Folate-Directed Shape-Transformative Synthesis of Hollow Silver

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Folate-Directed Shape-Transformative Synthesis of Hollow Silver Nanocubes: Plasmon Tunability, Growth Kinetics, and Catalytic Applications Bhavesh Kumar Dadhich, Bhavya Bhushan, Abhijit Saha, and Amiya Priyam ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01110 • Publication Date (Web): 26 Jul 2018 Downloaded from http://pubs.acs.org on July 27, 2018

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ACS Applied Nano Materials

Folate-Directed Shape-Transformative Synthesis of Hollow Silver Nanocubes: Plasmon Tunability, Growth Kinetics, and Catalytic Applications a

a

b

c,

Bhavesh K. Dadhich, Bhavya Bhushan, Abhijit Saha and Amiya Priyam * a

Department of Physics, School of Applied Sciences, KIIT, deemed to be University, Bhubaneswar-751024, India

b

UGC-DAE Consortium for Scientific Research, Kolkata Centre, III/LB-8, Bidhannagar, Kolkata-70098, India

c

Department of Chemistry, School of Physical and Chemical Sciences, Central University of South Bihar, Gaya823001, India *Corresponding Author: email: [email protected], Phone: +91-8521147173

KEYWORDS: Plasmonic nanomaterials, photocatalysis, plasmon tunability, hollow nanocubes, folate-capping, silver nanocrystals, growth kinetics ABSTRACT: A unique role of folate as a shape- and structure-directing agent has been found in nanosynthesis. Folatecapped Ag2O nanospheres transformed into hollow silver nanocubes (HAgNCs) having spherical void spaces during reduction with hydrazine hydrate (HH). HAgNCs with tunable plasmon peaks (λSPR) at 510, 550, 570, 590 and 630 nm were synthesized (hence named as HAgNC-510, HAgNC-550, HAgNC-570, HAgNC-590 and HAgNC-630, respectively). The corresponding edge-lengths were 33±4, 45±8, 60±8, 70±10 and 100±15 nm as determined by HRTEM and the aspect ratio (edge length/void diameter) remained constant at 2.3. The plasmon peak varied linearly while the molar extinction coefficient scaled exponentially with edge-length. The maximum red-shift was obtained with a molar ratio of 1:0.33:150 for Ag+:folate:HH at 50 oC with a stirring speed of 180 rpm. However, zero rpm synthesis yielded HAgNC-510 having lowest FWHM signifying high monodispersity. Within a short time span of 6-50 s, the particle-evolution was completed. It followed first-order kinetics with a faster reduction occurring at zero rpm. In addition, the HAgNCs were found to be good catalysts in dark as well as in sunlight, for the degradation of a model dye, methyl orange (MO). HAgNC-630 exhibited 3.3 times higher catalytic efficiency in sunlight as compared to solid silver nanospheres (λSPR=400 nm). Thus, the red-end of the visible solar spectrum displayed greater efficiency with HAgNCs as plasmonic photocatalysts.

1. INTRODUCTION Folic acid (FA), in the past two decades, has evolved as a versatile capping-agent in the synthesis of nanoparticles (NPs).1,2 Besides providing stability and biocompatibility, folate-capping imparts the ability to target cancer cells specifically as folate-receptors are over-expressed in such cells.3 Folic acid is also known to be a cell survival agent in human beings due to the requirement in the synthesis of nucleic acid and amino acids.4,5 It plays a crucial role in the prevention of neural-tube defects and has demonstrated a protective effect against oxidative stress.6,7 Here, we present a unique role of folate as a capping agent having a shape- and structure-directing ability. Using folate, a clear transformation of shape from spherical Ag2O template to cube-like silver nanocrystal with spherical void space has been found in our synthesis of hollow plasmonic nanostructures. Hollow noble metal nanostructures, i.e. nanoshells, nanocages, nanotubes, etc. are emerging as a new class of optical nanomaterials as they possess superior plasmonic properties in comparison to the solid nanostructures i.e. nanoparticles, nanocubes, nanorods, etc.8,9 Solid plasmonic nanocrystals have been used for

numerous applications such as catalysis,10,11 bioimaging,12,13 biosensing,14–16 SERS,17,18 drug delivery,1,19–21 photothermal therapy,22,23 thin film solar cells,24 storage media25 and optoelectronics.26 However, plasmon hybridization27 in the hollow nanostructures makes them a better candidate over the solid nanoparticles for biomedical applications due to enhanced tunability range from visible to near-IR. In addition, they are also found to have a higher absorption cross-section and therefore could be employed as light-harvesting centers in various photoactivable devices. Due to the tunable SPR absorption within the biologically transparent window (650-1200 nm) and a high light-toheat conversion efficiency, these nanomaterials are paving the way for the development of light-induced, noninvasive theranostic techniques such as dark-field imaging,12,28 photothermal29,30 and photodynamic therapy.31–33 Besides the therapeutic applications, the photothermal effect has also been employed recently to drive the chemical reactions which are otherwise difficult to perform under ambient conditions. The rate of such reactions can be substantially enhanced by local heating and activation of reactant molecules adsorbed on the surface.34,35 The plasmonic nanocrystals also have the ability to efficiently couple the thermal energy and low-intensity visible pho-

