Photosensitization of TiO2 microspheres by novel Quinazoline-derivative as visible-light-harvesting antenna for enhanced Rhodamine B photodegradation

Water pollution is one of the global threats severely affecting our planet and human health. Organic textile dyes are one of the common organic water pollutants that are presentient to degradation by traditional physical methods. Semiconductor-assisted photocatalysis is considered a green, efficient, and sustainable technology for wastewater treatment. To maximize the efficient utilization of solar radiation, it is of pivotal significance to explore novel organic molecules to be employed as efficient dye sensitizers for wide-bandgap semiconductors to extend their performance to the Visible-light region. Hence, in this work, we are proposing the design and synthesis of novel structures of QAD molecule as a dye photosensitizer with extended visible light absorptivity due to the extended π–π/n–π conjugations, to promote the performance of TiO2 nanoparticles to the visible-light region and enhance the charge separation. The physicochemical characterizations confirmed the successful synthesis of QAD, TiO2, and QAD/TiO2 samples with the proposed structures. The anchoring of QAD molecules on the surface of TiO2 caused a substantial improvement in the optical characteristics of TiO2 as well as overcoming its common drawbacks by decreasing its bandgap energy to 2.6 eV, a remarkable reduction of PL intensity indicating reducing the e–h recombination and enhancing the charge separation, and creation of efficient visible light-harvesting antenna in the range of 400–600 nm. Besides, the QAD/TiO2 sample achieved a 3-fold enhancement in the observed rate constant of the photodegradation of Rhodamine B dye compared to the bare TiO2. The parameters affecting the photodegradation process were optimized and the sample displayed outstanding stability after 4 consecutive cycles. Finally, the effect of the scavengers was investigated and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{O}}_{2}^{\cdot -}$$\end{document}O2·- was proposed to be the most reactive species and the mechanism of the enhancement was suggested based on the electron injection from the QAD’s HOMO level to the TiO2’s CB. Finally, this work opens the door for various studies for the investigation of the proposed structures or similar structures in various photocatalytic/biomedical applications.


Experimental methods
Materials.Titanium tetra-(isopropoxide), methyl viologen (MV), p-benzoquinone (p-BQ), and iso-propyl alcohol (i-PrOH) were purchased from Sigma Aldrich.Rhodamine B dye (RB) was obtained from Merck, while methyl iodide was obtained from Prolabo.All other chemicals were pure and employed without the aid of any additional purifying processes.Distilled water was utilized for preparing the aqueous solutions of RB.
Synthesis of bare TiO 2 nanoparticles.TiO 2 nanoparticles were synthesized through a facile templatefree sol-gel route, according to our earlier work with some modifications 19,20 .Typically, 50 mL of Titanium tetra-(iso-propoxide) were dissolved in 0.5 L of iso-propyl alcohol then the solutions were magnetically stirred for 1 h.Then, appropriate amounts of distilled water were added dropwise for the conversion of titanium-alkoxide precursor into titanium hydroxide colloidal sol until a milky-like solution was obtained.This solution was magnetically stirred for 2 h, then kept for aging for 2 days to form the TiO 2 gel.After that, the gel was further purified by filtration and washing numerous times with isopropanol and water, then the powder was collected and dried at 100 ˚C and calcined at 500 ˚C for 5 h.Finally, the obtained powders were denoted as "bare TiO 2 ".Synthesis of QAD-sensitized TiO 2 nanoparticles.The QAD-sensitized TiO 2 nanoparticles were synthesized through a facile wet impregnation scheme 19,20 .Typically, 1%QAD/TiO 2 is prepared as follows: 0.05 g of QAD was dissolved in dimethyl sulfoxide (DMSO) and then 5 g of TiO 2 nanoparticles were added to this solution.Then, the above solution was magnetically stirred for 4 h and left overnight for aging and evaporation of the solvent.Finally, the solids were obtained and further purified through filtration and washing numerous times with methanol and water, and then dried in an electric oven at 80 ˚C, and the as-synthesized sample was denoted as "QAD/TiO 2 ".

