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ZnO Layered Structure on Silk Textile for Flexible Device Applications
Electrochimica Acta 253 (2017) 39–46
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Fabrication and Photocatalytic Performance of Au/ZnO Layered Structure on Silk Textile for Flexible Device Applications Wan-Ting Chiua,b , Chun-Yi Chena,b,* ,1, Tso-Fu Mark Changa,b,* ,1, Yuma Taharac , Tomoko Hashimotoc, Hiromichi Kurosuc , Masato Sonea,b,1 a b c
Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan CREST, Japan Science and Technology Agency, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan Department of Clothing Environmental Science, Nara Women's University, Kitauoya Higashimachi, Nara 630-8506, Japan
A R T I C L E I N F O
Article history: Received 27 July 2017 Received in revised form 6 September 2017 Accepted 7 September 2017 Available online 8 September 2017 Keywords: ZnO/Au/silk composite Supercritical CO2 catalyzation Electroless plating Cathodic deposition Photocatalytic activity
A B S T R A C T
This study integrated ZnO/Au layered structure on silk textiles to realize a flexible photocatalytic composite material for wearable devices. Supercritical carbon dioxide promoted electroless plating and cathodic deposition were used to metallize and deposit photocatalytic metal oxides on silk textiles. Silk was used as the substrate for its flexibility and stretchability. Au was selected due to its high biocompatibility, electrical conductivity, and ductility. ZnO was chosen for its photocatalytic activity and biocompatibility to fulfill the application. Surface morphology, composition, crystal structure, electrical resistance and photocatalytic activity assessments were evaluated. With the assistance of supercritical carbon dioxide, smooth, strong adherence and uniform coverage of Au layer was successfully electroless plated on the silk. ZnO was homogeneously deposited on the Au metal layer. The coatings on the silk have been confirmed to be metallic Au phase and ZnO wurtzite phase. The ZnO modified specimen with 0.2 wt. % H2O2 in the electrolyte showed the highest crystallinity and performed 11.5 times enhancement in the photocurrent density when comparing to the H2O2-free one. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction The global market of wearable devices is predicted to keep prospering in the future [1]. In the meanwhile, wearable devices are diversified into different functions such as biomedical devices, supercapacitors, and photocatalytic devices [2] due to diverse requirements in the next-generation technology. The integration of functional and flexible materials turns out to be the most critical step in fabrication of the component used in wearable devices. For example, integration of photocatalytic, electrical conductive, and flexible materials is needed to realize wearable devices capable of harvesting solar energy. Flexible materials such as cloth textiles are usually nonconductive, and the flexible material can be made electrically conductive by depositing metal coatings on the non-conductive material through electroless plating [3]. Electroless plating
* Corresponding authors at: Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan. E-mail addresses: [email protected] (C.-Y. Chen), [email protected] (T.-F.M. Chang). 1 ISE member. http://dx.doi.org/10.1016/j.electacta.2017.09.041 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
consists of a pretreatment step to clean the substrate, a catalyzation step to activate the substrate, and a metallization step to metallize the substrate. In the conventional catalyzation step, catalyst-contained corrosive aqueous solution is used to activate the substrate by depositing the catalyst on the substrate; meanwhile, the substrate structure is often damaged during the process. Due to the high surface tension and polarity of the aqueous catalyzation solution, the catalyst is only inlaid on the substrate surface resulting in poor adhesive property. On the other hand, supercritical carbon dioxide (sc-CO2) can be introduced to the catalyzation step to enhance the plating characteristics [4,5]. Carbon dioxide develops into supercritical phase owning properties in the midway of gas phase and liquid phase when both the temperature and pressure go beyond its critical point [6]. In this case, sc-CO2 can be used as the solvent, and an organic-metal complex would be used as precursor of the catalyst. Due to the affinity to non-polar material [7], low surface tension [8], and high self-diffusivity [6] of sc-CO2, sc-CO2 thus can convey the organicmetal complex into the textile structure while remains the substrate structure intact, and the catalyst could be formed after a reduction process. Regarding the photocatalytic functionality, there are various synthetic methods to deposit photocatalytic metal oxides, which
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include high-temperature vapor deposition [9], polymer-mediated chemical reaction [10], hydrothermal-annealing process [11], and one-pot solution-based method [12]. Among the synthetic techniques, cathodic deposition allows a simple and effective process to produce metal oxides [13–15]. In addition, cathodic deposition can cooperate with lithography to allow further applications such as patterning [16]. In this study, the photocatalyst/metal/cloth textile flexible composite material was realized by sc-CO2 assisted electroless plating and cathodic deposition for applications in wearable devices. A slight disturbance from the rigid component in a wearable device can be annoying to the user, and hence a flexible material is preferred in wearable devices. Silk, a common clothing material, was chosen in this study due to its flexibility and stretchability. Since wearable devices come into contact with the skin frequently during the employment, a thin Au layer, which is known to be highly biocompatible, was selected as the electrically conductive layer. Moreover, Au also shows high ductility [17], which is an essential requirement for wearable devices. There are many materials that can play the role as the photocatalyst in the composite material, such as TiO2 [18], CuO [19], and ZnO [20]. ZnO was designated as the photocatalyst due to its comprehensive applications, requirement of low fabrication temperature to have high crystallinity, and simple fabrication process. There are several reports on preparation of flexible photocatalyst materials, such as metal-organic chemical vapor deposition of GaN on graphene film [21] and electrospinning combined with hydrothermal method to deposit TiO2 on polyacrylonitrile [22]. However, due to the aforementioned requests in next generation wearable devices, flexible composite materials composed of highly biocompatible cloth textile, conductive metallic layer, and photocatalyst are needed. Here, we report a facile fabrication process in realization of a flexible and photocatalytic ZnO/Au/silk layered composite material toward applications such as solar energy harvesting wearable devices. Fundamental properties required for wearable devices such as electrical resistance and photocatalytic activity evaluations are demonstrated.
2. Experimental 2.1. Chemicals A piece of silk in the dimension of 2 cm 4 cm was used as the substrate, and Pd(II) acetylacetonate (Pd(acac)2) (98.0%, Tokyo Chemical Industry Co., Ltd., Japan) was used as the organic-metal complex. Carbon dioxide gas (99.99%, Nippon Tansan Gas Co., Ltd. Japan) was used as the source of sc-CO2. A commercially available acidic Ni-P electroless plating electrolyte (Okuno Chemical Industries Co., Ltd., Japan) consisted of nickel chloride (1.8 wt.%), sodium hypophosphite (2.4 wt.%), complexing agent (2.4 wt.%), and ion-exchanged water (93.4 wt.%) was used in this study. The Au electroless plating electrolyte used in this study was a commercially available electrolyte provided by MATEX JAPAN Co., Ltd. The Au electroless plating electrolyte was composed of ammonium gold sulfite (0.2 wt.%), reducing agent (0.3 wt.%), complexing agent (0.3 wt.%), and ion-exchanged water (99.2 wt.%). The electrolyte for cathodic deposition of ZnO comprised of 0.25 wt.% ZnCl2 (98.0%, Sigma-Aldrich, USA), 3.0 wt.% NaCl (99.0%, Sigma-Aldrich, USA), 1.5 wt.% NaNO3 (99.0%, Sigma-Aldrich, USA), 0–0.3 wt.% H2O2 (30%, Sigma-Aldrich, USA), and ion-exchanged water. 2.2. Sc-CO2 assisted catalyzation No pretreatment was carried out before the sc-CO2 catalyzation step. Pd(acac)2 was used as the precursor in formation of the Pd catalyst seed used in metallization of a Ni-P layer on the silk, and 25 mg of the Pd(acac)2 catalyst was used with respect to a 50 ml reaction cell to maintain the Pd(acac)2 concentration at the saturation point throughout the process. A piece of the silk textile was hooked from inner ceiling of the 50 ml stainless reaction cell [23], and sc-CO2 and the Pt(acac)2 were introduced into the reaction cell afterward. A high-pressure apparatus (Japan Spectra Company, Japan) was used to pressurize CO2 into the reaction cell, and the details could be found in a previous study [23]. The catalyzation was executed at 80 1 C and 15 0.1 MPa with
Fig. 1. (a) OM image and (b) XRD pattern of the as-received silk, and (c) OM image and (d) XRD pattern of the catalyzed silk (diamond symbols: Pd(acac)2).
