Een gemakkelijke eenpotsynthese van biomimetische fotokatalysator Zn(II)-porfyrine-gevoelige 3D TiO2-holle nanoboxen en synergetisch verbeterde degradatie van zichtbaar licht

Resultaten en discussie

TEM- en SEM-afbeeldingen

De morfologie en kristalvlakken van TiOF₂ , TiO₂ -HNB's en ZnTCP@TiO₂ -HNBs-monsters werden waargenomen door TEM geïllustreerd in Fig. 1. Figuur 1a laat zien dat TiOF₂ onthult uniform solide kubieke vorm en glad oppervlak met een gemiddelde grootte van 250 nm. Zoals weergegeven in Fig. 1b, is te zien dat de morfologie van TiO₂ -HNB's getransformeerd in een uniforme holle doosachtige vorm bestaande uit zes geordende vlakken met een gemiddelde zijlengte van 260 nm, wat consistent was met de kubieke TiOF₂ sjabloon. Als we goed kijken naar Fig. 1c-f, kunnen we zien dat 3D holle structuren van ZnTCP@TiO₂ -HNB's vertoonden nog steeds een goed gedefinieerde doosvormige morfologie tijdens de fluoride-geïnduceerde zelftransformatie kubische TiOF₂ tot ZnTCP-gesensibiliseerde TiO₂ holle nanoboxen-proces, die goed gevormd waren met enkele harige tentakels van ZnTCP bedekt op het oppervlak van holle dozen [39]. HRTEM-afbeeldingen (zie het inzetstuk van Fig. 1g) werden verkregen om te bevestigen dat de roosterranden ongeveer 0,235 nm waren, wat goed overeenkwam met het hoogenergetische {001} kristallietfacet van anatase TiO₂ . Het oppervlak van ZnTCP@TiO₂ -HNB's roosterdiagram vertoonde 3D hiërarchische holle nanoboxen bestaande uit zes geordende dominante blootgestelde {001} facetten nanosheets.

TEM-afbeeldingen van gesynthetiseerde katalysatoren:a TiOF₂ , b TiO₂ -HNB's, c T-1p, d T-2p, e T-3p en f , g T-5p

De oppervlaktemorfologie van TiOF₂ , TiO₂ -HNB's en ZnTCP@TiO₂ -HNB's werden geïdentificeerd aan de hand van de SEM-afbeeldingen geïllustreerd in Fig. 2. Figuur 2a bestaat uit uniform en volledig kubisch TiOF₂ in overeenstemming met de TEM-afbeeldingen. Zoals weergegeven in Fig. 2b, zijn de afbeeldingen van TiO₂ -HNB's vertoonden goed gevormde morfologie van holle dozen, die is afgeleid van de sjabloon TiOF₂ kubussen in de topologische transformatieprocedure en omsloten door zes geordende TiO₂ nanosheets-arrays. Zoals weergegeven in Fig. 2c-f, alle ZnTCP@TiO₂ -HNBs-monsters vertoonden ook volledig holle doosachtige structuren met harige tentakels van Zn (II) porfyrine die op het oppervlak van nanoboxen bedekten, wat goed consistent was met TEM-resultaten, wat het succesvolle topologische transformatieproces in één pot van kubieke TiOF impliceert 2 naar TiO₂ holle nanoboxen met ZnTCP in situ deelname.

SEM-afbeeldingen van voorbereide katalysatoren:a TiOF₂ , b TiO₂ -HNB's, c T-1p, d T-2p, e T-3p en f T-5p

Zoals beschreven in Fig. 3, het waarschijnlijke vormingsmechanisme van Zn(II)-porfyrine-gesensibiliseerde 3D TiO₂ holle nanoboxen (ZnTCP@TiO₂ -HNB's) is voorgesteld via een proces van topologische transformatie waarbij sjabloonparticipatie betrokken is. Ethanol zorgde voor milde reactieomstandigheden door een oplosmiddel te gebruiken, waarvan de dehydratatiereactie (Vgl. 1, hoofd) en esterreactie tussen ZnTCP en ethanol (Vgl. 2) H₂ leverden O en bevorderde de hydrolyse van kubische TiOF₂ , die topologisch kan transformeren naar anataas TiO₂ holle doosvormige nanokristallen tijdens solvotherme reactie (vergelijking 3). Omdat de adsorptie van F ⁻ ion op het oppervlak van TiO₂ nanokristallen kunnen de oppervlakte-energie van (001) facetten sterk verminderen, fluoride-ionen vergemakkelijken de vorming van hoogenergetische anatase TiO₂ nanosheets en het is begrijpelijk dat voorloper TiOF₂ kan transformeren in TiO₂ holle nanoboxen assemblage door zes geordende nanosheets met blootgestelde hoogenergetische (001) facetten. De perifere tetracarboxylgroep van ZnTCP vormde de sterke esterbinding gecombineerd met de drager TiO₂ -HNB's (Vergelijking 4).