Physicochemical characterizations.
Concerning the organic compounds, all melting points were measured on a GALLENKAMP electric melting point apparatus and are uncorrected. 1H NMR and 13 C NMR spectra (δ, ppm) were run at 300 and 75 MHz on a GEMINI NMR spectrometer (GEMINI, Manufacturing & Engineering Inc., Anaheim, CA, USA) utilizing tetramethyl silane (TMS) as an internal standard in deuterated dimethyl sulfoxide.The crystalline parameters of the as-fabricated QAD/TiO 2 and bare TiO 2 samples were explored by the X-ray diffraction (XRD) patterns measured by Panalytical X′pert PRO MPD X-ray Diffractometer with Cu Kα radiation (30 mA, 40 kV, λ = 0.15406 nm) in the range of 2θ from 5˚ to 90˚ with a continuous scanning mode, step size of 0.02˚, and step time of 2 s.The morphology images of the as-fabricated materials were investigated using Zeiss SEM Ultra 60 field emission scanning electron microscope (FESEM, 5 kV).The FT-IR spectra were measured by Thermoscientific Pye-Unicam SP-3-300 infrared spectrophotometer using the KBr-discs-based method.The optical absorbance of the as-fabricated materials was studied by measuring the diffuse reflectance spectra (DRS) by Shimadzu UV-2600 UV-Visible spectrophotometer while the measurement mode was absorption in the wavelength range from 200 to 800 nm with a scan step size of 2 nm and barium sulfate was used as a reference sample.The room-temperature photoluminescence (PL) spectra of the as-fabricated QAD/TiO 2 and bare TiO 2 samples were measured using a ThermoScientific Lumina fluorometer with the utilization of different excitation wavelengths (300-500 nm).

Photocatalytic measurements.
Aqueous solutions of RB dye were used as a typical representative model of textile dyes as the major organic pollutants to estimate the photocatalytic activities of the as-synthesized QAD/TiO 2 sample as well as the bare TiO 2 sample.A commonly used cylindrical batch reactor containing a certain catalyst dose (0.25-3.00 g/L) of the employed catalyst, and 0.1 L of RB (0.5 × 10 -5 -2.0 × 10 -5 M) solution was employed with adjusting the pH in the range 2-12 using diluted HCl and NaOH solutions.Initially, the reactor was exposed to ultrasonication for 5 min to guarantee the good dispersion of the photocatalysts in the RB solution, then the suspensions were magnetically stirred at 600 rpm for 60 min in the dark (without irradiation) to guarantee the establishment of adsorption-desorption equilibria.Then, the suspensions were exposed to UV irradiation (15W Sylvania UV-A lamp) or visible irradiation (15W Philips Fluorescent lamp) with continued stirring at 600 rpm.Through the irradiation time, 5 mL samples were withdrawn from the suspensions at various time intervals and centrifuged at 2000 rpm for 10 min and the filtrates were separated.Finally, the UV-Visible absorption spectra of the collected filtrates were recorded by Thermoscientific Evolution 300 UV-Visible spectrophotometer, where the RB photodegradation % was assessed from Eq. (1): where ' A o and A t ' are the absorbance values of the RB dye at λ max = 554 nm at equilibrium and at a time 't' of irradiation, respectively.
The stability of the as-synthesized QAD/TiO 2 photocatalyst was investigated by exploring the effect of recyclability of the as-synthesized QAD/TiO 2 photocatalyst on its photocatalytic activity toward RB photodegradation under UV-A irradiation.The experimental parameters were as follows: [Cat] = 1 g/L, pH 4, [RB] = 1 × 10 -5 M, dark time = 1 h, UV-irradiation time = 3 h for each cycle.Then, the powder was filtered and washed several times with water and then dried at 60 ˚C.Finally, the samples are re-weighed and dispersed in a fresh RB solution to maintain the same catalyst dose and other conditions are kept constant.
The effects of scavengers were studied to elucidate the reactive species using specific scavengers including methyl iodide (MeI), iso-propyl alcohol (i-PrOH), methyl viologen (MV), and p-benzoquinone (p-BQ) were separately examined.The concentrations of the scavengers were kept constant (1 × 10 -5 M) upon the comparison while keeping the other parameters to be constant ([Cat] = 1 g/L, pH 4, [RB] = 1 × 10 -5 M).The effect of oxygen bubbling was studied using the bubbling of oxygen to the solution for 1 h during the dark time only, before the irradiation at the operating conditions ([Cat] = 1 g/L, pH 4, [RB] = 1 × 10 -5 M).

