Hydrogen generation under visible light using nitrogen doped titania anodesH. Lin, A. K. Rumaiz, M. Schulz, C. P. Huang, and S. Ismat Shah Citation: Journal of Applied Physics 107, 124305 (2010); doi: 10.1063/1.3428514 View online: http://dx.doi.org/10.1063/1.3428514 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/107/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Mesoporous coupled ZnO/TiO2 photocatalyst nanocomposites for hydrogen generation J. Renewable Sustainable Energy 5, 033118 (2013); 10.1063/1.4808263 Visible light photocatalytic degradation of 4-chlorophenol using vanadium and nitrogen co-doped TiO 2 AIP Conf. Proc. 1512, 280 (2013); 10.1063/1.4791020 Epitaxial Rh-doped SrTiO3 thin film photocathode for water splitting under visible light irradiation Appl. Phys. Lett. 101, 033910 (2012); 10.1063/1.4738371 Energy-level and optical properties of nitrogen doped TiO2: An experimental and theoretical study Appl. Phys. Lett. 99, 221909 (2011); 10.1063/1.3664104 Epitaxial growth and characteristics of N-doped anatase TiO 2 films grown using a free-radical nitrogen oxidesource J. Appl. Phys. 97, 123511 (2005); 10.1063/1.1929889

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Hydrogen generation under visible light using nitrogen doped titaniaanodes

H. Lin,1 A. K. Rumaiz,2 M. Schulz,1 C. P. Huang,3 and S. Ismat Shah1,4,a�

1Department of Materials Science and Engineering, University of Delaware, Newark, Delaware 19716, USA2National Synchrotron Light Source, Brookhaven National Laboratory, Upton, New York 11973, USA3Department of Civil and Environmental Engineering, University of Delaware, Newark, Delaware 19716,USA4Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, USA

�Received 24 August 2009; accepted 15 April 2010; published online 16 June 2010�

Hydrogen is among several energy sources that will be needed to replace the quickly diminishingfossil fuels. Free hydrogen is not available naturally on earth and the current processes require afossil fuel, methane, to generate hydrogen. Electrochemical splitting of water on titania proposed byFujishima suffers from low efficiency. The efficiency could be enhanced if full sun spectrum can beutilized. Using pulsed laser deposition technique we synthesized nitrogen doped titanium dioxide�TiO2−xNx� thin films with improved visible light sensitivity. The photoactivity was found to be Nconcentration dependent. Hydrogen evolution was observed under visible light irradiation�wavelength�390 nm� without the presence of any organic electron donor. © 2010 AmericanInstitute of Physics. �doi:10.1063/1.3428514�

I. INTRODUCTION

Titanium dioxide �TiO2� was first found to be capable ofsplitting water molecules into oxygen and hydrogen gases in1972 by Fujishima and Honda.1 However, owing to its rela-tively large band gap �3.0 and 3.2 eV for rutile and anatase,respectively�, photovoltaic application of this photocatalystis limited to the use with ultraviolet portion of the solarspectrum. The quantum conversion efficiency of TiO2 can beenhanced if its band gap could be tailored to absorb in thevisible range.

The band gap is governed by the electronic structure.Theoretical studies based on first-principles orthogonalizedlinear-combinations of atomic-orbitals2 and full-potential lin-earized augmented plane-wave3 have revealed that the con-duction and valance bands are mainly constructed withTi 3d states and O 2p states, respectively. In principle, ifnew electronic states could be introduced within its forbid-den band and overlap sufficiently with TiO2 states withoutgreatly sacrificing its exciton lifetime, visible light quantumefficiency can be enhanced due to the reduction in the effec-tive band gap.4 The most promising method to reduce theeffective band gap of TiO2 is through the doping of impuri-ties into the TiO2 lattice to modify its electronic structure.4–12