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Fig. 2. OM images of the (a) Ni-P metallized silk at 4 min and the Au metallization at (b) 3 min, (c) 5 min, and (d) 80 min.
agitation for 2 hours [24]. No post treatments were carried out after the catalyzation step. 2.3. Metallization A layer composed of a less noble metal is needed to initiate deposition of the Au layer. Hence, Ni-P metallization was conducted on the catalyzed silks at 70 C and atmospheric pressure using the Ni-P electroless plating electrolyte. Metallization time at 4 min was conducted to ensure the full coverage [25]. Likewise, Au metallization process was executed after the Ni-P metallization step for various lengths of metallization times (t = 3, 5, 10, 20, 40, 60, and 80 min). The Au plating bath was set at 70 C and atmospheric pressure. Specimens showing a full Au coverage were selected to proceed with deposition of the ZnO layer. 2.4. ZnO cathodic deposition The cathodic deposition was run at a constant current density of 0.010 A/cm2 at 70 C for 2 h without agitation. Surface area of the working electrode, that is the contact area between the Au layer and the electrolyte, was estimated to be 10 cm2, which was calculated by estimating surface area of every bundle of the Au metallized silk in a piece of the specimen. A piece of Pt foil was used as the counter electrode. The cathodic depositions were performed by using a Potentiostat/Galvanostat (1287A, Solartron Analytical, UK). No post treatment was conducted after the ZnO cathodic deposition step. 2.5. Characterization
Fig. 3. XRD patterns of the Au metallization at (a) 5 min, (b) 20 min, and (c) 80 min and (d) the composition revolution with the Au metallization time.
The surface morphology, cross-section and composition were examined by an optical microscope (OM; VHV-5000, KEYENCE, Japan), a scanning electron microscope (SEM; JSM-7500, JEOL, Japan), and energy-dispersive X-ray spectroscopy (EDX; EMAX Evolution, HORIBA, Japan). Thickness of the Au layer was estimated by observing cross-section of the Au metallized silk from the SEM image and ImageJ software (National Institutes of Health, USA).
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3. Results and discussion 3.1. Surface morphology and structures of the Au layer
Fig. 4. Plot of the metallization thickness versus the metallization time. An OM image of cross-section of the composite material is inserted.
Fig. 5. (a) Electrical resistances of the Au metallized silk at various metallization times, and SEM images of the Au metallized silk at (b) 20 min and (c) 80 min of the Au metallization time (hollow symbols: Au-free; solid symbols: Au metallized).
The crystal structures and phases were identified by an X-ray diffractometer (XRD; Ultima IV, Rigaku, Japan). The electrical resistance was measured by a four-point probe (Mitsubishi Chemical Analytech Co., Ltd., MCP-T370, Japan). Adhesive test of the Au layer was conducted by a repeating peeling test using the 3 M tape (3 M adhesive tape #810, 3 M, USA) [24]. The electrical resistance was measured again after the adhesive test to evaluate the reliability. The photoelectrochemical measurement was conducted in a three-electrode cell consisting of a Pt counter electrode, a Ag/AgCl reference electrode, and a piece of the ZnO/Au on silk as the working electrode. The chronoamperometric It curves were recorded at 0 V vs Ag/AgCl using the Potentiostat/ Galvanostat under white light illumination (500 W xenon lamp, with a light intensity of 100 mW/cm2). Na2SO4 (0.5 M) solution was used as the electrolyte.
3.1.1. Sc-CO2 assisted catalyzation Fig. 1 shows OM images and XRD patterns of the as-received silk and the catalyzed silk. The crossed-linked see-through bundles shown in Fig. 1(a) imply the silk textile, and the yellow flakes in Fig. 1(c) specify the Pd(acac)2. A broad peak at 2u = 20 degree can be observed in Fig. 1(b), which reveals an amorphous structure of the silk textile. On the other hand, in Fig. 1(d), diffraction peaks labelled by diamond symbols can be indexed to Pd(acac)2 [26,27], and the results indicated the catalyst precursors were successfully deposited on the substrate. As shown in the OM images, the silk substrate remained intact after the catalyzation step due to the low surface tension, low viscosity, and non-corrosive properties of scCO2. The sc-CO2 assisted catalyzation step demonstrated a significant improvement while comparing to the conventional catalyzation process which was shown in a previous study [25]. Pd (acac)2 acts as an activation site after it was reduced to Pd metal by the reductant in the metallization solution. The Pd metals can play the role as the catalyst in the following metallization step. 3.1.2. Metallization Development of the Au layer is shown in Fig. 2. Fig. 2(a) shows morphology of the specimen with the Ni-P metallization time at 4 min before the Au metallization. Fig. 2(b-d) demonstrate the Au metallization time at 3 min, 5 min, and 80 min, respectively. At 3 min of the Au metallization (Fig. 2(b)), part of the Ni-P layer was still not enclosed showing a gray luster. As the Au metallization times were extended to 5 min and longer (Fig. 2(c, d)), the Ni-P layer was completely concealed by the Au layer. The XRD patterns at various Au metallization time are shown in Fig. 3(a–c). Five apparent diffraction peaks shown in Fig. 3(a–c) locate at 2u = 38.2, 44.4, 64.6, 77.6, and 81.7 degree can be indexed to (111), (200), (220), (311), and (222) planes of the FCC structure for Au (JCPDS #65-2870), respectively. In Fig. 3(a), at 5 min of the Au metallization time, only a thin layer of Au was metallized on the silk, and the broad diffraction peak of silk at around 20 degree was still detectable. Ni-P diffraction peaks were not found in Fig. 3(a), since the Ni-P layer is designed as the sacrifice layer for the Au metallization. In addition, it owns an amorphous structure [25], and the diffraction peaks might be overlapped by the high intensity Au diffraction peaks. As the Au metallization proceeded, no diffraction peaks from the silk were found in the XRD patterns (Fig. 3(b-c)) since the Au layer is thick enough to suppress the diffraction peaks from the silk. Fig. 3(d) shows the composition changed with the Au metallization time. In the early stage, due to the insufficient thickness of Au, the Ni-P layer was detectable by the EDX. On the contrary, the Ni-P was no longer detectable as the Au metallization time extended to 80 min. Phosphorus was not detected owing to the insufficient amount [25]. Fig. 4 illustrates growth of the Au layer thickness with the Au metallization time. There is no obvious partition of the Ni-P and Au layer shown in the inserted SEM image in Fig. 4. A positive correlation was found between the Au layer and the Au thickness presenting the average growth rate at 0.48 mm/h. The growth rate slowed down slightly from 60 min to 80 min of the Au metallization time due to the consumption and the reduction in concentration of the metal ion and reducing agent as the reaction proceeded. 3.2. Electrical conductivity and adhesive tests The electrical resistances at various Au metallization times are shown in Fig. 5. The electrical resistance dropped down in the early stage and increased after reaching a minimum point. Since the Ni-P
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Fig. 6. SEM images of the specimens deposited in (a) free of H2O2, (b) 0.1 wt.%, (c) 0.2 wt.%, and (d) 0.3 wt.% H2O2, and the OM images in (e) free of H2O2, (f) 0.1 wt.%, (g) 0.2 wt. %, and (h) 0.3 wt.% H2O2.
