In Situ Decoration of ZnSnO3 Nanosheets on the Surface of Hollow Zn2SnO4 Octahedrons for Enhanced Solar Energy Application.

Hierarchical ZnSnO3/Zn2SnO4 porous hollow octahedrons were constructed using the method of combining the acid etching process with the in situ decoration technique for photovoltaic and photocatalytic applications. The composite was used as photoanode of the dye-sensitized solar cells (DSSCs), an overall 4.31% photovoltaic conversion efficiency was obtained, nearly a 73.1% improvement over the DSSCs that used Zn2SnO4 solid octahedrons. The composite was also determined to be a high-performance photocatalyst for the removal of heavy metal ion Cr (VI) and antibiotic ciprofloxacin (CIP) in single and co-existing systems under simulated sunlight irradiation. It was remarkable that the composite displayed good reusability and stability in a co-existing system, and the simultaneous removal performance could be restored by a simple acid treatment. These improvements of solar energy utilization were ascribed to the synergetic effect of the hierarchical porous hollow morphology, the introduction of ZnSnO3 nanosheets, and the heterojunction formed between ZnSnO3 and Zn2SnO4, which could improve light harvesting capacity, expedite electron transport and charge-separation efficiencies.

1. Introduction

Synthesis of ternary metal oxides with controllable sizes and shapes has been an active research field in the past two decades, because of their size- and shape- dependent physical, chemical, optical, electronic, and catalytic properties [1,2,3]. In particular, the two zinc stannates, Zn2SnO4 and ZnSnO3, have caused considerable attention due to their wide applications in dye-sensitized solar cells (DSSCs) [4,5], gas sensors [6,7], lithium-ion batteries [8,9], and photocatalysts [10,11]. Zn2SnO4 and ZnSnO3 are both photovoltaic and photocatalytic materials with band gaps of about 3.6 eV and 3.2 eV, and the Femi energy level of ZnSnO3 is higher than that of Zn2SnO4 [12,13]. When ZnSnO3 is coupled with Zn2SnO4, the heterojunction would form at the interface between the two ternary metal-oxide semiconductors, and the electrons would transfer from the conduction band of ZnSnO3 to that of Zn2SnO4 until the system reaches equilibrium of Fermi energy level [13]. This process will promote the separation of photogenerated electron–hole pairs and thus increase the performance of the DSSCs and photocatalysts. Therefore, the composite consisting of Zn2SnO4 and ZnSnO3 has been successfully synthesized to improve device performance [13]. For example, the ZnSnO3/Zn2SnO4 acted as photocatalyst for degradation of Intracron Blue, and achieved high performance under UV illumination due to lower recombination of electron-hole pairs [14]. The mixed phases of ZnSnO3 and Zn2SnO4 were used as photoanodes in DSSCs, and it was found the ferroelectric polarization would aid in obtaining relatively high fill factors and open-circuit voltage values in the devices [15].

The published research studies have demonstrated that the three-dimensional hierarchical architectures assembled from the one- or two- dimensional nanostructured building blocks are an interesting class of materials with an abundant variety of tunable physicochemical properties [16]. Hierarchical porous hollow microstructures composed of single-crystalline two-dimensional nanosheets can provide low material density, good surface permeability, high surface area and electron transport rate, as well as high light-harvesting efficiency for application areas of catalysis and photovoltaic devices [17,18]. In this structure system, the material with micrometer-sized diameters can act as light scattering centers to increase the light harvesting of the photoanodes and photocatalysts, which is one of the crucial factors of improving the photovoltaic conversion efficiency of DSSCs and photocatalytic activity [19]. The multiple-reflection inside hollow interior can provide more opportunities for the photoanodes and photocatalysts to trap the incident light for an enhanced light utilization [20,21]. The porous hollow structures possess a large surface area, and to a great extent enhance the mass transportation and the diffusion of liquid reactant in the active layers, meanwhile, the high porosity can provide rich channels with the sizes from nanometer to micron for the surface accessibility [22]. In addition, nanosheets can offer direct pathways for photogenerated electrons, which will result in enhanced electron transport rates [23]. Hence, the hierarchical porous hollow semiconductors were acknowledged as the ideal candidates for the high-performance DSSCs and photocatalysts. However, as far as we know now, there was scarcely any previous research on hierarchical porous hollow ZnSnO3/Zn2SnO4 with the view of the photovoltaic and photocatalysis fields simultaneously.

