ZnS and ZnO Nanosheets from ZnS(en)0.5 Precursor: Nanoscale

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ZnS and ZnO Nanosheets from ZnS(en)0.5 Precursor: Nanoscale Structure and Photocatalytic Properties Lucia Nasi, Davide Calestani, Tullo Besagni, Patrizia Ferro, Filippo Fabbri, Francesca Licci, and Roberto Mosca* IMEM-CNR, Parco Area delle Scienze 37a, I-43124 Parma, Italy ABSTRACT: Wurtzite ZnS and ZnO porous nanostructures have been obtained by annealing ZnS(en)0.5 nanosheets in air at different times and temperatures. The evolution of the morphological and structural transformation has been investigated at the nanoscale by transmission and scanning electron microscopy (TEM and SEM) analysis. At the annealing temperature of 400 °C, the ZnS(en)0.5 hybrid decomposes by a topotactic transformation, giving ZnS nanosheets. By increasing the annealing time, the gradual transformation ZnS→ZnO is observed to take place at the nanoscale. The transformation completes with the formation of highly porous ZnO crystals when the annealing temperature is increased to 600 °C. Remarkably, all the structural and morphological transformation steps during the annealing preserve the nanosheet crystalline character. These processes have been related to the materials photocatalytic activity in methylene blue degradation under UV−visible irradiation. The ZnS(en)0.5 precursor is the weakest photocatalyst, whereas porous ZnO is the strongest. Samples treated at 400 °C show degradation efficiencies that are intermediate between ZnS(en)0.5 and ZnO platelets and decrease with increasing annealing time due to the formation of ZnS/ ZnO heterojunctions.

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INTRODUCTION ZnS and ZnO are important wide bandgap semiconductors whose nanostructures have attracted considerable interest due to their sizes, morphology-related properties, and wide-ranging applications (e.g., sensing, gas storage, catalysis, energy transformation, and storage, among others).1,2 Although much attention has been devoted to one-dimensional nanoscale materials (nanowires, nanorods, nanotetrapods),3−5,8 nanostructures such as nanosheets6,7 also represent an important category of nanostructured materials, especially when monocrystallinity is combined with porosity, i.e., with a high surfaceto-volume ratio. The potential of porous nanosheets has been pointed out in photocatalysis for both ZnS and ZnO9 and in gas sensing for ZnO,10 but other applications can be envisaged (e.g., dye-sensitized solar cell). ZnS nanosheets were obtained by a template method where ZnS(en)0.5 (en = ethylenediamine) platelets prepared by a solvothermal method were employed as the precursor. In particular, wurtzite ZnS sheet-like nanocrystals were obtained when the ethylenediamine template was removed by thermal decomposition at 250−500 °C in Ar atmosphere11 or under vacuum.12 Wurtzite ZnO flake-like dendrites were achieved by oxidation of ZnS nanosheets.11 A similar template method was used by Ni et al.,13who decomposed ZnS(en)0.5 cuboids at 450 °C in vacuum and 650 °C in air, thus obtaining ZnS cuboids and ZnO nearly spherical grains, respectively. By studying the degradation of safranine T they found that ZnS was the strongest photocatalyst and ZnS(en)0.5 the weakest. This result was explained by the © 2012 American Chemical Society

different specific surfaces deriving from changes in morphology and particle size that are associated with the degradation of the ZnS(en)0.5 precursor. The changes in the crystal structure of the ZnS(en)0.5 precursor when it undergoes thermal treatments were studied in detail by Jang et al.9 who calcined the precursor at temperatures from 400 to 600 °C. They showed that at 400 °C only wurtzite ZnS is obtained, while at intermediate temperatures ZnS coexists with ZnO, and at 600 °C only wurtzite ZnO is observed. Nanoplate morphology is conserved during the ZnS(en)0.5 → ZnS → ZnO transformation, whose topotactic nature was demonstrated. Additionally, ZnS and ZnO platelets have a “highly porous single crystal” character, but their photocatalytic activity under visible light irradiation could not be related to the surface area. The formation of ZnS/ ZnO heterojunctions during thermal treatments was pointed out, but the possible influence of these heterojunctions on the photocatalytic efficiency was not discussed, even if coupling ZnO with ZnS has been addressed as an efficient method to enhance photocatalytic activities in ZnO−ZnS colloids9 and nanoparticles,14 as well as in ZnO rods15 and nanofibers16 coated by ZnS (ZnO/ZnS structures). In all these cases, the efficiency enhancement was explained by the reduction of the photoexcitation threshold to lower photon energy, and the relative positions of conduction and valence bands of the two Received: November 22, 2011 Revised: February 10, 2012 Published: February 23, 2012 6960

