Synergistic Effect of Mo + Cu Codoping on the Photocatalytic Behavior

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Synergistic Effect of Mo+Cu Co-Doping on the Photocatalytic Behavior of Metastable TiO Solid Solutions 2

Aakanksha Chaudhary, Mandali Poshit Nag, Narayanan Ravishankar, Tiju Thomas, Manish Jain, and Srinivasan Raghavan J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 24 Nov 2014 Downloaded from http://pubs.acs.org on November 28, 2014

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The Journal of Physical Chemistry

Synergistic Effect of Mo+Cu Co-doping on the Photocatalytic Behavior of Metastable TiO2 Solid Solutions Aakanksha Chaudhary1, M.Poshit Nag2, N.Ravishankar1, Tiju Thomas1*, Manish Jain2 & Srinivasan Raghavan1,3* 1

Materials Research Centre, Indian Institute of Science, Bangalore-560012

2

Department of Physics, Indian Institute of Science, Bangalore-560012

3

Centre for Nanoscience and Engineering, Indian Institute of Science, Bangalore560012 * [email protected]; [email protected] Abstract: Co-doping with Cu and Mo is shown to have a synergistic effect on the photocatalytic activity of TiO2. The enhancement in activity is observed only if the synthesis route results in TiO2 in which (Cu, Mo) co-dopants are forced into the TiO2 lattice. Using x-ray photoelectron spectroscopy, Cu and Mo are shown to be present in the +2 and +6 oxidation states respectively. A systematic study of the ternary system shows that TiO2 containing 6 mole % CuO and 1.5 mole % MoO3 is the most active ternary composition. Ab-initio calculations show that co-doping of TiO2 using (Mo, Cu) introduces levels above valence band, and below conduction band, resulting in a significant reduction in the band gap (~0.8 eV). However, co-doping also introduces deep defect states, which can have a deleterious impact on photoactivity. This helps rationalize the narrow compositional window over which the enhancement in photocatalytic activity is observed. Keywords: TiO2 photocatalysis; Mo,Cu co-doping; shallow and deep level defects; Ab-initio calculations. 1. Introduction: TiO2 is by far the most attractive and well studied photocatalytic system because of its robust surface chemistry, significant recyclability, non-toxicity, relative abundance and low cost.1-7 The efficiency of a photocatalyst is dictated by a multitude of factors such as optimal loading levels, crystalline phase, surface area, surface chemistry, catalyst morphology, defects and dopants.6, 8-11 This current paper pertains to the last of these effects, doping. TiO2 has been doped practically with almost every transition metal ion in the periodic table.6,

12-13

However unifying

conclusions on the role of dopant ions on the photocatalytic performance of TiO2 are 1

ACS Paragon Plus Environment


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difficult to make.8 To add to the complexity, the properties of semiconductor oxides are very sensitive to the process and the processing conditions under which these modified oxides are synthesized. This makes direct comparison between two different materials, with the same nominal composition significantly problematic. The influence of dopants on the photoactivity of TiO2 is often reasoned on the basis of introduction of oxygen vacancies14-17 and defect levels in the band gap12-13, 18 (and hence modification of the electronic structure). These defect levels can be shallow donor and acceptor levels, or deep recombination centres.12, 19 The very same defect states that can enhance absorption due to availability of transitions requiring lower energy than the band edge transitions can also act as traps when shallow and therefore delay recombination. On the other hand defect centers that are adequately deep can aid recombination.12,

19-20

Thus, an increase in absorbance in the visible

spectrum does not necessarily translate to enhanced photoactivity.13, 21 In addition to absorbance, prevention of e-h+ recombination plays a crucial role in the photocatalytic activity.3 Recombination reduces the availability of photogenerated charge carriers, which in turn reduces the possibility of charge transfer reaction. This means the recombination centers have a detrimental impact of the photocatalytic behavior.3, 22

Doping TiO2 with Mo has proven to be effective in improving its photoactivity.23-25 The observed enhancement is attributed to creation of defect levels just below the ••

