Au-TiO2 loaded cubic g-C3N4 nanohybrids for photocatalytic and

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Au-TiO2 loaded cubic g-C3N4 nanohybrids for photocatalytic and volatile organic amines (VOAs) sensing applications Ritu Malik, Vijay K. Tomer, Nirav J Joshi, Torben Dankwort, Liwei Lin, and Lorenz Kienle ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08091 • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 10, 2018

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ACS Applied Materials & Interfaces

Au-TiO2 loaded cubic g-C3N4 nanohybrids for photocatalytic and volatile organic amines (VOAs) sensing applications

Ritu Malik†, Vijay K. Tomer‡,*, Nirav Joshi‡, Torben Dankwort† , Liwei Lin‡,* and Lorenz Kienle†,*

†

Synthesis & Real Structure Group, Institute of Materials Science, Kiel University, 24143 Kiel, Germany ‡

Berkeley Sensor & Actuator Center (BSAC), University of California, Berkeley, CA-94720, USA

Keywords: Nanocasting, Mesoporous, Photocatalysis, Triethylamine, graphitic carbon nitride

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Abstract Green and efficient approach for efficient nanohybrid photocatalysts in extending the light response to the visible spectrum is a hot research topic in sustainable energy technologies. In this work, novel Au-TiO2@m-CN nanocomposite was synthesized using hard template of cubic ordered mesoporous KIT-6 via the nanocasting process. The as-prepared Au-TiO2@m-CN nanohybrids exhibit enhanced photocatalytic activities with improved stability and reusability using methyl orange (MO) dye. The enhanced photocatalytic performance is a result from the conjugated effect of catalytic active Au and TiO2 nanoparticles supported on highly efficient visible light sensitizer, graphitic carbon nitride (mCN or g-C3N4) and ordered mesoporous morphology. Besides, the sensing performance of AuTiO2@m-CN nanohybrids was also tested for the detection of amine gases wherein, a significant response was reported for triethylamine (TEA) at low operating temperatures. This study reveals a simple and scalable methodology to design and develop next generation of layered mesoporous materials for multifunctional applications. Introduction In the last decade, several research attempts have been dedicated to semiconducting photocatalysts that degrade environmental pollutants and split water under the solar light irradiation.1-3 A suitable redox reaction potential and strong light absorption are required for photocatalytic reaction which most of semiconducting photocatalysts do not possess.4,5 In this context, TiO2 has gained significant attention as a semiconducting photocatalyst with great stability, low cost, high activity and nontoxicity, yet, its high energy band gap (Eg ∼ 3.6 eV) limits its functionality to mainly absorb ultraviolet (UV) light which represents to only 4% of the solar spectrum.1,6,7 Hence, in order to maximize the absorption of solar light and transportation of charge, TiO2 nanohybrids have been extensively explored in the recent years.8,9 Suwarnkar et al. prepared Ag doped TiO2 and observed 99% of degradation of Methyl orange dye within 90 min.10 Sakthivel et al. reported noble metallic ACS Paragon Plus Environment

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nanoparticles as dopants to boost the properties of TiO2 by increasing the charge carrier transportation.11 They observed that due to the excellent electronic conductivity and catalytic properties, the metallic (Ag, Au) nanoparticles could provide new opportunities for realizing high degradation performances. Yet, the achievements made are far from the idea goal of harnessing the abundant visible light. Recently, graphitic carbon nitride (g-CN), which is a metal-free polymeric with semiconductor properties and visible light activated band gap (2.70 eV) has been proposed as photocatalyst in the applications involving water splitting and decomposition of organic pollutants for advantages of earth abundance, appropriate reduction potential, exceptional physical properties, unique electronic structure, and outstanding visible light absorption.12-18 However, the quick recombination of photogenerated electron and hole pairs and small surface area limit the photocatalyst performance of g-CN materials. Hitherto, a variety of techniques involving template inversion of mesoporous silica (KIT6, SBA-15, colloidal silica) and exfoliation were employed in increasing the surface area of the gCN for sensing, hydrogen production, and visible light photocatalysis applications.13,14,19 The negative potential (-1.12 eV vs NHE) of g-CN implies that it can easily form heterojunctions with TiO2 to attain superior visible light degradation performance.20-24 In this context, Shi et al. have synthesized nitrogen doped TiO2/g-C3N4 photocatalysts through a cyanamide precursor by utilizing in-situ impregnation calcination method. Under the visible light (λ ≥ 420 nm), the nanocomposite was able to produce hydrogen in the absence of noble metal as co-catalyst.20 The group of Wei et al. have prepared noble metal (Ag, Pt, Au) doped TiO2/g-C3N4 heterostructured nanofibers by a combination of Electrospinning and thermal oxidation/reduction process and reported the excellent photocatalytic activity.21 Similarly, Shen et al. have synthesized TiO2 nanobelts laminated on g-C3N4 nanosheet and demonstrated excellent degradation performance of methyl orange (95%) and hydrogen evolution.22 Wei et al. have prepared mesoporous TiO2/g-C3N4 microspheres using facile


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nanocoating process using cyanamide as g-CN precursor. They have reported excellent phenol degradation using the photocatalyst.23 Albanna et al. have synthesized metal free nanocomposite (gC3N4 nanosheets/TiO2 mesocrystals) and realized that the presence of TiO2 increases the photocatalytic efficiency for H2 evolution.24 The results endorses that the g-CN and TiO2 heterojunctions exhibited higher degradation performance for Azo dye than pristine TiO2 or g-CN. In addition, TiO2 has also remained as an important counterpart material in designing high performance gas sensing material.25,26 The key requisite for gas sensors remains the high intrinsic surface area of the material which improves the sensing performance by providing enormous surface active sites for gaseous molecules to desorb upon and also enhances the rate of adsorption, transportation and desorption of charge carriers across the sensor surface. TiO2 also has been utilized to form composites with 2D materials for achieving superior gas sensing performances.25-29 Till date, however, nanocomposites of TiO2 and g-CN have been prepared using conventional methods for water splitting20-23,30 and electrode material applications,31 but this novel cubic mesoporous nanocomposite using KIT-6 template has not been reported for photocatalysis and gas sensing applications. In this work, we report a novel two step method (hydrothermal and nanocasting) to prepare AuTiO2 loaded cubic g-CN nanohybrids using hard template of ordered mesoporous silica, KIT-6 to obtain high photodegradation efficiency (90.4%) towards the organic pollutant methyl orange (MO) dye under 90 min irradiation of visible light and optimized conditions of dye concentration, pH and catalyst dosage. Comparative degradation studies on organic dyes including cresol red (CR), acid orange (AO7), direct blue (DB) and methylene red (MR) have also been performed. The prepared catalyst shows excellent reusability without noticeable losses, suggesting that excellent applicability of Au-TiO2@g-CN nanohybrids in degrading aqueous dye solutions. Meanwhile, the Au-TiO2@gCN nanonybrids respond to a variety of volatile organic amines, such as triethylamine, butylamine, triethanolamine and benzylamine, for potential sensing applications. Au nanoparticles, because of


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their exceptional catalytic nature and sensing characteristics,32,33 were employed for enhancing the performance of the TiO2@g-CN nanocomposite. The novel outcomes of this study can be highly useful in designing 2D layered materials and also shows promising glimpse in designing mesoporous materials with extended multi-functionalities. Experimental Materials Preparation: In this study, the hard template of mesoporous KIT-6 was utilized for the preparation of mesoporous g-C3N4 (m-CN), TiO2@m-CN, Au-TiO2@m-CN and Au/TiO2. The KIT-6 and nanocasted m-CN were prepared as per our previous report.16 The mesoporous TiO2@m-CN was obtained by the addition of aqueous TiCl4 (1 ml, Titanium tetrachloride, Merck) in solution of CH2N2 (1g/ml, cyanamide, Sigma Aldrich). The solution was then added drop-wise to 0.6 g KIT-6 to produce a thick solution and stirred at room temperature for 1 h to induce homogeneity in the solution. The obtained solution was then dried at 80 °C for 4 h and later transferred in a closed crucible and heated for 6 h at 550 °C. The yellow powder was then stirred for 24 h with 2 M and 50 ml NH4HF2 (Ammonium hydrogen difluoride, Sigma Aldrich) solution to eliminate the silica template. The resulting bright yellow products were washed with excess of distilled water followed-up by drying in an oven for 4 h at 100 °C to recover TiO2@m-CN nanostructures. In a similar process, the mesoporous Au-TiO2@m-CN was synthesized by the addition of HAuCl4•4H2O (14.75 mg, Tetrachloroauric acid, Fisher Scientific) to 1 ml TiCl4 aqueous solution and stirred for 1 h. Later, the solution was added to the aqueous cyanamide solution and a similar synthesis protocol was practiced as utilized for TiO2@m-CN above. The effect of m-CN matrix in the Au-TiO2@m-CN nanocomposite was studied by preparing pure mesoporous Au-TiO2 was also synthesized by using KIT-6 but in the absence of m-CN. For it, 1g TiCl4 was added to 20 ml ethanol and stirred for half hour followed by the addition of 14.75 mg


