Sn(IV)porphyrin-Anchored TiO 2 Nanoparticles via Axial-Ligand Coordination for Enhancement of Visible Light-Activated Photocatalytic Degradation

: A visible-light-active photocatalyst, SnP/AA@TiO 2 , was fabricated by utilizing the coordination chemistry between the axial hydroxo-ligand in the ( trans -dihydroxo)(5,10,15,20-tetraphenyl-porphyrinato)Sn(IV) complex (SnP) and adipic acid (AA) on the surface of TiO 2 nanoparticles. The SnP center was strongly bonded to the surface of the TiO 2 nanoparticles via the adipic acid linkage in SnP/AA@TiO 2 , as conﬁrmed by various instrumental techniques. SnP/AA@TiO 2 exhibited remarkably enhanced photocatalytic activity toward the degradation of rhodamine B dye (RhB) in aqueous solution under visible-light irradiation. The RhB degradation efﬁciency of SnP/AA@TiO 2 was 95% within 80 min, with a rate constant of 0.0366 min − 1 . The high degradation efﬁciency, low catalyst loading and high reusability make SnP-anchored photocatalysts more efﬁcient than other photocatalysts, such as TiO 2 and SnP@TiO 2 .


Introduction
With the increasing development of civilization and industrialization, environmental pollution has become a serious problem affecting ecosystems, biodiversity and human health worldwide. Every year, the leather, textile, drugs and paper printing industries discharge large amounts of toxic compounds, such as organic dyes, pesticides, herbicides, pigments, plasticizers, biphenyls, phenols, nitro and amino compounds, into water bodies. These organic chemicals not only deteriorate the potability of these water sources but also have negative effects on aquatic life (flora and fauna) [1,2]. This has led to increasing scientific efforts to develop strategies for environmental remediation by wastewater treatment in the environmental and research fields [3,4]. Currently, various physicochemical methods, including adsorption [5], filtration [6], chemical coagulation [7], precipitation [8], bacterial treatment [9], electrochemical methods [10] and advanced oxidation processes (AOPs) [11], are utilized for the removal of these hazardous compounds from wastewater. Undoubtedly, AOPs are the most promising techniques for the remediation of wastewater due to their simple operation, low cost and high efficiency in degrading hazardous pollutants to less-toxic CO 2 and H 2 O without the generation of secondary pollution. In AOP methods, visible light is absorbed by a photocatalyst, which subsequently generates reactive oxygen species (ROS) in situ, thereby accelerating the degradation of pollutant chemicals. Light absorption and ensuing electron transfer are the key factors for obtaining photocatalysts with efficient solar energy conversion [12,13].
In general, nanoscale materials display distinctive optical and electronic properties, depending on the size and shape of the photo-functional materials. Considering the advantages of photocatalytic processes, various organic and inorganic-based nanomaterials, such as ZnO [14], TiO 2 [15], graphitic carbon nitride (g-C 3 N 4 ) [16], zeolites [17], metalorganic framework (MOF) [18], bismuth-based photocatalysts [19], carbon quantum dots (CQDs) [20], fullerene [21], graphene oxide or reduced graphene oxide (GO and RGO) [22] and porphyrin-based nanostructures [23], have been used to remove toxic compounds from wastewater. Among them, TiO 2 and related materials have earned distinction as promising photocatalysts due to their high stability, low toxicity, low cost and high catalytic efficiency in degrading hazardous chemicals in water. The large bandgap (~3.2 eV) of these catalysts restricts the absorption of the solar spectrum in the visible region and results in low photocatalytic efficiency. A large amount of catalyst is required to achieve effective degradation rates in the photocatalytic process [24].
On the other hand, because of their high absorption coefficient in the regions of 400-440 nm (Soret band) and 500-720 nm (Q bands), porphyrinoids (free-base porphyrins or metalloporphyrins) are capable of absorbing light over a wider range (UV and visible), which facilitates the generation of ROS through a spin-forbidden intersystem crossing mechanism [25,26]. The inherent aromatic electronic features and rigid structural skeleton of porphyrinoids facilitate their self-aggregation in solution [27,28]. The use of porphyrinoid compounds in photocatalysis is limited by the proneness of these compounds to fast deactivation, easy agglomeration in solution and poor reusability. The development of new photocatalysts that overcome all of the above-mentioned drawbacks remains a challenge. For this purpose, porphyrinoid compounds have been immobilized on solid supports, such as matrices [29], hydrogels [30], zeolites [31] and nano-or microparticles [32][33][34][35], where the resulting hybrid materials show outstanding photocatalytic activity compared to that of the starting porphyrinoid compounds. After entrapping porphyrinoid compounds on solid supports, the resultant hybrid materials exhibit increased solar absorption/harvesting capabilities, as well as enhanced electronic conductivity mediated by intimate chemical contacts. The rigid framework of these hybrids is resistant to hydrolysis and increases the chemical and physical stability for further use.
