CsPbBr₃ nanocrystals supported on a partially oxidized Ti₂N MXene for photothermal CO₂ conversion

Abstract

The development of new photo-thermal catalysts for the transformation of CO₂ into fuels is of great interest, offering a clean and sustainable approach to reducing the carbon footprint. Herein, we present a novel hybrid material composed of a nanocrystalline metal halide perovskite (CsPbBr₃) supported on a two-dimensional titanium nitride (Ti₂N) MXene. Additionally, we demonstrate the importance of forming an external TiO₂ layer through partial oxidation of the MXene (POM–Ti₂N), which introduces catalytic centers and enhances photogenerated charge separation. Remarkable activity in the formation of CH₄ and CO was observed, with yields of 321 µmol g⁻¹ and 480 µmol g⁻¹, respectively. The selectivity of the reaction was found to be temperature dependent. The mechanism was thoroughly investigated using XPS and photoluminescence studies. XPS analysis revealed a significant chemical interaction between the CsPbBr₃ nanocrystals and the POM–Ti₂N MXene after the formation of the composite. Photoluminescence measurements revealed a considerably shorter emission lifetime for the hybrid catalyst (τ_ave = 1.73 ns) compared to that of the CsPbBr₃ nanoparticles (τ_ave = 25.32 ns), indicating strong interaction with the MXene. Furthermore, this research highlights the potential of combining metal halide perovskites with MXenes and the importance of controlling their interface for photo-thermal reactions.

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1. Introduction

Researchers are making significant efforts to address climate change by developing CO₂ utilization technologies that convert this molecule into chemicals. Solar-driven photothermal catalysis is a compelling and sustainable approach that harnesses light and heat energy to synergistically convert CO₂ into fuels and valuable chemicals.^1,2 There are many photothermal catalytic pathways that offer sustainable routes for CO₂ valorization,^3,4 such as the Sabatier reaction, which has been one of the most studied reactions for over a century since its discovery. This reaction consists of the hydrogenation of CO₂ to CH₄, which is thermodynamically favorable but is hindered by slow reaction kinetics; therefore, the use of high temperatures is necessary to activate the CO₂ molecule. The photothermal catalytic mechanism includes multiple proton-coupled electron transfer to CO₂ to form CH₄, which requires control of intermediate species on catalytic surfaces, often resulting in reduced selectivity and lower overall activity.⁵

In the classical thermal methanation process, the most studied metallic active centers are Ni, Ru, and Rh, which require high temperatures (300 to 400 °C) and pressures (5–20 bar) to achieve high efficiencies. However, the photothermal pathway can promote the Sabatier reaction under milder conditions, employing the same active sites supported on a semiconductor, with metal oxides being the most commonly used due to their robustness under reaction conditions. Recent studies showed that nickel (Ni) has emerged as an optimal compromise, delivering Ru-like activity when properly nanostructured on TiO₂ or CeO₂ supports.^6–8 Other researchers used single atoms and nanoparticles of ruthenium supported on CeAlO_x, achieving formidable catalytic performances.⁹ Although a huge number of materials have been already explored to boost the photocatalytic side of the photothermal reaction (e.g. metal oxides, MOFs, COFs, metal complexes, C₃N₄…),^10–13 there is still a need to boost this reaction at lower temperatures (<250 °C).

Metal halide perovskite nanocrystal (NC) semiconductors have been extensively studied because they offer a wide range of applications due to their excellent optoelectronic properties, such as broad absorption in the UV-vis spectrum, tunable light absorption range, long charge diffusion length, and high molar extinction coefficients.¹⁴ All these properties make them potential candidates for multiple photocatalytic applications. However, to improve the photocatalytic activity of pristine CsPbBr₃, it has been combined with other materials, on the one hand, to improve charge separation properties through charge migration and, on the other hand, to add better active catalytic sites.^15,16 In this regard, MXene materials as a new class of two-dimensional (2D) structures have recently attracted great attention due to their interesting properties such as rich surface chemistry, modifiable electronic structure and thermal stability. MXenes are transition metal carbides or nitrides with the general formula M_n+1X_nT_x (n = 1, 2 or 3), where M corresponds to a transition metal, X can be C or N, and T_x indicates the surface functional groups (usually –F, –O or –OH). These functional groups play an important role in the adsorption/desorption of molecules and electron transfer properties. Currently, in photothermal reduction of CO₂, there are a few examples in the literature studying the catalytic properties of MXene carbides containing perovskite NCs. For instance, aerogels of a CsPbBr₃/Ti₃C₂T_x MXene showed that the Ti₃C₂T_x MXene's conductive network promotes charge separation from photoexcited CsPbBr₃, while its surface terminations (–O, –F, –and OH) improve CO₂ adsorption, outperforming individual components.¹⁷

In addition, partially oxidized 2D transition metal carbide MXenes also offer the possibility to decorate the titanium nitride MXene with TiO₂ moieties. According to the work of Pazniak et al.,¹⁸ the enhanced charge carrier exchange at the TiO₂/MXene interface enhances the sensing properties due to the formation of a depletion region between the metal and the semiconductor interface.¹⁹

The novelty of this work lies in the use of the titanium nitride (Ti₂N) MXene in the Sabatier reaction for the first time. Therefore, within this context, we have been the first investigating the formation of a novel hybrid material composed of nanocrystalline CsPbBr₃ supported on a two-dimensional titanium nitride (Ti₂N) MXene for photo-thermal CO₂ conversion. To date, there has been no report describing this specific combination. Although bare metal nitride MXenes can exhibit certain limitations in this reaction, we addressed this by forming external TiO₂ moieties through partial oxidation of the Ti₂N MXene and combining it with the perovskite nanocrystals. We evaluated catalytic activity under different conditions and assessed its reusability, concluding that this hybrid is capable of performing CO₂ revalorization. XPS analysis and photophysical studies were carried out. Finally, we propose a working mechanism based on the formation of a heterojunction architecture, featuring an interlayer that inhibits direct charge recombination.

