Ultrasensitive Gold NanostarâPolyaniline Composite for Ammonia

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Ultrasensitive gold nanostar-polyaniline composite for ammonia gas sensing Vished Kumar, Vithoba Patil, Amey Anant Apte, Namdev Harale, PS Patil, and Sulabha K. Kulkarni Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03018 • Publication Date (Web): 01 Nov 2015 Downloaded from http://pubs.acs.org on November 2, 2015

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Ultrasensitive gold nanostar-polyaniline composite for ammonia gas sensing

Vished Kumar 1, Vithoba Patil 2, Amey Apte 1, Namdev Harale 2, Pramod Patil2, and Sulabha Kulkarni 1,*

1

Indian Institute of Science Education and Research, Dr.Homi Bhabha Road, Pashan, Pune-

411008, India 2

Thin Film Materials Laboratory, Department of Physics, Shivaji University, Kolhapur-416004,

India

KEYWORDS: gold nanostars, polyaniline, nanocomposite, gas sensing, resistive device.

ABSTRACT

Gold in the form of bulk metal mostly does not react with gases or liquids at room temperature. On the other hand, nanoparticles of gold are very reactive and useful as catalysts. Reactivity of

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nanoparticles depends on the size and the morphology of the nanoparticles. Gold nanostars with copper in it have rough surface and large number of active sites due to tips, sides, corners and large surface area-to-volume ratio due to their branched morphology. Here the sensitivity of gold nanostars-polyaniline composite (average size of nanostars ~ 170 nm) towards ammonia gas has been investigated. For 100 ppm ammonia, the sensitivity of the composite increased to 52% from a mere 7% value for pure polyaniline. The gold nanostar-polyaniline composite even showed a response time as short as 15 seconds, at room temperature. The gold nanostars act as catalyst in the nanocomposite. The stability and sensitivity at different concentrations and selectivity for ammonia gas were also investigated.

TEXT 1. Introduction

Nanomaterials have revolutionized many of our concepts related to various properties of materials. Bulk materials do not have size and shape dependent mechanical, optical, chemical etc. properties but nanomaterials indeed through their large surface to volume ratio and/or quantum confinement effects have size and shape dependent properties 1–5.

Therefore nanomaterials are useful to solve many challenges in different fields. A striking example in which bulk and nanomaterials differ a lot is gold. Gold is well known as a noble metal. It is unreactive and retains its optical, mechanical, electrical and other properties for extended period of time due to its completely filled 4f and 5d electron shells. However, research has shown that very small nanoparticles of gold (< 10 nm) have size and shape dependent


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chemical activity which can be used in various important catalytic oxidation reactions like carbon monoxide oxidation

6–11

. Reactivity of gold nanoparticles along with large surface to

volume ratio increase with decreasing particle size is often attributed to the quantum effect related to size 12. At extremely low sizes (< 6 nm or so) ‘quantum size effects’ are responsible for very high activity of gold nanoparticles. This has to do with the opening of an energy gap at small size of gold nanoparticles. Unlike in a bulk metal, nanoparticles are then suitable for catalytic oxidation of carbon monoxide as an example. Morphology of the particles also is often important as larger number of surface active atoms can be exposed through tips, edges, kinks, steps, high index planes etc.13, 14 We have also provided the comparison of sensitivity towards ammonia of gold nanostars, gold nanorods and gold nanospheres embedded in polyaniline (PANI). Best results are obtained with gold nanostars. Interestingly, the alloys of gold also show catalytic activity.15-18 Bimetallic nanoparticles of AuCu alloys have been found to be very good catalysts

19–21

in various oxidative reactions. It was

shown in an earlier experiment by Kim et al. that Au-Cu alloy composition could be critical in catalytic action

21

. Many research groups are trying to understand the shape and size related

phases, segregation and build up correlations with the basic thermodynamic and electronic structure theories.22,23 Even bigger gold nanostars (up to ~ 200 nm) have such favourable morphology in which not only the small size but surface roughness, corners, tips, edges and high index planes all are present which contributes to their sensing and reaction capability.14, 24,25 It is therefore possible that in future even larger Cu-Au nanoparticles could be useful as illustrated by He et al 14.


