Shape-selective formation and characterization of catalytically active iridium nanoparticles

Journal of Colloid and Interface Science 354 (2011) 597–606

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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Shape-selective formation and characterization of catalytically active iridium nanoparticles Subrata Kundu ⇑, Hong Liang ⇑ Materials Science & Mechanical Engineering, Texas A&M University, College Station, TX 77843-3123, USA

a r t i c l e

i n f o

Article history: Received 23 September 2010 Accepted 11 November 2010 Available online 16 November 2010 Keywords: Ir nanoparticles (NPs) CTAB UV-photoirradiation Nano-needles Catalysis

a b s t r a c t Iridium (Ir) nanoparticles (NPs) of variable shapes have been synthesized via the reduction of Ir(III) ions in CTAB micellar media containing alkaline 2,7-DHN under 4 h of UV-irradiation. The one-step process generates different shapes, such as nano-spheres, nano-chains, nano-flakes, and nano-needles. The synthesized Ir NPs are stable for more than a month in ambient conditions. The particles’ morphology can be tuned by simply changing the surfactant-to-metal ion molar ratios and altering other reaction parameters. The mechanisms of the Ir particle formation and effects of different reaction parameters were studied in detail. The Ir nano-needles serve as a good catalyst for the reduction of organic dye molecules in presence of NaBH4. The catalysis rate was compared by considering the electron transfer process during the reduction of the dye molecules. The present method would lead to a quick process for the synthesis of other mono-metallic, composite, and semiconductor particles with variable shapes. The Ir NPs will find promising applications in different types of organic and inorganic catalysis reactions, nanoelectronics, and biomedical applications. Ó 2010 Elsevier Inc. All rights reserved.

1. Introduction In recent years, nanoparticle (NP) research has received enormous attention in the field of science and technology (chemistry, biology, materials science, etc.). NPs in their nanoscale dimensions exhibit variety of unique electronic [1], spectroscopic [2], and chemical [3] properties due to their small size and large surfaceto-volume ratio. NPs have found various applications in the field of chemistry [4], physics [5], biology [6], medicine [7], materials science [8], and other different fields in engineering [9]. Among the different metals, novel metal NPs show particular interests because of their closely packed structures and valence bands in which electrons have free movement. This movement of electrons generates surface plasmon bands which depend upon particles size, shape, and other parameters [10]. Recently, the possibility of size and shape-controlled synthesis of nanomaterials like nanorods [11], nanocubes [12], nanowires [13], nano-flakes [14], and so forth has become increasing promising due to their wide variety of applications. Among the different synthetic routes, the chemical reduction remains the more advantageous over the physical process in terms of particles size and shape control. In the chemical reduction method, the corresponding metal salt is reduced in presence of a capping/stabilizing agent which prevents the metal NPs from unwanted agglomeration/aggregation. ⇑ Corresponding authors. Fax: +1 979 845 3081. E-mail addresses: [email protected] (S. Kundu), [email protected] (H. Liang). 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.11.032

Recently, nanoscale metal particles have been widely considered as effective catalysts because of their high surface area and high density of active sites. Among the different transition metals as catalyst, iridium (Ir) NPs were found an outstanding candidate due to their high activity, stability and selectivity under different reaction conditions [15–18]. The synthesis of size and shape controlled NPs is a challenging task and there is few report on the shape control of Ir NPs compared to other noble metals [19–21]. It has been reported earlier that Ir NPs can be used as excellent catalysts in many catalysis reaction like oxygen evolution reaction (OER) [22], oxygen reduction reaction (ORR) [23], and for direct methanol fuel cell reactions [24] for methanol crossover problems. Moreover, the supported Ir NPs have been used as catalysts in other reactions like hydrogenation of a,b-unsaturated aldehydes [25,26], water electrolysers [22], and activation of intermolecular C–H bonds for both saturated and unsaturated hydrocarbons [27–29]. The catalytic rate of NPs strongly depends upon the particles size and shapes. With the change in particles sizes and shapes, the catalytic rate is changed accordingly. Previously we reported the shape-selective catalysis of aromatic nitro compounds using Au and Rh NPs as catalyst [30,31]. For Au, we have found that nanoprisms acted as a better catalyst compared to nanorods [30]. Earlier, El-Sayed group reported that Pt NPs with tetrahedral shapes were better catalyst compared to spheres and cubes due to availability of more catalytically active surface atoms [32]. The Ir NPs with spherical shapes were synthesized by Watzky and Finke [33] in 1997 and stabilized them using polyoxoanion and Bu4N+. Yee et al. synthesized thiol stabilized Ir NPs using lithium