Het voorgestelde vormingsproces van Zn(II)-porfyrine maakte TiO₂ . gevoelig -HNB's (ZnTCP@TiO₂ -HNB's)

XRD-analyse

Het is algemeen aanvaard dat de kristalvlakken van TiO₂ -gebaseerde katalysatoren spelen een zeer belangrijke rol vanwege hun fotokatalytische eigenschappen. Afbeelding 4 toont de XRD-patroonanalyse van alle voorbereide monsters. Kijkend naar de curve ten opzichte van TiOF₂ voorloper, {100}, {200} en {210} kristalvlakken van TiOF₂ overeenkomend met 2θ =-24,05°, 48,34° en 54,45° werden duidelijk waargenomen. Voor alle TiO₂ monsters, een brede piek bij 2θ = 25,37° werd waargenomen, wat overeenkomt met de {101} vlakdiffractie van anatase TiO₂ (JCPDS No. 21-1272), anders dan 24,05°, die wordt toegeschreven aan de diffractiepiek van TiOF₂ , gaf aan dat alle monsters het topotactische transformatieproces met TiOF₂ . met succes hebben voltooid als sjabloon [40]. Vanwege F ⁻ ionen als vormcontrolemiddel, er was geen rutiel TiO₂ patronen gevormd tijdens het gesynthetiseerde proces. Vergeleken met TiO₂ -HNB's (T0p), de diffractiepieken rond {101} en {200} kristalvlak van Zn(II)-porfyrine-gesensibiliseerd TiO₂ -HNBs-monsters worden scherper, impliciet hoger in de mate van kristallisatie, wat wordt toegeschreven aan het feit dat de toename van de hoeveelheid Zn(II)-porfyrine leidde tot de concentratietoename van chemische esterbindingen tussen de TiO₂ oppervlakte-hydroxylgroep en perifere tetracarboxylgroep van ZnTCP. De andere karakteristieke diffractiepieken van TiO₂ monsters in XRD-patronen vertoonden geen verschuivingen of veranderingen in piekvorm, wat erop wees dat ZnTCP-modificatie en sensibilisatie de kristalstructuur van TiO₂ niet veranderde -HNB's.

XRD-patronen van voorbereide monsters

Om de kristallietgroottes van ZnTCP@TiO₂ . verder te onderzoeken -HNBs-monsters, werd de Scherrer-vergelijking gebruikt op basis van hun diffractiepiek [41, 42]. Zoals beschreven in tabel 1, gaven alle monsters hun gemiddelde deeltjesgrootte aan bij ongeveer 260 nm volgens structurele analyse, in overeenstemming met de SEM- en TEM-metingen. Echter, de “d ” ruimtewaarde van ZnTCP@TiO₂ -HNBs-monsters namen niet toe in vergelijking met TiO₂ -HNB's, wat de eenheidsceldimensie van de voorbereide ZnTCP@TiO₂ impliceert -HNBs-katalysator zou waarschijnlijk niet veranderen tijdens Zn(II)-porfyrine-gesensibiliseerde procedure.