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of MB solutions were measured by a Jasco UV−vis V-530 spectrometer with 10 mm quartz cells. The degradation efficiency (D%) of MB was calculated by the formula D% = (A0 − A)/A0·100, where A0 and A are the absorbances measured at the characteristic wavelength of 664 nm on the primal and remaining MB, respectively. Commercial ZnO nanopowders (99.9% purity, APS = 90 nm, Chempur) were used for comparison in the photocatalysis investigation.

semiconductors that favor the transfer of photogenerated electrons and holes to ZnO and ZnS, respectively, thus increasing carrier lifetime and favoring their interaction with dyes adsorbed on the photocatalyst surface.17 Thus the possible influence of ZnS and ZnO coexisting in nanosheets derived from ZnS(en)0.5 platelets should be better understood. On the other hand, since shape, porosity, and crystal phase of nanosheets significantly affect their potential applications,18−22 studying the morphology and crystalline structure of these systems as a function of the process parameters is necessary to achieve a deeper control of the process itself, and a prerequisite to fully exploit the features of these materials. In this paper we study the ZnS(en)0.5 → ZnS → ZnO transformation by annealing the ZnS(en)0.5 precursor in air at different times and temperatures. The evolution of the structural and morphological transformation is investigated on the atomic scale for the first time. In light of the results obtained, we discuss the activity of the different nanosheets in the photodegradation of methylene blue (MB) under UV-visible (UV−vis) light.

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RESULTS AND DISCUSSION Figure 1 shows that as-synthesized samples consist of plate-like sheets with micrometric width and thickness of about 100 nm.

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EXPERIMENTAL SECTION The ZnS(en)0.5 hybrid was synthesized solvothermally by adding S powder (3.8 mmol), ZnCl2 (1.9 mmol), and ethylenediamine (30 mL) in a commercial Teflon-lined autoclave with a capacity of 45 mL. The autoclave was sealed, put in an oven, maintained at 180 °C for 12 h, and finally allowed to cool naturally to room temperature. The product was filtered, washed with ethanol and deionized water, dried overnight in an oven at 65 °C, and finally collected. Further details about ZnS(en)0.5 hybrid preparation are reported in ref 23, where nanocrystals are shown to have plate-like behavior. The solvothermally synthesized ZnS(en)0.5 hybrid was annealed in air at 400 °C for 1 h (sample 400 °C-1 h), 2 h (sample 400 °C-2 h), 24 h (sample 400 °C-24 h), and 50 h (sample 400 °C-50 h). An aliquot of this last sample was further calcined at 600 °C for 2 h (sample 600 °C-2 h). Following this last procedure, ZnO platelets were prepared using smaller ZnS(en)0.5 nanosheets obtained as discussed in ref 23 (sample 600 °C-2h_S). The crystal phase of each sample was determined by powder X-ray diffraction (XRD) and selected area electron diffraction (SAED) in a Jeol 2200FS transmission electron microscope (TEM). Morphological and structural characterization was performed by scanning electron microscopy (SEM) using a Jeol JSM-6400F, high-angle annular-dark-field (HAADF) scanning transmission electron microscope, and high-resolution transmission electron microscopy (HRTEM). The photocatalytic activity of nanosheets was studied by using MB (Riedel-de-Haen). Reaction suspensions were prepared in an opened Pyrex vessel having 50 mL capacity. Five milligrams of nanosheets were dispersed in 5 mL of MilliQ water and sonicated for 15 min. After sonication, 5 mL of a MB aqueous solution (6.25 × 10−5 M) was added, and the suspension was magnetically stirred in the dark for 15 min to establish an adsorption/desorption equilibrium. Photocatalytic degradation was achieved under continuous magnetic stirring by an Osram Ultravitalux300 lamp positioned 30 cm above the vessel containing the mixture. Irradiation time was 50 min. A quartz beaker containing water was set between lamp and vessel to eliminate the infrared light. In order to monitor the MB photocatalytic degradation, 2 mL of suspension was collected by a syringe before and after irradiation and immediately centrifuged to remove catalyst particles. The absorbance spectra

