Tunable Chemical Sensing Performance of Black Phosphorus by

Article pubs.acs.org/cm

Tunable Chemical Sensing Performance of Black Phosphorus by Controlled Functionalization with Noble Metals Soo-Yeon Cho,†,‡ Hyeong-Jun Koh,†,‡ Hae-Wook Yoo,†,‡ and Hee-Tae Jung*,†,‡ †

Department of Chemical and Biomolecular Engineering (BK-21 Plus), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea ‡ KAIST Institute for Nanocentury, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea S Supporting Information *

ABSTRACT: In this work, the effects of noble metal (Au and Pt) incorporation into black phosphorus (BP) were first investigated. Several important sensing results were observed as a result of the incorporation of Au or Pt into the BP surface. First, prior to incorporation, pristine BP only detects paramagnetic molecules, e.g., NO2 or NO. However, after incorporation with Pt, low concentration of H2 can be detected with high response amplitude. Furthermore, the H2 sensing performance reported in this study was found to be most sensitive as compared with that observed for a previously reported 2D H2 gas sensor. The second significant result was obtained after incorporation with Au, where the work function of BP was varied by the transfer of electrons from the Au nanoparticles, thereby inducing the effects of n-doping on p-type pristine BP. Accordingly, the response behavior of BP to oxidizing gas changed from a p-type response (negative resistance variation) to an n-type response (positive resistance variation). In addition, a highly stable, low noise baseline was achieved in the Au-incorporated BP channel material. Finally, because of the high chemical and ambient stability of the Au or Pt, the synthesized Au- or Pt-incorporated BP systems exhibited long-term stability, which is difficult to achieve when using other doping strategies. Overall, a significant step was taken toward the efficient control of the electrical/chemical sensitization level of BP and significant enhancement of superior chemical sensing performance of BP.

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materials, e.g., graphene and MoS2.7 BP has also been reported to exhibit superior chemical sensing performance. In particular, BP is selective for the detection of paramagnetic molecules, e.g., NO2, in addition to high sensitivity at a limit of detection (LOD) of ppb levels, an ultrafast response time, and roomtemperature operation.10 This superior and unique chemical sensing performance of BP makes it suitable as a channel material over other sensing materials, e.g., graphene, transition metal dichalcogenides (TMD), and metal oxide semiconductors. Nevertheless, to better control and further improve the performance of the BP-based electronic sensing devices, it is crucial to effectively tune the n- or p-type doping level of the

INTRODUCTION Black phosphorus (BP), a layered material of elemental phosphorus, is a rapidly emerging two-dimensional (2D) material and a potential candidate for various applications, e.g., nanoscale optoelectronics, rechargeable ion batteries, electrocatalysts, solar cells, thermoelectrics, and sensors.1−3 Among these applications, BP is particularly interesting for use in electronic sensing devices not only because of its unique electrical properties, e.g., a direct bandgap for all number of layers, a small intrinsic band gap, and anisotropic electrical or thermal conductance,4−6 but also because of its high chemical adsorption energy, less out-of-carrier conductance, and large adsorption sites caused by a puckered surface structure.7−9 Recent density functional theory (DFT) studies conducted on the molecular adsorption energy of BP suggest the use of BP as a high-performance chemical sensor because of its superior adsorption energy, which even surpasses that of other 2D © 2017 American Chemical Society

Received: April 2, 2017 Revised: July 31, 2017 Published: July 31, 2017 7197

DOI: 10.1021/acs.chemmater.7b01353 Chem. Mater. 2017, 29, 7197−7205

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Chemistry of Materials

Figure 1. Functionalization with noble metals (Au or Pt) and characterization of BP. (a−d) Schematic for the incorporation of Au or Pt into chemically exfoliated BP with a few layers using redox reactions and a device setup. Photographs of the (e) dispersions of a few layers and (f) vacuum-filtered films of Au/BP and Pt/BP. The films were transferred onto two-probe resistor-type devices via vacuum filtration (the artwork of photo is used with permission from KAIST). (g) UV−vis absorbance and (h) XRD spectrum demonstrate that high-resolution Au and Pt nanoparticles are finely incorporated into the BP system with a few layers.

