Grey Rutile Tio2 with Long-Term Photocatalytic Activity Synthesized Via Two-Step Calcination

nanomaterials

Communication Grey Rutile TiO2 with Long-Term Photocatalytic Activity Synthesized Via Two-Step Calcination

Yan Liu, Ping Chen, Yaqi Fan, Yanfei Fan, Xifeng Shi, Guanwei Cui * and Bo Tang *

College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized Probes for Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education, Shandong Normal University, Jinan 250014, China; [email protected] (Y.L.); [email protected] (P.C.); [email protected] (Y.F.); [email protected] (Y.F.); [email protected] (X.S.) * Correspondence: [email protected] (G.C.); [email protected] (B.T.); Tel.: +86-135-8906-3951 (G.C.)

Received: 29 March 2020; Accepted: 5 May 2020; Published: 9 May 2020

Abstract: Colored titanium oxides are usually unstable in the atmosphere. Herein, a gray rutile titanium dioxide is synthesized by two-step calcination successively in a high-temperature reduction atmosphere and in a lower-temperature air atmosphere. The as-synthesized gray rutile TiO2 exhibits higher photocatalytic activity than that of white rutile TiO2 and shows high chemical stability. This is attributed to interior oxygen vacancies, which can improve the separation and transmission eﬃciency of the photogenerated carriers. Most notably, a formed surface passivation layer will protect the interior oxygen vacancies and provide long-term photocatalytic activity.

Keywords: gray color; rutile titanium dioxide; oxygen vacancy; photodecomposition; surface passivation

1. Introduction

Among titanium oxides, TiO2 is well investigated in the photocatalysis research field because of its high chemical stability, low cost, and nontoxicity [1]. However, it can only absorb ultraviolet light, resulting in low photocatalytic efficiency. To expand its light absorbance range and enhance the separation efficiency of the photogenerated carriers, many efforts, such as doping with other elements, sensitizing with dyes, and coupling with metal or nonmetal nanoparticles or different semiconductor materials, have been made to solve the aforementioned problems [2–7]. Very recently, TiO2 nanotubes synthesized via the electrochemical anodization of titanium foil exhibited visible light response characteristics for the photodecomposition of formaldehyde [8]. It has been reported that when TiO2 is partially reduced by H2 or CO, or bombarded by high-energy + particles (laser, electron, or Ar ), the obtained colored TiO2 powers show visible light photocatalytic activity. In 2010, a blue titanium dioxide with a mixture of anatase and rutile phase was synthesized via hydrolysis and the reduction of isopropyl titanium, showing a higher photocatalytic activity than that of commercial anatase TiO2. The higher photocatalytic activity was attributed to the presence of Ti3+ in the interior of the titanium dioxide crystal [9]. In 2011, Giamello et al. used an isotope labeling method to study the existence of Ti3+ in rutile titanium dioxide in detail [10]. In the same year, a black TiO2 with a strong absorption of visible light was synthesized via a high-temperature hydrogenation reduction of P25 TiO2 by Chen et al. The obtained higher photoactivity of the black TiO2 was attributed to the reduced band gap of titanium dioxide caused by the generation of the surface disordered structure [11]. Another kind of simple method to produce colored TiO2 is the addition of fluorine species during TiO2 preparation [12,13]. In 2014, Xu et al. synthesized stable blue TiO2 nanoparticles with a non-stoichiometric TiO2 x core and stoichiometric TiO2 shell structure for the − photodecomposition of methylene blue (MB) dyes under visible light irradiation [12]. In addition to

Nanomaterials 2020, 10, 920; doi:10.3390/nano10050920 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 920 2 of 11

the black- or blue-color TiO2, Ye et al. found that TiO2 nanocrystal assemblies show a yellow color, caused by the interfacial Ti–Ti electronic bonding. Until now, many outstanding works on colored TiO2 have been reported [14–28]. They usually showed a broader light absorbance range and higher photocatalytic activity than that of white TiO2. However, the mechanisms for the higher photocatalytic activity still remain a controversy. Some studies have suggested that it is ascribed to a surface disorder; other reports suggest that it is caused by the “oxygen vacancy” states associated with the Ti3+ within the band gap of the TiO2 [16,29,30]. Most notably, the reported colored titanium oxides are usually unstable in the atmosphere because of a large number of oxygen vacancies and the presence of Ti3+ with poor stability [24]. It is still a challenge to synthesize a stable colored TiO2 photocatalyst. In contrast to anatase TiO2, rutile TiO2 has attracted less attention in the photocatalytic research ﬁeld because of its low photocatalytic activity [31,32]. Herein, gray rutile (GR) titanium dioxide particles (marked as TiO2-GR), which are composed of microcrystals with oxygen vacancies on the surface, are synthesized by two-step calcination successively performed in a high-temperature reduction atmosphere and in a lower-temperature air atmosphere. The as-synthesized gray rutile TiO2 exhibits a higher photocatalytic activity than that of white rutile (WR) TiO2 (marked as TiO2-WR). This is attributed to the presence of oxygen vacancies, which can improve the photogenerated carrier separation and transmission eﬃciency. Most notably, the formed surface passivation layer will protect the interior oxygen vacancies and provide long-term photocatalytic activity.

2. Materials and Methods

2.1. Materials Hexanoic acid (HA), tetrabutyl titanate (TBOT), methylene blue (MB) dye, and glucose were purchased from Sinopharm Chemical Reagent Company. All chemicals were of AR grade. The ultrapure water used in the experiment was obtained from a Mill-Q (electric resistivity 18.2 MΩ cm) water · puriﬁcation system.

2.2. Synthesis of TiO2-GR and TiO2-WR

First, uniform spherical anatase TiO2 particles (Figure1a and Figure S1a) with a diameter of 200–300 nm were synthesized via a previous reported method [33]. In a typical process, hexanoic acid (0.46 g) dissolved in ethanol (230.0 mL), and TBOT (1.70 g, 10% ethanol solution) was mixed by stirring at room temperature. Then, 35.0 mL H2O was dropped into the mixture with vigorous stirring for 12 h at room temperature. The products were obtained after centrifugal separation and were then ready for use for the next two-step calcination procedure. Second, the as-prepared TiO2 nanosphere was ﬁrstly calcinated in a tubular high-temperature furnace with continuous argon ﬂow at 900 ◦C for 3 h. Then, it was further calcined at 500 ◦C in air atmosphere for 10 h, and gray rutile TiO2 particles with polyhedron morphology were obtained (TiO2-GR, Figure1b and Figure S1b). A white rutile TiO 2 used as a reference sample (TiO2-WR, Figure1c and Figure S1c) was prepared by the calcination of the as-prepared TiO 2 nanosphere at 900 ◦C for 3 h in an air atmosphere. The as-prepared photocatalysts were stored in an air atmosphere at room temperature.

