Dual-sensitized modification engineering with enhanced photocatalytic degradation for organic dye

Organic pollutant is an environmentally harmful and ubiquitous aquatic pollutant with extensive production and application. In this study, a unique novel TiO2-based dual-sensitized heterojunction photocatalyst (PbS/CdS/TiO2) was successfully obtained by one step hydrothermal method, chemical bath deposition (CBD) method and ionic layer adsorption and reaction (SILAR) method. Scanning electron microscopy, X-ray photoelectron spectroscopy, transmission electron microscopy and energy dispersive spectrometer characterized the composition. PbS/CdS was successfully decorated on the surface of TiO2 nanosheet array. As a stable catalyst with wide-spectrum (300–800 nm) optical response, excited electron–hole pairs of PbS/CdS/TiO2 could apply in degrading organic pollutants. Successfully, the degradation efficiency of PbS(5 C)/CdS(30 min)/TiO2 nanocomposites reached 99.9% under visible light, which was 5 times over pure TiO2. This ternary heterostructured materials will be well-promising photocatalysts.


Introduction
With the demand of development in science and technology, storage of fossil energy is nearly exhausted, and numerous organic toxins are produced and released into water through the process of industrialization simultaneously [1][2][3]. Therefore, the strategy of seeking clean energy and degradation of organic pollutants recently been widely investigated by multitudinous scientists [4][5][6]. Nowadays, semiconductor materials have been considered more due to their attractive applications including energy conversion, storage, solar fuel, photocatalytic and medical [7,8]. They are regarded as potential catalysts for wastewater management and water splitting, which can directly decompose the pollutants absorbed on the surface through redox processes [9,10].
TiO 2 is thought to be a suitable photocatalyst. Although, it has played an important role in stability, cost effectiveness and inert nature, it is still encounters the problems of large bandgap (* 3.2 eV), high recombination rate of photogenerated electron-hole pairs and narrow light response range [11][12][13][14][15].
Numerous researchers have studied that combining the photoprocess with physical or chemical methods was an effective way to improve the efficiency of photodecomposition over TiO 2 [16][17][18][19]. Among them, it is one of the most effective way that building multiple heterojunction between semiconductors [7,[20][21][22].
As an important visible light-sensitive semiconductor, cadmium sulfide nanocrystal (CdS NCs) is the most investigated among the metal chalcogenides due to its direct band energy of 2.42 eV [23][24][25]. Therefore, CdS is easy enough for photoexcitation of electrons and used light to a larger extent [26]. Moreover, lead sulfide nanocrystal (PbS NCs) can be excited with less light, due to its bandgap is only as narrow as 0.4 eV [27][28][29][30]. And it is not difficult to find that its light absorption edge is in the near infrared region [31]. As far as we know, there were only a few studies that CdS/PbS co-sensitized vertically aligned TiO 2 nanosheets(TiO 2 NSs) array films for photocatalytic degradation [32]. Hence, the study of CdS/ PbS/TiO 2 NSs nanocomposites is of remarkable interest and challenging.
In our work, we study a kind of unique recyclable heterogeneous PbS/CdS/TiO 2 sheet nanocomposite photocatalyst. CdS and PbS play three roles in photocatalyst: (1) PbS/CdS can affect the energy band structure for nanocomposite; (2) the increasing number of quantum dots increases the specific surface area of the material; (3) building multiple heterojunction between semiconductors. Simultaneously, it may lead to a new alternative or potential technology for future photocatalyst design and improvement in practical applications.  2 and Na 2 S were employed for synthesis of PbS NCs, which were obtained from Tianjin Institute of Guangfu Fine Chemicals and Xilong Chemical Co., Ltd.

TiO 2 NSs
Ti(OC 4 H 9 ) 4 (1 ml) and (NH 4 ) 2 TiF 6 (0.5 g) were added into a solution, including deionized water (30 ml) and hydrochloric acid (30 ml). Then, the precursor solution was transferred into hydrothermal reactor (100 ml) with FTO substrates, reacted for 12 h at 170°C. Finally, the obtained product was washed with deionized water several times and dried in the air.