''

conduction band (CB) of TiO2. The MoTi -Vo defect pair acts as an electron donor (~ 0.58 eV) below the conduction band as reported in literature.25 This reduction in band gap leads to activation of the catalyst under visible light. Similarly, Cu doping has also been reported to have a positive effect in enhancing the photocatalytic 2


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behaviour of TiO2 under visible light illumination.26-28 Doping TiO2 with Cu introduces new states at top of the valence band (VB) and narrows the band gap of the material.29 Given the perks of doping TiO2 with Cu and Mo, co-doping seems a logical step to boost the photocatalytic performance even further. Cu and Mo (in +2 and +6 states respectively) cation pair is effective in maintaining the charge balance when doped in TiO2. Also the ionic radius of Cu (0.73 Å) and Mo (0.62 Å) is similar to that of Ti (0.61Å) and hence doesn’t result in significant lattice distortions.

The above mentioned reports and facts suggest that co-doping with Cu and Mo should have a synergistic effect on the photocatalytic activity of TiO2 particularly in the visible region as one would expect a narrowing of the gap. However, it has not been attempted so far. This article highlights the role of co-doping TiO2 with Cu and Mo. It is shown that co-doping has a synergistic effect on the photoactivity of TiO2 in the UV only; this happens when both Cu and Mo oxides, which are otherwise insoluble in anatase titania, are brought into solution using some non-equilibrium processing techniques such as sol-gel synthesis. The observed photoactivities are rationalized on the basis of electronic structure calculations performed within density functional theory. In relaxed co-doped (Cu,Mo): TiO2 systems, we notice evidence of charge transfer between Mo and Cu, and a concomitant introduction of electronic states below CB and above VB. We also show that this co-doping results in deep level defect states that are very likely deleterious to photocatalytic activity. Hence there exists a narrow compositional window over which (Cu,Mo): TiO2 is expected to perform well; this is consistent with our experimental results. Thus, while absorbance in the visible is found to increase dramatically it is not accompanied with a concomitant improvement in activity in the visible spectrum. 3



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2. Experimental Section

2.1 Sol-Gel method: The first approach adopted to obtain doped systems involved sol-gel (sg) hydrolysis and precipitation method. In this method, titanium(IV) isopropoxide, Cu(NO3)2.3H2O, (NH4)6Mo7O24.4 H2O and DI water were mixed in absolute ethanol with hydrochloric acid in molar ratio of Ti : Cu : Mo: C2H5OH : H2O : HCl = 1 : 0.06 : 0.01 : 10 : 6: 0.2 while being stirred vigorously30. The sols were aged for 10 hr and then dried at 80 ˚C for 4 hr. Post drying, these powders were crushed in a mortar and pestle. These crushed powders were then calcined at 500˚C for 2 h. The same procedure was also used to obtain pure TiO2 (TiO2)SG and (TiO2)6%Cu and (TiO2)1.5%Mo (henceforth labelled as (TiO2)Cu and (TiO2)Mo respectively. Similarly, different (TiO2)%Cu/%Mo ternary compositions were obtained where (TiO2)%x represents the mole % of dopant x in TiO2. These ternary compositions are depicted on the ternary plot as shown in Fig 1.

2.2 Synthesis of reference oxides using microwave synthesis: TiO2 and CuO nanopowders were synthesized using microwave synthesis. For synthesis of (TiO2)MW powders, 8 mL titanium isopropoxide was added to 15.5 mL glacial acetic acid. The mixture was magnetically stirred at 400 rpm. Next, 170 mL deionized (DI) water was added to this mixture.31 The solution was stirred until a clear sol was obtained. The resulting sol was heated to 100˚C using a MARS microwave system (2.45 GHz, 300 W, CEM Corporation) for 6 minutes. The resulting white solid was filtered and washed with DI water several times until the pH of the decanted solution

