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Environment Sensitive Photoresponse of Spontaneously Partially Oxidized TiCT MXene Thin Films 3
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Sergii Chertopalov, and Vadym N. Mochalin ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b02379 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 9, 2018
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Environment Sensitive Photoresponse of Spontaneously Partially Oxidized Ti3C2 MXene Thin Films Sergii Chertopalov1), Vadym N. Mochalin1) 2) *
1
Department of Chemistry, Missouri University of Science & Technology, Rolla, MO 65409,
USA 2
Department of Materials Science & Engineering, Missouri University of Science & Technology,
Rolla, MO 65409, USA ABSTRACT: A large family of two-dimensional transition metal carbides and nitrides (MXenes) has increasingly raised interest for electronic and optoelectronic applications due to their high electrical conductivity, potentially tunable electronic structure, non-linear optical properties, and ability to be manufactured in the thin film state. During delamination and storage in ambient air environment, spontaneous oxidation of MXene flakes leads to formation of titanium oxide, a process that, as we demonstrate here, can be harnessed for manufacturing MXene-titania composites for optoelectronic, sensing, and other applications. We show that partially oxidized MXene thin films containing the in-situ formed phase of titanium oxide have a significant photoresponse in the UV region of the spectrum. The relaxation process of photoexcited charge carriers takes a long time (~24 hours) but can be accelerated in the presence of oxygen and water vapor in the atmosphere. These properties of spontaneously formed MXene-titania thin films make them attractive materials for photoresistors with memory effect and sensitivity to the environment, as well as many other photo- and environment-sensing applications. KEYWORDS: MXene, Ti3C2, two-dimensional materials, titanium oxide, photoresponse, sensors, photoresistors. A recent and quickly growing family of two-dimensional (2D) early transition metal carbides/nitrides, MXenes, has been extensively studied for a wide range of applications including supercapacitors,1-4 Li ion batteries,5,6 field-effect transistors,7-10 triboelectric nanogenerators,11 transparent electrodes,4,12-14 passive photonic diodes15 and nonlinear photonics,16 electromagnetic 1
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shielding,17-19 sensors,20-22 catalysts,23-25 composites,26-28 water purification and desalination,29-35 CO2 capture,36 etc. Full control of composition and thickness of a monolayer within the same family of materials is a distinct advantage of MXenes over other 2D materials. Their outstanding and tailorable electronic properties render MXenes suitable for pairing with other materials in various electronic devices, such as photo- and light emitting diodes, transistors, sensors, and optoelectronic components. MXene is produced from a bulk MAX phase by wet-chemical etching with hydrofluoric acid (HF),37 fluoride salts with hydrochloric acid,1,8,38 and bifluoride-based etchants, such as NH4HF2.39 In contrast to HF etching, a mix of lithium fluoride (LiF) with hydrochloric acid (HCl) yields larger and less defective MXene flakes while eliminating the need for organic solvents, such as DMSO40 or amines,41,42 in the MXene delamination step. This so-called MILD8,39 etchant yields a stable aqueous colloidal dispersion of individual MXene flakes terminated by fluorine and oxygen containing functional groups. According to first principles calculations43,44 the work functions (ϕ) of bare, −OH, −O, and −F terminated Tin+1Cn MXenes are controlled by the induced dipole moments due to charge transfer between the functional group and MXene, and the changes in the total surface dipole moments caused by surface relaxation. OH-terminated MXenes have ultralow work functions between 1.6 and 2.8 eV while O-terminated MXenes have high work functions between 5.75 and 6.25 eV. The ability to change the work function in a wide range by tailoring surface chemistry opens many avenues for applications of MXenes in electronics. It is known that single MXene flakes are not stable towards oxidation in colloidal state.45,46 Oxidation of Ti3C2 MXene is significantly suppressed when the content of oxygen in the environment is reduced. The spontaneous oxidation of MXene was first mentioned in publication45 and further studied in other papers.46-49 Provided enough time, MXene oxidation results in complete conversion of the material into TiO2 accompanied by changes in the appearance of colloidal solution from black to white. Initiated at the edges or defects, the oxidation proceeds to other parts of the flake and ends up with the formation of TiO2 nanocrystals.46,48,50 Although this spontaneous process is considered as undesirable and leading to degradation of MXene, it can be harnessed for spontaneous formation of MXene - nano-TiO2 composites. In a broader sense, combinations of MXenes with semiconducting nanoparticles, such as zinc oxide,18 titanium oxide,51 nickel oxide,52 tin oxide53 raise significant interest for optoelectronic, energy storage, (electro)chemical sensing, and biomedical applications.