BP channel by controlling the carrier concentration and functionalizing BP with chemical sensitization.11−13 If the carrier concentration and chemical sensitization levels are efficiently controlled, the selective analysis of target chemicals using a multichannel array with considerable functionalization can be realized.14,15 Despite the importance of adopting a customized doping strategy in BP-based optoelectronic devices, only a few materials, e.g., cesium carbonate (Cs2CO3) or molybdenum trioxide (MoO3)16 and Cu adatoms,17 as well as doping with potassium,18 have been incorporated into BP. The ambipolar characteristics of BP transistors with few layers were effectively modulated by the deposition of Cs2CO3 and MoO3 onto BP surfaces, inducing both electrons and holes caused by Cs2CO3 and MoO3, respectively. Cu adatoms on BP may also be used to render n-doping effects for BP with a few layers. Futhermore, the bandgap of BP with a few layers has been tuned by using a potassium surface-doping technique in which the vertical electric field originating from the potassium dopant modulates the band gap and tunes BP from a moderate-gap semiconductor to a band-inverted semimetal. Despite the recent progress in the field of BP doping, research has remained confined to the investigation of the carrier concentration and the n- or p-doping level of BP with a simple device structure. Studies on the effects of doping on the electronic sensor ability of a BP system have not been reported. In this study, the effects of Au and Pt incorporation on BP were first investigated. Incorporation with Au and Pt significantly improves the chemical sensing ability of the BP system. During the solution mixing of an aqueous solution (ethanol) of gold(III) chloride hydrate or platinum(IV) chloride, the metal precursors were reduced as a result of the

charge transfer of electrons from the BP nanoflakes, which were subsequently incorporated into the BP surface because of the difference in the reduction potential between the Au (or Pt) precursor and the BP work function. Several important sensing results were observed as a result of the incorporation of Au or Pt into the BP surface. First, prior to incorporation, pristine BP only detects paramagnetic molecules, e.g., NO2 or NO. However, after functionalization with Pt, low concentration of H2 gas can be easily detected; this result is not possible using pristine BP. It was found to be comparable to the sensing performance of metal oxide semiconductors (MOSs). The second significant result was obtained after doping with Au, where the work function of BP was varied by the transfer of electrons from the Au nanoparticles, thereby inducing the effects of n-doping on p-type pristine BP. Accordingly, the response behavior of BP to oxidizing gas changed from a p-type response (negative resistance variation) to an n-type response (positive resistance variation). In addition, a highly stable, low noise baseline was achieved in the Au-incorporated BP (Au/ BP) channel material. Finally, because of the protection of active BP surface from oxygen and humidity by Au and Pt showing high chemical and ambient, the synthesized Au- or Ptincorporated BP (Pt/BP) systems exhibited long-term stability, which is difficult to achieve when using other doping strategies, e.g., molecular physisorption or chemical functionalization. Thus, the approach used in this study may significantly enhance the superior chemical sensing ability of BP, demonstrating promise for their use as high-performance chemical sensing channels. 7198


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Figure 2. Morphological characterization of Au/BP and Pt/BP nanosheets. SEM images of (a) Au/BP and (b) Pt/BP systems with a few layers show that nanoparticles are selectively and densely decorated along the edge sites of the BP. TEM and HRTEM images of (c−f) Au/BP and (g−j) Pt/BP show that 1−10 nm sized nanoparticles are well decorated on the monolayer BP with a highly crystalline lattice configuration.

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1486.6 eV) were employed to investigate the morphology and chemical composition of the materials. Device Fabrication. Dispersions of pristine BP, Au/BP, and Pt/ BP were filtered by vacuum filtration through a porous alumina membrane filter (Whatman, 20 nm pore size, diameter of 47 mm). After a few hours of drying, the filtered films were directly placed on a 3 M NaOH solution to dissolve the alumina membrane. Once the thin films of the Au/BP and Pt/BP were allowed to float on the NaOH solution surface, the solution was replaced with deionized water by recirculation until the pH was around 7. A μ-electrode-printed substrate was then immersed in the water. With the drainage of water, the floating films were allowed to slowly descend and attach onto the electrode. Gas Delivery System and Resistance Measurement.37 Au electrodes with a thickness of 70 nm (5 nm Cr was predeposited as an adhesion layer) with a spacing of 100 μm were deposited by e-beam evaporation using a customized SERS mask to measure the resistance signal of the uploaded noble-metal-incorporated BP channel. A fabricated sensor was loaded inside of a homemade gas-sensing chamber. Test analytes (NO2, H2, ethanol, toluene, hexane, acetone, and acetaldehyde) used in this study were N2-based gases with concentrations of 100 to 10000 ppm. MFCs (Brooks, 5850E) with Viton or Karlez sealing (depending on the type of VOC) were used to control the flow rate of each analyte and inert N2. The resistance signal of the mounted sensor was measured by a data acquisition module (Agilent 34970A), while the controlled analyte gas was passed through

