# Dual solar-driven hydrogen evolution and alcohol oxidation with CdS quantum dot-sensitized photocatalysts prepared by the SILAR methodology

Vasilis Nikolaou, Deborah Romito, Clément Maheu, Jonathan Hamon, Eric Gautron, Florian Massuyeau, Pierre-Emmanuel Petit, Stéphane Jobic, Fabrice Odobel

► To cite this version:

Vasilis Nikolaou, Deborah Romito, Clément Maheu, Jonathan Hamon, Eric Gautron, et al. Dual solar-driven hydrogen evolution and alcohol oxidation with CdS quantum dot-sensitized photocatalysts prepared by the SILAR methodology. Journal of Materials Chemistry A, 2024, 12 (44), pp.30885-30891. 10.1039/d4ta03559c. hal-04798888

HAL Id: hal-04798888

https://hal.science/hal-04798888v1

Submitted on 21 Aug 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Dual Solar-Driven Hydrogen Evolution and Alcohol Oxidation with CdS Quantum Dot-Sensitized Photocatalysis Prepared by SILAR Methodology

Received 00th January 20xx,
Accepted 00th January 20xx

DOI: 10.1039/x0xx00000x

Vasilis Nikolaou,⁺ª Deborah Romito,⁺ª Clément Maheu,ᵇ Jonathan Hamon,ᵇ Eric Gautron,ᵇ Florian Massuyeau,ᵇ Pierre-Emmanuel Petit,ᵇ Stéphane Jobic,\*ᵇ Fabrice Odobel\*ª

Quantum dots (QDs) have gained significant attention as efficient photosensitizers for light-harnessing applications. While quantum dots are extensively employed in dye-sensitized solar cells (DSSCs), their exploration as sensitizers on metal oxide nanoparticles, such as TiO₂ for dye sensitized photocatalytic systems (DSPs) remains relatively unexplored. The successive ionic layer adsorption and reaction (SILAR) method offers a promising solution to prepare QDs on metal oxide nanoparticles, since it is a simple, mild, and cost-effective approach. In this study, we present the first successful utilization of the SILAR methodology for growing cadmium sulfide (CdS) QDs on TiO₂ nanoparticles. The structural characterization of CdS-TiO₂ hybrids was conducted using Transmission Electron Microscopy (TEM), Energy-Dispersive X-ray Spectroscopy (EDS), and X-ray Photoelectron Spectroscopy (XPS). The photocatalytic performances of CdS-TiO₂ NPs were thoroughly examined for light driven simultaneous benzyl-alcohol oxidation into aldehydes and H₂ evolution. Remarkably, the CdS-TiO₂ nanoparticles operate without any catalyst and sacrificial electron donor. Furthermore, these composite species demonstrate excellent stability, retaining over 95% of their initial efficiency after three recycling cycles.

## Introduction

International agreements have been established to develop carbon-neutral and sustainable technologies, aiming to mitigate global warming and our profound reliance on fossil fuels.¹, ² Among the array of renewable energy sources, Sol. Energy emerges as a frontrunner, distinguished by its cleanliness and limitless availability. Solar-driven hydrogen (H₂) production, in particular, represents a compelling avenue toward the generation of green fuel, poised to catalyze the decarbonization efforts across various industrial sectors.³⁻⁵ Green H₂, characterized by its environmental friendliness, high energy density, and storage capability, holds immense potential as an energy carrier, heralding a pivotal transition towards a sustainable energy landscape.⁶ The seminal work of Fujishima and Honda in 1972⁷ ignited a fervent wave of research activity aimed at harnessing photocatalytic systems based on inorganic

metal oxide semiconductors, notably with titanium dioxide (TiO₂). However, TiO₂, characterized by its wide bandgap, predominantly absorbs ultraviolet (UV) light, necessitating sensitization with visible light-absorbing materials to extend its photoactivity across a broader spectrum of solar irradiance. In this field, three primary types of photochemical devices have been devised: dye-sensitized solar cells (DSSCs)⁸⁻¹⁰ for photovoltaic applications, and dye-sensitized photoelectrochemical cells (DSPECs)¹¹⁻¹⁵ alongside dye-sensitized photocatalysis (DSP)¹⁶, ¹⁷ for solar fuel production.

