# Unraveling the Role of Interfacial Charge Transfer on Photoactivity and Anomalous Luminescence Quenching of V4C3Tx/Protonated g-C3N4 Heterostructures

Muhammad Abiyyu Kenichi Purbayanto, Madhurya Chandel, Michal Makowski, Muhammad Danang Birowosuto, Verónica Montes-García, Kaitlyn Prenger, Artur Ciesielski, Michael Naguib, Agnieszka Maria Jastrzębska

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Muhammad Abiyyu Kenichi Purbayanto, Madhurya Chandel, Michal Makowski, Muhammad Danang Birowosuto, Verónica Montes-García, et al. Unraveling the Role of Interfacial Charge Transfer on Photoactivity and Anomalous Luminescence Quenching of V4C3Tx/Protonated g-C3N4 Heterostructures. ACS Applied Materials & Interfaces, 2025, 17 (11), pp.17454 - 17464. 10.1021/acsami.4c19729. hal-04996760

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Submitted on 19 Mar 2025

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# Unraveling the Role of Interfacial Charge Transfer on Photoactivity and Anomalous Luminescence Quenching of V₄C₃Tₓ/Protonated g-C₃N₄ Heterostructures

Muhammad Abiyyu Kenichi Purbayanto,* Madhurya Chandel, Michał Makowski, Muhammad Danang Birowosuto, Verónica Montes-García, Kaitlyn Prenger, Artur Ciesielski, Michael Naguib, and Agnieszka Maria Jastrzębska*

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Cite This: ACS Appl. Mater. Interfaces 2025, 17, 17454–17464

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Supporting Information

**ABSTRACT:** Two-dimensional van der Waals heterostructures with exotic quantum phenomena have garnered a huge surge in the field of optoelectronic devices. Herein, we report spectroscopic evidence of efficient interfacial charge transfers at the interface of a novel 2D/2D V₄C₃Tₓ MXene/protonated g-C₃N₄ (PCN) heterostructured thin film, demonstrating robust photosensitivity and a large exciton activation energy of 139.5 meV. Through temperature-dependent photoluminescence (PL) and time-resolved PL spectroscopy, we unravel the photophysical mechanism driving efficient charge transfer and photosensitivity in V₄C₃Tₓ/PCN heterostructures. These heterostructures exhibit superior photosensitivity to white and UV light compared with either PCN or V₄C₃Tₓ pristine materials. Additionally, we

observed significant PL quenching with unusual negative thermal quenching and extended charge carrier lifetime in the V₄C₃Tₓ/ PCN heterostructures across a broad temperature range of 70–370 K. Notably, at the elevated temperature of 370 K, the carrier lifetime was enhanced by more than 2-fold, making the heterostructures promising for optoelectronic applications. This work provides critical insight into the charge transfer mechanism between V₄C₃Tₓ MXene and PCN, opening a new avenue for rationally designing g-C₃N₄-based heterostructures for highly photosensitive optoelectronic devices.

**KEYWORDS:** protonated g-C₃N₄, V₄C₃Tₓ, MXenes, optoelectronics, heterostructures

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## 1. INTRODUCTION

Optoelectronic devices have catalyzed substantial technological advancements across various fields, including information, medicine, aerospace, defense, and environmental monitoring.¹ The discovery of graphene has been instrumental in unlocking the potential of two-dimensional (2D) materials for use in optoelectronic applications, owing to their remarkable physical properties, such as pronounced quantum confinement effects.²,³ Moreover, the atomically thin structure of 2D materials facilitates easy integration into devices and ensures compatibility with current silicon photonic technology.⁴ Beyond graphene, a wide range of 2D layered materials has been identified, spanning from metallic to insulating types, such as transition-metal dichalcogenides (TMD), black phosphorus, transition-metal carbides/nitrides (MXenes), transition-metal borides (MBenes), hexagonal boron nitride (h-BN), and polymeric graphitic carbon nitride (g-C₃N₄).⁵,⁶ Among them, g-C₃N₄ has attracted significant attention as an optically active material, particularly following the seminal work by Wang et al., which demonstrated its potential as a

metal-free photocatalyst operating under visible light irradiation.⁷ The use of g-C₃N₄ offers several advantages, including its intrinsic bandgap of 2.7 eV, high excitonic binding energy (~328 meV), low cost, tunable emission properties, and sensitivity across the blue-to-green light spectrum.⁸⁻¹¹ While much of the research on g-C₃N₄ has focused on its photocatalytic and photochemical applications, its optoelectrical properties offer substantial potential for exploration in next-generation optoelectronic devices.⁹ Moreover, despite its promising characteristics, only a few fundamental studies have explored the photophysics and carrier recombination processes in g-C₃N₄, which are crucial for the advancement of optoelectronic technologies.⁸,⁹,¹² Additionally, owing to its

**Received:** November 12, 2024

**Revised:** January 24, 2025

**Accepted:** February 10, 2025

**Published:** March 10, 2025

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layered van der Waals structure, g-C₃N₄ can be easily exfoliated into 2D nanosheets, considerably enhancing charge carrier density and quantum confinement.¹³

