fossil 
 fuels have been the primary energy source of mankind since industrialization , 
 while in the last decades sustainable and environmentally benign alternative 
 energy sources attract increasing attention in research . solar light 
 can become one of the main sources of sustainable energy in the future . however , the intermittent nature of solar energy requires scalable 
 technologies for its harvesting and converting into storable and transportable 
 energy carriers . in this regard , production of liquid or gaseous chemical 
 fuels ( e.g. , h2 , formic acid , alcohols , or light hydrocarbons ) 
 by solar - energy - driven water splitting and carbon dioxide reduction 
 is seen as a promising option . production of these 
 so - called solar fuels can be facilitated by a suitable photocatalytic 
 material that combines light harvesting and redox functions . typical 
 photocatalysts are inorganic semiconductors , and titanium dioxide 
 is the most widely studied material . this wide band 
 gap ( 3.03.2 ev ) oxide finds application in water splitting , dye - sensitized solar cells , and photocatalytic remediation of pollutants . the energy conversion efficiency of most 
 semiconductor - based systems remains low ( in most cases < 1% ) due 
 to a mismatch between the lifetime of photogenerated charge carriers 
 ( femtoseconds to nanoseconds ) and slow kinetics of the redox reaction 
 ( milliseconds to minutes ) , resulting in severe electron  hole 
 recombination . therefore , understanding of the semiconductor bulk 
 and interfacial charge carrier dynamics and how they couple to surface 
 redox processes is key to optimizing photocatalytic systems . over 
 the last decades , titania has become a  guinea pig  
 of photocatalytic research , and the mixed - phase commercial pigment 
 aeroxide p25 ( ca . 7580% anatase , 2520% rutile ) is 
 an example of an extensively studied tio2-based material 
 that is frequently used as benchmark photocatalyst . the charge carrier dynamics in tio2-based materials have 
 been investigated by different spectroscopic techniques such as transient 
 microwave conductivity ( trmc ) , photoluminescence 
 ( pl ) , transient absorption spectroscopy 
 ( tas ) , electronic paramagnetic resonance 
 ( epr ) , and stimulated desorption . these methods provide information about charge 
 carrier trapping and recombination ( trmc , tas , stimulated desorption ) 
 as well as the nature ( epr ) and depth ( pl ) of the trap states . among them , infrared spectroscopy has proven to be a versatile tool 
 since it can simultaneously assess light - induced changes of surface 
 groups and adsorbates and the majority charge carriers . over the last decades , steady - state and time - resolved ir 
 spectroscopy was used to study electrons in n - type tio2 . both free conduction band and 
 shallow trapped electrons ( cbe and ste , respectively ) interact with 
 electromagnetic irradiation and give rise to specific electronic absorption 
 in the ir region . absorption ( a ) due to cbe is featureless , 
 and its intensity exponentially increases with the decrease of the 
 probing light energy:1where  
 is the frequency of the probing light in wavenumbers , k is the proportionality coefficient , and n is an 
 exponent which can take values between 1.5 and 3.5 depending on the 
 electron scattering mode . optical transitions 
 due to ste can be described , for instance , as excitation of electrons 
 from shallow donor states into the conduction band continuum:2where e is the energy of ir irradiation in electronvolts , k is the proportionality coefficient , eop is the energy of the ir light required to excite an 
 electron from the shallow state into the conduction band continuum , 
 and ethermal is the thermal ionization 
 energy of the donor state . even though charge 
 carrier dynamics in tio2-based materials have been extensively 
 studied over the last decades , there are still some discrepancies regarding 
 the role of surface hydroxyls and adsorbed water in trapping and recombination 
 of photogenerated charge carriers . for instance , it has been reported 
 that dehydroxylation of tio2 at elevated temperatures slows 
 down the decay of photogenerated electron absorption from minutes 
 to hours . on the contrary , a recent time - resolved 
 ir study revealed that water adsorbed on nanocrystalline anatase increases 
 the yield of photogenerated electrons and slows down their recombination 
 at the picosecond to nanosecond time scale due to trapping of photogenerated 
 holes by adsorbed water . in keeping with this , showed that surface - trapped holes in tio2 only exist in the presence of adsorbed water . in this work we investigated how hydration of tio2 p25 affects the dynamics of photogenerated electrons at the 
 millisecond to minute time scale in order to better understand its 
 effect on charge carrier trapping and recombination . to this end , 
 we used rapid scan time - resolved diffuse - reflectance infrared fourier 
 transform spectroscopy ( drifts ) . we found that dehydration of tio2 at t > 423 k accelerates charge carrier 
 recombination and leads to a reversible hysteresis of charge carrier 
 dynamics . oxidative treatment altered the hydration state of tio2 p25 and induced formation of a prominent ste signal and slowed 
 down its decay rates . temperature - dependent measurements revealed 
 low activation energies of the electron absorption decay rates in 
