Cite this: DOI: 10.1039/c3ce26909d

Received 23rd November 2012,Accepted 1st February 2013

Rational design of oxynitride materials: From theory toexperiment

DOI: 10.1039/c3ce26909d

www.rsc.org/crystengcomm

Jakub Szlachetkoab and Jacinto Sa*a

In this communication, we present an elegant and cost effective

strategy for the rational design of novel materials used in the

conversion of solar energy into chemical bonds. The strategy

relies on the combination of theory and the latest developments

in high-resolution XAS/XES at synchrotrons, which are able to

map the material electronic structure with high precision,

requiring a very limited amount of sample (experimental work)

or none (theory).

Demand for goods at affordable costs has increased with increasesin population and life quality standards, however this is not alwaysconciliated with environmental protection. Catalysis is a keytechnology in chemical and pharmaceutical industries but alsoextremely important for the production of clean energy andenvironmental protection. Photocatalysis is an emerging strategythat can help solve the energy crisis by converting solar radiationinto chemical bonds. TiO2 is the most commonly usedphotocatalyst but it requires excitation with UV light due to itswide band gap (3.2 eV).1 A common strategy to improve visiblelight absorption is to reduce the band gap energy by doping with3d elements, which shifts the of conduction band downwards inenergy,2 and/or light elements such as N, C and S able to shift thevalence band upwards in energy.3 The portfolio of reactions that asemiconductor can perform is determined by the position ofreaction redox potential in respect to the band gap, i.e., in order tooccur the reaction redox potential needs to be within the band gapenergy.4 If one considers water splitting as the primary technologyto produce carbon free molecular oxygen and hydrogen,5 it is clearthat there is more leeway to shift the valence band position ratherthan conduction band.6

Currently, the discovery and/or improvement of catalysts isperformed primarily by trial and error, a costly, wasteful and slowstrategy. Competitive research on future catalyst generation canonly be attained when the mentioned drawbacks are successfullyaddressed. This demands a dramatic change of scientist’s research

philosophy, especially when it comes to catalyst formulation andscreening, which some industrial7 and academic groups8 haveembraced. Theoretical studies in heterogeneous catalysis are oftenfocused on mechanistic understanding not on the catalystformulation. There is a historical resilience in accepting theoryas a reliable tool to direct catalyst formulation, in part due to thetime consuming nature of detailed calculations. However, as a pre-screening step, calculations can be carried out in a short period oftime. Theoretical prediction is an economical solution since itsaves costs related to energy inputs and raw materials. The othersolution is to use characterization techniques able to determinematerials electronic structure from a small amount of sample,such as X-ray absorption and emission spectroscopy (XAS/XES)combination.

In the case of photocatalysts and in particular N-doped TiO2,the key aspects for a good material are:

Good overlap between N and O orbitals (synergy effect) and;Dense and wide orbital structure in the conduction (charge

mobility).The first aspect defines how well mixed the doped system is, a

key parameter towards achieving the desired synergetic effect. Inthe worst case scenario the material is composed of two differentsemiconductors solid solution, namely TiO2 and TiN. The secondaspect determines charge mobility. Charge carriers move throughthe material via a trapping and detrapping mechanism. Chargemobility depends on the number of available energy levels(subbands) in the conduction and valence band, e.g., a verynarrow conduction band composed of a few orbitals results in lowcharge mobility.

FEFF is a readily available code for ab initio multiple scatteringcalculations of X-ray Absorption Fine Structure (XAFS), X-rayAbsorption Near-Edge Structure (XANES) and various other spectrafor clusters of atoms.9 FEFF enables the estimation of the densityof states (DOS), which reveals materials band structure andposition in respect to a known starting point. In the case of TiO2,FEFF calculations revealed a conduction band dominated by theempty Ti d-band and a valence band composed of the occupied Op-band and Ti d-band. The accurate determination of the edgepositions as a function of doping level enables the estimation of

aPaul Scherrer Institute (PSI), 5232 Villigen-PSI, Switzerland.