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In this work we harness the spontaneous oxidation of thin Ti3C2Tx MXene films to fabricate Ti3C2Tx-TiO2 composites that change their electrical resistance in response to UV irradiation. We find that the parameters of photoresponse are sensitive to the environment. Photocurrent relaxation in these composite films takes a long time, which can be useful, e.g., for photoresistors with memory effect. Sensitivity to the environment renders these films prospective materials for photoresistive sensing applications. RESULTS The electrical properties of MXene films were tested using a two-terminal method (Figure 1 and Figure S1). Because we are interested only in the relative changes of these properties with and without UV irradiation or in different environments, and since the resistance of our Ti3C2Tx films is larger than 10 kOhm, greatly exceeding the resistance of the connected wires, the use of the two-terminal method is justified in our case. Moreover, as demonstrated below, the contacts between silver paste and MXene are of the Ohmic type and therefore the influence of the Schottky barriers does not need to be eliminated.
Figure 1. a) Schematics of Ti3C2Tx thin film on cover glass with silver paint contacts; b) thin film cross section showing MXene flakes and TiO2 nanoparticles (green color) between and on the surface of the MXene flakes; c) atomistic model of Ti3C2Tx MXene monolayer illustrating typical surface functional groups (mainly –OH with some amount of –F); d) photographs of a reference glass slide (1) and glass slide coated with ~16 nm thick MXene thin film before (2) and after (3) oxidation in air. Missouri S&T logo reproduced with permission from Missouri University of Science & Technology. Crystalline structure and morphology of Ti3C2Tx thin films XRD pattern of a freshly made interfacial Ti3C2Tx film on glass shows only one peak at 2θ = 5.98° (Figure 2a). 3
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Figure 2. a) XRD patterns of the MAX powder and Ti3C2Tx thin film on glass. b) typical UVVis spectra of 16 nm and 38 nm thick Ti3C2Tx films used in this study.15 As expected, the flakes in the film are preferentially oriented such that the (001) MXene plane is parallel to substrate surface. For MXene film, no peaks were detected in the 30 – 70° 2θ range, in contrast to Ti3AlC2 MAX phase. Also, higher order (00l) peaks are absent and (002) peak is downshifted from 2θ = 9.6° (MAX phase) to 2θ = 5.98° in the Ti3C2Tx film, in agreement with literature.8,17,39 The large downshift of (002) peak is indicative of complete exfoliation of Ti3C2Tx. We didn’t observe reflexes from TiO2 in XRD pattern, probably due to a low fraction of crystallites and their small size (see Figure 3). However, formation of TiO2 on MXene was very well documented before49,54 and after a month long exposure of our Ti3C2Tx films to air, the TiO2 (anatase) peak could be clearly detected by Raman spectroscopy (Figure S2). At the same time, even after several months storage in the ambient air, the films retain high electrical conductivity. To illustrate changes in visual appearance of the films, we performed their accelerated oxidation in air at 225° C for 1 hour. However, even after that the visual differences in color or transparency are marginal (Figure 1d). Nevertheless, increased TiO2 formation over time can be clearly seen in UV-Vis spectra of the Ti3C2Tx MXene thin film prepared on quartz support and oxidized in the same accelerated conditions (Figure S3). The structure and composition of the film are further confirmed by SEM and AFM, which show (Figure 3 a-d) that the MXene film consists of horizontally oriented thin overlapping flakes with lateral sizes 0.5-2 µm. High magnification SEM (Figure 3c) as well as AFM (Figure 3d) show titania nanoparticles located near the edges and on top of MXene flakes, similar to previous reports.50 The thickness of our Ti2C2Tx MXene interfacial films on glass has been reported before15 and is within 5-70 nm. 4
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Figure 3. SEM and AFM images of partially oxidized Ti3C2Tx film. SEM images (a-c) show a large area of the sample (a) nearly completely covered with MXene flakes forming the film. A selected area of the film (dashed square) is shown at increasing magnification in (b, c). Glass charging artefacts and small white particles of TiO2 preferentially located at the edges of MXene flakes can be seen.50 AFM image (d) shows a few nearly polygon shaped 0.5-2 μm MXene flakes with white nanoparticles of TiO2 that are also mainly located near the edges of the flakes. Optical properties of Ti3C2Tx thin films Prior studies15,55 showed that Ti3C2Tx MXene thin films demonstrate interesting non-linear optical properties and ability to sustain a much higher laser radiation power as compared to graphene. We observed that the Ti3C2Tx MXene thin films are semitransparent and conductive. Transmittance of our samples was 80-86 % (for 16 nm thick films) and 65-75 % (for 38 nm thick films) in 450-1000 nm wavelength range (Figure 2b, Figure S3). The broad absorbance peak at 750-800 nm is due to plasmon resonance in Ti3C2Tx MXene.14 A sharp drop in transmittance below 5
