Photocatalytic Deposition of Nanostructured CsPbBr3 Perovskite Quantum Dot Films on Mesoporous TiO2 and Their Enhanced Visible-Light Photodegradation Properties.

Herein, we report the in situ photocatalytic deposition of cesium lead bromide (CsPbBr3) perovskite quantum dots on mesoporous TiO2-coated fluorine-doped tin oxide (FTO/TiO2) electrodes. The mesoporous TiO2 layer is used as a photocatalyst to promote the following: (1) the Pb deposition from a Pb2+ aqueous solution and (2) the in situ Pb conversion into CsPbBr3 perovskite in the presence of a CsBr methanolic solution without any organic capping agent. Both steps are carried out under ultraviolet light irradiation under ambient conditions without any post-treatment. The obtained FTO/TiO2/CsPbBr3 film was characterized by UV-vis diffuse reflectance spectroscopy, X-ray diffraction, photoluminescence spectroscopy, scanning electron microscopy, and transmission electron microscopy. The FTO/TiO2/CsPbBr3 heterojunction exhibited enhanced visible-light photodegradation activity demonstrated for the oxidation of curcumin organic dye as a model system. The novel and simple approach to fabricating a supported photocatalyst represents a scalable general method to use semiconductors as a platform to incorporate different perovskites, either all-inorganic or hybrid, for optoelectronic applications. The perovskite deposition method mediated by the UV light at room temperature could be further applied to flexible and wearable solar power electronics.

Introduction

Metal-halide perovskites have attracted great attention in the past two decades for many applications,[1−3] such as solar cells,[4−6] light-emitting diodes,[7,8] photodetectors,[9,10] and, most recently, photocatalysis,[11−13] due to their unique optoelectronic properties including tunable band gap, low exciton binding energy, and long carrier lifetime, along with simple preparation techniques and low material cost. All-inorganic halide perovskite quantum dots (QDs) have emerged in the past decade with promising stability if compared with their hybrid organic–inorganic perovskite counterparts.[14−16]

Titanium dioxide (TiO2) is a semiconductor extensively used for the electron transport layer on perovskite solar cells due to the favorable band alignment for electron transfer and its good electron transport properties.[17,18] TiO2 sensitized with halide perovskite QDs has been applied in visible-light-driven photocatalysis.[19−24] Schünemann et al. reported the use of a CsPbBr3/TiO2 composite for visible-light-driven photocatalytic benzyl alcohol oxidation.[19] Guo et al. investigated a CsPbBr3/TiO2 photocatalyst for visible-light photodegradation of rhodamine with better efficiency than a commercially available TiO2 catalyst (P25) used as the reference material.[22] Also, the CsPbBr3 QDs have been applied to photocatalytic degradation of tobacco tar alongside other semiconductors, like Bi2WO6.[25]

Both physical and chemical deposition methods were employed to sensitize TiO2 electrodes with halide perovskite QDs films.[26,27] The chemical deposition methods, also known as solution-processing methods, are a more facile approach with greater possibilities for scale-up manufacturing. A typical deposition method is spin coating, where the halide perovskite bulk material precursors are dissolved in a polar solvent (often DMF, DMSO) followed by spin coating in a sequential way that allows in situ perovskite formation. The process involves an additional temperature annealing step to improve the perovskite crystallinity.[27] The spin coating methods are raising safety concerns because of the waste of the precursors containing Pb and toxic solvents.[28] Also, when spin coating is used for greater areas, the perovskite film thickness often is compromise.[29]

Electrodeposition methods for perovskites on TiO2 electrodes have been proposed, especially for the preparation of the mixed halide perovskites.[30−34] Usually this method has two or more steps; in the first step, the Pb precursor is electrodeposited as PbO2[31] or metallic Pb[34] by oxidative or reductive electrodeposition technique, respectively. After the electrodeposition step, the lead-containing layer is chemically[31] or electrochemically[34] converted to lead halide and subsequently converted to the desired halide perovskite. Electrodeposition is a reproducible and relatively low-cost method, suitable for industrial large area coatings, and also avoids the waste of lead compounds because the deposited Pb is precisely controlled by the electrochemical parameters.

