ZnO whiskers were added to a neutral solution of HAuCl₄ and the suspension was heated at reflux at 100 °C for 48 h before filtration. The treated whiskers were thoroughly washed with deionized water, dried and calcined at 300 °C for 4 h. The ZnO whiskers then changed color from white to pink-purple, as shown in Figure 1(a,b). Furthermore, transmission electron microscopy (TEM) analysis suggested that many NPs with a particle size less than 5 nm were synthesized on the surfaces of the ZnO whiskers (Figure 1(b)). Subsequently, the HAuCl₄-treated whiskers were added to an aqueous solution of AgNO₃ and the suspension was mixed with aqueous solutions of sodium citrate and hydroquinone for 1 h before filtration. The obtained whiskers were thoroughly washed with deionized water and dried at room temperature for 24 h to afford a yellow-brown product (Figure 1(c)). As shown in Figure 1(d), high-resolution TEM imaging of the resulting NPs indicates that there are electron-dense and electron-lucent layers inside and outside the NPs, respectively. The TEM images suggest that many NPs with core-shell nanostructures were synthesized on the ZnO whiskers.

Energy dispersive X-ray spectroscopy (EDS) analysis of the NPs confirms the coexistence of Au and Ag (Figure 2).

Figure 3 shows the X-ray photoelectron spectroscopy (XPS) spectra for the original ZnO whiskers, ZnO whiskers treated with HAuCl₄ and ZnO whiskers treated with HAuCl₄ and AgNO₃.

When the ZnO whiskers were treated with HAuCl₄, the Au4f_7/2 peak at ca. 83.8 eV, assigned to the characteristic peak of Au(0) [36], appears (Figure 3(b)), which indicates the NPs shown in Figure 1(b) correspond to AuNPs. On the basis of previous reports for the synthesis of Au on metal oxides, via a deposition-precipitation method [37,38], it is proposed that the formation of AuNPs on the ZnO whiskers proceeds as follows: (1) Au(III) complex anions in an aqueous solution of HAuCl₄ change from [AuCl₄]⁻ to [Au(OH)₄]⁻ at pH 7.5–8.0; (2) the [Au(OH)₄]⁻ ions are adsorbed onto the surfaces of the positively charged ZnO whiskers under neutral conditions [39] due to electrostatic interactions; (3) thermal treatment at 100 °C leads to the formation of Au(OH)₃ on the surfaces of the ZnO whiskers; and (4) the Au(OH)₃ is thermally reduced to AuNPs under calcination treatment at 300 °C. When the AuNPs^@ZnO whiskers are treated with AgNO₃, sodium citrate and hydroquinone, the two peaks for Ag3d_3/2 and Ag3d_5/2 at ca. 373.9 eV and ca. 367.8 eV, respectively, assigned to the characteristic peaks of Ag(0) [40], appear, and the intensity for the Au(0) peak at ca. 83.8 eV decreases (Figure 3(c)). This indicates the formation of an Ag layer on the surfaces of the AuNPs. Hydroquinone has a weak redox potential (E° = −0.7 V vs. NHE) [41], and is therefore unable to reduce the Ag⁺ ions that are isolated in solution (Ag⁺/Ag⁰, E° = −1.8 V). It can, however, reduce Ag⁺ in the presence of Ag⁰ clusters or NPs (E° = +0.799 V vs. NHE) [42,43]. Furthermore, Lim et al. have recently reported the successful formation of an Ag_shell layer on Au_core NPs using hydroquinone as the reducing agent [44]. On the basis of these results and previous reports, it is proposed that Au_core-Ag_shell bimetallic NPs, as shown in Figure 1(d), were successfully synthesized on the surfaces of ZnO whiskers via a multi-step process.

As described above, very small Au_core-Ag_shell bimetallic NPs were successfully immobilized onto the ZnO whiskers. However, these as-prepared AuAgNPs^@ZnO whiskers were somewhat difficult to handle, since the ZnO whiskers are fine fibers (Figure 1). The proposed on-paper synthesis technique is a promising strategy to solve this issue [30]. In this study, the on-paper synthesis of AuAgNPs was carried out; ZnO whiskers were initially incorporated into an easy-to-handle paper matrix, upon which the AuAgNPs were synthesized in situ. Firstly, the ZnO whiskers were embedded into a paper matrix consisting of ceramic, glass and pulp fibers using our established papermaking technique [45]. The dual polyelectrolyte retention system, which involves the sequential addition of cationic and anionic polyelectrolytes, allowed a quantitative retention of the added materials, which included the fine ZnO whiskers. The paper composite was then thermally treated at 700 °C for 30 min to remove the organic pulp fibers and enhance the physical strength by sintering the glass fibers. Subsequently, the AuAgNPs were synthesized in situ on the as-prepared ZnO paper in a multi-step preparation process, similar to that for the AuAgNPs^@ZnO whiskers. Figure 4 shows optical images for the original ZnO paper, AuNPs^@ZnO paper and AuAgNPs^@ZnO paper.

