Phase controlled synthesis of bifunctional TiO2 nanocrystallites viad-mannitol for dye-sensitized solar cells and heterogeneous catalysis

The crystal architecture of TiO2 was successfully tailored via a low-temperature (≤200 °C) hydrothermal process in the presence of d-mannitol for feasible applications in dye-sensitized solar cells (DSSCs) and heterogeneous catalysis. In the development of anatase-TiO2 (A-TiO2), d-mannitol does not merely acts as a complexing agent to manage the zigzag chains of octahedral TiO62− with dominant edge sharing but also performs as a capping agent by influencing the hydrolysis process during nucleation, as confirmed by Fourier-transform infrared spectroscopy and dynamic light scattering studies. After physical measurements, the as-synthesized nanocrystallites (NCs) of A-TiO2 were used in DSSCs, where a fascinating power conversion efficiency (PCE) of 6.0% was obtained, which showed excellent performance compared with commercial anatase-TiO2 (CA-TiO2: 5.7%) and rutile-TiO2 (R-TiO2) obtained without d-mannitol (3.7%). Moreover, a smart approach was developed via the A-TiO2 catalyst to synthesize pharmaceutically important C-3 alkylated 4-hydroxycoumarins through different activated secondary alcohols under solvent-free, and heat/visible light conditions. In addition, the catalytic activity of the so-produced A-TiO2 catalyst under solvent-free conditions exhibited remarkable recyclability with up to five consecutive runs with negligible reduction, which is superior to existing reports, and clearly reveals the novelty, and green, sustainable nature of the as-synthesized A-TiO2 catalyst. A plausible reaction mechanism of both coupling partners was activated through the interaction with the A-TiO2 catalyst to produce valuable C-3 alkylated 4-hydroxycoumarins with 95% yield and high selectivity.