Aiming to reduce even more the perovskite processing costs and the development of greener processes, in this work, we have applied a photocatalytic deposition strategy to the direct fabrication of all-inorganic halide perovskites on a TiO2 semiconductor scaffold for the first time. This simple and versatile method takes advantage of the photocatalytic properties of the TiO2 and Pb redox behavior for depositing the metallic Pb precursor onto the mesoporous TiO2 layer by photoreduction using only UV light without any electrochemical input. In the second step, the deposited Pb is converted to CsPbBr3 perovskite by an in situ photocatalytic reaction mediated by TiO2 photocatalysts and UV light. The FTO/TiO2/CsPbBr3 heterojunction obtained here presents enhanced visible-light photodegradation properties. This fast fabrication method has a great scalability potential and can be completely carried out in air at room temperature, using greener solvents and without any post-treatment. The proof-of-concept developed here may be applied to the photocatalytic deposition of perovskites for different optoelectronic applications and may use the solar energy not just for energy conversion in the solar cell but also for the perovskite sensitizer layer synthesis itself, reducing the overall cost to produce renewable energy.

Materials and Methods

Materials

All chemicals used in the experiments were used as received, without further purification. Lead(II) acetate hexahydrate [Pb(Ac)2, purity ≥ 98%], sodium acetate (NaAc), and cesium bromide (CsBr) were purchased from Alfa Aesar. Dimethylacetamide [DMAc, CH3CN(CH3)2, ≥ 99.8%], dichloromethane (DCM, CH2Cl2 ≥ 99.0%), and hydroxypropyl cellulose, 80,000 MW, were ordered from Across Organics. Acetic acid, glacial (certified ACS), 2-propanol (C3H7OH, 99%), toluene, (C7H8, 99.5%), and methanol (CH3OH, 99.8%) were bought from Fisher Scientific. Fluorine-doped tin oxide (FTO) glass with a sheet resistance of 15 Ω/sq was purchased from Hartford. Tetrabutylammonium hexafluorophosphate (NBu4PF6), for electrochemical analysis, ≥99.0% was bought from Millipore Sigma and the curcumin dye from TCI America. TiO2 nanoparticles (15 nm) were purchased from US Research Nanomaterials, Inc.

Preparation of FTO/TiO2 Electrodes

The mesoporous TiO2-coated fluorine-doped tin oxide (FTO/TiO2) electrodes were prepared using planar FTO with a sheet resistance of 15 Ω/sq. The mesoporous nanoTiO2 films were fabricated on top of FTO as reported in the literature.[35] Briefly, TiO2 nanoparticles (anatase, 15 nm diameter) were dispersed in a solution made of 5% hydroxypropyl cellulose (80,000 MW) in 2-propanol.

The dispersion was doctor-bladed onto the planar FTO substrate. After drying in air, the films were annealed at 400 °C for 3 h in ambient atmosphere.

Photocatalytic Deposition of CsPbBr3 Perovskite QDs on FTO/TiO2 electrodes

The FTO/TiO2/CsPbBr3 QDs films were obtained using a photocatalytic two-step method (Figure ). In the first step, metallic Pb was deposited on the mesoporous TiO2 surface of the FTO/TiO2 electrode by photocatalytic reduction of Pb2+ aqueous solution. In a typical experiment, the FTO/TiO2 electrode was introduced into a clear vial containing 10 mM Pb2+ solution prepared in 0.1 M acetate buffer, with pH = 4.8. The electrode was then irradiated with a UV light (UVP EL Series, 4 W-302 nm) placed 1 cm away for 1, 3, 5, 10, 15, and 30 min in a series of different experiments. As a control experiment, an FTO/TiO2 electrode was kept in the 10 mM Pb2+ solution for 30 min in the dark to compare with the samples obtained under UV light irradiation.

Figure 1

Schematic representation of the photocatalytic two-step method for CsPbBr3 QDs deposition on FTO/TiO2 electrodes. (See Supporting Information for SEM–EDX elemental mapping composition after both steps.)

In the second step, an FTO/TiO2/Pb sample obtained using 1 min irradiation was converted into CsPbBr3 QDs by an in situ photocatalytic conversion method. Briefly, a freshly prepared FTO/TiO2/Pb electrode was immersed in a 0.05 M CsBr methanol solution with 2% (v/v) of dimethylacetamide (DMAc) and irradiated with the same UV light source placed 1 cm away, for different times (1, 3, 5, 10, and 30 min). The DMAc was used to improve the crystallinity of the perovskite film.[36] The obtained FTO/TiO2/CsPbBr3 was rinsed with 2-propanol, dried in air, and stored in the dark for further material and photoelectrochemical (PEC) characterizations.

To prove that ultraviolet irradiation is required, in the second step, we designed two different control experiments. For the first, we kept the FTO/TiO2/Pb in the CsBr solution in the dark for 30 min to monitor the occurrence of any reaction. In the second control experiment, a photomask was used, enabling the UV light to pass only through the open window to the FTO/TiO2/Pb sample (FIU design) while blocking it in the masked area. This experiment aims to monitor any reaction occurring in the non-illuminated area of the sample.