To evaluate the photocatalytic activity for the degradation of organic dyes under visible-light irradiation, we consider rhodamine B (RhB) dye as a model pollutant in this study. RhB is a water-soluble, organic amino xanthene dye. It is widely used as a coloring agent in the printing, textile, leather and cosmetic industries. It is one of the 20 dyes detected most frequently in wastewater, and it irritates the skin and eyes and damages the respiratory, reproductive and nervous systems. Even at low concentrations, RhB dye is carcinogenic. RhB dye is very stable in aqueous solutions and non-biodegradable. It is imperative to find an effective oxidation system for removing this pollutant dye from wastewater [55][56][57][58][59][60][61][62][63][64][65]. We investigate the photocatalytic degradation of RhB using Sn(IV)porphyrinanchored TiO 2 photocatalysts. The degradation efficiency of SnP/AA@TiO 2 is compared with that of SnP@TiO 2 , and the kinetics and the mechanism of the degradation process are also discussed.

Fabrication and Characterization of Photocatalysts
The synthesis of SnP/AA@TiO2 has been described in the experimental section, where the surface of the TiO2 nanoparticles was firstly functionalized with adipic acid, after which SnP was anchored by forming a chemical Sn-O2C bond through the interaction of the axial OH ligand of the Sn(IV)porphyrin and COOH group of adipic acid to afford the SnP/AA@TiO2 composite. The above reaction was also performed without treating the nanoparticles with adipic acid, affording the SnP@TiO2 composite. The surface modifications for generating SnP/AA@TiO2 and SnP@TiO2 were confirmed by analyzing the zeta potential, which is calculated by the degree of electrostatic repulsion of the composite surface. The average zeta potentials of pure TiO2 and the SnP@TiO2 and SnP/AA@TiO2 composites were measured in aqueous solutions at pH 7, with respective values of −6.97, −4.87 and 7.02 mV. The more negative value for pure TiO2 is mainly due to the presence of electronegative oxygen atoms on the surface. In contrast, the high positive zeta potential of SnP/AA@TiO2 compared to that of pure TiO2 and SnP@TiO2 indicates that Sn(IV)porphyrins sufficiently covered the surface of TiO2 over a large area in the former. As expected, this immobilization was successfully achieved through chemical anchoring via the adipic acid linkage. On the other hand, the zeta potential of SnP@TiO2 was less negative than that of pure TiO2. The presence of hydroxyl groups on the surface of TiO2 is essential for the formation of Sn-O-Ti bonds in SnP@TiO2. This is because hydroxo-Sn(IV)porphyrin complexes undergo dehydrating condensation with OH or COOH groups to yield covalently bonded moieties, such as Sn-O-Ti. However, the main linkages on the surface of TiO2 are O-Ti-O groups, which remain intact or may physically adsorb SnP species via weak hydrogen bonding to form species such as Ti-O···HO-SnP. The amount of adsorbed porphyrin (SnP) was estimated from the Sn content of the hybrid composites (SnP/AA@TiO2 or SnP@TiO2) by ICP analysis. The determined amount of SnP in SnP/AA@TiO2 and SnP@TiO2 was 0.149 and 0.042 mmol/g, respectively.
The FT-IR spectra of SnP, TiO2, SnP@TiO2 and SnP/AA@TiO2 are depicted in Figure  1. The FT-IR spectrum of SnP shows vibrational peaks at 790 and 1024 cm −1 , attributed to the out-of-plane bending vibration of C-H and the in plane bending vibration of C-H in the benzene ring, respectively. The peaks at 1406, 1591 and 3384 cm −1 are designated to the stretching vibrations of C-N, C=C and C-H in the pyrrole ring, respectively. The peak at 3601 cm −1 is assigned to the stretching vibration of the OH of the axial hydroxyl group in

Fabrication and Characterization of Photocatalysts
The synthesis of SnP/AA@TiO 2 has been described in the experimental section, where the surface of the TiO 2 nanoparticles was firstly functionalized with adipic acid, after which SnP was anchored by forming a chemical Sn-O 2 C bond through the interaction of the axial OH ligand of the Sn(IV)porphyrin and COOH group of adipic acid to afford the SnP/AA@TiO 2 composite. The above reaction was also performed without treating the nanoparticles with adipic acid, affording the SnP@TiO 2 composite. The surface modifications for generating SnP/AA@TiO 2 and SnP@TiO 2 were confirmed by analyzing the zeta potential, which is calculated by the degree of electrostatic repulsion of the composite surface. The average zeta potentials of pure TiO 2 and the SnP@TiO 2 and SnP/AA@TiO 2 composites were measured in aqueous solutions at pH 7, with respective values of −6.97, −4.87 and 7.02 mV. The more negative value for pure TiO 2 is mainly due to the presence of electronegative oxygen atoms on the surface. In contrast, the high positive zeta potential of SnP/AA@TiO 2 compared to that of pure TiO 2 and SnP@TiO 2 indicates that Sn(IV)porphyrins sufficiently covered the surface of TiO 2 over a large area in the former. As expected, this immobilization was successfully achieved through chemical anchoring via the adipic acid linkage. On the other hand, the zeta potential of SnP@TiO 2 was less negative than that of pure TiO 2 . The presence of hydroxyl groups on the surface of TiO 2 is essential for the formation of Sn-O-Ti bonds in SnP@TiO 2 . This is because hydroxo-Sn(IV)porphyrin complexes undergo dehydrating condensation with OH or COOH groups to yield covalently bonded moieties, such as Sn-O-Ti. However, the main linkages on the surface of TiO 2 are O-Ti-O groups, which remain intact or may physically adsorb SnP species via weak hydrogen bonding to form species such as Ti-O···HO-SnP. The amount of adsorbed porphyrin (SnP) was estimated from the Sn content of the hybrid composites (SnP/AA@TiO 2 or SnP@TiO 2 ) by ICP analysis. The determined amount of SnP in SnP/AA@TiO 2 and SnP@TiO 2 was 0.149 and 0.042 mmol/g, respectively.