These paper composites are cardboard-like materials. Even after sonication in water for 30 min, no fiber components, including the ZnO whiskers, fell from the paper composites. This suggests that the AuAgNPs^@ZnO whiskers are strongly incorporated in the paper matrix and the AuAgNPs^@ZnO paper has a high physical strength. The metal NPs^@ZnO paper composites have a highly porous structure; the peak pore size and the porosity are ca. 15 μm and ca. 80%, respectively [30]. Thus, the AuAgNPs^@ZnO paper was porous, flexible and easy-to-handle, allowing excellent practical utility.

4-NP is known to be an anthropogenic pollutant, since it is carcinogenic, mutagenic, and cyto- and embryonic-toxic. Hence, it is desirable to develop an effective method for the removal of 4-NP. A promising approach involves the catalytic reduction of 4-NP to 4-AP, an important intermediate for the manufacture of analgesic and antipyretic drugs [46]. In this study, the catalytic performances of the AuAgNPs^@ZnO whiskers and paper were investigated for the aqueous reduction of 4-NP to 4-AP with NaBH₄. Figure 5(a) shows the UV-vis absorption spectra recorded throughout the catalytic reduction of 4-NP over the AuAgNPs^@ZnO whiskers.

The characteristic peak for 4-NP at ca. 400 nm, ascribed to the 4-nitrophenolate ion [3,10], gradually decreases, while a new peak at ca. 300 nm, assigned to 4-AP [3,10] appears. The reaction with the AuAgNPs^@ZnO whiskers was almost complete within 360 s under continuous stirring. Figure 5(b) shows the ln(A_t/A₀) (A_t: absorbance at 400 nm at the reaction time t, A₀: absorbance at 400 nm at the initial stage) as a function of reaction time for the AuAgNP^@ZnO whiskers, AuNPs^@ZnO whiskers, AgNPs^@ZnO whiskers and original ZnO whiskers. In each case, a linear correlation was observed, which suggests that the 4-NP reduction, with an excess amount of NaBH₄, follows pseudo-first-order kinetics [10]. The pseudo-first-order reaction rate constants (k) for the AuAgNPs^@ZnO whiskers, AuNPs^@ZnO whiskers, AgNPs^@ZnO whiskers and ZnO whiskers were estimated from each slope to be 7.0 × 10⁻³ s⁻¹, 1.1 × 10⁻³ s⁻¹, 1.0 × 10⁻⁴ s⁻¹ and 3.0 × 10⁻⁶ s⁻¹, respectively. The original ZnO whiskers exhibit an extremely low catalytic efficiency, indicating that the metal NPs synthesized onto the ZnO whiskers play a key role in this reaction. The AuNPs^@ZnO whiskers demonstrate a higher catalytic efficiency than the AgNPs^@ZnO whiskers, suggesting the AuNPs have a superior catalytic activity to the AgNPs. It is noteworthy that the AuAgNPs^@ZnO whiskers had the highest catalytic efficiency among the test samples; the k value for the AuAgNPs^@ZnO whiskers (7.0 × 10⁻³ s⁻¹) was 1.2–3.3 times larger than those for the many metal nanocatalysts previously reported, such as AuNP^@cellulose single nanofibers (5.9 × 10⁻³ s⁻¹) [3], dendric Ag/Au bimetallic nanostructures (6.07 × 10⁻³ s⁻¹) [19], AuNPs/poly(amidoamine) dendrimer (3.7 × 10⁻³ s⁻¹) [47] and spongy Au nanocrystals (2.1 × 10⁻³ s⁻¹) [48]. Figure 6 shows influence of the Au/Ag ratio on the turnover frequency (TOF) for the AuAgNPs^@ZnO whiskers in the reduction of 4-NP.

Although the total amount of Au and Ag was constant (0.1 μmol) in all test samples, the Au_core-Ag_shell bimetallic NPs exhibited a much higher TOF than the monometallic equivalents at all Au/Ag ratios (7/3, 5/5, 4/6, 2/8) studied. The maximum TOF (ca. 1400 h⁻¹) was achieved with an Au/Ag ratio of 7/3, and was 5−40 fold higher those for the monometallic NPs. Esumi et al. have proposed that the AuNPs-catalyzed 4-NP reduction, with an excess of NaBH₄, proceeds in two steps: (1) the diffusion and adsorption of 4-NP to catalyst surface; and (2) an electron transfer mediated by the catalyst surface from BH₄⁻ to 4-NP [47,49]. They concluded that the first step is the rate limiting step in the 4-NP reduction. Thus, the interesting phenomenon observed in this study may be attributed to the electronic ligand effect between the core and shell atoms [21,22,24]. Since the ionization potentials for Ag and Au are 7.58 eV and 9.22 eV, respectively, an electronic transfer could occur from Ag_shell to Au_core [25], resulting in a decrease in electron density on the shell surfaces of the Au_core-Ag_shell bimetallic hybrid NPs. Such electron-deficient surfaces would favor the adsorption of 4-nitrophenolate anions, and lead to excellent 4-NP reduction efficiencies.

