Synthesis of N-Doped Tio2 for Efficient Photocatalytic Degradation

catalysts

Article Synthesis of N-Doped TiO2 for Efﬁcient Photocatalytic Degradation of Atmospheric NOx

Tamal Tahsin Khan 1,2, Gazi A. K. M. Raﬁqul Bari 1 , Hui-Ju Kang 1 , Tae-Gyu Lee 1 , Jae-Woo Park 1 , Hyun Jin Hwang 3 , Sayed Mukit Hossain 4 , Jong Seok Mun 1 , Norihiro Suzuki 5 , Akira Fujishima 5, Jong-Ho Kim 3, Ho Kyong Shon 4,* and Young-Si Jun 1,3,*

1 Department of Advanced Chemicals & Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Korea; [email protected] (T.T.K.); graﬁ[email protected] (G.A.K.M.R.B.); [email protected] (H.-J.K.); [email protected] (T.-G.L.); [email protected] (J.-W.P.); [email protected] (J.S.M.) 2 Department of Materials Science and Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Korea 3 School of Chemical Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Korea; [email protected] (H.J.H.); [email protected] (J.-H.K.) 4 Faculty of Engineering and IT, University of Technology, Sydney, P.O. Box 123, Broadway, NSW 2007, Australia; [email protected] 5 Photocatalysis International Research Center (PIRC), Research Institute for Science and Technology (RIST), Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan; [email protected] (N.S.); [email protected] (A.F.) * Correspondence: [email protected] (H.K.S.); [email protected] (Y.-S.J.); Tel.: +61-2-95142629 (H.K.S.); +82-62-530-1812 (Y.-S.J.)

Abstract: Titanium oxide (TiO2) is a potential photocatalyst for removing toxic NOx from the atmosphere. Its practical application is, however, signiﬁcantly limited by its low absorption into

visible light and a high degree of charge recombination. The overall photocatalytic activity of TiO2 Citation: Khan, T.T.; Bari, remains too low since it can utilize only about 4–5% of solar energy. Nitrogen doping into the TiO2 G.A.K.M.R.; Kang, H.-J.; Lee, T.-G.; lattice takes advantage of utilizing a wide range of solar radiation by increasing the absorption Park, J.-W.; Hwang, H.J.; Hossain, S.M.; Mun, J.S.; Suzuki, N.; Fujishima, capability towards the visible light region. In this work, N-doped TiO2, referred to as TC, was synthesized by a simple co-precipitation of tri-thiocyanuric acid (TCA) with P25 followed by heat A.; et al. Synthesis of N-Doped TiO2 for Efﬁcient Photocatalytic treatment at 550 degrees C. The resulting nitrogen doping increased the visible-light absorption and

Degradation of Atmospheric NOx. enhanced the separation/transfer of photo-excited charge carriers by capturing holes by reduced Catalysts 2021, 11, 109. https:// titanium ions. As a result, TC samples exhibited excellent photocatalytic activities of 59% and 51% in doi.org/10.3390/catal11010109 NO oxidation under UV and visible light irradiation, in which the optimum mass ratio of TCA to P25 was found to be 10. Received: 15 December 2020 Accepted: 12 January 2021 Keywords: N-doped TiO2; precipitation; NOx removal Published: 14 January 2021

Publisher’s Note: MDPI stays neutral with regard to jurisdictional clai- 1. Introduction ms in published maps and institutional afﬁliations. Atmospheric pollution resulting from photochemical oxidants, such as nitrogen oxides (NOx; NO+NO2) and ozone, has been considered one of the critical concerns in urban areas. [1]. NOx, a by-product resulting from fossil fuel incineration and photochemical conversion in nature, is mainly considered a dominant contributor to environmental Copyright: © 2021 by the authors. Li- problems such as photochemical smog, acid rain, haze, etc. [2,3]. The atmospheric NOx censee MDPI, Basel, Switzerland. concentration (annual mean concentrations of 0.01–0.05 ppm in urban areas over the This article is an open access article world [4]) has drastically increased over the past few decades due to industrial genesis distributed under the terms and con- and automobile development [2]. NOx has stimulating effects on the environment, human ditions of the Creative Commons At- and animal health, and plant vegetation [5]. Consequently, over the past few decades, the tribution (CC BY) license (https:// demand to mitigate the detrimental effects of NOx has increased in light of sustainable creativecommons.org/licenses/by/ climate policy, and this has inspired increased focus on NO efﬁcacy. Therefore, researchers 4.0/). x

Catalysts 2021, 11, 109. https://doi.org/10.3390/catal11010109 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, 109 2 of 13

have manifested various schemes such as direct decomposition [6], selective catalytic/non- catalytic reduction [7], solid–liquid adsorption [8], plasma-assisted catalytic reduction [9], and photocatalytic oxidation (PCO) for effective removal of NOx from the atmosphere. However, most of those propositions are not commercially feasible for NOx removal at low concentration levels and require intricate devices, engineering, or relatively high temperatures. Among various approaches, PCO is a potential and cost-effective method to manage atmospheric pollutants [10]. The PCO of NOx is an effective process due to its ability to use enormous, inexhaustible solar energy, moderate reaction conditions appropriate for cheap and large-scale application [11,12]. TiO2 has gained significant attention as a photocatalyst with the breakthrough of photocatalytic water splitting capability. What is more, TiO2 features a strong photocatalytic oxidation competence, photostability, non- toxicity, and cost-effectiveness [13]. Usually, from theoretical and experimental studies, it has been demonstrated that the dominant anatase facets have better photoactivity than rutile [14]. Under UV light irradiation, TiO2 generates electrons and holes in the conduction band and valence bands, respectively, facilitating the oxidation-reduction (redox) reactions for pollutant degradation [15,16]. However, the photocatalytic activities of anatase TiO2 are restricted by their large energy bandgap (~3.6 eV) and immediate recombination − + of photogenerated e /h pairs [17]. Consequently, typical TiO2 primarily exhibits low quantum efficiency under UV irradiation with a wavelength of <387 nm. Therefore, TiO2 can utilize a maximum of 4–5% of the solar radiation during photocatalysis [17], mostly wasted for recombination in their bulk phase. So far, researchers have proposed many strategies for the improvement of photocatalytic activity of TiO2 in NO oxidation by varying their crystal phase [18], engineering the energy bandgap to shift its absorption edge into the visible region [19,20], increasing specific surface area via porosity introduction [21], and doping of anions such as nitrogen and carbon in TiO2 [22–24]. In this study, nitrogen is considered to be an effective dopant because of its similar size (155 pm) to oxygen (152 pm) as well as similar ionization energy (first ionization energy 1402.3 kJ/mol) compared to oxygen (first ionization energy 1313.9 kJ/mol) [25]. N-doped TiO2 also has an improved photocatalytic activity in the visible light region, similar to oxidation of CO, C2H6, and ethylene [26,27]. It is well established that the doping of nitrogen into the TiO2 lattice produces a narrower band gap by coupling N 2p states with O 2p states and creates donor states over the valence band (VB). This process attenuates the energy band gap of TiO2 and generates oxygen vacancy, both of which enhance the absorbance of visible light [28]. Therefore, in this study, N-doped TiO2 powder was fabricated by a simple co-precipitation of TCA and TiO2 P25, followed by calcination at ◦ 550 C for 4 h in N2 atmosphere. This is the novel strategy to synthesize N-doped TiO2 from TCA as a nitrogen precursor. In general, the substitution of nitrogen with oxygen in TiO2 ◦ happens above 675 C under N2 atmosphere using cyanamide, melamine, and melem as nitrogen sources. Nevertheless, in this study, using TCA lowers the temperature to 550 ◦C since the SH group acts as a leaving group, and at lower temperatures polycondensation takes place, which facilitates doping of nitrogen into TiO2 [29,30]. The synthesized N- doped TiO2 materials showed a pronounced ability to absorb visible light and improved the bandgap structure with more negative (0.73 eV) conduction band (CB) potential than · − O2/ O2 potential of −0.33 eV for NO oxidation. Thus, with such a narrow energy bandgap and broad absorption ability of visible light, the synthesized N-doped TiO2 materials exhibited enhanced photocatalytic oxidation of NO under both UV and visible light irradiation. This study governs a new method for fabricating N-doped TiO2 for efficient photocatalytic degradation of noxious atmospheric NOx.