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In the catalytic reactions or sensing, a common point of basic importance is the reactivity of the material which depends upon the availability of surface area and active sites 26,27. For the purpose of application, the sensing time, ability to detect low concentration of the analyte, fast recovery, stability, operating temperature and selectivity are most important aspects besides portability and ease of operation of the final sensing device 28–30. Recently, gold nanostars also have been used as sensors. In the sensing applications of gold nanostars their optical properties associated with surface plasmon resonance and Surfaceenhanced Raman Spectroscopy (SERS) due to intense electric field at tip apexes has been used.14,31-35 Gold nanostars were also embedded in Polydimethylsiloxane for ammonia gas sensing where optical changes were monitored.36,37 In the present work we have made a gold nanostar and highly insulating PANI composite. Gold nanostars are having high percentage of copper. Here gold nanostars act as bridging components or catalysts to increase the sensitivity of insulating PANI by a factor of ~7. It can be seen that gold nanostar-PANI nanocomposite substrates can act as ultrasensitive ammonia sensors. Ammonia and its compounds are widely used in pharmaceutical, fertilizer, cosmetics, and food packaging industries. Leakage of ammonia is quite hazardous to health 38–40. In small dosage its smell is not only irritating but can lead to temporary respiratory problems or fatigue. In extreme cases, inhalation of a large amount of ammonia can lead to blindness or even death. Therefore detection of ammonia (NH3) gas is considered to be very important. There are many attempts to fabricate NH3 sensors with high sensitivity, selectivity, small response time, low power consumption, small size etc. which use optical, electrochemical or resistive methods to detect the molecules.28,41-43 Amongst the various solid state sensors, nanoparticle-based sensing is gaining more attraction because of their ease and low cost of fabrication, ultrasensitive character, room


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temperature operation, stability and robustness, as well as small size. There are reports in which variety of nanoparticles or their composites with carbon nanotubes, graphene, polymers etc. are used in the detection of NH3.44-51 In this work we use chemically synthesized gold nanostars as catalysts and show that they enhance the sensing activity of insulating PANI thin-film. It will also be shown that the use of gold nanostars has increased sensitivity for the same concentration level of ammonia, compared to gold nanorods and spherical gold nanoparticles. Gold nanostars (average size ~ 170 nm) were synthesized via a colloidal chemical method, and constituted with polyaniline solution for making a thin film on glass substrate by spin coating. Ammonia sensors employing nanoparticlepolymer composites are also reported earlier sensing.is reported

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36,52,53

. Even gold-bio composite used in ammonia

. Here, the sensing action based on a change in the resistance of the nano-

composite film due to exposure to ammonia has been used, thus making it simpler to fabricate and use inexpensively. PANI was used as it is a low cost, readily available, biocompatible and easy to process, polymer. We demonstrate that such a gold nanostar-PANI composite is an ultrasensitive material for detecting NH3 gas, with fast response and recovery time along with high stability. Moreover the presence of copper in the gold nanostars also reduces the cost to some extent compared to pure gold nanoparticles.14 2. Methods 2.1 Synthesis of gold nanostars For the synthesis of gold nanostars (referred to as ‘Au NS’), cupric chloride hydrate (CuCl2, .2H2O, 99%), gold chloride trihydrate (HAuCl4.3H2O, 99%), D-(+)-glucose (C6H12O6, 99.5%) and hexadecylamine (98%) were purchased from Sigma Aldrich. Milli-Q water was used as


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solvent during the synthesis. Synthesis protocol was followed from He et al.14 Briefly, 100 mM of 0.5 ml of HAuCl4.3H2O and 100 mM of 0.5 ml CuCl2.2H2O were mixed with 1M of 0.47 ml glucose. This solution was then added to 6.5 ml of Milli-Q water containing 70 mg hexadecylamine. The above solution was stirred for 24 h at room temperature in a glass vial which was later kept in an oil bath for heating at 100oC for 40 minutes with continuous stirring. During heating, the solution changed the colour from green to blue and finally to dark violet (see Fig. S1 in Supplementary Information). The solution was centrifuged at 10,000 rpm for 10 minutes and washed thrice with Milli-Q water and twice with ethanol. Finally, the Au NS could be dispersed in either ethanol or water. 2.2 Synthesis of gold nanospheres Gold nanospheres (referred to as Au NP), were synthesized following Bastús et al.55. Gold chloride trihydrate (HAuCl4.3H2O, 99%) purchased from Sigma Aldrich and trisodium citrate dihydrate (Na3C6H5O7.2H2O, 99%) purchased from Alfa Aesar were used as the precursors. Milli-Q water was used as solvent. Initially the gold seeds were prepared in a three neck 250 ml round bottom flask, fitted with a condenser. First 150 ml of 2.2 mM trisodium citrate dihydrate (TSC) in milli-Q water was heated at ~100 OC and continuously stirred. While the solution boiled, 1ml of 25mM HAuCl4 was added to it. After ~15-20 minutes the colour of the solution changed to the light pink colour, indicating the formation of Au seeds. Solution was then cooled to 90OC. Then 1ml of 60 mM TSC was added followed by addition of 1 ml of 25 mM HAuCl4 after 2 minutes. Heating and stirring was continued for 30 minutes. The addition of TSC and HAuCl4 was repeated (identical fashion) with further 30 min of heating and stirring. After the solution was cooled to room temperature, 1ml of the above solution was centrifuged at 8500 rpm for 10 minutes and the precipitate was collected for further use (UV-Vis spectrum in Fig. S2 (a)).


