Nanofilms
Controllable Fabrication and Photoelectrochemical Property of Multilayer Tantalum Nitride Hollow Sphere-Nanofilms Rui Gao, Linfeng Hu, Min Chen, and Limin Wu*
Fabrication of two- and three-dimensional ordered architectures in thin-film form using low-dimensional nanostructures as building blocks is a burgeoning field in the past decades, because these ordered film materials are expected to have the enhanced even novel properties and potential applications in catalysts,[1] gas sensors,[2] optical devices,[3] electronics,[4] opto-electronic devices,[5] surface-enhanced Raman scattering devices,[6] solar cells,[7] high-performance display units,[8] capacitors, and transistors.[9] Many physical and chemical methods have been developed to fabricate these ordered superstructures, including electron-beam lithography, micro-contact printing, and selfassembly of colloidal particles.[10] Recently, an oil-water interfacial self-assembly is considered an ideal strategy for the assembly of various low-dimensional nanostructures into nanofilms.[11] The low-dimensional nanostructures are well dispersed in water, and then an oil phase is added to form an oil-water interface. After the addition of an appropriate amount of inducer, the decreased interfacial energy causes the nanostructures to self-assemble into closely packed monolayer nanofilms at the interface.[12] Very recently, we have developed this self-assembly procedure to fabricate monolayer nanofilm-based devices, such as ultraviolet (UV)light photodetectors and electrical resistive switching memory devices.[5,13,14] The simplicity of the interfacial assembly technology significantly decreases the cost of fabrication, and the high quality of the as-assembled monolayer films can ensure that the performance of the nanodevices exceeds that of those constructed from individual nanostructures. Recent studies have shown that the multilayer assembly of nanostructures is an effective means of creating films with enhanced or even novel, useful properties.[15] However, the big challenge is growing high-quality multilayer nanofilms with uniform coverage ratios and smooth surfaces suitable for use in optoelectronic and microelectronic devices. More recently, we successfully fabricated two-layer semiconducting
Dr. R. Gao, Dr. L. Hu and Dr. M. Chen, Prof. L. Wu Department of Materials Science and State Key Laboratory of Molecular Engineering of Polymers Fudan University Shanghai 200433, China E-mail: [email protected] DOI: 10.1002/smll.201303873 small 2014, DOI: 10.1002/smll.201303873
ZnS/ZnO hollow sphere nanofilms through oil-water interfacial self-assembly strategy with organic/inorganic core-shell composite particles as building blocks and subsequent calcination. We observed markedly enhanced photoresponsive behavior of the two-layer-nanofilm-based nanodevice as compared with the corresponding monolayer-based one.[16] However, hollow sphere-nanofilms fabricated by more than 2 layers with precisely controllable numbers of layer cannot be fabricated using this method due to the instability and inhomogeneity of core-shell composite particles or by any other methods. Although the preparation of nanofilms with precisely tunable numbers through the self-assembly strategy has been reported,[17] the fabricated nanofilms were mainly inverse-opal structured, it is impossible to use these methods reported so far to fabricate hollow sphere-based nanofilms with precisely tunable layer numbers. In this communication, we have for the first time successfully fabricated hollow sphere-nanofilms with precisely tunable numbers of layers using Ta3N5 hollow sphere-nanofilms as an example, through the oil-water interfacial selfassembly combined with the control of the sol-gel reaction of precursors. Ta3N5 is an important photocatalyst suitable for solar hydrogen production because it has a narrow band gap of 2.1 eV, making it suitable for the collection of more than 45% of incident solar energy.[18] And the photoelectrodes based on Ta3N5 thin films, nanoparticles, nanotube arrays and nanorod arrays have displayed excellent visible-light activity for photoelectrochemical (PEC) water splitting.[19] The present Ta3N5 hollow sphere-nanofilms based photoelectrodes exhibited significantly enhanced visible-light water splitting ability and superior stability, and the PEC performance of the as-obtained hollow sphere-nanofilms was found to be highly dependent on the numbers of layers. Scheme 1 summarizes the fabrication procedures of multilayer Ta3N5 hollow sphere-nanofilms. First, well-defined poly(styrene-co-acrylic acid) (PSAA) colloidal particles were self-assembled on smooth substrates using oil-water interfacial self-assembly strategy to form polymer nanofilms.[5,11–14] Because of the monodispersibility and stability of polymer colloidal particles, high-quality multilayer polymer particle nanofilms were more easily controlled than organic/inorganic core-shell composite particles as building blocks.[16] Tantalum ethoxide ethanol solution was then dipping-infiltrated into the voids between PSAA colloidal particles by capillary force and its hydrolysis and condensation reactions
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Scheme 1. Schematic illustration for the formation of multilayer Ta3N5 hollow sphere- and inverse opal- nanofilms.
were performed during drying. The strong interactions between polymer particles and the inorganic phase are very important. Because the monodisperse PSAA colloidal particles were composed of hydrophobic PS-enriched cores and carboxylic-acid-group-enriched shells, they quickly captured small Ta2O5 nanoparticles derived from the sol-gel reaction of tantalum ethoxide.[1] The sol-gel reaction conditions of tantalum ethoxides during room-temperature drying are also crucial. Well-defined PSAA/Ta2O5 core-shell sphere nanofilms were produced at high humidity (80% or higher). Accordingly, Ta3N5 hollow sphere-nanofilms with precisely tunable layers were easily fabricated through calcination in ammonia (Route A in Scheme 1). However, at low humidity (e.g., 10% or less), the Ta2O5 sols formed closely packed three-dimensional skeletons due to the low rate of the sol-gel reaction. After calcination, Ta3N5 monolayer- and multilayerordered pore structures (inverse opal-nanofilms) were produced (Route B in Scheme 1). Figure 1a–c shows typical scanning electron microscopy (SEM) images of the as-obtained PSAA colloidal particles-, PSAA/Ta2O5 core-shell spheres-, and Ta3N5 hollow spheresmonolayer nanofilms, respectively. The monodisperse PSAA colloidal particles which had an average diameter of about 280 nm, were assembled into a high-quality nanofilm consisting of hexagonally packed spheres, although few microcracks appeared due to the decrease in the hydrodynamic size of the polymer particles (Figure 1a). During dried at high humidity, the surfaces of the polymer particles were coated with Ta2O5 through the interactions between the carboxylic groups of polymer particles and the hydroxyl groups of inorganic nanoparticles, forming PSAA/Ta2O5 core-shell spheres with a mean diameter of 303 nm and core-shell sphere-nanofilms
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(Figure 1b). Fourier transform infrared (FTIR) spectrum clearly indicated a peak at 1540 cm−1 for carboxylate groups at the cost of carboxylic groups (at 1704 cm−1) and the typical broad peak at 624 cm−1 for Ta2O5 (Figure S1), indicating strong interactions between the surface carboxyl groups and the hydroxyl groups of Ta2O5. High humidity caused fast hydrolysis and condensation reactions of tantalum ethoxide, yielding large numbers of Ta2O5 nanoparticles, which are quickly captured by the carboxylic-group-enriched surfaces of the PSAA colloidal particles, forming complete core-shell sphere nanofilms. This film was subjected to calcination in ammonia at 1100 °C for 10 h, producing Ta3N5 hollow sphere-nanofilms, with about 10% shrinkage of the sphere diameter (Figure 1c). Well-maintained Ta3N5 hollow sphere based nanofilms with different sizes were also produced by simply controlling the sizes of the template spheres, as shown in Figure S2. Figure 1d shows the hollow structures and porous shells of the Ta3N5 spheres. The shells are 11-nm thick. N2 adsorption-desorption (Figure 1g) indicates that the hollow sphere samples have a high surface area of 37 m2 g−1 and unique meso-microporous shell structures due to the profound decrease in volume during the transformation from Ta2O5 to Ta3N5.[20] The selected area electron diffraction (SAED) pattern (Figure 1e), indicates the polycrystalline nature of the Ta3N5 hollow spheres. In addition, the high-resolution transmission electron microscopy (HRTEM) image clearly shows the (002) atomic planes with a lattice spacing of 0.51 nm (Figure 1f). This suggests that the samples had good crystallinity, which is important for their photoelectrochemical properties. In the X-ray diffraction (XRD) pattern (Figure 1h), all the reflections can be readily indexed to orthorhombic Ta3N5 phase (JCPDS 79–1533)
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small 2014, DOI: 10.1002/smll.201303873
Controllable Fabrication and Photoelectrochemical Property of Multilayer
Figure 1. Typical SEM images of (a) PSAA colloidal particles-, (b) PSAA/Ta2O5 core-shell spheres-, and (c) Ta3N5 hollow spheres, (d) Ta3N5 hollow sphere-nanofilms at higher magnification. The inset shows a TEM image of a single Ta3N5 hollow sphere with a scale bar of 100 nm. (e and f) Corresponding SAED pattern and HRTEM image of Ta3N5 hollow spheres. (g, h and i) Nitrogen adsorption-desorption isotherm, XRD and EDX patterns of the sample in Figure 1c.
