Cu and Ag modified mesoporous TiO2 nanocuboids for visible light driven photocatalysis

Nano-Structures & Nano-Objects 21 (2020) 100420

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Cu and Ag modified mesoporous TiO2 nanocuboids for visible light driven photocatalysis M.P. Nikhila a , Deepthi John b,c , Mrinal R. Pai d , N.K. Renuka a ,

∗

a

Department of Chemistry, University of Calicut, Kerala 673 635, India Department of Chemistry, Deva Matha College, Kuravilangad, Kottayam, Kerala, India c School of Environmental Studies, Cochin University of Science and Technology, Kerala, India d Chemistry Division, Bhabha Atomic Research Centre, Mumbai, India b

article

info

Article history: Received 22 August 2019 Received in revised form 10 December 2019 Accepted 19 December 2019 Keywords: Mesoporous titania Heterojunction Surface plasmons Solar energy harvesting

a b s t r a c t Visible light driven photocatalysis by Cu and Ag incorporated mesoporous titania nanocuboids is monitored in this work. The mesoprous titania is obtained by using non-ionic triblock copolymer P123 as the structure-directing agent, which yields self-organized meso structured titania nanocuboids. The catalysts are well characterized using XRD, SEM, TEM, N2 adsorption study, DR-UV, FTIR, photoluminescence, Raman and X-ray Photoelectron Spectral analysis. The co-exposed 101 and 001 planes of cuboids are observed to be favourable for the photo catalytic activity of anatase titania. Both CuO and Cu2 O are identified in Cu doped titania, while silver in zero oxidation state is observed in Ag modified system. The absorbance of the parent titania is extended to visible region by the incorporation of these co-catalysts, and the visible light driven photocatalytic efficiency of the system is commendably improved, as confirmed by methylene blue and phenol degradations and hydrogen production through water splitting. The copper doped mesoporous assembled titania nanostructures promised itself to be the best candidate among the series, which excelled almost all other TiO2 based systems that operated under identical conditions. © 2019 Elsevier B.V. All rights reserved.

1. Introduction The innovative discovery of photoelectrochemical water splitting at TiO2 electrode by Fujishima and Honda [1] has enabled the recognition of an efficient photocatalyst. Though the literature is immensely rich with titania based photocatalysts, the environmental concern and commercial applications keep the research area ever green. However, the universal acceptability of TiO2 is limited by its restricted absorbance in the UV region that regulates its service in visible light induced catalysis, which is a more practical window of application. Besides, the high recombination rate of photogenerated electron–hole pairs also sets back the application potential of pure titania. To head off these problems, many approaches have been thought of by the researchers [2–7]. Among these, tuning the morphology of photocatalysts has attracted considerable attention, because change in morphology can alter charge carrier diffusion pathways. Efforts have been made to develop highly efficient materials, with superior morphologies [8–10] as well as exposed high energy faceted nanostructures [11–16]. In addition, a common strategy that has been studied was chemical reduction, which helps to develop ∗ Corresponding author. E-mail address: [email protected] (N.K. Renuka). https://doi.org/10.1016/j.nanoso.2019.100420 2352-507X/© 2019 Elsevier B.V. All rights reserved.

defect centres such as Ti3+ and oxygen vacancy, species identified as the control centres of photocatalytic efficiency. Along this line, dedicated attempts are being made for the modification of the crystal structure and surface defect states thereby achieving longer wavelength absorption. Enhanced visible light induced catalytic efficiency of TiO2 is achieved by doping the photocatalysts with species like metals, non-metals, carbonaceous materials, semiconductor metal oxides etc. [17–25]. In the present work, mesoporous assembled titania nanostructures (MT) are synthesized using non-ionic triblock copolymer P123 (EO20 PO70 EO20 ) as the structure-directing agent, which can promote the co-operative assembly with relatively weak interactions at the titanium/surfactant interface, and yield self-organized meso structured titania nanocuboids with good thermal stability. The co-exposed 101 and 001 planes of cuboids are observed to be favourable for the enhanced photocatalytic activity of anatase titania. Silver and copper are the chosen additives in TiO2 , so as to tune the absorption to visible and near IR spectral range. The supporting effects of these co-catalysts have already been proven in photocatalysis [26–29]. Degradation of organic contaminants and hydrogen generation by water splitting have been monitored to examine the Photocatalytic efficiency of the catalysts under visible light. The reactions selected deserve attention in the sense that they address environmental pollution issues and energy crisis, in the respective order. Here, Ag nanoparticles, due to their

