Figure1 shows the fabrication process of TiO2 NSs, CdS/TiO2 NSs and PbS/CdS/TiO2 NSs. X-ray diffraction (XRD) measurement for TiO2 NSs, CdS/TiO2 NSs and PbS/CdS/TiO2 NSs were performed as shown in Figure2a, which showed diffraction peaks at 25.34°, 37.83°, 48.10° and 55.06° corresponding to the (101), (004), (200) and (211) crystal planes of anatase TiO2 phase (JCPDS#21-1272)(32, 33). Meanwhile, the blue curve presents new diffraction peaks at 43.97° and 52.08°, assigning to the (220) and (311) planes of cubic phase CdS NCs. Beyond that, the new diffraction peaks appear at 30.13° and 68.88° in green curve, assigning confirmed to (111) and (200) crystal planes of PbS NCs (JCPDS#65-2935). TEM, HTEM and SAED patterns (FigureS1) further confirmed that the samples were synthesized, successfully. The x-ray photoelectron spectroscopy (XPS) spectrum of PbS/CdS/TiO2NSs is shown in Figure2b. FigureS2 present the characteristic peaks position of Ti2p, Cd3d, Pb4f and S2p. The peak positions of Ti 2p3/2 and Ti 2p1/2 are located at 458.3 and 464.0 eV(34). The peaks of Cd 3d5/2 and Cd 3d3/2 are 405.0 and 411.7 eV, respectively. In the meantime, the peaks of S2p locate at 161.2 eV(35). The Pb4f is observed in FigureS2d, which is at about 138.3 eV as revealed in the previous literature(36). Therefore, the above results are confirmed that the preparation of the three-way PbS/CdS/TiO2NSs catalyst is successful.
The morphology images of samples were conducted by FESEM (Figure3). Figure 3a-3b show the morphology of bare TiO2NSs, inset picture is the cross-section of bare TiO2NSs. It can be seen that the dense and uniform TiO2NSs grow on the FTO substrate, vertically. The length and the thickness of TiO2NSs are about 1.5-2.1 μm and 200-240 nm, respectively. Figure3c-3f show the images of CdS/TiO2NSs composites with more active sites. The increase of CdS NCs is growing with the longer chemical bath deposition time (10-40 min). The more CdS NCs are growing, the more roughness of CdS/TiO2NSs composites is improving. Figure3h-3j present the morphology of PbS/ CdS/TiO2NSs with different cycles. The number of PbS NCs is growing with the increase in experimental cycles. The more PbS NCs are growing, the specific surface of PbS/CdS/TiO2NSs is increasing. Therefore, a larger specific surface area is beneficial for photocatalysts. The EDS and EDS mapping spectrum of PbS/CdS/TiO2NSs are shown in FigureS4-5. The elements of Pb, Cd and S are homogenously distributed on the TiO2NSs surface. The atomic ratio of each element is consistent with expectations.
Figure4a illustrates the UV–vis absorption spectra of TiO2NSs and CdS(10-40 min)/T. Meanwhile, Figure4c shows the UV–vis absorption spectra of PbS(3-5 C)/CdS(10-40 min)/T. Pure TiO2NSs exhibited visible light absorption with absorption band edges at 380 nm. In Figure4c, the absorption edge of PbS(7C)/ CdS(40 min)/ TiO2NSs is widened to 800 nm(32, 36). As shown in Figure4b and Figure4d, the UV–vis absorption spectra of PbS(3-5 C)/CdS(10-40 min)/T are red shifted as the band gap (Eg) gets narrow. Eg was computed as stated by Kubelka–Munk function (a), where c is proportionally constant, α and ν are the absorption coefficient and the frequency, respectively. See formula 2 in the supplementary files.
The EIS measurement and steady-state photoluminescence (PL) were conducted to explore the charge transfer dynamics. Figure5a shows the EIS curves of TiO2NSs, CdS(30)/T and PbS(5C)/CdS(30)/T. The TiO2 NSs could exhibit higher carrier resistance than PbS(5C)/CdS(30)/T. It demonstrates that PbS(5C)/CdS(30)/T has lower electron transport resistance. Therefore, PbS(5C)/CdS(30)/T is beneficial to electrons and holes transportation. The separation of electron-hole pairs plays a key role in the photocatalytic activity of the catalyst(37, 38). To further explore the charge recombination dynamics, the steady-state photoluminescence (PL) was performed and the PL intensity of PbS(5C)/CdS(30)/T demonstrates an obvious decrease compared with TiO2 NSs (Figure5b). It can be seen that the PL is quenched to some extent after the introduction of PbS and CdS. The PL quenching effect of PbS(5C)/CdS(30)/T indicates that electrons and holes can separate, effectively. It is consistent with the EIS results (Figure5a). The transient photocurrent response is shown in Figure5c. The photoelectrochemical performance of samples has been tested for analyzing the number of photogenerated electron-hole pairs. A higher current density was achieved by PbS(7C)/CdS(30)/T than the other samples, implying the enhanced electron-hole pairs generation. Moreover, the reason of much more electron-hole pairs is due to the proper energy band matching between PbS, CdS and TiO2. In particular, the transmitting procedure of electrons and holes is obviously shown in Figure5d.