CdS NCs
The CdS NCs were synthesized by the method of chemical bath deposition (CBD). Firstly, Cd(NO 3 ) 2-4H 2 O (0.12 g), KOH (0.5611 g), NH 4 NO 3 (2.412 g) and CH 4 N 2 S (0.30448 g) were added into deionized water (80 ml). Then, the TiO 2 NSs were put into beakers with mixed solutions. Finally, the beakers were transferred into thermostat water bath, and then reacted at 80°C for 10-40 min. The obtained products were abbreviated as CdS(10-40)/T [33].

PbS NCs
The PbS NCs were synthesized by successive ionic layer adsorption and reaction (SILAR) method, which were assembled onto CdS(10-40)/T. The detailed synthetic process are as follows: Firstly, the CdS(10-40)/T were put into 0.02 M Pb(CH3COO) 2 ethanol solution(100 ml) and kept for 5 min. The CdS(10-40)/T were cleaned by methanol and dried several minutes. Then, the obtained products were put into 0.02 M anhydrous Na 2 S methanol solution (100 ml) and kept for 5 min. After that, the final products were cleaned by methanol and dried several minutes. The process of fabrication was regarded as one cycle. And the final products were abbreviated as PbS(nC)/CdS(10-40)/T [32].

Characterization
All samples were tested in air condition. The measurements of X-ray diffraction were performed on Rigaku D/max-2500 diffratometer (k = 0.154056 nm) with the scanning rate of 0.3°s -1 in the 2h range from 20°to 80°. UV-vis curves were obtained by an UV-3150 with double-beam spectrophotometer. The samples of morphology, size and crystallographic directions images were obtained by SEM (FEI MAGELLAN 400 Scanning Electron Microscope) and the transmission electron microscope (TEM, JEM-2100F, 200 kV). The element composition and distribution were measured by EDS. The X-ray photoelectron spectra (XPS) and valence band X-ray photoelectron spectra (VB-XPS) were obtained by the ESCALAB-250 photoelectron spectrometer. Steadystate photoluminescence (PL) spectrum was recorded on a Ramascope System (Renishaw, London, UK) with an excitation laser at wavelength of 473 nm. The light excitation current curve of the samples was obtained by a three-electrode test system. Electrochemical impedance spectroscopy (EIS) was tested by SI 1296 electrochemical interface & SI 1260 interface / grain phase analyzer.

Photocatalytic test
The test of photocatalytic activity was performed on a self-built photocatalytic reaction system, a mid-pressure Hg lamp (300 W) and a xenon lamp (XLS-150A) were selected as the light source. The catalysis (size: 3 cm Â 1.5 cm) was added into the dye solution (700 ml, 2 Â 10 À5 M) and stirred at 25°C under UVvis light. The method was used to evaluate the degradation efficiency of organic pollutants. Before irradiation, the suspension was stirred in the dark to reach the adsorption-desorption equilibrium on the surface of the photocatalyst. Then, the samples (5-8 ml) were selected at the same time interval, according to the photocatalytic degradation of organic pollutants. The absorption peaks, which included different concentration of the RhB or MB solution, were determined by a UV-vis spectrophotometer. Ultimately, according to the formula [ C C 0 Â 100 0 = 0 ] (C 0 , initial concentration; C, reaction concentration), the photocatalytic degradation efficiency was calculated.  [33,34]. Moreover, the blue curve presents new diffraction peaks at 43.97°a nd 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 (Fig. S1) further confirmed that the samples were synthesized, successfully. The X-ray photoelectron spectroscopy (XPS) spectrum of PbS/ CdS/TiO 2 NSs is shown in Fig. 2b. Fig. S2 shows the characteristic peaks position of Ti2p, Cd3d, Pb4f and S2p. The peak positions of Ti 2p 3/2 and Ti 2p 1/2 are located at 458.3 and 464.0 eV [35]. The peaks of Cd 3d 5/2 and Cd 3d 3/2 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 Fig. S2d, which is at about 138.3 eV as revealed in the previous literature [37]. Therefore, the above results confirm that the preparation of the three-way PbS/CdS/TiO 2 NSs catalyst is successful.