In our recent work, well-defined octahedral Zn2SnO4 was synthesized through a chemical solution route, and these Zn2SnO4 micro-crystals were converted to hollow octahedrons by an acid etching process. Whereafter, by means of an in situ growth method, ZnSnO3 nanosheets were intentionally introduced on the surface of hollow Zn2SnO4 octahedrons to construct the hierarchical porous hollow ZnSnO3/Zn2SnO4 octahedrons [12] (Scheme 1). In this paper, the as-prepared hollow ZnSnO3/Zn2SnO4 octahedrons were used as photoanodes of DSSCs to evaluate their photovoltaic performance. Meanwhile, the photocatalytic properties of the composite under simulated sunlight irradiation were evaluated by the removal of antibiotic ciprofloxacin (CIP) and heavy metal ion Cr(VI) in single and co-existing systems. Significantly, the ZnSnO3/Zn2SnO4 composite consisting of the two ternary metal oxides presented distinct advantages and had a higher photovoltaic conversion efficiency and better removal performance than solid and hollow Zn2SnO4 octahedrons. In addition, the morphology, crystal structure, optical properties and photocatalytic stability of the composite were investigated via various characterization methods, and the causes of enhanced photovoltaic and photocatalytic performance were proposed and discussed in detail.

Scheme 1

Schematic illustration for the formation of the three samples.

2. Materials and Methods

2.1. Materials Synthesis

All chemicals were purchased from Shanghai Aladdin Bio-Chem Technology Co. Ltd. (Shanghai, China) and used without purification. Zn2SnO4 solid octahedron (S1), Zn2SnO4 hollow octahedron (S2), hierarchical ZnSnO3/Zn2SnO4 hollow octahedron (S3), and ZnSnO3 nanosheets were prepared according to a reported procedure in our previous paper [12], and more details are described in the “Materials Synthesis” section in the Supplementary Materials.

2.2. Materials Characterization

The scanning electron microscopy (SEM, FEI NOVA NanoSEM230) and transmission electron microscopy (TEM, JEOL 3010) images were collected to obtain the morphology information of the samples. The compositions were analyzed by X-ray diffraction (XRD) (Rigaku, Tokyo, Japan) with Cu Ka radiation (λ = 1.54056 Å).

2.3. Preparation of the Electrode

An amount of 1 g of as-prepared sample powder and a certain amount of absolute ethanol were put into a mortar and stirred for 30 min. An amount of 4 g of the ethanol solution of two ethyl celluloses and 2 g of anhydrous terpineol were added, and the homogenization was performed using a sonicator (Vibra cell 72408, Bioblock Scientific, Wasserbad, Germany). The contents were concentrated by evaporator with 120 mbar at 38 °C and the pastes were finalized. Fluorine-doped tin oxide (FTO) conductive glass was cleaned with dilute acid and alkali solution, ethanol and deionized water several times in the ultrasonic cleaner. A doctor-blade technique was employed to coat paste onto the surface of FTO. The film was dried at 125 °C, annealed at 450 °C for 30 min and sintered at 500 °C for 15 min in the air to form the working electrodes. Typically, to improve the connection among ZnSnO3/Zn2SnO4 hollow octahedrons, the as-prepared working electrodes were immersed into ZnSnO3 sol for 30 min, then sintered at 500 °C for another 30 min.

A magnetron sputter platinum mirror acted as the counter electrode. Dye sensitization was accomplished by immersing a film in 0.3 mM dye solution (solvent mixture of tert-butyl alcohol and acetonitrile in a volume ratio of 1:1) at 80 °C over a set length of time. Here, the dye used in the DSSCs was cis-bis (isothiocyanato) bis (2,2′-bipyridyl-4,4′-dicarboxylato) ruthenium-(II) bis-tetrabutylammonium (also known as N719) [24]. The electrolyte contained 0.5 M tert-butylpyridine, 0.5 M LiI and 0.05M I2 in acetonitrile. The FTO substrate, film, and counter electrode constituted a sandwich-like cell structure (Figure S1).