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2.3 Synthesis of gold nanorods The procedure used for the synthesis of gold nanorods has been reported earlier by Nikoobakht et al.56. The Gold chloride trihydrate (HAuCl4.3H2O), Ascorbic acid, Cetyl trimethyl ammonium bromide (CTAB) and benzyl dimethyl hexadecyl ammonium chloride (BDAC) procured from Sigma Aldrich and Sodium borohydride (NaBH4) from SRL Chem Pvt. Ltd and Silver nitrate (AgNO3) from Merck were used in the synthesis of gold nanorods. Solutions were all made in Milli-Q water. First the GNR seeds were formed for which HAuCl4 (5 × 10−4 M, 5 ml) was mixed with CTAB (0.2 M, 5 ml) to which ice cold NaBH4 (0.01 M, 0.6 ml) was added. After stirring the solution for few minutes, it became brownish yellow. The growth solution was prepared adding AgNO3 (0.004 M, 0.2 ml) into CTAB (0.2 M, 2.5 ml) and BDAC (0.15 M, 2.5 ml). HAuCl4 (0.001 M, 5 ml) was then mixed, which changed the solution colour to golden yellow. This was followed by the addition of ascorbic acid (0.0788 M, 0.07 ml) which made the solution colourless. After adding 20 µl of the seed solution to the growth solution and leaving it undisturbed overnight, the latter developed a dark magenta colour. 1 ml of this solution was centrifuged at 9600 rpm for 15 minutes. The precipitate gave the gold nanorods (UV-Vis spectrum in Fig. S2 (b)). 2.4 Synthesis of polyaniline Polyaniline (PANI) salt was prepared by a modified procedure adopted from Stejskal et al.57 In a 250 ml round-bottom flask maintained at a temperature of 0°C, 10 ml of water was added followed by 0.1 ml of distilled aniline. The solution was stirred well before adding 0.05 ml of (35%) HCl dissolved in 50 ml water, followed by stirring for 30-40 min. To the above solution, 2.3 g of ammonium peroxydisulphate (APS) dissolved in 10 ml of water was slowly added. After


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some time, the colourless solution turned into dark green solution. The solution was stirred for six hours and then it was left for ~ 12 hrs without stirring, for complete polymerisation. The precipitate obtained was then filtered and washed with methanol and 5% HCl followed by drying under vacuum to obtain PANI in powder form. 2.5 Preparation of gold nanostructures and polyaniline (Au + PANI) nanocomposite thinfilms 2.5.1 Au NS + PANI thin-films: PANI solution was made by dissolving 2 mg of PANI salt in 1 ml of Dimethyl Formamide (DMF). 30 µl of PANI solution was put at the center of a glass slide (mounted on the spinning podium) and then it was set to spin at 1200 rpm for 1 minute. At the end of the spinning, the solution made a very thin layer over the glass slide. To increase the thickness of the PANI film, this step of spinning 30 µl of PANI solution was repeated 10 times. Next, to a washed Au NS precipitate, 200 µl of absolute ethanol and then 100 µl of above PANI solution was added. This was mixed well and then spin-coated on the PANI-coated glass slide, similarly by putting 2-3 drops of Au NS + PANI mixture using a 100 µl micropipette and coating at a speed of 1200 rpm for 1 min. This step was repeated 5 times so to get embedded stars in PANI layer. FESEM image in Fig. S3 shows that the stars are embedded in PANI thin film. To another washed Au NS precipitate we added 100/200/300µl of absolute ethanol. This solution was then used similarly to coat the above Au NS + PANI coated glass slide 20 times at 1200 rpm for 1 minute. (For morphology dependence study, the above Au NS solution was taken in 200 µl of milli-Q water, followed by similar spin-coating for 20 times.) 2.5.2 Au NR + PANI thin-films: PANI solution deposition was identical to that mentioned above. Further, to a washed Au NR precipitate, 200 µL of milli-Q water and then 100 µL of