with a high intensity, indicating the high purity and good crystallinity of the products. EDX measurement, combined with the XRD results, show the atomic ratio of Ta/N to be about 3:5 (Figure 1i). However, during low-humidity drying, the Ta2O5 sols due to the suppressed sol-gel reaction of tantalum ethoxide, tended to form closely packed three-dimensional skeletons by filling the voids and produced monolayer Ta3N5 inverse opal-nanofilm after nitridation (Figure S3). Several layers of polymer nanofilms were fabricated easily due to the stable, monodisperse PSAA colloidal particles, accordingly, the Ta3N5 hollow sphere-nanofilms were produced easily, and the number of layers was precisely controllable, as shown in Figure 2. For the sake of comparison, the Ta3N5 inverse opal-nanofilm with 4 layers was also fabricated and shown in Figure 2. This method could also be extended to the fabrication of other multilayer semiconductor hollow sphere-nanofilms, such as multilayer TiO2 hollow spherenanofilms, as shown in Figure S4. Figure 3a shows the current-potential curves of the photoelectrodes based on Ta3N5 hollow sphere-nanofilms of 4 layers nitrided at different temperatures in an aqueous 0.5 M Na2SO4 solution (pH = 13) under chopped visible small 2014, DOI: 10.1002/smll.201303873
light irradiation (>400 nm). Before the PEC measurements, all the electrodes were modified with IrO2, which is usually used as an effective co-catalyst for water oxidation to improve charge separation and to decrease the overpotential for corresponding redox processes.[21] This modification indeed significantly enhanced the photocurrent of the Ta3N5 hollow sphere-nanofilm electrodes (Figure S5a). The Ta3N5 hollow sphere-nanofilms nitrided at 1100 °C exhibited a high and fast photoresponsive anode current (17.9 mA cm−2 at 0.65 V vs. Ag/AgCl and 5.2 mA cm−2 at 1.21 V vs. RHE, respectively), indicating that the Ta3N5 hollow sphere- nanofilms are very sensitive to visible light and that the contacts between the nanofilms and the Ta substrates and between the spheres inside the nanofilms are very good, facilitating the electron transport within the photoelectrodes.[19c,f] The incident photon-to-current conversion efficiency (IPCE) recorded at 1.23 V vs. RHE for the Ta3N5 hollow spherenanofilms with 4 layers were about 43.5% at 470 nm. Both the photocurrent density and the IPCEs show that the asobtained Ta3N5 hollow sphere-nanofilms based photoelectrodes occupy superior PEC performance to those previously reported on other Ta3N5 (Table 1), and produce a violent gas
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Figure 2. SEM images of Ta3N5 hollow sphere-nanofilms with different layers: (a) bilayer, (b) trilayer, (c) four-layer, (d) five-layer, and (e) eight-layer nanofilms. (f) SEM image of Ta3N5 inverse opal-nanofilms with 4 layers. The scale bars are 500 nm.
bubbles at atmospheric pressure by the illumination of visible light (Figure S5b). The nitridation temperature has a strong influence on the PEC performance of Ta3N5 hollow sphere-nanofilms. The photocurrent density of the hollow sphere-nanofilms nitrided at 1100 °C is 6 and 1.6 times of those nitrided at 700 °C and 900 °C, respectively. This result is consistent with that of the Ta3N5 nanorod arrays reported previously.[19c] Increase of the nitridation temperature from 700 to 1100 °C, not only remarkably decreased the resistance of the Ta3N5 electrodes, but also greatly improved the crystallinity of the Ta3N5 hollow spherenanofilms, as shown in Figure S6 and S7. It is also interesting to note that the apparent photocurrent increase was achieved for the whole applied potential region, while the abrupt photocurrent increase was observed only at high potential region for the Ta3N5 nanorod arrays.[19c] This may be ascribed to the porous hollow sphere array structure, which offers large effective surface area in close proximity with the electrolyte, thus facilitating diffusive transport of photogenerated holes to oxidizable species in the electrolyte.[22] However, further increasing the temperature to 1200 °C, decreased the photocurrent density. This is probably attributed to the decrease of surface area and distortion of hollow sphere structure.[23] The photocurrent was found to be strongly dependent on the numbers of layers and nanostructures of Ta3N5 nanofilms. Figure 3b shows the photocurrent-potential curves for Ta3N5 hollow sphere-nanofilms with different numbers of layers. The monolayer Ta3N5 hollow sphere-nanofilm showed a photocurrent density of 6.1 mA·cm−2 at 0.65 V. As the number of layers increased, the photocurrent densities of the Ta3N5 hollow sphere-nanofilms first increased because of the increased light adsorption.[19f] With the further increase of the layer number, the recombination of photogenerated electrons and holes became more apparent because of the
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decreasing electrical field and increasing film resistance.[19a,22] This caused a decrease in photocurrent. Maximum photocurrent density was detected at 4 layers of hollow sphere-nanofilm, showing ca. 17.9 mA·cm−2 at 0.65 V as discussed above. This photocurrent density was 4.2 and 1.7 times as high as that of Ta3N5 thin films with the same thickness (Figure S8) and the Ta3N5 inverse opal- nanofilms with the same layer number (Figure 2f), respectively (Figure 3c). The reason might be due to the different nanostructures among these photoelectrodes. For the Ta3N5 thin films, the photogenerated carriers should travel a long distance before reaching the surface active sites, which increases the bulk charge recombination and thus limits the photoactivity.[19c] However, for the Ta3N5 hollow sphere-nanofilms, the thin shell thickness (about 11 nm) can offer a short distance for the photoexited charges to travel to the surface. Moreover, the high surface area provides a high density of active sites for photocatalytic reaction, and the porous hollow structure is beneficial for light harvesting through multireflection and scattering, thus enabling more photons be captured for photocatalytic reaction. Therefore, the PEC performances of the Ta3N5 hollow sphere-nanofilms are far superior to those of Ta3N5 thin films. Because the fabrication process of both Ta3N5 hollow sphereand inverse opal- nanofilms is nearly the same, differing only in drying conditions, no obvious differences in film quality, such as the quality of crystal structure and surface area were detected in the two types of Ta3N5 nanofilms (Figure S3c and S3d). In this way, the superior PEC activity of Ta3N5 hollow sphere-nanofilms over Ta3N5 inverse opal-nanofilms can be attributed to the fact that the hollow sphere films have much greater visible-light harvesting efficiency, as shown in Figure S9a. The Ta3N5 inverse opal-nanofilms have an open, continuous void structure, while the Ta3N5 hollow-sphere nanofilms have interior holes sealed by the porous shells.
