Electrocatalytic and Optoelectronic Characteristics of the Two

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Functional Nanostructured Materials (including low-D carbon)

Electrocatalytic and Optoelectronic Characteristics of the Two-Dimensional Titanium Nitride TiNT MXene 4

3

x

Abdoulaye Djire, Hanyu Zhang, Jun Liu, Elisa M. Miller, and Nathan R. Neale ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01150 • Publication Date (Web): 04 Mar 2019 Downloaded from http://pubs.acs.org on March 5, 2019

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ACS Applied Materials & Interfaces

Electrocatalytic and Optoelectronic Characteristics of the Two-Dimensional Titanium Nitride Ti4N3Tx MXene Abdoulaye Djire1, Hanyu Zhang,1 Jun Liu2, Elisa M. Miller1, Nathan R. Neale1* 1 Chemistry

and Nanoscience Center, National Renewable Energy Laboratory, Golden, CO, 80401, USA 2 Materials Science Center, National Renewable Energy Laboratory, Golden, CO, 80401, USA E-mail: [email protected]

Abstract A relatively new class of two-dimensional (2D) materials called MXenes have garnered tremendous interest in the field of energy storage and conversion. Thus far nearly all MXenes reported experimentally have been described as metals, with a lone report of a mixed-metal carbide phase exhibiting semiconducting character. Here, we report the optical, electrocatalytic and electrical properties of the 2D Ti4N3Tx MXene (Tx = basal plane surface terminating groups) and show this material exhbits both metallic and semiconducting behavior. We provide complete structural characterization of exfoliated Ti4N3Tx MXene and assign Tx = O and/or OH and find that this material is susceptible to surface oxidation. Optical experiments indicate that the exfoliated Ti4N3Tx MXene forms a hybrid with a thin surface oxide layer resulting in visible light absorbtion at energies greater than ~2.0 eV and an excitation wavelength-dependent defect-state emission over a broad range centered at ~2.9 eV. As an electrocatalyst for the hydrogen evolution reaction, the exfoliated Ti4N3Tx shows an overpotential of ~300 mV at –10 mA cm–2 and a Tafel slope of ~190 mV/decade. Finally, we observe a clear semiconductor-to-metal transition at ~90 K from temperature-dependent transport measurements under 5 T magnetic field likely resulting from the thin oxide layer. These results unveil the intriguing optical, electrocatalytic, and electrical properties of this 2D Ti4N3Tx MXene that expands the potential of these new 2D materials into electrocatalysis and (opto)electronic applications.

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ACS Applied Materials & Interfaces 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

Keywords: Nitride MXenes, 2D materials, semiconductors, water splitting, energy conversion

1. Introduction MXenes are a new family of two-dimensional (2D) materials comprised of layers of transition metal carbides and nitrides. The 2D material is produced by etching the “A” layer from MAX phases, which are a large family of hexagonal layered ternary transition metalbased structures, where M is an early transition metal (Ti, V, Nb, Mo, etc.), A denotes a main group element (Al, Ga, In, Si, etc.), and X is a non-metal such as carbon or nitrogen. The general formula for MXenes is Mn+1Xn where n = 1, 2 or 3 depending on the structure. Like graphene and other 2D layered structures, MXenes have the potential to deliver fast charge transport kinetics1-9 and, therefore, are being explored in numerous applications including Liand Na-ion batteries, supercapacitors, fuel cells, electromagnetic interference shielding, and water purification.1,2,7,9-26 Distinct from the hydrophobic sp2-hybridized carbon atoms making up the sheets of graphene, however, MXenes show hydrophilic behavior that allows for easy dispersion in aqueous solutions owing to the presence of chemical functional groups terminating the surface of the basal planes.[1] These terminating groups are termed Tx and provide coordinative as well as electronic saturation of the transition metal elements that make up the outer-most layers in these structures. Theoretical calculations and experimental data both show that the fundamental properties such as hydrogen evolution reaction activity (HER), metallic or semiconducting character, as well as chemical, physical, and structural properties of the MXene depends on the nature of the surface termination groups Tx (typically O, OH, and/or F). [2,3,5,10,11,24,25,27-32] Thus far, nearly all experimental data has suggested MXenes are metallic compounds even when terminated with Tx, including Ti4N3Tx.2,10,11,24,27 The lone experimental data of a MXene exhibiting semiconducting behavior is from the mixed-metal Mo-Ti MXene Mo2TiC2Tx.28 In 2 ACS Paragon Plus Environment

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addition, several studies have shown that nitrides offer many potential advantages over their carbide counterparts including high conductivity and stability in aqueous media.[6,10,27,28] Whereas carbide MXenes have been investigated extensively, only three reports exist on nitride MXenes,6,27,29 and the electrochemical and optical behaviors remain unexplored. Herein, we report on the synthesis and the first optical, electrochemical, and electrical characterization of Ti4N3Tx to show this 2D layered hybrid MXene exhibits both metallic and semiconducting behavior. We synthesize Ti4N3Tx MXene from the Ti4AlN3 MAX phase— prepared via a high temperature-pressure solid-state reaction between TiH2, AlN, and TiN (see Experimental Methods)—by extracting the A-site Al3+ cation separating Ti4N3 layers in the MAX phase via a temperature-programmed reaction with molten fluoride salts (as illustrated in Figure 1). Acid etching of residual aluminum fluoride salts and exfoliation provides a phase-pure compound of single- and/or few-layer Ti4N3Tx MXene sheets as confirmed by power X-ray diffraction (XRD), nitrogen physisorption, and X-ray photoelectron (XPS), Raman and Fourier-transform infrared (FTIR) and selected area X-ray (SAED) spectroscopies as well as scanning electron (SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron (TEM) and atomic force (AFM) microscopies. Additionally, we characterize the Ti4N3Tx MXene optically using absorption and emission spectroscopies and electrochemically using linear sweep voltammetry (LSV) to show that this hybrid MXene is an effective dark HER electrocatalyst. Finally, we perform temperaturedependent resistivity measurements with and without a magnetic field on exfoliated Ti4N3Tx films that show evidence of a low-temperature semiconductor-to-metal transition under a magnetic field. These studies provide the first comprehensive characterization of the intriguing optical, electrocatalytic, and electrical behavior of the nitride MXene Ti4N3Tx.



Figure 1. Schematic illustration of the synthesis of Ti4N3Tx MXene via eutectic molten salt treatment of the parent MAX phase Ti4AlN3 at 550 ºC for 7 h under static argon, followed by aluminum fluoride salt etching in 4 M H2SO4 to produce multilayered Ti4N3Tx. Exfoliation of multilayered Ti4N3Tx is accomplished via sonication in tetrabutylammonium hydroxide (TBAOH) solution to produce single- and/or few-layered Ti4N3Tx MXene as a pale, pinkishpeach aqueous colloid. Shown is Tx = OH and O based on Raman and FTIR analysis of exfoliated Ti4N3Tx MXene, though Tx = F may exist in the multilayered Ti4N3Tx prior to TBAOH treatment.

