DOI: 10.1126/science.1061051
, 269 (2001);
293 Science , et al.
R. Asahi Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides
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␣S,were obtained from P.T.Lansbury [J.C.Rochet et al.,Biochemistry 39,10619(2000)].
21.
The silver-stained band at 22kD in an HP2A immu-noprecipitate of normal human brain and a co-mi-grating silver-negative slice from an adjacent lane of a parkin-preabsorbed HP2A precipitate were excised,trypsin digested and subjected to blind analysis by mass spectrometry at MDS Proteomics,Inc.(Toronto,ON)[A.Shevchenko et al.,Anal.Chem .68,850(1996)].The HP2A-specific 22-kD protein yielded tryptic peptides corresponding to aa 13–21,44–58,46–58,59–80,61–80,81–96,and 81–97of human ␣S (GenBank accession #L08850),each ending with a lysine,as expected.No ␣S sequence was detected in the co-migrating slice from the preabsorbed lane.22.Y.Mizuno,N.Hattori,H.Mori,Biomed.Pharmaco-ther.53,109(1999).
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Human UbcH7and UbcH8were expressed li harboring the Lac -repressor expressing plasmid pREP4,using the QIA express system (QIAGEN,Va-lencia,CA)with an NH 2-terminal 6xHis tag.Proteins
were purified on nickel NTA-agarose (QIAGEN).En-zymatic activity was tested in Ub shift assays.
29.
Parkin was immunoprecipitated (yielding IP-parkin)from frontal cortex homogenates or HEK293cells transiently transfected with myc-parkin cDNA (10g)(13).IP parkin was incubated at 37°C in 50l of re
action buffer containing ATP (4mM ATP in 50mM Tris-HCl,pH 7.5,2mM MgCl 2),100ng of recombi-nant human E1,2g of UbcH7(E2),and 2g His-Ub (all from Affinity Research Products,Exeter,UK).The reaction was terminated by adding 20l of 4X sample buffer,and 25l aliquots of the reaction mixtures were electrophoresed and immunoblotted.30.
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For enzymatic digestion of HP2A precipitates,N -glycosidase,sialidase A,endo-O-glycosidase [with bovine fetuin as a control protein (ProZyme,San Leandro,CA)],and protein phosphatase-1(Sigma)were used as per manufacturers’instructions.
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41.We thank H.Mori,Y.Mizutani,and K.Yamane for ARPD
reaction to a book or an articlebrain specimens;J.Chan for providing normal human brain;and the patients’families for the donation of
tissue.We also thank K.Wynn and P.Howley for anti-E6-AP (JH-16),T.Suzuki and K.Tanaka for a parkin cDNA-containing vector,P.Lansbury for reagents and critical review of our manuscript,and M.Medina for experimental advice.H.S.is supported by a Pergolide Fellowship (Eli Lilly,Japan K.K.).M.G.S.is supported by the Grass Foundation (Robert S.Morison Fellowship),the Lefler Foundation,and the NIH (NS 02127).M.P.F.is supported by a Beeson Scholar Award.ived grants from the National Bank of Austria.Supported by the Morris R.Udall Center of Excellence in PD (NS 38375)at Brigham and Women’s Hospital (M.P.F.,K.S.K.,and D.J.S.).
12March 2001;accepted 6June 2001Published online 28June 2001;10.1126/science.1060627
Include this information when citing this paper.
Visible-Light Photocatalysis in Nitrogen-Doped Titanium
Oxides
R.Asahi,*T.Morikawa,T.Ohwaki,K.Aoki,Y.Taga
To use solar irradiation or interior lighting efficiently,we sought a photocatalyst with high reactivity under visible light.Films and powders of TiO 2-x N x have revealed an improvement over titanium dioxi
de (TiO 2)under visible light (wavelength Ͻ500nanometers)in optical absorption and photocatalytic ac-tivity such as photodegradations of methylene blue and gaseous acetaldehyde and hydrophilicity of the film surface.Nitrogen doped into substitutional sites of TiO 2has proven to be indispensable for band-gap narrowing and photocat-alytic activity,as assessed by first-principles calculations and x-ray photo-emission spectroscopy.