E-mail: jacinto.sa@psi.chbInstitute of Physics, Jan Kochanowski University, 25-406 Kielce, Poland

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the effective band gap of the semiconductor. Crucially thecalculation can be reproduced experimentally by high-resolutionspectroscopy XES and XAS measurements [combination RIXS map(resonant inelastic X-ray scattering)] and vice-versa. This is apowerful strategy because one can optimize between theory andcharacterization depending on, which one is revealed to be themost cost effective. The combination of high-resolution XES andXAS enables the mapping of electronic band structure of occupiedand unoccupied states, respectively. Fig. 1 contracts the strategyproposed above. It encompasses the physical process used todetermine experimentally the effect on TiO2 band structure andthe theoretical strategy to estimate the effect of atom substitution,which can be carried out prior to synthesis and therefore help withthe rational design of materials.

Experimental technique

The experiments were performed at the superXAS beamline ofSwiss Light Source, Switzerland. The X-ray beam delivered by asuper-cooled 2 T bending magnet was monochromatized bymeans of a channel-cut Si(111) monochromator. The higherharmonics of the monochromator were rejected by means of a Simirror operated at a 3 mRad incident angle. Downstream themonochromator, the X-ray beam was focused down to a 100 6100 mm2 by means of a Pt-coated torroidal mirror. For X-rayenergies around the Ti K-edge (y5 keV) the setup provides anenergy resolution for incoming X-rays of about 1 eV and a flux of 56 1011 photons per second.

The X-ray fluorescence from the sample was recorded with avon Hamos type spectrometer operating in Ge(400) crystaldiffraction and at Bragg angle of 62u. The detailed description ofthe spectrometer setup can be found elsewhere.10 In the presentsettings, the spectrometer provides an energy resolution fordetected X-rays of about 1 eV and an energy bandwidth for singleXES acquisition around 80 eV. The energy bandwidth of thespectrometer provides a unique possibility of simultaneousdetection of many decay channels and therefore access to severalfinal electronic states within a single XES measurement. Asrecently reported, this property may be of great advantage forsingle-shot XES11 and time resolved RIXS (4D RIXS) studies underin-situ conditions.12

In the present experiment, the unoccupied electronic states ofTiO2 anatase (Sachtleben Hombikat UV100) were probed byexciting the 1s electron above the Fermi level and the low energyoccupied electronic states were detected from the following 3 A 1s

and valence A 1s decay transitions. By scanning the incidentbeam energy around K absorption edge, a RIXS map can be thusobtained that represents a p-projected picture of the electronicstates of the studied system. The Kb and valence-to-core (v2c) RIXSexperiment provides an enhanced chemical sensitivity due to theelectron–electron interaction between 1s-excited and 3p/valence-decay electrons. Moreover, thanks to the penetrating properties ofX-rays, the RIXS experiments can be performed in-situ and/orunder reaction conditions giving v2c-RIXS a great advantage incomparison to the electron-based spectroscopies.

TiO2 RIXS outcomes were compared to the theoretical resultscalculated using FEFF8.4 code. For comparison and validationpurposes we calculated the orbitals associated to TiN, a structureknown to have a smaller band gap than TiO2 but a similar crystalstructure. The strategy was adapted to predict the band structureof substitutional N-doped TiO2. The aim is to exploit this tool forthe rational design of novel materials and only synthesize the onesthat reveal the band structure of interest.

Results and discussion

To access the low energy electronic states in TiO2 (anatase), RIXSplane around the Ti K-edge of Kb1,3 and valence-to-core transitionswere collected. As aforementioned, the highest occupied andlowest unoccupied electronic states can be accessed simulta-neously. The recorded TiO2 RIXS plane is depicted in Fig. 2.

The RIXS plane consists of well-separated pre-edge structures atbeam energies between 4968–4975 eV and post edge features athigher excitation energies. The HR-XAS (high-resolution – X-rayabsorption spectroscopy) spectrum (Fig. 2, right) was obtained at afixed emission energy of 4931.7 eV, while the TFY-XAS (totalfluorescence yield – X-ray absorption spectroscopy) was con-structed by integrating the XES intensities over the entiremeasured emission energy range. As shown, the spectra aresignificantly different in terms of structure and intensity,especially in the pre-edge region. The TFY-XAS curve shows threepre-edge components, while HR-XAS only detects two pre-edgefeatures. The discrepancy can be explained by careful analysis ofthe two-dimensional RIXS plane. As shown, the HR-XAS cut does

Fig. 1 Schematic representation of combined theory and experimentalapproach for the design of improved doped semiconductors.

Fig. 2 TiO2 anatase RIXS plane. (top) Non-resonant XES spectrum; (right) TFY-XAS versus HR-XAS extracted at constant emission energy (4931.7 eV).

