strontium titanate 
 ( srtio3 , sto ) , the prototypical perovskite 
 oxide , has long been known to show interesting effects taking place 
 on its surfaces .
for example , it is commonly used as a substrate for 
 the growth of complex oxides , and it supports 
 the formation of a two - dimensional electron gas on its surfaces .
furthermore , srtio3 was one of the first materials to 
 show a higher photocatalytic water - splitting efficiency than the classical photocatalyst tio2 .
what makes a material a good 
 photocatalyst are an appropriate band 
 gap allowing harvesting most of the solar spectrum ( maximum intensity 
 at around 2.4 ev ) and a suitable alignment of the valence band maximum 
 ( vbm ) and conduction band minimum ( cbm ) with respect to the redox 
 potentials of water . as was found by scaife , for most semiconducting transition metal oxides , the ( h2o / h2 ) redox potential of water lies around 3 v above the 
 vbm , i.e. , inside the band gap for srtio3 ( eg = 3.2 ev ) .
this means that band engineering toward an 
 efficient photocatalyst should decrease the magnitude of the band 
 gap while retaining the position of the cbm .
such band engineering 
 was already successfully demonstrated by 
 konta et al . by doping sto with rhodium .
the rhodium dopants introduce an impurity level located 2.3 ev below 
 the cbm of sto .
visible light can therefore excite electrons from 
 the dopant levels into the conduction band of sto , and the photocatalytic 
 process is initiated .
recently , we have studied the adsorption 
 of water on the pristine 
 srtio3(110)-(4  1 ) surface .
this reconstruction is formed to compensate for the polarity of 
 the sto(110 ) surface .
the structure consists of six- 
 and ten - membered rings of tio4 tetrahedra , sitting on a 
 bulk - truncated sto(110 ) plane , in which titanium is octahedrally coordinated . 

we found that this surface is remarkably inert toward the interaction 
 with water , providing a very good starting point for studying the 
 effect of promoters .
the photocatalytic 
 activity of the srtio3:nio system 
 has already been demonstrated successfully by townsend and co - workers . in their study ,
townsend et al . showed that 
  niox - sto is more likely a three - component 
 ni - sto - nio catalyst , in which sto absorbs the light , ni reduces protons , 
 and nio oxidizes water  . the present study is intended to provide 
 fundamental information on the srtio3:nio system .
we have 
 performed x - ray absorption near - edge structure ( xanes ) , x - ray and 
 ultraviolet photoelectron spectroscopy ( xps / ups ) , and scanning tunneling 
 microscopy ( stm ) to determine the effects of adsorption of nio onto 
 the sto(110)-(4  1 ) surface .
nio was prepared by either ( i ) 
 postoxidation of submonolayer amounts of ni deposited by molecular 
 beam epitaxy ( mbe ) in ultrahigh vacuum or ( ii ) reactive mbe growth 
 of metallic ni in a molecular oxygen background .
stm reveals that 
 the nanometer - sized nio patches that develop on sto(110)-(4  
 1 ) distort the local surface structure independently of the preparation 
 method .
consistently , xanes indicates a distortion / transformation 
 of the surface tio4 tetrahedra upon nio growth .
band bending 
 is observed in both xps and ups , which increases with the amount of 
 nio .
the onset of the vbm is effectively shifted upward , while minimal 
 changes are observed on the cbm , as seen in the xanes results . combining 
 xps , ups , and low - energy ion scattering spectroscopy ( leis ) , a clear 
 signature of dissociative adsorption of water
nb - doped ( 0.5 wt % ) srtio3 single crystals 
 with polished 
 ( 110 ) surface were purchased from mateck ( germany ) and crystec ( germany ) . 

