Research Article pubs.acs.org/journal/ascecg
Facile Aqueous Phase Synthesis of (200) Faceted Au-AgCl Cubes Using Bael Gum and Its Activity Toward Oxidation and Detection of o‑PDA Sathiya Balasubramanian, Srinivasa Rao Bezawada, and Raghavachari Dhamodharan* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India S Supporting Information *
ABSTRACT: A new, facile, one step, single-pot, green synthesis of (200) faceted Au-AgCl cubes, at room temperature, is reported. The simultaneous addition of aqueous solutions of HAuCl4 and AgNO3, at room temperature, to an aqueous solution of bael gum results in the formation of nanocubes consisting of Au and AgCl, as evidenced by a combination of characterization techniques such as UV−visible spectroscopy, powder X-ray diffraction, XPS, SEM, and STEM. The formation of silver chloride with (200) as the preferred surface facet might arise from the controlled release of chloride ions, which in turn could arise from the slow reduction rate of HAuCl4 by bael gum. This claim is supported by the preparation of Au-AgCl cubes having no specific orientation synthesized by the successive addition of HAuCl4 followed by AgNO3, where the release of the chloride ion is not controlled. The Au-AgCl cubes exhibit selective activity toward the oxidative dimerization of ortho-phenylenediamine (o-PDA), in the absence of H2O2. The time required for this conversion is less than 30 min. This in turn enables the detection of o-PDA in a trace amount with the minimum being 2 ppm via fluorescence and 15 ppm via UV−visible spectroscopy. Control experiments suggest the specific and rapid activity shown by the (200) facetoriented Au-AgCl cubes might arise from its smaller size and morphology. KEYWORDS: Green synthesis, One-pot synthesis, Single-step synthesis, Ambient temperature reduction, (200) Surface facet, AgCl cubes, Bael (Aegle marmelos) gum, o-PDA oxidation
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INTRODUCTION The distinct nature of nanoparticles (NPs) when compared to their bulk arises principally from their high surface energy and area. Recently, it was realized that the surface facets of nanocrystals can also dramatically influence their performance. For example, the synthesis of Pt−Pd nanocrystals with (100) rich crystal facets exhibiting better electrocatalytic activity toward oxidation of methanol and (111) rich facets possessing better durability was reported.1 Pd nanocrystals of different shapes (such as cubes, tetrahedron, and near spherical) were synthesized, and their catalytic activity was shown to follow the order: tetrahedron > near spherical > cubes.2 This observation was justified in terms of number of surface atoms, which will ultimately decide their catalytic activity.2 The tuning of size and morphology of noble metal NPs such as gold (Au) and silver (Ag) alters surface plasmon absorption (SPA) and surface-enhanced Raman scattering (SERS), while enabling certain specific applications such as catalysis, sensing, and photothermal therapy.3−5 The immobilization of noble metals such as Au and Ag on the surface of semiconductors enhances their photocatalytic activity, which is attributed to the absorption of light in the visible region. In this context, it is pertinent to mention that several Au-modified semiconductor © XXXX American Chemical Society
surfaces have been synthesized and proven to have excellent photocatalytic activity. For example, Au-modified AgCl,6 TiO2,7 CdS,8 ZnS,9 Fe/TiO2,10 and Ta2O5/Ta3N511 have been shown to exhibit very good photocatalytic activity compared with their counterpart without Au. In a recent study, both Au and Ag NPs-modified AgCl electrodes were prepared in which Aumodified AgCl was shown to exhibit an increased photoelectrochemical property compared with the latter.6 It was speculated that the superior activity of Au-AgCl arose from the stable and oxidation-resistant nature of the Au NPs, which could favor the best charge transfer process at the semiconductor/electrolyte interface. Very recently, the effect of surface facets on photocatalytic semiconducting materials was initiated. The synthesis of (001) facet BiVO4 semiconductor nanoplates were reported, and it was shown to exhibit enhanced photocatalytic activity toward the degradation of rhodamine dye.12 Similarly, it was shown that (110) faceted Ag3PO4 rhombohedra exhibited better photocatalytic activity compared with the (100) faceted cube.13 Received: October 13, 2015 Revised: April 27, 2016
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DOI: 10.1021/acssuschemeng.5b01279 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Figure 1. UV−visible spectra of the reaction mixture HAuCl4 + AgNO3 + BG with reaction time and the precipitate formed after prolonged reaction time (a), before and after HNO3 treatment (b).