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small 2014, DOI: 10.1002/smll.201303873
Controllable Fabrication and Photoelectrochemical Property of Multilayer
Figure 3. Photocurrent vs. potential curves. (a) The Ta3N5 hollow sphere-nanofilms with 4 layers nitrided at different temperatures. (b) The Ta3N5 hollow sphere-nanofilms with different numbers of layers nitrided at 1100 °C. (c) The comparison between the Ta3N5 hollow sphere-nanofilms (1), Ta3N5 inverse opal-nanofilms (2), and IrO2-Ta3N5 thin films (3), the inset is the schematic illustration of multireflections inside the Ta3N5 hollow sphere-nanofilms. (d) The current-time evolution measured at 0.1 V vs. Ag/AgCl at pH 13 for the IrO2-modified Ta3N5 hollow sphere nanofilms (1), the IrO2-modified Ta3N5 thin films (2) and the bare Ta3N5 hollow sphere nanofilms (3).
The gaps between the Ta3N5 hollow spheres in the nanofilms and large interior voids in the porous hollow spheres may enhance multiplereflection and scattering of light,[24] as illustrated in the inset of Figure 3c, which allows the Ta3N5 hollow sphere-nanofilms to harvest more visible light than inverse opal-nanofilms with the same number of layers. Thus the Ta3N5 hollow sphere-nanofilms exhibit better PEC properties than Ta3N5 inverse opal-nanofilms under visible light irradiation, although fabricating inverse opal array structures
are widely considered as an effective way of enhancing the photocatalytic activities of semiconductor-based photoelectrodes because of their high light harvesting capacity and decreasing recombination with photoexcited charges.[1,7,25] The photochemical stability of the photoelectrode is another key factor in water splitting, besides a high photocatalytic activity. Figure 3d shows the current-time evolutions of the Ta3N5 hollow sphere-nanofilms compared with the Ta3N5 thin films. After modification with IrO2, Ta3N5
Table 1. Comparison of the critical parameters of the present Ta3N5 hollow sphere-nanofilm- and other Ta3N5-based photoelectrodes. Photoelectrodes
Photocurrent density (mA cm−2)
Light source
Light intensity (mW/cm2)
IPCE (%)
References
Ta3N5 nanotubes
_
_
_
5.3 (at 450 nm)
[19a]
Ta3N5 nanotubes
0.6 (at 1.23 VRHE)
A 150 W Xe lamp
110
10 (at 400 nm)
[19b]
Ta3N5 nanorods
3.8 (at 1.23 VRHE)
AM 1.5G simulated sunlight
100
41.3 (at 440 nm)
[19c]
Ta3N5 nanorods
2.8 (at 1.23 VRHE)
AM 1.5G simulated sunlight
100
37.8 (at 480 nm)
[19d]
Ta3N5 Thin films
3.1 (at 1.2 VRHE)
AM 1.5G simulated sunlight
100
40 (at 400 nm)
[19e]
Ta3N5 Thin films
3.6 (at 1.2 VRHE)
A 300 W Xe lamp fitted with a cutoff filter (λ > 400 nm)
_
39.5 (at 400 nm)
[19f]
Nanostructured Ta3N5 Films
0.5 (at 1.25 VRHE)
A Xe lamp
73
12.7 (at 400 nm)
[19g]
Ta3N5 hollow sphere-nanofilms
5.2 (at 1.21 VRHE)
A 100 W Xe lamp fitted with a cutoff filter (λ > 400 nm)
100
43.5 (at 470 nm)
This work
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hollow sphere-nanofilms still remained as much as 70% of the initial photocurrent even after 60 min, while the Ta3N5 thin films decreased to 5% of its initial value after 20 min time period of irradiation. The enhancement of the stability can be attributed to the porous hollow sphere structure of Ta3N5 hollow sphere-nanofilms. After adsorption/desorption in the IrO2 sol, more IrO2 nanoparticles were adsorbed by the Ta3N5 hollow sphere-nanofilms than by the Ta3N5 thin films (Figure S9b). Porous hollow spheres have large surface area and the through-holes to connecting the inner and outer spaces are particularly suitable for the adsorption of nanoparticles.[23b,c,d] Moreover, the micropores can act as the molecular sieve to seize the IrO2 nanoparticles. Thus the meso-micro porous hollow sphere structures can guarantee the efficient and homogeneous modification of IrO2 nanoparticles.[26] As shown in Figure S10, these IrO2 nanopartilces with 1–2 nm in diameter were homogeneously dispersed on the hollow sphere structures with no obvious aggregation, and well protected the surfaces of the Ta3N5 hollow sphere-nanofilms.[19f,21] In summary, we have presented a facile self-assembly strategy for the fabrication of high-quality Ta3N5 hollow sphere-nanofilms with tunable numbers of layers. The controllable polymer nanofilms, strong interactions between the surfaces of polymer colloidal particles and inorganic particles, and the relatively high hydrolysis and condensation rates of the tantalum precursors were found to be vital to the formation of high-quality hollow sphere multilayer nanofilms. The as-obtained Ta3N5 hollow sphere-nanofilms exhibited significantly enhanced PEC water splitting efficiency, and the photoelectrochemical behavior strongly depended on the layer numbers. This method can be easily extended to the fabrication of other multilayer semiconductor hollow spherenanofilms, such as Nb2O5, TiO2, and ZrO2 nanofilms.