2.4. Photovoltaic Characterization

Electrochemical impedance spectroscopic (EIS) measurements were carried out with a PAR2273 workstation (Princeton Applied Research, Oak Ridge, TN, USA). The current–voltage (J-V) characteristics were measured with an active area of 0.16 cm2 on an Oriel 92251A-1000 on a Keithley 236 source measurement unit. An Oriel 92251A-1000 sunlight simulator was used as the light source (AM1.5, 100 mW cm−y). Intensity-modulated photocurrent/photovoltage spectra (IMPS/IMVS) were recorded on an electrochemical workstation (Zahner, Zennium, Kansas City, MO, USA). The incident photon-to-current efficiency (IPCE) was investigated on a PEC-S20 instrument (Peccell, Yokohama-shi, Japan). The film thickness was measured by a computerized profilometer (Dektak 3, Ulvac, Chigasaki, Japan). The dye-loading amount of photoanodes was ascertained by desorbing the dye into 10 mL of 0.1 M NaOH water solution and quantified by measuring N719 optical adsorption peak intensity.

2.5. Photocatalytic Measurements

The photocatalytic activities of the different photocatalysts were evaluated by the removal of CIP and Cr(VI) using a 500 W xenon lamp as an irradiation source. Photocatalyst (75 mg) was added into 150 mL of CIP (10 mg/L) or/and K2Cr2O7 (50 mg/L) single contaminant or contaminant mixture. To achieve adsorption-desorption equilibrium, the suspension was continuously stirred for 24 h at room temperature in the dark. During the catalytic reaction, at the given intervals, 5 mL of suspension was collected and centrifuged with 4000× g rpm for 5 min to remove the photocatalyst. The concentration of Cr(VI) was estimated using the standard diphenylcarbazide analytical method at 540 nm with a UV-Vis spectrometer. The contents of CIP were analyzed by detecting the values of feature absorbance of 277 nm on a UV–Vis spectrometer. To study the reusable ability of the photocatalysts, the used samples were collected and washed with deionized water to remove the residual materials, followed by drying at 60 °C, and then the cycling experiments were conducted at optimal operating conditions.

3. Results and Discussion

3.1. Structure Characterization

The SEM image in the Figure S2a clearly demonstrated that the typical S1 showed the octahedral morphology with an average size of ~1.2 μm and possessed smooth surfaces. After a simple acid etching process, these Zn2SnO4 octahedrons still maintained their original size and shape whilst also having rough surfaces (Figure S2b). As with design, S1 converted into hollow structure, which could be confirmed by the TEM image of Figure S2c, to gain the sample S2. As can be seen from the Figure S2d, the ZnSnO3/Zn2SnO4 composite remained octahedron-like in structure, and the nanosheets of ~50 nm length and ~15 nm thickness distributed on each facet of the octahedron to form a hierarchical structure. Additionally, the TEM image revealed that the hollow structure was not damaged through the solvothermal process of ZnSnO3 nanosheet growth (Figure S2e).

Figure S3 presented the XRD spectra of the three samples. All the diffraction peaks of S1 and S2 could be assigned to the cubic phase Zn2SnO4 (JCPDS: 24-1470), this result illustrated that no change of composition and crystal form transformation emerged during the acid etching process. For the cases of S3, the additional XRD peaks characteristic of ZnSnO3 (JCPDS No. 28-1486) confirmed that S3 was ZnSnO3/Zn2SnO4 composites.

To construct an energy band diagram for S1, S2 and S3, the band gap energies (Eg) and flat-band potentials (Efb) of the three samples should be determined. As shown in Figure 1a, the wavelength threshold of the pristine solid and hollow Zn2SnO4 octahedrons located at 340 nm and 360 nm, separately, and ZnSnO3/Zn2SnO4 hollow octahedrons displayed the red-shifted light absorption in comparison with pure Zn2SnO4, which was helpful to extend the light absorption. The Eg was estimated according to the Kubelka-Munk function equation to be around 3.83 eV for S1, 3.59 eV for S2 and 3.43 eV for S3 (the inset of Figure 1a). The positive slopes for Mott–Schottky plots indicated the characteristics of n-type semiconductors for the three samples, and the Efb values were calculated to be −1.15, −1.15 and −1.29 V for S1, S2 and S3 (vs. Ag/AgCl, pH = 7), respectively, from the tangential intercept of the curves (Figure 1b). Then, the conduction band (CB) potentials of S1, S2 and S3 could be determined to be −0.93, −0.93 and −1.07 V (vs NHE, pH = 7), respectively, from the formula (Equation (1)) [25]: where Eθ and EAg/AgCl represent standard electrode potentials and the Ag/AgCl electrode potential at pH = 7 (0.197 V), respectively. According to the formula (EVB = ECB + Eg), the valence band (VB) potentials of S1, S2 and S3 were calculated to be 2.90, 2.66 and 2.36 V, respectively. Based on the Efb, ECB and EVB values of the three samples (Table S1), their energy band constructions were depicted in Scheme 2a. For the three materials, the bottom of CB and the top of VB were lower than lowest unoccupied molecular orbital (LUMO, −1.5 eV vs. NHE) and the highest occupied molecular orbital (HOMO, +1.1 eV vs. NHE) energy levels of N719 dye, respectively, and match well with the redox couple in the electrolyte, ensuring the operation of the DSSCs [26].