layer is not fully covered by Au at 3 min of the Au metallization time (solid symbols) (Fig. 2(b) and Fig. 5(a)), it showed high electrical resistance close to the Au-free specimen at 0 min of the
Au metallization (hollow symbols) (Fig. 2(a) and Fig. 5(a)). An inserted graph in Fig. 5(a) shows results of the Au metallization time between 0 to 5 min. The electrical resistances obtained from 0
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minimum at the Au metallization time of 20 min due to the complete coverage, smooth surface, and sufficient Au thickness (Fig. 5(b)). From the beginning of the full coverage to the region near the lowest electrical resistance can be sorted to the second stage (from the dash line at 5 min to the other one at 40 min). The electrical resistance increased slightly as the Au metallization time increased beyond the minimum point. The surface grew coarser (pointed by the arrows in Fig. 5(c)) with the elongated metallization time, and the electrical resistances were deteriorated due to more electron scattering [28]. The region of worsened electrical resistance and bumpy surface can be categorized into the third stage. The electrical resistances after the adhesive tests are shown in Fig. 5(a). No significant impact was observed in all stages since the silk was completely enclosed by the Ni-P and Au, and the adhesions between the silk and the Ni-P and between the Ni-P and the Au both showed sturdy adhesive property. 3.3. ZnO cathodic deposition and photocatalytic activity assessment
Fig. 7. General scheme of the solute concentration profiles.
Fig. 6 shows SEM images (Fig. 6(a–d)) and OM images (Fig. 6(e– h)) of the pristine ZnO deposited with H2O2 concentration varied from 0 to 0.3 wt.%. Tower-like ZnO nanowires are observed in all of the SEM images (Fig. 6(a-d)). ZnO coverage on the Au layer was processed and calculated by a built-in software of the OM. The ZnO coverages were at 63%, 62%, 41%, and 32% for the specimens deposited with free of H2O2 to 0.3 wt.% H2O2. Morphology of cathodically deposited ZnO is often affected by Cl in the solution. The Cl can act as a capping agent adsorbing on the (0001) plan of ZnO to stabilize the surface and leads to formation of nano-rod like structures. On the other hand, when concentration of the Cl is low, tower-like structures instead of nano-rod structures are formed [29]. Furthermore, size of the ZnO particle increased as the H2O2 concentration increased from 0 to 0.2 wt.% as shown in the SEM images. Similar results on increase in the ZnO particle size as the H2O2 concentration increased were reported in a literature [30]. However, in Fig. 6(d), the particle size decreased when the H2O2 concentration went beyond 0.2 wt.%. ZnO deposition equations are shown in the following [31]: NO3 + H2O + 2e ! NO2 + 2OH
(1)
H2O2 + 2e ! 2OH
(2)
Zn2+ + 2OH ! ZnO + H2O
(3)
Fig. 8. (a) XRD patterns and (b) voltage-time plot of the specimens fabricated with electrolytes containing various H2O2 concentration. (star symbols: Au and hexagon symbols: ZnO).
to 5 min (the dash line shown in Fig. 5) of the metallization time, which merely showed a slight difference from the Au-free one, are classified into the first stage. Full coverage was constructed as the Au metallization time reached 5 min, and the electrical resistance was improved accordingly. The electrical resistance reached its
OH generated on the substrate from Eqs. (1) and (2) are expected to react with Zn2+ in the electrolyte to form ZnO on the substrate. As shown in Eq. (2), generation rate of the OH would be increased with an increase in H2O2 concentration. Since the diffusivity of Zn2+ in aqueous solution is four times slower than that of H2O2, diffusion of Zn2+ to the substrate surface cannot catch up with the generation of OH as the H2O2 concentration increased [32]. When the amount of H2O2 was increased to 0.3 wt.%, generation rate of the OH could be too high,and lead to accumulation of the OH near the electrode surface. A general scheme of the solute concentration profiles in the electrolyte is shown in Fig. 7. OH are generated on the substrate, hence the concentration is gradually lowered toward the bulk solution direction. Zn2+ would diffuse from the bulk solution to the substrate surface to react with the OH, therefore, Zn2+ concentration is lowered as it approaches the substrate surface. Depends on the OH generation rate, local OH concentration, or pH, on the substrate could vary. ZnO is formed at a pH range of ca. 8 to 12, and ZnO would dissolve away to form ZnO22 or HZnO2 when the pH is higher than 12 [33,34]. The
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Fig. 9. Photocurrent density of the ZnO/Au/silk hybrid materials with (a) noncorrected (an inserted figure shows the flexibility of the composite material) and (b) corrected by the ZnO coverage.
