UV Illumination Enhanced Molecular Ammonia Detection Based On

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UV Illumination Enhanced Molecular Ammonia Detection Based On Ternary Reduced Graphene Oxide-Titanium Dioxide-Au Composite Film at Room Temperature Yong Zhou, Xian Li, Yanjie Wang, Huiling Tai, and Yongcai Guo Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b04347 • Publication Date (Web): 31 Dec 2018 Downloaded from http://pubs.acs.org on January 1, 2019

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Analytical Chemistry

UV Illumination Enhanced Molecular Ammonia Detection Based On Ternary Reduced Graphene Oxide-Titanium Dioxide-Au Composite Film at Room Temperature Yong Zhoua, Xian Lib, Yanjie Wanga, Huiling Taic, Yongcai Guoa aKey

Laboratory of Optoelectronic Technology and System of

Ministry of Education, College of Optoelectronic Engineering, Chongqing University, Chongqing 400044, PR China bAgricultural

Information Institute, Chinese Academy of Agricultural

Sciences, Key Laboratory of Agricultural Information Service Technology of Ministry of Agriculture, Beijing 100081, China cState

Key Laboratory of Electronic Thin Films and Integrated

Devices, School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, China

Corresponding author. Email address: [email protected] (Y. Zhou).

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ABSTRACT In this work, we report on UV illumination enhanced roomtemperature trace NH3 detection based on ternary composites of reduced graphene oxide nanosheets (rGO), titanium dioxide nanoparticles (TiO2) and Au nanoparticles as the sensing layer, which is firstly reported by far. The effect of UV state as well as componential combination and content on the sensing behavior disclosed that, rGO nanosheets served as not only a template to attach TiO2 and Au, but an effective electron collector and transporter; TiO2 nanoparticles acted as a dual UV and NH3 sensitive material; Au nanoparticles could increase the sorption sites and promote charge separation of photoinduced electron-hole pairs. The as-prepared rGO/TiO2/Au sensors were endowed with a sensing response of 8.9% toward 2 ppm NH3, a sensitivity of 1.43×10-2/ppm within the investigated range, nice selectivity, robust operation repeatability and stability, which was fairly competitive in comparison with previous work. Meanwhile, the experimental results provided clear evidence of inspiring UV-enhanced gas detection catering for the future demand of low power-consumption and high sensitivity. Key words: Ammonia; Gas sensor; Reduce graphene oxide; Titanium dioxide; Au nanoparticle; UV illumination; Room temperature.


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INTRODUCTION With the ever-increasing expansion of human activities especially in the agricultural field, unreasonable farming management has caused a majority of nitrogen (NH3 as a main reactive nitrogen) to be released outside, inducing an urgent necessity of sensitive monitoring. As one of the most harmful and toxic pollution gases, NH3 was extremely detrimental to ecological harmony and human health. NH3 can bring irritation to skins, eyes, respiratory tracts as well as lung diseases arising from particulate matter less than 2.5 μm (PM2.5) formed when NH3 reacted with other air pollutants (NOx or SOx). National Institute for Occupational Safety and Health (NIOSH) in the United States has established a recommended exposure limit for NH3 of 35 ppm in 15 min and 25 ppm in 8 hours.1 Hence, effective supervision of NH3 emission at a very low dose is of great importance. Among various NH3 detection technologies, chemiresistive sensors2-5 prevail over their optical,6,

7

electrochemical,8,

9

and mass-

sensitive counterparts10, 11 due to the distinct merits of handy measurement, easy fabrication and high cost-effectiveness. In the case of chemiresistive sensors, two-dimensional graphene and its derivatives have exhibited a vast potential as a molecular recognition platform on account of large specific surface area, excellent conductivity and low noise level,12,

13

but at the expense of tiny response and poor 3

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selectivity particularly for weakly reducing NH3 molecules. Another class of NH3 sensitive materials is metal oxides represented by TiO2 owing to its chemical

activity,

eco-friendliness

and

photocatalysis

properties.