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above PANI solution was added. This mixture was mixed well and then spin-coated on the PANI coated Glass slide similarly by putting 2-3 drops of Au NR/ PANI mixture using a 100 µL micropipette and coating at a speed of 1200 rpm for 1 min. This step was repeated 5 times. To another washed Au NR precipitate, add 200µL of milli-Q water. This solution was then used similarly to coat the above Au NR+PANI coated glass slide 20 times at 1200 rpm for 1 minute. 2.5.3 Au nanospheres + PANI thin-films: For the thin-films of Au nanospheres and PANI, the procedure followed was identical to the one with Au NRs, except that the Au nanospheres solution was used for deposition. 2.5.4 Varying Gold concentration and PANI Thickness: Thin films of Au NS in PANI were prepared using different concentration of Au NS to get films with varying thickness. Au NS solutions with varying concentrations were made by dispersing equal amount of washed Au NS precipitate in varying amount of absolute ethanol i.e. three different solutions were made by adding 100, 200 and 300 µl of EtOH separately (so as to get different concentration in gold nanostar solutions). Using these 3 solutions, three different samples were prepared by following the similar procedure as mentioned above, i.e. coating a glass slide 10 times using PANI solution (2 mg, in 1mL DMF) followed by 5 times coating with Au NS + PANI solution. In the next step, the 3 different concentrations of Au NS in EtOH were spin coated on Au NS + PANI coated glass slides, 20 times each, at 1200 rpm for 1 minute. Samples were also prepared by varying the thickness of initial PANI layer; these were prepared by varying the number of coatings of PANI solution on the glass slide (so as to vary the PANI layer thickness). But no change was done in the Au

NS concentration.

For varying the PANI thickness, three samples were prepared using PANI solution (2mg PANI salt in 1 ml DMF) with 5, 10 and 15 times of coating. Further these samples were coated


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similarly (as discussed in the section 2.5) by Au NS + PANI solution 5 times, followed by Au NS Solution (in 200 µl EtOH) 20 times each.

2.6 Characterization of Au NS, PANI, and Au NS + PANI samples Morphologies of Au NS and Au NS + PANI nano-composite films were investigated using a ZeissTM Ultra Plus Field Emission Scanning Electron Microscope (FESEM) equipped with an Energy Dispersive X-ray (EDAX) spectrometer. The Au NS and Au NS+ PANI nano-composite samples were prepared on clean Si wafers and glass slides (respectively) and the latter was sputter-coated with a thin conductive ~ 5 nm layer of gold for clear imaging. The FESEM was operated at 3 KV for the imaging and 15 KV for EDAX analysis. X-ray photoelectron spectroscopy (XPS) was performed using an ESCA 3000 spectrometer from VG Microtech with an Al Kα source (1486.6 eV). The pass energy for the measurements was 50 eV with a resolution of 0.9 eV. X-ray Diffraction (XRD) of the samples was carried out on a Bruker AXS D8 Advance powder X-ray diffractometer with Cu Kα radiation (λ = 1.5417 Å). Samples were drop casted on clean glass slides and diffraction patterns were recorded from 2θ = 20° to 80° with step size 0.01°. Fourier Transform Infrared spectra were recorded on a Bruker Alpha FTIR spectrophotometer, and UV-Vis spectra were recorded on a Thermo Scientific Evolution 300 UV-Vis spectrophotometer. 2.7 Gas sensing measurements The gas-sensing characteristics of ‘PANI’ and ‘Au NS + PANI’ nano-composite thin films were measured by using computer-controlled static gas sensing unit. The gas sensing unit consisted of an airtight stainless steel test chamber having a volume of 250 cc with a provision


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for gas inlet through a rubber septum. The computer controlled Keithley 6514 system electrometer with data acquisition system was used for measurement of electrical conductivity with respect to time. The electrical contacts were made using silver paste. The ﬁlms with dimension of 10 mm × 10 mm were used for the sensing element fabrication. The target gas from a standard canister (of 1000 ppm concentration) was injected through a syringe so as to yield desired gas concentrations in the gas chamber. The gas response of the sensor was measured after achieving stable response upon exposure to targeted gas. The electrical resistance of thin film sensors in air (Ra) and in the presence of NH3 (Rg) was measured to evaluate the gas response (S). The gas response (S) is calculated using the following equation:

3. Results and Discussion Gold and silver nanoparticles in various morphologies and core-shell structures have proved to be excellent sensors sometimes achieving molecular level detection58

