Synergistic Excited State involved Catalytic Reduction of (NH3-trz)[Fe(dipic)2] Complex by SnO2/TiO2 Nanocomposite

Abstract The photocatalytic activity of well-fabricated, economical SnO 2 /TiO 2 nanocomposite synthesized via hydrothermal route was validated using the (NH 3 -trz)[Fe(dipic) 2 ] complex under ultra-violet illumination. The structural features of (NH 3 -trz)[Fe(dipic) 2 ] complex was explored in detail. The catalysts were systematically examined with XRD, SEM, FT-IR UV-vis, PL, micro-Raman, VSM, AFM, HRTEM. The photoreactivity of the model compound (NH 3 -trz)[Fe(dipic) 2 ] in water/binary solvent systems was investigated. The rate of photoreaction ( k ) of nanocomposite (0.1432 sec -1 ) is higher than the SnO 2 (0.0373 sec -1 ) and TiO 2 (0.1422 sec -1 ) in H 2 O:Pr i OH (70:30%) than the rest of the solvents system. The pathways, mechanistic feature of accumulated reactive species on nanocomposite to induce adherent [Fe(dipic) 2 ] - anion and photo-reductive products were studied. The generation of hydroxyl radical on the surface of each catalyst can be identified as 7-hydroxycoumarin and discussed Supplementary data of UV-Visible absorption spectrum, photoluminescence, Raman and FT-IR spectra of iron(III) complex. Electronic absorption curves with various conditions, together with geometric and Cyclic voltammograms parameters. UV-vis DRS spectra, Photoluminescence spectra and FT-IR spectra of nanocomposites and its bare components. M-H loop curve parameters, intensity values of hydroxyl trapping experiments and efficiency data of Fe(II) formation.


Introduction
In the past years, nanoscale heterostructures have been rapidly developed through research objective due to their favorable performance in various fields. The impacts are more obvious and precise in high-technology applications and the dimension dependence of nanoparticles brings novel electronic, optical, magnetic, and mechanical properties [1][2][3][4]. Research investigations combined that the construction of semiconductor heterostructures proved to be an excellent method for improving the photosynthetic function of a material. [5]. More formulations have been emphasized on hybrid heterostructure systems in recent years by coupling two/more metal oxides and the resulting new nanocomposites exhibit high photocatalytic reactivity compared with bare components [6][7][8]. Tin dioxide (SnO2) is one of the acceptable n-type semiconductors with chemical and thermal stability (wide bandgap = 3.6 eV at 300 K) and it was used in transparent conductive electrodes, electronics, solar cells, energy storage, gas sensors, and photocatalysis [9,10]. Tin oxide is a good electron acceptor comparing anatase TiO2 (wide bandgap = 3.2 eV at 300 K) photocatalyst because SnO2 has a more positive conduction band edge and makes it a suitable candidate for the preparation of heterostructure [11,12]. It is worthy enough to associate SnO2 with TiO2 to obtain a unique heterostructure to achieve the synergistic effect between the two semiconductors. This modifies the electronic states of the composite, allows the transfer of photogenerated charge carriers to be switched between the two semiconductors, promotes electron/hole pair separation, and changes the lifetime of the carrier. Nanocomposite SnO2/TiO2 engineering using SnO2 with TiO2 becomes the anticipated best material from the point of matching bandgap, low in toxicity, good durability, high stability, promising in the separation of photogenerated electrons/holes, and results in highly efficient photocatalyst [13,14]. It is reasonable to exploit the photocatalytic efficiency of SnO2/TiO2 nanocomposite, the effect would be higher than that of individual SnO2 and TiO2 nanoparticles. Second, synthesis of this nanocomposite with a tunable and uniform composition comprising a tight interface [15][16][17]. The hybrid formation of TiO2 with SnO2 modifies the electronic structure and can be used to control and enhance the surface chemical and physical properties of SnO2/TiO2 nanocomposites [18][19][20]. The immobilizing of nanocomposite with the synthesized coordination compound can be considered as a good way for additional processes such as photocatalyst recovery and recycling [21,22]. Kinetically labile, high spin, hexacoordinated iron(III) complex shows enormous aspects in photophysics and photochemistry [23]. Transition metal complexes with nitrogen donor can provide versatile properties such as limited steric hindrance, structural lability, and sensitivity to surface platforms as functional materials [24][25][26]. The spin cross over (SCO) is possible in d 5 system of the metal complex at the metal centre. Due to external stimuli such as irradiating light source, temperature and pressure changes, it affects the spin states to switch over between high spin state to low spin state or vice versa. This induces notable modifications in the structural, vibrational, magnetic, or optical properties of the complex [27]. In the present piece of investigation, we report for the first time to our knowledge the controlled synthesis of SnO2/TiO2 nanocomposite photocatalyst coupled with an iron(III) complex to induce photoreduction activities probed under UV illumination.

