Research method
X-ray Photoelectron Spectroscopy (XPS)
17 studies
X-ray photoelectron spectroscopy measures binding energies of core electrons ejected from the near-surface of a solid (typically the top few nanometres). Peak positions report oxidation state and liganding (S 2p of thiolate versus unbound thiol; Ni⁰ versus Ni–N single-atom signatures; Ni³⁺/Ni²⁺ ratios), and survey scans give atomic percentages. In this library XPS is used to argue what the surface is — a SAM, a carbon-coated Ni particle, a reconstructed LDH, a pyrolysis-free Fe site — not to measure a faradaic efficiency by itself.
Electrochemists reach for XPS when bulk diffraction cannot tell whether the working skin is metal, oxide, single atoms, or leftover ligand. It answers 'what oxidation states and heteroatoms sit in the photoelectron escape depth?' Its main limitation is that a surface ratio is not a catalytic cycle, Ti-supported films can confound O/C, and the absence of a Ni–N peak is an argument against one structural hypothesis, not a complete operando mechanism.
Studies
17
Findings
5
6 supporting · 0 challenging · 0 qualifying citations
Open tensions
2
Currently
What we know
- XPS can prove chemisorption of a thiol sugar on gold and still not give a binding constant. On ~15 nm citrate AuNPs and sputtered Au films, S 2p at 162 eV marks Au–S; C–O (286.5 eV) rises about six-fold on films after 1-β-D-thio-glucose. Nanoparticles retain ~30% unbound thiol and lower thioglucose density, hence weaker maltose-binding-protein uptake; SIMS peaks at 358 and 375 u mark TG–pocket complexes. No calibrated Kd is reported.
- XPS plus STEM can decide nanoparticle versus single-atom nickel for CO₂-to-CO electrolysis. Carbon-coated Ni on N-doped carbon (loading ~3.6–4.0 wt%) shows Ni⁰ and no Ni–N single-atom peak; HAADF-STEM sees 10–100 nm particles. CO faradaic efficiency reaches ~94% at −0.7 V vs RHE, with 22.7 mA cm⁻² at −1.1 V. Long-term MEA stacks and C2+ products are not claimed.
- Valence ratios from XPS/XAS can be the design handle for OER. NaBH₄ reduction of NiFe LDH raises Ni³⁺/Ni²⁺ from 0.39 to 1.32 and Fe²⁺/Fe³⁺ from 0.40 to 1.49, with oxygen vacancies near Fe by EXAFS. O₂ faradaic efficiency is 95.9% (7.48×10⁻⁵ mol O₂ at 50 mA for 10 min). A full electrolyzer lifetime and a complete microkinetic OER cycle are not provided.
- XPS also reports how much metal is actually on a molecular or pyrolysed catalyst. A pyrolysis-free Fe–PIPhen on Vulcan XC-72 has 0.748 at% Fe, onset ~0.85 V, average n = 3.94 at 1600 rpm, and H₂O₂ yield 1.6–3.1% (k₁/k₂ = 30–50). A Co–N–C material identified as isolated Co²⁺ in a CoN₄C₈-1-2O₂ motif (XPS with XANES/EXAFS/STEM) gives 97% azo yield and TOF 35.9 h⁻¹ — still without in-operando proof of the hydrogenation cycle.
- On photoelectrodes, XPS sits beside XRD/SEM/UV-vis rather than replacing them. AACVD BiOCl/Br/I films are phase-pure tetragonal matlockite; untreated BiOCl (p-type) and BiOI (n-type) are unstable for PEC, while BiOBr reaches ca. 0.38 mA cm⁻² at 1 V vs Ag/AgCl without a sacrificial donor. XPS does not by itself explain why Cl and I films fail stability.
Largest unresolved question
XPS is a surface vote; XRD/STEM decide how that vote relates to the bulk. The Ni CO₂ paper uses missing Ni–N XPS intensity plus 10–100 nm STEM particles to reject a single-atom picture; the Co–N–C paper uses XPS with EXAFS to assert isolated CoN₄. Same spectroscopy, opposite structural conclusions, because the materials differ — you cannot treat 'XPS showed the metal state' as portable.
Procedure
How it works
The main stages of the method. Specific protocols and analysis choices vary between studies.