The FT-IR spectra of SnP, TiO 2 , SnP@TiO 2 and SnP/AA@TiO 2 are depicted in Figure 1. The FT-IR spectrum of SnP shows vibrational peaks at 790 and 1024 cm −1 , attributed to the out-of-plane bending vibration of C-H and the in plane bending vibration of C-H in the benzene ring, respectively. The peaks at 1406, 1591 and 3384 cm −1 are designated to the stretching vibrations of C-N, C=C and C-H in the pyrrole ring, respectively. The peak at 3601 cm −1 is assigned to the stretching vibration of the OH of the axial hydroxyl group in the Sn(IV) porphyrin. The strong peaks at 855-515 cm −1 are characteristic peaks of Ti-O-Ti stretching in TiO 2 . The FT-IR spectrum of SnP@TiO 2 showed the characteristic band of Ti-O-Ti stretching at 595 cm −1 . A very broad band was also observed at 3450 cm −1 , corresponding hydrogen-bonded OH groups. All other peaks remained unchanged or were slightly modified relative to those of the starting components. For SnP/AA@TiO 2, the characteristic carboxylate stretching band of AA shifted from 1685 to 1690 cm −1 upon complexation with SnP. All other peaks remained unchanged or were slightly changed relative to those of SnP@TiO 2 . These observations indicate that the characteristic linkages of SnP and TiO 2 remained intact in the composites (SnP/AA@TiO 2 or SnP@TiO 2 ). A very broad band was also observed at 3450 cm −1 , corresponding hydrogen-bonded OH groups. All other peaks remained unchanged or were slightly modified relative to those of the starting components. For SnP/AA@TiO2, the characteristic carboxylate stretching band of AA shifted from 1685 to 1690 cm −1 upon complexation with SnP. All other peaks remained unchanged or were slightly changed relative to those of SnP@TiO2. These observations indicate that the characteristic linkages of SnP and TiO2 remained intact in the composites (SnP/AA@TiO2 or SnP@TiO2).
(a) (b) Solid-state UV-vis spectroscopy was used to evaluate the light absorption properties of SnP, TiO2, SnP@TiO2 and SnP/AA@TiO2 ( Figure 2). The spectrum of SnP shows a strong absorption band at 427 nm, assigned as the Soret band, and two weak absorption bands at 566 and 606 nm corresponding to the Q bands. The spectrum of pure TiO2 showed only one strong absorbance band at 359 nm in the UV region. In the visible light region, a strong and very broad absorption band was centered at 440 nm in the spectrum of SnP/AA@TiO2, which is the Soret band, along with four weak bands at 523, 565, 603 and 626 nm, which are the Q bands. The absorption pattern of SnP@TiO2 was similar. Accordingly, SnP/AA@TiO2 and SnP@TiO2 are expected to have enhanced light-harvesting ability in most of the visible light regions. The bandgap energy (Eg) calculated from the Tauc plot was ~2.42 eV for SnP/AA@TiO2, which is narrower than that of SnP (~2.96 eV), TiO2 (~3.12 eV) and SnP@TiO2 (~2.75 eV). The enhanced light absorption and narrower bandgap of SnP/AA@TiO2 can effectively improve solar energy utilization to generate more photogenerated reactive species that can participate in the photocatalytic degradation of organic pollutants. Solid-state UV-vis spectroscopy was used to evaluate the light absorption properties of SnP, TiO 2 , SnP@TiO 2 and SnP/AA@TiO 2 ( Figure 2). The spectrum of SnP shows a strong absorption band at 427 nm, assigned as the Soret band, and two weak absorption bands at 566 and 606 nm corresponding to the Q bands. The spectrum of pure TiO 2 showed only one strong absorbance band at 359 nm in the UV region. In the visible light region, a strong and very broad absorption band was centered at 440 nm in the spectrum of SnP/AA@TiO 2 , which is the Soret band, along with four weak bands at 523, 565, 603 and 626 nm, which are the Q bands. The absorption pattern of SnP@TiO 2 was similar. Accordingly, SnP/AA@TiO 2 and SnP@TiO 2 are expected to have enhanced light-harvesting ability in most of the visible light regions. The bandgap energy (E g ) calculated from the Tauc plot was~2.42 eV for SnP/AA@TiO 2 , which is narrower than that of SnP (~2.96 eV), TiO 2 (~3.12 eV) and SnP@TiO 2 (~2.75 eV). The enhanced light absorption and narrower bandgap of SnP/AA@TiO 2 can effectively improve solar energy utilization to generate more photogenerated reactive species that can participate in the photocatalytic degradation of organic pollutants.