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As mentioned earlier, while the Au-modified semiconductor is shown to exhibit better catalytic activity, the influence of surface facets on catalytic activity has not been studied well. In addition, in all the above-mentioned literature, the preparation of particular faceted crystals used synthetic chemicals or involved more than a one-step procedure or used high temperature. A milder preparation procedure using a green reagent would be a useful addition to the preparation of noble metal NPs with specific surface facets. Here, we report the synthesis of (200) faceted Au-AgCl crystals using a natural product, i.e., bael fruit based gum, in a single step, at room temperature. The synthesis of nanoparticles using natural products has been reported extensively.14−17 However, there are fewer reports on shape-selective synthesis.18−23 The shapeselective synthesis of nanoparticles using natural products offers few advantages such as an environmentally friendly nature and a low cost. The bael tree belongs to the Rutaceae family and is well known for its medicinal values.24−26 The structure of the polysaccharide present in bael fruit was elucidated and found to consist of repeat units such as galactose, rhamnose, arabinose, and galactouronic acid.27 In addition, the gum has been reported to consist of alkaloids and coumarins.28 The use of bael leaf extract and fruit in the synthesis of nanoparticles has also been reported.16,29 These reports establish the reducing property of the bael gum. However, details about the working of the bael gum in the reduction of noble metal salts have not been studied in any detail. In this work, bael gum was used as a reducing agent cum steric stabilizing agent to prepare Au-AgCl cubes with a preferred (200) surface facet, at room temperature, by reducing chloroauric acid (HAuCl4) in the presence of silver nitrate (AgNO3). It is demonstrated that the as synthesized Au-AgCl cubes oxidize o-phenylenediamine (o-PDA) more rapidly than Au-AgCl cubes without any specific surface orientation. Although o-PDA is used for many applications, such as the preparation of conducting composites,30 ligands in organometallics,31 etc., it is classified as highly toxic, hazardous water pollutant and typical carcinogenic substance, according to European Union regulations. Therefore, identifying its presence in low concentration and rapid destruction is very important for the protection of the environment and aquatic life.
EXPERIMENTAL SECTION
Isolation of Bael Gum. Bael fruit (collected from the bael tree at the IIT Madras campus during the months of November and December) was cut into two equal halves, and the amber-colored viscous gummy substance obtained with seeds was filtered (to eliminate seeds) and subsequently dried in a vacuum oven at 70 °C for 24 h. The dry gum was ground into a fine powder (bael gum or BG). Synthesis of Au-AgCl Cubes. Au-AgCl cubes were prepared by both the simultaneous and successive addition of metal salts to aqueous BG. In the simultaneous addition method, 25 mg of BG was dissolved in 8 mL of deionized double-distilled water. Then, 1 mL of 0.2 wt % of aqueous HAuCl4 was added. Initially, the solution was colorless, and after the addition of HAuCl4, a slight yellow coloration was observed. Then, 0.2 wt % of freshly prepared AgNO3 solution was added, upon which the color of the solution turned deep yellow. Within 10 min of addition, a slight turbidity (but no precipitate formation) was observed. After 2 h, the solution was completely transparent and changed into a lilac color. After 2 days, some precipitate formation was also observed. The supernatant was removed simply by decantation, and the precipitate was again, suspended in 5 mL of water and used for further analysis. In the successive addition method, spherical Au nanoparticles were synthesized in the first step, as follows. A mixture of HAuCl4 and BG in the weight ratio of 1:12.5 was dissolved in 10 mL of deionized distilled water. This was placed in an oil bath maintained at 90 °C for 24 h. It was then allowed to cool to room temperature and used as such for further analysis. Then, 1 mL of 0.2 w % AgNO3 was added, and the mixture was left at room temperature. After a day, the precipitate formed was centrifuged and analyzed. Preparation of AgCl powder. AgCl powder was prepared by the addition of concentrated HCl to a freshly prepared aqueous solution of AgNO3, in a dark room. The precipitate obtained was rinsed several times with milli-Q water until the pH of the solution obtained on rinsing was neutral. The AgCl precipitate was placed in a scintillation vial, wrapped with black color tape, and used in all further applications/reactions. Oxidation of o-phenylenediamine (o-PDA) by Au-AgCl cubes. Initially, the Au-AgCl cubes prepared by the simultaneous addition were dispersed in 5 mL of deionized water. Then, 0.5 mL of this solution was transferred to 10 vials. The total volume in each vial was made up to 3 mL using deionized water followed by the addition of different concentrations (from 1 μM to 1000 μM) of o-PDA, exactly. Though immediate color change (from lilac to yellow) was observed, all the solutions were left to react at room temperature for B