2. Results and Discussion The crystal phase structure of the prepared materials was investigated using XRD. Figure1a compares the XRD patterns of pure TiO 2 P25 and all the prepared N-doped TiO2 materials (TCX; TC means N-doped TiO2, X is wt% of TCA on TiO2). It showed that Catalysts 2021, 11, x FOR PEER REVIEW 3 of 13

materials (TCX; TC means N-doped TiO2, X is wt% of TCA on TiO2). It showed that both Catalysts 2021, 11, 109 3 of 13 the anatase phase (JCPDS21-1272) and the rutile phase (JCPDS21-1276) of TiO2 were present in pure TiO2 as well as N-doped TiO2 materials. This is consistent with the biphasic structure of commercial TiO2 P25. The XRD patterns also indicate that there were no other both the anatase phase (JCPDS21-1272) and the rutile phase (JCPDS21-1276) of TiO2 were characteristic peaks frompresent impurities. in pure TiO2 Toas well further as N-doped investigate TiO2 materials. N doping This effects is consistent on crystallite with the biphasic size, the calculated crystalstructure size of commercial of pure TiO22P25. P25 The and XRD the patterns synthesized also indicate N-doped that there TiO were2 are no other given in supporting informationcharacteristic peaks in Table from S1. impurities. The crystal To further size investigateof synthesized N doping materials effects on was crystallite determined from half-widthsize, the calculated of (101) peak crystal using size of Scherrer’s pure TiO2 P25 formula. and the The synthesized crystallite N-doped size of TiO 2 are given in supporting information in Table S1. The crystal size of synthesized materials was pure TiO2 P25 was founddetermined at 16.60 from nm half-width [31]. The of (101) crys peaktallite using size Scherrer’s increas formula.ed up Theto 18.54 crystallite nm size of 3− 2− with doping of largerpure atomic TiO2 P25 radius was found nitrogen at 16.60 (N nm 170 [31]. Å) The into crystallite oxygen size increased(O 140 upÅ) to [32]. 18.54 In nm with this study, though thedoping shifting of larger of peak atomic posi radiustion nitrogen was not (N3 −observed170 Å) into significantly, oxygen (O2− 140 the Å) crys- [32]. In this tallite size increased study,with nitrogen though the doping. shifting ofTh peakis phenomenon position was not can observed be explained signiﬁcantly, by the the fact crystallite size increased with nitrogen doping. This phenomenon can be explained by the fact that that the nitrogen dopingthe nitrogen may dopingsharpen may the sharpen XRD thepeak XRD by peak enhancing by enhancing the the growth growth rate rate ofof crystal crystal and increase andthe increasecrystallite the crystallitesize. size.

FigureFigure 1. 1.(a )( XRDa) XRD patterns patterns and (b and) FT-IR (b) spectra FT˗IR of spectra pure TiO of2 P25pure and TiO the2 synthesizedP25 and the N-doped synthesized TiO2 materials N-doped (TCX; TC meansTiO2 materials N-doped TiO (TCX;2, X is TC wt% means of TCA N-doped on TiO2). TiO2, X is wt% of TCA on TiO2).

FT-IR spectra of pure TiO2 P25 and the synthesized N-doped TiO2 materials are ˗ 2 2 FT IR spectra ofillustrated pure TiO in FigureP25 and1b. Thethe absorptionsynthesized peak N-doped in the region TiO between materials 650 cm are−1 and illus- 800 cm−1 −1 −1 trated in Figure 1b. Thewas identiﬁedabsorption for peak the Ti–O in andthe O–Ti–Oregion bondbetween vibrations 650 cm for bothand the 800 N-doped cm was TiO 2 and −1 −1 identified for the Ti–Opure and TiO O–Ti–O2 P25 [33]. bond The broad vibrations absorption for peakboth betweenthe N-doped 3000 cm TiOand2 and 3600 pure cm was TiO2 P25 [33]. The broadattributed absorption to the N-H peak stretching between vibration 3000 cm of the−1 and residual 3600 N-H cm group−1 was or attributed O-H of absorbed −1 H2O[34]. The small absorption peak at 1050 cm for the N-Ti-O bond was observed for to the N-H stretching vibration of the residual N-H group or O-H of absorbed H2O [34]. all the synthesized N-doped TiO2 materials, conﬁrming the nitrogen incorporation into -1 The small absorptionTiO peak2 P25 at [35 1050]. cm for the N-Ti-O bond was observed for all the synthesized N-doped TiO2 Thematerials, morphological confirming structure the of nitrogen pure TiO2 P25incorporation and N-doped into TiO 2TiOwas2 assessedP25 to [35]. determine whether the particle size changed with nitrogen doping using SEM and HR- TEM (Figure2). There was no particular particle size change on nitrogen doping until The morphological structure of pure TiO2 P25 and N-doped TiO2 was assessed to TC50 compared to TiO2 P25, where the spherical nanoparticles were observed (Figure2a–e) . determine whether theThough particle the change size ofchang particleed size with was nitrogen not clearly doping observable using from SEMSEM images, and HR- the surface TEM (Figure 2). Therearea was was investigatedno particular by anparticle N2 adsorption size change desorption on nitrogen isotherm (Figuredoping S2). until The BET TC50 compared to TiOsurface2 P25, area where and the porespherical volume nanoparticles for pure TiO2 wereP25 and observed synthesized (Figure N-doped 2a– TiO2 e). Though the changematerials of particle are given size in Tablewas not S1. From clearly N2 adsorptionobservable desorption from SEM isotherm images, it is clearly the seen that the surface area decreased with nitrogen doping, suggesting the doping of nitrogen surface area was investigated by an N2 adsorption desorption isotherm (Figure S2). The into TiO2. Figure2f depicts the HR-TEM image of TC10. The lattice fringe with d-spacing 2 2 BET surface area andof the 0.350 pore nm andvolume 0.240 for nm pure was observed TiO P25 due and tothe synthesized anatase (101) N-doped lattice planes TiO of TiO2 materials are given innanoparticles Table S1. From and 0.320 N2 nmadsorption due to the desorption rutile (110) lattice isotherm plane of it pureis clearly TiO2 P25 seen [36 ]. that the surface area decreased with nitrogen doping, suggesting the doping of nitrogen into TiO2. Figure 2f depicts the HR-TEM image of TC10. The lattice fringe with d-spacing of 0.350 nm and 0.240 nm was observed due to the anatase (101) lattice planes of TiO2 nanoparticles and 0.320 nm due to the rutile (110) lattice plane of pure TiO2 P25 [36]. Catalysts 2021, 11, 109 4 of 13 Catalysts 2021, 11, x FOR PEER REVIEW 4 of 13

