Porous Flower Aggregates: Selective Photocatalytic Reversibility for

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Defect Rich Brown TiO Porous Flower Aggregates: Selective Photocatalytic Reversibility for Organic Dye Degradation Sanjay Gopal Ullattil, and Resmi M. Ramakrishnan ACS Appl. Nano Mater., Just Accepted Manuscript • Publication Date (Web): 24 Jul 2018 Downloaded from http://pubs.acs.org on July 24, 2018

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ACS Applied Nano Materials

Defect Rich Brown TiO2-x Porous Flower Aggregates: Selective Photocatalytic Reversibility for Organic Dye Degradation Sanjay Gopal Ullattil, *,†,‡ and Resmi M. Ramakrishnan *,† a

Department of Chemistry, Sree Neelakanta Government Sanskrit College, Pattambi, Kerala,

India-679306. ABSTRACT A low temperature, Mn (II) assisted sol-solvothermal strategy has been developed for the synthesis of positively surface charged defective brown TiO2-x flower aggregates with porous nature. The porous structure possessed enormous surface defect states such as trivalent titanium ion (Ti3+) and oxygen vacancy (Vo) sites. The defect states present in the brown TiO2-x facilitated enhanced absorption even in the NIR region of the solar spectrum, whereas for the negatively surface charged TiO2 sample, synthesized in the absence of Mn (II), the absorption was limited in the visible region. Obviously, the band gap narrowing was occurred for brown TiO2-x as compared to the yellow TiO2 synthesized in the absence of Mn (II). Interestingly the photocatalytic efficiency of these materials have been carried out using methyl orange-methylene blue (MO-MB) dye mixture model system under solar illumination resulted in the selective photocatalytic reversibility such that MB and MO photodegradation have been selectively performed by yellow TiO2 and brown TiO2-x respectively. After all, this is the first time report on surface charged brown TiO2-x with porous flower aggregate morphology for selective photocatalysis. KEY WORDS Brown TiO2-x, Oxygen richness, oxygen vacancy richness, anatase phase purification, porous flower aggregates, surface charge, selective photocatalysis

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INTRODUCTION Unrivalled superiority of TiO2 in photocatalytic applications attracted by the whole scientific world, except their limited absorption in the UV region and thereby limited access to the universal energy source, sunlight. The limited access is due to the colourless nature of titania offers the utilization of only UV energy from the sun which is 5% of the solar spectrum. 1 To harvest solar energy more effectively metal doping and non-metal doping came in to act but the absorption of coloured resultant materials paved the way to capture energy from the visible region, which covers 43% of the spectrum.1 Now the question arises regarding the rest of the 52%. If a material is black coloured it can absorb energy even from the IR region of the solar spectrum. So research works have been done on one of the most efficient material TiO2 for converting it to its coloured versions.2 Chen et al. were successful in synthesizing black titania in 2011 and they implemented the material in photocatalysis.3 Since then several research on the same material has been performed in several applications such as photocatalysis,4 dye sensitized solar cells,5 Li ion battery,6 capacitors,7 photothermal therapy8 etc. and at the same time, new strategies have been coming into the research ground for elucidating the simplified version of its formation. However blue,9,10 brown11,12 and red13,14 TiO2 had also been synthesized nowadays and found efficiently suitable for harvesting solar energy.15 Dedicated attempts are therefore being made for the modification of the crystal structure, surface defect states and thereby long wavelength absorption.2 Various approaches such as H2, Ar, N2 treatment, laser ablation, ultrasonication etc. have been implemented.4,6 In addition, a common strategy that has been implemented was the chemical reduction which led the way to defect centres such as Ti3+ and Vo responsible for various applications, especially photocatalysis. 16,17,18 Our recent reports for the synthesis of black and brown TiO2 suggested that Mn (II) reduction is one of