DOI: 10.1021/acssuschemeng.5b01279 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering 30 min. The same procedure was also followed for oxidation of o-PDA with Au-AgCl cubes. Control Experiments. The oxidation of o-PDA was conducted as reported above under the following conditions: (i) in the presence of Au NPs without AgCl powder, (ii) in the presence of AgCl powder without Au NPs, (iii) in the presence of a physical mixture of Au NPs and AgCl powder, (iv) in the presence of added H2O2 with Au-AgCl cubes, (v) in the absence of light (in a dark room with the vials wrapped around in black tape) with Au-AgCl cubes, and (vi) in N2 atmosphere with Au-AgCl cubes. Characterization. The UV−visible spectra were recorded using a JASCO UV-530 spectrophotometer (Japan). The fluorescence of the product of o-PDA oxidized was measured using a JASCO FP-6300 spectrofluorimeter. The powder XRD were recorded using Bruker D8 Advance Powder X-ray diffractometer (λ of X-ray = 1.54 Å). The TEM images were recorded using a Philips CM12 transmission electron microscope (electron acceleration voltage was 120 kV). STEM image and elemental mapping were done usinga JEOL JEM-2100 transmission electron microscope. The SEM images were recorded using an FEG Quanta 400 scanning electron microscope (electron acceleration voltage was 3 kV). X-ray photoelectron spectroscopy (XPS) was carried out using a Thermofisher Scientific instrument with an Al anode (monochromatic Kα X-rays; courtesy of IISc, Bangalore) as the source.
nanoparticles. The deviation in the absorption maxima of AgCl from the reported values might be due to the effect of size and shape of the particles. The deviation could also arise due to the formation of Ag nanoclusters of smaller size. To confirm the presence or absence of Ag nanoclusters, a few drops of concentrated nitric acid (HNO3) was added to the aqueous dispersion of Au-AgCl, and this was allowed to react at room temperature for 15 min. The UV−visible spectrum of this solution (Figure 1b), before and after oxidation, overlapped exactly in the visible region. This suggested the absence of Ag(0) clusters on the surface of the cubes (if a small amount of Ag(0) was present on the surface of Au-AgCl, it should have been oxidized by HNO3, resulting in significant change in the absorbance in the visible region). The sharp peak at λmax ∼ 300 nm, after HNO3 treatment, arises from the unreacted HNO3. The broad peak observed in the visible region for the dispersion might arise from the scattering of visible light. To confirm the findings from the UV−visible spectroscopy studies, the powder X-ray diffraction of the precipitate formed in the reaction was obtained. This exhibited weak diffraction peaks for Au NPs and peaks with high intensity for AgCl as shown and assigned in Figure 2. Upon comparing the XRD
RESULTS AND DISCUSSION It was observed, during the initial experiments, that BG could reduce tetrachloroauric acid, at room temperature, but not silver nitrate even after prolonged exposure on the order of days. This might be due to the higher potential required to reduce AgNO3 vis-à-vis HAuCl4. Therefore, it was expected that the concurrent addition of HAuCl4 and AgNO3 in the presence of BG could result in the initial formation of Au NPs with the release of chloride ions, which can combine with silver ions resulting in the subsequent formation of AgCl that could coat the Au NPs. It was surprising to observe the formation of (200) faceted cubes of Au-AgCl in this process, the detailed characterization of which is discussed below. Characterizations of Au-AgCl Cubes. The simultaneous addition of HAuCl4 and AgNO3 to aqueous BG, at room temperature, was followed by UV−visible spectroscopy. The spectra of the reaction mixture, at different reaction times, are shown in Figure 1a. It can be observed from this figure that following 5 h of the reaction, a small hump characteristic of Au SPA is observed at 550 nm, due to the formation of Au NPs, as expected (reduction potential of Au3+ in HAuCl4 is far more positive than Ag+). With increasing reaction time, the intensity of this peak increased due to the formation of a higher concentration of Au NPs. It may be noted that the surface plasmon peak does not shift to lower wavelength with increasing reaction time and that the surface plasmon peak due to Ag nanoparticles (∼420 nm) is also not seen. This suggests that Ag NPs are not formed under these reaction conditions. It was observed that after 3 days of reaction, the bigger-sized particles settled out, while the supernatant exhibited the color typical of Au NPs (deep purple). The precipitate was isolated by decantation and dispersed well, again, in water. The UV− visible spectrum of this dispersion in water is also presented in Figure 1a. This exhibited peaks at 300 and 410 nm and a broad absorption in the visible region. It is known from the literature that AgCl can display two defined peaks in the UV region (i.e., at 241 and 382 nm) associated with its direct and indirect band gap (5.15 and 3.25 eV), respectively.32 Therefore, the first two peaks could be from the deposition of AgCl on Au
Figure 2. Powder XRD pattern of the precipitate formed in the reaction mixture HAuCl4 + AgNO3 + BG. Black color indicates the diffraction planes of AgCl, and red color represents different diffraction planes of Au NPs.
