Hierarchical Assembly of SnO2/ZnO Nanostructures for Enhanced Photocatalytic Performance

SnO2/ZnO hierarchical heterostructures have been successfully synthesized by combining electrospinning technique and hydrothermal method. Various morphologies of the secondary ZnO nanostructures including nanorods (NRs) and nanosheets (NSs) can be tailored by adding surfactants. Photocatalytic performance of the heterostructures was investigated and obvious enhancement was demonstrated in degradation of the organic pollutant, compared to the primary SnO2-based nanofibers (NFs) and bare ZnO. Furthermore, it was found that the H2 evolution from water splitting was achieved by photocatalysis of heterostructured nanocomposites after sulfurization treatment. This synthetic methodology described herein promises to be an effective approach for fabricating variety of nanostructures for enhanced catalytic applications. The heterostructured nanomaterials have considerable potential to address the environmental and energy issues via degradation of pollutant and generation of clean H2 fuel.

On the other hand, one dimensional (1D) nanomaterials are ideal material to serve as the backbone in hierarchical heterostructures due to the enhanced charge carrier mobility and large surface area [13][14][15] . Electrospinning technique used to fabricate continuous 1D nanofibers (NFs) has shown its high efficiency and convenience 16 . Considerable attempts have been made to synthesize a variety of continuous semiconductor metal oxide NFs with the flexible diameter control from nanometers to a few micrometers [17][18][19] . These works imply that it is possible to fabricate the semiconductor-semiconductor hierarchical heterostructures with enhanced photocatalytic activity from NF backbones.
In this work, we present a facile strategy for fabricating SnO 2 /ZnO hierarchical nanostructures for enhanced photocatalytic performance, the overall procedure was shown in Fig. 1. First, the SnO 2 -based NFs used as backbone in hierarchical heterostructure were prepared by electrospinning method. Subsequently, after dip-coating with ZnO seed nanoparticles, ZnO nanorod (NR) and nanosheet (NS) secondary structures were assembled directly to the backbones of nanofibers by hydrothermal process. It is found that the SnO 2 /ZnO heterostructural nanocomposites exhibit excellent photocatalytic activity superior to the individual SnO 2 and ZnO via the photocatalytic degradation of dye wastewater, which could be ascribed to the formation of heterojunction of SnO 2 -ZnO. Finally, after sulfurization treatment, these sulfurized heterostructured nanocomposites were extended to apply to solar hydrogen (H 2 ) evolution, manifesting the favorable capacity and stability for photocatalytic reactions. Our demonstration of rational fabrication of hierarchical nanostructures may provide the possibility for design and preparation of photocatalysts with tunable morphology and improved photocatalytic properties applied in environmental and energy sectors. Figure 2a and b show the SEM images of the as-spun SnO 2 /ZnO NF composite which is referred to as SZ-10 before and after annealing. The Sn-Zn-PVP NFs with average diameter of ca. 150 nm interweave randomly, constructing a three-dimensional (3D) porous mat. After annealing, PVP was removed and at the same time Sn-Zn precursors were converted to the SnO 2 /ZnO NFs. Like the pure SnO 2 NFs ( Supplementary Fig. S1), the fibrous structure of SZ-10 NFs is well preserved with an average diameter exhibited shrinkage to ca. 90 nm (Fig. 2b inset) resulting from the decomposition of PVP. The typical TEM images of SZ-10 NFs were illustrated in Fig. 2c,d. It could be seen that the SZ-10 NFs were composed of interconnected nanoparticles. In Fig. 2d, many nanopores were observed within the spaces between adjacent nanoparticles.