Introduction
Titanium dioxide (TiO 2 ) mainly exists in three polymorphs: anatase (tetragonal), i.e., A-TiO 2 ; brookite (orthorhombic), and rutile (tetragonal), i.e., R-TiO 2 ; these are regarded as different orientations of the shared edges/corners of the [TiO 6 2À ] octahedra. 1 The TiO 2 polymorphs play an important role due to their outstanding electrical, optical, and catalytic properties, as well as their chemical stability under both acidic and alkaline conditions, which have enabled their applications in energy storage, photovoltaic cells, and photocatalysis in the last few decades. 2 For instance, A-TiO 2 is largely preferred in photovoltaics and photocatalysis applications over the other two polymorphs, due to its abundance, environmental friendliness, low cost, excellent electron mobility, catalytic activity, and availability of several cleanroom-free chemical/physical synthesis methods. Generally, the higher performance of the A-TiO 2 over R-TiO 2 because of its maximum dye-loading capacity and superior incident photocurrent conversion efficiency. Moreover, the high refractive index and UV absorptivity of R-TiO 2 make it a potential candidate in the application of optical communication devices. 3 The crystal phase transition of TiO 2 polymorphs using simple, cost-effective, and low-temperature solution-based methods with different morphologies has attracted considerable research interest for technologically important and practical applications. Normally, peptization, a modier/surfactant/chelating agent, capping agent, cohesive agent, and annealing temperature play crucial roles during the phase transition pathways of these TiO 2 polymorphs, in addition to intrinsic parameters for instance pore size, concentration of the Ti precursor, energy on the surface, aggregation rate, pH value of solution, and crystal growth dynamics. 4,5 Many solution-based methods have been reported for the synthesis of different TiO 2 polymorphs such as solvothermal, wet chemical, and sol-gel. 6 Among these, hydrothermal synthesis is used to obtain phase homogeneity and morphological control at low temperatures. 7 Zhou et al. reported the hydrothermal crystallization process for the synthesis of TiO 2 via a dissolution-crystallization mechanism. 8 At a specic temperature, hydrated Ti ions begin to dissolve to produce Ti 4+ hydrated octahedral monomers, which, eventually, form polymers via a condensation reaction. Consequently, connection with octahedral monomers, the formation and growth of the TiO 2 nuclei are initiated. 9 Thus, the architecture of Ti 4+ hydrated octahedral monomer and nucleation process under hydrothermal conditions are major components involved in the fabrication of TiO 2 polymorphs. Recently, Oskam et al. reported the nucleation and growth mechanism of different TiO 2 polymorphs by applying precursor chemistry, which was mainly dependent on the utilization of reactants. 10 Joo et al. studied the crystal structures of TiO 2 polymorphs that are controlled from rutile to anatase without and with D-sorbitol acting as a complexing agent. 11 Yanqing et al. examined the impact of the TiCl 4 concentration and pH of solution on nucleation processes of TiO 2 polymorph. 12 Until now, the formation of TiO 2 polymorphs with a denite crystal structure, shape, surface orientation, and nucleation process under hydrothermal conditions remain uncharted; thus, the correlation between crystal structure formation, and the hydrolysis ratio needs to be investigated for various applications. 13 In the present work, we successfully synthesized pure phase A-TiO 2 NCs via a facile, low-temperature, and economical hydrothermal method in the presence of D-mannitol, which play role of both capping as well as complexing agent. The D-mannitol was selected due to its non-toxic biological origin, environmental friendliness, low-cost, capping ability, and capability to form an intermediate complex. The possible reaction mechanism to the formation of A-TiO 2 is based on the different orientation of shared corners and edges of the TiO 6 2À octahedral. The nucleation process of TiO 2 has been derived by [Ti(OH) x (D-mannitol) y Cl z ] nÀ , where, 4 # x + y + z # 6 during the hydrolysis process, and the relationship between the crystal structure formation and hydrolysis process has been explored. Furthermore, for comparison, we conrmed that the reactions that proceed without D-mannitol result in a phase of pure R-TiO 2 rather than A-TiO 2 . The effect of the hydrothermal reaction conditions on the crystal architecture formation as well as the photovoltaic and photocatalytic properties was investigated, where A-TiO 2 , synthesized using D-mannitol, exhibited an enhanced DSSCs performance compared to that of the R-TiO 2 prepared without it. The presented method for obtaining both A-TiO 2 and R-TiO 2 polymorphs is facile, environmentally friendly, exhibits long-term chemical stability, and involves a single-step. The TiO 2 polymorphs synthesized through this method are used in various applications, including in DSSCs and heterogeneous catalysis.
Hydrothermal synthesis of A-and R-TiO 2 The NCs of A-and R-TiO 2 polymorphs were produced by a simple and elegant one-step hydrothermal route. Typically, for the A-TiO 2 NCs, 1 M TiCl 4 and 2.5 g of D-mannitol were dissolved in DI-water water (DI, Milli-Q water; 18.2 MU cm). The reaction mixture was transferred to high pressure Teon reactor and kept at constant temperature of 180 C for 24 h. The as-procured yellowish-white precipitate was separated on centrifugation at 4000 rpm for 10 min, followed air annealing at 550 C for 30 min to get white powder of A-TiO 2 NCs (Scheme 1). A similar experimental condition was applied for the synthesis of R-TiO 2 NCs, except there was no D-mannitol in the precursor solution. A 100% A-TiO 2 paste of $20 nm sized NCs, named CA-TiO 2 , was purchased from ENB Korea and used without any modication as the reference.

Fabrication of the DSSCs
A uorine-tin-oxide substrate (FTO, TEC15, Pilkington, 15 ohm/ sq.) was sonicated through detergent in DI-water, acetone, and isopropanol for 20 min, in sequence, and dried under nitrogen gas ow, followed by 15 min ultraviolet-ozone treatment to remove presence of any unwanted residues. Typically, the TiO 2 paste was made by mixing of 1.0 g of TiO 2 NCs, 2.0 g of aterpineol, and 0.3 g of ethyl cellulose in 5.0 mL of ethanol with continuous stirring to obtain homogeneous paste for 6 h. Subsequently, the as-prepared NCs of A-, R-, and CA-TiO 2 pastes were coated on the FTO substrate via a simple doctor-blade The hydrothermal synthesis process of A-TiO 2 in presence of D-mannitol for DSSCs and heterogeneous catalysis applications. method using an adhesive tape on both side as a spacer. About eight-ten-micron thickness TiO 2 lms for each case were prepared and annealed at 550 C in a muffle furnace under an air environment at a constant heating 5 min À1 rate for 30 min. For sensitization, these TiO 2 photoanodes were heated to 60 C and kept for adsorbing 0.3 mM N-719 dye solution of 1 : 1 tertbutanol : acetonitrile for 24 h at 27 C in the dark. The Pt counter electrode having mirror nish, deposited by DC magnetron sputtering on the conducting FTO substrate, was used as the counter electrode. The two electrodes, i.e., TiO 2 with N-719 dye and Pt were clipped together, and to prevent the leakage of the iodine electrolyte solution heat resistant sublimation tape was used as a sealant. The purpose of mask is to allow light to fall only onto a active area of all the devices, i.e. the area was 0.5 cm 2 dened by the mask.