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HAuCl4•4H2O (1wt% Au) in the prepared solution. The solution was then tipped drop-wise upon on 1 g KIT-6 and the obtained slurry solution was stirred at 50 °C to allow the complete ethanol evaporation. Afterwards, the product was dried for 1 h at 80 °C and later calcined for 4 h at 550 °C. In the end, the pure mesoporous Au-TiO2 was obtained by the removal of KIT-6 through NaOH (2M) solution and following the processes of washing and filtration and later drying the sample for 6 h at 80 °C. Characterization of Materials Both low angle and wide angle X-rays diffraction data was recorded on Bruker D8 advance diffractometer using CuKα monochromatic radiation having λ = 0.154 nm at 40 kV and 50 mA. The FEI Tecnai F30 G2 transmission electron microscope (TEM) was used to determine pore morphology of the prepared samples at an acceleration voltage of 300 kV (field emission gun). The elemental mapping and overall structure of the as-synthesized sample was examined by scanning electron microscopy (SEM) which is equipped with an energy-dispersive x-ray spectroscopy (SEMEDX, FEI QUANTA 200F). The N2 sorption isotherms were measured on Autosorb iQ2 instrument, (Quantachrome) at 77 K to estimate the pore dimensions and surface area of the materials. The photocatalytic properties were evaluated on UV-vis spectrophotometer (Shimadzu UV-2401). The X-ray photoelectron spectroscopy (XPS) measurements were obtained on PHI 5000 instrument using Al Kα (1486.6 eV) radiation. The photoluminescence (PL) spectra were obtained on Perkin Elmer LS-55 Fluorescence spectrometer at room temperature conditions. Evaluation of photocatalytic performance The photocatalytic activity of the prepared samples was evaluated by monitoring the removal of MO dye by utilizing the visible light irradiation. Typically, 0.2 g/L of catalyst, H2O2 solution and 100 ml of 100 ppm MO dye solution were mixed thoroughly followed by stirring for 15 min in dark to acquire an adsorption/desorption equilibrium between catalysts and MO dye. After that, the mixture was irradiated perpendicularly using a Xe lamp (300W) from a distance of 10 cm. After every 5 min, ACS Paragon Plus Environment

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2 ml aliquots were collected and centrifuged in triplicated followed by filtration. The solution was moved instantly in a quartz corvette to record the absorbance spectra in the 300–800 nm range. The effect of pH on the degradation was studied by varying the pH value using 1 M solution of NaOH and HCL. The catalytic performance was also studied by varying the catalysts dosages and dye concentration. The influence of catalyst presence was studied by performing the experiments in the absence of catalysts. The degradation performance (D %) was evaluated by using the equation, =

( )

× 100

(1)

where, C0 (mgL-1) is the dye initial concentration (ppm) and C (mgL-1) is the dye concentration at any time ‘t’. The mineralization of dye was studied by total organic carbon (TOC) analysis on a multi N/C 2000 Analytic Jena analyzer. Fabrication of sensing elements and sensing measurements The sensing performance was monitored using Ag-Pd Interdigitated electrodes (IDE, electrodes separation distance and electrode width of 0.2 mm) which were printed on ceramic substrate (13.4mm x 7mm x 0.5mm). A dilute paste of sample: ethanol (1:20) was poured upon ceramic substrate by using a pipette (10 µL) and dried for 2h at 60 °C. The sensing experiments were performed in a customized gas sensing set-up as per our earlier report.16,17 Results and Discussion Characterization The crystallographic structure of KIT-6, m-CN, TiO2@m-CN, Au-TiO2@m-CN and Au-TiO2 samples is shown in Figure 1(A). A wide peak at 2θ = 22° was observed for KIT-6 which is a standardized peak for pristine materials.35,36 The nanocasted samples with –CN content (m-CN, TiO2@m-CN, Au-TiO2@m-CN) displays two distinct peaks at 12.5° ([100] plane) and 27.5° ([002] plane) corresponding to the reflections of CN.12,13 For TiO2 loaded samples (TiO2@m-CN, AuTiO2@m-CN and Au-TiO2), the peaks centered at 25.3°, 37.8°, 48.1°, 53.9°, 55.1°, 62.7° and 68.7°


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denotes the presence of TiO2 in anatase crystalline phase ((JCPDS no. 21-1272). For Au-TiO2@mCN and Au-TiO2 nanohybrids, three distinct peaks for Au at 2θ = 38.1°, 44.2° and 64.3° corresponding to (111), (200) and (220) respectively, were observed (JCPDS no. 04-07834) and the average crystallite size calculated from the Scherrer’s equation for metallic Au nanoparticles was 3.18 nm. Besides, the Au peaks confirm the presence of Au nanoparticles in the TiO2@m-CN nanohybrids. The small angle X-ray scattering (SAXS) pattern of the as-synthesized KIT-6, m-CN, TiO2@m-CN, Au-TiO2@m-CN and Au-TiO2 materials are shown in Figure 1(B). The peak at 2θ = 0.84° corresponds to the (211) plane represents the characteristic peak for the mesoporous materials. Besides, the presence of two shoulder peaks between 2θ = 1−2° which indicates the ordered structure of KIT-6 with the Ia3d symmetry.16 However, these shoulder peaks were absent in the nanocasted samples which suggest that mesoporous ordering was reduced in the process of template removal step. The prominent peak corresponding to (211) plane was found to be slightly shifted towards higher angles in the nanocasted samples (as compared to the KIT-6) due to the decrease in scattering contrast amid the mesoporous framework and pore channels. The N2 sorption isotherms and pore measurement curves for as-prepared materials are shown in Figure 2(A) and 2(B), respectively. The KIT-6 silica shows a type IV adsorption curve with H1 type hysteresis loop which denotes to the process of capillary condensation within the mesoporous framework of KIT-6 template.37 For nanocasted samples (m-CN, TiO2@m-CN, Au-TiO2@m-CN and Au-TiO2), a typical hysteresis loop of H3 type was observed which shows non-uniform pores and reduced interparticle porosity of nanocasted materials.38 Figure 2(B) shows the pore size distribution for all the materials. As can be clearly seen that the mesoporous KIT-6 exhibits a typical pore diameter (Dp) of 8.5 nm whereas the nanocasted samples show smaller pore diameter than the KIT-6 template. When the TiO2 and Au nanoparticles were loaded in the m-CN and TiO2@/m-CN,