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not include the first pre-edge resonance observed at an incidentbeam energy of 4968 eV. This resonance is located at an emissionenergy of 4929.9 eV, i.e. 1.8 eV lower than the maximum of themain Kb1,3 emission line for which the HR-XAS spectrum isconstructed. This observed energy shift is due to the interaction ofthe core hole on delocalized 3d orbitals, in agreement with pastobservations.13 The extracted non-resonant XES spectrum isplotted in Fig. 2 (top). The spectrum consists of a main peaklocated around 4931 eV corresponding to the Kb1,3 emission line.Weak structures around 4947 eV and 4965 eV are also observedthat are related to the transition of valence electrons to the 1score–hole.

The valence and conduction band electronic states wereextracted from the measured RIXS plane based on the non-resonant XES spectra and HR-XAS, respectively. The extractedcurves were scaled to the Fermi energy by shifting the energy axisof each spectrum by a value of 4965.4 eV, extracted from thevalence-to-core transition inflection point (dashed line in Fig. 2).The recalibrated and renormalized spectra are plotted in Fig. 3(top). The spectra are compared to the density of states calculatedwith the FEFF8.4 code.9 The Ti and O DOS for TiO2 are plotted inFig. 3 (middle).

The DOS computation shows that the pre-edge structuremainly consists of Ti d-states and the contribution of O p-states isnegligibly small. For the occupied electronic states just below theFermi level an equal contribution of O-p and Ti-d orbitals isobserved. Additionally, a weak structure can be seen at ca. 215 eVdue to oxygen 2s orbital. As shown, the measured profiles are wellreproduced by theoretical DOS calculations, and thus providedetailed information about the energies and density of theelectronic structure. Based on the results, we have derived theband gap of the material directly, following the proceduredescribed by Chiou et al.14 It is important to note that theexperiment was performed around the K-absorption edge of Tiand therefore the measured electronic density of states isp-projected. For this reason, the amplitudes of measured andcalculated DOS structures may differ due to the excitation anddecay probabilities involving 1s electron–hole. Moreover, theexperimental spectrum is much broader as compared to thetheoretical one, due to the experimental resolution (~1 eV) as wellas due to the initial and final electronic state broadenings. Thelatter broadenings vary from sub-eV up to a few eV depending onthe electronic transitions involved, and thus sets the limits forultimate spectral resolution of the v2c RIXS spectroscopy.

Based on the calculated DOS structure, the orbital contribu-tions are marked on the RIXS plane of Fig. 3 (top). As shown, thedetected pre-edge structure around the incident beam energy of4970 eV consists mainly of d-states of Ti. For the occupiedelectronic states the two structures are seen in the RIXS plane. Amixture of O p-electrons and Ti-d electrons corresponds tofeatures located at an X-ray emission energy of 4965 eV whilethe O s-electron contribution is detected at the emitted X-rayenergy of 4947 eV. In summary, the unoccupied and occupiedstates were probed within the same experiment and thus underthe same experimental/in-situ conditions. The combination oftheoretical calculations and experimental results demonstrates a

usefulness of Kb and v2c RIXS spectroscopy to study the electronicstructure of materials.

As aforementioned, semiconductor band gaps are directlydetermined by RIXS planes analysis, i.e., from the energies andintensities of the electronic structure. The direct outcome of sucha premise is that this methodology can be use to determine theeffects induced by dopants. Nitrogen doping of TiO2 is one of themost common strategies to improve visible light absorption.3 Theeffect of replacing O by N on the electronic structure of Ti wascalculated and plotted in Fig. 3 (bottom), assuming a 2% N-dopantlevel. In the calculations, O was randomly replaced by a N atom inthe TiO2 anatase structure. The DOS of the resultant structure wascomputed for each atom and repeated until reaching the desireddopant level. The unoccupied electronic states of TiO22xNx consistof Ti-d orbitals, similar to the TiO2, with negligible contributionsfrom N and O p-states. Direct comparison of TiO22xNx with TiO2

Fig. 3 Electronic band structure of TiO2 and TiN. (top) Valence and conductionband electronic states extracted from measured RIXS plane; (below) calculatedTi, O, and N DOS for TiO2, TiN and TiOxNx, where x amounts to 2% N-dopantlevel.