region suitable for deposition of ZnO is defined as region I, and the region leads to dissolution of ZnO is denoted as region II as shown in Fig. 7. The region near surface of the substrate would be in region I when the H2O2 concentration was lower than 0.2 wt.%. Therefore, an increase in the H2O2 concentration led to an increase in the particle size (Fig. 6(a-b)) and the coverage (Fig. 6(e-f)). When the H2O2 concentration was increased to 0.3 wt.%, both the particle size (Fig. 6(d)) and the coverage (Fig. 6(h)) were decreased, which suggest the pH near the substrate surface could be higher than 12. The results suggest the region near the substrate surface is in region II when the H2O2 concentration is higher than 0.3 wt.%. Fig. 8(a) shows XRD patterns of the ZnO deposited with various H2O2 concentrations in the electrolyte. Only the diffraction peaks of Au (star symbols) and ZnO (hexagon symbols) were observed. No secondary phase was found in the XRD patterns after the ZnO deposition within the detection limitation. A relative crystallinity of ZnO on the Au layer was roughly estimated by calculating ratio of the Au (111) intensity to the ZnO (100) intensity with consideration of the ZnO coverage. The ratios were at 0.10, 0.11, 0.34, and 0.16 from free of H2O2 to 0.3 wt.% H2O2. The ratios indicated the electrolyte containing 0.2 wt.% H2O2 produced ZnO with the best crystallinity, and the crystallinity became worse when 0.3 wt.% H2O2 was used. Another explanation concerning crystallinity of the ZnO is provided here. Crystallinity of the ZnO is reported to be dependent on concentration of the Zn2+ [35], which defects and deteriorated crystallinity are introduced to the ZnO when the Zn2+ concentration is low. In this study, when the H2O2 concentration reached 0.3 wt.%, local concentration of OH near the electrode surface would be high, and higher diffusion rate of OH to the bulk
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solution was built. Then, pH far away from the substrate surface became suitable for formation of ZnO and consumed great amount of the Zn2+. In this case, amount of the Zn2+ diffused to the substrate surface was lowered and resulted the deteriorated crystallinity. Fig. 8(b) shows the voltage-time curves during the ZnO deposition. While there is H2O2 in the electrolyte, the potential drop decreased as the H2O2 concentration increased. The potential drop here refers to the potential difference between the potential at ca. 0 sec of the reaction time and the potential as the reaction gradually stabilized with time. No obvious difference was found in the specimens deposited with free of H2O2 and 0.1 wt.% H2O2, which are in good agreement with the ZnO coverage in Fig. 6(e) and Fig. 6(f) revealing that 0.1 wt.% H2O2 has limited influence on the ZnO deposition. The potential fluctuation increased with an increase in the H2O2 concentration, which is because more N2 gas is emitted at high pH [36]. To assess the applicability as photocatalytic materials, photocurrent generated from photoelectrochemical water splitting reaction using the as-deposited sample as the photoanode was evaluated. Fig. 9 shows the chronoamperometric It curves of the ZnO/Au/silk layered composite materials recorded in 0.5 M Na2SO4 electrolyte under chopped light illumination. The current density was corrected by considering coverage of the ZnO on the Au layer, which the ZnO coverages are provided in the beginning of this section. Fig. 9(a) shows the as obtained current density, which is before the correction, while Fig. 9(b) shows the corrected values. All the specimens showed an increased in the current density when the illumination is on, which imply the effective generation, transfer and collection of the electrons during the water splitting process. The ZnO deposited with 0.2 wt.% H2O2 showed the highest current density in both Fig. 9(a) and (b) since it owns the highest crystallinity. It has been reported that the high crystallinity can assist in suppressing the possible photocorrosion effect [37]. In addition, high crystalline ZnO leads to efficient transportation of the excited electrons. Photocurrent density of the ZnO fabricated with 0.2 wt.% H2O2 showed 11.5 times enhancement when comparing to the one deposited without H2O2. The greater photoelectrochemical performance can be attributed to the facile carrier transport resulting from the oriented crystallinity. In this study, high H2O2 concentration led to the high photocurrent owing to the high crystallinity; however, excess OH resulted in the dissolution of ZnO and deteriorated crystallinity, which resulted a deteriorated photocurrent density. A digital photo is inserted into Fig. 9(a) to show flexibility of the ZnO/Au/silk layered composite material. Various evaluations were conducted to reveal properties of the silk/Au/ZnO composite materials in this study. The composite materials possessed flexibility, high electrical conductivity, and high adhesive firmness. Photocurrent density was enhanced significantly by synthesizing ZnO via the optimized electrolyte. Therefore, the results showed the potential of the silk/Au/ZnO composite materials in applications in flexible wearable devices. 4. Conclusions ZnO/Au layered structure was successfully fabricated on the silk textile by sc-CO2 assisted electroless plating and cathodic deposition. Common problems encountered in the conventional electroless plating were solved by the introduction of sc-CO2. With the help of sc-CO2, silk was catalyzed without defects and the adhesive property between the silk and the metallized layer was enhanced at the same time. Surface coverage and thickness of the Au layer increased with the Au metallization time. The lowest electrical resistance was achieved as the Au metallization time extended to 20 min. On the other hand, ZnO was deposited on the
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Au metallized silk via the cathodic deposition. The coatings on the silk have been confirmed to be metallic Au phase and ZnO wurtzite phase. ZnO deposited with 0.2 wt.% H2O2 showed the best crystallinity and the highest photocurrent density. The ZnO modified specimen with 0.2 wt.% H2O2 performed 11.5 times enhancement in the photocurrent density when comparing to that without H2O2. Acknowledgment This work is supported by Japan Society for the Promotion of Science (17J07395), the Grant-in-Aid for Scientific Research (B) (JSPS KAKENHI Grant Number 26282013), CREST Project (#14531864) operated by the Japan Science and Technology Agency (JST), and research grant from The Murata Science Foundation. References [1] J. Gozalvez, 5G tests and demonstrations [mobile radio], IEEE Veh. Technol. Mag. 10 (2015) 16–25. [2] B.R. Sankapal, H.B. Gajare, S.S. Karade, R.R. Salunkhe, D.P. Dubal, Zinc oxide encapsulated carbon nanotube thin films for energy storage applications, Electrochim. Acta 192 (2016) 377–384. [3] G.O. Mallory, J.B. Hajdu, Electroless plating: fundamentals and applications, William Andrew, Amsterdam, 1990. [4] H. Adachi, K. Taki, S. Nagamine, A. Yusa, M. Ohshima, Supercritical carbon dioxide assisted electroless plating on thermoplastic polymers, J. Supercrit. Fluids 49 (2009) 265–270. [5] J.J. Watkins, T.J. McCarthy, Polymer/metal nanocomposite synthesis in supercritical CO2, Chem. Mater. 7 (1995) 1991–1994. [6] G. Brunner, Gas extraction: an introduction to fundamentals of supercritical fluids and the application to separation processes, Springer Science & Business Media, Berlin, 2013. [7] A. Clifford, T. Clifford, Fundamentals of supercritical fluids, Oxford University Press, Oxford, 1999. [8] S.P. Sawan, S.P. Sawan, Supercritical fluid cleaning: fundamentals, technology and applications, Elsevier, Amsterdam, 1998. [9] C. Gorla, N. Emanetoglu, S. Liang, W. Mayo, Y. Lu, M. Wraback, H. Shen, Structural optical, and surface acoustic wave properties of epitaxial ZnO films grown on (0112) sapphire by metalorganic chemical vapor deposition, J. Appl. Phys. 85 (1999) 2595–2602. [10] B. Panigrahy, M. Aslam, D. Misra, D. Bahadur, Polymer-mediated shapeselective synthesis of ZnO nanostructures using a single-step aqueous approach, CrystEngComm 11 (2009) 1920–1925. [11] C.-H. Ku, J.-J. Wu, Aqueous solution route to high-aspect-ratio zinc oxide nanostructures on indium tin oxide substrates, J. Phys. Chem. B 110 (2006) 12981–12985. [12] Q. Zhang, C. Tian, A. Wu, T. Tan, L. Sun, L. Wang, H. Fu, A facile one-pot route for the controllable growth of small sized and well-dispersed ZnO particles on GO-derived graphene, J. Mater. Chem. 22 (2012) 11778–11784. [13] S. Peulon, D. Lincot, Mechanistic study of cathodic electrodeposition of zinc oxide and zinc hydroxychloride films from oxygenated aqueous zinc chloride solutions, J. Electrochem. Soc. 145 (1998) 864–874. [14] S. Peulon, D. Lincot, Cathodic electrodeposition from aqueous solution of dense or open-structured zinc oxide films, Adv. Mater. 8 (1996) 166–170. [15] C.-C. Hu, H.-C. Hsu, K.-H. Chang, Cathodic deposition of TiO2: effects of H2O2 and deposition modes, J. Electrochem. Soc. 159 (2012) D418–D424. [16] T.-F.M. Chang, T. Sato, M. Sone, Fabrication of TiO2 micro-structures by cathodic deposition, Microelectron. Eng. 121 (2014) 80–82. [17] H.M. Lee, M. Ge, B. Sahu, P. Tarakeshwar, K.S. Kim, Geometrical and electronic structures of gold, silver, and gold-silver binary clusters: Origins of ductility of gold and gold-silver alloy formation, J. Phys. Chem. B 107 (2003) 9994–10005.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Fabrication and Photocatalytic Performance of Au/ZnO Layered Structure on Silk Textile for Flexible Device Applications Wan-Ting Chiua,b , Chun-Yi Chena,b,* ,1, Tso-Fu Mark Changa,b,* ,1, Yuma Taharac , Tomoko Hashimotoc, Hiromichi Kurosuc , Masato Sonea,b,1 a b c
Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan CREST, Japan Science and Technology Agency, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan Department of Clothing Environmental Science, Nara Women's University, Kitauoya Higashimachi, Nara 630-8506, Japan
A R T I C L E I N F O
Article history: Received 27 July 2017 Received in revised form 6 September 2017 Accepted 7 September 2017 Available online 8 September 2017 Keywords: ZnO/Au/silk composite Supercritical CO2 catalyzation Electroless plating Cathodic deposition Photocatalytic activity
A B S T R A C T
This study integrated ZnO/Au layered structure on silk textiles to realize a flexible photocatalytic composite material for wearable devices. Supercritical carbon dioxide promoted electroless plating and cathodic deposition were used to metallize and deposit photocatalytic metal oxides on silk textiles. Silk was used as the substrate for its flexibility and stretchability. Au was selected due to its high biocompatibility, electrical conductivity, and ductility. ZnO was chosen for its photocatalytic activity and biocompatibility to fulfill the application. Surface morphology, composition, crystal structure, electrical resistance and photocatalytic activity assessments were evaluated. With the assistance of supercritical carbon dioxide, smooth, strong adherence and uniform coverage of Au layer was successfully electroless plated on the silk. ZnO was homogeneously deposited on the Au metal layer. The coatings on the silk have been confirmed to be metallic Au phase and ZnO wurtzite phase. The ZnO modified specimen with 0.2 wt. % H2O2 in the electrolyte showed the highest crystallinity and performed 11.5 times enhancement in the photocurrent density when comparing to the H2O2-free one. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction The global market of wearable devices is predicted to keep prospering in the future [1]. In the meanwhile, wearable devices are diversified into different functions such as biomedical devices, supercapacitors, and photocatalytic devices [2] due to diverse requirements in the next-generation technology. The integration of functional and flexible materials turns out to be the most critical step in fabrication of the component used in wearable devices. For example, integration of photocatalytic, electrical conductive, and flexible materials is needed to realize wearable devices capable of harvesting solar energy. Flexible materials such as cloth textiles are usually nonconductive, and the flexible material can be made electrically conductive by depositing metal coatings on the non-conductive material through electroless plating [3]. Electroless plating
* Corresponding authors at: Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan. E-mail addresses: [email protected] (C.-Y. Chen), [email protected] (T.-F.M. Chang). 1 ISE member. http://dx.doi.org/10.1016/j.electacta.2017.09.041 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
consists of a pretreatment step to clean the substrate, a catalyzation step to activate the substrate, and a metallization step to metallize the substrate. In the conventional catalyzation step, catalyst-contained corrosive aqueous solution is used to activate the substrate by depositing the catalyst on the substrate; meanwhile, the substrate structure is often damaged during the process. Due to the high surface tension and polarity of the aqueous catalyzation solution, the catalyst is only inlaid on the substrate surface resulting in poor adhesive property. On the other hand, supercritical carbon dioxide (sc-CO2) can be introduced to the catalyzation step to enhance the plating characteristics [4,5]. Carbon dioxide develops into supercritical phase owning properties in the midway of gas phase and liquid phase when both the temperature and pressure go beyond its critical point [6]. In this case, sc-CO2 can be used as the solvent, and an organic-metal complex would be used as precursor of the catalyst. Due to the affinity to non-polar material [7], low surface tension [8], and high self-diffusivity [6] of sc-CO2, sc-CO2 thus can convey the organicmetal complex into the textile structure while remains the substrate structure intact, and the catalyst could be formed after a reduction process. Regarding the photocatalytic functionality, there are various synthetic methods to deposit photocatalytic metal oxides, which
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include high-temperature vapor deposition [9], polymer-mediated chemical reaction [10], hydrothermal-annealing process [11], and one-pot solution-based method [12]. Among the synthetic techniques, cathodic deposition allows a simple and effective process to produce metal oxides [13–15]. In addition, cathodic deposition can cooperate with lithography to allow further applications such as patterning [16]. In this study, the photocatalyst/metal/cloth textile flexible composite material was realized by sc-CO2 assisted electroless plating and cathodic deposition for applications in wearable devices. A slight disturbance from the rigid component in a wearable device can be annoying to the user, and hence a flexible material is preferred in wearable devices. Silk, a common clothing material, was chosen in this study due to its flexibility and stretchability. Since wearable devices come into contact with the skin frequently during the employment, a thin Au layer, which is known to be highly biocompatible, was selected as the electrically conductive layer. Moreover, Au also shows high ductility [17], which is an essential requirement for wearable devices. There are many materials that can play the role as the photocatalyst in the composite material, such as TiO2 [18], CuO [19], and ZnO [20]. ZnO was designated as the photocatalyst due to its comprehensive applications, requirement of low fabrication temperature to have high crystallinity, and simple fabrication process. There are several reports on preparation of flexible photocatalyst materials, such as metal-organic chemical vapor deposition of GaN on graphene film [21] and electrospinning combined with hydrothermal method to deposit TiO2 on polyacrylonitrile [22]. However, due to the aforementioned requests in next generation wearable devices, flexible composite materials composed of highly biocompatible cloth textile, conductive metallic layer, and photocatalyst are needed. Here, we report a facile fabrication process in realization of a flexible and photocatalytic ZnO/Au/silk layered composite material toward applications such as solar energy harvesting wearable devices. Fundamental properties required for wearable devices such as electrical resistance and photocatalytic activity evaluations are demonstrated.
2. Experimental 2.1. Chemicals A piece of silk in the dimension of 2 cm 4 cm was used as the substrate, and Pd(II) acetylacetonate (Pd(acac)2) (98.0%, Tokyo Chemical Industry Co., Ltd., Japan) was used as the organic-metal complex. Carbon dioxide gas (99.99%, Nippon Tansan Gas Co., Ltd. Japan) was used as the source of sc-CO2. A commercially available acidic Ni-P electroless plating electrolyte (Okuno Chemical Industries Co., Ltd., Japan) consisted of nickel chloride (1.8 wt.%), sodium hypophosphite (2.4 wt.%), complexing agent (2.4 wt.%), and ion-exchanged water (93.4 wt.%) was used in this study. The Au electroless plating electrolyte used in this study was a commercially available electrolyte provided by MATEX JAPAN Co., Ltd. The Au electroless plating electrolyte was composed of ammonium gold sulfite (0.2 wt.%), reducing agent (0.3 wt.%), complexing agent (0.3 wt.%), and ion-exchanged water (99.2 wt.%). The electrolyte for cathodic deposition of ZnO comprised of 0.25 wt.% ZnCl2 (98.0%, Sigma-Aldrich, USA), 3.0 wt.% NaCl (99.0%, Sigma-Aldrich, USA), 1.5 wt.% NaNO3 (99.0%, Sigma-Aldrich, USA), 0–0.3 wt.% H2O2 (30%, Sigma-Aldrich, USA), and ion-exchanged water. 2.2. Sc-CO2 assisted catalyzation No pretreatment was carried out before the sc-CO2 catalyzation step. Pd(acac)2 was used as the precursor in formation of the Pd catalyst seed used in metallization of a Ni-P layer on the silk, and 25 mg of the Pd(acac)2 catalyst was used with respect to a 50 ml reaction cell to maintain the Pd(acac)2 concentration at the saturation point throughout the process. A piece of the silk textile was hooked from inner ceiling of the 50 ml stainless reaction cell [23], and sc-CO2 and the Pt(acac)2 were introduced into the reaction cell afterward. A high-pressure apparatus (Japan Spectra Company, Japan) was used to pressurize CO2 into the reaction cell, and the details could be found in a previous study [23]. The catalyzation was executed at 80 1 C and 15 0.1 MPa with
Fig. 1. (a) OM image and (b) XRD pattern of the as-received silk, and (c) OM image and (d) XRD pattern of the catalyzed silk (diamond symbols: Pd(acac)2).