2.3. Photocurrent Measurements The photocurrent measurements were carried out on an electrochemical analyzer (CHI660D Instruments, Shanghai Chenhua Instrument Co., Ltd., Shanghai, China) using a standard three-electrode system. The as-prepared samples, a commercial Pt gauze electrode (Gaoss Union Technology Co., Ltd., Wuhan, China, 2 cm 2 cm, 60 mesh), and saturated calomel electrode were used × as working electrodes, counter electrode, and reference electrode, respectively. The working electrode was prepared as follows: 0.05 g of the sample was ground with 0.10 g terpinol for 10 min to make uniform slurry. Then, the slurry was evenly dripped onto a 4.0 cm 1.0 cm indium tin oxide-coated × Nanomaterials 2020, 10, x FOR PEER REVIEW 3 of 11 smoothed by a doctor’s blade. Therefore, the formed film about had a thickness of 0.5 mm. Next, these electrodes were dried in an oven and were calcined at 350 °C for 30 min in an air atmosphere. The electrode was immersed in a 0.10 M NaClO4 aqueous solution to measure the transient photocurrent under a 300 W Xe arc lamp irradiation with an incident light power density of 130 mW/cm2 at 0.4 V vs. the saturated calomel electrode.

2.4. Photoactivity Measurements The photocatalytic discoloration of MB dyes was performed on a reformative XPA-7 photocatalytic reaction instrument（）Xujiang Electromechanical Plant, Nanjing, China . The incident light power was 162 mW/cm2, which was measured by a handheld Optical Power Meter (Newport 1916-R, Newport Corporation, California, CA, USA). The light exposure area of the quartz bottle was about 19.1 cm2. The discoloration effect was measured using the absorption spectroscopic technique. In the typical process, an aqueous solution of the MB dyes (10.0 mg/L and 30.0 mL) and 20.0 mg of the as-prepared photocatalysts were mixed in a 50 mL cylindrical quartz tube and left overnight in darkness to reach the adsorption equilibrium for the MB dyes. Then, the mixture was exposed to 1000 W Xe lamp irradiation with or without the light cutoff filters (λ > 420 nm), under ambient conditions and magnetic stirring. At given time intervals, the reaction solution was sampled and analyzed by a UV-visible spectrophotometer (UV 2250, Shimadzu, SHIMADZU (CHINA) Co., Ltd., Shanghai, China). Nanomaterials 2020, 10, 920 3 of 11 3. Results and Discussion

A spherical anatase TiO2 (Figure 1a and Figure S1a) in a white color was fabricated as the raw glass (ITO glass) electrode masked by an adhesive tape with thickness of 0.5 mm and smoothed by a material for the gray TiO2-GR via the hydrolysis of TBOT in the presence of alkyl chain carboxylic doctor’s blade. Therefore, the formed ﬁlm about had a thickness of 0.5 mm. Next, these electrodes acids [33]. It was determined that alkylchain carboxylic acids remained on the surface of the TiO2 were dried in an oven and were calcined at 350 ◦C for 30 min in an air atmosphere. The electrode was nanospheres, which were used as a reductant for the subsequent high-temperature reduction of immersed in a 0.10 M NaClO4 aqueous solution to measure the transient photocurrent under a 300 W titanium dioxide [4]. Both the gray rutile TiO2 and the reference sample (TiO2-WR) exhibited Xe arc lamp irradiation with an incident light power density of 130 mW/cm2 at 0.4 V vs. the saturated polyhedron morphology (TiO2-GR, Figure 1b and Figure S1b). calomel electrode.

Figure 1. Morphology of spherical anatase TiO (a), TiO -gray rutile (TiO -GR) (b), and TiO -white Figure 1. Morphology of spherical anatase TiO22 (a), TiO22-gray rutile (TiO22-GR) (b), and TiO22-white rutile (TiO -WR) (c). rutile (TiO2-WR) (c). 2.4. Photoactivity Measurements After calcination at 900 °C in an Ar atmosphere, it can be seen from the High Resolution TransmissionThe photocatalytic Electron Microscope discoloration (HRTEM) of MB dyes pattern was performedthat a surface on alayer reformative composed XPA-7 of a photocatalyticlarge number ofreaction microcrystals instrument(Xujiang surrounded by Electromechanical a disordered structure Plant, formed Nanjing, on the China). obtained The gray incident TiO2 (Figure light power S2a). 2 Thewas disordered 162 mW/cm structure, which is wasbelieved measured to be mainly by a handheld caused by Opticalthe presence Power of Meteroxygen (Newport vacancies, 1916-R, which areNewport response Corporation, for the black California, color of CA, TiO USA).2 [14, 30]. The Then, lightexposure after further area calcination of the quartz at bottle500 °C was in an about air 19.1 cm2. The discoloration effect was measured using the absorption spectroscopic technique. In the typical process, an aqueous solution of the MB dyes (10.0 mg/L and 30.0 mL) and 20.0 mg of the as-prepared photocatalysts were mixed in a 50 mL cylindrical quartz tube and left overnight in darkness to reach the adsorption equilibrium for the MB dyes. Then, the mixture was exposed to 1000 W Xe lamp irradiation with or without the light cutoff filters (λ > 420 nm), under ambient conditions and magnetic stirring. At given time intervals, the reaction solution was sampled and analyzed by a UV-visible spectrophotometer (UV 2250, Shimadzu, SHIMADZU (CHINA) Co., Ltd., Shanghai, China).