Catalyst activity
A combination of 4-hydroxycoumarin (1 mmol) and 4-methylphenylethanol (1 mmol) and A-TiO 2 NCs powder (20 mg) was stirred at 70 C for 5 h. Aer completion of the reaction (monitored by TLC) ethyl acetate was added to the reaction mixture and the catalyst was collected by ltration. The organic solvent was concentrated under reduced pressure. Aer purication by recrystallization with hot ethanol, C-3 alkylated 4hydroxycoumarins were obtained. Moreover, to achieve the best reaction conditions, the effects of various amount of catalyst, reaction time, optimal temperature, and solvent were meticulously investigated. A model reaction between 4-hydroxycoumarin and activated alcohols was monitored to ascertain the optimal reaction parameters. Aer completing reaction, obtained products were conrm and characterized by 1 H NMR spectral data.

Measurements and characterizations
The morphology observation of A-TiO 2 and R-TiO 2 were conrmed from the surface digital images by eld emission scanning electron microscopy (FE-SEM, FEI Nova NanoSEM). High-resolution transmission electron microscopy and selected area electron diffraction (SAED) measurements analyses were conducted by (HR-TEM, FEI TECNAI G2 20 S-TWIN) by using a 200 kV accelerating voltage. The phases of the TiO 2 NCs were validated by powder X-ray diffraction patterns (Shimadzu, XRD-6000, Japan) obtained using l ¼ 0.1542 nm. The phase was conrmed by Raman microscopy (Renishaw, inVia Raman microscope, UK) also. The laser beam (l ¼ 532 nm) was focused using a lens to produce a spot on the TiO 2 lm surfaces. The Fourier-transform infrared (FTIR) spectroscopy spectra were obtained to conrm intermediate reaction state from 500 to 4000 cm À1 using Nicolet, iS10, Smart MIRacle, Thermo Scien-tic. X-ray photoelectron spectroscopy (XPS) spectra were acquired using a PHI 5000 VersaProbe (Ulvac-PHI) to conrm the surface elements and the chemical states. The binding energies were used for internal calibration of C 1s peak (284.6 eV). The specic surface areas of A-TiO 2 and R-TiO 2 powders were measured through Brunauer-Emmett-Teller spectra on nitrogen physisorption instrument (BET, Belsorp II, BEL Japan INC). The amount of dye adsorbed over A-, R-, and CA-TiO 2 photoelectrode was conrmed by desorbing in a 0.1 M NaOH water-ethanol (v/v ¼ 1 : 1) mixed solution, followed by UV-Vis spectrophotometer (Varian Cary 5000 spectrophotometer) measurement by UV-Vis spectrophotometer (Shimadzu UV-2450) at 540 nm. Simulated sunlight provided by a solar simulator (Sun 2000 ABET 5 Technologies, USA) tted with an A.M. 1.5G lter was used, and the intensity of one sun (100 mW cm À2 ) was calibrated on standard silicon solar cell. A Keithley 2400 source meter was used to estimate the current densityapplied voltage (J-V) spectra of the A-, R-, and CA-TiO 2 photoanodes. The computer-controlled electrochemical impedance spectroscopy (EIS) measurement spectra were recorded on IviumStat Technologies, Netherland at a bias voltage of 0.70 V in a frequency range of 150 kHz to 0.1 Hz under dark condition.