Characterization

The crystal structure and purity of prepared FTO/TiO2/CsPbBr3 films were determined by X-ray powder diffraction (XRD) using a Rigaku MiniFlex diffractometer (Cu Kα radiation, λ = 1.5405 Å). The FTO/TiO2/CsPbBr3 film absorption was calculated using the Kubelka–Munk function from the diffuse reflectance spectra (DRS) obtained with a Shimadzu UV-3600 Plus UV–vis–NIR spectrophotometer. BaSO4 was used as a reference standard for the DRS measurements. The photoluminescence (PL) characterization was carried out with an Edinburgh Instruments FS5 spectrofluorometer. The morphology and size of the synthesized CsPbBr3 were determined by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), using an FEI Talos F200X equipment; scanning electron microscopy (SEM) images were collected on a JEOL 6330F. Both the JEOL 6330F scanning electron microscope and FEI Talos F200X transmission electron microscope are equipped with energy dispersive spectroscopy (EDS), which enabled the determination of elemental distribution in the FTO/TiO2/CsPbBr3 samples.

Electrochemical experiments were performed with a WaveNow Potentiostat (PINE research) using a three-electrode system in one compartment cell. The pristine FTO/TiO2 or FTO/TiO2/Pb samples were used as the working electrode, a graphite rod was used as a counter electrode, and Ag/AgCl (saturated KCl) was used as a reference electrode for the aqueous experiments.

For the photodegradation experiments, 3 mL of 10 ppm curcumin solution in toluene was placed in a quartz vial together with the photocatalyst FTO/TiO2/CsPbBr3 electrode and irradiated with visible light for 120 min. Before the visible-light irradiation, the sample was kept in the dark for 20 min to ensure that the adsorption–desorption equilibrium was reached. The visible-light source used was a white light lamp (UVP EL Series) with an intensity of 50 mW/cm2 quantified using a Thor Labs silicon photodiode. At 20 min intervals, the UV–vis spectra of the solution were measured by UV–vis–NIR, using a Shimadzu UV3600 instrument to monitor the absorption maxima of curcumin at 417 nm. The ratio of curcumin concentration present in the solution at each time (C) to the initial concentration (C0), that is, C/C0, was determined from the ratio of the absorbance values measured in each interval and initial absorbance. PEC experiments were carried out using the same white light lamp as the visible-light source, placed at 1 cm of the working electrode. The working electrode was the pristine FTO/TiO2 or FTO/TiO2/CsPbBr3 samples, a graphite rod was used as a counter electrode, and Ag wire was used as a pseudo-reference for the experiments. NBu4PF6 dissolved in DCM was used as the organic electrolyte.

Results and Discussion

Photocatalytic Deposition of Pb on TiO2 Electrodes

Cyclic voltammetry (CV) was used to initially investigate the electrochemical behavior of Pb2+ at FTO/TiO2 electrodes. Figure a shows a CV for 10 mM Pb(Ac)2 on FTO/TiO2 electrode in 0.1 M acetate buffer, pH 4.8, as the electrolyte. The potential of the electrode was scanned negatively from 1.2 to −1.0 V versus Ag/AgCl and back again to the initial voltage at a scan rate of 20 mV/s. The forward scan of the CV showed a broad reduction shoulder around −0.5 V versus Ag/AgCl corresponding to the two-electron reduction of Pb2+ to metallic Pb(0).[37]

Figure 2

(a) CV from a Pb2+/Pb redox couple on an FTO/TiO2 electrode in 0.1 M acetate buffer (pH = 4.8). Scan rate: 20 mV/s. (b) LSV curves at 50 mV/s for the stripping of the photocatalytic deposited Pb after different UV irradiation times (scale bottom-left). Amount of Pb in μg/cm2 deposited on the FTO/TiO2 electrode after different UV irradiation times (scale up-right).

The reverse scan of CV also showed a very broad anodic peak current at approximately 0.6 V versus Ag/AgCl corresponding to the reoxidation of Pb(0) on the electrode surface back to the solution as Pb2+.

It is well known that TiO2 can be used as a photocatalyst for Pb2+ removal from the aqueous solution by reductive deposition of metallic Pb on the TiO2 surface using light.[38−40] We took advantage of this property to deposit the metallic Pb on FTO/TiO2 electrodes by only UV irradiation without applying any electrochemical potential, thus saving energy in the process. The photocatalytically deposited Pb serves as precursor to be subsequently converted to CsPbBr3 perovskite QDs using also a photocatalytic method mediated by the mesoporous TiO2 surface.