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the easiest routes for developing defect rich black TiO2.5,11,19 Our previous report on brown TiO2 equipped with noticeable surface defect states was employed in sodium hybrid capacitor. Wang et al. synthesized brown TiO2 at low temperature (150 °C) by NaH treatment and utilized in photodegradation of phenol under UV and Visible light illumination.12 In another work, brown TiO2 had been synthesized at 140 °C and implemented in the photodegradation of methylene blue.20 Considering all these reports including ours, we have tried to develop a defect rich TiO2 at low temperature, 90 °C (less than the previous reports) and it ended up in the formation of a defective brown TiO2-x porous flower aggregates. Thus, here it has been confirmed that Mn (II) can act as a reducing agent as well as a template for the synthesis of broad and longer wavelength absorbing materials for photocatalysis. Furthermore, H2O2 is an inevitable part of the synthetic strategy, because of its ability to act as a template remover that enables the removal of any organic moieties that could be able to suppress the photocatalytic efficiency by forming an organic cloud.21 Organic dyes are integral parts of industrial effluent whose production is about 450000 tons annually and more than 11% among them is wastes. Since many of these dyes are potential carcinogens and affects human body in so many ways, the selective photocatalytic decomposition of them has great importance. Different adsorbents/photocatalysts has been reported till date for the selective adsorption and photodegradation of cationic and anionic dyes based on the surface charge of the adsorbents and their electrostatic interactions with the dye molecules.22,23,24 The photocatalytic phenomenon could be further extended to the affinity of certain dyes toward TiO2, in order to increase the dye adsorption and thereby maximizing the photon to electricity conversion efficiency of dye sensitized solar cells.25 Furthermore, selective positioning of different dyes on crystalline/semicrystalline/amorphous TiO2, could also be possible, which in turn could lift the efficiency of dye sensitized solar cells.26



Here we have developed a sol-solvothermal strategy for the synthesis of brown TiO2-x flower aggregates (SBT) using Mn (II) as reducing agent as well as a template and yellow TiO2 spherical aggregates (SYT) in the absence of Mn (II) at a temperature of 90 ºC for selective photocatalysis. SBT possessed enormous defect states such as trivalent titanium and oxygen vacancies which are the control centers for photoactivity. Investigation of selective photocatalysis using these materials under MO-MB model system have been done because of the assumption that, excess surface charges would be available for photocatalysis and it was proved after performing the experiments. Distinctly, SYT performed the degradation of MB, where MO was intact and when SBT was used the performance was reversed. EXPERIMENTAL Materials Titanium(IV) butoxide, 97% (Sigma-Aldrich), isopropanol, extra pure, AR (Merck), hydrogen peroxide, 30% (Merck), manganese acetate tetrahydrate, extra pure, AR (Sisco Research Laboratories, India), methylene blue (Merck) and methyl orange (Merck) were used as received without further purification. Synthetic Strategy For the synthesis of brown TiO2-x porous sphere aggregates (SBT), 6.8 g of titanium butoxide was dissolved in 100 mL of isopropanol (0.2 M) at room temperature and continuously stirred at 1200 rpm for 30 minutes followed by quick addition of 0.02 M manganese acetate solution (0.2451 g in 50 ml of H2O2). Under the same conditions, the stirring was continued again for 30 minutes. The reaction mixture was then sealed in a Teflon lined autoclave and heated at 90 °C for 16 hours. The so obtained product was then cooled to 50 °C. For the synthesis of yellow TiO2 spherical aggregates (SYT), 50 ml of H2O2 was added alone by replacing 10 mol% manganese acetate