4


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was almost neutral. The solid was dried for 30 min in air and then calcined at 500˚C for 2 h. (CuO)MW nanopowders were synthesized using an aqueous solution of CuSO4.5H2O. 140 mL of 0.5 M NaOH solution was added to 80 mL of 0.3 M CuSO4.5H2O while stirring (200 rpm).32 The (CuO)MW precursor was microwaved at 100˚C for 7 minutes (2.45 GHz, 100 W). The precipitated (CuO)MW nanopowder was washed with ethanol and dried at 40˚C for 4 hrs followed by calcining at 500˚C for 2 h. MoO3 was obtained by thermal decomposition of ammonium heptamolybdate ((NH4)6Mo7O24.4H2O) at 350˚C for 5 hrs.33 These powders were used in the solid state synthesis as described below. 2.3 Mixing and calcination of metal oxides: The second approach adopted to obtain doped systems was to mix the individual oxides together in the desired amounts in a mortar and pestle. These powders were then calcined at 500˚C for 2 h. Different compositions synthesized were 2, 5 and 10 mole % CuO mixed with TiO2 labelled as (TiO2)2CuO, (TiO2)5CuO and (TiO2)10CuO respectively. Likewise (TiO2)2MoO3, (TiO2)5MoO3 and (TiO2)10MoO3 were also made which have different mole percent Mo. Similarly a ternary composition (TiO2)6CuO,1.5MoO3 (herein (TiO2)Ternary) was synthesized.

2.4 Materials characterization: XRD was performed out using high resolution Rigaku smart lab diffractometer with Cu Kα radiation. The scan was carried out for a range of 20˚- 80˚ with a scan rate of 1.5 ˚/min. XPS was performed using Thermo Fischer Scientific Multilab 2000 using monochromatic Al K radiation. The binding 5



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energies were corrected relative to graphitic carbon peak at 284.6 eV. Raman spectroscopy was carried out using LabRAMHR800 operating at a wavelength of 514 nm. Photoluminescence (PL) of the solid samples was measured using a Perkin Elmer LS 55 luminescence spectrophotometer (325 nm laser source).

2.5 Photoactivity assay: Photodegradation of methyl orange was used as a probe to evaluate the photocatalytic activity of the samples. The experiments were carried out in a jacketed quartz vessel reactor with a water circulation system. The irradiation was carried out using a 125 W high pressure Hg lamp (HPL-N, Philips) which has maximum emission at 365 nm. For powder samples, 0.2 g was dissolved in 75 mL of methyl orange solution with initial concentration of 20 g/L. The distance between the dye solution and the lamp source was fixed at 10 cm. The photolytic degradation in the absence of the catalyst is < 5 %. The solution was vigorously stirred to eliminate diffusion effects. The solution was stirred in dark for 60 min to establish adsorptiondesorption equilibrium and then irradiated. Aliquots of the solution were collected at different time intervals and centrifuged. The concentration of the dye was determined by measuring the absorbance using a UV-Vis Hitachi U-3000 spectrophotometer. For the best performing catalyst, an optimum catalytic loading was established to be 2 g/L.

2.5 Computational Methods:

The ground state total energy and density of states calculations for doped and pure anatase TiO2 were performed using density functional theory within the generalized gradient approximation (GGA), as implemented in Quantum ESPRESSO package.346


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We chose Purdew-Burke-Ernzerhof exchange-correlation functional37 for all the