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Figure 2. SEM images of (a) pure TiO2 P25, (b) TC7.5, (c) TC10, (d) TC25, (e) TC50, and (f) HRTEM image of TC10 sample. Figure 2. SEM images of (a) pure TiO2 P25, (b) TC7.5, (c) TC10, (d) TC25, (e) TC50, and (f) HRTEM image of TC10 sample.

XPS was carried out to investigate the surface chemical composition and the elec- XPS was carried out to investigate the surface chemical composition and the electronic Figure 2. SEM images of (atronic) pure TiOstate2 P25, of (eachb) TC7.5, element. (c) TC10, The (d )XPS TC25, survey (e) TC50, scan and spectra (f) HRTEM and image elemental of TC10 composition sample. of state of each element. The XPS survey scan spectra and elemental composition of pure pure TiO2 P25, TC10, and TC50 are provided in Supporting Information Figure S1 and XPS was carried out to investigate the surface chemical composition and the elec- TiOTable2 P25, S2. TC10, The photoelectron and TC50 are peaks provided on the in surv Supportingey spectra Informationat around the Figure binding S1 energies and Table S2. tronic state of each element. The XPS survey scan spectra and elemental composition of Theof photoelectron285, 399, 459.5, peaksand 530.5 on theeV surveywere assigned spectra to at C1s, around N1s, theTi2p, binding and O1s, energies respectively, of 285, 399, pure TiO2 P25, TC10, and TC50 are provided in Supporting Information Figure S1 and 459.5,indicating and 530.5 the presence eV were of assigned nitrogen in to the C1s, N-doped N1s, Ti2p, TiO2 andmaterials. O1s, respectively,Here, the nitrogen indicating con- the Table S2. The photoelectron peaks on the survey spectra at around the binding energies presencetents increased of nitrogen with inincreasing the N-doped TCA contents TiO2 materials. in Table S2, Here, suggesting the nitrogen incorporating contents nitro- increased of 285, 399, 459.5, and 530.5 eV were assigned to C1s, N1s, Ti2p, and O1s, respectively, withgen increasing into TiO2 lattice. TCA The contents N1s spectrum in Table in S2,Figure suggesting 3a consists incorporating of three peaks at nitrogen 399.6, 397.3, into TiO2 indicating the presence of nitrogen in the N-doped TiO2 materials. Here, the nitrogen con- lattice.and 395.8 The N1seV. The spectrum peak at in around Figure 399.63a consists eV is characterized of three peaks by the at 399.6,absorbed 397.3, NH andx at the 395.8 eV. tents increased with increasing TCA contents in Table S2, suggesting incorporating nitro- Thesurface peak of at the around particle. 399.6 The eV peak is characterizedat 397.3 eV is assigned by the absorbedto Ti-O-N NHbond;at it theis due surface to the of the gen into TiO2 lattice. The N1s spectrum in Figure 3a consists of three peaksx at 399.6, 397.3, interstitial position of N into TiO2 lattice. Furthermore, around 395.8 eV peaks are at- particle.and 395.8 The eV. peak The at peak 397.3 ateV around is assigned 399.6 eV to is Ti-O-N characterized bond; it by is duethe absorbed to the interstitial NHx at the position tributed to the Ti-N-Ti bond due to N substitutional doping on the oxygen site into the ofsurface N into of TiO the2 lattice.particle. Furthermore,The peak at 397.3 around eV is 395.8assigned eV to peaks Ti-O-N are bond; attributed it is due to to the the Ti-N-Ti TiO2 lattice [37]. Figure 3c shows the O1s spectra of pure TiO2 P25, TC10, and TC50. The bondinterstitial due to position N substitutional of N intodoping TiO2 lattice. on the Furthermore, oxygen site around into the 395.8 TiO eVlattice peaks [37 are]. Figureat- 3b peak at 529.4 eV is marked for the Ti-O bond [38]. The minor peak at 531.32 eV is possibly showstributed the to O1s the spectra Ti-N-Ti ofbond pure due TiO to NP25, substitu TC10,tional and doping TC50. Theon the peak oxygen at 529.4 site into eV isthe marked characterized by the impurity hydrocarbon2 C=O bond [39]. The substitutional Ti-N-Ti TiO2 lattice [37]. Figure 3c shows the O1s spectra of pure TiO2 P25, TC10, and TC50. The forbond the Ti-O was bondalso confirmed [38]. The from minor the peak Ti2p atXPS 531.3 spectrum. eV is possibly Figure 3c characterized illustrates the Ti2p by the spec- impurity peak at 529.4 eV is marked for the Ti-O bond [38]. The minor peak at 531.3 eV is possibly hydrocarbontra of pure TiO C=O2 P25 bond compared [39]. The to substitutionalas-prepared samples Ti-N-Ti of TC10 bond and was TC50. also confirmedFor pure TiO from2 the characterized by the impurity hydrocarbon C=O bond [39]. The substitutional Ti-N-Ti Ti2pP25, XPS at 463.9 spectrum. eV and Figure 458.3 3eVc illustratesthe peak positions the Ti2p confirm spectra the of Ti2p pure1/2 TiOand 2Ti2pP253/2, compared which to bond was also confirmed from the Ti2p XPS spectrum. Figure 3c illustrates the Ti2p spec- as-preparedaffirms the samplespresence of of Ti TC10 in the and Ti4+ TC50.speciesFor [33]. pure At 463.6 TiO eV2 P25, and at458.2 463.9 eV, eVminor and peaks 458.3 in eV the tra of pure TiO2 P25 compared to as-prepared samples of TC10 and TC50. For pure TiO2 4+ peakN-doped positions TiO confirm2 are mainly the due Ti2p to1/2 Ti2pand1/2 and Ti2p Ti2p3/2,3/2 which of Ti2O affirms3, which the are presence usually found of Ti indue the Ti P25, at 463.9 eV and 458.3 eV the peak positions confirm the Ti2p1/2 and Ti2p3/2, which speciesto oxygen [33]. vacancy At 463.6 creation eV and on 458.2 the lattice eV, minor structure peaks [39]. inIn N-dopedcomparison TiO to pureare mainlyTiO2 P25, due to affirms the presence of Ti in the Ti4+ species [33]. At 463.6 eV and 458.2 eV, minor2 peaks in Ti2pthe N-dopedand Ti2p TiO2 hadof Ti a shiftO , whichof binding are usuallyenergy to found higher due energy to oxygen for both vacancy O1s and creation Ti2p on N-doped1/2 TiO2 are3/2 mainly2 due3 to Ti2p1/2 and Ti2p3/2 of Ti2O3, which are usually found due thepeaks lattice due structure to the interaction [39]. In between comparison titanium to pure and nitrogen TiO P25, [40]. the N-doped TiO had a shift to oxygen vacancy creation on the lattice structure [39].2 In comparison to pure TiO2 2 P25, ofthe binding N-doped energy TiO2 tohad higher a shift energyof binding for energy bothO1s to higher and Ti2penergy peaks for both due O1s to theand interactionTi2p betweenpeaks due titanium to the interaction and nitrogen between [40]. titanium and nitrogen [40].