Results and discussion
After the secondary growth of ZnO NRs, the initially pure SnO 2 ( Supplementary Fig. S2) and SZ-10 NFs (Fig. 3a,b) are evenly branched out, forming hierarchical structures. The ZnO NRs with the length of ca. 450 nm and diameter of ca. 50 nm stand uniformly and densely on surfaces of the SZ-10 NFs after 2 h hydrothermal treatment (Fig. 3a,b). However, bare ZnO NR clusters ( Supplementary Fig. S2a) as well as sparsely distributed ZnO NRs on the SnO 2 NFs (Supplementary Fig. S2b) are observed, which may attribute to the less ZnO seed nanoparticles comparing to SZ-10 NFs. Furthermore, in addition of sodium citrate in precursor solution of SZ-10 NFs case, ZnO NSs generated instead of ZnO NRs. As shown in Fig. 3e,f, core-shell structure with a diameter of 800 to 1000 nm can be clearly observed, which is significantly larger than that of the plain SZ-10 NFs backbone. According to the calculation, the thickness of the sheath is around 400 nm. The corresponding TEM images reveal the detailed morphology and structure of SZ-10/ZnO NRs (Fig. 3c,d) and SZ-10/ZnO NSs (Fig. 3g,h). Both in SZ-10/ ZnO NRs and SZ-10/ZnO NSs cases, SZ-10 NFs is the structure core, while secondary ZnO NRs fully covered and firmly anchored on the SZ-10 NFs, which suggests high structural stability. In addition, the high resolution TEM image of the junction between SZ-10 NFs and ZnO NR branch was taken and shown in the Supplementary Fig. S3. Both the branch and backbone display the clear lattice fringes. The measured lattice distances of 0.319 nm correspond to the SnO 2 backbones and 0.512 nm belong to the secondary ZnO NRs 12,20,21 . Besides that, the lattice spacing of 0.325 nm in the backbone attribute to ZnO which consist in the SZ-10 NFs 12,20 .
The EDX spectra in Fig. 4a,g show that the Sn L1 peak at 3.06 keV, Lα peak at 3.44 keV, Lβ peak at 3.68 keV and Lβ 2 peak at 3.90 keV, as well as O Kα peak at 0.52 keV are uniform both in SZ-10/ZnO NRs and NSs, which reveal that the ZnO growth process does not affect the SnO 2 structures or properties 22,23 . Zn Lα 1,2 peak at 1.01 keV, Kα 1 peak at 8.63 keV and Kβ 1 peak at 9.57 keV were observed indicating the successful synthesis of ZnO 9,12 . Elemental mapping images presented in Figs 4b-f and h-l demonstrate the well-defined spatial distribution of Zn (Fig. 4d,j), Sn (Fig. 4e,k) and O (Fig. 4f,l). The signal of Sn element is seen mainly coming from the backbone portion, while the signals of Zn and O are uniformly distributed throughout the entire heterostructure, confirming the ZnO NRs branches and NSs homogeneously covered on the SZ-10 NFs backbone.
The crystal structures of SZ-10 NFs, SZ-10/ZnO NRs and SZ-10/ZnO NSs composites were also investigated by XRD and the patterns are shown in The corresponding UV-vis absorption measurements of SZ-10/ZnO NRs and NSs were carried out and the spectra are shown in Supplementary Fig. S5. In general, the absorbance intensities increase after the growth of ZnO hierarchical structures while the SZ-10 NFs exhibits the lowest intensity. The absorbance of SZ-10/ZnO NRs is higher than that of SZ-10/ZnO NSs, which may lead to better photocatalytic performance.