Synthesis of Tin Oxide Nanospheres
All the chemicals used in this experiment were AR grade. Tin oxide nanoparticles were synthesized with a modified route using dimethyl oxalate as the precipitator [29].

Synthesis of SnO2/TiO2 Nanocomposite
Nanocomposite formation is a complex process that consists of dissolution, precipitation, reorganization, and hydrothermal process [30].

Materials and instrumentation
The structural and phase identification of nanomaterials were carried out employing X-ray diffractometry (PXRD patterns: the range of 2θ = 10-80°, step scanning using 2θ increments of 0.02° and a fixed counting time of 5 secs/step) with Cu-Kα radiation (wavelength of the X- Lifetimes were determined by fitting the data to exponential decay models using software packages. Cyclic voltammetry data were obtained using Autolab interface electrochemical analyzer Single crystal of the complex was mounted on an Oxford Diffraction Xcalibur diffractometer with an Eos (Nova) detector consists of ω and φ scan modes to obtain single crystal analysis.
All diffraction measurements were performed at 293(2) K using graphite monochromated Mo-Kα radiation (λ = 0.71073 Å). The structure was solved by direct methods and refinement by full-matrix least-squares on F2 using 32-bit Olex 2-1. and 80-1400 K temperature). with a Shimadzu spectrophotometer. OH coumarin, which is a highly fluorescent product. A certain quantity of coumarin (1 x 10 -3 M) was dissolved in 100 ml of water and 300 mg of catalyst dispersed, the mixture was placed under ultra-violet irradiation for 60 min. After irradiation, the mixture was filtered and measured by fluorescence spectra at excitation wavelength of 450 nm.

Description of crystal structure
As presented from X-ray analysis crystal data, the complex ( axis [32,33]. The crystal structure data, structural refinement, and geometric parameters are presented in Table 1 [37]. The existence of weaker π….π stacking relationships between the pyridine rings also leads to the crystal structure stability.

UV-Vis Spectrum
The appeared at the range between 296-390 nm with a high-intensity energy shoulder of ligands [39]. Fe(III) ion is coordinated with ligands, the emission bands at 378 nm with slight redshift and weak intensity shoulder enhancement were observed for the emission band of (NH3trz)[Fe(dipic)2] complex [40]. The bands appeared in a blue region between 412 to 486 nm due to intra-ligand -* transition. respectively [41]. The metal ion is substituted on the two carboxylic acid results in the disappearance of the characteristic symmetric and asymmetric vibrations of carboxyl groups.

Raman Spectrum
The band appears at 648 and 682 cm -1 can be attributed to vibrations of Fe-N and O-Fe-N groups respectively [42].

Electrochemical measurements
The redox behavior of (NH3-trz)[Fe(dipic)2] complex was investigated by cyclic voltammetry in acetonitrile at room temperature. The experiments were carried out using a three-electrode system (working electrode as platinum foil, counter electrode as platinum rod, and Ag/AgCl in KCl (sat.) as reference electrode) and tetrabutylammonium perchlorate as the supporting electrolyte. A cyclic voltammogram of (NH3-trz)[Fe(dipic)2] complex is displayed in Fig. 3 and exhibits one quasi-reversible reduction region and one irreversible region and the potential values are presented in the Table T2 at

FT-IR Analysis
Fourier transform infra-red spectroscopy reveals vibrations of functional groups such as the N-H group coordinated to 4-amino-4H-1,2,4-trizol-4-ium cation, nitrogen atom and aromatic CHs of pyridine dicarboxylate groups and binding interaction of metal to the ligand in the complex.
As depicted in Fig. S4, the strong bands at 1650 and 1380 cm -1 can be attributed to asymmetric and N-H of the inorganic cation of counter-ion of 4-amino-1,2,4-triazole moiety as disclosed by the X-ray crystal structure [48,49].