Quantum dots (QDs), nanocrystals of inorganic semiconductors, have attracted significant attention as efficient light harvesters for these applications.¹⁸, ¹⁹ Leveraging the quantum confinement effect, QDs offer tunable bandgaps, enabling high absorption coefficients across a wide range of visible wavelengths merely by varying their diameter. Furthermore, QDs exhibit superior stability compared to many molecular sensitizers and their stability can be enhanced with passivation strategies.²⁰, ²¹ Additionally, QDs can be synthesized cost-effectively and in large quantities, underscoring their potential for deployment in Sol. Energy conversion technologies. While the utilization of QDs is prevalent in DSSCs, commonly referred to as QDSSCs,⁸⁻¹⁰, ¹⁹, ²², ²³ their exploration in DSPECs remains limited, especially on TiO₂.¹¹⁻¹⁵, ²⁴ In contrast, there has been more extensive research on NiO (p-type

<footnote>ª Nantes Université, CNRS, Chimie Et Interdisciplinarité Synthèse Analyse Modélisation, CEISAM, UMR 6230, F-44000 Nantes, France.</footnote>

<footnote>ᵇ Nantes Université, CNRS, Institut des Matériaux de Nantes Jean Rouxel, IMN, F-44000 Nantes, France.</footnote>

<footnote>† These authors have equally contributed</footnote>

<footnote>Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/x0xx00000x</footnote>
DSPEC)²⁵⁻²⁹, and surprisingly, their application in DSPs has rarely been reported,¹⁶, ¹⁷, ³⁰ despite their significant potential. QDs, both alone and in combination with catalysts, have been used for H₂ photocatalysis,³¹, ³² however rarely to sensitize a wide bandgap metal oxide such as TiO₂, or NiO. ¹⁶, ¹⁷, ³⁰ In a typical QD-sensitized DSPEC configuration, a mesoporous TiO₂ film is coated with QDs and immersed in a sulfur-based electrolyte (containing typically sulfide or sulfite anions) with a platinum-based counter electrode facilitating hydrogen evolution. Noteworthy, sulfur-based anions most often serve as redox mediator or as sacrificial electron donors in QDSSCs and QD-sensitized DSPECs respectively, albeit posing potential contamination risks to metal-based catalysts, including platinum. The elimination of sacrificial electron donors (SEDs) in photocatalytic systems represents a significant step toward practical applications.³³ When a reagent is used as SED, it is consumed without producing recoverable products, resulting mostly in waste. In contrast, if the oxidized reagent is converted into a valuable product, such as an aldehyde, it can be recovered and has a higher economic value than the original reagent. Furthermore, in the case of organic molecule oxidation, the product can be easily separated from the reduction product (H₂) since they exist in different phases: the oxidation product remains in solution, while H₂ is in the headspace. This phase separation is advantageous as it avoids the need for an additional purification step, which would otherwise be required to separate the flammable mixture of H₂ and O₂ generated in water-splitting processes. These differences underscore the economic and environmental benefits of using benzyl alcohols as an alternative to conventional SEDs.