-61

. SERS harnesses the

enhancement due to electromagnetic field and chemical factors. One can develop simple, robust, sensitive and inexpensive chemiresistors for practical gas sensing applications to be used in household, agricultural or industrial environments. There are many attempts to develop nanoparticle-based solid state resistive sensors

30,62,63

. Nanoparticles have large surface area-to-

volume ratio which provides many active sites for interaction between molecules and the substrate material. Nanoparticles with different morphologies can be produced in large quantities and rather inexpensive chemical routes. In this work, we have used Au NS which can provide very active sites at the sharp tips, edges and corners due to their unique morphology 14. Au NS


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can be synthesized either by seed mediated or seedless route as reported in the literature 35,37,44,64 . Often Au NS have multiple branches. The synthesis route adopted by us gives rise to welldefined pentacle nanostars and incorporates copper along with gold14. Details of growth mechanism as well as morphology details were also reported for the Au NS. However, our EDAX and XPS analysis shows some differences which also could be partially due to size of the (bigger in our case) Au NS. Fig. 1 (a), (b), and (c) show the FESEM images of the PANI, Au NS, and Au NS + PANI thinfilm, spin coated on glass substrates respectively. The inset in Fig. 1(b) shows a single Au NS. It is quite clear that most of the Au NS are well formed with five arms. The star tips are quite sharp. The average lengths (inset) of the five arms (centre to tip) are ~ 90 nm and the inter-arm angles are ~ 71°. In Fig. 1 (d), a single Au NS is mapped for the elemental distribution of Au and Cu. The mappings in Figs. 1 (e) and (f) clearly show that Au and Cu are uniformly spread in the Au NS. The EDAX results of Au NS, and Au NS + PANI samples are shown in Fig. S5 in Supplementary Information. By comparing only Au and Cu from spectrum in Fig. S5 (a), we find that overall atomic percentage of Au is 91.7% and Cu is 8.3%. It should be however noted that besides Au and Cu, C, and N also show up due to the presence of hexadecylamine as a capping agent of Au NS, whereas Cl originates from the Au and Cu precursor salts. The relative atomic percentages of Au and Cu from the spectrum of ‘Au NS + PANI’ in Fig. S5 (b) were found to be reduced to 74% and Cu increased relatively to 26%. We further tried to determine the concentration of Au and Cu at the centre, edge and tip of Au NS (Fig. S6 (a-c)). From this analysis it was clear that Au was dominant at the centre and the tips but rather less at the edges. However, the XPS analysis which gives only surface sensitive analysis shows nearly the same


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percentage of Au and Cu in Au NS sample but ~ 55% Au and ~ 45% Cu in Au NS + PANI sample. The UV-Vis spectra, EDAX full spectra, and composition from EDAX of selected regions of stars, XPS and XRD results of the samples are shown in Figs. S4, S5, S6, S7 and S8 respectively. Tables 1 and 2 in Supplementary Information give XPS and XRD analysis. We found a small peak corresponding to AuCu (111) as illustrated in the XRD pattern in Fig. S8. This peak may be due to some small clusters of AuCu alloy formed at some places in the Au NS (more details in Fig. S9). EDAX analysis depth is typically ~ 1 to 2 µm. However, the surface atoms are directly involved in the sensing. Therefore we carried out XPS analysis, which can measure the signals from a depth of ~ 5 nm. Fig. S7 shows the survey scans, C 1s, N 1s, O 1s, Cu 2p and Au 4f regions for ‘Au NS’ and ‘Au NS + PANI’ samples respectively. The XPS composition analysis shows that Au and Cu in ‘Au NS’ are almost equal (49.8% and 50.2%). For ‘Au NS + PANI’, it is 55.1% and 44.9% respectively, which is understandable

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. For the detailed composition see

Table 1 in the Supplementary Information. From XPS analysis very different composition is observed for the surface of gold nanostars compared to EDAX. Thus according to the surface analysis using XPS, Cu has either post diffused to the surface or formed after the gold seed of the NS is shaped. Unfortunately due to the large spot-size (1 cm2) of the X-ray beam of the Al target, it is not possible here to scan the Au NS point by point and decide the composition at the apex, sides and centre. However, from EDAX and XPS we can safely conclude that Au is the major player in the chemical activity at the surface even in the composite with PANI. EDAX however points out that tips (apex), edges and centre could be considered to be reactive in the decreasing order respectively. Hexadecylamine with chemical formula CH3 (CH2)15NH2 can interact with Au surface through nitrogen.


