Letter www.acsami.org
Hierarchically Structured Macro with Nested Mesoporous Zinc Indium Oxide Conducting Film Susanta Bera, Moumita Pal, Saswati Sarkar, and Sunirmal Jana* Sol−Gel Division, CSIR-Central Glass and Ceramic Research Institute, Jadavpur University, 196 Raja S.C. Mullick Road, Kolkata 700032, West Bengal, India S Supporting Information *
ABSTRACT: Fabrication of homogeneously distributed (HD) macropores by breath figure process is an active research area. Adopting the process, for the first time, we report the fabrication of HD macro with nested meso (hierarchical) porous nanocrystalline zinc indium oxide conducting sol−gel thin film on glass by dip-coating at 45−50% room relative humidity (RH) from a solution in ethanol-2-butanol (1:1, w/ w) medium with a 1:1, Zn:In ratio. In this process, solution composition and RH are found to play key roles on HD macropore generation. The film is highly promising toward visible-light-driven photoelectrochemical water splitting.
KEYWORDS: hierarchical porous film, sol−gel, breath figure process, zinc indium oxide, photoelectrochemical activity conducting thin film be fabricated by a facile sol−gel route involving BRF and EISA, the material can be highly useful for photoelectrochemical (PEC) based water splitting in the area of solar energy conversion. However, the study on PEC activity of ZI thin film is scanty17 and at the same time, the fabrication strategy of hierarchical porous ZI conducting thin film is not yet reported. Herein, adopting the BRF process, we report for the first time, the fabrication of hierarchically structured macro with nested mesoporous nanocrystalline zinc indium oxide (ZI) conducting sol−gel based thin film (conductivity, ∼ 0.65 S.cm−1 for single layer film with physical thickness ∼180 nm; Figure S1 and Table S1) on silica glass cured at 450 °C (Figure S2). The film has been deposited from surfactant-free precursor solution containing Zn to In, 50:50 (atomic ratio) in ethanol-2butanol (1:1, w/w) medium in the presence of acetylacetone (see the Experimental Section in the Supporting Information). The solution composition (Zn to In atomic ratio, RC; ethanol to 2-butanol weight ratio, RS; content of nitrates) and room relative humidity (RH) are found as key factors of the overall fabrication process. A schematic diagram (Figure 1a) presenting the formation, growth and self-organization of water-droplet arrays which subsequently, act as template for generation of HD macropores in ZI conducting thin film. Optical microscope image of ∼60 min aged solution film (SZI) clearly shows (Figure 1b) the presence of arrays of
ierarchical porous films consisting homogeneously distributed (HD) macro with mesopores have been extensively elicited owing to their amazing properties and versatile applications in energy conversion and storage, sensors, separations, catalysis etc.1−4 To generate HD macropores, the breath figure process is very fast, nonpolluting, and costeffective compared to lithography, templates, colloidal crystals, and emulsions processes.5,6 In this process, the arrays of selforganized water droplets (called breath figure, BRF) form on cold solution film surface by evaporation of volatile solvent, can act as template of the macropores.7−10 On the other hand, in solution process, the evaporation induced self-assembly (EISA) is a well-reported process for development of mesoporous films.11−13 In the past decades, the BRF process had been utilized particularly in organic polymers with a few success noticed in single metal oxide films.4,14,15 Regarding the stabilization of BRF in organic polymers, the interfacial chemistry of water droplets and solution film had found to play a major role7−10 but in the case of inorganic oxide materials, it is still under study. Zinc indium oxide (ZI) is a functional material with high chemical and thermal stability as well as superior electrical conductivity.16,17 It is known that ZnO and In2O3 can form a series of homologous compounds, In2O3(ZnO)k (k = integer), possess wide band gap energy with excellent structural and chemical stability suitable for many promising applications.18−20 Recently, ordered macroporous conducting graphene based films1 by BRF process have been reported as potential materials for advanced applications in energy conversion and storage, catalysis and sensors.1−4 Therefore, it is expected that if hierarchical porous zinc indium oxide
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© XXXX American Chemical Society
Received: October 16, 2016 Accepted: January 23, 2017 Published: January 23, 2017 A
DOI: 10.1021/acsami.6b13143 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 1. (a) Schematic presentation of the fabrication process of HD macroporous zinc indium oxide thin film. (b) Optical microscope image of precursor solution (SZI) film surface at room temperature (RH, 45−50%), displaying the formation of water droplets after ∼60 min of the film deposition. (c−e) FESEM images with different magnifications of ZI thin film, confirms the formation of HD macropores. (f) AFM 3D image with height profile (pore depth, 110 ± 10 nm), measured along the dotted line. The height profile exhibits nearly periodic arrangement of macropores (periodicity, ∼860 nm).
