Article pubs.acs.org/Langmuir
Light-Reducible Dissipative Nanostructures Formed at the Solid−Liquid Interface Tetsuro Soejima,*,†,∥ Yuta Amako,† Seishiro Ito,† and Nobuo Kimizuka*,‡,§,∥ †
Department of Applied Chemistry, Kinki University, 3-4-1 Kowakae, Higashi-osaka, Osaka 577-8502, Japan Department of Chemistry and Biochemistry, Graduate School of Engineering, §Center for Molecular Systems (CMS), and ∥JST CREST, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan
‡
S Supporting Information *
ABSTRACT: Dissipative structures are macroscopic or even larger ordered structures that emerge under conditions far from thermodynamic equilibrium. In contrast, molecular self-assembly has been investigated near at the thermodynamic equilibrium, which provides basically smaller, nano-to-micron sized structures. In terms of the formation principles, there exists an essential gap between the dissipative structures and molecular self-assemblies. To fill this gap, molecular self-assembly of light-reducible organic−inorganic ion pairs was investigated under far-from-equilibrium conditions. When solid films of tetraalkylammonium hexafluorophosphate were immersed in aqueous Au(OH)4− and immediately photoirradiated, gold nanowires are formed at the solid−aqueous interface. On the other hand, such nanowires were not formed when the photoirradiation was conducted for the specimens after a prolonged immersion period of 60 min. These observations indicate spontaneous growth of dissipative nanofibrous self-assemblies consisting of light-reducible ion pairs [tetraalkylammonium ion][Au(OH)4−] at the interface and their photoreduction to give developed nanowires. These nanowires are not available by the photoreduction of Au(OH)4− ions under conditions near at the thermodynamic equilibrium. A picture for the dissipative nanostructures is obtained: the formation of amphiphilic light-reducible nanowire structures is based on the static self-assembly near at the thermodynamic equilibrium, whereas their spontaneous, anisotropic growth from the interface to the aqueous phase is directed by dynamic, dissipative self-assembly phenomena under the far-from-equilibrium conditions. Thus, the both elements of dissipative self-assembly (dynamic) and static molecular self-assembly fuse together at the nanoscale, which is an essential feature of the dissipative nanostructures.
■
INTRODUCTION The self-organized structures that emerge under conditions far from the thermodynamic equilibrium by continuous energy supply and dissipation are referred to as dissipative structures, which are of pivotal importance because they provide fundamental concept to understand the universe as well as living matter.1 In general, the dissipative structures in nonequilibrium systems emerge as macroscopic or even larger structures. In the field of chemistry, these dissipative structures gained prominence by the Belousov−Zhabotinsky reaction,2,3 Turing structures,4,5 and Bénard convection6 which exhibit unique spatiotemporal patterns that appear in the macroscopic scale. In contrast, molecules self-assemble when the system reaches an energy minimum at the thermodynamic equilibrium, giving ordered nano- to microscopic structures in solution or at the interface. These self-assembly phenomena have been often classified into the dynamic and static self-assemblies,1 and lying in between them is an essential gap in terms of formation principles and their existing size. This is partly because the molecular assembly under far-from-equilibrium has been a less developed area of research. Although formation of myelin figures, i.e., the dynamic growth of water-swollen tubules of surfactants,7−12 lipids,13 liquid crystals,14 diblock copolymer amphiphiles,15 and recently polyoxometalate (POM),16−18 has © 2014 American Chemical Society