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We also recorded the FTIR spectra (Fig. 2) for hexadecylamine (HDA), Au NS (which has HDA capping), PANI (powder form obtained according to Section 2.2), and Au NS + PANI (mixture of solutions obtained respectively according to Sections 2.1 and 2.2) samples respectively. The peaks at 2956 cm-1, 2914 cm-1 and 2846 cm-1 are due to the C-H stretches of terminal CH2 and CH3 of HDA, whereas those at 1600 cm-1 1545 cm-1, and 1464 cm-1 are due to NH2 bending, C=N or C=C quinoid stretch, and CH2-CH3 asymmetric bending modes respectively. The modes at 1300 cm-1 and 1239 cm-1 in PANI may be due to CN stretching (of aromatic amine group), and C-C or C-H stretching. The peak at 1137 cm-1 could arise due to C-H in-plane bending (aromatic group). The peaks at 1164 cm-1, 1058 cm-1 and 996 cm-1 are mostly due to C-N=C stretch, arising out of the dynamic nature of N-H bonds while being capped to the Au surface (the 1164 cm-1 peak is stronger when HDA is capped to Au NS). The peaks at 919 cm-1 and 722 cm-1 could arise due to aliphatic chain vibrations and out-of-plane bending mode of –CH2 bonds respectively, while those at 793 cm-1 and 627 cm-1 are due to the C-H in- and out-plane bending, and NH2 wagging modes respectively. It can be seen that Au NS embedded PANI predominantly shows the Au NS dominated spectrum. Thus we can anticipate that the sensing results presented below could be mostly due to the catalytic presence of gold nanostars. Ammonia gas sensing being very important in industries, laboratories etc., has been widely investigated with the aim to obtain highly sensitive and selective ammonia sensor. The literature suggests that silver nanoparticles, gold nanoparticles, NiO porous structures have been used in the past

46,49,51

. The highest sensitivity reported at 100 ppm so far is 36%

51

. The NH3 gas

sensitivity of the PANI and Au NS + PANI thin film sensors was tested as a function of the time. Fig. 3 shows the gas sensing performance of Au NS + PANI thin film sensor for fixed concentration of 100 ppm NH3 gas. It is observed from Fig. 3 that the maximum gas response of


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Au NS + PANI is ~ 52% with an extremely short response time of 15 seconds at room temperature. It will be seen in all the NH3 sensing experiments shown here that when NH3 gas is injected into the gas testing chamber (gas on), the resistance increases from its base resistance (Ra). After some time it tends to saturate and attain a stable value (Rg). As shown in the enlarged view in Fig. S10. NH3 gas vented after the saturation is attained. Subsequently NH3 molecules get desorbed and electrical resistance starts decreasing with time and eventually attain the original value Ra. Pandey et al. reported ~ 50 seconds response time for their silver nanoparticles in gum sample51. The Au NS + PANI thin film sensor also shows enhanced gas response in comparison with PANI thin film sensor which reaches ~7% sensitivity. In our case due to highly insulating PANI matrix, PANI sensor shows less gas response but when Au NS are embedded in PANI film, the composite shows enhanced gas response due to highly catalytic activity of the Au NS, which can be explained by the reversible binding of ammonia molecules to PANI (see Fig. 4); in this case PANI is present in the insulating form. Gold nanostars are embedded within PANI in a sort of contiguous manner that facilitates the formation of a semi-covalent dative bond between the PANI nitrogen atoms and the gold atoms. This is even more likely due to presence of kinks and sharp tips of the gold nanostars. As a result of the dative bonding, the nitrogen atoms of PANI are rendered electron deficient. This can be corroborated, for instance, from the XPS plot in Fig. S7 (c), which shows a shift towards increasing binding energy of the N 1s peak, which indicates a positive charge on nitrogen atoms. As for the strong gas response of Au NS + PANI towards ammonia, the incoming NH3 (gas) molecules result in the formation of dative bonds between the lone pairs of the ammonia nitrogen atoms and the positively charged nitrogen atoms of the PANI structures, following a reversible type of Lewis acid-base chemistry