Despite its excellent optoelectrical properties, g-C₃N₄ faces several challenges, including rapid of electron-hole pair recombination, limited light absorption, poor electrical conductivity, and difficulties in postsynthesis processing due to its insolubility.¹⁴,¹⁵ To address these limitations, the fabrication of 2D/2D hybrid heterostructures has emerged as a highly effective strategy. Such heterostructures can significantly enhance charge transfer and separation through the built-in electrical field at the atomically thin interfaces.¹⁶ In this context, MXenes are considered as ideal building blocks for 2D/2D heterostructures because of their high electrical conductivity, excellent dispersibility, and strong optical absorption.⁵,¹⁷ In recent years, MXenes, a large family of transition-metal carbides, nitrides, and carbonitrides, have been shown to enhance the photocatalytic activity of g-C₃N₄. For example, hybridizing g-C₃N₄ with Ti₃C₂Tₓ MXene has been reported to improve light absorption and simultaneously reduce charge recombination, as evidenced by an increase in the average fluorescence lifetime from 8.20 ns (for pristine g-C₃N₄) to 9.32 ns (in the g-C₃N₄/MXene heterostructure).¹⁸ Furthermore, a close interfacial connection was achieved through electrostatic interactions between positively charged protonated g-C₃N₄ (PCN) and negatively charged MXenes.¹⁹,²⁰ The intimate contact between PCN and Ti₃C₂Tₓ MXene leads to separation of photogenerated charge carrier at the interface and suppresses recombination of photogenerated electrons and holes.²⁰ While extensive research has focused on g-C₃N₄/MXene heterostructures, particularly for photocatalytic applications, their fundamental optoelectronic properties and photophysics remain poorly understood. Recently, V₄C₃Tₓ MXenes have emerged as a promising heterostructure component due to their larger interlayer spacing, superior saturable optical absorption, greater structural and thermal stability, abundance valence states (from V²⁺ to V⁵⁺), and richer surface chemistry compared to Ti₃C₂Tₓ.²¹⁻²⁵ Notably, the extinction coefficient of V₄C₃Tₓ reaches 33.8 L g⁻¹ cm⁻¹ at 808 nm, much higher than Ti₃C₂Tₓ (25.2 L g⁻¹ cm⁻¹), making it an excellent candidate for photon absorption.²⁶

Building on recent advancements in the optoelectronic applications of g-C₃N₄ and V₄C₃Tₓ MXenes, we have successfully fabricated an ultrathin 2D/2D V₄C₃Tₓ/PCN heterostructure employing a straightforward electrostatic assembly method. We conducted an in-depth investigation of the photogenerated charge carrier behavior and excitonic dynamics of the heterostructure through temperature-dependent photoluminescence (PL) and time-resolved PL spectroscopy. To the best of our knowledge, this is the first study to explore photophysical phenomena in MXene/g-C₃N₄ heterostructures by using these characterization techniques. Interestingly, the optimized V₄C₃Tₓ/PCN heterostructures demonstrated efficient separation of photogenerated charge carriers and extended exciton lifetime across a broad temperature range (70–370 K). This was highlighted by a 2.3-fold increase in PL lifetime compared to that of pristine PCN at 370 K. Furthermore, the heterostructure exhibited superior photosensitivity to white light compared to that of pristine V₄C₃Tₓ or PCN. These findings may pave the way for the advancement of PCN-based heterostructures in optoelectronic devices.

## 2. EXPERIMENTAL SECTION

**2.1. Materials.** Hydrofluoric acid (HF, 48 wt %), hydrochloric acid (HCl, 35–38%), urea, Na₂SO₄ (99%), and isopropanol (99.5% purity) were acquired from Warchem. Tetramethylammonium hydroxide (TMAOH, 25 wt % aqueous solution), and Hellmanex III cleaning concentrate were purchased from Sigma-Aldrich. The glass substrate (15 × 15 mm size) was obtained from Chemland. The indium tin oxide (ITO)-coated glass substrate was purchased from 3D Nano, Poland. Double-distilled water (DDW) was used in this study.

**2.2. Preparation of V₄AlC₃ MAX Phase.** The V₄AlC₃ MAX phase was synthesized similarly to previously reported studies.²⁷ In short, powders of vanadium (Alfa Aesar, -325 mesh, 99.5%), aluminum (Alfa Aesar, -325 mesh, 99%), and carbon (graphite, Alfa Aesar, APS 7–11 µm, 99%) were mixed in a ratio of 4.00:1:50:3:00 in an HDPE jar with 10 mm yttrium-stabilized zirconia balls in a Turbula T2F mixer for 3 h at ~56 rpm. The mixed powders were pressed into 1 in. pellets of ~10 g each, placed into an alumina crucible, and then heated in a SentroTech tube furnace. The heating cycle was 10 °C/min to 1000 °C and then 5 °C/min to 1500 °C, held at 1500 °C for 2 h, and then the furnace was allowed to cool to room temperature. All heating and cooling steps were performed under flowing argon. After being cooled, the pellets were crushed using a mortar and pestle and sieved to obtain -325 mesh particles.

**2.3. Synthesis of V₄C₃Tₓ MXene Nanoflakes.** First, 500 mg of V₄AlC₃ was pretreated by stirring (100 rpm) in 5 mL of 9 M HCl. The pretreatment was done overnight to ensure the removal of oxide layers on the V₄AlC₃ surface. The treated V₄AlC₃ was washed with a repeating cycle (1500 RCF, 5 min) with double-distilled water (DDW) until pH ~ 6. Next, V₄AlC₃ was added slowly into 48 wt % HF in an ice bath, and the mixture was stirred at 100 rpm for 96 h at room temperature. After the etching process, the sedimented V₄C₃Tₓ was washed several times with DDW until pH ~ 7. At this stage, we obtained multilayered (ML) V₄C₃Tₓ. To further transform ML into delaminated V₄C₃Tₓ nanosheets, we carried out a delamination process by using a tetramethylammonium hydroxide (TMAOH, 25 wt %, Sigma-Aldrich) solution in a ratio of 10 mL per 500 mg of MXene. The mixture was stirred for 24 h at room temperature. Next, the mixture was washed with DDW (10,000 RCF for 5 min) until the pH ~ 7. Furthermore, the MXene was left to sediment for 1h, and the supernatant consisting of delaminated V₄C₃Tₓ nanosheets was collected, while the sediment containing ML V₄C₃Tₓ was discarded. In this work, the delaminated V₄C₃Tₓ will be referred to as V₄C₃Tₓ for simplicity.

**2.4. Synthesis of Protonated Graphitic Carbon Nitride (PCN).** Graphitic carbon nitride (g-C₃N₄) is typically synthesized by heating 20 g of urea in air in a ceramic crucible covered with aluminum foil at 550 °C for 2 h. The heating rate was set at 10 °C/min, followed by natural cooling overnight. The obtained bulk g-C₃N₄ was further ground using a mortar to obtain fine powders. Next, 250 mg of bulk g-C₃N₄ was stirred in 5 mL of 12 M HCl for 1h at 300 rpm to obtain protonated g-C₃N₄ (PCN). Then, PCN was washed with DDW at 10,000 RCF for 5 min until the pH reached ~7. To obtain a 2D nanosheet structure, we bath-sonicated PCN for 1 h. At the end, we separated PCN nanosheets by centrifugation at 1500 RCF for 10 min and collected the supernatant.