the sto(110)-(4  1 ) surface was prepared by cycles of sputtering 
 ( ar , 1 kev , 5 a , 10 min ) and annealing ( 900 c , 
 3  10 mbar o2 , 1 h ) .
the samples 
 were heated either by passing alternating current ( 7.6 v , 1.75 a ) 
 through the samples or by bombarding them with electrons from the 
 back ( 900 v , 15 ma ) .
the sto(110 ) surface exhibits a series 
 of well - defined reconstructions , which can be tuned by the sr / ti stoichiometry 
 in the near - surface region . for the present 
 investigation
the surface structure was adjusted to yield the 4  
 1 reconstruction , as checked by low - energy 
 electron diffraction ( leed ) .
nickel metal ( purity of 99.999% ) was 
 deposited onto the surface from an electron beam evaporator ( omicron 
 efm3 ) , with a typical growth rate of 0.1  / min , as measured in 
 uhv via a home - built quartz - crystal microbalance .
the oxidation of 
 metallic ni was achieved either by reactively depositing ( rd ) ni in 
 5  10 mbar of o2 at different 
 temperatures ( room temperature [ rt ] , < 300 c , and 355 c ) 
 or by postoxidizing ( po ) the submonolayer ni amounts ( deposited in 
 ultrahigh vacuum at rt ) in 5  10 mbar of 
 o2 for 20 min . in the latter case ,
the substrate temperature 
 was adjusted to be slightly below the onset of the ir pyrometer reading 
 ( 300 c ) , unless otherwise stated . in the following ,
synchrotron 
 radiation photoemission spectroscopy ( pes ) and x - ray absorption spectroscopy 
 ( xanes ) experiments were performed at beamline i311 at the max - lab 
 laboratory .
the base pressure of the 
 beamline end station was below 1  10 mbar . 

all synchrotron - based photoemission spectra ( scienta ses200 analyzer ) 
 were acquired with a takeoff angle for the photoelectrons of approximately 
 55 away from the surface normal and with suitable kinetic energies 
 ( see spectra for photon energies ) to ensure highest surface sensitivity . 

only one set of photon energy measurements was carried out for each 
 sample treatment by measuring the fermi - edge position on the ta sample 
 holder .
therefore , uncertainties of binding energies are stated for 
 the shown synchrotron - based spectra .
work functions (  ) were 
 determined from a straight line tangent to the leading edge of the 
 low kinetic - energy cutoff of the secondary electrons , as measured 
 at normal emission with a sample bias of 10 v. x - ray absorption 
 was performed in two different modes .
auger - electron yield ( aey ) spectra 
 were acquired by measuring the intensity of the ti l2m2,3m2,3 auger peak , while sweeping the photon energy 
 across the ti l2,3 edge . in secondary - electron yield ( sey )
, 
 the increase of low - kinetic - energy electrons ( ekin  60 ev ) was measured as an indication of the absorption 
 of x - rays .
stm experiments were performed in a separate uhv system 
 with a specs aarhus stm at room temperature ( rt ) , using electrochemically 
 etched w tips .
all stm images were taken in constant - current mode 
 with approximately + 2 v sample bias voltage ( empty states ) and a tunneling 
 current of around 0.2 na .
xps ( omicron x - ray source with al anode , h = 1486.7 ev ) and leis ( specs iq12/38 , 1000 ev he ions , 5.8  10 he/[cms ] , and total scattering angle  = 127 ) 
 experiments were performed with a specs phoibos 100 hemispherical 
 analyzer in the same analysis chamber with a base pressure below 6 
  10 mbar . because of the small uncertainties 
 in the photon energy calibration for the experiments performed at 
 max - lab synchrotron radiation facility , measurements of band bending 
 as a result of different surface treatments were repeated in the home 
 laboratory , providing a well - defined photon energy .
such band bending 
 was derived from the binding energy of fitted xps core - levels ( o 1s 
 and ti 2p3/2 ) . samples were prepared in a connected preparation 
 chamber with a base pressure of 3  10 mbar . 

deionized water and isotopically labeled h2o ( sigma - aldrich ) were cleaned by repeated freeze  pump 
thaw 
 cycles and dosed onto the sample in the preparation chambers through 
 high - precision leak valves . peaks in ion scattering spectra have been 
 assigned by calculating the kinetic energy of he ions 
 elastically scattered by o , o , ti , ni , and 
 sr atoms , considering a 127 total scattering angle .
figure 1a shows 
 photoemission spectra ( raw data ) of the valence band region of differently 
 treated srtio3(110)-(4  1 ) surfaces .
the inset shows 
 a zoom into the gap region with the spectra aligned at the o 2s peak at eb  22 ev 
 ( not shown ) .