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surface plasmon resonance, and CuO, due to its narrow band gap (1.7 eV), act as co-catalysts and induce an activity under visible region irradiation. 2. Experimental 2.1. Materials The chemicals used in this study include Titanium dioxide anatase (Sigma Aldrich), Titanium butoxide (Aldrich), Acetyl acetone (Sigma Aldrich), P123(Sigma-Aldrich), sodium hydroxide (Merck, 98% purity), Hydrochloric acid (Merck), Ethanol (Merck), Silver nitrate and Copper nitrate (Merck) were used as-received without further purification. 2.2. Synthesis of mesoporous assembled titania nanostructures Reported procedure was adopted for the synthesis of mesoporous assembled titania [29]. To a premixed ethanol (Merck) P123 (Sigma Aldrich) solution, HCl (Merck) was added. Tetrabutyltitanate (Sigma Aldrich) and acetylacetone (Sigma Aldrich) were added to the above solution under vigorous mechanical stirring, followed by addition of DI water after 30 min. The molar ratio P123/TBOT/Ethanol/Water/Acetylacetone/HCl was maintained as 0.025:1:28.5:30:0.5:0.005. The solution was stirred for 6 h at room temperature followed by ageing at 40 ◦ C for 9 days. The gel was collected, dried overnight at 110 ◦ C, and calcined at 500 ◦ C for 4 h to obtain mesoporous assembled titania nanostructures, denoted as MT. 2.3. Preparation of silver and copper doped titania nanostructures While screening the activity, the optimum loading with maximum activity among the doped analogues was observed to be 0.5 wt% among the selected compositions (0.2, 0.5 and 0.8 and 1.1 wt% of Cu/Ag in TiO2 ). Hence the activity studies are carried out using the above-mentioned sample of TiO2 . Required quantities of titania nanostructures were dispersed in water and ethanol mixture in the ratio (2:1) and stirred for 15 min under UV radiation to remove any adsorbed impurities on the surface. Sufficient amount of precursor nitrate solutions of the metals (Merck) (so as to achieve 0.5Wt% doping) were added, sonicated for 5 min and again illuminated with UV light for 2 h with continuous stirring. The precipitates obtained after filtration were washed with water and dried for 12 h at 80 ◦ C. 2.4. Material characterizations The catalysts were characterized using X-ray diffraction (XRD), Fourier Transform infrared (FTIR), ultraviolet–visible (UV–Vis) spectroscopy, scanning electron microscopy (SEM), N2 adsorption study, transmission electron microscopy (TEM) and Photoluminescence spectroscopy. Rigaku D/MAX-diffractometer with CuKα radiation was used for recording in the wide angle powder Xray diffraction (XRD) pattern of the samples in the scan range of 2θ , 20–80◦ . Raman spectra of the samples were recorded using Bruker multi Ram FT-NIR spectrometer with Nd-YAG Laser source (1064 nm). The morphology of the samples was investigated using Transmission electron microscopy, TEM (FEI TECNAI 30 G2, 300 kV), and SEM (JEOL, Model JSM-6390LV). N2 adsorption study was conducted on a Micromeritics Gemini surface area analyser using a static adsorption procedure of N2 at 77 K. The specific surface area of the system was calculated using BET equation for the data in the p/p0 range between 0.05–0.299. Jasco FTIR-4100 spectrometer was used for recording the FTIR spectra of samples using the KBr disc method. Jasco V-550 spectrophotometer recorded the UV–visible diffuse reflectance spectra, with BaSO4 as the reference.