Figure 1

(a) UV−_vis absorption spectra; (b) Mott−Schottky plots of the samples; Inset of (a): the corresponding Tauc’s curves.

Scheme 2

(a) Energy band construction of three samples; (b) Schematic diagram of the band structure and charge separation in ZnSnO3/Zn2SnO4 composite.

3.2. Photovoltaic Characteristics of Zn2SnO4 and ZnSnO3/Zn2SnO4

3.2.1. Photovoltaic Characteristics of Zn2SnO4

Four DSSCs based on the Zn2SnO4 solid octahedrons photoanodes with the same thickness of ~10 µm were sensitized with various sensitization durations to examine the optimum immersion time in N719 dye solution (Table S2). Under 1 sun AM 1.5 illumination, the short-circuit current (Jsc) increased with the sensitization time in the initial 6 h, while the open-circuit voltage (Voc) and fill factors (FF) remained stable, then the photovoltaic conversion efficiency (η) reached the maximum 2.35% at 6 h. However, a decrease in the FF, Jsc and η were observed at 8 h later probably because of the formation of aggregates, as in the case of Zn2SnO4 [27]. Reduced Voc was largely due to the formation of Zn2+/dye complex. The immersion time was longer, a complex layer could be observed and formed a thick covering layer, which was therefore inactive for electron injection [28]. These results suggested that the optimal sensitization time was six hours in the acidic dye molecules. To explore the effect of film thickness on η of DSSCs, solid Zn2SnO4 films with the different thicknesses of 6, 9, 12, 15 and 18 μm were prepared and the detailed photovoltaic parameters of DSSCs based on these photoelectrodes are summarized in Table S3. In effect, when the thickness varied from 6 μm to 12 μm, the thicker film could adsorb much more dye to enhance the light harvesting ability of the photoanode, and then Jsc significantly increased from 4.17 to 5.65 mA cm−2. As the film thickness increased continuously to 15 μm or thicker, the more trapping sites and electron recombination centers would generate, so the more serious electron recombination would lower the electrons concentration, and hence lead to the decline of Jsc, η [29] and Voc [30]. For the optimized DSSCs based on Zn2SnO4 solid octahedrons, the η, Voc, Jsc and FF were 2.49%, 0.64 V, 5.65 mA cm−2 and 68.79%, respectively.

To study the effect of the hollow structure on the photovoltaic properties of DSSCs, Zn2SnO4 hollow octahedron photoelectrodes were prepared by depositing 12 μm film on FTO glass, and photovoltaic properties were tested by constructing DSSCs. The characteristic J-V curves and their photovoltaic parameters are given in Figure 2a. It can be seen that the Voc and FF of S2-based DSSCs were similar to those of S1-based DSSCs, the distinct photovoltaic behavior of S2 was larger Jsc (7.61 mA cm−2) compared to S1 (5.65 mA cm−2). The relatively high Jsc of S2 can be considered following the three combined effects: (1) Higher dye loading, due to larger specific surface area. The measured dye-loading amount for the S2 film was around 4.23 × 10−7 mol·cm−2 μm−1, and was more than that of the S1 film (2.87 × 10−7 mol·cm−2 μm−1). The surface area of S2 was measured as 37.7 m2 g−1 (Figure S4), 70.6% higher than that of S1 (22.1 m2 g−1). So, the larger specific surface area enabled S2 to adsorb more dye to improve Jsc of DSSCs. (2) Stronger light-harvesting efficiency, due to the hollow structure. The corresponding UV–vis absorption spectra of solid and hollow Zn2SnO4 octahedrons displayed a strong absorption peak in the ultraviolet region, and the absorption intensity of the latter was stronger than that of the former (Figure 1a). Because of the same crystal structure, the enhanced light absorption of S2 was presumably due to its hollow structure, as explained in Figure S5. Obviously, in case of Zn2SnO4 hollow octahedrons, the empty internal structure could offer more interfaces to considerably increase the chance of light reflections, leading to the more efficient utilization of the incident light and increased light-harvesting efficiency, and then more photogenerated electron-hole pairs would generate when S2 was under light irradiation [31,32]. (3) Efficient electrolyte diffusion into the interior of the hollow octahedrons, due to mesoporous wall. The results of N2 adsorption/desorption confirmed the existence of the mesoporous microstructure for Zn2SnO4 hollow octahedrons (Figure S4). The pore-size distribution indicated that the average pore size for S2 was about 3.6 nm, which would promote electrolytes to transport in the photoelectrode and enter the internal area of hollow octahedrons to increase the contact probability between electrolyte and electrodes to improve the η of DSSCs.