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450 nm can be assigned to TiOx light absorption.56 Due to a strong absorbance in the UV and transparency in the visible light combined with good electrical conductivity, one could envision applications of these films in various optoelectronic devices, e.g., photodiodes operating in the UV range where the MXene films can potentially be used as transparent electrodes, similar to ITO. Electrical properties of Ti3C2Tx thin films MXenes are electrically conductive8,14,57-59 and, accordingly, our Ti3C2Tx thin film samples demonstrate low electrical resistance: ~80 kOhm/□ for a 16 nm thick film and ~13 kOhm/□ for a 38 nm thick film. The current-voltage (IV) characteristics of Ti3C2Tx thin films (Figure 4a) are linear. Therefore, only the Ohmic contacts exist between the MXene flakes within the film, as well as between the MXene film and silver paste. UV irradiation reversibly reduces the film resistance (slope of the IV line in Figure 4a) but does not change its linear behavior. The photocurrent through the films at 1 V DC is ~a half of the dark current. The Nyquist plots (Figure 4b) reveal co-existence of real (resistance of resistor) and imaginary (reactance of capacitor) components of the film impedance. This leads to an equivalent circuit shown as inset in Figure 4b, consisting of a resistor and a capacitor whose characteristics calculated from the Nyquist plots are provided in Table S1. UV irradiation results in lower resistance (R1) and higher capacitance (C1) of the sample. a
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Figure 4. a) IV characteristics of a Ti3C2Tx thin film (16 nm) with and without UV irradiation measured in Ar atmosphere. b) Nyquist plots of a Ti3C2Tx thin film (16 nm) with (violet circles) and without (red squares) UV irradiation measured in Ar atmosphere at a bias potential 1 V and sinusoidal amplitude 20 mV. The inset shows the equivalent circuit of the Ti3C2Tx film. Fitting parameters of the Nyquist curves and corresponding errors are presented in Table S1 in Supporting Information. According to a schematic structure of the film in Figure 1b, which follows form published studies50 and our SEM and AFM images in Figure 3, the current can flow through the overlapping 6
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MXene flakes, as well as TiO2 nanocrystals. Ti3C2 terminated with −OH, −H, and −F (Figure 1c) has metallic conductivity.6,44,60 However, surface modification may lead to changes in the electronic density of states of MXenes. The wide range of work functions allows to use Tin+1Cn MXenes as substitutes of precious metals (Au, Pd, Pt) with high work functions, as well as of alkali and alkali earth metals with low work functions. In our MXene thin films we expect Ti3C2 terminated primarily with −OH and some amount of –F.10 The Ohmic contacts between MXene and TiO2 will form when the Fermi levels of MXene and n-type TiO2 are equalized due to electrons flowing from MXene into TiO2 at the interface when the two materials are brought in contact. The flow of electrons from MXene charges the flakes positively (see Figure S4) resulting in bending of TiO2 bands downward, just enough for the energy barrier at the interface to vanish so that the current can flow in either direction. Effect of environment on photoresponse of MXene thin films To investigate the photoresponse in different atmospheres, we kept a sample of MXene thin film in a controlled environment chamber with applied voltage 1 V (DC) while measuring current through the film. A stable ~8.5 μA current was recorded for a sample that was kept in Ar atmosphere over 6 days after its fabrication, indicating a good stability of MXene films in an inert environment (Figure 5a, red squares). After that, the sample was exposed to ambient air (20-30% humidity) resulting in a slow but significant increase of resistance over time (Figure 5a, blue circles). The slow drop of current through this sample is in striking contrast to the freshly made sample that was exposed to ambient air (20-30% humidity) from the beginning over the same period of time (Figure 5a, blue triangles) and showed ~3 times higher apparent current decay rate (equations in Figure 5a). These results are in agreement with a recent study49 and confirm that freshly prepared MXene thin films can be quickly oxidized when exposed to air forming TiO2, a process that leads to a rapid increase in the resistance of the films. What is important, however, is that the apparent rate of oxidation of thin MXene films that were kept in Ar atmosphere during the first 6 days of the experiment prior to exposure to ambient air, is much lower than for the films exposed to ambient air right from the beginning.