Figure 7 shows the 4-NP reduction performance for the AuAgNPs^@ZnO paper.

The reusability of the AuAgNPs^@ZnO paper was evaluated from the changes in TOF over a five-cycle test: the sequential procedure for the 4-NP reduction reaction involved washing with water and drying at room temperature between each cycle of the test. The TOF value for the AuAgNPs^@ZnO paper was ca. 120 h⁻¹ in the first cycle, which is lower than that for the AuAgNPs^@ZnO whiskers. This may be because the accessibility for the reactants to the AuAgNPs^@ZnO paper was insufficient under continuous stirring due to its large size (ca. 15 mm in diameter) in comparison to the fine AuAgNPs^@ZnO whiskers (fiber length: 2−50 μm, fiber diameter: 0.2−3.0 μm), although the unique porous microstructure of the metal NPs^@ZnO paper, with an open porous network throughout, was favorable for the efficient transport of the 4-NP to the NPs surfaces under static conditions [30,35]. While the AuAgNPs^@ZnO whiskers exhibited a higher catalytic performance, they remain difficult to handle and hard to separate from the reaction system owing to their fine size (Figure 1(c)). Conversely, the AuAgNPs^@ZnO paper is easy-to-handle in practical terms due to its simple paper form (Figure 4(c)), allowing a high reusability. In fact, the AuAgNPs^@ZnO paper was easily recovered from the reaction solution after each performance test, and demonstrated a TOF of ca. 90 h⁻¹ after the fifth cycle. The atomic absorption analysis confirmed that no AuAgNPs dropped out of the paper composite during the catalytic reaction. Thus this decrease in the catalytic efficiency may be due to the inevitable aggregation of metal NPs during the 4-NP reduction [10,35] and/or the degradation of an Au_core-Ag_shell nanostructure. It is difficult to fully suppress the deterioration of the metal NPs after repeated use, but the immobilization of the AuAgNPs on a paper matrix via our on-paper synthesis technique has provided a high degree of practical utility for the very small AuAgNPs. Furthermore, the highly porous structure of the paper composites provide a great potential for application to flow-type processes, which can be applied to continuous processes, regardless of liquid- [50] and gas-phase reactions [28–30,33,34,45]. Thus the AuAgNPs^@ZnO paper with paper-like characteristics is a promising practical material for a variety of catalytic processes.

Ceramic fibers (SiO₂: 52 wt%, Al₂O₃: 48 wt%) and ZnO whiskers (fiber length: 2–50 μm, fiber diameter: 0.2–3.0 μm) were obtained from IBIDEN, Ltd. (Tokyo, Japan) and Matsushita Amtec, Ltd. (Osaka, Japan) respectively. Pulp fibers, as a matrix component in the paper fabrication process were prepared by refining a commercial bleached hardwood kraft pulp to a Canadian Standard Freeness of 300 mL with a Technical Association of the Pulp and Paper Industry standard beater. Glass fibers (CMLF208, Nippon Sheet Glass, Ltd., Tokyo, Japan) were used as a binding component to enhance the physical strength of the paper composite following calcination. Two types of flocculants, namely cationic poly(diallyldimethylammonium chloride) (PDADMAC; molecular weight ca. 3 × 10⁵ g mol⁻¹; charge density 5.5 meq g⁻¹) and anionic polyacrylamide (A-PAM, HH-351; molecular weight ca. 4 × 10⁶ g mol⁻¹; charge density 0.64 meq g⁻¹), were purchased from Aldrich, Ltd. (Sheffield, UK) and Kurita, Ltd. (Tokyo, Japan), respectively. AgNO₃ (99.8+ % purity) and trisodium citrate dihydrate were obtained from Wako Pure Chemical Industries, Ltd. HAuCl₄·3H₂O (99.9% purity), hydroquinone (>99% purity), 4-NP (99% purity) and sodium borohydride (NaBH₄, >95% purity) were purchased from Aldrich, Ltd. (Sheffield, UK). All other chemicals were of reagent grade and used without further purification.