Figure 1. Typical SEM image of ZnS(en)0.5 precursor platelets.

Figure 2. XRD patterns of the samples (a) ZnS(en)0.5 precursor, (b) 400 °C-1 h, (c) 400 °C-2 h, (d) 400 °C-24 h, (e) 400 °C-50 h, and (f) 600 °C-2 h.

Their XRD patterns (Figure 2) match those reported in previous studies,9−12 which revealed that ZnS(en)0.5 hybrids are layered complexes with orthorhombic structure consisting of inorganic two-dimensional (2D) ZnS sheets connected by ethylenediamine molecules along the a-axis. The nanosheets change their crystal structure upon thermal annealing while retaining the same external shape, as shown by XRD and SEM analysis. Figure 2 shows the XRD patterns of the calcined samples. After 1 and 2 h of thermal annealing at 400 °C in air,24 all the diffraction peaks fit quite well the standard wurtzite structure of ZnS (JCPDS no. 36-1450, a = 0.382 nm and c = 0.626 nm). By increasing the annealing time to 24 h and 50 h (Figure 2b,c) the ZnS phase progressively 6961

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Figure 3. SEM images of the different annealed samples: (a) 400 °C-2 h; (b) 400 °C-24 h; (c) 400 °C-50 h; (d) 600 °C-2 h. A large area view of the platelets is shown in the insets.

Figure 4. HAADF images with the respective SAD patterns of the annealed samples: (a) 400 °C-2 h; (b) 400 °C-24 h; (c) 400 °C-50 h; (d) 600 °C2 h.

Figure 5. HRTEM micrographs of samples (a) 400 °C-24 h, (b) 400 °C-50 h, and (c) 600 °C-2 h. In the top insets: the FFT and the grayscale maps of the inverse FFT showing the ZnS (dark) and ZnO (bright) domains. Bottom inset of panel a: Fourier-filtered HRTEM image of the interface between ZnS and ZnO showing the presence of misfit dislocations (marked by arrows).

transforms into wurtzite ZnO (JCPDS no. 36-1451, a = 0.325 nm and c = 0.521 nm). By further increasing the calcination temperature to 600 °C for 2 h, only peaks of ZnO were detected (Figure 2d), indicating that ZnS is completely transformed into ZnO. The SEM investigation of the calcined samples (Figure 3) evidences that the shape and size of the plate-like crystals do not change upon annealing, while the morphology at the nanoscale exhibits dramatic changes. No significant change in morphology has been observed after 1 h (not shown here) and 2 h of thermal treatments at 400 °C (Figure 3a). Increasing the annealing time to 24 h and 50 h results in a significant increase of granularity (Figure 3b,c) which yields an increase of the exposed surface area of the platelets. Highly porous morphology is finally obtained by increasing the calcination temperature to 600 °C, as shown in Figure 3d. The HAADF micrographs of the annealed products with the corresponding SAED patterns are shown in Figure 4. Platelets appear more rough and porous in the HAADF images than revealed by SEM analysis (Figure 3). This difference can be

explained taking into account that the whole thickness of the platelets and not only the surface contributes to the HAADF contrast, thus revealing the presence of a granular structure in the film volume. The SAED analysis shows that the disassembly of the hybrid structure by thermal decomposition of the ZnS(en)0.5 precursor leads to (2−10)-oriented hexagonal ZnS nanosheets with the c axis in the layer plane. The ZnS → ZnO transformation under annealing revealed by XRD is confirmed. The SAED patterns reveal that the ZnS and ZnO phases exhibit a topotactic crystal orientation, which results in [001]ZnS||[001]ZnO and (2−10) ZnS||(2−10) ZnO relationships. In addition, the measured d spacings from the diffraction patterns suggest that the ZnO regions on the ZnS matrix are completely relaxed. In order to gain deep insight into the mechanisms of the phase and morphological transformation, HRTEM analysis was performed. Figure 5 reports the HRTEM images of the annealed samples with their fast Fourier transforms (FFT) in the insets. In the samples where both ZnO and ZnS were present, the amplitude grayscale maps resulting from the 6962