Figure 2

(a) J−V curves for the DSSCs based on the different electrodes; (b) Transport time, (c) Lifetime and (d) the charge collection efficiency of the different DSSCs.

3.2.2. Photovoltaic Characteristics of ZnSnO3/Zn2SnO4

To investigate the influence of the ZnSnO3 nanosheets on the electron transfer and recombination behaviors, the IMPS/IMVS measurements of S2 and S3 were performed. The IMPS/IMVS measurements could provide direct evidence of the electron transport time (τt) and electron recombination time (τr) through the equations [33]: where ft (fr) is the minimum frequency of the IMPS(IMVS) imaginary component. Here, τt and τr characterize the electron transport properties in photoanode films and recombination with redox species in the electrolyte, respectively. Hence, smaller τt and larger τr mean a faster transport rate and slower recombination rate. Examinations of Figure 2b,c clearly showed that the τt (τr) of the DSSCs based on S3 photoanodes was shorter (longer) than that of S2 over light intensities from 30 to 150 W m−2. This finding was a consequence of the following reasons: first, the engineered interfacial design of band-structure-matched ZnSnO3/Zn2SnO4 composite photoanodes. A schematic illustration of the energy levels of ZnSnO3 and Zn2SnO4 is shown in Scheme 2b, the CB edge of ZnSnO3 was determined to be −0.99 V (Figure S6), which was more negative than that of Zn2SnO4 (−0.93 V, Table S1), such an energy band matching composite photoanode would serve as the “bridge” to expedite electron transport from excited ZnSnO3 to FTO resulting in shorter τt [34]. The band-structure-matched ZnSnO3/Zn2SnO4 photoanodes, meanwhile, to some extent avoid any extra internal traps, and could effectively suppress the recombination between the electron in the CB of composite photoanodes and I3− in the electrolyte, which rationalizes the reasons for the increase in τr. Second, the two-dimensional ZnSnO3 nanosheets in the composite photoanodes caused direct electron transport paths and were beneficial for faster electron transport. Besides, it is well known that the less trap sites there are, the slower charge recombination occurs [35]. Compared to the nanoparticles with abundant grain boundaries, ZnSnO3 nanosheets exhibited a reduced recombination loss, which can contribute to longer τr. Third, interconnected contact between the ZnSnO3 nanosheets of neighboring octahedrons made the contact area larger and prolonged the electron transport channel of the injected electrons, so lead to fast electron transport to the back contact, thus a lower recombination rate [36]. These results verified the reasonableness of the promoting the electron transfer of the ZnSnO3 nanosheets in the composite film.

To evaluate the scattering effect of the ZnSnO3 nanosheets, the diffuse reflection spectra of the films made from S2 and S3 were measured (Figure 3a). Compared to the Zn2SnO4 film, the composite film had higher diffuse reflection ability in the wavelength region from 380 to 800 nm, suggesting that the vertical ZnSnO3 nanosheets would lead to more significant incident light scatter within the composite films. A possible explanation can be that S3, owing to the surface ZnSnO3 nanosheets, can scatter the incident light to all directions, as schematically illustrated in Scheme 3. Thereby, the composite film utilized the incident light to a higher degree than that in the case of S2. At the same time, this scattering capacity could provide more chances for the N719 molecules on the composite area to trap photons, and then turn into electrons [26]. Otherwise, compared with S2, the absorption edge (~390 nm) of S3 exhibited red shift (Figure 1a), implying more holes and electrons can be generated after ZnSnO3 incorporation, which probably was attributed to the interaction between ZnSnO3 and Zn2SnO4. These results confirmed the positive impact of the ZnSnO3 nanosheets in the composite film on the light harvesting efficiency.

Figure 3

(a) Diffuse reflectance spectra and (b) IPCE spectra of the different electrodes.