Reaction mechanism
The TiO 6 2À octahedral with different orientations of shared edges and corners are responsible for affording A-and R-TiO 2 . Basically, R-TiO 2 is connected by ten octahedra, in which eight are corner-shared and two are edge-shared; the crystal nuclei for rutile TiO 2 is made up of linear chains of TiO 6 2À octahedra connected by corner sharing. 14 In contrast, A-TiO 2 is connected by eight octahedra, in which four are corner-shared and four are edge-shared, making zigzag chains of TiO 6 2À octahedra that are linked to each other through edge-sharing bonding. 15 The density of A-TiO 2 (3.84 g cm À3 ) less than R-TiO 2 (4.26 cm À3 ) due to more edge-shared, and larger interstitial spaces between the octahedra. 16 Notably, the crystal structure of TiO 2 can be agglomerated through different edge-shared and corner-shared TiO 6 2À octahedra orientations during the condensation processes. Generally, when Ti precursor reacts with water molecules, the coordination number of Ti 4+ increases from four to six, owing to the acceptance of oxygen lone pairs from nucleophilic ligands through its vacant d-orbitals. These sixfold octahedra structural units can be combined into the nal crystal structure formation through condensation. 17 Scheme 2 has illustrated the formation of A-TiO 2 reaction mechanism. It was found that D-mannitol anions can be substituted for Cl anions during the hydrolysis process to form [Ti(OH) x (Dmannitol) y Cl z ] nÀ intermediate complexes during crystallization. Similarly, Chen et al. demonstrated that BiOCl surface selectively adsorb molecules of D-mannitol to from hydrogen bonds with exposed oxygen atoms to decrease the surface energy during the crystal growth formation. 18 Effect of D-mannitol on the crystal structure formation and nucleation process of A-TiO 2 is worth for future investigations. In addition, the hydrochloric acid present in the TiCl 4 aqueous solution could act as a catalyst not only facilitating the formation of crystal growth via condensing the intermediate complex but also nucleation process and related species. 19 Intermediate complexes

Hydrolysis rate estimation
The effect of D-mannitol (0.1 M) on the hydrolysis process of TiCl 4 (1 M) was measured through dynamic light scattering technique. As shown in Fig. 1a, in the presence of D-mannitol, TiCl 4 in aqueous medium under goes hydrolysis to develop slow particle agglomeration with a small size distribution of $18 nm. TiCl 4 in aqueous solution, in the absence of Dmannitol undergoes hydrolysis to form fast particles agglomeration with $70 nm size distribution (Fig. 1a), conrming the interaction of D-mannitol with TiCl 4 that prevents the rapid hydrolysis (Fig. 1b). The coordination of D-mannitol anions with TiCl 4 inhibit the crystal growth resulting the formation of small-sized A-TiO 2 NCs. For an improved understanding of the hydrolysis reaction, we maintained both samples for more than thirty days (TiCl 4 and TiCl 4 -0.1 M D-mannitol) at room temperature (Fig. 1c). Aer two days, aqueous medium of TiCl 4 solution changed to white coloured precipitate with sediments. This white coloured precipitate subsequently agglomerated, and settle down slowly on the surface of bottom. However, the TiCl 4 -0.1 M D-mannitol solution remained transparent without precipitated even keep at one month, which is also supported by the dynamic light scattering method, where to preventing the fast hydrolysis process Ti cations could bond with D-mannitol anion at room temperature. We assumed that the slow hydrolysis process taking place between TiCl 4 -0.1 M D-mannitol solutions plays important role in the formation of small particles size, which ultimately supports the phase transformation process, from rutile to anatase. Though, to initiate the nucleation process during the hydrolysis of A-TiO 2 NCs from the D-mannitol comprising solution, suitable temperature is required. Xiong et al. reported, in their BiOCl study, that D-mannitol not only behaved as a cohesive agent to prompt the assembly of the structural morphology but also acted as a capping agent to prevent the crystal anisotropic growth. 22 To validate the comparison, the acidity (pH ¼ 0.6) of both solutions were kept the same for the dynamic light scattering measurement.