The substitutional doping of transition metals5,7,10,13 and an-ion impurities into TiO2 catalysts4,6,8–10,14,15 have oftenproven to enhance the photocatalytic performance of TiO2.However, transition metal dopants often create localizeddeep d states in the TiO2 band gap which are generally re-sponsible for reducing charge carrier lifetime due to increaserecombination rate. Among all dopants that have beenreported, nitrogen is consistently recognized as oneof the most effective dopants, both theoretically andexperimentally.4,9–12,14,16,17 The electronic structure study of

N doped TiO2 by Asahi et al.4 based on the density of states�DOS� calculations reveal that the substitutionally dopedTiO2−xNx creates localized N 2p states just above the val-ance band edge. This results in a mild reduction in the bandgap ��0.3 eV� and redshifts the optical absorption edge byabout 100 nm beyond the UV region. Theoretical studies ofN doped TiO2 by Valentin et al.16 concluded that the bandgap reduction is due to the insertion of nitrogen 2p stateswithout shifting the O 2p derived valance band edge, andvisible light sensitivity is only pronounced under the condi-tions that: �1� the material is primarily anatase and �2� thereis high atomic concentration ��5%� of N. It is also impor-tant to note that N doping in TiO2 lattice �interstitial andsubstitutional� generates distinct electronic structures.4,11

Valentin et al.18 have reported N 2p states positioned at 0.14and 0.73 eV above the valance band edge for substitutionallyand interstitially N doped TiO2, respectively. The later con-figuration is generally believed to cause reduction in the pho-toactivity as a consequence of potential increase in chargecarrier trapping at deep N 2p states.11,14 Depending on thesynthesis methods and N sources, the binding energies of Natoms in TiO2 matrix have been reported to be �396 and400 eV for substitutionally and interstitially doped TiO2,respectively.19–21

Another crucial fact is that the generation of oxygen va-cancies in N doping process is inevitable due to great reduc-tion in its formation energy when oxygen vacancies aretrapped at N impurity sites.11,22 A more recent theoreticalinvestigation performed by Finazzi et al.23 has shown thatsubstitutional N doping is more favorable under oxygen de-ficient conditions while interstitial doping of N in TiO2 isfavored under oxygen rich conditions. Based on the heat offormation calculations for three conditions: �1� substitu-tional, �2� interstitial, and �3� mixed substitutional and inter-stitial in anatase, their results showed that formation of oxy-a�Electronic mail: ismat@udel.edu.

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gen vacancies is thermodynamically favorable under verybroad oxygen pressure range �i.e., high vacuum to 1 atmregime�.4,23

Although theoretical studies of N doped TiO2 have beenwell documented,4,11,16,23 the observation of the relationshipbetween N impurity and the induced vacancies are seldomreported. In the majority of the reported studies, the band gapof N doped TiO2 was determined based on optical absorptionmeasurement, which provides information in terms of theenergy gap between localized N 2p states and the bottom ofthe conduction band. Such measurements do not give theresolution of energy states between the O 2p-derived val-ance band maximum and localized N 2p states, nor the po-sition of the defect states positioned deeply within the for-bidden band.

Energy translation between the absolute vacuum andnormal hydrogen electrode scales is offset by �4.5 eV atnormal condition.24,25 The direction of electron flow in asemiconductor/liquid junction is dictated by the relative en-ergy positions of the semiconductor band edges �i.e., conduc-tion �EC� and valance �EV� band edges� and the Fermi energyof the redox species �EF redox

o � at the solid/liquid interface.The greater oxidation potential relies on the greater differ-ence in �EF redox

o −EV�. By introducing N states into the bandgap, oxidation potential is certainly reduced �EF redox