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triethyl borohydride (superhydride) as a reducing agent [34]. Williams et al. synthesized Al2O3 supported Ir NPs as catalyst for the liquids phase hydrogenation of benzonitrile [35]. Yinghuai et al. synthesized Ir NPs using ethylene glycol and stabilized them in an ionic liquid [36]. The synthesized Ir NPs have been used as a catalyst for catalytic phenylborylation reaction. Recently, Redel et al. synthesized Ir NPs in ionic liquid and used them for the catalytic hydrogenation of cyclohexene [37]. Zheng et al. synthesized nafion stabilized Ir NPs for direct use in fuel cells and water electrolysers [38]. All the above synthetic methods for Ir NPs generated mostly spherical shapes and most of them required a long reaction time, needed multiple steps, or produced non-uniform particles sizes with low yields. In the present work, we report the shape-selective synthesis and catalytic application of Ir NPs using a photochemical approach. The reduction of Ir salts was done in CTAB (cetyltrimethyl ammonium bromide) surfactant media under the presence of a reducing agent alkaline 2,7-dihydroxynaphthalene (2,7-DHN). The reaction mixture was UV-irradiated for 4 h continuously. The process exclusively generated Ir nano-spheres, Ir nano-chains, Ir nano-flakes, and Ir nano-needles, just by altering the metal ion-to-surfactant-molar ratios and changing other reaction parameters. The synthesized Ir nano-needles found to serve as an effective catalyst for the reduction of several organic dye molecules in the presence of NaBH4 in a short time. To the best of our knowledge, the shape-controlled synthesis of Ir NPs using a simple photochemical method and their catalytic activity have not been explored before. The synthesis of Ir NPs and the catalysis study is simple, reproducible, and cost effective. 2. Experimental 2.1. Reagents Cetyltrimethyl ammonium bromide (CTAB, 99%), Iridium chloride, trihydrate (IrCl3
3H2O), and sodium hydroxide (NaOH) were purchased from Sigma–Aldrich. The 2,7-dihydroxynapthalene (2,7-DHN), 1,2-dihydroxynapthalene (1,2-DHN), and 2-naphthol (2-N) (Sigma–Aldrich) were recrystallized in hot water before use. Three different dye molecules, Methylene Blue (C16H18N3SCl, MB), Rhodamine B (C28H31N2O3Cl, RhB), and Rose Bengal (C20H2Cl4I4Na2O5, RB) (Sigma–Aldrich) were used as received. Sodium borohydride (NaBH4) (Sigma–Aldrich) was used fresh daily. De-ionized (DI) water was used for the entire synthesis process.

photoelectron spectroscopy (XPS) analysis was carried out using a Kratos Axis Ultra Imaging X-ray photoelectron spectrometer with monochromatic Al Ka line (1486.7 eV). The instrument integrates a magnetic immersion lens and charge neutralization system with a spherical mirror analyzer, which provides real-time chemical state and elemental imaging using a full range of pass energies. The emitted photoelectrons were detected by the analyzer at a passing energy of 20 eV with energy resolution of 0.1 eV. The incident X-ray beam is normal to the sample surface and the detector is 45° away from the incident direction. The analysis spot on the sample is 0.4 mm by 0.7 mm. A xenon lamp from Newport Corporation at a wavelength of 275 nm on the sample was used for UV-photoirradiation. The approximate intensity of the irradiation was 22 lW and the distance of the sample from the light source was 18 cm. The sample was placed over a wooden box with a stand to make the light shine on it directly. 2.3. Photochemical synthesis of shape-selective Ir NPs Shape-selective Ir NPs were synthesized by varying the concentration of CTAB with the Ir(III) ions as well as changing the concentration of 2,7-DHN and NaOH. For a typical synthesis 50 mL of CTAB (0.1 M) was mixed with 6 mL of IrCl3
3H2O (10 2 M) solution. The solution mixture was stirred well and finally 6 mL of 2,7-DHN (10 2 M) and 0.6 mL of NaOH (1 M) were added. The reaction mixture was further stirred for half a minute using a magnetic stirrer. Then the whole reaction mixture was continuously UV-irradiated for about 4 h. The method exclusively generates Ir nano-needles. For the synthesis of other shapes like Ir nano-spheres, Ir nanochains, and Ir nano-flakes, we varied the concentration of CTAB relative to the concentration of Ir(III) ions. The final concentration of all the reaction parameters used, time of UV-irradiation is given in Table 1. After mixing all the reagents, the resulting solution mixture has no noticeable color. The reaction mixture became slightly greenish after 10–15 min, turned to deep greenish after 1–2 h, and finally blackish green color after 4 h of UV-irradiation. After the completion of the irradiation process the solution mixture was centrifuged at 6000 rpm for 15 min and 4000 rpm for 8 min to remove excess surfactant and other chemicals from the solution mixture. The precipitated Ir NPs were blackish brown in color and re-dispersed in DI water. The synthesized Ir NPs solutions were found to be stable for more than a month in the dark and stored at 4 °C in a refrigerator. 2.4. Reduction of organic dye molecules using Ir nano-needles as catalyst

2.2. Instruments A high resolution-transmission electron microscope (HR-TEM) (JEOL JEM 2010) was used at an accelerating voltage of 200 kV. The energy dispersive X-ray spectrum (EDS) was recorded with an Oxford Instruments INCA energy system connected with the TEM. The UV–visible (UV–vis) absorption spectra were recorded in a Hitachi (model U-4100) UV–vis–NIR spectrophotometer equipped with a 1 cm quartz cuvette holder for liquid samples. The X-ray

We study the reduction of three different organic dye molecules like MB, RhB and RB in presence of NaBH4 using Ir nano-needles as catalyst. A stock solution of three different dyes (10 3 M) was prepared and a fresh stock solution of NaBH4 (0.1 M) was made fresh daily. The NaBH4 solution was stored in the refrigerator in the dark. For the reduction reaction, the dye solution was diluted to a

Table 1 The final concentrations of all the reaction parameters, time of UV-irradiation and particles size and shapes distribution for the formation of Ir NPs. Set no.