**2.5. Preparation of V₄C₃Tₓ/PCN Heterostructures.** The V₄C₃Tₓ/PCN heterostructures were formed by a facile electrostatic self-assembly method. Specifically, V₄C₃Tₓ and PCN aqueous dispersions (each 2 mg/mL) were stirred together at 300 rpm for 1 h at room temperature. We varied the ratio of V₄C₃Tₓ to PCN components in the heterostructure, i.e., 1:3, 1:1, and 3:1 by mass.

**2.6. Characterization.** The scanning electron microscopy (SEM) and scanning transmission electron microscopy images (STEM) of the V₄C₃Tₓ/PCN heterostructure were studied by field emission scanning microscopy (FE-SEM, Hitachi S5500, Hitachi, Tokyo, Japan). The images were acquired by mounting the samples on a carbon-coated copper grid (Ted Pella Inc., Redding, CA, USA). The same FE-SEM setup enables energy-dispersive X-ray spectroscopy (EDS). Raman spectra were recorded using an in situ Raman setup
<figure><img src="image_9.png" /><figcaption>**Figure 1.** Schematic illustration of the V₄C₃Tₓ/PCN synthesis.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 2.** HR-SEM images of (a) V₄C₃Tₓ, (b) protonated g-C₃N₄ (PCN), and (c) V₄C₃Tₓ/PCN 1:1 heterostructure. (d) STEM images and (e) EDS elemental mapping of the V₄C₃Tₓ/PCN 1:1 heterostructure.</figcaption></figure>

(ReactRaman, Mettler-Toledo, US), operating at 785 nm, a power of 400 mW, and an acquisition time of 30 s. The Raman spectra were obtained by measuring the samples in aqueous dispersion. X-ray diffraction patterns (XRD) were obtained from drop-cast samples using a BrukerD8 X-ray diffractometer. The zeta potential of PCN and V₄C₃Tₓ was performed by a Zetasizer NANO ZS ZEN3500 and Advance Series - Lab (Blue Label), Malvern, respectively, using a plastic micro cuvette from Malvern Panalytical.

The chemical composition of the samples was studied by using X-ray photoelectron spectroscopy (XPS). XPS analyses were carried out using a Thermo Scientific KALpha X-ray photoelectron spectrometer with an aluminum X-ray source (energy 1.4866 keV) and working at a pressure of 10⁻⁸–10⁻⁹ mbar in the main chamber. The X-ray spot size was settled at 400 µm. The samples were prepared by drop-casting onto conductive copper tape and dried under nitrogen (N₂). All XPS spectra were calibrated by using the C₁s peak at 284.8 eV as a reference.

UV-visible (UV-vis) diffuse reflectance spectra of the samples were measured with a UV-vis spectrometer (Evolution 220, Thermo Scientific) equipped with an integrated sphere. The spectra were recorded with an integration time of 0.3 s, a wavelength resolution of 1 nm, and a scanning speed of 200 nm/min.

Evaluation of photoluminescence (PL) and time-resolved PL (TRPL) was done by employing a picosecond 375 nm laser diode, an objective microscope, a 405 nm upper pass filter, and a high-sensitivity visible light spectrometer. For TRPL measurements, we shifted the laser diode from continuous wave (CW) mode to pulsed mode at a 10 MHz repetition rate. The emitted PL signal after filtering was directed to a single-photon avalanche photodiode.

The temporal characteristics of this signal were examined by using time-correlated single-photon counting electronics. Temperature-dependent PL and TRPL measurements were conducted using a Linkam HFS600E-PB4 cryostat with liquid nitrogen cooling, facilitating measurements from 70 to 370 K in 50K steps.

## 2.7. Optoelectronic Measurement.
We investigated the optoelectronic properties of V₄C₃Tₓ/PCN thin films by fabricating drop-cast thin films on glass substrates. The thin films were prepared as follows: first, glass substrates were immersed in Hellmanex cleaning concentrate (2 vol %) for 20 min, followed by rinsing with isopropanol and DDW. The substrates were then dried on a hot plate at 150 °C for 3 min. Before coating, the substrates were treated with UV-O₃ for 30 min to promote surface hydrophilicity. Next, 150 µL of the V₄C₃Tₓ/PCN aqueous solution (2 mg/mL) was drop-casted onto the substrates by a micropipette and allowed to dry overnight at room temperature. To improve the interfacial adhesion of the film and the substrate, the obtained coating was sintered at 110 °C for 1h. Here, V₄C₃Tₓ and V₄C₃Tₓ/PCN heterostructured thin films were fabricated without polymeric binders. The edges of the thin films were cast with silver paste and spaced 1 cm apart, providing an electrical connection. Current-voltage curves were measured using a source/meter unit (Keithley SMU 2450, USA), within a measurement range of -5 to 5 V. For studying the photoresponsive properties of the films, the samples were placed in a photochemical LED reactor (Photocube Photochemical reactor, ThalesNano, Budapest, Hungary) and irradiated with white (400-700 nm, luminous flux 5920 lm) or UV light (365 nm, radiant flux of 44.8 W). Furthermore, the multicolor photocurrent density of the samples was measured using a digital multimeter (Keithley DAQ 6510, USA), with a bias voltage of
<figure><img src="image_11.png" /><figcaption>**Figure 3.** (a) XRD of V₄C₃Tₓ/PCN, and V₄C₃Tₓ/PCN heterostructures. High-resolution XPS spectra of (b) V 2p, (c) C 1s, and (d) N 1s for V₄C₃Tₓ/PCN, and V₄C₃Tₓ/PCN 1:1. (e) Tauc plots of PCN and V₄C₃Tₓ/PCN heterostructures. (f) Steady-state photoluminescence spectra of PCN and V₄C₃Tₓ/PCN heterostructures at 270 K.</figcaption></figure>

1.5 V and five alternating light on-and-off cycles (each cycle was carried out for 60 s).

**2.8. Electrochemical Impedance Spectroscopy (EIS) Characterization.** EIS characterizations were conducted in three-electrode systems using a Metrohm Dropsens potentiostat (μStat-i 400s), with 0.5 M Na₂SO₄ electrolyte. Ag/AgCl (in saturated KCl) and platinum wire were used as the reference and counter electrodes, respectively. Moreover, the previously prepared V₄C₃Tₓ/PCN heterostructured thin films grown on ITO substrates were used as working electrodes via a drop-casting method similar to that explained before. EIS measurements were performed within the frequency range of 0.1–100 kHz with an AC voltage of 10 mV at the open-circuit potential in the dark condition.