Figure 3. XPS spectra of pure TiO2 P25, TC10, and TC50 (a) N1s, (b) O1s, and (c) Ti2p.

Figure 3. XPS spectra of pure TiO2 P25, TC10, and TC50 (a) N1s, (b) O1s, and (c) Ti2p. Figure 3. XPS spectra of pure TiO2 P25, TC10, and TC50 (a) N1s, (b) O1s, and (c) Ti2p. CatalystsCatalysts 20212021, ,1111, ,x 109FOR PEER REVIEW 5 5of of 13 13

Photoluminescence (PL) analysis of pure TiO2 P25 and the prepared N-doped TiO2 Photoluminescence (PL) analysis of pure TiO2 P25 and the prepared N-doped TiO2 forfor different different nitrogen nitrogen contents contents was was perform performeded at at an an excitation excitation wavelength wavelength (266 (266 nm) nm) to to investigateinvestigate the the rate rate of of recombination recombination and and sepa separationration efficiency efficiency of of photo-excited photo-excited electrons electrons andand hole hole pairs. pairs. The The PL PL intensity intensity mainly mainly resulted resulted from from the the recombination recombination of e of-/h e+ −pairs./h+ pairs. The lowerThe lower intensity intensity suggests suggests a lower a lower recombination recombination rate rateand andhigher higher separation separation efficiency efficiency of − + − + 2 eof/h e pairs/h [41].pairs Figure [41]. Figure4a demonstra4a demonstratestes the PL thespectra PLspectra of pure of TiO pureP25 TiO and2 P25the synthe- and the sizedsynthesized N-doped N-doped TiO2. The TiO PL2. Thepeaks PL were peaks found were at found around at around 522 nm 522 for nm pure for TiO pure2 P25, TiO 2pos-P25, siblypossibly due to due the to partially the partially reduced reduced titanium titanium ion, surface ion, surface nitrogen nitrogen species, species, and oxygen and oxygen va- canciesvacancies [42]. [42 In]. pure In pure TiO TiO2, usually2, usually the the energy energy levels levels of ofoxygen oxygen vacancies vacancies are are below below the the CB CB ofof TiO TiO2.2 The. The photogenerated photogenerated electrons electrons in in the the CB CB of of TiO TiO2 2cancan fall fall into into the the oxygen oxygen vacancies vacancies byby the the non-irradiation non-irradiation process process and and then then are are easily easily recombined recombined with with photogenerated photogenerated holes holes onon the the VB VB generated generated by by fluorescence fluorescence emissions emissions [43]. [43 In]. this In this study, study, the thePL intensity PL intensity of N- of dopedN-doped TiO TiO2 decreased2 decreased and, and, thus, thus, increased increased the the separation separation efficiency efficiency of of photogenerated photogenerated e−e/h−+/h pairs+ pairs due due to tocapturing capturing of ofthe the photogenerated photogenerated holes holes by by reduced reduced titanium titanium ion ion (Ti (Ti3+3+). ). UV–VisUV–Vis DRS DRS spectroscopy spectroscopy analysis analysis was was perf performedormed to to determine determine the the estimated estimated energy energy bandgapbandgap of of pure pure TiO TiO2 2P25P25 and and synthesized synthesized N-doped N-doped TiO TiO22 materials.materials. Figure Figure 4b4b illustrates illustrates thethe absorbance absorbance spectra spectra of of TiO TiO2 2andand N-doped N-doped TiO TiO2 2forfor different different nitrogen nitrogen contents. contents. In In this this study,study, the the N-doped N-doped TiO22 materials comparedcompared to to pure pure TiO TiO2 showed2 showed yellow yellow color color(Figure (Figure2b) , 2b),suggesting suggesting the the ability ability to to absorb absorb light light in in the the visible visible region region [[40].40]. The N-doped TiO TiO22 withwith increasingincreasing nitrogen nitrogen content content exhibited exhibited a asignificant significant red-shift red-shift of of the the absorption absorption edge edge into into thethe visible visible light light region. region. This This result result sugg suggestsests that that nitrogen nitrogen doping doping into into the the TiO TiO2 2latticelattice can can successfullysuccessfully narrow narrow the the bandgap bandgap energy energy due due to to modifying modifying the the band band structure structure [44]. [44].

Figure 4. (a) PL spectra and (b) UV–Vis DRS spectra of pure TiO2 P25 and synthesized materials. Figure 4. (a) PL spectra and (b) UV–Vis DRS spectra of pure TiO2 P25 and synthesized materials.

TheThe energy energy bandgaps bandgaps of of the the pure pure TiO TiO2 2P25P25 and and synthesized synthesized N-doped N-doped TiO TiO2 2materialsmaterials withwith different different nitrogennitrogen contentscontents were were determined determined using using the Kubelka–Munkthe Kubelka–Munk function function (F(R)). (F(R)).Extrapolating Extrapolating (F(R)h (F(R)hυ)1/2 vs.ʋ)1/2 h υvs.plot hʋ isplot used is used to calculate to calculat the indirecte the indirect semiconductor semiconductor energy energybandgap. bandgap. The estimated The estimated energy energy band band gaps ga wereps were 3.25, 3.25, 3.22, 3.22, 3.18, 3.18, 3.14, 3.14, and and 3.10 3.10 eV eV for forpure pure TiO TiO2 P25,2 P25, TC7.5, TC7.5, TC10, TC10, TC25, TC25, andand TC50,TC50, respectivelyrespectively (Figure5 5a).a). The estimated energyenergy bandgap bandgap also also revealed revealed the the substitu substitutionaltional doping doping of of nitrogen nitrogen into into the the TiO TiO2 2latticelattice sincesince interstitial interstitial nitrogen nitrogen doping doping narrows narrows the the energy energy bandgap bandgap significantly significantly more more than than substitutionalsubstitutional doping doping does does [4 [5].45]. Figure Figure 5b5b demonstrates demonstrates th thee Mott–Schottky Mott–Schottky plot plot of of pure pure TiOTiO2 2P25P25 and and N-doped N-doped TiO TiO22 withwith different nitrogennitrogen contentscontents at at 495.16 495.16 Hz. Hz. In In this this study, study, the theflat-band flat-band potential potential (Efb) (Efb) was was determined determined by theby Mott–Schottkythe Mott–Schottky plot plot as conduction as conduction band bandminimum minimum (CBM), (CBM), and and the valencethe valence band band maximum maximum (VBM) (VBM) was was estimated estimated based based on on the thedifference difference from from the the CBM CBM using using the the “optical “optical bandgap bandgap energy” energy” determined determined by theby the Tauc Tauc plot. plot.The SchottkyThe Schottky formula formula can be can applied be applied to measure to measure the flat-band the flat-band potentials potentials and the conduction and the conductionband edge band of pure edge TiO of2 and pure N-doped TiO2 and TiO N-doped2, since thin TiO compact2, since thin TiO 2compactfilm behaves TiO2 film similarly be- havesto conventional similarly to macroscopic conventional semiconductor macroscopic electrode semiconductor [46]. TiO electrode2 is an n-type [46]. semiconductor,TiO2 is an n- Catalysts 2021, 11, 109 6 of 13

