alkylsilanes 
 are known to self - assemble at the surface of oxide 
 materials .
this ability has been widely exploited , for example , to 
 tune the wetting and electronic transport properties of the surface .
furthermore , alkylsilanes are often used as coupling agents for 
 attaching organic moieties or nanoparticles 
 to the oxide surface , forming tailored 
 three - dimensional supramolecular arrays . in this respect ,
aminoalkylsilanes , 
 such as ( 3-aminopropyl)triethoxysilane ( aptes , scheme 1 ) , have been extensively employed .
the ability to control the 
 surface properties of technologically 
 relevant materials , such as tio2 , opens 
 new perspectives in cutting - edge industrial sectors , such as microelectronics , sensors , and photovoltaics .
furthermore , surface functionalization of tio2 has been 
 recently employed to immobilize specific biomolecules , paving the 
 way toward the design of novel chemical sensors and biosensors , drug release systems , and materials with increased biocompatibility . to this end , alkylsilanes such as aptes are ideal linker candidates 
 because they can be selectively photodegraded to prompt the controlled 
 release of proteins , enzymes , and bioactive molecules .
moreover , by irradiating the silane - functionalized tio2 surface through a patterning mask , a 
 site - selective degradation of the organic layer can be obtained , for 
 example producing patterns with wetting contrast ; the resulting patterns can be used to assemble complex , 
 hierarchical structures where metal nanoparticles , polymers , or biological 
 molecules are chemisorbed to specific surface sites . light irradiation 
 of silane - functionalized tio2 can also be used as a trigger 
 to activate the on - demand controlled release of active substances 
 from stimulus - responsive pickering emulsions . despite the increasing interest in aptes - functionalized tio2 , no general consensus exists on the structure of the organic  inorganic 
 interface .
several models have been proposed , mostly in the case of 
 sio2 substrates , such as hydrogen bond - mediated coupling 
 of nh2 ( or nh3 ) 
 and surface oh ( or  o ) groups or 
 direct chemisorption via the silane headgroup , with the further complication 
 of the possible formation , to various extents , of o  si  o  si 
qualitative estimates of the relative number of various aptessurface 
 interaction modes are usually drawn by comparing the intensities of 
 xps peaks in the n 1s region . however , the presence of surface contaminants makes an accurate quantification on this basis a very difficult 
 task .
also , the exact attribution of the measured xps chemical shift 
 to the putative state of the surface nitrogen species is a debated 
 aspect . the silane 
surface 
 interaction mode is a crucial aspect for the development of advanced 
 composite materials because it determines whether nh2 terminal groups are available to react with electrophilic species , 
 allowing further functionalization through standard chemical procedures , 
 and it has been shown to depend crucially on the substrate type and 
 functionalization conditions . in this work , 
 we aim to shed light on the preferred interaction 
 modes of aptes molecules with the tio2 surface .
synchrotron 
 xps and nexafs experiments were complemented by afm , hr - tem , and ft - ir 
 measurements on pristine and aptes - functionalized nanocrystalline 
 tio2 .
high - level plane - wave ( pw ) density functional theory 
 ( dft ) calculations were also performed to simulate possible arrangements 
 of the chemisorbed aptes molecules at the tio2 surface . 

finally , afm , ftir , xps , and nexafs measurements on ex situ solar - irradiated 
 samples on the time scale of an hour allowed us to go a step further 
 with respect to a purely static picture , providing information on 
 the photoinduced oxidation mechanism of the organic layer .
it is worth 
 noting that the silane oxidation pathway has been scarcely investigated 
 in the literature , whereas its study represents a crucial issue for 
 a series of applications involving a light - triggered response , such 
 as the use of these hybrids as nanocarriers for drug release .
all 
 chemicals were purchased 
 from sigma - aldrich with reagent - grade purity and used as received ; 
 doubly distilled water passed through a milli - q apparatus was utilized .
the tio2 substrate 
 film was deposited on a commercial ito layer ( sigma - aldrich , 
 812 /sq ) as reported elsewhere .
the procedure was repeated two times to obtain a 150-nm - thick 
 tio2 film . as specified elsewhere ,
the grazing incidence x - ray diffraction confirmed 
 that it consisted mainly of the anatase polymorph . to remove surface 
 contaminants by photocatalytic oxidation and promote extensive surface hydroxylation , the bare tio2 film underwent extensive photo - oxidation 
 by uv irradiation through an iron - halogenide lamp ( jelosil hg500 , 
 effective power density 58 mw cm between 300 and 
 400 nm ) .
water contact angle measurements served as a probe for the 
 completeness of the oxidation process : uv irradiation was stopped 
 when complete wetting was observed .
finally , the film was placed , 
 together with a teflon cup filled with aptes diluted in toluene ( 150 
 l of 2 m solution ) , in a glass bottle under a dry n2 atmosphere to avoid aptes gelation .
the bottle was kept in an oven 
 at 80 c for 2 h. subsequently , the film was sonicated in toluene 
 for 15 min and then blown dry with an n2 gas stream .
the 
 final water contact angle , as determined by a krss easy drop 
 apparatus , was 55  5. the photocatalytic 
 degradation of the aptes layer was studied by irradiating aptes - tio2 films with simulated solar light using a halogen lamp ( lot 
 oriel , effective power density 15 mw cm in the 
 range of 300800 nm ) .
the degradation of the hydrophobic layer 
 was monitored by water contact angle measurements ( krss easy 
 drop ) .
three different irradiation times ( t = 0 , 
 30 , and 60 min , corresponding to initial , intermediate , and ca .