Experimental Section Synthesis of PSAA colloidal spheres: Monodisperse PSAA colloidal spheres were synthesized by soap-free emulsion polymerization as follows: Styrene (10.0 g), acrylic acid (1 g), H2O (150 g) were charged into a 250 mL four-necked flask equipped with mechanical stirrer, a thermometer with a temperature controller, a N2 inlet, a Graham condenser, and a heating mantle. This mixture was deoxygenated by bubbling nitrogen gas at room temperature for 60 min under a stirring rate of 200 rpm and then heated to 70 °C followed by injection of ammonium persulfate solution (0.1 g in 5 mL of H2O). The reaction was conducted for about 10 h to finish the polymerization. The as-obtained PSAA spheres were centrifuged and washed three times with deionized water, and then redispersed into H2O with a solid content of 10 wt% for further use. Fabrication of polymer particle-nanofilms: Ta sheets used for the experiments were first washed with acetone, ethanol, and deionzed water, and then dried by flushing with Ar gas. The PSAA colloidal particle-nanofilms were prepared on smooth substrates by oil-water interfacial self-assembly strategy. Briefly, 0.1 mL of PSAA colloidal particles was dispersed into 30 mL of H2O by sonication for 5 min, followed by addition of 15 mL of hexane to form a
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hexane-water interface. Then 1.5 mL of ethanol was slowly added to the interface by a syringe at a very low rate (50 µL/min). The PSAA particles were gradually trapped at the interface forming closely packed nanofilms. After removal most of the hexane, the assembled nanofilms were transferred to the substrates sheets by a lifting method, and then dried at room temperature. Using the same strategy, multilayer nanofilms of PSAA colloidal particles were successfully fabricated by repeating the above procedure. Fabrication of Ta3N5 nanofilms: Briefly, the as-obtained polymer crystal nanofilms on the Ta sheets were partially dipped into a Ta precursor solution (0.2 mL of tantalum ethoxide dispersed in 5 mL of ethanol containing 20 mg acetic acid) for 30 s, and then left for hydrolysis and condensation reactions in wet air at room temperature for 10 min to form PSAA/Ta2O5 core-shell spherenanofilms. After calcination at 1100 °C in NH3 for 10 h, highquality Ta3N5 hollow sphere multilayer nanofilms were fabricated. When dried in very low humidity air, ordered macroporous (inverse opal-) Ta3N5 nanofilms were obtained. The Ta3N5 thin films were prepared by oxidizaing Ta foil and following the nitridation of the resulting films for the sake of comparison. Synthesis and loading of IrO2 colloids: The IrO2 colloids were synthesized by the Murray method as follows:[27] an aqueous solution of Na2IrCl6 (25 mL, 2.0 mM), whose pH was adjusted to 13 using NaOH, was heated to 90 °C. After stirring for 20 min, the resultant solution was kept in an ice bath for 1 h and then was adjusted to the pH 1 by addition of HNO3. After stirring for another 2 h, a deep blue solution was obtained. The final pH of the solution was adjusted to 7 by addition of NaOH and stored in a refrigerator at 2 °C for further use. The as-obtained Ta3N5 electrodes were immersed into the IrO2 colloids solution (pH = 7) for 4 h and washed three times with water. The electrodes were then dried at room temperature. Characterizations: The morphologies and structures of the asprepared products were observed by high-resolution transmission electron microscopy (TEM, Philips XL30) and scanning electron microscopy (SEM, Philips XL30). The chemical compositions were analyzed with energy-dispersive X-ray spectroscopy (EDX) attached to the SEM. X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max-kA diffractometer with Cu Kα radiation. FTIR analysis was carried out with a Nicolet Nexus 470 FTIR spectrometer. The UV-vis adsorption spectra were obtained with Hitachi U-4100 spectrophotometer. Nitrogen adsorption-desorption isotherms were determined at 77 K using an ASAP 2010 analyzer. The surface area was calculated according to the Brunauer-Emmett-Teller (BET) method. Photoelectrochemical measurements: The photoelectrochemical properties of the Ta3N5 hollow sphere-nanofilms based electrodes were analyzed in a quartz electrochemical cell with a platinum coil as the counter electrode, an Ag/AgCl as the reference electrode, and aqueous 0.5 M Na2SO4 solution as the electrolyte, under potentiostat control. The Ta3N5 hollow sphere-nanofilms used for PEC measurements were all fabricated on the Ta substrates. The area of Ta3N5 hollow sphere-nanofilms was 1 cm2. The pH was adjusted to 13 by addition of NaOH solution. The electrolyte solution was purged by Ar gas for 30 min prior to the measurements. A 100 W xenon lamp fitted with a cutoff filter (λ > 400 nm) was used as visible-light irradiation source with average light density of 100 mW cm−2 measured by an Optical Power Meter. For the sake of comparison, the photoelectrochemical properties of
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Controllable Fabrication and Photoelectrochemical Property of Multilayer
the Ta3N5 inverse opal nanofilm- and Ta3N5 thin film- electrodes were also tested in the same condition. The IPCE was determined by employing 470 nm monochromatic light-emitting diodes (LED) with light density of 15 mWcm−2. According to the Nernst equation, ERHE = EAg/AgCl + 0.059pH + 0.197. The IPCE can be calculated from the following equation:
IPCE (%) =
1240 × photocurrent density ( mAcm −2 ) × 100% wavelength( nm ) × photonflux ( mWcm −2 )
(1)
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgment Financial support was received from the National Natural Science Foundation of China (Grants 51133001 and 21374018), National “863” foundation, the Science and Technology Foundation of Ministry of Education of China (20110071130002), and Science and Technology Foundation of Shanghai (12nm0503600, 13JC1407800).
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[13] L. F. Hu, R. Ma, T. C. Ozawa, T. Sasaki, Angew. Chem., Int. Ed. 2009, 48, 3846. [14] a) L. Hu, L. Wu, M. Liao, X. Fang, Adv. Mater. 2011, 23, 1988; b) L. Hu, M. Chen, L. Wu, X. Fang, Chem. Soc. Rev. 2012, 41, 1350. [15] Y.-S. Yoon, S.-H. Byeon, I.-S. Lee, Adv. Mater. 2010, 22, 3272. [16] L. Hu, M. Chen, W. Shan, T. Zhan, M. Liao, X. Fang, X. Hu, L. Wu, Adv. Mater. 2012, 24, 5872. [17] a) F. Sun, W. Cai, Y. Li, L. Jia, F. Lu, Adv. Mater. 2005, 17, 2872; b) J. M. Lee, D. G. Lee, J. H. Kim, Macromolecules 2007, 40, 9529; c) T. Ding, K. Song, K. Clays, C. H. Tung, Langmuir 2010, 26, 11544. [18] a) G. Hitoki, A. Ishikawa, T. Takata, J. N. Kondo, M. Hara, K. Domen, Chem. Lett. 2002, 736; b) W. J. Chun, A. Ishikawa, H. Fujisawa, T. Takata, J. N. Kondo, M. Hara, M. Kawai, Y. Matsumoto, K. Domen, J. Phys. Chem. B. 2003, 107, 1798; c) M. Hara, E. Chiba, A. Ishikawa, T. Takata, J. N. Kondo, K. Domen, J. Phys. Chem. B. 2003, 107, 13441; d) A. Ishikawa, T. Takata, J. N. Kondo, M. Hara, K. Domen, J. Phys. Chem. B. 2004, 108, 11049. [19] a) X. Feng, T. J. LaTempa, J. I. Basham, G. K. Mor, O. K. Varghese, C. A. Grimes, Nano Lett. 2010, 10, 948. b) Y. Cong, H. Park, S. Wang, H. Dang, F. R. F. Fan, C. B. Mullins, A. J. Bard, J. Phys. Chem. C. 2012, 116, 14541; c) Y. Li, T. Takata, D. Cha, K. Takanabe, T. Minegishi, J. Kubota, K. Domen, Adv. Mater. 2013, 25, 125; d) C. Zhen, L. Wang, G. Liu, G. Lu, H. Cheng, Chem. Commun. 2013, 49, 3019; e) M. Liao, J. Feng, W. Luo, Z. Wang, J. Zhang, Z. Li, T. Yu, Z. Zou, Adv. Funct. Mater. 