2.5. Photocatalytic activity study Methylene blue dye degradation Photocatalytic activities of the synthesized nanostructures were compared using the degradation of methylene blue dye at room temperature. In a typical experiment, 75 mL of 2.325 × 10−5 M dye solution was stirred continuously with catalyst in a Luzchem LZC 4X model Photoreactor (98 W, λ = 340 nm). The irradiation experiments under sunlight were done for the doped systems between 12.00 am to 2.00 pm in the month of March and April. The intensity of sunlight was recorded using a Digital Luxmeter (Smart Sensor, AR823). Before switching on the irradiation, all the reaction solutions were stirred in dark for 30 min in order to reach adsorption desorption equilibrium. The catalyst dosage chosen was commendably low (0.25 g/L). By sampling a small amount of reaction mixture at regular intervals of time and centrifuging at 5000 rpm for 5 min, the left over methylene blue dye concentrations were examined with the help of a UV–Visible spectrophotometer. Absorbance at 663 nm is correlated with concentration using the standard curve in desired concentration range. Photocatalytic water splitting The activity of the materials towards photocatalytic reduction of water was examined using methanol as a hole scavenger, which serves as sacrificial reagent. Photogenerated holes irreversibly oxidizes methanol instead of water and hence no oxygen evolved as a result of oxidation of water. Irradiation is done with a medium pressure mercury lamp (Hg, Ace GlassInc., 450 W) in a closed rectangular quartz cell having both sampling and evacuation ports. The irradiated catalysts were placed in an outer irradiation quartz cell with the lamp surrounded with water circulation jacket to absorb IR radiation. The lamp produces wide range emission spectra with maxima at both UV and visible range [16% UV–84% visible region]. The catalytic activity experiments were conducted in static mode with water in methanol (2:1 v/v%) mixture, with well dispersed catalyst. The reaction products were analysed with an interval of 2 h over a period of 6 h. A gas chromatograph (Netel (Michro-1100), India) equipped with a thermal conductivity detector (TCD) and a molecular sieve column (4 m length) with argon as the carrier gas was employed in the isothermal temperature mode (50 ◦ C). Measurement of light intensity (Flux) Silicon diode based light metre LX 1108; Lutron Electronic measures the flux of medium pressure mercury lamp. The number of photons falling on the reaction cell or flux of medium pressure mercury lamp was determined to be 19 × 104 lx or 278.2 W m−2 in horizontal geometry irradiation of UV–Visible photoirradiator. 3. Results and discussion 3.1. Characteristics of mesoporous titania cuboids The morphology of the prepared parent mesoporous system (MT) was investigated through Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy analysis (TEM). FESEM image revealed more or less uniform (10–15 nm sized) units assembled together showing specific growth in upward direction, resembling floral petal development, which confirms certain axis directed growth (Fig. 1a). Generation of voids due to the peculiar arrangement is quite evident in the microscopy image. Fig. 1b represented low resolution TEM image, in which the piling of the primary cuboid blocks of size 10–15 nm by sharing specific crystal faces was noticed. The clear contrast in the TEM image

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Fig. 1. FESEM image (a) and TEM images (b, c) of parent mesotitania, MT; Representation of cuboidal arrangement in the meso structure (d).

