Summary Since their discovery in 2011, MXenes have garnered worldwide interest. Given their 2D structure, surface, or termination, chemistries play a vital role in most applications. X-ray photoelectron spectroscopy (XPS) is one of the most common characterization tools for quantifying surface terminations and overall chemistry. Herein, we critically review the XPS fitting models proposed for Ti3C2Tz MXene in the literature and make the case that they are at best incomplete and at worst contradictory. We propose a new fitting algorithm based on all the data obtained from previously published studies and propose a new method for quantifying the surface terminations in Ti3C2Tz. In our approach, we assign the Ti 2p peak at 455.1 eV to the C–Ti–O\O\O, and the peaks at 456.0, 457.0, 457.9, and 459.6 eV are assigned to C–Ti–O\O\F, C–Ti–O\F\F, C–Ti–F\F\F, and TiO2-xF2x, respectively. The first four represent possible Ti atom terminations; the last is an oxyfluoride. The C 1s peak at 282 eV, ascribed to C atoms surrounded by 6 Ti atoms, is so universal that it can almost be used as a reference.

/pdf/a-critical-analysis-of-the-x-ray-photoelectron-spectra-of-13slm0rx6c.pdf

A critical analysis of the X-ray photoelectron spectra of Ti3C2Tz MXenes

Two-dimensional transition-metal carbides and nitrides (MXenes) are a large family of materials actively studied for various applications, especially in the field of energy storage. MXenes are commonly synthesized by etching the layered ternary compounds, called MAX phases. We demonstrate a direct synthetic route for scalable and atom-economic synthesis of MXenes, including compounds that have not been synthesized from MAX phases, by the reactions of metals and metal halides with graphite, methane, or nitrogen. The direct synthesis enables chemical vapor deposition growth of MXene carpets and complex spherulite-like morphologies that form through buckling and release of MXene carpet to expose fresh surface for further reaction. The directly synthesized MXenes showed excellent energy storage capacity for lithium-ion intercalation. Description MXenes without MAX phases Two-dimensional metal carbides and nitrides, or MXenes, have several potential applications in energy storage and electronics. They are usually made by delamination of a layered parent compound, a MAX phase, in a harsh etching step. Wang et al. synthesized one of the most widely used MXenes, Ti2CCl2, as well as MXenes not available from MAX phases, using chemical vapor deposition (see the Perspective by Robertson and Tolbert). The reaction of methane and titanium tetrachloride on a titanium surface led to the growth of Ti2CCl2 sheets perpendicular to the metal. Under some growth conditions, the sheets could buckle off the surface to form vesicles. —PDS Solid-state reactions formed vertical carpets of 2D metal carbides and nitrides on metal substrates.

/pdf/direct-synthesis-and-chemical-vapor-deposition-of-2d-carbide-30o651v7.pdf

Direct synthesis and chemical vapor deposition of 2D carbide and nitride MXenes

http://lib.tkk.fi/Diss/2005/isbn9512278804/article5.pdf

Chemical state quantification of iron and chromium oxides using XPS: the effect of the background subtraction method

A complete and self-consistent evaluation of XPS spectra of TiN

A strategy to synthesize a 2D graphenic but ternary monolayer containing atoms of carbon, nitrogen, and boron, h-BCN, is presented. The synthesis utilizes bis-BN cyclohexane, B2N2C2H12, as a precursor molecule and relies on thermally induced dehydrogenation of the precursor molecules and the formation of an epitaxial monolayer on Ir(111) through covalent bond formation. The lattice mismatch between the film and substrate causes a strain-driven periodic buckling of the film. The structure of the film and its corrugated morphology is discussed based on comprehensive data from molecular-resolved scanning tunneling microscopy imaging, X-ray photoelectron spectroscopy, low-energy electron diffraction, and density functional theory. First-principles calculations further predict a direct electronic band gap that is intermediate between gapless graphene and insulating h-BN.

Graphene-like Boron–Carbon–Nitrogen Monolayers

Photoelectron peak areas are measured using various background removal methods. Linear, horizontal, Shirley's and Tougaard's backgrounds have been tested on many spectra for different pass energies in the spectrometer. These four values, obtained for each photoelectron line, are compared together and it is shown that the first three methods give proportional results. These values are then compared to theoretical intensitities for five elements (Au, Ag, Cu, Ni, Cr) and one oxide (Al2O3). These theoretical calculations are based on the knowledge of the spectrometer transmission, photoionization cross-sections and inelastic mean free path, the product of which is commonly called the relative sensitivity factor. The best agreement is obtained when Tougaard's background is removed.

Comparison of background removal methods for XPS

In the present interlaboratory comparison, the consistency and validity of the Shirley, straight-line and Tougaard methods for determination of peak intensities from measured XPS spectra have been studied by analysis of data on Au and Ni taken at eight laboratories. The reproducibility of the individual measuring systems was determined to be 2–6%. The RMS scatter around their mean values of the ratio of the peak intensity to the Au 4d peak intensity was 6% for the Tougaard method, 9–10% for the Shirley method and 8–9% for the straight-line method. The increase in root mean square (RMS) scatter of data due to data being recorded in different laboratories amounts to 3–4%, independent of the method applied for background correction. The RMS deviation from theory of intensity ratios of peaks from Ni and Au to the Au 4d peak intensity is 14% for the Tougaard method, 33% for the Shirley method and 31% for the straight-line method. The absolute accuracy of the methods cannot be established to better than the accuracy on the theoretical peak intensity ratios and the uncertainty in the energy dependence of the analyser response function. It is, however, reasonable to expect the combined uncertainty of these factors to be at least 10–15%. The peak intensity ratios obtained with the Tougaard method then falls within the expected accuracy of the theoretical ratios.

Intercomparison of algorithms for background correction in XPS

Determination of the intensity–energy response function of the ISA Riber MAC 2 electron analyser

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Comparison of background removal methods for XPS

Intercomparison of algorithms for background correction in XPS

Determination of the intensity–energy response function of the ISA Riber MAC 2 electron analyser