Results and discussion
The morphology images of samples were conducted by FESEM (Fig. 3). Figure 3a Fig. 4c, the absorption edge of PbS(7 C)/ CdS(30 min)/ TiO 2 NSs is widened to 800 nm [33,37]. As shown in Fig. 4b The EIS measurement and steady-state photoluminescence (PL) were conducted to explore the charge transfer dynamics. Figure 5a shows the EIS curves of TiO 2 NSs, CdS(30)/T and PbS(5C)/ CdS(30)/T. It demonstrates that bare TiO 2 NSs could exhibit higher carrier resistance than PbS(5C)/ CdS(30)/T. Therefore, heterojunction is beneficial to electrons and holes transportation. The separation of electron-hole pairs plays a key role in the photocatalytic activity of the catalyst [38,39]. 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 TiO 2 NSs (Fig. 5b). 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 (Fig. 5a). The transient photocurrent response is shown in Fig. 5c. The photoelectrochemical performance of samples has been tested for analyzing the number of photogenerated electron-hole pairs. However, no obvious current signal is observed for TiO 2 , a higher current density was achieved by PbS(7C)/CdS(30)/T, implying the enhanced electron-hole pairs generation. Moreover, the reason of much more electronhole pairs is due to the proper energy band matching between PbS, CdS and TiO 2 . In particular, the transmitting procedure of electrons and holes is obviously shown in Fig. 5d.
The photocatalytic performance of the samples over RhB and MB was studied under UV-vis light.  obvious that all of the composites sensitized with CdS quantum dots show better photocatalytic activity.
We made another experiment under visible light using a xenon lamp simulator. Clearly, according to the analysis in Fig. 6a, the appropriate addition of PbS NCs could enhance the catalytic property. After 160 min, 97.1% of RhB could be removed by PbS(5C)/CdS(30)/TiO 2 . The other samples, such as PbS(7C)/CdS(30)/TiO 2 , show lower photocatalytic activity, which 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 TiO 2 NSs to broad the light utilization range. Establishing heterogeneous nodes could accelerate the photocatalysis process and better explain the enhancement of photocatalytic efficiency. Interestingly, as confirmed by Fig. 5a, the electrochemical impedance spectroscopy (EIS) analysis could also agree with this.
Those response energy about photocatalytic degradation for RhB of samples were greatly fitted with pseudo-first-order energy module: -ln(C 0 /C t )-= Kt, where C 0 is the concentration at the initial, C t is the concentration at a reaction time t, and K is the rate constant. As shown in Fig. 6b, PbS(5C)/ CdS(30)/ TiO 2 has a maximum K value that is much higher than the other samples. It demonstrates that the establishment of heterostructures between semiconductors may be conducive to the realization of highefficiency catalytic activity, and heterostructures make more free superoxide molecules appear on the surface to enhanced 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)/TiO 2 NSs was conducted by 5 recycling experiments in Fig. 7a. Specifically, there was a little drop in degradation ability after five cycles, PbS(5C)/CdS(30)/TiO 2 NSs composites could still degrade RhB at 86.64%. As the outermost layer of the sample, lead sulfide prevented cadmium sulfide from direct light exposure, reduced the photo-corrosion of cadmium sulfide, and improved 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 Fig. 7b, 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. Therefore 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 Fig. 8. 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 TiO 2 (CB), which follows the law of conservation of energy. Meanwhile, h ? on the surface of TiO 2 (VB) should be transferred 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 ÁO 2 and ÁOH for degradating organic pollutants.

Conclusion
In conclusion, XRD, XPS and EDS tests proved that a novel ternary PbS/CdS/TiO 2 NSs heterojunction photocatalyst was successfully prepared by one-step hydrothermal method, chemical bath deposition (CBD) and successive ionic layer adsorption and reaction (SILAR). Transient photocurrent response indexes that the heterojunction of the ternary PbS/ CdS/TiO 2 NSs composites demonstrate the ability to efficiently separate photogenerated electrons and holes and make photocatalytic activity improved nearly 5 times. Absorption spectra experiment indicates that the absorption edge of samples has extended from 375 to 800 nm, the light utilization rate increased significantly. Thus, a catalyst featuring heterojunction as the bridge between the photogenerated electrons and holes could degrade 99.9% RhB and MB solution. We believe that the novel three-way PbS/CdS/TiO 2 NSs catalyst could have great potential in the field of energy and environmental protection. This work could provide a dual-sensitized modification engineering to construct ternary catalysts improving the degradation efficiency.