0 
 water contact angle values ) were selected as relevant benchmarks to 
 study the reaction . to ensure consistency ,
all of the irradiated samples 
 were obtained from the same functionalized tio2 film , which 
 was cut in pieces , and each piece was irradiated for a different time .
the bare 
 and aptes - functionalized tio2 films were analyzed by atomic 
 force microscopy ( afm ) with a ntegra aura ( nt - mdt ) device in tapping 
 mode , equipped with nsc35/aibs tips ( masch ) .
the adsorbed species 
 at the surface of aptes - functionalized anatase tio2 during 
 photocatalytic degradation were analyzed by fourier transform infrared 
 ( ft - ir ) spectroscopy using a perkinelmer spectrum 100 ft - ir ( atr ) 
 spectrometer .
x - ray spectroscopy ( xps ) and near - edge x - ray absorption fine structure 
 ( nexafs ) experiments were carried out at the bending magnet source 
 of the materials science beamline at the elettra synchrotron in trieste . 

full technical details are reported in section s1.1 of the supporting information , si . the adsorption 
 geometries of aptes molecules at the most stable ( 101 ) anatase tio2 surface were investigated by 
 dft + u calculations using the generalized 
 gradient approximation ( gga ) with the 
 perdew  burke 
ernzerhof ( pbe ) parametrization of the 
 exchange and correlation functional , as 
 implemented by the vasp plane - wave program .
afm scans on unfunctionalized 
 tio2 ( figure 1a ) show a homogeneous surface with low roughness ( estimated root - mean - square , 
 rms , on a 10  10 m area < 1 nm ) .
aptes - bearing 
 surfaces ( figure 1b ) 
 show a homogeneous coverage consistent with the formation of an aptes 
 monolayer ( see also section 3.2 ) .
the few nanometric aggregates in figure 1b are attributable to contamination 
 and residues from the preparation process .
the effect of simulated 
 solar light irradiation was also investigated ( figure 1c , d ) .
after 30 min , the surface appears less 
 homogeneous , with appreciable aggregates ( figure 1c ) whose presence might be related to the 
 occurrence of either vertical polymerization among physisorbed aptes 
 or their degradation residues .
the increased hydrophilicity of tio2 upon irradiation can easily result in the presence of adsorbed 
 water molecules , which might promote 
 such polymerization .
prolonged irradiation ( figure 1d ) results instead in a more homogeneous 
 and polished surface , as shown by rms values , which might be indicative 
 of ( almost ) complete aptes degradation .
afm topography images 
 of bare tio2 ( a ) , aptes - functionalized 
 tio2 ( b ) , and aptes - tio2 after 30 min ( c ) and 
 60 min ( d ) of light irradiation .
the corresponding rms roughness estimates 
 are ( a ) 0.56 , ( b ) 0.75 , ( c ) 1.46 , and ( d ) 1.18 nm .
reference 
 unfunctionalized 
 tio2 was first analyzed to investigate the effect of surface 
 contaminants on the recorded signals . to this end , xps spectra were 
 collected before and after sputtering by ar ions for 10 
 min . comparing
the xps spectra of the as - synthesized and sputtered 
 tio2 layers shows that adventitious surface c and n species 
 are indeed present , which are almost entirely removed after sputtering 
 ( figure s2 ) .
these species are likely ascribable 
 to the fast adsorption of atmospheric contaminants at the bare tio2 surface because of its well - known photoinduced amphiphilic 
 behavior . in the unfunctionalized 
 substrate , before sputtering ( figure 2a ) the ti 2p region is characteristic of ti(iv ) ( ti 
 2p3/2 458.7 ev ,  = 5.7 ev ) .
the o / ti ratio is close 
 to the theoretical value of 2 ( table 1 ) . upon surface functionalization with aptes ( figure 2a ) , the ti signals 
 shift to a slightly lower b.e .
(  = 0.3 ev ) , which is 
 indicative of more electron - rich ti atoms .
furthermore , in agreement 
 with previous reports , the ti peaks also 
 show a slight decrease in intensity .
if significant , the latter might 
 be attributed to the attenuation of the signal due to the limited 
 inelastic mean free path of electrons , i.e. , the probe depth of xps , 
 because in functionalized samples they have to cross the adsorbed 
 alkylsilane layer before reaching the detector .
it should be underlined that the detected xps signals contain 
 information on both the aptes layer and the underlying oxide because 
 they show both organic species and surface ti ions .
this is expected 
 on the basis of the probe depth of the emitted photoelectrons considered 
 here ( 5.36.5  for kinetic energies in the range 
 of 50170 ev ) .
note that these probe depths are comparable 
 to the monolayer thickness in the limiting case of fully grafted aptes 
 molecules normal to the surface at the maximum extension of the linear 
 hydrocarbon chain ( see also section 3.5 ) .