indicated the presence of void spaces existed in the nanostructures. Structural mesoporosity evolution of these cuboids with co-exposed facets have been reported earlier [30]. The interconnected fabrication of individual building cuboid structure is attributed to the solvent effect. Generally, the nano units may have a strong interaction with water due to the existence of large amount of hydroxyl groups on the metal oxide surface, which leads to the formation of hydrogen bonds. However, when using the mixed solvent system (water–ethanol) as the reaction medium, the interaction between hydroxyl groups on the TiO2 surface via hydrogen bonds is weakened because of the introduction of alkyl group, thus leading to random aggregation. Fig. 1c reveals the lattice fringes of the cuboids, where two sets of lattice fringes taken at two different places of cuboid are shown. Lattices with interfringe spacing value of 0.35 nm corresponds to the 101 plane of anatase titania and 0.23 nm signifies the 001 plane. Oriented attachment growth mechanism, in which the spontaneous self-organization of adjacent nanounits occurs by sharing a common crystallographic orientation is proposed as the pathway to these cuboids in our previous study [30]. Aggregation-based growth is driven by the reduction in surface free energy. P123 molecules selectively adsorb onto {001} plane of TiO2 nanocrystals to suppress their intrinsic crystal growth along that faces. Thus P123 stabilizes the structure by lowering its surface energy because of the coverage of P123 on crystal surface [31,32]. Fig. 1d illustrates a two dimensional view of the piling of individual units in the oxide. For convenience, cuboidal geometry has been chosen for the pictorial representation [30]. Fig. 2a confirms the preferential orientation of certain planes. Diffraction peaks at 25.22, 37.8, 47.9, 53.9, 54.9, 62.6, 68.74, 70.12, and 75.02 degrees correspond respectively to (101), (004), (200), (105), (211), (204), (116), (220) and (215) planes of TiO2 in anatase phase ( JCPDS card, No. 21–1272). Considerably enhanced peaks of (200) reflection confirms a preferred orientation. The diffraction peaks from (103) and (105) planes that are parallel or quasi-parallel to the (001) plane were relatively small or not observed. When compared to (101) and (200) peaks, (004) and (105) peaks are broader indicating small crystal size along 001 plane as well as oriented growth along special axis. In particular, the relative intensity ratio of (004) and (200) peak (I(004) /I(200) ),

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is found to be 0.936, indicating the strong [001] oriented planes. The average crystallite size calculated using the Scherrer equation is found to be 11.17 nm, which is in agreement with the TEM results [30]. The Raman spectral lines of mesoporous assembled titania (Fig. 2b) showed all the characteristic raman active modes [144 cm−1 (Eg), 399 cm−1 (B1g), 514 cm−1 (A1g+B1g), 639 cm−1 (Eg)] of anatase titania, supporting the conclusion from XRD analysis. The UV–Visible absorption spectrum of the prepared nanostructure is presented in Fig. 2c. The band gap energy of the samples determined from the reflectance DR UV spectra (Fig. 2c) using the KM (Kubelka–Munk) formalism and the Tauc plot was observed to be 3.22 eV [30]. The Nitrogen adsorption–desorption studies (Fig. 2e, f) and the low angle XRD pattern (Fig. 2d) confirm the mesoporous character of the synthesized sample. Fig. 2e showed the nitrogen adsorption desorption isotherm of mesoporous assembled titania nanostructure. The sample shows hysteresis loops at relative pressures close to unity, the type IV isotherm, characteristic of mesoporous materials with H1 hysteresis loop. Huge and steep condensation/evaporation steps with P/P0 beyond 0.8 indicated the presence uniform and large mesopores and high pore volume. The pore distribution curve (Fig. 2f) again confirmed that pores are within the mesoporous range (6.2 nm–27.6 nm). A high surface area, 93.54 m2 /g, noted for the system is highly beneficial for catalysis, and the pore volume for the system is obtained to be 0.407 cm3 /g. The single strong diffraction peak in the low-angle region in Fig. 2d indicated the presence of meso-structure [30]. Mesopores packed at random with more or less regular diameter often display a single peak in low angle XRD. XPS measurement provides idea about the surface composition and the elemental chemical states of the catalyst. Fig. 2g and 2h show the X-ray photoelectron spectrum of pure titania. Ti2p spectrum (Fig. 2g) is characterized by two peaks: the peak at 458.80 eV is assigned to Ti 2p3/2 and the one at 464.3 eV to Ti 2p1/2 , which shows that titanium exists mostly as Ti4+ species. Fig. 1h displays O 1s spectrum in which the peak at 530.5 eV and 532.1 eV are assigned to lattice O2− and surface oxygens, respectively. 3.2. Characterization of co-catalyst doped TiO2 In the FTIR spectra (Fig. 3a), the band at 1631 cm−1 and 3415 cm−1 are due to the bending vibrations of H-O-H and stretching vibrations of O-H bonds which suggest the presence of adsorbed water and surface hydroxyl group. The band observed in the lower frequency range (400–900 cm−1 ) is due to the Ti-O-Ti and O-Ti vibrations. After doping with co-catalysts, the OH bands shift to lower wavenumber region. Cu and Ag doped analogues of mesoporus assembled titania, i.e., CuMT and AgMT, respectively, do not show any other specific peak in the XRD patterns (Fig. 3b), which can be attributed to the low doping content (0.5 wt%; probably below the detection limit of the X-ray diffractometer), and also shows high dispersion of components in the samples. Fig. 4a displays the TEM image and the corresponding SAED pattern (insets) of copper decorated mesoporous structure (Cu/ MT). The concentric rings in SAED pattern revealed the polycrystalline nature of the system. HRTEM image of copper doped mesoporous assembled titania and inset [Fig. 4b] showed the inter fringe spacing to be 0.24 nm, assigned to the [001] plane of anatase titania which appeared as square toped in HRTEM as reported earlier exposing the active facet [33]. Interfringe spacing of 0.182 nm which corresponds to (200) facet of metallic copper is also observed. The TEM image of silver doped mesoporous assembled nanostructures along with the SAED pattern has been portrayed in Fig. 4c. The interfringe spacing of 0.35 nm observed (inset) corresponds to the (101) plane of anatase titania, as depicted in Fig. 4d. The surface area, pore volume and pore