vbm were determined from the intersection of the horizontal 
 ( zero counts ) line with a straight segment tangent to the leading 
 edge of the ups spectra , as represented in the inset of figure 1a . for the clean surface ( black 
 curve ) ,
the onset of the valence band is located at approximately 
 3.2 ev below the fermi level , in nice agreement with the n - type doping 
 of the sto crystals and a reported band gap of 3.2 ev , i.e. , a flat - band situation .
the gap - region 
 of the pristine surface does not show any detectable density of states , 
 meaning that no in - gap states are introduced by the nb dopants in 
 the sto single crystal .
black , red , and blue spectra correspond to the 
 pristine surface , the surface after deposition of 0.1  ni , and 
 after postoxidizing the same ni amount at 300 c , respectively . 

the inset shows a zoom into the gap region 
 with the spectra aligned to the o 2s feature at 22 
 ev .
( b ) ni 2p core levels before and after oxidizing 0.1  ni . ( c ) 

the insets show the o 1s high binding - energy region ( c ) and the ti 2p3/2 low binding - energy region ( d ) after aligning the spectra to the 
 corresponding main peak .
( e ) band bending as measured from the o 1s 
 and ti 2p peak positions with al k radiation as a function 
 of the rd - deposited nio deposition amount ( quartz crystal microbalance 
 readings assuming ni density ; error bars represent 99% confidence 
 intervals on the fitted peak positions ) .
evaporation of ni ( red curve ) shifts all spectral features 
 to lower 
 binding energies ( by 0.22  0.15 ev ) and an in - gap state is formed . 
 the adsorption of single ni adatoms at the srtio3(110)-(4 
  1 ) surface has been studied recently , showing that isolated ni adatoms are formed up to a coverage of 
 0.05  [ see figure 2b ] . upon postoxidizing the ni adatoms , the valence band shifts 
 to even 
 lower binding energies by 0.38  0.15 ev compared to the pristine 
 surface , and the in - gap state is shifted downward , forming a shoulder 
 of the valence band . in the case of 0.1
 ni postoxidized , the 
 onset of the valence band is shifted to lower binding energies by 
 around 1.5 ev [ inset of figure 1a ] .
the upward bending of the energy bands as a result of 
 nio adsorption is consistent with the higher workfunction of nio compared 
 to the pristine sto surface ( sto(110)-(41 )  4.9 ev , nio = 6.26.7 ev ) and the measured increase of the workfunction 
 of the nio - loaded surface ( sto+0.1  nio  5.3 ev ) .
figure 1b shows 
 the ni 2p core - level spectra measured on as - deposited 0.1  ni 
 ( red ) and after postoxidizing the sample ( blue ) .
the as - deposited 
 ni exhibits the typical metallic ni 2p core - level features with a 
 binding energy of the j = 3/2 component of 853.6 
  0.3 ev and a 17.1 ev spin  orbit - split ( sos ) 2p1/2 peak ( compared to 2p3/2 photoemission from pure metallic 
 ni at eb = 852.7 ev with a sos of 17.3 
 ev ) . upon postoxidizing the nickel ,
the 
 ni 2p core - level features change to the typical nio line shape , with 
 a binding energy of the j = 3/2 component of 855.9 
  0.3 ev and a sos of 17.6 ev ( compared to literature values 
 of eb = 853.8 ev of 2p3/2 and 
 sos = 17.5 ev for bulk nio ) .
the 
 o 1s core - level [ figure 1c ] shows only a very weak change .
the presence of ni adatoms 
 causes the main o 1s spectral feature to be damped and a small shoulder 
 at higher binding energy is formed .
this shoulder is slightly diminished 
 upon postoxidation [ see inset of figure 1c , in which the spectra are shifted for clarity , 
 aligning the main o 1s feature ] .
the effect of nickel adatoms binding 
 to the surface oxygen can also be seen in the ti 2p core - level spectra 
 in figure 1d . here , 

the pristine surface ( black ) shows the commonly observed ti 2p features 
 of ti ions ( ti 2p3/2eb = 459  0.15 ev with a sos of 5.7 ev ) . upon deposition 
 of ni adatoms , a shoulder at lower binding energies
is formed , as 
 it can be seen in the inset of figure 1d .
