Room-temperature gas sensing of laser-modified anatase TiO2 decorated with Au nanoparticles

Journal Pre-proofs Full Length Article Room-Temperature Gas Sensing of Laser-Modified Anatase TiO2 Decorated with Au Nanoparticles Neli Mintcheva, Parthasarathy Srinivasan, John Bosco Balaguru Rayappan, Aleksandr A. Kuchmizhak, Stanislav Gurbatov, Sergei A. Kulinich PII: DOI: Reference:

S0169-4332(19)33986-8 https://doi.org/10.1016/j.apsusc.2019.145169 APSUSC 145169

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

9 October 2019 28 November 2019 23 December 2019

Please cite this article as: N. Mintcheva, P. Srinivasan, J. Bosco Balaguru Rayappan, A.A. Kuchmizhak, S. Gurbatov, S.A. Kulinich, Room-Temperature Gas Sensing of Laser-Modified Anatase TiO2 Decorated with Au Nanoparticles, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.145169

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2019 Published by Elsevier B.V.

Room-Temperature Gas Sensing of Laser-Modified Anatase TiO2 Decorated with Au Nanoparticles

Neli Mintchevaa,c, Parthasarathy Srinivasanb, John Bosco Balaguru Rayappanb, d,e

d,e

Aleksandr A. Kuchmizhak , Stanislav Gurbatov , Sergei A. Kulinich

a

b

a,d,f,

*

Research Institute of Science and Technology, Tokai University, Hiratsuka, Kanagawa 259-1292, Japan

School of Electrical and Electronics Engineering, SASTRA Deemed University, Thanjavur, Tamil Nadu 613 401, India c

Department of Chemistry, University of Mining and Geology, Sofia 1700, Bulgaria d

e

Far Eastern Federal University, Vladivostok 690041, Russia

Institute of Automation and Control Processes, Far Eastern Branch of the Russian Academy of Science, Vladivostok 690091, Russia f

Department of Mechanical Engineering, Tokai University, Hiratsuka, Kanagawa 259-1292, Japan

Abstract. This article reports on fabrication of activated TiO2 and gold decorated TiO2 prepared by irradiation with millisecond laser and on their gas sensing properties. Commercially available anatase TiO2 was used as starting material whose nanoparticles were modified and decorated upon irradiating its water suspension by pulsed laser. Formation of Au nanoparticles was achieved by laser-induced reduction of Au(III) ions, followed by their deposition onto TiO2 nanoparticle surface. In order to evaluate the effect of gold concentration in Au@TiO2 nanocomposites on their sensing properties, two gold-containing samples with 1 and 5 wt % of Au were prepared. All nanomaterials were characterized by XRD, XPS and SEM methods and then tested as gas sensing devices towards a number of volatile compounds. XPS analysis revealed formation of oxygen vacancies and Ti3+ ions on the TiO2 surface upon laser irradiation, while the latter defects disappeared after decoration with gold. Depending on the degree of Au loading, the samples demonstrated selectivity towards ammonia, acetaldehyde or benzene. Keywords: laser irradiation, TiO2, Au@TiO2, gas sensing * Corresponding author. Email: [email protected]

Page | 1

Highlights

o Au-decorated TiO2 nanoparticles prepared by laser beam irradiation o Decoration of TiO2 surface with different amount of Au o Sensing response of Au@TiO2 nanomaterials towards different gases o Au-nanoparticle dependent selectivity

1. Introduction Titanium dioxide is a widely investigated semiconducting metal oxide with applications in optoelectronics, photocatalysis, and gas detection, to name just a few [1-4]. Its photocatalytic activity and gas sensing properties are well-known strongly to correlate with material’s surface structure, grain size and morphology, and phase composition [1-3]. The gas sensing properties of TiO2 nanomaterials can be enhanced by changing their electric conductivity, through enlarging surface area, controlling surface and sub-surface defects, smaller nanoparticle (NP) size, and use of different particle morphologies (nanowires, nanorods, nanotubes, nanosheets, microspheres, etc.) [1-7]. For this purpose,

various

synthetic

methods

(sol-gel,

hydrothermal,

chemical

vapor

deposition,

electrodeposition, etc.) were previously demonstrated [3]. For example, anatase NPs, originated from thermolysis of titanium glycolate, showed high sensitivity and selectivity towards acetone at low concentration (0.5 ppm) at 270 oC [4]. Nanoporous anatase TiO2 synthesized by hydrothermal method and having high surface area and average crystallite size of 12 nm, demonstrated good selectivity towards acetone, as well as long-term stability, quick response and recovery time [5]. Yang et. al. observed crystal-facets-dependent gas sensing of hierarchical TiO2 structures using TiO2 microspheres that consisted of truncated octahedron-shaped single crystals with enhanced gas sensing properties towards 100 ppm of acetone [6,7]. A study of different crystal phases of TiO2 nanotubes revealed that the gas sensing response of anatase towards volatile alcohols such as methanol, ethanol and isopropyl alcohol was higher than that of rutile [8]. At the same time, both sensors demonstrated the highest sensitivity to isopropyl alcohol [8]. Carbon monoxide and hydrogen gas were often reported as target gases for TiO2-based sensors [9-11], while SO2 and triethylamine were rarely sensed examples [12,13]. Page | 2

Gas sensing properties of TiO2 can be improved by incorporation of dopant atoms or by decoration of semiconductor surface with metal NPs, in particular noble metals. Pt-decorated mesoporous TiO2 was found to have superior gas sensing over pristine TiO2 [14]. The Pt-loaded TiO2based sensor not only showed the highest room-temperature sensitivity to acetaldehyde among other tested compounds (such as ethanol, acetone, water, tetrachloromethane, hexane and benzene), but also exhibited higher rate of adsorption and larger adsorption uptake of acetaldehyde compared to nondecorated mesoporous TiO2 [14]. Selective adsorption of target gas molecules on Au NPs was found to promote the selectivity and sensitivity of metal-semiconductor sensors. Such surface Au NPs supported on TiO2 are believed to act as favorable binding sites for CO, H2 or NO2 molecules, which provides strong interaction between gaseous molecules and metal lattice [15,16]. DFT studies of Au/TiO2 sensor for NO2 revealed that both O and N atoms of NO2 could bind to Au atoms, with more energetically stable chemical bond forming with oxygen [16]. This agrees well with the finding of Chomkitichai et al., who concluded that the increase in Au loading from 0 to 0.75 at.% of Au on TiO2 enhanced the H2 gas sensing performance of the material in terms of sensitivity and faster response and recovery times [17]. Although gas sensing performance of various TiO2-based nanomaterials was reported by many groups [4-12], materials working at room temperature and showing high selectivity and sensitivity are still rare and thus highly desired [11,18]. From this point of view, laser-processed nanomaterials show promise, as recently several reports appeared on metal oxide nanomaterials generated via laser ablation in liquid (LAL) and detecting gases at room temperature [19-21]. LAL is a simple, costefficient and environmentally friendly method for preparing diverse NPs with different sizes, chemical composition and morphologies [19–42]. It uses a laser beam ablating solid (typically metal) target immersed into liquid medium to give rise to NPs, the latter often having unique surface states and phases because of huge temperature gradients and quenching rates during processing [20,25-33]. As a result, such NPs have defect-rich surfaces, which is believed to be beneficial for gas sensing. In our recent works, we reported on ZnO, SnOx and ZnO-SnOx NPs produced by nanosecond and millisecond-pulsed lasers that detected ethanol or ammonia at room temperature with good selectivity [22-24]. As an extensively investigated semiconductor metal oxide, TiO2 was also prepared by means of LAL, for which various pulsed lasers were applied to ablate Ti metal plates in liquid media [37-47]. LAL-generated TiO2 NPs with ultra-small size [37,38], nonstoichiometric composition and defectrich surface [39,40], were reported by different groups, which makes such nanomaterials promising as photocatalysts and gas sensors. Depending on laser parameters and liquids used, TiO 2 in its main crystal forms (anatase, rutile and brookite) could be produced, with both mono- or multi-phase composition [39-43]. Moreover, phase transformation caused by post-irradiation of TiO2 colloid, Page | 3