2. Results and Discussion The precursor Ti4AlN3, MAX phase, is produced by mixing TiH2, AlN, and TiN powders. The mixture was hot pressed for 24 h at 1275 °C and 70 MPa in a graphite die within an Ar atmosphere hot press, and residual graphite was removed mechanically to give a metallic monolith (Figure S1). Next, the synthesized Ti4AlN3 MAX phase is ground to a powder and then sieved through a 400 mesh to collect particles smaller than 37 μm. Figure 1 shows a schematic of the subsequent synthesis of multilayered and exfoliated Ti4N3Tx MXene from Ti4AlN3 MAX. First, the A-site Al3+ cation in the MAX phase is etched using a temperatureprogrammed reaction via eutectic molten salt treatment. Second, fluoride salts are removed by 4 ACS Paragon Plus Environment

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acid treatment to give multilayered Ti4N3Tx MXene. Third, exfoliation is accomplished by sonication in aqueous tetrabutylammonium hydroxide (TBAOH) solution. The resulting exfoliated Ti4N3Tx MXene is separated from excess hydroxide and resuspended in water for characterization and deposition on a substrate for futher characterization.

Full details

regarding the synthesis procedure can be found in the Experimental Methods. The powder XRD patterns for the Ti4AlN3 MAX phase precursor, molten salt-treated Ti4AlN3 (prior to 4 M H2SO4 etching), and exfoliated Ti4N3Tx final product (MXene phase) are shown in Figure 2a. The XRD pattern for Ti4AlN3 is consistent with those reported previously,4,29 with trace amounts of unreacted TiN precursor as an impurity. Following molten salt treatment, the expected aluminum fluoride compounds are detected (Figure S2), including K2Na[AlF4]3, K2NaAlF6, K2Li[AlF6], KAlF4, and Na3AlF6, all of which are soluble in concentrated sulfuric acid solution.29 The presence of aluminum fluoride-based compounds and absence of Ti-containing fluorides in the molten salt treated sample indicates successful depletion and selective etching of Al3+ from the Ti4AlN3 precursor. We note that Al3+ etching is accomplished only after 7 h of heating at 550 °C, with shorter heating times resulting in significant amounts of unreacted Ti4AlN3 MAX phase (Figure S3).



a Ti4N3Tx

(002)

(004)

molten salt treated Ti4AlN3 TiN Si sub

20

30

Ti4AlN3

40

50

Pore Volume (cm3/g)

Intensity (a.u.)

b

10

12 Ti4N3Tx

10

Ti4AlN3

8

c

60

4

0

70

2

4

1

ω5/ω6

10

TiO2

Intensity (a.u.)

ω3 ω ω2

8

d

252

ω4

6

Pore Size (nm)

608

ω7

3.8 m2g-1

2

428

145

32 m2g-1

6

2�(deg.) Intensity (a.u.)

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

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Ti4N3Tx ω9 ω 10 ω8 Ti4AlN3

TiN Ti4N3Tx Ti-OH Ti4AlN3

3750 3000 2250 1500

Raman Shift (cm-1)

750

Wavenumber (cm-1)

Figure 2. Characterization of Ti4AlN3 and Ti4N3Tx. (a) Powder XRD patterns for Ti4AlN3 (blue), molten salt-treated Ti4AlN3 (green), and exfoliated Ti4N3Tx (red). The XRD peaks at ~30°, ~43°, and ~66° on the diffraction patterns for Ti4N3Tx come from the Si substrate. Full peak assignments from the molten salt-treated sample are listed in Figure S2. The arrows in (a) indicate shifts of the (002) and (004) peaks (labeled with the symbol  and emphasized by the green and tan shaded boxes, respectively) towards lower angles upon conversion from MAX to MXene phases. (b) Pore size distributions with surface areas for Ti4AlN3 (blue) and multilayered Ti4N3Tx (purple). (c) Raman spectra of Ti4AlN3 conducted in air (blue) and Ti4N3Tx measured in nitrogen (red). (d) FTIR spectra for Ti4AlN3 (blue) and exfoliated Ti4N3Tx (red), with reference spectra TiN (light green) and anatase TiO2 (magenta) where the dashed lines represent the center location of their peaks. 6 ACS Paragon Plus Environment

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The formation of the Ti4N3Tx MXene following fluoride molten salt etching is confirmed by shifts toward lower angles and broadening of the (002) reflection (from 7.5° for Ti4AlN3 to 6.1° for multilayer Ti4N3Tx) and the (004) reflection (from 15.1° for Ti4AlN3 to 12.8° for multilayer Ti4N3Tx) as shown by the black arrows and shaded boxes in Figure 2a and in the Rietveld refinement analysis in Figure S4. Such shifts to lower angle represent an increase in the c-lattice parameter (c-LP) of the unit cell. Specifically, the observed shift in the (002) reflection from 7.5° for Ti4AlN3 MAX to 6.1° for Ti4N3Tx MXene corresponds to an increase in the c-LP from 2.3 nm (for Ti4AlN3) to 2.9 nm (for Ti4N3Tx), which is consistent with previous c-LP results.4,29 Along with the peak broadening, these changes in the XRD pattern are characteristic of formation of multilayered MXenes relative to their parent MAX phases.810,18,30,31

Following acid treatment and exfoliation, the (002) reflection does not exhibit any

additional shift to lower angle but broadens further. In contrast, the intensity of the (004) reflection is diminished significantly upon acid treatment and exfoliation. The absence of the (004) reflection and retention of the (002) reflection suggest a decrease in degree of overlapping MXene sheets, which is consistent with exfoliation into primarily single- or fewlayer structures. Additional features in the XRD pattern of the exfoliated sample include a peak assignable to the Ti4N3Tx MXene reflection at 31.7° and a broad feature centered at 16.7° possibly due to a surface or amorphous oxide (i.e., Ti–O and/or Ti–OH).32 Additional features below from the Raman, FTIR, and XPS results suggest formation of a surface oxide layer, Tx. Further evidence of multilayer Ti4N3Tx formation from the Ti4AlN3 MAX phase was provided by N2 physisorption, where removing the Al3+ A-site cation results in an increase in the physical surface area from 3.8 to 32 m2g–1 (Figure 2b). As expected, the pore volume and pore size distribution increase significantly in the multilayered Ti4N3Tx MXene with pore diameters measuring ~2 and ~3.3 nm, which is due to new pores generated by voids between the multilayer sheets (Figure 2b). 7 ACS Paragon Plus Environment