The photocatalytic performances of the samples over RhB and MB were studied under UV-vis light. The details of reaction experiments were shown in Supplementary FigureS6. FigureS6a and FigureS6b show the UV-Vis absorption of RhB and MB dye solution degraded by the sample (30-CdS/TiO2NSs). As shown in FigureS6c, the concentration of the organic solution did not change significantly during the dark treatment (-30-0 min). It illustrates the catalyst has no catalytic effect without driven force of solar power. Outstandingly, the degradation rate of 30-CdS/TiO2NSs for RhB was 99.8 % within 40 min. The degradation rate of pure TiO2 for RhB was only 23.3 % under the UV light. It is due to the smaller number of light-excited oxidative holes and the higher electron hole combination rate of bare TiO2NSs. Meanwhile, FigureS6d shows a degradation pattern for MB analogous to that of FigureS6c, it is obvious that all of the composites sensitized with CdS quantum dots show better photocatalytic activity.
We made a comparison experiment under visible light by using a xenon lamp simulator. Clearly, according to the analysis in Figure7a, the appropriate addition of PbS QDS could enhance the catalytic property. After 160 min, 97.1 % of RhB could be removed by PbS(5C)/CdS(30)/TiO2. The other samples, for example PbS(7C)/CdS(30)/TiO2, show lower photocatalytic activity, it maybe owing to the increasing number of defects, recombination center and smaller surface area. Acting as recombination center of photoelectrons and holes, the defect has a negative effect on catalysis. In conclusion, CdS and PbS could effectively help TiO2NSs to broad the light utilization range. Establishing heterogeneous nodes could accelerate the photocatalysis process. Interestingly, as confirmed by Figure5a, the electrochemical impedance spectroscopy (EIS) analysis could also agree with this.
Those response energy about photocatalytic degradation from RhB by samples sort of impetuses were greatly fitted with pseudo-first-order energy module: -ln(C0/Ct)=Kt, where C0 is the concentration at the initial, Ct is the concentration at the reaction time t, and K is the rate constant. As shown in Figure7b, PbS(5C)/ CdS(30)/TiO2 has a maximum K value that is much higher than the other samples. It demonstrates that the multiple heterostructure photocatalyst has high-efficiency photocatalytic activity.
In fact, the excellent recycling property and stability of photocatalysts can effectively cut the waste water treatment cost and avert secondary pollution. The stability of PbS(5C)/CdS(30)/TiO2NSs was conducted by 5 recycling experiments in Figure7a. Specifically, there was a little drop in degradation ability after five cycles, PbS(5C)/CdS(30)/TiO2NSs composites could still degrade 86.64% RhB. As the outermost layer of the sample, lead sulfide avoided direct light exposure to cadmium sulfide to reduce the photo-corrosion of cadmium sulfide and improve the stability of the sample. In order to investigate the sample changes before and after the reaction, X-ray diffraction patterns were analyzed on the samples after the photocatalytic reaction. According to Figure7b, the diffraction peak of the photocatalyst has almost no change, and the sample maintains its original composition. As we expected, this result indicates that the sample exhibits excellent long-term stability. It is hopeful to achieve large-scale use without any additional pollution in the future.
The mechanism of photodegradation of dye over the catalyst is shown in Figure8. PbS is excited to produce photoelectrons under visible light irradiation. The electrons on the PbS (CB) jump to the CB of CdS and continue to transfer to TiO2 (CB), which follows the law of conservation of energy. Meanwhile, h+ on the surface of TiO2 (VB) should transfer to VB of CdS. Holes don’t rest on CdS (VB) and jump to the valence band of PbS. The isolated electrons and h+ react with water to produce large quantities of highly oxidizing ·O2- and ·OH for degradating organic pollutants.
PbS+UV light→PbS(e-)+PbS(h+) (1)
OH+RhB/MB→degraded or mineralizedproducts (7)
CdS(h+)+RhB/MB→degraded or mineralized products (8)
PbS(h+)+RhB/MB→degraded or mineralized products (9)
As discussed, the degradation reactions can be described as the above equations (1-9).