Eject core electrons from the near-surface
Soft X-rays illuminate the solid in vacuum; photoelectrons escape from typically the top few nanometres. Anything deeper than the escape depth is invisible, which is why XPS is a surface method rather than a bulk assay.
Convert kinetic energy to binding energy
The spectrometer measures kinetic energy; binding energy follows once the photon energy and a charge-reference are set. Charging, adventitious carbon, and the reference choice can shift apparent peak positions.
Assign oxidation state and liganding
Peak positions and shapes are fitted to species — thiolate versus unbound thiol, Ni⁰ versus Ni–N, Ni³⁺/Ni²⁺. Overlapping envelopes and shake-up structure make those fits model-dependent, not unique fingerprints.
Report atomic percentages from survey scans
Survey intensities are turned into atomic percent of the analysed surface. That composition is not a count of catalytic sites, and a support such as Ti can confound O/C so that a film is not a copy of the colloid.
Read the skin, not the catalytic cycle
XPS answers which oxidation states and heteroatoms sit in the photoelectron escape depth. Absence of a Ni–N peak is an argument against one site hypothesis, not a claim that nitrogen is absent from the support, and a surface ratio is not a faradaic efficiency or an operando mechanism.
Common misconceptions
If XPS shows Ni⁰ and no Ni–N peak, the catalyst cannot have any nitrogen and must be single-phase metal.
The CO₂-to-CO paper argues metallic Ni nanoparticles (3.6–4.0 wt%) on an N-doped carbon support whose coating plus N suppress HER. Absence of a Ni–N single-atom signature is a site assignment, not a claim that nitrogen is absent from the support.
An XPS atomic percent of Fe is the active-site density that produces the RRDE electron number.
Fe–PIPhen reports 0.748 at% Fe and n = 3.94 with 1.6–3.1% H₂O₂. Loading is low and part of the polymer is microcrystalline; there are no PEMFC data. XPS percent is composition of the analysed surface, not a count of catalytic turnovers.
S 2p at 162 eV on gold nanoparticles means every thiol is bound and the SAM is identical to a flat-gold film.
NPs retain ~30% unbound thiol and lower thioglucose density than sputtered films, where C–O intensity rises ~6-fold. Ti-supported NP films also confound O/C, so film XPS is not a copy of the colloid surface.
Related
Claim ledger
What the evidence shows
Drawn from 17 studies in this library. Mix labels say which citation roles are present; they are not a strength score. Supports means evidence for a finding; Challenges means evidence against a stated position; Qualifies marks scope.
XPS can prove chemisorption of a thiol sugar on gold and still not give a binding constant. On ~15 nm citrate AuNPs and sputtered Au films, S 2p at 162 eV marks Au–S; C–O (286.5 eV) rises about six-fold on films after 1-β-D-thio-glucose. Nanoparticles retain ~30% unbound thiol and lower thioglucose density, hence weaker maltose-binding-protein uptake; SIMS peaks at 358 and 375 u mark TG–pocket complexes. No calibrated Kd is reported.
XPS plus STEM can decide nanoparticle versus single-atom nickel for CO₂-to-CO electrolysis. Carbon-coated Ni on N-doped carbon (loading ~3.6–4.0 wt%) shows Ni⁰ and no Ni–N single-atom peak; HAADF-STEM sees 10–100 nm particles. CO faradaic efficiency reaches ~94% at −0.7 V vs RHE, with 22.7 mA cm⁻² at −1.1 V. Long-term MEA stacks and C2+ products are not claimed.
Valence ratios from XPS/XAS can be the design handle for OER. NaBH₄ reduction of NiFe LDH raises Ni³⁺/Ni²⁺ from 0.39 to 1.32 and Fe²⁺/Fe³⁺ from 0.40 to 1.49, with oxygen vacancies near Fe by EXAFS. O₂ faradaic efficiency is 95.9% (7.48×10⁻⁵ mol O₂ at 50 mA for 10 min). A full electrolyzer lifetime and a complete microkinetic OER cycle are not provided.