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reveals that N-doping does not affect the unoccupied statesstructure, i.e., does not affect the conduction band. However,N-doping markedly affects the occupied electronic side, i.e.,valence band. The exchange of O by N atoms leads to theappearance of N 2p-orbital (grey dashed line) at higher bindingenergy. The new orbitals are hybridized with O 2p-states, asexpected when the N-doping is substitutional.15 The N-s orbitalsare shifted by about 5 eV to a higher energy with respect to the O-sstates, however these orbitals do not contribute to the valencestates since they are too deep. The result shows that the N-dopingdoes not affect the unoccupied electronic states, thus the cause forband gap reduction is the hybridization of O-p and N-p orbitals.15

For comparison the orbital contributions to the electronicstructure in TiN were also calculated. The unoccupied electronicstates of TiN consist of Ti-d orbitals, similar to the TiO2, withnegligible contribution from the N p-states. Direct comparison ofTiN with TiO2, reveals a change in the shape minimum energyband position of Ti d-DOS positions. The occupied N-p orbitals areshifted down in energy of ca. 1 eV as compared to the TiO2 case,maintaining the good energy overlap between Ti-d and N-porbitals. From an electronic point of view the smaller band gap ofTiN should translate to higher catalytic performance in watersplitting to produce hydrogen under visible light radiation,however, the TiN conduction band position shifts to low energy,meaning that TiN is no longer able to reduce protons to molecularhydrogen. An opposite scenario was observed with N-doping ofTiO2. In this case the conduction band position is not affected andtherefore the improvement of photocatalytic reduction of protonsis enhanced under visible irradiation. The predicted changescaused by the exchange of O by N are large, thus we can exploitthem to estimate theoretically the dopant levels, their effect in theband gap energy and orbital overlap.

Based on above calculations, the N-doping of TiO2, would leadto a shift of the v2c structure detected at 4947 eV by about 5 eV asmarked on the RIXS plane in Fig. 2. Moreover, changes in the lineshape and position of the states lying just below the Fermi level(around 4965 eV) is expected due to the appearance of a new Np-orbital. The superposition of v2c structure and pre-edge featurescan thus provide a direct probe on the band gap energy inducedby the doping. Therefore, we foresee a broad application of v2c-RIXS to study the doping effects on the electronic structure ofmetal-sites. The application of hard X-rays provides a bulksensitivity while the high-energy resolution spectroscopy guaran-tees the sensitivity on detected electronic structure. Combinationof both may well be employed for in-situ studies to control andfollow, at any stage, the preparation of doped materials. Deeperinsight knowledge on the electronic structure may thus beobtained and exploited for further improvements of materialdesign.

Finally, detailed information about the final state distributionand electron–electron interaction can be extracted from the pre-edge structure of Kb-RIXS plane. The emitted photon energies ofthe measured Kb-RIXS pre-edge region were scaled by incomingbeam energy providing thus the so-called energy transfer RIXSwhich is plotted in Fig. 4. The three resonances denoted as A1, A2,A3 and highlighted by the white dashed lines were detected. The

profiles at constant beam energy and constant energy transfer forthe strongest A2 resonance are plotted by blue and red curvesrespectively. The A1, A2 and A3 resonances have maxima at beamenergies of 4968.4, 4971.1, 4973.7 eV and at energy transfer, i.e.final state energy of 38.6 eV, 39.3 eV and 41.8 eV, respectively,which are in good agreement with other experimental data.13 TheA1 resonance corresponds to the 1s A 3d excitation with the 3dorbital being delocalized, while A2 and A3 resonances areassociated to the 1s excitation into the 3d localized orbital. Fromthe experiment we found that the energy difference betweendelocalized and localized orbitals in the vicinity of the 1s core holeincreases by 1.8 eV. This implies that the effects of core–holeinteraction with the outer shell electronic orbital can be probeddirectly from RIXS measurements and detailed information aboutthe energies levels can be extracted.

Summary

The presented work shows the strength of combination theorywith state-of-the-art high-resolution XAS/XES measurements avail-able at synchrotrons for the determination of the electronicstructure of materials used for the conversion of solar energy intochemical bonds. The proposed strategy is a very cost effectiveapproach for the rational design of materials. We foresee a drasticincrease in user numbers as the technique becomes more andmore accessible at synchrotron light sources around the world,which increased significantly in number over the last decade.

Acknowledgements

Experiments were performed on the SuperXAS beamline at theSLS-PSI.

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