Journal ofMaterials Chemistry A

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aEnergy and Environmental Division, Kore

Technology, Seoul 153-801, Republic of KorbCorporate R&D, LG Chem/Research Park, DcDepartment of Chemistry and Nano Scienc

750, Republic of KoreadDepartment of Materials Science and Engine

and Technology and Center for Nanomateria

305-701, Republic of KoreaeNeutron Science Division, Korea Atomic En

Republic of KoreafDepartment of Materials Science and Engin

Republic of Korea

† CCDC 908784. For crystallographic dataDOI: 10.1039/c3ta00936j

‡ First two authors contributed equally tco-rst authors.

Cite this: DOI: 10.1039/c3ta00936j

Received 1st November 2012Accepted 14th January 2013

DOI: 10.1039/c3ta00936j

www.rsc.org/MaterialsA

This journal is ª The Royal Society of

Preparation and visible-light photocatalysis of hollowrock-salt TiO1�xNx nanoparticles†

Seul Gi Seo,‡af Cheol-Hee Park,‡b Ha-Yeong Kim,c Woo Hyun Nam,d Mahn Jeong,a

Yong-Nam Choi,e Young Soo Lim,*a Won-Seon Seo,a Sung-Jin Kim,c Jeong Yong Leed

and Yong Soo Chof

A simple method for the preparation of hollow rock-salt TiO1�xNx nanoparticles and their visible-light

photocatalytic activities are presented. By thermal annealing of TiO2 nanoparticles under NH3/N2 gas

flow at 800 �C, a rock-salt TiO phase was achieved far below its transition temperature to a monoclinic

TiO phase (1250 �C). The composition of the nanoparticles was revealed to be Ti0.7(O0.67N0.33)1 by

detailed structural and compositional characterizations, and their structural stability was critically

determined by the formation of vacancies and the incorporation of N. The hollow rock-salt TiO1�xNx

nanoparticles presented efficient visible-light photocatalytic activity, and their origins were discussed.

Introduction

Since the rst discovery of its photocatalytic effect by Fujishimaand Honda, TiO2 has been extensively investigated for diverseuses in energy and environmental applications.1–3 Within a widespectrum of solar light, however, the photocatalysis of TiO2 islimited mainly to the ultraviolet (UV) region due to its wideband gap. Therefore, band gap narrowing for the efficientutilization of solar light, especially in the visible range, has beena critical issue for expanding the photocatalytic applications ofTiO2.4,5 Asahi et al. successfully realized the visible-light pho-tocatalysis in N-doped TiO2 for the rst time due to the bandgap narrowing by the hybridization of N 2p with the O 2p-derived valence band.4 Meanwhile, there is a variety of titaniumsuboxides, such as Magneli phases (TinO2n�1) and titaniummonoxide (TiO).6,7 Among them, because the nitride compoundof titanium, TiN (a0 ¼ 4.24 A), has the same rock-salt structureas TiO (a0 ¼ 4.18 A), rock-salt TiO is expected to easily

a Institute of Ceramic Engineering and

ea. E-mail: yslim@kicet.re.kr

aejeon 305-380, Republic of Korea

e, Ewha Womans University, Seoul 120-

ering, Korea Advanced Institute of Science

ls and Chemical Reactions, IBS, Daejeon

ergy Research Institute, Daejeon 305-353,

eering, Yonsei University, Seoul 120-749,

in CIF or other electronic format see

o this work and should be considered

Chemistry 2013

accommodate N atoms in oxygen vacancy sites.8 In the litera-ture, band structure calculations based on different theoreticalapproaches consistently revealed that the band gap of TiO issuitable to absorb visible-light.8–10 By ab initio density functionaltheory calculations, Graciani et al. showed that the band gap ofTiO1�xNx between hybridized O/N 2p and Ti 3d bands decreasesas the concentration of N increases.8 Because its Fermi level issignicantly higher than the conduction band minimum, theconducting behavior of TiO can facilitate the stabilization ofN3� ions in the oxygen sites by the electronic transfer of Ti 3delectrons.9,10 Furthermore, although a tiny amount of methyleneblue (MB, 1 mmol L�1) was photodegraded within quite a shortirradiation time (<4 min) in their experiments, Simon et al.reported a signicant red-shi of the absorption threshold andenhanced photocatalytic activity in N-doped rock-salt TiOnanoparticles which were prepared by the laser pyrolysismethod.11