In the context of DSPs sensitized with QDs, there are very few examples so far. One notable exception, is the work by Zhang *et al.*, which describes H₂ evolution using CdSe QDs sensitized TiO₂ nanoparticles.¹⁷ In addition, two recent reports demonstrate CO₂ reduction by CdSe or InP QDs attached to TiO₂ NPs.¹⁶, ³⁰

The fabrication of QD-sensitized TiO₂ relies predominantly on three strategies (Fig. 1).¹⁵, ¹⁸, ¹⁹ The most common synthetic protocol is the chemical bath deposition (CBD). This method involves submerging a TiO₂ mesoporous film into a solution of the metal salt (typically Cd(OAc)₂) in the presence of a sulfide anion precursor, such as a basic solution of thiourea. This immersion process initiates the precipitation of QD, which are finally crystallized by sintering at temperatures ranging from 300 to 400°C.¹⁵, ¹⁸ The second approach starts with the separate synthesis of a colloidal solution of QDs, typically using the hot injection method, which is followed by their grafting onto TiO₂ surfaces using bifunctional linkers such as 3-mercaptopropionic acid (Fig. 1).¹⁸, ¹⁹ Alternatively, the successive ionic layer adsorption and reaction (SILAR) method offers a simple, flexible, and cost-effective approach, enabling room temperature deposition without necessitating sophisticated equipment.¹⁸, ¹⁹, ³⁴ The general SILAR method involves immersing the TiO₂ mesoporous electrode into a solution of the metal precursor (typically a nitrate or acetate metal salt, such as Cd(NO₃)₂), rinsing it with a solvent, then immersing it into another solution containing the chalcogenide anion (typically a

sodium salt of the chalcogenide, such as Na₂S) and rinsing it again. This deposition process is repeated several times to ensure optimal QD loading. SILAR has been frequently employed to fabricate quantum dots (QDs) on TiO₂ mesoporous films, notably for applications in quantum dot-sensitized solar cells (QDSSC) and quantum dot-sensitized photocatalysis (QDSPEC). Two studies report the use of SILAR method to coat TiO₂ NPs with CdS QD for dye photodegradation.³⁵, ³⁶ However, the application of this strategy in the context of DSPs for solar fuel production, such as H₂, remains relatively uncommon and warrants further investigation, which is the focus of the present study (Fig. 1). CdS QDs have been studied extensively as a visible light-absorbers;¹⁵ hence, they were selected as the model material for this study to fabricate the first QD-based DSPs for H₂ evolution using the SILAR method. The resulting CdS-TiO₂ nanoparticles were evaluated as photocatalysts for solar-driven H₂ production. In most cases, QD-based DSPs have been investigated as photocatalysts in the presence of an external sacrificial electron donor (SED) to regenerate the oxidized QDs.³⁷⁻⁴⁰ However, the development of DSPs that operate without noble metal catalysts and without SED is needed to make photocatalytic systems more economically viable. SEDs are expensive chemicals and potentially toxic, which are only used to consume photogenerated holes and to avoid fast charge recombination reaction. As an alternative, solar-driven H₂ evolution could be combined with a profitable oxidation reaction, namely alcohol oxidation.⁴¹⁻⁴⁴

<figure><img src="image_2.png" /><figcaption>**Fig. 1** Schematic illustration of the two usual methodologies to prepare QDs on TiO₂ mesoporous films (for DSSCs and DSPECs, upper</figcaption></figure>
part) vs. our approach in this work (CdS-TiO₂ NPs for DSPs, lower part).

Specifically, the oxidation of benzyl alcohols to the respective benzaldehydes offers a promising alternative to the use of SEDS, since aldehydes could be utilized as starting materials in pharmaceuticals and chemical industries.45-49 Currently, the selective oxidation of benzyl alcohols is feasible in high yields and selectivity, however using high temperature, noxious chemical oxidants, or noble-metal catalysts.47, 50, 51 Hence, concomitant solar-driven H₂ evolution and alcohol oxidation using greener and milder conditions could be an encouraging solution. Only recently, a similar approach was presented in the literature. Nevertheless, in this example: i) CdS QDs were synthesized using solvothermal synthesis (high temperature, high pressure, low yield), ii) the functionalization of the CdS QDs with cobalt as co-catalyst was critical for achieving alcohol oxidation, and iii) only one benzyl alcohol was tested.52 In stark contrast, we describe the successful fabrication of a fast, easy, and low-cost methodology at room temperature and atmospheric pressure for preparing CdS QDs on TiO₂ NPs that demonstrate good performance for hydrogen evolution and benzyl alcohol oxidation in aqueous and visible-light illumination, without the need for external catalysts or sacrificial reagents.