 hydrated samples . ab initio dft+u analysis of the 
 holes trapped on anatase tio2(101 ) revealed that they are 
 more stable and mobile on hydrated surface than in the dehydrated 
 system . these findings demonstrate that adsorbed associated water 
 stabilizes surface - trapped photogenerated holes in tio2 p25 and suppresses charge carrier recombination at the millisecond 
 to minute time scale characteristic for chemical reactions . commercial mixed - phase tio2 ( aeroxide p25 , evonik industries ) was used in all experiments . the 
 morphology of the material was characterized by bright field transmission 
 electron microscopy with an fei technai g2 ( type sphera ) electron 
 microscope operated at 200 kv . x - ray diffractograms were collected 
 on a bruker endeavor d2 powder diffractometer equipped with a bragg brentano 
 goniometer , a cu cathode , and a 1d lynxeye detector ( nickel - filtered 
 k , 1.0 mm primary beam slit and 3.0 mm beam knife 
 height ) . diffuse - reflectance uv  vis spectra were recorded with 
 a shimadzu uv-2401pc uv  vis spectrometer equipped with an integrating 
 sphere accessory , using baso4 as reference . a bruker vertex 70v ftir spectrometer 
 equipped with a praying mantis diffuse - reflectance accessory ( harrick 
 scientific ) and a liquid - nitrogen cooled mct detector was used for 
 the diffuse - reflectance infrared fourier transform spectroscopy ( drifts ) 
 measurements . the ir measurements and sample treatment were carried 
 out in an in situ drifts low - temperature reaction chamber ( harrick 
 scientific ) equipped with two kbr and one fused - silica windows for 
 ir signal collection and sample irradiation , respectively . the cell 
 was connected to a home - built gas delivery system . during all experiments , the temperature of the cell exterior was maintained at 293 k by cooling 
 water . the sample compartment of the spectrometer was purged with 
 dry nitrogen during all experiments . the spectra were recorded at 
 4 cm resolution in the spectral range of 3950600 
 cm . steady - state spectra reported in this work 
 are averages of 100 scans . a low - pass ir filter ( cutoff frequency 
 3950 cm ) was placed in front of the detector compartment 
 to block stray light and reflected laser irradiation . kbr ( sigma - aldrich , 
 ir grade ) was used as reference for the survey drift spectra . typically , 
 the spectrum of the sample in the dark was used as the reference for 
 difference spectra and in time - resolved measurements . the following 
 settings were used : 4 cm spectral resolution , 
 40 khz scanner velocity , acquisition of single - sided interferograms 
 at forward and backward mirror movement with automatic splitting of 
 the interferograms by bruker opus 7.5 software . with these settings , temporal profiles of the ir signal at particular wavenumbers 
 were extracted by bruker opus 7.5 from the 3d data blocks consisting 
 of transient spectra stacked against the delay time . the signal - to - noise 
 ratio was poor , results of several independent measurements were averaged 
 during data processing . o2 ( 99.95% ) , h2 ( 99.999% ) , and ar ( 99.999% ) gases were supplied 
 by linde and passed through moisture and/or oxygen filters ( agilent 
 technologies ) . exposure of the sample ( 60 s for time - resolved measurements ) was controlled 
 by a software script and an optical shutter ( thorlabs ) . more details on data conversion 
 and processing can be found in the supporting information ( cf . also figure s1 ) . untreated tio2 ( sample i ) was prepared 
 by keeping as - delivered tio2 p25 in the spectroscopic cell 
 under dynamic vacuum ( p < 10 mbar ) for 45 h at 293 k prior to the experiments . room - temperature 
 dehydrated tio2 ( sample ii ) was prepared by keeping as - delivered 
 tio2 p25 at 293 k under dynamic vacuum for 36 h. dehydrated 
 titania ( sample iii ) was prepared by heating tio2 to 623 
 k for 12 h under dynamic vacuum followed by cooling to room 
 temperature under static vacuum . rehydrated sample ( sample iv ) was 
 prepared with the same procedure as sample iii but the sample was 
 kept for 16 h in 300 mbar dry ar after cooling down to 293 k. residual 
 water in the ar stream ( < 0.1 ppm ) and water desorbed from the cell 
 interior rehydrated the sample during this hold time . the oxidized 
 sample ( sample v ) was prepared by heating titania to 523 k under dynamic 
 vacuum , followed by irradiation with a 325 nm laser ( 10 mw / cm ) under static vacuum for 1 h. subsequently , the reaction 
 chamber was filled with 50 mbar o2 and the sample was irradiated 
 with the uv light again until a stable spectrum was obtained . after 
 this , the reaction chamber was evacuated , refilled with 50 mbar o2 , cooled to room temperature , and then the sample was exposed 
 to uv light for 30 min in the presence of oxygen . the resulting material 
 ( sample v ) was stored in 300 mbar o2 overnight in dark 
 prior to further spectroscopic experiments . the color of tio2 samples did not turn blue or gray upon prolonged evacuation and/or 
 heating , which indicated that no substantial reduction of the studied 
 materials took place under the chosen experimental conditions . all calculations were performed 
 using the vienna ab initio simulation package ( vasp ) . the ion electron interactions were represented by 
 the projector - augmented wave ( paw ) method and the electron exchange - correlation by the generalized gradient 