Structural elucidation
XRD patterns were used to characterize the crystallite structures, phases, and average sizes of the as-obtained A-and R-TiO 2 . The XRD trends of the as-synthesized TiO 2 NCs with and without D-mannitol are respectively shown in Fig. 2a      adding D-mannitol. The average crystal size was calculated using the Debye-Scherrer equation. For the as-synthesized A-, CA-, and R-TiO 2 , the average crystallite sizes were 12.0, 16.2, and 19.5 nm, respectively. Raman spectroscopy is an effective tool for identifying the molecules in A-TiO 2 NCs obtained from Dmannitol. In Fig. 2b, the major Raman shi peaks at 144, 199, 400, 519, and 635 cm À1 were attributed to the E g , E g , B 1g (B 1g / A 1g ), and E g symmetries of A-TiO 2 . 25 In the Raman spectrum, no additional peaks, except distinctive vibrations, were found, which conrms the formation of pure phase A-TiO 2 . The Raman spectrum demonstrated at 142, 235, 443, and 610 cm À1 are attributed to the B 1g , two-phonon scattering, E g , and A 1g modes of the rutile phaseof TiO 2 NCs, respectively (Fig. S2b †). The spin-orbital splitting of Ti 2p 1/2 and Ti 2p 3/2 photoelectrons corresponds to a doublet situated at binding energies of 463.9 and 458.1 eV, respectively, as shown in Fig. 2c. The peak separation of Ti 2p 1/2 and Ti 2p 3/2 was 5.8 eV, conrming the Ti 4+ chemical state which is in magnicent consensus with the literature data for A-TiO 2 ,. 26 The O 1S singlet of oxygen at 529.3 eV is shown in Fig. 2d. The Lorentzian-Gaussian mix function value was employed in the curve resolution of the individual O 1S peaks in the two spectra. The curve resolved O 1S signal revealed small sized secondary peak that is positioned at a binding energy of 531.86 eV for -OH species present on the surface. 27

Surface morphology evolution
The surface morphology evolution of A-TiO 2 NCs is shown in Fig. 3a. The FE-SEM images of A-TiO 2 conrmed the formation of small-sized spheres, where all the grains are well-interlinked to one another, which would offer various facile transporting channels for electrons (or charge carriers) through the TiO 2 NCs. The FE-SEM images demonstrate the well-interconnected $60 nm average diameter of nanocrystallites, when Dmannitol was absent in the precursor solution as shown in Fig. S2c. † The elemental stoichiometry determined by EDX mapping (Fig. S3 †) (Fig. 3b). The 0.36 nm interplanar distance in compliance with the (101) plane of A-TiO 2 NCs (Fig. 3c), and 0.32 nm interplanar distance in compliance with the (110) plane of R-TiO 2 NCs (Fig. S2d †) were approved. 28 The (101) facet of the A-TiO 2 NCs has a relatively low surface energy; thus, it has better stability than those of other facets, and the HR-TEM image also has evidenced clear and sharp rings in reference to the (101), (004), (200), (105), and (211) lattice planes of A-TiO 2 in the SAED pattern (Fig. 3d).

Specic surface area and pore-size distribution measurements
The specic surface area and porosity of both R-TiO 2 and A-TiO 2 NCs were investigated from the BET and BJH spectra. A type-IV isotherm (Brunauer-Deming-Deming-Teller classication) was due to their mesoporous nature. The hysteresis loop of type H1 suggested that the porosity of A-TiO 2 is derived from interconnected NCs. 29 Fig. 4 indicates 77.8 m 2 g À1 as the specic surface area of A-TiO 2 NCs. Furthermore, its narrow pore-size distribution centered at 13.0 nm was ascribed to the presence of relatively small inner pores between NCs and a large pore constructed by an independent structure, which agrees with the FE-SEM and HR-TEM analyses as well. Additionally, the standard multi-point BET technique was employed to calculated the specic surface area of 14.2 m 2 g À1 for R-TiO 2 , which was only one-h that of A-TiO 2 . The narrow pore-size distribution exhibited in the as-prepared R-TiO 2 was centered at 60.2 nm. Basically, both DSSCs and heterogeneous catalyst applications are porosity dependent due to the increase in the dye adsorption and active centers.