o −EN�under irradiation of visible photons. The band gap of TiO2

can be reduced through doping of the impurities. However,to obtain a meaningful band gap reduction, namely, a realenhancement of the photoactivity, there are other factors thatneed to be considered: �i� impurity states introduced by thedopants should overlap with TiO2 band states to form a con-tinuum in order to allow the photoinduced electrons to reachthe conduction band, and electrons from the redox species toreach the hole states of the catalyst within their lifetime,4 �ii�impurity states should overlap with Ti 3d-derived conduc-tion band for oxidation reactions and O 2p-derived valanceband for reduction applications. Clearly, from reportedstudies,4,16,22 only factor �i� is satisfied since the anticipatedenhancement in oxidation performance from substitutionallyN doped TiO2 fails to satisfy the condition �ii�. This impliesthat redshift on photoabsorption spectra and the decrease inphotoactivity �oxidation reaction� for N doped TiO2 can beanticipated. Therefore, certain energy alignment between thelocalized N 2p states and the redox potential of targetchemicals in solution is necessary to take full advantage ofthe band gap reduced titania.

As mentioned above, N doping of TiO2 changes its elec-tronic properties. The position of the N 2p states within theTiO2 band gap, configuration, and concentration of N atomsin TiO2 lattice, and doping induced oxygen vacancies couldgreatly affect its optoelectronic properties and subsequentlydetermine its photoactivity for practical applications. In thispaper, we present a detailed study of the N doped anataseTiO2 thin films with substitutional N concentration rangingfrom 0%–4.42%. To experimentally probe the possible elec-tronic configuration from mid-gap to valance band of TiO2,we have performed photoemission and photoelectrochemicalmeasurements to provide different insights on this secondgeneration photocatalyst.

II. EXPERIMENTAL

A reactive pulsed laser deposition system was modifiedfor the synthesis of TiO2−xNx thin films. A KrF gas excimerlaser system �Lambda Physik LPX 305, �=248 nm� wasused for the deposition of TiO2−xNx thin films. The basepressure in deposition chamber was kept at 1.0�10−6 Torrby operating a turbo molecular pump and a mechanical pumpin series. To obtain anatase phase, two 500 W halogen lampswere used as irradiative heating source to control the sub-strate temperature at 600 °C. Target rotation speed was 15rpm. A solid TiO2 target was prepared by pressing highlypure �99.999% Sigma-Aldrich� titanium dioxide powder intoa 5 cm diameter and 0.5 cm thick disk. The incident laserbeam was maintained at a 45° angle to the target surfaces.Laser pulse frequency was set at 15 Hz with a calculatedlaser beam fluence of 1.8 J /cm2. The deposition rate of TiO2

thin films was about 0.09 Å per laser pulse. Indium doped tinoxide �ITO� coated quartz with the sheet resistance of10�1 � /� �SPI supplies Inc., PA, USA� was used as thesubstrate for deposition. Prior to the deposition, all ITOcoated quartz substrates were cleaned ultrasonically in pureacetone solution and triple rinsed with deionized water�18.0 M ��. With increase in reactive gas �N2� to buffer gas�1:1 Ar and O2� ratio from 0 to 10, we were able to obtaincrystalline TiO2 thin film with the atomic N doping concen-tration from 0.0% to 4.4%. The chemical structure of thefilms was determined based on x-ray photoelectron spectros-copy �XPS� measurements. An SSI-M probe XPS systemwas used employing Al K� �h�=1486.6 eV� as excitationsource. High resolution XPS spectra were collected at 26 eVpass energy with a dwell time of 100 ms per point. Peakpositions were referenced to the C 1s peak at 284.6 eV. Per-kin Elmer UV-Vis spectrophotometer �model Lambda 35�equipped with an integrating sphere was used for recordingthe absorbance/transmittance and diffuse reflectance spectra.A photoelectrochemical system consists of a three-electrodepotentiostat �model AFRDE 4, Pine Instrument Inc., USA�, acustom made polytetrafluoroethylene cell with fused silicawindow, and a monochromatic excitation source �ModelRF-5301, Shimadzu, Japan� was employed. TheTiO2−xNx/ITO/quartz slides were fabricated as the photoan-ode, a platinized platinum wire was used as the counter elec-trode, and a saturated calomel electrode �SCE� was selectedas the reference electrode for this study. NaOH �0.01 M� wasused as electrolyte for the photocurrent measurements. Cy-clic voltammetry was recorded within a sweeping voltage of�2.0 V �versus SCE�. For the quantum efficiency study, themonochromatic light source mentioned above was used. Inhydrogen generation and flatband potential studies, a 350 Whalogen light bulb was used as the solar simulation lightsource. For both experiments, power flux was recorded by aradiant power energy meter �model 70260, Newport Corp.,USA�.