Scheme 3

Schematic representation of (left) a DSSC with ZnSnO3/Zn2SnO4 hollow octahedron film, (center) facile penetration of electrolyte through the hollow octahedrons and light scattering effect by the octahedrons, and (right) a single octahedron showing penetration of an electrolyte through it.

To evaluate the beneficial impact of the addition of the ZnSnO3 nanosheets on the photovoltaic conversion properties, the composite was applied as photoanodes of DSSCs. Four ZnSnO3/Zn2SnO4 DSSCs with the same thickness (~12 µm) were fabricated with various sensitization durations. The η gradually increased with the sensitization time and reached the maximum at 4 h (Table S4). Figure 2a shows the J–V curves of the DSSCs based on the optimized composite photoanode film DSSCs. Compared with DSSCs based on S2, the addition of ZnSnO3 nanosheets to Zn2SnO4 cells resulted in considerable improvement in Voc, Jsc and FF, so the DSSCs with S3 exhibited an enhanced η (4.31%) value by around 27.9% as compared with S2 DSSCs (3.34%). It can be seen that the Voc of the S3 (0.66 V) was higher than that obtained from the S2 (0.64 V). The Voc is determined by the chemical potential of the electrolyte and the Fermi level of the semiconductor oxide [37]. As seen in Scheme 2a, the S3 showed a more negative Efb value, so the difference between the Fermi level of S3 and the redox potential of I−/I3− would be larger, and the corresponding Voc will be higher. On the other hand, through the analysis of the IMVS and IMPS dates above, the introduction of ZnSnO3 nanosheets in the Zn2SnO4 film can inhibit electron recombination and prolong electron lifetime, which could increase the Voc. So, the enhanced Voc with ZnSnO3 nanosheets added should offer the combined impact of the elevated CB edge and the suppressed interfacial charge recombination.

The J can be estimated using the following formula [38]: where q and I0 are the elementary charge and light flux. Hence, the Jsc is directly proportional to the light harvesting efficiency (ηlh), electron injection efficiency (ηinj) and electron collection efficiency (ηcc) of DSSCs. First, the ηlh is determined by both the dye loading amount and/or the light scattering capability [39]. The absorption quantity of N719 on S3 with the surface area of 41.3 m−2 g−1 (4.49 × 10−7 mol·cm−2 μm−1) was slightly larger than that of S2 (4.23 × 10−7 mol·cm−2 μm−1). Meanwhile, as discussed above, the addition of the ZnSnO3 nanosheets favored enhanced light-harvesting efficiency and originated from light scattering and the red shift of the absorption edge. To further obtain the detailed information on the light harvest of the DSSCs, the IPCE measurements were performed, as shown in Figure 3b. The IPCE value of the S3-cell was higher than that of the S2-cell in whole wavelength range from 400 to 800 nm, indicating that the quantum efficiency of the S3-cell was greater than that of the S2-cell in the visible-light wavelength region [40]. The higher IPCE values for the S3-cell at short wavelengths reflected on its stronger dye-loading capacity. In the longer wavelength region from 600 to 750 nm, the increased IPCE values of the S3-cell could be explained by the enhanced light scattering efficiency, which promotes the light harvesting of the N719 dye in this region. Further, the IPCE measurements also validated the estimated Jsc values (Figure S7) and correlated well with those obtained from the J-V curves (Figure 2a). Those studies indicated superior light harvesting efficiency of S3 films compared to S2 films. Second, φinj is the probability of electron injection from the dye-excited state into the CB of the semiconductor. The CB was moving upward in the S3 film, which meant that charge injection from LUMO of N719 dye to the CB of S3 would be limited, thus reducing φinj. Third, the ηcc can be estimated according to τt and τr [33].

Figure 2d showed the ηcc of the S3-cell was superior to the S2-cell under various light intensities, which is attributed to the incorporation of ZnSnO3 nanosheets enhancing the electron transfer. In summary, although the φinj values would proportionally decrease, the S3 showed an increased Jsc because the ηlh and ηcc were raised more significantly than the decrease in φinj.