We next analyze the Raman spectra to evaluate the vibrational signatures of the Ti4AlN3 precursor and exfoliated Ti4N3Tx MXene. The ten Raman-active modes—3A1g (ω4, ω7, and ω8), 3E1g (ω2, ω5, and ω10), and 4E2g (ω1, ω3, ω6, and ω9)—predicted by theory33,34 for the Ti4AlN3 precursor MAX phase are all present in our Ti4AlN3 sample (blue spectrum, Figure 2c), confirming the high-quality of our Ti4AlN3 MAX phase precursor. As can be seen from the red spectrum in Figure 2c, the Raman spectrum changes significantly upon removal of the A-site Al3+ cation. We assign vibrational modes based on assignments from the reported nitride MXene Ti2N. In that work, Soundiraraju and George found that the E2g vibrational modes for Ti–Al in the Ti2AlN parent MAX phase were removed and replaced by the shear vibration of Ti–Tx in Ti2NTx.6 We also observe the same elimination of E2g Ti–Al (ω1, ω3, ω6, and ω9) and appearance of shear Ti–Tx vibrational modes at 145 cm–1, 252 cm–1 , 428 cm–1, and 608 cm–1 consistent with the four similar Raman signatures at 143, 234, 426, and 606 cm–1 observed in the lone prior report on Ti4N3Tx MXene.29 Importantly, we find that this characteristic Raman spectrum of the Ti4N3Tx MXene is observed only by collecting spectra under inert atmosphere environment. The high power of the Raman laser causes the Ti4N3Tx MXene to oxidize under air, and the resulting Raman spectrum shows vibrational signatures consistent with anatase TiO2 (Figure S5). Since all processing is performed in aqueous environment in air, including the exfoliated sample used to collect the Raman spectrum in Figure 2c, bulk oxidation is highly unlikely except under the most extreme energetic stress. In contrast, surface oxidation is highly probable and likely contributes to some of the optoelectronic and electrical properties described below. The differences in the chemical bonding between the Ti4AlN3 MAX and exfoliated Ti4N3Tx MXene phases are highlighted in the FTIR spectra, where the commercial TiN and anatase TiO2 are provided as references (green and magenta traces, respectively, Figure 2d). The Ti4AlN3 MAX precursor phase (blue trace) exhibits a broad peak at ~960 cm–1, which is attributed to a Ti–N vibration given its similarity to the main peak at ~905 cm–1 from TiN 8 ACS Paragon Plus Environment

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(green dashed line). Molten fluoride salt extraction of Al3+ and TBAOH treatment to form the exfoliated Ti4N3Tx MXene (red trace) results in a red shift of this broad spectral feature to 840 cm–1, which is between the energies of the Ti–N stretch in TiN (905 cm–1, green dashed line) and the Ti–O stretch in TiO2 (730 cm–1, magenta dashed line). This energy could result from a convolution of Ti–N and Ti–O stretches in Ti4N3Tx, where Tx = O and/or OH. Notably in the exfoliated Ti4N3Tx spectrum, a new broad feature centered at ~3300 cm–1 is present, which is characteristic of a hydroxyl O–H stretch. Combined with the XRD (004) peak intensity decrease upon exfoliation and Raman modes associated with a Ti–O and/or Ti–OH vibration in the exfoliated Ti4N3Tx MXene, these data provide strong evidence that the surface terminating groups Tx are comprised largely of titanium oxo and/or hydroxide chemical species. Next, we use XPS to characterize the chemical composition and environment of the MAX and MXene phases. XPS is a surface sensitive measurement that detects the top 5–10 nm. Distinct differences in the spectral regions for Al 2p, N 1s, and Ti 2p are observed for the Ti4AlN3 MAX precursor and exfoliated Ti4N3Tx MXene phases as shown in Figure 3a–c. First, the Al peak position (~74.5 eV) is consistent with an Al3+ environment in the precursor Ti4AlN3 MAX phase. After conversion to exfoliated Ti4N3Tx MXene, this Al signal is no longer present (Figure 3a). Second, the N 1s spectrum for the Ti4AlN3 MAX precursor exhibits an intense peak at 397.3 eV and a shoulder at 396.3 eV that are both consistent with peaks present from a metal nitride bond (blue spectrum, Figure 3b). Converting this material to the exfoliated Ti4N3Tx MXene results in a significant decrease of the higher electron binding energy peak at 397.3 eV and a more homogeneous N environment than in the MAX precursor as evidenced by the single, dominant peak at 396.2 eV (red spectrum, Figure 3b). The slight asymmetry to the high energy side of this broad peak suggests that it is likely a convolution of two or more similar but distinct N electronic environments, as would be expected for the three very similar nitrogen layers in the 7-atom layer structure in exfoliated 9 ACS Paragon Plus Environment


Ti4N3Tx (see schematic in Figure 1). Next, the Ti:N ratio of 1.3 in the Ti4AlN3 MAX precursor is consistent with the expected value. A slightly higher Ti:N ratio (1.5) is observed for the exfoliated Ti4N3Tx MXene that is still very close to the expected value (see Table 1). More information about the chemical environment within the exfoliated Ti4N3Tx MXene are found upon close examination of the Ti 2p spectrum (Figure 3c). The spin-orbit splitting of Ti 2p results in two transitions split by 6.17 eV; therefore, the peaks in the range of 454–460 eV are due to the Ti 2p3/2 and the peaks from 460–466 eV are due to the Ti 2p1/2. The Ti4AlN3 MAX precursor phase exhibits peaks at 455.0 eV, which is similar to the Ti2+ environment in TiN, 456.9 eV (Ti3+ environment), as well as a strong peak at 458.6 eV (Ti4+ environment).35 Following fluoride salt extraction of Al3+ and formation of exfoliated Ti4N3Tx MXene, the original Ti2+ feature is no longer present, and the peaks at 457.0 and 458.5 eV are attributable to the Ti3+ and Ti4+ environments. Finally, the 457.0 eV Ti3+ peak in the exfoliated Ti4N3Tx MXene spectrum becomes much more prominent compared to the MAX precursor (red and blue spectra, Figure 3c). These results, in particular the loss of Ti2+ intensity at 455.0 eV, are in line wtih the Raman and FTIR data showing that surface oxidation has occurred. This surface oxidation likely explains the slightly higher than expected Ti:N ratio of 1.5 in the exfoliated MXene (Table 1), where some of the near-surface N atoms are lost due to surface oxidation (i.e., Ti–N–Ti–O(H) bonding structure becomes Ti–N(O)–Ti–O(OH)). Additional discussion of the XPS spectra including fitting of the Ti 2p spectrum (Figure S6) are included in the Supporting Information. In sum, these chemical composition changes clearly establish that the Ti4AlN3 MAX precursor is converted to the Ti4N3Tx MXene with surface termination group Tx = O and/or OH upon molten fluoride salt treatment and exfoliation.


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Table 1. Summary of XPS data for Ti4AlN3, exfoliated Ti4N3Tx, and TiN, where the error in raw atomic composition is +/- 5%. Compound

Ti

Al

N

Ti:N

Ti4AlN3

45

22

33

1.3

Ti4N3Tx

61

~0

39

1.5

TiN

53

-

47

1.1



Figure 3. XPS spectra for Ti4AlN3 (blue trace) and exfoliated Ti4N3Tx (red trace), showing intensity for (a) Al 2p, (b) N 1s, and (c) Ti 2p. The spectrum for TiN (green, in b and c) is included for reference.