The fluorescence spectra are presented in Figure 3. The spectrum of SnP showed a single fluorescence peak at 654 nm, and that of the SnP/AA@TiO 2 composite exhibited two fluorescence peaks at 630 and 650 nm. On the other hand, the spectrum of SnP@TiO 2 showed a broad fluorescence peak that seems to be a combination of the peaks of SnP and SnP/AA@TiO 2 . Note that under excitation at 550 nm, pure TiO 2 showed no fluorescence signal. A comparison of the spectral features suggests that the immobilization mode of the SnP molecules in SnP/AA@TiO 2 and SnP@TiO 2 is different, which is consistent with the zeta potential analysis discussed above. The fluorescence spectra are presented in Figure 3. The spectrum of SnP showed a single fluorescence peak at 654 nm, and that of the SnP/AA@TiO2 composite exhibited two fluorescence peaks at 630 and 650 nm. On the other hand, the spectrum of SnP@TiO2 showed a broad fluorescence peak that seems to be a combination of the peaks of SnP and SnP/AA@TiO2. Note that under excitation at 550 nm, pure TiO2 showed no fluorescence signal. A comparison of the spectral features suggests that the immobilization mode of the SnP molecules in SnP/AA@TiO2 and SnP@TiO2 is different, which is consistent with the zeta potential analysis discussed above.   The fluorescence spectra are presented in Figure 3. The spectrum of SnP showed a single fluorescence peak at 654 nm, and that of the SnP/AA@TiO2 composite exhibited two fluorescence peaks at 630 and 650 nm. On the other hand, the spectrum of SnP@TiO2 showed a broad fluorescence peak that seems to be a combination of the peaks of SnP and SnP/AA@TiO2. Note that under excitation at 550 nm, pure TiO2 showed no fluorescence signal. A comparison of the spectral features suggests that the immobilization mode of the SnP molecules in SnP/AA@TiO2 and SnP@TiO2 is different, which is consistent with the zeta potential analysis discussed above.  The morphological developments of the synthesized composites were examined by field-emission scanning electron microscopy (FE-SEM). The morphologies of SnP@TiO 2 and SnP/AA@TiO 2, along with those of the starting compounds, are presented in Figure 4. SnP did not develop any well-defined nano-scale morphology (Figure 4a), whereas TiO 2 comprised regularly shaped particles with an average diameter in the range of 145-305 nm (Figure 4b). Compared to TiO 2 , the shape and size remained almost unchanged for SnP@TiO 2 (Figure 4c). In contrast, SnP/AA@TiO 2 had a flower-like, fused morphology, which is different from that of SnP, TiO 2 and SnP@TiO 2 . The elemental distribution in the composite (SnP/AA@TiO 2 ) was investigated using energy-dispersive X-ray (EDX) mapping ( Figure S1). Ti, O, Sn, N and C were evenly distributed in the composite, indicating that SnP was well immobilized on the TiO 2 nanoparticles.