2. Results and discussion

CsPbBr₃ perovskite NCs and the Ti₂N MXene were prepared separately according to the synthetic procedure described in detail in the Materials and Methods section (see SI) and illustrated in Scheme 1. The synthesis of CsPbBr₃ NCs was carried out by the hot injection method.²⁰Scheme 1a shows the conventional preparation of the NCs through three sequential steps: (i) mixing the precursors in a three-neck flask at elevated temperature, (ii) rapid injection of the cesium precursor, and (iii) immediate quenching in an ice-cold water bath.

Scheme 1 Synthetic procedure for the preparation of the catalyst. (a) Preparation of the CsPbBr₃ NCs, (b) formation of the Ti₂N MXene from the MAX phase and (c) the impregnation process for the deposition of the CsPbBr₃ NCs on the exfoliated Ti₂N MXene.

The preparation of the partially oxidized multilayer Ti₂N MXene (POM–Ti₂N) involves two main stages: (i) first the MAX phase was treated with CuCl₂ using the molten salt method at high temperature (650 °C), followed by copper removal using persulfate solution, and (ii) afterwards the solid was filtered and thoroughly washed with deionized water. It is during the purification with persulfate salts at 70 °C that a thin layer of small nanoparticles of TiO₂ was grown on top of the surface resulting in the partial oxidation of the Ti₂N MXene. (see Scheme 1b). Actually, TiO₂'s presence is residual if the purification is carried out during the same time and at room temperature.

Finally, the perovskite NCs were supported on POM–Ti₂N by dispersing POM–Ti₂N by ultrasonication in hexane and adding dropwise a concentrated suspension of NCs. After some time, the solid was recovered by filtration. More details are described in the Materials and methods section in the SI (Scheme 1c).

The synthesis of M–Ti₂N was carried out according to the publication by Xinbo Liu et al., following the molten salt method, in which molar ratios of 1 : 3 and 1 : 4 with respect to Ti₂NAl : CuCl₂ were used. In our case, a ratio of 1 : 4 was chosen to synthesise the multi-layered T₂N MXene.²¹ In fact, analysis carried out with X-ray diffraction spectroscopy shown in Fig. S4 confirms that the titanium nitride MXene synthesized by the molten salt method leads to a nitride with the crystal structure of α-TiN with characteristic peaks at 37.9°, 43.3° and 62.7° corresponding, respectively, to the planes (004), (103), and (110).^22,23

A slight modification of the purification procedure during the bath with persulphate salts, such as raising the temperature of the process or increasing the concentration of these salts, leads to modification of the chemistry of the surface of the MXene layers by superficially oxidising the M–Ti₂N layers as can also be seen in Fig. S4 with the appearance of new TiO₂-related signals at 2θ = 25.3°, 47.9°, and 54.4° and 2θ = 27.4° and 43.3° assigned to (101), (202), and (211) anatase and (110) and (111) rutile crystalline planes.

Furthermore, XRD analyses were performed for all the materials. The XRD results were consistent with the ICSD data file #204325,²⁴ which confirms the cubical crystalline structure of the CsPbBr₃ NCs, evidenced by the presence of the peaks at the angles 15° and 30°, which represent the (001) and (002) crystalline planes of the cubic perovskite crystals (Fig. 1b). Once the NCs@POM–Ti₂N composite has been formed, it is not possible to visualise the signals due to the low concentration of NCs in the sample (<5% by weight) and the small amount of material used during the analysis (Fig. 1a).

Fig. 1 (a) Comparative XRD patterns of CsPbBr₃ NCs (blue), POM–Ti₂N MXene (red), NCs@POM–Ti₂N before (green) and after the catalytic reaction (black), (b) CsPbBr₃ NCs compared with reference data from the ICSD #204325 data file.

Moreover, high-resolution transmission electron microscopy (HR-TEM) was used to study the CsPbBr₃ NCs and the Ti₂N MXene independently. Fig. 2a shows that the perovskite NC particles have a similar size and crystallinity. The particle size distribution of NCs was estimated using HR-TEM, yielding an average size of 5 nm (Fig. 2b).

Fig. 2 (a) HR-TEM image showing the lead halide perovskite (CsPbBr₃) nanocrystals. (b) Histogram for the size distribution of the CsPbBr₃ NCs, the inset shows enlarged individual NCs, (c) HR-FESEM image of the Ti₂N MXene, (d) HR-FESEM picture of POM–Ti₂N, (e) HR-TEM of a layer of POM–T₂N with different interplanar distances, (f) HR-FESEM images of the composite NCs@POM–Ti₂N. (g) TEM image showing CsPbBr₃ NCs distributed across the Ti₂O/Ti₂N MXene. (h) Corresponding high-resolution (HR-TEM) image of NC@POM–Ti₂N displaying the lattice fringes assigned to anatase TiO₂ (3.50 Å, (101)), Ti₂N MXene (2.70 Å and 2.10 Å, (0110)), and CsPbBr₃ nanocrystals (3.00 Å, (200)), confirming the formation of the Ti₂O/Ti₂N heterostructure decorated with CsPbBr₃ NCs.