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Fig. 2. OM images of the (a) Ni-P metallized silk at 4 min and the Au metallization at (b) 3 min, (c) 5 min, and (d) 80 min.
agitation for 2 hours [24]. No post treatments were carried out after the catalyzation step. 2.3. Metallization A layer composed of a less noble metal is needed to initiate deposition of the Au layer. Hence, Ni-P metallization was conducted on the catalyzed silks at 70 C and atmospheric pressure using the Ni-P electroless plating electrolyte. Metallization time at 4 min was conducted to ensure the full coverage [25]. Likewise, Au metallization process was executed after the Ni-P metallization step for various lengths of metallization times (t = 3, 5, 10, 20, 40, 60, and 80 min). The Au plating bath was set at 70 C and atmospheric pressure. Specimens showing a full Au coverage were selected to proceed with deposition of the ZnO layer. 2.4. ZnO cathodic deposition The cathodic deposition was run at a constant current density of 0.010 A/cm2 at 70 C for 2 h without agitation. Surface area of the working electrode, that is the contact area between the Au layer and the electrolyte, was estimated to be 10 cm2, which was calculated by estimating surface area of every bundle of the Au metallized silk in a piece of the specimen. A piece of Pt foil was used as the counter electrode. The cathodic depositions were performed by using a Potentiostat/Galvanostat (1287A, Solartron Analytical, UK). No post treatment was conducted after the ZnO cathodic deposition step. 2.5. Characterization
Fig. 3. XRD patterns of the Au metallization at (a) 5 min, (b) 20 min, and (c) 80 min and (d) the composition revolution with the Au metallization time.
The surface morphology, cross-section and composition were examined by an optical microscope (OM; VHV-5000, KEYENCE, Japan), a scanning electron microscope (SEM; JSM-7500, JEOL, Japan), and energy-dispersive X-ray spectroscopy (EDX; EMAX Evolution, HORIBA, Japan). Thickness of the Au layer was estimated by observing cross-section of the Au metallized silk from the SEM image and ImageJ software (National Institutes of Health, USA).
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3. Results and discussion 3.1. Surface morphology and structures of the Au layer
Fig. 4. Plot of the metallization thickness versus the metallization time. An OM image of cross-section of the composite material is inserted.
Fig. 5. (a) Electrical resistances of the Au metallized silk at various metallization times, and SEM images of the Au metallized silk at (b) 20 min and (c) 80 min of the Au metallization time (hollow symbols: Au-free; solid symbols: Au metallized).
The crystal structures and phases were identified by an X-ray diffractometer (XRD; Ultima IV, Rigaku, Japan). The electrical resistance was measured by a four-point probe (Mitsubishi Chemical Analytech Co., Ltd., MCP-T370, Japan). Adhesive test of the Au layer was conducted by a repeating peeling test using the 3 M tape (3 M adhesive tape #810, 3 M, USA) [24]. The electrical resistance was measured again after the adhesive test to evaluate the reliability. The photoelectrochemical measurement was conducted in a three-electrode cell consisting of a Pt counter electrode, a Ag/AgCl reference electrode, and a piece of the ZnO/Au on silk as the working electrode. The chronoamperometric It curves were recorded at 0 V vs Ag/AgCl using the Potentiostat/ Galvanostat under white light illumination (500 W xenon lamp, with a light intensity of 100 mW/cm2). Na2SO4 (0.5 M) solution was used as the electrolyte.
3.1.1. Sc-CO2 assisted catalyzation Fig. 1 shows OM images and XRD patterns of the as-received silk and the catalyzed silk. The crossed-linked see-through bundles shown in Fig. 1(a) imply the silk textile, and the yellow flakes in Fig. 1(c) specify the Pd(acac)2. A broad peak at 2u = 20 degree can be observed in Fig. 1(b), which reveals an amorphous structure of the silk textile. On the other hand, in Fig. 1(d), diffraction peaks labelled by diamond symbols can be indexed to Pd(acac)2 [26,27], and the results indicated the catalyst precursors were successfully deposited on the substrate. As shown in the OM images, the silk substrate remained intact after the catalyzation step due to the low surface tension, low viscosity, and non-corrosive properties of scCO2. The sc-CO2 assisted catalyzation step demonstrated a significant improvement while comparing to the conventional catalyzation process which was shown in a previous study [25]. Pd (acac)2 acts as an activation site after it was reduced to Pd metal by the reductant in the metallization solution. The Pd metals can play the role as the catalyst in the following metallization step. 3.1.2. Metallization Development of the Au layer is shown in Fig. 2. Fig. 2(a) shows morphology of the specimen with the Ni-P metallization time at 4 min before the Au metallization. Fig. 2(b-d) demonstrate the Au metallization time at 3 min, 5 min, and 80 min, respectively. At 3 min of the Au metallization (Fig. 2(b)), part of the Ni-P layer was still not enclosed showing a gray luster. As the Au metallization times were extended to 5 min and longer (Fig. 2(c, d)), the Ni-P layer was completely concealed by the Au layer. The XRD patterns at various Au metallization time are shown in Fig. 3(a–c). Five apparent diffraction peaks shown in Fig. 3(a–c) locate at 2u = 38.2, 44.4, 64.6, 77.6, and 81.7 degree can be indexed to (111), (200), (220), (311), and (222) planes of the FCC structure for Au (JCPDS #65-2870), respectively. In Fig. 3(a), at 5 min of the Au metallization time, only a thin layer of Au was metallized on the silk, and the broad diffraction peak of silk at around 20 degree was still detectable. Ni-P diffraction peaks were not found in Fig. 3(a), since the Ni-P layer is designed as the sacrifice layer for the Au metallization. In addition, it owns an amorphous structure [25], and the diffraction peaks might be overlapped by the high intensity Au diffraction peaks. As the Au metallization proceeded, no diffraction peaks from the silk were found in the XRD patterns (Fig. 3(b-c)) since the Au layer is thick enough to suppress the diffraction peaks from the silk. Fig. 3(d) shows the composition changed with the Au metallization time. In the early stage, due to the insufficient thickness of Au, the Ni-P layer was detectable by the EDX. On the contrary, the Ni-P was no longer detectable as the Au metallization time extended to 80 min. Phosphorus was not detected owing to the insufficient amount [25]. Fig. 4 illustrates growth of the Au layer thickness with the Au metallization time. There is no obvious partition of the Ni-P and Au layer shown in the inserted SEM image in Fig. 4. A positive correlation was found between the Au layer and the Au thickness presenting the average growth rate at 0.48 mm/h. The growth rate slowed down slightly from 60 min to 80 min of the Au metallization time due to the consumption and the reduction in concentration of the metal ion and reducing agent as the reaction proceeded. 3.2. Electrical conductivity and adhesive tests The electrical resistances at various Au metallization times are shown in Fig. 5. The electrical resistance dropped down in the early stage and increased after reaching a minimum point. Since the Ni-P
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Fig. 6. SEM images of the specimens deposited in (a) free of H2O2, (b) 0.1 wt.%, (c) 0.2 wt.%, and (d) 0.3 wt.% H2O2, and the OM images in (e) free of H2O2, (f) 0.1 wt.%, (g) 0.2 wt. %, and (h) 0.3 wt.% H2O2.