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the pore diameter of resultant nanocomposite decreases sequentially. The information obtained from N2 sorption isotherms and pore size distribution curves is listed in Table-1. The morphologies of mesoporous KIT-6 and Au-TiO2@m-CN as obtained from HRTEM results are shown in Figure 3. As observed, the KIT-6 material shows long range of 3D meso-ordered pore channels with pore diameter of ~ 8.45 nm, well agreeing with the results obtained from pore size distribution curves. In addition, the nanocasted Au-TiO2@m-CN shows (Figure 3(c,d)) a similar mesopores morphology even after the process of heat treatment and KIT-6 removal, thus revealing the excellent restoration of the original structure of parent template. However, as evident from the physico-chemical properties, listed in Table-1, a slight reduction in pore attributes was observed for the nanocasted Au-TiO2@m-CN due to the shrinkage of mesopores architecture on account of nanocasting procedures. The HRTEM image in Figure S1 shows the presence of Au nanoparticles inside the pore channels of Au-TiO2@m-CN nanohybrid. Figure 3(e) shows the SEM image of the Au-TiO2@m-CN nanocomposite. As can be seen, several round shape particles having size between 100-300 nm were observed. In Figure 3(f-j), the elemental mapping results reveals the uniform presence of C, N, Ti, O and Au elements in the Au-TiO2@m-CN nanocomposite. The XPS spectrum for C 1s in Au-TiO2@m-CN nanocomposite in the energy range of 282-291 eV is shown in Figure S2. The two peaks recorded at 284.6 eV and 287.8 eV can be respectively related to the C-C bonding and sp2 hybridization of C-atoms to the three N-atoms (C–(N)3) in the gCN material.39 The N 1s spectrum demonstrates three distinct peaks at 398.6 eV, 399.8 eV and 401.5 eV which respectively corresponds to the nitrogen bonding of sp2 hybridized aromatic nitrogen in C=N−C, ternary nitrogen in N-(C)3) and heptazine bonding with –NH or –NH2 groups.40 The XPS spectra of Ti 2p illustrates that the peak at 458.4 eV and 463.9 eV which respectively denotes the binding energies for Ti 2p1/2 and Ti 2p3/2 electrons and justifies the existence of Ti4+ in the TiO2 crystal lattice.41 The Au 4f core level XPS spectrum displays doublet peaks at 83.2 eV and 87.4 eV,


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which respectively corresponds to the Au 4f7/2 and Au 4f5/2. Moreover, the splitting energy between the two peaks is equal to 3.2 eV which denotes to the presence of metallic gold (Au0) according to the standard spectrum of Au which is in good agreement with the Handbook of X-ray Photoelectron Spectroscopy.42 The O 1s spectrum shows an asymmetric profile and the single peak at 531.1 corresponds to the existence of physically adsorbed oxygen on Au-TiO2@m-CN material. The UVvis diffuse reflectance spectrum in Figure S3a shows that the absorption edges of Au-TiO2@m-CN positions in the visible region. The tauc plot (inset) demonstrates a band energy gap of ~3.0 eV which further concludes that the Au-TiO2@m-CN material can absorb large amounts of visible light and can be used as a visible-light irradiation photocatalyst. The photo-luminescence intensity is a useful tool for determining the recombination rate of electron and hole. A higher intensity denotes to the faster recombination rate while lower intensity illustrates the trapping and transferring of electrons across the interface.12 Figure S3b shows the photoluminescence (PL) spectra of asprepared samples under the excitation at 330 nm. As can be seen, a broad emission peak was noted for m-CN at ~458 nm which is approximately equals to the band-gap of pure g-C3N4. On coupling of m-CN with Au-TiO2, the peak intensity of Au-TiO2@m-CN decreases which indicated that the nanohybrid had lower recombination rate of photo-generated charge carriers. Evaluation of photocatalysts The photodegradation performance of m-CN, TiO2@m-CN, Au-TiO2@m-CN and Au-TiO2 for AOPs were evaluated under visible-light irradiation by evaluating the time-dependent degradation profile of methyl orange (MO). The stability of the dye was also evaluated by performing degradation experiments without the use of Au-TiO2@m-CN photocatalyst), effect of absence of light irradiation (Au-TiO2@m-CN photocatalyst was present) and importance of H2O2 (light was irradiated but catalyst was not present) in photocatalytic reaction system (Figure 4(A)). The degradation profiles in Figure 4(A) are the results of photocatalytic experiments performed under the


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conditions of catalyst dosage of 0.2g/L, with 100 ppm MO concentration at pH 4. As clearly observed in Figure 4(A), MO showed excellent stability for 15 min under dark conditions. In 90 min of irradiation time, the pure m-CN displays only 32% degradation of MO dye. Nevertheless, the addition of TiO2 in the m-CN matrix increases the photocatalytic degradation performance (to 52%). The addition of TiO2 nanoparticles enables the photo-generated electron to get transferred from the surface of m-CN to TiO2 thereby increasing the photocatalytic efficiency of the TiO2@m-CN nanocomposite. The pure Au-TiO2 catalyst (without the presence of m-CN) shows only 70% degradation of MO dye in 90 min. The photocatalytic degradation rate was further enhanced with the addition of Au nanoparticles in the TiO2@m-CN material. The Au nanoparticles reduces the rate of recombination of hole (h+) and electron (e-) which allows more dopant nanoparticles to involve in the redox reaction thereby forming active oxygen species. It was also observed that the AuTiO2@m-CN catalyst doesn’t show any apparent degradation activity towards MO when no light was irradiated, contrarily ~80% of the MO dye degraded within 90 min when irradiated with light. The degradation performance by using Au-TiO2@m-CN catalyst (without H2O2) demonstrates the 75.6% degradation of MO dye under the same experimental conditions. Overall, the Au-TiO2@mCN exhibits the maximum degradation efficiency among all the prepared materials due to the presence of both catalytically active Au and TiO2 nanoparticles in the mesoporous m-CN matrix. The first order kinetic results (Langmuire Hinshewood) for determining the apparent rate constant of degradation process of dye were calculated by using the equation,22

ln =

(2)

where C is the MO concentration at time t, C0 denotes the initial concentration of MO and k represents the first-order rate constant. As can be seen, Figure 4(B) represents the linear transformation of Fig. 4(A) curve. The degradation spectra of MO dye in Figure 4(C) exhibited the maximum absorption peak λmax at 550 nm. A decrement in λmax was clearly observed with the time ACS Paragon Plus Environment

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which is directly related to the reduction of MO dye. The equilibrium adsorption capacity was estimated by using the formula,20 =

(3)

where, C0 (mg/l), Cf (mg/l), m (g) and V respectively denotes to the initial and final concentrations of dyes, weight of the photocatalyst and volume of the original mixture. The degradation and absorption performance of mesoporous Au-TiO2@m-CN nanohybrid over the MO dye were determined by varying the pH of the dye solution as shown in Figure 5(A). The results reveal to a little absorption of the MO dye and so a less degradation performance was observed under the alkaline condition (pH 8). In the aqueous suspension, g-CN is amphoteric with its isoelectric point (IEP) at pH 2.0034.12 This reveals that the surface of g-CN is mainly negatively charged when the pH is greateer than IEP and positively charged when the pH is lesser than IEP.12 In the reaction system, the dissolved O2 reacts with the first photoinduced electron (e–) to produce moderate oxidant ●O2– (eqn 4) which on reacting with the H+ and second e– forms H2O2 (eqn 5).14,20,22,43 Due to the appropriate band positions of g-CN, the produced H2O2 reacts with the third photoinduced e– to produce most reactive ●OH radical (eqn 6) which later assists in achieving a higher degradation efficiency of 85.8% for Au-TiO2@m-CN nanohybrid at pH 2. Also, the positively charged surface of Au-TiO2@m-CN nanohybrid readily absorbs the negatively charged dye molecules of MO due to electrostatics interaction. O2 + e– → ●O2–

(4)

●O2– + 2H+ + e– → H2O2

(5)

H2O2 + e– → ●OH + ●OH–

(6)

Figure 5(B) shows the effect of concentration of MO dye on its photocatalytic degradation activity. As can be seen, a reduction in degradation efficiency from 85.8 to 72% was observed when initial concentration of MO dye was increased from 100 to 500 ppm. This could be due to the fact ACS Paragon Plus Environment