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inverse FFT generated by selecting the different diffraction spots of ZnS and ZnO are shown in the insets:25 bright and dark areas refer to ZnO and ZnS, respectively (Figure 5a,b). The HREM analysis revealed that the transformation takes place at the nanoscale. In particular, the substitution of S by O atoms during the oxidation process does not occur through the formation of a ternary ZnSxO(1−x) alloy but with the nucleation of pure ZnO nanoscale domains, which starts at the crystal edges (Figure 5a) and spreads over the whole crystals as the annealing time increases (Figure 5b). The FFT spot patterns indicate a topotactic relationship between ZnS and ZnO coexisting in the platelets despite the different lattice parameters of the completely relaxed materials. Misfit dislocations are present at the ZnS/ZnO grain interfaces as a consequence of the strain relaxation of the materials. These defects are clearly visible in the Fourier-filtered HRTEM image of the grain borders (bottom inset of Figure 5a) as extra half planes in the material with the smaller lattice parameter (ZnO). The evolution of the structural properties upon annealing clarifies the morphology change observed in the calcined nanosheets. In fact, considering the lower lattice parameters of wurtzite ZnO with respect to those of wurtzite ZnS, the formation of ZnO grain domains of nanometric dimension is accompanied by a lattice contraction, which results in the granular morphology shown in Figure 4b,c, in agreement with that proposed by Jang et al.9 Finally, when the ZnS is completely transformed in ZnO, nanopores form between grains, as shown in Figure 4d. The nanopore formation is the direct consequence of the volume contraction during the process and can be regarded as a mechanism to release the tensile stress accumulated in the matrix. It is worth mentioning that the formation of pores leaves the crystal structure and orientation unchanged from the nano- to microscale, as shown by the HRTEM investigations (Figure 5c and confirmed by the spot FFT and SAED patterns. This can be taken as the experimental evidence that the final products of the transformation are highly porous monocrystalline ZnO nanosheets. On the basis of the coexistence of ZnS and ZnO pointed out above, platelets annealed at 400 °C could be expected to exhibit improved photocatalytic performances, as photoexcited carriers are separated by transferring electrons in the ZnO conduction band and holes in the ZnS valence band (Z-scheme mechanism).17 The photocatalytic activities of the platelets obtained by the different thermal processes were analyzed as described in Section 2. Before irradiation, optical absorption measurements show in all the cases the typical spectrum of MB aqueous solutions, with a maximum absorption at about 664 nm. The presence of different platelets during irradiation gives rise to a reduction of absorbance and a shift of the peak maximum (Figure 6) that originates from the degradation of the MB chromophore and demethylation, respectively.26 The ZnS(en)0.5 precursor is the weakest photocatalyst, since the value of D% ≈ 4% is comparable with that obtained without catalysts (Figure 7). The low activity of ZnS(en)0.5 is consistent with the results of Ni et al.13 and can be explained considering that the number of photons available for carrier photoexcitation is smaller in ZnS(en)0.5 than in ZnS and ZnO platelets because the absorption edge is blue-shifted9 by the quantum confinement effect27 present in the layered organic−inorganic structure. Samples annealed at 400° show degradation efficiencies that decrease when increasing treatment duration. In particular, the relatively high degradation efficiency measured for short

Figure 6. Absorption spectra of the MB aqueous solution after irradiation without any catalyst (○) and in the presence of different catalysts: ZnS(en)0.5 (●), 400 °C-1 h (△), 400 °C-2 h, (▼), 400 °C-2 h (□), 400 °C-50 h (■), and 600 °C-2 h (▲).

