Plasmon assisted synthesis of TiN-supported single-atom nickel catalysts

We report the deposition of single atom nickel catalyst on refractory plasmonic titanium nitride (TiN) nanomaterials supports using the wet synthesis method under visible light irradiation. TiN nanoparticles efficiently absorb visible light to generate photoexcited electrons and holes. Photoexcited electrons reduce nickel precursor to deposit Ni atoms on TiN nanoparticles’ surface. The generated hot holes are scavenged by the methanol. We studied the Ni deposition on TiN nanoparticles by varying light intensity, light exposure time, and metal precursor concentration. These studies confirmed the photodeposition method is driven by hot electrons and helped us to find optimum synthesis conditions for single atoms deposition. We characterized the nanocatalysts using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), energy dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). We used density functional theory (DFT) calculations to predict favorable deposition sites and aggregation energy of Ni atoms on TiN. Surface defect sites of TiN are most favorable for single nickel atoms depositions. Interestingly, the oxygen sites on native surface oxide layer of TiN also exhibit strong binding with the single Ni atoms. Plasmon enhanced synthesis method can facilitate photodeposition of single atom catalysts on a wide class of metallic supports with plasmonic properties. Supplementary Information The online version contains supplementary material available at 10.1186/s11671-024-03992-z.


Introduction
Single-atom catalysts (SACs) have attracted extensive attention due to their high atom efficiency and excellent catalytic performance.Water-gas shift reaction, Fischer-Tropsch synthesis, and alkynes selective hydrogenation, CO oxidation, CO 2 reduction, oxygen reduction reaction, NO reduction and hydrocarbon oxidation are very few examples of that [1].SACs possess not only 100% atom efficiency, but also possess high activity due to the low-coordination environment of metal centres in the small HOMO-LUMO bandgap.There are several methods proposed for single atom catalysts synthesis including bottom-up strategies and top-down approach.The top-down strategies such as high-temperature atom trapping approach and in-situ pyrolysis methods can afford single atom catalysts with precise structure and high loading and yield.However, these methods require extremely high temperature limiting their wide scale applications.Mass-selected soft-landing techniques, atomic layer deposition (ALD), and wet chemical routes, are some common bottom-up synthesis methods for single atoms catalysts.Among these methods, the wet chemical method is advantageous for large scale applications as this method doesn't require specialized equipment and can be performed in wet chemistry lab [1][2][3][4][5][6][7][8].Recently, photoreduction of metal salts on photocatalytic substrate such as TiO 2 is demonstrated as an effective method to prepare relatively high loading stable single atom catalysts [9,10].Photochemical synthesis of single atom catalysts using wet chemical method where the reduction step can be done using light eliminating the need of high temperature, has potential to implement in the industry scale.It provides the advantages of photo-induced processing, that is, clean process, high spatial resolution, and convenient useful.However, this method requires the support to be photocatalytic limiting the choice of substrates to be semiconducting materials.Here for the first time, we deposit a single atom of nickel catalyst by reducing metal precursor salts with the photo-excited electrons generated on plasmonic nanomaterials.Plasmonic nanoparticles sustain strong charge oscillations when Illuminated with the right wavelength, which are commonly refer to as plasmons.These excitations interact strongly with light and produce a large concentration of the electromagnetic field in nanoscale volumes.After excitation, plasmons can decay following two different pathways: (i) remitting light as scattering, or (ii) absorbing light to generate a transient population of non-equilibrium (hot) charge-carriers that eventually release their energy to the structure in the form of heat, thus increasing the particle temperature.The photoexcited carriers can be transferred to reduce the adsorbed metal precursor when there is a favorable energetic alignment among them [11].Our recently published paper on visible light mediated plasmon enhanced deposition of nickel and platinum nanoparticles on titanium nitride nanomaterials provides the evidence of that.[11] In addition to providing all the advantages of photochemical synthesis method, plasmon enhanced deposition method can extend the advantages of photodeposition to wide class of metallic supports with high conductivity.Additionally, it offers several other advantages: (i) concentrating electromagnetic field in the nanoscale, hence can drive precursor reduction reaction right on the substrate surface; (ii) visible light induced reactions providing opportunity to develop solar light enhanced processes; (iii) photothermal effects of plasmonic nanomaterials provide additional pathways to improve metal precursor reductions; (iv) the wavelength and intensity of absorbed light as well as hot electron generation efficiency of plasmonic nanoparticles can be optimized by tuning their shapes and size; (v) solution phase and single pot synthesis can be easily implemented in any wet lab.
Here, we present a combined experimental and theoretical study to deposit single atom nickel catalysts on refractory plasmonic titanium nitride supports.Transition metal nitride nanomaterials such as titanium nitride were considered as promising candidates for the support materials for transition metal catalysts due to their good electrical conductivity and its high resistance to corrosion and acid environment [12][13][14][15][16][17].Due to their plasmonic properties they can efficiently absorb and concentrate light at the nanoscale to generate intense electromagnetic fields [18,19].The photoexcited electrons can undergo nonradiative decay, generating highly energetic electrons.These photoexcited electrons can reduce metal precursors adsorbed on the surface to deposit metal atoms on the surface [11].Titanium nitride (TiN) displays tunable stoichiometry of Ti and N and also presence of oxygen on the surface, which can provide favourable binding sites for single atoms.TiN has been identified as a promising candidate for catalyst support for different metal catalysts including Ni, Pt and Co [12,13,20,21].Deposition of single atom transition metal catalysts such as Ni and Pt on titanium nitride is particularly advantageous due to their strong electronic interaction with the supported transition metal catalysts.TiN supported catalysts have shown significant promise for direct hydrogen and methanol fuel cells, electrocatalytic oxidation of small organic molecules and electrochemical oxygen reduction reactions [22].In particular, titanium nitride supported nickel catalysts exhibited enhancement in different important reactions including hydrogenolysis of aryl ether, dry reforming of methane, photoreduction of bicarbonate.[11,[22][23][24] To understand the mechanism of plasmon-enhanced deposition, we studied the Ni deposition on TiN nanoparticles by varying the synthetic conditions such as light intensity, light exposure time and metal precursor concentration.The understanding developed from the studies helped us to achieve optimum conditions for Ni single-atom catalysts depositions on TiN nanoparticles.We used Density Functional Theory (DFT) based calculations to understand interactions of Ni atoms with the anchoring surface atoms of TiN to predict the favourable deposition sites and stability as well as their electronic interactions.The outcome of this study will guide the synthesis of single atom catalysts on wide range of catalytic support materials including many metals and few transition metal nitrides and carbides, chalcogenide, and oxide nanostructures, which exhibit plasmonic properties.