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Next, we use SEM to detail the topological features of the different phases. The SEM micrograph of the multilayer Ti4N3Tx MXene displays an accordion-like structure with visible open layering—characteristic of MXenes—compared with that of the parent MAX structure (Figure 4a and 4b). Further morphological analysis using energy-dispersive X-ray spectroscopy (EDS) of the multilayered Ti4N3Tx MXene shows a negligible amount of residual Al (Figure S7). The elemental compositions are shown in Table S1. The ratio of Ti:N is comparable to that obtained from XPS (~1.5), and the F:Ti ratio of 0.05–0.10 shows that most of the surface fluoride groups are converted to O and/or OH following sulfuric acid treatment (full coverage is a F:Ti ratio of 0.50). However, EDS is unable to fully resolve the Ti, N, and O peaks (Figure S7). Therefore, we turned to scanning transmission electron microscopy with electron energy loss spectroscopy (STEM-EELS, Figure 4c) to provide a more detailed analysis of the elemental composition of the exfoliated Ti2NTx MXene. One key difference of STEM-EELS over XPS is the ability to analyze the local (~nm scale) elemental composition rather than average area (~mm scale); specifically, we are able to measure the composition of a single MXene flake with STEM-EELS. Thus, this technique complements the averaged chemical information obtained by XPS by providing a more detailed analysis of the elemental composition of the exfoliated sample at specific sites in the images. Consistent with the results from XRD, Raman, XPS, and EDS data, the STEM-EELS spectra exhibit a strong Al signal in the Ti4AlN3 MAX precursor, whereas no Al signal is present in the exfoliated Ti4N3Tx MXene (Figure 4c). Slight changes to the N and Ti STEMEELS spectra confirm changes in the Ti and N surroundings upon conversion of the MAX to MXene phase (Figure S8).



Ti4AlN3

a

STEM-HAADF

+

1 μm

100 nm

b

Ti4N3Tx

STEM-HAADF

+

1 μm

50 nm

c 1.0

Intensity (a.u.)

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

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Ti4N3Tx

0.8

Ti4AlN3

0.6

Al-L2,3

0.4

0.2

0.0

65

70

75

80

85

Energy (eV)

90

95

Figure 4. SEM images for (a) Ti4AlN3 MAX and (b) multilayer Ti4N3Tx MXene samples. (c) STEM-EELS spectra of Ti4AlN3 MAX and exfoliated Ti4N3Tx MXene. STEM-EELS spectra are acquired from the region marked with a “+” in the STEM images in (a,b) showing significant Al signal (Al-L2,3 edge) from the Ti4AlN3 MAX and no signal from the Ti4N3Tx MXene in the diagnostic Al-L2,3 region near 75 eV. The intensity in the range of 65–70 eV in the exfoliated sample is from the background.


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Interestingly, further TEM and electron diffraction data along with AFM topography of the exfoliated Ti4N3Tx MXene show that the exfoliation results in single and/or multilayer sheets. Figure 5a shows a TEM image of two partially overlapping Ti4N3Tx MXene flakes, which are delineated visually by the colored outlines. Selected area electron diffraction (SAED) patterns are collected from each of the two individual Ti4N3Tx flakes as well as their region of overlap and are shown in Figures 5b–d. The Ti4N3Tx structure of each flake is confirmed from the SAED patterns that exhibit a single diffraction system of a typical hexagonal structure viewed normal to the basal (002) plane. In contrast, in the overlap area, the SAED pattern shows two hexagonal diffraction systems that exist at different orientations relative to each other (Figure 5d). This could result either from overlapping single-layer sheets or crystallographically aligned few-layer monoliths. Finally, AFM topography experiments confirm the presence of few-layer Ti4N3Tx MXenes generated by the exfoliation process. Figure S9 shows that the thickness of one individual flake (monolith) ranges from ~6 to ~9 nm, corresponding to 2 or 3 unit cells, respectively. The dark and light contrast difference of the flakes may be due to surface roughness and/or different layer thicknesses, potentially resulting from surface oxidation, as observed by TEM data (Figure 5a).4 Finally, lateral dimensions of several hundred nm are observed for the majority of flakes, with some larger (~700  ~600 nm) and smaller (~100  ~100 nm) flakes (Figure S9). We note that the AFM topography image in Figure S9a shows a small feature with a thickness of ~1.5 nm that is the thickness expected for single-layer Ti4N3Tx.



a

b

d Overlap c

100 nm

Figure 5. (a) TEM image and (b, c, d) corresponding SAED for regions on the image for exfoliated Ti4N3Tx MXene. The inner faded dots in the SAED are reflections of the crystallographic plane. The dark and light contrast difference of the flakes may be due to surface roughness and/or different layer thicknesses as observed by AFM data (Figure S9).

These structural, chemical, and imaging analyses represent the most comprehensive characterization of any nitride MXene and clearly demonstrate that the Ti4N3Tx MXene is successfully prepared and exfoliated into few-layer flakes. The prior report of Ti4N3Tx MXene used spin-polarized partial density of states (PDOS) calculations to suggest that Ti4N3Tx is a metallic conductor irrespective of the nature of the Tx termination chemistry (Tx = F, O, and OH).29 Given the recent report that the mixed-metal carbide Mo2TiC2Tx MXenes is a semiconductor with a narrow bandgap28, the lack of experimental data on the properties for nitride MXenes, and the rich electrochemistry of MXenes generally,1 we endeavored to characterize our exfoliated Ti4N3Tx MXene optically, electrochemically, and electrically. By exfoliating the multilayer Ti4N3Tx MXene in hydroxide solution, we tune the surface terminating functional group from F to O and/or OH as evidenced by Raman and FTIR spectroscopies. The resulting exfoliated Ti4N3Tx MXene dispersed in ultra-pure water (Millipore, 18 M), forms a pale, pinkish-peach, optically transparent colloid that only shows 16 ACS Paragon Plus Environment

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signs of precipitation after two months. This Ti4N3Tx MXene colloid exhibits an absorption onset in the visible region (red curve, Figure S10) as evidenced by the broad peak in the range of 350–550 nm with an absorption edge of ~620 nm (~2.0 eV). The absorption spectra in either ethanol or dimethylformamide exhibit comparable features to that in water (Figure S10). Figure 6a shows the Tauc plot of exfoliated Ti4N3Tx MXene suspended in water, where a best fit is obtained using the square of the product of the absorption intensity and light energy plotted as a function of energy. This plot provides a linear region giving an absorption onset of ~2.0 eV that is reminiscent of the behavior observed for direct bandgap semiconductors. Interestingly, the photoluminescence (PL) spectrum of the exfoliated Ti4N3Tx MXene exhibits a broad emission peak centered at ~2.9 eV (~430 nm) covering a range of 3.5–2.0 eV (355– 620 nm, Figure 6b) when illuminated from 300–400 nm (~4.1 to ~3.1 eV). The emission spectrum shifts to higher energy with greater excitation wavelength, which suggests that the observed PL does not derive from a band-to-band semiconductor transition, but instead stems from defect-state emission from the Ti4N3Tx MXene. This excitation-dependent PL behavior is commonly reported for fluorescent nanomaterials such as graphene quantum dots,36-39 transition metal oxides,40-42 and in 2D transition metal dichalcogenide materials.[37-42,44-47] While the exact nature of these defects is beyond the scope of this paper, the defect structure had been mapped out only recently for the well-studied carbide MXene Ti3C2Tx,43 and likely resulted from the surface oxidation upon synthesis. We also note that a similar optical feature at 2.87 eV had been reported from a (001)TiO2/Ti3C2 hybrid structure intentionally generated by hydrothermal treatment of the Ti3C2 MXene.44