### 3. RESULTS AND DISCUSSION

The step-by-step synthesis of the V₄C₃Tₓ/PCN heterostructure is illustrated in Figure 1. The ratio of V₄C₃Tₓ to PCN components in the heterostructure formulation was varied (i.e., 1:3, 1:1, and 3:1 by mass) to optimize the intrinsic optoelectronic properties. The starting V₄AlC₃ exhibits a bulk structure (Figure S1a). After 96 h of HF etching was performed, the accordion-like structure of multilayered (ML) V₄C₃Tₓ was formed, as revealed in the SEM image (Figure S1b). After delamination, we obtained ultrathin V₄C₃Tₓ nanosheets with smooth edges and wrinkled structures, as depicted in Figure 2a. The energy-dispersive X-ray spectroscopy (EDS) elemental analysis confirms the successful Al etching in V₄C₃Tₓ, as evidenced by reduced Al to trace amounts (Figure S2). We observe the presence of V and C atoms, the main components in V₄C₃Tₓ, as well as O and F, which are associated with the surface functional groups of MXene, such as -F, -OH, and -O.²² The second building block of the heterostructure, PCN, exhibits a crumpled nanosheet structure with a few hundred nanometer lateral sizes (Figure 2b). The thin-layer structure of both V₄C₃Tₓ and PCN facilitates 2D/2D interfacial contacts, which may be

advantageous for designing heterostructures with tight interfacial connections.¹⁸

The protonation of g-C₃N₄ into PCN can tune its surface charge from a negative value (−10.76 mV) to a positive zeta potential value (+6.92 mV) (Figure S3). Therefore, a highly negative surface charge of V₄C₃Tₓ MXene (−44.36 mV) and a positive surface charge of PCN (+6.92 mV) facilitate the formation of V₄C₃Tₓ/PCN heterostructures via electrostatic interactions. The SEM image of the 1:1 V₄C₃Tₓ/PCN heterostructure shows a well-stacked 2D/2D nanosheet morphology (Figure 2c). We further confirm the interfacial connection between V₄C₃Tₓ and PCN via scanning transmission electron microscopy (STEM). Here, we can observe the intimate interfacial contact between V₄C₃Tₓ and PCN, where PCN with smaller nanosheet dimensions uniformly decorates the transparent V₄C₃Tₓ nanosheets (Figure 2d). Well-defined EDS elemental mapping of V₄C₃Tₓ/PCN 1:1 further confirms that PCN successfully decorated the V₄C₃Tₓ nanosheets, as evidenced by the homogeneous distribution of V, C, N, O, and F elements (Figure 2e).

The crystal structure of the synthesized samples was analyzed via X-ray diffraction (XRD). The precursor V₄AlC₃ MAX phase shows an intense peak of (0010) located at 39.75°, which is in good agreement with the literature.²⁸ After performing etching and delamination, we successfully transformed V₄AlC₃ into V₄C₃Tₓ MXene, as evidenced by the absence of the (0010) peak of the MAX phase and the broadening and shifting of the (002) peak to lower energy (from 7.80° to 5.29°) (Figure S4). This shifting indicates a larger interlayer spacing in V₄C₃Tₓ MXene compared to V₄AlC₃ due to the removal of the Al layer.²⁸,²⁹ The XRD pattern of PCN shows typical (100) and (002) peaks, which can be assigned to the intraplanar structural packing of s-triazine units and interlayer π-π stacking of g-C₃N₄, respectively.³⁰ Compared to bulk g-C₃N₄, PCN shows a weakening of the (100) peak, indicating a decrease in the
planar size of polymeric s-triazine units (Figure S5).³¹ Interestingly, the overall crystal structure of PCN is still maintained, suggesting that the protonation treatment does not compromise the structural integrity of g-C₃N₄.³² Importantly, the XRD patterns of V₄C₃Tₓ/PCN heterostructures show the coexistence of each heterostructure components, i.e., the (002) peak of V₄C₃Tₓ and the (100) and (002) peaks of PCN (Figure 3a).

Raman spectra further support the successful formation of V₄C₃Tₓ/PCN heterostructures (Figure S6). As the PCN content increases, the baseline of the Raman spectra rises, attributed to the fluorescence effect of PCN.³³ Moreover, after PCN hybridization with V₄C₃Tₓ, we observe several Raman-active peaks. The peaks located at 691 cm⁻¹ can be assigned to the C–N ring breathing modes of s-triazine.³⁴ The peaks in the range of 1200–1700 cm⁻¹ originate from C–N stretching vibrations, resembling typical D- and G-bands of graphitic carbon-based materials.³⁵

X-ray photoelectron spectroscopy (XPS) was used to investigate the surface chemistry and chemical composition of the samples. Figure S7 shows the XPS survey spectra of V₄C₃Tₓ, PCN, and V₄C₃Tₓ/PCN heterostructures. Notably, we observe the peaks of V 2p, C 1s, F 1s, and O 1s in the XPS survey spectra of V₄C₃Tₓ. The atomic concentration determined from XPS measurements is given in Table S1. The absence of peaks corresponding to Al and other impurities confirms the high purity of the synthesized MXene. After the hybridization of V₄C₃Tₓ with PCN, a new peak corresponding to N 1s emerges in the XPS survey spectra, while the intensity of the V 2p peak weakens. This result confirms the uniform surface coverage of V₄C₃Tₓ nanosheets by PCN, as observed in SEM images.