Synthesis of TiN_Ni nanocomposites
400 mg of TiN nanoparticles dispersed in 20 mL of 35% HCl solution was sonicated for 10 min at room temperature in a sealed container.The above sonicated solution was then kept in a water bath at 65 °C for 1 h under a continuous N 2 (g) flow.After 1 h of heating, the mixture was filtered and washed with 1:1 water: ethanol solution.Again, it was washed twice with 40 mL of anhydrous ethanol.Obtained TiN was concentrated into 5 mL of ethanol.Then the above concentrate was dried in an oven at 80 °C overnight.[11] A 1.6 mM TiN solution was prepared in 50 mL of 1:1 (v/v) water: methanol solution.The above solution was sonicated for 10 min.Then NiCl 2 (aq) was introduced to the above TiN solution where the final concentration of NiCl 2 (aq) is varied.This mixture was then stirred and illuminated with different wavelengths and intensities of light for different durations.An LED light source (X-Cite TURBO XT600-T LED, Canada) is used to illuminate the reaction mixtures with different wavelength light.A Fiber Lite DC-950 Halogen lamp (made in the USA) is used as a broad-spectrum white light source.At the end of the reaction, the reaction mixture was centrifuged and rinsed twice with deionized water to collect the nanoparticles in a solid form.During this study the light source, light intensity, Ni precursor mole fraction with respect to TiN, and the light irradiation time were systematically varied to study their effects.