Figure 6. (a) Tauc analysis plot of exfoliated Ti4N3Tx MXene in water (red) showing an absorption onset of ~2.0 eV based on the fit of the linear region of the spectrum (dashed black line). (b) PL intensity for exfoliated Ti4N3Tx MXene in water, illuminated from ~4.1 to ~3.1 eV. The apparent discrepancy between the emission energy (~2.9 eV) and absorption onset (~2.0 eV)which would be expected to be at the same energy for an ideal semiconductorand the shift in the emission spectrum with excitation energy suggest that the observed emission is likely from surface oxidation. The emission peak at high energy is the water Raman band indicated by the black arrow. We next perform a series of (photo)electrochemical experiments on the exfoliated Ti4N3Tx MXene in film form. Chopped light photocurrent measurements are performed in 10 18 ACS Paragon Plus Environment

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mM ascorbic acid aqueous solution; ascorbic acid is a well-known rapid hole scavenger that can be used to mitigate oxidation in aqueous solution. The working electrode is prepared by membrane filtration and transfer of the exfoliated Ti4N3Tx MXene onto a fluorine-doped tin oxide (FTO) transparent conductive oxide substrate. Pt foil and Ag/AgCl are used as counter and reference electrodes, respectively, and all experiments are performed using a singlewavelength light source at 2.3 eV (530-nm, 200 mW) at the open circuit potential (0.49 V vs RHE, Figure 7a and 7b). We observe a clear anodic photocurrent response from exfoliated Ti4N3Tx MXene under light in the presence of ascorbic acid hole scavenger (red trace, Figure 7a). For comparison, the chopped light photocurrent response of a blank FTO substrate exhibits no photoresponse (black trace, Figure 7a). Also, bulk TiO2 does not exhibit photoresponse at this excitation energy (data not shown). Next, incident photon-to-current efficiency (IPCE) experiments are performed under identical conditions to learn more about the observed photoresponse from the exfoliated MXene. IPCE spectra of exfoliated Ti4N3Tx MXene at the same potential and in the same ascorbic acid electrolyte shows a weak response at energies higher than ~2 eV (red spectrum, Figure 7b), which is consistent with the chopped light experiment as well as the absorption data (Figures 6a and S8). A control experiment with blank FTO corroborates the chopped light photocurrent experiments, with no IPCE response observed down to 3.54 eV (350 nm, black spectrum, Figure 7b). These experiments show that photogenerated holes are injected from the exfoliated Ti4N3Tx MXene into the ascorbic acid electrolyte and conclusively demonstrate the semiconducting behavior of this 2D oxynitride MXene. Chopped light linear sweep voltammetry (LSV) scans over a wide voltage window (– 0.2 to 0.7 V vs RHE; scan rate of 5 mV s–1) using the same ascorbic acid electrolyte show similar onset for anodic photocurrent for the Ti4N3Tx MXene and blank FTO at ~0.5 V vs RHE (Figure S11) with photoresponse only from the exfoliated MXene.



Figure 7. (a) Chopped-light photocurrent for exfoliated Ti4N3Tx MXene on FTO (red) and blank FTO (black) in 10 mM ascorbic acid at 0.49 V vs RHE. (b) IPCE data for exfoliated Ti4N3Tx MXene on FTO (red) and for blank FTO (black) collected in 10 mM ascorbic acid at 0.49 V vs RHE. To probe the (photo)electrocatalytic behavior of the Ti4N3Tx MXene further, we conduct a similar LSV in 0.5 M H2SO4 (pH 0.3) deaerated electrolyte at a scan rate of 5 mV s–1 using glassy carbon as the current collector (FTO is unstable in strong acidic conditions under negative bias). The exfoliated Ti4N3Tx MXene is dropcast onto the glassy carbon electrode (mass loading: 0.2 mg cm–2geo, where the subscript “geo” refers to geometric surface area) using Nafion 117 solution as a binder (see Experimental Methods). The bare glassy carbon and the Ti4N3Tx-coated glassy carbon electrodes are examined in a three-electrode 20 ACS Paragon Plus Environment

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electrochemical cell using a rotating disk electrode apparatus at 1600 rpm. In this experiment, we switch the counter electrode to RuO2 mesh as Pt dissolution in acidic electrolyte under negative potentials can interfere with the electrochemical performance of the Ti4N3Tx MXene.45 The first LSV scan is conducted in the dark (solid blue trace, Figure 8a) and a subsequent second LSV scan conducted under illumination is nearly identical to the dark LSV (solid red trace, Figure 8a). One possible explanation for the lack of photoresponse under negative bias in sulfuric acid electrolyte is poor hole transport kinetics in the exfoliated MXene film. Still, both LSV scans, highly reproduceable, show significant cathodic current of –10 mA cm–2 at –0.3 V vs RHE (solid blue and red traces, Figure 8a) in stark contrast to the response observed using glassy carbon, which requires over –0.7 V vs RHE to achieve the same current density (black trace, Figure 8a). The slight increase in cathodic current from 0 V to –0.2 V in the Ti4N3Tx MXene LSVs is attributed to hydrogen adsorption on the surface of the Ti4N3Tx electrode.46 The corresponding Tafel plots for the dark and light LSVs for exfoliated Ti4N3Tx MXene on glassy carbon and blank glassy carbon electrodes in 0.5 M H2SO4 electrolyte are shown in Figure 8b. Compared to the glassy carbon electrode, exfoliated Ti4N3Tx MXene shows considerably lower Tafel slopes. However, ~190 mV/decade value is higher than those reported for the optimized electrocatalysts Mo2CTx and Ti2CTx, which exhibited Tafel slopes of 70 and 124 mV/decade, respectively.15 A more relevant comparison is perhaps the Ti3C2Tx MXene, which exhbited Tafel slopes ranging from 128–190 mV/decade depending on the exfoliation etchant used.47 For the exfoliated Ti4N3Tx MXene electrocatalyst studied here, the relatively high Tafel slope suggests that the mechanism for HER involves various reaction pathways and may not be explained by the conventional Volmer-Tafel or Volmer-Heyrovsky mechanisms.48 In sum, these electrochemical results show that the exfoliated Ti4N3Tx MXene bahaves as a dark electrocatalyst for HER with a relatively low overpotential in strong acidic



electrolyte, which adds to the growing number of carbides, nitrides, and 2D materials that have been invoked as HER electrocatalysts.49-54

Figure 8. (a) Dark and light LSVs and (b) corresponding Tafel plots for exfoliated Ti4N3Tx MXene on glassy carbon and blank glassy carbon electrodes in 0.5 M H2SO4 electrolyte. The mass loading of Ti4N3Tx on the glassy carbon is 0.2 mg cm–2geo for the HER experiment. The Tafel slopes are reported based on the fit of the linear region of the LSVs (over 200 mV, green lines, and the dashed lines are the extrapolated fits). The data are iR corrected.

