Aqueous solution synthesis of SnO nanostructures with tuned optical absorption behavior and photoelectrochemical properties through morphological evolution

Abstract

We have studied the aqueous solution synthesis of divalent tin oxide (SnO) nanostructures, changes in their optical absorption behavior, and their photoelectrochemical properties. A number of SnO nanostructures including sheets and wires, and their composite morphologies were obtained in aqueous solution containing urea at low temperatures. Parallel control of both oxidation state and morphology was achieved through the urea-mediated solution process. Nanoscale morphological variation facilitated changes in optical absorption behavior and the generation of a photocurrent. As for the nanostructured SnO, the absorption of visible light decreased and absorption in UV region increased. In contrast, bulk black SnO crystals showed strong absorption over the entire range of UV to visible light. A photocurrent was generated from the SnO nanostructures with irradiation of UV and visible light.

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Introduction

Aqueous solution synthesis, a bottom-up approach, is a promising route for the preparation of functional nanomaterials under ambient conditions near room temperature.¹ A wide variety of fine nanostructures can be obtained through aqueous solution processes because thermal treatments resulting in collapse of nanostructures with sintering are not required.^1–6 Fine tuning of nanostructures through aqueous solution routes will lead to the development of functional materials with new emergent properties. Here, we have synthesized a number of divalent tin monoxide (SnO) nanomaterials including sheets, wires, and their composite morphologies in an aqueous solution containing urea. The resultant SnO nanostructures possessed remarkable changes in their optical absorption behavior and photoelectrochemical properties through morphological evolution. Morphological control exerts an effect on the optical absorption behavior and photoelectrochemical properties of the metal oxide nanostructures.

Metal oxides with low-dimensional nanostructures such as zero, one, and two dimensions have been synthesized through solution processes.^6–13 As well as morphological control, one of the next important challenges is control over oxidation states and polymorphs of metal oxides. In nature, living organisms form biominerals with controlled oxidation states and polymorphs through aqueous solution processes.^14–17 If we can achieve fine control of both nanostructures and their oxidation states in an aqueous solution, functional nanomaterials with new properties will be developed under mild conditions. Tin oxides have two oxidation states including divalent SnO and tetravalent tin dioxide (SnO₂).^18–36 A variety of SnO₂ nanostructures have been synthesized for applications such as anode materials for lithium-ion batteries, gas sensors, and photoelectrochemical devices.^18–27 In contrast, the synthesis and applications of SnO have not been as fully studied,^28–34 even though previous works have suggested SnO would have a high theoretical reversible capacity for lithium-ion batteries and a high hole mobility as a p-type semiconductor.^35,36 It is not easy to synthesize SnO crystals through solution processes because the divalent tin ion (Sn²⁺) is easily oxidized to the tetravalent state. Our target here is the selective synthesis and morphological control of SnO nanostructures leading to the development of new properties. We previously reported the formation of a SnO nanosheet thin film through an aqueous solution process.³⁷ In the present work, a number of SnO nanostructures including sheets, wires, and their composite morphologies have been synthesized with the change in urea concentration in an aqueous solution at low temperature. Moreover, we have found interesting changes in the optical absorption behavior and photoelectrochemical properties originating from the nanostructures.

Electron transitions in semiconductor materials include direct and indirect transitions from valence to conduction bands.^38,39 These electron transitions involve optical and electronic properties. Direct-bandgap materials allow the minimum energy transition of an electron from the valence to the conduction band without a change of wave vector (k). On the other hand, indirect electron transitions require a change of k with the assistance of a phonon. In general, indirect-bandgap semiconductors are not as suitable for many applications compared to direct-bandgap materials because of their low efficiency in excitation and recombination processes. If we can tune the optical absorption behavior through morphological control on the nanoscale, new functions can be generated. For example, Si is a typical indirect-bandgap semiconductor having a direct bandgap of 3.4 eV and an indirect bandgap of 1.1 eV in bulk crystals.⁴⁰ Bulk Si absorbs the entire range of visible light based on its indirect bandgap. In contrast, it has been reported that a variety of Si nanostructures show significant changes in their optical absorption.⁴¹ As for nanostructured Si, absorption in visible-light region decreases and absorption in UV-light region becomes apparent. These changes in optical absorption behavior lead to the emission of photoluminescence from Si nanostructures in the visible-light region.⁴² SnO is a semiconductor having an indirect bandgap of 0.7 eV (corresponding wavelength of the absorption edge: 1770 nm) and a direct bandgap of 2.7 eV (corresponding wavelength of the absorption edge: 460 nm).^36a,b A powder of bulk SnO crystals is black due to its indirect bandgap in the visible-light region. The strong absorption of visible light originating from its indirect bandgap limits the potential of SnO crystals for photoelectrochemical applications. If we can achieve fine control of SnO nanostructures, new properties and functions can be developed as observed in Si nanomaterials. In general, it is not easy to tune the optical absorption behavior of metal oxide semiconductor nanomaterials by changing their crystal morphologies. In the present work, the optical absorption behavior of SnO nanostructures was changed with morphological variation including sheets, wires, and their composite structures, while SnO microcrystals are black with absorption over the entire range of visible light. Along with the changes in the morphology and optical absorption, photocurrent was generated by the SnO nanostructures with irradiation of light.