EXPERIMENTAL SECTION

BP Exfoliation and Noble Metal Incorporation. First, bulk black phosphorus (3 mg, Smart elements) was immersed in 30 mL of distilled water. Second, the prepared materials were ultrasonically treated in ice water for 6 h using a horn probe sonic tip (Sonic VCS 750). Pulsed ultrasonic irradiation was performed for 6 s on and 2 s off to avoid damage to the sonic processor and to reduce solvent heating and the resulting degradation of the materials. To incorporate BP with Au and Pt, controlled molar concentrations of gold(III) chloride hydrate (HAuCl4·3H2O) (Sigma-Aldrich, 99.999%, trace metals basis) and platinum(IV) chloride (Sigma-Aldrich, >99.99%, trace metals basis) were injected into a chemically exfoliated BP solution at room temperature and stirred for 30 min. For functionalization with Pt, annealing of the vial at 60 °C in the oil bath was subsequently carried out for 12 h. Characterization. SEM (Sirion FE-SEM, FEI) was employed to obtain images of the materials. The incident energy of the electron beam was between 1 and 10 kV. For TEM sample preparation, a dispersed solution of the noble-metal-incorporated BP system with a few layers was drop-casted onto a lacey-carbon-coated TEM grid and dried in an oven for several hours. UV−vis spectroscopy (92-570, JASCO), X-ray diffraction (RIGAKU, D/MAX-2500, Cu Kα target at 40 kV and 200 mA (λ = 1.5406 Å)), Raman spectroscopy (Horiba Jobin Yvon ARAMIS, 514 nm wavelength), and XPS (Sigma Probe, Thermo VG Scientific, incident wavelength 0.83386 nm, energy 7199


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Figure 3. Characterization of Au/BP and Pt/BP with XPS and Raman spectroscopy. XPS surface analysis of BP before and after functionalization with Au or Pt, binding energy peaks of (a) P 2p1/2 and 2p3/2, (b) Au 4f5/2 and 4f7/2, and (c) Pt 4f5/2 and 4f7/2. (d) Raman spectrum of the six spots of exfoliated pristine BP flakes. Shift behaviors of three representative Raman peaks of BP (A1g, B2g, and A2g) by functionalization with (e) Au and (f) Pt. the sensing chamber. Figure S1 shows the schematic of the gas delivery system.

Finally, gas sensors from the noble-metal-incorporated BP were loaded onto a homemade gas sensing channel containing multiarray systems, and the dynamic sensing responses were measured using various target analytes (Figure 1d). The film transfer and device fabrication are described in detail in the Experimental Section. Figure 1e shows photographs of a BP dispersed solution with a small number of layers obtained by the centrifugation of the pristine BP dispersion, Au/BP dispersion (AuCl4−/BP molar ratios of 1:2), and Pt/BP dispersion ([PtCl4]2−/BP molar ratios of 1:2). Upon doping, obvious color changes were observed as the yellow pristine BP solution changed to purple (for Au/BP) and brown (for Pt/ BP), caused by the surface plasmon resonance (SPR) phenomena of the Au and Pt nanoparticles. Figure 1f shows a photograph of the resistor-type chemical sensor fabricated with the 100 μm spacing Au/Ti electrode and BP channels. The UV−vis absorption spectra of each solution clearly demonstrated the change in the optical characteristics of the Au/BP and Pt/BP (Figure 1g). The pristine BP dispersed solution exhibited a representative absorption peak around 300 nm. With the doping of Au or Pt nanoparticles into the exfoliated BP layers, new absorption peaks corresponding to the Au SPR band at approximately 550 nm and the Pt SPR band at approximately 300 nm were observed, indicating the formation of high-resolution Au and Pt nanoparticles. The Xray diffraction (XRD) patterns of Au/BP demonstrate that the Au precursor is perfectly reduced into Au crystal state, corresponding to a typical crystal lattice with Au(111) and Au(200) planes (Figure 1h). Conversely, Pt/BPs did not exhibit representative Pt XRD peaks, corresponding to Pt(111) and Pt(200) planes, because of the ultrasmall size of the reduced Pt nanoparticles (1 to 5 nm in diameter).