Results and discussion
Characterization of novel Quinazoline-derivative (QAD).Characterizations of the novel Quinazoline derivatives demonstrated the successful synthesis of the proposed structure named

Physicochemical characterizations
Optical absorption of QAD.The UV-Visible absorption spectrum of the as-synthesized QAD molecules was compared to the spectra of QAD-Start and QAD-Intermediate samples as shown in Fig. 2. It was found that the QAD-Start and QAD-Intermediate exhibit absorption peaks in the UV region only, around 270 nm and 350 nm, where they demonstrated negligible absorption capability in the visible region.However, the proposed www.nature.com/scientificreports/novel structure of QAD-Final (QAD) exhibits the same peaks in the UV region in addition to new 2 shoulder peaks at about 390 nm and 410 nm beside an additional broad absorption peak in the visible range from 440 to 600 nm and centred at about 500 nm.These new peaks and the outstanding peaks of QAD-Final in the visible region can be accredited to the extended conjugations due to the addition of 1,3-diphenylpyrazole-4-carbaldehyde that extended the π-π/n-π conjugation through the aromatic heterocyclic ring and the two phenyl substituents as well as the extended (-N-Ph) and (-C=N-N=C-) bonds added to the QAD-intermediate structure.The significantly extended visible-light absorbance of the proposed QAD molecule indicated the successful synthesis of novel organic dye with visible-light absorption capability.Besides, this suggested the expected outstanding efficiency of QAD as a visible-light photosensitizer to wide bandgap semiconductors in general and specifically for TiO 2 nanoparticles in this work.

Crystalline parameters.
The crystalline nature of the as-synthesized samples, as well as their crystalline parameters, were investigated in detail to confirm the successful synthesis of TiO 2 nanoparticles as well as to inspect the influence of the incorporation of QAD molecules on the surface of TiO 2 nanoparticles.where 'k' is the shape factor of the nanoparticles and set as 0.9, 'λ' is the wavelength of the X-rays (λ = 0.15406 nm), 'β' is full width at half maximum (FWHM) of the diffraction peak, and 'θ' is the angle of diffraction.It is clearly noticed that the attachment of QAD molecules on the TiO 2 surface led to a little reduction in the estimated crystalline size in the QAD/TiO 2 sample when compared to the bare TiO 2 sample as stated in Table 1.
Furthermore, it was found that the lattice strain% at the (101) plane of TiO 2 has been slightly increased in the QAD/TiO 2 sample which can be attributed to the anchoring of the QAD molecules on this plane.Hence, all these findings confirmed that the QAD molecules led to the occurrence of distortions and disorders in the lattice structure of the Anatase TiO 2 phase.As reported earlier 15,18,36 , it could be assumed that the increase in strain could substantially enhance the electronic and optical properties of the as-fabricated heterojunction in the as-synthesized QAD/TiO 2 nanoparticles compared to the bare TiO 2 nanoparticles.

Structural properties.
The structural characteristics of the fabricated QAD/TiO 2 heterostructure were investigated by FTIR spectra via the KBr-discs-based method.Figure 4 shows the FTIR spectra of both bare TiO 2 and QAD/TiO 2 samples measured in the wavenumber range from 400 to 4000 cm −1 to inspect the type of the bonds in the as-synthesized QAD/TiO 2 sample compared to the bare TiO 2 sample and to confirm the attachment of the QAD molecules on the surface of TiO 2 nanoparticles.Concerning the bare TiO 2 sample, the broad peaks at 3380 and 1620 cm −1 can be ascribed to the stretching vibrations of O-H groups and the bending vibrations of H-O-H for the adsorbed water molecules, respectively 15,37 .The broad band in the range from 400 to 820 cm −1 could be assigned to those characteristic vibrations of the metal oxide bonds such as the Ti-O-Ti bridging and Ti-O stretching 4 .Similarly, the FTIR spectrum of the as-fabricated QAD/TiO 2 sample demonstrates both assigned groups of TiO 2 nanoparticles and QAD molecules with small shifts confirming the successful fabrication of the proposed heterostructure.

Morphological properties.
The microstructure and the morphology of the as-prepared QAD/TiO 2 sample have been investigated by FE-SEM as shown in Fig. 5.The FE-SEM images demonstrate the assembly of the TiO 2 nanoparticles in the form of microsphere morphology and just a few distortions have been observed after the incorporation of QAD molecules on the surface of TiO 2 microspheres. (2) The crystalline parameters at the main Anatase plane (101) and the optical bandgap energy of the as-synthesized QAD/TiO 2 and bare TiO 2 photocatalysts.