accompanied by change in morphology, size and structure, was also reported [44,45]. Irradiation of anatase suspension by unfocused laser beam with wavelength 355 nm was observed to result in formation of rutile and hollow NPs [46,47]. Fragmentation mechanisms of TiO2 NPs under laser irradiation in liquid (LIL) was also attempted, showing that the size, phase and inner structure strongly depended on laser fluence applied [48]. Decoration of LAL-produced TiO2 nanostructures with noble-metal NPs (Pt, Ag) was also attempted, which was realized either by successive ablation of platinum plate in fresh TiO2 colloid or by simultaneous laser irradiation of both (Ag and Ti) metal plates in water [49,50]. As-prepared Pt@TiO2 and Ag@TiO2 nanomaterials demonstrated enhanced photocatalytic and antibacterial activity, respectively, compared with non-decorated TiO2 product [49,50]. The higher performance of such TiO2 nanomaterials decorated with noble-metal NPs was one of motivations in this study which aimed at developing a new low-cost, simple and effective method for decoration of metal oxide with gold. By using microscale of soluble complex salt of gold added to TiO2 NPs, we applied LIL to achieve Au@TiO2 nanocomposites. Thus, in this work, commercially available anatase TiO2 nanopowder was LIL-treated in water by infrared millisecond-pulsed laser to yield activated Ti3+doped TiO2 NPs. Addition of different amounts of aqueous AuCl4- into such LIL-processed titania suspension led to Au@TiO2 materials with different gold loading. Upon annealing at 400oC for 2 h, the materials were characterized and, being drop-cast onto interdigitated electrodes, tested as gas sensors working at room temperature. It was found that, depending on the gold content, such materials demonstrated high selectivity towards different analytes, showing promise as sensors towards ammonia, acetaldehyde or benzene.

2. Experimental section 2.1 Decoration on TiO2 nanoparticles with gold Titania nanopowder used in this work was purchased from Wako Chemical, Japan. Sodium tetrachloroaurate (III) dihydrate, NaAuCl4˙2H2O (purity 95 %), from Wako was used without further purification, from which an aqueous solution of NaAuCl4 with concentration 0.01 M was prepared. Suspensions of TiO2 with concentration 0.1 mg/mL were prepared in deionized water and sonicated prior to use. The suspension (15 mL in volume) was placed into a quartz cuvette, with size dimensions 30 x 30 x 50 mm3 and wall thickness of 2 mm, and irradiated by unfocused beam of a millisecond pulsed Nd:YAG laser, while being rigorously agitated by magnetic stirrer. The laser parameters were: wavelength 1064 nm, pulse peak power 5.0 kW, pulse width 1.0 ms (which resulted in pulse energy equal to 5.0 J/pulse), and repetition rate 5 Hz. Page | 4

Sample A was non-decorated titania laser-irradiated for 30 min. Samples B and C were LILtreated following the same procedure, after which they were decorated with Au NPs. More specifically, at first 15 mL of TiO2 suspension was irradiated for 30 min, then 8 or 40 μL of 0.01 M NaAuCl 4 solution were added to samples B and C, respectively, followed by irradiation for another 15 min. The samples briefly described in Table 1. The amounts of Au added to the suspensions were calculated to be 1 and 5 wt% with respect to TiO2 NPs in samples B and C, respectively (see Table 1). The suspension was centrifuged, supernatant was removed and NPs were washed with water twice by consecutive sonication and centrifugation. Then NPs were dispersed in water again and dropcast on copper grid, on Si wafer and on the interdigitated electrode for X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), X-ray diffraction (XRD) and gas sensing test, respectively. Additionally, samples for gas sensing and XPS were annealed at 400 oC for 2h. To evaluate particle mean diameters and size distribution, SEM micrographs of the samples were processed by means of the Image J software, for which at least 100 NPs were analyzed for each sample.

2.2 Characterization The as-prepared nanomaterials were characterized by XRD, SEM, and XPS. The XRD patterns were measured by diffractometer D8 Discover from Bruker. The SEM micrographs were taken on a Hitachi S-4800 FE-SEM tool. The XPS measurements were performed on X-ray photoelectron spectrometer Quantum 2000, ULVAC-PHI. Binding energies of all peaks were corrected using C1s signal at 284.8 eV.

2.3 Gas sensing measurements Gas sensing measurements were carried out using high resistance electrometer (Keithley 6517B) integrated with the customized gas sensing chamber [51,52]. Gold contacts were established on the gas sensing elements with interdigitated electrode pattern as reported in our previous works [52,53]. Static liquid-gas distribution method was used for determining the concentration of test gas (Eq.1) [52,54]:

𝐶𝑝𝑝𝑚 =

𝛿 × 𝑉𝑟 × 𝑅 × 𝑇 𝑀 × 𝑃𝑏 × 𝑉𝑏

(1)

where C is the concentration of test vapor (ppm), δ is the density of test vapor (g/mL), Vr is the volume of injected vapour (µL), R is the universal gas constant (8.3145 J/mol K), T is the absolute

Page | 5

temperature (K), M is the molecular weight, Pb is the chamber pressure (atm) and Vb is the volume of the chamber (L). The response (S) of the sensors [55] was calculated using Eq.2:

𝑆=

𝑅𝑎 𝑅𝑔

𝑅𝑎 ≫ 𝑅𝑔

(2)

where, Ra is the baseline resistance and Rg is the gas resistance.

3. Results and discussion

3.1. Morphology and particle size Figure 1 shows morphology and particle size distribution as evaluated by FE-SEM for the assupplied (non-irradiated) TiO2 material (a,c) and sample A (b,d). It is clearly seen that before irradiation the particles have irregular shapes, with dimensions in a wide range from 50 to 320 nm. The applied laser energy causes NP reshaping, smaller sizes and narrower size distribution, as wellseen in panels (b,d). The particles turned to more spherical ones and the size decreased being between 50 to 180 nm and with average diameter ~106 nm. After irradiation, bigger NPs with sizes 200-300 nm are not observed in Fig.1d, implying their fragmentation via melting/vaporization caused by laser photons. It was previously reported that laser fluence needed to melt anatase TiO2 NPs depends on their size, so that lower energy is required for particles with larger diameter [46]. A heating-melting-evaporation mechanism was proposed to be responsible for reshaping of anatase TiO2 NPs LIL-treated with nanosecond pulsed laser with wavelength of 355 nm [46,48]. Although much longer pulses and longer wavelength were applied in the present study, the results presented in Fig.1 are similar. It is believed that the NPs with diameters 200-300 nm absorbed photons more efficiently, which caused melting of the surface and vaporization, thus resulting in rounding and smoothening of their shapes, decrease in size and formation of surface defects (oxygen vacancies and Ti3+ centers, as discussed below). Additionally, surface etching could also take place due to redox and acid-based reactions between Ti4+ and O2- ions, formed within upper layers, and laser-induced water species such as H• and OH• radicals, H2O2, H+, OH- (see Section 3.4), which would reduce the NP size. Figures 2a,b display SEM images of the Au-decorated samples B and C and reveal the deposition of monodispersed Au NPs onto their surface. As expected, larger number of Au nanoparticles are observed for sample C in comparison with sample B (Fig.2a,b) as more Aucontaining salt was used for its fabrication. Size distribution diagrams for B and C are shown on Fig.2c and Fig.2d, respectively. Gold NPs are within a narrow range of 8-19 nm for sample B and 617 nm for sample C. The average size of gold NPs that decorated samples B and C was found to be 14.4±2.7 nm and 10.8±2.4 nm, respectively. In both Au-decorated TiO2 materials, the size of TiO2 is Page | 6

in the range 50-170 nm (with mean diameters 94 and 92 nm for samples B and C, respectively), which coincides with non-decorated sample A shown in Fig.1d.

3.2. Crystal structure and phase composition XRD patterns of produced nanomaterials TiO2 and Au@TiO2 (samples A – C) are shown in Fig. 3. All peaks are indexed to the anatase phase of TiO2 according to the standard card (JCPDS no. 21-1272). The sharp XRD peaks show good crystallinity of the TiO2 nanoparticles. No traces of rutile phase were observed, indicating that no phase transformation takes place during irradiation with 1064 nm laser beam. In contrast, irradiation with second or third harmonic lasers, which ensure much higher energy provided size, morphology and phase changes of TiO2 by pulse laser induced melting of anatase in liquid and crystallisation to rutile in different shapes depending on laser fluence, time irradiation and so on [46-48]. Additionally, no peaks representative of Au NPs was observed in Fig.3 due to a low gold content in the samples. Taking into account the fact that the most intensive peak of cubic Au phase at 38.10° (JCPDS no. 002-1095) would overlap with titania peaks, this result was expected.