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RESULTS AND DISCUSSION Noble Metal Nanoparticle Incorporation on BP. Figure 1a−d shows the schematic for the fabrication of Au/BP or Pt/ BP. First, BP crystals were chemically exfoliated in distilled water (H2O). Immersed BP was ultrasonically treated in an ice bath for an appropriate amount of time using a horn probe sonic tip. Dynamic light scattering characterizations demonstrate that bulk crystals are exfoliated to few layers of BP with average size of 391.5 nm and standard deviation of 276.4 nm (Figure S1). For incorporation of Au into the exfoliated BP layers, an aqueous solution (ethanol) of gold(III) chloride hydrate (HAuCl4·3H2O) (Sigma-Aldrich, 99.999%, trace metals basis) was injected into a chemically exfoliated BP solution at a controlled molar concentration (Figure 1a). After simple stirring with heating, high-resolution Au nanoparticles were spontaneously decorated onto the BP layers because of a redox reaction between the BP and noble metal ions (Figure 1b). Regarding functionalization with Pt, a similar process was conducted using the platinum(IV) chloride (Sigma-Aldrich, >99.99%, trace metals basis) precursor. The work function of exfoliated BP was around 4.5 eV;19 hence, the Fermi level of BP is well above the reduction potentials of AuCl4− and [PtCl4]2− (+1 V and +0.73 V versus standard hydrogen electrode (SHE), respectively) (Figure 1c). Therefore, BP/AuCl4− and BP/ [PtCl4]2− should theoretically form a redox pair, allowing for the spontaneous transfer of electrons from BP to the Au or Pt ions, which in turn results in the formation of Au or Pt nanoparticles. The Au/BP and Pt/BP dispersion was filtered under vacuum on an anodic aluminum oxide (AAO) film, with a small pore size, and transferred onto a μ-electrode-printed substrate, dissolving the AAO film with a NaOH solution.20 7200


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Figure 4. Chemical sensing performance of Au/BP and Pt/BP. (a) Electrical resistance variation of Au/BP and Pt/BP at various incorporation concentrations. (b) Resistance variation of Pt/BP and pristine BP with various injected gases (1000 ppm toluene, hexane, ethanol, acetaldehyde, 1% H2, and 100 ppm of NO2). (c) Gas response of Pt/BP and pristine BP toward various H2 concentrations. (d) Resistance variation of Au/BP and pristine BP toward various injected gas molecules. (e) Tunable gas response behavior of Au/BP achieved by controlling Au incorporation concentrations. (f) Summary of gas response of the pristine BP, Au/BP, and Pt/BP for various target chemical analytes.

Morphological Characterization of BP Functionalized with Noble Metals. Figure 2a−b shows the scanning electron microscopy (SEM) images of the Au/BP and Pt/BP flakes exfoliated by mechanical cleavage. Simply dropping the noble metal precursor solution onto the mechanically exfoliated BP results in the formation of Au nanoparticles and Pt nanoparticles, with a size of less than or equal to tens of nanometers, onto the BP layers. Notably, the reduced Au and Pt nanoparticles were selectively and densely decorated along the edge sites of BP, indicating that the redox reaction preferentially occurs at sites of highly energetic defects on the edge sites of the BP layers.21,22 To investigate the lattice configuration and decoration trends of the noble metals onto a BP system with a few layers, transmission electron microscopy (TEM) images were recorded (Figure 2c−i). A number of Au nanoparticles with diameters ranging from 5 to 10 nm were observed on the surface of the BP system with few layers (Figure 2c,d). Figure 2e shows a high-resolution TEM (HRTEM) image of Au nanoparticles on the BP layer: an inplane lattice spacing of 2.3 Å was observed, corresponding to the Au (111) plane. This observation clearly demonstrates that Au nanoparticles are well decorated on a BP system with a few layers via solution-based reduction. As can be observed in the HRTEM image of the white-dashed square region in the phosphorus region of Figure 2d, a measured lattice spacing of 3.1 Å was observed with a hexagonal crystal structure, corresponding to the (012) plane of BP (Figure 2f). For Pt/ BP, Pt nanoparticles (1 to 5 nm in diameter), significantly less than that of the Au nanoparticles, were densely decorated onto the BP system with few layers (Figure 2g−h). In the HRTEM images of the Pt nanoparticles and BP host layer, measured lattice spacings of 2.2 and 3.1 Å, corresponding to the Pt (111)