Results and discussion

Aqueous solution synthesis of SnO nanostructures

A precursor aqueous solution containing 50 mM divalent tin fluoride and 1 to 5 M of urea was maintained for 24 h at 60 °C. The precipitates were washed and collected by centrifugation. All the X-ray diffraction (XRD) patterns of the resultant precipitates agree with those of tetragonal SnO (Fig. 1).

Fig. 1 XRD patterns of the resultant precipitates prepared with 1 M (A), 2 M (B), 4 M (C), and 5 M (D) of urea in the precursor solutions.

As the concentration of urea increased from 1 M to 5 M, the morphologies were changed from nanoscale sheets and wires to composite structures (Fig. 2). Nanosheets ca. 10 nm in thickness and ca. 500 nm in width were obtained from the precursor solution containing 1 M of urea [Fig. 2(a) and (b)] As the concentration of urea increased up to 2 M, the morphology changed to that of nanowires ca. 50 nm in thickness, 200–500 nm in width, and several micrometres in length [Fig. 2(c) and (d)]. A further increase in urea concentration to 4 M led to the formation of composite structures consisting wires with attached sheets [Fig. 2(e) and (f)]. The thickness and size of the attached sheets on the composite structures were increased at a urea concentration 5 M [Fig. 2(g) and (h)]. In this way, nanostructured SnO crystals were selectively obtained through a urea-mediated aqueous solution process.

Fig. 2 FE-SEM images of the SnO nanostructrures: (a), (b) nanoscale sheets prepared with 1 M of urea; (c), (d) wires formed with 2 M of urea; (e)–(h) composite structures of the wires and attached sheets obtained at urea 4 M (e), (f) and 5 M (g), (h).

The SnO nanosheets were surrounded by large (001) faces. The intensified peak of the (001) plane on the XRD patterns indicates that the nanosheets exhibiting the large (001) faces were preferentially arranged parallel to the sample holder (Fig. 1A). The diffraction spots corresponding to the (110) and (200) planes were observed on the selected-area electron diffraction (SAED) patterns of the field-emission transmission electron microscopy (FE-TEM) images [Fig. 3(a)]. The lattice fringes corresponding to two pairs of (110) planes were observed on the high-resolution transmission electron microscopy (HRTEM) images and its fast Fourier-transform spectrum [Fig. 3(b)]. SnO nanosheets formed on a glass substrate also showed the large (001) faces in our previous study.³⁷ These results suggest that the SnO nanosheet is a single crystal surrounded by large (001) faces. The SAED pattern of the nanowires shows the diffraction spots assigned to the (110) and (101) planes [Fig. 3(c)]. The lattice fringes of the (110) planes were observed parallel and perpendicular to the edge of the nanowires [Fig. 3(d)]. Based on the FE-TEM analyses, the SnO nanowire is a single crystal exhibiting (001) faces and elongated in the <110> direction [Fig. 3(c) and (d)]. The composite structures are formed with the combination of the wires and the attached sheets having the same crystallographic orientation [Fig. 2(e)–(h)]. Residual urea and the related products were not clearly detected by Fourier-transform infrared spectroscopy (FT-IR) and thermogravimetry (TG) (Fig. S1 of the ESI†). Therefore, we used these SnO nanostructures for the characterization of their optical absorption behavior and photoelectrochemical properties.