3. Results and Discussion

A spherical anatase TiO2 (Figure1a and Figure S1a) in a white color was fabricated as the raw material for the gray TiO2-GR via the hydrolysis of TBOT in the presence of alkyl chain carboxylic acids [33]. It was determined that alkylchain carboxylic acids remained on the surface of the TiO2 nanospheres, which were used as a reductant for the subsequent high-temperature reduction of titanium dioxide [4]. Both the gray rutile TiO2 and the reference sample (TiO2-WR) exhibited polyhedron morphology (TiO2-GR, Figure1b and Figure S1b). After calcination at 900 ◦C in an Ar atmosphere, it can be seen from the High Resolution Transmission Electron Microscope (HRTEM) pattern that a surface layer composed of a large number of microcrystals surrounded by a disordered structure formed on the obtained gray TiO2 (Figure S2a). The disordered structure is believed to be mainly caused by the presence of oxygen vacancies, which are response for the black color of TiO2 [14,30]. Then, after further calcination at 500 ◦C in an air atmosphere, Nanomaterials 2020, 10, 920 4 of 11 Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 11 atmosphere,a dense layer a with dense ordered layer latticewith ordered was formed lattice by was the refilling formed ofby oxygen the refilling atoms of into oxygen the oxygen atoms vacancies into the oxygenon the outmost vacancies layer on ofthe TiO outmost2 particles layer (Figure of TiO S2b).2 particles The formed (Figure dense S2b). layer The withformed the dense size of layer 2–5 nmwith is theon thesize outermost of 2–5 nm layer is on of the TiOoutermost2-GR particle, layer of which the TiO would2-GR act particle, as a surface which passivation would act layer as a to surface hinder passivationthe further di layerffusion to hinder and infiltration the further of oxygendiffusion molecules and infiltration into the interiorof oxygen oxygen molecules vacancies. into the As interior a result, oxygenthe interior vacancies. lattice disorderedAs a result, structure the interior would lattice be retained.disordered The structure lattice width would of be the retained. surface passivationThe lattice widthlayer is of 0.21 the nm, surface which passivation is ascribed tolayer the (210)is 0.21 crystal nm, which faces of is the ascribed rutile TiO to 2the(JCPDS (210) 21-1276;crystal faces Figure of S2b). the rutileHowever, TiO2 no(JCPDS such 21-1276; surface layerFigure structures S2b). However, wereobserved no such surface on the layer surface structures of the TiO were2-WR observed particles on (Figurethe surface S2c). of The the lattice TiO2-WR widths particles are 0.32 (Figure nm and S2c). 0.25 The nm, lattice which widths belong toare the 0.32 (110) nm and and (101) 0.25 crystal nm, which faces belongof the rutile to the TiO (110)2, respectively and (101) crystal (JCPDS faces 21-1276; of the Figurerutile TiO2a).2 The, respectively X-ray di ff (JCPDSraction 21-1276; (XRD) peaks Figure of 2a). the Thegray X-ray TiO2 centereddiffraction at (XRD) 2θ = 27.75 peaks◦, 36.4 of the◦, 39.45 gray◦ ,TiO 41.552 centered◦, 44.35◦ at, 54.6 2θ ◦=, 27.75°, 56.9◦, 63.05 36.4°,◦, 39.45°, 64.35◦, 41.55°, 69.25◦, 44.35°, 70.05◦, 54.6°,and 82.6 56.9°,◦ are 63.05°, ascribed 64.35°, to the 69.25°, (110), (101),70.05°, (200), and (111),82.6° (210),are ascribed (211), (220), to the (002), (110), (310), (101), (301), (200), (112), (111), and (210), (321) (211),crystal (220), planes (002), of the (310), rutile (301), TiO2 ,(112), respectively and (321) (Figure crystal2b), planes which of is consistentthe rutile TiO with2, therespectively referenced (Figure white 2b),rutile which TiO2 .is Both consistent of the XRDwith peaksthe referenced of TiO2-GR white and rutile TiO2 -WRTiO2. areBoth similar of the to XRD that peaks of the of standard TiO2-GR rutile and TiO2-WR(PDF# are 87-0710). similar to The that Rietveld of the standard analysis rutile (TOPAS TiO V2 (PDF# 6.0) of 87-0710). the XRD The patterns Rietveld shows analysis that TiO(TOPAS2-GR Vhas 6.0) an of average the XRD particle patterns size shows of 48.5 that nm. TiO This2-GR result has an is average different particle from that size intuitively of 48.5 nm. observed This result from is differentthe HRTEM from patterns, that intuitively which is observed attributed from to thethe diHRTEMfferent patterns, detection which areas betweenis attributed XRD to andthe different HRTEM. detectionHerein, the areas HRTEM between patterns XRD are and mainly HRTEM. afforded Herein, the surfacethe HRTEM layer patterns crystal structure are mainly of TiO afforded2 particles. the Therefore,surface layer it cancrystal be inferred structure that of theTiO as-synthesized2 particles. Therefore, TiO2-GR it can nanoparticles be inferred are that mainly the as-synthesized composed of TiOmicrocrystals2-GR nanoparticles with an average are mainly size ofcomposed about 48.5 of nm, microcrystals while their with surface an average layers are size composed of about of 48.5 smaller nm, whilemicrocrystals. their surface Additionally, layers are the composed analysis resultsof smaller indicate microcrystals. that the broadening Additionally, of the the di ffanalysisraction peakresults is indicatemainlydue thatto the grain broadening refinement, of the and diffraction there is nopeak existing is mainly microstrain. due to grain However, refinement, compared and there with is theno existingcell parameters microstrain. of the However, standard rutilecompared TiO2 with(PDF# th 87-0710),e cell parameters both TiO 2of-GR the and standard TiO2-WR rutile show TiO a2 (PDF# lattice 87-0710),expansion, both with TiO average2-GR and lattice TiO distortions2-WR show ofa lattice 0.11% expansion, and 0.13%, with respectively. average Itlattice is proposed distortions that of this 0.11% is andmainly 0.13%, caused respectively. by the diff Iterent is proposed treatments that during this is the mainly high-temperature caused by the calcination different treatments process. No during peaks thecentered high-temperature at 2θ = 25.9 ◦calcination, ascribed toprocess. carbon No (JCPDS peaks 26-1079),centered at were 2θ = observed 25.9°, ascribed in the to XRD carbon patterns (JCPDS of 26-1079),gray TiO2 were[34]. observed in the XRD patterns of gray TiO2 [34].

Figure 2. HRTEM ( a) and XRD patterns (b) of gray rutile TiO22.

The chemical state of the surface species of TiO -GR and TiO -WR was determined by X-ray The chemical state of the surface species of TiO22-GR and TiO22-WR was determined by X-ray photoelectron spectroscopy (XPS), which was further analyzed by an XPS peak-ﬁttingpeak-fitting program (version 4.0, Hong Kong, China). The C1s XPS peaks of TiO -GR centered at 284.6 eV (FWHM = 4.55 eV) (version 4.0, Hong Kong, China). The C1s XPS peaks of TiO2 2-GR centered at 284.6 eV (FWHM = 4.55 and 282.9 eV (FWHM = 1.96 eV), which is similar to that of TiO -WR, were ascribed to the *C and eV) and 282.9 eV (FWHM = 1.96 eV), which is similar to that of TiO2 2-WR, were ascribed to the *C and (*CO)Ti species caused byby thethe carboncarbon contaminantcontaminant (Figure(Figure3 3a,b)a,b) [[35,36].35,36]. It It was was reported reported that if a carbon atom was doped in the crystal lattice of TiO , a bonding energy peak ascribed to C* or Ti*–C carbon atom was doped in the crystal lattice of TiO2, a bonding energy peak ascribed to C* or Ti*–C emerged atat 281.6281.6 eV eV or or 454.90 454.90 eV, eV, respectively respectively [37 ,38[37,38].]. However, However, no such no such carbon carbon bonding bonding energy energy peaks were observed for either TiO -GR or TiO -WR. This indicates that there were no carbon atoms doped peaks were observed for either2 TiO2-GR 2or TiO2-WR. This indicates that there were no carbon atoms doped in the crystal lattice of the prepared gray TiO2, which means that the gray color did not originate from the carbon residues. As shown in Figure 3c,d, the Ti2p XPS peaks of TiO2-GR were