such a shoulder is normally assigned to reduced 
 ti species , as described elsewhere . upon oxidation , this shoulder vanishes , and the spectrum 
 resembles the pristine surface , except for a rigid shift of 0.2  
 0.15 ev to lower be .
band bending is found to progressively increase 
 with the successive deposition of nio , attaining a saturation value 
 of approximately 0.8 ev at deposited amounts above 1  nio [ see figure 1e ] .
the ( 4 
  1 ) reconstructed surface of srtio3(110 ) , 
 having a unit cell size of 15.6   5.52  , appears 
 in stm images as periodic double lines of weak maxima along the [ 110 ] 
 direction [ see figure 2a ] .
these maxima correspond to tio4 tetrahedra , which 
 form six- and ten - membered rings via corner - sharing oxygen atoms .
bright protrusions on the ( 4  1 ) rows [ a 
 few marked by green dots in figure 2a ] are assigned to single strontium adatoms , which 
 are stabilized on antiphase domain boundaries of the reconstruction , 
 ensuring polarity compensation .
we have recently investigated the adsorption of nickel on this 
 surface , showing that isolated single 
 ni adatoms [ red stars in figure 2b ] are stabilized when submonolayer ni amounts are 
 deposited on sto at rt .
ni adatoms can adsorb either at the center 
 of ( 4  1 ) rows ( six - membered tio4 rings ) or on their 
 sides , in proximity of the dark trenches ( ten - membered tio4 rings ) . upon postoxidizing 0.1 
  ni in 5
 10 mbar of o2 at 375 c , the ni adatoms coalesce into 
 irregularly shaped patches [ highlighted by the dashed blue line in figure 2c ] , extending over 
 several adjacent ( 4  1 ) lines , and not showing any apparent 
 preferential adsorption site .
no clear ordering of atomic - scale features 
 could be observed on such patches , likely indicating the formation 
 of a locally defective nio structure .
similar morphological and structural 
 features are observed when growth is carried out by reactively depositing 
 ni in o2 atmosphere [ figure 2d , e ] , and no significant improvement of the measurable 
 atomic - scale structure is obtained upon increasing the deposited ni 
 amount [ cf .
the area of nio patches progressively extends with increasing 
 the deposited amount , and 0.3  nio ( not shown ) nearly fully 
 cover the sto surface , with only little ( 4  1 ) structure visible 
 in stm .
leed images acquired on the samples at different nio coverages 
 ( not shown ) do not exhibit any additional features in the diffraction 
 pattern , but an overall increase of the inelastic background upon 
 increasing the deposited material .
stm images [ ( a  d ) 20  20 
 nm , ( e ) 16  
 16 nm , ( f ) 30  30 nm ] of differently 
 treated srtio3(110)-(4  1 ) surfaces .
( b ) pristine 
 surface after deposition of 0.05  ni . panels c and d show 0.1  ni postoxidized ( po ) and reactively 
 deposited ( rd ) , respectively .
( e ) reactively deposited 0.2  
 ni . ( f ) 0.1  ni after postoxidation at 600 c .
tunneling 
 parameters for panels a  f : us = 
 + 2.0 to + 2.5 v , it = 0.02 to 0.3 na .
it is worth pointing out that 
 a local distortion of the ( 4  
 1 ) rows is visible in stm in close proximity of the nio patches .
this 
 is mostly evident wherever nio patches extend up to the dark trenches 
 separating adjacent dotted rows , as indicated by the white arrows 
 in figure 2c , d . in 
 such regions
the dark trenches appear wider in the in - plane direction , 
 and a deeper corrugation ( by 1020 pm ) is measured in stm .
figure 2f shows 
 the sample surface with 0.1  nio postoxidized at 600 c . 