Figure 2. (a) FESEM image (magnified from Figure 1c) of 450 °C cured single-layered ZI film, distinctly shows the presence of macropores with their pore walls (white color, thickness ∼100 nm). (b) TEM image of the film also supports the FESEM observation. (c) Another TEM image of the sample also shows the pores and existence of nanoparticles (average size, 14.4 nm; also see Figure S4b for more details). (d) Mesopores are clearly visible from the higher magnification TEM image. (e) TEM-SAED pattern measured from the microstructure as shown in c. (f) HRTEM image of the nanoparticles in the film (inset shows the FFT from marked portion of f). (g, h) Display FESEM images of double and four layered ZI films, respectively (inset shows the corresponding magnified image). (i) AFM image of four layered ZI film. (j, k) Display FESEM crosssectional views of ZI films with double and four layers, respectively.
spherical-shaped water droplets on the SZI film surface. It is worthy to note that the solution film has been deposited from ∼24 h aged precursor solution by dip coating (lifting speed, 18 cm/min) in a 10 000+ clean room (temperature, 23 ± 2 °C, RH, 45−50%). At this condition, the transformation of atmospheric moisture into water droplets took place due to rapid evaporation of volatile solvent at the air/solution film interface.7−10,14 Moreover, the water droplets are found to grow to a self-limiting size distribution21 and they mostly selforganized into a hexagonal lattice.22 The existence of macropores (diameter, 700−900 nm; wall thickness ∼100 nm for single layer film) is confirmed from the FESEM image (Figure 1c−e) of 450 °C cured film. The height profile obtained from the 3D AFM image (Figure 1f) evidence an existence of macropores (depth, ∼110 nm). Details on pore domain and nanoparticles in the thin film are shown in Figure 2a−c. The presence of mesopores (pore diameter 10−15 nm) in the assembly of nanoparticles (average size, ∼14 nm) is clearly visible (Figure 2d). Moreover, TEM-SAED (Figure 2e) and HRTEM (Figure 2f) images confirm the formation of cubic indium oxide (c-In2O3) and a compound, In2O3(ZnO)k (k = 3) ∼Zn3In2O6 nanocrystals,19,20 as obtained by GIXRD (Figure S3) and TEM (Figure 2 and Figure S4) results. Multipoint BET N2 adsorption−desorption isotherm measurement of 450 °C cured scratched off film material, highlights an IUPAC type IV isotherms with H-2 type hysteresis loop23 (Figure S5a). This
result also implies that the system is mesoporous in nature having narrow necks and wide bodies of interconnected pore networks.23 The measured average pore size and specific surface area of the material are ∼11.3 nm and ∼80 m2/g, respectively. The mesopores are possibly formed by EISA process where 2-butanol, a short chain alcohol may create micelles,12,13 upon evaporation of ethanol. It is noted that the textural property is found to be different from the SZI precursor solution derived zinc indium oxide bulk powder cured at 450 °C (Figure S5b). We also successfully fabricated a thicker (∼600 nm, Figure 2g−k) hierarchical porous film via layer-by-layer deposition (up to four layers) under the controlled condition as adopted for the single-layer film. Stability of HD macropores in ZI film is found to be dependent upon the curing temperature as observed from FESEM study (Figure S6). On increasing the curing temperature to 600 °C, the shape of macropores changes remarkably and the film shows an interconnected pore assemblies, consisting larger pores (1.3−1.7 μm) surrounded by smaller pores (160−180 nm). With the pore architecture, the film B