The high-resolution V 2p XPS spectra of V₄C₃Tₓ can be fitted by three doublet components of V 2p₃/₂ and V 2p₁/₂ (Figure 3b).²²,²⁷ The peaks at 513.5 and 521.0 eV can be attributed to the V–C (V²⁺),³⁶,³⁷ while those at 514.6 and 522.1 eV can be attributed to V³⁺. The highest binding energy peaks, i.e., 516.1 and 523.7 eV, are assigned to V⁴⁺. Here, V³⁺ and V⁴⁺ may arise from the surface oxidation of V₄C₃Tₓ.²⁷ For the V₄C₃Tₓ/PCN heterostructure, the V–C peak shifts slightly to 513.7 and 521.2 eV, and the V³⁺ peak is not observed. In contrast, the V⁴⁺ peak is enhanced and shifts to higher binding energies (516.7 and 524.3 eV). These shifts imply strong interfacial interactions and charge transfer between V₄C₃Tₓ and PCN, with electrons transferring from V₄C₃Tₓ to PCN.³⁸,³⁹

Figure 3c shows the C 1s XPS spectra of the V₄C₃Tₓ and V₄C₃Tₓ/PCN heterostructures. Four components fit the spectra of V₄C₃Tₓ C 1s. The peak at 282.5 eV can be attributed to the C–V bond of V₄C₃Tₓ.²⁷ The peaks located at 284.7, 286.3, and 288.4 eV can be assigned to C–C/C=C, C–O, and O–C=O, respectively.²² In the case of V₄C₃Tₓ/PCN, a new intense peak located at 288.1 eV can be observed, which can be assigned to sp²-hybridized C bonded to N in triazine rings (C≡N), as can also be seen in C 1s of PCN (Figure S8).⁴⁰ The C–V component cannot be clearly identified in the V₄C₃Tₓ/PCN heterostructure due to the surface coverage of V₄C₃Tₓ by PCN. Additionally, a carbon atom is located at the inner layer compared to a vanadium atom in the V₄C₃Tₓ layered structure. The N 1s spectrum of PCN can be fitted with three components (Figure 3d). The peaks located at 398.6 and 400.3 eV can be assigned to C–N=C and N–(C)₃ groups, respectively.³² Furthermore, the

peak at 404.5 eV appears because of the successful protonation treatment, which makes the CN heterocycles positively charged, consistent with the zeta potential results.³² In the V₄C₃Tₓ/PCN heterostructure, we observe a similar N 1s spectrum in agreement with PCN. Therefore, XRD, Raman, and XPS results confirm the successful formation of the 2D/ 2D heterostructure via the electrostatic self-assembly method.²⁰,⁴¹

We then conducted optoelectrical characterizations of the as-synthesized V₄C₃Tₓ/PCN heterostructures. Examining these intrinsic properties is crucial for extending our understanding of g-C₃N₄-based heterostructures beyond photocatalysis applications. First, we examined the optical absorption of V₄C₃Tₓ, PCN, and V₄C₃Tₓ/PCN heterostructures via ultraviolet–visible (UV–vis) diffuse reflectance spectra (DRS) measurements; the measurements were taken in thin film form (Figure S9). V₄C₃Tₓ exhibits a strong broadband absorption in the measured wavelength range (300–1000 nm), spanning from UV to near-infrared regions (NIR). The absence of an absorption edge is related to its metallic band structure.²³ For PCN, we observe a distinct absorption edge at ~420 nm, related to the excitonic band-to-band electronic transition of typical semiconducting materials. This value is in agreement with a few-layer g-C₃N₄ nanosheet reported by Jiang et al.³⁰ We observe a similar absorption edge feature in all of the V₄C₃Tₓ/PCN heterostructures. Moreover, increasing the V₄C₃Tₓ content also increases the optical absorption of the heterostructure in the visible region. Therefore, using MXene into the PCN heterostructure is beneficial for improving its light absorption performance.¹⁸ Interestingly, V₄C₃Tₓ/PCN heterostructures can be used as soluble ink for postprocessing (Figure S10). Figure S11 shows the drop-casted thin films with color variations, where the V₄C₃Tₓ/PCN films get darker by adding more V₄C₃Tₓ, implying stronger light absorption properties. It should be noted that PCN exhibits an opaque white color, which differs from the well-known bulk g-C₃N₄ with a typical yellow color, which can be attributed to the transformation from bulk to few-layered nanosheets.⁴²

We used the Kubelka–Munk method to determine the optical bandgap values of PCN and V₄C₃Tₓ/PCN heterostructures (Figure 3e). The extracted bandgap of PCN is 3.01 eV, larger than the reported bulk g-C₃N₄ with a typical bandgap of 2.73 eV. However, the value is similar to 2D g-C₃N₄ formed by tris-triazine blocks.⁴³ The plausible reason for the obtained bandgap larger than the bulk counterpart is related to the strong quantum confinement effect in PCN nanosheets, shifting the conduction and valence bands to opposite directions.³⁰ The band gap of V₄C₃Tₓ/PCN shows a negligible change compared to pure PCN. This result suggests that the intrinsic bandgap of the heterostructure is attributed mainly to the electronic transition from the valence band to the conduction band of semiconducting PCN, without the introduction of new energy levels induced by V₄C₃Tₓ hybridization.¹⁸

To study the electronic structure and charge transfer of photogenerated charge carriers in V₄C₃Tₓ/PCN heterostructures, we performed temperature-dependent photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements. The steady-state PL spectrum of PCN shows broad visible luminescence in the range of 400–600 nm with observed peaks at 430 and 485 nm (Figure 3f). In PCN, the valence band is formed by the σ bond of C–N with sp³ hybridization, the π bond of C–N with sp² hybridization, and
<figure><img src="image_12.png" /><figcaption>**Figure 4.** PL contour maps at various temperatures from 70 to 370 K for (a) PCN, (b) V₄C₃Tₓ/PCN 1:3, and (c) V₄C₃Tₓ/PCN 1:1. PL lifetime of the (d) slow decay component, (e) fast decay component, and (f) average for PCN, V₄C₃Tₓ/PCN 1:3, and V₄C₃Tₓ/PCN 1:1 as a function of the temperature.</figcaption></figure>