the nio patches coalesce into rectangular islands , and several defect 
 structures are introduced in the sto substrate [ orange solid lines 
 in figure 2f ] .
one 
 type of defect appears as periodic , bright dots centered within the 
 ( 4  1 ) structure .
another type of defect is a cross - shaped vacancy 
 centered on one of the two rows of periodic dots , which build up the 
 ( 4  1 ) structure .
finally , localized depressions occasionally 
 appear next to nio patches and possibly correspond to missing units 
 of the surface structure .
it should be mentioned that despite the 
 relatively high temperature of 600 c , only a small decrease 
 ( < 6% ) of the ni 2p xps intensity was measured ( not shown ) , excluding 
 an extended intermixing of ni atoms into the sto crystal .
electrons from occupied core levels get excited into 
 unoccupied conduction band states , e.g. , ti 2p  ti 3d transitions . 
 as has been already pointed out
, the ( 4  1 ) surface consists 
 of tetrahedral tio4 units residing on bulk srtio3 consisting of octahedral tio6 units .
the spectral features 
 corresponding to these two configurations can be disentangled in the 
 xanes line shape , as has been recently shown [ wang et al . ,
the ti 3d levels split into subsets with t2g / eg or t / e symmetry in an octahedral ( oh ) or tetrahedral ( td ) crystal 
 field , respectively .
figure 3a shows xanes spectra of the ti l2,3 absorption 
 edge for differently treated srtio3(110 ) surfaces .
common 
 to all spectra are a strong absorption peak at a photon energy of 
 approximately 458 ev ( ti 2p3/2  ti 3d oh - t2 g ) followed by two overlapping peaks at 459 ev ( ti 2p3/2
 ti 3d td - e ) and 460 ev ( ti 2p3/2  
 ti 3d oh - eg ) and two broad overlapping features at 463 
 and 465 ev ( ti 2p1/2  ti 3d
oh - t2 g and 
 ti 2p1/2  ti 3d oh - eg ) .
note the 
 small shifts of the ti l2,3 peak at 458 ev 
 for different treatments of the surface .
these can not be ascribed 
 to band bending , which is caused by a local difference in electrostatic 
 potential and should therefore affect all core levels and conduction 
 band states equally .
the shifts are therefore related to changes in 
 the conduction band , i.e. , in the ti 3d levels .
this indicates that the cbm is only 
 minimally altered upon adsorption of nio , as compared to the change 
 in the vbm [ see inset of figure 1a ] .
a strong decrease of the tetrahedral signature 
 after adsorption 
 of ni and nio is visible in figure 3a .
this indicates that either the tetrahedral information 
 is damped by the adsorbates or that the latter distort the tio4 tetrahedra .
the latter possibility is consistent with the 
 distortion , and disruption of the surface structure , which was visible 
 in stm after preparing a nio - loaded surface at 300 c [ white 
 arrows in figure 2c , d ] 
 and 600 c [ figure 2f ] , respectively . in general ,
ti l2,3 xanes spectra acquired 
 in sey mode ( not shown ) show the same absorption peaks as aey with 
 the relative peak ratios dominated by the octahedral coordination 
 of ti in bulk sto .
the spectrum appears 
 similar for all surface treatments and acquisition modes ( aey and 
 sey ) , showing only minimal variations in the relative intensities 
 of the main features . because of the low signal , related to the small 
 amounts of ni deposited , the ni l2,3 xanes spectrum [ figure 3c ] was acquired in 
 secondary electron yield mode ( see materials and 
 methods section )
therefore , the use of sey mode is not 
 limiting the surface sensitivity of the measurement .
the photon energy 
 at which adsorption at the l3 edge occurs ( approximately 
 853 ev ) is consistent with previous reports for both metallic ni and 
 nio samples .
in the case of postoxidized 
 samples the main features of the characteristic line shape of nio 
 are observed .
namely , the intensity of the l2,3 white lines 
 with respect to the background is increased due to the oxidation of 
 metallic ni , resulting in an increased number of d holes for oxidized 
 ni species .
x - ray absorption spectra 
 of the pristine surface , with 0.1  
 ni adatoms , and after postoxidation to nio ( black , red , and blue curve , 
 respectively ) .
features related to ti coordinated in tetrahedral or octahedral environment 
 are indicated by td and oh , respectively .
here we studied the interaction of h2o with 
 the nio - loaded srtio3(110)-(4  1 ) surface by means 
 of ups / xps and leis .
the valence states of the adsorbed molecule allow 
 distinguishing whether water is adsorbed molecularly or dissociatively . 