2012, 22, 3066; f) M. Higashi, K. Domen, R. Abe, Energy Environ. Sci. 2011, 4, 4138; g) H. X. Dang, N. T. Hahn, H. S. Park, A. J. Bard, C. B. Mullins, J. Phys. Chem. C 2012, 116, 19225. [20] a) Q. Zhang, L. Gao, Langmuir 2004, 20, 9821; b) T. Takata, D. L. Lu, K. Domen, Cryst. Growth. Des. 2011, 11, 33; c) R. Gao, S. Zhou, M. Chen, L. Wu, J. Mater. Chem. 2011, 21, 17087. [21] a) A. Ishikawa, T. Takata, J. N. Kondo, M. Hara, H. Kobayashi, K. Domen, J. Am. Chem. Soc. 2002, 124, 13547; b) A. Kasahara, K. Nukumizu, G. Hitoki, T. Takata, J. N. Kondo, M. Hara, H. Kobayashi, K. Domen, J. Phys. Chem. A 2002, 106, 6750; c) A. Iwase, H. Kato, A. Kudo, Chem. Lett. 2005, 34, 946; d) B. H. Meekins, P. V. Kamat, J. Phys. Chem. Lett. 2011, 2, 2304. [22] J. H. Park, S. Kim, A. J. Bard, Nano Lett. 2005, 6, 24. [23] a) C. Grimes, J. Mater. Chem. 2007, 17, 1451; b) J. Hu, M. Chen, X. Fang, L. Wu, Chem. Soc. Rev. 2011, 40, 5472; c) J. Liu, F. Liu, K. Gao, D. Xue, J. Mater. Chem. 2009, 19, 6073; d) X. Lou, L. A. Archer, Z. Yang, Adv. Mater. 2008, 20, 3987. [24] a) H. Li, Z. Bian, J. Zhu, D. Zhang, G. Li, Y. Huo, H. Li, Y. Lu, J. Am. Chem. Soc. 2007, 129, 8406; b) Y. Kondo, H. Yoshikawa, K. Awaga, M. Murayama, T. Mori, K. Sunada, S. Bandow, S. Iijima, Langmuir 2008, 24, 547; c) J. Pan, X. Zhang, A. Du, D. Sun, J. O. Leckie, J. Am. Chem. Soc. 2008, 130, 11256; d) R. Gao, S. Zhou, M. Chen, L. Wu, J. Mater. Chem. 2011, 21, 17087; e) Y. Jiao, C. Peng, F. Guo, Z. Bao, J. Yang, L. Schmidt-Mende, R. Dunbar, Y. Qin, Z. Deng, J. Mater. Chem. 2011, 115, 6405; f) Z. Dong, X. Lai, J. E. Halpert, N. Yang, L. Yi, J. Zhai, D. Wang, Z. Tang, L. Jiang, Adv. Mater. 2012, 24, 1046. [25] a) X. Li, F. Tao, Y. Jiang, Z. Xu, J. Colloid. Interface Sci. 2007, 308, 460; b) I. Rodriguez, F. Ramiro-Manzano, P. Atienzar, J. M. Martinez, F. Meseguer, H. Garcia, A. Corma, J. Mater. Chem. 2007, 17, 3205. c) X. Chen, J. Ye, S. Ouyang, T. Kako, Z. Li, Z. Zou, ACS Nano 2011, 5, 4310. [26] a) L. Zhang, J. Yu, Chem. Commun. 2003, 2078; b) S. Ito, K. R. Thampi, P. Comte, P. Liska, M. Gratzel, Chem. Commun. 2005, 268. [27] T. Nakagawa, C. A. Beasley, R. W. Murray, J. Phys. Chem. C 2009, 113, 12958.
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Received: December 19, 2013 Revised: March 10, 2014 Published online:
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Controllable Fabrication and Photoelectrochemical Property of Multilayer Tantalum Nitride Hollow Sphere-Nanofilms Rui Gao, Linfeng Hu, Min Chen, and Limin Wu*
Fabrication of two- and three-dimensional ordered architectures in thin-film form using low-dimensional nanostructures as building blocks is a burgeoning field in the past decades, because these ordered film materials are expected to have the enhanced even novel properties and potential applications in catalysts,[1] gas sensors,[2] optical devices,[3] electronics,[4] opto-electronic devices,[5] surface-enhanced Raman scattering devices,[6] solar cells,[7] high-performance display units,[8] capacitors, and transistors.[9] Many physical and chemical methods have been developed to fabricate these ordered superstructures, including electron-beam lithography, micro-contact printing, and selfassembly of colloidal particles.[10] Recently, an oil-water interfacial self-assembly is considered an ideal strategy for the assembly of various low-dimensional nanostructures into nanofilms.[11] The low-dimensional nanostructures are well dispersed in water, and then an oil phase is added to form an oil-water interface. After the addition of an appropriate amount of inducer, the decreased interfacial energy causes the nanostructures to self-assemble into closely packed monolayer nanofilms at the interface.[12] Very recently, we have developed this self-assembly procedure to fabricate monolayer nanofilm-based devices, such as ultraviolet (UV)light photodetectors and electrical resistive switching memory devices.[5,13,14] The simplicity of the interfacial assembly technology significantly decreases the cost of fabrication, and the high quality of the as-assembled monolayer films can ensure that the performance of the nanodevices exceeds that of those constructed from individual nanostructures. Recent studies have shown that the multilayer assembly of nanostructures is an effective means of creating films with enhanced or even novel, useful properties.[15] However, the big challenge is growing high-quality multilayer nanofilms with uniform coverage ratios and smooth surfaces suitable for use in optoelectronic and microelectronic devices. More recently, we successfully fabricated two-layer semiconducting
Dr. R. Gao, Dr. L. Hu and Dr. M. Chen, Prof. L. Wu Department of Materials Science and State Key Laboratory of Molecular Engineering of Polymers Fudan University Shanghai 200433, China E-mail: [email protected] DOI: 10.1002/smll.201303873 small 2014, DOI: 10.1002/smll.201303873
ZnS/ZnO hollow sphere nanofilms through oil-water interfacial self-assembly strategy with organic/inorganic core-shell composite particles as building blocks and subsequent calcination. We observed markedly enhanced photoresponsive behavior of the two-layer-nanofilm-based nanodevice as compared with the corresponding monolayer-based one.[16] However, hollow sphere-nanofilms fabricated by more than 2 layers with precisely controllable numbers of layer cannot be fabricated using this method due to the instability and inhomogeneity of core-shell composite particles or by any other methods. Although the preparation of nanofilms with precisely tunable numbers through the self-assembly strategy has been reported,[17] the fabricated nanofilms were mainly inverse-opal structured, it is impossible to use these methods reported so far to fabricate hollow sphere-based nanofilms with precisely tunable layer numbers. In this communication, we have for the first time successfully fabricated hollow sphere-nanofilms with precisely tunable numbers of layers using Ta3N5 hollow sphere-nanofilms as an example, through the oil-water interfacial selfassembly combined with the control of the sol-gel reaction of precursors. Ta3N5 is an important photocatalyst suitable for solar hydrogen production because it has a narrow band gap of 2.1 eV, making it suitable for the collection of more than 45% of incident solar energy.[18] And the photoelectrodes based on Ta3N5 thin films, nanoparticles, nanotube arrays and nanorod arrays have displayed excellent visible-light activity for photoelectrochemical (PEC) water splitting.[19] The present Ta3N5 hollow sphere-nanofilms based photoelectrodes exhibited significantly enhanced visible-light water splitting ability and superior stability, and the PEC performance of the as-obtained hollow sphere-nanofilms was found to be highly dependent on the numbers of layers. Scheme 1 summarizes the fabrication procedures of multilayer Ta3N5 hollow sphere-nanofilms. First, well-defined poly(styrene-co-acrylic acid) (PSAA) colloidal particles were self-assembled on smooth substrates using oil-water interfacial self-assembly strategy to form polymer nanofilms.[5,11–14] Because of the monodispersibility and stability of polymer colloidal particles, high-quality multilayer polymer particle nanofilms were more easily controlled than organic/inorganic core-shell composite particles as building blocks.[16] Tantalum ethoxide ethanol solution was then dipping-infiltrated into the voids between PSAA colloidal particles by capillary force and its hydrolysis and condensation reactions
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Scheme 1. Schematic illustration for the formation of multilayer Ta3N5 hollow sphere- and inverse opal- nanofilms.