Undesirably, the crucial drawbacks encountered are high-temperature operation and severe cross-sensitivity, seriously hindering its further applications. To circumvent obstacles stated above, an alternative strategy is to incorporate graphene or TiO2 with other sensing elements particularly noble metal nanoparticles (i.e., Au, Ag, Pd, Pt) owing to the unique advantages of electronic and chemical sensitizations. Benefiting from the synergistic effect of multi-component composites, graphene derivatives, TiO2 and noble metals are routinely combined in the form of binary composites active in a multitude of areas such as photocatalytic

pollutant

degradation,14

energy

conversion,15,

16

electrochemical detection17, 18 and gas/vapor sensing.19-26 Analogously, a surge of ternary counterparts has been increasingly emerging in these domains during the last few years.27-33 Besides, auxiliary light activation can yet be regarded as an effective approach to improve the sensor performance. Recent reports on binary composites have provided clear evidence of UV illumination-enhanced sensor performance.34-36 In this aspect, however, ternary composites have been scarcely explored. Meanwhile, systematic research on the effect of material ingredients 4 ACS Paragon Plus Environment

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(componential combination and content) on sensor response is rarely conducted before to our best knowledge, especially for ternary composite film based sensors. In view of above facts, within this work we tried to synthesize ternary rGO/TiO2/Au composites, and then explored the NH3-sensing performance under UV illumination at room temperature (25 oC), which was reported for the first time. Thereinto, rGO served as an excellent conducting platform beneficial for resistance measurement without the need for heating elements, thereby lowering the power consumption. TiO2 materials displayed the dual functions of electronic reactivity and photocatalysis under UV illumination. Au nanoparticles were responsible for the electronic and chemical sensitizations. It is highly anticipated that a great performance improvement will be exhibited in comparison with other binary counterparts. Meanwhile, a detailed sensing mechanism involving photocatalysis and charge transfer is proposed as well, which we hope can provide a new viewpoint to enrich the applicable transduction mechanisms. EXPERIMENTAL SECTION Materials Preparation. Au nanoparticles were prepared by reducing HAuCl4.3H2O with sodium citrate via a solution method, which was discussed in Supporting Information. As shown in Figure S1, the mixture solution underwent a 5 ACS Paragon Plus Environment


gradual color variation from yellow to purple red. Furthermore, UV-vis absorption spectrum of the resulting solution (Figure S2) showed an absorption peak at about 527 nm. These facts verified the successful formation of Au nanoparticles. The obtained Au nanoparticle solution was kept at 4 oC for future use. Then rGO/TiO2 composites was synthesized via a hydrothermal method with GO and tetrabutyl titanate as the precursors of rGO and TiO2. The detailed preparation procedure was described in Supporting Information. Ternary rGO/TiO2/Au composites were obtained through mixing specific volume of Au nanoparticle solution into rGO/TiO2 composite solution by the ultrasonic cell crusher at a power of 150 W for 2 h. Measurement Setup and Procedure. The schematic test apparatus was illustrated in Figure S3. The employed planar interdigital electrodes (IDEs) were fabricated by photolithography and lift-off methods, with an active area of 1.1 cm×0.7 cm, both finger gap and width of 50 µm, and Au/Ti layer thickness of 120 nm/40 nm on SiO2/Si substrate, as shown in Figure 1.


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Figure 1. Planar IDEs: (a) schematic and (b) real images.

Additionally,

the

sensor

fabrication,

test

requirements,

characterization instruments and some measurement results were discussed in Supporting Information. The performance parameters including sensing response, response/recovery times and sensitivity were defined there. It should be stressed that N2 was utilized instead of air as the carrier and purification gas during the sensing tests, unless stated otherwise. Two aspects were considered for this treatment. On one hand, oxygen under continuing UV light illumination probably converts into ozone gas (O3) which remains elusive. On the other hand, within oxygen-enriched atmosphere, TiO2 material under UV illumination has been validated capable to oxidize NH3 into some intermediate products such as N2, NO, N2O, water, etc.37-41 To exclude the impact of the unexpected O3 and NH3 7 ACS Paragon Plus Environment


loss on the sensing behavior, high-purity dry nitrogen was adopted.