XPS also reports how much metal is actually on a molecular or pyrolysed catalyst. A pyrolysis-free Fe–PIPhen on Vulcan XC-72 has 0.748 at% Fe, onset ~0.85 V, average n = 3.94 at 1600 rpm, and H₂O₂ yield 1.6–3.1% (k₁/k₂ = 30–50). A Co–N–C material identified as isolated Co²⁺ in a CoN₄C₈-1-2O₂ motif (XPS with XANES/EXAFS/STEM) gives 97% azo yield and TOF 35.9 h⁻¹ — still without in-operando proof of the hydrogenation cycle.
Study Role Design N Population Outcome Pyrolysis-free Fe–PIPhen catalyzes alkaline ORR Supports Animal / in-vitroFe2+–PIPhen coordination polymer on Vulcan carbon with RRDE oxygen-reduction measurements RRDE/XPS/XRD catalyst characterization rather than a biological N Alkaline oxygen-reduction electrocatalyst films Nitrogen-rich phenanthroline–iron complex reduces O2 near 0.85 V with a four-electron pathway and low H2O2 yield Single-atom Co–N–C catalyzes nitroarene azo coupling Supports OtherCo–N–C single-atom catalyst identification by STEM/XAS and nitroarene hydrogenation tests Heterogeneous catalysis materials study — no sample N Co–N–C catalysts converting nitroarenes CoN4 site structure and azo-product yield/TOF On photoelectrodes, XPS sits beside XRD/SEM/UV-vis rather than replacing them. AACVD BiOCl/Br/I films are phase-pure tetragonal matlockite; untreated BiOCl (p-type) and BiOI (n-type) are unstable for PEC, while BiOBr reaches ca. 0.38 mA cm⁻² at 1 V vs Ag/AgCl without a sacrificial donor. XPS does not by itself explain why Cl and I films fail stability.
Debates
Tensions and limits
Some items are genuine disagreements on the same question. Others mark different assays, populations, or outcomes.
XPS is a surface vote; XRD/STEM decide how that vote relates to the bulk. The Ni CO₂ paper uses missing Ni–N XPS intensity plus 10–100 nm STEM particles to reject a single-atom picture; the Co–N–C paper uses XPS with EXAFS to assert isolated CoN₄. Same spectroscopy, opposite structural conclusions, because the materials differ — you cannot treat 'XPS showed the metal state' as portable.
XPS is a surface vote; XRD/STEM decide how that vote relates to the bulk. The Ni CO₂ paper uses missing Ni–N XPS intensity plus 10–100 nm STEM particles to reject a single-atom picture; the Co–N–C paper uses XPS with EXAFS to assert isolated CoN₄. Same spectroscopy, opposite structural conclusions, because the materials differ — you cannot treat 'XPS showed the metal state' as portable.
- Carbon-coated nickel nanoparticles that make CO from CO2
- Single-atom Co–N–C catalyzes nitroarene azo coupling
Study Role Design N Population Outcome Carbon-coated nickel nanoparticles that make CO from CO2 Supports OtherPyrolyzed carbon-supported Ni nanoparticles for aqueous CO2-to-CO electrolysis with DFT Electrocatalysis materials study — no sample N Ni nanoparticle catalysts on N-doped carbon supports CO faradaic efficiency and current density in CO2 electroreduction Single-atom Co–N–C catalyzes nitroarene azo coupling Supports OtherCo–N–C single-atom catalyst identification by STEM/XAS and nitroarene hydrogenation tests Heterogeneous catalysis materials study — no sample N Co–N–C catalysts converting nitroarenes CoN4 site structure and azo-product yield/TOF
PaperFren reads this as a limit on how far one study travels — different assays, populations, or outcomes — not a forced fight between papers.
A surface Ni³⁺/Ni²⁺ ratio is a descriptor, not the OER mechanism. NaBH₄-treated LDH correlates Ni³⁺ (0.39 → 1.32) with activity and reports 95.9% O₂ FE, but does not solve the atomic OER cycle. BiOBr's 0.38 mA cm⁻² photocurrent is a different electrochemical question (PEC of a halide film) that XPS characterisation does not make interchangeable with LDH OER.
A surface Ni³⁺/Ni²⁺ ratio is a descriptor, not the OER mechanism. NaBH₄-treated LDH correlates Ni³⁺ (0.39 → 1.32) with activity and reports 95.9% O₂ FE, but does not solve the atomic OER cycle. BiOBr's 0.38 mA cm⁻² photocurrent is a different electrochemical question (PEC of a halide film) that XPS characterisation does not make interchangeable with LDH OER.