DOI: 10.1021/acsami.6b13143 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces matrix also possesses the nanoclusters (size, 30−45 nm). However, an interesting film surface feature is observed when the film curing temperature reached to 900 °C. The film with the spherical shaped pores (diameter, 290−390 nm) and isolated channels is found to be cracked severely. Moreover, further growth of nanoclusters (size, 60−90 nm) is observed from FESEM study (Figure S6). This observation also supported the TEM analysis (Figure S4). In addition, the TEM analysis on the film cured at ≥600 °C confirms a major reduction in film mesoporosity, possibly due to formation of larger crystals.24 In this curing temperature, a significant decrease in the Zn to In ratio (RC) is found as confirmed from TEM-EDS (Figure S4) and XPS analyses (Figure S7). In this regard, from the EDS study, we estimated Zn to In ratios of 48.3:51.7, 32.5:67.5, and 13.6:86.4 for the film cured at 450, 600, and 900 °C, respectively. It is further noted that only Zn(II) and In(III) ions are evident from the XPS study of the films. The loss of zinc can be considered due to evaporation of ZnO24 from In2 O3 (ZnO)3 (Figure S3). However, the formation of a new crystal in 900 °C cured film is evident from its GIXRD pattern (Figure S8). In this film, nanosized rhomboherdal Zn2SiO4 [JCPDS Card 37−1485] along with cIn2O3 is found (Figure S8). Perhaps, zinc ions from In2O3(ZnO)3 compound interact with silica glass substrate forming Zn2SiO4 via ion diffusion process.20,25 This can be happened due to thermal instability 20 of nanosized In2O3(ZnO)3 (Figures S6 and S8) at higher curing temperature. Therefore, the synergic effect of significant decrease in Zn to In atomic ratio (RC), increase in clustered size and a dramatic change in crystal structure can lead to diminish the stability of macropores in ≥600 °C cured films. In the present work, many factors are found to play key roles7−10,14,15 on formation, growth and self-organization into arrays vis-à-vis stabilization of water droplets that subsequently act as template for generation of HD macropores. These are room RH and solution composition (RC, Zn to In atomic ratio; RS, ethanol to 2-butanol weight ratio; nitrate content). It is found that the change of room RH greatly influences on the formation of water droplets (Figure S9). At lower humidity, the availability of room moisture can be in-sufficient for generation of breath figures while at the higher humidity, water droplets of wide size range can grow due to their rapid coalescence from large quantity room moisture.9,14,26 Thus, the film deposited in lower humidity (say, 30−35%), the formation of macropores can be less while at the higher humidity (e.g., 65−70%), it can result a distorted larger sized macropores. Our experimental results also strongly justify the above statements (see Figure S9 for details). Therefore, an optimum RH, 45−50% has been chosen to obtain water droplets with narrow size distribution for HD macropore formation. At room atmosphere, ethanol is more volatile than 2-butanol (b.p., ethanol ∼80 °C; 2-butanol, ∼100 °C). Therefore, in this work, the water droplets formation can mostly be associated with the evaporation of ethanol whereas the 2-butanol can assist the self-organized water droplets to sink down into the solution film.14 For obtaining the HD macropores, the coexistence of ethanol and 2-butanol in 1:1 (w/w) (RS) is found to be an optimum in the precursor solution (Figure 3). Few water droplets are seen on the solution film surface when it is deposited from the precursor solution with 100% ethanol as solvent. As a result, the formation of irregularly spread macropores is noticed (Figure 3b). This type of water-droplet formation can possibly be related to the rapid evaporation of
Figure 3. (a) Schematic presentation for sinking down of developed water droplets upon solution film to form HD macropores; (b−f) photographs of 24 h aged precursors prepared by varying the proportion of ethanol to 2-butanol (w/w), Rs = (b) 1.0:0.0, (c) 1.0:0.5, (d) 1.0:1.0, (e) 0.5:1.0, and (f) 0.0:1.0 while fixing the Zn to In atomic ratio (RC), 50:50 with a fixed equivalent oxide content (6 wt %). Step 1 indicates film deposition by dip-coating with lifting speed, 18 cm/ min from the aged precursor solutions followed by keeping the ascoated solution films for ∼60 min in the coating room (temperature, 23 ± 2 °C) with RH, 45−50%. Step 2 describes thermal curing of aged solution films under identical condition as performed for the ZI film deposition.