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solution. Both the samples were washed using a 50 ml of H2O2, ethanol and 100 ml of deionized water. Photocatalysis 10-4 M of methyl orange, MO (0.0163 g) and methylene blue, MB (0.01599 g) were made up to 500 mL separately using H2O as solvent in a standard flask to get an intense orange and blue colored dye solutions respectively. 25 mL of the prepared MO and MB dye solutions were taken in a beaker and 0.1 g of the TiO2 photocatalyst was added to the dye solution prior to dark adsorption analysis (Fig. S1). A catalyst free photolysis was also performed under the same conditions and at the same time (Fig. S2). The photocatalytic measurements were executed under the natural sunlight and the intensity was found to be of 83000 - 92000 lux. The sunlight intensity27 was performed using a Lutron, LX-107HA lux meter at University of Calicut, Kerala, India (altitude: 11° 7’ 34” North 75° 53’ 25’ East, time: 12.30 - 2.30, temperature: 31 ± 1 °C) on Saturday, 20th January 2018. Characterization The yellow and brown TiO2 samples have been characterized using XRD, Raman, FTIR, XPS, UV-Visible spectroscopy, SEM and TEM. XRD patterns were recorded using a Rigaku Miniflex600 X ray diffractometer in the diffraction angle range of 2θ = 10 - 70° using Cu-Kα radiation. Raman analysis were performed using LabRAM HR, HORIBA JOBINYVON Raman spectrophotometer and the FTIR spectra measurements were obtained from a Jasco-FT/IR-4100 spectrophotometer. XPS measurements were performed using Axis Ultra, Kratos Analytical, UK with an Al-Kα (1486.6 eV) source. UV-Visible absorbance spectra were recorded using a JascoV-550-UV/VIS spectrophotometer. Particle morphologies were examined by SEM and TEM using



a Hitachi-Su field emission scanning electron microscope and a JEOL/JEM 2100 transmission electron microscope respectively. RESULTS AND DISCUSSION It has been early reported that manganese doping can drive spherical structures towards flower like architectures.28 Here TiO2 nanocrystallites of ~ 6 nm were assembled together to form spherical aggregates of micro dimension in the case of SYT. Under solvothermal condition, in presence of Mn (II) treatment at 90 °C the smaller nanocrystallites in spherical aggregates were started to dissolve into the mother solution and simultaneously, it gets nucleated driven by the thermodynamic force to reduce the total surface energy of the resulting structure which was ripened into porous flower aggregates.29 As shown in Scheme 1, the precursor dissolve almost completely in to the mother solution prior to nucleation followed by the anisotropic growth and Ostwald ripening. According to Zhang et al. partial dissociation of precursors may lead to the anisotropic regrowth of the TiO2 flower structures at the surface of TiO2 spheres to form flower like sphere structures.28 But here, due to the almost complete dissolution, the spherical surface is not available to maintain a spherical flower structure.28 The role of Mn is also important, as it has been reported as a perfect dopant for the formation of porous structures.30 After Mn incorporation in to the H2O2 can remove templates as well as organic functionalities, so that it can create high surface area and porosity.31 Thus it is a facile approach for template degradation with the advantages of economic, time saving and complete elimination of the organic moieties that can hinder the photocatalytic activity in terms of morphology.21 Therefore the porous as well as the non-porous parts are fully available for the degradation of organic pollutants.


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Scheme 1. Schematic illustration of the mechanism of formation of brown TiO2-x flower aggregates X ray Diffraction The XRD patterns of SYT and SBT confirm the formation of anatase TiO2 alone (JCPDS 75-1537) and from Fig. 1B the overall anatase peak texturing along all directions with respect to SYT (Fig. 1A) is observed along with new peak orientations in (103) and (213) directions in SBT. These new peaks with (103) and (213) orientations show the higher phase purity of brown anatase TiO2 as compared to the yellow anatase TiO2. This phenomena is due to the synergistic effects of the thermodynamic and kinetic factors which control crystal nucleation.19, 32 Thus Mn2+ incorporation into the anatase crystal lattice has directed towards the origin of such new anatase peaks and peak texturing simultaneously.19 Since the ionic radius of Mn2+ (0.83 Å)33 is greater than that of anatase Ti4+ (0.74 Å),34 the forceful displacement of lattice oxygen occurs leading to oxygen vacancies



with the help of Mn2+. In other words, the in situ incorporation of Mn2+ in to the lattice site of anatase TiO2 paved the way to oxygen vacancy richness. Furthermore, the Mn (II) incorporation was confirmed by the XRD peak shift of 0.52° to lower angle after Mn2+ modification (inset of Fig. 1).