High resolution field emission scanning electron microscopy (HR-FESEM) is a useful tool to distinguish between the formation of a multi-layered Ti₂N MXene (M–Ti₂N) and POM–Ti₂N. A soft purification treatment with 0.1 M sodium persulfate at room temperature leads easily to purification of M-Ti₂N with a minor structural defect, as shown in Fig. 2c, whereas the same concentration of persulfate, but treating the MXene at 70 °C, resulted in slight oxidation of the MXene surface, forming POM–Ti₂N (see Fig. 2d). Also, HR-FESEM images in Fig. S5 reveal the formation of rugosity on the surface and edges of the Ti₂N MXene layers with the appearance of small crystals of TiO₂ homogenously dispersed. Additionally, interplanar distances measured using HR-TEM on layers of the POM–T₂N sample confirm the simultaneous presence of Ti₂N and TiO₂ crystals (Fig. 2e) After the formation of the composite NCs@POM–Ti₂N no evident changes are seen by HR-FESEM in the partially oxidized MXene particle (Fig. 2f); however, the dispersion of NC particles on the POM–Ti₂N MXene is confirmed from the DF STEM image in Fig. S6. This image showed that elements that form the perovskite particles are well dispersed and distributed homogenously over the support. A clearer image that shows the physical presence of perovskite NCs on the POM–Ti₂N support is shown in Fig. S7, from which we can estimate a loading of 5% per weight of NCs on the POM–Ti₂N MXene.

To elucidate the fine structure of the hybrid, HR-TEM images of NCs@POM–Ti₂N are presented in Fig. 2g and h. As shown in Fig. 2g, TiO₂ nanoparticles are uniformly anchored onto the Ti₂N MXene sheets, forming a surface-decorated²⁵ hybrid. This observation confirms that the partial surface transformation of Ti₂N into TiO₂ occurs during processing, yielding a coherent TiO₂/Ti₂N heterostructure.²⁶

The distinct lattice fringes observed for both phases further verify the successful formation of the composite (Fig. 2h). Spacings of 3.50 Å and 2.70 Å, as well as 2.10 Å, correspond to the (101) planes of anatase TiO₂²⁶ and the (011̄0) planes of Ti₂N,^27,28 respectively. In addition, the lattice fringes of 3.00 Å are assigned to the (200) planes of CsPbBr₃ nanocrystals,²⁹ indicating their favourable deposition across the entire TiO₂/Ti₂N MXene surface.

Interestingly, the Ti₂N nanosheets appear partially intercalated among the surface-grown TiO₂ nanoparticles, generating a textured template onto which CsPbBr₃ NCs are subsequently anchored. This morphological arrangement is entirely consistent with the structural model illustrated in Fig. 8a for NCs@POM–Ti₂N. Such an architecture provides an enlarged interfacial contact area, thereby facilitating efficient charge-carrier migration throughout the heterostructure.

The thermal stability of CsPbBr₃ and the POM–Ti₂N MXene was investigated using thermogravimetric analysis (TGA) (Fig. S8). The results indicate that the POM–Ti₂N MXene exhibits high thermal stability, showing only a small percentage of weight loss over the entire temperature range studied. In addition, CsPbBr₃ (NCs) shows a sharp weight loss around 600 °C, indicating thermal stability up to this temperature. All of the above indicates the stability of the materials under the catalytic conditions studied.

The surface change induced by the partial oxidation of M–Ti₂N produces profound changes in the optical absorption of the material. As shown in Fig. S9, when titanium nitride is present, the absorbance spans the UV region to the entire visible range. However, when oxidation occurs, its absorbance becomes significantly more prominent in the UV region up to 380 nm due to the appearance of TiO₂ domains on the M–Ti₂N structure. For the sake of comparison, diffuse reflectance UV-vis spectra of perovskite NCs, the POM–Ti₂N MXene, and the composite NCs@POM–Ti₂N are also plotted in Fig. 3. Herein, we can see the POM–Ti₂N MXene UV-vis spectra exhibiting a strong band absorption with an edge at 380 nm, characteristic of TiO₂ absorption. In the case of CsPbBr₃ NCs, they display characteristic UV-vis absorption within the wavelength range of λ < 540 nm. The UV-vis spectrum also verifies the formation of the composite NCs and POM–Ti₂N MXene as the supported material combines both absorptions, expanding the photophysical response of the initial POM–Ti₂N in the visible region.

Fig. 3 (a) Diffuse reflectance UV-vis optical spectra of CsPbBr₃ NCs, the Ti₂N MXene and the supported material. The inset shows a magnification in the characteristic range of the CsPbBr₃ NCs absorption. (b) Schematic energy band level diagram of each component of the CsPbBr₃@POM–Ti₂N composite. The respective CB and VB values, as well as the optical band gaps (all in eV), are extracted from the XPS and UV-vis measurements respectively.

The optical band gaps (E_g) of the synthesized materials were determined using UV-vis diffuse reflectance spectroscopy (DRS), analyzed through Tauc plot extrapolations (Fig. S10, SI).³⁰ The extracted band gap values were found to be 2.28 eV³¹ for CsPbBr₃ NCs and 3.26 eV for POM–Ti₂N, respectively.