The growth process of ZnO NR branch was investigated by changing the hydrothermal treatment time. Supplementary Fig. S6 depicts the SEM images of hierarchical SZ-10/ZnO NRs nanostructures by increasing the hydrothermal time (0.5, 1, 1.5, 2 and 2.5 h). At the beginning, short and sparse ZnO NRs were grown from the nucleation sites of ZnO seed nanoparticles that were dip coated on the backbone of the nanofibers ( Supplementary Fig. S6a). Over time, the absorption of Zn 2+ and OH − from the feedstock solution increases the length and density of secondary ZnO NRs ( Supplementary Fig. S6b-e). However, when the reaction time was prolonged to 2.5 h, bare ZnO nanoclusters formed, as shown in Supplementary Fig. S6e inset, which are adverse to photocatalytic performance. By further measuring the diameter and length of the secondary ZnO NRs, we found that the diameter of the ZnO NRs increase slowly, from ca. 40 nm to ca. 53 nm, while the length grows dramatically, increasing from ca. 130 nm to ca. 500 nm ( Supplementary Fig. S6f). This formation mechanism could be explained by the general route to grow vertical ZnO NRs array on substrates using ZnO seeds. The SZ-10 NF backbones were dip coated with a layer of ZnO seed nanoparticles to form the nucleation sites, from where the randomly oriented crystals grew. In the absence of structure modifiers, the ZnO NRs nucleate and crystallize along the [001] direction which is the fastest growth orientation on these ZnO sites. Therefore, the branched nanostructure was obtained 9,12 .
Citrate anions as the structure modifying agent was applied to promote the alteration of the secondary ZnO nanostructure because it adsorbs strongly on metal and mineral surfaces and significantly changes the surface properties and mineral growth behavior [24][25][26] . Sodium citrate was used in our experiments, the molecules of which absorb preferentially on the [001] surfaces and thus inhibit the crystal growth along [001] orientations 12,20 . As a result, ZnO NSs were formed on the surface of the SZ-10 NFs backbones. Different concentrations of sodium citrate (0.3, 0.1, 0.05 and 0.025 mol/L) were added and the corresponding SEM images (Supplementary Fig. S7) indicate the optimum sodium citrate concentration is 0.05 mol/L.
To explore the photocurrents of the composites with hierarchical nanostructures, the PEC cells of SZ-10 NFs, bare ZnO and SZ-NFs/ZnO nanocomposites were employed under simulated sunlight glowed from Xe arc lamp and their amperometric I-t curves acquired with three light on/off cycles within 60 s are shown in Fig. 6. The photocurrents of both SZ-10 NFs and bare ZnO are relatively lower. The photocurrent of SZ-10 NFs is about 0.5 μ A cm −2 , while that of the SZ-10 NFs peaked at ca. 3.6 μ A cm −2 and then decayed to a steady state of ca. 2.3 μ A cm −2 after 30 s. However, when ZnO nanostructures are hierarchically assembled with SZ-10 NFs, the photocurrents are obviously improved comparing to that of individual NRs and NSs nanostructures. The initial photocurrents reached ca. 10 and 7.6 μ A cm −2 and eventually approached to a plateau at ca. 7.5 and 6.5 μ A cm −2 for SZ-10/ZnO NRs and SZ-10/ ZnO NSs, respectively. This improvement in the photocurrent output is mainly due to the heterojunction formation which promotes efficient separation of photo excited electron-hole pairs and thus enhances the photocatalytic stability [27][28][29] . Figure.   