Lastly, we perform temperature-dependent transport measurements with and without a 5 T magnetic field on the exfoliated Ti4N3Tx MXene to investigate its electrical behavior. Figure 9 shows the resistivity as a function of temperature for a film (~62 m thick) prepared 22 ACS Paragon Plus Environment

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by drop-casting exfoliated Ti4N3Tx aqueous colloid on a silicon patterned substrate and drying under vacuum at 80 C for 12 h to remove intercalated water molecules. As described above, the average lateral size of the exfoliated Ti4N3Tx MXene sheets was several hundred nm. Under no magnetic field, the resistivity decreases as the temperature is decreased from 302 K to 2 K (blue trace), a phenomenon that frequently is attributed to a metallic behavior where conduction is dominated by a single charge carrier (electrons for a metal). However, the resistivity of the Ti4N3Tx MXene film under 5 T magnetic field decreases as the temperature is decreased from 302 K down to ~90 K and then the resistivity increases for temperatures ~90 K down to 2 K. The magnetic field is applied normal to the charge transport direction and imposes a force (Lorentz force) on the charge carriers, forcing the positive and negative charges (if both are present) to move in opposite directions. Under a magnetic field, for a pure metal where conduction is dominated by electrons, the resistivity behavior should not change under magnetic field. In contrast, a pure semiconductor generally exhibits a continuous increase in resistivity with decreasing temperature since the thermalization of electrons into the conduction band (resulting in low resistivity) is restricted. However, for a material with mixed metallic- and semiconductor-like behavior, conduction can involve contributions from both electrons and holes that may be present in different concentrations and possess different mobilities, and the resistivity may increase or decrease with varying temperature depending on which charge carrier is dominating the conduction.55 Based on these results, our exfoliated Ti4N3Tx film sample may possess such mixed semiconductor-metal character. We speculate that a high density of inter-gap states in the MXene results in the observed metallic character, where the effect of surface oxidation to O and/or OH surface terminations previously has been shown to reduce but not completely eliminate inter-gap states even in the semiconducting Mo2TiC2Tx MXene.47 The semiconducting character may be a result of the surface oxidation upon synthesis. Such oxynitride surface states (i.e., N–Ti–O) could explain the apparently contradictory anodic 23 ACS Paragon Plus Environment


photocurrent response and the equivalent dark and light cathodic HER activity at room temperature (Figure 8a and 8b). We note that a very recent report demonstrated similar temperature-dependent resistance behavior in Ti3C2Tx and attributed the increase in resistivity at low temperature for gently vacuum annealed samples (125–200 °C) to a loss of intercalating ions and molecules (e.g., water) suggestive of a decrease in inter-flake resistivity as opposed to the metallic inter-flake conduction.56 Full investigation of the reasons for this optical, (photo)electrochemical, and electrical behavior as well as how processing affects the chemistry and ultimately fundamental properties are critical for the full understanding of this 2D nitride MXene.

Figure 9. Resistivity as a function of temperature for exfoliated Ti4N3Tx MXene film under no (blue trace) and 5 Tesla (red trace) magnetic field. The sample was prepared by dropcasting exfoliated Ti4N3Tx aqueous colloid on a silicon patterned substrate and dried under vacuum at 80 C for 12 h to remove intercalated water molecules. The thickness of the sample is ~62 m and the average lateral size of the flakes is several hundred nm (see Figure S9). 3. Conclusion We report complete characterization of the structural and chemical aspects of the titanium nitride MXene Ti4N3Tx. Exfoliation of molten fluoride salt-etched multilayers using simple 24 ACS Paragon Plus Environment

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sonication in hydroxide solution generates single-layer and/or multi-layer Ti4N3Tx flakes as evidenced by SAED, TEM, and AFM. XPS, EDS, and STEM-EELS spectroscopies confirm that molten fluoride salt etching and subsequent acid treatment results in complete abstraction of the Al3+ A-site cation from the Ti4AlN3 precursor MAX phase as well as provides insight about the chemical environment in exfoliated Ti4N3Tx MXene relative to its parent Ti4AlN3 MAX phase. Vibrational interrogation using FTIR and Raman spectroscopies characterize the bonding structure, including the presence of oxo and/or hydroxyl groups at the terminating surface sites Tx on exfoliated Ti4N3Tx MXene. Raman measurements under air result in bulk oxidation, showing the sensitivity of this MXene to oxidation under high energy conditions. Optical characterization reveals that the material absorbs light over a broad range in the visible region at energies greater than or equal to ~2.0 eV and also exhibits a wavelengthdependent defect-state emission over a broad range centered at ~2.9 eV likely resulting from surface oxidation (Tx = O and/or OH). (Photo)electrochemical measurements in both ascorbic acid and sulfuric acid electrolytes provide the first characterization of (photo)electrocatalytic properties and additional evidence for the semiconducting behavior of the exfoliated Ti4N3Tx MXene. The electrochemical data show that this new hybrid nitride MXene is an active electrocatalyst for HER with impressive overpotentials of ~0.3 V vs RHE at –10 mA cm–2 and a Tafel slope of ~190 mV/decade. Finally, temperature-dependent electrical measurements on exfoliated Ti4N3Tx MXene film with and without a magnetic field reveal evidence for both semiconducting and metallic behavior. These findings represent an important step in the characterization of nitride MXenes by unveiling the intriguing optical, (photo)electrochemical, and electrical properties of this class of 2D materials. Future work will focus on how chemical treatments during processing affects the chemistry and ultimately the fundamental semiconducting properties of this nitride MXene relevant for a number of energy applications.

4. Experimental Methods 25 ACS Paragon Plus Environment


4.1.

Material Synthesis

4.1.1.

Ti4AlN3 Precursor

The precursor Ti4AlN3, MAX phase, was produced by mixing TiH2 (TIMET, Henderson, NV; 99.3%, 325 mesh), AlN (Alfa Aesar, Ward Hill, MA; N 32.0% minimum, 2.5 to 4.0 μm), and TiN (Alfa Aesar, Ward Hill, MA; 99.8%, 2 to 5 μm) powders with a molar ratio of 2:1:2. These powders were mixed and ball milled with tungsten carbide balls for 8 h and hot pressed for 24 h at 1275 °C and 70 MPa in a graphite die within an Ar atmosphere hot press (2000C, 25 ton, Thermal Technologies 610G-25T, Santa Rosa, CA, USA) in the lab of Prof. Ivar Reimans (Colorado School of Mines, Golden, CO USA). Residual graphite from the die was removed by mechanical polishing using 60 grit sandpaper until a metallic luster was achieved and no remaining graphite was visible. The resulting Ti4AlN3 block (Figure S1) was broken into small pieces using a hammer. In some experiments, the Ti4AlN3 MAX phase pieces were ball milled with tungsten carbide balls. We found that this ball-milling process left residual WC that must be removed with concentrated nitric acid. Thus, an alternative method was to mechanically ground the Ti4N3Tx MAX material using a Cole-Parmer 6 in Agate Mortar and Pestle, 250 mL. The ground Ti4AlN3 MAX phase powder was sieved through a 400 mesh to ensure particles smaller than 37 μm. 4.1.2.