65

. This leads to the formation of a short-lived complex


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species with a reduced charge mobility which increases the electrical resistance 66. The existence of such a catalytic action of the gold nanostars is evident from the measured dramatic (nearly) seven-fold increase in the resistivity/ sensitivity of the ‘Au NS + PANI’ to ammonia exposure. For the pure ‘PANI’ substrate, the insulating form cannot facilitate such a dramatic reaction to ammonia gas, since the absence of gold nanostars cannot render the nitrogen atoms of PANI electron deficient; this leads to a substantially lower Lewis acid-base type interaction between PANI and NH3, thus leading to relatively diminished response in case of the pure PANI sample. Thus, gold nanostars embedded in the PANI film results in a very sensitive, robust and reversible response to ammonia detection. Morphology of gold nanoparticles can also affect their sensing. The sensing was checked at the fixed NH3 exposure viz. 60 ppm for gold nanostars, nanorods and nanospheres embedded in PANI using the same protocol for making the thin films. Fig. 5 shows the response of various morphologies to ammonia gas. It is well known that smaller the nanoparticles, more active they are compared to larger particles due to larger surface to volume ratio 4,5. It can be noticed that in spite of the fact that gold nanorods and gold nanoparticles are much smaller (gold nanorods have diameter ~ 12 nm and length ~ 50 nm, whereas gold nanospheres are ~ 22 nm in size), than gold nanostars (length from the centre ~ 92 nm) the response of gold nanostars is better than gold nanorods or nanoparticles. Moreover copper in Au NS also may be useful in sensing enhancement. However more experiments on this aspect would be needed in future. Au NS are having not only the higher response or sensitivity but also much lower response time. In order to check the reproducibility of the gold nanostar sensor, we have measured On-Off gas responses for a number of cycles. Fig. 6 shows On-Off response cycles of the 'Au NS + PANI' sensor at 40 ppm NH3 gas showing reproducibility of the sensor for four consecutive


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measurements. The real-time gas response-recovery curves of the composite sensor upon exposure to different gas concentrations (20 ppm to 250 ppm) of NH3 gas is shown in Fig. 7 (a). It can be seen that gas response increases with an increase in concentration of NH3 gas (detailed figures in Fig. S11). At higher gas concentration, the interaction between adsorbed gas molecules and the surface of the material is relatively more while at low concentration fewer amount of gas molecules adsorb on the sensing surface. Therefore gas response increases with increase in the gas concentration. The relationship between gas response and the gas concentration is shown in the Fig. 7 (b) with an approximate mathematical relation scaling linearly. Fig. 8 shows the gas response and recovery times of composite sensor for different concentrations of NH3 gas. The response time has decreased from 46 seconds to 7 seconds while the recovery time increased from 158 seconds to 526 seconds with an increase in NH3 gas concentration. At a higher concentration of NH3, the recovery time is long. It should be noted here that the stability of the films towards ammonia exposure was satisfactory as the FESEM images of the gold stars in PANI films, Fig. S12 and UV-Vis spectrum in Fig. S13 did not indicate any degradation of the Au NS or PANI. However, the long term stability (several months) needs to be investigated as Au NS and PANI may interact or get degraded in ambient. Above experiments were done for the fixed concentration of gold and PANI viz. Au NS (200 µl) and PANI (100 µl). In order to check how the sensitivity changes with film thickness and concentration of gold nanostars in the fixed amount of PANI some experiments were performed for 40 ppm NH3 exposure. In Fig. 9 (a), the sensitivity of NH3 is tested by changing the ‘Au NS’ amount (from 100 µl to 300 µl) in PANI (2 mg). Fig. 9 (b) shows that, as the PANI thickness is increased (amount of Au NS held constant, with 200 µl EtOH), the sensitivity increases. This is expected, since increased amount of PANI molecules facilitates higher sites of interaction with


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the adsorbed NH3, leading to increased sensor performance. Fig 9 (a) is counter-intuitive, since the sensitivity seems to slightly increase with decreasing Au concentration (i.e. higher dilution). This behaviour may be due to increased amount of EtOH (dispersing medium) leading to wider spreading of the Au NS over the PANI surface. The increased coverage leads to a synergistic effect, which peaks the sensitivity slightly above that of the sample with less EtOH dilution. For practical applications, the selectivity of gas sensor towards particular target gas is a significant factor. Fig. 10 shows a bar diagram of gas sensitivity to different reducing and oxidizing gases at a fixed concentration of 100 ppm, such as acetone, LPG, NH3, NO2 and SO2. The ‘Au NS + PANI’ composite sensor offers maximum response to NH3 gas as compared to other gases. When the ‘Au NS + PANI’ composite sensor is exposed to ammonia gas, PANI undergoes de-doping by deprotonation very fast rather than for other gases 67. However complete selectivity to ammonia gas is rather difficult

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. In our case, the selectivity of our ‘Au NS +