 approximation ( gga ) with the perdew  burke sham 
 valence states were expanded in a plane - wave basis set with a cutoff 
 energy of 400 ev . we have used the dft+u approach , 
 in which u is a hubbard - like term describing the 
 on - site coulombic interactions . this 
 approach improves the description of localized states in titania , 
 where standard lda and gga functionals fail . the values of u(ti ) = 4.2 ev and u(o ) = 5.25 ev were 
 applied to the ti 3d and o 2p states , which have been used previously 
 to successfully localize the electrons and holes in n- and p - type 
 defect states in titania , in agreement with experimental epr data . our calculated gga + u lattice parameters for anatase titania were a = 
 3.907  and c = 9.724  . the most 
 abundant and most stable facet exposed in tio2 p25was 
 modeled by a periodic slab using a 3  3 supercell consisting 
 of seven layers of in total 63 tio2 units . the bottom three layers were frozen to their 
 bulk position , and the four top ti  o twelve water molecules were adsorbed on anatase 
 tio2(101 ) surface to simulate the hydrated interface ( figure 
 s2 , supporting information ) . due to the 
 large cell size the climbing image nudged - elastic band ( ci - neb ) 
 algorithm was used to identify the transition states 
 for the trapped hole migration on the anatase tio2(101 ) 
 surface . the dark survey spectrum of untreated p25 tio2 ( sample i ) is shown in figure 1 . 3400 cm ( broad ) and 1622 cm correspond to the stretching 
 and bending vibrations of adsorbed associated water , respectively 
 ( cf . the presence of hydroxyls on the anatase and rutile 
 phases was evident from the bands at 3716 , 3693 , 3680 , 3658 , and 3631 
 cm . the bands in the 29802840 cm region indicate the presence of hydrocarbon moieties , while those 
 at 1554 and 1439 cm can be attributed to oxygenates 
 and/or carbonates . these organic compounds resulted from adsorption 
 of ambient contaminants on as - delivered tio2 p25 . left : 
 room - temperature dark drift spectra of tio2 p25 in static 
 vacuum : untreated sample i ( black ) , dehydrated sample iii ( red ) , oxidized 
 sample v ( blue ) . h stretching region . note : 
 the spectra are vertically offset for convenience . when the untreated 
 sample i was exposed to 325 nm irradiation , a broad featureless absorption 
 band developed in the 29001000 cm region 
 while the ir bands of adsorbed water and surface hydroxyls decreased 
 in intensity ( figure 2 ) . intensity of the 
 featureless absorption band increased toward lower wavenumbers and 
 could be reliably fit with function 1 characteristic for cbe . the best fit was achieved 
 with n = 1.6  0.1 in good agreement with previous 
 reports . this band was hardly affected 
 by addition of ar , while molecular oxygen substantially quenched its 
 intensity ( figure s3 , supporting information ) , which further confirms its assignment to photogenerated electron . ( sample 
 i ) in a static vacuum under 325 nm irradiation ( black curve ) and the 
 exponential fit of the experimental data ( red curve ) . h stretching region . similar to previous reports , the decay rates of the photogenerated electron absorption 
 band were wavenumber - independent in the 29001000 cm range . therefore , we monitored its transient behavior at 1205 cm , where no molecular bands were present ( figure 3 , black curve ) . the 
 electron absorption decay in the dark showed a complex behavior . within 
 the first minute the signal intensity decreased to ca . 30% of its 
 initial value and then decayed to zero at much slower rates . a reliable 
 fit of the initial part of the kinetic curve was achieved with either 
 a second - order decay or a sum of two exponents . both models gave similar r values and half - life times of the signal decay : 
 ( 11.8  0.3 ) s ( table 2 ) . 1 min onward followed first - order 
 kinetics with a half - life time of 2 min . no reliable fit of the kinetic 
 curves in the broad time range ( 5 min ) could be achieved when 
 only a second - order or a two - exponential model was used , but rather 
 a sum of first- and second - order decays or a three - exponential model 
 was needed tio2 p25 samples subjected to different 
 treatments : black , untreated sample i ; red , sample iii dehydrated 
 at 623 k ; blue , rehydrated sample iv ; green , oxidized sample v. inset : 
 normalized decay curves of untreated and rehydrated samples . temporal 
 profiles were recorded after 60 s of exposure to 325 nm irradiation 
 at 293 k under static vacuum . for details on sample treatment such a complex decay of the photogenerated electron 
 absorption may stem from different phenomena . first , if the observed 
 signal originates from well - defined single species ( i.e. , cbe ) , then 
 faster and slower decay components can correspond to at least two 
 different charge carrier recombination channels . for instance , a faster 
 second - order decay can correspond to bimolecular recombination of 
 trapped electrons and holes , while a slower first - order component 
 may be due to defect - assisted recombination . second , both cbe and ste may contribute to the broad absorption 
 band formed in the low - wavenumber region under uv irradiation ( figure 2 ) . in this case , faster and slower decay components may arise 
 from the recombination of photogenerated holes with cbe and ste , respectively . 
