Localized surface plasmon resonance H2 detection by MoO3 colloidal nanoparticles fabricated by the flame synthesis method

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Localized surface plasmon resonance H2 detection by MoO3 colloidal nanoparticles fabricated by the flame synthesis method A.R. Shafieyan, M. Ranjbar*, P. Kameli Department of Physics, Isfahan University of Technology, Isfahan, 84156-83111, Iran

article info

abstract

Article history:

Beside the noble metal nanoparticles (NPs), plasmonic semiconductors are being developed

Received 10 March 2019

nowadays for a variety of applications such as catalyst, phototherapy and sensing tech-

Received in revised form

nologies. In the present study, colloidal plasmonic MoO3 NPs with <20 nm mean size are

13 May 2019

prepared by flame synthesis as a rapid, cost effective, catalyst-free and atmospheric route.

Accepted 21 May 2019

A Mo rod was converted to oxide nanopowder in an oxyhydrogen flame and collected on a

Available online 13 June 2019

water-cooled rotating substrate. Colloidal solutions of these particles were made by

Keywords:

PdCl2 were added to the colloidal NPs to make them sensitive to hydrogen. Formation of

Flame synthesis

cubic Pd NPs in among the MoO3 NPs were recognized in TEM images for hydrogen-exposed

MoO3

samples. Upon hydrogen injection, a localized surface plasmon resonance (LSPR) absorp-

Colloidal nanoparticles

tion band was appeared in the UVeVis absorption spectra at 700e800 nm region, which

Localized surface plasmon reso-

began to decline by further exposure time. Within few seconds of gas exposure, samples

nance

turned blue and longer exposure made them brownish. The LSPR peaks exhibited a spectral

Hydrogen sensor

blue shift with a more hydrogenation, in agreement with the existing plasmonic model. In

TEM

addition, a correlation between optical band gap and LSPR intensity was found. The

dispersing the MoO3 NPs into DI water. As a palladium precursor, solutions of aqueous

colloidal Pd-MoO3 solutions then were employed for LSPR sensing of hydrogen at a concentration range of 0e5%. A pronounced LSPR linear sensitivity toward low concentration (<2.5%) of hydrogen was observed. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Recently, hydrogen has been considered as an alternative energy source in the human life as being clean, having high sustainability and energy density [1]. In addition, hydrogen gas is widely used in scientific researches, petroleum, steel, glass, semiconductor and food industries [2,3]. However, the use of hydrogen gas is particularly dangerous in enclosed

areas such as buildings and tunnels. Due to its high flammability, it can have many consequences in the production to consumption. Hydrogen is colorless, odorless and non-toxic gas, which require extreme caution during handling and usage [4]. Due to the high utilization of hydrogen in the industry, its high leakage and explosive potentials, the early detection of leaking hydrogen gas is important in preventing inconvenient events.