Fig. 3 FE-TEM analyses of the SnO nanosheets (a), (b) and nanowires (c), (d). (a), (c) Bright-field FE-TEM images with SAED patterns (inset). (b), (d) HRTEM images with FFT spectra (inset).

Morphological evolution of SnO nanostructures

Fig. 4 shows the time-dependent morphological evolution of the SnO nanostructures at a urea concentration of 5 M. At this condition, composite morphologies of the nanoscale wires and the attached sheets were formed after 24 h [Fig. 2(e) and (f)]. In the initial stage of crystal growth, comb-like morphologies of regularly arranged nanowires ca. 5 μm in length and ca. 100 nm in width were grown from the sheet-like morphologies after deposition for 6 h [Fig. 4(a) and (b)]. Then, elongated and dispersed nanowires ca. 20 μm in length and ca. 100 nm in width were observed after the deposition for 12 h [Fig. 4(c) and (d)]. After 24 h, composite structures were formed through deposition of the attached sheets onto the wires [Fig. 4(e) and (f)].

Fig. 4 Time-dependent morphological evolution of the SnO nanostructures at a urea concentration of 5 M. (a), (b) A comb-like morphology consisting of the sheets (dotted square) with anisotropic growth of wires formed after deposition for 6 h. (c), (d) The nanowires prepared after deposition for 12 h.

In the present study, urea plays multiple roles in the selective synthesis of SnO nanostructures with controlled oxidation state and morphology. Since the solubility of SnO is related to the pH of the precursor solution,^34a,36 the gradual increase in pH resulting from the decomposition of urea, changes of the degree of supersaturation.³⁷ The coordination of urea and/or related compounds to Sn²⁺ also influences the degree of supersaturation and the inhibition of oxidation from Sn²⁺ to Sn⁴⁺. The specific adsorption of urea and/or related compounds on the crystal faces contributes the changes of morphology.

Based on the results in Fig. 2 and 4, the morphological variation can be summarized as shown in Fig. 5. The morphological variation with increasing deposition time would be similar to that with increasing the initial concentration of urea in the precursor solution (Fig. 5). In both cases, the changes of pH, changing the degree of supersaturation, provide the same growth conditions in the solution. The low initial concentration of urea and the initial stage of the deposition provide low-pH conditions leading to crystal growth under moderate supersaturated conditions. Likewise, high-pH conditions are induced by the higher initial concentration of urea and the longer deposition time.

Fig. 5 Schematic models for morphological evolution of the SnO nanostructures with the changes of initial urea concentration and deposition time. (a) A nanosheet exhibiting (001) faces. (b) The anisotropic growth of the wires in the <110> direction from the nanosheets under diffusion-controlled conditions. (c) A nanowire formed through further growth. (d) Composite structures through deposition of the nanosheets onto the wires.

In the urea-mediated deposition processes, crystalline SnO is deposited prior to oxidation from Sn²⁺ to Sn⁴⁺ by oxygen dissolved in the precursor solution.^34a,43 It is inferred that the coordination of urea and related compounds inhibits oxidation in the solution. SnO nanosheets exhibiting (001) faces are obtained under low supersaturated conditions [Fig. 2(a) and (b) and 5(a)]. The formation of a 2D morphology is associated with the selective adsorption of urea and/or its decomposition product on the (001) face of SnO crystals. It has been reported that the formation of SnO microcrystals surrounded by (001) faces is ascribed to the slower growth rate of the (001) faces rather than that of the other faces.^31,34a The anisotropic growth from the nanosheets leads to the formation of nanowires under the higher supersaturated condition through the formation of the comb-like morphologies [Fig. 2(c) and (d), 4(c) and (d), and 5(b) and (c)]. The anisotropic growth of the wires from the sheets is ascribed to the instability of the growing surface under diffusion-controlled conditions at the higher degree of supersaturation.^44,45 After the formation of the wires, the growth condition is changed to that promoting the formation of the attached sheets because the consumption of Sn²⁺ for the growth of wires lowers the degree of supersaturation. Therefore, the formation of the sheets on the wires provides a composite morphology [Fig. 2(e) and (f) and 5(d)]. In the present work, urea, a simple organic molecule, plays multiple roles in the parallel control of oxidation states and morphologies in the solution process at low temperature. The urea-mediated process could be applied to the synthesis of other functional metal oxides for controlled oxidation states and morphologies.