these findings , together with the homogeneity 
 of the afm image of aptes - tio2 ( figure 1a ) , are consistent with the formation of 
 a monolayer .
high - resolution xp spectra of the pristine ( dotted line ) 
 and aptes - functionalized 
 tio2 ( solid line ) as a function of irradiation time ( 0 , 
 30 , and 60 min ) : ti 2p ( a ) and o 1s ( b ) .
estimated 
 standard deviations 
 are given in parentheses . upon ex situ light irradiation , a progressive shift of the ti 2p 
 peaks toward higher binding energy ( b.e . )
takes place along with an 
 increase in ti relative content , i.e. , the effects of aptes functionalization 
 are reversed .
after 60 min of irradiation , an almost full recovery 
 of the chemical states of near - surface transition - metal ions is observed . as for the o 1s signal ( figure 2b ) , surface
this is likely the signature of surface band bending , which determines 
 the formation of a negative space charge layer and implies that ti ions achieve a more reduced state , also according to paw - dft 
 outcomes ( section 3.5.2 ) .
furthermore , a marked increase 
 in the component at higher b.e . is appreciable , which seems to be 
 related to a component at 532.3 ev ( table s1 ) possibly ascribable to c  o bonds of surviving ethoxy moieties . 

accordingly , the o / ti ratio increases upon functionalization with 
 aptes from 2.2 to 2.7 ( table 1 ) .
this is due to the silane stoichiometry ( scheme 1 ) , which brings 3 atoms of 
 oxygen per si center , and implies an incomplete condensation of the 
 ethoxy functionalities with surface hydroxyl groups .
however , a higher 
 o content might imply either the presence of unhydrolyzed ethoxy groups 
 or uncondensed si 
furthermore , 
 upon light irradiation the o / ti ratio increases to 
 3.0 ( table 1 ) and the 
 intensity of the components ascribable to oh and c  o species 
 also increases with respect to the pure oxide ( table s1 ) .
these observations might be related to the expected 
 increase in surface hydroxylation upon light irradiation as well as to the progressive oxidation of the 
 alkyl chain of aptes . moreover , the presence of organic c 
o 
 features in the c 1s region , whose relative weight decreases upon 
 irradiation ( table s1 ) with respect to 
 the c = o peak ( figure s5 and table s3 ) . the si 2p region ( figure 3a ) in aptes - tio2 shows an 1.85 ev - wide 
 peak at 102.2 ev , which could be indicative of different environments 
 around the si atom .
the si / ti ratio is 0.6 ( table 1 ) , in agreement with previous 
 reports of aptes monolayers on tio2 . upon solar irradiation , the signal shifts to higher b.e . 
 as a result of further oxidation of the adsorbate .
interestingly , 
 the si / ti ratio remains almost unchanged with respect to the nonirradiated 
 sample .
figure 3b shows 
 the high - resolution xp spectra of the n 1s region for the pristine 
 and functionalized tio2 samples .
partial surface contamination 
 due to n - containing species is unavoidable , and indeed a weak n 1s peak at around 400.3 ev is already present 
 in the unfunctionalized tio2 film ( feature marked as i 
 in figure 3b ) . literature 
 studies report several conflicting attributions to this component , e.g. , surface n2 or amides .
when aptes is added , 
 however , organic nitrogen due to the aliphatic amine moiety is clearly 
 recognizable because two components neatly distinguishable from the 
 contaminant species appear ( marked as ii and iii in figure 3b , curve at t = 0 min ) , with a relative weight of 60:40 . in agreement 
 with previous reports on aptes - functionalized oxides ,
these features 
 can be attributed to free nh2 ( 399.6 ev ) and to either 
 h - bonded nh2 or nh3 ( 401.3 ev ) .
400.3 ev ) due to the reaction of the 
 amine group with atmospheric co2 has been previously reported 
 for amine - containing monolayers . in the 
 present case ,
however , the absence of significant amounts of amides 
 is confirmed by the very low c = o content observed in the c 
 1s xp spectrum of the nonirradiated sample ( table s3 ) as well as by ftir results ( vide infra ) .
high - resolution xp spectra 
 of the pristine ( dotted line ) and aptes - functionalized 
 tio2 ( full line ) as a function of irradiation time ( 0 , 
 30 , 60 min ) : si 2p ( a ) and n 1s ( b ) .
the c / n ratio is about 5 , i.e. , higher than the theoretical 
 value 
 of 3 expected in fully grafted aptes - tio2 ; this might be 
 related to adventitious carbon as well as to unhydrolyzed ethoxy groups . 

another possible reason for the higher - than - expected c / n ratio could 
 be that some aptes molecules might present a reversed or folded configuration . 
 as a result , the nitrogen signal would be attenuated by the limited 
 electron mean free path .
the latter hypothesis is also supported by dft calculations 
 and ftir results ( vide infra ) . as the solar irradiation proceeds , 

after 30 min , no further changes are detectable 
 in the n 1s xps signal ( compare the curves at t = 
 30 and 60 min in figure 3b ) , and the residual peaks are very broad and weak , similar to that 
 due to adsorbed impurities in pristine tio2 .
these findings are likely related to the disappearance of the aptes 
 nitrogen ( figure s6 ) .