The amount of Pb deposited onto the FTO/TiO2 electrode surface after UV irradiation at different times in a 10 mM Pb2+ acetate buffer solution was estimated by an electrochemical technique. Linear sweep voltammetry (LSV) from −0.1 to 1.2 V versus Ag/AgCl at 50 mV/s was acquired after each deposition time (Figure b). The oxidative sweep ensured the stripping of the photocatalytic deposited Pb onto the FTO/TiO2 electrode back to the solution as Pb2+. The charge (area under the peak) was determined from the LSV curves and plugged into Faraday’s Law to obtain the amount of Pb in μg/cm2 deposited on the FTO/TiO2 electrode surface, according to the following equationwhere Q is a charge (area under the peak) in C, M(Pb) = 207.2 g/mol is a Pb atomic mass, F = 96485 C/mol is Faraday’s constant, z = 2 is the number electrons transferred per Pb atom in the oxidation step, and A = 1 cm2 is a geometrical area of the FTO/TiO2 electrode.

Figure b (scale up-right) depicts the amount of Pb in μg/cm2 deposited on the FTO/TiO2 electrode after different times of UV irradiation estimated by the LSV curves. The graph shows that with increasing UV irradiation from 1 to 30 min, the amount of deposited Pb increases almost linearly from 0.07 to 3.07 μg/cm2 (Table S1), with a rate of 0.1 μg/cm2 per minute for our experimental conditions. As a control experiment, we kept the FTO/TiO2 in the 10 mM Pb2+ solution for 30 min in the dark, and no significant difference between the initial and final LSV was obtained. This observation means that no perceptible amount of Pb was deposited on the FTO/TiO2 electrode in the dark corroborating that the UV light irradiation is indispensable for the Pb2+ reduction reaction.

To corroborate the presence of Pb in the FTO/TiO2/Pb (obtained with 1 min of UV irradiation) sample, top-view SEM imaging with elemental mapping EDX was carried out (Figure S1). The EDX elemental mapping shows the presence of Pb homogeneously distributed on the FTO/TiO2/Pb sample together with the Ti and O elements, proving that the photoreduction of the Pb2+ in the first step is successful and occurs onto the entire irradiated area.

Pb deposition occurs as a result of a light-assisted process, entailing first the UV light absorption by the TiO2, which results in excited electrons and holes. Next, photogenerated electrons reduce the Pb2+ to Pb(0) and the photogenerated holes are further oxidizing H2O (O2 evolution). After 30 min of UV irradiation, bubbles were spotted on the surface of the FTO/TiO2 electrode, suggesting that the O2 evolution reaction happens together with the Pb deposition.

The experimental evidence leads us to propose the following mechanism for the photocatalytic deposition of Pb on the FTO/TiO2 electrodes, agreeing with previous studies.[40]

In Situ Photocatalytic Conversion of Pb into CsPbBr3 Perovskite QDs on FTO/TiO2 Electrodes

The second step of the reaction is the in situ photocatalytic conversion of Pb to CsPbBr3 perovskite QDs. Samples of FTO/TiO2/Pb obtained with 1 min of UV irradiation were selected for the study and optimization of the second step. The Pb deposition time was limited to 1 min to ensure that the Pb is fully consumed in the conversion to CsPbBr3. Different FTO/TiO2/Pb samples were subsequently in situ photocatalytic-converted to FTO/TiO2/CsPbBr3 by UV light irradiation in the presence of the CsBr precursor during different times (1, 3, 5, 10, and 30 min). The inset in Figure a shows a picture of the samples obtained with different UV irradiation times. The CsPbBr3 sample obtained with 1 min of UV irradiation shows a light-yellow color, indicating the formation of the CsPbBr3 QDs heterostructure perovskite. After increasing the UV irradiation time to 10 min, a more homogeneous and bright yellow color in the entire TiO2 electrode area is observable for the samples obtained with 3, 5, and 10 min of UV irradiation time.

Finally, for the 30 min UV irradiation time, a slight degradation of the perovskite layer occurs, as observed in the inhomogeneous, partially degraded yellow color film. According to literature, CsPbBr3 perovskite degradation could be ascribed to the longer exposure to methanol (polar solvent) together with the UV light irradiation for a longer time.[41,42]

The optical absorption of the samples was investigated by UV–Vis diffuse reflectance spectroscopy (DRS) (Figure a). The FTO/TiO2/CsPbBr3 samples show absorption in the visible region with an edge around 530 nm if compared with the pristine FTO/TiO2 that absorbs only in the UV region with an absorption edge around 380 nm. Figure b shows the absorbance at 450 nm versus the UV irradiation time for the different samples. It can be observed that the absorption of the samples increases until reaching 10 min of reaction, while for the 30 min of reaction time, the sample absorption is comparable with the 3 min sample. The quantitative analysis of the absorption confirms the qualitative trend that homogeneous CsPbBr3 is obtained within 5 to 10 min of UV irradiation.