RESULTS AND DISCUSSION Materials Characterizations. X-ray diffraction (XRD) spectra were conducted to investigate the componential information of the as-prepared rGO/TiO2 and rGO/TiO2/Au composites (Figure 2). In both samples, the diffraction peaks at 25.7o, 48.4 o,

56.3 o, 62.5 o and 69.1 o were indexed to (101), (200), (211), (118) and

(116) crystal planes of anatase TiO2 nanoparticles, respectively.42 Another character peak at 38.3o presented an enhanced intensity and sharpness after Au addition mainly due to the overlapped crystal surfaces of Au (111) and TiO2 (004), consistent with other reports.43, 44 No characteristic peaks for the carbon species were observed because of the relatively low diffraction intensity of rGO material.45


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Figure 2. XRD spectra of rGO/TiO2 and rGO/TiO2/Au composites.

Figure 3 showed transmission electron microscopy (TEM) features of the ternary composites. The panoramic image in Figure 3a displayed an extensive distribution of TiO2 nanoparticles (light black, particle diameter: ca.10 nm) onto the basal plane of rGO nanosheets (gray). A few Au nanoparticles (dark black, particle diameter: ca.20 nm) were discretely scattered on the TiO2 surface (Figure 3b). Furthermore, high-resolution transmission electron microscopy image (HRTEM) in Figure 3c revealed the lattice spacings of 0.34 nm, 0.24 nm and 0.20 nm corresponding to TiO2 (101) crystal surface as well as Au surfaces (111) and (200). Moreover, the selected area electron diffraction patterns (SAED) in Figure 3d showed a remarkable consistence with the XRD results.



Figure 3. TEM images of (a) ternary composites and (b) local magnification of (a), (c) HRTEM picture and (d) SAED patterns.

The detailed surface chemical composites and oxidation states of ternary composites were confirmed by X-ray photoelectron spectroscopy (XPS) in Figure S4. The full XPS curve (Figure S4a) verified the coexistence of Ti 2p, Ti 3p, O 1s and C 1s peaks. Among these peaks, C 1s resulted from rGO template with three peaks of C-C/C=C (283.3 eV), C-O (283.8 eV) and C=O (287.4 eV) (Figure S4b). O 1s (Figure S4c) could be deconvoluted into three separate peaks: chemisorbed oxygen (O−, 531.4 eV), lattice oxygen (O2−, 529.2 eV) and physisorbed oxygen (O2−, 532.7 10 ACS Paragon Plus Environment

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eV), demonstrating the oxygen adsorption on the material surface.46 Typical bonding of Ti-O at 457.8 eV (Ti 2p3/2) and 463.5 eV (Ti 2p1/2) in the Ti 2p spectrum (Figure S4d) indicated the state of Ti4+ in the sample. However, no obvious Au character peaks were recognized probably due to its small loading content. Figure S5 showed the Raman spectra of GO and rGO/TiO2 samples for two measurements. In GO material, two representative peaks at 1343 cm-1 and 1583 cm-1 were ascribed to D peak (a defect peak due to inter-valley scattering) and G peak (the graphitic hexagon-pinch mode). As for rGO/TiO2 sample, besides the character peak Eg(1) of anatase TiO2 at 151 cm-1,27 Raman blueshift of both D and G peaks meant a strong rGO-TiO2 interaction. Additionally, the larger intensity ratio of D and G peaks ID/IG for GO than rGO/TiO2 meant GO reduction to rGO in the hydrothermal process.47 Figure S6 showed Fourier transform infrared (FTIR) spectra of GO and rGO/TiO2 samples. The characteristic peaks of GO, containing O-H stretching at 3443 cm-1, skeletal vibration of unoxidized graphitic domains at 1635 cm-1, C-OH deformation at 1400 cm1,

and C-O stretching at 1069 cm-1, were clearly observed in the GO

spectrum. Regarding rGO/TiO2 sample, the peak intensity of each oxygencontaining group notably decreased in comparison with GO counterpart, indicating a successful reduction of GO to rGO as well. In addition, the broad band between 600cm-1~1000 cm-1 was characteristic of Ti-O 11 ACS Paragon Plus Environment


stretching.48 The peak at 472 cm-1 corresponded to Ti-O-C stretching of the anatase TiO2.49 Sensor Performance. Prior to target gas exposure, the time-resolved relative resistance variations of both rGO/TiO2 and rGO/TiO2/Au sensors were exhibited in Figure 4 under continual N2 purification and alternating UV on/off state. The initial UV on state ensured a steady baseline resistance. Distinctly, only rGO/TiO2/Au sensor displayed a periodical and reversible UV sensing over five cyclic periods, indicative of Au-incorporation enhanced charge separation.