To further elucidate the superior optoelectronic behavior of the perovskite material, the valence band edge positions (E^NHE_v) relative to the normal hydrogen electrode (NHE) were investigated using X-ray photoelectron spectroscopy (XPS).^32–34 The E^NHE_v values were calculated based on eqn (1):

E^NHE_v = E^f_v + φ_sp + E^NHE₀ (1)

where E^f_v is the Fermi level of the material determined from XPS measurements, φ_sp is the spectrometer work function (4.244 eV), and E^NHE₀ is the energy level of the standard hydrogen electrode relative to the vacuum level (−4.44 eV).

Based on the calculated valence band (VB) positions and the corresponding optical band gaps, the conduction band minimum (CB) energies were inferred for each material. The estimated VB potentials referenced to the NHE were 1.14 eV for CsPbBr₃ NCs and 2.30 eV for POM–Ti₂N, as shown in Fig. S11(a and b). Moreover, the CB values of CsPbBr₃ NCs and POM–Ti₂N are −1.14 and −0.96 eV, respectively. Regarding the POM–Ti₂N MXene material, we expect two distinct regions: an outer region corresponding to the partially oxidized Ti₂N MXene with TiO_x groups exhibiting electron-withdrawing behavior, and an inner region mainly of the Ti₂N MXene with a metallic character, which facilitates efficient electron extraction. Consequently, energy band diagrams were constructed, providing comprehensive insight into the electronic structure of the materials investigated (Fig. 3b).

With the aim to understand the mechanism of the catalytic process that occurs during the photo-thermal reaction, steady-state and lifetime emission were studied. The PL spectrum of the CsPbBr₃ NCs exhibits a strong emission peak at 530 nm; however, for the sample CsPbBr₃@POM–Ti₂N MXene, we observed a decrease in PL intensity along with a blue shift in the peak maximum to 513 nm. This shift to the blue can be attributed to the Burstein–Moss effect.³⁵ This effect is related to the change in carrier concentration caused by the electronic coupling between the perovskite nanocrystals and the MXene material. In addition, the decrease in emission intensity can be ascribed to the transfer of photogenerated carriers between the CsPbBr₃ NCs and the POM–Ti₂N MXene.³⁶

Time-resolved fluorescence spectroscopy (Fig. 4) was used to investigate the lifetime emission of pure CsPbBr₃ NCs and the supported CsPbBr₃@POM–Ti₂N MXene catalyst. The best fitting corresponds to a biexponential decay model, where the short lifetime is attributed to nonradiative recombination of electron–hole pairs and surface trap states and the long lifetime to the radiative recombination.^37,38

Fig. 4 (a) Photoluminescence spectra (λ_ex = 405 nm) of CsPbBr₃ nanoparticles (red line) and the CsPbBr₃@POM Ti₂N MXene catalyst (blue line). (b) Lifetime measurements registered at 530 and 513 nm under room conditions of CsPbBr₃ nanoparticles (blue line) and the CsPbBr₃@POM Ti₂N MXene catalyst (red line) respectively. The PL decay lifetimes were calculated from the best fitting to a biexponential decay model (y = A₁·e^−t/τ1 + A₂·e^−t/τ2), where A₁ and A₂ correspond to the amplitudes and τ₁ and τ₂ are the lifetimes of each component. The average lifetime was calculated using the formula: τ_average = A₁·τ₁² + A₂·τ₂²/A₁·τ₁ + A₂·τ₂.

The emission lifetime of the CsPbBr₃@POM Ti₂N MXene (τ_ave = 1.73 ns) was relatively shorter than that of the corresponding CsPbBr₃ nanoparticles (τ_ave = 25.32 ns). This difference in the emission lifetime between both materials indicates the emergence of a nonradiative pathway from the significant electronic interaction between CsPbBr₃ nanoparticles and the partially oxidized MXene.¹⁸ This provides an alternative interfacial pathway for the excited electrons of CsPbBr₃ to transfer to the oxidized MXene and relax non-radiatively, allowing them to participate in the catalytic reaction.^35,39

X-ray photoelectron spectroscopy (XPS) measurements were conducted to determine the chemical states of lead (Pb), bromine (Br), cesium (Cs), titanium (Ti), nitrogen (N) and oxygen (O) in the CsPbBr₃ NCs, POM–Ti₂N, and NCs@POM–Ti₂N samples (Fig. S12–S14 show the full survey scans). Fig. S12 displays the XPS core spectra of Cs 3d, Pb 4f, and Br 3d states, respectively, calibrated using the adventitious C 1s peak set to a binding energy (BE) of 284.8 eV.⁴⁰ In the case of CsPbBr₃ NCs, the BE peaks for Cs 3d at 724.0 and 738.0 eV correspond to the Cs 3d_5/2 and Cs 3d_3/2 signals (Fig. 5a), while the Pb 4f spectrum shows two peaks at 138.0 and 142.9 eV, attributed to the Pb 4f_7/2 and Pb 4f_5/2 levels respectively, of the Pb²⁺ state, consistent with previous literature reports^41,42 (Fig. 5b). Additionally, two Br 3d peaks are observed at 67.9 and 69.0 eV, corresponding to Br 3d_5/2 and Br 3d_3/2 contributions^43,44 (Fig. 5c). Further examination of the Ti 2p spectrum for the NCs@POM–Ti₂N composite reveals that the Ti 2p_3/2 and Ti 2p_1/2 levels corresponding to Ti–N in POM–Ti₂N appear at BE values of 458.5 and 464.2 eV, respectively. The Ti 2p_3/2 and Ti 2p_1/2 levels associated with Ti–O bonds characterize the POM–Ti₂N MXene, with various oxidation states from Ti²⁺ to Ti⁴⁺ presenting at BE values around 455.7–459.8 and 461–466 eV, respectively^45,46 (Fig. 5d).

