Enhanced Photocatalytic Activity of Cs4PbBr6/WS2 Hybrid Nanocomposite

Photocatalytic processes are among the prime means for mitigating the pollution caused by toxic effluents. In this context, photocatalysis presents a promising path and is undergoing rapid evolution. Halide perovskite‐nanocrystals (HP‐NCs) are excellent candidates due to their negative conduction band minimum and low work function, essential for photocatalysis. Yet, HP‐NCs face limitations within this domain because they are prone to chemical degradation when exposed to external factors like high temperature, polar solvents, oxygen, and light. A practical approach toward stabilizing HP‐NCs involves hybridizing them with a chemically inert material that can provide steric stabilization and act as a cocatalyst. Transition‐metal dichalcogenides emerge as outstanding candidates to sterically stabilize the HPs as they are stable, chemically inert, and can serve as co‐catalysts, enabling suppressed charge recombination. Herein, the photocatalytic performance of Cs4PbBr6/WS2‐nanocomposites towards organic dye degradation in polar solvents under visible light illumination is investigated. We found that the presence of WS2 nanostructures significantly stabilizes the HP‐NCs and promotes dye degradation rate compared to pristine Cs4PbBr6‐NCs. Using transient absorption measurements, we found that the WS2‐nanostructures act as an electron transport channel, effectively reducing charge recombination in the NCs. These findings pave the way for implementing Cs4PbBr6/WS2‐nanocomposites as stable and superior photocatalysts.


Introduction
Among the present environmental pollution issues, organic dyes are the primary source of pollution that causes a threat to human health.Thus, converting an organic species into a harmless substance is essential for human life. [1,2]][5][6][7][8] In recent years, halide perovskites (HPs) materials have attracted much attention due to their high photoluminescence quantum yield (PLQY) and versatile chemical processability.[11][12][13] Although there have been notable advancements, the real-world applications of HPs are hindered by their inherent phase instability and susceptibility to chemical degradation when exposed to high temperatures, polar solvents, and oxygen.
In general, the HPs can be expressed as A n BX 2þn .Here, A is either an organic or inorganic cation, B is a divalent metal ion (Pb 2þ or Sn 2þ ), and X is a halogen element.When n > 1, HPs are considered to be low dimension, such that n = 2 is 2D, n = 3 is 1D, and Photocatalytic processes are among the prime means for mitigating the pollution caused by toxic effluents.In this context, photocatalysis presents a promising path and is undergoing rapid evolution.Halide perovskitenanocrystals (HP-NCs) are excellent candidates due to their negative conduction band minimum and low work function, essential for photocatalysis.Yet, HP-NCs face limitations within this domain because they are prone to chemical degradation when exposed to external factors like high temperature, polar solvents, oxygen, and light.A practical approach toward stabilizing HP-NCs involves hybridizing them with a chemically inert material that can provide steric stabilization and act as a cocatalyst.Transition-metal dichalcogenides emerge as outstanding candidates to sterically stabilize the HPs as they are stable, chemically inert, and can serve as cocatalysts, enabling suppressed charge recombination.Herein, the photocatalytic performance of Cs 4 PbBr 6 /WS 2 -nanocomposites towards organic dye degradation in polar solvents under visible light illumination is investigated.We found that the presence of WS 2 nanostructures significantly stabilizes the HP-NCs and promotes dye degradation rate compared to pristine Cs 4 PbBr 6 -NCs.Using transient absorption measurements, we found that the WS 2 -nanostructures act as an electron transport channel, effectively reducing charge recombination in the NCs.These findings pave the way for implementing Cs 4 PbBr 6 /WS 2 -nanocomposites as stable and superior photocatalysts.n = 4 is 0D. [12,13]Between all inorganic halide perovskites, CsPbBr 3 and Cs 4 PbBr 6 stand out as the most extensively researched materials.Among HPs, all-inorganic HPswith B = Pb and X = Cl, Br, I have the highest moisture and oxidization resistance. [14]The stability is mainly attributed to the isolation of PbX 6 4À octahedra by a Cs þ ion from the crystal lattice. [15]nterestingly, the 0D/3D Cs 4 PbBr 6 /CsPbBr 3 composites have higher PLQY than the 0D or 3D counterparts, which indicates