were performed during drying. The strong interactions between polymer particles and the inorganic phase are very important. Because the monodisperse PSAA colloidal particles were composed of hydrophobic PS-enriched cores and carboxylic-acid-group-enriched shells, they quickly captured small Ta2O5 nanoparticles derived from the sol-gel reaction of tantalum ethoxide.[1] The sol-gel reaction conditions of tantalum ethoxides during room-temperature drying are also crucial. Well-defined PSAA/Ta2O5 core-shell sphere nanofilms were produced at high humidity (80% or higher). Accordingly, Ta3N5 hollow sphere-nanofilms with precisely tunable layers were easily fabricated through calcination in ammonia (Route A in Scheme 1). However, at low humidity (e.g., 10% or less), the Ta2O5 sols formed closely packed three-dimensional skeletons due to the low rate of the sol-gel reaction. After calcination, Ta3N5 monolayer- and multilayerordered pore structures (inverse opal-nanofilms) were produced (Route B in Scheme 1). Figure 1a–c shows typical scanning electron microscopy (SEM) images of the as-obtained PSAA colloidal particles-, PSAA/Ta2O5 core-shell spheres-, and Ta3N5 hollow spheresmonolayer nanofilms, respectively. The monodisperse PSAA colloidal particles which had an average diameter of about 280 nm, were assembled into a high-quality nanofilm consisting of hexagonally packed spheres, although few microcracks appeared due to the decrease in the hydrodynamic size of the polymer particles (Figure 1a). During dried at high humidity, the surfaces of the polymer particles were coated with Ta2O5 through the interactions between the carboxylic groups of polymer particles and the hydroxyl groups of inorganic nanoparticles, forming PSAA/Ta2O5 core-shell spheres with a mean diameter of 303 nm and core-shell sphere-nanofilms
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(Figure 1b). Fourier transform infrared (FTIR) spectrum clearly indicated a peak at 1540 cm−1 for carboxylate groups at the cost of carboxylic groups (at 1704 cm−1) and the typical broad peak at 624 cm−1 for Ta2O5 (Figure S1), indicating strong interactions between the surface carboxyl groups and the hydroxyl groups of Ta2O5. High humidity caused fast hydrolysis and condensation reactions of tantalum ethoxide, yielding large numbers of Ta2O5 nanoparticles, which are quickly captured by the carboxylic-group-enriched surfaces of the PSAA colloidal particles, forming complete core-shell sphere nanofilms. This film was subjected to calcination in ammonia at 1100 °C for 10 h, producing Ta3N5 hollow sphere-nanofilms, with about 10% shrinkage of the sphere diameter (Figure 1c). Well-maintained Ta3N5 hollow sphere based nanofilms with different sizes were also produced by simply controlling the sizes of the template spheres, as shown in Figure S2. Figure 1d shows the hollow structures and porous shells of the Ta3N5 spheres. The shells are 11-nm thick. N2 adsorption-desorption (Figure 1g) indicates that the hollow sphere samples have a high surface area of 37 m2 g−1 and unique meso-microporous shell structures due to the profound decrease in volume during the transformation from Ta2O5 to Ta3N5.[20] The selected area electron diffraction (SAED) pattern (Figure 1e), indicates the polycrystalline nature of the Ta3N5 hollow spheres. In addition, the high-resolution transmission electron microscopy (HRTEM) image clearly shows the (002) atomic planes with a lattice spacing of 0.51 nm (Figure 1f). This suggests that the samples had good crystallinity, which is important for their photoelectrochemical properties. In the X-ray diffraction (XRD) pattern (Figure 1h), all the reflections can be readily indexed to orthorhombic Ta3N5 phase (JCPDS 79–1533)
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small 2014, DOI: 10.1002/smll.201303873
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Figure 1. Typical SEM images of (a) PSAA colloidal particles-, (b) PSAA/Ta2O5 core-shell spheres-, and (c) Ta3N5 hollow spheres, (d) Ta3N5 hollow sphere-nanofilms at higher magnification. The inset shows a TEM image of a single Ta3N5 hollow sphere with a scale bar of 100 nm. (e and f) Corresponding SAED pattern and HRTEM image of Ta3N5 hollow spheres. (g, h and i) Nitrogen adsorption-desorption isotherm, XRD and EDX patterns of the sample in Figure 1c.
with a high intensity, indicating the high purity and good crystallinity of the products. EDX measurement, combined with the XRD results, show the atomic ratio of Ta/N to be about 3:5 (Figure 1i). However, during low-humidity drying, the Ta2O5 sols due to the suppressed sol-gel reaction of tantalum ethoxide, tended to form closely packed three-dimensional skeletons by filling the voids and produced monolayer Ta3N5 inverse opal-nanofilm after nitridation (Figure S3). Several layers of polymer nanofilms were fabricated easily due to the stable, monodisperse PSAA colloidal particles, accordingly, the Ta3N5 hollow sphere-nanofilms were produced easily, and the number of layers was precisely controllable, as shown in Figure 2. For the sake of comparison, the Ta3N5 inverse opal-nanofilm with 4 layers was also fabricated and shown in Figure 2. This method could also be extended to the fabrication of other multilayer semiconductor hollow sphere-nanofilms, such as multilayer TiO2 hollow spherenanofilms, as shown in Figure S4. Figure 3a shows the current-potential curves of the photoelectrodes based on Ta3N5 hollow sphere-nanofilms of 4 layers nitrided at different temperatures in an aqueous 0.5 M Na2SO4 solution (pH = 13) under chopped visible small 2014, DOI: 10.1002/smll.201303873
light irradiation (>400 nm). Before the PEC measurements, all the electrodes were modified with IrO2, which is usually used as an effective co-catalyst for water oxidation to improve charge separation and to decrease the overpotential for corresponding redox processes.[21] This modification indeed significantly enhanced the photocurrent of the Ta3N5 hollow sphere-nanofilm electrodes (Figure S5a). The Ta3N5 hollow sphere-nanofilms nitrided at 1100 °C exhibited a high and fast photoresponsive anode current (17.9 mA cm−2 at 0.65 V vs. Ag/AgCl and 5.2 mA cm−2 at 1.21 V vs. RHE, respectively), indicating that the Ta3N5 hollow sphere- nanofilms are very sensitive to visible light and that the contacts between the nanofilms and the Ta substrates and between the spheres inside the nanofilms are very good, facilitating the electron transport within the photoelectrodes.[19c,f] The incident photon-to-current conversion efficiency (IPCE) recorded at 1.23 V vs. RHE for the Ta3N5 hollow spherenanofilms with 4 layers were about 43.5% at 470 nm. Both the photocurrent density and the IPCEs show that the asobtained Ta3N5 hollow sphere-nanofilms based photoelectrodes occupy superior PEC performance to those previously reported on other Ta3N5 (Table 1), and produce a violent gas
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Figure 2. SEM images of Ta3N5 hollow sphere-nanofilms with different layers: (a) bilayer, (b) trilayer, (c) four-layer, (d) five-layer, and (e) eight-layer nanofilms. (f) SEM image of Ta3N5 inverse opal-nanofilms with 4 layers. The scale bars are 500 nm.