molecularly adsorbed h2o is characterized by its 1b1 , 3a1 , 
 and 1b2 valence orbitals , with typical binding energies of 7 , 10 , 
 and 14 ev , respectively . on the other 
 hand ,
oh species are characterized by their 1 ( 6 ev ) 
 and 3 ( 11 ev ) valence orbitals .
figure 4 shows valence band and o 1s core level photoemission spectra before 
 ( blue ) and after ( green ) dosing 50 langmuirs ( 1 langmuir = 1.33  
 10 mbar s ) of h2o on a ( 4  1 ) 
 surface with 0.1  nio .
an increased density of states is found 
 in regions commonly assigned to adsorbed oh , i.e. , the oh 3 
 state at 11 ev binding energy [ see inset of figure 4a , in which the spectra are aligned to the 
 o 2s peak at eb  22 ev to highlight 
 differences ] , and the high binding - energy shoulder of the o 1s core - level 
 [ inset of figure 4b , 
 again with aligned o 1s peak energies ] . after dosing 50 langmuirs 
 of h2o ,
( a ) valence band and ( b ) o 1s photoemission 
 spectra of nio - loaded 
 surfaces before ( blue ) and after ( green ) dosing 50 langmuirs of water 
 at rt .
the inset of ( a ) shows a zoom into the oh 3 region ( spectra 
 aligned to the o 2s peak at 22 ev ) .
inset of ( b ) shows the o 1s high 
 binding - energy region after aligning the spectra to the corresponding 
 main peak .
a different approach for detecting 
 adsorbed water is low - energy 
 he ion scattering ( leis ) .
leis is considered one of the 
 most surface sensitive spectroscopic techniques with the ability of 
 resolving small mass differences by choosing a suitable primary ion 
 mass .
figure 5 shows 
 ion scattering spectra of differently treated srtio3(110)-(4 
  1 ) surfaces .
the spectrum of the pristine ( 4  1 ) surface 
 ( black curve ) shows peaks related to he ions scattered 
 at o ( 400 ev ) , ti ( 710 ev ) , and sr ( 795 
 ev ) atoms .
it should be mentioned that upon impingement of 1000 ev 
 he ions a progressive degradation of the surface is observed , 
 most likely due to intermixing of the first few surface layers . in 
 particular ,
an inversion of the apparent ti to sr ratio is visible 
 already after 15 scans ( each scan takes about 1 min ; the ion currents 
 ranged from 15 to 23 na , which translates into a total ion fluence 
 of ( 1.11.7 )  10 ions / cm for 
 each spectrum ) .
in order to limit beam damage as much as possible 
 while still retaining good statistics , we used only the sum of the 
 first four scans for each leis spectrum in figure 5 . to investigate the adsorption of water 
 on the nio - loaded srtio3(110)-(4  1 ) surface
, we 
 performed leis after dosing up to 100 langmuirs ( 1.3  10 mbar  1000 s ) of isotopically labeled water 
 ( h2o ) . dosing up to 100 langmuirs of h2o onto the pristine surface does not introduce 
 any further features in the leis spectrum , consistent with the inert 
 nature of this surface .
we successively 
 deposited increasing amounts of nio onto the surface 
 and subsequently dosed up to 100 langmuirs of h2o , and each spectrum in figure 5 corresponds to a freshly prepared sample .
nickel introduces 
 a peak in leis spectra at a kinetic energy of approximately 770 ev . 

this 
 peak is assigned to he ions scattered at o belonging to adsorbed water .
the spectra acquired on samples with 
 different amounts of nio clearly show that water binds to the nio - loaded 
 surface , in agreement with the photoemission data in figure 4 .
the existence of a larger o peak in the case of 0.2  nio compared to the case 
 of 0.3  nio ( close to full coverage ) possibly indicates that 
 either h2o adsorption or dissociation takes place at the 
 nio / srtio3 boundary .
low - energy he ion scattering 
 spectra acquired on the 
 pristine srtio3(110)-(4  1 ) and after depositing 
 different amounts of nio with subsequent dosing of 100 langmuirs of 
 h2o .
the combination of various spectroscopic 
 techniques and direct - space 
 imaging provides new insights into the nio / sto system .