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that absorption of organic species on the surface of mesoporous Au-TiO2@m-CN nanohybrid increases when the concentration of dye is increased in the system. This results into a reduction in the photons path length which enters into the solution and slows down the formation rate of OH• radicals. In this case, the redox reaction and the degradation efficiency of the photocatalyst gets reduces. The effect of catalyst dosage on the degradation performance for MO dye was also studied by varying the catalyst amount (0.1 to 0.5 g/l). As can be seen in Figure 5(C), the dye degradation efficiency first increases with catalyst dosage (0.1 to 0.3 g/L) due to an increase in the HO• radical concentrations. However, the degradation efficiency decreases with further increase in catalyst amount (0.3 to 0.5 g/L) due to the accumulation of photocatalysts nanoparticles which increases the light scattering and opacity and causes a reduction in the availability of active surface sites. Therefore, the ideal concentration of catalyst was 0.3 g/L which degrades 90.4% MO dye within 90 min of irradiation time. The performance of mesoporous Au-TiO2@m-CN nanohybrid was also monitored by the degradation profile of various AZO dyes, such as cresol red, acid orange 7, direct blue, and methyl red under the same experimental condition. As can be seen in Figure 5(D), a degradation efficiency of 77%, 79%, 82% and 86% has been observed for CR, AO7, DB and MR, respectively for Au-TiO2@m-CN (0.3 g/L) at pH 2. A comparison of MO degradation performance using different materials in Table S1 shows that the Au-TiO2@m-CN has the best efficiency than earlier reported works. Mechanism for the degradation of MO A probable schematic diagram showing the process of charge transfer for mesoporous Au-TiO2@mCN nanohybrids has been illustrated in Figure 6. The high surface area of Au-TiO2@m-CN coupled with uniform pore structure results in its high absorptivity towards MO. In the dark condition, a small amount of MO dye was absorbed by Au-TiO2@m-CN nanohybrids to create as adsorptiondesorption equilibrium. The normal hydrogen electrode (NHE) vs band positions for Au-TiO2@m-


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CN nanohybrid is illustrated in Figure 6. Owing to the lesser work function (WF) of m-CN (~4.3 eV)12 than that of TiO2 (4.5 eV),44 the m-CN (h+) and m-CN (e-) were formed under the visible light irradiation. These excited electrons were transported from the conduction band of m-CN to that of TiO2 which later on get transferred to the Au surface. The absorbed oxygen gets reduced by these excited Au nanoparticles and superoxygen anionic free radicals (•O2-) are generated which further reacts with H2O to create •OOH active species by eqs (7) to (13) as follows: m-CN + visible light → m-CN (e-h+) -

(7) -

TiO2(m-CN(e )) + Au → TiO2@m-CN +Au(e )

(8)

Au(e-) + O2 (ads) → Au + •O2-

(9)

•O2- + H+ → •OOH

(10)

•OOH + H+ + e- → H2O2

(11)

H2O2 + e- → •OH + OH-

(12)

Au+(h+), TiO2(h+), m-CN(h+), •OH, •O2-, e- + MO → CO2 + H2O

(13)

As the removal of dye color alone does not determine the mineralization of the dye and the dye solution after decolorization can be more harmful than the dye itself, therefore, total organic carbon (TOC) analysis was carried out to determine the mineralization level of the MO dye during the photocatalytic process. For the analysis of TOC, the mesoporous Au-TiO2@m-CN nanohybrids were irradiated by visible light. The results in figure 7(A) demonstrate that the dye decolorization was faster than the degree of mineralization. The rate of TOC removal reaches around 69% after 2h. The reason for fast decolorization and high TOC value could be the cleavage of the azo bond which causes a reduction in the conversion of N atom of MO dye into oxidized nitrogen compounds due to the short life of the hydroxyl radicals and their negligible interaction with the aliphatic chain.45,46 The results revel that the degradation activity after 90 min leads to a complete mineralization of the MO dye. The recycling experiments were also performed in order to study the stability of mesoporous AuTiO2@m-CN nanohybrid for the degradation of MO dye under similar test conditions. After each ACS Paragon Plus Environment

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degradation experiment, the dispersed catalyst was recovered by following the process of centrifugation, washing (distilled water + ethanol) and drying (6 h at 50 ºC) and utilized for degradation of fresh MO dye solution under the similar experimental conditions. This process was performed 5 times and the results are revealed in Figure 7(B). As can be see, the degradation efficiency was minutely affected due to addition of organic species on the surface of Au-TiO2@mCN which directly affects the process of adsorption and reduction. Thus, it can be concluded that Au-TiO2@m-CN nanohybrid catalyst is reusable and stable over longer duration without much loss in the photocatalytic performance. The XPS spectrum of individual element of Au-TiO2@m-CN photocatalyst taken after 20 experiments of stability test (dotted line) is shown in Figure S2. As can be seen, all the elements show their peak intensity at respective binding energy and no apparent shifting in the binding energy was observed. The doublet peaks positions and peak distance for Ti and Au remain consistent after the stability test which denotes to the fact that the oxidation state of both the elements (Ti(4+) and Au(0)) remained intact after the photocatalytic test measurements. The long term stability test of the Au-TiO2@m-CN photocatalyst towards MO dye was recorded after a gap of 5 months. As can be seen in Figure S4, the photocatalyst shows excellent stability and the percentage of MO degradation remains larger than ~87% even for the 30th catalyst experiment cycle. This slight depreciation in the degradation efficiency could be due to the loss of Au-TiO2@m-CN photocatalyst while recovering it for new reactions. Gas Sensing Properties The multiple functionality of the Au-TiO2@m-CN nanohybrid in the chemical sensing regime is an important aspect of advanced functional nanomaterials for selectively detecting gases commonly present in the indoor climate. Volatile organic amines (VOAs) represent such a class of household gases which originate from dustbins, stored meat and sewers and owing to their toxic and stinking attributes, they are responsible for accute health problems.47-49 A typical fishy or sour/pungent/nasty


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odor is released by VOAs in the climate and when inhaled in higher quantity by human, they can cause reactions and can severely affect hematopoietic/nervous/urinary/respiratory systems, causes strokes and become carcinogenic when converted into nitrosamines.50-52 Considering the severe effect from these commonly and obviously present pollutants in the indoor climate, it becomes highly necessary to detect trace levels of organic amines with the help of ultrasensitive and selective chemical sensors. In this context, we have also investigated the prepared materials for the detection of VOAs. The optimized working temperature was determined by measuring the response of the prepared samples (g-CN, m-CN, TiO2@m-CN, Au-TiO2@m-CN and Au-TiO2) in temperature range 100-300 ºC towards 50 ppm TEA. Figure 8a shows that the response of sensing materials follows a “rise → maximum → decline” pattern with a rise in the temperature. The pure g-CN shows a very poor response (R = 6.6) across the temperature range, however, being of same composition, the ordered mesoporous m-CN, synthesized using hard template of KIT-6 shows highest response of 19.1 at 240 °C. When n-type TiO2 nanoparticles were loaded in mesoporous m-CN, a reduction of 40 ºC in the optimized working temperature was observed and a response of 62.5 was observed for TiO2@m-CN at 200 °C. A further decrease of 25 ºC in the optimized operating temperature was observed with the loading of Au nanoparticles and the Au-TiO2@m-CN shows a highest response of 78.9 at 175 C for 50 ppm TEA gas. On the contrary, the pure mesoporous Au-TiO2 (without m-CN) exhibits a response of 32.6 at 225 ºC, which elaborates the role (discussed later) of presence of mCN matrix in enhancing the response and decreasing the optimized operating temperature for detecting the TEA gas by the Au-TiO2@m-CN nanohybrid sensor. A variety of other VOAs including butylamine, triethanolamine and benzylamine were also detected using the Au-TiO2@mCN sensor. The response of Au-TiO2@m-CN sensor for the detection of VOAs in 1-500 ppm concentration range measured at 175 °C is shown in Figure 8b. As can be seen, the Au-TiO2@m-CN sensing responses increase with respect to an increment in the VOAs concentration. The high