 finally , a combination of both situations can not be excluded which 
 severely complicates the development of a reliable analytical model . 
 in this study we opted for the sum of first- and second - order decays 
 to fit the data over a broad time range , while the second - order decay 
 kinetics was used to fit fast initial decay of the signal ( for more 
 details on the fitting procedure see supporting information ) . have previously reported that dehydroxylation of tio2 p25 
 slows down the decay of photogenerated electrons from minutes to hours . on the other hand , a recent study of photogenerated 
 electron dynamics at the picosecond to nanosecond time scale shows 
 that adsorbed water suppresses charge carrier recombination in anatase 
 tio2 . in order to evaluate whether 
 adsorbed water has a positive or an adverse effect on the charge carrier 
 recombination at the millisecond to minute time scale , we studied 
 photogenerated electron absorption dynamics in the material which 
 was dehydrated under dynamic vacuum ( p < 10 mbar ) at 623 k ( sample iii ) . the room - temperature 
 dark drift spectrum of this sample is shown in figure 1 ( red curve ) . as expected , for this sample 
 the intensities of adsorbed water and surface hydroxyls bands were 
 substantially lower than for untreated tio2 . h 
 stretching region of sample iii contained two well - resolved bands 
 at 3716 and 3673 cm , while the bands at 3680 , 
 3658 , and 3631 cm were almost absent ( figure 1 , red curve ) . the 
 bending vibration band of adsorbed water at 1620 cm , which was absent in the spectrum recorded at 623 k ( figure s4 , supporting information ) , reappeared upon cooling , 
 but with lower intensity than before the treatment . the broad band 
 of hydrogen - bonded hydroxyls and associated water ( 3400 cm ) was diminished to a weaker well - resolved feature at 3460 cm . this band can be assigned to the asymmetric stretching 
 mode of water molecules present in the form of small clusters ( table 1 ) . the difference spectrum formed in sample iii under 
 325 nm irradiation is shown in figure 4 , black curve . uv excitation decreased intensities 
 of the bands at 3618 , 3460 , 3090 , and 1621 cm , 
 while a well - resolved band at 3690 cm and a broad 
 feature in the 1600900 cm region emerged . the band at 3618 cm was due to surface hydroxyls 
 of rutile tio2 , while the 3460 and 1621 cm bands corresponded to stretching and bending vibrations of adsorbed 
 water , respectively . the band at 3690 cm can be 
 ascribed to water dissociatively adsorbed on anatase and/or rutile 
 tio2 ( table 1 ) . the broad absorption feature formed in the 
 region below 1600 cm could be fit with function 1 , and its intensity 
 was quenched in the presence of o2 , confirming its cbe 
 origin . compared with sample i ( figure 2 ) , this band had lower intensity , its onset shifted 
 toward lower wavenumbers , and it decayed at substantially faster rates . the half - life decay time decreased from ( 11.8  0.3 ) s to ( 2.9 
  0.5 ) s upon tio2 dehydration ( table 2 ) . moreover , in the dehydrated sample cbe 
 absorption decayed to zero after already 30 s in the dark without 
 a prominent slow component ( figure 3 ) . , our results demonstrate the adverse effect of 
 dehydration on the charge carrier lifetime in tio2 p25 . difference 
 room - temperature drift spectra of dehydrated sample iii ( black ) and 
 rehydrated sample iv ( red ) tio2 p25 in static vacuum under 
 325 nm irradiation . when sample iii was left 
 in the in situ drifts low - temperature reaction chamber in 300 mbar 
 ar for 16 h , it was partially rehydrated by residual water in ar ( < 0.1 
 ppm ) and water desorbed from the internal cell walls ( sample iv ) . this was evident by the bands at 3675 , 3655 , 3630 , and 3400 cm which reappeared in the ir spectrum but with lower 
 intensities than before ( figure s5 , supporting information ) . both the difference spectrum produced in sample 
 iv by 325 nm irradiation ( figure 4 , red curve ) and the decay rates of the photogenerated 
 electron absorption ( figure 3 , inset ) were similar to those of the untreated material ( figure 2 ) , except for the 
 intensities of these spectral features . this suggests that the amount 
 of water adsorbed on titania strongly affects the apparent electron 
 absorption intensity ( i.e. , the number of photogenerated charge carriers ) 
 but not the room - temperature charge carrier dynamics . the reversible 
 effect of dehydration and slower charge carrier recombination rates 
 found for the hydrated samples ( table 2 ) demonstrate that adsorbed water substantially slows 