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where vacancies act as electron donors. In Mott–Schottky measurements, a semiconductor is in contact with a solution where a redox couple is present. An electron exchange can thentype occur semiconductor, to match the where two vacancies Fermi energies act as electron (Ef and donors. Eredox In ).Mott–Schottky Such electron measure- flow induces a ments, a semiconductor is in contact with a solution where a redox couple is present. An band bending. The flat band potential (Efb) is a counter-potential that flattens the potential backelectron again exchange by compensating can then occur themigrated to match the charge two Fermi capacity energies effects, (Ef whichand Eredox is). measured Such from electron flow induces a band bending. The flat band potential (Efb) is a counter-potential thethat M-S flattens plot. the Here, potential the measured back again flatby compensating band potential the migrated represents charge the capacity lower ef- edge of the conductionfects, which band, is measured which from is approximately the M-S plot. Here, the the Fermi measured energy flat value, band potential on an electrochemical repre- scalesents [47 ].the The lower figure edge showsof the conduction that the increase band, which in nitrogen is approximately content the resulted Fermi inenergy a steady shift of thevalue, estimated on an electrochemical flat-band potentials scale [47]. towardsThe figure more shows negative that the increase values. in The nitrogen calculated con- CB and VBtent potential resulted values in a steady are shift given of inthe Tableestimat S3,ed flat-band and the potentials synthesized towards sample more energynegative diagrams arevalues. given inThe Figure calculated6a. TheCB and potentials VB potential were values recalculated are given vs.in Table NHE S3, at and pH the 7 by synthe- the following sized sample energy diagrams are given in Figure 6a. The potentials were recalculated vs. equations [46]. NHE at pH 7 by the following equations [46]. V ≈ V = V + V − 0.059 ∗ (7 − 5.8) CB VFB(NHE,CB ≈ VFB(NHE, pH pH 7) 7) = VFB(Ag/AgCl,FB(Ag/AgCl, pH 5.8) pH + V 5.8) − 0.059 * (7 − 5.8) (1) VVB = VCB + Eg/e, (1) VVB = VCB + Eg/e,

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O2 + e− → O2− (3)

OH− + h+ → OH (4)

NO + O2− → NO3 (5)

NO + OH → NO2 (6)

− Figure 5. (a) Tauc plot and (b) Mott˗Schottky plot of pureNO TiO2 +2 P25OH and → NOsynthesized3 materials. (7) Figure 5. (a) Tauc plot and (b) Mott-Schottky plot of pure TiO2 P25 and synthesized materials.

The N-doped TiO2 with different nitrogen contents showed the steady shift of CB and VB potentials towards more negative and less positive directions (Figure 6a). The nitrogen incorporation into TiO2 lattice had the advantage to form new energy states since the N 2p position is more negative than the O 2p state, which thus decreases the energy bandgap of TiO2 and shifts the optical absorption towards the visible light region. There- fore, under visible light irradiation, the possibility of electron migration from the valence band to the conduction band increased, which leads to the activity of N-doped TiO2 materials into visible light [45,48]. A proposed schematic for the oxidation of NO is depicted in Figure 6b. The TiO2 incited under photo irradiation and generated e−/h+ pairs. The photogenerated e− can immediately reduce adsorbed O2 to O2− on CB, and the h+ can directly oxidize H2O/OH− to OH radicals on VB. Such O2− and OH radicals are the fundamental elements for the successful oxidation of NO to neutral component NO3− [49]. The more negative the CB band potential is (than the O2/O2− potential of −0.33 eV) and the more positive the VB band potential is (than OH−/OH− potential of 1.99 eV), the greater the oxidation of NO [33]. The reactions correspond to the formation of radicals, and a feasible pathway for NO oxidation is as follows [49]:

photocatalyst + hʋ → e− + h+ (2)

Figure 6. (a) Energy diagram with an energy bandgap, CB and VB edge potentials of pure TiO2 P25 and N-doped TiO2 Figure 6. (a) Energy diagram with an energy bandgap, CB and VB edge potentials of pure TiO2 P25 and N-doped TiO2 materials, and (b) schematic for NO oxidation mechanism of TiO2 under light irradiation. materials, and (b) schematic for NO oxidation mechanism of TiO2 under light irradiation.

Figure 7 demonstrates the time profile for NO, NO2, and NOx and the efficiency of NO removal, NO2 generation, NOx removal, and NO3− selectivity of pure TiO2 P25 and the synthesized N-doped TiO2 sample under both UV and visible light irradiation, respectively, for 1h. All samples compared to pure TiO2 P25 exhibited enhanced NO oxidation. Under UV irradiation (Figure 7a, Table S4), the TC10 sample exhibited the maximum NO removal with a superior efficiency of 59% compared to the pure TiO2 P25 (47%), approximately 1.26 times higher. The NOx removal efficiency of the TC10 (34%) was increased by 17% compared to TiO2 P25 (17%) under UV irradiation. The intensified NO expulsion could be clarified by the fact the narrow energy bandgap with more negative CB potentials facilitates the reduction of O2 to O2− radicals and, thus, intensifies the formation of the photo-oxidation product (i.e., NO3−). It is further validated by the decreasing trend of the NO2 generation rate. Here, NO2 generation was decreased (Figure 7c) over TC10 compared to TiO2, which leads to higher NOx removal. Under visible light irradiation (Figure 7b, Table S5), all the synthesized N-doped TiO2 materials exhibited significantly enhanced photocatalytic performance in both NO and NOx removal compared to pure TiO2 P25 samples, though the NO2 generation is relatively higher than pure TiO2. Among all the samples, TC10 showed maximum NO and NOx removals of 51% and 29%, respectively, almost 2.55 and 2.23 times higher than the pure TiO2 P25 of 20% and 13%. Nitrogen doping into the TiO2 lattice enhanced the absorption ability towards the visible light region and narrowed the energy bandgap with more negative CB potentials, which enhanced the formation of superoxide radicals and, therefore, oxidation of NO. For practical application, the goal is to produce materials with high se- Catalysts 2021, 11, 109 7 of 13