UV-vis-spectroscopie

Afbeelding 5 toont het UV-vis spectrum van de oplossing van ZnTCP in 5 × 10 ⁻⁴ mol/L DMF-oplossing (a) en de UV-vis diffuse reflectiespectroscopie (DRS) van ZnTCP@TiO₂ -HNBs-samples en BaSO₄ werd gebruikt als het referentiemonster (b). As Fig. 5a displayed, the strong 432 nm absorption peak was ascribed to the characteristic B-band absorption of porphyrin rings, whereas weaker 561 nm and 600 nm peaks in the visible region were attributed to the characteristic Q-band absorption of ZnTCP, implying the successful synthesis of porphyrin ring. Compared to Fig. 5a, the Q-band absorption peaks of ZnTCP was red-shifted to 660 nm into the longer wavelength visible region after sensitization to TiO₂ . Because there is a presented strong interaction force between zinc porphyrins and TiO₂ -HNBs after hydrothermal reaction, which provides favorable prerequisites for the degradation of organic dyes under the visible-light irradiation. It can be found that TiO₂ -HNBs displayed less absorption above its fundamental absorption edge (> 400 nm), while Zn(II) porphyrin-sensitized TiO₂ samples displayed an obvious broad visible-light absorbance, as described in Fig. 5b, and the absorption intensities of ZnTCP@TiO₂ -HNBs samples were gradually enhancing much stronger as Zn(II) porphyrin amount increased. In addition, the color of as-prepared samples become deepened gradually from white to blue purple with the increasing of mass ratio of ZnTCP/TiO₂ -HNBs, and it was widely accepted that the purple coloration cause Zn(II) porphyrin-sensitized samples resulted into enhanced visible-light absorption. To further investigate the bandgap in each sample, the energy gap (eV) threshold was obtained using the transversal method, and the graph of the transformed Kubelka-Munk function against the photon energy for samples T-2p and TiO₂ -HNBs are displayed in Fig. 5c. According to the transformed Kubelka-Munk function plot, the energy gaps of E _{g (T-2p)} en E _{g (TiO2-HNBs)} are 2.83 and 3.08 eV, respectively. The reduced bandgap arises because ZnTCP can form a local energy state between the valence and conduction bands as ZnTCP concentration increases, which reduce the forbidden band width and the electron transition energy. Meanwhile, we find that the ZnTCP self-doped samples also exhibit very strong UV absorption, which indicates ZnTCP self-doping not only enhances the visible-light response of the catalysts, but also improves their UV light absorption. Thus, it can be predicted that more photo-generated electrons and holes can be excited and can participate in photocatalytic reactions, which is attributed to the increased probabilities of activation by UV and visible light.

een UV-Vis spectra of ZnTCP in 5×10 ⁻⁴ mol/L DMF solution. b UV-vis diffuse reflectance spectra (DRS) of all the samples. (c ) Estimated optical absorption edges for the T-2p and TiO2-HNBs

FT-IR Spectra Analysis

FT-IR spectra analysis is a powerful tool to identify characteristic functional groups [43]. Figure 6 displays the FT-IR spectra of ZnTCP, TiO₂ -HNBs, and T-2p. Looking at the spectra relative to ZnTCP, the broad band at 3443 cm ⁻¹ is assigned to the stretching vibration of the -OH in the peripheral t carboxyl groups, the characteristic absorption peak of C=O at 1720 cm ⁻¹ . The stretching vibration of Ar-H bond is clearly observed at 2995 cm ⁻¹ . Furthermore, the spectra of ZnTCP show the characteristic absorption peak of benzene ring at 1620 and 1469 cm ⁻¹ relative to ZnTCP [44].

FT-IR spectra of TiO₂ -HNBs, ZnTCP, and T-2p sample

Seeing from the spectra relative to TiO₂ -HNBs, the stretching vibration and bending vibration of the absorption H₂ O and Ti–OH group on the surface of samples were found about 3446 cm ⁻¹ and 1627 cm ⁻¹ , respectievelijk. The stretching vibration of Ti-O-Ti is observed at 1388 cm ⁻¹ , and the adsorption peaks around 521 cm ⁻¹ is assigned to the bending vibrations of Ti-O bond.