plane and the (012) plane of BP, respectively, were observed (Figure 2i,j). Doping Effect Demonstrations of Noble Metal Functionalized BP. To better understand the transfer of charge between the incorporated noble metals and BP layers, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were carried out. Figure 3a−c shows the P 2p, Au 4f, and Pt 4f peaks before and after BP functionalization with Au (at a 1:2 Au to BP ratio) and Pt (at a 1:2 Pt to BP ratio). The XPS spectrum is calibrated with the C 1s peak of each sample. First of all, the relatively broad P 2p1/2 peak of the XPS spectrum ranging from 132 to 135 eV indicates that the phosphorus oxide (POx) is partially formed on the BP surfaces.23−26 However, partial oxidation of BP surfaces is not dominant compared to the original phosphorus chemistry, which can be confirmed with the above XRD data and following the Raman spectrum. In addition, a previous report demonstrates that the partial oxide form of BP surfaces does not show a significant effect on chemical sensing performance of BP.10 Noticeably, after functionalization of BP with Au, the P 2p peaks became broader, and positions of their maxima shifted toward higher values (red curve, Figure 3a). The upward shift in the maxima directly corresponds to n-doping, as it results in a shift of the Fermi level, wherein the zero energy lies toward the conduction band edge.27,28 This upward shift is consistent with that reported in previous studies related to the doping of TMDs. Considering functionalization with Pt (blue curve, Figure 3a), however, no observable shift in the peaks of the XPS spectrum was observed, indicating that Pt incorporation does not allow for the significant transfer of charge between Pt and BP. The appearance of the Au 4f and Pt 4f peaks after doping confirms the chemical bonding states of the Au and Pt nanoparticles on the BP host layers (Figure 3b,c). Raman spectroscopy was then 7201


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on BP that induces spin polarization, resulting in a spinpolarized current on the channel.7,10 Thus, pristine BP shows high selectivity only onto nitrogen containing molecules showing paramagnetic characteristics and no response to other reactive molecules including VOCs or acidic compounds (CO2, SO2). Interestingly, after incorporating Pt into BPs, the system exhibited a highly sensitive and selective response to H2 gas (approximately 1% concentration) with a resistance variation of approximately 500% in the channel (ΔR/Rb, where Rb and ΔR represent the baseline resistances of each sensor and the change in resistance after exposure to gas molecules, respectively) which cannot be achieved with the pristine BP channel. The mechanism of this selective H2 gas sensing is mainly caused by the change in the doping levels of BP. When H2 gases are injected and adsorbed on Pt nanoparticles, H2 molecules dissociate into atomic hydrogen on the Pt surface, and the resulting atomic hydrogen significantly decreases the work function of Pt.32,33 This in turn induces the transfer of electrons from Pt to BP and decreases the hole concentration of BP, resulting in increased channel resistance. Figure 4c shows the variation of resistance at various concentrations of H2 (ranging from 10 to 10000 ppm). Pt/BP exhibited an ultrasensitive response behavior at a low H2 concentration of 10 ppm with a resistance variation of 5%. The H2 gas sensing ability reported is very difficult to achieve when using pristine BP, other 2D materials, or metal oxide semiconductor sensors.34 Thus, the strong advantage of BP for chemical sensors, high selectivity, can be efficiently exploited in the H2 sensor with the Pt-incorporation strategy. The sensing behavior of Au/BP, however, was different. Figure 4d shows the resistance variation of pristine and Au/BP in response to various injected gases. Interestingly, Au/BP exhibited a highly stable baseline resistance with ultralow noise level (∼0.01%) and response to only to NO2 with a positive resistance variation of 0.4%. The baseline noise level is 50 times improved with Au/BP compared to that of pristine BP (∼0.5%). This system did not exhibit a response to either toluene or ethanol, both of which can be detected using pristine BP. Notably, the resistance variation changed from a negative to a positive response upon the reduction of gases (NO2), as the main charge carrier of the BP changed from holes to electrons because of the n-doping of Au nanoparticles. To investigate the effects of the Au incorporation concentration on the sensitivity variations of the BP, the resistance variation at various concentrations of NO2 (ranging from 1 to 50 ppm) was measured at a controlled Au incorporation concentration ratio ranging from 5:1 (BP:Au) to 2:1 (Figure 4e). When Au nanoparticles were incorporated into BP at a molar ratio of 5:1, the resistance variation to NO2 drastically decreased from −70% to −5% (at a concentration of 1 ppm of NO2), while negative resistance variation was maintained. This result is caused by the insufficient transfer of charge from Au to BP such that the main carrier concentration of BP is decreased as a result of the recombination of holes and electrons. However, when Au was incorporated at a higher concentration (2:1 molar ratio), the observed response direction was changed from negative to positive when the system consisted of oxidizing molecules (NO2) caused by the n-doped channel of the BP. In addition, because of the semimetallic characteristics of heavily n-doped BP, the resistance variation was observed to decrease from a scale of tens of a percentage to a 0.1% scale with a highly stable baseline. Figure 4f shows the chemical sensing performance of the noble-metal incorporated BP toward