that the 0D/3D has a higher density of charge carriers. [16,17]his is mainly due to the nonradiative recombinations in HPs. [18]Furthermore, the Cs 4 PbBr 6 has higher PLQY compared with Cs 4 PbI 6 and Cs 4 PbCl 6 perovskites. [19]][22] Hence, these materials might be suitable candidates for the degradation of organic dyes.However, their main drawbacks are stability, absorption range, and fast charge recombination.Nevertheless, the strong photoluminescence (PL) indicates higher charge recombination. [23]nother drawback of these 0D/3D composites is the 300-530 nm absorption range, which is similar to the 3D CsPbBr 3 . [24,25]High-performance catalytic reactivity requires a broad absorption range. [26,27]The possible solution to utilize that 0D/3D material's benefits is to add a tertiary, efficient cocatalyst.This additional material should be active in the wide visible range, i.e., low bandgap material, and suppress the fast charge recombination.30][31][32] TMDs are chemically expressed as MX 2 , where M is a group VI transition metal (Mo, W), and X is a chalcogen (S, Se, Te). [32]S 2 -nanostructures (NSs) like nanotubes (NTs) and nanoparticles (NPs) are made of S-M-S layers stacked together via weak van der Waals forces.Compared with other structures, the closed cage form of NSs like NTs and NPs is highly stable due to their low density of dangling bonds. [33,34]Moreover, the WS 2 and MoS 2 NSs were shown to support polaritonic modes at room temperature, which makes them optically active in a broader region (vis-infrared-infrared). [34,35] WS 2 is a compound with an indirect and direct bandgap of 1.2 and 2.01 eV. [28]There is a favorable band alignment between the conduction band (CB) of CsPbBr 3 and the valence band (VB) of WS 2 , i.e., the CB of CsPbBr 3 is À5.85 eV, and the VB of WS 2 is 5.9 eV enabling stable charge separation and reduced charge recombination in the composite of CsPbBr 3 /WS 2 . [36,37]TMDs nanosheets suffer from dangling bonds, and defects are thus easily oxidized. [28,29,38]sing closed-cage TMD NSs can solve these instabilities and make them almost completely inert during the photocatalytic process.Thus, hybridizing HP-NCs with WS 2 NSs can contribute steric stabilization to HPs, and function as co-catalysts, leading to suppressed charge recombination, and enhanced stability against photocatalytic degradation.
[41][42] Here, we report a successful synthesis of Cs 4 PbBr 6 -nanocrystals (NCs) and Cs 4 PbBr 6 /WS 2 nanocomposites and their superior photocatalytic pollutant refinements activity.The WS 2 NPs and NTs actively stabilize HPs-NCs within the nanocomposite, ensuring its stability in polar solvents even under oxidative conditions.The methylene blue (MB), methylene orange (MO), and bromocresol green (BCG) dyes were chosen as model pollutants.The influence of WS 2 features in all absorption measurements indicates that the nanocomposite has a higher absorbance cross-section than the pristine material.PL measurement shows intensity quenching for the nanocomposites.The quenching is attributed to the rapid charge transfer between Cs 4 PbBr 6 and WS 2 .Indeed, transient absorption (TA) and the reverse saturable absorption (RSA) measurements strongly indicate fast charge transfer between the WS 2 -NSs and the Cs 4 PbBr 6 -NCs.This ultrafast charge transfer process explains the superior photocatalytic performance of the nanocomposite. [43]Interestingly, despite the small amount of the WS 2 -NSs in the nanocomposite (≈11 wt%), its photocatalytic performance is yet superior to the pristine HP NCs.The kinetics of the degradation reactions were found to be the pseudo-first-order model.The mechanism of photocatalytic dye degradation was studied using the electron paramagnetic resonance (EPR) technique.We observe the formation of photoinduced OH • radicals, which confirms that the dye degradation process is radical-catalyzed.To the best of our knowledge, this is the first report on the synthesis of Cs 4 PbBr 6 /WS 2 nanocomposites and their implementation as superior photocatalysts for dye degradation in polar solvents, which is typically impossible.Due to their remarkable photocatalytic activities and excellent stability, these nanocomposites may serve as a new photocatalytic platform for decolorizing organic pollutants.The WS 2 NPs/ NTs, combined with HPs-NCs, stabilize the latter in polar solvents under oxidative conditions while simultaneously serving as co-catalysts and suppressing charge recombination, thereby promoting the commercialization of HPs-NCs in the field of photocatalysis.