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Fig. 4. TEM images of co-catalyst doped mesoporous assembled titania. Cu/MT (a), (b); Ag/MT (c), (d).

Fig. 2. XRD pattern (a), Raman spectrum (b), DR UV spectrum (c), low angle XRD pattern (d), Nitrogen adsorption–desorption isotherm (e) and the BJH pore distribution curve (e) of mesoporous assembled titania, MT. Ti 2p (g) and O 1s (h) X-ray photoelectron spectra. Fig. 5. XPS spectra of CuO doped mesoporous TiO2 system. Survey spectrum (a); Ti2p peaks (b); O1s peaks (c) Cu2p peaks (d).

Fig. 3. FTIR spectra (a) and XRD patterns (b) of doped mesotitania analogues.

size values of Cu/MT system are 93.23 m2 /g, 0.404 cm3 /g and 17.35 nm, respectively. The corresponding values for Ag/MT system were 92.63 m2 /g, 0.402 cm3 /g and 17.30 nm. Only a marginal difference in these values was noted, when compared with pure

titania, which can be ascribed to the low weigh percentage of the co-catalysts in titania. Fig. 5a shows the XPS survey spectrum of Cu/MT system, which proves the presence of Cu, O and Ti in the catalyst. The peaks for Ti 2p3/2 and Ti 2p1/2 , are located at 459 eV and 464.6 eV (Fig. 5b) respectively [34]. O 1 s spectrum shown in Fig. 5c consists of two peaks. The major peak at 530.0 eV corresponds to O2− in the lattice. The secondary peak at 532.6 eV is attributed to the oxygen species adsorbed on the surface [35]. The XPS peaks of Cu2p3/2 exhibited two peaks upon curve fitting (Fig. 5d). The one noted at binding energy value 933.1eV corresponds to Cu+ in Cu2 O, and the adjacent peak at 934.3 eV signifies Cu2+ in CuO species [36]. In Ag/MT sample, peaks corresponding to Ti, Ag and O are seen in the survey spectrum (Fig. 6a). The peaks of Ti 2p3/2 at

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Fig. 7. Photocatalytic degradation of methylene blue in presence of catalysts (left). Amount of hydrogen produced during water splitting reaction (right).