layer is not fully covered by Au at 3 min of the Au metallization time (solid symbols) (Fig. 2(b) and Fig. 5(a)), it showed high electrical resistance close to the Au-free specimen at 0 min of the
Au metallization (hollow symbols) (Fig. 2(a) and Fig. 5(a)). An inserted graph in Fig. 5(a) shows results of the Au metallization time between 0 to 5 min. The electrical resistances obtained from 0
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minimum at the Au metallization time of 20 min due to the complete coverage, smooth surface, and sufficient Au thickness (Fig. 5(b)). From the beginning of the full coverage to the region near the lowest electrical resistance can be sorted to the second stage (from the dash line at 5 min to the other one at 40 min). The electrical resistance increased slightly as the Au metallization time increased beyond the minimum point. The surface grew coarser (pointed by the arrows in Fig. 5(c)) with the elongated metallization time, and the electrical resistances were deteriorated due to more electron scattering [28]. The region of worsened electrical resistance and bumpy surface can be categorized into the third stage. The electrical resistances after the adhesive tests are shown in Fig. 5(a). No significant impact was observed in all stages since the silk was completely enclosed by the Ni-P and Au, and the adhesions between the silk and the Ni-P and between the Ni-P and the Au both showed sturdy adhesive property. 3.3. ZnO cathodic deposition and photocatalytic activity assessment
Fig. 7. General scheme of the solute concentration profiles.
Fig. 6 shows SEM images (Fig. 6(a–d)) and OM images (Fig. 6(e– h)) of the pristine ZnO deposited with H2O2 concentration varied from 0 to 0.3 wt.%. Tower-like ZnO nanowires are observed in all of the SEM images (Fig. 6(a-d)). ZnO coverage on the Au layer was processed and calculated by a built-in software of the OM. The ZnO coverages were at 63%, 62%, 41%, and 32% for the specimens deposited with free of H2O2 to 0.3 wt.% H2O2. Morphology of cathodically deposited ZnO is often affected by Cl in the solution. The Cl can act as a capping agent adsorbing on the (0001) plan of ZnO to stabilize the surface and leads to formation of nano-rod like structures. On the other hand, when concentration of the Cl is low, tower-like structures instead of nano-rod structures are formed [29]. Furthermore, size of the ZnO particle increased as the H2O2 concentration increased from 0 to 0.2 wt.% as shown in the SEM images. Similar results on increase in the ZnO particle size as the H2O2 concentration increased were reported in a literature [30]. However, in Fig. 6(d), the particle size decreased when the H2O2 concentration went beyond 0.2 wt.%. ZnO deposition equations are shown in the following [31]: NO3 + H2O + 2e ! NO2 + 2OH
(1)
H2O2 + 2e ! 2OH
(2)
Zn2+ + 2OH ! ZnO + H2O
(3)
Fig. 8. (a) XRD patterns and (b) voltage-time plot of the specimens fabricated with electrolytes containing various H2O2 concentration. (star symbols: Au and hexagon symbols: ZnO).
to 5 min (the dash line shown in Fig. 5) of the metallization time, which merely showed a slight difference from the Au-free one, are classified into the first stage. Full coverage was constructed as the Au metallization time reached 5 min, and the electrical resistance was improved accordingly. The electrical resistance reached its
OH generated on the substrate from Eqs. (1) and (2) are expected to react with Zn2+ in the electrolyte to form ZnO on the substrate. As shown in Eq. (2), generation rate of the OH would be increased with an increase in H2O2 concentration. Since the diffusivity of Zn2+ in aqueous solution is four times slower than that of H2O2, diffusion of Zn2+ to the substrate surface cannot catch up with the generation of OH as the H2O2 concentration increased [32]. When the amount of H2O2 was increased to 0.3 wt.%, generation rate of the OH could be too high,and lead to accumulation of the OH near the electrode surface. A general scheme of the solute concentration profiles in the electrolyte is shown in Fig. 7. OH are generated on the substrate, hence the concentration is gradually lowered toward the bulk solution direction. Zn2+ would diffuse from the bulk solution to the substrate surface to react with the OH, therefore, Zn2+ concentration is lowered as it approaches the substrate surface. Depends on the OH generation rate, local OH concentration, or pH, on the substrate could vary. ZnO is formed at a pH range of ca. 8 to 12, and ZnO would dissolve away to form ZnO22 or HZnO2 when the pH is higher than 12 [33,34]. The
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Fig. 9. Photocurrent density of the ZnO/Au/silk hybrid materials with (a) noncorrected (an inserted figure shows the flexibility of the composite material) and (b) corrected by the ZnO coverage.