Figure 1. XRD patterns of A) SYT and B) SBT (inset shows the (101) peak position difference indicating the incorporation of Mn2+) Raman Spectroscopy Raman spectra of SYT and SBT presented in Figure 2, confirm the anatase phase of the prepared TiO2 samples. All observed modes can be assigned to the Raman spectra of pure anatase TiO2. For SYT, 151 cm-1 (Eg), 400 cm-1 (B1g), 514 cm-1 (A1g+B1g) and 637 cm-1 (Eg) are seen and peak positions for SBT were 149 cm-1 (Eg), 199 cm-1 (Eg), 396 cm-1 (B1g), 514 cm-1 (A1g+B1g) and 636 cm-1 (Eg).35 The peak positions were almost the same for both SYT and SBT except, the presence of a very low intense peak at 199 cm-1 (Eg) only for SBT. This new peak formation is an after effect of the incorporation of the Mn (II) that acts as anatase phase purifier, which is evident from


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XRD. Although the spectra were measured under the same experimental conditions, Raman modes of SBT, the Mn treated sample has relatively strong intensities which are in agreement with the XRD results where the anatase peaks are more prominent for SBT. It has been reported in our previous report that Mn2+ incorporation into the crystal lattice leads to phase purification since Mn (II) is known as the anatase phase purifier.19

Figure 2. Raman spectra of A) SYT and B) SBT FTIR Spectroscopy The FTIR spectra (Fig. 3) show that there is a noticeable shift in all the existing peaks and thus a noticeable change in the IR spectra of SYT and SBT. A broad band at ~ 550 and ~547 cm-1 for SYT and SBT respectively further confirm the presence of anatase TiO2.36 Generally, the peaks for Ti-O-Ti and Mn-O stretching vibrations are present usually at 500 and 794 cm-1 respectively. In the FTIR spectrum of SBT, these bands are masked within a broad band present in the range of 400-900 cm-1.37 Similarly the generally assigned Ti-OH bands at 740 and 670 cm-1 may also be concealed within the same broad band.38 The -OH stretching vibration bands are present at 3404



and 3424 cm-1 for SYT and SBT respectively.39 In addition, the peaks present at 1629 and 1632 cm-1 are due to the -OH bending vibration.40 The peak at 1440 cm−1 could be assigned to the surface adsorbed CO2 stretching41 and the peak at 1018 cm-1 is ascribed to δ- (Ti–OH) deformation which is only present for SBT that uncovers the highly deformed crystal lattice of SBT.19,42

Figure 3. FTIR spectra of A) SYT and B) SBT UV-Visible Absorption Spectroscopy and Tauc plot UV-Visible spectroscopy reveals the substantial increased absorption coverage of SBT that to that of SYT. In SYT, the wavelength cut-off was found at ~ 480 nm (Fig. 4A.a) whereas the absorption of SBT (Fig. 4A.b and inset of 4A) is just beyond the near IR (NIR) region. The longer wavelength absorption ~780 nm of SBT is obviously due to the origin of surface oxygen vacancy states caused by Mn(II) incorporation and thereby the formation of Ti3+. These absorption features could have positive role in increased photocatalytic efficiency of the brown TiO2 flower aggregates. The presence of absorption humps for SBT in the range of 530-680 nm is associated with the d–d electronic transitions of Mn2+ in an octahedral environment and here this 4A1g(S) → 4T1g(G)


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transition indicates the incorporation of only Mn2+ into the TiO2 crystal lattice.43 For the Tauc plot (Fig. 4B) the band gap energy for SYT was found 3.02 eV which notably decreased to 2.76 eV for SBT. This could be ascribed to the high rate of reduction capability of Mn2+ which introduced defect richness into the crystal lattice of yellow TiO2 led the way to narrowed band gap for brown TiO2-x. The reducing agent Mn2+ enters into the yellow anatase titania, SYT for accepting excess oxygen leading to the formation of SBT, the Ti3+ doped brown anatase TiO2-x flower aggregates.