Molten Salt Treatment of Ti4AlN3

Phase pure Ti4N3Tx was prepared by etching the A-site Al3+ cation from Ti4AlN3 through a eutectic molten salt treatment (as illustrated in Figure 1) 29. The molten salt was a mixture of potassium fluoride (KF, Alfa Aesar), lithium fluoride (LiF, Alfa Aesar, 98.5 %, 325 mesh), and sodium fluoride (NaF, Alfa Aesar, Ward Hill, MA) with a mass ratio of 0.59: 0.29: 0.12, respectively, chosen because of their eutectic nature, which allows for the melt to form at relatively lower temperature (550 °C). The ground Ti4AlN3 MAX phase powder (95%) solution for 1 h under gentle heating (25–65 °C) while stirring with a Teflon-coated stir bar (200 rpm) in a ratio of 20 mL of 4 M H2SO4: 0.2 g Ti4N3Tx-fluoride salt mixture. The mixture was then washed with ultra-pure water (40 mL, Millipore, 18 M), centrifuged at 4200 rpm for 5 min and decanted to separate and dispose of the acid. Rinsing with ultra-pure water and centrifuging was repeated until the supernatant liquid in the centrifuge tube had a pH of at least 6. After the last decanting, the sediment (multilayer Ti4N3Tx powder) in the centrifuge tube was dried in a vacuum oven at 80 °C for 12 h. We noticed that at this stage of the processing, the color of the powder sample was not black anymore–a brown color reflects 27 ACS Paragon Plus Environment


successful multilayer Ti4N3Tx isolation, whereas a grey color indicates sign of oxidation as evidenced by XRD (data not shown). 4.1.4.

Exfoliated Ti4N3Tx

To exfoliate the multilayered Ti4N3Tx into few-layer and monolayer flakes, multilayer Ti4N3Tx powder was mixed with 40 wt% tetrabutylammonium hydroxide (TBAOH) (Acros Organics, Morris Plains, NJ; 40% in water) in a ratio of 20 mL TBAOH: 0.1 g Ti4N3Tx, mechanically shaken by hand for 5 min, then bath sonicated for 2 h using a Cole-Parmer Ultrasonic Cleaner (115 VAC, 50/60 Hz, 4.2 amps). This process is known to exfoliate multilayered flakes into single sheets as the TBAOH intercalates in between the layers during exfoliation29. To separate and remove the excess TBAOH, the powder was then washed with ultra-pure water (20 mL) and centrifuged at 4200 rpm for 15 min and the supernatant was decanted to remove the residual TBAOH. The residue powder was washed with ultra-pure water until the supernatant liquid in the centrifuge tube had a pH close to ~7. Ultra-pure water (40 mL) was added to the residue Ti4N3Tx powder (a pale, pinkish-peach) and bath sonicated for another 2 h to obtain exfoliated Ti4N3Tx as a suspension in water (see photograph in Figure 6a). A schematic of the synthesis if shown in Figure 1. 4.1.5.

Sample and Electrode Preparation

The working electrodes or photoelectrodes were prepared by membrane filtration process.57 In brief, the exfoliated Ti4N3Tx solution was slowly passed through a polycarbonate membrane (Whatman) with 50-nm pore size. The exfoliated Ti4N3Tx stayed on the membrane surface and formed a thin film. The exfoliated Ti4N3Tx film side of the wet membrane was placed against a silicon substrate (for AFM measurement) or FTO (for electrochemical measurements) until the water was completely evaporated (~15 min) in room conditions. The polycarbonate membrane filter on the silicon wafer/ FTO was emerged into 10 mL chloroform 3 times to be fully dissolved, the Ti4N3Tx thin film now on the silicon wafer or FTO surface was ready for Raman, AFM and electrochemical measurements. For the other 28 ACS Paragon Plus Environment

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experiments, unless otherwise stated, the sample was prepared by drop-casting 10–20 µL of solution on a substrate. 4.2.

Physical and Surface Characterization

4.2.1.

X-ray diffraction

X-ray diffraction (XRD) was used to characterize the chemical composition and phase purity at the different synthesis processes. To prepare the powder samples (Ti4AlN3 and molten salt treated Ti4AlN3), a piece of double-sided tape was taped onto the glass sample holder, on which the powder was sprinkled onto and spread out as much as possible to cover the entire area. The powder was pressed with a glass slide to make sure it stayed. The prepared sample was then mounted onto a multi-purpose stage for analysis. For the solution sample (exfoliated Ti4N3Tx), 10 µL of solution was drop-casted onto a zero-diffraction Si plate and dried in air. The sample was then placed into the XRD sample holder and mounted onto a multi-purpose stage for analysis. The XRD was carried out using a Rigaku Smartlab (Tokyo, Japan) diffractometer with Cu-Kα (λ = 0.15404 nm) radiation (40 kV and 250 mA); step scan 0.02°, 3°–70° 2 range, step time of 1 s, 10  10 mm2 window slit. The JADE 10.0 software was used for peak identification. 4.2.2.

N2 Physisorption

The surface area and pore size distribution of Ti4AlN3 and multilayered Ti4N3Tx were determined by N2 physisorption using a Micrometrics ASAP 2010 analyzer with the Brunauer-Emmett-Teller (BET) method and Barrett-Joiner-Helenda (BJH) method. Prior to analysis, the materials were degassed in vacuum at 350 ºC for 24 h. 4.2.3.

Fourier-transform infrared spectroscopy

Fourier-transform infrared spectroscopy (FTIR) measurements were performed on Ti4AlN3 and exfoliated Ti4N3Tx films on a Si window using a Thermo Nicolett 6700 from 6000 to 360 cm–1 at a resolution of 4 cm–1 for 128 scans. Background spectra were obtained on a clean Si



window of the same type. Note that peaks located at ~3000 cm–1 present in all the samples may come from trace hydrocarbon vapor present in the glove box atmosphere. 4.2.4.

Raman spectroscopy

Raman spectroscopy was performed on Ti4AlN3 and exfoliated Ti4N3Tx using a Renishaw inVia (Gloucestershire, UK) microspectrometer by using a 632.8 nm He-Ne laser focused through an objective lens with 100x magnification. The laser spot size in the focal plan was about 1 μm. The back-scattered light from the sample was directed to the CCD detector by a grating with 1800 lines/mm. The setup possesses a spectral resolution of 1 cm–1. 4.2.5.

X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) data were obtained on a Physical Electronics 5600 system using Al Kα radiation. The XPS setup was calibrated with Au metal, which was cleaned via Ar-ion sputtering. The energy uncertainty for the core level data is ± 0.05 eV. The atomic percentages have a +/- 5% error. Measurements were performed on Ti4AlN3 and exfoliated Ti4N3Tx samples. The samples were prepared by drop-casting 10–20 µL of exfoliated MXene solution onto Au substrates similar to sample preparation for the XRD measurements. 4.2.6.

Atomic force microscopy

Atomic force microscopy (AFM) was used to image the exfoliated Ti4N3Tx flakes that were deposited onto a silicon substrate via membrane filtration method as described in the sample preparation section. Briefly, the ambient environment AFM (Bruker Innova) is housed inside an acoustic box that is located on top of a vibration isolation table. 4.2.7.

Scanning electron microscopy

Scanning electron microscopy (SEM) was used to analyze the morphology of the Ti4AlN3 and multilayered Ti4N3Tx materials. SEM images were taken in a FEI Nova 630 system. The resolution was 1 nm. 4.2.8.