Catalyst 2θ (°) β (°) d-spacing (Å) D (nm) ɛ (%) E g (eV)
Bare TiO 2  As shown in Fig. S6, the EDX spectrum and the corresponding elemental mapping spectra of the bare TiO 2 sample show the existence of Ti and O elements only among the sample, and the estimated elemental weight and atomic percentages coincide with the typically reported ratios 19 .Besides, the EDX of the QAD/TiO 2 sample (Fig. S7) shows the presence of N and C elements in addition to the original Ti and O elements after the incorporation of QAD molecules.Additionally, the mapping spectra show the homogenous dispersion of C and N elements among the microspheres of TiO 2 ; the lower intensities of C and N elements are attributed to the low percentage of the anchored QAD molecules (~ 1 wt.%).Hence, these findings suggested the success of the employed simple wet-impregnation route to obtain a well-dispersion and homogenous distribution of the organic molecules over the TiO 2 surface.

Optical properties .
The optical properties of the as-synthesized novel QAD/TiO 2 heterostructure were studied by the inspection of its absorption profile and the estimation of its bandgap energy (E g ) and compared to those of the bare TiO 2 nanoparticles as well as investigating the photoluminescence (PL) data to evaluate the e-h recombination rates.Figure 6a showed the enhanced visible absorption of the QAD/TiO 2 sample compared to the bare TiO 2 sample, confirming the successful role of the incorporated QAD molecules as visible-light photosensitizer.Likewise, the assembled Tauc plots (Fig. 6b) revealed that the assessed bandgap energy of bare TiO 2 is 2.97 ± 0.05 eV which has been significantly decreased to 2.58 ± 0.05 eV after the incorporation of QAD molecules.Additionally, the anchoring of QAD on the TiO 2 surface not only did a remarkable shift in the absorbance edge of TiO 2 , but it also created an absorption tail and a wide absorption peak in the visible range from 400 to 650 nm.This confirmed the successful role of QAD as an outstanding visible antenna for TiO 2 to improve its light absorption capability in the visible light zone.
Photoluminescence (PL) is one of the potent techniques utilized for studying the optical properties of the semiconductors, especially their e-h recombination rates as well as their capabilities for charge separation, whereas the emission peaks are attributed to the recombination of the photoinduced electrons in the conduction band (CB) with the photoinduced holes in the valence band (VB) 17 .Figure 6c demonstrates the PL spectra of the as-synthesized materials measured at room temperature with λ ex. of 330 nm, where the bare TiO 2 sample demonstrated the highest PL intensity demonstrating their relatively fast e-h recombination rates.Nevertheless, the QAD/TiO 2 sample showed a remarkable reduction in the PL intensity proposing that the anchoring of QAD molecules on the TiO 2 surface could effectively achieve a drop in the recombination rate and enhancement in the charge separation rather than that of the bare TiO 2 nanoparticles.Besides, Fig. 6d depicts that the PL intensity of the QAD/TiO 2 is much lower than that of QAD confirming that QAD molecules and TiO 2 enhance the lifetime in the excited state of each other through the charge transfer and injection of electrons from the QAD molecules to the TiO 2 nanoparticles as described in detail in the mechanism section (Fig. 9).It is essential to confirm that the same trend and the same findings have been revealed upon the use of different excitation wavelengths as demonstrated in Fig. S8.Hence, we expect that the as-prepared QAD/TiO 2 nanoparticles would exhibit a significant enhancement in the photocatalytic performance rather than that of the bare TiO 2 nanoparticles.