## Results and discussion

The preparation of CdS-TiO₂ NPs was carried out following the protocol described in detail in the supporting information file (S1). The presence of CdS QDs on the surface of TiO₂ NPs clearly enhances the light harvesting efficiency in the visible spectrum as the onset of the absorption band of the CdS-TiO₂ composite is shifted until 517 nm (2.40 eV, Fig. S1), due to the appearance of an absorption band assigned to QDs just below the absorption threshold of TiO₂, located at ca. 388 nm (3.20 eV). Compared to CdS QDs prepared by hot injection method,[8] the full width at half maximum (FWHM) of the CdS absorption band appears broader here. It indicates that there is a distribution of QDs of different diameters. Based on the empirical fitting functions developed by Peng et coll.53 and taking a maximum absorbance at 456 nm, the mean diameter of CdS QDs is estimated at ca. 5.5 nm leading to a bandgap of 2.4 eV (Fig. S1). The steady-state emission spectrum of CdS-TiO₂ and CdS-SiO₂ (ESI for details) were measured and indicate a significant quenching of CdS emission; this is most likely due to efficient electron injection into the TiO₂ conduction band (Fig. S2).

We used X-ray powder diffraction (P-XRD) to identify the crystalline phases, determine their mass proportions, and estimate their crystallite sizes of the CdS-TiO₂ nanoparticles. To achieve this, P-XRD patterns of bare TiO₂ and CdS-TiO₂ were collected, and Rietveld refinement of the CdS-TiO₂ sample was performed for quantitative analysis (Fig. S3 and Table S1). The TiO₂ sample consists of a mixture of anatase and rutile, while the CdS-TiO₂ sample contains a combination of anatase, rutile, and CdS with the respective mass proportions of 48%, 8%, and 34%. The CdS phase is a mixture of two polymorphs: hawleyite (cubic CdS, sphalerite structure) and wurtzite (hexagonal CdS), with respective proportions of 20% and 14% (Table S1). The crystallite sizes of the cubic and hexagonal CdS phases are 3.4 nm and 3.7 nm, respectively, which is consistent with the UV-

Vis and Transmission Electron Microscopy (TEM) measurements (see below).

TEM and Energy-dispersive X-ray spectroscopy (EDS) were used to investigate the size and the distribution of the CdS QD on the TiO₂ nanoparticles. Dispersed bright particles with an average 5 nm diameter are visible on larger grains (few tens of nms large) on Scanning Transmission Electron Microscopy - High-Angle Annular Dark-Field (STEM-HAADF) images (Fig. 2). As the intensity in STEM-HAADF imaging is proportional to t.Zⁿ (with t the thickness, Z the atomic number and n a value ~1.6-1.9), these bright particles can be assigned to CdS particles deposited on TiO₂ grains. Their size is in good agreement with the one extrapolated from the absorption spectrum (see above). This is confirmed by the EDS maps presented in Fig. 3 where smaller particles contain both Cd and S. Atomic fraction determined by EDS over the whole area of Fig. 3 gives a Ti/(Cd+S) ratio around 4. This value is almost 3 times higher than the one measured by XPS (Fig. S4). It is in agreement with the higher depth of analysis of the EDS in comparison to the XPS one (below 10 nm) and confirms the deposition of the CdS nanoparticles at the surface of the TiO₂ ones. Images at higher magnification show that CdS particles are crystalline (Fig. 4). Fast Fourier Transform (FFT) performed on an individual particle can be indexed with the wurtzite phase along [001] zone axis (ICSD file: 154188).