bubbles at atmospheric pressure by the illumination of visible light (Figure S5b). The nitridation temperature has a strong influence on the PEC performance of Ta3N5 hollow sphere-nanofilms. The photocurrent density of the hollow sphere-nanofilms nitrided at 1100 °C is 6 and 1.6 times of those nitrided at 700 °C and 900 °C, respectively. This result is consistent with that of the Ta3N5 nanorod arrays reported previously.[19c] Increase of the nitridation temperature from 700 to 1100 °C, not only remarkably decreased the resistance of the Ta3N5 electrodes, but also greatly improved the crystallinity of the Ta3N5 hollow spherenanofilms, as shown in Figure S6 and S7. It is also interesting to note that the apparent photocurrent increase was achieved for the whole applied potential region, while the abrupt photocurrent increase was observed only at high potential region for the Ta3N5 nanorod arrays.[19c] This may be ascribed to the porous hollow sphere array structure, which offers large effective surface area in close proximity with the electrolyte, thus facilitating diffusive transport of photogenerated holes to oxidizable species in the electrolyte.[22] However, further increasing the temperature to 1200 °C, decreased the photocurrent density. This is probably attributed to the decrease of surface area and distortion of hollow sphere structure.[23] The photocurrent was found to be strongly dependent on the numbers of layers and nanostructures of Ta3N5 nanofilms. Figure 3b shows the photocurrent-potential curves for Ta3N5 hollow sphere-nanofilms with different numbers of layers. The monolayer Ta3N5 hollow sphere-nanofilm showed a photocurrent density of 6.1 mA·cm−2 at 0.65 V. As the number of layers increased, the photocurrent densities of the Ta3N5 hollow sphere-nanofilms first increased because of the increased light adsorption.[19f] With the further increase of the layer number, the recombination of photogenerated electrons and holes became more apparent because of the
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decreasing electrical field and increasing film resistance.[19a,22] This caused a decrease in photocurrent. Maximum photocurrent density was detected at 4 layers of hollow sphere-nanofilm, showing ca. 17.9 mA·cm−2 at 0.65 V as discussed above. This photocurrent density was 4.2 and 1.7 times as high as that of Ta3N5 thin films with the same thickness (Figure S8) and the Ta3N5 inverse opal- nanofilms with the same layer number (Figure 2f), respectively (Figure 3c). The reason might be due to the different nanostructures among these photoelectrodes. For the Ta3N5 thin films, the photogenerated carriers should travel a long distance before reaching the surface active sites, which increases the bulk charge recombination and thus limits the photoactivity.[19c] However, for the Ta3N5 hollow sphere-nanofilms, the thin shell thickness (about 11 nm) can offer a short distance for the photoexited charges to travel to the surface. Moreover, the high surface area provides a high density of active sites for photocatalytic reaction, and the porous hollow structure is beneficial for light harvesting through multireflection and scattering, thus enabling more photons be captured for photocatalytic reaction. Therefore, the PEC performances of the Ta3N5 hollow sphere-nanofilms are far superior to those of Ta3N5 thin films. Because the fabrication process of both Ta3N5 hollow sphereand inverse opal- nanofilms is nearly the same, differing only in drying conditions, no obvious differences in film quality, such as the quality of crystal structure and surface area were detected in the two types of Ta3N5 nanofilms (Figure S3c and S3d). In this way, the superior PEC activity of Ta3N5 hollow sphere-nanofilms over Ta3N5 inverse opal-nanofilms can be attributed to the fact that the hollow sphere films have much greater visible-light harvesting efficiency, as shown in Figure S9a. The Ta3N5 inverse opal-nanofilms have an open, continuous void structure, while the Ta3N5 hollow-sphere nanofilms have interior holes sealed by the porous shells.
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small 2014, DOI: 10.1002/smll.201303873
Controllable Fabrication and Photoelectrochemical Property of Multilayer
Figure 3. Photocurrent vs. potential curves. (a) The Ta3N5 hollow sphere-nanofilms with 4 layers nitrided at different temperatures. (b) The Ta3N5 hollow sphere-nanofilms with different numbers of layers nitrided at 1100 °C. (c) The comparison between the Ta3N5 hollow sphere-nanofilms (1), Ta3N5 inverse opal-nanofilms (2), and IrO2-Ta3N5 thin films (3), the inset is the schematic illustration of multireflections inside the Ta3N5 hollow sphere-nanofilms. (d) The current-time evolution measured at 0.1 V vs. Ag/AgCl at pH 13 for the IrO2-modified Ta3N5 hollow sphere nanofilms (1), the IrO2-modified Ta3N5 thin films (2) and the bare Ta3N5 hollow sphere nanofilms (3).
The gaps between the Ta3N5 hollow spheres in the nanofilms and large interior voids in the porous hollow spheres may enhance multiplereflection and scattering of light,[24] as illustrated in the inset of Figure 3c, which allows the Ta3N5 hollow sphere-nanofilms to harvest more visible light than inverse opal-nanofilms with the same number of layers. Thus the Ta3N5 hollow sphere-nanofilms exhibit better PEC properties than Ta3N5 inverse opal-nanofilms under visible light irradiation, although fabricating inverse opal array structures
are widely considered as an effective way of enhancing the photocatalytic activities of semiconductor-based photoelectrodes because of their high light harvesting capacity and decreasing recombination with photoexcited charges.[1,7,25] The photochemical stability of the photoelectrode is another key factor in water splitting, besides a high photocatalytic activity. Figure 3d shows the current-time evolutions of the Ta3N5 hollow sphere-nanofilms compared with the Ta3N5 thin films. After modification with IrO2, Ta3N5
Table 1. Comparison of the critical parameters of the present Ta3N5 hollow sphere-nanofilm- and other Ta3N5-based photoelectrodes. Photoelectrodes
Photocurrent density (mA cm−2)
Light source
Light intensity (mW/cm2)
IPCE (%)
References
Ta3N5 nanotubes
_
_
_
5.3 (at 450 nm)
[19a]
Ta3N5 nanotubes
0.6 (at 1.23 VRHE)
A 150 W Xe lamp
110
10 (at 400 nm)
[19b]
Ta3N5 nanorods
3.8 (at 1.23 VRHE)
AM 1.5G simulated sunlight
100
41.3 (at 440 nm)
[19c]
Ta3N5 nanorods
2.8 (at 1.23 VRHE)
AM 1.5G simulated sunlight
100
37.8 (at 480 nm)
[19d]
Ta3N5 Thin films
3.1 (at 1.2 VRHE)
AM 1.5G simulated sunlight
100
40 (at 400 nm)
[19e]
Ta3N5 Thin films
3.6 (at 1.2 VRHE)
A 300 W Xe lamp fitted with a cutoff filter (λ > 400 nm)
_
39.5 (at 400 nm)
[19f]
Nanostructured Ta3N5 Films
0.5 (at 1.25 VRHE)
A Xe lamp
73
12.7 (at 400 nm)
[19g]
Ta3N5 hollow sphere-nanofilms
5.2 (at 1.21 VRHE)
A 100 W Xe lamp fitted with a cutoff filter (λ > 400 nm)
100
43.5 (at 470 nm)
This work
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hollow sphere-nanofilms still remained as much as 70% of the initial photocurrent even after 60 min, while the Ta3N5 thin films decreased to 5% of its initial value after 20 min time period of irradiation. The enhancement of the stability can be attributed to the porous hollow sphere structure of Ta3N5 hollow sphere-nanofilms. After adsorption/desorption in the IrO2 sol, more IrO2 nanoparticles were adsorbed by the Ta3N5 hollow sphere-nanofilms than by the Ta3N5 thin films (Figure S9b). Porous hollow spheres have large surface area and the through-holes to connecting the inner and outer spaces are particularly suitable for the adsorption of nanoparticles.[23b,c,d] Moreover, the micropores can act as the molecular sieve to seize the IrO2 nanoparticles. Thus the meso-micro porous hollow sphere structures can guarantee the efficient and homogeneous modification of IrO2 nanoparticles.[26] As shown in Figure S10, these IrO2 nanopartilces with 1–2 nm in diameter were homogeneously dispersed on the hollow sphere structures with no obvious aggregation, and well protected the surfaces of the Ta3N5 hollow sphere-nanofilms.[19f,21] In summary, we have presented a facile self-assembly strategy for the fabrication of high-quality Ta3N5 hollow sphere-nanofilms with tunable numbers of layers. The controllable polymer nanofilms, strong interactions between the surfaces of polymer colloidal particles and inorganic particles, and the relatively high hydrolysis and condensation rates of the tantalum precursors were found to be vital to the formation of high-quality hollow sphere multilayer nanofilms. The as-obtained Ta3N5 hollow sphere-nanofilms exhibited significantly enhanced PEC water splitting efficiency, and the photoelectrochemical behavior strongly depended on the layer numbers. This method can be easily extended to the fabrication of other multilayer semiconductor hollow spherenanofilms, such as Nb2O5, TiO2, and ZrO2 nanofilms.