The photocatalytic performance of QAD/TiO 2 nanoparticles
Control tests.Aqueous solutions of RB dye were used as a representative simulated wastewater sample including a model of organic water contaminants to measure the photocatalytic activities of QAD/TiO 2 under UV-A/Visible irradiations.First, the control experiments including the degradation of RB via photolysis under UV-A/Visible irradiations (without catalyst) and the removal of RB dye in the dark via adsorption on the catalyst surface (without irradiation) were studied as shown in Fig. 7a.It was found that there were no changes in the RB absorbance were detected in the control experiments, confirming the photostability of the RB dye against photolysis under UV-A/Visible irradiations in addition to the insignificant role of the adsorption route.On the other hand, the exposure of the suspensions of QAD/TiO 2 and RB solution to UV-A/Visible irradiations achieved a significant drop in the absorbance of RB and its complete photodegradation signifying that the photodegradation process is attributed to the combination of both QAD/TiO 2 nanoparticles dispersed in RB solution and the UV-A/Visible irradiations; these results coincide with the earlier stated findings 17 .
QAD/TiO 2 vs bare TiO 2 .The incorporation of QAD molecules on the TiO 2 surface caused a double-fold enhancement in the photocatalytic activity of QAD/TiO 2 nano-heterostructure compared to the bare TiO 2 nanoparticles, where QAD/TiO 2 achieved almost complete photodegradation (~ 98%) of RB dye in 3 h only under UV-A irradiation.However, 5 h were required to achieve 93% degradation of RB using the bare TiO 2 sample while maintaining the other parameters (catalyst dose ([Cat]), dye concentration, pH, irradiation source, etc.) to be constant.Besides, the photodegradation process was found to follow first order kinetics as reported earlier 17,18 , and the estimated observed rate constants (k obs ) were found to be 0.0226 min −1 and 0.0090 min −1 for QAD/TiO 2 and bare TiO 2 , respectively (Fig. 7b); which means the k obs has been increased by about threefold enhancement, at the same conditions, due to the incorporation of QAD photosensitizer.Therefore, these results showed the outstanding effect of incorporation of a small amount of QAD molecules (less than 1%) on the TiO 2 surface to enhance their photocatalytic activity towards the photodegradation process and boost the kinetics of the reaction, which could be ascribed to the enhancing of the charge separation as suggested from the PL data mentioned above.Thus, starting from this point, in the following sections our story will focus on the proposed novel QAD/TiO 2 nanoparticles and the optimization of the parameters affecting their photocatalytic performance toward RB photodegradation and elucidating the mechanism of enhancement of TiO 2 by novel QAD photosensitizer.
UV and Visible photoactivity of QAD/TiO 2 .The photocatalytic activity of the as-fabricated QAD/TiO 2 heterojunction was investigated under visible-light irradiation to investigate their photocatalytic behaviour under visible irradiation compared to UV-A irradiation and to show whether the type of irradiation influences the degradation pathway or not. Figure 7c demonstrates the decline in the absorbance of RB at 554 nm, which is characteristic of the "n → π transitions" in the bathochromic bonds (C=N and C=O) in the structure of RB; this is an indication of the degradation of these bonds which caused the decolourization of the dye solution.In addition to this, the temporal broadening and vanishing of the peaks in the UV range confirmed the complete photodegradation of the nuclei of the aromatic rings in the RB dye, representing the entire mineralization of RB into H 2 O and CO 2 17,38,39 .On the other hand, upon investigating the photocatalytic performance of QAD/TiO 2 under visible irradiation (Fig. 7d), it was found that the RB removal% was found to be about 82% after 3 h and the observed rate constant has been reduced to 0.0146 min −1 under Visible irradiation compared to the UV-A irradiation at the same parameter, as shown in Fig. S9.This reduction in the observed RB removal% could be assigned to the fact of the higher energy of UV-A photons compared to the lower energy of the visible photons 14,20 .However, it was found that there are significant gradual blue shifts in λ max of RB dye altered from 554 to 498 nm after 7 h of exposure to visible irradiation under a fluorescent lamp.These blue shifts can be attributed to the N-deethylation of the amino groups in the RB structure, as stated earlier in the literature 16,20,40 .Thus, the mineralization of RB dye cannot be achieved under visible light irradiation due to the nature of the RB dye itself and the different mechanisms of its degradation under visible light.The above-mentioned findings revealed that the type of irradiation not only www.nature.com/scientificreports/can influence the photodegradation% and observed rate constant, but it also affects the reaction pathway as well as the degradation mechanism.
Factors affecting the photodegradation process.In general, there are many factors affecting the photodegradation processes starting from the nature of the photocatalyst, the nature of the pollutant itself (its chemical structure, being cationic or anionic, etc.), irradiation type (UV-A, UV-C, Visible, etc.), light source power, the distance between the reactor and the light source, reactor diameter, catalyst dose ([Cat]), initial pollutant concentration ([RB] 0 ), pH, etc. Herein, we are performing our experiments using the same photocatalyst (QAD/ TiO 2 nanoparticles), the same organic pollutant (RB dye), and the same light source (15 W UV-A irradiation where the distance between the reactor and the lamp was maintained to be 10 cm), and the same reactor diameter (10 cm).Thus, the three factors that are going to be optimized and investigated in the following subsections are the parameters of pH, catalyst dose, and initial dye concentration, as listed in Table 2.