3.3. Surface analysis The chemical composition and oxidation states of elements were analyzed by using XPS. The Ti2p, O1s and Au4f XPS spectra are given in Figs.4 and 5. In order to clarify the changes induced by laser irradiation, we compared irradiated samples A-C with their precursor, i.e. non-radiated commercial TiO2. Figures 4a-d show the Ti2p3/2 and Ti2p1/2 peaks of non-irradiated TiO2 (Fig.4a) and laser-irradiated samples A-C (Figs.4b-d). The signals at 458.8 eV and 464.5 eV (Fig.4a) correspond to the Ti2p3/2 and Ti2p1/2 levels of Ti4+ in TiO2 [40,44,56-58]. The spin orbital splitting between the Ti 2p peaks is 5.7 eV, confirming the anatase form of TiO2 [18]. The peak of non-decorated but laserirradiated sample A is seen in Fig.4b to be wider and shifted to smaller binding energies, that is why it was deconvoluted into two components. The stronger-intensity set, having Ti2p3/2 and Ti2p1/2 peaks at 458.2 eV and 463.8 eV, respectively was assigned to Ti3+ions [57]. This observation of Ti4+ to Ti3+ reduction is well-consistent with previous findings of others reported for rutile TiO2 suspension laserirradiated with 355-nm beam. [57]. The less intense peak set at 458.8 and 464.5 eV corresponds to Ti4+ ions in TiO2 [40,44,56-58]. The spectrum in Fig.4b implies that upon irradiation the Ti4+ to Ti3+ reduction occurs on NP surface, while the subsurface layers and bulk material remained intact TiO 2. Both constituent Ti2p3/2 peaks in Fig.4b were found to have the same FWHM (1.1 eV) and their area ratio allowed us to estimate the ratio of Ti3+ to Ti4+ species in the surface layer to be 83:17. Page | 7

The O 1s spectrum of non-irradiated TiO2 (Fig. 4e) consists of a main component located at 530.1 eV that corresponds to the lattice O2- in TiO2 (noted as O1) and a minor peak at 531.5 eV (noted as O2) attributed to surface oxygen vacancies [18,58]. The O 1s XPS spectrum of sample A was deconvoluted into 4 components (Fig. 4f). The peaks centered at 529.5 eV (O3) and 530.1 eV (O1) were assigned to oxide ions in Ti3+-O and Ti4+-O bonds, respectively. The component located at 531.5 eV and arising from oxygen vacancies (O2) is seen to be more pronounced after irradiation (compare O2 in Fig.4e and Fig.4f). As the titania NPs were irradiated by laser photons, the density of surface oxygen vacancies increased and the surface Ti4+ species were reduced to Ti3+ ions [20, 59]. The appearance of a new peak belonging to surface OH- groups (O4 in Fig.4f) was foreseen at 532.4 eV as surface etching and other reactions with water medium were expected under irradiation [38,39,44] (see section 3.4). The Ti 2p XPS spectra of both samples B and C (Figs.4c,d) are very similar, demonstrating Ti2p3/2 and Ti2p1/2 peaks at ~458.9 eV and ~464.5 eV, respectively. Importantly, only Ti4+ surface species are manifested in Figs.4c,d, which implies that during Au(III) to Au(0) reduction, the surface Ti3+ species were oxidized to Ti4+ state. In the O1s spectra of samples B and C displayed in Figs. 4g,h, components located at ~530.1 eV (O1), ~531.5 eV (O2) and 532.4 eV (O4) were found, while no component at ~529.5 eV was observed any more. Thus, the O1s spectra of samples B and C confirmed that after decoration with Au NPs, the surface of TiO2 NPs was somewhat more hydrated in comparison with the precursor material but was free of Ti3+ species. The formation of Au NPs was further confirmed by the Au 4f XPS spectra of samples B and C shown in Fig.5. The peaks at 83.3 eV and 87.0 eV account for Au 4f7/2 and Au 4f5/2, respectively, indicating gold in metallic state, Au(0). Our results are in agreement with the previously reported values for Au/TiO2 catalysts, where the binding energy of nano-sized Au NPs deposited onto TiO2 was typically lower than that of pure bulk gold [58]. This shift towards lower binding energies was explained via electron transfer from the support to metallic NPs, thus showing a crucial role of substrate in the gold-substrate interaction [60]. Theoretical calculations revealed that in case of TiO2, the electron transfer from Ti3+ to Au NPs occurs, and therefore the initial state of the support (availability of Ti3+) is important for effective and strong adsorption of Au NPs on oxygen vacancies [58,60]. Based on the SEM, XRD and XPS results presented above, the following conclusions can be drown: (i) upon laser irradiation of TiO2 suspension, the surface of the latter was covered by a large amount of Ti3+ ions; (ii) laser irradiation also induced surface oxygen vacancies and OH- groups on the anatase NP surface; (iii) upon irradiation, the titania NPs were activated and prepared for the reduction of Au(III) and formation of Au clusters on their surface (see section 3.4. below); (iv) after adding Au-containing salt, the laser-processed titania NPs were defect-rich and decorated with Au Page | 8

NPs; (v) the density of Au NPs decorating TiO2 NPs could be controlled by changing the amount of NaAuCl4 added into the system. It is also worth mentioning, as this is directly relevant for gas sensors, that the XPS spectra of both as-prepared and annealed TiO2 and Au@TiO2 nanomaterials did not show significant changes after heat-treatment in air at 400 oC for 2 h.

3.4. Formation of Au@TiO2 nanostructures As previously mentioned in section 2.1, the as-supplied TiO2 NPs were first activated via LIL, after which aqueous NaAuCl4 was added for their decoration. During LIL, irradiated NPs were previously reported to be subjected to intense photon bombardment [20,25-27,29,30,35]. With its band gap of around 3.2 eV, stoichiometric TiO2 is known to absorb mainly UV light, while its absorption in the near-infrared region is very low [61]. However, surface defects are able to create energy levels in the band gap and act as absorption centers for photons with larger wavelength. Upon absorption, such photons can elevate the temperature of the material and result in melting or vaporization of TiO2 NPs. This is expected to lead to local surface melting and/or vaporization, etching and modification of titania NPs, which was observed in Fig.1. Thus, even though we processed titania with an IR laser, because of their defect sites, corners and edges, the irradiated titania NPs were gradually melted, vaporized and modified, resulting in their gradual resizing and enrichment in defects [62]. In parallel, along with gradual surface modification, laser energy could also cause decomposition of H2O molecules, forming ions, radicals and other molecules (see Eq.3 below) [63]. The latter species, being strongly reactive, could initiate radical chain reactions and readily interact with other water molecules or O2- and Ti4+ ions from an already activated/melted surface layer of TiO2 NPs and modify their surface. For example, strongly oxidizing and reducing species such as OH•, H• radicals and H2O2 could generate oxygen vacancies on the NP surface by gaining electrons and binding oxygen atoms, while released electrons could reduce Ti4+ to Ti3+ species (see Eqs.4-7).

H2O ⇝ OH-, H3O+, H•, OH•, H2, H2O2

(3)

O2- ̵ 2e- → O  Ovac

(4)

O + H• → OH•

(5)

OH• + e- → OH-

(6)