used to investigate the effects of doping of noble metals on atomic vibrations. As shown in Figure 3d, pristine BP exhibited A1g (around 360 cm−1, out of plane), B2g (around 440 cm−1, in plane), and A2g (around 465 cm−1, in plane) modes, corresponding to three representative BP vibration peaks, respectively. Six Raman spectra recorded from six random spots on a single pristine BP flake showed identical peak locations indicating that the Raman characterization is well calibrated within the single BP flake. With the incorporation of Au and Pt nanoparticles into BP, clear Raman shifts (∼5 cm−1) of the A1g, B2g, and A2g peaks were observed (Figure 3e−f). When Pt or Au was incorporated onto the BP surfaces, the oscillation of the P atoms of BP was hindered to some extent, thus decreasing the corresponding Raman scattering energy and producing the red-shifts of the three Raman peaks of incorporated BP.29−31 Tunable Gas Sensing Performance of Noble Metal Functionalized BP. To determine the effects of doping on the electrical potentials of the BP channel, the resistance variation of the BP channel was investigated as a function of the dopant concentration (Figure 4a). A constant bias ranging from 0.5 to 1.5 V was automatically applied to the two-probe (Au/Cr) resistor-type sensor, and the electrical resistance of each channel was recorded via a data acquisition module (Agilent 34970A). For each noble metal, the incorporation concentration was controlled from a 5:1 to a 2:1 ratio of BP to noble metals and subsequently compared with the channel resistance of the pristine BP. For functionalization with Pt, the resistance of the BP channel did not exhibit a significant resistance variation even at a high incorporation concentration of Pt (at a BP to Pt molar ratio of 2:1), caused by the absence of charge transfer between the Pt and the host BP layers. However, functionalization with Pt induced significant chemical sensitization of the BP channel for the sensing of H2 target molecules without direct charge transfer, caused by the highly catalytic properties of Pt (the detailed mechanism is explained later). On the contrary, after Au incorporation, drastic reduction in the channel resistance was observed because of the significant ndoping effect provided by Au. When Au was densely incorporated into BP (at a BP to Au ratio of 2:1), the channel exhibited an almost metallic resistance level caused by the charge transfer of electrons from the Au nanoparticles to BP. To demonstrate the effects of functionalization with Au and Pt on the chemical sensing performance of the BP, the dynamic sensing responses of pristine BP, Au/BP, and Pt/BP sensors were investigated by the detection of VOCs, NO2, and H2 in N2 at room temperature. A constant bias was applied to the twoprobe resistor-type sensor, and the change in the electrical resistance of the sensor upon exposure to the analytes was monitored and recorded as the sensing signal. These samples were simultaneously loaded into a homemade gas-sensing chamber, and the sensing signals of each of the films were measured using multichannel sensing systems. Figure S2 in the Supporting Information shows the detailed gas delivery system, which was fabricated in-house. Figure 4b shows the resistance variation of Pt/BP and pristine BP with various injected gas molecules, including 1000 ppm (ppm) concentrations of toluene, hexane, ethanol, acetaldehyde, and 1% H2 and 100 ppm of NO2. The pristine BP sensor exhibited almost no response to VOCs (except ethanol, exhibiting a resistance variation of approximately 6%), while a significant response to NO2 was observed with an ultrafast response time. Typically, pristine BP exhibits a highly sensitive negative response to NO2, possibly caused by the adsorption of paramagnetic molecules 7202