* Corresponding author. E-mail address: [email protected] (M. Ranjbar). https://doi.org/10.1016/j.ijhydene.2019.05.171 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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Several mechanisms have been proposed to detect H2 concentration below Lower flammability limit (LFL) as involved in electrochemical [5], MEMS [6], thin film [7] and optical sensors [8]. Because of the high risk of explosion caused by electrical sparks or the hot zone in resistive sensors, it is preferable to use optical approaches in which there is no electrical current near the gas-sensing probe. Optical detecting devices are more safe and efficient at low gas concentrations and room temperature. Nowadays using optical properties of semiconductor metal oxide based method is one of the most reliable way to detect H2 gas [4,9,10]. Detection of hydrogen in aqueous media like water can be important in environmental sciences, biomedical sciences, biotechnology, research on photochemical water splitting and H2 enriched drinking water. For example, Drinking water enriched with dissolved hydrogen has been reported to have a positive effect on many health and well-being conditions. Therefore, water base liquid hydrogen detector, especially based on optical and vision methods, is of great important. MoO3, as a wide band gap semiconductor, has been widely used in catalysis, ion batteries and bio-sensing [11,12]. Recently, defective molybdenum oxide (MoO3-x) has been investigated as a novel plasmonic material, in which its optical absorption has a high sensitivity to the dielectric constants of the environment, particle size, shape and more importantly the free carrier concentration [13]. This feature has created great hopes in converting energy [14,15], enhanced catalytic activity [16,17], optical properties [18,19] surface enhanced Raman spectroscopy (SERS) [20], and treating serious diseases by photo-thermal therapy [21]. Introducing oxygen vacancies, doping of small ions such as Naþ and Hþ are the most effective ways to control the carrier concentration in the molybdenum oxide [22,23]. Therefore, molybdenum oxide has been one of the most important materials used as gas sensors, especially for H2 as a reducing gas [24]. The electrical resistance reduces [25] and the optical properties of molybdenum oxide change and it turns blue by hydrogenation due to the formation of defects [26e28]. Because of defect formation, much free carrier become available, which causes appearing of an LSPR optical absorption band in the visible or near-infrared (NIR) region, for which the peak position depends on particle aspect ratio [29]. In addition, since the defect concentration depends on hydrogen intercalation level, LSPR characteristics (peak position and intensity and total absorption) are very sensitive to the hydrogen concentration. So MoO3 NPs can be used to detect optically of hydrogen. A variety of synthesis techniques have been used for preparation of colloidal MoO3 NPs such as sol-gel [30], hydrothermal [31] and various chemical methods [32,33]. Nevertheless, these methods require expensive and hazardous metal-organic compounds, which are often inconvenient. In contrast, flame synthesis as an atmospheric, rapid rate growth, highly scalable, low cost and mass-productive technique for the synthesis of metal oxides [34,35], has been widely used in the industry to produce a wide range of materials such as carbon black, metal oxides and organic materials [36]. This method has been successfully used to produce various morphologies of molybdenum or tungsten oxide from 1D to monolayer to bulk structures [35]. The flame used in the flame synthesis methods essentially is made by combining a hydrocarbon or hydrogen fuel and an

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oxidizing gas such as air or oxygen, which requires different gas reservoir tanks that require a lot of space, much cost and have the risk of explosion. However, it is possible to produce oxygen and hydrogen simultaneously from water decomposition, which less attention has been paid to for flame synthesis method up to now. In spite of the fixed H2/O2 ratio in this method, the hydrogen flame production from water decomposition is simple, inexpensive without need to gas storage. This type of generators are often called oxyhydrogen, HHO, or brown gas generators [37]. With these generators, the mixture of hydrogen and oxygen created with a ratio of 2:1 undergoes rapid combustion within millisecond on the nozzle and a flame with a high temperature is formed. A Mo target is rapidly vaporized and oxidized in the oxyhydrogen high temperature flame and nucleates to form nanoparticles of molybdenum oxide leaving the target. Despite the successes in flame synthesis of Mo oxide [38], less study has been done to use this synthesis method for optical applications such as gasochromic and plasmonic phenomena. In this work, we have developed a MoO3 colloidal NP optical hydrogen sensor by dispersing of flamesynthesized MoO3 nanoparticles in DI water. Although there are numerous papers on the optical sensing of hydrogen gas using molybdenum oxide, the use of flame synthesis method for LSPR sensing has not yet been reported. Therefore, our work based on distinct advantages of flame synthesis as a cost effective and the mass-productive technique can be scientifically and practically important.