A comparison of photovoltaic performances between our assembled DSSCs and previously reported DSSCs is summarized in Table S5, including Jsc, Voc, FF and η. It can be seen that, the η of the DSSC based on ZnSnO3/Zn2SnO4 hollow octahedrons was not higher than that of Ba2+ ion doped Zn2SnO4 nanocrystalline [41], the cause was probably that the photo sensitizer was a mixture including N719 dye and D131 dye instead of only N719. The DSSC based on ZnSnO3/Zn2SnO4 hollow octahedrons showed superior photovoltaic performances compared to metallic silver decorated Zn2SnO4 spheres [42], Au inlaid Zn2SnO4 /SnO2 hollow rounded cubes [43], the binary metal oxide MgO passivation layers modified Zn2SnO4 nanoparticles [44], and ZnO/Zn2SnO4 core shell photoanodes [45]. In addition, the higher FF showed the advantage of ZnSnO3/Zn2SnO4 hollow octahedrons in practical application [46].

3.3. Photocatalytic Performance

In order to further demonstrate the beneficial effect of the ZnSnO3 nanosheets on photocatalytic activity, S2 and S3 were used to perform photocatalytic removal of CIP and Cr(VI) in single and co-existing systems under simulated sunlight. A blank control experiment was carried out to confirm that CIP would hardly self-degrade under the condition of no photocatalyst (Figure 4a). Initially, only 64.3% of CIP was degraded by S2 within 105 min of light irradiation. With the addition of ZnSnO3, the catalyzed degradation efficiency of the composites reached 94.3% within same irradiation time length. As shown in Figure 4b, the photodegradation of CIP catalyzed by S2 and S3 followed a pseudo-first-order rate law (Table S6), ln (C0/Ct) = kt, where k is the degradation rate constant, C0 and Ct are the concentrations of the pollutant solution before light irradiation and at irradiation time t, respectively. The k values of S2 and S3 were 0.010 and 0.026 min−1 (Figure 4b), respectively, and the photodegradation rate constant of S3 was around 2.6 times higher than that of S2. The solution of Cr(VI) was also treated to investigate the photoreduction performance of photocatalyst. As shown in Figure 4c, the reduction of Cr(VI) was almost negligible in the absence of the photocatalyst, and the Cr(VI) concentration was decreased by the processes of simulated sunlight. In 200 min, 60.5% and 87.1% photoreduction efficiency was observed using S2 and S3, respectively. The kinetic constant 0.010 min−1 of S3 was numerically about 2.50 times as high as S2 at 0.004 min−1 (Figure 4d). All these showed, compared with the S2 sample, that the S3 photocatalyst displayed stronger removal efficiency under illumination, which verified that the addition of ZnSnO3 nanosheets effectively enhanced system photocatalytic performance. For further research, the simultaneous photocatalytic CIP oxidation and Cr(VI) reduction over S3 were performed in the co-existing systems containing CIP (10 mg/L)/K2Cr2O7 (50 mg/L). S3 only removed a small amount of CIP and Cr(VI) under a no-light condition, which showed no simulated sunlight response (Figure 5a). Compared with the photodegradation results in Figure 4 and Figure 5b, the higher CIP removal performance presented after the introduction of Cr(VI) in the system, and the k value increased from 0.026 min−1 without Cr(VI) to 0.033 min−1 with co-existence of Cr(VI). Meanwhile, the removal rate of Cr(VI) increased with co-existence of CIP, and the k value of S3 in the mixed solution was 1.70 times as high as in the absence of CIP. The results of the experiment suggested the synergistic effect between photocatalytic CIP oxidation and Cr(VI) reduction.

Figure 4

Removal efficiency of (a) CIP and (c) Cr(VI); the first−order kinetics of (b) CIP and (d) Cr(VI) by different photocatalysts in single system.

Figure 5

(a) Removal efficiency of CIP and Cr(VI) in the mixed solutions by S3 sample; (b) the corresponding first-order kinetics and rate constants; (c) reusable degradation activity of S3 for three cycles; (d) simultaneous removal of CIP and Cr(VI) in the presence of S3 with and without acid treatment for nine cycles.

The quenching experiments were conducted to further explore the removal mechanism of Cr(VI) and CIP over S3 (Figure S8). In general, ammonium oxalate (AO), isopropyl alcohol (IPA), dimethyl sulfoxide (DMSO) and benzoquinone (BQ) were used to quench the active species photogenerated holes (h+), hydroxyl radicals (OH), photogenerated electrons (e−) and superoxide radicals (O2−), respectively [47]. It could be observed that the photodegradation efficiencies of CIP decreased evidently with the addition of AO and BQ to the co-existing system, reflecting that h+ and O2− had a significant effect in the removal reaction of CIP over S3. In contrast, when the IPA was introduced into this system, there was only a negligible suppression, indicating the influence of ·OH could be ignored. Therefore, h+ and O2− were the predominant reactive species in the CIP photocatalytic degradation process. Meanwhile, the photoreduction efficiencies of Cr(VI) obviously decreased from 92.9 to 47.6% with the addition of DMSO, it could be deduced that Cr(VI) was reduced by e− and the photogenerated electrons served as the main reaction species of the photocatalytic reduction of Cr(VI).