DSSC performance
The DSSC performance of CA-TiO 2 ($20 nm particle-size, 100% anatase, ENB Korea) NCs was also measured as a reference. The performances of A-and CA-TiO 2 NCs based DSSCs are given in Fig. 5. The UV-Vis absorption spectra of both photoanodes, with N-719 dye, were obtained in a wide visible wavelength range. The characteristic peak hump appeared at 530 nm was due to the N-719 dye (Fig. 5a). The N-719 dye adsorption over A-TiO 2 -NCs (0.86 Â 10 À7 mol cm À2 ) was higher than those over CA-TiO 2 (0.81 Â 10 À7 mol cm À2 ) and R-TiO 2 NCs (0.42 Â 10 À7 mol cm À2 ), which possibly as a result of the higher surface area and narrow particle size of A-TiO 2 . To quantify the dye-loading amounts over two photoanodes, these sensitized photoanodes were immersed into 0.1 M sodium hydroxide solution to desorb the adsorbed dye molecules, followed by measuring the UV-Vis absorbance spectra. Using the Beer-Lambert law, the amounts of dye absorbed over these electrodes were determined and are listed in Table 1. The current density-voltage curves of both DSSCs, i.e., CA-TiO 2 and, A-TiO 2 NCs photoanodes, under one sun illumination were measured and are plotted in Fig. 5b.  Table 1 presents a summary of the DSSCs outcome based on various parameters for both photoanodes, where, in both cases, the open circuit voltage (V OC ) of 0.70 V was nearly same because V OC mainly is determined by the difference between the oxidation potential of the electrolyte and the Fermi level of the photoelectrode. 30 Although, the as-obtained short-circuit current densities (J SC ) of both DSSCs conrmed higher performance in A-TiO 2 NCs photoanode compared to that in CA-TiO 2 , could be assigned to a narrow particle size as A-TiO 2 demonstrates more dye molecules than that of CA-TiO 2 . 31 The presence of an interconnected architecture of NCs in A-TiO 2 electrode could provide a long-range charge-transfer channels, resulting in 12.9 mA cm À2 J SC , 0.70 V V OC , 67% ff, and 6.0% PEC, which were considerably higher than those of CA-TiO 2 (12.0 mA cm À2 J SC , 0.70 V V OC , 65% ff, and 5.8% PEC) and R-TiO 2 electrodes (7.7 mA cm À2 J SC , 0.71 V V OC , 69% ff, and 3.7% PEC), Fig. 5. The signicant enhancement in the DSSCs performance of A-TiO 2 NCs photoanode could be due to its unique morphology, high electrical conductivity (2.96 Â 10 À7 U cm), relatively high specic surface area (77.8 m 2 g À1 ), and optimum electron mobility (0.124 cm 2 V À1 s À1 ). Fig. 5c reveals the incident photonto-electron conversion efficiency (IPCE) spectra of R-TiO 2 , A-TiO 2 , and CA-TiO 2 photoanodes, where two peaks, one at around 350 nm (TiO 2 ) and the other at around 520 nm, indicative of the absorbed N-719 dye, were observed. 32 The as-synthesized A-TiO 2 photoanode conrmed a maximum IPCE of 22.08%, which was 20.31% in CA-TiO 2 , and 15.12% in R-TiO 2 photoanodes, revealing a higher N-719 dye adoption of A-TiO 2 over CA-TiO 2 and R-TiO 2 . The open-circuit voltage decay plots of both electrodes measured under applying open circuit voltage conditions with respect to time are shown in Fig. 5d. At zero current, there was an exponential decrease in V OC , which supports the recombination of electrons. 33 In the graph, the A-TiO 2 photoanode exhibited a slow decrement in V OC , which is due to the electron recombination mitigation. The calculated lifetime of the electron, i.e., the time taken by an excess minority carrier to recombine, of A-TiO 2 , determined by the V OC decay method, was higher than those of the others, suggesting that the electrons are more likely to reach the working electrode than CA-TiO 2 and R-TiO 2 photoanodes. 34 Fig. 6a-d summarizes the average performances of CA-TiO 2 and A-TiO 2 photovoltaic devices. The as-synthesized A-TiO 2 photoanode exhibited a high reproducibility of J SC , V OC , ff, and PEC with a comparatively small standard deviation of 1.72%, which is substantially higher than those for the CA-TiO 2 photoanode-based DSSCs devices (1.20%), suggesting the possibility of its reusability compared to the case with others.