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In all the aforementioned syntheses of gold and silver nanoshells, versatile plasmon tunability has been achieved by changing the aspect ratio of the outer diameter and inner diameter, in a controlled fashion.8,42,43,49 In contrast, in this paper, we present hollow silver nanoshells in which the plasmon tunability has been found to be independent of the aspect ratio. These nanocrystals are cube-like nanoshells with inner spherical void space. The plasmon peak is tuned from 510 to 630 nm, but all of them have an aspect ratio of 2.3. Besides the peculiar folate-directed shape transformation, the temporal evolution of HAgNCs has also been investigated in two different ways: (i) by following the red-shifting plasmon peak and (ii) by following the NIR absorption of hydrazine hydrate in real-time. Molar extinction coefficient (εSPR) at the plasmon peak was found to increase exponentially with the size. Further, a unique catalytic effect, both in dark and sunlight, has also been observed for subpicomolar concentrations (ng/l) of HAgNCs. In this work, as-prepared HAgNCs have been used solely to understand the dynamics of plasmonic photocatalysis over the entire visible range which can change due to the variation in size, shape, and structure of the nanocrystals. By subtle tuning of SPR peak and molar extinction coefficient, the hollow nanostructures can be made to harvest most of the solar energies through the processes such as plasmonic photocatalysis. 2. EXPERIMENTAL SECTION

2.1 Materials. AgNO3 (Silver Nitrate) 99.5%, C19H19N7O6 (Folic Acid), N2H4.2H2O (Hydrazine hydrate) 99%, NaOH (Sodium Hydroxide) pellets and methyl orange dye (C14H14N3NaO3S) were purchased from Sigma-

2.2 Synthesis of HAgNCs. Folate-capped hollow silver nanocubes (HAgNCs) were synthesized in an aqueous medium by following our previously reported method8 with slight modifications. Firstly, silver oxide nanoparticles were synthesized at 50 0C by addition of aqueous solution of silver nitrate (150 μl, 10 mM) to 2.5 ml of Milli-Q water and folate (50 μl, 10 mM). Subsequently, NaOH solution (500 μl, 0.1 M) is added for adjusting the pH to 12. It results in the formation of a pale yellowish color colloidal solution of spherical Ag2O that acts as a template for HAgNCs. The Ag2O NPs are converted into HAgNCs by the addition of hydrazine hydrate (HH) (272 μl, 825 mM). The ensuing dissolution-diffusion-reduction (DDR) process leads to the formation of folate-capped HAgNCs. The schematic representation of the synthesis steps is shown in scheme 1.

HAgNCs

For the synthesis of nanoshells, the prevailing strategies are template-based methods,30,41 galvanic replacement,42,43 and nanoscale Kirkendall effect.9,44–46 The nanoscale Kirkendall effect is the most suitable technique due to fast synthesis and low cost with respect to others. The synthesis of hollow gold nanoshells42,43,47,48 have been widely reported, but there are only a few works on hollow silver nanostructures8,9,45,46 with tunable plasmonic properties. Most of these employ Ag2O as a template which on subsequent reduction yields hollow plasmonic 8,9,46 nanoshells. Recently, Kado et al. reported the synthesis of hollow silver nanoshells using silver thiocyanate as precursor instead of silver oxide.45

Aldrich and Spectrochem Pvt. Ltd. All the aforementioned chemicals have been used without further purification.

N2H4.H2O

ton flux which makes it more convenient to carry out the industrially relevant catalytic reactions.36 Although various aspects of plasmonic photocatalysis37 have been explored in literature, there are only a few reports38–40 on the use of hollow plasmonic nanostructures in such processes. The hollow plasmonic nanocrystals are less massive than similar sized solid particles and offer weight advantage as only a small amount needs to be added to the reaction system. However, to make it commercially viable, there is a need to further understand the photocatalytic behavior with respect to plasmon hybridization, extinction coefficient, and spectral tunability.

Ag2O

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Scheme 1. Illustration of the synthesis scheme for hollow silver nanocubes (HAgNCs). AgNO3 (10 mM, 150 μl) and folic acid (10 mM, 50 μl) were added to 2.5 ml of Milli-Q water. On addition of NaOH (0.1 M, 500 μl), spherical Ag2O NPs were formed which on subsequent reduction with hydrazine hydrate (10 mM, 272 μl) resulted in the formation of HAgNCs.

2.3 Effect of temperature, reagent concentration and stirring speed. The basic synthesis scheme was kept the same as mentioned above. The effect of temperature was studied by varying the temperature at the reduction step while keeping the Ag+: folate: HH molar ratio (1:0.33:150) and stirring speed (226 rpm) constant. Similarly, the effect of other parameters on synthesis was investigated, the details of which are given in supporting information (Section S1). 2.4 Spectroscopic measurements. As-prepared HAgNCs were taken in a quartz cuvette and UV-Vis-NIR spectra were recorded on Jasco V-770 spectrophotometer. FTIR spectroscopy was done on Perkin Elmer spectrophotometer in transmittance mode. Prior to the FTIR measurements, the colloidal solution was flocculated, centrifuged, washed and dried overnight. 2.5 Structural characterization. Few drops of the as-prepared nanocubes and Ag2O NPs were cast on the carbon-coated copper grids (Ted Pella, product code: 01800) for transmission electron microscopy (TEM). Further, the grids were dried in a vacuum desiccator. TEM images were recorded on JEOL-2100 microscope at an operating voltage of 200 kV to find the size, shape, and structure of the NPs. Selected area electron diffraction (SAED) analysis was also done on the samples to deter-

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mine the crystallinity. X-ray diffraction (XRD) patterns were recorded on Bruker-D8-Advance diffractometer with the Cu-Kα source (λ=1.5406 Å) after centrifugation and drying the samples in a vacuum desiccator. 2.6 Determination of molar extinction coefficient. The molar extinction coefficients (ε) of HAgNC samples at λmax were calculated by Lambert-Beer law

A = ε lC

1.0

Temp., λSPR(nm) o

10 C, 434

NVN A

(2)

2.8 Catalysis in sunlight and dark. Catalytic degradation of methyl orange (MO) dye by HAgNCs was analyzed in sunlight and dark. First of all, 100 ml stock solution of MO (10 ppm) was prepared. Two sets of 25 ml each were placed in sunlight and dark and used as controls. Out of the remaining MO solution, two sets of 25 ml each were taken and 1.5 ml of as-synthesized HAgNCs was added to both of them. One of the aforementioned solutions was kept in dark while the other was kept in sunlight. The ensuing reactions were monitored by following the decrease in MO absorption at 485 nm on UV-Vis-NIR absorption spectrophotometer (Jasco V 770) at regular time intervals.