Study Role Design N Population Outcome NaBH4 raises Ni3+ and speeds NiFe LDH OER Supports OtherNaBH4 reduction of NiFe LDH to raise Ni3+/Ni2+ and oxygen vacancies for OER Electrocatalysis materials study — no sample N NiFe layered double hydroxide electrocatalysts OER activity and O2 faradaic efficiency after hydride treatment Bismuth oxyhalide films as photoelectrodes Supports OtherAACVD BiOX film deposition with band-structure analysis and photoelectrochemical testing Materials photoelectrochemistry — no sample N BiOCl, BiOBr, and BiOI thin-film photoelectrodes Photoelectrochemical activity and band-edge positions of BiOX films
PaperFren reads this as a limit on how far one study travels — different assays, populations, or outcomes — not a forced fight between papers.
Timeline
How understanding moved
Study years are when the paper was published. Evidence edits are dated changes to this page's claims. Explanations are when PaperFren added a Discovery — not a claim that the science happened that day.
2026
Concept page published
X-ray Photoelectron Spectroscopy (XPS)
Change log
What changed
Dated edits to this page's evidence: studies added or removed from a claim, claims added or withdrawn, and new explanations tagged here. Coverage edits are distinct from reviewed research changes and corrections. Dates record PaperFren updates; study publication dates appear in the timeline.
- Coverage updated · Concept page published
Papers
17 studies in this library bear on X-ray Photoelectron Spectroscopy (XPS), ordered by citations.
- Single-atom Co–N–C catalyzes nitroarene azo coupling
A self-supporting Co–N–C single-atom catalyst with a CoN4C8-1-2O2 site hydrogenatively couples nitroarenes to azo compounds.
- Bismuth oxyhalide films as photoelectrodes
AACVD BiOX films show halide-tuned bandgaps; untreated BiOBr gives about 0.38 mA cm−2 photoanodic current without a sacrificial donor.
- Ten-gram grinding makes lanthanum coordination polymers
Manual grinding of LaCl3 with 5-sulfosalicylate and 8-hydroxyquinoline yields ternary CPs whose IR/XPS/MS match a chelated nanocomposite.
- XPS and SIMS map thioglucose SAMs on gold
Thio-glucose binds gold through Au–S; flat films pack better than citrate AuNPs, and ToF-SIMS sees TG–MBP pocket fragments.
- Carbon-coated nickel nanoparticles that make CO from CO2
N-doped carbon plus a carbon coat lets metallic Ni nanoparticles reach about 94% CO faradaic efficiency, suppressing hydrogen evolution.
- H2O2 is not born at the air–water droplet surface
Sensitive assays show microdroplet H2O2 comes from dissolved O2 reacting at solid–water interfaces, not from ultrahigh fields at the air–water surface.
- pH-switchable polymer that pulls down N-glycopeptides
A soluble poly(acrylic acid-co-hydrazide) captures glycopeptides in homogeneous solution and precipitates them by dropping pH for MS.
- NaBH4 raises Ni3+ and speeds NiFe LDH OER
Treating NiFe layered double hydroxide with NaBH4 lifts Ni3+/Ni2+ from 0.39 to 1.32 and gives 95.9% faradaic efficiency for O2.
Show 9 more studiesShow fewer studies
- Pyrite mesocrystals reconstruct into OER catalysts
Oleylamine-grown (NiFe)S2 porous cubes form by cluster aggregation; Fe-doped samples reach η10 < 260 mV after converting to S-doped (oxy)hydroxides.
- Stibnite in carbon sheets stores sodium better
Natural Sb2S3 anchored on sulfur-doped carbon sheets delivers much higher reversible sodium-storage capacity and cycle retention than raw stibnite.
- Magnetic Cu-on-pumice catalyst for nitro reductions
A cellulose-textured volcanic-pumice magnetite composite bearing copper nanoparticles reduces nitrobenzenes to anilines in eight minutes.