Compared to ZnTCP and TiO₂ -HNBs, the T-2p spectra features the characteristic stretching/bending vibration of the Ar-H bond at 2996 cm ⁻¹ and 1386 cm ⁻¹ attributed to Ti-O-Ti bond. Furthermore, the spectra of T-2p shows the characteristic absorption peak of phenyl ring at 1621 and 1468 cm ^− 1 corresponding to ZnTCP, indicating the existence of ZnTCP on the surface of sensitized TiO₂ [44]. However, the signal corresponding to -OH vibration peak disappeared, which demonstrates the formation of ester chemical bond between ZnTCP and TiO₂ . The stretching vibration of the C=O bond of ester group is clearly observed at 1713 cm ⁻¹ [45], further confirmed ZnTCP combined to TiO₂ -HNBs composites. The above information indicates that the TiO₂ -HNBs surface has been successfully functionalized with ZnTCP molecules. After sensitization, the spectra of T-2p show an increased intensity of the stretching vibration peak at 520 cm ⁻¹ and the characteristic absorption bands in the region of 1200~1060 cm ⁻¹ attributed to the CO-O-Ti bond resulting from the esterification between COOH of ZnTCP and OH of TiO₂ [46]. This reveals that the strong interaction (conjugated chemical bonds) between ZnTCP and TiO₂ -HNBs was established rather than simple physical adsorption. The results displayed on FT-IR spectra were in accordance with the photocatalytic performance of these catalysts, suggesting that the strong conjugated chemical bonds between ZnTCP and TiO₂ greatly contribute to the improvement of the photocatalytic activity as a transferring electrons bridge.

XPS Analysis

To further investigate chemical environments of as-prepared catalysts, the X-ray photoelectron spectra (XPS) measurements of TiO₂ -HNBs and T-2p were carried out [43]. As shown in full spectra of Fig. 7a, two elements including Ti and O were both observed at the corresponding positions, respectively. Figure 7b–f displayed the high-resolution XPS spectra of the corresponding element of two samples.

een The full spectra of all kinds of elements in sample TiO₂ -HNBs and T-2p; b the high-resolution Ti 2p XPS signals of TiO₂ -HNBs and T-2p; c the O1s XPS signals of TiO₂ -HNBs and T-2p; d the C1s O1s XPS signals of TiO₂ -HNBs and T-2p; e the N1s XPS signals of T-2p; v the Zn 2p XPS signals of T-2p; g the C XPS-peak-differentation-imitating analysis

The high-resolution Ti 2p XPS signals of TiO₂ -HNBs in Fig. 7b consisted of two binding energy levels of Ti 2p_1/2 and 2p_3/2 at 464.24 and 458.54 eV with a separation energy of about 5.70 eV. However, the binding energy levels of Ti 2p_1/2 and 2p_3/2 XPS peaks of T-2p exhibited at 465.08 and 459.38 eV, which shift 0.84 eV toward the lower energy region compared to TiO₂ , indicative of a partial charge transferred from the surface C=O ester moiety to Ti ⁴⁺ centers, which was probably due to the strong interactions between ZnTCP molecules and TiO₂ .

As shown in Fig. 7c, the O1s XPS signals of TiO₂ -HNBs and T-2p displayed at 529.72 and 529.91 eV respectively, with a deviation energy of nearly 0.19 eV. The shifting of lattice oxygen peaks in T-2p toward lower energy, compared to TiO₂ standard, further verified the different chemical environment due to ZnTCP sensitization. As displayed in Fig. 7d, the C 1s of T-2p appeared the distinct peaks at 284.6 eV, which were ascribed to C=O (and COO) bonds in the peripheral tetracarboxyl group, further confirmed that ZnTCP was well bonded on the surface of TiO₂ . It was found that the weak peak at 401.15 eV of T-2p in Fig. 7e was attributed to N1s of the N=N- bond in the porphyrin ring. The Zn 2p binding energy signals of T-2p appeared at 1021.5 eV and 1044.46 eV in Fig. 7f, which are basically consistent with the complex molecular structure of ZnTCP, further implying it well impregnated onto the surface of TiO₂ . Figure 7g shows the XPS spectra of C 1s region for samples. In Fig. 7g, the peak with a binding energy of 284.6 eV can be attributed to the C–C bonds, while the deconvoluted peaks centered at the binding energies of 286.5, 288.3, and 289.6 eV can be assigned to the C–O, C=O, and O=C–OH functional groups, respectively. C 1s spectra for the ZnTCP@TiO₂ -HNBs composite are shown in Fig. 7. In the spectrum of ZnTCP@TiO₂ -HNBs, all peaks from oxygen-containing functional groups decreased dramatically in intensity or even disappeared entirely, indicating a significant reduction of GO by solvothermal treatment. In addition, an additional shoulder peak was found, which was usually assigned to the formation of a chemical bond between a carbon atom and a titanium atom in the lattice of TiO₂ -HNBs, which resulted information of Ti–O–C bonds.