calculations. Ultrasoft pseudopotentials were used to describe the electron-ion interaction. In the construction of the ultrasoft pseudopotentials the following states were treated as valence: Ti -- 3s, 4s, 3p, 3d; O -- 2s, 2p; Cu -- 3d, 4s, 4p; Mo -- 4s, 5s, 4p, 5p, 4d. The kinetic energy cut off for plane-wave basis set was 35 Ry for Cu doped TiO2 and 50 Ry for Mo and (Cu,Mo): TiO2. Monkhorst-Pack k-point mesh38 of 2x2x1 was chosen for the structural optimization, while a 4x4x2 was chosen for the density of states (DOS) calculations of the doped systems. In order to determine the occupation of the d orbitals, we performed Löwdin population analysis. The theoretical lattice parameters (a and c) of bulk anatase were determined to be 3.78 Å and 9.6 Å respectively. These values are in excellent agreement with previous experimental and theoretical reports.39 We used a 2×2×2 anatase TiO2 supercell containing 96 atoms to simulate doped systems. We substituted one Ti atom with Cu or Mo in the case of mono-doped (corresponding to 3.125%) and replace two Ti atoms with Cu and Mo for co-doped system (Cu,Mo): TiO2 (corresponding to 3.125% of each dopant). Our calculations show that in the co-doped (Cu,Mo): TiO2 system, nearest neighbour Mo and Cu substitution is energetically favoured with respect to these atoms being far apart.

3. Results & Discussion:

The various (Cu,Mo): TiO2 compositions synthesized by the sol-gel method are shown in the ternary plot (Fig 1). Photocatalytic performance of the different ternary systems is compared in Fig 2. The degradation curves are presented as the logarithms of the normalized dye concentration which is given by the ratio of the dye 7



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concentration at any time t to the initial concentration (c/c0). Steeper slope reflects better catalytic performance. The degradation in the absence of any catalytic material i.e. the photolytic degradation is negligible (80 %. The anatase to rutile ratio is highly sensitive to the dopant chemistry and composition of the doped system. However for the samples obtained using this process, XRD patterns do not display any additional peaks corresponding to the crystalline Cu or Mo species. This is an indication of incorporation of these dopants in the lattice of TiO2. Furthermore, incorporation of dopants is confirmed by Raman spectroscopy as shown in Fig 5. Four active Raman modes of Eg (134 cm-1), B1g (388 cm-1), A1g (507 cm-1) and Eg (625 cm-1) are observed for anatase phase. While the peaks at 187 and 446 cm-1 correspond to rutile.26, 41 There are no characteristic peaks corresponding to Cu, Mo (or their oxides) found in the samples. This indicates incorporation of the dopant atoms into the lattice of TiO2 and is consistent with the XRD results. However, there is significant peak broadening and shift in the peak position for doped samples.

The binary phase diagrams of CuO and MoO3 do not suggest any solid solubility in TiO2 at temperature less than or equal to 500˚C. The possibility of the dopants getting incorporated into the lattice therefore arises from a metastable solution being formed through sol-gel process or of solubility being higher when a ternary system is 9



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used. As high as 8 mole % Cu is incorporated into TiO2 using this process. For dopant concentrations higher than 8 %, we begin to see peaks corresponding to crystalline Cu species as shown in Fig S4 (supplementary section). Fig 4 indicates that the binary and ternary compositions obtained using sol-gel synthesis is single phase. Consistent with this, there is a positive shift in the 2θ position of the (101) peak suggesting perturbation of the lattice. The average crystallite size is calculated using Scherrer’s equation. The crystallite size and phase fraction of various ternary compositions is listed in Table S2 (supplementary section).

Another

interesting

observation

for

the

best

performing

composition

(TiO2)6%Cu,1.5%Mo is the similar crystallite size of the two phases present (anatase and rutile) . The crystallite size calculated using Scherrer’s equation is found to be ~ 13 nm (for both anatase and rutile) as shown in Fig S5 (supplementary section). Accurate measurement of the crystallite size and morphology of (TiO2)6%Cu,1.5%Mo was further revealed by transmission electron microscopy (TEM) and high resolution TEM (HRTEM) imaging as shown in Figure S6. The nanoparticles are nearly spherical in shape as shown in Fig S6 (a) and S6 (c). The HRTEM image of the selected nanoparticles confirms the presence of both anatase and rutile in the best performing sample. Histogram for the particle size distribution gives an average particle size of 10 nm with a standard deviation of 3 nm. Whereas for other ternary compositions, anatase and rutile crystallite sizes is not similar. It might also be speculated that there is an interwoven hetero-interface between anatase and rutile which aids in further enhancing the photoactivity for this particular composition.42 This is not very well understood at this point but merits attention.