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Figure 5. Stability characteristics of Au/BP and Pt/BP. (a) Raman spectrum of Au/BP, Pt/BP, and pristine BP over a 30 day period. (b) Gas sensing amplitude ((ΔR/Rb)max (%)) of the Au/BP and Pt/BP sensor toward NO2 and H2 was intermittently tested over a 30 day period.

from the use of metals such as Au and Pt because of their characteristic high environmental resistance.29,33,36

various gases (namely, VOCs, NO2, and H2). Notably, the chemical selectivity of Pt/BP drastically enhanced to approximately 500% of the response, and the various response behaviors of BP can be realized with controlled Au functionalization. Stability Characteristics of Noble Metal Functionalized BP. Au/BP and Pt/BP were stable under ambient exposure conditions (laboratory ambient condition: ∼23 °C and ∼35% relative humidity). Figure 5 shows the long-term stability of Au/BP and Pt/BP gas sensors. To demonstrate the stability of these sensors, the Raman spectrum and gas response amplitude of each BP sensor were observed for a one-month period to investigate the chemical composition and device performance variation with exposure to the ambient environment. The gas sensing performances ((ΔR/Rb)max, measured as a percentage) of the Au/BP and Pt/BP sensors to NO2 and H2 were intermittently tested over a 30 day period. Interestingly, the Raman spectra of the noble-metal-incorporated BP systems exhibited three representative vibration peaks of BP (A1g, A2g, and B 2g , respectively) at the same positions; it also demonstrated relative peak intensity ratios similar to those of pristine BP following a one-month ambient exposure period (Figure 5a). However, for pristine BP after a one month exposure period, peak intensities of A1g, A2g, and B2g are significantly reduced due to surface degradation of BP.35 These Raman spectra serve to demonstrate the minimal chemical degradation of the multilayer BP flakes used in the present experiments under ambient exposure for a long period. Figure 5b shows the variation in the gas response amplitude of the Au/ BP and Pt/BP sensing channel. The H2 sensing ability of Pt/BP clearly remained stable at a (ΔR/Rb)max level of approximately 500%, while the sensing ability of Au/BP toward NO2 was stable at around a (ΔR/Rb)max level of 1% after a 30 day period. This result indicated that the noble metal incorporated BP sensor can be practically used under ambient conditions for a period of at least several months without obvious severe deformation of the devices. Highly dispersed and densely incorporated Au and Pt nanoparticles block the lone pair electron of the BP surfaces and prevent reaction of BP with oxygen and water, resulting in enhanced ambient stability of BP.29 These stable functionalization effects can be induced only

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CONCLUSION In conclusion, the effects of incorporation of highly stable noble metals (Au and Pt) into BP were investigated. Functionalization BP with noble metals significantly improves the gas sensing ability of BP. Pt/BP exhibited a highly sensitive and selective response to H2 gas of 1% concentration with a channel resistance variation (ΔR/Rb) of approximately 500%, a result that cannot be achieved using only a pristine BP sensor. These results are comparable to previously reported upon metaloxide-semiconductor-based H2 sensors. By functionalization BP with Au, a tunable response toward NO2 varied from negative to positive. In addition, a highly stable, low noise baseline of the BP sensor was achieved by functionalization with Au. By controlling the concentration of the incorporated noble metal into BP, the gas sensing performances of the BP system can be efficiently improved and diversely tuned. A highly stable device performance can be maintained by functionalization with noble metals because of their unique ambient and environmental resistance, a result that is difficult to achieve when using an alternative doping process. This research serves to benefit design strategies for BP-based electronics with tunable properties and improved performances.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01353. DLS characterization and TEM images of exfoliated BP and schematic diagram of the overall gas delivery system (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(H.-T.J.) E-mail: [email protected]. ORCID

Hee-Tae Jung: 0000-0002-5727-6732 7203


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Chemistry of Materials Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H.-T.J. acknowledges the financial support by the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT, and Future Planning, Korea (No. 2015R1A2A1A05001844, MSIP) and Global Frontier Research Center for Advanced Soft Electronics (No. 2014M3A6A5060937, MSIP).

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