Photocatalytic Activity of ZnTCP@TiO₂ -HNBs Catalysts

The photocatalytic activity of ZnTCP@TiO₂ -HNBs was evaluated by measuring the degradation with RhB as a probe molecule. Figure 8a exhibits the degradation curve of RhB under simulated sun light conditions with as-prepared ZnTCP@TiO₂ -HNBs catalysts. It is clearly observed that, after the adsorption-desorption equilibrium process, the self-degradation of probe molecule RhB can be negligible and the visible-light degradation activities of ZnTCP-sensitized TiO₂ -HNBs samples indicates much higher photocatalytic efficiencies than TiO₂ -HNBs and ZnTCP under visible-light irradiation (λ ≥ 420 nm), which depends on the concentration of RhB versus visible-light irradiation time. The T-2p sample shows highest photocatalytic activity with RhB degradation rate of 99% after 2 h under visible-light irradiation, resulting from its enhanced visible-light response. However, the TiO₂ -HNBs (T-0p) shows little decomposition rate of RhB under the same reaction conditions. In this process, the order of degradation rate is T-2p> T-3p> T-1p> T-5p> ZnTCP>TiO₂ -HNBs.

een The C _t /C₀ vs. time curves of RhB photodegradation under visible-light irradiation. b The apparent first-order rate constant of RhB photodegradation. c Cyclic degradation curve of T-2p. d The XRD before and after recycles of T-2p

It is well accepted that the slopes for the curve of RhB intensity versus illumination time (rate constant) represent the photodegradation activity of the samples. Therefore, the exponential curve of the degradation RhB under visible-light irradiation was fitted in Fig. 8b, and the degradation kinetics of RhB was described by the first-order kinetics curves. Degradation experiments followed the first-order kinetic equation ln(C₀ -C) = − Kt  + b (in which K is the apparent first-order rate constant and t is the reaction time). By comparing the rate constants (K values) of ZnTCP-sensitized TiO₂ -HNBs catalysts, we can see that the rate constants of these catalysts increase first and then decrease with the ZnTCP/TiO₂ -HNBs mass ratio increase. Observed from the rate constants histogram (Fig. 8b), we can find that T-2p reveals the highest visible-light photocatalytic activity (rate constant of 0.7139), which is 3.6 times higher than that of TiO₂ -HNBs (T-0p) (rate constant of 0.19841); furthermore, the photocatalytic rate constants of other ZnTCP@TiO₂ -HNBs samples are basically larger than that of TiO₂ -HNBs.

The possible reasons is that may be because ZnTCP-sensitized TiO₂ -HNBs can extend the absorption wavelength range of TiO₂ -based catalysts, reducing the bandgap of TiO₂ , and facilitate the effective separation of the electron-hole pairs generated by TiO₂ -HNBs after absorbing visible light, thereby improving the utilization of electron-hole pairs. However, too much ZnTCP-sensitized parts may result in the increase of defects on the surface of TiO₂ -based catalyst, resulting in a rapid recombination center of photogenerated electrons and holes, thereby decreasing the photocatalytic activity.

In order to investigate the long-term stability of as-prepared ZnTCP@TiO₂ -HNBs, the photocatalysts were recycled after each photocatalytic degradation experiment and then were reused in the next run after washing treatment. Figure 8c shows sample T-2p were recycled to 5 rounds after bleaching RhB under visible-light irradiation; as the number of photodegradation cycles increased, the photocatalytic rate of RhB still maintained a high catalytic activity, indicating that the active site of ZnTCP@TiO₂ -HNBs surface did not decrease, and the composite photocatalyst has good photocatalytic stability under visible-light irradiation. The strong bridging ester bond-linking leads to a slower decrease of the degradation efficiency for sample T-2p. These results further demonstrate that ZnTCP can establish a steady chemical bridging bond-linking interaction between ZnTCP and TiO₂ -HNBs, thus enhancing the visible-light photocatalytic performance, stability, and recyclability of ZnTCP@TiO₂ -HNBs. XRD shows that the T-2p is still anatase type and did not change the crystal type after many cycles, further verifying the stability of the catalyst (Fig. 8d).