( i ) before 
 irradiation , 40% of amine groups are involved in hydrogen 
 bonds or are protonated , which is in good agreement with previous 
 reports about sio2 and other oxides , showing a higher proportion of free amine 
 with respect to h - bonded / protonated nh2 .
( ii ) upon irradiation , 
 simulated solar light promotes extensive oxidation of the aptes alkyl 
 chain .
mineralization does not affect the si headgroup , as observed 
 for other silanes , whereas the alkyl 
 chain degradation leads to the cleavage of the amino group .
the degradation pathway 
 of aptes molecules adsorbed at the tio2 surface under simulated 
 solar irradiation was also studied via ft - ir spectroscopy .
the spectrum 
 of the bare tio2 reference can be found in figure s7 . to disentangle the ft - ir background 
 due to the tio2 substrate from the aptes signature , difference 
 spectra were analyzed as a function of the irradiation time ( figure 4 ) .
differential absorbance 
 ft - ir spectra obtained by subtracting the 
 curve of the pristine samples from those of aptes - functionalized tio2 at different irradiation times : 40002500 cm spectral range ( a ) and 1900900 cm spectral 
 range ( b ) .
vertical lines 
 are a guide to the eye and indicate the main peak positions at 0 min . despite a rather unfavorable signal - to - noise 
 ratio , the changes 
 observed as a function of the irradiation time are systematic and 
 have physical significance . the as - synthesized sample ( t = 0 min ) shows a clear envelope in the 30002850 cm spectral region , attributable to the c 
in particular , 
 bands at 2920 and 2864 cm are due to the asymmetric 
 and symmetric stretches of ch2 groups on the alkyl 
 chain , whereas no clear indication of ch3 asymmetric modes 
 is discerned .
previous 
 studies of surface functionalization by alkylsilanes of tio2 and other oxides reported the occurrence of negative 
 peaks in the oh ( surface hydroxyls ) and hoh ( undissociated 
 water molecules ) regions , which were attributed to the formation of 
 si  o  si bonds between the surface and alkylsilane molecules 
 with a consequent loss of surface hydroxyl groups and physisorbed 
 water . on the contrary , in the present case no negative peaks in these 
 ranges are observed .
reported 
 a slight increase in the width of the broad peak at 3500 cm for aptes - functionalized zeolites , which was attributed to the overlap 
 of the symmetric n 
it is worth noting that the 
 same band was reported at 3346 cm for 
 the free amine and at about 3305 cm for terminal 
 amine groups cross - linked with silanol groups . as for the low - frequency branch of the spectrum , the absorption 
 bands at 1562 and 1628 cm were 
 assigned to the scissor modes of nh2 groups and 
 to the asymmetric nh3 deformation mode , 
 respectively .
the corresponding symmetric nh3 deformation mode can be related to the component 
 at ca .
n stretching in amines . the overlap of the stretching 
 bands of si  o  si , si 
c 
 prevents a complete attribution of the peaks falling in the 9501250 
 cm region . the stretching mode 
 of si  o  si bonds has been reported in the range of 10101150 
 cm .
si 
 moieties is expected to fall at 1024 cm , and the si  o  c stretching modes 
 were also assigned at about 1050 cm .
solar - light irradiation ( figure 4 ) leads to the disappearance 
 of the bands related to 
 chx moieties ( especially the bands 
 at 30002850 cm ) , in agreement with xps 
 results . also , the peaks associated with nh2 bending 
 disappear as the irradiation time increases , while a peak at 1645 
 cm strengthens .
this peak can be assigned to the 
 in - plane hoh bending mode of water molecules .
an alternative attribution 
 of this peak to the c = o stretching mode of amides is contradicted 
 by the concomitant disappearance of nh2 bending signals . 

peaks in the 16001200 cm region disappear 
 in the first 20 min of irradiation , and a new peak appears at 1427 
 cm after a longer irradiation , which might be 
 attributed to c  o  h bending .
1100 and 1015 cm are still 
 appreciable even after prolonged irradiation ( 180 min ) , although they 
 shift to higher wavenumbers as the degradation proceeds and at the 
 same time their relative intensities change .
this evidence suggests 
 that the amine group is degraded very rapidly ( in the first 20 min ) , 
 as also supported by xps analyses , whereas the degradation of alkyl 
 chains requires longer times .
the silane headgroup is instead still 
 appreciable after prolonged irradiation , as confirmed by the persistence 
 of the two main peaks in the 11501000 cm region , even though the observed shift in the peak position and 
 the change in relative intensity suggest the occurrence of oxidation 
 ( i.e. , loss of the contributions due to si 
figures 5 and 6 show the nexafs 
 spectra of tio2-functionalized aptes surfaces at both grazing 
 and normal incidence geometries across the c k and n k 
 edges . prior to irradiation ,
the c k - edge signals ( figure 5a ) are dominated by two main 
 peaks , namely , a sharp resonance at 288.4 ev and a broader one at 
 292.5 ev .
c 
 ( or c  n )  * orbitals , and 
 the former could be due to c 1s  *(c  h ) .
it should be mentioned , however , that a c 1s 
  *(c = o ) resonance might be present at 289 
 ev , probably because of x - ray - induced oxidation of the alkyl chain .