The N-doped TiO2 with different nitrogen contents showed the steady shift of CB and VB potentials towards more negative and less positive directions (Figure6a). The nitrogen incorporation into TiO2 lattice had the advantage to form new energy states since the N 2p position is more negative than the O 2p state, which thus decreases the energy bandgap of TiO2 and shifts the optical absorption towards the visible light region. Therefore, under visible light irradiation, the possibility of electron migration from the valence band to the conduction band increased, which leads to the activity of N-doped TiO2 materials into visible light [45,48]. A proposed schematic for the oxidation of NO is depicted in Figure6b. The TiO 2 incited under photo irradiation and generated e−/h+ pairs. The photogenerated e− can · − + − immediately reduce adsorbed O2 to O2 on CB, and the h can directly oxidize H2O/OH · · − · to OH radicals on VB. Such O2 and OH radicals are the fundamental elements for the − successful oxidation of NO to neutral component NO3 [49]. The more negative the CB · − band potential is (than the O2/ O2 potential of −0.33 eV) and the more positive the VB band potential is (than OH−/·OH potential of 1.99 eV), the greater the oxidation of NO [33]. The reactions correspond to the formation of radicals, and a feasible pathway for NO oxidation is as follows [49]:

photocatalyst + hυ → e− + h+ (2)

− · − O2 + e → O2 (3) OH− + h+ → ·OH (4) · − NO + O2 → NO3 (5) · NO + OH → NO2 (6) · − NO2 + OH → NO3 (7)

Figure7 demonstrates the time profile for NO, NO 2, and NOx and the efficiency of NO − removal, NO2 generation, NOx removal, and NO3 selectivity of pure TiO2 P25 and the synthesized N-doped TiO2 sample under both UV and visible light irradiation, respectively, for 1h. All samples compared to pure TiO2 P25 exhibited enhanced NO oxidation. Under UV irradiation (Figure7a, Table S4), the TC10 sample exhibited the maximum NO removal with a superior efficiency of 59% compared to the pure TiO2 P25 (47%), approximately 1.26 times higher. The NOx removal efficiency of the TC10 (34%) was increased by 17% compared to TiO2 P25 (17%) under UV irradiation. The intensified NO expulsion could be clarified by the fact the narrow energy bandgap with more negative CB potentials · − facilitates the reduction of O2 to O2 radicals and, thus, intensifies the formation of the − photo-oxidation product (i.e., NO3 ). It is further validated by the decreasing trend of the NO2 generation rate. Here, NO2 generation was decreased (Figure7c) over TC10 compared to TiO2, which leads to higher NOx removal. Under visible light irradiation (Figure7b, Table S5), all the synthesized N-doped TiO 2 materials exhibited significantly enhanced photocatalytic performance in both NO and NOx removal compared to pure TiO2 P25 samples, though the NO2 generation is relatively higher than pure TiO2. Among all the samples, TC10 showed maximum NO and NOx removals of 51% and 29%, respectively, almost 2.55 and 2.23 times higher than the pure TiO2 P25 of 20% and 13%. Nitrogen doping into the TiO2 lattice enhanced the absorption ability towards the visible light region and narrowed the energy bandgap with more negative CB potentials, which enhanced the formation of superoxide radicals and, therefore, oxidation of NO. For practical application, the goal is to produce materials with high selectivity to − − NO3 . Comparing Tables S4 and S5, the selectivity of NO3 of pure TiO2 significantly differed and was much higher under visible irradiation (about 64%). This phenomenon can be explained by the energy band structure of pure TiO2 and N-doped TiO2. As we · − mentioned, the more negative the CB band potential than the O2/ O2 potential (−0.33 eV), and the more positive the VB band potential than OH−/·OH potential (1.99 eV), the greater Catalysts 2021, 11, x FOR PEER REVIEW 8 of 13

lectivity to NO3−. Comparing Tables S4 and S5, the selectivity of NO3− of pure TiO2 signif- Catalysts 2021, 11, 109 icantly differed and was much higher under visible irradiation (about 64%). This phenom-8 of 13 enon can be explained by the energy band structure of pure TiO2 and N-doped TiO2. As we mentioned, the more negative the CB band potential than the O2/O2− potential (−0.33 eV), and the more positive the VB band potential− than OH−/OH− potential (1.99 eV), the the oxidation of NO and the formation of NO3 . For− pure TiO2 the more positive valence greater the oxidation of NO and the formation of NO3 . For pure TiO2 the more− positive band (2.59 eV vs. NHE) than doped TiO2 (<2.53 eV vs. NHE) favors NO3 formation on valence band (2.59 eV vs. NHE) than doped TiO2 (<2.53 eV vs. NHE) favors NO3− for- the valence band (favors Equation (7)). However, in case of VIS experiment of TC50, a mation on the valence band− (favors Equation (7)). However, in case of VIS experiment of superior increase in NO3 selectivity was shown (77%). This might be due to the direct TC50, a superior increase in NO3− selectivity was shown (77%). This might be due to the conversion of NO to NO − on the more negative conduction band (−0.84 eV vs. NHE) direct conversion of NO to3 NO3− on the more negative conduction band (−0.84 eV vs. NHE) than pure TiO (−0.66 eV vs. NHE), which is more favorable for reduction of NO and than pure TiO22 (−0.66 eV vs. NHE), which is more favorable for reduction of NO and for- formation of NO − (favors Equation (5)). Compared to pure TiO P25, for the N-doped mation of NO3− (favors3 Equation (5)). Compared to pure TiO2 P25, for2 the N-doped TiO2 TiOsample,2 sample, the generation the generation of NO2 ofwas NO higher2 was (Figure higher 7d). (Figure This7 cand). be This explained can be explainedby the fact by thethat fact the N-doped that the N-dopedTiO2 showed TiO increased2 showed charge increased transfer/separation charge transfer/separation efficiency. Therefore, efﬁciency. Therefore,the formation the of formation OH radicals of OH on radicalsthe valence on band, the valence followed band, by formation followed byof NO formation2, was of NOincreased2, was under increased visible under light visible irradiation. light irradiation. The substitutional The substitutional nitrogen doping nitrogen into the doping TiO2 into thelattice TiO can2 lattice enhance can the enhance photocatalytic the photocatalytic activity of NO activity oxidation of NO by enhancing oxidation its by absorption enhancing its − + absorptionability towards ability the towards visible light thevisible region, light improving region, separation improving efficiency separation of efﬁciencye−/h+ pairs, of and e /h pairs,narrowing and narrowingthe energy theband energy gaps under band gapsboth UV under and both visible UV light and irradiation. visible light irradiation.

2, x re- FigureFigure 7. 7.Time Time proﬁlesprofiles of of NO, NO, NO NO 2,andand NO NO underx under (a) UV (a) and UV ( andb) visible (b) visible light irradiation; light irradiation; the profile the of proﬁle estimated of estimated NO moval, NO2, generation, NOx, removal, and NO3−selectivity of TiO− 2 and N-doped TiO2 with different N contents under (c) UV and (d) NOremoval, NO2, generation, NOx, removal, and NO3 selectivity of TiO2 and N-doped TiO2 with different N contents under (c) visible light irradiation, respectively. UV and (d) visible light irradiation, respectively.