Kinetics studies
Kinetics studies were carried out by studying the consumption of methanol in the system during the redox reaction by monitoring the IR peak for the C-O bond of methanol over time.The reaction mixtures were prepared as mentioned above.The main factors studied using kinetics studies are the impact of light intensity and the Ni 2+ mol fraction on the reduction rate of Ni 2+ .During the light intensity variation studies, a halogen lamp (Fiber Lite DC-950) was placed aside from the FTIR spectrophotometer (Thermo Scientific Nicolet iS50 FT-IR).~ 1 mL of the reaction mixture was placed on the sample holder.Using a Teflon ring, the liquid sample was stabilized on the detector.Additionally, a fused-silica slide was placed over the liquid reaction mixture to prevent evaporation.Then using a Fresnel lens and a 90° off-axis parabolic mirror (50.8 mm Dia.× 101.6 mm EFL, UV Enhanced Aluminium 50 Å 90° Off-Axis Mirror, Edmund Optics), the light beam from the halogen lamp was focused on to the sample mounted on the FTIR spectrophotometer.The data collection was performed every 34 s.

Characterization
Nanoparticles were characterized by UV-VIS spectroscopy for their extinction properties (Agilent Technologies: Cary 60, Malaysia), and by X-ray diffraction for their compositional information (X'Pert Pro XRD).They were further characterized with high-resolution transmission electron microscopy (HRTEM), scanning tunnelling electron microscopy (STEM), and energy-dispersive X-ray spectroscopy (EDS) using JEOL 2010 FEG STEM with Oxford EDS and JEOL Neo ARM 200CF TEM equipped with aberration correlation and Oxford Aztec EDS.X-ray photoelectron spectroscopy (XPS) measurements were taken using a Kratos AXIS ULTRA X-ray Photoelectron Spectrometer.XPS samples were prepared by drying the samples at 90 °C for 1 h and being collected in a powdered form.The samples were scanned, resulting in a full range scan as well as a high-resolution scan for Nitrogen, Oxygen, Titanium, and Nickel.Raw response data from the XPS were normalized via an assumption of summations between normal distributions and matched to existing libraries of binding energies.The Ni loadings on TiN were determined by inductively coupled plasma spectrometry-mass spectrometry (ICP-MS) using Agilent ICP-MS 7900.The samples for ICP-MS were prepared by drying the sample at 70 °C for 8 h under vacuum, the collected dry powder sample was then dispersed in 1% nitric acid solution at the concentration of 1 mg/L.

Computational methods
The Vienna Ab Initio Simulation Package (VASP) was used to carry out spin polarized density functional theory (DFT) calculations [25][26][27].The interactions between valence electrons and frozen cores were described by a 400 eV energy cut-off and the Perdew-Burke-Ernzerhof (PBE) form of the generalized gradient approximation (GGA) [28] of the projected-augmented wave method (PAW) [29,30].In all of our calculations, Van der Waal interactions were accounted for by a semi-empirical scheme proposed by Grimme (DFT-D2) [30].The Gaussian smearing method with a width of 0.05 eV around the Fermi level was used to facilitate convergence.The electronic energies were converged to 10 -6 eV and the ionic relaxations had a tolerance of 0.02 eV/Å for the residual forces on the atoms.The cell was periodic in the x and y directions.To ensure negligible interactions between periodic images, the z dimension was chosen to be 25 Å.The Brillouin zone was sampled using a 3 × 3 × 1 Monkhorst-Pack k-point mesh.
Comprised of 50 titanium and 50 nitrogen atoms, the titanium nitride (TiN) cell was 14.97 × 14.97 Å in the x and y directions (Fig. SI.1).This was chosen to represent a pristine TiN that was not oxidized.Same cell as TiN was used to simulate Ti x O y N z with the exception that there was a single oxygen substitution in place of a nitrogen in the TiN substrate to represent a partially oxidized surface (Fig.