indeed , the sample after 30 min of irradiation 
 ( figure 5b ) shows a 
 more intense peak at 288.6 ev , which could be related to the presence 
 of c 1s  *(c = o ) transitions , as also supported 
 by xps results showing an increased content of c = o species . 
 even though some artifacts are still appreciable in the pre - edge region , 
 probably because of auger transitions , the  * resonances at 
 292 ev in ni geometry are invariably stronger than in gi geometry , 
 which suggests a preferential horizontal orientation of aptes alkyl 
 chains with respect to the surface .
c k - edge nexafs spectra of aptes adsorbed 
 on tio2 , measured 
 at normal ( ni , solid lines ) and grazing ( gi , dashed lines ) x - ray incidence 
 as a function of different ex situ solar light irradiation times : t = 0 ( a ) and 30 min ( b ) .
n k - edge nexafs spectra of aptes adsorbed on tio2 , measured 
 at normal ( ni , full lines ) and grazing ( gi , dashed lines ) x - ray incidence 
 as a function of different ex situ solar irradiation times ( t = 0 , in black , and t = 30 min , in red ) .
previous studies reported a lack 
 of polarization dependence of 
 aptes monolayers , which was attributed to a completely disordered 
 layer structure .
our c k - edge nexafs 
 results seem instead to support the formation of a low - density monolayer 
 with aptes molecules tilted toward the surface , in agreement also 
 with dft calculations , which will be reported in section 3.5 . such a difference with 
 literature reports could be related to the preparation procedure adopted 
 here :
chemical vapor deposition has been previously reported to lead 
 to a lower degree of cross - linking with respect to wet procedures as a result of the absence of a prepolymerization 
 step in solution .
as for n k - edge spectra ( figure 6 ) , at t = 0 min a sharp 
 peak is present at 398399 ev , whose attribution is 
 debated : it has been assigned to either n = c or n 
c = c 
  * resonances originating from radiation damage or to n 1s  *(n  h ) resonances of 
 nh3 .
interestingly , 
 this feature vanishes in samples degraded for 30 min , consistent with 
 the disappearance of the aptes n 1s xps peak upon solar irradiation . 

the large shoulder at 401402 ev could be attributed to either 
  * n  h transitions or to  * nc resonances 
 arising from radiation damage ; however , it is still present after 
 the degradation of the silane ( red curves in figure 6 ) and is therefore attributable to either 
 aptes or adventitious nitrogen - adsorbed species .
this is not surprising because the amine end group 
 is expected to be conformationally more free than the whole hydrocarbon 
 chain , resulting in less preferred orientations for the nitrogen - bearing 
 groups .
the observed changes in the spectra upon irradiation indicate 
 possible changes in the intermolecular h bonding or even fragmentation 
 of the molecules , in agreement with xps and ft - ir results ( sections 3.2 and 3.3 ) .
accordingly , the n signal is more isotropic 
 ( red curves in figure 6 ) because there are fewer differences between the ni and gi k - edge 
 spectra .
the aptes adsorption on 
 a 11.5  21.0  ( 101 ) anatase tio2 surface was simulated by plane - wave ( pw ) dft calculations .
pw - dft 
 has already been successfully employed to study the adsorption of 
 amino - terminated silanes on a different class of substrates .
the ( 101 ) anatase tio2 surface was 
 selected according to hr - tem images ( figure s8 ) showing that the thermodynamic { 101 } facets are predominant in 
 our anatase tio2 .
this was expected on the basis of our 
 synthesis conditions , which allow crystallites to nucleate and grow 
 under thermodynamic control , as it is well known that the most stable 
 surface on anatase has a ( 101 ) orientation .
because the vasp program suite always applies 3-d periodic boundary 
 conditions , to reproduce a 2-d periodic slab , we introduced a vacuum 
 layer along the third dimension that was thick enough ( 14  ) 
 to avoid unphysical interactions between neighboring slabs ( figure s1 ) .
the accuracy of the functional 
 and of the chosen computational setup was verified through calculations 
 on isolated aptes molecules , i.e. , by comparing the vasp - predicted 
 molecular geometries and electronic structure in vacuo with geometries 
 optimized through atom - centered linear combinations of gaussian - type 
 functions ( lc - gtf ) .
the nwchem package was used to perform such benchmark calculations at the 6 - 311g(p , d)/pbe 
 theory level , i.e. , using the same exchange - correlation functional 
 employed in pw simulations ( table s4 and figure s9 ) .
the relative absolute differences in bond lengths among 
 the pw and lc - gtf models were invariably lower than 1% with very similar 
 electronic structures , so we feel confident that our plane - wave simulations 
 are able to reproduce with satisfactory accuracy the highly localized 
 states associated with the adsorbed silanes . given the good agreement 
 between the two types of calculations , we conclude that our computational 
 procedure is able to reproduce the correct features of aptes molecules 
 in both gas and gas  surface arrangements .
the theoretical 
 coverage corresponds to 0.41 ( 0.82 ) molecules / nm when 
 the aptes monomer ( dimer ) is adsorbed ( to be compared with an average 
 monolayer density on glass or silicon substrates of 2.14.2 
 molecules / nm ) . to better simulate 
 the adsorption process , a large slab of tio2 anatase ( 4 
  4  4 unit cell ) was modeled , i.e. , four o  ti 
the surface was hydroxylated , simulating a dissociative 
 water chemisorption process ( scheme 2 , figure 7 ) .