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response towards different gases can be explained on the basis of the C-H bond dissociation energies (BDE) of the respective gas molecules. As the BDE of triethylamine (91.2 kcal/mole)52 is less than that of butylamine (91.8 kcal/mole),53 triethanolamine (92.4 kcal/mol)52 and benzylamine (108 kcal/mole),54 the molecules of triethylamine dissociates easily at low operating temperature of 175 ºC. The stability of Au-TiO2@m-CN mesoporous sensor to different concentration (1-100 ppm) of TEA gas was also monitored over a span of 5 weeks and shown in Figure 8c. As observed, the sensor demonstrates good reproducibility over the measurement period which endorses the excellent stability of the Au-TiO2@m-CN sensor. The response/recovery times are among the important attributes of a gas sensor which determines its utility in the commercial applications.55,56 The response and recovery time of Au-TiO2@m-CN for all the tested VOAs (50 ppm) measured at 175 °C is shown in Figure 8d. As can be seen, an average response/recovery times of 9.5/7 s (triethylamine), 15/8.6 s (butylamine), 16/11 s (triethanolamine) and 10/10.3 s (benzylamine) were recorded for the Au-TiO2@m-CN sensor. The dynamic response/recovery curves of the Au-TiO2@m-CN sensor for detecting TEA gas (1- 50 ppm concentration range) at an operating temperature of 175 °C is shown Figure 8e. As can be seen, the sensor response increases with the increment of the TEA gas concentration and a respective response of 3.45, 19.2, 43.2, 79.8 and 147.8 was obtained for 1, 10, 25, 50 and 100 ppm TEA gas. The response/recovery curves also show the outstanding reversible nature of the sensor when quickly exchanged among TEA and fresh air. The possible reason could be the 2D layered mesoporous morphology of the Au-TiO2@m-CN sensor which allows the fast release of the TEA molecules from the surface of the sensor, and the response reaches to its baseline very quickly. The average time for response and recovery of Au-TiO2@m-CN sensor towards TEA gas respectively varies from 9-16 s and 6-12 s, in the 1-50 ppm range. The time response results suggest that the sensor take less time to recover to its initial position. When compared with the sensing results of earlier reported TEA sensor


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in Table S2, it was observed that the Au-TiO2@m-CN sensor owns significant higher response, quicker response/recovery time at comparatively lower working temperatures. The response of AuTiO2@m-CN sensor towards 50 ppm of tested gases measured at varied humidity conditions (10-95 %RH) is shown in Figure 8f. As can be seen, a very little change in the response of sensor was detected when the humidity conditions are low. The response decreases gradually by increasing the %RH contents in the test atmosphere, and a response of 68.5, 21.5, 12.3 and 6.6 was observed for triethylamine, butylamine, triethanolamine and benzylamine, respectively at 95 %RH conditions. The results concluded that the Au-TiO2@m-CN sensor responds to all tested VOAs even at highest %RH conditions. Figure 9 shows the selectivity of the Au-TiO2@m-CN sensor to the TEA gas among other VOAs (50 ppm) by randomly exposing the sensor at 175 °C. As observed, the response of the sensor quickly returns to its baseline state with the blow of the fresh air into the gas chamber. Inset of Figure 9 shows the results for relative sensitivity (RS = STEA/Stest gas) for the Au-TiO2@mCN sensor. As observed, the RS value of all test gases is higher as compared to the TEA gas, which endorses that the Au-TiO2@m-CN sensor is exceedingly selective to the TEA gas.57 TEA sensing mechanism On interaction of Au-TiO2@m-CN sensor with hot air at higher operating temperature (175 °C), the oxygen (O2) molecules present in air in the closed chamber traps the easily accessible electrons (e−) present in the conduction band of TiO2 and m-CN which results in the creation of negatively charged oxygen species (O2-, O- and O2-). The trapped electrons then lead to the formation of a charge depletion region, thus causing an increase in the resistance of the sensor. However, when TEA gas was exposed on to the surface of sensor inside the closed chamber at 175 °C, the O– discharges back the captured electrons (e−) to the sensor surface which decreases the depletion layer surrounding the and ultimately leads to an increase in the carrier concentration of Au-TiO2@m-CN. The interaction of gas molecules on the surface of sensor causes the TEA gas to get oxidized and leads to the


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formation of the iminium cations, free radical ions (TEA•+), and neutral amine radical of as shown in eqs (14) to eqs (19) as follows:48,59

TEA + O2 → O•− + TEA•+

(14)

O•− + TEA•+ → OH• + Et2−N−C•(H)−CH3

(15)

OH• + Et2−N−C•(H)−CH3 → OH- + Et2N+−CHCH3

(16)

Et2N+−(CH3)CH ↔ Et2−N−(CH2)CH (ads) + H(ads)

(17)

H (ads) + O- (ads) → e- + OH (ads)

(18)

H+ + OH− → H2O

(19)

In the end, TEA oxidizes into vinylamine and water as follows: TEA + O− (ads) → Et2N−CH−CH2 + H2O + e−, and the e− were freed from the adsorbed oxygen which return back to the conduction band of the Au-TiO2@m-CN nanocomposite which leads to an increment in the conductivity of the sensor. For the Au-TiO2@m-CN sensor, the enhanced response is governed by various factors including high surface area of the sensor, heterojunction formation between n-type TiO2 and m-CN materials and the availability of m-CN and highly catalytic metallic Au nanoparticles. The high surface area of the sensing material is directly related to the availability of the surface active sites.60,61 Besides, the ordered porous architecture of the sensor accelerates the processes of adsorption, transmission and desorption of the gas molecules across the sensor surface, thereby leading to an increase in the response of the sensor.62,63 In this work, the ordered mesoporous channels of Au-TiO2@m-CN sensor assists in the rapid adsorption, faster transmission and quick desorption of the TEA molecules from its surface which on the other hand helps in improving the response to TEA gas and also dictates the faster response-recovery time of the Au-TiO2@m-CN sensor. The heterojunction formation in semiconductor based sensors plays a crucial role in determining the sensor response. In the case of Au-TiO2@m-CN, two components (TiO2 and m-CN) are involved and so the electronic ACS Paragon Plus Environment

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movement occurs between two homojunction (TiO2 ↔ TiO2 and m-CN ↔ m-CN) and one heterojunction (TiO2 ↔ m-CN). Across the heterojunction, the electronic movement is directed from material possessing lower work function (WF) to the material having higher WF, and this process forms a potential barrier between the two components by achieving an equilibrium and causing the formation of another Fermi energy level.25,26,28 In the case of Au-TiO2@m-CN sensor, the electrons from the conduction band of m-CN (WF = 4.3 eV)12 are transferred to that of TiO2 (WF = 4.5 eV)43,44 by band bending which forms a potential barrier among them. This newly formed potential barrier obstructs the electronic motions thus causing the electrons which are present on surface of sensor, an obvious site for desorption of oxygen species thereby increasing the response towards TEA gas. The presence of high surface area, cubic ordered mesoporous m-CN material in Au-TiO2@m-CN provides long-range regular ordering of pore channels which contrarily helps in the smoother propagation of the charge carriers. Besides, the presence of ordered mesoporous m-CN matrix avoids agglomeration and assists in homogeneous dispersion of the dopants nanoparticles across the mesopores. While detecting TEA gas, the sensor Au-TiO2@m-CN (Ra/Rg = 79.8) shows 2.5 time better response than the sensor Au-TiO2 (Ra/Rg = 32.6) and also mark an impressive reduction of 50 °C in the optimized operating temperature. These important results highlight the usefulness of 2D mesoporous cubic m-CN matrix in sensing applications. It is known that Au nanoparticles have good catalytic and excellent conducting properties and because of spillover effect, the process of chemical sensitization and catalytic oxidation increases. This increases the rate of adsorption of active oxygen species on the surface of Au-TiO2@m-CN sensor. Specifically, the process of chemical sensitization increases the sorption rate of oxygen which captures electrons present in the conduction band of TiO2 and m-CN, and gets converted into oxygen ions (O−) as 2Au + O2(gas) → Au/O and O + e− → O− (ads). When the reducing TEA gas comes in contact to the sensor surface, the e− are transferred back to the surface of the sensor. As a result, the resistance decreases and the response of the AuACS Paragon Plus Environment

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TiO2@m-CN sensor increase. Thus it can be concluded that the unique ordered mesoporous morphology of Au-TiO2@m-CN plays a significant role in enhancing the photocatalyst performance and sensing response. Such type of multifunctional hybrids materials can become a smarter solution in terms of expense viability and performance for applications related to energy and environment domain. Conclusion In summary, we have prepared a novel nanohybrid consisting of g-C3N4 sheets well coupled with TiO2 and metallic Au nanoparticles, using hard template of mesoporous silica, KIT-6. Owing to its 3D cubic ordered mesoporous 2D layered architecture with high specific surface area and mesoordered homogeneous pore channels, the cubic mesoporous Au-TiO2@m-CN nanohybrid exhibited enhanced visible light photocatalytic degradation performance towards a variety of AZO dyes. The excellent surface area and pore properties of g-C3N4 sheets enhance the accessibility and dispersibility of TiO2 and Au nanoparticles, thereby promoting the transportation and diffusion of dye molecules. In addition, the Au-TiO2@m-CN nanohybrid unveiled excellent sensing performance towards detection of VOAs, commonly present in the indoor climate. The synergistic approach highlighted in this study could extend the utility of functional 2D layered materials in designing high performance visible light photocatalysts, gas sensors and battery materials which are not only economically viable but are also excellent performers.