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pattern of the precipitate with the JCPDS files of Au and AgCl, two important observations could be made. The full width at half-maximum (FWHM) was small, suggesting a larger size of the particles. The ratio of the intensities of the (200) and (111) diffractions associated with AgCl in the precipitate was around 30 times higher than that reported for AgCl (JCPDS No. 851355). This observation indicated the preferential growth of AgCl along the (200) direction. This, we believe, is the first report showing the much preferred growth along the (200) plane for AgCl, using a natural product-based gum. A powder XRD study further confirms that the quantity or the extent of Au present in the precipitate was small compared to AgCl. This data in combination with the UV−visible spectroscopy data (absence of Au SPA and absence of SPA absorption at lower wavelengths between 420 to 530 nm) suggests that Au is present along with AgCl, and the initially formed Au NPs could serve as a nucleus around which the AgCl could have grown. C
DOI: 10.1021/acssuschemeng.5b01279 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Figure 3. SEM images of Au-AgCl particles (cubes), at different magnifications, synthesized using bael gum (a, b) and its particle size distribution (c).
Figure 4. XPS (survey scan) of Au-AgCl cubes (a), expanded spectra of Au4f (b), Cl2p (c), and Ag3d (d).
the sensitivity factor for Au4f electrons is very high compared to Ag3d. To study the Au-AgCl interface in Au-AgCl cubes, STEMbased elemental mapping was performed. During the analysis, since the high energy electrons were passed through the sample, the AgCl was reduced into Ag metal, and the cubes could be observed to deform possibly due to the loss of chloride ions. The STEM image of a single cube and its corresponding elemental composition, along with mapping of individual elements such as Au, Ag, and Cl are shown in Figure 5. This figure suggests that Au (clusters or NPs) is spread throughout the cube, and the surface is mainly enriched by AgCl. The STEM analysis of another Au-AgCl cube is given in Figure S1 of the Supporting Information to show that the analysis is consistent from cube to cube. The STEM data in combination with the XPS data and the powder X-ray diffraction data suggest that Au is not only present on the surface but also in the bulk as a phase with significant concentration. It can also be inferred from the STEM data that the Au-AgCl cubes are not porous. The formation of Au-AgCl cubes requires some explanation, although the formation of Au and Ag NPs under similar conditions with neem leaf broth extract or seed extracts is known.34,35 It is known that when HAuCl4 is dissolved in water,
The SEM images of the Au-AgCl precipitate, at different magnifications, are shown in Figure 3a and b. This suggests the formation of well-defined cubes and some cuboids. The size of the particles varied from 180 to 360 nm with a mean value of 270 nm and standard deviation of 60 nm. The particle size distribution of these cubes is shown in Figures 3c. The Au-AgCl cubes were analyzed by XPS for the purpose of ascertaining the chemical composition of its surface. The survey X-ray photoelectron spectrum of Au-AgCl cubes is shown in Figure 4. This confirms the presence of Au, Ag, Cl, and other elements such as C, N, and O that might arise from the molecules present in BG adsorbed onto the Au-AgCl surface. The peaks observed at binding energies of 367.6 and 373.5 eV correspond to Ag 3d5/2 and 3d3/2 of Ag+ ions present in the cubes. This interpretation is also consistent with that reported for spherical AgBr particles.33 The ratio of Ag:Au observed is 175.5, suggesting the formation of a surface predominantly rich in AgCl. This is expected because a quantitative reaction of one Au should result in the release of four chloride ions and hence the formation of four AgCl (HAuCl4: AgNO3). Although the atom % of Au, as assessed by XPS and given in Table S1 of the Supporting Information, is very small, the alternate picture of a small quantity of Au dispersed in AgCl is not ruled out since D
DOI: 10.1021/acssuschemeng.5b01279 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Figure 5. STEM image of a single Au-AgCl cube (a), corresponding EDS analysis, shows the presence of Au, Ag, and Cl (b) and STEM-EDS mapping images of Au (c), Ag (d), and Cl (e).