this picture is probable 
 for defective surfaces such as our nanostructured system , where there 
 are numerous acid ti(v ) sites .
physisorbed water was 
 not included in our model because it is reversibly bound to the surface 
 and its interactions with aptes molecules are intrinsically dynamic , 
 i.e. , they produce an effect on a time - averaged basis . on the other 
 hand , strongly bound chemisorbed water
likely has a more significant 
 influence because it can form stable hydrogen bond networks also involving 
 the organic layer .
chemisorption geometries of aptes molecules ( a  d , scheme 2 ) at a ( 101 ) anatase 
 surface .
only the upper layers of the 4  4  4 supercell 
 are shown .
color code : ti ( light blue ) , o ( red ) , h ( white ) , si ( yellow ) , 
 n ( deep blue ) , and c ( brown ) .
o layers were relaxed to model 
 the 
 surface region perturbed by the interface and by the aptes molecular 
 adsorption .
the two bottom layers were instead kept fixed in order 
 to simulate the bulk tio2 .
indeed , silane binding onto 
 anatase has been reported to induce local conformational changes in 
 the tio2 structure , including small geometry rearrangements 
 of the ti and o ions next to the 
 molecular adsorbate .
six binding modes 
 between the silane group and the surface were modeled ( scheme 2 ) .
the single molecule was 
 chemisorbed with either one ( scheme 2a ) , two ( scheme 2b ) , or three si  o  ti covalent bonds , implying 
 the loss of one , two , or three ethoxy groups .
the adsorption of oligomers 
 was also taken into account : both a covalently bonded si
finally , reversed adsorption geometries , 
 i.e. , where the silane molecule interacts with the oxide surface via 
 the amino group , were considered : one geometry involving the formation 
 of covalent interactions ( scheme 2d ) .
aptes physisorption mediated by a h - bond 
 between the amine group 
 and a surface silanol was not simulated because of the intrinsic limit 
 of the dft procedure in modeling correlations among distant electrons 
 responsible for dispersive interactions .
it is also worth noting that , 
 upon solar irradiation , the si - containing headgroups seem to remain 
 grafted onto the surface , as shown by xps ( table 1 , rows 4 and 5 ) and ftir ( as appreciable 
 from peaks at 11501000 cm in figure 4 ) results .
this implies 
 that physisorption , if any , should be just a minority interaction 
 mode .
hence , the chemisorption modes shown in scheme 2a  c should adequately represent the 
 aptes  tio2 interface .
for each system , the corresponding 
 isolated aptes derivative with three , two , or one ethoxy group ( eto ) , 
 i.e. , si[(ch2)3nh2](oet)3 , si[(ch2)3nh2](oet)2oh , or si[(ch2)3nh2]oet(oh)2 , was considered for gas - phase benchmark calculations ( vide 
 supra ) . by comparing gas - phase and adsorbed analogues ( table 2 ) ,
the molecular distortion 
 taking place upon surface adsorption can be determined for the different 
 geometries .
oh bond 
 is present , then the average is given . ) o : bridging atom in the si  oet 
 bond .
o : bridging atom 
 in the si  o  si 
 system in the aptes dimer ( c ) .
the h atom involved 
 in the h - bonding 
 interaction is always considered .
if no relevant hydrogen bonds are 
 present , then the average value is given .
distance between the amine h atom 
 and the oxide o atom on the tio2 surface acting as a hydrogen 
 bond acceptor .
see scheme 2 and figure 7 for the meaning 
 of these labels . in the 
 adduct ( d ) , the amine hydrogen 
 is directly linked to the surface through a n  o bond , with dn  o = 1.2894  .
a first important result is that , 
 according to the calculations , 
 not all of the considered adsorption geometries are feasible . in particular , 
 the simulation of aptes chemisorption via three hydroxyl groups did 
 not lead to a converged electronic structure result , and we conclude 
 that a tripodal link to the surface is not feasible .
the formation 
 of three si  o  ti bonds between a single aptes molecule 
 and the oxide surface thus seems to be prevented by the bond lengths / angles 
 in the silane molecules in conjunction with the structure of the tio2 surface .
it is worth noting that such findings are in agreement 
 with previous ft - ir studies.figure 7 shows the corresponding 
 fully relaxed configurations of the geometries that reached a converged 
 electronic structure result .
ti 
 bond forms ( figure 7a ) , the aptes molecules lie 
 horizontally on the surface because of the formation of a hydrogen 
 bond interaction between the h of the aptes amine group and an o of 
 a surface hydroxyl group ( nho distance 1.42  , table 2 ) .
acceptor no distance 
 comparable to the length of charge - assisted bonds found between charged 
 groups in metalorganic complexes .