Optical absorption and photoluminescence of SnO nanostructures

Interestingly, the resultant SnO nanostructures possess remarkably different optical absorption from the bulk material through morphological variation (Fig. 6). UV–vis absorption spectra of the SnO nanostructures were measured using a diffuse-reflectance method. As a reference, commercial black SnO microcrystals absorb the entire range of visible light (sample E in Fig. 6). In contrast, the SnO nanosheets ca. 10 nm in thickness show a weakened absorption in the visible-light region and strong absorption at wavelengths shorter than 460 nm (Fig. 2(a) and (b) and sample A in Fig. 6). The morphological variation from the SnO nanosheets to the nanowires leads to an increase in absorbance in the visible-light region (Fig. 2(c) and (d) and sample B in Fig. 6). The composite nanostructures of the wires and the attached sheets show a slight increase in absorbance in the visible-light region (Fig. 2(e) and (f) and sample C in Fig. 6). When thicker and larger attached sheets were grown on the nanowires, a further increase in absorption was observed in the visible-light region (Fig. 2(g) and (h) and sample D in Fig. 6). The absorbance in the visible-light region is increased as the SnO nanostructures form larger architectures (Fig. 2 and 6). The optical absorption behavior including the color changes remarkably with morphological variation of the SnO nanostructures. We observed similar changes of optical absorption behavior by the other experimental demonstration (Fig. 7). A powder of SnO nanosheets was compacted into a pellet. The pellet was gray to the naked eye. The diffuse-reflectance spectrum of the pellet shows weakened absorption in the visible-light region (Fig. 7), even though the thick nature of the pellet causes an increase in absorption of the baseline. The pellet consisting of SnO nanosheets was sintered to ensure grain growth for 1 h at 400 °C in an argon atmosphere. The sharpened peak in the XRD pattern suggests that grain growth proceeded after the sintering (Fig. S2 of the ESI†). The color of the pellet turned to dark brown and the absorbance in the visible-light region increased after the sintering (Fig. 7).

Fig. 6 Macroscopic appearance (a) and diffuse-reflectance UV–vis spectra (b) of the SnO nanostructures prepared using: (A) 1 M urea (nanosheets); (B) 2 M urea (nanowires); (C) 4 M urea (composite structures); (D) 5 M urea (composite structures) and (E) commercial microcrystals.

Fig. 7 Macroscopic appearance and diffuse-reflectance UV–vis spectra of the pellet consisting of SnO nanosheets (A) and the sintered material (B).

The photoluminescence (PL) spectra of the SnO nanostructures were measured with UV-light excitation at 254 nm (Fig. 8). The PL spectra of the SnO nanostructures including sheets and wires are different from that of bulk crystals. While bulk SnO crystals show a broadened emission band from 300–400 nm (spectrum C in Fig. 8), different emission spectra was observed for the SnO nanostructures including sheets and wires (spectra A and B in Fig. 8). These PL spectra imply that the electronic transitions in the SnO nanostructures are different from those in bulk SnO crystals.

Fig. 8 PL spectra of SnO nanosheets (A), nanowires (B), and microcrystals (C).

These results indicate that the tuning of optical absorption properties can be ascribed to the formation of different SnO nanostructures. A powder of SnO microcrystals is black originating from the strong absorption of its indirect bandgap in the visible-light region (Fig. 6). In contrast, the absorption of visible light is weakened in the SnO nanostructures (Fig. 6). To the best of our knowledge, similar tuning of optical absorption based on morphological control has not been observed in metal oxides. In the present work, the changes in the optical absorption could be explained by the following three models. As proposed in Si nanostructures, the quantum-confinement effect in SnO nanostructures induces a blue-shift of the absorption band. In the other model, the optical absorptions based on the direct and indirect bandgaps are changed by the size effect through morphological evolution. As the third model,⁴¹ the morphological variation results in changes of the optical path length for the measurements of absorbance. Further studies including theoretical and computational approaches are required for the understanding of this behavior.