Au_core-Ag_shell bimetallic NPs were synthesized on ZnO whiskers via a multi-step process. Firstly, the synthesis of the AuNPs onto the ZnO whiskers was conducted using a deposition-precipitation method in line, to some degree to previous reports [37,38]. An aqueous solution of HAuCl₄ (0.2–0.5 mM, 25 mL) was adjusted to pH 7.5–8.0 with aqueous NaOH (100 mM). The ZnO whiskers (1.0 g) were then suspended in the solution, and the mixture was refluxed at 100 °C for 48 h with continuous stirring. The suspension was then filtered, washed with deionized water, dried at 80 °C for 1 h and calcined at 300 °C for 4 h. An Ag nanolayer was formed on the as-synthesized AuNPs as follows. The as-prepared AuNPs^@ZnO whiskers (400 mg) were suspended in an aqueous solution of AgNO₃ (0.06–0.55 mM, 5 mL) for 30 min. Aqueous solutions of sodium citrate (1.0% (w/v), 50 μL), as a stabilizer, and hydroquinone (30 mM, 200 μL), as a reducing agent, were then added to the mixture, followed by stirring for 1 h and filtration. Finally, the products were thoroughly washed with deionized water and dried at room temperature for 24 h. The AgNPs^@ZnO whiskers were also prepared according to our previous report [31].

The preparation of paper composites using organic and inorganic fibers, via a dual polyelectrolyte retention system, was conducted according to our previous reports [30,45]. In summary, an aqueous suspension of ceramic fibers, glass fibers and ZnO whiskers was mixed with PDADMAC (0.5 wt% of total solids) and A-PAM (0.5 wt% of total solids), in that order. This mixture was then added to a water suspension of pulp fibers, and solidified through dewatering using a 200-mesh wire. The wet-state handsheets were pressed at 350 kPa for 3 min and dried at 105 °C for 1 h. The resulting paper composite, with an area of 2 × 10⁴ mm² and a thickness of 1.4 mm, was composed of ceramic fibers (4.0 g), glass fibers (4.0 g), ZnO whiskers (1.0 g) and pulp fibers (1.0 g). The obtained paper composites were thermally treated at 700 °C for 30 min to remove any pulp fibers and to improve the physical strength through the sintering of glass fibers.

The in situ synthesis of AuAgNPs onto the ZnO paper was performed in a similar manner to that for the AuAgNPs^@ZnO whiskers. An aqueous solution of HAuCl₄ (0.03 mM, 80 mL) was adjusted to pH 7.5–8.0 using an aqueous solution of NaOH (100 mM). The ZnO paper was cut into circular pieces, each with an area of 1.8 × 10² mm² and a thickness of 1.4 mm. The four pieces were immersed in the solution and stirred at reflux for 48 h. The treated paper discs were removed from the solution using tweezers, thoroughly washed with deionized water, dried at 80 °C for 1 h and calcined at 300 °C for 4 h. A piece of the as-prepared AuNPs^@ZnO paper was then immersed in an aqueous solution of AgNO₃ (0.001 mM, 5 mL) for 30 min. Aqueous solutions of sodium citrate (1.0% (w/v), 50 μL) and hydroquinone (30 mM, 200 μL) were added to the mixture and stirred for 1 h. The obtained paper disc was thoroughly washed with deionized water and dried at room temperature for 24 h.

Catalytic performances towards the reduction of 4-NP were investigated in batch mode. An aqueous solution of 4-NP (0.1 mM, 150 mL) was initially mixed with NaBH₄ (3.0 mmol), as the reducing agent. The AuNPs^@ZnO whiskers, AgNPs^@ZnO whiskers or AuAgNPs^@ZnO whiskers were then added to the solution. In all cases, the total amount of Au and Ag remained constant at 0.1 μmol. Reactions were carried out at 25 °C with continuous stirring. At given times, samples (1.0 mL) were taken, filtered through a 0.2 μm membrane filter (Chromatodisk, GL Sciences, Ltd.) and analyzed via UV-vis. UV-vis spectra of the reaction solutions (1.0 mL) were recorded at room temperature using a U-3000 spectrophotometer (Hitachi, Japan). According to a previous report [10], the rate constants for the reduction process were determined by measuring the change in absorbance at 400 nm as a function of time. The reusability of the AuAgNPs^@ZnO paper was also evaluated from the changes in TOF value during a five-cycle test: the sequential procedures of the 4-NP reduction reaction, washing with water and drying at room temperature were conducted in each cycle of the test.

The levels of Au and Ag were determined using atomic absorption spectrophotometry on a Shimadzu AA-6600F instrument. TEM and EDS analyses were performed using a JEM-2010FEF instrument (JEOL, Ltd., Tokyo, Japan) at a 200 kV accelerating voltage. The chemical states of the component elements were analyzed by XPS (AXIS-HSi spectrometer, Shimadzu/Kratos, Ltd., Kyoto, Japan) using a monochromatic AlKα X-ray source (1486.6 eV) with a 12 kV voltage and 10 mA current. The binding energies for all spectra were determined with respect to the C1s reference signal (unoxidized C-C bond) at 285.0 eV.