Figure 7. Degradation efficiency of MB using the different samples as the catalysts.

treatment durations (D% ≈ 45% after 1 and 2 h) falls to D% ≈ 20% after 24 h treatment and further decreases after 50 h. An increased photocatalytic efficiency was reported for ZnS nanoparticles obtained by annealing at 400 °C for 2 h the ZnS·(piperazine)0.5 complex,28 whose layered organic−inorganic structure strictly resembles that of our ZnS(en)0.5 precursor. In that case, efficiency was ascribed mainly to the removal of organic residuals and increased N doping of ZnS. The same mechanisms can be assumed to originate the relatively high efficiency we observed after short annealing times. The efficiency decrease with increasing annealing time cannot be simply justified by the changes of surface area because SEM (Figure 3) and TEM (Figure 4) measurements show that platelets are compact after 2-h treatment, while they exhibit a granular structure after 24 h. The lack of correlation between surface area and photocatalytic activity was reported by Jang et al.9 for ZnS platelets obtained by treating the ZnS(en)0.5 precursors at different temperatures. They ascribed the degradation efficiency to amorphous ZnS(en)0.5 remnants that contribute to photocatalysis by extending the light absorption range, even if a definite correlation between light absorbance and photocatalytic activity was not found. They also pointed out the coexistence of ZnS and ZnO originated by the replacement of S atoms by O atoms during thermal treatments, but did not discuss the possible influence of the ZnS/ZnO heterojunctions on the photocatalytic efficiency. In our opinion, 6963

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expected to be enhanced by the direct Z-scheme mechanisms, but worsened by misfit dislocations present at the ZnS/ZnO interface and phenomena related to the ZnS coverage by ZnO. Thus in the samples considered here, the balance of these effects results in the D% decreasing with prolonging thermal treatments. This conclusion does not conflict with the enhanced photocatalytic performances reported for ZnS− ZnO systems made of ZnO and ZnS nanoparticles in intimate contact.14,32 Indeed, in those cases, the distribution of ZnS and ZnO can make the direct Z-scheme mechanisms dominant, since the detrimental effects of ZnO on ZnS are important only when ZnS is covered by ZnO. Finally, the most efficient degradation was measured using porous ZnO platelets obtained at 600 °C, as only about 1.5% of MB remained after irradiation. This high D% value can be ascribed not only to the smaller bandgap of ZnO compared to ZnS, but also to the surface increase originated by the sample porosity that guarantees a high surface area-to-volume (S/V) ratio despite platelets being relatively large. When the size of porous ZnO nanosheets is reduced as a consequence of the reduced ZnS(en)0.5 precursor size (sample 600 °C-2h_S), the degradation efficiency slightly decreases to D% ≈ 92%, which is comparable with that measured using commercial ZnO powders. We ascribe this decrease to the tendency of small particles to form agglomerates to reduce the energy associated with the high S/V ratio. Since in large platelets a significant contribution to the surface area originates from porosity, agglomeration may be less pronounced, thus making these nanosheets promising for surface functionalization processes.