Figure 3

(a) DRS spectra of the samples with different UV irradiation times. (b) F(R) at 450 nm for the samples with different UV irradiation times.

The band gap of the samples was calculated using a Tauc plot (Table S1).[43] For the pristine FTO/TiO2 sample, a band gap of 3.21 eV (using an indirect semiconductor plot) was estimated, corresponding to the anatase TiO2 layer.[44] In the case of the FTO/TiO2/CsPbBr3 samples, a band gap energy around 2.35 eV (using a direct semiconductor plot) was estimated, in agreement with literature-reported values for the CsPbBr3 perovskite layer.[3]

The samples obtained with 10 min of UV irradiation time were chosen for further characterization of FTO/TiO2/CsPbBr3 (unless otherwise stated).

Figure a shows the XRD pattern of the pristine FTO/TiO2 and the FTO/TiO2/CsPbBr3 samples. By comparison, it can be observed that after the in situ photocatalytic conversion, new peaks arise at 15.2, 21.6, 30.6, and 43.9° corresponding to the (100), (110), (200), and (220) crystal planes of cubic-phase CsPbBr3 NCs (JCPDS no. 54-0752). An average of 22 nm for the coherent diffraction crystallite domain sizes was obtained using the Scherrer equation for the (110) crystal face of cubic CsPbBr3.[45] The XRD results confirm the formation of CsPbBr3 perovskite nanocrystals on top of the FTO/TiO2 anatase electrode.

Figure 4

(a) XRD pattern of the samples. (b) PL spectra of the FTO/TiO2/CsPbBr3 sample at an excitation of 400 nm. Inset: image of the FTO/TiO2/CsPbBr3 sample under UV lamp excitation.

The PL spectrum (Figure b) of FTO/TiO2/CsPbBr3 sample shows a narrow emission at 525 nm with an FWHM of 25 nm at an excitation wavelength of 400 nm, agreeing with previous reports for CsPbBr3 perovskite materials.[6] Accordingly, the FTO/TiO2/CsPbBr3 sample showed a bright green fluorescence under a manual UV light lamp irradiation (inset of Figure b).

The ambient stability of the FTO/TiO2/CsPbBr3 sample without special storage conditions was monitored for 1 week by XRD and UV–vis absorption measurements (Figure S2). From the figure, we can observe that the XRD peaks of the cubic CsPbBr3 phase are still present after 7 days but with less intensity. Also, the UV–vis absorption peak after 7 days is similar in shape to the initial sample with a little less intensity. These results show that the obtained FTO/TiO2/CsPbBr3 perovskite sample exhibits good ambient stability within a week after synthesis, thus suitable for applications under ambient conditions.

Figure S3 shows the top-view SEM images of the pristine FTO/TiO2, FTO/TiO2/Pb, and FTO/TiO2/CsPbBr3 samples, respectively. In all the images, we can see the nanoparticles of TiO2 forming the mesoporous layer, but no significant difference is observed with the conversion of Pb to the CsPbBr3 perovskite. This result is expected as the perovskite forms an ultra-thin, nanostructure layer on the TiO2 surface. After the in situ photocatalytic conversion to the CsPbBr3 perovskite, we can observe in the EDX elemental mapping (Figure S3d–i) the presence of the Cs, Pb, and Br elements homogeneously distributed together with the matrix of Ti and O elements, suggesting that the conversion to the CsPbBr3 perovskite occurs in the second step of the reaction in the entire irradiated area where the Pb was previously deposited by photoreduction (in the first step of the process).

To investigate if the CsPbBr3 perovskite deposition takes place only on the top surface of the electrode or within the entire mesoporous TiO2 scaffold, cross-sectional SEM images together with EDX elemental mapping of the FTO/TiO2/CsPbBr3 sample were analyzed (Figure ).

Figure 5

(a–i) Cross-sectional SEM image and EDX elemental mapping of Si, Na, Sn, Ti, O, Cs, Pb, and Br for the FTO/TiO2/CsPbBr3 sample.