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For enhanced photocataytic behavior, it is critical that the dopant atoms get incorporated into the lattice of TiO2. We tried mixing the dopant oxides CuO/MoO3 with TiO2 in different proportions and heating them up. However this process does not result in a single phase and is ineffective in getting doped systems. These oxides are synthesized using microwave assisted synthesis (ref: Section 2.2) and the XRD patterns are presented in Figure 6. The XRD peaks for (TiO2)MW can be indexed to the tetragonal anatase phase without any indication of the rutile peak (JCDPS211272). While for (CuO)MW nanopowders, all the peaks are indexed to the monoclinic phase (JCPDS-80-0076). The XRD pattern of MoO3 is consistent with the orthorhombic phase (JCPDS-35-0609) and does not contain any precursor peak suggesting complete conversion. Depending on the composition of the binary or ternary mixture synthesized (ref: Section 2.3), there are peaks corresponding to either (CuO)MW or MoO3 seen in addition to the crystalline anatase phase (Fig 7). This means that neither Cu nor Mo gets incorporated into TiO2 structure. Hence the compositions obtained using this method is not single phase.

Fig 8 compares the photocatalytic performance of different multi-phasic mixtures (ref: Section 2.3) with (TiO2)MW. It is observed that none of these multi-phasic mixtures could outperform (TiO2)MW for the same catalytic loading. The poor performance of the mixtures could be attributed to the interfaces present between TiO2 and other oxides (CuO or MoO3). These hetero-interfaces can act as sites for recombination of the charge carriers and quench the catalytic performance. This method of mixing and annealing individual oxides to get systems with improved photocatalytic performance is inadequate and therefore discarded. Hence it is crucial 11



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for the dopants to form a solid solution and consolidate into the lattice for improved performance. A comparison of the best performing sol-gel sample and the best performing microwaved TiO2 (Fig S7, supplementary section) shows that the pure microwaved titania performs better than the sol-gel ternary composition. However, as discussed earlier a one-on-one comparion even between titanias with the same average composition but prepared by different methods is at first not valid comparison. Secondly, a verdict on the ability of ternary microwaved compositions would require the synthesis of a metastable ternary solid solution which has not yet been possible. If done, however, the current results indicate that they would outperform even the ternary sol-gel compositions.

The surface, sub-surface components and oxidation states of the best performing sample (TiO2)6%Cu,1.5%Mo are investigated using XPS analysis as shown in Fig 9. The XPS peaks at 464.6 and 458.9 eV correspond to Ti 2p1/2 and Ti 2p3/2; this is reasonable since Ti is expected to be in +4 state in TiO2. A spin orbit splitting of 5.7 eV further confirms the XPS signal from Ti4+ species.43 The peaks observed at 932.4 and 952.1 eV are attributed to Cu 2p3/2 and Cu 2p1/2 state respectively. In addition to these peaks, presence of Cu in +2 states is confirmed by the presence of the well known shake up satellites.29 Signals at 235.5 and 232.5 eV correspond to Mo 3d3/2 and Mo 3d5/2 respectively. These values along with the spin-orbit splitting of 3.1 eV corroborate the presence of Mo in Mo6+ state.

Fig 10 shows the diffuse reflectance UV-Vis absorbance spectra of different ternary compositions. For pure TiO2, the absorption edge is around 400 nm, characteristic of TiO2. For the doped systems, the absorption edge is observed to be red shifted with the absorption edge around 500 nm. The dopants introduce localized defect states 12