<figure><img src="image_3.png" /><figcaption>Fig. 2 STEM-HAADF image of CdS-TiO₂ NPs.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 3 EDS mapping of CdS-TiO₂ NPs.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 4** STEM-HAADF image of CdS-TiO₂ NPs (left). At higher magnification atomic columns are clearly seen on a CdS particle (FFT of this particle as an inset).</figcaption></figure>

The catalytic activity of CdS-TiO₂ NPs was evaluated by solar-driven H₂ evolution experiments in the presence of three different alcohols: *para*-methoxy benzyl alcohol (MeOBenzOH - CH₃OC₆H₄CH₂OH), *para*-trifluoromethyl benzyl alcohol (CF₃BenzOH - CF₃OC₆H₄CH₂OH) and benzyl alcohol (BenzOH - C₆H₅CH₂OH). All the details concerning the photocatalytic experiments are given in the SI. As illustrated in Table 1 (entries 1 and 2), initially bare TiO₂ NPs were tested in the presence of a SED (triethanolamine, TEOA) and MeOBenzOH (CH₃OC₆H₄CH₂OH). These control experiments demonstrated that no catalytic activity is observed in the absence of CdS or light irradiation. On the contrary, under the same experimental conditions, irradiation of CdS-TiO₂ NPs resulted in 19 µmol of H₂ (entry 3, Table 1) when using TEOA as SED and 22 µmol of H₂ with MeOBenzOH (entry 4, Table 1). Notably, the performance with MeOBenzOH was similar to that with TEOA as SED, and MeOBenzOH was quantitatively converted into the corresponding methoxy benzaldehyde (entry 4, Table 1). It is also important to mention that CdS-TiO₂ NPs showed no catalytic activity without irradiation with a LED light source (entry 5, Table 1). The ratio between the products (H₂ and methoxy benzaldehyde) was practically 1:1 (22 µmol vs. 20 µmol, entry 4, Table 1). The 2 µmol missing can easily be explained by the additional treatments (extraction filtration, drying) required to quantify the aldehyde. A comparison with the work of Zhang and coll.¹⁷ indicates that our system is only slightly less active (366 µmol/gTiO₂/h compared to 404.3 µmol/gTiO₂/h reported by Zhang). Nevertheless, our system does not consume a SED.

Moreover, to expand the scope of our work, two supplementary benzyl alcohols were tested using CdS-TiO₂ NPs. Namely, the CdS-TiO₂ NPs tested with BenzOH (entry 6, Table 1) and CF₃BenzOH (entry 7, Table 1), also resulted in H₂ production and conversion of alcohol into the respective benzaldehyde. In the case of BenzOH, 10 µmol of H₂ and 3 µmol of benzaldehyde were detected, whereas for CF₃BenzOH, 20 µmol of H₂ and 6 µmol of 4-(trifluoromethyl)benzaldehyde. Interestingly, despite CF₃BenzOH having a more positive oxidation potential compared to BenzOH, a greater amount of H₂ was produced. At this stage, the rationale behind this phenomenon remains unclear and is beyond the scope of this study. Nonetheless, this result is consistent with findings from other dye sensitized photoelectrochemical systems.³³, ⁵⁴ However, for both CF₃BenzOH and BenzOH, there is a discrepancy between the observed products (H₂ and aldehyde), unlike with MeOBenzOH (entries 4, 6, and 7, Table 1). Overoxidation of the aldehyde into carboxylic acid is ruled out, as no quantity of the acid was detected during GC analysis of the reaction mixture. Another more plausible explanation arises from the fact that the holes in CdS QDs are highly

oxidative and could potentially oxidize water into oxygen, particularly under basic conditions (pH = 8).⁵⁵ Since MeOBenzOH is easier to oxidize than BenzOH and CF₃BenzOH, it could have outcompeted water oxidation more effectively. However, we can exclude this hypothesis in our study, as no O₂ formation was detected in the headspace of the photoreactor.