The photocatalytic performance was demonstrated by photocatalytic degradation of MO, a common textile pollutant 32,33 . Figure 7a exhibits the content changes of MO in the absence and in the presence of different composites. A control experiment was carried out to show that photodegradation is not apparent in the absence of the photocatalyst. Also, photocatalytic degradation capabilities of SZ-10 NFs backbone and bare ZnO were tested and the results show 27.4% and 39.8% of MO molecules were degraded after 80 min of simulated sunlight irradiation respectively. In contrast, both SZ-10/ZnO NRs and SZ-10/ZnO NSs composites show appreciable photocatalytic activity under simulated sunlight and were able to fully degrade the MO dye after 60 and 80 min, respectively. The UV-Vis absorbance and digital photograph illustrating the time evolution photodegradation study using SZ-10/ZnO NRs are shown in Fig. 7c,d, which is clearly observed the MO solution recovered to clear after 60 min. Figure 7b shows the pseudo-first order kinetics of the MO degradation of the various photocatalysts. The efficiency of MO photodegradation by the composite was determined quantitatively using the pseudo-first order model 34 as follows:  Finally, in order to extend the application of photocatalysts to energy field, namely the solar hydrogen production, both SZ-10/ZnO NR and SZ-10/ZnO NS nanocomposites were sulfurized using Na 2 S solution, referred to as SZ-10/ZnO NRs (S) and SZ-10/ZnO NSs (S) respectively. Figure 8a,b show the SEM images of SZ-10/ZnO NRs (S) and SZ-10/ZnO NSs (S). It is found that SZ-10/ZnO NRs (S) can retain the overall morphology except that the secondary ZnO NRs structures are shorter in length and have rougher surface (Fig. 8a). However, SZ-10/ZnO NSs (S) loss their original morphology as the NSs collapsed and transformed into agglomerated nanoparticles sheath. (Fig. 8b) The elemental EDX examinations were carried out and corresponding spectra are shown in the Fig. 8c. Compared to the EDX spectra of SZ-10/ZnO NRs and SZ-10/ZnO NSs, additional sulfur elements appear in both spectra of SZ-10/ZnO NRs (S) and SZ-10/ZnO NSs (S), meanwhile, the signal intensity of oxygen element decreased relatively, which prove the sulfurization of the nanostructures. Figure 8d depicts the UV-vis absorbance spectra of SZ-10/ZnO NRs (S) and SZ-10/ZnO NSs (S). Compared to pristine samples, the absorbance of both sulfurized samples have broadened the absorption spectra and extended to the visible-light region 10 . Moreover, the absorbance property of SZ-10/ZnO NRs (S) is higher than that of SZ-10/ZnO NSs (S).
The results indicate that the sulfurized samples may show the enhanced photocatalytic activity in solar hydrogen generation and SZ-10/ZnO NRs (S) has a higher performance.
The H 2 evolution from water splitting was investigated under simulated sunlight by utilizing these nanocomposites as photocatalysts 37,38 . Figure 8e illustrates the time course of H 2 production of the SZ-10/ZnO NRs (S) and SZ-10/ZnO NSs (S). The SZ-10/ZnO NRs (S) shows a higher H 2 evolution rate of 202.2 μ mol g −1 h −1 than that of SZ-10/ZnO NSs (S) at the rate of 100.1 μ mol g −1 h −1 , which is in accordance with the UV-vis absorbance. The reusability of the photocatalyst for sacrificial water splitting by SZ-10/ZnO NRs (S) was demonstrated (Fig. 8f) hence suggesting that the structural stability of nanocomposite for promising solar hydrogen generation application.

Conclusion
In summary, SnO 2 /ZnO hierarchical nanostructures have been successfully fabricated by combining electrospinning technique and hydrothermal method. The NR and NS morphologies of secondary ZnO nanostructure can be tuned by adjusting the reaction time and modifying agents. The as-prepared SZ-10/ ZnO NR and NS composites have been explored for degradation of organic pollutant in water. Both NR and NS composites show much higher efficiency than primary SZ-10 NFs and bare ZnO, indicating the enhanced photocatalytic performance. More interestingly, the heterostructures can be extended to H 2 production from sacrificial water splitting after sulfurization treatment and show favorable reusability and structural stability. Therefore, such approach towards the growth of hierarchical nanostructures may offer an avenue for designing and fabricating other complicated nanostructures and nanocomposites, which might be useful for photocatalytic application in environment and energy field.