Ti4+ + e- → Ti3+

(7) Page | 9

Note that the presence of hydroxyl group OH- and both Ti3+ and Ovac species was confirmed by Ti 2p and O 1s XPS spectra presented in Figs.4b,f. Such oxygen vacancies are known to stabilize metallic NPs supported on metal oxide, acting as suitable adsorption sites and ensuring electron transfer from the oxide substrate to metallic NPs [64,65]. For example, Gong et al. demonstrated, both experimentally and through computations, that oxygen vacancies generated by electron irradiation of TiO2 significantly affected the growth and location of forming Au clusters, keeping them small and situated mainly on the NP wall [66]. Importantly, in the Ti 2p XPS spectra of samples B and C (Figs.4c,d), only peaks of Ti 4+ are observed, and in the O 1s XPS spectra (Figs.4g,h) the peak responsible for oxygen vacancies (component O2) is much lower than in irradiated TiO2 (Fig.4f). This implies that during decoration with Au NPs, aqueous Au(III) species were most likely reduced by surface Ti 3+ species with some involvement of oxygen vacancies and other laser-induced water species. Previously, Belloni and co-authors investigated the kinetics of Au(III) reduction into gold clusters under gamma and electron-pulse radiolysis in water and organic media and reported on several successive one-electron steps including reduction of Au3+ to Au2+, formation of dimer (Au2+)2 and its disproportionation into Au3+ and Au+, comproportionation of Au0 with Au3+, and reduction of stable Au+ ions by alcohol radicals [67]. The very negative potential of the Au+/Au0 couple is known to result in a high thermodynamic barrier for reduction of Au+ to Au0, which is a crucial step in gold cluster formation [67]. However, once radical-initiated reduction of initial metal ion (e.g., AuCl4-) begins, it is then followed by fast coalescence of Au atoms and cluster growth. It is also important that the redox potential of charged metallic clusters (Mn+/Mn) increases during their nucleation and, as a result, the reduction of metal ion attached to the cluster occurs easier than of an isolated ion, thus accounting for cluster growth [67]. In the complex redox system formed in our experiments upon adding AuCl4- species to laserirradiated TiO2, metallic gold atoms were believed to form initially by stepwise laser-induced reduction, after which they aggregated and grew very fast to gold clusters, Au n (Eqs.8-9). In parallel, the bonding between clusters and metal ions was also possible, thus forming charged cluster species Aun+1+ (Eq.10) which were likely to deposit onto the TiO2 surface where they were further reduced by Ti3+ species when the electrode potential of charged gold clusters, Aun+1+/Aun+1, became higher than that of the Ti4+/Ti3+ couple (Eqs.11-12). This process would continue until complete consumption of either AuCl4- or surface Ti3+ ions. Therefore, the size of Au NPs forming on TiO2 support must depend on the ratio between the concentration of Ti3+ ions and Au(III) species added into the system. When the concentration of the former Ti3+ ions is higher, the redox process is faster and slightly larger Au NPs are developed, as seen in Fig.2c (where the average diameter of Au NPs in sample B is 14.4

Page | 10

nm). Correspondingly, at relatively higher concentrations of Au(III) smaller, but more abundant, Au NPs were observed for sample C (with a mean size of 10.8 nm, Fig.2d).

Au3+ + e- → Au2+ + e- → Au+ + e- → Au0

(8)

n Au0 → Aun

(9)

Aun + Au+→ Aun+1+

(10)

Aun+1+ + e- → Aun+1

(11)

Ti3+ - e- → Ti4+

(12)

Considering the above, the different gas-sensing selectivity of samples A, B and C (see section 3.5 below) can be explained by their different Au content as one of key parameters. In case of sample A, the material was not decorated, and therefore LIL-induced Ti3+ defects remained on the surface (as well manifested in Fig.4b). In contrast, samples B and C show no presence of Ti 3+ as it was oxidized to Ti4+ during Au NP formation (see XPS spectra in Figs.4c,d). At the same time, addition of different amounts of NaAuCl4 not only resulted in different surface density and sizes of Au NPs decorating TiO2, but also could lead to Au NPs with different charges. More specifically, sample C is believed to have more non-reduced charged Au clusters remaining on the surface, as higher amount of Aucontaining precursor was added into the system during its preparation. As a result, sample C was found to be selective towards more electron-rich molecules such as benzene (see section 3.5 below). It is believed that its positively charged gold clusters could interact with electron-delocalized benzene rings, and thus its gold NPs could act as adsorption sites for C6H6 molecules, resulting in a higher selectivity towards benzene.

3.5 Gas-sensing tests at room temperature Sensing response of samples as chemiresistive sensors towards 200 ppm of various vapors was tested at room temperature (29 oC) and a relative humidity of 72 %. The non-decorated TiO2 (sample A) was found to be highly selective towards ammonia with a maximum sensing response of 64 (black bars in Fig.6). Since sample A was free of Au NPs, only direct oxidation of target gas molecules on its TiO2 surface by chemisorbed oxygen species could be expected, as given below in Eqs.13,14. In the ambient environment, chemisorption of oxygen on the surface of sensing semiconductor results in an increased space charge width as oxygen molecules consume electrons from the conduction band of the sensing nanomaterial, as expressed by Eq.13. As a result, the electrical resistance of TiO2 increases and stabilizes, which is observed as a baseline air resistance Page | 11

(Ra). When available in the atmosphere, the molecules of reducing target gas interact with such chemisorbed oxygen species, thereby reducing the space charge width, which leads to a decrease in resistance as a result of oxidation-reduction reactions, shown below by Eqs.14-16. The resultant resistance is observed as Rg. When the target gas is removed from the sensor surface, the latter surface attains its initial baseline resistance Ra.

O2 + 𝑒 − → O− 2(ads ) 4𝑁𝐻3 + 3𝑂2−

𝑠𝑎𝑚𝑝𝑙𝑒 𝐴

2𝐶𝐻3 𝐶𝐻𝑂 + 5𝑂2− 2𝐶6 𝐻6 + 15𝑂2−

(13) 2𝑁2 + 6𝐻2 𝑂 + 3𝑒 −

𝑠𝑎𝑚𝑝𝑙𝑒 𝐵

𝑠𝑎𝑚𝑝𝑙𝑒 𝐶

(14)

4𝐶𝑂2 + 4𝐻2 𝑂 + 5𝑒 −

(15)

12𝐶𝑂2 + 6𝐻2 𝑂 + 15𝑒 −

(16)

Sample A was found to have LIL-induced surface Ti3+ ions and oxygen vacancies, which was expected to enhance both adsorption of oxygen molecules and target-gas molecule - sensor interactions. Additionally, due to surface oxygen defects, titanium ions were coordinatively unsaturated, which is why they might interact with the lone electron pair of nitrogen atoms (from ammonia molecules) via the donor-acceptor mechanism, thus contributing to the selectivity of sample A towards ammonia [68]. Previously, high selectivity of titania quantum dots towards ammonia at room temperature was reported by others [68]. At the same time, sample B modified with Au NPs showed a maximum response of 115 towards acetaldehyde (red bars in Fig.6). This could be attributed both to the absence of surface Ti3+ species and modification with highly-conductive Au NPs. In this case, the enhanced gas-solid interaction is supported through the Au NPs, instead of direct oxidation of target gas on TiO2 surface. The decorating Au NPs may act as better adsorption sites for target gas, increasing the catalytic oxidation of acetaldehyde on the Au@TiO2 surface and determining the selective sensing of this gas (Eq.15). Interestingly, Pt-decorated TiO2 was also found by Bastakoti and co-workers to show a high sensitivity to acetaldehyde at room temperature [14]. Compared with sample B, sample C has a larger density of Au NPs on its surface (Fig.2). As mentioned in section 3.4, some of such Au NPs could be positively charged Au clusters, which is believed to be a key reason for shifting the selectivity of sample C towards electron-rich benzene molecules (blue bars in Fig.6). Such positively-charged metal clusters could act as electron acceptors for adsorbed C6H6 molecules. The electron transfer from benzene molecules to gold clusters can destabilize the π-electron system of benzene and facilitate its oxidation (Eq.16). Thus, the highly Page | 12

conductive Au NPs can act as a bridge for the release of electrons towards the conductive band of TiO2, enhancing its conductivity and decreasing resistance. Instead of direct oxidation of benzene on the TiO2 surface, gold-mediated benzene oxidation occurs and contributes to the resistance changes registered in Figs.8e,f [69]. When benzene concentration in air drops down, its molecules get oxidized or desorb from the surface of sample C, resulting in a rapid decrease in carrier concentration and recover of the initial baseline resistance of the sensing device (Fig.8e).

3.7. Response trend and transient response Response trends of all the sensors were studied as a function of varying concentration, as shown in Fig.7. The sensors’ response is seen to increase from lower to higher concentration of the corresponding target gases. After a certain concentration, a stable response was noted, which could be due to insufficient oxygen anions absorbed on the sensor surface. As, according to Eq.16, oxidation of C6H6 required much more surface O2- species, the response of sample C to benzene reached its plateau at smaller gas concentrations. The lowest detection limit (LOD) of 5 ppm (towards ammonia) was observed for sample A which was LIL-treated and non-decorated TiO2. At the same time, LOD values were 40 and 50 ppm towards acetaldehyde and benzene for sample B and sample C, respectively (Fig.8). The response and recovery times of sample A were observed to be 28 and 24 s, respectively, towards 200 ppm of ammonia at room temperature. The response and recovery times of sample B were observed to be 59 and 78 s, respectively, towards 200 ppm of acetaldehyde. And finally, sample C showed 39 and 26 s as response and recovery times towards 200 ppm of benzene at room temperature.