However, pure TiO has the rock-salt structure only above1250 �C and it is stable within the wide composition range fromTiO0.51 to TiO1.26.12 This large nonstoichiometry in rock-salt TiOis compensated by randomly distributed vacancies in both Tiand O sites. It is interesting that even stoichiometric TiO1.0

contains 15% disordered vacancies.9 Below the transitiontemperature, ordering of Ti and O vacancies leads to the slightdistortion of the disordered rock-salt structure, resulting in thephase transformation to an ordered monoclinic structure.9,12

Therefore, rapid quenching from high temperature has beenconventionally required to preserve the rock-salt TiO at roomtemperature.9,13 Although rock-salt TiO has been successfullyprepared by using non-conventional methods such as laserablation of metal titanium targets in deionized water, mecha-nochemical synthesis by using titanium and TiO2 powders, andlaser pyrolysis of titanium isopropoxide aerosol, the preparation

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by the thermal process, i.e., the equilibrium process, belowthe transition temperature of 1250 �C has not beenreported yet.11,14,15

In this study, we report a simple method for the preparationof N-incorporated hollow TiO nanoparticles with rock-saltphase and their visible-light photocatalysis. The hollow nano-particles were prepared by thermal annealing of TiO2 powder(P25, Degussa) under NH3/N2 ow at 800 �C (far below thetransition temperature of 1250 �C), and it was found to bemetal-decient TiO1�xNx. The structural stability of the rock-salt TiO phase and the formation mechanism of the hollowstructure were investigated by structural and compositionalcharacterizations. Compared to P25, the hollow rock-saltTiO1�xNx nanoparticles represented signicantly enhancedphotocatalytic performance, especially in the visible range, andtheir origins were discussed.

ExperimentalPreparation of hollow rock-salt TiO1�xNx nanoparticles

The starting material was a commercial TiO2 powder (P25,Degussa). 0.3 g of the TiO2 nanoparticles was loaded in a tubefurnace. Mixture gas (10% NH3 and 90% N2) was introducedinto the furnace for the reduction of TiO2 into TiO, and the gasow rate was controlled at 0.2 cc min�1. For the removal ofoxygen impurity from the mixture gas, 1.5 g of carbon blackpowder was located at the gas inlet as oxygen trap. Annealing ofthe P25 powder was carried out in the tube furnace for 3, 6 and9 h at 800 �C. The colors of the resulting powders changedfrom white to black aer 3 h reduction.

Structural characterizations

P25 and the reduced powders were characterized by using anX-ray diffractometer (XRD, D/Max 2500, Rigaku) with Cu Ka

radiation (l¼ 1.5406 nm) at 40 kV and 200 mA. Neutron powderdiffraction measurements were carried out by using the highresolution powder diffractometer at the 30 MW research reactor‘HANARO’ of the Korea Atomic Energy Research Institute.TiO1�xNx powder (1.00 g) lled into a vanadium canister wasput into a monochromatic neutron beam (l ¼ 1.8340 A, crosssection: 14 mm (width) � 50 mm (height)) obtained fromGe(331) monochromating crystals and the neutrons scatteredby the sample were detected by 32-channel 3He tube detectors.The structure determination and renement were performed byusing TOPAS soware (TOPAS 3, Bruker AXS).16 Microstructuralcharacterization was performed by using a transmission elec-tron microscope (TEM, JEM-4010, JEOL) operated at 400 kV andX-ray photoelectron spectroscopy (XPS) characterization wascarried out by using a PHI 5000 VersaProbe (ULVAC-PHI) withmonochromatic Al Ka X-rays (1486.6 eV). The Brunauer–Emmett–Teller (BET) specic surface area was determined byusing a Micromeritics ASAP 2420.

Fig. 1 XRD patterns of starting material (P25) and reduced powders.

Photocatalysis characterizations

MB and phenol were applied as target substrates for the pho-tocatalytic reactivity test of the TiO1�xNx and P25. Photocatalytic

J. Mater. Chem. A

reactions for degradation were carried out in the glass reactor(100 mL) with a quartz window, containing 50 mL of reactionslurry (100 mmol L�1). Agitation was provided by a magneticstirrer. The aqueous slurry, prepared with different kinds ofcatalysts and organic compounds, was stirred in the dark for60 min to ensure that the organic compound was adsorbed tosaturation on the catalysts. A 300W Xe arc lamp (Oriel) was usedas a light source. The light passed through a 10 cm IR waterlter and the cut-off lters, and then the ltered light wasfocused onto the reactor. Sample aliquots were withdrawn by a1 mL syringe intermittently during the illumination and lteredthrough a 0.20 mm PTFE lter (Millipore) to remove catalystparticles. The degradation of MB was monitored by measuringthe absorbance at l ¼ 664 nm as a function of irradiation timewith an UV-vis spectrophotometer (Cary 5000, Varian Technol-ogies). The degradation of phenol was also determined bymeasuring the absorbance at l ¼ 210 nm with the UV-visspectrophotometer.