Structural Analysis
The transmission electron microscope (TEM) image of a WS 2 -NT is presented in Figure 1a.The NTs have an average diameter of 50 nm and 1-5 μm in length. [35]The NTs are highly crystalline and multilayer in structure.Scanning transmission electron microscope (STEM) micrographs demonstrate that the Cs 4 PbBr 6 -NCs are aggregated into clusters (≈20-90 nm) (Figure 1b,c and Figure S1a and S2, Supporting Information).The average diameter of the spherical NCs within the clusters is in the range of 4-10 nm.STEM analysis shows that most of the NCs have well-defined rhombohedral structures.The lattice fringes spacings of the NCs were 0.40 and 0.30 nm, which corresponds to the (104) and (300) crystallographic planes of rhombohedral Cs 4 PbBr 6 -NCs, respectively (inset of Figure S1a, Supporting Information). [44]The TEM and STEM micrographs of the Cs 4 PbBr 6 /WS 2 -NTs indicate a nonuniform composition of the hybrid, i.e., the NTs and NCs are not distributed uniformly (Figure 1d and S1b, Supporting Information).However, most of the WS 2 -NTs are embedded into the aggregated clusters of the NCs.Detailed calculations on the particle size distribution and the cluster size distribution are presented in Figure S1c,d  pristine Cs 4 PbBr 6 -NCs, most of the diffraction peaks correspond to its rhombohedral phase. [9]However, two peaks at 15.21°and 21.45°are related to monoclinic CsPbBr 3 (100, 110). [1,2]The XRD pattern of the Cs 4 PbBr 6 /WS 2 -NT composite exhibited a pronounced peak at 14.16°(002) corresponding to WS 2 in addition to the peaks of CsPbBr Generally, to prepare the Cs 4 PbBr 6 /WS 2 composite, we dispersed the WS 2 NSs in ethanol and mixed it with the Cs 4 PbBr 6 -NCs dispersed in toluene (1: 2 ratio of ethanol: toluene).Consequently, with the introduction of WS 2 and ethanol, the Cs 4 PbBr 6 -NCs might be partially dissolved and then recrystallized as CsPbBr 3 -NCs.Thus, a possible explanation for the appearance of the CsPbBr 3 peaks is the dissolution of Cs 4 PbBr 6 and the subsequent formation of CsPbBr 3 during the nanocomposite preparation.The high surface area of the WS 2 nanocrystals provides further stabilization for the CsPbBr 3 -NCs and promotes and facilitates their arrangement on the surface of the NSs.

Optical Analysis (Absorbance and Photoluminescence)
Optical absorbance was measured using a UV-vis spectrophotometer equipped with an integrated sphere.We found two absorption edges of the dispersed NCs at 512 and 317 nm (Figure 2a-c and S4, Supporting Information) were revealed.The bandgaps of all the materials were calculated by the procedure described in the Supporting Information.The bandgaps that correspond to the absorption edges are 2.46 and 3.95 eV.That finding is somewhat surprising as the absorption edge of pure Cs 4 PbBr 6 is at 320 nm, and the bandgap is 3.87 eV. [45]he existence of an additional phase can explain this behavior.Namely, the 0D Cs 4 PbBr 6 often coexists with the CsPbBr 3 and/or CsBr phase, [11,20,46] which exhibits a strong, green luminescence and absorption at ≈520 nm.Thus, the absorption edge observed at 512 nm can be assigned to a stepwise decay through intermediate states. [47]That conclusion was supported by the XRD and Raman results, where we also observed the existence of residual CsPbBr 3 in the Cs 4 PbBr 6 -NCs (Figure S5, Supporting Information).Interestingly, the CsPbBr 3 phase was not detected in TEM, which implies that the fraction of CsPbBr 3 is low.