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within the band gap which is responsible for the red shift in the band gap transition. These defect states were also investigated with the help of photoluminescence (PL) spectroscopy as shown in Fig S8 (supplementary section) The PL spectrum of TiO2 shows two prominent features - a small hump near 425 nm corresponding to free exciton emission from TiO2 and a peak at ~ 520 nm corresponding to intrinsic oxygen vacancies/defects in the as synthesized material. For the best performing catalyst (TiO2)6%Cu,1.5%Mo , the luminescence spectrum is red shifted and there appears to be a shoulder around 600-650 nm. In terms of trends, this red shift in PL is in conformity with band gap narrowing as shown by the theoretical calculations. The charge transfer between these defect states and the VB/CB of TiO 2 results in visible light absorption. However, absorbance in the visible spectrum of sunlight doesn’t correlate to the rate of degradation as shown in Fig S9 (supplementary section). As discussed in the next section this is most probably due to deep level defects that are introduced on co-doping. Photocatalysis involves interplay between several parallel processes. One of the most important factors controlling photoactivity is the e--h+ separation and interfacial charge transfer efficiency.44-45 The ability of a defect state to act as an e-/h+ trap or a recombination centre depends on the energy levels within the TiO2 lattice. For defect states near the band edges i.e. shallow traps, electrons can be thermally excited into the CB and trapped again. For electrons trapped in defect states with energies near the middle of the band gap, recombining with a hole is more favourable than being thermally excited to the CB. These deep defects behave as recombination centres rather than trapping states. They therefore reduce the number of photogenerated free carriers that are available for transfer. Hence, while an optimum concentration of shallow states aid photocatalysis, deep level defects are always detrimental.20 The computational 13



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results support these arguments. This highlights the fact that defect levels responsible for visible light absorption could as well be potential recombination centres and hence quench the photocatalytic performance. Rendering these deep states occupied will prove to be beneficial from the point of reducing recombination of charge carriers.

Computational results:

As discussed in the previous section, experimentally we find Mo to be in the +6 oxidation state in the co-doped (Cu,Mo): TiO2 system. Our DFT calculations on the same system show the Mo 4d occupancy as 4.06, which is remarkably close to the Mo 4d occupancy in MoO3 (4d occupancy: 4.09). This indicates a +6 oxidation state of Mo in our calculations which is in good agreement with our experimental findings. On the other hand the Mo 4d occupancy in mono-doped TiO2 is 4.42. This suggests that Cu plays a crucial role in elevating the oxidation state of Mo in codoped (Cu,Mo): TiO2 system. In order to investigate this further, we calculated the 3d occupancy of Cu in Cu doped TiO2 and in co-doped (Cu,Mo): TiO2. We find these occupations to be 9.11 and 9.36 respectively. This, along with the 4d occupancies of Mo reported above suggest an electron transfer between the codopants in (Cu,Mo): TiO2. This in turn is responsible for an increase in the effective oxidation state of Mo to ~+6, in co-doped (Cu,Mo): TiO2 system. This charge transfer can also be seen in partial density of states (PDOS) calculation (Fig 11) as discussed below.

From PDOS (Fig 11) and the schematic (Fig 12), we find Cu 3d defect states above the valence band maximum (VBM) when TiO2 is doped with Cu. On the other hand, 14


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we find Mo 4d states below the conduction band minimum (CBM) in case of Mo doped TiO2. For co-doped (Cu,Mo): TiO2 system, as expected we find Cu 3d states both above the VBM and Mo 4d below the CBM. These Cu and Mo d states lead to a reduction of 0.8 eV in the Kohn-Sham gap of the co-doped (Cu,Mo): TiO2 system with respect to bulk TiO2 (see Table S3, supplementary section). In addition to these defect states, we also find deep lying Cu 3d defect states in the co-doped (Cu,Mo): TiO2 system. It is also important to note that unoccupied Cu 3d defect states in Cu doped TiO2, are partially occupied in the co-doped (Cu,Mo): TiO2 system. On the other hand, partially occupied Mo 4d states below CBM in Mo doped TiO2, are rendered empty in co-doped (Cu,Mo): TiO2.46 This is attributed to the charge transfer between Mo and Cu mentioned above.