3.8. Impact of relative humidity and stability The influence of relative humidity (%RH) on the gas sensing response was also studied, for which the level of humidity was varied by placing a corresponding saturated solution into the chamber [70,71]. An Arduino interfaced Digital Humidity Temperature sensor (DHT 11) integrated into the chamber was used to measure %RH. Figure 9a shows the variation in air resistance as a function of RH for all the samples. The baseline resistance for sample A is seen to decrease slightly with increase in %RH, while those for samples B and C first decrease and then increase when the percentage of RH increases from 32% to 72 % (Figs.9a,b). Table 2 shows the relative change (in %) of sensing response as a function of RH from the original sensing response observed at 72% RH. The response was observed to decrease as the %RH Page | 13

increased. This is believed to be due to a hindering effect of OH- ions on the oxygen ions chemisorbed on the surface. The hopping-charge transport of chemisorbed hydroxyl ions and Grottuss water physisorption are known to be the primary reasons behind the reduction of sensing response at lower and higher humidity levels, respectively [70,72-74]. Stability of samples A-C was tested over the period of 90 days with a time interval of 10 days (see Fig.10). All the sensors are seen to be stable towards 200 ppm of their corresponding target gases, well retaining their initial response values over time. Table 3 compares the sensing characteristics of the laser-generated nanomaterials developed in this study with other nanomaterials decorated with gold (or silver) NPs previously reported in the literature. It is seen that the laser-produced TiO2-based nanomaterials demonstrate good detection parameters and compare favorably with their competitors [75-82]. In addition, the present work demonstrates tunable selectivity which is achieved through varied concentration of Au NPs decorating TiO2 surface. Thus, such sensors based on Au@TiO2 nanomaterials produced by laser in liquid phase may be of interest for various applications such as safety control and environmental quality control.

4. Conclusions A series of nanomaterials was prepared via irradiation of TiO2 nanopowder by millisecond pulsed laser in water followed by decoration with Au nanoparticles. It was found that laser irradiation led to the formation of surface Ti3+ species and oxygen vacancies, which appeared to be further involved in nucleation and growth of Au nanoparticles that decorated titania support. Hybrid Au@TiO2 nanomaterials with different decoration degree were obtained by varying the amount of Au(III) species added into the system. Upon systematic characterization, three laser-irradiated TiO2 nanomaterials with varied level of gold decoration were tested as gas sensing devices at room temperature. Depending on decoration degree, selectivity towards different gases was demonstrated. More specifically, ammonia, acetaldehyde and benzene were selectively detected by non-decorated TiO2, titania with lower and higher degrees of gold decoration, respectively. Thus, laser-irradiated semiconductor nanomaterials are shown to be efficient gas sensors working at room temperature, while their sensitizing with gold nanoparticles is yet another approach not only to enhance their sensing response, but also to tune selectivity.

Page | 14

Acknowledgements: N.M. thanks Tokai University for the exchange research grant. P.S. and J.B.B.R. thank the Department of Science & Technology, New Delhi (grant no. SR/FST/ET-II/2018/221), SASTRA University, and the Council of Scientific and Industrial Research (grant no. HRDG- 09/1095/0016/2016EMR-I) for supporting gas-sensing experiments. S.A.K. acknowledges the support from the Amada Foundation (grant no. AF-2019225-B3). A.K. expresses gratitude to the Ministry of Science and Higher Education of the Russian Federation (grant no. МК-3258.2019.8). Laser-related experiments were supported by Russian Science Foundation (grant no. 19-79-00214).

5. References: [1] M.S. Kanan, M.O. El-Kadri, A.I. Abu-Yousef, C.M. Kanan, Semiconducting metal oxide based sensors for selective gas pollutant detection, Sensors 9 (2009) 8158–8196. doi:10.3390/s91008158. [2] Y. Wang, T. Wu, Y. Zhou, C. Meng, W. Zhu, L. Liu, TiO2-based nanoheterostructures for promoting gas sensitivity performance: Designs, developments, and prospects, Sensors 17 (2017) 1971. doi: 10.3390/s17091971. [3] J. Bai, B. Zhou, Titanium dioxide nanomaterials for sensor applications, Chem. Rev. 114 (2014) 10131– 10176. doi:10.1021/cr400625j. [4] S.T. Navale, Z.B. Yang, C. Liu, P.J. Cao, V.B. Patil, N.S. Ramgir, R.S. Mane, F.J. Stadler, Enhanced acetone sensing properties of titanium dioxide nanoparticles with a sub-ppm detection limit, Sens. Actuators B 255 (2018) 1701–1710. doi: 10.1016/j.snb.2017.08.186. [5] N. Chen, Y. Li, D. Deng, X. Liu, X. Xing, X. Xiao, Y. Wang, Acetone sensing performances based on nanoporous TiO2 synthesized by a facile hydrothermal method, Sens. Actuators B 238 (2017) 491–500. doi: 10.1016/j.snb.2016.07.094. [6] Y. Yang, J.X. Hu, Y. Liang, J.P. Zou, K. Xu, R.J. Hu, Z.D. Zou, Q. Yuan, Q.Q. Chen, Y. Lu, T. Yu, C.L. Yuan, Anatase TiO2 hierarchical microspheres consisting of truncated nanothorns and their structurally enhanced gas sensing performance, J. Alloy. Compd. 694 (2017) 292–299. doi: 10.1016/j.jallcom.2016.09.328. [7] Y. Yang, Y. Liang, G. Wang, L. Liu, C. Yuan, T. Yu, Q. Li, F. Zeng, G. Gu, Enhanced gas-sensing properties of the hierarchical TiO2 hollow microspheres with exposed high-energy {001} crystal facets, ACS Appl. Mater. Interfaces 7 (2015) 24902–24908. doi:10.1021/acsami.5b08372. [8] E. Sennik, N. Kilinc, Z.Z. Ozturk, Electrical and VOC sensing properties of anatase and rutile TiO 2 nanotubes, J. Alloy. Compd. 616 (2014) 89–96. doi: 10.1016/j.jallcom.2014.07.097. [9] E. Tolmachoff, S. Memarzadeh, H. Wang, Nanoporous titania gas sensing films prepared in a premixed stagnation flame, J. Phys. Chem. C 115 (2011) 21620–21628. doi: 10.1021/jp206061h. [10] B. Lyson-Sypien, M. Radecka, M. Rekas, K. Świerczek, K. Michalow, T. Graule, K. Zakrzewska, Grainsize-dependent gas-sensing properties of TiO2 nanomaterials, Sens. Actuators B 211 (2015) 67-76. doi:10.1016/j.snb.2015.01.050. [11] H. Milani Moghaddam, S. Nasirian, Hydrogen gas sensing feature of polyaniline/titania (rutile) nanocomposite at environmental conditions, Appl. Surf. Sci. 317 (2014) 117–124. doi: 10.1016/j.apsusc.2014.08.062. [12] H. Yang, X.L. Cheng, X.F. Zhang, Z. Zheng, X. Tang, Y.M. Xu, S. Gao, H. Zhao, L.H. Huo, A novel sensor for fast detection of triethylamine based on rutile TiO 2 nanorod arrays, Sens. Actuators B 205 (2014) 322–328. doi: 10.1016/j.snb.2014.08.092. [13] X. Zhang, J. Zhang, Y. Jia, P. Xiao, J. Tang, TiO2 nanotube array sensor for detecting the SF6 decomposition product SO2, Sensors 12 (2012) 3302–3313. doi:10.3390/s120303302.

