생체모방 광촉매 Zn(II)-포르피린에 민감한 3D TiO2 중공 나노박스의 손쉬운 원팟 합성 및 시너지 효과로 향상된 가시광선 분해

결과 및 토론

TEM 및 SEM 이미지

TiOF₂의 형태 및 결정면 , TiO₂ -HNB 및 ZnTCP@TiO₂ -HNB 샘플은 그림 1과 같이 TEM에 의해 관찰되었습니다. 그림 1a는 TiOF₂ 평균 크기가 250nm인 균일한 고체 입방체 모양과 매끄러운 표면을 나타냅니다. 그림 1b와 같이 TiO₂의 형태가 -HNB는 입방 TiOF₂와 일치하는 평균 측면 길이가 260nm인 6개의 정렬된 면으로 구성된 균일한 속이 빈 상자 같은 모양으로 변형되었습니다. 주형. 그림 1c-f에서 주의 깊게 보면 ZnTCP@TiO₂의 3D 중공 구조가 -HNB는 불소 유도 자기 변형 입방 TiOF₂ 동안 여전히 잘 정의된 상자 모양의 형태를 나타냅니다. ZnTCP에 민감한 TiO₂ 속이 빈 상자의 표면에 덮인 털이 많은 ZnTCP 촉수로 모양이 좋은 속이 빈 나노박스 프로세스[39]. 격자 무늬가 약 0.235nm임을 확인하기 위해 HRTEM 이미지(그림 1g의 삽입물 참조)를 얻었으며, 이는 아나타제 TiO₂의 고에너지 {001} 결정자 면과 잘 일치합니다. . ZnTCP@TiO₂의 표면 -HNB의 격자 다이어그램은 6개의 정렬된 지배적 노출면 나노시트로 구성된 3D 계층적 속이 빈 나노박스를 나타냅니다.

합성된 촉매의 TEM 이미지:a TiOF₂ , b TiO₂ -HNB, c T-1p, d T-2p, e T-3p 및 f , 지 T-5p

TiOF₂의 표면 형태 , TiO₂ -HNB 및 ZnTCP@TiO₂ -HNB는 그림 2에 표시된 SEM 이미지에서 식별되었습니다. 그림 2a는 균일하고 완전한 입방체 TiOF₂로 구성됩니다. TEM 이미지와 일치합니다. 그림 2b와 같이 TiO₂의 이미지는 -HNB는 TiOF₂ 템플릿에서 파생된 잘 형성된 중공 상자 형태를 보여주었습니다. 위상 변환 절차의 큐브 및 6개의 정렬된 TiO₂로 둘러싸여 있습니다. 나노시트 어레이. 그림 2c–f에 표시된 것처럼 모든 ZnTCP@TiO₂ -HNB 샘플은 또한 나노박스의 표면에 Zn(II) 포르피린의 털이 많은 촉수가 덮인 완전히 속이 빈 상자형 구조를 나타냈으며, 이는 TEM 결과와 잘 일치하여 입방체 TiOF_{로부터의 성공적인 원팟 토폴로지 변환 프로세스를 암시합니다. 2} TiO₂로 ZnTCP가 있는 속이 빈 나노박스.

준비된 촉매의 SEM 이미지:a TiOF₂ , b TiO₂ -HNB, c T-1p, d T-2p, e T-3p 및 f T-5p

그림 3에 설명된 것처럼 Zn(II)-포르피린에 민감한 3D TiO₂의 가능한 형성 메커니즘 속이 빈 나노박스(ZnTCP@TiO₂ -HNBs)는 템플릿 참여를 포함하는 토폴로지 변환 프로세스를 통해 제안되었습니다. 에탄올은 탈수 반응(식 1, 주)과 ZnTCP와 에탄올 사이의 에스테르 반응(식 2)이 H2를 공급하는 용매를 사용하여 온화한 반응 조건을 제공했습니다. O 및 입방 TiOF₂의 가수분해 촉진 , 위상적으로 아나타제 TiO₂로 변환할 수 있습니다. 용매열 반응 동안 속이 빈 상자 모양의 나노결정(식 3). F ^- 의 흡착 때문에 TiO₂ 표면의 이온 나노결정은 (001)면의 표면 에너지를 급격히 감소시킬 수 있으며, 불소 이온은 고에너지 아나타제 TiO₂의 형성을 촉진합니다. 나노시트이며 전구체 TiOF₂ TiO₂로 변형 가능 고에너지(001) 면이 노출된 6개의 정렬된 나노시트로 속이 빈 나노박스 어셈블리. ZnTCP의 주변 테트라카르복실기는 지지체 TiO₂에 결합된 강한 에스테르 결합을 형성했습니다. -HNB(Eq. 4).