Figure 4. A) UV-Vis spectra and B) Tauc plot of a) SYT and b) SBT (Inset of A shows the NIR wavelength cut off of SBT). Morphological Analysis The morphological analyses of the anatase TiO2 samples were carried out using SEM and TEM. SEM image shows that SYT (Fig. 5A) has a spherical aggregate morphology which changed to a bunch of flower aggregates after Mn (II) reduction (Fig. 5B). A magnified image of SBT is also shown in Fig. 5D. The HRTEM shows lattice fringes with 0.342 nm spacing corresponds to (101) plane of the anatase phase HRTEM.44 The HRTEM image (Fig 5C) shows a lot of breaks in the lattice fringe continuum which is an evidence for the defective surface layers of SBT and are



outlined by lined circles.40, 45 The introduction of surface disorder would have enhanced the visible and infrared absorption, with the additional benefit of carrier trapping.3 It could be the reason for the colour change of the brown TiO2-x porous flower aggregates. Large amounts of lattice disorder in semiconductors could yield mid gap band states, which can form a continuum extending to and overlapping with the VB and CB known as band tail states.3

Figure 5. SEM images of A) SYT B) SBT C) TEM image of SBT showing lattice fringe value and defect states and D) Zoomed SEM image of SBT XPS Analysis The Ti 2p XPS analysis of SYT (Fig. 6A and 6C) shows that the Ti 2p3/2 and Ti 2p1/2 peaks are at 459.5 and 465.2 eV. These values are reduced to 457.8 and 463.5 eV for SBT. For SYT, the Ti 2p3/2 peak at 459.5 eV is just greater than the conventional Ti 2p3/2 values and the positive shift may be due to oxygen rich nature at the surface of SYT.46 In the case of SBT, the same peak has


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been negatively shifted to 457.8 eV and is attributed to increased oxygen vacancy sites (Ti3+ doping). In the O 1s XPS spectrum (Fig. 6B and 6D), the Ti-O-Ti binding energy peak at 530.7 eV for SYT, is shifted to lower binding energy, 529.2 eV for SBT. This shift is because of the change in oxygen environment.47 The peaks at 533.2 eV for SYT indicates the presence of Ti–OH due to surface adsorbed –OH molecules.48 These peaks are shifted to 531.1 eV for SBT.48 The splitting between Ti 2p3/2 and Ti 2p1/2 is found to be 5.7 eV for SYT which is assigned to the anatase phase of TiO2.49 After the reduction of SYT using Mn2+, the peak separation was intact confirms the unchanged anatase phase.49 In the Mn 2p spectrum (Fig. 6E), the binding energy peak values are obtained as 640.5, 652 eV and a very low intense satellite peak at 647 eV displays the evidence for the presence of Mn2+ alone in trace amount into the crystal lattice of SBT and the amount of Mn in the final product was obtained was only 0.8% from survey XPS and 0.7% from high resolution spectra (Fig. S5).50 The existence of Mn2+ alone in the crystal lattice of TiO2 is responsible for the negative peak shifting of Ti 2p3/2 towards 457.8 eV in SBT and also proves the Ti3+ doped oxygen deficient environment. Simultaneously the oxidation state approaches to +3 from +4 for (Ti4+ → Ti3+).19 The survey XPS spectrum of both SYT and SBT are shown in Fig S3.



Figure 6. A) Ti 2p and B) O 1s of SYT C) Ti 2p and D) O 1s of SBT, E) Mn 2p of SBT and F) Valence Band XPS of SYT and SBT.