Final conc. of CTAB (M)

1 2

8.01  10 5.29  10

3

8.0  10

4

4 3

3

8.01  10

2

Final conc. of 2,7-DHN (M) 9.61  10 9.77  10

4

9.60  10

3

9.61  10

4

4

Final conc. of Ir(III) solution (M) 9.61  10 8.14  10

5

9.60  10

4

9.61  10

5

5

Final conc. of NaOH (M) 6.41  10 6.51  10 8.0  10

3 3

3

6.41  10

3

Time of UVirradiation (h)

Shape of the particles

Particles shape distribution

Diameter (D) and length (L) of the NPs

4 4

Spheres Chain-like

100% Spheres 100% Chains

4

Flake-like

100% Flakes

4

Needles-like

95% Needles

D = 3 ± 1 nm L = 1–3 lm and D = 35 ± 5 nm L = 0.8–1 lm and D = 145 ± 20 nm L = 2.5 ± 1 lm and D = 200 ± 15 nm

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3.0

A = only CTAB (water) B = only 2,7-DHN (water) C = only IrCl3 . 3H2O (water) D = mixture of CTAB, 2,7-DHN and Ir (III) E = mixture of CTAB, 2,7-DHN, Ir (III) and NaOH before UV irradiation F = Ir nanoparticles (needle shape) after UV-irradiation B N

2.5 2.0

Absorbance

concentration of 10 5 M. The whole reduction reaction was carried out using a quartz cuvette having a path length of 1 cm. For a typical reduction, 10 mL of (10 5 M) dye solution was mixed with 2 mL of (0.1 M) NaBH4 solution and the solution was mixed well by shaking in hand. Finally, 0.2 mL of Ir nano-needles was added and the progress of the reduction was monitored spectrophotometrically using an in situ UV–vis spectrophotometer. The reduction of all the dyes started within a couple of minute and completed in 90 min, as observed from the UV–vis spectrum. After the completion of the reduction, the bluish color MB, the pinkish color RhB and RB became colorless which was further confirmed from the UV–vis spectrum. After the complete reduction, the catalysis reaction rate was calculated and comparison studies were made.

1.5

F

1.0

D 0.5

C

E A

0.0

2.5. Preparation of samples for UV–vis, TEM, EDS, and XPS studies

3. Results and discussion 3.1. UV–vis spectroscopic study Shape-selective Ir NPs were synthesized by the reduction of Ir(III) ions in CTAB micellar media using alkaline 2,7-DHN as a reducing agent under 4 h of UV-irradiation. Fig. 1 shows the UV– vis spectra of the reaction mixture at the different stages of the synthesis process. Fig. 1, curve A shows the absorption spectra of an aqueous solution of CTAB which has no specific absorption bands in the visible region. A colorless aqueous solution of 2,7DHN shows two distinct absorption bands in the UV–vis region peaking at 281 nm and 321 nm (curve B, Fig. 1). This bands appear due to the presence of aromatic ring in 2,7-DHN. An aqueous IrCl3
3H2O solution shows two (curve C, Fig. 1) small humps at 328–335 nm and 384 nm respectively. The absorption band at lower wavelength region (328–335 nm) can be assigned to the spin allowed p ? p transition from water ligand [36,37]. The other small hump at 384 nm is probably due to spin allowed metal-to-ligand charge transfer (MLCT) spectra, i.e., Ir to chlorine ligand [39,40]. Similar types of absorption spectra were observed before by William’s et al. for their synthesis of dendrimer derived Ir NPs [35]. With the addition of aqueous 2,7-DHN to a mixture of CTAB and IrCl3
3H2O, the original peak of 2,7-DHN shifted a bit and the absorption value is also reduced (curve D, Fig. 1). This is probably due to the interaction of 2,7-DHN with either CTAB or with Ir(III) solution. After the addition of NaOH to the reaction mixture containing CTAB, Ir(III) and 2,7-DHN, a new peak appeared at a wavelength 340 nm (curve E, Fig. 1), which may be due to interaction of alkaline 2,7-DHN with either CTAB or with Ir(III) ions before UV-irradiation. After UV-irradiation of the solution mixture containing CTAB, Ir(III), 2,7-DHN, and NaOH, the solution color changed initially to light green, then deep green, then blackish green and finally blackish brown after 4 h of UV-irradiation.