PANI’ sensor to NH3 gas is around two to four times that of other tested gases (Fig. 10). 4. Conclusions In conclusion, we have synthesized gold nanostars using a wet chemical route. The XPS results showed a copper segregation on the gold nanostar surface, as compared to that in the bulk i.e. EDAX analysis. The nanostars were embedded in polyaniline in order to make a nano-composite which showed a seven-fold increase in sensitivity towards ammonia gas detection, as compared to pure polyaniline. Superiority of gold nanostars for ammonia sensing is established as compared to gold nanorods or spherical nanoparticles even of much smaller size compared to that of gold nanostars used here. In future, role of copper, amount of optimal concentration of copper, size effect of gold nanostars in ammonia sensing needs to be investigated. The


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sensitivity, selectivity, response time, stability and room temperature functionality of the gold nanostar-polyaniline composite sensor is attributed to the catalytic presence of gold nanostars in the polyaniline matrix. Thus potential of gold nanostars as sensors is explored.

ACKNOWLEDGMENTS One of the authors Mr. Namdev Harale is thankful to the DRDO, New Delhi, for the financial support through the project - DRDO/ERIP/ER/0803719/M/01/1343.

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FIGURES:

Figure 1 FESEM images of (a) PANI, (b) Au NS, (c) Au NS + PANI respectively (d) EDAX Elemental Composition of single Au NS showing (e) the spatial distribution of Au (red) and (f) Cu (green).


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Figure 2 FTIR spectra of hexadecylamine (HDA), gold nanostars (Au NS), polyaniline (PANI) and Au NS + PANI samples respectively.

Figure 3 Gas response of 'Au NS + PANI' sample towards 100 ppm NH3 gas (inset) gas response of ‘PANI’ sensor towards 100 ppm NH3 gas.


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Figure 4 Sensing mechanism: (a) Formation of Au-N dative bond rendering PANI N atoms electron deficient (b) Lewis acid-base binding between NH3 molecules and electro-positive N atoms of PANI (c) Formation of PANI-NH3 transition complex with increased resistivity (d) Unbinding of NH3 after chamber venting resulting in the recovery period.

Figure 5 (a) Morphology dependent sensitivity response to NH3 gas, of PANI composited with different Au nanostructures. FESEM images of the corresponding Au (b) nanostars, (c) nanorods, and (d) nanospheres, respectively.


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Figure 6 On-Off response cycles of the 'Au NS + PANI' sample towards 40 ppm NH3 gas. Circled region is enlarged in Fig. S10 (Supplementary Information).

Figure 7 (a) Gas response of the ‘Au NS + PANI’ sensor towards various ppm of NH3 gas as a function of time (b) relation between the sensor response and the PPM value of NH3 gas.


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Figure 8 Response and recovery times of the 'Au NS + PANI' composite sensor as a function of different gas concentration of NH3 gas.

Figure 9 Sensitivity variation of the Au NS + PANI sensor, as a function of (a) Au NS concentration, and (b) thickness of the PANI film (refer section 2.5.4 for details). All these measurements were made at 40 ppm NH3 gas.


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Figure 10 Selectivity of the 'Au NS + PANI' sensor towards different gases.


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ASSOCIATED CONTENT Supporting Information. UV-Vis spectra, Elemental compositions derived from EDAX and XPS data, XRD plots and related calculations. “This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources Defence Research and Development Organisation (DRDO), New Delhi, India for financial Support to Mr Namdev Harale: DRDO/ERIP/ER/0803719/M/01/1343.

ABBREVIATIONS Au NS, gold nanostars; PANI, polyaniline; Au NS + PANI, gold nanostar + polyaniline nanocomposite.


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Synopsis: Gold nanostars synthesized via wet, chemical method were integrated into insulating polyaniline to form a nanocomposite that was deposited in form of thin-films on glass substrate. Usage of this nanocomposite substrate for chemi-resistor based sensing of different gases showed very high sensitivity of ~52% for ammonia gas, with response times as fast as 15 seconds, at room temperature.


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Figure 4: Sensing mechanism: (a) Formation of Au-N dative bond rendering PANI N atoms electron deficient (b) Lewis acid-base binding between NH3 molecules and electro-positive N atoms of PANI (c) Formation of PANI-NH3 transition complex with increased resistivity (d) Unbinding of NH3 after chamber venting resulting in the recovery period. 496x220mm (150 x 150 DPI)


Ultrasensitive Gold NanostarâPolyaniline Composite for Ammonia

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Ultrasensitive Gold NanostarâPolyaniline Composite for Ammonia