Nanomaterials 2020, 10, 920 5 of 11

in the crystal lattice of the prepared gray TiO2, which means that the gray color did not originate Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 11 from the carbon residues. As shown in Figure3c,d, the Ti2p XPS peaks of TiO 2-GR were centered at 464.53 eV (FWHM = 2.84 eV) and 458.20 eV (FWHM = 3.82 eV), which are ascribed to Ti4+ 2p and centered at 464.53 eV (FWHM = 2.84 eV) and 458.20 eV (FWHM = 3.82 eV), which are ascribed to1 Ti/2 4+ Ti4+ 2P of TiO , respectively [39], which is similar to that of TiO -WR. Two reduced titanium ion 2p1/2 3and/2 Ti4+ 2P2 3/2 of TiO2, respectively [39], which is similar to that2 of TiO2-WR . Two reduced XPStitanium peaks centeredion XPS peaks at 462.43 centered eV (FWHM at 462.43= 3.91eV (FWHM eV) and = 455.923.91 eV) eV and (FWHM 455.92= eV2.63 (FWHM eV) were = 2.63 observed eV) forwere TiO 2-GR,observed which for could TiO2 be-GR, ascribed which tocould the lowbe valenceascribed state to the titanium low valence of nonstoichiometric state titanium TiO of2 x 3+ 2+ − (0 nonstoichiometric< X < 2), mainly TiO including2−x (0 < X Ti < 2),2p mainly1/2 of Tiincluding2O3 and Ti Ti3+ 2p2p1/2 3of/2 Tiof2O TiO3 and [40 Ti,412+ ],2p which3/2 of TiO is consistent[40,41], withwhich the Ois 1sconsistent XPS peak with results. the O However, 1s XPS peak the TiOresults.2-WR However, showed nothe reducedTiO2-WR titanium showed ionno XPSreduced peaks. Thetitanium O 1s XPS ion peaksXPS peaks. mainly The consisted O 1s XPS ofpeaks three mainly components consisted (Figure of three3e,f). components The two (Figure peaks centered3e,f). The at 529.47two peaks eV (FWHM centered= 3.30at 529.47 eV) andeV (FWHM 531.58 eV= 3.30 (FWHM eV) and= 4.48531.58 eV) eV were (FWHM ascribed = 4.48 toeV) the were lattice ascribed oxygen ofto the the stoichiometric lattice oxygen TiO of 2the[42 stoichiometric] and nonstoichiometric TiO2 [42] and TiO nonstoichiometric2 x (0 < X < 2) [43 TiO,442],−x respectively,(0 < X < 2) [43,44], and the − latterrespectively, may also and include the latter some may hydroxyl also include oxygen some species hydroxyl [45]. Theoxygen small species O 1s [45]. peak The centered small O at 1s 527.51 peak eV (FWHMcentered= 2.21at 527.51 eV) couldeV (FWHM be attributed = 2.21 eV) to could the attached be attributed ionic to oxygen the attached of CO ionic or O 2oxygen[46]. of CO or O2 [46].

Figure 3. C 1s XPS spectra of TiO2-GR (a) and TiO2-WR (b); Ti 2p XPS spectra of TiO2-GR (c) and Figure 3. C 1s XPS spectra of TiO2-GR (a) and TiO2-WR (b); Ti 2p XPS spectra of TiO2-GR (c) and TiO2- TiO2-WR (d); O 1s XPS spectra of TiO2-GR (e) and TiO2-WR (f). WR (d); O 1s XPS spectra of TiO2-GR (e) and TiO2-WR (f). It has been reported that the produced Ti3+ originated from the oxygen vacancies on the surface It has been reported that the produced Ti3+ originated from the oxygen vacancies on the surface of the gray TiO2. The removed oxygen atoms left behind two excess electrons per oxygen vacancy, of the gray TiO2. The removed oxygen atoms left behind two excess electrons per oxygen vacancy, which could be harvested by the neighboring Ti atoms, and induce the formation of Ti3+ ions showing which could be harvested by the neighboring Ti atoms, and induce the formation of Ti3+ ions showing EPREPR signals signals [47 [47].]. Therefore,Therefore, ElectronElectron Paramagnetic Resonance Resonance (EPR) (EPR) is isone one powerful powerful method method for for identifyingidentifying the the presence presence of of oxygen oxygen vacanciesvacancies in so solidlid materials. materials. A A low-fiel low-ﬁeldd signal signal with with a g-value a g-value closeclose to to the the free-electron free-electron value value ( g(g= = 2.0023) is generally generally attributed attributed to to an an unpaired unpaired electron electron trapped trapped on on anan oxygen oxygen vacancy vacancy site site [ 11[11].]. Herein,Herein, as shown in in Figu Figurere 4,4, an an EPR EPR signal signal with with a ag-value g-value of of1.997 1.997 is is 3+ attributedattributed to to the the Ti Ti3+centers centers inin thethe rutilerutile phase environment. environment. As As a acomparison, comparison, there there were were no noEPR EPR peakspeaks at at the the same same positionposition observed from from the the TiO TiO2-WR2-WR EPR EPR signals. signals. It is believed It is believed that the that surface the surface Ti3+ 3+ Ti wouldwould tend tend to adsorb to adsorb atmospheric atmospheric O2, which O2, which would would be reduced be reduced to O2− to, and O2− shows, and showsan EPR an signal EPR at signal g at ≈g 2.022.02 [11].[11 The]. absence The absence of such of a suchpeak in a peakthe TiO in2-GR the EPR TiO signals-GR EPR indicates signals that indicates after long that calcination after long ≈ 2 calcinationin an air atmosphere, in an air atmosphere, the surface oxygen the surface vacancies oxygen are vacanciesrefilled by areoxygen reﬁlled atoms by and oxygen the Ti atoms3+ is mainly and the Ti3present+ is mainly under present the formed under surface the formed passivation surface layer, passivation which is layer,proposed which as a is key proposed factor for as the a key observed factor for 2 theexcellent observed stability excellent of TiO stability-GR. of TiO2-GR.

Nanomaterials 2020, 10, 920 6 of 11 Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 11

Figure 4. EPREPR spectroscopy of TiO 2-GR-GR (red (red line) line) and and TiO TiO2-WR-WR (black (black line). line).

The photocatalytic activity of the as-prepared TiO2-GR-GR was was determined by the photocatalytic discoloration of the MB dyes. As As sh shownown in in Figure Figure 55a,b,a,b, thethe graygray TiOTiO2 showedshowed a a higher higher photoactivity photoactivity thanthan thatthat ofof TiOTiO2-WR2-WR under under visible visible light light or or full-spectrum full-spectrum light light irradiation. irradiation. Herein, Herein, different different from from most mostof the of previously the previously reported reported colored colored TiO2 TiOmaterials,2 materials, the as-synthesizedthe as-synthesized gray gray TiO TiO2 showed2 showed a long-life a long- lifephotocatalytic photocatalytic activity activity (Figure (Figure5c,d). 5c,d). The TiOThe 2TiO-GR2-GR samples samples retained retained their their color color and propertiesand properties even evenafter sixafter months six months of storage, of storage, and did and not did exhibit not exhi anybit reduction any reduction in their in photocatalytic their photocatalytic activity afteractivity six afterphotocatalysis six photocatalysis cycles. This cycles. indicates This indicates that the graythat the TiO gray2 has TiO an excellent2 has an excellent chemical chemical stability, stability, which is whichascribed is ascribed to the special to the special nanostructure nanostructure caused caused by the by two-step the two-step calcination calcination treatment. treatment. As As shown shown in inFigure Figure S2b, S2b, the the formed formed surface surface passivation passivation layer laye willr protectwill protect the interior the interior oxygen oxygen vacancies. vacancies. As a result, As a 3+ result,the T theon theT3+ moston the superficial most superficial layer will disappear,layer will whichdisappear, has been which confirmed has been by theconfirmed aforementioned by the aforementionedEPR results. The thermalEPR results. stability The of TiOthermal2-GR wasstability further of studied TiO2-GR by Thermogravimetric was further studied Analyzer by Thermogravimetric(TGA) in open air, as Analyzer shown in (TGA) Figure in S3. open The air, sample as shown was thermallyin Figure S3. stable The up sample to 650 was◦C in thermally open air, stablewith negligible up to 650 weight°C in open variation. air, with The negligible slight weight weight gain variation. and loss waveThe slight before weight 125 ◦C gain are ascribedand loss wave to the beforeadsorption 125 °C and are desorption ascribed to of the O2 adsorption, CO2, or H 2andO on desorption the surface of of O2 TiO, CO2-GR2, or inH2 theO on air. the A surface distinguishable of TiO2- GRweight in the loss air. from A distinguishable 380 ◦C is ascribed weight to the loss dissociation from 380 °C of theis ascribed surface to –OH. the Abovedissociation 650 ◦C, of thethe obvioussurface –OH.weight Above increase 650 °C, is ascribed the obvious to theweight refilled increase interior is ascribed oxygen to vacancies, the refilled indicating interior oxygen that the vacancies, surface indicatingpassivation that layer the would surface be passivation destroyed layer at this would temperature. be destroyed This showsat this thattemperature. the as-prepared This shows TiO2 that-GR thehas as-prepared high thermal TiO stability.2-GR has high thermal stability. A higher photocatalytic activity is attributed to the presence of oxygen vacancies that can create a higher light absorbance and improve the separation and transmission efficiency of photogenerated carriers, which is preliminarily confirmed by the UV-vis spectra, photocurrent, and photoluminescence spectra. The suggested photocatalysis mechanism is shown in Figure6a. It is proposed that the photocatalysis of TiO2-GR may undergo two different photogenerated carrier transfer pathways when pumped by UV light and visible light separately. It can be seen, in both pathways, that the oxygen vacancies all play a vital role. Compared with TiO2-WR, the as-prepared TiO2-GR exhibits a broad spectral absorption in the visible light region (Figure6b). This can be attributed to the transitions from the TiO2 valence band to the oxygen vacancy levels, or from the oxygen vacancies to the TiO2 conduction band pumped by visible light [30], which is responsible for the distinguishable higher photoactivity of TiO2-GR than that of TiO2-WR. These results are consistent with the photocurrent density results under visible light irradiation (Figure6c). This indicates that, in this case, the visible light absorption of TiO2-GR does lead to charge carrier generation and contributes directly to the photocurrent. However, as shown in Figure6c,d, the photocurrent density under a full-spectrum light condition is about 100 times that under a visible light condition, which indicates that the contribution of visible light to the improvement of the photocatalytic activity is very limited. This is consistent