Effect of pH.
Usually, the effluents of textile wastewater exhibit wide-ranging pH values, and it was reported that the pH parameter can play critical roles in wastewater treatment owing to its effects on (1) the features of effluents of wastewater, (2) the adsorption of the pollutants on the surface of the photocatalysts/adsorbents, and (3) the photo-induced production of reactive species 41 .Thus, the pH is considered one of the critical parameters influencing the interactions of the organic dye with the catalyst surface and accordingly affecting the photodegradation rates 19 .The change in the pH of the solution would adjust the charge of the catalyst surface; thus, we estimated the point of zero charges (PZC) of the as-fabricated QAD/TiO 2 photocatalyst using the facile pH drift method 16,42 .Figure S10 showed that the PZC of the as-synthesized QAD/TiO 2 exhibit a pH PZC = 5, revealing that the surface charge of the QAD/TiO 2 will be negative at pH > 5 and positive at pH < 5, which is in good accordance with the reported PZC range of TiO 2 in the literature 43 .Then, the effect of pH has been investigated in the range of pH 2-12 under UV-A irradiation, as shown in Fig. S11, while other factors remained constant; the observed rate constants (k obs ) and the corresponding correlation factors (R 2 ) are recorded in Table 2.It is clearly observed that the rate of RB photodegradation was reduced by increasing the pH above 4 in the range of 6-12;  this can be attributed to the repulsion between the negatively deprotonated COO -group (pK a = 3.7) and negative QAD/TiO 2 catalyst (pH PZC = 5) at this pH range.Besides, another valid reason for this decrease in the photodegradation rates is the fact of the formation of RB zwitterions and their aggregation 19,44 .The formation of these zwitterions can be attributed to the attractive electrostatic interaction between the negatively charged carboxyl groups (-COO − ) and the positively charged amino groups (-N + ) in the RB monomers, and consequently forming a dimer structure with a larger molecular form which hinders its interaction with the active sites of QAD/ TiO 2 photocatalyst 19,44 .On the other hand, at pH 2, both RB and QAD/TiO 2 are positively charged; thus, there would be an electrostatic repulsion between them, which reduces the photodegradation rates at very low pH values.However, the optimum pH value was pH 4, where there will be an attraction between the positively charged QAD/TiO 2 surface and the negatively charged RB molecules, which facilitates the approach of the RB molecules toward the active sites of photodegradation on the as-synthesized QAD/TiO 2 sample.Finally, it is important to state that there are no peaks corresponding to the QAD molecules have been observed in the UV-Visible spectra even at higher pH values indicating the stability of the QAD molecules under harsh conditions 19 .Hence, the following photodegradation experiments will be operated at the optimized pH value of 4.
Table 2.The influence of different factors affecting the photocatalytic activity of the as-fabricated QAD/TiO 2 toward RB photodegradation in UV-A irradiation.www.nature.com/scientificreports/Effect of catalyst dose.The effect of the photocatalyst doses can be considered one of the crucial parameters influencing the photodegradation processes to achieve the best efficiency relative to the employed catalyst loading.Thus, the photocatalytic performance of QAD/TiO 2 nanoparticles was studied under UV-A irradiation using different loadings from 0.25 to 3.00 g/L, as shown in Fig. S12; while other factors remained constant: pH 4 and [RB] = 1 × 10 -5 M. At the low loadings range (0.25-0.50 g/L), the observed rate constant was significantly increased by about threefold upon doubling the photocatalyst dose from 0.25 to 0.50 g/L, as recorded in Table 2.This can be attributed to the increase of the available active sites correlated to the loading amount of the catalyst with respect to the same amount of the target dye (RB).However, the observed rate constants and the corresponding photodegradation percentages are gradually increased during the subsequent increases of the catalyst dose from 0.50 to 2.00 g/L, as shown in Fig. S13.Besides, there was a decrease in the k obs constant and the corresponding RB photodegradation% after the increase of the photocatalyst dose from 2.00 to 3.00 g/L; this could be ascribed to the screening effect and the scattering of light caused by the excess QAD/TiO 2 nanoparticles which imped of the light penetration to other active sites 17,19,45 .Hence, the optimum catalyst dose range is 0.50-1.00g/L; and any increase in the catalyst doses not only can be considered a "dead mass", but also it has a negative influence on the photocatalytic activity of the as-synthesized QAD/TiO 2 photocatalyst.