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Figure 5. a) Chronoamperometry of Ti3C2Tx thin films (16 nm thick) at 1 V applied voltage (DC) in air (blue triangles) and Ar gas (red squares) followed by air (blue circles). b) Chronoamperometry of Ti3C2Tx thin films (16 nm thick) at 1 V applied voltage (DC) with and without UV irradiation in Ar gas (green areas mark the time when the UV lamp was on). Fitting parameters and errors are presented in Tables S2-S5 in Supporting Information. This observation is important because the ability to slow down the oxidation rate (by ~3 times in Figure 5a) provides a means to put the oxidation process under control and harness it to manufacture MXene-TiO2 composites. The differences in oxidation rate between the freshly made MXene films and the films kept in an inert atmosphere for long periods of time prior to their oxidation may be related to evaporation of residual water from between the flakes, leading to denser packing that inhibits oxygen access to MXene. This is supported by a significant increase of scratch resistance of the films dried in Ar atmosphere, noted when we tried to remove these films from glass substrates. For further experiments we used same setup and conditions, while exposing the samples to UV irradiation. A freshly made Ti3C2Tx thin film sample was mounted in a controlled environment chamber with continuous Ar flow. After measurement of the dark current (~1 minute from the start of experiment, Figure 5b) the UV lamp was turned on until saturation of photocurrent at 12 µA. While under UV irradiation, we observed a rapid and dramatic increase of current trough the sample. Switching the UV lamp off leads to a slow exponential decay of the current. The rate of increase of the photocurrent is an order of magnitude higher than its decay rate. Switching the UV lamp back on after a 340 s off period leads again to an increase of current until saturation at ~12 µA. Thus, a reversible and consistent photoresponse was detected for the MXene thin films in Ar atmosphere. Visible light does not produce any measurable photocurrent (see Methods). Based on literature, MXenes are metallic conductors44,60 and as such, should not produce a 8
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photoresponse. We hypothesize that the photoresponse in these films may be due to a UV induced electron transition from the valence band or defect levels to conductive band in TiO2 nanocrystals. The structure of spontaneously partially oxidized MXene, therefore, can be considered as in situ formed MXene-TiO2 composite, opening ways to fabrication of similar composite films with other semiconductor nanoparticles derived from the corresponding MXenes such as V2C, Nb2C, etc. A slow decay of photocurrent observed in Figure 5b is very interesting and may be used in photoresistors or sensors with memory effect. Taken in combination with the outstanding photostability of the MXene films,15 one could envision their applications in photodetectors or comparators, in particular, for high-power UV laser radiation, since MXenes can sustain higher laser powers than other 2D materials. Additional experiments with thicker (38 nm) MXene films (Figure 6a) have confirmed that in an inert atmosphere the photocurrent through the film slowly decayed from maximum to ~60% over ~two hours and then rapidly grew to almost its maximal value again over 20 minutes each time the UV lamp was switched on and off. The photoresponse in Ar atmosphere seems to be entirely reversible and reproducible over at least 5 on-off cycles that lasted ~10 h in total (Figure 6a). Increase in the MXene film thickness leads to an increase in photoresponse time and time of relaxation, thus providing additional means of tuning these parameters for practical applications. 0.16
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Figure 6. a) Cycling photoresponse of Ti3C2Tx thin film (38 nm thick) with and without UV irradiation in Ar gas. b) Photoresponse of Ti3C2Tx thin film (38 nm thick) with and without UV irradiation and exposure to ambient air during 30 minutes. However, this behavior changed dramatically when the gas was switched from Ar to ambient air (Figure 6b, blue curve). A high sensitivity of electrical current through TiO2 to oxygen in a wide temperature range has been reported before61-64 and explained due to chemisorption of oxygen on TiO2. For our MXene thin films, after their initial UV irradiation in Ar atmosphere 9