III. RESULTS AND DISCUSSIONS

Figure 1�a� compares the N 1s core level spectra ofTiO2−xNx at different doping concentrations. Two majorpeaks were observed at �400 and 396 eV. The peak at

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�400 eV is generally considered to be related to the chemi-sorbed N species �i.e., N–O�.19 This N peak was observedeven in the pure TiO2 thin film samples. Another N 1s peakat lower binding �396 eV is normally registered for thesubstitutional N atoms �i.e., Ti–N� in TiO2 matrix and isreported to be responsible for the enhancement of visiblelight sensitivity.19–21 Clearly, the results show that the substi-tutional N doping concentration increases with increase inthe N2/buffer gas ratio in reactive pulsed laser depositionprocess. The highest atomic N concentration achieved, asmeasured from the XPS survey spectrum, is 4.4%. The cor-responding Ti 2p core level peaks are compared in Fig. 1�b�.With the increase in N concentration, the Ti 2p spectra de-veloped shoulders at binding energy region lower than thatfor Ti3+ �i.e., 456.8 eV� confirming the formation oxygenvacancy.22,26 Obviously, the concentration of the oxygen va-cancy is doping concentration dependent. Similar result wasalso reported in N doped rutile TiO2.21 The deconvolution ofTi 2p spectra show that the atomic ratio of Ti�III�/Ti�IV� are0.06, 0.08, 0.10, and 0.29 for 0.6%, 2.1%, 4.4%, and 9.0%,respectively. The structure analyses from x-ray diffractionmeasurements show that all samples below 4.4% atomic con-centration have anatase structure. Above this doping concen-tration no crystalline TiO2 was observed. To the best of ourknowledge, 4.4% substitutional N doping concentration �i.e.,

N 1s at �396 eV� is the highest number in reported litera-tures and is very close to the theoretical limit of 5% proposedby Valentin et al.16

The indirect band gap of the TiO2−xNx films were ob-tained based on diffuse reflectance measurements usingKulbeka–Munk function and Tauc theory27 �Fig. 2�. It isclear that the band gap decreases with the increases in Ndoping concentration. The band gap for 0.0, 0.6%, 1.1%,2.1%, and 4.4% N samples are measured to be 3.29 eV, 3.21eV, 3.00 eV, 2.52 eV, and 2.43 eV, respectively. The insetshows the corresponding transmittance spectra for allsamples tested. The solid red-shifting in transmittance spec-tra at high N concentration�i.e., 2.1 and 4.4%� suggested thatthe inserted N 2p states overlapped with O 2p states suffi-ciently instead of being isolated near the valance band edgewhich yields high transmittance at lower wavelength regionfor samples with lower doping concentration �i.e., 0.6% and1.1%�. Due to large reduction in effective band gap for highconcentration samples, the wavelength of band gap threshold��bg� well exceeds the wavelength with the highest intensityin solar spectrum �i.e., 460 nm�. These high concentrationTiO2−xNx thin films certainly provide great opportunities formany potential optoelectronic and photovoltaic applications.