Effect of initial RB concentration.
The effect of the initial RB concentration was studied in the range of absorbance that obeys Beer-Lambert law with an absorption value below 2; thus, the suitable range of [RB] is from 0.5 × 10 -5 to 2 × 10 -5 M, while the other factors were maintained unchanged.As shown in Table 2 and Fig. S14, the increase of the [RB] 0 caused the decrease of the RB photodegradation% and the k obs constant was decreased accordingly.This behaviour is commonly noticed because of the reduction in the number of photons reaching the surface of the photocatalyst to photo-induce the generation of the reactive species because the high concentrations of RB dye behave as an inner-filter solution for the photons in addition to its blocking for more active sites of the catalyst compared to the case in the lower RB concentrations 17,19 .This is one of the general obstacles of all advanced oxidation processes (AOPs) and the practical/industrial application will require large facilities for dilution of the pollutant concentration to avoid a decrease in the activity; however, the other advantages of the AOPs outweigh this obstacle.
Stability of QAD/TiO 2 photocatalysts.One of the major obstacles to the practical application of organic dye sensitizers is the stability of the QAD molecules in the presence of highly reactive radicals and oxidative species.The stability of the as-fabricated QAD/TiO 2 sample was investigated over four consecutive cycles where the powder was washed after each cycle with distilled water numerous times and finally dried in an electric oven at 60 °C and re-employed in the next cycle with fresh RB dye solution with maintaining the same operating parameters.It was noticed that the photocatalytic activity was in the same range after four consecutive cycles as displayed in Fig. 8; the standard error calculations are demonstrated in Table S1.These findings signifying the stability of the as-fabricated QAD/TiO 2 photocatalyst that could be utilized in water treatment and reused for various cycles.Hence, all the above findings revealed the stability of the proposed photocatalyst.The influence of the types of different scavengers on the photocatalytic activity of QAD/TiO 2 nanoparticles was studied to evaluate the most reactive species involved in the photodegradation process.Thus, methyl iodide (MeI), iso-propyl alcohol (i-PrOH), methyl viologen (MV), and p-benzoquinone (p-BQ) were separately examined as specific scavengers for the trap of h + VB , OH • , e − CB , and O •− 2 , respectively 16,19,46 .The concentrations of the scavengers were kept constant (1 × 10 -5 M) upon the comparison, as displayed in Fig. 9a, while keeping the other parameters to be constant ([Cat] = 1 g/L, pH 4, [RB] = 1 × 10 -5 M).The standard error calculations are demonstrated in Table S2.It was found that the lowest activity was detected in the presence of p-BQ indicating the O •− 2 radicals were the most producible reactive species in the photodegradation of RB using the as-fabricated QAD/TiO 2 nanoparticles, as reported earlier in similar works 19,24 .