In the cross-sectional SEM image (Figure a), three well-defined layers could be observed, from left to right: a layer of the glass, which is the Supporting Information for the electrode, a 0.5 μm thick FTO layer, and the 5 μm thick mesoporous TiO2 layer. The EDX elemental mapping also corroborates the three well-defined layers (Figure b–f), First, from the left, we can observe the presence of the Si and Na elements, matching the composition of silica glass. Next, we can observe the Sn element matching the region of the FTO layer and the Ti element perfectly the mesoporous TiO2 layer. Lastly, we can observe the Cs, Pb, and Br elements from the CsPbBr3 perovskite homogeneously distributed across the entire cross section of the mesoporous TiO2 layer. The elemental composition of the TiO2/CsPbBr3 film, measured in the middle region of the cross section (2.5 μm), is shown in Table S3 (Supporting Information). This result proves that the CsPbBr3 QDs impregnate all the mesoporous TiO2 scaffolds and not only a few nanometers of the top surface, promoting a large surface area of the TiO2/CsPbBr3 heterojunction interface.

Figure a shows the TEM image of the TiO2/CsPbBr3 composite (sample scratched from the electrode) collected for studying the size and morphology of the CsPbBr3 nanocrystals. The image shows the larger TiO2 particles (around 20 nm) covered with very small (around 2 nm) CsPbBr3 nanocrystals. Given that the sample preparation involved probe ultrasonication of the scraped sample in hexanes before deposition of the particles on the TEM grid, the process likely separated potentially larger particles (suggested by the XRD evaluation) at the surface of the TiO2/CsPbBr3 particles, keeping only the immediately attached small ones on the surface of the TiO2.

Figure 6

(a) TEM, (b) HRTEM, and (c–h) STEM–EDX elemental mapping images of Ti, O, Cs, Pb, and Br for the TiO2/CsPbBr3 heterostructure.

HRTEM image of the TiO2/CsPbBr3 particles (Figure b) shows an interplanar distance of 0.35 nm corresponding to the (101) family of planes of anatase TiO2.[46]

The planes for the cubic CsPbBr3 perovskite were not identified in the HRTEM images due to the small size of the particles and also because the family of planes (111) with an interplanar distance of 0.34 nm (JCPDS no. 54-0752) is very similar to the (101) plane of anatase TiO2. The similarity of the d spacing and the small size of the CsPbBr3 NCs makes the proper identification very challenging.

Figure c–h shows the STEM–EDX elemental mapping images of Ti, O, Cs, Pb, and Br for the TiO2/CsPbBr3 heterostructure. We can observe that for the Ti and O elements, the corresponding colors are brighter and perfectly match the HAADF–STEM image, proving that the bigger particles correspond to the anatase TiO2. Also, Cs, Pb, and Br are present homogeneously throughout the sample, confirming that the impregnation of mesoporous TiO2 with CsPbBr3 QDs is successful over the entire surface area of the TiO2 mesoporous layer.

The different characterization techniques discussed above show that a CsPbBr3 perovskite QD film was obtained on FTO/TiO2 substrates via the in situ photocatalytic conversion method. While a homogeneous distribution is observed, the film appears to be formed of discrete QDs. The second step is based on the use of the TiO2 substrate as a photocatalyst to re-oxidize the deposited Pb in the first step back to Pb2+ under UV irradiation. Because the photocatalytic reaction occurs in the presence of CsBr, the Pb2+ rapidly reacts with Cs and Br and crystallizes in a CsPbBr3 perovskite cubic phase. The change in color from the white FTO/TiO2/Pb into a yellow FTO/TiO2/CsPbBr3 sample occurs in minutes. Importantly, the reaction method does not require post-processing (chemical treatment or thermal annealing) to stabilize the CsPbBr3 perovskite layer, and the whole process is conducted under ambient conditions using mostly green solvents.

To prove that the UV irradiation is also needed, in the second step, we designed two different control experiments. In the first, we kept the FTO/TiO2/Pb in the CsBr solution for 30 min in the dark, and no perceptible change in color was perceived.

This result suggests that without the UV light, no conversion to the CsPbBr3 perovskite takes place. To corroborate the above results, we carried out a second control experiment using a photomask (FIU pattern) where the UV light passed only through the letters, thus only selectively reaching the FTO/TiO2/Pb sample. After 5 min of UV irradiation using the photomask, we obtained a yellow FIU pattern (Figure ) at the center of the sample due to the in situ photocatalytic conversion of the Pb to the CsPbBr3 perovskite only in the irradiated parts of the FTO/TiO2/Pb sample.

Figure 7

Control experiment uses a photomask (only the unmasked areas of the sample are exposed to UV irradiation through the FIU letter slit).

The results obtained from both control experiments prove that UV light irradiation is needed in order to convert the Pb into the CsPbBr3 perovskite QDs. This is explained by an in situ photocatalytic mechanism mediated by the TiO2 semiconductor. When the FTO/TiO2/Pb sample is irradiated with UV light, some portion of the light will be absorbed by the TiO2 photocatalyst and generate electrons and holes.