ASSOCIATED CONTENT Supporting Information Figures showing HRTEM, XPS, UV-Vis, PL and photocatalyst stability of Au-TiO2@m-CN. Tables showing comparison of MO dye degradation and TEA gas sensing performance of Au-TiO2@m-CN with previously published works.


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AUTHOR INFORMATION Corresponding author [email protected] (V.K. Tomer) [email protected] (L. Lin) [email protected] (L. Kienle)

Conflicts of interest The authors declare no conflicts of interest. Acknowledgements R.M thanks CAU Kiel for visiting scientist position. VKT is thankful to United States India Education Foundation (USIEF) for Fulbright-Nehru Post-doctoral Fellowship. NJ thanks the São Paulo Research Foundation (FAPESP) for funding support (Grant No. 2016/23474-6). TD acknowledges financial support by the federal state Schleswig-Holstein. LK acknowledges the financial support from Deutsche Forschungsgemeinschaft (DFG), Germany under scheme PAK 902 (KI 1263/14-1).


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References (1)

Tian, J.; Zhao, Z.; Kumar, A.; Boughton, R. I.; Liu, H., Recent Progress in Design, Synthesis, and Applications of One-dimensional TiO2 Nanostructured Surface Heterostructures: A Review, Chem. Soc. Rev. 2014, 43, 6920-6937.

(2)

Ran, J.; Zhang, J.; Yu, J.; Jaroniec, M.; Qiao, S. Z., Earth-abundant Co-Catalysts for Semiconductor-based Photocatalytic Water Splitting, Chem. Soc. Rev. 2014, 43, 7787-7812.

(3)

Li, X.; Yu, J.; Low, J.; Fang, Y.; Xiao, J.; Chen, X., Engineering Heterogeneous Semiconductors for Solar Water Splitting, J. Mater. Chem. A 2015, 3, 2485- 2534.

(4)

Adhyapak, P.V.; Meshram, S. P.; Tomer, V. K.; Amalnerkar, D.; Mulla I. S., Effect of Preparation Parameters on the Morphologically Induced Photocatalytic Activities of Hierarchical Zinc Oxide Nanostructures, Ceram. Inter. 2013, 39, 7367-7378

(5)

Zhang, X.; Peng, T.; Song, S., Recent Advances in Dye-Sensitized Semiconductor Systems for Photocatalytic Hydrogen Production, J. Mater. Chem. A 2016, 4, 2365-2402.

(6)

Malik, R.; Tomer, V. K.; Chaudhary, V.; Dahiya, M. S.; Rana P. S.; Nehra, S. P.; Duhan, S., Facile Synthesis of Hybridized Mesoporous Au@TiO2/SnO2 as Efficient Photocatalyst and Selective VOC Sensor, Chem. Select 2016, 1, 3247–3258.

(7)

Luo, C.; Ren, X.; Dai, Z.; Zhang, Y.; Qi, X.; Pan, C., Present Perspectives of Advanced Characterization Techniques in TiO2-Based Photocatalysts, ACS Appl. Mater. Interfaces 2017, 9, 23265-23286.

(8)

Malik, R.; Rana, P. S.; Tomer, V. K.; Chaudhary, V.; Nehra, S. P.; Duhan, S., Visible Lightdriven Mesoporous Au-TiO2/SiO2 Photocatalysts for Advanced Oxidation Process, Ceram. Inter. 2016, 42, 10892-10901.

(9)

Yu, Y.; Wen, W.; Qian, X. Y.; Liu, J. –B.; Wu, J. –M., UV and Visible Light Photocatalytic Activity of Au/TiO2 Nanoforests with Anatase/Rutile Phase Junctions and Controlled Au Locations, Sci. Reports 2017, 7, 41253(1)-41253(13).

(10) Suwarnkar, M. B.; Dhabbe, R. S.; Kadam, A. N.; Garadkar, K. M., Enhanced Photocatalytic Activity of Ag doped TiO2 Nanoparticles Synthesized by a Microwave Assisted Method, Ceram. Int. 2014, 40, 5489-5496. (11) Sakthivel, S.; Shankar, M. V.; Palanichamy, M.; Arabindoo, B.; Bahnemann, D. W.; Murugesan, V., Enhancement of Photocatalytic Activity by Metal Deposition: Characterisation and Photonic Efficiency of Pt, Au and Pd Deposited on TiO2 Catalyst, Water Res. 2004, 38, 3001-3008.


23


Page 24 of 39

(12) Ong, W. –J.; Tan, L. –L.; Ng, Y. H.; Yong, S. –T.; Chai, S. –P., Graphitic Carbon Nitride (g-C3N4)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability?, Chem. Rev. 2016, 116, 7159-7329. (13) Tian, J.; Abdullah, Q. L.; Asiri, M.; Sun, X.; He, Y., Ultrathin Graphitic C3N4 Nanofibers: Hydrolysis-driven Top-down Rapid Synthesis and Application as a Novel Fluorosensor for Rapid, Sensitive, and Selective Detection of Fe3, Sens. Actuators B 2015, 216, 2015, 453460. (14) Tian, J.; Liu, Q.; Asiri, A. M.; Alamry, K. A.; Sun, X., Ultrathin Graphitic C3N4 Nanosheets/Graphene Composites: Efficient Organic Electrocatalyst for Oxygen Evolution Reaction, ChemSusChem 2014, 7, 2014, 2125-2130. (15) Tian, J.; Liu, Q.; Asiri, A. M.; Al-Youbi, A. O.; Sun, X., Ultrathin Graphitic Carbon Nitride Nanosheet: A Highly Efficient Fluorosensor for Rapid, Ultrasensitive Detection of Cu2+, Anal. Chem. 2013, 85 (11), 5595–5599. (16) Malik, R.; Tomer, V. K.; Dankwort, T.; Mishra, Y. K.; Kienle, L., Cubic Mesoporous PdWO3 Loaded Graphitic Carbon Nitride (g-CN) Nanohybrids: Highly Sensitive and Temperature Dependent VOCs Sensors, J. Mater. Chem. A 2018, 6, 10718-10730. (17) Malik, R.; Tomer, V. K.; Kienle, L.; Chaudhary, V.; Nehra, S. P.; Duhan, S., Ordered Mesoporous Ag-ZnO@g-CN Nanohybrid as Highly Efficient Bifunctional Sensing Material, Adv. Mater. Inter. 2018, 5, 1701357(1)-1701357(13). (18) Malik, R. Tomer, V. K.; Chaudhary, V.; Dahiya, M. S.; Sharma, A.; Nehra, S. P., Duhan, S.; Kailasam, K., Excellent Humidity Sensor Based on In-SnO2 Loaded Mesoporous Graphitic Carbon Nitride, J. Mater. Chem. A 2017, 5, 14134-14143. (19) Lee, E. Z.; Jun, Y. –S.; Hong, W. H.; Thomas, A.; Jin, M. M., Cubic Mesoporous Graphitic Carbon(IV) Nitride: An All-in-One Chemosensor for Selective Optical Sensing of Metal Ions, Angew. Chem. Int. Ed. 2010, 49, 9706-9710. (20) Shi, X.; Fujitsuka, M.; Lou, Z.; Zhang, P.; Majima, T., In Situ Nitrogen-Doped HollowTiO2/g-C3N4 Composite Photocatalyst with Efficient Charge Separation Boosting Water Reduction under Visible Light, J. Mater. Chem. A 2017, 5, 9671-9681. (21) Wei, X.; Shao, C.; Li, X.; Lu, N.; Wang, K.; Zhang, Z.; Liu, Y., Facile In Situ Synthesis of Plasmonic Nanoparticles-decorated g-C3N4/TiO2 Heterojunction Nanofibers and Comparison Study of their Photosynergistic Effects for Efficient Photocatalytic H2 Evolution, Nanoscale 2016, 8, 11034-11043.