Fig. 5 XPS spectra of CsPbBr₃ NCs, POM–Ti₂N (below), and NCs@POM–Ti₂N composites (top): (a) Cs 3d spectra, (b) Pb 4f spectra, (c) Br 3d spectra, (d) Ti 2p spectra, and (e) N 1s spectra.

Further evidence of the formation of the composite NCs@POM–Ti₂N is found in the XPS spectrum shown in Fig. 5, which shows the elemental XPS signals of CsPbBr₃ and the POM–Ti₂N MXene before and after the formation of the composite. Due to the effective formation of an interface between perovskites and the POM–Ti₂N MXene, subtle shifts in the Pb, Br, Ti, and N peak positions are observed, as shown below. Specifically, the binding energies for Pb 4f and Br 3d in the NCs@POM–Ti₂N composite shift towards more positive values, indicating a reduction in electron density on CsPbBr₃.⁴⁷ Conversely, the binding energies assigned to Ti 2p and N 1s (from Ti–N bonding) exhibit a negative shift, suggesting an increase in electron density on the POM–Ti₂N MXene particles (Fig. 5d and e).⁴⁸ The Ti 2p spectrum of the NCs@POM–Ti₂N composite shows a clear shift of the main peak positions to lower BE values compared to the POM-Ti₂N MXene alone, indicating a chemical interaction between the perovskite nanocrystals and the POM–Ti₂N MXene structure.

In addition, despite partial oxidation of the nitride MXene, Ti–N bonds characteristic of titanium nitride predominate, as evidenced by Ti 2p_3/2 and Ti 2p_1/2 signals at binding energies of approximately 458.5 eV and 464.2 eV, respectively. Additionally, the presence of an N 1s signal around 399.5 eV confirms the continued existence of N–Ti bonds,^49,50 and the peak at 400.8 eV may be associated with nitrogen atoms in the Ti(N, O) oxynitride lattice (Fig. 5e).

These findings indicate that, although some surface oxidation occurs, forming TiO₂, the internal structure of POM–Ti₂N predominantly retains its titanium–nitride bonds, thus facilitating an effective electronic interaction with the CsPbBr₃ nanocrystals. In summary, all these observations support the notion of electronic interaction between the orbitals that form CsPbBr₃ NCs and the POM–Ti₂N MXene.

To investigate the reducibility and metal–support interactions within the hybrid, H₂ temperature-programmed reduction (H₂-TPR) measurements were conducted (Fig. 6a). Pure TiO₂ exhibits only a weak, broad reduction event centred at ∼535 °C,⁵¹ characteristic of kinetically hindered lattice-oxygen removal and indicative of strongly bound oxygen species with low surface mobility. A similar trend has been observed for TiO₂ nanosheets by other authors.⁵² Conversely, the POM–Ti₂N composite exhibits a markedly different H₂ consumption behavior, revealing two pronounced low-temperature reduction peaks with maxima centered at 314 °C and 381 °C. The presence of these features at significantly lower temperatures reflects the formation of highly labile surface oxygen species generated at the Ti₂N/TiO₂ interface.⁵³ According to the reducibility criterion, where lower peak temperatures correspond to more weakly bonded oxygen species such as O⁻/O²⁻,⁵⁴ these events confirm the emergence of readily accessible active oxygen centres not present in pristine TiO₂. This enhanced reducibility is attributed to a strong metal–support interaction (SMSI) between the almost conductive Ti₂N core and the ultrathin surface-oxidised TiO₂ shell.⁵⁵ The Ti₂N framework promotes electron donation, activating H₂ molecules and facilitating the formation of hydrides, which collectively weakens the surface Ti–O bonds and triggers stepwise reduction of the MXene surface. Concurrently, the high-temperature feature centred at ∼530 °C originates from the reduction of oxidised TiO₂ moieties anchored on the POM surface, consistent with the consumption of more strongly bound lattice oxygen. Taken together, these redox characteristics demonstrate that the POM–Ti₂N hybrid possesses substantially improved oxygen lability and electronic coupling compared to pristine TiO₂. Such behaviour is essential for catalytic activation, as the generation of easily reducible oxygen species and the formation of a robust Ti₂N–TiO₂ heterojunction provide the active environment required for efficient H₂ dissociation, CO₂ adsorption, and subsequent photothermal reduction to CO and CH₄.

Fig. 6 (a) H₂-TPR analysis profiles of the tested materials: POM–Ti₂N (red line) and TiO₂ (blue line). (b) CO₂-TPD profiles obtained on POM–Ti₂N (red line) and TiO₂ (blue line) materials. In both studies, the ramp rate was 10 °C min⁻¹.