Nanomaterials 2020, 10, 920 7 of 11 with previously reported results [48]. Therefore, it is proposed that the main factor for the higher photocatalytic activity of TiO2-GR is the photogenerated carrier transfer path pumped by UV light. [49] In this process, the Vo is still proposed to be a key factor for the improvement of the separation efficiency of the photogenerated carriers. First, the Vo can act as a trap site for the temporary storage of electrons, which can be further pumped to the conduction band to react with the substrates, resulting in suppressed recombination of photogenerated carriers. [50]. Herein, the suppressed recombination of photogenerated carriers is preliminarily confirmed by the photoluminescence spectra. If the recombination of carriers was suppressed, the photoluminescence of semiconductor materials would be quenched to some degree [51,52]. As shown in Figure6e, compared with the reference TiO 2-WR sample, the TiO2-GR samples show a much lower photoluminescence intensity. This indicates that the as-prepared gray TiO2-GR has a much higher photoinduced charge separation efficiency than that of the white TiO2-GR materials. In addition, because of the presence of free electrons bound loosely to the titanium atom in the oxygen vacancies [47], the surface electric conductivity of TiO2-GR will be improved, as a result of improving the carriers’ transmission efficiency [26], which is also helpful for improving the photocatalytic activity. Nanomaterials 2020, 10, x FOR PEER REVIEW 7 of 11

FigureFigure 5.5. ((a) MethyleneMethylene blue blue (MB) (MB) dye dye photocatalytic photocatalytic discoloration discoloration plots plots of ofTiO TiO2-GR2-GR and and TiO TiO2-WR2-WR

samplessamples underunder full-spectrumfull-spectrum lightlight irradiation;irradiation; ((b)) MBMB dyedye photocatalyticphotocatalytic discoloration plots of TiO2--GR andGR and TiO 2TiO-WR2-WR samples samples under under visible visible light light irradiation; irradiation; ( c)) photocatalytic photocatalytic activity activity tests tests of ofTiO TiO2-GR2-GR samplessamples storedstored for different diﬀerent times times via via the the photocatal photocatalyticytic discoloration discoloration of ofMB MB dyes dyes under under full-spectrum full-spectrum lightlight irradiation;irradiation; ( d)) photocatalytic photocatalytic activity activity cycle cycle tests tests of of TiO TiO2-GR2-GR samples samples under under full-spectrum full-spectrum light light irradiation.irradiation. TheThe aqueousaqueous solutionsolution ofof thethe MB MB dyes dyes (10.0 (10.0 mg mg/L,/L, 30.0 30.0 mL) mL) without without photocatalysts photocatalysts was was used asused the as control the control sample sample marked marked as a blank as ina blank Figure in5a,b. Figure C /C 5a,b.0 is the C/C ratio0 is ofthe the ratio real-time of the concentration real-time toconcentration the initial concentration to the initial concentration of MB dyes. of MB dyes.

A higher photocatalytic activity is attributed to the presence of oxygen vacancies that can create a higher light absorbance and improve the separation and transmission efficiency of photogenerated carriers, which is preliminarily confirmed by the UV-vis spectra, photocurrent, and photoluminescence spectra. The suggested photocatalysis mechanism is shown in Figure 6a. It is proposed that the photocatalysis of TiO2-GR may undergo two different photogenerated carrier transfer pathways when pumped by UV light and visible light separately. It can be seen, in both pathways, that the oxygen vacancies all play a vital role. Compared with TiO2-WR, the as-prepared TiO2-GR exhibits a broad spectral absorption in the visible light region (Figure 6b). This can be attributed to the transitions from the TiO2 valence band to the oxygen vacancy levels, or from the oxygen vacancies to the TiO2 conduction band pumped by visible light [30], which is responsible for the distinguishable higher photoactivity of TiO2-GR than that of TiO2-WR. These results are consistent with the photocurrent density results under visible light irradiation (Figure 6c). This indicates that, in this case, the visible light absorption of TiO2-GR does lead to charge carrier generation and contributes directly to the photocurrent. However, as shown in Figure 6c,d, the photocurrent density under a full-spectrum light condition is about 100 times that under a visible light condition, which indicates that the contribution of visible light to the improvement of the photocatalytic activity is very limited. This is consistent with previously reported results [48]. Therefore, it is proposed that the main factor for the higher photocatalytic activity of TiO2-GR is the photogenerated carrier transfer path pumped by UV light. [49] In this process, the Vo is still proposed to be a key factor for the improvement of the separation efficiency of the photogenerated carriers. First, the Vo can act as a trap site for the temporary storage of electrons, which can be further pumped to the conduction band to react with the substrates, resulting in suppressed recombination of photogenerated carriers. [50]. Herein, the suppressed recombination of photogenerated carriers is preliminarily confirmed by the photoluminescence spectra. If the recombination of carriers was suppressed, the photoluminescence of semiconductor materials would be quenched to some degree [51, 52]. As shown in Figure 6e,

Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 11

compared with the reference TiO2-WR sample, the TiO2-GR samples show a much lower photoluminescence intensity. This indicates that the as-prepared gray TiO2-GR has a much higher photoinduced charge separation efficiency than that of the white TiO2-GR materials. In addition, because of the presence of free electrons bound loosely to the titanium atom in the oxygen vacancies Nanomaterials[47], the 2020surface, 10, 920 electric conductivity of TiO2-GR will be improved, as a result of improving 8the of 11 carriers’ transmission efficiency [26], which is also helpful for improving the photocatalytic activity.