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(“UV on” to “UV off” in Figure 6b) accompanied by a rapid and significant growth of the current through the sample with subsequent slow decay (from “UV off” to “Air on” in Figure 6b), switching the gas from Ar to ambient air during the photocurrent relaxation period resulted in a rapid drop of current (“Air on” to “Air off”). Finally, switching the gas from ambient air back to Ar (“Air off” and onward) completely restored the photoresponse behavior of the sample to what was observed above in the experiments with Ar (Figure 6b, second “UV on” and onward). These experiments demonstrate that the photoresponse of our partially oxidized Ti3C2Tx MXene thin films strongly and reversibly depends on the environment and the films can be used for sensing applications. Therefore, we studied the effect of other atmospheres, including H2 and water vapor on photocurrent relaxation. In an experiment performed similar to Figure 6b except with H2 instead of air, we found that H2 does not affect the photocurrent relaxation after UV excitation of the Ti3C2Tx film (Figure 7a). On the contrary, O2, ambient air (with 20-30% humidity), and water vapor significantly accelerate decay of the photocurrent, possibly by creating traps for electrons when the molecules such as O2 or H2O containing atoms with high electronegativity are adsorbed on the film surface (Figure 7b). a
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Figure 7. a) Photoresponse of Ti3C2Tx thin film (38 nm thick) with and without UV irradiation and in H2 gas. b) Photoresponse of Ti3C2Tx thin film (38 nm thick) with and without UV irradiation and exposure to Ar, O2, ambient air, and H2O vapor. Interestingly, the accelerated decay was more pronounced when ambient air was used as compared to O2, and it was most significant in the presence of water vapor: after a short exposure (~10 s) to water vapor, the current through the sample sharply dropped but quickly restored upon switching back from water vapor to O2 flow (Figure 7b). In addition to adsorption mechanism 10
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discussed above, this response of photoinduced current through partially oxidized Ti3C2Tx MXene thin film brings about another possible explanation of the effect of the environment: reversible intercalation/de-intercalation of MXene. It is known that Ti3C2Tx MXene is strongly hydrophilic and can be easily intercalated by H2O molecules, even when exposed to humid air.65 Intercalation will result in swelling of the film leading to poor electrical contacts between the flakes and reduced current through the sample. In addition, intercalation may change the dielectric constant of the interlayer space between MXene flakes in the film, which may also have an effect on current through the sample. H2O de-intercalation occurring when the gas flow is switched back to Ar or O2 will produce an opposite effect: compressing the film and leading to better electrical contacts and higher current through the sample. At this stage we cannot say which of the two mechanisms (adsorption of electronegative atoms on the surface or intercalation between the flakes) plays a dominant role in the observed effects of the environment on the decay of photoinduced current in MXene thin films. However, it is clear that these phenomena may be used for photo- and environment-sensing applications and deserve a follow-up in-depth study. CONCLUSIONS A reversible and reproducible photoresponse of Ti3C2Tx MXene films to UV irradiation has been experimentally demonstrated. Visible light does not result in any measurable photocurrent, thus the photoresponse has been associated with partially oxidized Ti3C2Tx containing spontaneously formed TiO2 nanoparticles. In an inert environment, the photoinduced current decay in these films takes very long time (~24 hours), rendering the films potential materials for photoresistors with memory effect for the use in optical detectors or comparators, especially with high intensity light, since the MXene has been demonstrated in literature to be more resistant to the light-induced damage compared to other 2D materials. On the contrary, in oxygen and water vapor containing atmospheres, the photocurrent decay is significantly accelerated. This process is reversible, opening avenues for applications of the Ti3C2Tx thin films in photo- and environmental-sensing. The accelerated photoinduced current decay observed with oxygen containing species present in the environment may be related to (i) adsorption of molecules containing electronegative atoms on the surface of the films, resulting in electron trapping or (ii) MXene intercalation and swelling, leading to poor electrical contacts between the MXene flakes within the film. Furthermore, we show the potential of spontaneous oxidation as a simple and inexpensive method of fabrication of MXene–semiconductor TiO2 nanocomposites. These
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findings may lead to applications of the MXene thin films in UV detectors, photoresistors, sensors, and optoelectronic devices where semitransparent conductive thin films whose resistivity is responsive to UV radiation and is sensitive to the environment are needed. METHODS Synthesis of Ti3AlC2 MAX phase. The Ti3AlC2 MAX phase was synthesized following a published procedure15,39 from the powders of titanium (