PL Analysis

In order to study the active species generated during photocatalytic degradation, the coumarin, 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl), and 1,3-diphenylisobenzofuran (DPBF) were used as the hydroxyl radical (•OH), superoxide radical (•O₂ ⁻ ), and singlet oxygen ( ¹ O₂ ) fluorescent probe molecules, respectively. Figure 9a displays the active species of hydroxyl radical detection map with T-2p as photocatalyst. As shown in Fig. 9a, the fluorescence emission wavelength was centered at 450 nm and the fluorescence intensity gradually increased, indicating the concentration of •OH group was increasing with the increase of illumination time. The reason is due to the fact that the continuously generated •OH active group and coumarin react to obtain 7-hydroxycoumarin, the amount of the reaction is gradually increased, and the fluorescence intensity is gradually increased [44, 45].

Fluorescent detection of active species •OH (a ), •O₂ ⁻ (b ), and ¹ O₂ (c ) with T-2p as photocatalyst

It is well accepted that quantification of superoxide ion concentration can be monitored its reaction with 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) by measuring its fluorescence emission intensity at 550 nm using an excitation wavelength of 470 nm. According to Fig. 9b, it can be found that the fluorescence emission wavelength was centered 550 nm and the fluorescence intensity gradually decreased with the increase of illumination time. In the presence of NBD-Cl, the degradation rate is not obviously increased. The results indicate that •OH radical is hardly generated, while •O₂ ⁻ is the significant active species generated in the system. More e ⁻ can react with O₂ to form superoxide radical •O₂ ⁻ instead of the rapid recombination with h ⁺ , which further improves the photodegradation activity and testifies the catalyst can generate the active species •O₂ ⁻ .

In addition, a typical fluorescence method for detecting singlet oxygen ( ¹ O₂ ,) by using 1,3-diphenylisobenzofuran (DPBF) as a probe molecule is proposed, which exhibited a strong fluorescence spectrum with a maximum at 455 nm when excited at 410 nm. As shown in Fig. 9c, the fluorescence emission wavelength was centered 455 nm and the fluorescence intensity gradually increased with the increase of illumination time. Based on the above observation, it can be concluded that the photocatalyst produces a certain amount of singlet oxygen in the light condition. To sum up, the ZnTCP@TiO₂ -HNBs catalyst can generate •OH, •O₂ ⁻ , and ¹ O₂ active species during the photodegradation process under visible-light irradiation.

Electrochemical Analysis

To attain more insights into photocatalytic transformation for oxidation degradation organic pollutants, the electrochemical properties of ZnTCP and ZnTCP@TiO₂ -HNBs (T-2p) in DMF were investigated with cyclic voltammetry (CV) on a glassy carbon electrode using tetrabutylammonium perchlorate as supporting electrolyte. It can be clearly seen that two complete oxidation and reductions processes for ZnTCP observed from the potential window are shown with blue dashed lines in Fig. 10. Furthermore, ZnTCP@TiO₂ -HNBs (T-2p) distinctly displayed similar two oxidation processes resulted from the ZnTCP center with red dashed lines [39]. The oxidation-reduction electrochemical behavior of ZnTCP complexes mainly occurred on the porphyrins ring and did not involve the change in the valence state of the central metal ions, which were good agreement with our previous reported [47]. However, the oxidation degradation reaction of ZnTCP@TiO₂ -HNBs (T-2p) was very similar as that of ZnTCP, which indicated the photocatalytic transformation mainly happened on the porphyrin ring, not relative to the center zinc ion and TiO₂ -HNBs. These results further confirm that the strong interaction, such as conjugated ester chemical bonds between TiO₂ -HNBs and ZnTCP, was established rather than simple physical adsorption interaction.