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


(22) Shen, L.; Xing, Z.; Zou, J.; Li, Z.; Wu, X.; Zhang, Y.; Zhu, Q.; Yang, S.; Zhou, W., Black TiO2 Nanobelts/g-C3N4 Nanosheets Laminated Heterojunctions with Efficient VisibleLight-Driven Photocatalytic Performance, Sci. Reports 2017, 7, 41978(1)-41978(11). (23) Wei, H.; McMaster, W. A.; Tan, J. Z. Y.; Cao, L.; Chen, D.; Caruso, R. A., Mesoporous TiO2/g-C3N4 Microspheres with Enhanced Visible-Light Photocatalytic Activity, J. Phys. Chem. C 2017, 121, 22114-22122. (24) Elbanna, O.; Fujitsuka, M.; Majima, T., Composite for H2 Evolution under Visible-Light Irradiation and Its Charge Carrier Dynamics, ACS Appl. Mater. Interfaces 2017, 9, 3484434854. (25) Malik, R.; Tomer, V. K.; Chaudhary, V.; Dahiya, M. S.; Nehra, S. P.; Rana, P. S.; Duhan, S., Ordered Mesoporous In-(TiO2/WO3) Nanohybrid: An Ultrasensitive n-Butanol Sensor, Sens. Actuators B 2017, 239, 364-373. (26) Bai, J.; Zhou, B., Titanium Dioxide Nanomaterials for Sensor Applications, Chem. Rev. 2014, 114, 10131-10176. (27) Gong, M.; Li, Y.; Guo, Y.; Lv, X.; Dou, X., 2D TiO2 Nanosheets for Ultrasensitive Humidity Sensing Application Benefited by Abundant Surface Oxygen Vacancy Defects, Sens. Actuators B 2018, 262, 350-358. (28) Tomer, V. K.; Duhan, S., Ordered Mesoporous Ag-doped TiO2/SnO2 Nanocomposite Based Highly Sensitive and Selective VOC Sensors, J. Mater. Chems. A. 2016, 4, 1033-1043. (29) Tomer, V.K.; Duhan, S.; Nano titania loaded mesoporous silica: preparation and application as high performance humidity sensor, Sens. Actuators B, 2015, 220, 192-200. (30) Zang, Y.; Li, L.; Xu, Y.; Zuo, Y.; Li, G., Hybridization of Brookite TiO2 with g-C3N4: A visible-light-driven Photocatalyst for As3+Oxidation, MO Degradation and Water Splitting for Hydrogen Evolution, J. Mater. Chem. A 2014, 2, 15774-15780. (31) Liu, Q.; Zhang, J., Graphene Supported Co-g-C3N4 as a Novel Metal–Macrocyclic Electrocatalyst for the Oxygen Reduction Reaction in Fuel Cells, Langmuir 2013, 29, 38213828. (32) Malik, R.; Rana, P. S.; Tomer, V. K.; Chaudhary, V.; Nehra, S. P.; Duhan, S., Nano Gold Supported on Ordered Mesoporous WO3/SBA-15 Hybrid Nanocomposite for Oxidative Decolorization of Azo Dye, Micropor. Mesopor. Mater. 2016, 225, 245-254 (33) Xing, R.; Li, Q.; Xia, L.; Song, J.; Xu, L.; Zhang, J.; Xie, Y.; Song, H., Au-modified Threedimensional In2O3 Inverse Opals: Synthesis and Improved Performance for Acetone Sensing Toward Diagnosis of Diabetes, Nanoscale 2015, 7, 13051-13060.


25


Page 26 of 39

(34) Kleitz, F.; Choi, S. H.; Ryoo, R., Cubic Ia3d Large Mesoporous Silica: Synthesis and Replication to Platinum Nanowires, Carbon Nanorods and Carbon Nanotubes, Chem. Commun. 2003, 2003, 2136-2137. (35) Tomer, V. K.; Devi, S.; Malik, R.; Duhan, S., Chapter in Mesoporous Materials & Their Nanocomposites, (Eds: P. M. Visakh, M. J. M. Morlanes), Wiley-VCH Verlag, Weinheim, Germany, 2016, DOI: 10.1002/9783527683772.ch7 (36) Tomer, V. K.; Jangra, S.; Malik, R.; Duhan, S., Effect of in-situ loading of nano Titania particles on structural ordering of mesoporous SBA-15 framework, Colloids Surf A Physicochem Eng Asp. 2015, 466, 160-165. (37) Tomer, V. K.; Singh, K.; Kaur, H.; Shorie, M.; Sabherwal, P., Rapid Acetone Detection using Indium loaded WO3/SnO2 Nanohybrid Sensor, Sens. Actuators B 2017, 253, 703-713. (38) Tomer, V. K.; Malik, R.; Kailasam, K., Near Room Temperature Ethanol detection using Ag-loaded Mesoporous Carbon Nitrides, ACS Omega 2017, 2, 3658−366. (39) Shi, L.; Liang, L.; Ma, J.; Wang F.; Sun, J., Enhanced Photocatalytic Activity Over the Ag2O–g-C3N4 Composite under Visible Light, Catal. Sci. Technol. 2014, 4, 758-765. (40) Yang, Y.; Guo, Y.; Liu, F.; Yuan, X.; Guo, Y.; Zhang, S.; Guo, W.; Huo, M., Preparation and enhanced visible-light photocatalytic activity of silver deposited graphitic carbon nitride plasmonic photocatalyst, Appl. Catal., B 2013, 142, 828 (41) Poonia, E.; Mishra, P. K.; Kiran, V.; Sangwan, J.; Kumar, R.; Rai, P. K.; Tomer, V. K., Aero-gel assisted synthesis of anatase TiO2 nanoparticles for humidity sensing application, Dalton Trans., 2018,47, 6293-6298 (42) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F., Handbook of X-ray Photoelectron Spectroscopy, Perkin Elmer, Eden Prairie, 1979, p. 81. (43) Cui, Y.; Ding, Z.; Liu, P.; Antonietti, M.; Fua. X.; Wang, X., Metal-free Activation of H2O2 by g-C3N4 under Visible Light Irradiation for the Degradation of Organic Pollutants, Phys. Chem. Chem. Phys. 2012, 14, 1455–1462. (44) Yin, X.; Battaglia, C.; Lin, Y.; Chen, K.; Hettick, M.; Zheng, M.; Chen, C. –Y.; Kiriya, D.; Javey, A., 19.2% Efficient InP Heterojunction Solar Cell with Electron Selective TiO2 Contact, ACS Photonics 2014, 1, 1245-1250. (45) Jiang, L.; Yuan X., Zeng, G.; Chen, X.; Wu, Z.; Liang, J.; Zhang, J.; Wang, H.; Wang, H., Phosphorus- and Sulfur-Codoped g-C3N4: Facile Preparation, Mechanism Insight, and Application as Efficient Photocatalyst for Tetracycline and Methyl Orange Degradation under Visible Light Irradiation, ACS Sustainable Chem. Eng. 2017, 5, 58315841. ACS Paragon Plus Environment