Experimental Sample preparation The experimental setup consisting of photographic images of a homemade oxyhydrogen generator, the produced flame and the schematic of MoO3 NPs synthesis process is shown in Fig. 1. The oxyhydrogen generator runs on 12 V and 30 A which produces H2:O2 molarity ratio ¼ 2 with a 0.3 l/min generation rate. A syringe needle (d ¼ 0.4 mm) as a nozzle, a Mo solid rod (99.9% purity, 2 mm width and 0.3 mm thickness) as the feeding material, and a rotating cylindrical substrate (12 rpm, 10 cm diameter) for collecting MoO3 nanopowder were used. The nozzle-to-target and target to collector distances were 1 and 9 cm, respectively and the total synthesis time was 1 min. By using cold water, the rotating substrate temperature remains constant at about 60e70  C. Fuel (H2) and oxidizing (O2) gases were premixed inside an oxyhydrogen generator, and the resulting flame is narrow (2e3 mm width) and long (10 cm). The flame temperature at the target position was about 1750  C. About 1 mg of the deposited MoO3 nanopowder on the substrate was dispersed in 80 ml DI water and a 12.5 mg/l colloidal solution of MoO3 was obtained which was highly clear. A great particle dispersity was observed next to the transfer the collected MoO3 powder to DI water, in which no precipitation could be observed days after the dispersion. In addition, the white color of flame-synthesized powder disappeared after dispersing so that a perfect transparent liquid is obtained indicating presence of NPs of a high dispersity probably due to hydrophilicity of MoO3 NPs. For

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Fig. 1 e The photographic images of the homemade oxyhydrogen generator, the narrow produced flame and the schematic representation of the MoO3 NPs generation by the flame synthesis method. The oxyhydrogen gas is produced with 0.3 l/min rate in an oxyhydrogen generator and injected through the nozzle. A Mo rod is placed within the flame which is oxidized and MoO3 clusters and NPs are forming. After collimating the synthesized MoO3 NPs, 1 mg is dispersed in 80 ml DI water to obtain a transparent colloidal solution of MoO3. The SEM image of the deposited powder on the rotating substrate is shown which shows a cauliflower-like morphology. Finally, the PdCl2 solution is added and PdCl2-MoO3 colloidal solutions are obtained (not shown here).

investigation of LSPR H2 detection ability of the obtained MoO3 colloidal solution, PdCl2 solution is added. The PdCl2 part is easily reduced to metallic Pd NPs which plays the role of hydrogen catalyst. Metallic Pd NPs act to dissociate hydrogen molecule to atomic hydrogen. The PdCl2 solution was prepared by adding 0.02 g of PdCl2 powder (99.999% purity sigma Aldrich) into 99.9 cc DI water and 0.1 cc HCl for more solvation. Then it was kept in ultrasonic until dissolved entirely and a uniform yellowish solution of 0.2 g/l PdCl2 was obtained. Various samples were obtained by adding 0.1, 0.2 and 0.4 ml of the PdCl2 solution to the 5 ml of the initial 12.5 mg/l MoO3 colloidal solution. The samples are denoted S0.1, S0.2 and S0.4, respectively.

Characterizations The optical absorption spectra of samples were measured on a PerkinElmer Lambda 25 UVeVis spectrophotometer using DI water as the reference sample. The crystalline structure of samples was characterized using XRD (Phillips XPERT with Cu-Ka radiation). The morphology of nanoparticles was investigated by scanning electron microscope (SEM), (Hitachi model S4460) and transmission electron microscope (TEM), (PHILIPS CM300). LSPR gas sensing investigations were performed using (10%H2)/Ar premixed gas for coloring which was bubbled or injected into a glass vessel containing the Pd-MoO3 colloidal solutions. After hydrogen gas exposure, the samples

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were allowed to remain in the laboratory for 5 min to stabilize before optical spectrophotometry. For hydrogen concentration detection, a gas concentration range of 0.5e10% was used.