Energy levels
To conrm the energy alignment of dye and TiO 2 , UPS measurement was performed, which is shown in Fig. 7a, where the various energy edge ranges such as the cut-off region, survey binding energy spectra, and onset region for A-TiO 2 NCs are clearly indicated. A schematic energy band diagram of the DSSCs based on the A-TiO 2 NCs photoanode charge transfer process is shown in Fig. 7b. The high binding energy cut-off (E cut-off ) of the A-TiO 2 NCs photoanode was 8.56 eV. The binding energy onset (E onset ) was À5.30 eV, relative to the Fermi level of Au (at 0 eV). The valence band minimum (VBM) calculated using VBM ¼ hn À (E cut-off À E onset ) equation, whereas, n is the frequency of vibration of light and h is the Planck's constant estimates VBM of À7.35 eV for A-TiO 2 NCs. The band gap energy denoted by (E g ) of the A-TiO 2 NCs electrode was estimated using the following equation: where, a is the absorption coefficient, and A is the proportionality constant. Fig. S4 † presents a UV-Vis spectrum of the A-TiO 2 photoanode, where 3.2 eV E g was obtained for A-TiO 2 NCs lm, which closely matches to the reported value. 35 Using this band  gap energy, the conduction band minimum (CBM) of A-TiO 2 was calculated using the following equation: The calculated CBM of the A-TiO 2 NCs electrode was À4.1 eV. To understand the electron transfer pathways of this photoanode, the energy band diagram of the FTO/A-TiO 2 /N-719 dye/ electrolyte/Pt DSSCs was also designed. In DSSCs, on capturing photons over the N-719 dye layer, the electron-hole pairs are created, and the excited electrons are injected into the CBM of the A-TiO 2 NCs, which are then collected at the FTO substrate of the À4.6 eV work function more easily. 36 Electrochemical charge transfer process The photogenerated charge transfer and recombination processes are crucial in DSSCs performance, which can be explained with EIS measurement operated bias potential 0.70 V under dark condition. Fig. 8 shows the Nyquist curve of the A-TiO 2 NCs photoanode which demonstrates one semicircle at high-frequency region (>1 kHz), and another semicircle at a lowfrequency region (100-0.1 Hz). A relatively small semicircle at high-frequency region corresponds to the charge-transfer resistance (R 1 ), and the capacitance (CPE 1 ) at the counter electrode (platinum)/redox electrolyte interface. The second semicircle, at the low-frequency range, corresponds to the charge-transfer interactions occurring at the photoanode-dye/electrolyte interface (R 2 ), and the chemical capacitance (CPE 2 ) at the TiO 2 /dye/ redox electrolyte interface. 37 The equivalent circuit and the corresponding values obtained by the Z view soware are shown as an inset of Fig. 8, and also in Table 2.
Effects of the catalyst amount, temperature, time, and solvent system For the synthesis of C-3 alkylated 4-hydroxycoumarins, we successfully analyzed the reaction conditions, for instance catalyst amount, temperature, and required time for the completion of the reaction (Scheme 3, Table 3). In rst, of A-TiO 2 catalyst amount was tested; among them a model amount of 20 mg exhibited a notable yield. Although, when the catalyst amount was increased to 30 mg, no obvious change in the yield was observed, this may be due to the exhaustion of the catalytic sites or the maximum conversion of the catalyst efficiency.
Under the optimized experimental conditions, we then performed the reactions with different solvents and varying reaction conditions (reux, 50 C, 70 C, and under visible light irradiation) for different time periods (1-10 h). In the presence of 20 mg of A-TiO 2 catalyst, at 70 C for a reaction time of 4.5 h, 95% yield was obtained, which is one of the maximum values, so far, reported in the literature. [38][39][40][41][42][43][44][45][46][47] It was observed that aer extended reaction time and high temperature, the reaction did not proceed in the forward direction. In our study, 20 mg of A-TiO 2 catalyst under heat/visible light and solvent free conditions demonstrated a notable product yield of 95% with stipulated time period of 4.5 h. Similarly, the nature of the solvent can interrupt the reaction environment by affecting the catalytic stability, solubility, and rate of the reaction. Aer careful examination of all the results, a solvent-free condition was preferred for the synthesis of further derivatives. Thereaer, this solvent-free specic condition was used for all the reactions. Table 3 summarizes the percentage yield of C-3 alkylated 4-hydroxycoumarins under distinct reaction condition i.e. catalyst amount, reaction temperature, various solvents, and time.