Characterization of TiN
The morphology of TiN nanoparticles characterized using high-resolution transmission electron microscopy (HRTEM) as shown in Fig. 1.It is well known that TiN nanoparticles develop 1-2 nm thick self-passivating oxide layer on the surface (Fig. 1b).However, as shown in the previous papers from our group, this thin oxide layer has negligible effect on the optical properties TiN nanoparticles [31,32].High resolution TEM image of TiN nanoparticles as shown in Fig. 1b indicates that the maximum thickness of the oxide layer on the surface of TiN nanoparticles is 2 nm.To understand the effect of oxide layer on the optical properties, we calculated absorption spectra of 20 nm TiN nanoparticle with different thickness of oxide layer using discrete dipole approximation method.As shown in Fig. SI.4, the 2 nm thick oxide layer on the surface of TiN nanoparticles doesn't significantly influence it's optical properties.The negligible effect of oxide layer is also evident by the UV-Vis absorbance peak of TiN nanoparticles which show usual plasmonic peak around 650 nm (Fig. 3a).However, our calculation suggests increasing the thickness of oxide layer gradually decreases the light absorbance of TiN nanoparticles and redshift its light absorption peak.We can see a significant decrease in light absorption when the oxide layer thickness reaches 10 nm.XPS analysis confirmed that the surface layer is a mixture of TiN, TiO 2 and titanium oxynitrides (Ti x O y N z ) (Fig. 1c and Fig. SI.5).Based on the Ti 2p high resolution XPS data for the nanoparticles, the nanoparticle surfaces are comprised of ~ 18% TiN, ~ 27% Ti x O y N z and ~ 55% TiO 2 .Similarly, the high resolution N 1 s XPS spectra of TiN_Ni indicate the presence of oxide layer on the surface.This is interesting as it provides different anchoring surfaces for Ni atoms.Additionally, formation of surface oxide layer on TiN can improve the lifetime of the hot electrons making it easier to harvest them for hot electron mediated reduction reactions [33].