Results and discussion Optical properties To examine the gasochromic hydrogen sensing capability of the MoO3colloidal samples containing palladium, optical absorption spectra of the samples were measured in the

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wavelength range of 190e1100 nm before and after hydrogen gas exposure. In Fig. 2, the initial unexposed PdCl2-MoO3 sample (sample S0.2 as a representative sample) is highly transparent without absorption, except a shoulder below 300 nm due to its optical band gap. In the presence of 10% H2 gas, it turns blue and reveals pronounce wide absorption band centered at 820 nm. This absorption is attributed to the LSPR effect induced by vibration of the conduction band electrons [39]. It has also been shown that for one or two-dimensional metal oxide NPs, with increasing particles aspect ratio the plasmonic absorption shift to shorter wavelength and visible LSPR occurs for too wide nanoparticles [Alsaif]. The TEM

Fig. 2 e (a) UVeVis optical absorption spectra and (b) corresponding Tauc plots of as-prepared MoO3 colloidal NPs (black line) and PdCl2-MoO3 colloidal NPs (blue bold line) after 10% H2 exposure. The LSPR peak appears at about 820 nm and optical band gap reduces. Right panel shows the photographic images of the as-prepared MoO3 NPs and hydrogen exposed PdCl2MoO3. The solution turns to a deep blue color. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 3 e (a) and (b) typical SEM and TEM images of as-prepared MoO3 NPs accumulated on the rotating substrate, respectively. (c) TEM image of PdCl2-MoO3 after hydrogen exposure. Formation of Pd NPs is recognizable as cubic particles with a lower contrast. SAED patterns indicating the presence of crystalline phases are also shown. Right panel shows the corresponding XRD patterns of each sample (up: as-prepared MoO3, bottom: hydrogen exposed sample S0.2). (0k0) orientation of a-MoO3 phase has been increased after gas exposure suggesting of delaminating of MoO3 NPs.

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images in next part support the NIR plasmonic peak of our samples. As the initial solution turns dark blue, the measured optical energy gap (Tauc method) has shifted from 3.4 to 3 eV. Indeed, hydrogen molecules generate oxygen vacancy in the MoO3 lattice through an intermediate Pd catalytic treatment. The increasing of oxygen vacancies results in a narrowing bandgap, increasing charge carries and as a result, the plasmonic resonance occurs [10,29,40e42].

Morphology and structure Typical SEM image of as-deposited MoO3 NPs on the rotating collector (Fig. 3(a)) represents porous agglomerates with cauliflower-like morphology. Because of its high effective surface area, this type of morphology is desirable for a gasochromic sensing device. TEM image and selected area electron diffraction (SAED) pattern in Fig. 3(b) provides more details of the individual particles as showing numerous

crystalline nano-flake structures of <20 nm dimension. Formation of nano-flakes is expected in a-MoO3 due to the presence of weak van der Waals forces along [0k0] direction [41]. XRD patterns examined the crystalline structure of drop-casted MoO3 NPs and H2 exposed PdCl2-MoO3 (sample S0.2 in Fig. 3 right panel). Unexposed MoO3 sample reveals main [0k0] ([020], [040] and [060]), [110] and [021] diffraction peaks in agreement with the orthorhombic a-MoO3 phase (JCPDS card no. 035-0609). Typical TEM images of hydrogen-exposed Pd-MoO3 (part(c)), demonstrate formation of regular multifaceted Pd NPs (shown by arrows) contacted to the MoO3 nano-flakes. The SAED pattern is more irregular than pristine sample confirming the exfoliation of the initial MoO3 nanocrystals in agreement with XRD results. After gas exposure the relative intensity of (0k0) peaks in XRD pattern (right panel bottom) increases suggesting that particles become layered more and more along the (0k0) direction. The sticking of