The morphological developments of the synthesized composites were examined by field-emission scanning electron microscopy (FE-SEM). The morphologies of SnP@TiO2 and SnP/AA@TiO2, along with those of the starting compounds, are presented in Figure 4. SnP did not develop any well-defined nano-scale morphology (Figure 4a), whereas TiO2 comprised regularly shaped particles with an average diameter in the range of 145-305 nm (Figure 4b). Compared to TiO2, the shape and size remained almost unchanged for SnP@TiO2 (Figure 4c). In contrast, SnP/AA@TiO2 had a flower-like, fused morphology, which is different from that of SnP, TiO2 and SnP@TiO2. The elemental distribution in the composite (SnP/AA@TiO2) was investigated using energy-dispersive X-ray (EDX) mapping ( Figure S1). Ti, O, Sn, N and C were evenly distributed in the composite, indicating that SnP was well immobilized on the TiO2 nanoparticles. The TGA curves of the samples are displayed in Figure S2. TiO2 exhibited only a slight weight loss (~1 wt%) between 50 °C and 600 °C, compared to 2.12 wt% for SnP@TiO2 and 3.26 wt% for SnP/AA@TiO2 in the range of 200-440 °C. The high thermal decomposition temperatures of the composites suggest a successful combination of the molecular components. SnP@TiO2 and SnP/AA@TiO2 possess moderate specific surface areas of 62.21 and 77.37 m 2 g −1 , respectively ( Figure S3). The composites also exhibit type-IV adsorption-desorption isotherms, demonstrating the mesoporous nature of the surface of the composites. The data indicate that SnP molecules were successfully anchored on the surface of the TiO2 nanoparticles to form SnP/AA@TiO2 and SnP@TiO2 composite structures with high thermal stability and a large surface area. The characteristics of these composite nanoparticles, revealed by the spectroscopic, SEM, TGA and surface area analyses, indicate their attractive potential as visible-light-activated photocatalysts. The TGA curves of the samples are displayed in Figure S2. TiO 2 exhibited only a slight weight loss (~1 wt%) between 50 • C and 600 • C, compared to 2.12 wt% for SnP@TiO 2 and 3.26 wt% for SnP/AA@TiO 2 in the range of 200-440 • C. The high thermal decomposition temperatures of the composites suggest a successful combination of the molecular components. SnP@TiO 2 and SnP/AA@TiO 2 possess moderate specific surface areas of 62.21 and 77.37 m 2 g −1 , respectively ( Figure S3). The composites also exhibit type-IV adsorption-desorption isotherms, demonstrating the mesoporous nature of the surface of the composites. The data indicate that SnP molecules were successfully anchored on the surface of the TiO 2 nanoparticles to form SnP/AA@TiO 2 and SnP@TiO 2 composite structures with high thermal stability and a large surface area. The characteristics of these composite nanoparticles, revealed by the spectroscopic, SEM, TGA and surface area analyses, indicate their attractive potential as visible-light-activated photocatalysts.

Photocatalytic Degradation of Rhodamine B (RhB)
The photocatalytic efficiency of SnP/AA@TiO 2 and SnP@TiO 2 was investigated in the degradation of an organic dye under visible-light irradiation in an aqueous medium. We selected rhodamine B (RhB) as the target pollutant dye for the photocatalytic degradation reaction. As shown in Figure S4, a period of~25 min was required to reach the adsorptiondesorption equilibrium, where 3, 14, 18 and 21% of RhB were respectively adsorbed by SnP, TiO 2 , SnP@TiO 2 and SnP/AA@TiO 2 . The degradation experiment indicates that the high surface area and porous nature of SnP/AA@TiO 2 and SnP@TiO 2 can enhance the adsorption of pollutants and can also promote mass diffusion. Time-dependent absorption spectra of the aqueous solution of RhB dye in the presence of SnP/AA@TiO 2 under visiblelight irradiation are shown in Figure S5. Negligible decay of RhB dye was observed in the absence of either visible light or the SnP/AA@TiO 2 photocatalyst ( Figure 5). Therefore, Inorganics 2023, 11, 336 7 of 15 visible light as well as a photocatalyst are required for the degradation of RhB dye. The absorbance of RhB dye at 554 nm decreased as the irradiation time increased ( Figure S5). As depicted in Figure 5, all precursors and as-prepared composite catalysts clearly showed significant effectiveness for the photo-degradation of RhB in aqueous solution.
SnP, TiO2, SnP@TiO2 and SnP/AA@TiO2. The degradation experiment indicates that the high surface area and porous nature of SnP/AA@TiO2 and SnP@TiO2 can enhance the adsorption of pollutants and can also promote mass diffusion. Time-dependent absorption spectra of the aqueous solution of RhB dye in the presence of SnP/AA@TiO2 under visible-light irradiation are shown in Figure S5. Negligible decay of RhB dye was observed in the absence of either visible light or the SnP/AA@TiO2 photocatalyst ( Figure  5). Therefore, visible light as well as a photocatalyst are required for the degradation of RhB dye. The absorbance of RhB dye at 554 nm decreased as the irradiation time increased ( Figure S5). As depicted in Figure 5, all precursors and as-prepared composite catalysts clearly showed significant effectiveness for the photo-degradation of RhB in aqueous solution. The degradation of RhB dye in the presence of a photocatalyst can be described by the degradation efficiency, (C0 − C)/C0, where C0 is the starting concentration of RhB dye and C is the concentration at time t. The respective degradation rates of RhB in the presence of SnP, TiO2, SnP@TiO2 and SnP/AA@TiO2 were 12, 37, 47 and 95% after 80 min of irradiation of visible light ( Figure 5). The SnP/AA@TiO2 photocatalyst shows superior performance for RhB dye degradation compared to SnP, TiO2 and SnP@TiO2. To further elucidate the reaction kinetics for the degradation of RhB dye, we applied the pseudofirst-order theory exhibited by the equation ln(C0/C) = kt, which is generally used in photocatalytic degradation experiments if the initial concentration of dye (C0) is low; k is the pseudo-first-order rate constant for degradation experiments. Based on the data The degradation of RhB dye in the presence of a photocatalyst can be described by the degradation efficiency, (C 0 − C)/C 0 , where C 0 is the starting concentration of RhB dye and C is the concentration at time t. The respective degradation rates of RhB in the presence of SnP, TiO 2 , SnP@TiO 2 and SnP/AA@TiO 2 were 12, 37, 47 and 95% after 80 min of irradiation of visible light ( Figure 5). The SnP/AA@TiO 2 photocatalyst shows superior performance for RhB dye degradation compared to SnP, TiO 2 and SnP@TiO 2 . To further elucidate the reaction kinetics for the degradation of RhB dye, we applied the pseudo-first-order theory exhibited by the equation ln(C 0 /C) = kt, which is generally used in photocatalytic degradation experiments if the initial concentration of dye (C 0 ) is low; k is the pseudo-first-order rate constant for degradation experiments. Based on the data plotted in Figure 5, Figure S6 presents the reaction kinetics of RhB dye degradation. The first-order degradation rate constant of RhB dye in the presence of SnP, TiO 2 , SnP@TiO 2 and SnP/AA@TiO 2 was calculated as 0.0016, 0.006, 0.0078 and 0.0366 min −1 , respectively ( Figure S6). The above results are encouraging compared with other reported rate constants for the degradation of RhB dye (Table 1).