Page | 15

[14] B.P. Bastakoti, N.L. Torad, Y. Yamauchi, Polymeric micelle assembly for the direct synthesis of platinumdecorated mesoporous TiO2 toward highly selective sensing of acetaldehyde, ACS Appl. Mater. Interfaces 6 (2014) 854–860. doi:10.1021/am4039954. [15] D. Buso, M. Post, C. Cantalini, P. Mulvaney, A. Martucci, Gold nanoparticle-doped TiO2 semiconductor thin films: Gas sensing properties, Adv. Funct. Mater. 18 (2008) 3843–3849. doi:10.1002/adfm.200800864. [16] A. Abbasi, J.J. Sardroodi, A novel nitrogen dioxide gas sensor based on TiO 2-supported Au nanoparticles: a van der Waals corrected DFT study, J. Nanostructure Chem. 7 (2017) 121–132. doi:10.1007/s40097-0170226-5. [17] W. Chomkitichai, N. Tamaekong, C. Liewhiran, A. Wisitsoraat, S. Sriwichai, S. Phanichphant, H 2 sensor based on Au/TiO2 nanoparticles synthesized by flame spray pyrolysis, NanoEng. J. 16 (2012) 135-142. doi:10.4186/ej.2012.16.3.135. [18] Z.P. Tshabalala, K. Shingange, B.P. Dhonge, O.M. Ntwaeaborwa, G.H. Mhlongo, D.E. Motaung, Fabrication of ultra-high sensitive and selective CH4 room temperature gas sensing of TiO2 nanorods: Detailed study on the annealing temperature, Sens. Actuators B 238 (2017) 402–419. doi: 10.1016/j.snb.2016.07.023. [19] S.C. Singh, R.K. Swarnkar, R. Gopal, Synthesis of titanium dioxide nanomaterial by pulsed laser ablation in water, J. Nanosci. Nanotechnol. 9 (2009) 5367-5371. doi: 10.1166/jnn.2009.1114 [20] D. Zhang, B. Gökce, S. Barcikowski, Laser synthesis and processing of colloids: Fundamentals and applications. Chem. Rev. 117 (2017) 3990-4103. doi: 10.1021/acs.chemrev.6b00468 [21] S.C. Singh, R.K. Kotnala, R. Gopal, Room temperature ferromagnetism in liquid-phase pulsed laser ablation synthesized nanoparticles of nonmagnetic oxides, J. Appl. Phys. 118 (2015) 064305. doi: 10.1063/1.4928312 [22] T. Kondo, Y. Sato, M. Kinoshita, P. Shankar, N.N. Mintcheva, M. Honda, S. Iwamori, S.A. Kulinich, Room temperature ethanol sensor based on ZnO prepared via laser ablation in water, Jpn. J. Appl. Phys. 56 (2017) 080304. doi:10.7567/jjap.56.080304. [23] N. Mintcheva, A.A. Aljulaih, S. Bito, M. Honda, T. Kondo, S. Iwamori, S.A. Kulinich, Nanomaterials produced by laser beam ablating Sn-Zn alloy in water, J. Alloy. Compd. 747 (2018) 166–175. doi: 10.1016/j.jallcom.2018.02.350. [24] M. Honda, T. Kondo, T. Owashi, P. Shankar, S. Iwamori, Y. Ichikawa, S.A. Kulinich, Nanostructures prepared via laser ablation of tin in water, New J. Chem. 41 (2017) 11308–11316. doi:10.1039/C7NJ01634D. [25] H. Zeng, X.W. Du, S.C. Singh, S.A. Kulinich, S. Yang, J. He, W. Cai, Nanomaterials via laser ablation/ irradiation in liquid: A review, Adv. Funct. Mater. 22 (2012) 1333–1353. doi:10.1002/adfm.201102295. [26] K.Y. Niu, J. Yang, S.A. Kulinich, J. Sun, X.W. Du, Hollow nanoparticles of metal oxides and sulfides: Fast preparation via laser ablation in liquid, Langmuir 26 (2010) 16652–16657. doi:10.1021/la1033146. [27] K.Y. Niu, J. Yang, S.A. Kulinich, J. Sun, H. Li, X.W. Du, Morphology control of nanostructures via surface reaction of metal nanodroplets, J. Am. Chem. Soc. 132 (2010) 9814–9819. doi:10.1021/ja102967a. [28] S.A. Kulinich, T. Kondo, Y. Shimizu, T. Ito, Pressure effect on ZnO nanoparticles prepared via laser ablation in water, J. Appl. Phys. 113 (2013) 033509. doi: 10.1063/1.4775733. [29] D. Zhang, J. Liu, C. Liang, Perspective on how laser-ablated particles grow in liquids, Sci. Chn. Phys. Mech. Astronom. 60 (2017) 074201. doi: 10.1007/s11433-017-9035-8. [30] J. Zhang, M. Chaker, D. Ma, Pulsed laser ablation based synthesis of colloidal metal nanoparticles for catalytic applications. J. Colloid Interface Sci. 489 (2017)138-149. doi: 10.1016/j.jcis.2016.07.050 [31] C.Y. Shih, M.V. Shugaev, C.P. Wu, L.V. Zhigilei, Generation of subsurface voids, incubation effect, and formation of nanoparticles in short pulse laser interactions with bulk metal targets in liquid: Molecular dynamics study. J. Phys. Chem. C 121 (2017) 16549-16567. doi: 10.1021/acs.jpcc.7b02301 [32] H.B. Wang, J.Q. Wang, N. Mintcheva, M. Wang, S. Li, J. Mao, H. Liu, C.K. Dong, S.A. Kulinich, X.W. Du, Laser synthesis of iridium nanospheres for overall water splitting. Materials 12 (2019) 3028. doi: 10.3390/ma12183028

Page | 16

[33] D. Zhang, J. Liu, P. Li, Z. Tian, C. Liang, Recent advances in surfactant-free, surface charged and defectrich catalysts developed by laser ablation and processing in liquids, ChemNanoMat 3 (2017) 512-533. doi:10.1002/cnma.201700079. [34] E.A. Gavrilenko, D.A. Goncharova, I.N. Lapin, A.L. Nemoykina, V.A. Svetlichnyi, A.A. Aljulaih, N. Mintcheva, S.A. Kulinich, Comparative study of physicochemical and antibacterial properties of ZnO nanoparticles prepared by laser ablation of Zn target in water and air. Materials 12 (2019) 186. doi: 10.3390/ma12010186 [35] S.C. Singh, S.K. Mishra, R.K. Srivastava, R. Gopal, Optical properties of selenium quantum dots produced with laser irradiation of water suspended Se nanoparticles, J. Phys. Chem. C 114 (2010) 17374-17384. doi: 10.1021/jp105037w [36] N. Mintcheva, A.A. Aljulaih, W. Wunderlich, S.A. Kulinich, S. Iwamori, Laser-ablated ZnO nanoparticles and their photocatalytic activity towards organic pollutants. Materials 11 (2018) 1127. doi: 10.3390/ma11071127 [37] M. Amin, J. Tomko, J.J. Naddeo, R. Jimenez, D.M. Bubb, M. Steiner, J. Fitz-Gerald, S.M. O’Malley, Laser-assisted synthesis of ultra-small anatase TiO2 nanoparticles, Appl. Surf. Sci. 348 (2015) 30–37. doi: 10.1016/j.apsusc.2014.12.191. [38] F. Barreca, N. Acacia, E. Barletta, D. Spadaro, G. Curro, F. Neri, Small size TiO 2 nanoparticles prepared by laser ablation in water, Appl. Surf. Sci. 256 (2010) 6408-6412. doi:10.1016/j.apsusc.2010.04.026. [39] C.N. Huang, J.S. Bow, Y. Zheng, S.Y. Chen, N. Ho, P. Shen, Nonstoichiometric titanium oxides via pulsed laser ablation in water, Nanoscale Res. Lett. 5 (2010) 972–985. doi: 10.1007/s11671-010-9591-4. [40] A. De Bonis, A. Galasso, N. Ibris, A. Laurita, A. Santagata, R. Teghil, Rutile microtubes assembly from nanostructures obtained by ultra-short laser ablation of titanium in liquid, Appl. Surf. Sci. 268 (2013) 571– 578. doi: 10.1016/j.apsusc.2013.01.015. [41] M. Boutinguiza, B. Rodriguez-Gonzalez, J. del Val, R. Comesaña, F. Lusquiños, J. Pou, Production of TiO2 crystalline nanoparticles by laser ablation in ethanol, Appl. Surf. Sci. 258 (2012) 9484–9486. doi: 10.1016/j.apsusc.2012.05.148. [42] T. Tsuji, M. Nakanishi, T. Mizuki, M. Tsuji, T. Doi, T. Yahiro, J. Yamaki, Preparation of nano-sized functional materials using laser ablation in liquids, Appl. Surf. Sci. 255 (2009) 9626–9629. doi: 10.1016/j.apsusc.2009.04.075. [43] S.M. Hong, S. Lee, H.J. Jung, Y. Yu, J.H. Shin, K.Y. Kwon, M.Y. Choi, Simple preparation of anatase TiO2 nanoparticles via pulsed laser ablation in liquid, Bull. Korean Chem. Soc. 34 (2013) 279-282. doi:10.5012/bkcs.2013.34.1.279. [44] G. García Guillén, S. Shaji, I. Mendivil, D. Avellaneda, G. Castillo, T. Roy, D. Garcia-Gutierrez, B. Krishnan, Effects of ablation energy and post-irradiation on the structure and properties of titanium dioxide nanomaterials, Appl. Surf. Sci. 405 (2017) 183-194. doi:10.1016/j.apsusc.2017.01.282. [45] A. Nath, S. Laha, A. Khare, Effect of focusing conditions on synthesis of titanium oxide nanoparticles via laser ablation in titanium–water interface, Appl. Surf. Sci. 257 (2011) 3118-3122. doi: 10.1016/j.apsusc.2010.10.126. [46] H. Wang, M. Miyauchi, Y. Ishikawa, A. Pyatenko, N. Koshizaki, Y. Li, L. Li, X. Li, Y. Bando, D. Golberg, Single-crystalline rutile TiO2 hollow spheres: room-temperature synthesis, tailored visible-light-extinction, and effective scattering layer for quantum dot-sensitized solar cells, J. Am. Chem. Soc. 133 (2011) 1910219109. doi: 10.1021/ja2049463. [47] H. Yu, X. Li, Y. Zhu, Z. Hao, X. Zeng, Investigation on the formation mechanism of hollow spheres prepared by pulsed laser selective heating colloidal nanoparticles in solution, J. Phys. Chem. C 121 (2017) 12469–12475. doi:10.1021/acs.jpcc.7b03464. [48] Y. Ishikawa, N. Koshizaki, A. Pyatenko, N. Saitoh, N. Yoshizawa, Y. Shimizu, Nano- and submicrometersized spherical particle fabrication using a submicroscopic droplet formed using selective laser heating, J. Phys. Chem. C. 120 (2016) 2439–2446. doi:10.1021/acs.jpcc.5b10691.