been reported, they are macroscopic, micron-sized structures which are spatiotemporally grown by the presence of osmotic pressures and convection at the solid−water interfaces. Thus, they are classified as static self-assemblies occurring under farfrom-equilibrium conditions. Meanwhile, dissipative structures on the nanoscale have not been recognized, until the report on ion pair self-assemblies formed from tetraalkylammonium ion and photoreducible Au(OH)4− ion at the organic liquid−water interface.19 They gave linear nanofiber assemblies which continuously grew from the liquid−water interface to the aqueous phase, which emerged under the spontaneous molecular flux of ion pairs produced by the difference in chemical potential across the interface.19 These dissipative nanostructures were in situ photoreduced to morphologically unique gold nanocrystals (ultralong holey nanowires and nanotubes) as observed by electron microscopy. Such dissipative nanostructures appears to be omnipresent, and therefore it is of paramount importance to generalize the concept since they provides new perspectives in the diverse research field of self-assembly as well as nanotechnology. Here, we report a new type of dissipative Received: September 12, 2014 Revised: November 2, 2014 Published: November 5, 2014 14219
dx.doi.org/10.1021/la5036568 | Langmuir 2014, 30, 14219−14225
Langmuir
Article
more hydrophilic, amphiphilic ion pair of [TBA+][Au(OH)4−] at the solid−water interface, as we observed at the organic−water interface.19 Meanwhile, upon photoirradiation of the TBAPF6 film immersed in aqueous Au(OH)4− for 30 min without stirring, surprisingly long nanowires were abundantly produced on the Si substrate (Figure 2a). Such nanowire structures were not
nanostructures emerged at the solid−liquid interface. The elements of static self-assembly under the thermodynamic equilibrium and dynamic self-assembly (dissipative self-assembly) are integrated at the nanolevel, providing a salient feature of the dissipative nanostructures.
■
EXPERIMENTAL SECTION
Materials and Methods. Tetrabutylammonium hexafluorophosphate (TBAPF6), tetrabutylammonium tetrafluoroborate (TBABF4), and tetrabutylammonium bromide (TBABr) were purchased from the Tokyo Chemical Industry Co., Ltd. HAuCl4·4H2O was purchased from the Kanto Chemical Co., Inc. A 50 mM TBAPF6 chloroform solution was dropped onto a piece (10 mm × 10 mm) of Si (100) wafer (n-type, mirror-finished surface) and dried at room temperature. An aqueous stock solution of AuCl4 was prepared by dissolving HAuCl4·4H2O (1 g) in deionized water (100 mL). The pH of a 24.3 mM aqueous AuCl4 solution was adjusted to 10.0 by adding appropriate amount of aqueous 1 M NaOH. The yellow color of aqueous AuCl4 disappeared as a consequence of the ligand exchange reaction to Au(OH)4− species.19,20 The Au(OH)4− ion is employed since it shows higher reactivity compared to AuCl4− ions toward photoreduction.19 The TBAPF6 films on Si wafer was immersed in the Au(OH)4− aqueous solutions, and the samples were photoirradiated using a high-pressure mercury lamp (Figure 1, Ushio OPM2-502HGC,
Figure 1. A schematic illustration of the experimental procedure. Cast films of TBAPF6 were prepared on Si wafers, which were placed in aqueous solution of Au(OH)4−. The dipped films were photoirradiated perpendicularly in the Au(OH)4− aqueous solution. Figure 2. (a) SEM image of gold nanowires formed on Si wafer. The TBAPF6 film/Si wafer was photoirradiated in aqueous Au(OH)4− solution. (b) TEM image of gold nanowires after dissolving the photoirradiated film in methanol.
λ > 300 nm, I330−390 = 5.0 mW cm−2). After the photoirradiation, the Si wafers were taken out from the aqueous solutions, washed with pure water, and were dried at room temperature. All the scientific glassware such as screw glass vials as reaction vessel were carefully washed with Milli-Q water and dried in drying oven before use. Characterization. The sample films on Si substrates were used for field-emission scanning electron microscopy (SEM) observation and X-ray photoelectron spectroscopy (XPS) measurement. SEM was performed using Hitachi S-4800 type II (accelerating voltage, 10 kV). XPS measurement was performed using a modified PHI model 5800 multiprobe spectrometer (X-ray source operated at 14 kV and 200 W). In transmission electron microscopy (TEM), the photoirradiated films were dissolved in methanol by ultrasonication for a few seconds, and the solution was dropped on carbon-coated copper grids. The specimens were then dried in ambient pressure. TEM and selected area electron diffraction were performed using a JEOL JEM-2010 (acceleration voltage, 120 kV), a JEOL JEM-2100F (acceleration voltage, 200 kV), and a JEOL JEM-3010 (accelerating voltage, 300 kV). High-resolution TEM (HRTEM) was performed using a JEM-4000EX (accelerating voltage, 400 kV).