Here, NOx removal performance was compared with some other progressive studies Here, NOx removal performance was compared with some other progressive studies (Figure 8). Nitrogen-doped with P25 using urea as a precursor achieved NOx removal (Figure8). Nitrogen-doped with P25 using urea as a precursor achieved NO x removal per- performances of approximately 18% and 8% at UV and visible regions, respectively formances of approximately 18% and 8% at UV and visible regions, respectively [19,50–52]. [19,50–52]. Composite of carbon nitride with anatase TiO2, composite of modified carbon Composite of carbon nitride with anatase TiO2, composite of modified carbon nitride with nitride with TiO2, and composite of hydroxyapatite with TiO2 achieved 18/17, 24/18, and TiO2, and composite of hydroxyapatite with TiO2 achieved 18/17, 24/18, and 27/0% NOx removal proficiencies in the UV/visible region, respectively [19,50–52]. In the present study, 34/28% NOx removal was attained in the UV/visible region, which indicates significant progress for the NOx removal strategy. The present study can be further improved by making a composite with carbon nitride with advanced synthesized N-doped TiO2 in the future. Catalysts 2021, 11, x FOR PEER REVIEW 9 of 13

27/0% NOx removal proficiencies in the UV/visible region, respectively [19,50–52]. In the present study, 34/28% NOx removal was attained in the UV/visible region, which indicates significant progress for the NOx removal strategy. The present study can be further im- Catalysts 2021, 11, 109 9 of 13 proved by making a composite with carbon nitride with advanced synthesized N-doped TiO2 in the future.

FigureFigure 8. 8.Comparison Comparison of NO of xNOxremoval removal performance performance in the UV in andthe UV visible and regions visible with regions some with some ad- advancedvanced studies. 3. Materials and Methods 3.1.3. Materials Preparation ofand N-Doped Methods TiO2 3.1.All Preparation the chemicals of N-Doped were used TiO as of2 analytical grade without further purifications. The N-doped TiO powder was synthesized as follows. Firstly, tri-thiocyanuric acid (TCA, All the2 chemicals were used as of analytical grade without further purifications. The Sigma-Aldrich, St. Louis, MO, USA; 95.0%) was dissolved into 10 mL dimethyl sulfoxide 2 (DMSO,N-doped Daejung, TiO Siheung,powder Korea, was 99.5%),synthesized and TiO as2 P25 follows. (Evonik Firstly, Degussa GmbH,tri-thiocyanuric Essen, acid (TCA, Germany)Sigma-Aldrich, was dispersed St. Louis, into 20 MO, mL ofUSA; H2O by95.0%) sonication was fordissolved 1h. Then, into the solution10 mL wasdimethyl sulfoxide ◦ mixed(DMSO, and keptDaejung, for 1 h followedSiheung, by Korea, filtering 99.5%), and drying and at 80TiOC.2 P25 The dried(Evonik powders Degussa were GmbH, Essen, ◦ then calcined in a tube furnace at 550 C for 4h under an N2 atmosphere. Different TCAs Germany) was dispersed into 20 mL of H2O by sonication for 1h. Then, the solution was (50, 25, 10, and 7.5 wt.%.) were mixed with TiO P25 and named as TC50, TC25, TC10, and mixed and kept for 1 h followed by filtering2 and drying at 80 °C. The dried powders were TC7.5, sequentially, to prepare the N-doped TiO2 powder. The pure TiO2 P25 was named TiOthen2. calcined in a tube furnace at 550 °C for 4h under an N2 atmosphere. Different TCAs (50, 25, 10, and 7.5 wt.%.) were mixed with TiO2 P25 and named as TC50, TC25, TC10, and 3.2. Characterization TC7.5, sequentially, to prepare the N-doped TiO2 powder. The pure TiO2 P25 was named The crystallinity of synthesized powders was evaluated using the X-ray diffraction TiO2. (XRD) technique (Rigaku D/max Ultima III, The Woodlands, TX, USA) with Cu Kα radiation at 40 kV and 40 mA. The synthesized powder surface morphology was assessed using3.2. Characterization scanning electron microscopy (FE-SEM, HITACHI, EX-200, Santa Clara, CA, USA). High-resolutionThe crystallinity transmission of electron synthesized microscopy powders (HR-TEM, was TECNAI evaluated F20 UT) using was used the toX-ray diffraction examine the N-doped TiO2 morphology’s fine details. A Fourier transform infrared (FT-IR) spectrometer(XRD) technique (FT-IR-4100, (Rig Jasco,aku D/max Portland, Ultima OR, USA) III, was The used Wood to identifylands, the TX, synthesized USA) with Cu Kα radi- sampleation chemicalat 40 kV bonds. and 40 X-ray mA. photoelectron The synthesized spectroscopy powder (XPS) surface was employed morphology using was assessed MultiLab2000,using scanning VG, USA,electron to identify microscopy the synthesized (FE-SEM, samples’ HITACHI, elements EX-200, and their chemicalSanta Clara, CA, USA). − + state.High-resolution The photogenerated transmission e /h pairs electron recombination microscopy rate of (HR-TEM, the synthesized TECNAI samples F20 UT) was used was evaluated using a photoluminescence (PL) measurement system (Spectrograph 500i, to examine the N-doped TiO2 morphology’s fine details. A Fourier transform infrared (FT- Acton Research Co., Acton, MA, USA) with an excitation wavelength of 266 nm. A UV–vis spectrometerIR) spectrometer (Lambda (FT-IR-4100, 365, PerkinElmer, Jasco, Waltham, Portland, MA, USA)OR, USA) equipped was with used an integrat-to identify the synthe- ingsized sphere sample accessory chemical was used bonds. to obtain X-ray the synthesizedphotoelectron sample spectroscopy UV–vis spectra (XPS) within was a employed us- rangeing MultiLab2000, of 200–800 nm. The VG, Kubelka–Munk USA, to identify function (F(R)) the wassynthesized used as follows: samples’ F(R) = K/S,elements and their − 2 Kchemical = (1 R) state.and S =The 2R, photogenerated where K, S, and R are e−/h the+ pairs molar recombination absorption coefficient, rate scatteringof the synthesized sam- factor, and reflectance, respectively, to determine the energy bandgap of the synthesized ples was evaluated using a photoluminescence (PL) measurement system (Spectrograph 500i, Acton Research Co., Acton, MA, USA) with an excitation wavelength of 266 nm. A UV–vis spectrometer (Lambda 365, PerkinElmer, Waltham, MA, USA) equipped with an integrating sphere accessory was used to obtain the synthesized sample UV–vis spectra Catalysts 2021, 11, 109 10 of 13

samples. The indirect energy bandgap was veriﬁed by extrapolation of (F(R)hυ)1/2 vs. hυ plot. The speciﬁc surface area was measured by N2 adsorption–desorption measurements at 77 K (Micromeritics new Tristar II surface area and porosity analyzer 3020, USA) using the Branauer–Emmet Teller (BET) method. The samples were degassed at 150 ◦C for 24 h before measurements.