0.6 0.4

(a)

-1

0.2

where NAg+ is the total number of Ag atoms, N is the average number of Ag atoms per nanocube, V (m3) is solution volume and NA is Avogadro’s constant. The detailed calculations for ε and C are given in Table S1. 2.7 Kinetics of the evolution of HAgNCs. The temporal evolution of HAgNCs was spectroscopically investigated using UV-Vis-NIR absorption spectrophotometer (Jasco V 770). For this purpose, in-situ syntheses were performed within the cuvette inside the thermo-stated and stirred sample compartment and spectra were recorded at high scanning speed (4000 nm/min) at regular time intervals. Four combinations of temperature and stirring speeds were used namely; zero rpm-25 oC, 180 rpm-25 oC, zero rpm-50 oC, and 180 rpm-50 oC.

15 C, 435 o 20 C, 437 o 25 C, 436 o 30 C, 444 o 40 C, 497 o 45 C, 511 o 50 C, 590

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300

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500

600

700

800

900 1000

Wavelength (nm) 40

40

b`

d= 30±4 nm

30

Frequency (%)

N Ag +

Extinction (cm )

o

(1)

Where A, ε, l and C are absorbance, molar extinction coefficient, light path length or cuvette width and the molar concentration of HAgNC samples, respectively. Molar concentration calculated as:

C=

Quantum yield (φ) is calculated by the ratio of the number of MO molecules decomposed and the number of photons incident.50,51 The calculated number of incident photons was found to be 18.37 × 1017 using a power meter.

Frequency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20 10 0

10

20 30 40 50 Void diameter (nm)

60

b``

l=70±10 nm

30 20 10 0

20

40 60 80 100 120 140 Edge length (nm)

(b) (c)

The following equation was used to calculate the rate constant (k) of MO degradation.

A ln o  At

  = kt 

2.0 Å

(3)

Here, Ao is the absorbance of MO at initially (t=0), At is the absorbance of MO at any time t (hrs) of irradiation. Degradation of the dye at any time t can be calculated by the following relation:

 ( Ao − At )   ×100  Ao 

Dye degradation (%) = 

(4)

Figure 1. (a) UV-Vis-NIR extinction spectra of hollow silver nanocubes synthesized at different temperatures ranging from 10 °C to 50 °C. The spectra for the samples synthesized at 55, 60 and 65 °C have been given in supporting information (Figure S5) alongwith respective deconvoluted peaks. + The molar ratio of Ag :folate: HH (1:0.33:150) and stirring speed (226 rpm) were kept constant. (b) TEM image of HAgNC-590. Size distribution histograms for (b`) void diam-

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ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

eter and (b``) edge length are shown in insets. l and d were found to be 70±10 and 30±4 nm. (c) HRTEM image of HAgNC-590. Zoomed-in view showing the lattice fringes, d: 2.0 Å, the scale is 2 nm (inset). 3. RESULTS AND DISCUSSION

Here, folate-capped hollow silver nanocubes (HAgNCs) have been synthesized using solid spherical Ag2O nanoparticles (NPs) as template and hydrazine hydrate as a reducing agent. In our synthesis strategy, folate plays crucial roles of stabilizer and growth moderator. In addition, it also acts as a shape and structure directing agent. Initially, the as-prepared Ag2O NPs formed are spherical in shape having a diameter of 14 nm (Figure S1). Subsequent addition of hydrazine hydrate leads to transformation of the spherical template into plasmonic nanocubes having spherical void space. The shape transformation was consistently observed under the wide range of reaction conditions which underscores the uniqueness of folate-capping. The conditions are as follows: (i) temperatures ranging from 20 to 50 oC, (ii) FA:Ag+ molar ratio ranging from 0.08:1 to 0.5:1, (iii) HH:Ag+ molar ratio ranging from 1:1 to 200:1, and (iv) stirring speed: ranging from zero rpm to 300 rpm. No other capping agent has shown such a distinct shape transformation for such extreme variations. The previous works done8,9,45 on such templates resulted in the predominant formation of spherical hollow silver nanocrystals even though different capping agents such as citric acid, glutathione, cysteine, PVP etc. were used in syntheses. Apparently, the shape transformation observed for folate-capping can be attributed to the unique molecular structure of folic acid which does not allow the crystal to grow equally along all directions. Although our previous work8 also showed cube-like hollow nanostructures, it was observed only in a specific case i.e., HH: Ag+ molar ratio of 137: 1 for citrate-stabilized samples. Interestingly, in this work, versatile plasmon tunability (510→630 nm) has been achieved although the aspect ratio was kept constant. This is in stark contrast to the prevailing strategy of changing the aspect ratio to tune the plasmonic properties in hollow nanostructures.8,9 The dependence of plasmonic and structural properties on temperature, stirring speed, and reagents’ molar ratio are examined in detail in the subsequent sections. 3.1 Effect of temperature. The effect of synthesis temperature on the optical and structural properties of HAgNCs is shown in Figure 1. As the temperature is initially raised from 10 to 30 °C, a nominal red-shift (10 nm) is obtained in the SPR peak (434→444 nm). For the next 20 °C rise in temperature (30→50 °C), a significant redshift of 150 nm is observed. The hollow nature of the nanocrystals was confirmed by TEM microscopy (Figure 1). Each of these nanostructures has a spherical void space inside the cube-shaped skeleton. Thus, a shape and structural transformation from solid spherical Ag2O nanoparticles to hollow cube-like silver nanocrystals are clearly

