Synthesis and Study of Optical Characteristics of Ti0.91O2/CdS Hybrid Sphere Structures

The optical properties of alternating ultrathin Ti0.91O2 nanosheets and CdS nanoparticle hybrid spherical structures designed by the layer-by-layer (LBL) assembly technique are investigated. From the photoluminescence (PL) spectral measurements on the hybrid spherical structures, a spectrum-shifted fluorescence emission occurs in this novel hybrid material. The time-resolved PL measurements exhibit a remarkably increased PL lifetime of 3.75 ns compared with only Ti0.91O2 spheres or CdS nanoparticles. The novel results were attributed to the enhanced electron-hole separation due to the new type II indirect optical transition mechanism between Ti0.91O2 and CdS in a charge-separated configuration. Electronic supplementary material The online version of this article (10.1186/s11671-018-2488-3) contains supplementary material, which is available to authorized users.


Background
Semiconductor composite nanostructures have attracted more attention due to the optimal assembling of conduction band and valance band for the photovoltaic applications and other optoelectronic devices [1][2][3][4]. Spatial separation of the electron and hole in the type II semiconductor composite nanostructures can result in a prolonged lifetime of charge carriers which has desirable optical characteristics for applications such as light sources [5,6], lasers [7][8][9], and photovoltaic devices [10,11]. Many studies of indirect optical transition (IOT) effect in the type II composite nanostructures have been reported over the past few years. For instance, IOT phenomenon has been reported in ultrathin hybrid sphere nanostructures including graphene oxide and TiO 2 nanosheets [12] or coupled quantum dots system [13]. In recent years, TiO 2 is an important optical materials which has been widely investigated owing to its outstanding optical properties for use in photocatalysis and solar cells, but the wide bandgap (3.2 eV) of TiO 2 limits its photocatalytic property in the UV region. In order to extensively exploit the optical activity in the visible light region, the surface of TiO 2 nanosheets coated with quantum dots has been investigated as a superior alternative for dye-sensitized solar cell [14][15][16][17][18]. Particularly important, the composite system of TiO 2 nanosheets coupled with CdS quantum dots (QDs) has been widely studied for various applications due to its suitable bandgap (2.4 eV) and excellent optical properties [19][20][21]. Combining these features, the TiO 2 /CdS hybrid structures have been recently highlighted as a unique system [22][23][24][25][26]. Moreover, the CdS nanoparticles coated with TiO 2 nanosheets can greatly improve its optical activity. So far, exciton separation and carrier extraction are the major bottleneck achieving highly efficient materialsensitized solar cells. However, fundamental studies on photoexcited carrier dynamics based on TiO 2 /CdS hybrid spheres are limited. Therefore, the photoluminescence (PL) properties and time-resolved PL decays of composite nanostructures consisting of alternating Ti 0.91 O 2 nanosheets and CdS nanoparticles are investigated in this paper. From the PL spectra and timeresolved PL decay measurements, the new type II indirect optical transition contributes to clarify the novel fluorescence emission mechanism of composite nanostructures consisting of Ti 0.91 O 2 nanosheets and CdS nanoparticles that are different from traditional TiO 2 / CdS fluorescence radiative transition systems. The excitation power-and excitation wavelength-dependent PL spectra and time-resolved PL decay measurements were also further investigated to affirm the recombination properties of charge carriers and elucidate the competition mechanism of different radiative transition pathways in Ti 0.91 O 2 /CdS composite nanostructure. These novel results provide a useful viewpoint for the design of charge separation and charge extraction in TiO 2 and CdS composite nanostructures for various optoelectronic device applications.

Synthesize Samples
The synthesis of Ti 0.91 O 2 nanosheets and CdS nanoparticles has been reported based on the layer-by-layer selfassembly technique [27]. The overall procedure for fabricating multilayer Ti 0.91 O 2 /CdS composite nanostructures is demonstrated as follows: the poly(methly methacrylate) (PMMA) solid spheres were completely diluted by the protonic polyethylenimine (PEI) aqueous solution, in order to ensure the saturated adsorption of PEI on PMMA solid spheres surfaces. The PMMA solid spheres coated with PEI are diluted with deionized water by ultrasonic treatment; then, negatively charged Ti 0.91 O 2 nanosheets were added to the hybrid PMMA coated with PEI solution under stirring, the PMMA combine with Ti 0.91 O 2 nanosheets due to the interior electrostatic interaction of the opposite charge. The above procedure was repeated. The multilayer PEI/ Ti 0.91 O 2 /PEI/CdS hybrid sphere nanostructures that have been deposited onto PMMA spheres were achieved based on the above repeated synthesis procedures. During microwave irradiation, PEI moiety was removed and PMMA particles were decomposed. After reaction, hollow spheres consisting of alternating Ti 0.91 O 2 nanosheets and CdS QDs were obtained, and trifle PMMA residue was removed with tetrahydrofuran (THF). Finally, the hybrid hollow spheres with multilayer Ti 0.91 O 2 /CdS nanostructures were obtained.