The electronic structure of predominantly O 2p derivedvalance band maximum were examined by XPS �Fig. 3�a��.Pure TiO2 primarily has a filled O 2p derived valence bandseparated from an empty Ti 3d, 4s, and 4p derived conduc-tion band by a bulk band gap of 3.2 eV.26 The valence bandspectra show the emission from O 2p band whose upperedge lies 3 eV away from the Fermi level. Visual examina-tion of Fig. 3�a� reveals two features in the N doped samplesas follows: a tailing of the valence-band maximum to lowerbinding energy and an impurity state just above this maxi-mum compared to undoped TiO2 for the sample with thehigh N doping. The tail-like state is attributed to the N 2plevel since the binding energy of N 2p is less than O 2pthus extending the valence-band maximum to a lower bind-ing energy.21,22 As we have discussed earlier, the oxygen

FIG. 1. �a� Comparison of N 1s core level XPS spectra for samples with Nconcentrations 0.0%, 0.6%, 1.1%, 2.1%, and 4.4%. �b� Comparison ofTi 2p core level XPS spectra of samples with N concentrations 0.6%, 2.1%,4.4%, and 9.0%. Ti 2p spectrum of the pure TiO2 sample was identical tothe 0.6% N sample and it is not shown.

FIG. 2. �Color online� Band gap approximation using the transformedKubelka–Monk function for samples with nitrogen concentration 0.0%,0.6%, 1.1%, 2.1%, and 4.4%. Inset contains the corresponding optical trans-mittance spectra for all samples.

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vacancy concentration increases with increase in N dopingconcentration. Removing an O atom from TiO2 frees a pairof electrons that potentially occupy the Ti 3d orbitals andform a more localized mid-gap states.28,29 In Fig. 3�a�, theadditional peak is noticeable only at the highest doping con-centration we have achieved �9.0%�. With reference to thesimilar study on N doped TiO2 by ion-implantation reportedby Batzil et al.,21 we suggest that these are the oxygen de-fects associated Ti 3d defect states.21,30 Recent results usinghard x-ray photoemission spectroscopy have also showed thedefect level as the occupied Ti 3d states.22 However, thestates at this binding energy is not clearly shown for sampleswith less than 9.0% doping concentration due to low sensi-tivity of XPS in this energy region. The theoretical DOScalculations have shown minimum to no shift of the conduc-tion band edge in N doped TiO2.4,16,23 Thus, the photon en-ergy used in quantum efficiency measurements is analogousto the energy below the conduction band minimum. In otherwords, the quantum efficiency spectrum is reasonably pro-

portional to the DOS of the TiO2 near the valance band edge.The quantum efficiency spectra are shown in Fig. 3�b�. Withthe increase in N doping concentration from 0.0% to 4.4%,quantum efficiency for water splitting reaction �described be-low� yields a peak at �2.8 eV that is also about 0.4 eVabove the valance band edge �3.2 eV�. This result is consis-tent with what we observed from photoemission studies �Fig.3�a��. It is worth mentioning that N 2p and O 2p statessuperimpose around the original valance band edge. Resultsbased on quantum efficiency profiling suggest that the degreeof isolation between O 2p and N 2p states are N concentra-tion dependent and N 2p states appear to overlap with O 2pstates when atomic N concentration exceeds 2.2%.

The water splitting study was performed using photo-electrochemical system shown in Fig. 4. In this study, twodifferent light sources were employed: 3.88 eV ��=320 nm� and 2.70 eV ��=460 nm�. Under the UV irradia-tion, it is observed that the photocurrent density decreaseswith increase in N doping concentration. Since the photonenergy from the former light source is higher than the bandgap of TiO2, the electron excited from the sates below thehighest occupied molecular orbital �HOMO� are potentiallytrapped by inserted N 2p states and the associated defectstates. Therefore, a decreased performance was observed forN doped TiO2. However, identical results were observed un-der irradiation with light source of �=480 nm �Fig. 4�b��.Since the energy of photons is lower than the band gap ofTiO2, the electron transition between O 2p �HOMO� andTi 3d lowest unoccupied molecular orbital �LUMO� shouldbe negligible. Under this condition, the photocurrent is con-tributed mainly by N 2p to Ti 3d interstates transition. In-terestingly, the photocurrent density rises at �0.25 and 1.5 Vapplied voltages: The photocurrent density first rises at ap-plied voltage �0.25 V and saturates until �1.5 V. Above1.5 V another increase in photocurrent is observed. For theformer case, the photocurrent density change is due to thefact that localized N 2p states above the O 2p derived val-ance band edge in the absolute energy scale, which subse-quently reduces the oxidation potential �energy gap betweenthe localized N 2p states and redox potential of H2O� of thewater splitting reaction. The applied voltage induces theband bending of the localized N 2p states that enlarges theoxidation potential and thermodynamically favors the watersplitting reaction. Continual increase in applied voltage im-plies that the oxidation potential for the water splitting reac-tion also increases. To a certain point, energy gap betweenthe defects derived Ti 3d states within the forbidden bandand the chemical potential of the water becomes largeenough to allow the reaction to occur due to the energyalignment. Thus, additional photocurrent rises beyond theapplied potential of 1.5 V. This is contributed by electrontransition between the defect derived Ti 3d states and theredox potential of water.