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evidenced. These hollow nanocubes, referred to as HAgNC-590 further in the article, have an edge length (l) of 70±10 nm and the diameter of inner spherical void (d) is 30±10 nm. Here we define the aspect ratio as the ratio of these two parameters, l/d. The mean aspect ratio was found to be 2.3. The size distribution histograms for the void diameter and edge length are shown in Figure 1b (insets b` and b`` respectively). The interplanar spacing as deduced from HRTEM image is 2.0 Å, which corresponds to 200 planes for fcc silver [JCPDS No. 04-0783]. Further, the SAED pattern also confirms that these samples are single crystalline (Figure S2). All HAgNCs are composed of elemental silver that is confirmed by XRD and EDX results (Figure S3 and S4, respectively). In XRD pattern, the peaks obtained at 2θ values of 38.24, 44.36, 64.61 and 77.69 degrees were assigned to reflections from 111, 200, 220 and 311 lattice planes of fcc silver, respectively (a=4.0862 Å, JCPDS file no. 04-0783). There were no peaks detected for oxygen in EDX and no peaks were found for silver oxide in XRD. The formation of hollow plasmonic nanocubes is a result of a subtle interplay of two oppositely mobile reaction fronts: 1. Dissolution of Ag2O to Ag+ ions and outward diffusion of Ag+ ion, and 2. Inward diffusion of hydrazine followed by the reduction of Ag+ ions. After the dissolution of Ag2O, Ag+ ions diffuse outward and the point at which they meet the inwardly diffusing HH molecules, reduction occurs followed by crystallization. We also note that the spherical void space (d=30 nm) formed within the HAgNCs are larger than the starting Ag2O NPs (d=14 nm). It suggests that the Ag+ ions move a larger distance at 50 oC before they encounter HH molecules. Since we got maximum red-shift in plasmon peaks at 50 °C, all further syntheses have been performed at this optimized temperature. This is in contrast to our previous report in which 20 °C was found to be the optimum temperature for synthesis of citrate-stabilized hollow 8 nanoshells. Folate vis-à-vis citrate is a bigger molecule and has several amine and amide groups in addition to carboxylate which can effectively coordinate to silver 2,52,53 ions. The bulkiness coupled with enhanced binding affinity in folate hinders the outward diffusion of Ag+ ions. Therefore, a higher temperature is necessitated to overcome the barrier and increase the rate of outward diffusion. Surprisingly, as the synthesis temperature is raised beyond 50 °C, the SPR peak apparently splits into two modes for all the three samples synthesized at 55, 60 and 65 °C. The UV-Vis-NIR spectra alongwith deconvoluted plasmon peaks are shown in supporting information (Figure S5). FESEM imaging technique revealed that these nanocrystals are spherical in shape with a hollow interior (Figure S6). One of the two peaks corresponds to the symmetric mode and the other corresponds to the quadrupole mode of plasmonic oscillations. The assignment of plasmon modes is in accordance with the previously developed theoretical model for hollow silver nanospheres.45

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3.2 Effect of stabilizer concentration. The molar ratio of silver ions to stabilizer molecules is one of the key parameters that has been thoroughly investigated, the results of which are shown in Figure 2. It was found that the absence of folate results in the formation of agglomerated solid silver nanoparticles. With the increase in FA: Ag+ molar ratio (Figure 2a), the absorbance of folic acid at 372 nm rises gradually. In all these studies, the concentration of reducing agent, hydrazine hydrate, and Ag+ ions have been kept constant at 825 mM and 1 mM respectively. The temperature has also been fixed at 50 °C. Initially, as the concentration of FA is doubled from 2.5 to 5 mM, or molar ratio of FA/Ag+ is doubled, no significant shift in SPR is observed. When FA concentration in solution increased from 5 to 10 mM, SPR shows a red-shift of 50 nm (540→590 nm). Interestingly, further increase of FA concentration by 5 mM (10→15 mM) brings about a blue-shift of 20 nm (590→570 nm) in the SPR.

tunability and low FWHM is achieved at a molar ratio of 150:1, which is then used as the optimized ratio for further synthesis. +

Molar ratio FA:Ag , λmax(SPR)

1.0

0.08:1, 540 0.16:1, 540 0.33:1, 590 0.50:1, 570

0.8 0.6 0.4

600

λmax (nm)

The plasmon peak for quadrupole mode remains centered around 450 nm while symmetric mode shows a gradual red-shift, 508→512→520 nm for temperatures, 55→ 60→ 65 °C, respectively.

-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Extinction (cm )

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0.2

(a)

0.0 300

400

585 570 555 540 0.1 0.2 0.3 0.4 0.5 Molar ratio (FA:Ag+)

500

600

700

800

900 1000

Wavelength (nm)

