68.37.Ef, 68.37.Ps
*adam.foster@tut.fi
†fbe@inano.dk
[1] G. H. Enevoldsen et al., Phys. Rev. Lett. 102, 136103
(2009).
[2] P.M. Kowalski, B. Meyer, and D. Marx, Phys. Rev. B 79,
115410 (2009).
[3] M. Calatayud et al., preceding Comment, Phys. Rev. Lett.
104, 119603 (2010).
[4] X.-L. Yin et al., Chem. Phys. Chem. 9, 253 (2008).
[5] G. H. Enevoldsen et al., Phys. Rev. B 78, 045416 (2008).
[6] G. Kresse and J. Furthmu¨ller, Comput. Mater. Sci. 6, 15
(1996).
[7] H. P. Pinto et al., Nanotechnology 20, 264020 (2009).
[8] J. Matthiesen et al., ACS Nano 3, 517 (2009).
FIG. 1 (color online). Calculated diffusion barriers for hydro-
gen migration from surface to subsurface site using (a) GGA and
(b) LSDAþU.
PRL 104, 119604 (2010) P HY S I CA L R EV I EW LE T T E R S
week ending
19 MARCH 2010
0031-9007=10=104(11)=119604(1) 119604-1  2010 The American Physical Society


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Reply to the comment on "Imaging of the Hydrogen Subsurface Site in Rutile TiO2''

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