Synthesis of substituted C-3 alkylated 4-hydroxycoumarins using the A-TiO 2 catalyst
Under the optimal re substituted C-3 alkylated 4-hydroxycoumarins using the A-TiO 2 catalyst (Table 4).  The plausible reaction mechanism A-TiO 2 -catalyzed synthesis of C-3 alkylated 4-hydroxycoumarins occurred by removal of a water molecule, which results in the formation of a stable carbocation, as alkylating species that were derived from alcohol on the Lewis acidic sites of A-TiO 2 (C-3 alkylation). The enolic hydroxy group of 4-hydroxycoumarin was activated by Ti metal via Lewis acid catalysis to increase the nucleophilicity of the 3-position of 4-hydroxycoumarin. The nal product C-3 alkylated 4-hydroxycoumarins was produced through an electrophilic attack activated secondary benzyl alcohols on the nucleophilic enolic hydroxy group of 4-hydroxycoumarin (Scheme 4).

Recyclability of the A-TiO 2 nanocatalyst
Aer evaluating the activity and exibility of the A-TiO 2 catalyst for different kinds of reactions, the recyclability of the catalyst was observed using the reaction between 4-hydroxycoumarin and 4-methyl-phenylethanol as a model reaction under the optimal conditions. Typically, aer completion of reaction, the A-TiO 2 catalyst was separated from the reaction mixture through a simple ltration method. The reusability of the catalyst was assessed aer activating the catalyst at 200 C for 1 h; the catalyst was employed for ve runs to test the consistency of its activity. Aer ve runs, the product yields obtained over the A-TiO 2 catalyst exhibited no noticeable decrease, suggesting a high consistency of the activity of the as-synthesized A-TiO 2 catalyst. This reusability study reveals the high stability and turnover of A-TiO 2 nanocatalysts under optimal operating conditions. The reusability of the catalysts is one and signicant advantage that makes them benecial for marketable applications.
Scheme 4 Plausible reaction mechanism for the synthesis of C-3 alkylated 4-hydroxycoumarins.

Conclusions
In summary, A-TiO 2 was successfully prepared via a simple, elegant and cost-effective hydrothermal method in the presence of D-mannitol. In this synthesis, D-mannitol plays a crucial role in the fabrication of A-TiO 2 NCs, as a; (a) complexing agent that regulates the zigzag chains of the TiO 6 2À octahedral by dominant edge sharing, and (b) capping agent that inuences the nucleation for NCs during the hydrolysis process. The FTIR spectrum impeccably suggests the formation of an intermediate complex structure between Ti 4+ and D-mannitol ions. A PCE of 6.0% is achieved from A-TiO 2 NCs photoanode, which is about 1.6 times higher than R-TiO 2 (3.7%); this may be due to the difference in the structure, surface area, dye intake capacity, and charge transfer resistance. In addition, A-TiO 2 catalyst was used for synthesizing of pharmaceutically signicant C-3 alkylated 4-hydroxycoumarins under solvent-free conditions. An excellent yield of 95% at very less reaction time of 4.5 h is obtained. The all synthesized compounds are conrmed by 1 H NMR spectroscopy. The catalytic activity of produced A-TiO 2 has remarkable recyclability up to ve consecutive runs with slight change in formed product yields, which clearly demonstrates the novel, efficient, recyclable, and economical nature of the as synthesized A-TiO 2 catalyst. Therefore, the A-TiO 2 catalyst can be a potential candidate, over other expensive and rare-earth catalysts, during organic transformation processes.

Conflicts of interest
There are no conicts to declare.