 down electron the above - discussed experimental results show that dehydration of 
 tio2 adversely affects both the intensity of photogenerated 
 electron absorption and its lifetime . on the other hand , adventitious 
 organic adsorbates the untreated tio2 p25 exhibited vibrational 
 bands at 29802840 , 1554 , and 1439 cm ( figure 1 ) due to hydrocarbons , 
 organic oxygenates , and/or carbonates ( table 1 ) . in order to remove these adventitious 
 adsorbates we oxidized the material at 523 k in 100 mbar o2 . when the sample was heated in the dark , the intensities of the 
 bands at 29802840 cm decreased , a new 
 band emerged at 2349 cm , and the bands at 1554 
 and 1439 cm became stronger . these spectral changes 
 evidenced oxidation of adsorbates containing hydrocarbon moieties 
 to gas - phase co2 and carboxylates ( table 1 ) . after about 1 h , the intensities of these 
 changes became stable and were not affected by the addition of oxygen 
 or increase of the treatment time . the remaining hydrocarbons and 
 formed carboxylates were further oxidized to co2 when the 
 material was irradiated with 325 nm light at 523 k in 100 mbar o2 . when the resulting sample was cooled to room temperature 
 either under static vacuum or in dry ar , its behavior under uv irradiation 
 did not differ from that of the non - oxidized dehydrated material . this suggests that adventitious organic adsorbates had no significant 
 effect on photogenerated electron dynamics at the millisecond to minute 
 time scale . unlike adventitious organic adsorbates , changes 
 of tio2 hydration had a very prominent effect on the photogenerated 
 electron absorption intensity , spectral appearance , and decay rates . when the oxidized sample was cooled to room temperature in 50 mbar 
 o2 instead of ar or vacuum ( see previous section ) , exposed 
 to 325 nm irradiation for 30 min , and stored overnight under the same 
 o2 atmosphere ( sample v ) , the survey drift spectrum exhibited 
 stronger bands of surface hydroxyls and adsorbed water compared with 
 untreated titania ( figure 1 , blue curve ) . h stretching region of sample 
 v showed a prominent band at 3693 cm of water 
 dissociatively adsorbed on tio2 . this band was also present in untreated titania but only as a shoulder 
 ( figure 1 ) . other differences 
 between samples i and v included a decreased intensity of the 3631 
 cm band and appearance of the band at 3670 cm . the latter band was rather broad and could include 
 superimposed 3680 and 3658 cm bands . these changes 
 agreed well with the notion that water adsorbs more easily on anatase 
 tio2 under uv irradiation in the presence of o2 than in an inert atmosphere . when sample v was exposed to 325 nm irradiation the bands of adsorbed 
 water ( 3400 , 1631 cm ) and surface hydroxyls ( 3693 , 
 3681 , 3670 , and 3634 cm ) decreased in intensity 
 ( figure 5 ) . this was 
 accompanied by the appearance of the band at 3734 cm which was attributed to terminal hydroxyls of the anatase tio2 and was not observed in the untreated sample 
 ( figure 2 ) . i the bands at 3633 and 
 3667 cm were affected the most , while in sample 
 v the most prominent intensity change was observed for the 3671 and 
 3693 cm bands ( insets in figures 2 and 5 , respectively ) . yet the most striking difference between these two samples was the 
 intensity and the structure of photogenerated electron absorption 
 bands . compared with the untreated material , the band formed in sample 
 v had higher intensity and did not follow exponential function 1 but almost linearly increased 
 toward lower wavenumbers evidencing formation of species other than 
 cbe . difference room - temperature drift spectrum of oxidized p25 tio2 ( sample v ) in static vacuum under 325 nm irradiation . the electron absorption formed in oxidized 
 sample v under uv irradiation ( figure 5 ) was similar to the bands previously observed by panayotov 
 et al . the authors ascribed such bands 
 to the overlapping absorption of the cbe and ste components . here , 
 we adopted the model developed by panayotov et al . to fit our experimental spectra ( figure s5 , supporting information ) . from this fitting , one 
 can see that a substantial part of the signal corresponds to ste . this accounted for the stronger photogenerated electron absorption 
 signal observed in sample v and may explain its slower decay rates 
 in comparison with untreated titania ( table 2 ) . despite the substantially different decay 
 rates , the absorption signal decay profiles of sample v exhibited 
 a complex behavior similar to that of the untreated and rehydrated 
 samples ( figure 3 ) . 