300

400

500

600

700

800

Wavelength (nm) Fig. 1. The UV–visible absorption spectra for the synthesis of Ir NPs at different reaction conditions of the synthesis process. (A) is the absorption spectrum of aqueous CTAB solution which has no specific absorption bands in the visible region; (B) is the absorption spectrum of aqueous 2,7-DHN solution in water showing two distinct bands peaking at 281 nm and 321 nm; (C) is the absorption spectrum of aqueous IrCl3
3H2O solution showing two very small intense humps at 328–335 nm and 384 nm; (D) is the absorption spectrum of the mixture of 2,7-DHN with CTAB and IrCl3
3H2O solution before UV-irradiation; (E) is the absorption spectrum of the mixture of 2,7-DHN with CTAB and IrCl3
3H2O solution and NaOH before UVirradiation showing a peak at a wavelength 340 nm; (F) is the absorption spectrum of Ir nano-needles. The inset shows the blackish brown color image of needleshaped Ir NPs solution indicated with ‘N’. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

The final product was centrifuged two times before re-dispersed in DI water. This solution contains Ir NPs of needle shapes. This needle-shaped Ir NPs solution (curve F, Fig. 1) shows a broad absorption bands in the region 470–660 nm and two others small humps at a wavelength 323 and 380 nm. These bands are due to the formation of Ir NPs having some similarities with previous reports [35,40]. The inset shows the blackish brown color image of needle-shaped Ir NPs solution indicated with ‘N’ after re-dispersion in aqueous solution. In Fig. 2, curve A–D shows the different optical absorption bands of various shaped Ir NPs. Curve A is the absorption bands 3.0

A = Ir nano spheres B = Ir nano chains C = Ir nano flakes D = Ir nano needles

2.5 2.0

Absorbance

The Ir NPs were characterized using UV–vis, TEM, EDS, and XPS measurements. The Ir NPs solution after successive centrifugation was re-dispersed in DI water and used for the measurement in UV–vis spectrophotometer. The samples for TEM and EDS were prepared by placing a drop of the corresponding Ir NPs solution onto a carbon coated Cu grid followed by slow evaporation of solvent at ambient conditions. For XPS analysis, glass slides were used as substrates for thin film preparation. The slides were cleaned thoroughly in acetone and sonicated for about 20 min. The cleaned substrates were covered with the Ir NPs solution and dried in air. After the first layer was deposited, subsequent layers were deposited by repeatedly adding more Ir NPs solution and drying. Final samples were obtained after 6–8 depositions and then analyzed using XPS techniques.

200

S

C

1.5

C

N

F

1.0

B

D

0.5

A

0.0 200

300

400

500

600

700

800

Wavelength (nm) Fig. 2. The UV–visible absorption spectra of different shaped Ir NPs. (A) is the absorption bands of Ir nano-spheres which has two small intense bands at 270 and 336 nm. (B–D) are the absorption bands for Ir nano-chains, Ir nano-flakes, and Ir nano-needles respectively having absorption maxima at 330 nm (for curve B), 330 and 496 nm (for curve C) and a broad band between 470 and 660 nm and two other small bands (for curve D) respectively. The inset of shows the image of four different shaped Ir NPs solution indicated with ‘S’, ‘C’, ‘F’ and ‘N’.

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Fig. 3. The transmission electron microscopy (TEM) images of different shaped Ir NPs were shown in (A–F). (A and B) are the low and high magnified TEM images of Ir nanospheres. The average diameters of the Ir nano-spheres are 3 ± 1 nm. The inset of (A) show the higher magnified images. (C and D) are the TEM images of the low and high magnified Ir nano-chains. The inset of both the image shows their corresponding higher magnified image. The lengths of the nano-chains are 1–3 lm with diameter 35 ± 5 nm. (E and F) are the TEM images of Ir nano-flake structures at low and high magnification. The inset of (F) shows the image of a single nano-flake. The average length of the flakes are 0.8–1 lm and the diameter 145 ± 20 nm. (G and H) are the low and high magnified TEM images of needle-shaped Ir NPs. The inset of (G) shows the corresponding higher magnified image. The average lengths of the needles are 2.5 ± 1 lm and the average diameter of the needles are 200 ± 15 nm.

of Ir nano-spheres which has two small intense bands at 270 and 336 nm having similarities with previous report [35]. Curves B–D

in Fig. 2 are the absorption bands for Ir nano-chains, Ir nano-flakes, and Ir nano-needles respectively. Curve B shows a band at 330 nm,

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curve C shows two bands at 330 and 496 nm. Curve D for nanoneedles shows a broad band between 470 and 660 nm and two other small bands as discussed already in curve F, Fig. 1. The inset of Fig. 2 shows the image of four different shaped Ir NPs solution indicated with ‘S’ for spheres, ‘C’ for chains, ‘F’ for flakes, and ‘N’ for needles. 3.2. Transmission electron microscopy (TEM) analysis Fig. 3 shows the TEM images of shape-selective Ir NPs synthesized after 4 h of UV-irradiation at different magnification. The synthesized NPs having different shapes like spheres, chains, flakes, and needles are formed in different reagents concentrations given in Table 1. Fig. 3A is the low magnified TEM image of Ir nano-spheres (corresponding to curve A, Fig. 2) and inset shows the corresponding higher magnified image. Fig. 3B shows the much higher magnified image of Ir nano-spheres. The average diameters of the Ir nanospheres are 3 ± 1 nm. Fig. 3C and D shows the lower and higher magnified TEM image of Ir nano-chains (corresponding to curve B, Fig. 2). The inset of both the image shows their corresponding higher magnified image. From the image it is clearly shown that the nanochains are bonded to each other having length 1–3 lm and diameter 35 ± 5 nm. There is no other shape observed and final yield is 100%. Fig. 3E and F shows the TEM images of Ir nano-flake structures at lower and higher magnifications (corresponding to curve C, Fig. 2). The inset of Fig. 3F shows the image of a single nano-flake.