Fig. 4 e UV_Vis optical absorption spectra at different hydrogen exposure times (from 30 s to 5 min) for PdCl2-MoO3 samples with different Pd:Mo ratio. (a) 0.1, (b) 0.2 and (c) 0.4 ml PdCl2. There are two absorption positions in the LSPR band of each samples located at 650-700 and 760e800 nm. The former is attributed to the resonance along the thickness axis and the later to the lateral axis. By increasing the PdCl2 amount the former peak intensity increases and a blue shift occurs which is shown by vertical lines. The Inside shows representative shapes of MoO3 NPs. The photographic image of sample with 0.4 ml PdCl2 is shown in the lower right panel for a long time hydrogen exposure indicating its reduction to brownish MoO2 phase. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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derived Pd NPs to MoO3 nano-flakes is expected to facilitate the hydrogen supply in the gasochromic process. Pd NPs formation of various shapes has been vastly reported through the reduction reaction of PdCl2 [43e45]. In our case, the catalyst Pd NPs are derived from PdCl2 cations reduction by hydrogen gas at the beginning of gas exposure as follows: PdCl2þH2/Pd (s)þ2HCl(g)

(1)

LSPR hydrogen sensing The LSPR sensing response of Pd-MoO3 colloidal samples to 10% H2/Ar gas was investigated for different PdCl2 concentrations via preparation of samples S0.1, S0.2 and S0.4. Then, the sensitivity behavior of selected optimal samples for various H2 concentrations was studied. The optical absorption spectra of the colloidal Pd-MoO3 sample next to 30 s, 1, 1.5, 2, 2.5, 3 and 5 min 10%H2 exposure times are shown in Fig. 4. There is a fundamental optical absorption edge below 450 nm for all the spectra, which corresponds to the wide optical band gap of MoO3. However, the effect of Pd amount can be understood from the LSPR absorption in 600e900 nm wavelength range. Before hydrogen exposure, samples are colorless without LSPR absorption. Upon hydrogen exposure, a blue coloration occurs, plasmonic absorption peaks arise, and the optical band-gap reduces. We explain the plasmonic absorption, optical bandgap and total absorption of each sample as below: In S0.1, upon gas exposure, a broad band consisting of one peak at 710 nm and a weak shoulder around 820 nm appears after a 1 min delay, which slightly intensify after 2 min but diminishes after 3 min. By increasing the PdCl2 amount, same behavior occurs for S0.2 but the 820 nm absorption peak somewhat dominates the first one due to gas exposure. Further increase in PdCl2 amount (S0.4) leads to a 2 min delay in appearing of LSPR absorption while no intensity drop is observed with increasing exposure time to 5 min. A long time hydrogen exposure to this sample reduces it to a brownish color better confirming the role of Pd, which could delay the reduction reaction time in this system. Therefor by varying the PdCl2 quantity one can tune the MoO3 plasmonic properties in a gasochromic coloration process. Alsaif and co-worker have attributed the origin of these two peaks to two optical modes [41]. The first peak is attributed to the thickness axis and the second to the lateral axis. Previously, we have also shown that PdCl2 addition to the orthorhombic MoOx colloidal solutions leads to an increase in the (0k0) diffraction peaks through a chemical exfoliation mechanism [46]. The observations in the present work suggest that by increasing the amount of PdCl2, the lateral axis (010) grows more than the thickness axis so that aspect ratio of the plates become bigger (shown schematically inside the Fig. 4) in agreement with XRD results. In the absorption spectra, there is another important point to be noted. A blue shift occurs with increasing PdCl2 concentration as shown by vertical red and blue lines in Fig. 4. This spectral shift, after deconvolution into different Gaussian peaks, is represented in more detail for two extreme

Fig. 5 e The spectral shift observed in Fig. 4, after deconvolution into different Gaussian peaks, is represented in more detail for two extreme conditions (lowest and highest intercalation level); (a) S0.1 with 1.5 min hydrogen exposure and (b) S0.4 with 5 min hydrogen exposure.

conditions (30 s: sample S0.1 and 5 min: sample S0.4 as lowest and highest intercalation cases) in Fig. 5. This observation can be described based on the LSPR mechanism. Higher H2 dissociation rate is expected to occur more easily in the presence of a greater amount of Pd catalytic NPs. More palladium leads to a more intercalation into MoO3 lattice. As a result, carrier concentration level increases due to formation of oxygen vacancies. This observation is in consistent with an existing LSPR models for resonance frequency as Eq. (2) [13]: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uLSPR ¼ 1=2p Ne2 =ðε0 me ðε∞ þ 2εm ÞÞ