N₂ lone pairs (LP). Meanwhile, the conduction band is the superposition of the excited σ and π bonds (σ* and π*).⁴⁴ To resolve the origin of the electronic transition in PCN, we performed Gaussian fitting (Figure S12). Here, the broad PL peak of PCN at 485 nm can be fitted by two components at 472 and 521 nm. Notably, the peak at 430 nm can be assigned to the transition from the σ* conduction band to the LP valence band. The second peak at 472 nm originated from the transition of the σ* conduction band to the π valence band. The shoulder peak at 521 nm is attributed to the transition of π* conduction band to the LP valence band.⁴⁵,⁴⁶ We further compare the PL spectra of PCN and V₄C₃Tₓ/PCN heterostructures at a temperature of 270 K. It is important to note that heterostructure formation may improve photo-generated charge transfers and exciton splitting, decreasing PL emission.⁴⁷ In the present case, the PL spectra of V₄C₃Tₓ/PCN heterostructures are quenched compared to pure PCN, where 1:1 V₄C₃Tₓ/PCN exhibits the weakest luminescence, suggesting PL quenching and efficient charge transfer across the 2D/ 2D interface, as supported by XPS results. Moreover, the PL emission of V₄C₃Tₓ/PCN 3:1 cannot be detected, which might be attributed to light-shielding effects from the excessive amount of metallic V₄C₃Tₓ in the heterostructure formulation.⁵,¹⁸ Thus, it is important to control the ratio between V₄C₃Tₓ and PCN to ensure that light can be absorbed by the photoactive material, i.e., PCN, in order to form excitons.

We then studied the temperature-dependent PL spectra of the samples to analyze the photogenerated charge carrier dynamics. To the best of our knowledge, this is the first study exploring the photophysics of g-C₃N₄-based heterostructures via a combination of temperature-dependent PL and TRPL. Figure S13 shows the temperature-dependent PL spectra (70–370 K) of PCN, V₄C₃Tₓ/PCN 1:3, and V₄C₃Tₓ/PCN 1:1. In all samples, we observe a major peak located at 430 nm, an asymmetrical peak at 485 nm, and a broad peak located at 700 nm. Interestingly, the peak at 700 nm gets stronger by

increasing V₄C₃Tₓ amount in V₄C₃Tₓ/PCN hybrids and can only be observed at low-temperature measurements (below 120 K). While we have discussed the origin of the PL peaks at 430 and 485 nm, the occurrence of the PL peak at 700 nm might be related to either radiative recombination of excitons bound to defects or hybridized states at the heterostructure interface. Furthermore, we observed that the emission centered at 485 nm dominated over the 430 nm emission by increasing the measurement temperature. One of the plausible reasons is the nonradiative defect electron trapping process, resulting in exciton quenching.⁴⁸ Next, we evaluated the stability of the 430 nm PL signal at room temperature by performing repeated heat treatments at 370 K (Figure S14). The PL signal is observable after performing heat treatment, indicating the decent thermal stability of the V₄C₃Tₓ/PCN heterostructure. It is known that V₄C₃Tₓ exhibits high thermal stability compared to other MXenes.²⁴

Interestingly, PCN shows noticeable negative thermal quenching (NTQ), where PL intensity increases in the temperature range of 70–270 K (Figure 4a), followed by a decrease in intensity due to typical thermal quenching above 270 K. In the case of V₄C₃Tₓ/PCN 1:3, both NTQ and typical thermal quenching are observed (Figure 4b). Similar temperature-dependent luminescence behavior is also observed in halide perovskite crystals and II–VI semiconductors.⁴⁹⁻⁵¹ The presence of NTQ can be related to defects or shallow trap states located near the band edge, acting as a barrier for carrier trapping.⁵² Godin et al. observed a significant density of trap states in g-C₃N₄ located at 1 eV below the band edges.⁵³ Furthermore, PL contour maps of V₄C₃Tₓ/PCN 1:1 (Figure 4c) exhibit different features, where only typical thermal quenching is observed without NTQ. We also observed that the peak located at 700 nm became more pronounced compared to PCN and V₄C₃Tₓ/PCN 1:3 at the measurement temperature below 120 K. This distinct feature implies that the addition of V₄C₃Tₓ in the V₄C₃Tₓ/PCN 1:1 formulation
<figure><img src="image_13.png" /><figcaption>**Figure 5.** (a) Measurement architecture used to assess the photosensitivity of the as-prepared samples. Current-voltage curves in the dark and under white or UV light irradiation (wavelength 365 nm) of (b) V₄C₃Tₓ and (c) V₄C₃Tₓ/PCN 1:1. (d) Nyquist plot of the V₄C₃Tₓ/PCN, and V₄C₃Tₓ/PCN heterostructures.</figcaption></figure>

promotes a different luminescence mechanism. Furthermore, the thermal quenching fitting results and the extracted parameters of PCN and V₄C₃Tₓ/PCN heterostructures are given in Figure S15 and Table S2, respectively. Interestingly, V₄C₃Tₓ/PCN 1:1 shows an activation energy of 139.5 meV, higher than the previously reported activation energy of g-C₃N₄ (73.58 meV).⁴⁶

Temperature-dependent TRPL decay curves of PCN and V₄C₃Tₓ/PCN heterostructures show similar behavior that can be fitted with a double exponential function (Figure S16). The extracted fitting parameters, i.e., weight contribution and lifetime, are presented in Tables S3–S5. The fast decay component (τ₁) is related to direct exciton recombination, whereas the slow decay component (τ₂) is originated from the recombination of electrons trapped by defects.⁵⁴ Figure 4d shows the PL lifetime of the slow decay component for PCN and V₄C₃Tₓ/PCN heterostructures. Notably, V₄C₃Tₓ/PCN 1:1 and V₄C₃Tₓ/PCN 1:3 show prolonged lifetimes compared to PCN at all measured temperatures. The same trend is observed in the fast decay component (Figure 4e). Based on the fitting parameters, besides the prolonged PL lifetime, the hybridization of PCN by V₄C₃Tₓ caused the contribution of the slow decay component to become more prominent, implying that the emission mechanism can be influenced by the occurrence of trap states, promoting long interfacial charge separation times.⁵¹,⁵⁵