Magnetic property
The magnetic property of Fe 3+ ion in (NH3-trz)[Fe(dipic)2] complex was measured at room temperature by vibrating sample magnetometer as presented in Fig. 4a.

PXRD analysis
The crystal structures and phase purity of hydrothermally prepared bare SnO2, TiO2 nanospheres, and SnO2/TiO2 nanocomposite were characterized by powder X-ray (XRD) spectroscopy. Fig. 5 exhibits all the diffractions peaks well indexed to the pure tetragonal phase (space group P42/mnm) of SnO2 (JCPDS card No. 041-1445) [51]. The XRD pattern of SnO2/TiO2 nanocomposite can be easily distinguished with two separate phases from the mixture, the smaller particle size of the anatase phase of TiO2 combined with the slightly larger particle size of the tetragonal phase of SnO2 [52,53]. After annealing at ~500°C for 3 h, the absence of impurities or secondary phase was inducted as supported in the XRD analysis. All  Table 2. Consequently, it could be deduced that the SnO2 tetragonal phase and TiO2 anatase phase coexisted in these nanocomposites, in good agreement with the SEM and TEM results.

Raman Analysis
Raman spectroscopy is one of the versatile characterization tool for hydrothermally prepared SnO2/TiO2 nanocomposite, bare SnO2 and TiO2 nanospheres at the excitation wavelength of 784 nm as shown in Fig. 6 and presented in Table 2. As can be seen, the rutile phase of SnO2 has a D4h symmetry with two formula units per primitive cell and lattice constants a = b = 4.7373 Å and c = 3.1864 Å [55]. The Raman scattering spectra of SnO2 has three vibrational active modes A1g, Eg, and B2g were identified and corresponding to 445, 624, and 775 cm -1 modes respectively. The anatase phase of TiO2 has a tetragonal symmetry with point group 4/mmm, four active modes are observed 141 cm -1 (Eg), 394 cm -1 (B1g), 514 cm -1 (A1g), 637 cm -1 (Eg) [56]. The anatase modification has six Raman active modes (A1g + 2B1g + 3Eg).

Photoluminescence Spectra and Life time studies
Photoluminescence (PL) spectra of nanostructured materials convey the charge separation and photogenerated electron/hole recombination characteristics of the photocatalyst. The steadystate photoluminescence (PL) spectra of nano SnO2, SnO2/TiO2 nanocomposite, and nano TiO2 exhibit predominant emission peak at λ ~ 400 nm, two sub-peaks at λ ~ 467 and 521 nm, and additionally very weak peaks as depicted in Fig. 11a. It is imperative to understand that the emission is caused by the recombination, and free excitons as well as bound excitons (selftrapped excitons). It becomes also clear that the sub-peaks observed are induced surface defect states if compared with energetic positions. It is well known that lowering peak intensity refers to delay in electron/hole recombination, higher charge separation efficiency, and resulted in better photocatalytic activity [62]. The intensity of emission decreased in the order: nano SnO2 (421 nm) > nano TiO2 (398 nm) >> SnO2/TiO2 nanocomposite (397 nm). Table 2 illustrates the high-intensity emission band at ~ 400 nm (weak ultraviolet emission) is assigned to selftrapped excitons and ~ 467 nm free excitons [63]. The ~520 nm (green emission) is due to band-band transition along with a set of weak intensity emission bands mentioning the formation of a significant number of trapped states, which are responsible for various PL signals in the visible region [64]. Currently, an important experimental technique, time-  Table 2. This is equated with the efficiency of the various

Magnetic property analysis
The magnetic properties of the as-prepared nanocomposite were also explored for their  Table T3. The saturation magnetization of nanocomposites is much more prominent than the bare TiO2 and SnO2 nanospheres because of the spin reorientation emerging from the spin-spin interface between Sn-O and Ti-O dipoles [70]. Among the various solvent proportions, the impact of dimethyl sulfoxide (DMSO) gave much higher than the other medium, due to more interaction of complex and nanocomposites. In addition, the effects of solvent on the less formation of hydrogen-bond interaction between template molecules to interact with the surface of the nanoparticles [71][72][73], could also be the reason that led to the different adsorption capacity and selectivity as presented in Table 3, T4.