601

The mono-dispersed nano-flakes are well separated with each other and no other shapes are found. The nano-flakes yielded at the production rate of 100%. The average length of the flakes are 0.8– 1 lm and the average diameter 145 ± 20 nm. Fig. 3G and H shows the low and high magnified TEM images of needle-shaped Ir NPs (corresponding to curve C, Fig. 2). The inset of Fig. 3G shows the corresponding higher magnified image. From the image it is seen that more than 95% NPs formed the needle shape and few other anisotropic shapes were formed. The average length of the needles was 2.5 ± 1 lm and the average diameter of the needles was 200 ± 15 nm. According to the image it is clear that most surfactant molecules were removed from the NPs solution and the yields of different shapes were high. 3.3. Energy dispersive X-ray spectroscopy (EDS) analysis Fig. 4 shows the results obtained from the energy dispersive X-ray spectroscopic (EDS) analysis. The EDS spectrum was used for the determination of elements presented in the reaction product. The EDS spectrum consisted of different peaks for Ir, C, Cu, O, Cl and Br. The large intense Ir peak came from the Ir NPs and Cu peak came from the Cu grid used for TEM analysis. The C peak also came from the carbon-coated Cu-TEM grid used for analysis. The small intense Cl peak came from the iridium salts and Br peak from the surfactant CTAB used for stabilization of the particles during the synthesis of Ir NPs. The inset shows the same image with enlarge Ir peak. 3.4. X-ray photoelectron spectroscopy (XPS) analysis Fig. 5 shows the XPS spectra of Ir NPs. The Ir (4f) region is characterized by a doublet which arises from spin–orbit coupling of Ir 4f5/2 and Ir 4f7/2. The two peak positions are 60.7 eV (Ir 4f7/2) and 63.6 eV (Ir 4f5/2) respectively. This two binding energies of Ir 4f7/2 and Ir 4f5/2 in the XPS spectra of the CTAB coated particles corresponds to Ir in the zero valent (Ir0) state. These values are consisted with the literature report [41]. 3.5. Study of other reaction parameters In our synthesis we have conducted some control experiments to check the effects of other reaction parameters. We tested our reaction with different concentrations of CTAB, Ir(III) ion, 2,7-DHN, NaOH, and varied UV-irradiation time. The shape-selective Ir NPs like nano-spheres, nano-chains, nano-flakes, and EDS spectrum of Iridium NPs

Fig. 3 (continued)

Fig. 4. The energy dispersive X-ray spectroscopic (EDS) analysis of the Ir NPs shows different peaks for Ir, C, Cu, O, Cl and Br. The inset shows the enlarge spectrum with high intense Ir peak.

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Fig. 5. The X-ray photoelectron spectrum (XPS) of the Ir NPs for the Ir (4f) region. Fig. 6. The transmission electron microscopy (TEM) images of spherical shaped Ir NPs synthesized using 2-N as reducing agent.

nano-needles are formed at a specific concentration given in Table 1. From Table 1, it is clearly seen that the nano-spheres are formed at a lower CTAB concentration where as other anisotropic shapes were formed at higher CTAB concentration. When Ir(III) concentration is low (610 5 M), a longer UV-irradiation time is necessary. Where at higher Ir(III) concentrations (P10 2 M), the particles were formed with either mixed shapes or no specific shapes. We changed the concentrations of 2,7-DHN and observed that at lower 2,7-DHN concentration (610 5 M), the formation of particles took longer time and at high 2,7-DHN concentrations (P5  10 2 M), the particles formed but precipitated after sometime (1 h) of synthesis. We also tested the varied concentration of NaOH and seen that at very high concentration of NaOH (P10 1 M), the particles formed but undefined shapes and gets precipitated immediately after synthesis. Thus a proper concentration of all the reagents is necessary for the formation of definite shaped Ir NPs. In our proposed reactions we also varied the UV-exposure time. We have seen that 4 h irradiation time is sufficient for the complete formation of different shaped Ir NPs. Shorter exposure time (90 min or less) yields non-uniform particles whereas longer exposure time (7 h or more) yields the precipitation of the particles with mixture of different shapes. We do some preliminary study with other hydroxyl compounds like 2-naphthol (2-N) and 1,2dihydroxy naphthalene (1,2-DHN) having similar structure with 2,7-DHN. The initial results yields the formation of particles mostly spherical shapes as shown in Fig. 6 using 2-N as reducing agent. The detailed synthesis using these hydroxyl compounds will be discussed in near future. All the above control experiments summarized as a general idea about the reaction process. 3.6. Mechanism of the Ir NPs formation The shape-selective formation of Ir NPs was initiated by the reduction of aqueous Ir(III) ions in CTAB micellar media in presence of alkaline 2,7-DHN under UV-irradiation. The overall synthesis was conducted at room temperature and ambient environment. In our present synthesis, the presence of CTAB and alkaline 2,7DHN is extremely necessary for the formation of definite shaped particles. In the absence of CTAB but keeping the other reaction parameters same, the Ir particles formed but immediately became precipitated due to absence of specific stabilizer. In the absence of alkaline 2,7-DHN, no particles formed due to absence of any reducing agent in the reaction media. In the alkaline condition at a pH  8–9.5, the rate of NPs formation enhanced. It was reported earlier that a proper concentration of hydroxyl ions can enhanced