The photogenerated holes are used to oxidize the deposited Pb back to Pb2+, which rapidly reacts with the Cs+and Br– precursors to form the CsPbBr3 perovskite nanocrystals.

The proposed mechanism (the oxidative part) for the second step of the in situ photocatalytic conversion of Pb in the CsPbBr3 perovskite in the FTO/TiO2 substrates is shown below (eqs7–9)

PEC Activity of FTO/TiO2/CsPbBr3 Heterojunction

The FTO/TiO2/CsPbBr3 samples were further photoelectrochemically characterized to study the potential application of the heterojunction as a visible-light photocatalyst. Figure a shows the current–voltage (I–V) curves of the pristine FTO/TiO2, FTO/TiO2/Pb, and FTO/TiO2/CsPbBr3 samples. The LSV experiments were carried out in the 0.1 M NBu4PF6/DCM electrolyte solution at 20 mV/s, in the dark and under white light irradiation.

Figure 8

(a) Current–potential (I–V) curves for the different samples in dark and under white light irradiation. (b) Current–potential (I–V) curve under on–off cycles of white light irradiation for the FTO/TiO 2/CsPbBr3 sample. (c) Photodegradation curves of curcumin by the different samples and experimental conditions. (d) Pseudo-first-order rate kinetics for the curcumin photodegradation with the different samples.

The results show that for the pristine FTO/TiO2 sample, both I–V curves are very similar with a small capacitive current for both experimental conditions. The I–V curve shows that the FTO/TiO2 sample does not develop any photocurrent under white light irradiation, making the sample inactive for visible-light photocatalysis. The above result was expected because the anatase TiO2 (band gap 3.21 eV) absorbs light only in the ultraviolet region. For the FTO/TiO2/Pb sample, we can observe for both experimental conditions a fast increase in the current around 0.75 V versus Ag/AgCl related to the Pb oxidation peak. The oxidation current is expected due to the Pb present in the sample after the photocatalytic reduction of the Pb2+ carried out in the first step of the reaction. This sample also does not show visible-light photocatalytic properties. On the other hand, for the FTO/TiO2/CsPbBr3 sample, the characteristic diode curve for photoactive semiconductor materials under visible-light irradiation can be observed.[11] In the I–V curve, the photocurrent arises above the dark current level starting at −0.21 V and reaching a plateau (∼40 μA/cm2) in the 0.2–0.9 V potential region. After 0.9 V we can see an increase in the current that matches with the I–V curve taken in the dark and also in the same region for the FTO/TiO2/Pb sample. The increasing current at 0.9 V is related to the remnant Pb oxidation (Pb in excess, which does not react with the CsBr precursor and is not converted to the CsPbBr3 perovskite).

The obtained characteristic diode curve (−0.2 to 0.9 V) proves that the FTO/TiO2/CsPbBr3 heterojunction is photoactive under visible-light irradiation, enhancing the performance of the pristine FTO/TiO2 sample. This behavior is owed to the yellow CsPbBr3 perovskite layer, absorbing visible light and generating electron/hole pairs, which are efficiently separated through the applied potential and instantly measured as a generated photocurrent.

Figure b shows the I–V curve for the FTO/TiO2/CsPbBr3 sample under an on–off regime of white light irradiation. From the figure, we can observe that when the light is on, the current density increases almost instantly and drops again almost instantly to the dark current level when the light is off. This result confirms that the current is related to the visible-light stimuli and that the photo-response of the FTO/TiO2/CsPbBr3 electrode is very efficient.

The above PEC characterizations corroborate that the TiO2 and CsPbBr3 semiconductors form a heterojunction with an efficient interface for the electron transfer from the CsPbBr3 nanocrystals to the mesoporous TiO2 layer and thereafter to the FTO current collector, closing the circuit with the graphite counter electrode. The enhanced PEC properties of the FTO/TiO2/CsPbBr3 sample toward a visible-light excitation make the heterojunction interesting for photocatalytic and solar cell applications.

Photodegradation Activity of FTO/TiO2/CsPbBr3 Samples

To study the organic photodegradation activity of the FTO/TiO2/CsPbBr3 sample with visible-light irradiation, curcumin, a natural dye, dissolved in toluene (yellow solution) was chosen as a model molecule. The curcumin dye has strong absorption in the visible domain of the spectrum with a maximum peak (λmax) around 417 nm.[47] In a typical photodegradation experiment, the reduction of the maximum absorption peak was monitored as an indication of the degradation (decolorization) of the curcumin molecule in the toluene solution.