region suitable for deposition of ZnO is defined as region I, and the region leads to dissolution of ZnO is denoted as region II as shown in Fig. 7. The region near surface of the substrate would be in region I when the H2O2 concentration was lower than 0.2 wt.%. Therefore, an increase in the H2O2 concentration led to an increase in the particle size (Fig. 6(a-b)) and the coverage (Fig. 6(e-f)). When the H2O2 concentration was increased to 0.3 wt.%, both the particle size (Fig. 6(d)) and the coverage (Fig. 6(h)) were decreased, which suggest the pH near the substrate surface could be higher than 12. The results suggest the region near the substrate surface is in region II when the H2O2 concentration is higher than 0.3 wt.%. Fig. 8(a) shows XRD patterns of the ZnO deposited with various H2O2 concentrations in the electrolyte. Only the diffraction peaks of Au (star symbols) and ZnO (hexagon symbols) were observed. No secondary phase was found in the XRD patterns after the ZnO deposition within the detection limitation. A relative crystallinity of ZnO on the Au layer was roughly estimated by calculating ratio of the Au (111) intensity to the ZnO (100) intensity with consideration of the ZnO coverage. The ratios were at 0.10, 0.11, 0.34, and 0.16 from free of H2O2 to 0.3 wt.% H2O2. The ratios indicated the electrolyte containing 0.2 wt.% H2O2 produced ZnO with the best crystallinity, and the crystallinity became worse when 0.3 wt.% H2O2 was used. Another explanation concerning crystallinity of the ZnO is provided here. Crystallinity of the ZnO is reported to be dependent on concentration of the Zn2+ [35], which defects and deteriorated crystallinity are introduced to the ZnO when the Zn2+ concentration is low. In this study, when the H2O2 concentration reached 0.3 wt.%, local concentration of OH near the electrode surface would be high, and higher diffusion rate of OH to the bulk
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solution was built. Then, pH far away from the substrate surface became suitable for formation of ZnO and consumed great amount of the Zn2+. In this case, amount of the Zn2+ diffused to the substrate surface was lowered and resulted the deteriorated crystallinity. Fig. 8(b) shows the voltage-time curves during the ZnO deposition. While there is H2O2 in the electrolyte, the potential drop decreased as the H2O2 concentration increased. The potential drop here refers to the potential difference between the potential at ca. 0 sec of the reaction time and the potential as the reaction gradually stabilized with time. No obvious difference was found in the specimens deposited with free of H2O2 and 0.1 wt.% H2O2, which are in good agreement with the ZnO coverage in Fig. 6(e) and Fig. 6(f) revealing that 0.1 wt.% H2O2 has limited influence on the ZnO deposition. The potential fluctuation increased with an increase in the H2O2 concentration, which is because more N2 gas is emitted at high pH [36]. To assess the applicability as photocatalytic materials, photocurrent generated from photoelectrochemical water splitting reaction using the as-deposited sample as the photoanode was evaluated. Fig. 9 shows the chronoamperometric It curves of the ZnO/Au/silk layered composite materials recorded in 0.5 M Na2SO4 electrolyte under chopped light illumination. The current density was corrected by considering coverage of the ZnO on the Au layer, which the ZnO coverages are provided in the beginning of this section. Fig. 9(a) shows the as obtained current density, which is before the correction, while Fig. 9(b) shows the corrected values. All the specimens showed an increased in the current density when the illumination is on, which imply the effective generation, transfer and collection of the electrons during the water splitting process. The ZnO deposited with 0.2 wt.% H2O2 showed the highest current density in both Fig. 9(a) and (b) since it owns the highest crystallinity. It has been reported that the high crystallinity can assist in suppressing the possible photocorrosion effect [37]. In addition, high crystalline ZnO leads to efficient transportation of the excited electrons. Photocurrent density of the ZnO fabricated with 0.2 wt.% H2O2 showed 11.5 times enhancement when comparing to the one deposited without H2O2. The greater photoelectrochemical performance can be attributed to the facile carrier transport resulting from the oriented crystallinity. In this study, high H2O2 concentration led to the high photocurrent owing to the high crystallinity; however, excess OH resulted in the dissolution of ZnO and deteriorated crystallinity, which resulted a deteriorated photocurrent density. A digital photo is inserted into Fig. 9(a) to show flexibility of the ZnO/Au/silk layered composite material. Various evaluations were conducted to reveal properties of the silk/Au/ZnO composite materials in this study. The composite materials possessed flexibility, high electrical conductivity, and high adhesive firmness. Photocurrent density was enhanced significantly by synthesizing ZnO via the optimized electrolyte. Therefore, the results showed the potential of the silk/Au/ZnO composite materials in applications in flexible wearable devices. 4. Conclusions ZnO/Au layered structure was successfully fabricated on the silk textile by sc-CO2 assisted electroless plating and cathodic deposition. Common problems encountered in the conventional electroless plating were solved by the introduction of sc-CO2. With the help of sc-CO2, silk was catalyzed without defects and the adhesive property between the silk and the metallized layer was enhanced at the same time. Surface coverage and thickness of the Au layer increased with the Au metallization time. The lowest electrical resistance was achieved as the Au metallization time extended to 20 min. On the other hand, ZnO was deposited on the
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Au metallized silk via the cathodic deposition. The coatings on the silk have been confirmed to be metallic Au phase and ZnO wurtzite phase. ZnO deposited with 0.2 wt.% H2O2 showed the best crystallinity and the highest photocurrent density. The ZnO modified specimen with 0.2 wt.% H2O2 performed 11.5 times enhancement in the photocurrent density when comparing to that without H2O2. Acknowledgment This work is supported by Japan Society for the Promotion of Science (17J07395), the Grant-in-Aid for Scientific Research (B) (JSPS KAKENHI Grant Number 26282013), CREST Project (#14531864) operated by the Japan Science and Technology Agency (JST), and research grant from The Murata Science Foundation. References [1] J. Gozalvez, 5G tests and demonstrations [mobile radio], IEEE Veh. Technol. Mag. 10 (2015) 16–25. [2] B.R. Sankapal, H.B. Gajare, S.S. Karade, R.R. Salunkhe, D.P. Dubal, Zinc oxide encapsulated carbon nanotube thin films for energy storage applications, Electrochim. Acta 192 (2016) 377–384. [3] G.O. Mallory, J.B. Hajdu, Electroless plating: fundamentals and applications, William Andrew, Amsterdam, 1990. [4] H. Adachi, K. Taki, S. Nagamine, A. Yusa, M. Ohshima, Supercritical carbon dioxide assisted electroless plating on thermoplastic polymers, J. Supercrit. Fluids 49 (2009) 265–270. [5] J.J. Watkins, T.J. McCarthy, Polymer/metal nanocomposite synthesis in supercritical CO2, Chem. Mater. 7 (1995) 1991–1994. [6] G. Brunner, Gas extraction: an introduction to fundamentals of supercritical fluids and the application to separation processes, Springer Science & Business Media, Berlin, 2013. [7] A. Clifford, T. Clifford, Fundamentals of supercritical fluids, Oxford University Press, Oxford, 1999. [8] S.P. Sawan, S.P. Sawan, Supercritical fluid cleaning: fundamentals, technology and applications, Elsevier, Amsterdam, 1998. [9] C. Gorla, N. Emanetoglu, S. Liang, W. Mayo, Y. Lu, M. Wraback, H. Shen, Structural optical, and surface acoustic wave properties of epitaxial ZnO films grown on (0112) sapphire by metalorganic chemical vapor deposition, J. Appl. Phys. 85 (1999) 2595–2602. [10] B. Panigrahy, M. Aslam, D. Misra, D. Bahadur, Polymer-mediated shapeselective synthesis of ZnO nanostructures using a single-step aqueous approach, CrystEngComm 11 (2009) 1920–1925. [11] C.-H. Ku, J.-J. Wu, Aqueous solution route to high-aspect-ratio zinc oxide nanostructures on indium tin oxide substrates, J. Phys. Chem. B 110 (2006) 12981–12985. [12] Q. Zhang, C. Tian, A. Wu, T. Tan, L. Sun, L. Wang, H. Fu, A facile one-pot route for the controllable growth of small sized and well-dispersed ZnO particles on GO-derived graphene, J. Mater. Chem. 22 (2012) 11778–11784. [13] S. Peulon, D. Lincot, Mechanistic study of cathodic electrodeposition of zinc oxide and zinc hydroxychloride films from oxygenated aqueous zinc chloride solutions, J. Electrochem. Soc. 145 (1998) 864–874. [14] S. Peulon, D. Lincot, Cathodic electrodeposition from aqueous solution of dense or open-structured zinc oxide films, Adv. Mater. 8 (1996) 166–170. [15] C.-C. Hu, H.-C. Hsu, K.-H. Chang, Cathodic deposition of TiO2: effects of H2O2 and deposition modes, J. Electrochem. Soc. 159 (2012) D418–D424. [16] T.-F.M. Chang, T. Sato, M. Sone, Fabrication of TiO2 micro-structures by cathodic deposition, Microelectron. Eng. 121 (2014) 80–82. [17] H.M. Lee, M. Ge, B. Sahu, P. Tarakeshwar, K.S. Kim, Geometrical and electronic structures of gold, silver, and gold-silver binary clusters: Origins of ductility of gold and gold-silver alloy formation, J. Phys. Chem. B 107 (2003) 9994–10005.
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