h covalent 
 bond in the grafted molecule ( dn  h = 1.13  ) with respect to the isolated one ( dn  h = 1.02  ) while at the same time the si 
o 
 bond with the surface shortens and strengthens ( table 2 ) . when two si  o 
ti bonds form 
 between the si headgroup and the surface ( figure 7b ) , the aptes molecule shows similar folding , 
 with the amino group approaching the surface similarly to what was 
 observed with a single si  o  ti bond . however , in this 
 case , the distance between the n 
h group and the surface o 
 is higher ( 1.73  ) . as for the reversed attachment configuration 
 ( scheme 2d , figure 7d )
, the direct interaction 
 between the n of the aptes molecule and a surface oxygen ( figure 7d ) was taken into 
 account . in this configuration ,
the aptes molecule is perpendicular 
 to the tio2 surface , showing no interactions between the 
 silane head and the surface .
the adsorption of partially hydrolyzed 
 oligomers was also modeled . 
 in this case ,
as done for the isolated monomers , we performed benchmark calculations 
 on the isolated dimer , as reported in section s5 .
figure 7c reports the adsorption geometries of a representative aptes dimer 
 at the tio2 surface .
when two aptes molecules bond with 
 neighboring surface hydroxyls , the formation of an intermolecular 
 si  o  si bond results in a 20 tilt of the aptes 
 alkyl chain with respect to the surface .
the interaction of the amino 
 group with the surface is still substantial , although the hydrogen - acceptor 
 distance ( dho = 1.942.16 
  ) is now much longer than in the case of a single monomer ( figure 7a , b ) .
moreover , the 
 si  o  si bond is highly strained , as seen from bond angle 
 values ( 167 ) .
some attempts were also carried out to simulate 
 larger oligomers 
 chemisorbed on the surface , e.g. , aptes trimers in various configurations . 

for example , we tried to simulate an aptes trimer similar to the system 
 shown in scheme 2c , 
 where three r 
si bonds . however , 
 these calculations invariably led to aptes trimers bound to the surface 
 only through two si  o 
we believe that this 
 evidence is consistent with the high bond strain detected from geometrical 
 bond angles associated with the si center , very far from those expected 
 for an ideal sp geometry .
such strain could be related 
 to the reported limited occurrence of covalent interactions between 
 silane headgroups in ordered monolayers and hydroxyl groups on the 
 oxide surface .
this high strain might 
 also favor the chemisorption of aptes molecules at the tio2 surface as monomers or very short oligomers .
figure 8 reports the density of states 
 ( dos ) of systems ( a  d ) . for the sake of comparison ,
the corresponding 
 plot for the bare , fully hydroxylated tio2 ( 101 ) surface 
 is reported in figure s10 . in agreement 
 with previous literature findings ,
the 
 only effect that hydroxylation has on the anatase band structure is 
 the generation of a few empty states just above the valence band ( vb ) 
 edge .
figure 8a shows 
 that in the single - bonded monomer structure ( scheme 2a , figure 7a ) , oxygen atoms inject new states at the top of the 
 vb , which are likely contributed from the silane moiety , whereas n 
 is close enough to the surface to be able to add shallow midgap empty 
 states just above the fermi level ( figure 8a , magenta line ) .
the electronic structure 
 of the double - bonded monomer ( scheme 2b , figure 7b ) , reported in figure 8b , is pretty similar , although the different folding and chemical 
 state of the organic system lead to a few differences . in particular , 
 the amino group is now more distant from the surface ( table 2 ) , so n midgap states are less 
 dense , although still significant in terms of the dos intensity .
second , 
 the contribution to deep vb states from oxygen increases , as two oxygen 
 atoms are now acting as bridges between si and ti .
( a ) single - bonded monomer 
 ( scheme 2a and figure 7a ) .
( b ) double - bonded 
 monomer ( scheme 2b 
 and figure 7b ) .
( d ) reversed ( n  o  ti ) chemisorption ( scheme 2d and figure 7d ) .
the vertical dashed blue line represents 
 the fermi energy , which is set as the zero of the energy .
light curves 
 are enhanced atom - projected partial doss ( pdoss ) ; magenta , n ; blue , 
 o atoms bound to si ( a  c ) or n ( d ) ; green , ( a ) oxide o interacting 
 with n through the hydrogen bond ; orange , ( c ) bridging oxygen between 
 two aptes monomers ( main text ) .
the pdos of all atoms is enhanced 
 by a factor of 100 in order to aid the interpretation of the results . in the aptes dimer ( scheme 2c , figure 7c ) ,
it is 
 worth noting , however , that the 
 oxygen atom involved in the si  o  si bridge ( orange line 
 in figure 8c ) provides 
 states more deeply buried in the vb with respect to those involved 
 in si 
finally , as expected , the 
 electronic structure is markedly different when the reversed chemisorption 
 geometry ( scheme 2d , figure 7d ) is considered .
figure 8d shows that two 
 shallow states , due to the overlap of localized states on the bridging 
 n  o system , appear in the vicinity of the top of the vb .
they 
 are very close in energy ( < 1 ev ) , and indeed they are most likely 
 understandable as a bonding / antibonding couple associated with localized 
 n  o orbitals .
this implies that the reversed configuration 
 should be immediately destroyed at finite temperature by visible / thermal 
 electron excitation . in conclusion , upon aptes functionalization , 
 both ti 2p and o 1s 
 xps signals shift toward lower b.e .