Reduced recombination of charge carriers is an important criterion from the point of view of designing new materials for photocatalysis. However, equally important is the suitable alignment of the VB and CB edges with respect to the redox potential of water.47 It is important to highlight that co-doping TiO2 with Cu and Mo does not disturb the way the redox potentials of water straddle the CBM and VBM of the material. However the band structure modification does ensure a substantial improvement in absorption of the solar spectrum owing to a band gap reduction of 0.8 eV (Table S3, supplementary section). Photocatalysis is a complex process and involves several competing pathways. There is a sweet spot where there is enhanced e--h+ separation and interfacial transfer where encouraging results are obtained.

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Conclusions:

We show that co-doping TiO2 with Cu and Mo significantly improves its photocatalytic activity, when compared to mono doping (using either Cu or Mo). The enhancement in activity is observed only if the synthesis route results in titania in which dopants have been forced into the TiO2 lattice. Cu and Mo are shown to be present in the +2 and +6 oxidation states respectively by XPS; this trend is entirely consistent with DFT calculations reported here. A systematic study of the ternary system shows that TiO2 containing 6 mole % CuO and 1.5 mole % MoO3 is the most active ternary composition with a rate constant of 1.2× 10-2 min-1. Using DFT calculations we show that co-doping of TiO2 using (Cu,Mo) introduces levels above VB, and below CB, resulting in a significant reduction in the band gap. This is consistent with the reported absorption spectrum. However co-doping also introduces deep defect states, which can have a deleterious impact on photoactivity. This explains the confined compositional space over which promising results are achieved. Supporting Information: Recyclability and stability of the best performing catalyst (TiO2)6%Cu,1.5%Mo, degradation of cationic dye (methylene blue) using (TiO2)6%Cu,1.5%Mo, XRD pattern for the composition (TiO2)9%Cu,1.5%Mo, crystallite size for various ternary compositions obtained using sol-gel, comparison of photocatalytic degradation of (TiO2)SG,(TiO2)MW and (TiO2)MW-Ternary, comparison of photocatalytic activity of

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(TiO2)6%Cu,1.5%Mo under UV and visible light and variation in VBM (ΔEv), CBM (ΔEc) and energy band gap (ΔEg) of doped TiO2 system with respect bulk TiO2.

Acknowledgement:

The authors would like to acknowledge SERIIUS (Solar

energy research Institute for India and United States) for the financial support. Computational resources for this work were provided by Center for Development of Advanced Computing, Pune and Supercomputing Education and Research Center, IISc. We would like also like to thank Micro and Nano Characterization Facility (MNCF) at CeNSE for the characterization facilities. Tiju Thomas would like to thank the Department of Science and Technology for financial support in the form of grant no. DST 01117.

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Figure Captions: Figure 1. Ternary plot showing various compositions obtained using sol-gel method. (1) (TiO2)1%Cu,1.5%Mo (2) (TiO2)1%Cu,2%Mo (3) (TiO2)2%Cu,1.5%Mo (4) (TiO2)1%Cu,3%Mo (5) (TiO2)2%Cu,2%Mo (6) (TiO2)3%Cu,1.5%Mo (7) (TiO2)1%Cu,4%Mo (8) (TiO2)4%Cu,1.5%Mo (9) (TiO2)1%Cu,5%Mo (10) (TiO2)3%Cu,3%Mo (11) (TiO2)5%Cu,1.5%Mo (12) (TiO2)1%Cu,6%Mo(13) (TiO2)6%Cu,1%Mo (14) (TiO2)4%Cu,4%Mo (15) (TiO2)6%Cu,2%Mo. The green box shows the composition of the best performing sample which is (TiO2)6%Cu,1.5%Mo. Figure 2. Comparison of photocatalytic degradation of various ternary compositions (TiO2)1%Cu,1.5%Mo, (TiO2)1%Cu,2%Mo, (TiO2)1%Cu,5%Mo, (TiO2)5%Cu,1.5%Mo, (TiO2)6%Cu,1.5%Mo, (TiO2)7%Cu,1.5%Mo, (TiO2)8%Cu,1.5%Mo and (TiO2)3%Cu,3%Mo. (TiO2)6%Cu,1.5%Mo is the most potent combination with a rate constant of 1.2 × 10-2 min-1. Figure 3. Enhanced photoactivity of co-doped (TiO2)6%Cu,1.5%Mo over monodoped (TiO2)6%Cu and (TiO2)1.5%Mo showing a synergistic effect between Cu and Mo.