FigureFigure 6. (6.a) ( Schematica) Schematic illustration illustration of of the the photocatalysis photocatalysis mechanismmechanism of of TiO TiO22-GR;-GR; (b (b) )UV-vis UV-vis diffuse diﬀuse reﬂectancereflectance spectra spectra of of TiO TiO2-GR2-GR and and TiO TiO2-WR;2-WR; ( c(c)) photocurrent photocurrent densitydensity of TiO TiO22-GR-GR and and TiO TiO2-WR2-WR under under visiblevisible light light irradiation; irradiation; (d ()d photocurrent) photocurrent density density of of TiO TiO22-GR-GR andand TiO2-WR-WR under under full-spectrum full-spectrum light light irradiation;irradiation; (e) (e) photoluminescence photoluminescence spectra spectra of of TiO TiO2-GR2-GR andand TiOTiO22-WR with with an an excitation excitation wavelength wavelength of of 380380 nm. nm.

4. Conclusions4. Conclusion InIn summary, summary, gray rutile rutile titanium titanium dioxide dioxide was synthesized was synthesized via two-step via calcination, two-step performed calcination, performedsuccessively successively in a high-temperature in a high-temperature reduction reduction atmosphere atmosphere and andin a in lower-temperature a lower-temperature air air atmosphere.atmosphere. The The results results indicate indicate that, that, compared compared with wi theth white the white rutile titaniumrutile titanium dioxide, dioxide, the as-prepared the as- prepared gray titanium dioxide exhibits the typical characteristics of black- or blue-color TiO2, such gray titanium dioxide exhibits the typical characteristics of black- or blue-color TiO2, such as the presenceas the ofpresence Ti ions of in Ti a lowions valence in a low state, valence surface state, disorder surface structure,disorder structure, and oxygen and vacancies, oxygen vacancies, which are causedwhich by are the caused loss of by oxygen the loss atoms of oxygen under atoms reduction under reactionreduction conditions. reaction conditions. According According to previous to 3+ reportsprevious [14], reports it is proposed [14], it is that proposed the presence that the of presence Ti3+ or a of surface Ti or disorder a surface structure disorder isstructure mainly inducedis mainly by induced by oxygen vacancies. The as-synthesized gray titanium dioxide exhibits a higher oxygen vacancies. The as-synthesized gray titanium dioxide exhibits a higher photocatalytic activity photocatalytic activity than does white rutile TiO2. This is attributed to the interior vacancies, which than does white rutile TiO . This is attributed to the interior vacancies, which can create a higher can create a higher light2 absorbance and improve the separation and transmission efficiency of light absorbance and improve the separation and transmission eﬃciency of photogenerated carriers. photogenerated carriers. Most notably, it is proposed that the two-step calcination can produce a Most notably, it is proposed that the two-step calcination can produce a surface passivation layer on surface passivation layer on the surface of gray titanium dioxide particles, as a result of protecting thethe surface interior of grayoxygen titanium vacancies, dioxide which particles, provides as long-term a result ofphotocatalytic protecting the activity. interior This oxygen study provides vacancies, whicha considerable provides long-term reference for photocatalytic the design and activity. synthesi Thiss of study other providessemiconductor a considerable photocatalysts reference rich in for theoxygen design vacancies, and synthesis with ofhigh other activity semiconductor and high stability. photocatalysts rich in oxygen vacancies, with high activity and high stability. Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1: Figure S1: Morphology Supplementaryof spherical anatase Materials: TiO2 The(a), TiO following2-GR (b), are and available TiO2-WR online (c). Figure at http: S2:// HRTEMwww.mdpi.com of TiO2-only/2079-4991 calcination/10/ 5at/920 900/s1 : Figure S1: Morphology of spherical anatase TiO (a), TiO -GR (b), and TiO -WR (c). Figure S2: HRTEM of °C in Ar atmosphere (a), TiO2-GR (b), and TiO2-WR2 (c). Figure2 S3: TGA curve in2 open air for the TiO2-GR sample. TiO2-only calcination at 900 ◦C in Ar atmosphere (a), TiO2-GR (b), and TiO2-WR (c). Figure S3: TGA curve in open air for the TiO2-GR sample. Author Contributions: Conceptualization, G.C. and B.T.; methodology, G.C. and Y.L.; validation, Y.F. (Yanfei Fan), and X.S.; investigation, Y.L., P.C., and Y.F. (Yaqi Fan); writing (original draft preparation), Y.L.; writing (review and editing), G.C. and B.T.; supervision, B.T.; project administration, B.T.; funding acquisition, B.T. All of the authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (21927811, 21575082, 21535004, 91753111, and 21976110) and the Key Research and Development Program of Shandong Province (2018YFJH0502), the development plan of science and technology for Shandong Province of China (2013GGX10706), and a project of the Shandong Province Higher Educational Science and Technology Program (J13LD06). Nanomaterials 2020, 10, 920 9 of 11

Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

References

1. Chen, X.; Mao, S.S. Titanium Dioxide Nanomaterials: Synthesis, Properties, Modiﬁcations, and Applications. Chem. Rev. 2007, 107, 2891–2959. [CrossRef][PubMed] 2. Kubacka, A.; Fernández-García, M.; Colón, G. Advanced Nanoarchitectures for Solar Photocatalytic Applications. Chem. Rev. 2012, 112, 1555–1614. [CrossRef][PubMed] 3. Xie, M.; Fu, X.; Jing, L.; Luan, P.; Feng, Y.; Fu, H. Long-Lived, Visible-Light-Excited Charge Carriers of

TiO2/BiVO4 Nanocomposites and their Unexpected Photoactivity for Water Splitting. Adv. Energy Mater. 2014, 4, 1300995. [CrossRef] 4. Cui, G.; Wang, W.; Ma, M.; Zhang, M.; Xia, X.; Han, F.; Shi, X.; Zhao, Y.; Dong, Y.; Tang, B. Rational design of

carbon and TiO2 assembly materials: Covered or strewn, which is better for photocatalysis? Chem. Commun. 2013, 49, 6415. [CrossRef] 5. Linh, V.T.N.; Xiao, X.; Jung, H.S.; Giannini, V.; Maier, S.A.; Kim, D.-H.; Lee, Y.-I.; Park, S.-G. Compact

Integration of TiO2 Nanoparticles into the Cross-Points of 3D Vertically Stacked Ag Nanowires for Plasmon-Enhanced Photocatalysis. Nanomaterials 2019, 9, 468. [CrossRef] 6. Zhang, Z.; Liu, H.; Wang, X.; Zhang, J.; Yu, M.; Ramakrishna, S.; Long, Y. One-Step Low

Temperature Hydrothermal Synthesis of Flexible TiO2/PVDF@MoS2 Core-Shell Heterostructured Fibers for Visible-Light-Driven Photocatalysis and Self-Cleaning. Nanomaterials 2019, 9, 431. [CrossRef]