observed when an aqueous Au(OH)4− solution containing a bare Si wafer was photoirradiated, which only gave irregular nanocrystals deposited on the surface (Figure S2 in Supporting Information). Apparently, all of the elementsTBAPF6 film, aqueous Au(OH)4−, and photoirradiationare required to produce developed nanowires. To observe nanowires in more detail by transmission electron microscopy (TEM), the TBAPF6 film after photoirradiation was dispersed in methanol by ultrasonication. Interestingly, flexible, vine-like thin nanowires with the thickness of 3.3−17.1 nm are abundantly seen in the TEM image (Figure 2b). These nanowires show distribution in diameter as shown in Figure S3 of the Supporting Information (average thickness ∼5.52 nm). In X-ray photoelectron spectrum of the nanowire/Si wafer sample, sharp peaks were observed at binding energies of 84.0 and 87.9 eV (Figure 3a and Figure S5 in Supporting Information). They are assigned to emissions from 4f7/2 and 4f5/2 electrons of gold(0), respectively. A highresolution TEM (HRTEM) image of the nanowire showed fringes with a spacing of 0.23 nm, which corresponds to the (111) reflection of the gold crystal (Figure 3b). Moreover, the selected-area electron diffraction pattern showed polycrystalline rings, which are assignable to the (111), (200), (220), and (311) planes of face-centered cubic gold (Figure 3c). These observations confirm that the nanowires are made of gold. This is further
■
RESULTS AND DISCUSSION A scanning electron micrograph (SEM) of the as-prepared TBAPF6 film on Si wafer is shown in Figure S1a (Supporting Information). A smooth surface was observed, while upon immersing the film in aqueous Na+Au(OH)4− solution for 30 min, some cracks were found on the film surface (Figure S1b in Supporting Information). As TBAPF6 is hydrophobic and only slightly soluble in pure water, the observed changes in the film structure reflect dissolution of TBA+ ions to the aqueous phase. This process would be promoted by the formation of 14220
dx.doi.org/10.1021/la5036568 | Langmuir 2014, 30, 14219−14225
Langmuir
Article
Figure 3. (a) XPS spectrum of the as-deposited gold nanowires on Si wafer. HRTEM images (b, d) and SAED pattern (c) of gold nanowires.
supported by a TEM-EDS measurement (Figure S4 in Supporting Information). Interestingly, the HRTEM image of the gold nanowire in Figure 3d clearly showed multicrystallinity: twinning boundaries were clearly observed, and single-crystalline domains were fused to form continuous nanowire structures. The multicrystalline structure of gold nanowires would be responsible for the distinct flexible morphology observed in Figure 2b. In the course of HRTEM observation, we noticed the presence of nano-sized pale contrasts in thicker gold nanowires, as shown in Figure S6a (indicated by arrows, in Supporting Information). As shown in Figure S6b (in Supporting Information), annual ring-like fringes were observed, indicating deformation of the gold crystal structure around the pale contrast. Such a pale contrast indicates the presence of nanocavities, as we reported for thicker gold nanowires (diameter > ∼15 nm) obtained at the water−organic solvent interface.19 We assume that TBA ions may serve as sacrificial agent to reduce Au(OH)4− ions, and these cavities would be formed by lateral segregation of such oxidized TBA species in the course of interfacial photoreduction process.19 In the present system, gold nanowires formed at the solid−water interface are remarkably thinner (the mean diameter, 5.52 nm) compared to those reported for the liquid−liquid interface system, and the yields for thick nanowires (d > 15 nm) were very low (yield
Light-Reducible Dissipative Nanostructures Formed at the Solid−Liquid Interface Tetsuro Soejima,*,†,∥ Yuta Amako,† Seishiro Ito,† and Nobuo Kimizuka*,‡,§,∥ †
Department of Applied Chemistry, Kinki University, 3-4-1 Kowakae, Higashi-osaka, Osaka 577-8502, Japan Department of Chemistry and Biochemistry, Graduate School of Engineering, §Center for Molecular Systems (CMS), and ∥JST CREST, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan
‡
S Supporting Information *
ABSTRACT: Dissipative structures are macroscopic or even larger ordered structures that emerge under conditions far from thermodynamic equilibrium. In contrast, molecular self-assembly has been investigated near at the thermodynamic equilibrium, which provides basically smaller, nano-to-micron sized structures. In terms of the formation principles, there exists an essential gap between the dissipative structures and molecular self-assemblies. To fill this gap, molecular self-assembly of light-reducible organic−inorganic ion pairs was investigated under far-from-equilibrium conditions. When solid films of tetraalkylammonium hexafluorophosphate were immersed in aqueous Au(OH)4− and immediately photoirradiated, gold nanowires are formed at the solid−aqueous interface. On the other hand, such nanowires were not formed when the photoirradiation was conducted for the specimens after a prolonged immersion period of 60 min. These observations indicate spontaneous growth of dissipative nanofibrous self-assemblies consisting of light-reducible ion pairs [tetraalkylammonium ion][Au(OH)4−] at the interface and their photoreduction to give developed nanowires. These nanowires are not available by the photoreduction of Au(OH)4− ions under conditions near at the thermodynamic equilibrium. A picture for the dissipative nanostructures is obtained: the formation of amphiphilic light-reducible nanowire structures is based on the static self-assembly near at the thermodynamic equilibrium, whereas their spontaneous, anisotropic growth from the interface to the aqueous phase is directed by dynamic, dissipative self-assembly phenomena under the far-from-equilibrium conditions. Thus, the both elements of dissipative self-assembly (dynamic) and static molecular self-assembly fuse together at the nanoscale, which is an essential feature of the dissipative nanostructures.
■
INTRODUCTION The self-organized structures that emerge under conditions far from the thermodynamic equilibrium by continuous energy supply and dissipation are referred to as dissipative structures, which are of pivotal importance because they provide fundamental concept to understand the universe as well as living matter.1 In general, the dissipative structures in nonequilibrium systems emerge as macroscopic or even larger structures. In the field of chemistry, these dissipative structures gained prominence by the Belousov−Zhabotinsky reaction,2,3 Turing structures,4,5 and Bénard convection6 which exhibit unique spatiotemporal patterns that appear in the macroscopic scale. In contrast, molecules self-assemble when the system reaches an energy minimum at the thermodynamic equilibrium, giving ordered nano- to microscopic structures in solution or at the interface. These self-assembly phenomena have been often classified into the dynamic and static self-assemblies,1 and lying in between them is an essential gap in terms of formation principles and their existing size. This is partly because the molecular assembly under far-from-equilibrium has been a less developed area of research. Although formation of myelin figures, i.e., the dynamic growth of water-swollen tubules of surfactants,7−12 lipids,13 liquid crystals,14 diblock copolymer amphiphiles,15 and recently polyoxometalate (POM),16−18 has © 2014 American Chemical Society
been reported, they are macroscopic, micron-sized structures which are spatiotemporally grown by the presence of osmotic pressures and convection at the solid−water interfaces. Thus, they are classified as static self-assemblies occurring under farfrom-equilibrium conditions. Meanwhile, dissipative structures on the nanoscale have not been recognized, until the report on ion pair self-assemblies formed from tetraalkylammonium ion and photoreducible Au(OH)4− ion at the organic liquid−water interface.19 They gave linear nanofiber assemblies which continuously grew from the liquid−water interface to the aqueous phase, which emerged under the spontaneous molecular flux of ion pairs produced by the difference in chemical potential across the interface.19 These dissipative nanostructures were in situ photoreduced to morphologically unique gold nanocrystals (ultralong holey nanowires and nanotubes) as observed by electron microscopy. Such dissipative nanostructures appears to be omnipresent, and therefore it is of paramount importance to generalize the concept since they provides new perspectives in the diverse research field of self-assembly as well as nanotechnology. Here, we report a new type of dissipative Received: September 12, 2014 Revised: November 2, 2014 Published: November 5, 2014 14219