Furthermore, the CO₂ adsorption behaviour of POM–Ti₂N and TiO₂ was further examined by CO₂ temperature-programmed desorption (CO₂-TPD), as illustrated in Fig. 6b. POM–Ti₂N and TiO₂ exhibit a desorption peak in the low-temperature region (∼105 °C),⁵⁶ which can be attributed to physisorbed CO₂ weakly interacting with surface hydroxyl groups. In the case of TiO₂, this low-temperature feature dominates the profile, indicating a limited population of strongly adsorbed carbonate species.⁵⁷ In contrast, POM–Ti₂N displays a markedly enhanced desorption signal extending into the high-temperature region (350–700 °C), with a pronounced maximum centred at approximately 525 °C. This peak reflects the formation of strongly bound carbonate species, resulting from the interaction of CO₂ with basic surface sites generated during the formation of the TiO₂ moieties within the POM–Ti₂N heterostructure. The significantly increased desorption intensity suggests an enlarged density of active sites capable of stabilising chemisorbed CO₂ species. These results collectively indicate that the incorporation of Ti₂N and its partial oxidation to TiO₂ enhance both the strength and capacity of CO₂ adsorption.

Periodic DFT calculations were performed with VASP (PAW-PBE including dispersion corrections;^58,59 full computational details are provided in the SI) on three representative Ti-based supports designed to mimic the POM–Ti₂N composite: a Ti₂N MXene monolayer (conductive nitride core), a rock-salt TiN(100) slab (nitride surface), and rutile TiO₂ (110) surfaces with and without a surface oxygen vacancy, V_O (partially oxidized TiO₂/TiO_x domains) (Fig. S15). CO₂ and H₂ were individually adsorbed at low coverage on each model, exploring several initial geometries. Among these, Ti₂N exhibited the largest thermodynamic affinity for CO₂ (E_ads = −3.32 eV),^60,61 whereas stoichiometric TiO₂(110) bound to CO₂ only weakly (E_ads = −0.10 eV), in line with previous DFT and surface-science studies.^62,63 Introducing a surface oxygen vacancy on TiO₂(110) markedly strengthened CO₂ adsorption (E_ads = −1.11 eV) and led to bent, vacancy-anchored configurations, consistent with the enhanced reactivity of defective rutile surfaces.^64–66 In contrast, molecular H₂ adsorption was found to be very weak on all Ti-based supports (|E_ads(H₂)| ≤ 0.05 eV), while an artificially initialized, dissociated H₂ state on Ti₂N was strongly stabilized (E_ads = −7.57 eV per H₂), indicating that nitride sites act as a deep thermodynamic sink for atomic hydrogen rather than as strong chemisorption sites for molecular H₂.⁶⁷ Taken together with the H₂-TPR and CO₂-TPD data, these DFT trends support a bifunctional picture in which oxygen-deficient TiO₂/TiO_x regions and TiO₂/Ti₂N interfacial sites provide the main CO₂-binding and activation centres, whereas the TiN/Ti₂N scaffold supplies electronic/photothermal conductivity and preferentially stabilizes dissociated hydrogen species, as described in detail in the DFT analysis (see SI).

Catalytic activity of the NCs@POM–Ti₂N material

First, the catalytic activity of NCs@POM–Ti₂N was studied at temperatures ranging between 160 and 250 °C, either in the dark (thermal catalysis) or under light irradiation (photo-thermal catalysis). Fig. 7 and Fig. S16 and S17 summarize the catalytic results obtained under light and dark conditions. In both cases, under light or dark conditions, at low temperatures selectivity towards CH₄ is 100% but as temperature increases selectivity shifts to CO. Additionally, maximum conversion is achieved above 200 °C, whereas at 250 °C the activity drops. This fact may be due to changes happening on the support as the colour of POM–Ti₂N shifted from light grey to dark grey at these temperatures. On the other hand, as shown in Fig. 7, under light irradiation production of CO₂-derived products is always higher compared to dark conditions. These differences in the catalytic activity indicate that light effectively assists the CO₂ conversion reaction in NCs@POM–Ti₂N.

Fig. 7 Photocatalytic activity of CsPbBr₃(5 wt%)@POM–Ti₂N MXene under different conditions of temperature (a) in the dark and (b) under light irradiation. Reaction conditions: CsPbBr₃(5 wt%)@POM–Ti₂N MXene (10 mg), P_H2 = 1.2 bar, P_CO2 = 0.3 bar, at 23 h reaction time, simulated sunlight irradiation (226 mW cm⁻²), and temperature as indicated.

A deeper analysis of catalytic data summarized in Table 1 brings further arguments for the photo-thermal catalytic pathway, especially, which is more evident at lower temperatures. A comparison of CO₂ conversion or CH₄ production at temperatures of 160 °C or 180 °C leads to conversions 5 to 4-fold higher when photo-thermal conditions are applied. Interestingly, both thermal and photothermal conditions share similar trends displaying higher conversions at 220 °C; in both cases, CO production starts at 200 °C and has a Gaussian plot in which 220 °C seems to be the optimum temperature conditions. It seems that at higher temperatures than 180 °C desorption of reaction intermediates is favored and the complete hydrogenation of CO₂ to CH₄ is difficult.