Deposition of Ni on TiN
Titanium nitride nanomaterials can strongly absorb broad spectrum solar light.Under visible light irradiation TiN can generate hot electrons both due to interband and intraband transitions as its interband transition (N p-band to Ti d-band) is ~ 2 eV (Fig. 2) [34].One important factor for photodeposition of metals on support materials is the favorable positioning of the energy bands and the potential values.Interestingly, the presence of oxide layer on the surface of titanium nitride nanoparticles can increase the work function.For example, while the work function for pristine stoichiometric TiN is found to be 2.6 eV, the presence of oxide layer on the surface can increase the work function to 3 eV.[32] However, even with the presence of oxygen the Fermi energy level of TiN is such that the reduction of Ni precursor is energerically favorable both for the excited electrons generated from interband and intraband transitions (Fig. 2b).However, methanol can mostly scavenge the holes in N p-band mainly generated by the interband transitions.
An experiment with in situ FTIR studies of reactions under light is developed to study the rate of the consumption of methanol in the reaction mixture by monitoring the depletion of the FTIR peak for methanol C-O bond at ~ 1000 cm −1 (See SI for the detailed information, Fig. SI.7).While direct reduction of metal cations could be challenging to study with FTIR, the consumption of methanol is used to indirectly probe the Ni atom depostion rate via Ni precursor reduction (Please see SI for the details of these experiments).We studied the effect of different parameters such as light wavelengths, light intensity, and the metal precursor concentrations on the reaction.The purpose of these studies were two fold.First to unravel the mechanism of plasmon enhanced deposition and second to understand the effect of above parameters on Ni atom deposition rate to find an optimum condition for single atom deposition.The deposition of Ni on TiN only occurred in the presence of light.Additionally, there was no formation of Ni nanostructures in the absence of TiN.These confirm that the Ni deposition process is light mediated and driven by plasmonic properties of TiN.Several possible contributions for plasmonic enhanced reactions are enhanced near-fields, photoexcited charge carriers transfer and local photothermal heating [35].It's important to unravel the particular mechanism which enhance the plasmon enhanced deposition to fully exploit the advantages of such method.First, we pursued an wavelength dependent studies to confirm that the deposition of Ni on TiN is indeed due to the reduction of Ni precursors by photoexcited charge carriers generated on TiN (Fig. 3a).
We studied the photonic efficiency (ɳ) of the reaction at different excitation wavelength to understand the effect of photoexcited electrons generated from interband or intraband transition on Ni deposition.Photonic efficiency (ɳ) is the ratio between rate of the reaction of interest (r i ) and the photon flux (ρ) Eq. ( 1).Photon flux is the number of photons irradiated on the sample for a unit time.We found that when irradiating the reactant solution with shorter wavelengths like 385 nm, we see highest photonic efficiency (40%).Following the TiN absorbance spectrum, it goes down to 20% at 450 nm and again increased at 575 nm to 30%.Interestingly at 630 nm, the efficiency is at its minimum (~ 10%) (Fig. 3a).The SPR excitation of TiN nanoparticles results in both intraband and interband transitions.
The wavelengths above 600 nm mostly excite intraband transitions as the interband transition energy of TiN is ~ 2 eV.Both intraband and interband transitions can generate photoexcited electrons suitable to reduce Ni precursors (Fig. 2b).Intraband transition from Fermi energy level can generate higher energy hot electrons; however, the holes generated by this process cannot be scavenged by methanol.Additionally, the recombination rate for excited electrons due to intraband transitions is inherently higher than the electrons generated from the interband transitions [36].While the formation of surface oxide layer on TiN can improve the lifetime of the hot electrons making it easier to harvest them for hot electron mediated reduction reactions, it may still not be as effective as the electrons generated from interband transitions.This may be the reason for the decreased effectiveness of 630 nm wavelength light to reduce Ni precursors.On the other hand, the interband transition from N p-band to Ti d-band can generate larger population of relatively lower energy electrons as the holes generated in the N p-band can be easily scavenged by methanol.
The wavelength dependency of photonic efficiency of reaction (Fig. 3a) confirms that the reaction is driven by photogenerated hot electrons.It should be noted that in the absence of light we did not observe any evidence of Ni deposition on TiN (Fig. SI.8).
Additionally reaction rate dependency on light intensity is investigated to understand whether the reaction is driven solely by hot electrons or some other factors such as photothermal heat plays in role.(Fig. 3b) The effect of light intensity on reaction kinetics was measured by varying the light intensity of the halogen lamp, while maintaining the Ni precursor mole fraction at 30% with respect to TiN.The methanol concentration obtained from FTIR data vs time plots were fitted for zero order reaction kinetics to extract rate constant.Figure 3b, shows a linear trend for the reaction rate increase with the increase of incident light intensity up to 575 mW/cm 2 then plateaus.The initial linear trend confirmed the reaction is driven by plasmonically generated hot electrons.The electron hole pair generation rates is directly proportional to the incident light intensity; hence, we see linear increase in reaction rate with the light intensity at the initial stage.However, the recombination rate is also significantly higher at higher intensity and if the hole scavenging rate can not keep up with the recombination rate the effect tapers off [36].It should be noted that if the reaction is thermally driven, the trend for reaction rates with increasing light intensities should be exponential due to the Arrhenius equation.Additionally, we control the temperature of solution at room temperature using a cooling fan.At the range of light intensities used in our experiments, the difference between the global temperature of the solution and the local temperature of nanoparticles is expected to be negligible [37].Therefore, the photodeposition can not be due to photothermal effects.
We also study effect of metal precursor concentration on the recation rate.The impact of Ni precursor concentration on kinetics were studied by changing the Ni precursor mole fraction 10%, 30%, 50% and 70% with respect to the amount of TiN, while maintaining the light intensity at a constant value of 576 mW cm −2 .Figure 3c showed an increase of reaction rate with the increase of Ni 2+ mol fraction.It confirms that with higher intensities and higher mole fractions of Ni 2+ precursor it is possible to reach a higher Ni 2+ reduction rate.This is an important knowledge for optimizing the conditions for single atoms depositions as higher precursor reduction rate favours deposition of single atoms on the supports [4,5].We used ICP-MS to confirm the Ni loading on TiN at different intensities.As it can be seen for Fig. 3d, in general Ni loading on TiN increases with the increase in light intensity and then tapers off at the highest intensity 975 mW/cm 2 .The trend matches with what we see for methanol oxidation rate (Fig. 3b 4d).We have identified different forms of Ni deposited on TiN by measuring their size using ImageJ software.The size of Ni single atoms, dimers and trimers are 0.25 nm, 0.5 nm and 0.75 nm respectively (considering atomic radius of Ni is 0.124 nm).Any particles with the size above that is considered to be nanoparticles.
With the increase of light intensity, we can see a higher percentage of single atoms deposition (Fig. 4b, c and d).This agrees with the fact that higher intensities of light lead to elevated reduction rates of Ni 2+ precursor enabling more single-atom deposition.Rapid rates of metal precursor reduction at the beginning can introduce a large amount of Ni 0 available to deposit as single atoms on the favourable sites.This is mainly possible if the cation reduction rate is higher than the nucleation rates of metal atoms on different sites.On the other hand, slower metal precursor reduction rates SI.2).The anatase titanium dioxide (TiO 2 ) cell was 15.23 × 15.23 Å in the x and y directions and comprised of 64 titanium and 128 oxygen atoms (Fig. SI.3).This was chosen to represent a fully oxidized surface of TiN.
Fig. SI.6 depicts the x-ray diffractograms (XRD) data for TiN.TiN mostly shows the rock-salt crystal structure of osbornite.