 first , the signal decayed to ca . 25% of its initial value following 
 the apparent second - order decay kinetics with 0.5 = ( 62  2 ) s. then , the remaining absorption relaxed to zero 
 in an apparent first - order process with 0.5 = ( 3.6 
  0.5 ) min ( table 2 ) . previous sections show 
 that both the intensity of photogenerated electron absorption and 
 its decay rates depend on tio2 hydration . however , room - temperature 
 measurements alone are insufficient to derive a reliable conclusion 
 about the role of adsorbed water in the photogenerated charge carrier 
 dynamics . this is because both cbe and ste can contribute to the apparent 
 electron absorption ( figure s5 , supporting information ) . moreover , the studied samples exhibited a number of stretching 
 o  h bands whose positions and intensities were obscured by 
 hydrogen bonding ( table 1 ) . in order to understand how adsorbed water affects photogenerated 
 charge carrier dynamics we studied the electron absorption decay rates 
 as a function of temperature and pretreatment ( figure 6 ) . temperature dependence of the characteristic 
 electron absorption decay times found for the tio2 p25 
 samples subjected to different treatments : ( a ) untreated sample i ; 
 ( b ) rehydrated sample iv ; ( c ) oxidized sample v ; ( d ) sample ii dehydrated 
 at room temperature for 36 h ; ( e ) sample iii dehydrated at 623 k ; 
 ( f ) untreated sample i at t < 273 k. values in 
 electronvolts represent apparent activation energies derived from 
 the linear regions of the plots ( red lines ) . independent from the treatment , all hydrated samples showed 
 two distinct regimes of the electron absorption decay . in the 293423 
 k temperature range , the apparent decay rates exhibited arrhenius - type 
 behavior , while they did not vary with temperature above 423 k ( figure 6a  d ) . it is 
 worthwhile to mention that , above 423 k , the bands of associated water 
 at 3400 and 1622 cm , as well as the o h 
 bands at 3632 and 3683 cm , substantially decreased 
 in intensity in comparison with lower temperatures ( figure s4 , supporting information ) . the dehydrated sample 
 iii showed no apparent temperature dependence of the charge carrier 
 recombination rates in the temperature range of 323723 k ( figure 6e ) . above 523 k the 
 drift spectra of sample iii exhibited mainly the bands of terminal 
 hydroxyls at 3720 and 3670 cm . below this temperature 
 the 3460 cm band emerged in the survey drift spectra . this band was attributed to stretching vibrations of water adsorbed 
 in the form of small clusters , and it 
 was stable for at least 34 h after cooling to 293 k ( figure 
 s6 , supporting information ) . eventually , 
 the broad absorption feature of associated water with the maximum 
 at 3400 cm and the bands at 3693 , 3670 , and 3630 
 cm reappeared in the survey drift spectra evidencing 
 rehydration of titania . rehydration restored properties of the material 
 under 325 nm irradiation both at 293 k ( see section 3.2 ) and elevated temperatures ( figure 6b ) . hence , we surmise that 
 the hysteresis of the photogenerated charge carrier dynamics observed 
 in tio2 upon its heating above 423 k related to the association 
 of adsorbed water . this is because sample iii exhibited fast temperature - independent 
 charge carrier recombination ( figure 6e ) while its survey drift spectra evidenced the presence 
 of terminal hydroxyls and small clusters of adsorbed water ( figure 
 s6 , supporting information ) but not the 
 associated water band . the latter indicates that neither terminal 
 hydroxyls nor small water clusters can efficiently suppress electron these findings agree with a recent epr study showing that 
 surface trapping of photogenerated holes is inefficient in anatase 
 tio2 dehydrated at 423 k. from figure 6 , one 
 can see that the apparent activation energies of the photogenerated 
 electron absorption decay were almost identical for the untreated 
 and rehydrated samples ( 0.080.09 ev ) and slightly lower for 
 the sample dehydrated at room temperature ( 0.06 ev ) . although oxidized 
 titania exhibited the slowest electron absorption decay rates and 
 the highest apparent activation energy of 0.17 ev in the 293423 
 k temperature range , charge carrier recombination was temperature - independent 
 above 423 k ( figure 6c ) similar to other hydrated samples . consequently , differences in 
 apparent activation energies and the electron absorption decay rates 
 can be attributed to different hydration of the materials rather than 
 their intrinsic properties . from the 
 temperature - dependent plots ( figure 6a  d ) one can expect that cooling of the hydrated 
 samples below 293 k can further slow charge carrier recombination . contrary to this , we found that the photogenerated electron absorption 
 decay rates were temperature - independent between 263 and 153 k , yet 
 slightly faster than at 293 k ( figure 6f ) . the intensity of electron absorption at these temperatures 
 was substantially lower than at 293 k. besides this , steady - state 
 spectra of untreated tio2 changed when it was cooled below 
 273 k. the absorption bands at 3400 and 1620 cm became weaker , the shoulder at 3695 cm disappeared , 
 and the 3720 cm band grew in intensity compared 
 with the sample at 293 k ( figure s8 , supporting information ) . as water could not desorb from titania under 
 these conditions , the aforementioned spectral changes can be explained 
 by perturbed interaction between the oxide surface and adsorbed water . for instance , the increase of the terminal hydroxyls band intensity 
 at 3720 cm can be interpreted as weakened hydrogen 
 bonding between adsorbed water and surface hydroxyls . even though the apparent activation energies 
 obtained in this work are difficult to assign to a specific process , 
 these values , when combined with the effect of dehydration and low 
 temperature , provide valuable phenomenological insights about the 
 interplay of surface hydration and charge carrier dynamics in tio2 p25 at the millisecond to minute time scale . first of all , 
 these apparent activation energies are unlikely to directly correspond 
 to water desorption . this is because the desorption energy is at least 