Fig. 6. XPS spectra of Ag doped MT system. Survey spectrum (a); Ti2p peaks (b); O1s peaks (c) and Ad 3d peaks (d).

458.8 eV and Ti 2p1/2 at 464.5 eV (Fig. 6b) are consistent with the typical TiO2 . O 1 s spectrum (Fig. 6c) shows two peaks, at 530.0 eV corresponding to lattice O2− and at 531.5 eV, indicating the oxygen species adsorbed on the surface. The Ag 3d core peaks split into two components, Ag 3d5/2 and Ag 3d3/2 , due to spin– orbit coupling (∆BE(Ag 3d5/2−3/2 ) = 6 eV), and the core peaks at 368.1 and 374.1 eV (Fig. 6d) are attributed to 3d5/2 and 3d3/2 of metallic silver in zero oxidation state [37]. 3.3. Visible light driven photocatalytic activity Dye degradation Methylene blue, a heterocyclic aromatic cationic dye was chosen as the target pollutant to study the photocatalytic activity of the prepared nanostructures in visible light. Fig. 6a represents the rate of degradations of methylene blue dye in presence of various photocatalysts, which clearly indicated decrease in concentration of dye with increase in irradiation time. Under visible light, the photoactivity of the pure titania is usually very low because the illumination energy is lower than the band gap energy. It was observed that the activity of the nanostructures prepared was higher when compared to the commercially available Degussa P25 (Fig. 7a). The time taken for complete degradation was 90 min and 60 min, respectively for the former and the latter respectively, which justifies the fabrication of titania in specific morphology. Particularly, the efficiency of parent meso assembled titania cuboids can be explained as follows. The synergy of 101 and 001 planes of the cuboid structure is favourable for the enhanced photocatalytic activity of anatase titania. When the sample is irradiated with light, electrons migrate to {101} plane (acts as reduction site) and holes to {001} plane (acts as oxidation sites) of anatase titania. The selective passage of photo generated charge carriers to the specific exposed crystal facets results in the spatial separation of redox sites on anatase. This reduces the recombination of photo generated electrons and holes, leading to enhanced photocatalytic activity. The study of the reaction in presence of radical scavengers (Supplementary information, Figure S2) gives idea about the reaction pathway involved in photocatalysis by pure TiO2 . As seen from Fig. 7a, the copper and silver doped nanostructures showed better dye degradation ability when compared to the parent nanostructures. The parent TiO2 mesoporous structure

took nearly an hour for the complete degradation while the copper doped mesoporous nanostructures degrade the dye within 25 min. The silver analogue took 35 min for achieving the same result. It is clear that Cu/MT is more efficient in degradation reaction. It is worthy to mention here that the catalyst dosage chosen is 0.25 g/L, which is commendably low when compared with previous reports using titania based systems under sunlight, under identical experimental conditions [38–44]. It is believed that the presence of mesoporosity and exposed facets on the catalyst surface provide more accessibility of active sites for the reactants and products to diffuse through, resulting in increased activity [45]. The improved reaction rate by the incorporation of co-catalysts is further confirmed by studying the decomposition of phenol also (Supplementary information). Water splitting reaction The appreciable photodegradation efficiency of the doped systems in visible light towards methylene blue dye indicated the potential of the materials in photocatalytic hydrogen evolution. The detailed experimental aspects of the same have been described in detail in the characterization section. Control experiments were conducted and found that there is no appreciable hydrogen evolution without either photocatalyst or irradiation. Fig. 7b shows the hydrogen yield in presence of mesoporous assembled structures and their copper and silver doped analogues. The photocatalytic hydrogen yield is proportionally enhanced with light irradiation time. It was noticed that pure parent titania is not active for hydrogen production under visible light irradiation, due to their very large band gap which is suitable for UV absorption. Upon doping, a substantial improvement in activity is observed in the case of mesoporous assembled structures. Maximum hydrogen production of 7432.7 micromoles per gram and 16 673.2 micromoles per gram is observed for silver and copper doped samples respectively in 6 h, with no signs of decay. The higher activity of copper doped mesoporous structures again highlights its usage as a less expensive catalyst for future hydrogen energy production. Under identical conditions, this catalyst out performs the mesoporous assembled structures by a factor of 18. The activity of copper doped mesoporous assembled structures was found to be significant when compared to various reports under more or less similar experimental conditions. [46– 56]. Enhancement of photocatalytic activity of titania upon modification with adequate dopants has been well established [57–59], and is ascertained by the above mentioned results. Enhanced Photocatalytic Activity It is worth noting from the TEM images that the effective interaction between the dopants and TiO2 existed in these systems. Such a close interconnection is particularly important for photocatalytic applications where the interfacial electron transfer from dopants to TiO2 is believed to enhance the charge carrier separation and, thus, the photocatalytic efficiency. The higher activity of the doped systems when compared with the undoped ones is