Among the developed photocatalysts, SnP/AA@TiO 2 possesses the highest photodegradation rate and can remove~95% of RhB dye within 80 min. To confirm the effect of TiO 2 -immobilized SnP on the photocatalytic activity of the SnP/AA@TiO 2 composite, the weight percentage of SnP in the composite was serially varied and the effect on the degradation of RhB dye was evaluated ( Figure S7). The photodegradation of RhB by SnP/AA@TiO 2 containing 30 wt% SnP proceeded much more efficiently than that in the presence of pure SnP or pure TiO 2 . When the mass ratio of SnP to TiO 2 in the composite was increased, the dye degradation rate increased and reached a maximum of 30%. The degradation rate increased slightly to 40% or 50%. When the wt% of SnP in SnP/AA@TiO 2 Inorganics 2023, 11, 336 8 of 15 is higher than 30%, SnP molecules agglomerate easily on the surface of TiO 2 to some extent. This reduced the number of reactive sites, restrained the photogenerated charge separation and slowly lowered the degradation rate. This trend suggests that the synergistic effects between SnP and TiO 2 are responsible for the enhanced photocatalytic efficiency of the SnP/AA@TiO 2 composite. Because the reusability of photocatalysts is very important for practical application, we evaluated the recyclability of SnP/AA@TiO 2 in RhB dye degradation ( Figure S8). Even after ten consecutive cycles, SnP/AA@TiO 2 still maintained a high degradation efficiency, with only a 3% reduction, indicating that the SnP/AA@TiO 2 photocatalyst possesses remarkable stability. The stability of this photocatalyst was further confirmed by examining the structure of SnP/AA@TiO 2 after the degradation reaction. The XRD ( Figure S9) and FE-SEM ( Figure S10) data for used SnP/AA@TiO 2 were almost the same as that of the pristine state, indicating that the properties of this photocatalyst remained intact during the photocatalytic reaction. Moreover, SnP/AA@TiO 2 was easily recovered from the reaction mixture through a successive filter-wash-dry procedure.
To optimize the reaction conditions (RhB dye/photocatalyst ratio, temperature and solution pH), the photodegradation of RhB was performed under several different conditions. Degradation experiments were performed at various temperatures to analyze the effect of temperature on the decay of RhB dye by SnP/AA@TiO 2 . Over the temperature range of 290-330 K, the degradation occurred consistently without a significant decrease ( Figure S11). Initially, the RhB dye solution was prepared with distilled water at pH 7. The pH of the aqueous RhB dye solution had a notable effect on the degradation rate of RhB dye. It is evident from Figure S12 that the rate of degradation increased when the pH was raised from pH 2 to 7 but decreased when the pH was further increased to 12. Basic pH had a greater effect on the degradation compared to acidic pH. Additionally, the effect of the RhB dye/catalyst ratio on the degradation of RhB dye was examined using various concentrations of RhB solution (10 to 80 mg L −1 ) with a constant amount of photocatalyst SnP/AA@TiO 2 (50 mg was used in every experiment). Most of the RhB dye was degraded at concentrations of 10-30 mg L −1 , and approximately 70% of the dye was degraded even at 80 mg L −1 ( Figure S13).