Despite the small ratio of WS 2 -NSs (≈11 wt%), the absorbance spectra of the Cs 4 PbBr 6 /WS 2 -NS composite are entirely different from that of the pure Cs 4 PbBr 6 .Moreover, at the range of 500-800 nm, the spectral features are very similar to the absorbance of pristine WS 2 -NT and NP.Additionally, there is a minute difference between the bandgaps of the composites compared to the pristine WS 2 -NTs.This behavior can be ascribed to the high refractive index and absorption cross-section of the latter compared to CsPbBr 3 or Cs 4 PbBr 6 . [47]teady-state PL was studied for Cs 4 PbBr 6 -NCs and Cs 4 PbBr 6 / WS 2 nanocomposites; the results are presented in Figure 2d and Figure S6 and S7, Supporting Information.The PL of Cs 4 PbBr 6 -NCs exhibits a strong emission peak at 518.5 nm.Remarkably, the PL intensity of all nanocomposites is quenched by ≈97%.This reduction in the PL intensity can be attributed to the effective charge transfer between Cs 4 PbBr 6 and WS 2 -NT. [48]ere, the Cs 4 PbBr 6 -NCs are excited at energies higher than the bandgap and donate the free charges to the WS 2 -NTs, which act as an acceptor for the charges.Namely, when the NCs in the nanocomposite absorb photons, photoexcitation occurs while leaving a hole in the VB.The photoexcited electrons are transferred to the WS 2 -NT; this behavior is discussed in detail in the later section.Figure 2e illustrates the charge transfer process between Cs 4 PbBr 6 -NCs and the WS 2 -NTs.

TA Spectroscopy
We have used femtosecond TA spectroscopy to investigate the charge transfer mechanism further.We excite Cs 4 PbBr 6 NCs, WS 2 NT, and Cs 4 PbBr 6 /WS 2 -NT hybrid nanocomposite by 3.10 eV pump excitation energy at a constant pump fluence of 230 μJ cm À2 .A time-delayed broadband probe beam (1.83-2.49eV) was used to measure TA signals.Figure 3a-c represents the spectral evolution of the TA spectra at various probe delays for Cs 4 PbBr 6 NCs, WS 2 NT, and Cs 4 PbBr 6 / WS 2 -NT hybrid nanocomposite, respectively.Figure 3a shows an instantaneous bleach at the excitonic position of ≈2.34 eV in Cs 4 PbBr 6 -NCs due to Pauli blocking.Photo-induced absorption bands P1 and P2 centered at 2.44 and 1.99 eV appear on the side of the exciton position due to excitonic Stark-shift and higher energy excitons. [49]][52] Strikingly, Cs 4 PbBr 6 /WS 2 -NTs hybrid nanocomposite shows temporal and spectral profile differences, see Figure 3c.We observed bleach at A exciton, initially at 1.90 eV, which was slightly redshifted ≈10 meV in the hybrid nanocomposite due to charge gained from Cs 4 PbBr 6 , indicating charge transfer from Cs 4 PbBr 6 -NCs to the WS 2 -NT.Moreover, the bleach amplitude at the A exciton position in the hybrid nanocomposite is substantially enhanced compared to the pristine NTs.These results unambiguously indicate charge transfer from Cs 4 PbBr 6 -NCs to the WS 2 -NT.The observed quenching in the PL measurements further supports the charge transfer.For a deeper understanding of the charge transfer dynamics observed in TA measurements, we analyzed the bleach decay kinetics for WS 2 -NT and Cs 4 PbBr 6 /WS 2 -NT at ≈1.91 eV (A exciton) probe energy, as illustrated in Figure 3d.The bleach maximum of WS 2 -NTs in the hybrid nanocomposite (τ 1 = 5.5 AE 0.8ps) decays much slower than the pristine NTs (τ 1 = 0.4 AE 0.1 ps) due to the charges gained from the Cs 4 PbBr 6 .