 0.6 ev and increases with decreasing water coverage ( i.e. , hydration ) which does not agree with our findings . the 
 hysteresis of the charge carrier dynamics observed upon dehydration / rehydration 
 of titania and rather low apparent activation energies suggest that 
 association of the oxide with adsorbed water can account for these 
 phenomena . when temperature increases , hydrogen bonding among adsorbed 
 water molecules and between water and 
 oxide becomes weaker . this destabilizes trapped charge carriers , increases 
 their recombination rates , and results in the apparent activation 
 energy . the absence of apparent activation energy for the electron absorption 
 decay at temperatures below 273 k ( figure 6f ) can be understood by assessing the strength 
 of hydrogen bonding through the shifts of the stretching o  h 
 frequencies . as the o h bands 
 positions and intensities of sample i were almost identical at 153 
 and 263 k ( figure s8 , supporting information ) , hydrogen bonding is expected to be similar in both cases . when 
 association does not change with temperature the electron our experimental results showed that the decay 
 rates of photogenerated electron absorption in tio2 p25 
 depend strongly on the hydration state of titania and hydrogen bonding 
 between the oxide and adsorbed water . these findings suggest that 
 the slowed down charge carrier recombination rates in hydrated n - type 
 tio2 originate from the interaction of adsorbed water with 
 surface - trapped holes rather than with electrons . this is because 
 all tio2 samples exhibited hysteresis of the electron absorption 
 dynamics at temperatures above 423 k irrespective of the nature of 
 photogenerated electrons ( figure 6 ) . for instance , the photogenerated electron absorption 
 signal in the oxidized sample v was dominated by ste up to 523 k. 
 nonetheless , the electron absorption decay rates in this sample were 
 temperature - independent above 423 k. this hysteresis can be understood 
 by taking into account the recent finding of panarelli et al . , who 
 observed that the surface - trapped holes disappear from the epr spectra 
 when water was desorbed from tio2 at 423 k. this was interpreted in terms of stabilization 
 of surface - trapped holes by adsorbed water present on the oxide . suppressed 
 charge carrier recombination at the picosecond time scale due to trapping 
 of photogenerated holes by water adsorbed on anatase nanoparticles these observations agree with the expected interaction 
 of adsorbed water molecules with photogenerated holes that are driven 
 to the surface in hydrated tio2 . in order to better understand how associated adsorbed water 
 interacts with photogenerated holes in the studied titania samples 
 and to this end , we studied the migration 
 of a photogenerated hole trapped on the anatase tio2(101 ) 
 surface by an ab initio dft+u analysis . 84 wt % of used 
 tio2 p25 and ( 101 ) is the most stable and abundant anatase 
 facet . the dehydrated sample was represented by a clean tio2(101 ) surface while the hydrated material was modeled by adding 12 
 associated water molecules to the system . some of these water molecules 
 adsorbed dissociatively forming surface hydroxyls . the ( 101 ) surface 
 exhibits three types of oxygen atoms : twofold - coordinated bridge oxygen 
 ( o2c ) , threefold - coordinated surface ( o3c(a ) ) , and threefold - coordinated subsurface ( o3c(b ) ) atoms , 
 as well as sixfold ( ti6c)- and fivefold ( ti5c)-coordinated titanium ( figure s2 , supporting information ) . the surface - trapped hole ( h ) the formed charge was compensated with a uniform background 
 of charge of the opposite sign , and then the system was allowed to 
 relax . in the dehydrated system the hole localized on o2c had 0.28 ev lower energy than on a o3c(b ) site , while 
 no stable configuration was found for the hole trapped on the o3c(a ) atom . for the hydrated system the situation was substantially 
 different . namely , the most energetically favorable localizations 
 of the hole were o2c ( figure 7 , parts a and d ) and o3c(b ) ( figure 7 , parts c and e ) 
 atoms ( the latter was only 0.03 ev lower in energy ) , while the hole 
 localized on o3c(a ) was ca . 0.5 ev higher in energy than 
 either of these two sites . when the hole was localized on the o3c(b ) atom , the corresponding ti5c  o bond 
 broke , which resulted in the formation of a fourfold - coordinated ti 
 atom . in the dehydrated system this atom was thermodynamically unstable . on the other hand , water adsorbed on the ti5c site stabilized 
 this undercoordinated ti atom in the hydrated system . the bond rearrangement 
 upon migration of h from the o2c site to o3c(b ) site in the hydrated system is shown in figure 7 , parts d and e. the ti5c o3c(b ) bond length was 2.08 and 2.31  
 when the hole localized on the o2c and o3c(b ) sites , respectively . these differences in hole localization on clean 
 and hydrated anatase tio2(101 ) agreed well with the works 
 of selloni and co - workers . migration of the hole 
 ( h ) trapped at the anatase tio2(101 ) surface 
 along the transport coordinate as defined in figure s2b . ( a and c ) spin density of h localized into 
 the o2c and o3c(b ) sites . ( b ) spin density of 
 the transition state for h migration from o2c to o3c(b ) site . ( d and e ) configurations of h localized into the o2c and o3c(b ) sites . ( f ) 
 potential energy surfaces of h migration on the tio2(101 ) surface with and without adsorbed water . in order to evaluate whether adsorbed water affects 
 mobility of the hole trapped on the anatase tio2(101 ) surface we chose this trajectory 
 because the alternative path would involve the o3c(a ) atom 
 which was energetically substantially less favorable for the h localization on both clean and hydrated surfaces . first of all , we found that the transition state 
 was shared between o2c and o3c(b ) atoms ( figure 7b ) , which agreed 
 well with a previous study of the hole transfer from anatase tio2(101 ) to a surface hydroxyl . the 
 barriers for the hole migration from o2c to o3c(b ) in dehydrated and hydrated systems were 0.31 and 0.17 ev , respectively . 