Photocurrent generation in SnO nanostructures

The changes in optical absorption behavior inspired us to study the photoelectrochemical properties of the SnO nanostructures. A cathodic photocurrent was generated from the SnO nanostructures in response to irradiation using UV and visible light (Fig. 9). In contrast, a cathodic photocurrent was never detected on SnO microcrystals under the same conditions. The intensity of the photocurrent is related to the UV–vis absorption spectra as shown in Fig. 6. The photocurrent increased with the morphological changes leading to a decrease of absorbance in the visible-light region (Fig. 9). Therefore, the largest cathodic photocurrent was obtained on the SnO nanosheets having the weakest absorbance in the visible-light region (sample A in Fig. 9). As the absorption in the visible-light region increased through morphological variation (Fig. 6), the photocurrent decreased relative to that of the nanosheets (samples B and C in Fig. 9). It is inferred that the decay of photocurrent over time is caused by the presence of rate-determining steps in the reaction processes involving the photocurrent generation. The rate-determining steps lead to the recombination of the photoexcited electrons and holes. Therefore, the current density gradually decreased with time. Based on our previous work,³⁷ the photocurrent originates from absorption at wavelengths shorter than about 500 nm corresponding to the direct bandgap energy of bulk SnO crystals. In this way, photocurrent generation has been achieved by the morphological evolution of SnO nanostructures. Based on the approach in the present study, photocurrent may be generated from the other metal oxide nanostructures.

Fig. 9 Photocurrent measurement of the SnO nanosheets (A), nanowires (B), composite structures (C), and microcrystals (D) with UV and visible light under application of −0.5 V vs. Ag/AgCl.

Conclusions

SnO nanostructures including sheets, wires, and their composite architectures were synthesized in an aqueous solution at low temperature. Urea-mediated synthesis facilitated the parallel control of their oxidation states and morphologies. The optical absorption behavior in the visible-light region decreased with formation of the SnO nanostructures while bulk SnO crystals absorb the entire range of visible light. PL measurements imply changes of electronic transition behavior in the SnO nanoarchitectures. Photocurrent generation was observed on the SnO nanostructures with irradiation of UV and visible light. These results suggest that fine control over the nanostructures leads to the emergence of optical absorption behavior and photoelectrochemical properties. This approach could be applied to other metal oxides.

Experimental

Preparation of the SnO nanostructures

A stock solution containing 1–5 M of urea (CH₄N₂O, Junsei Chemical, 99.0%) and 100 mM of stannous fluoride (SnF₂, Wako Pure Chemical, 90%) was prepared with 40 cm³ of purified water in poly(propylene) bottles. The sample bottles were maintained at 60 °C for 24 h. The resultant precipitates were centrifuged and rinsed with purified water. Then, the resultant powders were dried at 60 °C in air. Commercial SnO powder was used as a microcrystal reference (Wako Pure Chemical, 99.9%).

Characterization

The crystal phases of the resultant precipitates were identified by using a XRD (Bruker D8 Advance and Rigaku miniFlex II with Cu Kα radiation). The morphologies of the resultant materials were observed using an FE-SEM (FEI Sirion and Hitachi S-4700 operated at 2.0 or 5.0 kV) and an FE-TEM (FEI, Tecnai F20 operated at 200 kV). The remaining organic molecules in the nanosheets were analyzed by FT-IR (Bruker, FT-IR ALPHA) and TG (Seiko, TG-DTA 6200).

Optical absorption and photoluminescence spectroscopy

The optical absorption of the SnO nanostructures was analyzed using a UV–vis absorption spectrophotometer (JASCO, V-560). For the SnO nanostructures and microcrystals (Wako Pure Chemical, 99.9%), diffuse reflectance spectra were obtained by using an integrating unit measurement. PL spectra were observed using a spectrofluorophotometer at room temperature (JASCO, FP-6500) with UV-light excitation at 254 nm.

Photoelectrochemical properties

The working electrode for the photocurrent measurement consisted of 90 wt% SnO nanosheets and 10 wt% poly(tetrafluoroethylene) was pasted on a nickel mesh which served as a current collector. The photocurrent was measured in 1 M of LiClO₄ in ethylene carbonate (EC) and diethylene carbonate (DEC) (EC:DEC = 1 : 1 by volume). The counter and reference electrodes were platinum and Ag/AgCl, respectively. A xenon lamp was used as the light source.

Acknowledgements

This work was partially supported by Grant-in-Aid for Scientific Research (B, 20360302) (HI) and Young Scientists (A, 22685022) (YO) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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Footnote

† Electronic supplementary information (ESI) available: FT-IR spectra and TG curves of the resultant SnO nanostructures, XRD patterns before and after the sintering of the SnO nanosheet pellets. See DOI: 10.1039/c0nr00370k

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