To study the charge transfer dynamics in the Cs 4 PbBr 6 /WS 2 -NT hybrid, we used a standard open-aperture Z-scan method, which measures the total transmittance as a function of incident laser intensity. [53,54]Figure 3d shows the Z-scan peak trace of Cs 4 PbBr 6 -NCs, WS 2 -NTs, and Cs 4 PbBr 6 /WS 2 -NTs at the intensity of 0.35 GW cm À2 for 532 nm excitation.The normalized transmittance of Cs 4 PbBr 6 /WS 2 -NT shows a strong RSA of 0.252 AE 0.005 cm GW À1 , which is another indication of the charge transfer mechanism. [55]The strong RSA of the nanocomposite is in stark contrast to the saturable absorption (SA) of Cs 4 PbBr 6 -NCs and the weak RSA of WS 2 -NT.
For Cs 4 PbBr 6 -NCs, the SA originates from the depletion of the ground state because the excitation rate is much faster than the relaxation rate to the ground state. [56]The pristine WS 2 -NT shows weak RSA due to two single-photon absorption. [57]Namely, since the excitation energy (2.32 eV) is greater than the bandgap of  WS 2 -NT (1.9 eV), a somewhat broad ns pulse induces successive two-step, two single-photon absorption.At the leading edge, the pulse excites the ground-state carriers to the corresponding excited state and then decays to the band edge.In contrast, the trailing edge of the pulse causes the WS 2 -NTs' excited-state absorption of the species to a higher state.
In the case of hybrid Cs 4 PbBr 6 /WS 2 -NT, the NCs undergo a single-photon absorption process, generating free electrons, and working as electron donors, as seen in Figure 2e.The excited electrons in Cs 4 PbBr 6 -NCs are transferred to the lower energy level of WS 2 -NTs, and from there, they absorb another photon to go to the higher energy level (more details can be found in the Supporting Information).This charge transfer pathway from the excited state of Cs 4 PbBr 6 -NCs to the lower excited state of WS 2 -NTs prevents the saturation of Cs 4 PbBr 6 -NCs absorption and results in RSA. [58]

Photocatalytic Dye Degradation
Following the strong evidence of charge transfer phenomena, we studied the photocatalytic degradation activity toward several pollutants such as MB, MO, and BCG (Figure 4 and S8-S12, Supporting Information).The summary of the degradation efficiency, quantum efficiency (QE), and the reaction rate constant of all the dye degradation reactions are presented in Table 1.The results of all the photocatalytic dye degradation activity show that the nanocomposites of Cs 4 PbBr 6 /WS 2 -NTs exhibited superior photocatalytic activity.The reaction rate constant, QE, and degradation efficiency were higher for the Cs 4 PbBr 6 /WS 2 -NTs nanocomposites compared to the Cs 4 PbBr 6 .Notably, the degradation efficiency varies for the nanocomposites (MB-84%, MO-51%, and BCG-100%).This behavior can be explained by the different the lowest unoccupied molecular orbitals of the various dyes. [59]n the following discussion, we show the BCG as a typical example.The characteristic absorption peak of BCG at 620 (AE20) and 414 nm was chosen to monitor its photocatalytic degradation.Without any photocatalyst, the BCG did not degrade during 60 min of irradiation (Figure S8, Supporting Information).Generally, the excitation of the composite by visible light initiates the formation of hydroxyl radical (•OH), as found by EPR and shown in Figure 5.This radical most likely attacks the -C=C bond connecting the aromatic rings in BCG (dianionic form).As a result, the BCG molecule is divided into 2,6-dibromo hydroquinone (corresponding to absorption at 290 and 320 nm) and (2-(3,5-dibromo-4-hydroxy-2-methylbenzoyl)benzenesulfonate (corresponding to absorption at ≈260 nm).Subsequently, the intermediate products further react with •OH to generate CO 2 , H 2 O, and aliphatic acid. [60]Namely, when the BCG is partly decomposed, the absorption of the intermediates does not overlap with the main peaks of its spectrum.