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substantiated by the UV Visible diffuse reflectance and photoluminescence (PL) spectra [Fig. 8]. For the undoped nanostructures, the reflectance onset is sharp while its variation is gradual for the doped systems (Fig. 8a). UV–Vis spectral data reveal the substantially increased absorption coverage of co-catalyst doped systems compared to that of pure mesotitania. It is evident that the introduction of dopants in titania shifts the absorption of light to higher wavelength. Increased visible light and near IR absorption were noticed in the systems. Band gap values obtained by the extrapolation of hν vs. (α hν )2 (Fig. 8b) were notably lower for and Cu/MT (2.96 eV) and Ag/MT (3.0 eV) when compared to the parent system, MT (3.22 eV). This in turn assures their suitability as visible light photocatalyst. The wavelength cut off was observed at ∼400 nm for the mesoporous system, corresponding to intrinsic inter-band absorption of titania. M/TiO2 absorbs in the visible and near IR regions. Two maxima of localized surface Plasmon resonance (LSPR) peak was noticed for Ag/MT, which indicates nanoparticles of silver (LSPR at ∼415 nm) and larger particles (LSPR at ∼590 nm) [60–62]. Silver nanoparticles exhibit a LSPR with a maximum at around 410 nm in water, which is susceptible for change depending on the environment and the support. Therefore, a red-shift of LSPR is usually noticed as a result of the coupling between the metal nanoparticles and TiO2 which possessed a high reflective index (the refractive index and absorption coefficient for anatase at a wavelength of 380 nm are 2.19 and 90 cm−1 , respectively) [63]. The broad peak from 400 to 800 nm in Cu/MT system is caused by: (i) the interfacial charge transfer (IFCT) from the valence band of titania to the Cux O (x:1; 2), corresponding to 400 to 500 nm, and (ii) the interband absorption of Cu(I)oxide at 500 to 600 nm. The absorption band in the near IR is observed in the case of Cu loading, and this is attributed to 2Eg → 2T2 g inter band transitions in the Cu clusters deposited on different sites of TiO2 , due to strong interaction with the support. They represent transitions of Cu2þ located in the perfect/ distorted octahedral symmetry at 600 to 800 and 740 to 800 nm, respectively [64–66]. The grafting of Cu(II) clusters increases the absorption intensities in the 420–550 and 700–800 nm wavelength regions [67]. The slight increase in the former region can be assigned to interfacial charge transfer (IFCT) of valence band (VB) electrons to surface-grafted Cu(II) nanoclusters, and the latter is attributed to d–d transition in Cu(II). The reduced recombination of e− - h+ pairs due to the presence of co-catalysts is indicated by the decrease in PL intensity of the doped systems (Fig. 7c). The emission of all of the samples was recorded at an excitation wavelength of 300 nm, under similar excitation conditions. PL signals were at the same positions for both the parent and doped systems except that the photoluminescence intensity is decreased in the region of 455–550 nm in the case of doped series. The peak region represents the excitonic PL indicating the presence of surface oxygen vacancies and defects in the solid. It is clear that the electron–hole pairs are well separated due to the participation of additional levels introduced by the dopants, thus reducing the charge carrier recombination. Usually, when irradiated under UV light, metallic Ag nanoparticles and CuO clusters act as electron traps when doped in titania, leading to decreased recombination process and an apparent increment in the quantum yield, thus enhancing the activity. However, under visible irradiation, the above said mechanism is immaterial, as negligible absorption is expected by pure mesoporous titania under visible light excitation, which possessed a large band gap. As per the reports of Medrano et al. under visiblelight excitation, Ag nanoparticles, due to their surface plasmon resonance, and Cux O (p-type semiconductors, due to their narrow band gap with band gap energies of 2.1 eV and 1.7 eV, respectively for Cu2 O and CuO), induce an activity in the visible