dx.doi.org/10.1021/la5036568 | Langmuir 2014, 30, 14219−14225
Langmuir
Article
more hydrophilic, amphiphilic ion pair of [TBA+][Au(OH)4−] at the solid−water interface, as we observed at the organic−water interface.19 Meanwhile, upon photoirradiation of the TBAPF6 film immersed in aqueous Au(OH)4− for 30 min without stirring, surprisingly long nanowires were abundantly produced on the Si substrate (Figure 2a). Such nanowire structures were not
nanostructures emerged at the solid−liquid interface. The elements of static self-assembly under the thermodynamic equilibrium and dynamic self-assembly (dissipative self-assembly) are integrated at the nanolevel, providing a salient feature of the dissipative nanostructures.
■
EXPERIMENTAL SECTION
Materials and Methods. Tetrabutylammonium hexafluorophosphate (TBAPF6), tetrabutylammonium tetrafluoroborate (TBABF4), and tetrabutylammonium bromide (TBABr) were purchased from the Tokyo Chemical Industry Co., Ltd. HAuCl4·4H2O was purchased from the Kanto Chemical Co., Inc. A 50 mM TBAPF6 chloroform solution was dropped onto a piece (10 mm × 10 mm) of Si (100) wafer (n-type, mirror-finished surface) and dried at room temperature. An aqueous stock solution of AuCl4 was prepared by dissolving HAuCl4·4H2O (1 g) in deionized water (100 mL). The pH of a 24.3 mM aqueous AuCl4 solution was adjusted to 10.0 by adding appropriate amount of aqueous 1 M NaOH. The yellow color of aqueous AuCl4 disappeared as a consequence of the ligand exchange reaction to Au(OH)4− species.19,20 The Au(OH)4− ion is employed since it shows higher reactivity compared to AuCl4− ions toward photoreduction.19 The TBAPF6 films on Si wafer was immersed in the Au(OH)4− aqueous solutions, and the samples were photoirradiated using a high-pressure mercury lamp (Figure 1, Ushio OPM2-502HGC,
Figure 1. A schematic illustration of the experimental procedure. Cast films of TBAPF6 were prepared on Si wafers, which were placed in aqueous solution of Au(OH)4−. The dipped films were photoirradiated perpendicularly in the Au(OH)4− aqueous solution. Figure 2. (a) SEM image of gold nanowires formed on Si wafer. The TBAPF6 film/Si wafer was photoirradiated in aqueous Au(OH)4− solution. (b) TEM image of gold nanowires after dissolving the photoirradiated film in methanol.
λ > 300 nm, I330−390 = 5.0 mW cm−2). After the photoirradiation, the Si wafers were taken out from the aqueous solutions, washed with pure water, and were dried at room temperature. All the scientific glassware such as screw glass vials as reaction vessel were carefully washed with Milli-Q water and dried in drying oven before use. Characterization. The sample films on Si substrates were used for field-emission scanning electron microscopy (SEM) observation and X-ray photoelectron spectroscopy (XPS) measurement. SEM was performed using Hitachi S-4800 type II (accelerating voltage, 10 kV). XPS measurement was performed using a modified PHI model 5800 multiprobe spectrometer (X-ray source operated at 14 kV and 200 W). In transmission electron microscopy (TEM), the photoirradiated films were dissolved in methanol by ultrasonication for a few seconds, and the solution was dropped on carbon-coated copper grids. The specimens were then dried in ambient pressure. TEM and selected area electron diffraction were performed using a JEOL JEM-2010 (acceleration voltage, 120 kV), a JEOL JEM-2100F (acceleration voltage, 200 kV), and a JEOL JEM-3010 (accelerating voltage, 300 kV). High-resolution TEM (HRTEM) was performed using a JEM-4000EX (accelerating voltage, 400 kV).