ups and xanes 
 results indicate that the deposition of nio onto the sto(110)-(4  
 1 ) surface effectively reduces the band gap of the system from 3.2 
 to approximately 1.7 ev .
this change is visible in the altered onset 
 of the valence band when nio is present at the surface as well as 
 in the relatively stable positions ( within 0.2 ev ) of ti l2,3 xanes peaks ( probing the cbm ) with and without nio . assuming that 
 this vbm shoulder is hybridized with the o 2p valence band , these 
 results suggest a considerable reduction of the band gap of the system 
 nio / sto , allowing harvesting of visible light ( 1.7 ev corresponding 
 to 729 nm , i.e. , near - infrared ) .
although the present data do not 
 provide information on hybridization of these states , it is known 
 from experiments with n - doped tio2 that such vbm shoulders can considerably 
 enhance the visible - light response in photocatalysis experiments .
in addition , a pronounced ( up to 0.8 ev ) upward 
 band bending is observed for the nio - loaded surface .
photogenerated 
 excitons can effectively dissociate in the electrostatic potential 
 gradient related to the band bending , which drives electrons toward 
 the bulk of the substrate and holes to the very surface , provided 
 that extended states exist to allow charge transport before electron 
the excess holes located at the surface can 
 then participate in the catalytic splitting of adsorbed h2o . combining stm and xanes , a 
 disruption of the reconstructed surface 
 lattice
the resulting defects 
 probably represent favorable adsorption sites for water , which is 
 also consistent with ion scattering results .
leis shows a higher water 
 signal for the surface with 0.2  nio compared to the nearly 
 full coverage obtained upon depositing 0.3  nio .
this indicates 
 that the adsorption may take place at the circumference of the nio 
 patches , i.e. , at the triple phase boundary nio / sto / uhv .
no interdiffusion 
 of ni was detected ; i.e. , the two oxide phases are well separated . 

it should be mentioned that the patches observed in stm possibly consist 
 of a mixed ni 
ti oxide phase , since the amount of reactively 
 deposited ni resulting in a nearly full coverage ( 0.3  ) approximately 
 corresponds to 33% of a single nio(110 ) layer .
such a mixed oxide 
 phase might contribute to the damping of the tetrahedral feature in 
 xanes . in a previous study
, we argued about the inertness of 
 the pristine 
 ( 4  1 ) surface to be caused by the undistorted tetrahedra forming 
 the surface reconstruction . therefore , 
 the distortion or disruption of these tetrahedra could be a reason 
 for the enhanced reactivity of the surface toward water adsorption .
we have studied the morphology and electronic structure of the 
 nio - modified srtio3(110)-(4  1 ) surface .
an in - gap 
 state , formed by the deposition of metallic ni onto this surface , 
 is transformed into a valence band shoulder upon oxidation of the 
 ni into nio .
this effectively increases the vbm with the cbm being 
 unchanged , as seen in xanes , and therefore decreases the band gap 
 of the system .
furthermore , xanes results indicate that 
 surface tio4 tetrahedra get considerably distorted or disrupted 
 upon bonding with nio , as is also evident from stm images of nio - covered 
 surfaces .
ups and leis experiments confirm that the nio activates 
 the surface toward dissociative adsorption of h2o , possibly 
 involving sites at the interface between nio patches and the substrate .
 nickel 
 oxide ( nio ) , deposited onto the strontium titanate ( srtio3 ) ( 110)-(4  1 ) surface , was studied using photoemission 
 spectroscopy ( pes ) , x - ray absorption near edge structure ( xanes ) , 
 and low - energy he+ ion scattering ( leis ) , as well as scanning 
 tunneling microscopy ( stm ) . 
 the main motivation for studying this 
 system comes from the prominent role it plays in photocatalysis . 
 the 
 ( 4  1 ) reconstructed srtio3(110 ) surface was previously 
 found to be remarkably inert toward water adsorption under ultrahigh - vacuum 
 conditions . 
 nickel oxide grows on this surface as patches without 
 any apparent ordered structure . 
 pes and leis reveal an upward band 
 bending , a reduction of the band gap , and reactivity toward water 
 adsorption upon deposition of nio . 
 spectroscopic results are discussed 
 with respect to the enhanced reactivity toward water of the nio - loaded 
 surface . 