Mechanism of the
Moreover, to further confirm that the O •− 2 radicals are the most reactive species, and the effect of O 2 bubbling on the photocatalytic performance of the QAD/TiO 2 photocatalyst was studied.It was noticed that the bubbling of O 2 gas to the photocatalyst/RB suspensions during the dark time achieved a significant enhancement in the observed rate constant by about 70% (Fig. S15) confirming that the dissolved O 2 is participating in the RB photodegradation process.This double enhancement can be attributed to the increase in the dissolved O 2 concentration due to the O 2 bubbling process, which facilitates the formation of O •− 2 radicals and consequently increase the photocatalytic performance.Hence, this supported that the O •− 2 radicals were the most reactive radicals in the photodegradation process using the as-fabricated QAD/TiO 2 nanoparticles under UV-A irradiation.
The proposed mechanism.According to the effect of scavenges on the RB photodegradation and the above-mentioned PL and DRS data, the mechanism of the QAD/TiO 2 photocatalyst was suggested and associated with that of the bare TiO 2 sample, as shown in Fig. 10.In the bare TiO 2 sample, after the excitation of the sample by highly energetic photons with energy exceeding 2.97 eV; i.e.UV photons only, the e-h pairs are formed and the e-h recombination is the most predominant pathway.However, upon the exposure of the QAD/ TiO 2 photocatalyst exposed to the light photons, the TiO 2 with narrower bandgap energy (2.58 ± 0.05 eV) can be excited by the visible-light photons and the QAD molecules anchored on the TiO 2 surface are excited to be QAD * where the electrons can transfer from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of QAD.Then, the photoexcited electrons from QAD * can be introduced into the CB of TiO 2 beside the photoinduced e − CB which leads to the increase of the concentration of electrons in the CB of TiO 2 ; this assumption of injection is supported by the reduction of PL intensity of QAD in the QAD/TiO 2 compared to the bare QAD sample.Subsequently, the high concentration of the reactive photoinduced electrons in the CB of QAD/TiO 2 increased the probability of the reduction of the dissolved O 2 molecules to form O 2 .− radicals, which are responsible for the photodegradation of RB via a reductive degradation pathway, as supported by the results mentioned above regarding the effects of scavengers and O 2 bubbling.
Moreover, the photoinduced positive holes on the VB of the excited TiO 2 can spontaneously transfer to the HOMO of QAD leading to the reduction of the tendency of the excited electrons in the CB to return to the VB.Alternatively, the electrons have a new level, HOMO of QAD, which is available for relaxation rather than the original VB of TiO 2 leading to the increase in the probability of availability of oxidative species to degrade the RB molecules.Hence, the as-fabricated heterojunction upon the anchoring of QAD molecules on the surface of TiO 2 provided different pathways for enhanced charge separation rather than the recombination route as evidenced by the reduction of PL of QAD/TiO 2 when compared to the bare TiO 2 and pure QAD samples.This mechanism can interpret the enhanced optical properties and the improved photocatalytic activity of the as-synthesized QAD/TiO 2 compared to the bare TiO 2 nanoparticles signifying the outstanding role of the efficient proposed structure of QAD as a novel organic dye sensitizer.Future outlook.The findings and outputs of this research article state that the efficient photosensitization of TiO 2 using the novel QAD molecules confirmed their role in enhancing the optical properties of TiO 2 by decreasing its band gap energy, reducing the e-h recombination rates, and creating a visible-light antenna to extend its activity to the visible region.Hence, these findings will open the door for investigating numerous organic sensitizers based on Quinazoline derivatives and other heterocyclic compounds by tuning the heterocyclic structure via various cycloaddition reactions to further extending of the light absorption capability of the organic photosensitizers toward the visible region.Finally, the proposed QAD/TiO 2 samples are expected to exhibit exceptional photocatalytic performance when utilized in the photodegradation of further organic water pollutants, other photocatalytic applications, and/or antibacterial applications.

Conclusion
A novel organic compound (Quinazoline-derivative, QAD) has been synthesized to be used as a novel organic dye sensitizer for TiO 2 nanoparticles.The FTIR, NMR, and UV-Visible spectra confirmed the successful synthesis of the proposed structure of novel QAD.Anatase-phase TiO 2 nanoparticles had been synthesized through a simple template-free sol-gel approach with a microsphere morphology as suggested by the XRD and SEM techniques.The as-synthesized novel QAD photosensitizer was anchored on the surface of TiO 2 and made slight distortions in the crystal structures, a remarkable reduction in the bandgap energy of its band gap energy to 2.58 ± 0.05 eV, and sufficient reduction of the e-h recombination rates.Consequently, the photocatalytic activity of the as-fabricated QAD/TiO 2 sample has been enhanced and k obs increased by about 3-folds towards the photodegradation of RB dye.The effects of the scavengers and oxygen-bubbling experiments suggested that the superoxide anion radicals ( O •− 2 ) were the most reactive radicals in a reductive degradation pathway.Finally, the mechanism of the enhancement of the photocatalytic performance of QAD/TiO 2 photocatalyst has been suggested to depend on the injection of electrons from the HOMO of QAD to the CB of TiO 2 and enhancing the charge separation and consequently boosting of the photocatalytic activity compared to the bare TiO 2 nanoparticles.

Figure 3 .
Figure 3.The XRD patterns of the as-synthesized QAD/TiO 2 and bare TiO 2 samples.

Figure 4 .
Figure 4.The FTIR spectra of the as-synthesized QAD/TiO 2 and bare TiO 2 samples compared to that of the QAD molecules.

Figure 5 .
Figure 5. FE-SEM images and the elemental mapping spectra of the QAD/TiO 2 sample.

Figure 10 .
Figure 10.The typical mechanism of the bare TiO 2 photocatalysts and the proposed mechanism of the enhanced photocatalytic performance of QAD/TiO 2 based on the enhancement of the charge separation as supported by PL results.(The bandgap energies are deduced from the DRS findings, and the relative positions of QAD's HOMO/LUMO levels compared to TiO 2 ' s VB/CB are proposed based on the PL findings).