Photocatalytic Reduction
It was explored to deduce the apparent kinetic model to estimate the photocatalytic reduction of (NH3-trz)[Fe(dipic)2] complex by SnO2, TiO2 nanospheres, and SnO2/TiO2 nanocomposite in water and binary solvent media under ultra-violet irradiation. Fig.13a SnO2/TiO2 heterostructures showed superior photocatalytic activity rather than bare SnO2 and TiO2 nanospheres. In a coupled system, efficient separation of photogenerated electron-hole pairs and less recombination played a significant role in the reduction of the coordination complex. It can be observed that the photo efficiency of SnO2, TiO2, SnO2/TiO2 nanocomposite are 49%, 88%, and 91% in H2O/Pr i OH (70/30% (v/v)) at 18 min time duration is higher than the other medium are presented in Table T5. The apparent photoreaction rate (k) of nanocomposite is 0.1432 sec -1 , which is higher than the SnO2 (0.0373 sec -1 ) and TiO2 (0.1422 sec -1 ) bare components in H2O:Pr i OH (70:30%) than the other solvents medium as shown in  were able to trap more number of hydroxyl radical, thus enhancing the availability of electrons to reduce coordination complex more effectively. After photocatalytic reduction of complex, the efficiency of catalyst decreased in the presence of scavengers. It is confirmed that, the reactive species involved in the photocatalytic reduction reactions and the importance of hydroxyl radicals and electrons in the environmental remediation.

Mechanism of photodegradation
The photoconversion of Fe(III) centre into Fe(II) due to effective separation of electron-hole separation in SnO2/TiO2 nanocomposite is illustrated in Fig. 14. The mechanism of electron flow in the heterojunction of the photocatalyst was calculated by using the EQs (1) and (2) with the band positions of the conduction band (ECB) and valence band (EVB) of SnO2 and TiO2 semiconductors.

EVB = ECB + Eg
where ECB and EVB are the CB and VB band edge potential, respectively;  is the electronegativity of the semiconducting material, which is the geometric mean of the electronegativity of the constituent atoms; E e is the energy of free electrons on the hydrogen scale (4.5 eV), and Eg is the bandgap energy of the semiconductor. The SnO2 and TiO2 conduction band (CB) edges are located at -0.13 eV and -0. 33 TiO 2 (h + ) + OH   TiO2 + OH  TiO 2 (h + )+ OH  +  O2  +  TiO2 + CO2+ H2O (8) -+ H2O  + H + H2O2 (10) Reactions (3) - (11) imply that the possible pathway of the photocatalytic process.

Hydroxyl radical study
In order to investigate the progress of hydroxyl radicals (  OH) on the surface of photocatalyst during the photocatalytic reaction. The generation of reactive organic species of each catalyst has been investigated to the photochemical reduction of the (NH3-trz)[Fe(dipic)2] complex [75]. In a typical process, coumarin used as the  OH-trapping agent, allowing the measurement of photoluminescence intensity with an excitation wavelength at 332 nm are depicted in Fig.   15. The characteristic photoluminescence (PL) emission peak at about 450 nm of 7hydroxycoumarin (7-OH coumarin) for all catalyst is measured for 60 min are presented in Table T7. Nanostructured TiO2 offered much higher PL intensity than SnO2/TiO2 nanocomposites and bare SnO2 nanoparticles. The results revealed that the TiO2 nanoparticles generate more number of hydroxyl radicals on the surface of the catalyst rather than nanocomposites and SnO2 nanoparticles.

Conclusion
By integrating nano TiO2 and nano SnO2 into a single-step hydrothermal process, a progressive hybrid design was successfully evolved. In comparison to pure SnO2 and TiO2 nanostructures,          See image above for gure legend.  See image above for gure legend.    See image above for gure legend.

Supplementary Files
This is a list of supplementary les associated with this preprint. Click to download.