Results and discussion

Fig. 1 shows XRD patterns of P25 (starting material) and thereduced powders under NH3/N2 ow at 800 �C. The molarfractions of each phase in the powders were determined byRietveld renements, and the results are summarized inTable 1. The P25 powder was composed of mainly the anataseTiO2 phase (�88%) and partially the rutile TiO2 phase (�12%),which agrees well with the reported values in the literature.17 Asreduction time increased, diffraction peaks of the anatase phasewere drastically decreased. The weight percent of the anatasephase was rapidly decreased to �4% in the 3 h reduced powder,and the anatase phase was not detected in the diffractionpattern of the 6 h reduced powder. In the case of the rutilephase, the fraction was increased up to �26% aer 3 h reduc-tion. However, the diffraction intensities of the rutile phasewere decreased by further reduction and hardly observed aer9 h reduction.

This journal is ª The Royal Society of Chemistry 2013

Table 1 Weight percents of titanium oxide phases (anatase TiO2, rutile TiO2 androck-salt TiO phases) in P25 and reduced powders which were identified by XRDin Fig. 1

Powdersamples

AnataseTiO2 phase

RutileTiO2 phase

Rock-salt TiOphase

P25 87.7 12.3 03 h-reduced 4.3 25.6 70.16 h-reduced 0 16.6 84.39 h-reduced 0 1.5 98.5

Fig. 2 (a) XPS N 1s and (b) Ti 2p3/2 lines of 9 h-reduced powder (TiO1�xNx).

Fig. 3 Neutron Rietveld refinement profiles of 9 h-reduced powder (TiO1�xNx).Calculated (red line), observed (open circles), and difference (bottom line)patterns are shown together with Bragg positions.

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On the other hand, diffraction intensities of the rock-salt TiOphase were drastically increased with reducing time. At theinitial stage of reducing P25, a rapid decrease of the anatasephase and also a rapid increase of the rock-salt TiO phase weresimultaneously observed. This result indicates that there waspreferential phase transformation of the anatase TiO2 phaseinto the rock-salt TiO phase. Furthermore, almost completephase transformation of all TiO2 phases (anatase and rutile)into the rock-salt TiO phase was observed aer 9 h. Althoughthere have been some reports on the preparation of rock-saltTiO nanoparticles by non-thermal processes, such as laserablation, mechanochemical synthesis, and spray pyrolysis, theTiO phase in this experiment was obtained by a simple thermalannealing process for the rst time.11,14,15 This result impliesthat the annealing condition under NH3/N2 gas ow provides acertain equilibrium condition for the formation of the rock-saltTiO phase at 800 �C even though the monoclinic TiO phase isthermodynamically stable below 1250 �C.

In the rock-salt TiO structure, the oxygen vacancy is a ther-modynamically stable site for accommodating N.8 Therefore, asignicant amount of N was inevitably incorporated into therock-salt TiO during the reduction process under NH3/N2 gasow. The existence of N in the 9 h-reduced powder was char-acterized by using XPS, and the ratio of O to N was characterizedto be �0.7 : 0.3. In Fig. 2(a), major Ti–N bonding in titaniumoxynitride andminor chemisorbed N–O– and/or N–O2– bondingin N 1s core levels were located at 396.2 and 401.7 eV, respec-tively.11 Therefore, the rock-salt TiO phase aer 9 h reductionwas found to be rock-salt titanium oxynitride, i.e., TiO1�xNx. Inthe case of Ti 2p3/2 levels, binding energies of reduced Ti2+ andTi3+ were located at 455.5 and 457.2 eV, respectively (Fig. 2(b)).11