the reducing power of hydroxyl compounds and facilitate the growth of anisotropic shapes [31,42]. Other hydroxyl compounds, such as ascorbic acid [43], TX-100 [44], dendrimers [45], poly(vinyl) alcohol (PVA) [46] having hydroxyl group on the structure, can act as a reducing agent to reduce the metal ion and formed the NPs. In the presence of UV-light, these hydroxyl compounds underwent hydroxylic cleavage to generate radical species. This UV photo-generated radical species acted as a reducing agent for the reduction of metal ions to metal0. Our group reported earlier that hydroxylic compounds reduce Pt(IV), Au(III) ions to produce their corresponding NPs in different shapes [14,47]. Recently we also reported that deoxyribonucleic acid (DNA) having hydroxyl group on their aromatic sugar part could act as a reducing agent to produce metal nanowires [13,48]. In our present work, we assumed that the hydroxyl radical formed from the mixed solution containing alkaline 2,7-DHN facilitated the reduction of Ir(III) ions to Ir(0). The UV-irradiation was done at a wavelength of 275 nm which was close to the absorption bands of 2,7-DHN. To confirm the radical species formed in our reaction we conducted one control experiment. We have irradiated our solution mixture far below (180 nm) and far above (375 nm) the absorption band of 2,7DHN. The reaction did not result the formation of Ir NPs in the experimental time scale. Moreover, when the mixed solution was heated by a conventional heater, no particles were generated because only heating might not able to cleave the hydroxyl bond. So the control experiment proved that the formation of radical species was responsible for the reduction of Ir(III) ions. Initially during the reduction process, the Ir(III) was reduced to form Ir(0) particles; i.e., they nucleated to form Ir(0) seeds and with the increase in time smaller Ir seeds grew to form larger Ir NPs. As soon as the Ir NPs formed in the solution, it catalyzed the reduction of other Ir ions presented in the solution and accelerated the reduction process. The CTAB molecules adsorbed on the surface of the Ir NPs and reduced the growth rate of different crystal planes. It was assumed that the growth of Ir NPs took place in multiple steps and finally formed the specific shaped particles. It was reported previously that the formation of specific shape mainly depended upon two important factors [49]. One is the growth kinetics of Ir, i.e., the rate of supply of Ir(0) to the different crystallographic planes and the other is faceting tendency of the stabilizing agent, i.e., the surfactant CTAB. At a lower CTAB concentration, mostly spherical Ir NPs formed whereas at higher CTAB concentration mostly anisotropic shapes are formed. Our group and Murphy’s group reported earlier that at high CTAB concentration, mostly

S. Kundu, H. Liang / Journal of Colloid and Interface Science 354 (2011) 597–606

the anisotropic shapes were formed [31,50]. The formation of nano-chain occurred at 10 3 (M) CTAB concentration, nano-flake at 10 2 (M) concentration, where as nano-needles at 10 1 (M) concentrations as shown in Scheme 1. It was accepted that at a higher surfactant concentration (P10 3 M), CTAB formed worm like or rod-shaped micellar template [50]. Murphy’s group reported earlier that rod-shaped micellar template facilitated the formation of Au nano rods [50]. In our case, at a higher CTAB concentration, it facilitated the formation of flake-like or needlelike nano structures. It was reported that a proper concentrations of CTAB and metal ion played a crucial role for the formation and stabilization of definite shapes in aqueous solution. At a low CTAB concentration (10 4 M), lots of seed particles are formed and they inhibited the growth of others and formed smaller size spherical NPs. At a moderate concentration, they probably grew on the worm like micellar structure and produced the nano-chain structure. The nano-flake structure was formed at a higher CTAB concentration (10 2 M) and higher Ir(III) concentrations (10 3 M). The nano-needle structures were formed at very high CTAB concentrations (10 1 M) and moderately low Ir (III) ion concentrations (10 4 M). At this point we are not fully aware about the detailed growth mechanism of different anisotropic shape. More experiments are needed for thorough understanding in the shape control process. After the synthesis of four different shapes, we have studied the catalytic activity for the reduction of three different organic dye molecules using NaBH4 as a reducing agent. As an example, we have studied details about reduction of dyes using Ir nano-needles as an effective catalyst.