To facilitate easier comparison, we plot the average PL lifetime of PCN, V₄C₃Tₓ/PCN 1:3, and V₄C₃Tₓ/PCN 1:1 in one frame, as depicted in Figure 4f. The average PL lifetime of PCN within the temperature range of 70–370 K ranges at 1.88–2.71 ns. The short photogenerated charge carrier lifetime is a well-known bottleneck in developing g-C₃N₄ as optoelectronic devices and photocatalysts.⁵⁶ The PL lifetime is prolonged in V₄C₃Tₓ/PCN heterostructures, with a magnitude of 1.99–4.19 and 2.24–3.86 ns for V₄C₃Tₓ/PCN 1:3 and 1:1, respectively, at 70–370 K. This improvement can be connected to the efficient interfacial charge transfer at the

2D/2D interface of PCN and V₄C₃Tₓ, inducing electrons to travel longer to recombine, subsequently getting electrons trapped into defect states.⁵⁷ Interestingly, the PL lifetime of V₄C₃Tₓ/PCN 1:1 remains stable at all evaluated temperatures, indicating this sample's good stability and the potential to be used as a building block of optoelectronic devices operating at elevated temperatures (e.g., ~370 K).⁵⁸ Here, the optimum ratio between PCN and V₄C₃Tₓ, i.e., 1:1, creates new emission pathways, i.e., interfacial charge transfers, resulting in a stable PL lifetime across various temperatures (i.e., 70–370 K). However, in the case of PCN and V₄C₃Tₓ/PCN 1:3, the lifetime decreases by increasing the temperature as PL relaxation dynamics become faster.⁴⁵ Whereas, in the case of V₄C₃Tₓ/PCN 3:1, the excess amount of V₄C₃Tₓ leads to light-shielding effects, which is not beneficial for light-emitting applications. By comparing the PL lifetime at the elevated temperature (370 K), V₄C₃Tₓ/PCN 1:1 exhibits a 2.3-fold longer lifetime than pristine PCN. Based on PL and TRPL analyses, V₄C₃Tₓ/PCN 1:1 shows interesting intrinsic optoelectronic properties, benefiting from the synergistic combination of V₄C₃Tₓ and PCN.

The photosensitivity of V₄C₃Tₓ/PCN heterostructures was evaluated using a straightforward metal-semiconductor-metal device, fabricated by drop-casting V₄C₃Tₓ/PCN aqueous ink onto a glass substrate. This thin film was prepared without the need for an additional polymeric binder. Figure 5a shows the architecture employed to measure the optoelectronic properties of the samples. The samples were exposed to UV or white light, and their corresponding current-voltage (I-V) curves were recorded. V₄C₃Tₓ exhibits an ideal ohmic contact with no photosensitivity toward UV and white light, as evidenced by negligible current changes in Figure 5b. This behavior is expected because nonoxidized V₄C₃Tₓ MXene acts as a metallic material,²³ resulting in limited light-matter interactions for effective light-harvesting.⁵,¹⁷ To investigate the effects of oxidation on the optoelectronic properties, a 9-month-aged V₄C₃Tₓ sample was tested. Figure S17 shows the
UV-vis transmittance evolution of V₄C₃Tₓ after aging, with an increase in the 400-800 nm region indicating oxidation. Simultaneously, a decrease in the UV-region peak suggests the presence of vanadium oxide species. Subsequently, the photoactivity of the aged V₄C₃Tₓ was tested to assess the role of surface oxides. The aged samples demonstrated a lower dark current compared to that of fresh V₄C₃Tₓ, implying degradation due to oxidation into vanadium oxide species. However, the oxide species contributed minimally to the photoresponse under UV and white light irradiation, as shown in Figure S18. Therefore, the superficial oxide had only a minor impact on the light absorption of V₄C₃Tₓ.

Therefore, we expect that upon the hybridization of V₄C₃Tₓ with PCN, the light-matter interaction in V₄C₃Tₓ/PCN heterostructures improves. PCN (Figure S19a) and V₄C₃Tₓ/PCN 1:3 (Figure S19b) exhibit poor electrical conductivity, exceeding the lowest measurement limit of the sourceometer. In the case of V₄C₃Tₓ/PCN 3:1, the current slightly increases with UV and white light irradiation (Figure S19c). However, the photocurrent enhancement is not strong enough, which can be caused by the light-shielding effects of excessive V₄C₃Tₓ content, as we discussed before. Interestingly, high photosensitivity with a linear curve is observed in 1:1 V₄C₃Tₓ/PCN, as can be seen in Figure 5c. Notably, V₄C₃Tₓ/PCN 1:1 shows photosensitivity toward white light as well as UV light, which is beneficial for practical purposes as a large fraction of sunlight lies within the visible light region.⁵⁹ A high photocurrent in 1:1 V₄C₃Tₓ/PCN indicates a low probability of charge recombination and low charge transfer resistance across the interfaces, which are in line with PL and TRPL findings. Therefore, V₄C₃Tₓ/PCN 1:1 is the optimum formulation to get sufficient electrical conductivity coupled to good photosensitivity.

We further evaluated the photocurrent of V₄C₃Tₓ/PCN with various excitation wavelengths, spanning from red (625 nm) to UV light (365 nm). A high photocurrent is observed when the wavelengths reach blue light (457 nm), whereas a low photocurrent is acquired when the wavelengths are below this threshold (Figure S20). This photocurrent response agrees well with the optoelectronic characterization of V₄C₃Tₓ/PCN, where the semiconducting PCN exhibits an absorption edge around this region. Electrochemical impedance spectroscopy (EIS) through the Nyquist plot was used to study the charge transfer resistance of V₄C₃Tₓ/PCN, and V₄C₃Tₓ/PCN heterostructures. The experimental results are well-fitted with the equivalent circuit model in Figure S21. In detail, PCN shows a high charge transfer resistance (R<sub>CT</sub>) with a magnitude of 27.58 kΩ (Table S6). The value of R<sub>CT</sub> is significantly lower in the heterostructures, reaching R<sub>CT</sub> of 14.50 kΩ (V₄C₃Tₓ/PCN 1:3), 11.02 kΩ (V₄C₃Tₓ/PCN 1:1), and 4.88 kΩ (V₄C₃Tₓ/PCN 3:1) (Figure 5d). Here, V₄C₃Tₓ lowers the resistance and promotes high electron mobility in the heterostructures, as V₄C₃Tₓ has the smallest semicircle among all samples, indicating a low R<sub>CT</sub>. However, it is important to note that in the case of excess V₄C₃Tₓ (V₄C₃Tₓ/PCN 3:1), the R<sub>CT</sub> is close to the pure V₄C₃Tₓ due to a strong metallic characteristic, which explains why we did not observe the luminescence in this sample. Therefore, we showcase that 1:1 V₄C₃Tₓ/PCN became the optimized sample, showing excellent photoactivity and luminescence properties. Our results show that V₄C₃Tₓ/PCN heterostructures can effectively promote efficient charge transfer across the interfaces and prolong photogenerated charge carrier lifetimes, which is beneficial to fabricate high-performing optoelectronic devices.