The observed blue-shift in the SPR band is attributed to the reduction in the size of HAgNCs. TEM image of HAgNC-590 and HAgNC-570 are shown in Figure 1b, c, and 2b, c respectively. HAgNC-570 was found to be smaller in size with an edge length of 60±8 nm and void diameter of 26±4 nm (Figure S7). However, the aspect ratio for HAgNC-570 is found to be nearly the same as that of HAgNC-590. The binding of folate to the surface of nanocubes was examined by FTIR spectroscopy. On comparing the FTIR spectra of pristine folate and folate-capped HAgNCs (Figure S8a), the peak corresponding to OH-stretching vibrations was found to blue-shift by 6 cm-1 with a concomitant increase in relative intensity. Furthermore, the peak also becomes spectrally narrower, i.e., FWHM decreases by 23 cm-1 (Figure S8b). These results clearly indicate that folate is bound to the surface through OH groups of pteridine ring (Figure S9). 3.3 Effect of reductant concentration. The effect of concentration of reducing agent on the synthesis of HAgNCs has been shown in Figure 3. Here, HH: Ag+ molar ratio was varied from 1:1 to 200:1. During this study, folate: Ag+ molar ratio (1:3) and synthesis temperature (50 °C) were kept constant. A gradual blue-shift of 100 nm, 600 →550 →520 →500 nm, in SPR peak position was observed with increasing HH: Ag+ molar ratio, 1:1 →50:1 →75:1 →100:1, respectively. Interestingly, a red-shift of 90 nm (500 →590 nm) in SPR peak position was noticed when the molar ratio is increased 1.5 times from 100:1 to 150:1. Further increase in reductant concentration causes no shift in SPR peak position; it remains constant at 590 nm from 150:1 to 200:1 molar ratio. Although the equimolar HH:Ag+ ratio yields HAgNCs with most red-shifted SPR peak, the associated peak-width (FWHM) is also large. Therefore, a delicate balance between enhanced SPR

(b)

(c) Figure 2. (a) The effect of stabilizer concentration on the UVVis-NIR extinction spectra of HAgNCs. Here, synthesis tem+ perature (50 °C), Ag : HH molar ratio (1:150) and stirring speed (226 rpm) were kept constant. Size distribution for

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ACS Applied Nano Materials HAgNC-570 is shown in Figure S7. l and d were found to be 60±8 and 26±4 nm, respectively. Molar ratio + HH:Ag , λSPR

(a)

1:1, 600 nm 50:1, 550 nm 75:1, 520 nm 100:1, 500 nm 150:1, 590 nm 200:1, 590 nm

0.8 0.6 0.4 0.2 0.0 300

400

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Wavelength (nm)

1000

300

600 585

270

570

240

555

210

540

180

525 510 495 480

150

(b) 0

120 25

50

75

Peak-width, FWHM (nm)

-1

Extinction (cm )

1.0

SPR peak, λSPR(nm)

100 125 150 175 200

Molar ratio (HH/Ag+)

all samples. (c) TEM and (d) HRTEM image of HAgNC-550. Size distribution histograms for edge length and void diameter are shown in Figure S10. l and d were found to be 45±8 nm and 19±4 nm, respectively.

The effect of reducing agent concentration on the structural properties could be explained by the analysis of TEM images of HAgNC-590 (150:1) and HAgNC-550 (50:1), as shown in Figure 1c and 3d, respectively. Most of the particles have cube-like shapes with the spherically hollow interior. It was also observed that some of these hollow structures collapsed as the electron beam impinged on them during the TEM imaging. Some of the residues of collapsed particles can also be seen in the Figure 1b and 3c. As discussed in previous sections, HAgNC590 has an average edge length of 70 nm while HAgNC550 has an average edge-length of 45 nm. The size distribution histogram of HAgNC-550 is shown in Figure S10. Again, we find that the aspect ratios of both nanocubes, HAgNC-590 and HAgNC-550, are nearly the same, 2.3. For three times increase in reductant concentration, the size of HAgNCs increases by 1.5 times (l, 45→70 nm). These structural changes are responsible for the red-shift in SPR peak (λSPR, 550→590 nm). 3.4 Effect of stirring. The effect of stirring speed on plasmonic properties has been studied in a wide range spanning from zero rpm (no stirring) to 1000 rpm (vigorous stirring). During these studies, the reagent ratio and synthesis temperature were kept constant. As shown in Figure 4, the SPR peak could be tuned in a wide range of 180 nm (450→630 nm) simply by changing the stirring speed. The variation in stirring speed mainly alters the double diffusion process which in turn modifies the plasmonic and structural properties.

(c)

Stirring Speed, λSPR

Extinction (cm-1)

1.0

(d)

zero rpm,510 nm 90 rpm, 524 nm 135 rpm, 530 nm 180 rpm, 630 nm 226 rpm, 590 nm 271 rpm, 541 nm 317 rpm, 527 nm 1000 rpm, 453nm

0.8 0.6 0.4 0.2

(a) 0.0 300

400

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900 1000

Wavelength (nm) 640

330

620

Figure 3. (a) UV-Vis-NIR extinction spectra for HAgNCs with + a change in HH:Ag molar ratio (b) Variation of SPR peak + position and FWHM with a change in HH:Ag molar ratio. Here, the synthesis temperature (50 °C), molar ratio of + Ag :folate (1:0.33) and stirring speed (226 rpm) were fixed for

SPR Peak FWHM

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(b)

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Peak-width, FWHM (nm)

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100 150 200 250 300 350 400

1000

Stirring speed (rpm)

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Figure 4. (a) UV-Vis-NIR extinction spectra for HAgNCs synthesized at different stirring speeds (b) SPR peak position and FWHM as a function of stirring speed.

means the distribution of sizes is least and the uniformity of shape maximum. This is further corroborated by HRTEM images shown in Figure 5. HAgNC-510 (zero rpm) has an edge length of 33±4 nm and it shows a standard deviation of 12 % in the size distribution histogram (Figure S11). Further, as the stirring speed is gradually increased, the inward diffusion of reductant and outward diffusion of Ag+ ions both are enhanced. However, this enhancement is more pronounced in the latter case because it also involves the prior dissolution of Ag2O NPs, which also gets assistance from the mechanical agitation during stirring. Together, the dissolution and diffusion, show greater sensitivity towards the increase in stirring speed in the initial part. Taking Ag2O NPs as our frame of reference, the outwardly mobile Ag+ ions move faster and the reaction front is formed at a larger distance where it meets the inwardly mobile reductant molecules. Thus, the size of void space is gradually increased, (14→43 nm) as the stirring is increased (0→180 rpm). At 180 rpm, the two oppositely mobile fronts are evenly balanced and produce the largest void space resulting in maximum red-shifted SPR peak. Beyond this point, any further increase in stirring speed, i.e., 180 rpm to 1000 rpm, causes the blue-shift in SPR peaks from 630 to 450 nm.