ethanol.27,28 Although, the precursor solution (SZI45) with RC, 55:45 shows the generation of sufficient number of water droplets but the macropores in the ZI45 film may not exist because of development of islandlike particles of wide size range (1.5−7.5 μm) (Figure S10,). However, a different phenomenon is found to be happened in ZI55 film deposited from the solution with RC, 45:55. In this case, the existence of macropores (Figure S10) is not found because of rarely formed water droplets on SZI55 solution film surface as revealed from optical microscope image of the film. It is seen that the viscosity measured at 23 ± 2 °C of SZI55 precursor solution is slightly higher (∼3.3 cP) compare to SZI45 solution (∼3.1 cP). The increased viscosity may indicate the formation of enhanced metal oxygen cross-linkages via condensation polymerization.29 Therefore, the sinking down of water droplets onto higher viscous solution film surface is less feasible.14,27 Stabilization of BRF is absolutely necessary for the formation HD macroporous film.10 In this regard, for the solution film, an important role of nitrate content on the stabilization of BRF is found while changing the nitrate concentration in the precursor (Figure S11 and Table S2). It is also noted that we have used acetylacetone (acac) as a solution stabilizer, which can form a partial complex with In(III) ions as revealed from UV−vis spectra of the solution film (Figure S12). Moreover, the existence of free and complexed nitrates is also evident from the FTIR spectra (Figure S13 and Table S3) of the SZI film. Thus, an optimum content of nitrates is highly required for stabilization of BRF. Possibly, the nitrates as interfacially active inorganic species can adsorb at the interface of solution film and water droplets.8,10 The species may stabilize the BRF from coalescence. We also deposited the hierarchical porous ZI film on conducting ITO coated glass (Figure S14). Moreover, photoC
DOI: 10.1021/acsami.6b13143 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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porous film matrix. It is noted that the hierarchical pores are found to be restored even in the multilayer films made via layer by layer successive deposition (Figure 2h, i). This significant improvement of PEC activity can also demonstrate the existence of more accessible pore openings and available internal surface area (see Figure S18) in hierarchically structured macro with nested mesoporous zinc indium oxide thin film.11 In summary, for the first time, adopting breath figure process, we report the fabrication of hierarchically structured macro with nested mesoporous zinc indium oxide conducting sol−gel thin film on silica glass substrate from surfactant-free precursor solution in the presence of acetylacetone. Arrays of selforganized water droplets formed on cold solution film surface act as template for generation of macropores whereas mesopores are possibly developed by evaporation induced self-assembly. Solution composition (Zn to In atomic ratio, ethanol to 2-butanol weight ratio, content of nitrates) and room RH are examined as key factors for the formation of macropores. The hierarchical porous film is found to be promising for photoelectrochemical water splitting under visible-light irradiation and can have substantial opportunity in solar energy conversion.
electrochemical (PEC) activities of In2O3 and ZI films (Figure 4) have been studied under visible-light irradiation. It is seen
Figure 4. Photoelectrochemical activities of pristine In2O3 and ZI samples: (a) Linear sweep voltammetry (LSV) curves measured under dark condition and visible light exposure, (b) photocurrent response through on−off cycles at 0.26 V vs Ag/AgCl/3 M KCl, (c) change of current density with time (I−t curves) recorded under visible light irradiation on four layer ZI film, and (d) electrochemical impedance (EIS) Nyquist plots of electrodes of different samples (In2O3 and ZI thin films) under visible light illumination. Equivalent circuit simulation is embedded in the figure (more details are given in the Supporting Information).
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b13143. Fabrication and characterization details, XRD, FESEM, TEM, UV−vis, FTIR, PL, band gaps, XPS, BET N2 adsorption−desorption, nitrogen content, thermogravimetry, conductivity, CV (PDF) Video S1, showing the ZI film as photoanode can efficiently perform PEC water splitting toward generation of H2 and O2 (AVI)
that with increasing the layer thickness by successive deposition, the photocurrent density (PCD) is found to increase gradually (Figure 4a and Table S4). To further investigate the PEC activity, we have also performed the photo response at 0.26 V vs Ag/AgCl [1.23 V vs RHE (reversible hydrogen electrode)] (Figure 4b). With the similar film thickness, the double-layered ZI film shows more than 8 times higher PCD value than pristine In2O3 thin film and also higher than indium-oxide-based materials (Table S5). A significant long-term PEC stability against photo-oxidation of four layer ZI film at 0.26 V vs Ag/AgCl is also found (Figure 4c). Electrochemical impedance spectra (Figure 4d) displays a gradual decrease in Rct1 (see the Supporting Information) that resulted from the increase in porosity30 in the ZI films upon increasing the number of layers, as evident from the FESEM and AFM analyses (Figure 2). Therefore, the ZI film as photoanode can efficiently perform PEC water splitting toward generation of H2 and O2 (see Video S1). This can be attributed to the presence of hierarchically structured macro with nested mesoporous architecture that can provide large number of active sites for ionic diffusion and transportation.11 Thus, the film may be useful for efficient solar light harvesting as the band gap of the film is found to be narrow (Figure S15) that caused by the surface oxygen vacancies as evidenced from photoluminescence spectral study and XPS analysis of the film (Figures S7 and S16). Moreover, the decrease in UV PL intensity (Figure S16) of In2O3 in the ZI film can imply an occurrence of effective electron transfer, supposed to be happened from In2O3 to In2O3(ZnO)3 ≈ Zn3In2O6 semiconductors (Figure S17). This process can decrease the recombination rate of photogenerated charge carriers and also can diminish an accumulation of larger electron density of the
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] or
[email protected] ORCID
Sunirmal Jana: 0000-0003-4573-4465 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS S.B. thankfully acknowledges CSIR, India, for providing his doctoral research fellowship. This work has been done under 12th Five Year Plan Project of CSIR (ESC0202).
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REFERENCES
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