The water splitting studies under visible light irradiation��=460 nm� was carried out using the photoelectrochemicalsystem. The overall photoconversion efficiency is calculatedby measuring the ratio of the maximum energy output to theenergy supplied in the form of light. It is presumed that allthe carriers are utilized only for generating hydrogen/

FIG. 3. �Color online� �a�. Valance band XPS spectra of undoped and Ndoped TiO2 �The inset is the magnified spectra of valance band edge regionwhich shows the occupied Ti 3d states�. �b�. Quantum efficiencies ofsamples at N concentrations 0.0%, 1.1%, 2.1%, and 4.4%. 0.01 M NaOHwas used as electrolyte. Film thicknesses were kept at 800 nm and a Pt wirewas used as cathode.

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oxygen, i.e., 100% Faradic conversion. In such case, eitherthe total amount of hydrogen and oxygen produced can bemeasured and converted into total current or, conversely andmore conveniently, the anode current could be measured andused, along with the applied bias, for the calculation of thepower output. The later strategy is utilized in the currentwork. The photoconversion efficiencies at different N dopingconcentration are compared in Fig. 4�c�. The highest photo-conversion efficiency of 3.15% was obtained from the 4.4%N doped sample. The undoped TiO2 sample yields negligibleefficiency. The quantum efficiency of these TiO2−xNx thinfilms under broader range of the photospectrum can be im-proved by depositing another layer of pure TiO2 on top of Ndoped layer. In this arrangement, UV photons will be con-verted before entering the N doped layer and will not causecharge carrier trapping under UV irradiation.

In conclusion, we have prepared high quality TiO2−xNx

thin films by reactive pulsed laser deposition with wide rangeof control in N doping concentration. Diffuse reflectancespectroscopy show a systematic reduction in the band gap asthe N doping increased. Valence band measurements fromXPS show the N 2p level to be just above the O 2p level. Aphotoconversion efficiency of 3.15% was achieved under ir-radiation of photons with energy at main peak of the solarspectrum. It was also shown that N doping also leads to theformation of oxygen vacancies and, at high N doping, weobserved that the vacancies populate the otherwise emptyTi 3d states. Due to the electronic structure of TiO2−xNx thinfilms, the UV, and visible light sensitivity appear to be afunction of doping concentration which makes it potentiallyfeasible for variety of optoelectronic and photovoltaic appli-cations.

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FIG. 4. �Color online� �a� I–V plots of samples with N concentrations 0.0%,1.1%, 2.1%, and 4.4% for water splitting reaction under UV light withincident photon energy of 3.88 eV ��=320 nm�. �b� I–V plots of sampleswith N concentrations 0.0%, 1.1%, 2.1%, and 4.4% for water splitting reac-tion under visible light with incident photon energy of 2.70 eV ��=480 nm�. �c� Photoconversion efficiency of samples with N concentrations0.0%, 1.1%, 2.1%, and 4.4%for water splitting reaction under conditionsdescribed in Fig. 4�b�.

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