Besides decomposition, the BCG might undergo deprotonation or protonation, i.e., a transformation from a dianionic form to an anionic one (Scheme S3, Supporting Information).Generally, in 60 min, the BCG degradation by pristine Cs 4 PbBr 6 -NCs exhibits 100% degradation efficiency for the anionic form (corresponding to the absorption at 620 nm) and ≈40% for the dianionic (414 nm).Namely, the BCG is only partly degraded (by 40%) and mostly deprotonated (60%) in the presence of pristine Cs 4 PbBr 6 -NCs.The anionic form is yellowish, as can be observed in Figure 4a inset.Remarkably, the degradation efficiency of Cs 4 PbBr 6 /WS 2 -NTs nanocomposites reached 100% in 30 min into the dianionic and anionic forms, i.e., the BCG degraded utterly.The complete transparency of the final degradation product further reinforces this claim (Figure 4b Inset).The results of the photocatalytic performance indicate the catalytic superiority of the Cs 4 PbBr 6 /WS 2 -NT nanocomposites over the pristine Cs 4 PbBr 6 -NCs.The formation of synergetic interaction between the WS 2 -NTs and Cs 4 PbBr 6 -NCs provided an efficient platform for vectorial charge transfer and enhanced photocatalytic reactivity.The photocatalytic degradation efficiency of Cs 4 PbBr 6 /WS 2 -NTs nanocomposite was ≈72% higher compared with previously reported CsPbBr 3 -TMD composites. [2]fter the photocatalytic decolorization reaction, we centrifuged the final solution to assess the stability of Cs 4 PbBr 6 -NCs and Cs 4 PbBr 6 /WS 2 .Then, we analyzed the residue using STEM (Figure S13a,b, Supporting Information).In the case of the pristine NCs, we detected only very few NSs, indicating their   degradation/dissolution during the photocatalytic process.However, in the nanocomposite, the NCs remained attached to the NTs and appeared intact, demonstrating their endurance during the dye degradation process in the polar solvent.
To investigate the kinetics of the BCG degradation process and to identify whether the process obeys pseudo-first-order kinetics, we plot the ratio of actual to initial concentration (ln(C 0 /C t )) versus irradiation time (Figure 4d).Indeed, a linear correlation in the kinetic plot indicates that the BCG degradation reaction follows the pseudo-first-order model. [43]Additionally, the dye degradation mechanism of Cs 4 PbBr 6 -NCs and Cs 4 PbBr 6 /WS 2 -NTs follows a type-1 photocatalytic mechanism, as shown in Scheme S5, Supporting Information.The apparent rate constant (k) was calculated by: ln(C 0 /C t ) = kt. [9]The calculated apparent reaction rate constant of Cs 4 PbBr 6 and Cs 4 PbBr 6 /WS 2 -NT are 0.087 and 0.119 min À1 , respectively.Compared to the pristine NCs, these reaction rates are ≈1.3 times higher for Cs 4 PbBr 6 / WS 2 -NTs.
[63] A reported method was used to calculate the QE of all the degradation reactions, as shown in Table 1.The calculated QE of BCG degradation is 2.73 Â 10 À2 and 6.82 Â 10 À2 molecules photon À1 for Cs 4 PbBr 6 -NCs and Cs 4 PbBr 6 /WS 2 -NTs, respectively.The detailed calculations of QE can be found in the Supporting Information.The Cs 4 PbBr 6 /WS 2 -NTs nanocomposite exhibits enhanced photocatalytic efficiency, QE, and the highest reaction rate constant.

QE ¼
The number of pollutant molecules reduced ðor degradedÞin a given time Total number of photons absorbed by the catalyst to be activated (1)

Assessment of the Recyclability and Stability of the Photocatalysts
We conducted a preliminary recyclability test for the pristine Cs 4 PbBr 6 -NCs and Cs 4 PbBr 6 /WS 2 -NTs nanocomposites as inferred from the photodegradation of the BCG dye.After the first cycle, the photocatalyst was collected and used for the subsequent cycle.A detailed description of the cyclic experiments is presented in the supporting information.The Cs 4 PbBr 6 / WS 2 -NTs nanocomposites exhibited 100-90% dye degradation efficiency up to three cycles by degrading BCG (Figure S10, Supporting Information).In contrast, the dye degradation efficiency of the Cs 4 PbBr 6 -NCs was just 40% in the first cycle and ≈0% in the following cycles.This latter observation indicates the complete degradation of the pristine NCs during the photocatalytic dye degradation.The results of the recyclability test imply further that adding WS 2 -NTs boosts the photocatalytic activity and the stability of the Cs 4 PbBr 6 -NCs.