Experiment Apparatus
The sample images of solid Ti 0.91 O 2 /CdS hybrid spheres and hollow Ti 0.91 O 2 /CdS hybrid spheres were measured by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. The appropriate amounts of solid Ti 0.91 O 2 /CdS hybrid spheres and hollow Ti 0.91 O 2 /CdS hybrid spheres were diluted by deionized water to have lower sample densities. Diluted samples were spin-coated on silica coverslip to prepare thin films for optical measurement with the 266 and 400 nm excitation. The optical measurements of all samples were carried out at room temperature. For PL spectral measurements, the 800 nm ps Ti:Sapphire laser with 76 MHz repetition rate was used to generate the 266 and 400 nm wavelength pulse laser based on secondharmonic and third-harmonic conversion technique, respectively. Two hundred sixty-six nanometer and 400 nm pulse laser at an incident angle of~45°relative to the vertical direction was focused onto the sample surface with a power density of~100 W/cm 2 . The PL from samples was collected vertically by a ×60 objective and sent to the spectrometer, and the emission PL spectra were recorded with a monochromator (Acton SP-2500i, 0.5 m, 150 lines mm − 1 grating, blazed at 500 nm) fitted with a Princeton Instruments liquid-nitrogencooled charge-coupled device (CCD) camera. For timeresolved PL decay measurements, the PL from the samples was collected by the same objective and then detected by the single photon counting system with the 250 ps time resolution. Moreover, the corresponding 450, 500, and 550 nm band-pass filter with a 10-nm bandwidth was used to effectively measure the different wavelength PL lifetimes. Under the 266 nm excitation, the fluorescence emission peak around 530 nm from CdS nanoparticles embodies smaller energy bandgap than that of CdS (2.48 eV). We suppose that the nonradiative transition of excited electrons from the conduction band bottom to different defect states level occurs in the CdS nanoparticles. However, the fluorescence emission peak shifts to 500 nm when the Ti 0.91 O 2 /CdS hybrid structure excited at 266 nm. If we exclude the contribution of either Ti 0.91 O 2 or CdS to the blue-shifted spectra emission; then, this fluorescence mechanism attributes to an indirect optical transition (IOT) in hybrid interface of Ti 0.91 O 2 /CdS system. In the traditional type II TiO 2 /CdS composite nanostructure, light excitation of TiO 2 and CdS will transfer electrons from the higher conduction band of CdS to the lower conduction band of TiO 2 and generated holes from the lower value band of TiO 2 to  removing the organic surfactants from the quantum dots (QDs) surface. The photoluminescence (PL) spectra and PL decay lifetime are shown in Fig. 3a, b,  By tuning the excitation wavelengths to 400 nm at higher excitation power, the PL spectra and transient timeresolved PL decay dynamics were measured. The results show that the feeble PL spectra with 10 times integrated time are shown in Fig. 3c, and the average PL lifetime (0.59 ns) of Ti 0.91 O 2 /CdS solid hybrid structures is shorter than the PL lifetime (0.45 ns) of Ti 0.91 O 2 /CdS hollow hybrid structures as shown in Fig. 3d, suggesting that CdS has higher electron transfer rate toward Ti 0.91 O 2 according to the traditional type II Ti 0.91 O 2 /CdS heterostructure. Compared with the case of 266 nm excitation, the shorter PL lifetime with 400 nm excitation indicates that PL quenching effect is further enhanced due to optical recombination between electrons and holes in the Ti 0.91 O 2 /CdS system or the wastage of holes for photocorrosion in the CdS nanoparticles. Therefore, the Ti 0.91 O 2 /CdS hollow hybrid spheres show weak optical activity under a 400-nm laser excitation, and no obvious sensitization emerges in the Ti 0.91 O 2 /CdS hybrid spheres.