The mechanism of photocatalytic degradation of dye by SnP@TiO 2 is somewhat different from that of SnP/AA@TiO 2 . In the case of SnP@TiO 2 , when visible light is absorbed, excited electrons can easily move to the conduction band of TiO 2 . These reactive electrons in the conduction band can react with oxygen to produce superoxide radical anions (O 2 −• ) and degrade RhB dye. TiO 2 holes can react with water to generate hydroxyl radicals ( • OH) and degrade RhB dye. On the other hand, when the surface of TiO 2 is modified with SnP via AA, SnP/AA@TiO 2 can behave as a single molecule with lower band gap energy [66,67]. Therefore, the degradation mechanism of SnP/AA@TiO 2 is very similar to other porphyrin-based materials [42][43][44][45]. In step 1, the Sn(IV)porphyrin-based photocatalyst (P cat ) in an aqueous solution of RhB dye is activated by absorbing visible light. After crossing the bandgap, the valence band (VB) electrons are promoted to the conduction band (CB). This leads to the creation of electron-hole pairs (e − /h + ) pairs on the surface of the photocatalyst. These photo-generated holes (h + ) subsequently react with H 2 O to produce highly reactive hydroxyl radicals ( • OH) (step 2). The excited electron reacts with O 2 (dissolves in water) to produce highly reactive superoxide radical anions (O 2 −• ) in step 3. The above photo-generated highly reactive species ( • OH and O 2 −• ) react with RhB dye and degrade it into smaller molecules, eventually forming CO 2 and H 2 O (steps 4 and 5).
The surface modification of TiO 2 through AA bridging enhanced the adhesive strength of SnP photosensitizers and optimized its surface properties. During the photocatalytic process, it is possible that the bridging-anchoring groups strongly inhibited the detachment of SnP from the surface of modified TiO 2 . And the robustly strong adhesion between SnP and AA@TiO 2 not only increases the amount of SnP attached to the surface of AA@TiO 2 but also accelerates electron transfer from excited SnP to the conduction band of TiO 2 .
To detect the photo-generated reactive species during the photocatalytic degradation of RhB dye, we used radical trapping experiments [68,69]. For this purpose, tert-butanol ( t BuOH) was used to capture hydroxyl radicals ( • OH); para-benzoquinone (p-BQ) was used for superoxide radical anions (O 2 −• ); NaN 3 for singlet oxygen; and ethylenediaminetetraacetic acid disodium (Na 2 -EDTA) was used to capture photogenerated holes (h + ) during the photodegradation of RhB dye in the presence of SnP/AA@TiO 2 . Figure S14 reveals that the RhB dye degradation rate was critically affected in the presence of t BuOH, Na 2 -EDTA and p-BQ. The degradation of RhB dye was not affected by the presence of NaN 3 or singlet oxygen. Photogenerated holes (h + ) are the major reactive species compared to hydroxyl radicals ( • OH) or superoxide radicals (O 2 −• ), which are responsible for the catalytic degradation of RhB dye in aqueous solution. The photocatalytic activity of SnP/AA@TiO 2 in RhB degradation under irradiation with monochromatic light at different wavelengths was also investigated ( Figure S15). The variation in the wavelength-dependent photodegradation of RhB dye demonstrated that optical absorption makes a significant contribution to solar energy conversion and photocatalytic performance. SnP/AA@TiO 2 showed some degradation ability even at λ > 700 nm.
To identify the degradation products of RhB dye after visible-light irradiation in the presence of SnP/AA@TiO 2 , the reaction mixture was withdrawn after 40 min for each photo-degradation experiment and analyzed by ESI-mass spectrometry ( Figure S16). The new peaks appearing in the mass spectra confirmed the degradation of RhB dye to new, smaller molecules [70]. Based on the mass spectrum in Figure S16, possible intermediates for the degradation of RhB dye are shown in Figure 6. The initially observed base peak (m/z = 443.2; a cationic form of RhB) belongs to the RhB dye. RhB dye can undergo N-de-ethylation and fragment into smaller parts (m/z 415.2 and m/z 387.2). Finally, all four N-de-ethylation steps lead to the product with m/z 331.1. This cationic species fragmented further after cleavage of the chromophoric group to smaller molecules (m/z 115.0, 166.9, 110.0). These low-molecular-weight aromatic compounds can undergo successive ring cleavage and hydrolysis, thereby forming low-molecular-weight compounds (m/z 119.0, 105.0). Finally, all these intermediate molecules were further disintegrated and mineralized into CO 2 and H 2 O. Furthermore, the total organic carbon (TOC) was estimated to calculate the removal of RhB dye by the photocatalysts [71]. The TOC removal percentage obtained with SnP/AA@TiO 2 was only 81%. The TOC removal was 78% for SnP@TiO 2 .