Figure 4. Time-resolved relative resistance variation of both rGO/TiO2 and rGO/TiO2/Au sensors within N2 atmosphere during UV on/off switching.

To unveil the effect of UV light on NH3 sensing, rGO/TiO2/Au sensor 12 ACS Paragon Plus Environment

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was then exposed to NH3 gas (10 ppm as a case study) in the absence/present of UV light. It was worth noting that rGO/TiO2/Au sensors exhibited a resistance drop within NH3 atmosphere irrespective of the UV state (Figure. S7a and S7b), signifying n-type semiconducting properties of the sensing layer. After illumination, the baseline resistance decreased dramatically due to abundant photogenerated electrons. Moreover, a better performance (Figure 5a) was manifested in terms of enhanced sensor response (17.8% vs. 15.7%), accelerated response speed (244 s vs. 478 s) and improved recovery level (75% vs. 32%) than the dark case. In a typical assay, we investigated the response kinetics of four individual sensors with different component combinations (denoted as rGO, rGO/Au, rGO/TiO2 and rGO/TiO2/Au sensors) toward 4 ppm NH3 under UV illumination (Figure 5b). Each recipe solution of equal volume and settled component amount (rGO: 0.1 mg/mL; TiO2: 1:96; Au: 200 μL) was sprayed onto IDEs to prepare the respective sensors. Herein, the proportion 1:96 about TiO2 amount represented the weight ratio of GO powder and tetrabutyl titanate before the hydrothermal reaction. It was found that rGO and rGO/Au sensors showed a resistance increase after NH3 adsorption while rGO/TiO2 and rGO/TiO2/Au counterparts an opposite one, representing the corresponding p and n-type characters. It has been well documented that rGO always acts as a p-type material due to molecular adsorption of 13 ACS Paragon Plus Environment


ambient oxygen and water vapor, and shows a negligible gas response (0.5% here). After Au decoration, a five-time response enhancement was achieved with unchanged semiconductor polarity. However, the opposite polarity appeared only after TiO2 incorporation, reflecting the change of prominent conducting paths from p-type rGO to n-type TiO2. In addition, the response of both rGO/TiO2 and rGO/TiO2/Au sensors attained 9%, nearly four times larger than rGO/Au one.

Figure 5. (a) The impact of UV state on the sensing response of rGO/TiO2/Au sensor, (b) ingredient combination-dependent sensor response on exposure to NH3 gas under UV illumination.

With the issue that how the componential amount (especially Au) 14 ACS Paragon Plus Environment

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impacted the sensor performance under UV illumination, the relevant experiments were subsequently conducted along with fixed rGO content for the sake of convenient comparison analysis. Firstly, TiO2-content dependent sensor response was explored (Figure 6a). Same as stated above, the polarity of resistance variation changed after TiO2 incorporation. A nearly four-fold larger response was obtained for rGO/TiO2/Au sensor (1:48) than rGO/Au one. With growing TiO2 amount, the response initially increased (1:48 to 1:96), and then saturated (1:144) similar to 1:96 case. Thus, excess TiO2 seemed adversely favorable for response enhancement arising from the confined gas accessibility into rGO/TiO2 interfaces. Subsequently, different Au amounts (0, 50, 100, 150 and 200 μL) were blended into rGO/TiO2 solution (TiO2 amount: 1:96) to prepare a series of sensors (Figure 6b). On exposure to 10 ppm NH3, the sensor with 50 μL Au showed a larger response than rGO/TiO2 one (17.5% vs. 10%). However, the response just began to increase (20.6%) till Au amount was added to 150 μL. Further Au augment (200 μL) inversely weakened the response to 10% comparable to rGO/TiO2 one. Following these results, Au decoration could sensitize the gas-solid reaction just at an optimal content while excess content hindered this behavior by occupying the sorption sites. We then adopted 0.1 mg/mL rGO, 150 μL Au and 1:96 TiO2 as the componential content to prepare ternary composite sensor in the following 15 ACS Paragon Plus Environment