- Microwave phage carbons glow and sense Fe3+
M13 bacteriophage carbonized in a 700 W microwave gives fluorescent nanoparticles (QY 14.8%) that quench with Fe3+ down to an 8.0 μM LOD.
- Pyrolysis-free Fe–PIPhen catalyzes alkaline ORR
An Fe2+ coordination polymer of a nitrogen-rich phenanthroline ligand on carbon reduces O2 near 0.85 V with a four-electron pathway.
- Plasmonic SiO2/TiO2 aerogels degrade RhB
Anatase TiO2 grown in silica aerogels, then Ag@Au nanoshells, adsorbs and photocatalyzes rhodamine B far better than TiO2 powder.
- Sputtering Pt onto PEG to make fuel-cell catalysts
Pt sputtered onto liquid PEG, then heat-transferred onto carbon, gives bimodal nanoparticles whose larger population drives ORR activity.
- Al3+ lights up Ag/Au clusters to sense fluoroquinolones
Dithioerythritol Ag/Au nanoclusters aggregate with Al3+ and then report ciprofloxacin, norfloxacin and enrofloxacin by ratiometric fluorescence down to a few nanomolar.
- Paper titanyl sensor sees H2O2 vapor at 0.04 ppb
Ammonium titanyl oxalate on cellulose turns yellow as a Ti(IV)–peroxide complex, detecting hydrogen peroxide vapor down to about 0.04 ppb without fancy instrumentation.
Compare studies
Select 2–10 studies. Design and N are labels, not a ranking.
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Questions
What is still open
XPS is a surface vote; XRD/STEM decide how that vote relates to the bulk. The Ni CO₂ paper uses missing Ni–N XPS intensity plus 10–100 nm STEM particles to reject a single-atom picture; the Co–N–C paper uses XPS with EXAFS to assert isolated CoN₄. Same spectroscopy, opposite structural conclusions, because the materials differ — you cannot treat 'XPS showed the metal state' as portable.
A surface Ni³⁺/Ni²⁺ ratio is a descriptor, not the OER mechanism. NaBH₄-treated LDH correlates Ni³⁺ (0.39 → 1.32) with activity and reports 95.9% O₂ FE, but does not solve the atomic OER cycle. BiOBr's 0.38 mA cm⁻² photocurrent is a different electrochemical question (PEC of a halide film) that XPS characterisation does not make interchangeable with LDH OER.
Ask PaperFren about X-ray Photoelectron Spectroscopy (XPS)
Study this conceptflashcards and short-answer questions
Ni 2p XPS shows Ni⁰ and no Ni–N single-atom peak, while STEM shows 10–100 nm particles and CO FE is ~94% at −0.7 V vs RHE. What hypothesis has been argued against, and what has not been shown?
A dominant Ni–N single-atom (or nitride) active-site picture for this material. Not shown: long-term industrial MEA performance or C2+ products. Carbon coating and N-doped support are invoked to suppress HER, which is a catalytic argument beyond the XPS peak table.
Ni³⁺/Ni²⁺ rises from 0.39 to 1.32 after NaBH₄ treatment and O₂ FE is 95.9%. Why is that not a solved OER mechanism?
XPS/XAS correlate a higher Ni³⁺ (and Fe²⁺) fraction plus oxygen vacancies with faster OER. Correlation of a surface ratio with current and a 10 min O₂ quantification (7.48×10⁻⁵ mol at 50 mA) does not specify the elementary proton-coupled steps or electrolyzer lifetime.
Why can S 2p at 162 eV be true for both a dense thioglucose SAM on a gold film and a weaker MBP-binding AuNP, and what extra XPS number separates them?
162 eV reports Au–S chemisorption in both cases. Nanoparticles keep ~30% unbound thiol and lower TG density; films show a ~6-fold rise in C–O at 286.5 eV. Coverage and unbound fraction, not the mere presence of thiolate, track protein uptake.
Co–N–C is assigned as CoN₄C₈-1-2O₂ isolated Co²⁺ and gives 97% azo yield (TOF 35.9 h⁻¹). What can XPS not replace in that assignment?
HAADF-STEM isolation of atoms and XANES/EXAFS coordination (four pyridinic N plus weakly bound axial O). XPS is one surface-sensitive vote; the paper still lacks in-operando proof of the working hydrogenation cycle.