Fabrication of SnO 2 -based NFs. SnO 2 -based composite NFs were prepared by the electrospinning technique 8,30,39 . Typically, 0.9 g SnCl 2 ·2H 2 O and 0.09 g Zn(NO 3 ) 2 ·6H 2 O were added into the 5 g of ethanol and 6 g DMF solution under vigorously stirring for 3 h at 60 °C. Then, 1.2 g of PVP was added and the mixture was subsequently magnetic stirred for more 3 h at the same temperature to obtain a homogenous and clear Sn-Zn-PVP precursor solution. Subsequently, electrospinning was carried out at an applied voltage of 18 kV and a flow rate of 1 mL h −1 . The distance between needle tip and aluminum foil collector was 15 cm. Finally, the as-spun NFs were calcined in air at 500 °C for 2 h in a furnace, at temperature ramp rate of 5 °C min −1 to obtain SnO 2 /ZnO NFs.
Synthesis of the SZ-10/ZnO hierarchical structure. SZ-10/ZnO NRs were prepared by a hydrothermal process 12 . First, the SZ-10 NFs were coated with ZnO nanoparticles which were prepared according to a previous work 40 as seeds by a dip-coating technique. Subsequently, the SZ-10 NFs were dispersed into a 30 mL aqueous solution of equimolar Zn(NO 3 ) 2 ·6H 2 O and HMT. The hydrothermal treatment was conducted at 90 °C for 2 h. After reactions, the products were separated and washed with deionized water by centrifugation for 3 times. The resulting precipitates were dried in the drying oven. SZ-10/ZnO NSs were also synthesized as described above by adding sodium citrate as the surfactant into the hydrothermal solution and heated for 6 h.
Characterization of hierarchical structure. The fabricated products were analyzed using an X-ray diffractometer (XRD, Philips X-ray diffractometer with Cu Kα radiation at λ = 1.541 Å) to obtain crystallographic information. Morphology and structural characteristics were studied using a field-emission scanning electron microscope (FESEM, JEOL FEG JSM 7001F) and a transmission electron microscope (TEM, Phillips FEG CM300), respectively. The elements were analyzed by energy-dispersive X-ray spectroscopy (EDX, Oxford Instruments). The absorption spectra of photocatalysts were obtained using a UV-visible spectrophotometer (UV-vis, Shimadzu UV-3600). Photocurrent test of photoelectrochemical (PEC) cells. The PECs were realized by coating a layer of bare ZnO, SZ-10 NFs, SZ-10/ZnO NR and NS nanocomposites on fluorine doped tin oxide (FTO) glasses (1.5 × 2 cm) and calcined for 10 min on a 450 °C hotplate. With the 0.1 mol/L Na 2 SO 4 solution as electrolyte, the sample as working electrode and Pt as the reference electrode, the amperometric curves I-t were recorded under an illumination with the simulated sunlight glowed from 300 W Xe arc lamp of intensity 100 mW cm −2 for three 60 s light-on-off cycles.
Degradation of MO solution. The photocatalytic reactions of 15 ml of 10 mg L −1 MO aqueous solution was carried out based on 15 mg of under light irradiation with the same Xe arc lamp. The concentration of MO was analyzed using a UV-Vis spectrophotometer and the maximal absorbance peak value was noted to plot the amount of MO degraded and thus, determine the photodegradation activity of the composite.
Sulfurization of SZ-10/ZnO heterostructures. In order to prepare sulfurized SZ-10/ZnO heterostructures, the as-prepared SZ-10/ZnO NR and NS nanocomposites (20 mg) were dispersed in 10 mL of 1M Na 2 S solution and sonicated for 10 min. The mixtures were then sealed in a glass bottle before being heated at 70 °C for 4 h. After the reaction, the products were collected by the rinse-centrifugation process with deionized water for several times. The obtained products were dried thoroughly at 55 °C in oven for 4 h 41 .
Photocatalytic water splitting. The photocatalytic reactions were conducted in a quartz cylindrical reaction cell of 25 mL in volume. Typically, 5 mg of photocatalyst, 9 mL deionized water and 1 mL methanol were mixed and stirred for 30 min to form a homogeneous suspension. The reactor was purged with argon (Ar) gas for 10 min prior to illumination with the same Xe arc lamp. Gas samples were analysed using a gas chromatograph (Shimadzu GC-2014AT).