Since photoinduced decomposition of water on TiO 2electrodes was discovered (1),semicon-ductor-based photocatalysis has attracted exten-sive interest.One particular focus has been on applications in which organic molecules are photodegraded,such as water and air purifica-tions (2–4).Most of the investigations have focused on TiO 2(5–7),which shows relatively high reactivity and chemical stability under ul-traviolet (UV)light [wavelength ()Ͻ387nm],whose energy exceeds the band gap of 3.2eV in the anatase crystalline phase.
The development of photocatalysts that can yield high reactivity under visible light (Ͼ
380nm)should allow the main part of the solar spectrum,and even poor illumination of interior lighting,to be used.One approach has been to dope transition metals into TiO 2(8–10),and another has been to form reduced TiO x photo-catalysts (11,12).However,doped materials suffer from a thermal inst
ability (9),an increase of carrier-recombination centers,or the require-ment of an expensive ion-implantation facility (10).Reducing TiO 2introduces localized oxy-gen vacancy states located at 0.75to 1.18eV below the conduction band minimum (CBM)of TiO 2(12),so that the energy levels of the optically excited electrons will be lower than the redox potential of the hydrogen evolution (H 2/H 2O)located just below the CBM of TiO 2(13)and that the electron mobility in the bulk region will be small because of the localization.We have considered whether visible-light
activity could be introduced in TiO 2by doping,and we set the following requirements:(i)dop-ing should produce states in the band gap of TiO 2that absorb visible light;(ii)the CBM,including subsequent impurity states,should be as high as that of TiO 2or higher than the H 2/H 2O level to ensure its photoreduction ac-tivity;and (iii)the states in the gap should overlap sufficiently with the band states of TiO 2to transfer photoexcited carriers to reactive sites at the catalyst surface within their lifetime.Conditions ii and iii require that we use anionic species for the doping rather than cationic met-als,which often give quite localized d states deep in the band gap of TiO 2and result in recombination centers of carriers.We have cal-culated densities of states (DOSs)of the substi-tutional doping of C,N,F,P,or S for O in the anatase TiO 2crystal,by the full-potential lin-earized augmented plane wave (FLAPW)for-malism (14,15)in the framework of the local density approximation (LDA)(16)(Fig.1).The s
ubstitutional doping of N was the most effective because its p states contribute to the band-gap narrowing by mixing with O 2p states.Although doping with S shows a similar band-gap narrowing,it would be difficult to incorporate it into the TiO 2crystal because of its large ionic radius,as evidenced by a much larger formation energy required for the substi-tution of S than that required for the substitution of N (17).The states introduced by C and P are too deep in the gap to satisfy condition iii.The calculated imaginary parts of the dielectric functions of TiO 2-x N x indeed show a shift of the absorption edge to a lower energy by the N doping (Fig.2A)(18).Dominant transitions at the absorption edge have been identified with
Toyota Central R&D Laboratories,Nagakute,Aichi 480-1192,Japan.
*To whom correspondence should be addressed.E-mail:jp
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those from N 2p to Ti d xy ,instead of from O 2p as in TiO 2(19).Jansen and Letschert dem-onstrated tunable colorings in modified perov-skite oxinitrides,which suggested a similar band-gap control by N,although they did not
address the detailed electronic states in these pigments (20).
Photocatalysts for the present purpose,however,require more elaborate controls for doping besides the optical ,
conditions ii and iii.To this end,we have made a theoretical comparison among three systems:substitutional N doping,interstitial N doping,and both types of doping in the anatase TiO 2.In optimizing the positions of N in the eight TiO 2units per cell,we ob-served molecularly bonding states—NO and N 2—for the last two cases;the obtained bond lengths of N–O and N–N were 1.20and 1.16Å,respectively,which are compared with those of the NO molecule (1.15Å)and the N 2molecule (1.10Å).Such molecularly existing dopants give rise to the bonding states below the O 2p valence bands and antibonding states deep in the band gap (Fig.1B,N i and N i ϩs ).However,these states are well screened and hardly interact with the band states of TiO 2,and thus are unlikely to be effective for photocatalysis because of con-dition iii.The importance of substitutional site N doping is emphasized in the experi-mental results discussed below.