Table 1 Comparison of the CH₄ and CO selectivity of CsPbBr₃(5 wt %)@POM–Ti₂N MXene at different temperatures and under different conditions (dark or light)

T reaction (°C)	Conditions	Selectivity for CH4 (%)	Selectivity for CO (%)
160	Dark	100	0
	Light	100	0
180	Dark	100	0
	Light	100	0
200	Dark	70	30
	Light	60	40
220	Dark	36	64
	Light	28	72
250	Dark	22	78
	Light	15	85

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For comparison, we studied the photo-thermal activity for CO₂ reduction using CsPbBr₃ nanocrystals and CsPbBr₃@POM–Ti₂N MXene at 200 °C. (Fig. 8). As can be seen, our CsPbBr₃@POM–Ti₂N catalyst presents the highest activity for CH₄ production compared to pristine CsPbBr₃ NCs or POM–Ti₂N MXene alone. In fact, while the components of the composite CsPbBr₃ and M–Ti₂N MXene are not as active as the pristine POM–Ti₂N we assume that the catalytically active sites are present in the oxidized side of POM–Ti₂N. (Fig. 8a) However, selectivity changes occurred when POM–Ti₂N was loaded with the perovskite due to the suppression of CO production and a change in selectivity towards CH₄. This indicates a synergistic effect between both materials when they form a combined material.

Fig. 8 (a) Comparison of photocatalytic activity for different samples. (b) Recycling of the photocatalyst CsPbBr₃(5 wt%)@POM–Ti₂N MXene. Reaction conditions: photocatalyst (10 mg), P_H2 = 1.2 bar, P_CO2 = 0.3 bar, at 23 h reaction time, simulated sunlight irradiation (226 mW cm⁻²), and 200 °C.

In addition, the recyclability of CsPbBr₃@POM–Ti₂N MXene was evaluated over five consecutive cycles under identical reaction conditions, as illustrated in Fig. 8b. The results indicate a stable CH₄ production yield for the first three cycles (only a 3% reduction in activity per cycle), suggesting desirable photostability of the catalyst, even with each cycle lasting 23 hours. Interestingly, while CH₄ remained the primary product, CO was observed as a byproduct of the reaction. We attribute this emergent CO formation to surface restructuring and the gradual alteration of active sites, likely due to subtle changes in the surface composition or morphology of CsPbBr₃@POM–Ti₂N during the initial reaction cycle. This shift in selectivity, with CO generation emerging, may be explained by changes in the binding and activation of CO₂ at newly exposed or modified active sites on the catalyst surface. This argument is reinforced by XPS data shown in Fig. 5 which reveal the electronic polarization in Cs and Ti atoms after the formation of CsPbBr₃@POM–Ti₂N, suggesting that perovskite NCs are not randomly deposited, if not interacting somehow. Photocatalytic CO₂ reduction involves a series of complex, multi-step processes, including light absorption, charge separation, reactant adsorption, and catalytic reduction at active sites.⁶⁸ Minor modifications in any of these steps, particularly in the structure of surface-active sites, can significantly impact product selectivity. In this case, the MXene support likely plays a dual role: enhancing electron–hole separation and simultaneously protecting CsPbBr₃ nanocrystals from rapid degradation, as confirmed by the absence of notable changes in XRD patterns (Fig. 1, black line) and consistent XPS spectra. Nevertheless, with continued cycling, slight restructuring of the surface may expose new catalytic sites that favor CO production. By the fourth cycle, a reduction of approximately 30% in CH₄ yield was observed, possibly due to gradual surface degradation or partial deactivation of active sites, while CO production remained stable. This indicates that, despite a decrease in CH₄ yield, the modified surface continues to facilitate CO₂ reduction to CO, suggesting persistent reactivity at specific catalytic sites that favor CO production over CH₄. In fact, while CsPbBr₃@POM–Ti₂N has higher selectivity towards CH₄, its individual components (the M–Ti₂N MXene, the POM–Ti₂N MXene, and CsPbBr₃ nanocrystals) are also selective to CO. This change in catalytic activity agrees with superficial chemicophysical changes on the POM–Ti₂N surface. This matches with the observation made using XPS in terms of electronic densities of atomic elements that compose CsPbBr₃ NCs and POM–Ti₂N. These findings underscore the importance of optimizing both the initial surface properties and long-term stability of active sites to maintain selective CO₂ photoreduction over extended reaction cycles.

Proposed mechanism (photocatalytic CO₂ reduction)

According to XPS and UV-vis measurements, the energy alignments of the CB and VB of CsPbBr₃ NCs, POM–Ti₂N, and CsPbBr₃@POM–Ti₂N were calculated. From a thermodynamic point of view, the reduction of CO₂ to CO and CH₄ occurs in all cases, see Fig. 9b. However, CO₂ conversion increases considerably in the case of the CsPbBr₃@POM-Ti₂N composite, especially upon UV-vis light exposure. This fact, plus the intrinsic heterojunction morphology of the composite (see Fig. 9a), suggests that photogenerated electrons can flow from the conduction band (CB) of the perovskite to the CB of the TiO₂ layer that eventually will pass to Ti₂N, thus inducing effective charge separation enabled by a type II heterojunction formed between the CsPbBr₃ NCs and the POM-Ti₂N MXene structure. Additionally, the difference in potential between the two materials generates an internal electric field (IEF) that further enhances charge separation and transfer efficiency.⁶⁹ The direction of the IEF is shown by XPS results (Fig. 5).

Fig. 9 (a) The schematic diagram of the CsPbBr₃@POM-Ti₂N composite. (b) Diagram of CH₄ and CO production by the CsPbBr₃@POM-Ti₂N composite system under sunlight irradiation and charge migration and separation. (1) Upon illumination, the perovskite absorbs photons, generating electron–hole pairs. (2) Internal electric fields (IEF) push electrons toward TiO₂ and then to the MXene. (3) Electrons accumulate at the MXene surface, where they reduce CO₂ to CO and CH₄.