EPR Spectroscopy for Elucidation of the Photocatalytic Mechanism
EPR measurements were undertaken to elucidate the organic dye degradation mechanism and the path for forming radicals.Here, the 5-tert-Butoxycarbonyl-5-methyl-1-pyrroline-N-oxide (BPMO) was added to the photocatalyst solution as a "spin-trap" to detect the formed radicals more precisely.Specifically, the dispersed NCs and the nanocomposites were mixed with BMPO, and the detected entities were nitroxide radicals.The measurements were performed at dark and under illumination (tungsten lamp) at room temperature.The detected EPR signals are presented in Figure 5.The Cs 4 PbBr 6 -NCs and the Cs 4 PbBr 6 /WS 2 -NT nanocomposite produce a weak EPR signal in the absence of light.This behavior indicates that no radicals are formed in a dark environment.A strong EPR signal of •OH (hydroxyl) radicals was detected upon illumination of the samples.The detected EPR signal of the nanocomposite is slightly higher than for the pristine NCs.This result indicated that both the NCs and nanocomposites are photoactive.However, the addition of WS 2 -NTs exhibits a beneficial impact on the radical formation under illumination, which fastens the degradation of the organic dyes.The fact that the radicals formed mainly under illumination indicates that the composites are promising candidates for organic dye degradation and photocatalysis in general.
To derive the mechanism for all the dye degradation reactions, let us recall that the charge transfer process suppresses the charge recombination in the nanocomposites and thus creates more active sites for radical formation.The results of the EPR, TA, and PL measurements support these statements.Hence, it can be concluded that the photogenerated electrons in the CB can be absorbed by oxygen and converted to •O 2 À free radicals. [43]The photogenerated holes in the VB can convert OH-to •OH radicals.The •OH and •O 2 À radicals react with the organic dyes and induce their degradation.The decolorization mechanism can be derived by combining all the present findings with the EPR measurements.

Conclusion
In summary, we have studied the photocatalytic activity of Cs 4 PbBr 6 -NCs and Cs 4 PbBr 6 /WS 2 nanocomposites under visible light.We demonstrated the superior ability of the nanocomposites toward the degradation of organic dye.The PL measurements reveal that upon mixing WS 2 -NSs with Cs 4 PbBr 6 , there is a 96% quenching of the PL intensity That phenomenon can be attributed to charge transfer occurring upon the nanocomposite formation.The TA results further confirm the charge transfer between Cs 4 PbBr 6 -NCs and WS 2 -NTs.Here, the NTs serve as an electron transport channel and reduce charge recombination, thus significantly improving photocatalytic dye degradation activity and the stability of the NCs.Indeed, the photocatalytic results prove the superior photocatalytic activity of Cs 4 PbBr 6 /WS 2 -NTs (degradation efficiency 100% in 30 min) nanocomposites for dye degradation.The kinetics of the degradation reactions follow the pseudo-first-order model.
In this work, we demonstrated the superior photocatalytic activity of Cs 4 PbBr 6 /WS 2 toward organic dye degradation in polar solvents, which is typically unachievable due to the instability of Cs 4 PbBr 6 .Our results pave the way for using
[66] In brief, Cs 4 PbBr 6 -NCs were synthesized using CsBr and PbBr 2 precursors (0.2 mmol) dissolved in DMF (3 mL) under nitrogen flow.In less than 20 min, the solution turned orange.Then, this solution was heated up to 150 °C for 30 min, and 1 mL DMF was added.
Here, the precursor becomes colorless.Then, the precursor was added dropwise into the solution of ODE (1.25 mL), OLA (0.125 mL), and OA (0.125 mL) under vigorous stirring.Subsequently, 5 mL of acetone was quickly added to this mixture.At this point, the color of the solution turns white and then slowly green, indicating the formation of Cs 4 PbBr 6-NCs.The synthesized NCs were centrifuged at 3500 rpm for 5 min and redispersed in 4 mL toluene.The NCs were separated into four sections for additional characterization and degradation tests, with one part used for comparison and the other three parts used to make composites with WS 2 -NSs.