Results and Discussion
To further investigate the charge carrier relaxation pathways in Ti 0.91 O 2 /CdS hollow hybrid interface, the excitation intensity-dependent PL spectra in the Ti 0.91 O 2 / CdS hybrid spherical structures were investigated under a 266-nm laser excitation. Under a 266-nm low-excitation intensity, we first observed that the 475 nm peak is dominant in the PL spectrum. With increasing the excitation power, the corresponding PL spectra intensity varied as a function of the excitation power ranging from 300 to 1000 W/cm 2 and the central peak wavelength of PL spectrum shift from 475 to 560 nm as shown in Fig. 4a. We tentatively attributed to electron transfer from conduction band of Ti 0.91 O 2 to conduction band of CdS when Ti 0.91 O 2 / CdS hybrid nanostructures were excited by higher power 266 nm laser; then, the electron-hole recombination occurs between electrons in conduction band of CdS and holes in the valence band or the defect level of CdS nanoparticles according to type I recombination mechanism as shown in Fig. 4b. These varied PL spectra show that the red shift occurs with increasing excitation power. Such results confirm the different nature and origin of the emissions wavelength at 475 and 560 nm, respectively. Thus, the 475-nm emission wavelength indicates the type II emission property and the 560-nm emission wavelength reflects the type I emission property. The spectra shifted with excitation power indicate the competition mechanism between spatially direct and indirect recombination channels in Ti 0.91 O 2 /CdS composite interfaces. With the continuous by increasing the excitation power, more electrons with high-power excitation transfer from the conduction band of Ti 0.91 O 2 to the conduction band of CdS nanoparticles, leading to a strongly increasing intensity ratio between central wavelength 560 and 475 nm, and the photoluminescence intensity ratio of two emission peaks can reach to 3.5 as shown in Fig. 4c. However, the weak photoluminescence intensity implies that the electron transfer from the conduction band of Ti 0.91 O 2 to the conduction band of CdS nanoparticles only plays a minor role in the appearance of PL emission.
To further verify the two kinds of transition mechanisms with different excitation power in the Ti 0.91 O 2 / CdS hollow spheres, the probing wavelength-dependent time-resolved photoluminescence (TRPL) experiment was performed with different excitation power density. It is suitable to monitor the charge carrier transfer or electron-hole recombination process in the Ti 0.91 O 2 /CdS interface. The TRPL lifetimes of Ti 0.91 O 2 /CdS were measured with different probe wavelengths at 450, 500, and 550 nm, respectively. And the corresponding 450, 500, and 550 nm band-pass filter with a 10-nm bandwidth was used. The TRPL give longer decay lifetimes (3.72 ns) at shorter wavelength (450 nm) in the Ti 0.91 O 2 /CdS interface as shown in Fig. 4d because of the spatial separation of the charge carriers in the composite structures with the electrons in the conduction band of Ti 0.91 O 2 nanosheets and holes in valence band of CdS nanoparticles. This type II hybrid structures reduce the PL intensity due to the smaller overlap between electron and hole wave functions and consequently enhances PL recombination lifetimes. However, the PL lifetimes (1.61 ns) at longer wavelength (550 nm) become faster due to enhancing the wave function overlap between the electron of conduction band (CB) and hole of valence band (VB) in the CdS nanoparticles as shown in Fig. 4d. This findings clearly reveal that the photoexcited carriers in the Ti 0.91 O 2 /CdS make a significant contribution to the longer PL lifetimes. This evidence further confirms that the dominant PL is from the recombination between the electron in the CB of Ti 0.91 O 2 and hole in the VB in of CdS nanoparticles. These findings confirm that electrons in the conduction band of Ti 0.91 O 2 nanosheets recombine with holes in the valence band of CdS nanoparticles through indirect optical transition that is different from traditional TiO 2 /CdS system. These prolonged carrier lifetime makes the Ti 0.91 O 2 /CdS composite nanostructure most suitable for photovoltaic applications. To characterize the ability of the synthetic samples, linear J −V curves were recorded as shown in Additional file 1: Figure S4. The great enhancement of the photocurrent after CdS sensitization shows the advantage of the Ti 0.91 O 2 /CdS compared to the Ti 0.91 O 2 with light illumination. Therefore, a higher loading of the photosensitizer will lead to a higher photocurrent density.

Conclusions
In summary, we have detected novel indirect optical transition (IOT) properties in the multilayer PEI/Ti 0.91 O 2 /PEI/ CdS hybrid nanostructures from the PL spectra and timeresolved PL measurements. From the PL spectral and TRPL measurement, the red-to-blue shift light emission emerges in this novel composite material. And prolonged photoluminescence lifetime of Ti 0.91 O 2 /CdS composite nanostructure compared with only Ti 0.91 O 2 spheres or CdS nanoparticles was found. These results demonstrate new photoluminescence recombination mechanism due to the optical recombination between holes in the value band level of CdS and electrons in the conduction band level of Ti 0.91 O 2 that is different from traditional TiO 2 /CdS composite system. By tuning the excitation wavelengths and excitation power, the PL spectra and PL lifetimes of Ti 0.91 O 2 / CdS hybrid structures exhibit an excitation wavelengthand excitation power-dependent behavior. From the bandgap configurations, the IOT for Ti 0.91 O 2 /CdS hybrid structure which lead to prolonged carrier lifetime make for charge carrier separation and extraction for the important applications in photovoltaic system.