(a)

(b) 2.3 Å

(c) Figure 5. (a) TEM and (b) HRTEM images of HAgNC-510. Size distribution histograms are shown in Figure S11. (l:33±4; d:14±2 nm) (c) HRTEM image of HAgNC-630. Zoomed-in view showing the lattice fringes (inset), d: 2.3 Å, scale 2 nm.

At zero rpm, the HAgNCs formed have a sharp plasmon peak at 510 nm. Amongst all samples, the HAgNC-510 has the lowest FWHM of SPR peak which

On analyzing the TEM images of HAgNC-630 and HAgNC-590 formed at 180 and 226 rpm, respectively, we find that the average edge length has decreased from 100 to 70 nm and the void has shrunk by 30 %. Apparently, in this stirring range (226 to 1000 rpm) the intense stirring has a more pronounced effect on the diffusion of inwardly mobile reductant molecules. Due to their faster movement, the reaction front is formed at a much shorter distance (taking Ag2O NPs as a reference frame), resulting in shrinking of the void space. 3.5 Empirical scaling law and the molar extinction coefficient. The uniqueness of work is that all the HAgNCs synthesized have the same aspect ratio of 2.3 (Figure 6a), yet they show a wide tunability of SPR peaks ranging from 510 to 630 nm. Thus, the void diameter of HAgNCs increases with enhancement in edge length in similar proportions. This is in stark contrast to the previously reported works on hollow plasmonic nanostructures in which SPR tunability was found to depend on the aspect ratio.8,43 For hollow nanostructures, SPR is known to be a sensitive function of electromagnetic coupling between the inner surface cavity plasmons and the outer surface plasmons.8,43,54 Similar mechanism operates in the present case, but the HAgNCs are also different in the structural sense, i.e., the exterior solid is cube-like while the interior void space is spherical. Therefore, the correlation of SPR peaks to the structure has to be worked out differently. Based on our results, we have developed an empirical scaling law for these HAgNCs. A linear relation between the SPR peak position and the edge length (l) of HAgNCs is established.

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ACS Applied Nano Materials Without the void space, the solid nanocubes have also been shown to exhibit the edge length dependent plasmonic properties as reported by Siekkinen et al.55 However, the effect was much less pronounced and the maximum red-shifted peak was obtained at 500 nm for an edge length of 65 nm. Here, by creating a spherical void space, we have obtained significant red-shift of 90 nm, (500→590 nm) for the similarly-sized hollow silver nanocubes HAgNC-590 (70±10 nm).

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found for HAgNC-630, which is the largest in size. Thus, the ability to tune the molar extinction coefficient adds another dimension to the applications of plasmonic nanomaterials in photocatalysis or photovoltaics. 3.6 Mechanistic aspects of growth. An interesting observation in the evolution of these hollow nanostructures is that the size of HAgNCs is always found to be significantly larger than the starting Ag2O template NPs. This aspect of particle evolution is quite different from the previously reported ones8,9,45 in which the size of hollow nanostructures is comparable to the template size. A 14 nm-sized Ag2O NPs is composed of 5.3×104 Ag atoms whereas the smallest HAgNC synthesized, HAgNC-510, is composed of 2.02×106 Ag atoms. 1200 Coefficient of correlation, R=0.999

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Figure 6. (a) Variation of void diameter, d (∙) and SPR peak (∎) as a function of edge length, l. The inverse of the slope of d vs l plot gives the aspect ratio (2.3). (b) Variation of the molar extinction coefficient at the SPR peak, εSPR, with edge length, l, of HAgNCs. Inset: Linear variation of ln(εSPR) with

ln(l). The molar extinction coefficients (εSPR) of HAgNC-samples at their respective SPR peak were determined by the Lambert-Beer law. From Figure 6, it can be further noted that the increase in edge length of HAgNCs causes an exponential increase in the extinction coefficients. The inset shows the linear variation of ln(ε) with ln(l). A similar correlation between extinction coefficient and size was also obtained for solid spherical gold nanocrystals.56 No such correlation has been established previously for hollow plasmonic nanostructures although extinction coefficients for a few samples of Au and Ag-Au alloy nanocages were determined.57,58 As can be seen in Figure 6, the red-shift in SPR peak is also accompanied with an increase in extinction coefficients. Such HAgNCs could be employed as efficient light-harvesting centers. The highest value of ε has been

HAgNC

Figure 7. (a) Variation of NAg(HAgNC)/NAg(Ag2O) ratio with edge length (l). [NAg(HAgNC) → No. of Ag atoms per HAgNC; NAg(Ag2O) → No. of Ag atoms per Ag2O NP] (b) Scheme of formation of HAgNCs showing occurrences of various events: 1. Dissolution-diffusion-reduction, 2. Nucleation of Ag0 atoms and coalescence of void spaces and, 3. Expansion of central void and crystal growth. To understand this aspect better, the ratio of the number of Ag atoms per HAgNC[NAg(HAgNC)] and the number of Ag atoms per Ag2O NP [NAg(Ag2O)], has been plotted as a function of the edge length of the nanocubes. As seen in Figure 7, it follows an exponential growth profile. The ratio, [NAg(HAgNC)/NAg(Ag2O)] is 33 for the HAgNC510 which increases to 1054 for HAgNC-630. It implies 33 times greater number of Ag atoms are required to constitute a single HAgNC-510 as compared to a single Ag2O NP. All these extra Ag atoms are sourced from Ag2O NPs