603

as a reducing agent. For the catalysis study, the Ir nano-needles solution was centrifuged for 3–4 times to remove the excess surfactant and finally re-dispersed in DI water. It is accepted that in any types of homogeneous or heterogeneous catalysis the catalyst particles surface should be clean for fast electron transfer. If there is any other chemicals like polymers, surfactant, etc., rapped the particle surface, the catalytic rate will be slow. It is needed to have a fresh surface for greater catalytic efficiency. In the present study, we have selected three different dye molecules having organic skeleton with different chemical structures. We chose one cationic dye, MB and two anionic dyes RhB and RB for the catalysis study. All three dye molecules were used as commercial colorants in dye industry and they were highly soluble in water. It is well known that most of the dye molecules mixed with water to create a major problem in environmental pollution [51]. Scientists are searching a suitable method to degrade these dyes to prevent the waste water contamination. There are a few methods reported in literature for the degradation of dye molecules [52,53]. Most of the methods require harsh condition for the reduction, multiple steps, longer time, and more over do not seem to be environmental friendly. There are several groups studied the dye reduction in surfactant media using UV-irradiation [53,54]. Here in our study, we reduced the dye molecules using NaBH4 in presence of Ir nano-needles as a catalyst. The reduction was done at room temperature and the major advantage was that we could follow the reaction steps using UV–vis spectrophotometer. The pH of aqueous MB, RhB, and RB (10 5 M) solutions are 7.56, 6.80, and 6.84 respectively. The pH of 0.1 (M) NaBH4 is 10.5 and for Ir nano-needles solution is 6.84. The dye reduction using spherical metal NPs as a catalyst has been reported before [52,55]. In our study, the reduction of these dyes using NaBH4 is slow in absence of Ir nanoneedles. On the other hand the reduction of the dyes using Ir nano-needles in absence of NaBH4 does not take place at all even after a couple of days. So a proper concentration of dye molecules,

3.7. Catalytic reduction of organic dye molecules using Ir nano-needles as catalyst The catalytic efficiency of Ir nano-needles was tested for the reduction of three different organic dye molecules using NaBH4

A

B 2,7-DHN/ NaOH CTAB + Ir (III) soln.

Ir seed particles A = Very Low CTAB conc. ~ 10-4 M B = Low CTAB conc. ~ 10-3 M C = Medium CTAB conc. ~ 10-2 M D = High CTAB conc. ~ 10-1 M

C

D

UV light

Ir nano-spheres

Ir nano-chains

Ir nano-flakes

Ir nano-needles

Scheme 1. Schematic presentation for the formation of Ir NPs.

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NaBH4 and the Ir nano-needles is important. The absorption maxima of MB, RhB, and RB are peaked at 664, 552, and 548 nm in the UV–vis spectrum. The dye solution, after mixing with NaBH4 and catalytic amounts of Ir nano-needles, the reduction started spontaneously which could be easily monitored using a in situ

UV–vis spectrophotometer. Fig. 7A shows the successive reduction of MB curve with time and the reduction is completed after an hour. Fig. 7B shows the corresponding ln(Abs) vs. time (T, min) plot for the same and the rate constant value for the reduction is 4.07  10 2 min 1. Similarly Fig. 7C and D shows the successive

2.0

1.0

Absorbance

1.5

ln (Abs) vs Time plot for the catalytic reduction of MB using Ir nano-needles as catalyst

664 nm 0.5

01 min 07 min 13 min 21 min 28 min 35 min 42 min 48 min 54 min 60 min

MB 1.0

0.5 Oxidized Form

MB 0.0

ln (Abs)

Reduction of MB using Ir nano-needles as catalyst

-0.5 -1.0 -1.5

Reduced Form

R = 0.967 SD = 0.214 -2

-1

k = 4.07 x 10 min

0.0 -2.0 400

500

600

700

800

0

900

10

20

30

Wavelength (nm)

Time (min)

A

B

40

50

60

1.8

0.9 0.6 Oxidized Form

0.3

ln (Abs) vs Time plot for the catalytic reduction of RhB using Ir nano-needles as catalyst

0.5

01 min 03 min 05 min 07 min 09 min 12 min 15 min 18 min 21 min 24 min

RhB

1.2

Absorbance

552 nm

0.0

RhB

ln (Abs)

Reduction of RhB using Ir nano-needles as catalyst

1.5

-0.5 -1.0 -1.5

R = 0.998 SD = 0.057

Reduced Form -1

k = 1.18 x 10 min -2.0

0.0 400

450

500

550

600

650

700

0

5

10

Wavelength (nm)

Time (min)

C

D

1.8

15

20

1.0

548 nm

Reduction of RB using Ir nano-needles as catalyst

1.2

0.5

01 min 10 min 20 min 30 min 40 min 50 min 60 min 70 min 80 min 90 min

RB

0.9 0.6 0.3

Oxidized Form

ln (Abs) vs Time plot for the catalytic reduction of RB using Ir nano-needles as catalyst

RB

0.0

ln (Abs)

1.5

Absorbance

-1

-0.5 -1.0

R = 0.984 SD = 0.151

-1.5

k = 2.9 x 10 min

Reduced Form -2

-1

0.0 -2.0 400

450

500

550

600

650

700

0

20

40

Wavelength (nm)

Time (min)

E

F

60

80

Fig. 7. The UV–vis absorption spectrum for the reduction of three different dye molecules with NaBH4 using Ir nano-needles as catalyst. (A) is the UV–vis spectrum for the successive reduction of MB at 664 nm and (B) is the ln(Abs) vs. time (T) plot having the rate constant value of 4.07  10 2 min 1. (C and D) are the UV–vis spectrum for the successive reduction of RhB at 552 nm and ln(Abs) vs. time (T) plot respectively having rate constant value of 1.18  10 1 min 1. Similarly, (E and F) are the successive reduction for RB at 548 nm and ln(Abs) vs. time (T) plot respectively having rate constant value of 2.90  10 2 min 1. The insets of (A, C and E) show the corresponding oxidized form of the dye images and (B, D and F) show their colorless reduced product.