We prepared TiO 2-x N x films by sputtering the TiO 2target in an N 2(40%)/Ar gas mixture.After being annealed at 550°C in N 2gas for 4hours,the films were crystalline,with features assignable to a mixed structure of the anatase and rutile crystalline phases,as determined by x-ray diffraction (XRD).The TiO 2-x N x films were yellowish and transparent.TiO 2films were prepared in a similar fashion by sputtering the TiO 2target in an O 2(20%)/Ar gas mixture and subsequently annealing it at 550°C in O 2gas for 4hours.XRD showed that the homo-geneous anatase crystalline phase was formed.Optical absorption spectra (Fig.2B)show that the TiO 2-x N x films noticeably absorb the light at less than 500nm,whereas the TiO 2films do not,which is in good agreement with the theo-retical results.
Photocatalytic activity was evaluated by measuring decomposition rates of methylene blue as a function of the cutoff wavelength of the optical high-path filters under fluorescent light (Fig.3A).Substantial photocatalytic
ac-
Fig.1.(A )Total DOSs of doped TiO 2and (B )the projected DOSs into the doped anion sites,calculated by FLAPW.The dopants F,N,C,S,and P were located at a substitutional site for an O atom in the anatas
e TiO 2crystal (the eight TiO 2units per cell).The results for N doping at an interstitial site (N i -doped)and that at both substitutional and interstitial sites (N i ϩs -doped)are also shown.The energy is measured from the top of the valence bands of TiO 2,and the DOSs for doped TiO 2are shifted so that the peaks of the O 2s states (at the farthest site from the dopant)are aligned with each other.Arb.unit,arbitrary
units.Fig.2.Optical proper-ties of TiO 2-x N x (thick lines)compared with TiO 2(thin lines).(A )Calculated imaginary parts of the dielectric functions (2),which are averaged over three (x ,y ,and z )po-larization vectors.(B )Experimental optical absorption spectra of TiO 2-x N x and TiO 2
films.
Fig.3.Photocatalytic proper-ties of TiO 2-x N x samples (solid circles)compared with TiO 2samples (open squares).(A )Decomposition rates [measur-ing the change in absorption of the reference light (⌬abs)]of methylene blue as a func-tion of the cutoff wavelength of the optical high-path filters under fl
uorescent light with the integrated photon flux of 2.45ϫ10Ϫ9einstein (E)s Ϫ1cm Ϫ2between 350and 520nm,compared with the re-sults under BL illumination
with the integrated photon flux of 3.51ϫ10Ϫ9E s -1cm -2in the UV range.(B )CO 2evolution as a function of irradiation time (light on at zero)during the photodegradation of acetaldehyde gas [with an initial concentration of 485parts per million (ppm)]under UV irradiation (BL with a peak at 351nm and the light power of 5.4mW cm Ϫ2)and visible irradiation [fluorescent light cut by the optical high-path filter (SC42,Fuji Photo Film),with a peak
intensity at 436nm and a light power of 0.9mW cm Ϫ2].(C )Contact angles of water as a function of time under interior lighting (with light powers of 28.5and 159.4W cm Ϫ2in the UV and visible ranges,respectively).All these light powers were measured by the UV radiometer (UVR-2,TOPCON,Tokyo,Japan)with detectors (UD-36for the UV range and UD-40for the visible range).
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tivity under 500nm has been observed in TiO 2-x N x films,and the cutoff wavelength for photocatalytic activity corresponds well with the optical absorption spectra (21,22).Both TiO 2-x N x and TiO 2films reveal similar activity under UV light [represented by the results un-der black light (BL)illumination].