(2)

where uLSPR is the resonance angular frequency, N is free carrier concentration, e is the electron charge, ε0 is vacuum dielectric constant, εm is medium dielectric constant, e∞ is high frequency dielectric constant and me is electron effective mass. This equation predicts that as carrier concentration increases the LSPR peak blue shifts. Moreover, our investigations showed that there is a significant correlation between the total optical absorption and the optical band gap with increasing gas exposure time (Fig. 6). It should be noted that the LSPR bandwidth varies during the gas exposure times as it could be seen in Fig. 4. Therefore, for a

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more meaningful analysis of the optical spectra data, total absorption values are calculated by integrating over each observable width of the LSPR absorption band. The values of the optical band gap are also calculated from the Tauc plots [47] (the Tauc curves are not shown here). In Fig. 6, the optical band gap of the as-prepared MoO3 NPS was found to be about 3.4 eV, close to the values reported for MoO3 [48,49], which drops to smaller values by increasing hydrogen exposure time. However, the total absorption integral increases to a maximum that occurs for all samples when the band gap is about 2.7e2.8 eV (shown by dashed red line) then drops upon

further exposure time. Although this total absorption drop is not seen in S0.4, the same decrease is predicted by increasing time for more than 5 min. These data also suggest that one can tune the optical band gap of MoOx by varying the gas exposure time at a given sample. The color change of the samples, shown in the bottom panel of Fig. 6, also confirms the optical gap variations at different stages of gas exposure times. The samples are first transparent, then dark blue and after exceeding the optical gap of 2.8 eV, where the maximum total absorption occurs, turns brownish. The brownish color and the measured ~2.8 eV optical band gap both are attributed to

Fig. 6 e Optical bandgap and total absorption of LSPR peak for sample (a) S0.1, (b) S0.2 and (c) S0.4 as a function of 10% H2 exposure. LSPR total absorptions exhibit a maximum at which the bandgap is about 2.7e2.8 eV. Bottom panel indicates the photographic images of a typical sample (S0.2) at different gas exposure times. The color changes from blue to brown due to formation of MoO2 phase. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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formation of a MoO2 phase. It has been shown that hydrogen intercalation into MoO3 actually leads to initial formation of HxMoO3 which decomposes into MoO2 and H2O by further intercalation [24]. Although for more accurate analysis, more varied data is needed, but we attempted to describe the process based on the optical data. Based on the suggested model shown schematically in Fig. 7, the initial transparent MoO3 as a nonplasmonic phase reduces to plasmonic MoO3-x by the injection of hydrogen, while the optical gap decreases as well. This is due to the increase in charge carrier concentrations. With further increase in the hydrogen intercalation level,

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MoO3-x reduces more and more until MoO2 phase is obtained which is a non-plasmonic semiconducting phase. The TEM image in Fig. 3(c) revealed a few cubic Pd NPs attached to MoO3 NPs. Fig. 7 up panel suggest a model in which those MoO3 NPs attached to a Pd NP begin to turn blue and, by further exposure time, they turn brown. However, if no Pd NPs is in intimate contact with a MoO3 NP, it remains unaffected hence the final solution is a mixture of MoO3 and MoO2 that both of which are non-plasmonic phase and having no absorption. Moreover, it has been known that without Pd or Pt NPs no coloration occurs, as hydrogen needs to dissociate by a catalyst prior to injection into the MoO3 lattice. This is

Fig. 7 e Schematic representation of the coloration mechanism for low and high PdCl2 concentrations. At low concentrations, some of the MoO3 NPs remain intact, and some are reduced to a MoO2. These two phases are mixed and both are non-plasmonic. At a high PdCl2 concentration, all particles have access to Pd NPs and will be reduced, but need more gas exposure time.