Cyclic voltammetry analysis of 1.0 × 10 ⁻³  mol dm ⁻³ of ZnTCP and T-2p on Pt in 0.1 mol dm ⁻³ TBAP/DMF vs. SCE. Scan rate:0.1 V s ⁻¹

Proposed Probable Photocatalytic Mechanism

Much effort has been devoted to illustrate the photocatalytic mechanism for oxidation degradation organic pollutants. In order to speculate the photodegradation mechanism, the main oxidative species in the photocatalytic process was firstly detected through radical and hole trapping experiments by using EDTA (hole h ⁺ scavenger), p -benzoquinone (BZQ, superoxide radical •O₂ ⁻ scavenger), iso -propyl alcohol (IPA, hydroxyl radical •OH scavenger) and 1,3-diphenylisobenzofuran (DPBF, singlet oxygen ¹ O₂ scavenger). Figure 11 shows the Ct/C0 vs. time curves of different reactive species scavengers on the photodegradation of RhB under visible-light irradiation.

The C _t /C₀ vs. time curves of different reactive species scavengers on the photodegradation of RhB under visible-light irradiation

As displayed in Fig. 11, the photocatalytic degradation of RhB was apparently restrained after the injection of the active species scavenger. Taking the decreasing photodegradation rate into consideration, it can be found that RhB degradation rate with ZnTCP as a photocatalyst mainly depended on active species of ¹ O₂ group according to the 1,3-diphenylisobenzofuran (DPBF) detection results. Furthermore, RhB photodegradation rate with TiO₂ -HNBs as catalysts mostly depended on active species of h ⁺ and •OH group. However, it is very clear that RhB photodegradation rate with ZnTCP@TiO₂ -HNBs (T-2p) as catalysts was influenced by all active species of hole (h ⁺ ), hydroxyl radical (•OH), superoxide radical (•O₂ ⁻ ), and singlet oxygen ( ¹ O₂ ), further confirming ZnTCP@TiO₂ -HNBs (T-2p) can generate these active species during the photodegradation process.

The probable photodegradation mechanism of organic dyes with ZnTCP-sensitized TiO₂ -HNBs as catalysts under visible-light irradiation is shown in Fig. 12. The degradation process generally consists of three moieties. The first part mainly involves the generation of singlet oxygen ( ¹ O₂ ). When irradiated by simulated sunlight, the inspired electrons of ZnTCP are transferred from the ground state of porphyrin [Pp] to the excited singlet state ¹ [Pp]* [47], and then, relaxation of the singlet excited state generates the triplet excited state ³ [Pp]* through a process of intersystem crossing. Electrons from ¹ [Pp]* and ³ [Pp]* excited states are trapped by the adsorbed O₂ to form singlet oxygen ( ¹ O₂ ), which can cause RhB degradation into pieces of small organic molecule or mineralization to CO₂ .

Proposed probable photocatalytic mechanism of ZnTCP@TiO₂ -HNBs

In contract, superoxide radical (•O₂ ⁻ ) can be generated in the second moiety. Biomimetic catalytic moiety of ZnTCP is not only responsible to generate singlet oxygen ( ¹ O₂ ), but also acts as electron bridges from ¹ [Pp]* and ³ [Pp]* excited states transferred to conduction band (CB) of TiO₂ -HNBs, which can be trapped further by the adsorbed O₂ , resulting in formation of an amount of •O₂ ⁻ causing photodegradation of RhB present on the surface of TiO₂ -HNBs.

On the other hand, in the sensitization of porphyrins, the electrons of TiO₂ are excited from valence band (VB) to conduction band (CB) forming electron-hole pairs (e ⁻ -h ⁺ ) during the visible-light irradiation [48, 49]. The photogenerated holes transfer to the surface of TiO₂ reacted with H₂ O or OH ⁻ , resulting in formation of hydroxyl radical (•OH). The photogenerated electrons are transferred to the surface of TiO₂ trapped by the adsorbed oxygen molecule, obtained an amount of superoxide radical (•O₂ ⁻ ), and these active species (•O₂ ⁻ , •OH) mainly implement the degradation of organic dyes by its successive formation of intermediaries participated.

Thus, overall, a cooperative mechanism is proposed for the degradation of organic dyes involving three components of photocatalytic system. The enhanced visible-light photodegradation activities of Zn(II)TCP@TiO₂ -HNBs compared to ZnTCP and TiO₂ -HNBs might be related to synergetic generation of three active species (•O₂ ⁻ , •OH, and ¹ O₂ ), resulting in more higher efficient photocatalytic activities.