The ratio of the concentration of total curcumin present in the solution at any time (C) to the initial concentration (C0), that is, C/C0, was determined from the ratio of the absorbance values measured at 417 nm at different times during 120 min. The adsorption (dark condition) and photodegradation (white light irradiation) results obtained for the different experimental conditions are shown in Figure c.

By comparing the samples under dark conditions, we can observe that the pristine FTO/TiO2 sample presents a greater adsorption capacity (48.2%) than the FTO/TiO2/CsPbBr3 sample (21.8%) due to the higher affinity of the curcumin to form bidentate (O–Ti–O) bonds with the titanium sites on the TiO2 surface.[48] In the case of the FTO/TiO2/CsPbBr3 sample, the lower adsorption capacity is because of fewer Ti sites available on the TiO2 surface now covered by the CsPbBr3 perovskite nanocrystals. On the other hand, when we look at the curves under visible-light irradiation, first in the absence of any photocatalyst, the curcumin photodegradation is about 32.1%. The degradation of the curcumin by visible light is expected and reported for different solvents elsewhere.[47] Second, when the pristine FTO/TiO2 was used as a photocatalyst, the photodegradation of the curcumin was 57.2%, only 1.2 times greater than the adsorption result from the dark experiment. This result suggests that the FTO/TiO2 is not efficient as a visible-light-driven photocatalyst in agreement with the PEC results. Finally, when the FTO/TiO2/CsPbBr3 perovskite sample was used as a photocatalyst, the photodegradation of the curcumin was 91%. This value is 4.2 times greater than the value measured in the dark and 2.8 times greater than the photodegradation of the organic curcumin with only the visible-light irradiation. The above results prove that the FTO/TiO2/CsPbBr3 sample is efficient as a visible-light-driven photocatalyst for the photodegradation of curcumin in toluene solvent.

Figure d shows the photodegradation curves under visible-light irradiation adjusted to the pseudo-first-order kinetic model. The kinetic constants extracted from the slope of the linear curves are shown in the graph (0.003, 0.004, and 0.018 min–1 for the self-degradation, pristine FTO/TiO2, and FTO/TiO2/CsPbBr3 samples, respectively). The curcumin photodegradation rate using the FTO/TiO2/CsPbBr3 perovskite sample is 6 times higher than the self-degradation of the curcumin and 4.5 times higher than for the pristine FTO/TiO2 sample, validating the performance of the FTO/TiO2/CsPbBr3 heterojunction sample as an efficient photocatalyst for organic degradation under visible-light irradiation as showed before through the PEC characterization. We posit that the mechanism for the visible-light-driven photodegradation of curcumin using the TiO2/CsPbBr3 heterostructure as a photocatalyst is aligned with previous literature findings. A schematic of the proposed mechanism is shown in Figure S4 (Supporting Information).[11,19,49]

The results described in this work proved the successful deposition of the CsPbBr3 all-inorganic perovskite QD films onto FTO/TiO2 substrates, using a novel in situ photocatalytic method, without the use of any post-treatment. The two-step method uses the mesoporous TiO2 layer as a photocatalyst to enable Pb precursor deposition under ultraviolet irradiation in the first step. Subsequently, in the second step, the deposited Pb is converted to CsPbBr3 under UV light irradiation in the presence of the CsBr methanolic solution by an in situ photocatalytic reaction mediated by the TiO2 photocatalyst. The obtained FTO/TiO2/CsPbBr3 heterojunction shows enhanced properties toward organic molecule photodegradation and photocurrent generation under visible-light irradiation.

Conclusions

The present work provides a new, simple, and scalable strategy to deposit CsPbBr3 all-inorganic perovskite QDs onto mesoporous TiO2 films, using an in situ photocatalytic deposition method. In summary, the two-step photocatalytic method uses TiO2 as a platform to photo-reduce the aqueous Pb2+ to metallic Pb and subsequently convert Pb to the CsPbBr3 perovskite QDs film using UV light irradiation in both steps. The overall process takes place under ambient conditions and room temperature without the use of any post-treatment and any major toxic solvent, making the process a greener alternative to other perovskite deposition methods.

The FTO/TiO2/CsPbBr3 sample has successfully been applied as a visible photocatalyst for organic curcumin photodegradation, showing enhanced efficiency if compared with the pristine FTO/TiO2. The FTO/TiO2/CsPbBr3 heterojunction also produces a higher photocurrent density than the pristine FTO/TiO2 under visible-light irradiation, due to the improved electron excitation in the visible region, charge carrier separation, and transport efficiency. The concepts developed here can be extended to the photocatalytic deposition of different types of perovskite nanocrystals on top of different semiconductors on both rigid and flexible substrates, opening avenues for a myriad of applications in optoelectronics and solar photovoltaics.