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The narrowing of band gap occurred due to the transformation of SYT to SBT (from 3.02 to 2.76 eV) is due to the formation of mid-gap band states, those are formed either above the valence band (VB) or below the conduction band (CB). These mid gap band states will overlap with the respective bands such as VB or CB leads to band gap narrowing. Here the valence band edge is observed at 2.56 eV for SYT and a VB edge of 1.39 and a band tailing of -1.09 eV is observed for SBT below the Fermi energy confirm the noticeable downward binding shift by introducing oxygen richness and simultaneously Ti3+ doping into the rich oxygen environment (Fig. 6F). Since the optical band gap of SYT is 3.02 eV, the CB minimum (CBM) would occur at −0.46 eV as depicted in the Density of States (DOS) diagram (Fig. 7). DOS of SBT shows that the narrowed band gap energy of SBT (2.76 eV obtained from the Tauc plot, Fig. 4B) is due to the existence of mid gap band states in between the VB and CB.2 The mid gap band states due to the VB maximum (VBM) and CBM are mainly due to O 2p and Ti 3d orbitals, respectively.3 From the DOS diagram (Fig. 7), it can be easily perceive that the main absorption onset of SBT is located at 1.39 eV. In addition, the maximum energy associated with the band tail, shifted to lower binding energy, resulting in the VBM at -1.09 eV. Furthermore, there may be CB band tail states arising from Ti3+ defect states that extend below the CBM. In this way, the CB minimum would be observed at -3.85 eV.51



Figure 7. Density of Sates of A) SYT and B) SBT Selective Photocatalysis The remarkable materials SBT and SYT have been employed in selective photocatalysis under solar illumination. It is well known that one of the major constituents in industrial effluent are organic dyes. Therefore the selective adsorption of dyes has prime importance with regard to their degradation. As, both SYT and SBT have been synthesized at low temperature, the possibility for the presence of high degree of surface charges cannot be eliminated. This assumption led the way to carry out selective photocatalytic experiments using a mixture of anionic methyl orange (MO) and cationic methylene blue (MB) solution in equal amount viz. 25 ml of MO and 25 ml of MB with 10-4 M concentration. The MO-MB system has been utilized for the selective photocatalytic evaluation before23 and to the best of our knowledge, the photodegradation reversibility in terms of surface charge using TiO2 was reported earlier only once by Liu et al.24 They prepared the material at high calcination temperature (600 °C) and achieved partial selectivity for the degradation of MO and MB in a MO-MB mixture using fluorinated TiO2 and the NaOH treated


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fluorinated TiO2. But the selectivity was not much promising, since they used high temperature processing for the synthesis of hollow TiO2 microspheres that led the way towards the unavailability of sufficient surface charges. In the present study, high rate of complete selective degradation of MB and MO occurred using SYT and SBT respectively (Fig. 8). From XPS, it has been confirmed that the surface of SYT is oxygen rich and that of SBT is oxygen deficient. It has been reported early that, using H 2O2, oxygen richness can be introduced at the surface of TiO2 and here due to the presence of excess H2O2 in the SYT system, H2O2 accept a photoinduced electron from the CB of SYT leading to the formation of one hydroxyl radical and one hydroxyl ion (equation 1).52 H2O2 + eCB¯ → •OH + OH¯

(1)

Figure 8. A) UV-Visible spectra of selective photodegradation of MO and MB using A) SYT & B) SBT and photodegradation kinetics using C) SYT and D) SBT



Therefore it can be derived that the oxygen richness may be due to the increased concentration of •OH and OH¯ accumulated at the surface of SYT due to excess H2O2 present in the system after utilization of H2O2 for precipitation, i.e. high concentration of negative charge. This highly negatively charged surface would have an affinity (electrostatic interaction) towards MB, the positively charged dye present in the photocatalytic model system. 53 As shown in figure 8A and 8C, SYT has done the selective photocatalytic degradation of MB, where the absorption peak of MO was found almost intact even after 120 minutes. In other words, the selective photodegradation of MO was occurred by taking 120 minutes in the presence of the oxygen rich photocatalyst, SYT. On the other hand, the oxygen vacancy richness in SBT is due to the effect of Mn (II) that displace, lattice and surface oxygen from TiO2. The excess amount of positively charged surface oxygen vacancies and low concentration of Mn2+ present at the surface of brown TiO2-x paved the way to an exciting photocatalytic reversibility of TiO2, i.e. the selective photodegradation of MO and the main absorption peak of MB was photodegradation time was changed to 90 minutes (Fig. 8B and 8D). In general both the catalysts SYT and SBT have very small average crystallite size (~ 6 nm), which means both these catalysts have powerful redox ability because of the quantum size effect. Furthermore, such catalysts with smaller crystallite sizes are beneficial for the separation of the hole and electron pairs.54 Specifically, in the case of brown TiO2-x, morphology would have played an important role in the high rate of photocatalytic activity in addition to all the aforementioned qualities of catalysts. The flower like structure with a porous structure having surface positive charge lead to the diffusion of a large number of organic molecules, specifically, the oppositely (negatively) charged dye molecules, MO to be attached on the catalyst surface and also a number of catalytic adsorption/desorption sites are available for photocatalysis.55 It could be positively contributed towards the reaction faster in addition to the advantages gathered due to excess oxygen