observed when an aqueous Au(OH)4− solution containing a bare Si wafer was photoirradiated, which only gave irregular nanocrystals deposited on the surface (Figure S2 in Supporting Information). Apparently, all of the elementsTBAPF6 film, aqueous Au(OH)4−, and photoirradiationare required to produce developed nanowires. To observe nanowires in more detail by transmission electron microscopy (TEM), the TBAPF6 film after photoirradiation was dispersed in methanol by ultrasonication. Interestingly, flexible, vine-like thin nanowires with the thickness of 3.3−17.1 nm are abundantly seen in the TEM image (Figure 2b). These nanowires show distribution in diameter as shown in Figure S3 of the Supporting Information (average thickness ∼5.52 nm). In X-ray photoelectron spectrum of the nanowire/Si wafer sample, sharp peaks were observed at binding energies of 84.0 and 87.9 eV (Figure 3a and Figure S5 in Supporting Information). They are assigned to emissions from 4f7/2 and 4f5/2 electrons of gold(0), respectively. A highresolution TEM (HRTEM) image of the nanowire showed fringes with a spacing of 0.23 nm, which corresponds to the (111) reflection of the gold crystal (Figure 3b). Moreover, the selected-area electron diffraction pattern showed polycrystalline rings, which are assignable to the (111), (200), (220), and (311) planes of face-centered cubic gold (Figure 3c). These observations confirm that the nanowires are made of gold. This is further
■
RESULTS AND DISCUSSION A scanning electron micrograph (SEM) of the as-prepared TBAPF6 film on Si wafer is shown in Figure S1a (Supporting Information). A smooth surface was observed, while upon immersing the film in aqueous Na+Au(OH)4− solution for 30 min, some cracks were found on the film surface (Figure S1b in Supporting Information). As TBAPF6 is hydrophobic and only slightly soluble in pure water, the observed changes in the film structure reflect dissolution of TBA+ ions to the aqueous phase. This process would be promoted by the formation of 14220
dx.doi.org/10.1021/la5036568 | Langmuir 2014, 30, 14219−14225
Langmuir
Article
Figure 3. (a) XPS spectrum of the as-deposited gold nanowires on Si wafer. HRTEM images (b, d) and SAED pattern (c) of gold nanowires.
supported by a TEM-EDS measurement (Figure S4 in Supporting Information). Interestingly, the HRTEM image of the gold nanowire in Figure 3d clearly showed multicrystallinity: twinning boundaries were clearly observed, and single-crystalline domains were fused to form continuous nanowire structures. The multicrystalline structure of gold nanowires would be responsible for the distinct flexible morphology observed in Figure 2b. In the course of HRTEM observation, we noticed the presence of nano-sized pale contrasts in thicker gold nanowires, as shown in Figure S6a (indicated by arrows, in Supporting Information). As shown in Figure S6b (in Supporting Information), annual ring-like fringes were observed, indicating deformation of the gold crystal structure around the pale contrast. Such a pale contrast indicates the presence of nanocavities, as we reported for thicker gold nanowires (diameter > ∼15 nm) obtained at the water−organic solvent interface.19 We assume that TBA ions may serve as sacrificial agent to reduce Au(OH)4− ions, and these cavities would be formed by lateral segregation of such oxidized TBA species in the course of interfacial photoreduction process.19 In the present system, gold nanowires formed at the solid−water interface are remarkably thinner (the mean diameter, 5.52 nm) compared to those reported for the liquid−liquid interface system, and the yields for thick nanowires (d > 15 nm) were very low (yield