7. Jiang, X.; Xu, W.; Yu, L. Photocatalytic Decomposition of Gaseous HCHO over Ag Modiﬁed TiO2 Nanosheets at Ambient Temperature. Nanomaterials 2019, 9, 338. [CrossRef] 8. Sahrin, N.T.; Nawaz, R.; Kait, C.F.; Lee, S.L.; Wirzal, M.D.H. Visible Light Photodegradation of Formaldehyde

over TiO2 Nanotubes Synthesized via Electrochemical Anodization of Titanium Foil. Nanomaterials 2020, 10, 128. [CrossRef] 9. Zuo, F.; Wang, L.; Wu, T.; Zhang, Z.; Borchardt, D.; Feng, P. Self-Doped Ti3+ Enhanced Photocatalyst for Hydrogen Production under Visible Light. J. Am. Chem. Soc. 2010, 132, 11856–11857. [CrossRef] 10. Livraghi, S.; Maurelli, S.; Paganini, M.C.; Chiesa, M.; Giamello, E. Probing the Local Environment of Ti3+ 17 Ions in TiO2 (Rutile) by O HYSCORE. Angew. Chem. Int. Ed. 2011, 50, 8038–8040. [CrossRef] 11. Chen, X.; Liu, L.; Yu, P.Y.; Mao, S.S. Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals. Science 2011, 331, 746–750. [CrossRef][PubMed]

12. Zhu, Q.; Peng, Y.; Lin, L.; Fan, C.-M.; Gao, G.-Q.; Wang, R.-X.; Xu, A.-W. Stable blue TiO2 x nanoparticles for − eﬃcient visible light photocatalysts. J. Mater. Chem. A 2014, 2, 4429–4437. [CrossRef] 13. Bellardita, M.; Garlisi, C.; Ozer, L.Y.; Venezia, A.M.; Sá, J.; Mamedov, F.; Palmisano, L.; Palmisano, G.

Highly stable defective TiO2 x with tuned exposed facets induced by ﬂuorine: Impact of surface and bulk − properties on selective UV/visible alcohol photo-oxidation. Appl. Surf. Sci. 2020, 510, 145419. [CrossRef] 14. Naldoni, A.; Allieta, M.; Santangelo, S.; Marelli, M.; Fabbri, F.; Cappelli, S.; Bianchi, C.L.; Psaro, R.; Santo, V.D.

The eﬀect of nature and location of defects on bandgap narrowing in black TiO2 nanoparticles. J. Am. Chem. Soc. 2012, 134, 7600–7603. [CrossRef][PubMed] 15. Hamdy, M.S.; Amrollahi, R.; Mul, G. Surface Ti3+-Containing (blue) Titania: A Unique Photocatalyst with High Activity and Selectivity in Visible Light-Stimulated Selective Oxidation. ACS Catal. 2012, 2, 2641–2647. [CrossRef]

16. Hu, Y.H. A Highly Eﬃcient Photocatalyst-Hydrogenated Black TiO2 for the Photocatalytic Splitting of Water. Angew. Chem. Int. Ed. 2012, 51, 12410–12412. [CrossRef] 17. Grabstanowicz, L.R.; Gao, S.; Li, T.; Rickard, R.M.; Rajh, T.; Liu, D.-J.; Xu, T. Facile Oxidative Conversion of 3+ TiH2 to High-Concentration Ti -Self-Doped Rutile TiO2 with Visible-Light Photoactivity. Inorg. Chem. 2013, 52, 3884–3890. [CrossRef] 18. Zhou, W.; Li, W.; Wang, J.; Qu, Y.; Yang, Y.; Xie, Y.; Zhang, K.; Wang, L.; Fu, H.; Zhao, D. Ordered Mesoporous

Black TiO2 as Highly Eﬃcient Hydrogen Evolution Photocatalyst. J. Am. Chem. Soc. 2014, 136, 9280–9283. [CrossRef] Nanomaterials 2020, 10, 920 10 of 11

19. Liu, N.; Schneider, C.; Freitag, D.; Hartmann, M.; Venkatesan, U.; Müller, J.; Spiecker, E.; Schmuki, P. Black

TiO2 Nanotubes: Cocatalyst-Free Open-Circuit Hydrogen Generation. Nano Lett. 2014, 14, 3309–3313. [CrossRef] 20. Zhao, Z.; Zhang, X.; Zhang, G.; Liu, Z.; Qu, D.; Miao, X.; Feng, P.; Sun, Z. Eﬀect of defects on photocatalytic

activity of rutile TiO2 nanorods. Nano Res. 2015, 8, 4061–4071. [CrossRef] 21. Liu, N.; Haublein, V.; Zhou, X.; Venkatesan, U.; Hartmann, M.; Mackovic, M.; Nakajima, T.; Spiecker, E.;

Osvet, A.; Frey, L.; et al. “Black” TiO2 Nanotubes Formed by High-Energy Proton Implantation Show Noble-Metal-co-Catalyst Free Photocatalytic H2-Evolution. Nano Lett. 2015, 15, 6815–6820. [CrossRef] [PubMed] 3+ 22. Chen, J.; Song, W.; Hou, H.; Zhang, Y.; Jing, M.; Jia, X.; Ji, X. Ti Self-Doped Dark Rutile TiO2 Ultraﬁne Nanorods with Durable High-Rate Capability for Lithium-Ion Batteries. Adv. Funct. Mater. 2015, 25, 6793–6801. [CrossRef] 23. Tian, M.; Mahjouri-Samani, M.; Eres, G.; Sachan, R.; Yoon, M.; Chisholm, M.F.; Wang, K.; Puretzky, A.A.;

Rouleau, C.M.; Geohegan, D.B.; et al. Structure and Formation Mechanism of Black TiO2 Nanoparticles. ACS Nano 2015, 9, 10482–10488. [CrossRef][PubMed] 24. Chen, S.; Tao, J.; Tao, H.; Wang, C.; Shen, Y.; Jiang, J.; Zhu, L.; Zeng, X.; Wang, T. One-Step Solvothermal

Synthesis of Black TiO2 Films for Enhanced Visible Absorption. J. Nanosci. Nanotechnol. 2016, 16, 3146–3149. [CrossRef][PubMed] 25. Chen, X.; Zhao, D.; Liu, K.; Wang, C.; Liu, L.; Li, B.; Zhang, Z.; Shen, D. Laser-Modiﬁed Black Titanium Oxide Nanospheres and Their Photocatalytic Activities under Visible Light. ACS Appl. Mater. Interfaces 2015, 7, 16070–16077. [CrossRef][PubMed] 26. Lv, X.; Chen, A.; Luo, Y.; Lu, P.; Dai, Y.; Enriquez, E.; Dowden, P.; Xu, H.; Kotula, P.G.; Azad, A.K.; et al.