Page | 17

[49] K. Siuzdak, M. Sawczak, M. Klein, G. Nowaczyk, S. Jurga, A. Cenian, Preparation of platinum modified titanium dioxide nanoparticles with the use of laser ablation in water, Phys. Chem. Chem. Phys. 16 (2014) 15199–15206. doi:10.1039/C4CP01923G. [50] A. Hamad, L. Li, Z. Liu, X. Zhong, T. Wang, Picosecond laser generation of Ag–TiO2 nanoparticles with reduced energy gap by ablation in ice water and their antibacterial activities, Appl. Phys. A 119 (2015) 1387-1396. doi: 10.1007/s00339-015-9111-6. [51] P. Shankar, J.B.B. Rayappan, Racetrack effect on the dissimilar sensing response of ZnO thin film—An anisotropy of isotropy, ACS Appl. Mater. Interfaces 8 (2016) 24924–24932. doi:10.1021/acsami.6b05133. [52] P. Srinivasan, J.B.B. Rayappan, Growth of eshelby twisted ZnO nanowires through nanoflakes & nanoflowers: A room temperature ammonia sensor, Sens. Actuators B 277 (2018) 129–143. doi: 10.1016/j.snb.2018.09.003. [53] P. Shankar, J.B.B. Rayappan, Monomer: design of ZnO nanostructures (nanobush and nanowire) and their room-temperature ethanol vapor sensing signatures, ACS Appl. Mater. Interfaces 9 (2017) 38135–38145. doi:10.1021/acsami.7b11561. [54] S. Parthasarathy, V. Nandhini, B.G. Jeyaprakash, Improved sensing response of photo activated ZnO thin film for hydrogen peroxide detection, J. Colloid Interface Sci. 482 (2016) 81–88. doi:10.1016/j.jcis.2016.07.066. [55] Y. Vijayakumar, G.K. Mani, M.V.R. Reddy, J.B.B. Rayappan, Nanostructured flower like V2O5 thin films and its room temperature sensing characteristics, Ceram. Int. 41 (2015) 2221–2227. doi:10.1016/j.ceramint.2014.10.023. [56] M. Hussain, M. Ahmad, A. Nisar, H. Sun, S. Karim, M. Khan, S.D. Khan, M. Iqbal, S.Z. Hussain, Enhanced photocatalytic and electrochemical properties of Au nanoparticles supported TiO 2 microspheres, New J. Chem. 38 (2014) 1424–1432. doi:10.1039/C3NJ01525D. [57] S.S. Pan, W. Lu, Y.H. Zhao, W. Tong, M. Li, L.M. Jin, J.Y. Choi, F. Qi, S.G. Chen, L.F. Fei, S.F. Yu, Self-doped rutile titania with high performance for direct and ultrafast assay of H 2O2, ACS Appl. Mater. Interfaces 5 (2013) 12784–12788. doi:10.1021/am4045162. [58] N. Kruse, S. Chenakin, XPS characterization of Au/TiO2 catalysts: Binding energy assessment and irradiation effects, Appl. Catal. A: Gen. 391 (2011) 367–376. doi: 10.1016/j.apcata.2010.05.039. [59] B. Bharti, S. Kumar, H.-N. Lee, R. Kumar, Formation of oxygen vacancies and Ti 3+ state in TiO2 thin film and enhanced optical properties by air plasma treatment, Sci. Rep. 6 (2016) 32355. doi: 10.1038/srep32355 [60] S. Arrii, F. Morfin, A.J. Renouprez, J.-L. Rousset, Oxidation of CO on gold supported catalysts prepared by laser vaporization: direct evidence of support contribution, J. Am. Chem. Soc. 126 (2004) 1199-1205. doi: 10.1021/ja036352y. [61] H. Yang, W. Liu, C. Xu, D. Fan, Y. Cao, W. Xue. Laser sintering of TiO 2 films for flexible dye-sensitized solar cells. Appl. Sci. 9 (2019) 823. doi:10.3390/app9050823. [62] H. Zeng, S. Yang, W. Cai, Reshaping formation and luminescence evolution of ZnO quantum dots by laserinduced fragmentation in liquid. J. Phys. Chem. C 115 (2011) 5038–5043. doi:10.1021/jp109010c. [63] E. Gachard, H. Remita, J. Khatouri, B. Keita, L. Nadjo, J. Belloni. Radiation-induced and chemical formation of gold clusters. New J. Chem. 22 (1998) 1257-1265. doi: 10.1039/A804445G [64] C. T. Campbell, C. H. F. Peden. Oxygen vacancies and catalysis on ceria surfaces. Science 309 (2005) 713. doi: 10.1126/science.1113955. [65] L. Giordano, J. Goniakowski, G. Pacchioni. Characteristics of Pd adsorption on the MgO(100) surface: Role of oxygen vacancies. Phys. Rev. B 64 (2001) 075417. doi:10.1103/physrevb.64.075417. [66] X.Q. Gong, A. Selloni, O. Dulub, P. Jacobson, U. Diebold. Small Au and Pt clusters at the anatase TiO2(101) surface: Behavior at terraces, steps, and surface oxygen vacancies. J. Am. Chem. Soc. 130 (2008) 370-381. doi: 10.1021/ja0773148. [67] J. Belloni, J.L. Marignier, M. Mostafavi. Mechanism of metal nanoparticles nucleation and growth studied by radiolysis. Radiat. Phys. Chem. review in press. doi:10.1016/j.radphyschem.2018.08.001