Zn(II) 포르피린 감작 TiO₂의 제안된 형성 과정 -HNB(ZnTCP@TiO₂ -HNB)

XRD 분석

TiO₂의 결정면이 -계 촉매는 광촉매 특성에 있어 매우 중요한 역할을 합니다. 그림 4는 준비된 모든 샘플의 XRD 패턴 분석을 보여줍니다. TiOF₂에 상대적인 곡선 보기 TiOF₂의 전구체, {100}, {200} 및 {210} 결정면 2θ에 해당 =24.05°, 48.34° 및 54.45°가 명확하게 관찰되었습니다. 모든 TiO₂용 샘플, 2θ에서 넓은 피크 =25.37°가 관찰되었으며, 이는 아나타제 TiO₂의 평면 회절에 해당합니다. (JCPDS No. 21–1272), TiOF₂의 회절 피크에 기인하는 24.05°와 다름 , 모든 샘플이 TiOF₂를 사용하여 토포택틱 변환 프로세스를 성공적으로 완료했음을 나타냅니다. 템플릿으로 [40]. F ⁻ 로 인해 형상 조절제로 이온, 금홍석 TiO₂ 없음 합성 과정에서 형성된 패턴. TiO₂와 비교 -HNB(T0p), Zn(II) 포르피린 감응 TiO₂의 {101} 및 {200} 결정면에 대한 회절 피크 -HNB 샘플은 더 날카로워지며 결정화 정도가 높아짐을 암시하는데, 이는 Zn(II) 포르피린 양의 증가가 TiO₂ 사이의 에스테르 화학 결합의 농도 증가를 초래했다는 사실에 기인합니다. ZnTCP의 표면 수산기 및 주변 테트라카르복실기. TiO₂의 다른 특징적인 회절 피크 XRD 패턴의 샘플은 피크 모양의 이동이나 변화를 나타내지 않았으며, 이는 ZnTCP 수정 및 민감화가 TiO₂의 결정 구조를 변경하지 않았음을 나타냅니다. -HNB.

준비된 샘플의 XRD 패턴

ZnTCP@TiO₂의 결정자 크기를 더 조사하려면 -HNB 샘플, Scherrer 방정식은 회절 피크를 기반으로 사용되었습니다[41, 42]. 표 1에 설명된 바와 같이 모든 샘플은 SEM 및 TEM 측정과 잘 일치하는 구조 분석에 따라 약 260nm에서 평균 입자 크기를 나타냅니다. 그러나 "d ” ZnTCP@TiO₂의 공간 값 -HNB 샘플은 TiO₂에 비해 증가하지 않았습니다. -HNB, 준비된 ZnTCP@TiO₂의 단위 셀 치수를 의미 -HNB 촉매는 Zn(II) 포르피린 감작 절차 동안 변경될 가능성이 없습니다.

UV-vis 분광법

그림 5는 5 × 10 ⁻⁴ 에서 ZnTCP의 솔루션 UV-vis 스펙트럼을 보여줍니다. mol/L DMF 용액(a) 및 ZnTCP@TiO₂의 UV-vis 확산 반사 분광법(DRS) -HNB 샘플 및 BaSO₄ 참조 샘플(b)로 사용되었습니다. 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. 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)} 및 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.

아 UV-Vis spectra of ZnTCP in 5×10 ⁻⁴ mol/L DMF solution. ㄴ UV-vis diffuse reflectance spectra (DRS) of all the samples. (ㄷ ) 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 , 각각. 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.

아 The full spectra of all kinds of elements in sample TiO₂ -HNBs and T-2p; ㄴ the high-resolution Ti 2p XPS signals of TiO₂ -HNBs and T-2p; ㄷ the O1s XPS signals of TiO₂ -HNBs and T-2p; d the C1s O1s XPS signals of TiO₂ -HNBs and T-2p; 이 the N1s XPS signals of T-2p; 에 the Zn 2p XPS signals of T-2p; 지 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.

아 The C _t /C₀ vs. time curves of RhB photodegradation under visible-light irradiation. ㄴ The apparent first-order rate constant of RhB photodegradation. ㄷ 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 (ㄷ ) 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.