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(46) Tian, J.; Ning, R.; Liu, Q.; Asiri, A. M.; Youbi, A. O. A; Sun, X. Three-Dimensional Porous Supramolecular Architecture from Ultrathin g-C3N4 Nanosheets and Reduced Graphene Oxide: Solution Self-Assembly Construction and Application as a Highly Efficient MetalFree Electrocatalyst for Oxygen Reduction Reaction, ACS Appl. Mater. Interfaces, 2014, 6 (2), 1011–1017. (47) Gao, T.; Tillman E. S.; Lewis, N. S., Detection and Classification of Volatile Organic Amines and Carboxylic Acids Using Arrays of Carbon Black-Dendrimer Composite Vapor Detectors, Chem. Mater. 2005, 17, 2904-2911. (48) Duhan, S.; Tomer, V.K., Mesoporous Silica: Making “Sense” of Sensors, in “Advanced Sensor and Detection Materials”, Wiley-Scrivener, U.S.A, 149-192, (2014) (49) Guo, T.; Luo, Y.; Zhang, Y.; Lin, Y. –H.; Nan, C.-W., ZnO-NiO Hetero-nanostructures as Highly Sensitive and Selective, Triethylamine Sensor, J. App. Phy. 2014, 116, 044309(1)044309(6). (50) Ju, D.; Xu, H.; Qiu, Z.; Guo, J.; Zhang, J.; Cao, B., Highly Sensitive and Selective Triethylamine-sensing Properties of Nanosheets Directly Grown on Ceramic Tube by Forming NiO/ZnO PN Heterojunction, Sens. Actuators B 2014, 200, 288-296. (51) Sui, L.-L.; Xu, Y.-M.; Zhang, X.-F.; Cheng, X.-L.; Gao, S.; Zhao, H.; Cai, Z.; Huo, L.-H., Construction of Three-dimensional Flower-like α-MoO3 with Hierarchical Structure for Highly Selective Triethylamine Sensor, Sens. Actuators B 2015, 208, 406. (52) Lalevée, J.; Allonas, X.; Fouassier, J. –P., N−H and α(C−H) Bond Dissociation Enthalpies of Aliphatic Amines, J. Am. Chem. Soc. 2002, 124, 9613-9621. (53) Wayner, D. D. M.; Clark, K. B.; Rauk, A.; Yu, D.; Armstrong, D. A., C−H Bond Dissociation Energies of Alkyl Amines: Radical Structures and Stabilization Energies, J. Am. Chem. Soc. 1997, 119, 8925-8932. (54) http://digitool.library.mcgill.ca/webclient/StreamGate?folder_id=0&dvs=1522461177604~8 18 (55) Singh, H.; Tomer, V. K.; Jena, N.; Bala, I.; Sharma, N.; Nepak, D.; Sarkar, A. D.; Kailasam, K.; Pal, S. K., Truxene based Porous, Crystalline Covalent Organic Frameworks and it’s Applications in Humidity Sensing, J. Mater. Chems. A 2017, 5, 21820-21827. (56) Tomer, V. K.; Duhan, S.; Sharma, A. K.; Malik, R.; Jangra, S.; Nehra, S. P.; Devi, S.; Humidity Sensing Properties of Ag0 Nano Particles Supported WO3-SiO2 with Super Rapid Response and Excellent Stability, Europ. J. Inorg. Chems. 2015, 31, 5232–5240 (57) Qu, F.; Yuan, Y.; Guarecuco, R.; Yang, M., Low Working-Temperature Acetone Vapor Sensor Based on Zinc Nitride and Oxide Hybrid Composites, Small 2016, 12, 3128-3133. ACS Paragon Plus Environment

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(58) Fuente, J. R.; Jullian, C.; Saitz, C.; Neira, V; Poblete, O.; -Sanchez, E. S., Unexpected Formation of 1-diethylaminobutadiene in Photosensitized Oxidation of Triethylamine Induced by 2, 3-dihydro-oxoisoaporphine Dyes. A 1H NMR and isotopic exchange study, J. Org. Chem. 2005, 70, 8712-8716. (59) Xu, L.; Song, H.; Hu, J.; Lv, Y.; Xu, K., A Cataluminescence Gas Sensor for Triethylamine Based on Nanosized LaF3–CeO2, Sens. Actuators B 2012, 169, 261-266. (60) Joshi, N.; Silva L. F. da.; Jadhav, H. S.; Shimizu, F. M.; Suman, P. H.; M’Peko, J. –C.; Orlandi, M. O.; Gil Seo, J.; Mastelaro V. R.; Oliveira Jr O. N., Yolk-shelled ZnCo2O4 Microspheres: Surface Properties and Gas Sensing Application, Sens. Actuators B 2018, 257, 906-915. (61) Joshi, N.; Silva, L. F. da; Jadhav, H. S.; M'Peko, J. –C.; Torres, B. B. M.; Aguir, K.; Mastelaroa, V. R.; Oliveira Jr O. N., One-step Approach for Preparing Ozone Gas Sensors Based on Hierarchical NiCo2O4 Structures, RSC Adv. 2016, 6, 92655-92662. (62) Tomer, V. K.; Thangaraj, N.; Gahlot, S.; Kailasam, K., Cubic mesoporous Ag@CN: a high performance humidity sensor, Nanoscale, 2016,8, 19794-19803 (63) Malik, R.; Tomer, V. K.; Chaudhary, V.; Dahiya, M. S.; Nehra, S. P.; Duhan, S.; Kailasam, K., A Low temperature, Highly Sensitive and Fast Response Toluene Gas Sensor based on In(III)-SnO2 Loaded Cubic Mesoporous Graphitic Carbon Nitride, Sens. Actuators B 2018, 255, 3564-3575.


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Sample KIT-6 m-CN TiO2@m-CN Au-TiO2@m-CN Au/TiO2 a

SBET (m2/g)a 842 185 143 118 64

DP (nm)b

VP (cm3/g)c

8.6 5.8 5.1 4.7 4.2

1.29 0.96 0.75 0.66 0.48

SBET: Total surface area; b DP: Pore size; c VP: Pore volume

Table 1: Physiochemical properties of KIT-6, m-CN, TiO2@m-CN, Au-TiO2@m-CN and AuTiO2 nanohybrids


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Figure 1: (A) Wide angle-XRD and (B) Low angle-XRD spectra of (a) KIT-6, (b) m-CN, (c) TiO2@m-CN (d) Au-TiO2@m-CN and (e) Au-TiO2 nanohybrids


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Figure 2: (A) N2 adsorption-desorption isotherms curves and (B) BJH Pore size distribution of (a) KIT-6, (b) m-CN, (c) TiO2@m-CN, (d) Au-TiO2@m-CN and (e) Au-TiO2 nanohybrids


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Figure 3: HRTEM image showing uniform channels with long range order of, (a,b) KIT-6, (c,d) Au-TiO2@m-CN, (e) SEM image and (f-j) corresponding elemental distribution in Au-TiO2@mCN material


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Figure 4: (a) The comparison of visible light photocatalytic activity in terms of degradation of MO dye and (b) First order kinematics study for g-CN, m-CN, TiO2@m-CN, Au-TiO2@m-CN and Au-TiO2 nanohybrid and (c) Photodegradation of MO dye over Au-TiO2@m-CN


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Figure 5: (a) Effect of pH, (b) dye concentration, (c) catalyst concentration and (d) comparison with other AZO dyes for Au-TiO2@m-CN nanohybrid


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Figure 6: The proposed mechanism for the degradation of MO dye over Au-TiO2@m-CN nanohybrid


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Figure 7: (a) Dye and TOC degradation of various AZO dyes for Au-TiO2@m-CN and (b) Reusability/ stability of Au-TiO2@m-CN in five cycles of continuous degradation loops.


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Figure 8: (a) Responses of as-prepared materials to 50 ppm TEA gas at different operating temperatures (100–300 °C) and (b) Responses of the Au-TiO2@m-CN sensor to different VOAs in the range from 1–500 ppm at 175 °C; (c) The long-term stability of mesoporous Au-TiO2@m-CN sensor to 50 ppm TEA gas measured for 5 weeks at 175 °C; (d) The response and recovery times of mesoporous Au-TiO2@m-CN to all the tested VOAs (50 ppm) at 175 °C; (e) Typical real time response curves of Au-TiO2@m-CN sensor when exposed to different concentrations of TEA (1100 ppm) gas at 175 °C; (f) Response of Au-TiO2@m-CN sensor towards VOAs (50 ppm) measured at different relative humidity conditions (10-95 %RH).


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Figure 9: Dynamic sensing performance of Au-TiO2@m-CN sensor toward 50 ppm of various gases. (Inset) ratio between the response of TEA and to other test gases


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Table of contents:


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Au-TiO2 loaded cubic g-C3N4 nanohybrids for photocatalytic and

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