on the contrary, these heterojunctions contribute to the decrease of the degradation efficiency we observe with increasing annealing time. Indeed, defects originated at the ZnS/ZnO interface by lattice mismatch (e.g., inset in Figure 5a) may act as recombination centers that limit the carrier lifetime and entangle their transfer across the interface, thus counteracting the beneficial effect of the Z-scheme mechanism (i.e., the simultaneous migration of electrons toward ZnO and holes toward ZnS) originated by the ZnS and ZnO coexistence. As a consequence, we could ascribe the D% decrease observed when annealing time increases (Figure 6) to the prevailing effect of dislocation-related defects over the Z-scheme mechanism. However, such a conclusion would seem to contrast the observation that the heterojunction improves photocatalytic activity in ZnO/ZnS core/shell structures.15,29 We thereby suggest that, even if both the direct Z-scheme mechanism and recombinations due to misfit dislocations are expected to be present at the interface between ZnS and ZnO, no matter if ZnS overlies or underlies ZnO, the two cases show significant differences. Lahiri et al.30 showed that covering ZnO by a submonolayer film of ZnS improves the photocatalytic activity by narrowing the band gap at the surface and modifying other surface properties such as work function and band bending. If that mechanism is adapted to the present case (i.e., ZnO on ZnS), the even partial ZnO coverage could be expected to affect the surface properties (i.e., decrease the work function) in such a way that degradation efficiency worsens. Additionally, the presence of the interface between ZnO and ZnS has other implications on the photocatalytic efficiency of ZnS/ZnO systems, since the number of photons available for ZnS photoexcitation is reduced by a ZnO coating due to its lower bandgap and higher refractive index that favor photon absorption by ZnO and light reflection at the ZnO−ZnS interface, respectively. The influence of the ZnO coating can be evaluated considering that, at the ZnS band gap energy (3.77 eV),1 the ZnO absorption coefficient is31 ∼1.6 × 105 cm−1, so that ∼21% of the incident photons are absorbed by a 15 nm thick ZnO layer, which is a ZnO thickness consistent with TEM observations (Figure 5a). As a consequence, when ZnS is coated by ZnO, a large photon fraction is absorbed in the ZnO layer where the increased carrier concentration enhances direct recombination, which allows photoexcited carriers to recombine before they are separated by the direct Z-scheme mechanism. This phenomenon is more relevant the thicker the ZnO coverage, and is expected to deteriorate the photocatalytic efficiency of our ZnS/ZnO systems. Another effect of the ZnS/ZnO heterojunctions can be devised considering that in our samples ZnO formation starts at the platelet surface and spreads over the whole crystals as the annealing time increases, so that ZnS platelets are coated by ZnO, although not uniformly. Since the coverage increases with annealing time, the exposed ZnS surface as the oxidation site decreases. In these conditions, holes present in the ZnS valence band may contribute to dye degradation only after they reach the platelet surface, and this may happen either by tunnelling through the ZnO layer, or by diffusing to the areas where ZnS is uncovered by ZnO. Prolonging the annealing increases the ZnO coverage and prevents more and more holes from reaching the surface, thus deteriorating the photocatalytic performances of the platelets. In light of the above discussion, we conclude that, due to the coexistence of ZnS and ZnO, the degradation activity is

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CONCLUSIONS The thermal disassembly of the ZnS(en)0.5 hybrid complex in air leads to porous ZnS and ZnO nanosheets via topotactic transformation. Upon annealing at 400 °C, the platelike ZnS(en)0.5 precursors at first transform to ZnS, then ZnO forms so that ZnS and ZnO domains coexist at the nanoscale. A further increase of the annealing temperature to 600 °C transforms the nanosheets into porous ZnO single crystals. The structural and chemical transformation of the nanosheets is accompanied by a significant morphological change from compact to highly porous material, due to the lattice contraction experienced by the newly formed ZnO domains. Photocatalytic activity investigations show that the ZnS platelets obtained by annealing the ZnS(en)0.5 precursor at 400 °C have degradation efficiencies that decrease when annealing time increases. We suggest that the presence of misfit dislocations at the ZnS/ZnO interface and the partial ZnS coverage by ZnO may counteract the direct Z-scheme mechanisms, thus reducing the beneficial effects expected from the ZnS/ZnO heterojunctions. In spite of the large platelet size, the photocatalytic activity of ZnO porous nanosheets is slightly larger than that of commercial ZnO nanopowders, likely due to the large surface area to volume ratio. Porosity and monocrystallinity make these nanosheets very interesting for different applications, especially those where large surface area must be combined with high conductivity,33 such as gas sensors and dye-sensitized solar cells.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +39 0521269214; FAX: +39 0521269206; e-mail: [email protected]. 6964

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Notes

(32) Rabani, J. J. Phys. Chem. 1989, 93, 7707−7713. (33) Suresh, S.; Pandikumar, A.; Murugesan, S.; Ramaraj, R.; Raj, S. P. Sol. Energy 2011, 85, 1787−1793.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the MIST E-R Consortium. REFERENCES

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