region [63]. Copper oxide gets excited, and generate e− -h+ pairs in respective CB and VB, respectively and the electrons from conduction band are transferred to the same level of parent n-type titania. This in turn creates an internal electric field across the interface, which limits the charge recombination. Along with this, increase in the life time of the charge carrier; and enhancement of the interfacial charge transfer efficiency to adsorbed substrates are also realized in such type II heterojunctions [67,68]. The holes in CuO remain there, as the valence band of CuO is more cathodic than that of TiO2 . Dang et al. proposed that the co-existence of CuO and Cu2 O favoured the activity than either of the single component [69]. The situation matches with the present case, as evident from the results of XPS study. Surface Plasmon resonance of silver contributes in the same way to the activity of the support. The existence of surface plasmon resonance band absorbing in the visible region leads to efficient transfer of the photoexcited electrons from silver particles to the conduction band of TiO2 . This results in electron-deficiency in metal and electron-richness in TiO2 , and therefore, the direct photocatalytic oxidation occurs on the metal surface rather than on TiO2 [70]. These are represented in Scheme 1. Copper oxide is able to activate TiO2 in a wider range of wavelengths under visible-light irradiation, compared to the activation achieved by the presence of silver, indicating a higher concentration of charge carriers in the matrix, when irradiated with visible light. . Conclusion In summary, mesoporous assembled nanotitania structures are prepared by sol-gel method, and decorated with silver and copper adopting photo-deposition method. Both CuO and Cu2 O are identified in Cu doped titania, while metallic silver in zero oxidation state is observed in Ag modified system. The photocatalytic activities of the systems are examined towards degradation of methylene blue and phenol degradations and Hydrogen production by photocatalytic water splitting. The co-exposed 101 and 001 planes of cuboids are observed to be favourable for the enhanced photo catalytic activity of anatase titania. Visible light driven photocatalytic efficiency is commendably improved in presence of co-catalysts. The favourable activity tuning is attributed more to the presence of co-catalysts which bestows energy absorption in a wider range, rather than the creation of intrinsic defects in titania due to doping. Copper oxide is able to activate TiO2 in a wider range of wavelengths under visible-light irradiation, compared to the activation achieved by the presence of silver, indicating increased amount of charge carriers in the matrix to a greater extent when irradiated with visible light. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement M.P. Nikhila: Investigation, Writing - original draft. Deepthi John: Investigation. Mrinal R. Pai: Investigation. N.K. Renuka: Conceptualization, Supervision, Writing - review & editing. Acknowledgements Nikhila M. P. gratefully acknowledges the financial assistance received from UGC, India and also University of Calicut for providing research facilities.

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Fig. 8. UV-DRS spectra (a) Kubelkamunk plot (b) photoluminescence spectra (c) of the synthesized samples.

Scheme 1. Representation of degradation pathway in modified TiO2 in presence of visible light.

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