3.3. Electrochemical Measurements

The electrochemical performance of the synthesized N-doped TiO2 was examined using Bio-logic VSP potentiostat equipped with a handmade three-electrode cell containing conductive fluorine-doped tin oxide (FTO) as a working electrode, Pt electrode as a counter, and Ag/AgCl KCl (3 M) electrode as a reference electrode, respectively. The electrode materials were prepared by dispersing 50 mg of investigated materials in 2 mL solution containing 1.8 mL of ethanol (DAEJUNG) and 0.2 mL of Nafion (Sigma Aldrich, St. Louis, MO, USA 5 wt% in ethanol), followed by sonication and stirring. After that, 100 µL of the dispersion was cast on the FTO electrode, followed by drying at 60 ◦C for 8 h. The electrolyte used in the experiment was 0.2 M sodium sulfate (Na2SO4, Sigma Aldrich, 99%, Darmstadt, Germany). Before measurements, N2 gas was purged for 30 min. The experiment was performed at room temperature. These measurements were performed to calculate the CB edge potential of synthesized samples. While the CB edge potentials and flat band potentials practically coincide for n-type semiconductors, the Mott–Schottky relation can be used for the determination of VFB [46]. 1 1 kT = ( − − ) 2 V VFB (8) C εε0eND e

Here, C is the space charge capacitance, ε and ε0 are the dielectric constants of the semiconductor and free space, e is the charge of the elementary electron, ND is the dopant density, k is the Boltzmann constant, and T is the absolute temperature. The ﬂat band potential VFB is determined by intercepting the plot’s linear part with the potential axis.

3.4. Photocatalytic Activity Measurements

The NOx removal efficiencies of pure TiO2 P25 and the synthesized N-doped TiO2 samples were determined using a NOx removal photocatalytic analyzer (KS ISO 22197- 1 standard) under both UV and visible light irradiation. The emission spectra of UV and visible light sources are given in Figure S3 in the Supporting Information. According to ISO protocol, the reaction parameters were maintained at 1 ± 0.003 ppm initial NO concentration, 3 L/min gas flow, relative humidity of 50%, and reaction temperature of 25 ◦C. The samples were prepared by casting a slurry of catalyst powders and water on a glass plate, and the exposed catalyst surface had an area of 40 cm2. The loading mass for each sample was 5 mg cm−2. During the measurement procedure, the UV and visible light intensities were 1.00 ± 0.30 W m−2 and 6000 ± 300 lux. All the experiments were performed for 100 min, where 20 min was used for adsorption of gas and 60 min for light irradiation. To measure the NO and NO2 concentrations, a NOx analyzer (Sernius 40, Ecotech, Melbourne, Australia) was fixed at the reactor’s outlet. The rates of NO − removal, NO2 generation, NOx removal, and nitrate (NO3 ) selectivity were calculated by the following equations: NOin − NOout NOremoval = (9) NOin

NO2, out − NO2, in NO2, generation = (10) NOin

NOx, in − NOx, out NOx, removal = (11) NOin

− NOx, removal NO3 selectivity = × 100 (12) NOremoval Catalysts 2021, 11, 109 11 of 13

4. Conclusions

In the present study, nitrogen-doped TiO2 was prepared in a straightforward way by calcination of precipitates from the solution mixture of tri-thiocyanuric acid in DMSO and TiO2 P25 in H2O. The optimized TC10 materials showed impressive light absorption capability in the visible region by narrowing the bandgap (TC10: 3.18 eV; TiO2 P25: 3.25 eV). The ratio of nitrogen doping assisted in tuning the conduction band and valance band positions. Most signiﬁcant, the electron and hole separation efﬁciencies were improved for nitrogen doping on the titanium oxide. Consequently, the TC10 (N doped TiO2 P25) removed 51% NO and 28% NOx, which are 2.55 and 2.23 times higher than that for pure TiO2 P25 in the visible region. Similarly, in the UV region, TC10 removed 59% NO and 34% NOx and was 1.26 and 2 times higher than that for TiO2 P25. N-doped titanium oxide (TC10) is a unique material that offers excellent potential to remove noxious nitrogen oxide (NOx) from the environment and maintain sustainable urban life for the well-being of human continued existence.

Supplementary Materials: The following are available online at https://www.mdpi.com/2073-4 344/11/1/109/s1, Figure S1: The broad scan survey spectra of pure TiO2 P25 and N-doped TiO2 samples with 10 wt% and 50 wt% nitrogen contents. Figure S2: N2 adsorption desorption isotherm of pure TiO2 P25 and synthesized N-doped TiO2. Figure S3: Emission spectra of (a) UV, and (b) visible light sources. Table S1: Estimated FWHM and crystal size of the prepared samples along with the anatase (101) plane of TiO2, and BET surface area and pore volume from N2 adsorption desorption isotherm. Table S2: XPS elemental analysis of purer TiO2 P25 and N-doped TiO2 samples with 10wt% and 50wt% nitrogen contents. Table S3: The energy band gap, CB and VB edge potential of pure TiO2 P25 and N-doped TiO2 samples. The CB and VB edge potentials was measured vs Ag/AgCl at pH 5.8 and calculated vs NHE at pH 7. Table S4: Photocatalytic efficiency of samples in NO removal, − NO2 generation, NOx removal and NO3 selectivity under UV irradiation. Table S5: Photocatalytic − efficiency of samples in NO removal, NO2 generation, NOx removal and NO3 selectivity under visible light irradiation. Author Contributions: Conceptualization, T.T.K., and Y.-S.J.; methodology, T.T.K., and Y.-S.J.; formal analysis, T.T.K., T.-G.L., J.-W.P., H.J.H., J.S.M.; investigation, T.T.K., G.A.K.M.R.B., H.-J.K., T.-G.L., and Y.-S.J.; data curation, T.T.K., G.A.K.M.R.B., H.-J.K., T.-G.L., and Y.-S.J.; writing—original draft preparation, T.T.K.; writing—review and editing, T.T.K., G.A.K.M.R.B., T.-G.L., H.-J.K., H.K.S., J.-H.K., S.M.H., N.S., A.F., and Y.-S.J.; supervision, Y.-S.J.; funding acquisition, J.-H.K., and Y.-S.J. All authors have read and agreed to the published version of the manuscript. Funding: This research was financially supported by the R&D Convergence Program of NST (Na- tional Research Council of Science & Technology) of the Republic of Korea (CAP-15-02-KBSI), the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2019R1C1C1007745), and the National Research Foundation of Korea (NRF) grant funded by the Korean government (Ministry of Science, ICT & Future Planning) (No. 2019R1A4A2001527). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available within the article and the supplementary materials. Acknowledgments: This study was partly supported by the Joint Usage/Research Program of the Photocatalysis International Research Center, Research Institute for Science and Technology, Tokyo University of Science, Japan. Conflicts of Interest: The authors declare no conflict of interest.

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