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why the LSPR peak drops and disappears at higher exposure times. By increasing the number of Pd NPs (high PdCl2 content), the MoO3 NPs are more likely to be exfoliated and chemically reduced, so a longer time is needed to get enough hydrogen to all of them. Eq. (3) shows the possible chemical reactions: MoO3 ðNon  plasmonicÞþH2 /MoO3x ðplasmonicÞþyH2 O MoO3x ðplasmonicÞþH2 /MoO2 ðNon  plasmonicÞþyH2 O (3)

Hydrogen concentration sensing To examine the detection capability for H2 concentration below the explosion limit (4%), H2/Ar gas with different concentrations from 0 to 5% was confined by injection into the container of Pd-MoO3 colloidal solutions. The LSPR absorption spectra were recorded 5 min after each injection process for the best two stable samples (S0.2 and S0.4). To avoid data mixing between different spectra we show the spectra in two separate sections, as depicted for 0e2.5% and 2.5e5%,

respectively. Fig. 8(a and b) shows these series of subsequent curves for samples S0.2 and parts (d, e) for sample S0.4. The corresponding LSPR peak intensity and total absorption graphs, as a calibration curve, are shown in terms of the gas concentration in parts (c) and (f), respectively. As the H2 concentration increases from 0 to 2.5%, the LSPR peak of both the samples grows linearly then stables and drops slightly by going to 5% H2. The most important point to be noted is that in the linear part of the calibration curves a slight change in the concentration (for example from 0.5 to 0.75% H2) causes a significant change in the LSPR peak strength. This observation shows that the LSPR intensity is very sensitive to measure the low concentrations of hydrogen gas. Therefore, flame synthesized Pd-MoO3 colloidal system is expected to detect hydrogen concentration with large quantities of points. Accordingly, the flame synthesis method based on a homemade oxyhydrogen generator as a low cost and massproductive technique is able to generate nanoparticles with plasmonic properties and high sensitivity against H2 gas. It is worth noting that almost no spectral shift is observed with

Fig. 8 e The variation of LSPR peak of PdCl2-MoO3 colloidal NPs at different hydrogen concentration from (0.5e5%). (a) S0.2 for 0.5e2.5% H2, (b) S0.2 for 2.5e5% and (c) peak intensity versus gas concentration. (a) S0.4 for 0.5e2.5% H2, (b) S0.4 for 2.5e5% and (c) peak intensity versus gas concentration.

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increasing gas concentration. This issue needs to be further investigated, as it shows that the concentration of carriers is not likely to increase, but the number of nanoparticles that are being injected by hydrogen is increasing.

[8]

[9]

Conclusion In this work, we report successful synthesis of MoO3 nanoparticles by a simple and low cost flame synthesis method based on a homemade oxyhydrogen generator. XRD confirmed generation of a-MoO3 nanoparticles and SEM revealed cauliflower structures, that according to TEM, they consist of nanoparticles with 5 nm average size. The MoO3 were activated for the gasochromic hydrogen sensing by adding PdCl2 solution. Pd nanoparticles were easily derived from hydrogen-reduction of PdCl2. Hydrogen exposure leads to blue coloration of the colloids, lamination of MoO3 nanoflakes and appearance of a LSPR band consisting of two peaks around 700 and 800 nm. The gas sensing capability of flame synthesized Pd-MoO3 was explored, and it was determined that the ratio of palladium to molybdenum should be optimized. Moreover, a high sensitivity for hydrogen detection below 2.5% in the experimental conditions of this paper was observed. In general, a cost-effective synthesis method was proved with a desirable hydrogen sensing capability based on LSPR effect of hydrogen.

[10]

[11]

[12]

[13]

[14]

[15]

[16] [17]

Acknowledgment The authors would like to thank the Iranian National Science Foundation (INSF) for their financial support.

[18]

[19]

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