Transmission electron microscopy 30 ACS Paragon Plus Environment

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Transmission electron microscopy (TEM) images for exfoliated Ti4N3Tx were obtained on FEI Tecnai ST30 at an accelerating voltage of 300 kV. Samples were prepared by dipping (three times) the ultrathin carbon film on lacey carbon support film (400 mesh, Copper) into the solution containing the material, and then dried under ambient air. The resolution was 0.1 nm. Additionally, electron energy loss spectroscopy (EELS) was performed on both Ti4AlN3 and exfoliated Ti4N3Tx flakes. EELS provides a detailed analysis of the elemental composition and purity of the exfoliated sample and information such as electron binding energy of specific atoms at specific sites in the images. 4.3.

Optical Characterization

4.3.1.

UV-Vis spectroscopy

UV-Vis spectroscopy was conducted on films deposited on FTO as described in the sample preparation section. The absorption data were collected from UV-Vis-NIR measurements, obtained on a Varian Cary 500 spectrophotometer from 1100 to 200 nm in 1 or 2 nm increments at a rate of 600 nm/min with a bandwidth of 2 nm. Note that absorption experiments were also conducted on exfoliated Ti4N3Tx suspended in water, ethanol, and dimethylformamide (DMF). 4.3.2.

Photoluminescence spectroscopy

Emission experiments were conducted at room temperature on exfoliated Ti4N3Tx in water using

the

reflectance

mode

in

a

modified

Horiba

Jobin-Yvon

Fluoromax-4

Spectrofluorometer (model FLUOROMAX-4 S/N: 0366C-3808). The sample was excited from 300 to 400 nm with an increment of 2 nm (50 spectra) using a 300 W xenon lamp, with wavelength selection provided by a double-grating spectrometer (grating specs, 1200 grooves/mm, blazed at 500 nm), and fluorescence from the samples was detected with a liquid-nitrogen-cooled CCD detector coupled to a single-grating iHR320 imaging spectrometer (grating specs, 150 grooves/mm, blazed at 500 nm). All spectra were corrected



for the response of the detection system. Each spectrum was integrated over 0.1 s with an excitation slit of 10 nm and an emission slit of 5 nm and averaged over 10 scans. 4.4.

Photoelectrochemical Characterization

4.4.1.

Incident photon-to-current efficiency and photocurrent

Incident photon-to-current efficiency (IPCE) was performed on the exfoliated Ti4N3Tx MXene deposited via drop-casting an aqueous solution onto a FTO substrate. Data were collected in a three-electrode configuration, with Ag/AgCl as the reference electrode and a Pt foil counter electrode, in 10 mM ascorbic acid aqueous solution at 0.49 V vs RHE. Further detail about the set-up of the instrument is reported elsewhere.58 The sequence at each wavelength was 2 s of dark following by 2 s of illumination. The current was collected at 10 points per s, with the final 10 points of each light and dark cycle averaged. For each data point at one wavelength, the photocurrent was obtained by subtracting the dark current from the light current. Sample photocurrent data were normalized to the output of a calibrated silicon photodiode (Hamamatsu, A02 S1336-8BQB) outside the solution. Each plot was obtained by the average value of two experiments. For the raw photocurrent measurement, performed in 10-mM ascorbic acid, a 530-nm light source was used as the excitation wavelength. Light was switched on and off as the photoelectrode was held at an equilibrium potential and the corresponding photocurrent was measured as a function of time. 4.4.2.

(Photo)electrochemical HER

(Photo)electrochemical measurements were conducted using linear sweep voltammetry (LSV). The LSV data were collected in 0.5 M H2SO4 aqueous electrolyte (pH = 0.3) at 20 mVs–1 using an EG&G Princeton Applied Research VersaStat II potentiostat with a 300 W Xe-arc lamp (Newport). Intensity of light was calibrated by using GaInP2 photodiode (Eg = 1.83 eV) whose short-circuit current was measured under AM1.5G. Measurements were carried out in a standard three-electrode electrochemical cell using a rotating disk electrode (Pine Research Instrumentation). The working electrode (mass loading: 0.2 mg cm-2geo) was prepared by 32 ACS Paragon Plus Environment

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mixing the Ti4N3Tx solution (500 L in water) and Nafion 117 solution (~ 5%; 5 µL) using sonication (30 min), followed by drop-casting onto glassy carbon disk and drying at room temperature. RuO2 mesh and Ag/AgCl (in 3M NaCl) were used as the counter and reference electrodes respectively. All displayed LSVs and Tafel plots are iR-corrected to account for any uncompensated resistance and transport issues. The bare glassy carbon and Ti4N3Tx working electrodes were rotated at 1600 rpm in a 0.5 M H2SO4 electrolyte that was sparged with N2 by bubbling through the solution for 30 min prior to measurements, and the headspace of the cell was continuously purged with N2 during measurements. 4.5.

Electrical Characterization

4.5.1.

Temperature-dependent resistivity measurements

Temperature-dependent resistivity measurements were performed on exfoliated Ti4N3Tx film of ~62 m thickness (measured via cross-sectional SEM, not shown) using a Dynacool PPMS equipped with High-Field Module from Quantum Design (1.8 K to 400 K, 14 Tesla Magnet). The sample was prepared by drop-casting the exfoliated Ti4N3Tx solution in water onto a silicon patterned substrate with electrical gold contacts (10 m spacing between contacts), then dried under vacuum at 80 C for 12 h. Resistance and magnetoresistance were measured in a two-probe configuration in a temperature range from room temperature (302 K) down to 2 K in a magnetic field up to 5 Tesla.

Supporting Information. Pictures of Ti4AlN3 MAX monolith; XRD spectrum of molten salt treated Ti4AlN3 with complete peak assignments; XRD spectra of molten salt treated Ti4AlN3 and Ti4N3Tx MXene following a short 30 min heat treatment; Raman spectra for Ti4AlN3 and exfoliated Ti4N3Tx collected in N2 and air, peak fits of Ti 2p XPS spectrum of exfoliated Ti4N3Tx MXene; N and Ti STEM-EELS spectra of Ti4AlN3 MAX and exfoliated Ti4N3Tx MXene; AFM topography 33 ACS Paragon Plus Environment


and additional TEM images of exfoliated Ti4N3Tx MXene flakes; and absorption intensity of exfoliated Ti4N3Tx MXene in water, ethanol and DMF as well as the Ti4AlN3 MAX precursor suspended in ethanol. Chopped light linear sweep voltammetry of exfoliated Ti4N3Tx MXene and blank FTO in ascorbic acid electrolyte.

Acknowledgment We are grateful to Prof. Ivar Reimanis and Dr. Aaron Miller for use of the hot press at the Colorado School of Mines. We are also grateful to Dr. Noah Bronstein for assistance with some TEM, to Bobby To for SEM measurements, to Dr. Joseph J. Berry for temperaturedependent resistivity measurements, and Dr. Jao van de Lagemaat for helpful discussions. This work was conducted by all co-authors, employees of the Alliance for Sustainable Energy, LLC, the manager and operator of the National Renewable Energy Laboratory for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding provided by the U.S. DOE, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, Solar Photochemistry Program.

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TOC Graphic H2

Ti4N3Tx e-

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x=

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H+H+

DA h+

T

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

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Electrocatalytic and Optoelectronic Characteristics of the Two

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