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3.7 Temporal evolution of HAgNCs. To further understand the synthesis process, the temporal evolution of HAgNCs was investigated in-situ by following the SPR peaks in real-time. The effect of stirring speed and temperature on the kinetics of formation of hollow nanocrystals has also been studied. The appearance of a valley around 370 nm in absorption spectra is due to the high scan speed (4000 nm/min) employed for this study. Figure 8 shows the variation of extinction spectra with time at two temperatures, 25 °C (room temp.) and 50 °C (optimum temp.) and two stirring speeds, zero rpm, and 180 rpm. In general, the reaction kinetics at room temperature is slower as compared to 50 °C. However, the effect of stirring is different at two temperatures. At room temperature, zero rpm leads to a faster growth of HAgNCs as compared to 180 rpm. In contrast, zero rpm vis-à-vis 180 rpm causes a slower growth at 50 °C. Additionally, the maximum red-shift in SPR peaks during the synthesis is obtained at 50 °C, 180 rpm. Initially, the SPR appears at 474 nm which gradually shifts to 558 nm within 6 seconds. It also implies that the crystal growth occurs concomitantly with the expansion of void space at comparable rates resulting in the constancy of aspect ratio. We also note that the reduction of Ag+ to Ag0 is an event that precedes crystal growth and it is also a major component determining the overall kinetics of the particle evolution. Therefore, we made an attempt to comprehend the reduction kinetics by following the most intense absorption of the reductant, hydrazine hydrate, at 1067 nm which has no interference with SPR band of the evolving nanocrystals (Figure S13). The decrease in hydrazine concentration with time can give the rate of reduction of Ag+ ions.

Furthermore, the amount of hydrazine consumed is far greater than the amount of hydrazine required for the reduction of Ag+ ions. In all the four sets of syntheses, the amount of Ag+ ions is the same, 1.5 µmol which requires one-fourth of hydrazine hydrate, 0.375 µmol for complete reduction.59 4Ag+ + N2H4.H2O→4Ag0 + N2 + 4H+ + H2O 0

RT (25 C), Zero rpm

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The above mechanism also explains the observation of multiple voids of smaller sizes in some of the HAgNCs shown in the TEM images (Figure 3c and 5a). In an ideal case, coalescence of voids should be completed before the crystallization process. However, in some cases, crystallization may get completed prior to coalescence of voids and the fluidity of the system is lost. This is the kind of situation in which the system remains trapped in the intermediate state (Figure 7b) and multiple voids appear in a single nanocube.

Figure 9 shows the time-course measurement at 1067 nm for different combinations of temperatures and stirring speeds. Exponential decay profile is obtained for all the conditions of reduction. We also note that the time scale for hydrazine decay curves to become asymptote is much longer, in the range of 300 s to 600 s whereas the time-scale for completion of nanocrystal evolution is in the range of 6 to 50 s.

Extinction (cm -1)

only, which is the starting material. Thus, several Ag2O NPs likely interact as they undergo the dissolution, diffusion and reduction process. Several small void spaces created initially in the process coalesce to form a bigger void space. Concomitantly, the outwardly mobile Ag+ ions are reduced and the so formed Ag0 atoms undergo nucleation and crystal growth. These processes, eventually lead to the formation of HAgNCs of much bigger size. The mechanism for such a synthesis is illustrated in the scheme as given in Figure 7b. The HRTEM image (Figure S12) taken at an intermediate stage shows multiple voids coalescing together.

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Time (second) Figure 8. High scan-speed extinction spectra recorded to follow the temporal evolution of HAgNCs for different combinations of stirring speed and temperature. (a) zero rpm at 25 °C, (b) zero rpm at 50 °C, (c) 180 rpm at 25 °C, (d) 180 rpm 50 °C, (e) Change in SPR peak position with respect to time.

However, the actual decrease in hydrazine concentrations is in the range of 750-1125 µmol. This suggests that the HAgNCs formed act as a catalyst for further deg-

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ACS Applied Nano Materials radation of hydrazine in presence of oxygen by the reaction:60 N2H4 + O2→ 2H2O + N2

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Following the above discussion, we restrict ourselves to the time scale of 6-50 s for determining the rates of Ag+ reduction. We also get point of inflection corresponding to this time scale in the 1st derivative plots for hydrazine decay (Figure S14). In this time range, Ag+ reduction follows 1st order kinetics as shown in Figure 9 (inset). From this rate constants were calculated to be 9.11×10-3, 7.46×10-3, 260×10-3, 13.5×10-3 s-1 for zero rpm 25 °C, 180 rpm 25 °C, zero rpm 50 °C, 180 rpm 50 °C respectively. Thus, at both the temperatures, zero rpm stirring results in a faster rate of reduction. 0.40

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The photocatalytic process was found to follow first-order decay kinetics and accordingly, the quantitative estimations were made by following the absorbance of the dye at 485 nm. It is worth noting that nearly 82% dye-degradation was achieved in sunlight using HAgNCs as photocatalysts. In contrast, only 18% degradation is achieved in the dark in the same time frame. The rate constants for the dark- and photo-catalyzed reactions have been found to be 0.031 and 0.281 hr-1, respectively (Figure 10 and Table S2). So, the sunlight catalyzed reaction is nearly 9 times faster. On replacing the HAgNCs with spherical solid AgNPs (λSPR= 400 nm), the darkcatalyzed reaction remains nearly unaffected (Figure S18) but the rate of the sunlight-catalyzed reaction decreases by more than 70%. For further comparison of photocatalytic efficiencies, photochemical quantum yield51 (φ) was also determined, which was found to be 40% and 10% for HAgNC-630 and solid Ag NPs, respectively.

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Figure 9. Time course measurements of HH absorption at 1067 nm with different combinations of stirring speed and temperature: Plot of ln(A0/At) with time (inset).