The TiO₂ layer also reduces direct recombination between the perovskite and the MXene, forming an additional small electric field that promotes electron flow into the MXene and avoids recombination. The introduction of such an interlayer to reduce direct recombination is a well-known strategy in the field of dye-sensitized solar cells (DSSCs) and hybrid solar cells.^70,71

Thus, these electrons that are injected into the MXene can reduce CO₂ into either CO or CH₄ through multielectron transfer pathways:⁷²

CO₂ + 2H⁺ + 2e⁻ → CO + H₂O (E⁰_redox = −0.53 V) (2)

CO₂ + 8H⁺ + 8e⁻ → CH₄ + 2H₂O (E⁰_redox = −0.24 V) (3)

Moreover, these reactions proceed concurrently, with their predominance governed by the interplay between thermodynamic feasibility and kinetic accessibility. Thermodynamically, the formation of CH₄ is favored; however, kinetically, CO production is more accessible as it requires fewer electrons (2e⁻vs. 8e⁻ for CH₄). Experimental data from Table 1 and Fig. 7 confirm this duality. At lower temperatures (160–180 °C), CH₄ selectivity remains at 100%, indicating a thermodynamically driven process. However, as the temperature increases (200–250 °C), a significant shift in selectivity toward CO is observed, dropping CH₄ selectivity to 15% under illumination at 250 °C. These results suggest that at higher reaction temperatures, kinetic factors increasingly govern product distribution.

Therefore, the temperature-dependent behavior observed in the CsPbBr₃@POM-Ti₂N composite under light irradiation illustrates a nuanced balance between thermodynamics and kinetics.⁷³

In summary, the superior photocatalytic activity of CsPbBr₃@POM-Ti₂N can be attributed to several synergistic factors. First, the composite exhibits enhanced light absorption across the visible spectrum compared to the individual components (Fig. 8a), generating more charge carriers. Second, the heterojunction architecture suppresses electron–hole recombination, thereby ensuring greater availability of charge carriers for surface redox reactions. Lastly, the 2D/0D structural configuration—where zero-dimensional CsPbBr₃ NCs are uniformly distributed over a two-dimensional layers of POM–Ti₂N support—maximizes the active surface area and facilitates efficient interfacial charge migration (Fig. 9).

3. Conclusions

In conclusion, this study highlights the successful synthesis and performance of a novel hybrid heterojunction composed of CsPbBr₃ nanocrystals supported on a partially oxidized Ti₂N MXene (POM-Ti₂N), specifically engineered for efficient photo-thermal CO₂ conversion into CH₄ and CO. Preparation of the hybrid material was achieved by simple ultrasonic impregnation, enabling the uniform deposition of CsPbBr₃ NCs onto the surface of the 2D POM–Ti₂N sheets. The partial oxidation of Ti₂N was fundamental to introducing oxygen-containing functional groups, which improved surface hydrophilicity and provided anchoring sites for the nanocrystals. This facilitated strong interfacial interactions, critical for enhancing the overall photocatalytic activity.

The proposed mechanism involves a synergistic interplay between light absorption, charge separation, thermodynamic, and kinetic effects. CsPbBr₃ NCs serve as efficient light harvesters due to their strong absorption in the visible range and excellent charge transport properties. Upon irradiation, electron–hole pairs are generated in the perovskite, with electrons being efficiently transferred to the POM-Ti₂N support. The partially oxidized MXene acts both as a photothermal catalyst and an electron sink, avoiding charge recombination to promote the catalytic reduction of CO₂, while simultaneously accelerating charge separation and transport. The presence of oxygen vacancies and various oxidation states from Ti²⁺ to Ti⁴⁺ on the POM–Ti₂N surface further facilitates CO₂ adsorption and activation, which are essential for driving the CH₄ and CO formation pathway with high selectivity.

In terms of recyclability, the hybrid catalyst demonstrated excellent structural integrity, maintaining its catalytic activity over multiple reaction cycles. Post-reaction analyses showed minimal degradation of the CsPbBr₃ NCs, retaining the interfacial architecture and confirming the robustness of the catalyst under operational conditions. Overall, the CsPbBr₃@POM–Ti₂N hybrid catalyst demonstrates a promising strategy for the design of stable and highly active photocatalysts, combining the tunable optoelectronic properties of perovskite NCs with the multifunctionality of 2D MXenes. The enhancement in CH₄ and CO production under light-driven conditions underscores the potential of this hybrid catalyst in advancing solar-to-fuel technologies.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting the results of this study are available from the corresponding author upon reasonable request.

Additional supporting information can be found online in the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5ma01122a.

Acknowledgements

This research was funded by the Spanish Ministry of Science and Innovation through MCIN/AEI/10.13039/501100011033/and “FEDER a way of making Europe” and the Severo Ochoa Centre of Excellence program (CEX2021-001230-S). K. M. acknowledges the PRE2022-104119 grant for the project “Catalytic processes: catalysis for energy and process intensification by applying instrumental tools to monitor chemical processes.” P. A. received funding from the PID2021-123163OB-I00 grant. H. G. B. was supported by the RYC2022-037287-I grant, PAID-06-23. Also, this research leading to these results has received funding from the European Union's Horizon Europe research and innovation programme (Project DAM4CO2, Double-Active Membranes for a sustainable CO₂ cycle; HORIZON-EIC-2022-PATHFINDERCHALLENGES-01-Number: 101115488). We thank the technical team at the Instituto de Tecnología Química (ITQ) for their support in facilitating the characterization.

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