Figure 6. Powder XRD pattern of Au-AgCl cubes synthesized using bael gum at different time intervals (a). Inset shows the expanded portion of Au present in the Au-AgCl cubes. TEM images of Au-AgCl formed after 4 h (b) and 10 h (c).
NPs, the remaining chloride ions will be relinquished gradually. This rate would depend on the rate of reduction of chloroauric acid and consequently on the concentration of BG (reducing agent). Once Au particles are formed (nanocluster or NPs), their high surface areas could attract silver chloride on their surfaces, which results in the deposition of AgCl on the surfaces of Au clusters. The growth of the Au-AgCl cubes at different time intervals was monitored by the analysis of the reaction mixture (precipitate obtained was used for TEM and XRD analysis). From the XRD data (shown in Figure 6a), it can be inferred that the preferred growth of AgCl along the (200) direction was imprinted from the initial stage itself, in contrast to the method involving the preparation of AgCl cubes [in which HCl is added to AgNO3 and a high temperature reduction is effected
it undergoes hydrolysis with a consequent decrease in the pH of the solution, and the extent of the hydrolysis depends on the pH of the reaction medium.36 Especially, under a highly acidic condition, aqueous HAuCl 4 exists in the form of [AuClx(OH)4‑x]− where (x ≥ 2). The aqueous solution of BG is also acidic (pH 5.6). Therefore, upon hydrolysis, one HAuCl4 molecule can result in the formation of four chloride ions and therefore four AgCl molecules. In the synthesis of AuAgCl cubes, an equal weight percentage of HAuCl4 and AgNO3 was taken (that is, the ratio of silver and chloride ions was approximately 1:1.5). Due to the hydrolysis of HAuCl4, roughly one-quarter of its chloride ions will be released instantly, and around 40% of the Ag+ ions present in the solution will be converted into AgCl. This explains the turbidity observed initially. As the chloroauric acid is progressively reduced to Au E
DOI: 10.1021/acssuschemeng.5b01279 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Figure 7. Powder XRD pattern of Au-AgCl cubes synthesized by successive addition method (a). Red color indicates the diffraction planes of Au NPs, and black color represents different diffraction planes of AgCl. TEM image of spherical Au nanoparticles synthesized at high temperature (b). SEM image of Au-AgCl cubes with rounded corner (c).
Figure 8. UV−visible spectra of o-PDA-oxidized product using Au-AgCl cubes (a). UV−visible spectra from control experiments using Au NPs, AgCl precipitate, Au NPs + AgCl physical mixture, under a N2 atmosphere, in the absence of light and in the presence of H2O2 (b).