Materials and Methods
TiO 2 (P-25, Degussa, average particle size 100-300 nm) was purchased from commercial suppliers. All purchased chemicals were used without further purification unless otherwise specified. Toluene and pyrrole were distilled from a solution of calcium hydride. (trans-Dihydroxo)(5,15,10,20-tetraphenylporphyrinato)tin(IV) (SnP) was synthesized according to our previously reported procedure [36]. Fourier-transform infrared (FT-IR) spectra (KBr) were measured using a Shimadzu FTIR-8400S spectrophotometer (Shimadzu, Tokyo, Japan). Steady-state UV-vis spectra were measured on a Shimadzu UV-3600 spectrophotometer (Shimadzu, Tokyo, Japan). Fluorescence spectra were measured with a Shimadzu RF-5301PC fluorescence spectrophotometer (Shimadzu, Tokyo, Japan). Thermogravimetric analysis (TGA) was recorded using an Auto-TGA Q500 instrument (TA Instruments, New Castle, DE, USA) at a scan rate of 10 • C/min in the range of 30-600 • C, under an N 2 atmosphere. The Brunauer−Emmett−Teller (BET) surface area was calculated with an analyzer (BELSORP-mini volumetric adsorption equipment) by acquiring N 2 adsorption isotherms at 77 K. The data for the catalyst surface were obtained by using an Autosorb-iQ and Quadrasorb SI apparatus. Powder X-ray diffraction (PXRD) patterns were determined on a Bruker AXS D8 Advance powder X-ray diffractometer (Bruker, Billerica, MA, USA). The morphology and elemental distribution of the synthesized samples were examined using field-emission scanning electron microscopy (FE-SEM) (MAIA III, TES-CAN, Brno, Czech Republic) with energy dispersive X-ray spectroscopy (EDS). All samples were deposited on the surface of copper tape by using the drop-casting method. The zeta potential was measured using an Otsuka Electronics ELSZ-2 instrument. Inductively coupled plasma (ICP) analyses were performed on an ICP-Spectrociros CCD instrument.

Synthesis of SnP/AA@TiO 2
TiO 2 (1.0 g, 12.5 mmol) was added to a solution of adipic acid (1.83 g, 12.5 mmol) dissolved in tetrahydrofuran (THF, 20 mL) and stirred for 6 h at room temperature. The solid materials were filtered and washed with THF and then dried under vacuum for over 2 h at 70 • C. The resulting solid was added to a solution of SnP (0.30 g, 0.39 mmol) dissolved in CH 2 Cl 2 (40 mL). The reaction mixture was stirred for 12 h at 25 • C. After that, the solid was filtered, washed with CH 2 Cl 2 and dried in a vacuum oven for 6 h at 90 • C. SnP/AA@TiO 2 powder was obtained with a yield of 1.101 g.

Synthesis of SnP@TiO 2
TiO 2 (1.0 g, 12.5 mmol) was added to a solution of SnP (0.30 g, 0.39 mmol) dissolved in CH 2 Cl 2 (40 mL). After that, the reaction mixture was stirred for 12 h at 25 • C. The solid was filtered, washed with CH 2 Cl 2 and dried in a vacuum oven for 4 h at 80 • C. SnP@TiO 2 was obtained in a yield of 1.122 g.

Photocatalytic Degradation Experiment
The photocatalytic efficiency of the synthesized SnP/AA@TiO 2 or SnP@TiO 2 was investigated in the degradation of RhB dye in aqueous solution. The photodegradation reaction of this dye was carried out under irradiation with a 150 W xenon arc lamp with a UV cut-off filter (ABET Technologies, Old Gate Lane Milford, CT, USA) at room temperature (298 K). In a typical procedure, 50 mg of the photocatalyst was added to a 100 mL aqueous solution of RhB (40 mg L −1 , distilled water at pH 7) with stirring at room temperature. The reaction mixture remained in the dark for 25 min to reach adsorption-desorption equilibrium. After irradiation with visible light, 3 mL of the suspension was collected at regular intervals. The photocatalyst was collected from the solution by centrifugation followed by filtration using filter paper. The concentration of RhB was examined by determining the absorbance at 554 nm using a UV-vis spectrophotometer.

Conclusions
A visible-light-active photocatalyst, SnP/AA@TiO 2 , was fabricated by utilizing the coordination chemistry between the axial hydroxo-ligand in the (trans-dihydroxo)(5,10,15,20tetraphenylporphyrinato)Sn(IV) complex (SnP) and adipic acid (AA) on the surface of TiO 2 nanoparticles. The SnP center was tightly coupled to the TiO 2 nanoparticles via an adipic acid linkage in SnP/AA@TiO 2 , as judged by various instrumental techniques. Compared to SnP, TiO 2 and SnP@TiO 2 , SnP/AA@TiO 2 exhibited remarkably enhanced photocatalytic efficiency for the degradation of rhodamine B dye (RhB) under visible-light irradiation in aqueous solution. The high dye degradation efficiency, low catalyst loading and high reusability make this photocatalyst more efficient than other conventional photocatalysts, such as TiO 2 and ZnO. This work provides a new route for the design of high-efficiency porphyrin-based photocatalytic systems and holds great importance for extending applica-tions in the field of environmental remediation. The catalyst can prospectively be used as an alternative to so-called conventional photocatalysts.