( figure 2 ) , likely because of chemisorption - induced 
 band bending . the silane molecule should 
 act as an electron donor , implying charge transfer to the surface . as a consequence ,
this is in fairly good agreement 
 with the pw - dft results : all of the si  o  ti grafted 
 silanes ( scheme 2a  c ) 
 undergo a significant negative charge displacement toward the surface , 
 as can be inferred from the n empty states that invariably appear 
 near the top of the vb .
this picture is also consistent with the optimized 
 bond lengths ( table 2 ) calculated for aptes - tio2 hybrids and , in particular , 
 with ( i ) the significant shortening ( lengthening ) of the si 
o 
 ( n  h ) bonds of the silane ; ( ii ) the presence of very strong 
 nho bonds , with ho distances fully 
 comparable to known cases of charge - assisted hydrogen bonds ; and ( iii ) 
 the significant rearrangement of the ti  o coordination geometry 
 close to the chemisorption site .
in this work , bare and aptes - functionalized tio2 was 
 extensively characterized both experimentally ( xps , nexafs , afm , ft - ir , 
 hr - tem ) and theoretically ( pw - dft ) to clarify the structure of the 
 organic layer at the tio2 surface .
afm , ftir , xps , and 
 nexafs analyses on ex situ solar - irradiated samples at various degrees 
 of reaction extent also provided information on the photoinduced oxidation 
 mechanism of the organic layer .
the combined experimental and 
 theoretical evidence suggests that 
 aptes chemisorption on tio2 mainly implies one or two si  o 
ti 
 bonds involving the si headgroup , whereas a tripod configuration seems 
 unfeasible as highly strained .
other interaction modes , such as physisorption 
 through terminal amino groups , are expected to be considerably more 
 labile .
although dimerization of the silane at the surface through 
 si  o  si bonds is possible , further polymerization appears 
 to be less probable .
chemisorption of individual monomers leaves the 
 aptes terminal amino groups available for hydrogen bonds with surface 
 hydroxyl groups of tio2 .
these interactions are rather 
 strong because , on the basis of xps and dft results , they are reinforced 
 by a probable charge displacement from the organic molecules toward 
 the surface . in nexafs spectra ,
the strong c k - edge  * resonances 
 at normal incidence of the x - rays imply a preferential orientation , 
 where alkyl chains are bent toward the surface .
we therefore expect 
 that nho interactions should constitute a preferential 
 aptes  substrate secondary mode of interaction , especially at 
 low surface coverage , as in our specimens 40% of the amine 
 groups were estimated to be involved in some kind of strong hydrogen 
 bond .
the latter could hamper the availability of the amine as a nucleophilic 
 center for further functionalization of the adsorbed alkylsilane molecules . 

indeed , under these conditions strong nho interactions 
 pose a kinetic barrier against the chemical availability of the amine .
we also found compelling evidence that solar irradiation promotes 
 extensive oxidation of the aptes alkyl chain , rapidly leading to the 
 cleavage of the amino group . on the contrary , the oxidation process 
 does not destroy si head moieties , which remain bound to the titania 
 surface in a higher oxidation state .
this implies that the tio2 photocatalyst surface is not pristine at the end of the degradation 
 process .
such an effect could hamper the tio2 photocatalytic 
 behavior , with notable implications for 
 applications , but it could also be exploited , for instance , to produce 
 sio2/tio2 composite surfaces for advanced optics 
 as narrow bandpass filters .
moreover , 
 amino group availability and lability are both essential factors for 
 further functionalization of the oxide surface . indeed , because of 
 the rapid solar - induced n 
c cleavage , chemical functionalization 
 of aptes by small - to - medium organic molecules seems to be a suitable 
 step toward the development of effective drug - carrier devices for 
 disease prophylaxis and health care purposes .
 the 
 surface functionalization of tio2-based materials 
 with alkylsilanes is attractive in several cutting - edge applications , 
 such as photovoltaics , sensors , and nanocarriers for the controlled 
 release of bioactive molecules . 
 ( 3-aminopropyl)triethoxysilane ( aptes ) 
 is able to self - assemble to form monolayers on tio2 surfaces , 
 but its adsorption geometry and solar - induced photodegradation pathways 
 are not well understood . 
 we here employ advanced experimental ( xps , 
 nexafs , afm , hr - tem , and ft - ir ) and theoretical ( plane - wave dft ) tools 
 to investigate the preferential interaction mode of aptes on anatase 
 tio2 . 
 we demonstrate that monomeric aptes chemisorption 
 should proceed through covalent si  o  ti bonds . although 
 dimerization of the silane through si  o  
 si bonds is 
 possible , further polymerization on the surface is scarcely probable . 
 
 terminal amino groups are expected to be partially involved in strong 
 charge - assisted hydrogen bonds with surface hydroxyl groups of tio2 , resulting in a reduced propensity to react with other species . 
 
 solar - induced mineralization proceeds through preferential cleavage 
 of the alkyl groups , leading to the rapid loss of the terminal nh2 moieties , whereas the si - bearing head of aptes undergoes 
 slower oxidation and remains bound to the surface . 
 the suitability 
 of employing the silane as a linker with other chemical species is 
 discussed in the context of controlled degradation of aptes monolayers 
 for drug release and surface patterning . 
Introduction
Materials and Methods
Results and Discussion
Conclusions