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reduction of RhB absorption maxima (at 552 nm) with time and corresponding ln(Abs) vs. time (T, min) plot respectively. The reduction is completed just after 24 min and the rate constant value for the reduction is 1.18  10 1 min 1. Fig. 7E and F shows the successive reduction curve for RB with change in time and the corresponding ln(Abs) vs. time (T, min) plot respectively. The complete reduction for RB took longer than other two dyes and completed after 90 min. The first order rate constant value for the reduction of RB was 2.90  10 2 min 1. The overall concentrations of the dye molecules, concentration of NaBH4, volume of catalyst added, reduction time, and rate constant values were summarized and are listed in Table 2. From the Table 2 it is clear that the reduction is faster with RhB, medium with MB, and the slowest with RB. Two of three processes maintained the first order reduction kinetics. In the reduction, NaBH4 molecules transfer electron to the dye molecule via the Ir nano-needles and the dye molecule reduced and became colorless. The picture of the oxidized form of the dye molecule is shown in insets of Fig. 7A, C and E for MB, RhB, and RB respectively. The reduced colorless forms of dyes are shown

in the insets of Fig. 7B, D, and F for MB, RhB, and RB respectively. The fastest reduction for RhB is probably due to better adsorption of the dye on the surface of the Ir nano-needles. The RhB structure contains both N and O on its skeleton which can absorb strongly with the NPs surface resulting fast electron transfer. The dye reduction process is schematically shown in Scheme 2. It is important to mention that the reduction is a little faster when we used fresh NPs compared to the NPs after 10–15 days of synthesis. This is probably due to some surface oxidation of the NPs. The medium reduction rate is observed for MB. MB structure contains N and S on its skeleton. Metal NPs have good affinity to the N and S compounds and the electron transfer became fast. In case of RB, we observed the slowest rate probably due to presence of bulky halide ions which prevented the easy adsorption of the dye in the NPs surface. At this point, we are not fully confirmed about the different reduction rates for three different dyes. Further study is required in order to understand clear mechanisms. As a summary, using our above procedure it is possible to reduce organic dyes that will find important applications in environmental studies.

Table 2 The final concentrations of the dye molecules, concentration of NaBH4, volume of NPs added, reduction time and rate constant values for the reduction of dye molecules using Ir nano-needles as catalyst. Name of the dye molecules

Final conc. dye (M)

Rhodamine B (RhB) Methylene Blue (MB) Rose Bengal (RB)

8.92  10 8.92  10 8.92  10

Final conc. of NaBH4 (M) 6 6 6

8.92  10 8.92  10 8.92  10

3 3 3

Volume of Ir nanoneedles added (mL)

Time for full reduction (min)

First order rate constant (k) (min

0.2 0.2 0.2

24 60 90

1.18  10 1 4.07  10 2 2.9  10 2

No NPs

DYE molecules

1

)

Correlation coefficient (R)

Standard deviation (SD)

0.998 0.967 0.984

0.057 0.214 0.151

No Reduction

NaBH4

Ir NPs

Oxidized form, Colored

Ir NPs Colorless Reduced Dye

NaBH4 Ir NPs RhB

Fast NaBH4

Colored

Colorless

Ir NPs MB

Medium NaBH4 Ir NPs

RB

Slow

Scheme 2. Schematic presentation of the dye reduction process using Ir nano-neddles as catalyst.

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4. Conclusion In conclusion we have described a new synthetic route for the synthesis of shape-selective Ir NPs. The different shaped Ir NPs were synthesized via reduction of Ir(III) ions in CTAB micellar media containing alkaline 2,7-DHN as reducing agent under 4 h of UVirradiation. The synthesis process exclusively generated Ir NPs of different shapes such as nano-spheres, nano-chains, nano-flakes, and nano-needles. The nano-needles show excellent catalytic activity for reduction of different organic dye molecules in presence of NaBH4. The Ir nano-needles participate during the electron transfer process from the BH4 ions to the oxidized form of the dye molecules. In future, the above synthesis process and catalytic study are expected to find wide applications for other NPs synthesis and different organic catalysis reaction. Acknowledgments This research was in part sponsored by the NSF-0506082; the Department of Mechanical Engineering, Texas A&M University; and the Texas Engineering Experiments Station. Supports from the Microscopy Imaging Center (MIC) by Dr. Zhiping Luo and Materials Characterization Facility (MCF), at the Texas A&M University were greatly appreciated. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

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