The fractions of Ti2+, Ti3+, and Ti4+ determined by using thedeconvoluted spectrum were 10.5, 50.5 and 39.0%, respectively.Because the stability of the monoclinic TiO phase is mainlyattributed to the metal–metal bonding between Ti 3d orbitals,the incorporation of N can induce the instability of the mono-clinic structure due to the less populated electrons in Ti 3d bythe charge transfer for the stabilization of N3�.18,19

In order to understand the crystal structure and compositionof the 9 h-reduced TiO1�xNx powder in detail, neutron scat-tering experiments were carried out. The neutron scatteringcoherence length of the O atom (5.803 fm) is much differentfrom that of the N atom (9.360 fm), so it is possible to distin-guish between the O atom and the N atom.20 Rietveld rene-ment of the neutron data was carried out using the rock-salt TiOstructure as a starting model, resulting in good prole tting as

This journal is ª The Royal Society of Chemistry 2013

shown in Fig. 3. The rened atomic positions and occupanciesof each atom are given in Table 2. The ratio of O to N atoms inthe O sites of the rock-salt structure was determined as0.67 : 0.33, which is quite similar to the observation in XPS. Theoccupancy for Ti sites relative to O sites of rock-salt TiO was 0.7and any kind of interstitial or ordering of O and N atoms wasnot observed.

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Table 2 Atomic parameters for TiO1�xNx determined from neutron Rietveldrefinementa

Elements x y z Bisob (A2) Occupancy

Ti 0.5 0.5 0.5 1.61 (0.05) 0.702 (0.007)O 0 0 0 1.23 (0.03) 0.67 (0.02)N 0 0 0 1.23 (0.03) 0.33 (0.02)

a Space group Fm�3m: a ¼ 4.1987 (1) A. b Biso ¼ 8p2u2 where u2 is themean square displacement of an atom from its mean position.

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Because the occupancy of Ti sites was �0.7, and because theratio of O to N was characterized to be �0.67 : 0.33, thecomposition of the observed TiO1�xNx aer 9 h reduction can beestimated as metal-decient Ti0.7(O0.67N0.33)1. Considering therelative fraction of Ti2+ (10.5%), Ti3+ (50.5%), and Ti4+ (39.0%) inXPS (Fig. 2), the oxidation number of Ti can be evaluated as 0.7�[(+2) � 0.105 + (+3) � 0.505 + (+4) � 0.39] z (+)2.3. It is worthnoting that 67% of O2� and 33% of N3� give the oxidationnumber of �(�)2.3. This result clearly demonstrates that themetal-decient rock-salt Ti0.7(O0.67N0.33)1 prepared by the directreduction of P25 at 800 �C is a stable phase and that the stabilityis essentially determined by both the incorporation of N and thepreferential formation of Ti vacancy.

Microstructures of the starting material and the reducedpowders were characterized by using TEM, and the results areshown in Fig. 4. The P25 powder was composed of nanoparticleswhose size is within the range of 20–30 nm as shown in Fig. 4(a),and its corresponding electron diffraction pattern (inset)revealed that the main phase in the P25 powder is anatase TiO2.However, the microstructures of the reduced powders weretotally different from that of the starting material. Aer 3 hreduction, strange hollow nanoparticles were observed inFig. 4(b), while nanoparticles with their original shapes couldnot be easily identied. The crystal structure of the hollownanoparticles was proven to be the rock-salt TiO phase by elec-tron diffraction patterns (insets). When we consider the XRD

Fig. 4 Bright-field TEM micrographs of (a) P25 and (b) 3 h-, (c) 6 h- and (d) 9 h-reduced powders and their corresponding electron diffraction patterns (insets). A,R, and M in the insets indicate the anatase TiO2, rutile TiO2, and rock-salt titaniummonoxide phases, respectively.

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results in Fig. 1, these results indicate that the hollow rock-saltTiO1�xNx nanoparticles are produced by the direct reduction ofanatase TiO2 nanoparticles. Besides, a few rutile nanoparticleswere also produced in the 3 h-reduced powder and their sizeswere larger than those of original nanoparticles in P25. There-fore, it was found that the anatase TiO2 nanoparticles werepreferentially reduced into hollow nanoparticles with rock-saltstructure, and also partially transformed into rutile TiO2 nano-particles via themerging or growth process in this initial stage.21