Page | 18

[68] H. Liu, W. Shen, X. Chen, J.-P. Corriou, A high-performance NH3 gas sensor based on TiO2 quantum dot clusters with ppb level detection limit at room temperature, J. Mater. Sci. Mater. Electron. 29 (2018) 18380–18387. doi:10.1007/s10854-018-9952-9. [69] Y. Nakamura, Y. Ishikura, Y. Morita, H. Takagi, S. Fujitsu, Photo-assisted aromatic VOC sensing by a pNiO : Li / n-ZnO transparent heterojunction sensor element, Sens. Actuators B 187 (2013) 578–585. doi: 10.1016/j.snb.2013.04.105. [70] G.K. Mani, J.B.B. Rayappan, Novel and facile synthesis of randomly interconnected ZnO nanoplatelets using spray pyrolysis and their room temperature sensing characteristics, Sens. Actuators B 198 (2014) 125–133. doi: 10.1016/j.snb.2014.02.101. [71] G.K. Mani, J.B.B. Rayappan, A highly selective and wide range ammonia sensor - Nanostructured ZnO:Co thin film, Mater. Sci. Eng. B 191 (2015) 41–50. doi: 10.1016/j.mseb.2014.10.007. [72] N. Barsan, M. Schweizer-Berberich, W. Göpel, Fundamental and practical aspects in the design of nanoscaled SnO2 gas sensors: a status report, J. Anal. Chem. 365 (1999) 287–304. doi: 10.1007/s002160051490. [73] N. Barsan, U. Weimar, Conduction model of metal oxide gas sensors, J. Electroceram. 7 (2001) 143–167. doi: 10.1023/A:1014405811371. [74] W.M. Sears, The effect of oxygen stoichiometry on the humidity sensing characteristics of bismuth iron molybdate, Sens. Actuators B 67 (2000) 161–172. doi: 10.1016/S0925-4005(00)00395-6. [75] A.N. A. Anasthasiya, R.K. Kampara, P.K. Rai, B.G. Jeyaprakash, Gold functionalized ZnO nanowires as a fast response/recovery ammonia sensor, Appl. Surf. Sci. 449 (2018) 244-249. doi: 10.1016/j.apsusc.2017.11.072. [76] M. Gautam, A.H. Jayatissa, Ammonia gas sensing behavior of graphene surface decorated with gold nanoparticles, Solid. State. Electron. 78 (2012) 159–165. doi: 10.1016/j.sse.2012.05.059. [77] N. Singh, R.K. Gupta, P.S. Lee, Gold-nanoparticle-functionalized In2O3 nanowires as CO aas sensors with a significant enhancement in response, ACS Appl. Mater. Interfaces 3 (2011) 2246–2252. doi: 10.1021/am101259t. [78] A.S.M.I. Uddin, G.-S. Chung, Fabrication and characterization of C2H2 gas sensor based on Ag-loaded vertical ZnO nanowires array, Procedia Eng. 120 (2015) 582–585. doi: 10.1016/j.proeng.2015.08.730. [79] A.S.M.I. Uddin, U. Yaqoob, D.-T. Phan, G.-S. Chung, A novel flexible acetylene gas sensor based on PI/PTFE-supported Ag-loaded vertical ZnO nanorods array, Sens. Actuators B 222 (2016) 536–543. doi: 10.1016/j.snb.2015.08.106. [80] A.S.M.I. Uddin, D.-T. Phan, G.-S. Chung, Low temperature acetylene gas sensor based on Ag nanoparticles-loaded ZnO-reduced graphene oxide hybrid, Sens. Actuators B 207 (2015) 362–369. doi: 10.1016/j.snb.2014.10.091. [81] A.S.M.I. Uddin, K.-W. Lee, G.-S. Chung, Acetylene gas sensing properties of an Ag-loaded hierarchical ZnO nanostructure-decorated reduced graphene oxide hybrid, Sens. Actuators B 216 (2015) 33–40. doi: 10.1016/j.snb.2015.04.028. [82] B. Wang, H.T. Jin, Z.Q. Zheng, Y.H. Zhou, C. Gao, Low-temperature and highly sensitive C2H2 sensor based on Au decorated ZnO/In2O3 belt-tooth shape nano-heterostructures, Sens. Actuators B 244 (2017) 344–356. doi: 10.1016/j.snb.2016.12.044.

Page | 19

Table 1. Description of samples tested in this study. Particle

Amount of

Irradiation

Sample

composition

Au (wt.%)

time (min)

A

TiO2

0

30

B

Au@TiO2

1

30+15

C

Au@TiO2

5

30+15

Table 2. Relative change (in %) of response as a function of RH with reference to response observed at 72% RH. %RH

%

change

in

the % change in the response %

response of sample A

of sample B

change

in

the

response of sample C

32

36.55

15.17

55.11

52

16.73

8.18

29.80

84

14.00

15.21

22.82

Page | 20

Table 3. Comparison of present results with the literature. Nanostructures

Target gas

Operating temperature (°C)

Maximum response (S) 64 115 19.76

Detection range (ppm) 5-200 40-200 50-200

Response time (s)

Recovery time (s)

Referen ce

TiO2 or Au@TiO2 nanoparticles

Ammonia Acetaldehyde Benzene

29 °C

28 59 39

24 78 26

Present work

Au-functionalized ZnO nanowires Au-decorated graphene Au-functionalized In2O3 nanowires Ag-loaded ZnO nanowires Ag-loaded vertical ZnO nanorod array Ag-loaded ZnO-rGO hybrid

Ammonia

32 °C

~14

0.5-100

66

38

[75]

Ammonia

25 °C

8%

15-58

-

-

[76]

Carbon monoxide

RT

104

0.2- 5

130

50

[77]

Acetylene

220 °C

30.8

1- 1000

43

-

[78]

Acetylene

200 °C

27.2

3-1000

62

39

[79]

Acetylene

150 °C

21.2

1-1000

25

80

[80]

Ag-loaded ZnO nanostructures- rGO hybrid Au-decorated ZnO/In2O3 heterostructure

Acetylene

200 °C

12.3

3-1000

57

90

[81]

Acetylene

90 °C

5

25-500

8.5

-

[82]

Page | 21

(a)

(b)

Figure 1. SEM images of initial non-irradiated TiO2 material (a) and sample A (b), and size distribution histograms of non-irradiated material (c) and sample A (d).

Page | 22

(a)

(b)

(c)

(d) T iO 2 NP s

7 8 9 10 11 12 13 14 15 16 17 18 19 20

Diameter (nm)

40

60

80

100 120 140 160 180

Diameter (nm)

T iO 2 NP s

c ou n ts

Au NP s

c ou n ts

c ou n ts

c ou n ts

Au NP s

5 6 7 8 9 1011121314151617181920

Diameter (nm)

40

60

80

100 120 140 160 180

Diameter (nm)

Figure 2. SEM images of samples B (a) and C (b), and size distribution histograms for both Au and TiO2 NPs in samples B (c) and C (d). The mean size of Au NPs is 14 and 11 nm in samples B and C, respectively.

Page | 23

C

In te n s ity (a .u .)

*

*

B

*

A

JCPDS 21-1272

20

30

40

50

60

70

2 theta (degree)

Figure 3. XRD patterns of samples A, B and C. Expected peak for Au NPs is denoted by solid circle (● ). Signal resulting from Si substrate is marked by asterisk. Pattern from anatase TiO2 (JCPDS 21-1272) is also given for comparison.

Page | 24

Figure 4. XPS Ti 2p (a-d) and O 1s (e-h) spectra of non-irradiated TiO2 (a,e) and samples A (b,f), B (c,g) and C (d,h).

Page | 25

Figure 5. XPS Au 4f spectra of samples B (a) and C (b).

Page | 26

Figure 6. Selectivity of the sensors towards 200 ppm of different vapors.

Page | 27

Figure 7. Response trend of the sensors as a function of varying concentrations of target gas.

Page | 28

Figure 8. Transient responses towards varying concentrations of different VOCs and responserecovery times towards 200 ppm of different target gases for samples A (a,b), B (c,d) and C (e,f).

Page | 29

Figure 9. (a) Variations of resistance in air (Ra) of the sensors as a function of %RH. (b) Variations in response of the sensors as a function of %RH.

Page | 30

Figure 10. (a) Stability of sensing performance of the samples over a period of 90 days towards 200 ppm of acetaldehyde (red), ammonia (black) and benzene (blue symbols).

Page | 31

Neli Mintcheva: conceptualization; experiments and analyses; draft preparation; manuscript polishing Parthasarathy Srinivasan: gas sensing experiments; draft of relevant section John Bosco Balaguru Rayappan: supervising gas sensing experiments; interpreting results; manuscript preparation and polishing Aleksandr A. Kuchmizhak: laser related experiments; interpretation of results; manuscript preparation Stanislav Gurbatov: laser related experiments; interpretation of results; sample characterization Sergei A. Kulinich: overall supervision of project; analysis of results; manuscript preparation and polishing

Conflict of interest The authors declare no conflict of interest.

Page | 32

Graphical Abstract

Page | 33

Highlights

o Au- decorated TiO2 nanoparticles prepared by laser beam irradiation o Decoration of TiO2 surface with different amount of Au o Sensing response of Au@TiO2 nanomaterials towards different gases o Au-nanoparticle dependent selectivity

Page | 34