We also evaluated the photodecomposition of gaseous acetaldehyde of TiO 2-x N x powder samples that were prepared by treating anatase TiO 2powder (ST01,Ishihara Sangyo Kaisha,Osaka,Japan)in the NH 3(67%)/Ar atmosphere at 600°C for 3hours.The Brunaer-Emmett-Teller surface areas of the TiO 2-x N x and TiO 2powders were measured as 67and 270m 2/g,respectively.Figure 3B shows CO 2concentra-tions,evolved as a result of the photodecompo-sition of acetaldehyde,as a function of irradia-tion time.The photocatalytic activity of the TiO 2-x N x sample is superior to that of the TiO 2sample in the visible range of irradiation,whereas both samples yield similar UV activity.We used the photodegradation process of acet-aldehyde proposed in (23)to estimate quantum yields for the CO 2evolution,based on the number of incident photons being 0.42%(TiO 2-x N x )and 0.14%(TiO 2)at 436nm,and 3.0%(TiO 2-x N x )and 2.6%(TiO 2)at 351nm.Our final evaluation of the photocatalytic activity was to measure contact angles of water on the sample films under interior lighting.The so-called photoinduced hydrophilic surface is known as an important application of TiO 2(24,25).In this experiment,we used the TiO 2-x N x and TiO 2films,on which SiO 2with a nominal thickness of 5nm wa
s deposited to hold ad-sorbed water (25).The results show an excel-lent hydrophilic surface of the SiO 2/TiO 2-x N x film,which maintained a contact angle of 6°even after 30days,in contrast to the SiO 2/TiO 2film,whose contact angle constantly increased with time (Fig.3C).As with usual TiO 2spec-imens,our TiO 2-x N x samples resisted the attack of acid and alkaline solvents such as H 2SO 4,HCl,H 2O 2,and NaOH at the ambient temper-ature,and their photocatalytic performance was
stable during successive use under 100-W mer-cury lamp irradiation for more than 3months.To investigate N states in TiO 2-x N x ,we measured N 1s core levels with x-ray photo-emission spectroscopy (XPS).Three peak structures at the binding energies of 402,400,and 396eV were observed for the TiO 2-x N x films (Fig.4A).We observed similar XPS spectra for the TiO 2-x N x powder samples.The TiO 2films also included a small amount of N;however,the peak at 396eV was not observed.Saha et al .(26)investigated the N 1s XPS spectra during the oxidation process of TiN and assigned the peaks as atomic -N (396eV)and molecularly chemisorbed ␥-N 2(400and 402eV).To elucidate relations to photocatalytic activity systematically,we prepared TiO 2-x N x samples by annealing the TiO 2powder in the NH 3(67%)/Ar atmo-sphere at 550°to 600°C.The powder samples were used here,because the reaction temper-ature and the N concentration in the samples can be easily controlled by changing temper-ature and time of the NH 3treatment rather than by changing the
sputtering conditions.Figure 4B shows the decomposition rates of methylene blue under visible light (Ͼ400nm)as a function of the ratio of the decom-posed area in the XPS peak at 396eV to the total area of N 1s .Each powder sample includ-ed the total N concentration of about 1atomic %.An increase of photocatalytic activity with an increase of the component of N with the XPS peak at 396eV is clearly observed.The decrease in photocatalytic activity observed in the powder sample (e)may be attributed to a change in the crystal structure of the sample caused by the high doping of N in the crystal,although we did not find a noticeable change in XRD.An optimum concentration of N with the XPS peak at 396eV,which may depend on the preparation process,was found at ϳ0.25atomic %(read from Fig.4B and the total concentra-tions of N in the samples)or TiO 1.9925N 0.0075.All these results,combined with the theoretical analyses described before (along with Fig.1),
consistently show that the active sites of N for photocatalysis under visible light are the substi-tutional ones that can be identified with the atomic -N states peaking at 396eV in the XPS spectra.
The active wavelength of TiO 2-x N x ,of less than 500nm,promises a wide range of applications,as it covers the main peak of the solar irradiation energy beyond Earth’s atmo-sphere (around 460nm)and an excellent light source peaking at 390to 420nm,provided by recently developed light-emitting indium gal-lium nitride diodes (27).
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28.
We thank A.J.Freeman and W.Mannstadt for their continuous support with the FLAPW code,and K.Tanaka and N.Isomura for their experimental sup-port.
28December 2000;accepted 25May
2001
Fig.4.(A )N 1s XPS spectra of the (upper lines)TiO 2-x N x and (lower lines)TiO 2films.(B )Decomposi-tion rates (measuring the change in absorp-tion of the reference light after 10hours)of methylene blue in the aqueous solution un-der visible light (the same light source as the visible irradiation in Fig.3B)as a func-tion of the ratio of the decomposed area in
the XPS spectra with the peak at 396eV to the total area of N 1s .The total N concentrations for the powder samples were evaluated to be 1.0atomic %,a;1.1atomic %,b;1.4atomic %,c;1.1atomic %,d;and 1.0atomic %,e.
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