The rutile TiO2 nanoparticles also became hollow rock-saltTiO1�xNx structures by further thermal reduction as shown inFig. 4(c) and (d). Also, the electron diffraction patterns in theinsets reveal that the rutile TiO2 nanoparticles experienced thephase transformation into the hollow nanoparticles with rock-salt structure during the reduction process. Aer 9 h, all rutileTiO2 nanoparticles were reduced to be hollow nanoparticleswith rock-salt structure in this TEM observation, and this resultis quite consistent with the XRD results in Fig. 1. These resultsstrongly imply that the formation of the hollow structure isclosely related to the phase transformation of TiO2 phases(anatase and rutile) into the rock-salt phase. A mechanism forthe formation of the hollow structure can be proposed in termsof oxygen vacancy diffusion. During the reduction process,oxygen atoms at the surface of TiO2 would be preferentiallyremoved as NOx by the reaction with NH3. Although a signi-cant amount of oxygen vacancies could also be occupied by N(�33%), the oxygen vacancies should diffuse into the innerregion due to the outward diffusion of oxygen atoms for thesuccessive reduction process. Moreover, a large amount ofintrinsic oxygen vacancies in the TiO phase, which are trans-formed from the TiO2 phase in the surface region, can accel-erate this process. The inwardly diffused oxygen vacanciesmerge, and eventually this process can produce a hollowstructure in the nanoparticle.

Fig. 5(a) represents a high-resolution TEM (HRTEM) micro-graph of a hollow rock-salt TiO1�xNx nanoparticle obtained

Fig. 5 (a) A HRTEMmicrograph of a hollow rock-salt TiO1�xNx nanoparticle with{111} facets and (b) its enlarged micrograph. (c) A HRTEMmicrograph of a hollowrock-salt TiO1�xNx nanoparticle which shows {111} facets even at the innersurface.

This journal is ª The Royal Society of Chemistry 2013

Fig. 6 Photocatalytic degradation of (a) MB and (b) phenol by P25 and TiO1�xNx

powders. Squares and circles represent P25 and TiO1�xNx, respectively (open: UV-visible, filled: visible irradiation).

Table 3 Specific surface areas of P25 and reduced powders characterized byusing the BET method

Powder samplesSpecic surfacearea (m2 g�1)

P25 91.83 h-reduced 35.86 h-reduced 30.39 h-reduced (TiO1�xNx) 39.6

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along the h110i zone axis. The angle of 70.5� indicates that thenanoparticle with rock-salt structure has {111} facets. Anenlarged HRTEM micrograph in Fig. 5(b) clearly shows theinterplanar distance between {111} planes. Interestingly, the{111} facets were repeatedly observed even at the inner surfaceas shown in Fig. 5(c). In general, the {111} plane in the rock-saltstructure based on ionic bonding is a polar plane, so that thesurface energy should be signicantly higher than the {100} and{110} non-polar planes. However, Vaz et al. also observed thepreferentially grown {111} planes in sputter-grown TiO1�xNx

thin lms.22 Furthermore, the incorporation of N into rock-saltTiO leads to the enhancement of the covalent bonding char-acter.8 Therefore, the observed close-packed {111} facets indi-cate the strengthened covalent bonding character in the hollowrock-salt TiO1�xNx nanoparticle due to the incorporation of N.

In the literature, the instability of the rock-salt TiO phase isstrongly inuenced both by the enhanced Ti–Ti bonding inter-actions through and around the oxygen vacancies and by theelectrostatic stabilization through the signicant changes in theelectron distribution between oxygen and titanium vacancies.19

Therefore, the preferential consumption of oxygen vacancies inthe rock-salt TiO phase for the formation of the hollow structurecan be one origin of the structural stability. Based upon density-functional calculations, Graciani et al. showed that rock-saltTi1(OxN1�x)1 can be stable up to x ¼ 0.6 even at a very lowtemperature of 10 K due to the depopulated Ti 3d band result-ing from the incorporation of N atoms.8 In this experiment, thecomposition of the nanoparticles was characterized to beTi0.7(O0.67N0.33)1 and this value (x ¼ 0.67) was quite comparablewith the theoretical result in spite of titanium deciency.Furthermore, the balanced oxidation number between titanium(�(+)2.3) and oxygen sites (�(�)2.3) in Fig. 2 and 3 revealed thatthe incorporation of N atoms and the formation of titaniumvacancies are critical to stabilize the rock-salt phase. Therefore,it can be concluded that the annealing of TiO2 nanoparticlesunder NH3/N2 gas ow provides an equilibrium condition forthe formation of the rock-salt TiO phase at a relatively lowtemperature in this experiment.