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vacancy richness. Photolysis of the dye mixture in the absence of any catalyst has also been done and it has been noted that no noticeable degradation was occurred (Fig. S2). It is also noted that MB degradation was more as compared to MO in the absence of photocatalysts. It is an evidence for the high influence of SBT in selective photocatalysis as compared to SYT. In other words the synergistic effect with the sunlight works more for SBT as compared to SYT. As a whole, the remarkable selective photocatalytic activity of SBT is due to synergistic effects of brown colour and thereby long wavelength absorption, narrowed band gap, defective surface with enormous positive charges and obviously the porous flower aggregates for large amount of dye diffusion. CONCLUSIONS Brown TiO2-x flower aggregates (SBT) with porous structure and yellow TiO2 (SYT) with spherical aggregate morphology have been synthesized by sol-solvothermal method at low temperature (90 °C) in the presence and absence of Mn (II) respectively. SYT possessed oxygen rich environment and SBT exhibited enormous surface defect states such as trivalent titanium ions (Ti3+) and oxygen vacancy sites (Vo) available for promising selective photocatalysis using methylene blue-methyl orange (cationic-anionic) dye model system. Yellow TiO2 with negatively charged surface, has shown a high rate of selective photocatalytic activity viz. the selective photocatalysis of the cationic dye, methylene blue, which was reversed using surface positively charged defective brown TiO2-x, such that, the methyl orange dye degradation has alone occurred. For the future, it can be expected that defect states, surface charges, porous structure and small crystallite size will pave the way to its application especially in photocatalytic water splitting even in the absence of any co-catalysts.



ASSOCIATED CONTENT Supporting Information UV-Vis spectra of before and after Dark Adsorption Analysis (DAA), Catalyst free photolysis of MO-MB model system, Survey XPS of SYT and SBT, SEM of SBT at different magnifications/resolutions. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected], [email protected] ORCID Sanjay Gopal Ullattil: 0000-0001-5252-2896 Present Address ‡

Department of Applied Chemistry, National Chiao Tung University, Hsinchu 300, Taiwan

Notes Authors declare no competing financial interest. ACKNOWLEDGEMENTS SGU is thankful to Kerala State Council for Science, Technology and Environment (KSCSTE), India for the post-doctoral fellowship and Dr. Pradeepan Periyat for his support for doing the experiments. REFERENCES 1

Jiang, C.; Moniz, S. J.; Wang, A.; Zhang, T.; Tang, J. Photoelectrochemical Devices for Solar Water Splitting-Materials and Challenges. Chem. Soc. Rev. 2017, 46, 4645-4660.

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Ullattil, S. G.; Narendranath, S. B.; Pillai, S. C.; Periyat, P. Black TiO2 Nanomaterials: A Review of Recent Advances. Chem. Eng. J., 2018, 343, 708-736.

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Chen, X.; Liu, L.; Peter, Y. Y.; Mao, S. S. Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals. Science 2011, 331, 746-750. 20 ACS Paragon Plus Environment

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Ullattil, S. G.; Thelappurath, A. V.; Tadka, S. N.; Kavil, J.; Vijayan, B. K.; Periyat, P. A Solsolvothermal Processed ‘Black TiO2’as Photoanode Material in Dye Sensitized Solar Cells. Sol. Energy 2017, 155, 490-495.

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