preferential growth for Au-AgCl cubes along the (200) direction that was observed in the simultaneous addition method (Figure 3). This observation is consistent with that observed in the preparation of AgBr-Ag nanoparticles by the direct addition method involving the addition of NaBr to AgNO3 in the presence of morphology directing polymer.33 Activity of Au-AgCl Cubes for o-PDA Oxidation. The activity of Au-AgCl cubes was studied by the oxidation of oPDA, a standard substrate reported in the literature. In addition, o-PDA is a highly toxic water pollutant and is also considered to be a carcinogenic substance. Its LD50 (lethal dose 50%), nearly the same as that of mercury(II) chloride. Thus, sensing the presence of o-PDA, even in micromolar quantity, is very important. In the literature, three different methods are reported for the oxidation of o-PDA. The first method is a direct simple oxidation catalyzed by a metal ion in the presence of molecular oxygen in which a metal salt such as Cu(OAc)2,38 AgNO3,39,40 FeCl3,41 K2Cr2O7,42 (NH4)2S2O8,43,44 and HAuCl445 and vanadium salt46 will be reduced during the oxidation. The second method involves the use of different catalysts such as Fe3O4,47 metal organic frameworks (MOF),48 carbon nanotubes,49 and iron (iii) complexes50 but in the presence of hydrogen peroxide (H2O2). The third method involves the use of NPs such as Au along with H2O2 as the oxidizing agent and horseradish peroxidase (HRP) or a peroxidase as electron transport
in the presence of ethylene glycol and poly(vinylpyrrolidone)], where no preferred orientation is reported.37 The appearance of the Au (111) plane (shown in inset of Figure 6) peak at 2θ = 38.2 confirms the presence of Au clusters. The intensity of the peak due to Au (111) is not observed to increase with reaction time, suggesting that HAuCl4 reduced by bael gum might serve as seeds and direct the growth of AgCl with time. The deposition of AgCl upon the high surface area Au seeds resulted in many smaller irregular-shaped aggregates as observed in TEM (Figure 6b). With respect to time, the smaller worm-like spherical AgCl nanocrystals can adsorb on the surface of small Au clusters. The slow growth rate of AgCl could be responsible for the polydispersity in size of the particles and also for the specific formation of (200) surface-faceted AgCl cubes. This in turn could be due to the slow release of Cl− ions that could control the orientation of AgCl cubes. To support this claim, the successive addition method was followed in which HAuCl4 was reduced into spherical Au nanoparticles, initially, (TEM shown in Figure 7b) at high temperature. When freshly prepared AgNO3 solution was added to this solution, immediate precipitation of AgCl was observed since all chloride ions were present in its completely ionized form. The SEM image of the Au-AgCl formed was observed to exhibit cubical morphology with rounded corners as shown in Figure 7c, but the powder XRD pattern (Figure 7a) confirmed the lack of F
DOI: 10.1021/acssuschemeng.5b01279 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Figure 9. Fluorescence spectra of product formed by the oxidation of o-PDA using Au-AgCl (200) faceted cubes (a) and Au-AgCl cubes having no preferred orientation (b). Plot of fluorescence intensity of product vs concentration of o-PDA as a result of oxidation carried out using Au-AgCl cubes prepared by the successive and simultaneous addition methods (c).
facilitator.51 In this work, the Au-AgCl cubes are used for oxidation similar to the first method. In this oxidation process, o-PDA is converted into its dimer or polymer or both, which can be visibly seen by the color change (i.e., from lilac to yellow color). The conversion can be quantitatively monitored using UV spectroscopy as the o-PDA has the λ max at 285 nm and the product exhibits λ max at around 430 nm.52 Also, the product is highly fluorescent with strong emission maximum at 560 nm, which can be sensed through fluorescence spectroscopy.53 The UV−visible spectrum of o-PDA before and after oxidation is shown in Figure 8a. From this figure, it is noted that at low concentrations of o-PDA (i.e., from 1 to 100 μm solutions) there was no detectable peak at 430 nm corresponding to the oxidized product. Instead, a slight increase and a small blueshift in the absorbance of Au-AgCl cubes were noticed. This suggests that at low concentration of the substrate, the product formed (i.e., dimer) may have adsorbed to the Au-AgCl shell surface as a very thin film so that the absorbance is slightly blueshifted. At higher concentrations of o-PDA, a new peak at 430 nm is seen to appear. The disappearance of the peak arising from the Au-AgCl cubes and the appearance of the peak due to the oxidized product might be due to the thicker dimer shell formation, as reported earlier.53 Even at higher concentrations of o-PDA, the presence of a Au-AgCl core is still reflected by the redshift in the absorbance of the dimer product. To investigate the specific origin of activity of the (200) faceted Au-AgCl cubes, several control experiments (shown in Figure 8b) using specific concentrations of o-PDA (300 μL) were carried out. The substrates used were Au NPs (synthesized using bael gum), AgCl powder, and a physical mixture of Au NPs and AgCl powder. In the case of the physical mixture, although there was no change at lower concentrations of o-PDA (100 μL), a measurable change was noticed at higher concentrations (500 μL and above). This result could be due to the presence of Au NPs in the proximity of AgCl, but the activity of the physical mixture was much lower than the Au-AgCl cubes. It was also observed that the oxidation of o-PDA by Au-AgCl cubes in the dark was equally facile (Figure 8b). This observation eliminates the role of light (AgCl as photocatalyst or Au-AgCl plasmonic catalyst) and suggests that the presence of Au might be essential for the facilitation of