Fig. 1 aFig. 2 a
Fig. 1 a HRTEM of TiN nanoparticles b magnified image to show amorphous oxide layer on the surface of TiN nanoparticles.c XPS Ti 2p data analysis of TiN samples

∶Fig. 3 a
Fig. 3 a Action spectra with TiN extinction spectra for TiN_ Ni synthesis at 95.4 mW cm −2 illumination light intensities b Methanol consumption rate on different intensities of broad-spectrum halogen light.c Dependence of methanol consumption rates on mole fraction of Ni precursor.d Ni loading (wt%) on TiN nanostructures as determined by ICP-MS Figure4a-cshow the images collected using High-angle annular dark-field imaging -scanning transmission electron microscope (HAADF-STEM) of Ni/TiN samples deposited under different light intensities of broad-spectrum halogen light.The percentage of Ni deposited as single atoms, dimers, trimers, and nanoparticles under different light intensities is determined from the HAADF-STEM images of Ni deposited on TiN (Fig.4d).We have identified different forms of Ni deposited on TiN by measuring their size using ImageJ software.The size of Ni single atoms, dimers and trimers are 0.25 nm, 0.5 nm and 0.75 nm respectively (considering atomic radius of Ni is 0.124 nm).Any particles with the size above that is considered to be nanoparticles.With the increase of light intensity, we can see a higher percentage of single atoms deposition (Fig.4b, c and d).This agrees with the fact that higher intensities of light lead to elevated reduction rates of Ni 2+ precursor enabling more single-atom deposition.Rapid rates of metal precursor reduction at the beginning can introduce a large amount of Ni 0 available to deposit as single atoms on the favourable sites.This is mainly possible if the cation reduction rate is higher than the nucleation rates of metal atoms on different sites.On the other hand, slower metal precursor reduction rates

Fig. 4 a
Fig. 4 a-c show the images collected using High-angle annular dark-field imaging -scanning transmission electron microscope (HAADF-STEM) of Ni/TiN samples deposited under different light intensities of broad-spectrum halogen light after 5 h. a 192 mW/cm 2 , b 384 mW/ cm 2 , and c 576 mW/cm 2 .To guide the eye, some of the single atoms are circled in blue.d Percentage of deposited Ni single-atoms, atom dimers, atom trimers and particles on TiN under different conditions

Fig. 5 Fig. 6 a
Fig. 5 Aggregation energy investigated for formation of surface Ni clusters.Positive values signify the thermodynamic preference for atomically dispersed configurations.Orange circle, red square and blue triangles are for Ni/TiO 2 , Ni/pristine TiN and Ni/defect TiN, respectively