## 4. CONCLUSIONS

In conclusion, we have fabricated the 2D/2D V₄C₃Tₓ/PCN heterostructure via an electrostatic self-assembly technique and studied its photophysics and intrinsic optoelectronic properties. The effect of V₄C₃Tₓ addition to tune the optical properties of PCN was studied comprehensively by temperature-dependent PL and TRPL. The optimized sample (V₄C₃Tₓ/PCN 1:1) showed efficient charge transfer across the 2D/2D interface, as demonstrated by PL quenching. We also observed a prolonged PL lifetime compared to pristine PCN at 70-370 K, with a 2.3-fold longer lifetime at elevated temperatures (370 K). Additionally, we observed high photosensitivity of V₄C₃Tₓ/PCN 1:1 toward white and UV light, which is beneficial for harvesting a large portion of solar energy. The ability to achieve photosensitivity to both UV and white light without the need for additional polymeric binders is particularly encouraging and could be further refined through interfacial engineering. In the case of V₄C₃Tₓ/PCN 1:3, we observed low electrical conductivity and photosensitivity toward light, indicating low photoactivity. The observed absence of light-emitting properties in the V₄C₃Tₓ/PCN 3:1 configuration, attributed to light-shielding effects, further underscores the importance of optimizing material composition. The fabricated V₄C₃Tₓ/PCN heterostructures open the potential for usage in advanced optoelectronic devices such as photodetectors, solar cells, solar batteries, and light-emitting devices. By addressing interfacial effects, such as structural defects, facet mismatching, and charge polarization, the performance of these devices can be substantially enhanced.

### ■ ASSOCIATED CONTENT

#### ● Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.4c19729.

SEM images and EDS spectra of V₄AlC₃ MAX phase; zeta potential of bulk g-C₃N₄, PCN, and V₄C₃Tₓ aqueous dispersion; XRD patterns of V₄AlC₃, V₄C₃Tₓ bulk g-C₃N₄, and PCN; absorption spectra and digital photograph of V₄C₃Tₓ PCN, and V₄C₃Tₓ/PCN heterostructures; temperature-dependent PL and time-resolved PL spectra of V₄C₃Tₓ PCN, and V₄C₃Tₓ/PCN heterostructures; additional current-voltage curve data of V₄C₃Tₓ PCN, and V₄C₃Tₓ/PCN heterostructures; equivalent circuit model used for fitting the Nyquist plot; atomic concentrations of V₄C₃Tₓ PCN, and V₄C₃/PCN heterostructures obtained from XPS; fitting parameters of thermal quenching by integrating excitonic peak; extracted fitting parameters of temperature-dependent TRPL; and equivalent circuit parameters obtained from EIS fitting (PDF)

### ■ AUTHOR INFORMATION

#### Corresponding Authors

Muhammad Abiyyu Kenichi Purbayanto - Warsaw
University of Technology, Faculty of Mechatronics, 02-525 Warsaw, Poland; ● orcid.org/0000-0002-0271-220X;
Email: muhammad_abiyyu.kenichi.dokt@pw.edu.pl

Agnieszka Maria Jastrzębska - Warsaw University of Technology, Faculty of Mechatronics, 02-525 Warsaw, Poland; ● orcid.org/0000-0001-8900-1859;
Email: agnieszka.jastrzebska@pw.edu.pl
## Authors

Madhurya Chandel - Warsaw University of Technology,
Faculty of Mechatronics, 02-525 Warsaw, Poland

Michal Makowski - Łukasiewicz Research Network—PORT
Polish Center for Technology Development, 54-066 Wrocław,
Poland; ● orcid.org/0000-0001-7758-2326

Muhammad Danang Birowosuto - Łukasiewicz Research
Network—PORT Polish Center for Technology
Development, 54-066 Wrocław, Poland; ● orcid.org/0000-
0002-9997-6841

Verónica Montes-García - Université de Strasbourg, CNRS,
ISIS, 67000 Strasbourg, France; ● orcid.org/0000-0002-
3235-0545

Kaitlyn Prenger - Department of Physics and Engineering
Physics, Tulane University, New Orleans, Louisiana 70118,
United States

Artur Ciesielski - Université de Strasbourg, CNRS, ISIS,
67000 Strasbourg, France; Center for Advanced
Technologies, Adam Mickiewicz University, 61-614 Poznań,
Poland

Michael Naguib - Department of Physics and Engineering
Physics, Tulane University, New Orleans, Louisiana 70118,
United States; ● orcid.org/0000-0002-4952-9023

Complete contact information is available at:
https://pubs.acs.org/10.1021/acsami.4c19729

## Author Contributions

M.A.K.P. designed the experiments, prepared and characterized the samples, collected and analyzed the results, prepared figures, and wrote the original manuscript draft. M.C. helped electrochemical measurements. M.M. and M.D.B. performed temperature-dependent PL and TRPL measurements. V.M.-G. and A.C. performed XRD and XPS measurements. M.N. and K.P. prepared MAX phase and corrected the original manuscripts. A.M.J. supervised the preparation of the manuscript, coordinated the research, and corrected the original manuscript. All the authors contributed to writing of the manuscript.

## Notes

The authors declare no competing financial interest.

## ACKNOWLEDGMENTS

This work was funded by the National Science Centre (NCN)
within the framework of the research projects "PRELUDIUM-
21" (UMO-2022/45/N/ST5/02472). M.M. and M.D.B.
acknowledge the OPUS-24 grant (2022/47/B/ST5/01966).
Figure 1 and graphical abstract are partly created with
BioRender.com.

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