3.8 Catalysis in sunlight and dark. In order to explore the possibility of using such uniquely structured hollow plasmonic nanocrystals for photocatalytic applications, methyl orange (MO) was chosen as a model system and degradation of the dye was investigated in sunlight and dark. The reaction system was prepared by adding 1.5 ml of HAgNC-630 solution to 25 ml aqueous solution of MO (10 ppm). The solution was then irradiated under sunlight for 6 hours and another reaction set prepared in a similar manner was kept in dark for the same duration. Two additional reaction sets were also prepared in which nanocrystals were not added and these acted as control sets. One was kept in sunlight and another one in dark for the same duration. For the control solution of MO dye, initially, the maximum absorbance is found at 485 nm (Figure S15). For control sets, no change in absorbance and wavelength were found on irradiation in sunlight or incubation in dark. Thus, methyl orange, alone, does not degrade in sunlight or dark. We also found that catalytic degradation is affected even if the HAgNC is added in the dark (Figure S16), although the efficiency is relatively low with degradation occurring up to 18% (Figure S17). To the best of our knowledge, this is the first report on darkcatalysis of dye-degradation by the hollow silver nanostructures.

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Figure 10. The plot of ln(A0/At) vs time for HAgNC-catalyzed MO degradation (λ=485 nm). Rate-constant is calculated by the slope of the plot.

The above observations indicate that the darkcatalyzed reactions remain unaffected by the change in structure and shape of the silver nanocrystals. There is no role of plasmons in the dark-catalysis. The spherical solid Ag NPs have an SPR peak around 400 nm (Figure S19) whereas HAgNCs absorb at 630 nm. So, both extremes of the visible spectrum have been employed, but the red-end shows greater solar-catalytic efficiency. Further, recycle experiments were performed to test the reusability of the photocatalysts. Robust catalytic activity has been observed for three cycles. On going from 1st to 3rd cycle, a nominal decrease in rate constant by 10% has been observed (Figure S20). Here, we also note that a good catalytic efficiency has been achieved for HAgNCs, even without the aid of electron donor species such as NaBH4 or semiconductor nanoparticles which is in contrast to the earlier reports61– 64 In this way, the multitude of reagents is reduced and the process gets simplified. It also helps understand the dynamics of plasmonic photocatalysis in a better way. Further, the scaling of concentration of catalysts is different. We used sub-picomolar concentrations (ng/l), precisely, 0.6 pM of HAgNCs, whereas the previous workers51,61,65 used concentrations on the order of mg/l for dif-

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ferent types of photocatalysts. Thus, the weight advantage of hollow vis-à-vis solid NPs is quite evident as the concentration of photocatalyst is reduced substantially by a factor of 106. In addition, we found that folate-capping imparts greater stability to the hollow nanostructures in aqueous solutions. As shown in Figure S21, the SPR peak position remains constant for over two weeks’ time. The SPR intensity shows a minimal decrease of 5% for the same time period. 4. CONCLUSION

Through this work, a unique shape- and structuredirecting ability of folate molecules has been discovered in the course of the synthesis of HAgNCs. It would be interesting to see if similar transformation can also be effected in other nano-systems using folate. A key finding of the work is the versatile plasmon tunability (510→630 nm) that can be achieved in hollow nanostructures without changing the aspect ratio. In this context, the role of stirring speed also gets underscored. Zero rpm stirring was found to be best suited for obtaining spectrally narrow HAgNCs while mild stirring of 180 rpm resulted in most-red-shifted SPR peak. The application of these hollow nanocrystals as robust catalysts at subpicomolar concentrations (ng/l) in dark and sunlight has been vividly demonstrated. The red-end vis-à-vis blue-end of the visible spectrum turned out to be more efficient for photocatalysis. This goads us to ask ourselves whether the catalytic efficiency would be even better if the SPR peak is tuned to the near-IR range. For designing of better plasmonic photocatalysts, the twin aspects of SPR wavelength and molar extinction coefficient need to be taken into consideration. This will help harvest the solar energy effectively across the visible and near-IR region for the light-driven processes such as photocatalysis and photovoltaics.

2. Nano-Mission, Govt .of India Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT One of the authors (AP) gratefully acknowledges the financial support from DST, Govt. of India, under the Nano Mission (ref. no. SR/NM/NS-1047/2012) and Fast-Track scheme (ref. no. SB/FT/CS-84/2011). AP also thanks UGCDAE CSR, Kolkata Centre for funding under CRS scheme. One of the authors (BKD) is thankful to CSIR, New Delhi for the award of the Senior Research Fellowship (SRF). ABBREVIATIONS HH, Hydrazine Hydrate; SPR, Surface plasmon resonance; MO, Methyl Orange; HAgNC, hollow silver nanocubes; NP, nanoparticle; FA, Folic acid; SERS, Surface-enhanced Raman spectroscopy; HRTEM, Highresolution transmission electron microscopy. DDR;

dissolution-diffusion-reduction. REFERENCES (1)

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ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. It includes synthesis condition details, TEM of Ag2O, FESEM, XRD, SAED, EDX, size distribution histogram, FTIR & deconvolution spectra, Near-IR absorption spectra of hydrazine hydrate, first derivative plot of time-course absorption measurements at 1067 nm, timedependent absorption spectra of methyl orange on addition of HAgNCs in dark and sunlight, extinction spectra of HAgNCs with time to show the stability. AUTHOR INFORMATION

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Corresponding Author * Email: [email protected], Phone: +91-8521147173.

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Author Contributions All authors have given approval to the final version of the manuscript.

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Funding Sources. 1. SERB DST, Govt. of India

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Folate-Directed Shape-Transformative Synthesis of Hollow Silver

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