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part. In the research of repeatability (Figure 6c), four successive responses toward 10 ppm NH3 showed a tiny perturbation (18.7%, 18.3%, 18.2% and 18.1%) with a mean square error (mse) of 2.6‰. Noteworthy was that the recovery degree improved with increasing cycles. As the greater probability of irreversible occupation of high-energy adsorption sites occurred over more cycles, an easier NH3 desorption from low-energy ones was

thus

undertaken

under

reestablished

adsorption/desorption

equilibrium. Then this test period was conducted within seven days (Figure 6d). The small response discrepancy (18.8%, 18.6%, 19.4% and 19.8%, mse=5.5‰) demonstrated an inspiring operation stability. The prepared TiO2 material here exhibited a primary low-energy (101) crystal facet,50, 51 thereby resulting in the robust sensor reliability.


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Figure 6. The effect of (a) TiO2 and (b) Au content on response, (c) repeatability and (d) stability of rGO/TiO2/Au sensor toward 10 ppm NH3 under UV illumination.

As shown in Table S1, we summarized some relevant reports on chemiresistive NH3 sensors using binary or ternary composites of carbon nanomaterials, noble metal and metal oxide, and compared the sensor performance in terms of operation temperature (OT), gas concentration and sensing response. When the OT was set at room temperature or below, most sensors exhibited a rather weak response even at higher concentration.19, 20, 53-56 Although sporadic reports showed a comparable response with this work, larger concentration and elevated OT were simultaneously required.52 Considering all these parameters, rGO/TiO2/Au sensors presented an inspiring prospect on trace NH3 detection. 17 ACS Paragon Plus Environment


Then the dynamic response of rGO/TiO2/Au sensor as a function of NH3 concentration under UV illumination was researched in Figure 7a. The monotonous response increase with NH3 concentration (2, 4, 6, 8 and 10 ppm) exhibited a sensitivity of 1.43×10-2/ppm and a nice linearity r2=0.992 (Figure 7b). Moreover, the ternary sensor displayed an excellent specificity toward NH3 in comparison with other reducing interference species including 10 ppm H2S, CO, HCHO and H2 (Figure 7d). Thereinto, the response curves toward CO and H2S revealed smaller response and larger noise level than NH3 (Figure 7c). As for HCHO vapor (Figure S8), the abnormal response was stemmed from photocatalytic vapor degradation by TiO2 nanoparticles into CO2 and H2O.57 The good selectivity toward NH3 was mainly ascribed into three aspects. Firstly, the surface acidity of TiO2 nanoparticles containing Lewis acid sites (Ti4+) and Brønsted acid sites (TiOH) were favorable to boost molecular adsorption of basic ammonia.20 Secondly, the interaction between basic NH3 molecules and hydroxyl groups on TiO2 surface produced strong chemisorption of coordinative type while H2S was too difficult to form hydrogen bonds with these hydroxyl groups. Finally, it is well known that the adsorption affinity increases with the increasing electron-donor ability. In this case, the electron-donor ability of NH3 is stronger than H2S. As for CO and H2, TiO2 based sensors were always operated at elevated temperature (>120 oC) 18 ACS Paragon Plus Environment

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even simultaneously with the aid of UV illumination.58-60

Figure 7. (a) Time-resolved response of rGO/TiO2/Au sensor as a function of NH3 concentration spanning from 2 ppm to 10 ppm under UV illumination, (b) linearity, (c) NH3 response compared with CO and H2S, (d) selectivity.