Synthesis of WS 2 -NSs: WS 2 -NTs and WS 2 -NPs were synthesized using the procedures reported in refs.[67,68].In detail, the WS 2 -NTs were synthesized by the vapor-gas-solid high-temperature reaction of WO 3 nanoparticles with H 2 and H 2 S gases.The reaction consists of several steps, including WO 3 reduction to volatile suboxide, condensation to oxide nanowhiskers (a few microns in length and tens of nm in diameter), and sulfurization of these whiskers into WS 2 nanotubes.The WS 2 nanoparticles were synthesized via a solid-gas reaction of WO 3 spherical nanoparticles with H 2 and H 2 S gases.The sulfurization starts from the surface of oxide nanoparticles and continues outward and inward until the complete conversion of tungsten oxide to sulfide and the formation of hollow WS 2 nanoparticles. ., Synthesis of Nanocomposites: WS 2 -NTs, WS 2 -NPs, and WS 2 -bulk, with Cs 4 PbBr 6 -NCs: The WS 2 -NTs (2.5 mg), WS 2 -NPs (2.5 mg), and WS 2 -bulk (2.5 mg) were dispersed in ethanol (3 mL) by sonicating for 5 min.Then, the dispersed WS 2-NTs, WS 2 -NPs, and WS 2 -bulk were added into the second, third, and fourth portions of Cs 4 PbBr 6 -NCs, respectively.The mixtures were then stirred for 10 min and sonicated for another 10 min to increase the yield and improve the dispersion.Schematic illustrations of NCs and nanocomposite preparation are presented in Scheme S1, Supporting Information.
Cs 4 PbBr 6 -NCs and WS 2 -NSs Hybrid Nanocomposite Weight Ratio: The final weight of the as-synthesized Cs 4 PbBr 6-NCs was ≈116 mg.That amount was divided into four parts, such that each sample contained ≈29 mg.The total amount of WS 2-NSs added to each composite was 2.5 mg, so the w/w% of WS 2 was ≈11%.The preliminary steps of composite preparation included the examination of different ratios of the WS 2 and Cs 4 PbBr 6 .Generally, our primary objective was to identify conditions that could promote steady-state charge separation and improve the stability of the NCs.To achieve this, we used PL to optimize the WS 2 content, as the notable 90% reduction in the PL intensity indicates the effectiveness of charge transfer.Namely, we varied the amount of WS 2 to achieve the highest PL reduction (Figure S7, Supporting Information).
, Supporting Information.X-Ray diffraction (XRD) patterns of the examined materials are presented in Figure1e, S3, Supporting Information.For

Figure 4 .
Figure 4.The absorption spectrum of BCG and its decolorization process a) by Cs 4 PbBr 6 -NCs and b) by Cs 4 PbBr 6 /WS 2 -NT nanocomposite; insert images are photographs of the degraded BCG after every 10 min, c) calculated degradation efficiency of Cs 4 PbBr 6 -NCs and Cs 4 PbBr 6 /WS 2 -NSs nanocomposites at 620 nm peak point, and d) plots of the kinetics of all the decolorization reaction at 620 nm peak point.

Table 1 .
Summary of results for all the dye degradation for Cs 4 PbBr 6 -NCs and Cs 4 PbBr 6 /WS 2 -NT nanocomposite.
)2.7.EPR Spectroscopy for Assessment of the Stability of the PhotocatalystWe examined the stability of both pristine Cs 4 PbBr 6 -NCs and Cs 4 PbBr 6 /WS 2 nanocomposites in a toluene:ethanol (2:1) solution under visible light illumination using time-dependent EPR measurement (FigureS14, Supporting Information).At t = 0 min, both the NCs and the nanocomposites exhibit a signal characteristic of the hydroxyl radicals.After 7 min of illumination, the signal Cs 4 PbBr 6 /WS 2 remained unchanged, while the NCs showed additional peaks, which can be ascribed to photogenerated superoxide radicals.The superoxide radicals are formed through the decomposition of the nanocrystals and their interaction with ethanol.Thus, the appearance of the extra picks is indirect evidence of the Cs 4 PbBr 6 -NC's degradation.In contrast to the pristine NCs, the EPR signal of Cs 4 PbBr 6 /WS 2 remains stable even after 11 min.The results of the timedependent EPR measurements further corroborate that the addition of WS 2 -NTs stabilizes the Cs 4 PbBr 6 -NCs.