Based on the above results and discussion, a photocatalytic mechanism was proposed (Scheme 2b). When the light irradiated on the photocatalyst, the ZnSnO3/Zn2SnO4 composite could be excited by the photon, and equivalent energy would generate the electron-hole pairs. The conduction band of ZnSnO3 (−0.99 V) was more negative than that of the hollow Zn2SnO4 (−0.93 V), the photogenerated electrons would easily inject from the former into the latter through the heterostructure between ZnSnO3 and Zn2SnO4. Therefore, the photogenerated electron-hole pairs were spatially separated by this heterostructure to restrain the recombination of charges. Cr(VI) was a strong oxidant and could serve as an electron acceptor to consume these generated electrons. Because the ECB of Zn2SnO4 was more negative than the O2/·O2− potential (−0.33 eV) [48], and the electrons would react with the dissolved O2 to produce O2−. Meanwhile, the valence band of Zn2SnO4 (2.66 V) was more positive than that of ZnSnO3 (2.23 V), and the holes would migrate simultaneously from VB of Zn2SnO4 to ZnSnO3 in the opposite direction, whereas, h+ at VB of ZnSnO3 was not more positive than the H2O/·OH (2.40 eV) [48], and not enough to oxidize H2O to ·OH. Hence, the O2− and h+ acted as the main oxidants to decompose CIP into small molecules. So, in this co-existing system, the faster photoreduction rate of Cr(VI) and photodegradation efficiency of CIP might be due to the synchronized actions of the CIP as an electron donor and Cr(VI) as an electron acceptor. The oxidation process of CIP and the reduction process of Cr(VI) consumed the photo-excited holes and electrons, respectively. These two processes effectively promoted the separation of photogenerated electrons and holes, and resulted in much more electrons for Cr(VI) reduction and holes for CIP oxidation, and the recombination of photogenerated carriers would be prohibited so as to enhance the photocatalytic efficiency [47].

To evaluate the reusable ability of S3 in the co-existing systems, the recycling experiment was conducted under the same conditions. As shown in Figure 5c, after the three cycles of operation, the removal efficiencies changed from 92.7% to 86.1% for Cr(VI) and 91.1 to 83.2% for CIP, and the photocatalytic performance showed less than 9% loss, which confirmed that the ZnSnO3/Zn2SnO4 composite possessed outstanding reusability. To further investigate the repeatable removal performance, the number of repeated uses was increased from three to nine, and the photocatalytic efficiency decreased to 71.8% for Cr(VI) and 75.3% for CIP. Borrowed from the previous method [49], after three consecutive cycles, S3 was cleaned with 0.1 mM HCl once to remove the adsorbed pollutant molecules, it was found that S3 with the acid-treatment preserved 88.5 and 89.2% of the initial efficiency for Cr(VI) and CIP in the ninth cycle, and greater than 15.6 and 10.4% photocatalytic efficiency without the acid-treatment, respectively (Figure 5d). So, the hierarchical ZnSnO3/Zn2SnO4 porous hollow octahedrons could be applied in the simultaneous removal of antibiotic and heavy metal pollutants from contaminated water for environmental protection.

4. Conclusions

To summarize, hierarchical ZnSnO3/Zn2SnO4 porous hollow octahedrons were designed by structure and composition evolution. According to IMVS/IMPS, reflectance spectra, IPCE and Mott-Schottky plots measurements, ZnSnO3 nanosheets introduced on the surface of hollow Zn2SnO4 octahedrons could provide a direct pathway for electron transport, act as light-scattering centers to improve light harvesting, and induce the flat-band potential to shift negatively. As a result, as-prepared ZnSnO3/Zn2SnO4 hollow octahedrons could enhance the overall photovoltaic conversion efficiency by 73.1% compared to that of the Zn2SnO4 solid octahedrons. Meanwhile, the composite also exhibited higher removal activity of Cr(VI) and CIP in single and co-existing systems under simulated sunlight irradiation. In addition, the composite photocatalyst showed good durability and stability in a co-existing system, suggesting its potential practical applications in the simultaneous removal of heavy metal and antibiotic pollutants from contaminated water.