oxidation as discussed in the Introduction. While carrying out the reaction under nitrogen, the observed lower activity suggests that oxygen could be an active participant in the oxidation reaction, although more detailed experiments have to be conducted to confirm this observation. The best activity was observed in the presence of the well-known oxidizing agent H2O2 as expected. This brings up the issue as to the possible mechanism of oxidation. The oxidation of o-PDA as evidenced by the formation of a dimer with well-known fluorescence suggests that a parallel process of reduction might be taking place. Since Au is present in the zerovalent state, it is reasonable to assume that AgCl might be undergoing reduction. If so, this should result in the formation of Ag in the zerovalent state. The conversion of AgCl to Ag in the presence of Au could be catalyzed by oxygen as observed from the control experiments. To confirm this assertion, the XPS of the Au-AgCl cubes, postreduction (and oxidation of o-PDA), was recorded, and the spectrum is shown in Figure S2 of the Supporting Information. The quantitative analysis of each element present in the cube before and after oxidation reaction shown in Table S1 of the Supporting Information reveals, clearly, the following: The Ag/ Cl ratio is higher (2.7) compared to what it was (1:1) before oxidation of o-PDA, and the binding energy of the elements were lowered. For example, the binding energy of Ag3d is increased from 367.3 and 373.2 eV to 367.6 and 373.5 eV. This is consistent with the formation of Ag (0) as reported earlier.33 The binding energy of C1s is also reduced from 273.6 eV (binding energy of C 1s on a conducting substrate is 273.4 eV). This suggests, unambiguously, the formation of a more conducting surface, which could only imply the formation of Ag from Ag+ ions. To address the specific importance of (200) faceted AgCl cubes toward the oxidation of o-PDA, its activity was compared with that of Au-AgCl cubes prepared by successive addition but having no preferred orientation. The fluorescence spectra of the o-PDA oxidized product are shown in Figure 9. Although, the Au-AgCl cubes prepared by successive addition could catalyze the oxidation of o-PDA, even at low concentrations of o-PDA, the best catalytic activity is observed for (200) oriented cubes. This can be clearly viewed by plotting the intensity of the oxidized product versus the concentration of o-PDA as shown in Figure 9c. From the fluorescence spectra, it is shown that oG
DOI: 10.1021/acssuschemeng.5b01279 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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PDA of concentration as low as 2 ppm could be easily detected using (200) faceted Au-AgCl cubes.
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CONCLUSION In conclusion, a new green synthetic route for the synthesis of (200) surface-faceted Au-AgCl cubes, at room temperature, using bael (Aegle marmelos) fruit gum is presented. The bael gum is observed to act as the reducing and capping agent. We believe that the slow and controlled release of Cl− ions by the initial reduction of HAuCl4 might be responsible for the preferential formation of (200) faceted AgCl cubes. This claim is substantially proved by synthesizing Au-AgCl cubes (lacking preferred orientation) by the successive addition method in which the release of chloride ions is uncontrolled. The Au-AgCl cubes are observed to exhibit oxidative activity in the absence of hydrogen peroxide, as demonstrated by the oxidation of o-PDA. In addition, o-PDA, a carcinogen, can be detected with lower limits of detection of 2 ppm by fluorescence spectroscopy and 15 ppm by UV−visible spectroscopy. The oxidative activity is observed to be facilitated by the presence of Au. The influence of particular surface facets on the oxidation is also demonstrated by comparing the catalytic activity with AuAgCl cubes with no specific preferred surface facet.
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ASSOCIATED CONTENT
S Supporting Information *
STEM image, XPS spectrum and quantitative analysis of elements based on XPS survey spectrum. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01279. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank IIT Madras for the support of this research work. Ms. Sathiya thanks UGC for the Fellowship to pursue her Ph.D. The authors thank Prof. S. Sankaran of the Department of Materials and Metallurgical Engineering of IIT Madras for his extensive support in training and acquiring TEM images. We gratefully acknowledge the Sophisticated Analytical Instruments Facility (SAIF) of IIT Madras for providing the SEM facility. We thank NCCR, IIT Madras, for providing the STEM facility. We also thank Prof. G. Mohan Rao, IISc Bangalore, for providing the XPS facility.
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