Sensing Mechanism. From Figure 3a and 3b, we found that in the ternary composites Au nanoparticles were directly contacted with TiO2 nanoparticles and seldom with rGO nanosheets. Thus, both rGO/TiO2 and TiO2/Au heterojunctions were considered. For rGO/TiO2 and rGO/Au sensors, only rGO/TiO2 or rGO/Au junction was taken into account. The role of Au nanoparticles was explained as follows. In rGO/Au composites, free electrons transfer from rGO to Au through the contact interfaces owing to the work function difference (4.5 eV vs. 5.1 eV)36, 19 ACS Paragon Plus Environment


61leaded

to p-doping of rGO material. On TiO2 surface, pre-adsorbed

ambient O2 molecules consumed the surface electrons and created a nearby depletion region. In either group contrast of rGO vs. rGO/Au sensors or rGO/TiO2 vs. rGO/TiO2/Au sensors, Au incorporation could effectively adsorb and dissociate more O2 molecules, further doping rGO and/or TiO2. For rGO and rGO/Au sensors upon adsorbing NH3 gas, p-type rGO withdrew electrons from reducing NH3 molecules accompanied with the resistance increase. Hence, rGO/Au sensor with stronger p-type properties owned a larger response than rGO one. For rGO/TiO2 and rGO/TiO2/Au sensors, a mass of p-n rGO/TiO2 heterojunctions resulted in extended depletion region beneficial for NH3 adsorption. Additional TiO2/Au heterojunctions in rGO/TiO2/Au sensor made electrons flow from TiO2 to Au in dark condition prior to NH3 exposure, leading to thicker depletion region than rGO/TiO2 one. Under UV illumination, Au captured more UV lights. As the optical energy of UV light (365 nm, i.e., 3.4 eV) exceeded the bandgap of anatase TiO2 (3.2 eV), incident photons were converted into electron-hole pairs in TiO2 material. Then photogenerated electrons transferred from valence band to conduction band while the holes moved toward the surface detaching the O2- (ad) on the surface along with the discharge of oxygen gas O2 (g), as described in eq. (1-1). 20 ACS Paragon Plus Environment

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O2- (ad) + h+ ⇋ O2 (g) (1-1) With that, the conduction electrons flew to Au as the conduction band potential of TiO2 (-4.2 eV) was larger than Au, inducing effective charge separation. Hence, free electrons interacted more sufficiently with NH3 molecules, producing a larger response for rGO/TiO2/Au sensor than rGO/TiO2 counterpart. It should be stressed that no sustaining oxygen molecules were supplemented in N2 atmosphere and permanent discharge of abundant O2 (g) from the film surface would result in a significant response attenuation. However, a nice response repeatability appeared. It indirectly implied that only a tiny fraction of O2 (g) was detached during cyclic NH3 exposures, and reversible gas-solid reactions occurred with the condition of the interconversion between O2 (g) and O2- (ad) on the surface. As a highly conductive template, rGO nanosheet offered a TiO2 nucleation plane to alleviate the aggregation behavior, and simultaneously served as an effective electron collector and transporter in the sensing processes. Additionally, the recombination probability of electron-hole pairs within each sensing layer under illumination followed the relationship rGO>rGO/TiO2>rGO/TiO2/Au, bringing about a quick response speed and large recovery degree for rGO/TiO2/Au sensor verified in Figure 5b.27

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In conclusion, we prepared ternary rGO/TiO2/Au sensors to detect NH3 gas under UV illumination at room temperature. The sensor performance in terms of response, sensitivity, linearity, repeatability, stability and selectivity indicated an encouraging detection capability of trace NH3 gas of weak polarity. The detailed investigations of UV illumination, ingredient composition, and componential content on sensor response provide a clear strategy of rational design of composite materials applied for molecular recognition. In addition, the joint actions of photocatalysis effect and charge transfer strategy favorably induce novel multi-function optoelectronic devices. ASSOCIATED INFORMATION Supporting Information The

device

fabrication,

test

requirements,

materials

characterization and some sensing behaviors were discussed in Supporting Information. AUTHOR INFORMATION Corresponding Authors *Email:

[email protected].

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS 22 ACS Paragon Plus Environment

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This work was partially supported by National Natural Science Foundation of China (Grant No. 61704014).



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