Photocatalytic activities of the hollow rock-salt TiO1�xNx

nanoparticles and P25 were characterized by using photo-degradation experiments of MB as shown in Fig. 6(a). Thedegradation of MB wasmonitored bymeasuring the absorbanceat l ¼ 664 nm as a function of irradiation time with a UV-visspectrophotometer. Time-dependent photodegradation of MBwas carried out under UV-visible and visible irradiation by usingcut-off lters up to 3 h. Prior to the photodegradation experi-ments, specic surface areas of the hollow rock-salt TiO1�xNx

and P25 powders were determined by the BET method to ndthe effects of the surface area. Although the surface area ofTiO1�xNx was smaller than that of P25 as summarized in Table3, the hollow rock-salt TiO1�xNx nanoparticle shows superiorphotocatalytic properties to P25. Aer 3 h irradiation of the UV-visible light (l > 295 nm), 90% of MB was decomposed byTiO1�xNx, while 48% of MB was decomposed by P25 under thesame experimental conditions. Therefore, the surface areaeffect was overwhelmed by the high photocatalytic performanceof TiO1�xNx.

This journal is ª The Royal Society of Chemistry 2013

Moreover, the hollow rock-salt TiO1�xNx nanoparticlesshowed strong visible-light photocatalytic activity. When weexcluded UV-light by using a cut-off lter (lcut-off ¼ 420 nm),45% of photodegradation was achieved by the hollow rock-saltTiO1�xNx nanoparticles aer 3 h irradiation. Meanwhile, due toits wide band gap, P25 exhibited weak photodegradation(�10%) under the visible-light irradiation. This high photo-catalytic performance of TiO1�xNx in the visible range is mainlyattributed to its narrow band gap. Although the optical bandgap of TiO1�xNx could not be characterized in this experimentdue to its black color, it has been theoretically reported that thenarrow band gap (#2 eV) of TiO can be further reduced by theincorporation of N due to the hybridized valence band of O/N 2porbitals.4,8–10 When we consider the UV-visible photo-degradation of MB by TiO1�xNx, half of the total photocatalyticactivity was obtained only by the absorption of visible-light.

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The visible-light photocatalysis of the hollow rock-saltTiO1�xNx nanoparticles was also tested by photodegradationexperiments of phenol. As shown in Fig. 6(b), P25 did not showmeaningful photodegradation of phenol under visible-lightirradiation. However, signicant visible-light photodegradationby the hollow rock-salt TiO1�xNx nanoparticles was clearly ach-ieved up to 3 h under the same experimental conditions.Although the visible-light photocatalysis of N-incorporated rock-salt TiO nanoparticles prepared by non-thermal processes hasbeen reported in the literature, only 1 mmol L�1 of MB was usedfor their photodegradation experiments within a few minutes.11

In this experiment, the concentration of MB was 100-fold higherand the efficient photocatalytic performances of the hollowrock-salt TiO1�xNx nanoparticles were persistently observedwithin our irradiation time of 3 h. Furthermore, the visible-lightphotocatalysis was manifested by the photodegradation of acolorless organic compound, phenol. Therefore, it was eluci-dated that the hollow rock-salt TiO1�xNx nanoparticles preparedby the equilibrium process has efficient visible-light photo-catalytic activity and promising structural stability.

Conclusions

In summary, hollow rock-salt TiO1�xNx nanoparticles wereprepared for the rst time by thermal annealing of P25 underNH3/N2 ow at 800 �C for 9 h. Their unique structural propertieswere characterized by using XRD, XPS, TEM, and neutronscattering, and the results demonstrated that metal-decienthollow rock-salt Ti0.7(O0.67N0.33) nanoparticles were producedby the direct reduction process. The signicance of the incor-poration of N and the vacancy formation on the structuralstability of this new material was discussed, and the formationmechanism of the hollow structure was also proposed. Thehollow rock-salt TiO1�xNx nanoparticles showed strong visible-light photocatalytic performances due to their narrow band gapand it was manifested by photodegradation experiments of MBand phenol. With these results, it can be concluded that a newmaterial for the efficient utilization of visible light in photo-catalytic applications was discovered.

Acknowledgements

This research was supported by the Nano Material TechnologyDevelopment Program through the National Research Foun-dation of Korea (NRF) funded by the Ministry of Education,Science and Technology (2011-0030147) and also supported bythe Korea Institute of Ceramic Engineering and Technology.

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