 beside the higher barrier , hole migration on the clean tio2(101 ) surface involved the unfavorable o3c(b ) site ( figure 7f ) . from this , one 
 can expect a substantially higher mobility of the surface - trapped 
 holes in hydrated titania compared with dehydrated samples . this higher 
 mobility can also account for lower charge carrier recombination rates 
 observed for hydrated tio2 . besides this , the difference between the theoretical barriers of 
 the hole migration in dehydrated and hydrated systems was ca . 0.10.2 
 ev ( figure 7f ) , in 
 keeping with the experimental apparent activation energies ( figure 6 ) . these results 
 support our hypothesis that the apparent activation energies obtained 
 in the present work for the hydrated samples originate from the hydrogen - bond 
 interaction between the adsorbed water and titania . moreover , the 
 difference between the surface - trapped hole mobility in dehydrated 
 and hydrated systems can account for the hysteresis of the charge 
 carrier dynamics upon dehydration of tio2 ( figure 6 ) . the influence of tio2 p25 hydration on the decay rates 
 of photogenerated electron absorption at the millisecond to minute 
 time scale was investigated by rapid scan time - resolved drift spectroscopy . dehydrated titania exhibited a weaker electron absorption signal which 
 decayed at higher rates compared with hydrated samples . the slowest 
 electron absorption decay was found for tio2 after an oxidation 
 treatment . the photogenerated electron absorption feature formed under 
 325 nm uv laser irradiation in untreated and oxidized tio2 was dominated by contributions from conduction band electrons ( cbe ) 
 and shallow trapped electrons ( ste ) , respectively . charge carrier 
 recombination rates in hydrated samples increased with temperature 
 in the 293423 k range with low apparent activation energies 
 between 0.06 and 0.17 ev . the electron absorption decay rates became temperature - independent 
 at t > 423 k and t < 273 k. 
 at high temperature a perturbed interaction between adsorbed water 
 and the oxide surface resulted in the temperature - independent charge 
 carrier recombination , while below 273 k this was due to constant 
 strength of hydrogen bonding between titania and adsorbed water . the 
 experimental results were complemented by an ab initio dft+u analysis of surface - trapped hole migration on dehydrated 
 and hydrated anatase tio2(101 ) . this analysis revealed 
 that adsorption of water stabilizes holes localized on surface oxygen 
 atoms and lowers hole migration barriers . these effects are expected 
 to increase charge carrier mobility and with that suppress electron we conclude that the apparent activation energies obtained in this 
 study originate from hydrogen - bonding interactions between adsorbed 
 water and tio2 , which in turn are involved in the stabilization 
 of surface - trapped holes leading to the observed prolonged lifetime 
 of photogenerated electrons .
overall photocatalytic 
 water splitting is one of the most sought after processes for sustainable 
 solar - to - chemical energy conversion . 
 the efficiency of this process 
 strongly depends on charge carrier recombination and interaction with 
 surface adsorbates at different time scales . here , we investigated 
 how hydration of tio2 p25 affects dynamics of photogenerated 
 electrons at the millisecond to minute time scale characteristic for 
 chemical reactions . 
 we used rapid scan diffuse - reflectance infrared 
 fourier transform spectroscopy ( drifts ) . 
 the decay of photogenerated 
 electron absorption was substantially slower in the presence of associated 
 water . 
 for hydrated samples , the charge carrier recombination rates 
 followed an arrhenius - type behavior in the temperature range of 273423 
 k ; these became temperature - independent when the material was dehydrated 
 at temperatures above 423 k or cooled below 273 k. a dft+u analysis revealed that hydrogen bonding with adsorbed water stabilizes 
 surface - trapped holes at anatase tio2(101 ) facet and lowers 
 the barriers for hole migration . hence , hole mobility should be higher 
 in the hydrated material than in the dehydrated system . 
 this demonstrates 
 that adsorbed associated water can efficiently stabilize photogenerated 
 charge carriers in nanocrystalline tio2 and suppress their 
 recombination at the time scale up to minutes .