Experimental Section Synthesis of PSAA colloidal spheres: Monodisperse PSAA colloidal spheres were synthesized by soap-free emulsion polymerization as follows: Styrene (10.0 g), acrylic acid (1 g), H2O (150 g) were charged into a 250 mL four-necked flask equipped with mechanical stirrer, a thermometer with a temperature controller, a N2 inlet, a Graham condenser, and a heating mantle. This mixture was deoxygenated by bubbling nitrogen gas at room temperature for 60 min under a stirring rate of 200 rpm and then heated to 70 °C followed by injection of ammonium persulfate solution (0.1 g in 5 mL of H2O). The reaction was conducted for about 10 h to finish the polymerization. The as-obtained PSAA spheres were centrifuged and washed three times with deionized water, and then redispersed into H2O with a solid content of 10 wt% for further use. Fabrication of polymer particle-nanofilms: Ta sheets used for the experiments were first washed with acetone, ethanol, and deionzed water, and then dried by flushing with Ar gas. The PSAA colloidal particle-nanofilms were prepared on smooth substrates by oil-water interfacial self-assembly strategy. Briefly, 0.1 mL of PSAA colloidal particles was dispersed into 30 mL of H2O by sonication for 5 min, followed by addition of 15 mL of hexane to form a
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hexane-water interface. Then 1.5 mL of ethanol was slowly added to the interface by a syringe at a very low rate (50 µL/min). The PSAA particles were gradually trapped at the interface forming closely packed nanofilms. After removal most of the hexane, the assembled nanofilms were transferred to the substrates sheets by a lifting method, and then dried at room temperature. Using the same strategy, multilayer nanofilms of PSAA colloidal particles were successfully fabricated by repeating the above procedure. Fabrication of Ta3N5 nanofilms: Briefly, the as-obtained polymer crystal nanofilms on the Ta sheets were partially dipped into a Ta precursor solution (0.2 mL of tantalum ethoxide dispersed in 5 mL of ethanol containing 20 mg acetic acid) for 30 s, and then left for hydrolysis and condensation reactions in wet air at room temperature for 10 min to form PSAA/Ta2O5 core-shell spherenanofilms. After calcination at 1100 °C in NH3 for 10 h, highquality Ta3N5 hollow sphere multilayer nanofilms were fabricated. When dried in very low humidity air, ordered macroporous (inverse opal-) Ta3N5 nanofilms were obtained. The Ta3N5 thin films were prepared by oxidizaing Ta foil and following the nitridation of the resulting films for the sake of comparison. Synthesis and loading of IrO2 colloids: The IrO2 colloids were synthesized by the Murray method as follows:[27] an aqueous solution of Na2IrCl6 (25 mL, 2.0 mM), whose pH was adjusted to 13 using NaOH, was heated to 90 °C. After stirring for 20 min, the resultant solution was kept in an ice bath for 1 h and then was adjusted to the pH 1 by addition of HNO3. After stirring for another 2 h, a deep blue solution was obtained. The final pH of the solution was adjusted to 7 by addition of NaOH and stored in a refrigerator at 2 °C for further use. The as-obtained Ta3N5 electrodes were immersed into the IrO2 colloids solution (pH = 7) for 4 h and washed three times with water. The electrodes were then dried at room temperature. Characterizations: The morphologies and structures of the asprepared products were observed by high-resolution transmission electron microscopy (TEM, Philips XL30) and scanning electron microscopy (SEM, Philips XL30). The chemical compositions were analyzed with energy-dispersive X-ray spectroscopy (EDX) attached to the SEM. X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max-kA diffractometer with Cu Kα radiation. FTIR analysis was carried out with a Nicolet Nexus 470 FTIR spectrometer. The UV-vis adsorption spectra were obtained with Hitachi U-4100 spectrophotometer. Nitrogen adsorption-desorption isotherms were determined at 77 K using an ASAP 2010 analyzer. The surface area was calculated according to the Brunauer-Emmett-Teller (BET) method. Photoelectrochemical measurements: The photoelectrochemical properties of the Ta3N5 hollow sphere-nanofilms based electrodes were analyzed in a quartz electrochemical cell with a platinum coil as the counter electrode, an Ag/AgCl as the reference electrode, and aqueous 0.5 M Na2SO4 solution as the electrolyte, under potentiostat control. The Ta3N5 hollow sphere-nanofilms used for PEC measurements were all fabricated on the Ta substrates. The area of Ta3N5 hollow sphere-nanofilms was 1 cm2. The pH was adjusted to 13 by addition of NaOH solution. The electrolyte solution was purged by Ar gas for 30 min prior to the measurements. A 100 W xenon lamp fitted with a cutoff filter (λ > 400 nm) was used as visible-light irradiation source with average light density of 100 mW cm−2 measured by an Optical Power Meter. For the sake of comparison, the photoelectrochemical properties of
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2014, DOI: 10.1002/smll.201303873
Controllable Fabrication and Photoelectrochemical Property of Multilayer
the Ta3N5 inverse opal nanofilm- and Ta3N5 thin film- electrodes were also tested in the same condition. The IPCE was determined by employing 470 nm monochromatic light-emitting diodes (LED) with light density of 15 mWcm−2. According to the Nernst equation, ERHE = EAg/AgCl + 0.059pH + 0.197. The IPCE can be calculated from the following equation:
IPCE (%) =
1240 × photocurrent density ( mAcm −2 ) × 100% wavelength( nm ) × photonflux ( mWcm −2 )
(1)
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgment Financial support was received from the National Natural Science Foundation of China (Grants 51133001 and 21374018), National “863” foundation, the Science and Technology Foundation of Ministry of Education of China (20110071130002), and Science and Technology Foundation of Shanghai (12nm0503600, 13JC1407800).
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: December 19, 2013 Revised: March 10, 2014 Published online:
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