Conducting Interface in Oxide Homojunction: Understanding of Superior Properties in Black TiO2. Nano Lett. 2016, 16, 5751–5755. [CrossRef] 3+ 27. Zhou, Y.; Chen, C.; Wang, N.; Li, Y.; Ding, H. Stable Ti Self-Doped Anatase-Rutile Mixed TiO2 with Enhanced Visible Light Utilization and Durability. J. Phys. Chem. C 2016, 120, 6116–6124. [CrossRef]

28. Chen, S.; Xiao, Y.; Wang, Y.; Hu, Z.; Zhao, H.; Xie, W. A Facile Approach to Prepare Black TiO2 with Oxygen Vacancy for Enhancing Photocatalytic Activity. Nanomaterials 2018, 8, 245. [CrossRef] 29. Chen, X.; Liu, L.; Liu, Z.; Marcus, M.A.; Wang, W.; Oyler, N.A.; Grass, M.E.; Mao, B.; Glans, P.-A.; Yu, P.Y.; et al. Properties of Disorder-Engineered Black Titanium Dioxide Nanoparticles through Hydrogenation. Sci. Rep. 2013, 3, 1510. [CrossRef]

30. Yu, P.; Zhang, J. Some interesting properties of black hydrogen-treated TiO2 nanowires and their potential application in solar energy conversion. Sci. China Chem. 2015, 58, 1810–1815. [CrossRef]

31. Li, X.; Xiong, Y.; Li, Z.; Xie, Y. Large-Scale Fabrication of TiO2 Hierarchical Hollow Spheres. Inorg. Chem. 2006, 45, 3493–3495. [CrossRef][PubMed] 32. Andersson, M.; Osterlund, L.; Ljungstrom, S.; Palmqvist, A. Preparation of Nanosize Anatase and Rutile

TiO2 by Hydrothermal Treatment of Microemulsions and Their Activity for Photocatalytic Wet Oxidation of Phenol. J. Phys. Chem. B 2002, 106, 10674–10679. [CrossRef] 33. Liu, S.; Han, G.; Shu, M.; Han, L.; Che, S. Monodispersed inorganic/organic hybrid spherical colloids: Versatile synthesis and their gas-triggered reversibly switchable wettability. J. Mater. Chem. 2010, 20, 10001–10009. [CrossRef]

34. Li, Y.; Zhu, S.; Liu, Q.; Gu, J.; Guo, Z.; Chen, Z.; Feng, C.; Zhang, D.; Moon, W.-J. Carbon-coated SnO2@C with hierarchically porous structures and graphite layers inside for a high-performance lithium-ion battery. J. Mater. Chem. 2012, 22, 2766–2773. [CrossRef] 35. Xie, Y.; Sherwood, P.M.A. Ultrahigh Purity Graphite Electrode by Core Level and Valence Band XPS. Surf. Sci. Spectra 1992, 1, 367. [CrossRef]

36. Ocal, C.; Ferrer, S. The strong metal–support interaction (SMSI) in Pt–TiO2 model catalysts. A new CO adsorption state on Pt–Ti atoms. J. Chem. Phys. 1986, 84, 6474. [CrossRef] 37. Liu, G.; Han, C.; Pelaez, M.; Zhu, D.; Liao, S.; Likodimos, V.; Ioannidis, N.; Kontos, A.G.; Falaras, P.; Dunlop, P.S.M.; et al. Synthesis, characterization and photocatalytic evaluation of visible light activated

C-doped TiO2 nanoparticles. Nanotechnology 2012, 23, 294003. [CrossRef] 38. Galuska, A.A.; Uht, J.C.; Marquez, N. Reactive and nonreactive ion mixing of Ti ﬁlms on carbon substrates. J. Vac. Sci. Technol. A 1988, 6, 110. [CrossRef] Nanomaterials 2020, 10, 920 11 of 11

39. Dementjev, A.P.; Ivanova, O.P.; Vasilyev, L.A.; Naumkin, A.V.; Nemirovsky, D.M.; Shalaev, D.Y. Altered layer as sensitive initial chemical state indicator. J. Vac. Sci. Technol. A 1994, 12, 423. [CrossRef] 40. Gonbeau, D.; Guimon, C.; Pfister-Guillouzo, G.; Levasseur, A.; Meunier, G.; Dormoy, R. XPS study of thin films of titanium oxysulfides. Surf. Sci. 1991, 254, 81–89. [CrossRef] 41. Franzen, H.F.; Umana, M.X.; McCreary, J.R.; Thorn, R.J. XPS spectra of some transition metal and alkaline earth monochalcogenides. J. Solid State Chem. 1976, 18, 363–368. [CrossRef] 42. Haukka, S.; Lakomaa, E.-L.; Jylha, O.; Vilhunen, J.; Hornytzkyj, S. Dispersion and distribution of titanium species bound to silica from titanium tetrachloride. Langmuir 1993, 9, 3497–3506. [CrossRef] 43. Kuznetsov, M.V.; Zhuravlev, J.F.; Gubanov, V.A. XPS analysis of adsorption of oxygen molecules on the surface of Ti and TiNx films in vacuum. J. Electron. Spectrosc. Relat. Phenom. 1992, 58, 169–176. [CrossRef] 44. Kuznetsov, M.V.; Zhuravlev, J.F.; Zhilyaev, V.A.; Gubanov, V.A. XPS study of the nitrides, oxides and oxynitrides of titanium. J. Electron. Spectrosc. Relat. Phenom. 1992, 58, 1–9. [CrossRef] 45. Tan, B.J.; Klabunde, K.J.; Sherwood, P.M.A. X-ray photoelectron spectroscopy studies of solvated metal atom dispersed catalysts. Monometallic iron and bimetallic iron-cobalt particles on alumina. Chem. Mater. 1990, 2, 186–191. [CrossRef] 46. Bukhtiyarov, V.I.; Boronin, A.I.; Prosvirin, I.P.; Savchenko, V.I.J. Stages in the Modification of a Silver Surface for Catalysis of the Partial Oxidation of Ethylene: II. Action of the Reaction Medium. J. Catal. 1994, 150, 268–273. [CrossRef] 3+ 47. Deskins, N.A.; Rousseau, R.; Dupuis, M. Distribution of Ti Surface Sites in Reduced TiO2. J. Phys. Chem. C 2011, 115, 7562–7572. [CrossRef] 48. Wang, G.; Wang, H.; Ling, Y.; Tang, Y.; Yang, X.; Fitzmorris, R.C.; Wang, C.; Zhang, J.Z.; Li, Y. Hydrogen-treated

TiO2 nanowire arrays for photoelectrochemical water splitting. Nano Lett. 2011, 11, 3026–3033. [CrossRef] 49. Wheeler, D.A.; Ling, Y.C.; Dillon, R.J.; Fitzmorris, R.C.; Dudzik, C.G.; Zavodivker, L.; Rajh, T.; Dimitrijevic, N.M.; Millhauser, G.; Bardeen, C.; et al. Probing the nature of bandgap states in hydrogen-treated

TiO2 nanowires. J. Phys. Chem. 2013, 117, 26821–26830. [CrossRef] 50. Wheeler, D.A.; Wang, G.; Fitzmorris, R.C.; Adams, S.A.; Li, Y.; Zhang, J.Z. Ultrafast charge carrier dynamics

and photoelectrochemical properties of hydrogen-treated TiO2 nanowire arrays. MRS Proc. 2012, 1387, e04–e07. [CrossRef]

51. Li, H.; Liu, G.; Chen, S.; Liu, Q. Novel Fe doped mesoporous TiO2 microspheres: Ultrasonic–hydrothermal synthesis, characterization, and photocatalytic properties. Physica E 2010, 42, 1844–1849. [CrossRef] 52. Acharya, K.P.; Khnayzer, R.S.; Connor, T.; Diederich, G.; Kirsanova, M.; Klinkova, A.; Roth, D.; Kinder, E.; Imboden, M.; Zamkov, M. The Role of Hole Localization in Sacriﬁcial Hydrogen Production by Semiconductor–Metal Heterostructured Nanocrystals. Nano Lett. 2011, 11, 2919. [CrossRef][PubMed]

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