Figure 4. XRD pattern of the ternary compositions (TiO2)1%Cu,1.5%Mo, (TiO2)1%Cu,2%Mo, (TiO2)1%Cu,5%Mo, (TiO2)5%Cu,1.5%Mo, (TiO2)6%Cu,1.5%Mo, (TiO2)7%Cu,1.5%Mo, (TiO2)8%Cu,1.5%Mo and (TiO2)3%Cu,3%Mo obtained using sol-gel can be indexed to pure anatase peaks. These ternary compositions are single phase and as high has 8% Cu has been incorporated into TiO2 lattice.

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Figure 5. Raman spectrum of (TiO2)SG, (TiO2)6%Cu , (TiO2)1.5%Mo and (TiO2)6%Cu,1.5%Mo indicating the incorporation of dopant atoms into the lattice of TiO2. Figure 6. XRD patterns of (TiO2)MW and CuO synthesized using microwave synthesis. Also shown is the XRD for MoO3 obtained using thermal decomposition of ammonium molybdate. Figure 7. XRD pattern of multi-phasic mixtures obtained by simple mixing and sintering of oxides (Section: 2.3). XRD of (TiO2)5CuO, (TiO2)5MoO3

and

(TiO2)Ternary shows peaks in addition to that of TiO2 (T). Peaks corresponding to CuO (C) and MoO3 (M) are obsereved in the multi-phasic mixtures. Figure 8. Photocatalytic performance of various multi-phasic mixtures (TiO2)2CuO, (TiO2)5CuO, (TiO2)10CuO, (TiO2)2MoO3, (TiO2)5MoO3 and (TiO2)10MoO3 compared with (TiO2)MW and Degussa P-25 for the same catalytic loading.

Figure 9. XPS spectrum for the best performing (TiO2)6%Cu,1.5%Mo indicating the presence of Cu and Mo in +2 and +6 state respectively. Figure 10. UV-Visible absorbance spectrum of various ternary mixtures shows red shift in absorbance for doped systems. Figure 11. a) Total DOS of bulk TiO2 and PDOS of (b) O 2p state, and (c) Ti 3d state in bulk TiO2 (d) Mo 4d state in Mo:TiO2 and in (e) (Cu, Mo):TiO2. Cu 3d state in (f) (Cu,Mo):TiO2 and in (g) Cu:TiO2 system. The top of the bulk TiO2 valence band is set to be zero. The rest of the PDOS are aligned such that the Ti 3s states are 25



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at the same energy as in bulk TiO2.The dashed red line indicate Fermi energy in the corresponding system. Figure 12. Relative positions of CBM and VBM (dotted lines), the Fermi levels (dashed red lines), and defect states (wherever relevant) of doped and undoped TiO2 are shown. The conduction and valence bands of materials studied are juxtaposed against the redox potential of water. (a) redox potential of water at normal hydrogen conditions (b) bulk TiO2 (c) Cu: TiO2 (d)Mo: TiO2 (e) (Cu,Mo): TiO2. Deep level defect states which are potential recombination centres, are seen in the forbidden gap in (Cu,Mo): TiO2.

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Figures Figure 1:

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Figure 2

Figure 3

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Figure 4

Figure 5

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Figure 6

Figure 7

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Figure 8

Figure 9

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Figure 10

Figure 11

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Figure 12

Graphical Abstract

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Synergistic Effect of Mo + Cu Codoping on the Photocatalytic Behavior

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