<table><caption>**Table 1** Photocatalysis results using TiO₂ and CdS-TiO₂ NPs in the presence of different alcohols (50 mM). All the experiments in Table 1 were conducted using a blue-LED lamp (~ 450 nm, 34 mW/cm²).</caption><thead><tr><th>Entry</th><th>Sample</th><th>SED</th><th>Alcohol</th><th>H<sub>2</sub><sup>a</sup></th><th>Ald<sup>a</sup></th></tr></thead><tbody><tr><td>1</td><td>TiO<sub>2</sub></td><td>TEOA</td><td>-</td><td>-</td><td>-</td></tr><tr><td>2</td><td>TiO<sub>2</sub></td><td>-</td><td>MeOBenzOH</td><td>-</td><td>-</td></tr><tr><td>3</td><td>CdS-TiO<sub>2</sub></td><td>TEOA</td><td>-</td><td>19 ± 1.5</td><td>-</td></tr><tr><td>4</td><td>CdS-TiO<sub>2</sub></td><td>-</td><td>MeOBenzOH</td><td>22 ± 1.2</td><td>20 ± 2.1</td></tr><tr><td>5</td><td>CdS-TiO<sub>2</sub><sup>b</sup></td><td>-</td><td>MeOBenzOH</td><td>-</td><td>-</td></tr><tr><td>6</td><td>CdS-TiO<sub>2</sub></td><td>-</td><td>BenzOH</td><td>10 ± 2.0</td><td>3 ± 0.4</td></tr><tr><td>7</td><td>CdS-TiO<sub>2</sub></td><td>-</td><td>CF<sub>3</sub>BenzOH</td><td>20 ± 1.6</td><td>6 ± 1.1</td></tr></tbody></table>

ᵃH₂ and aldehyde values are reported in µmol, upon 6h of irradiation;

ᵇWithout light irradiation.

Photo-corrosion and poor photostability during photocatalysis are major issues in this field.³⁷, ³⁸, ⁵⁶⁻⁶⁰ In an effort to examine the stability of this SILAR-made CdS NPs, cycling photocatalytic experiments for 6 h were performed. Upon the completion of each experiment the CdS-TiO₂ NPs were filtered, dried, and kept in the dark overnight. The next day, the NPs were re-dispersed in a fresh borate buffer solution containing 50mM of MeOBenzOH and irradiated for 6 h. Remarkably, the solar-driven H₂ evolution experiments illustrated that the NPs retained their catalytic activity and also possess great recyclability (Fig. 5), since the H₂ production remained at the same levels. More specifically, we observed a slight catalytic activity decrease of approximately 5% (23 µmol of H₂ the 1st day vs. 22 µmol of H₂ the 3rd day). It is worth noticing that another cycle of photocatalysis (4th day) could not be performed due to loss of NPs during the filtration step (see SI for details).

The photocatalytic stability and recyclability of non-modified CdS NPs in the literature is certainly an unusual feature. For this reason, methodologies including chemical doping, co-catalysts, and surface functionalization are employed to increase the photostability in similar systems. However, in our work the diverse preparation procedure of CdS-TiO₂ NPs, namely the SILAR methodology, clearly improved both the photocatalytic stability and recyclability of these NPs.

<figure><img src="image_6.png" /><figcaption>**Fig. 5** Comparison of cycle stabilities of CdS-TiO₂ using 50 mM of MeOBenzOH in borate buffer pH=8.</figcaption></figure>
X-ray photoelectron spectroscopy (XPS) was finally used to characterize the SILAR-made CdS-TiO₂ nanoparticles and more particularly to determine the relative energy position of CdS and TiO₂ conduction band. However, the characteristic energy difference of 456.0 eV between Ti 2p level and the valence band (VB) onset of TiO₂ (Fig. S5) is in agreement with previous works on n-TiO₂ anatase.⁶¹,⁶² The 403.8 eV energy difference between Cd 3d and the VB onset of the CdS-TiO₂ composite is slightly higher (Fig. S5) than the previously reported 403.5 eV for n-CdS,⁶³ or 403.4 eV,⁶⁴ indicating a priori less n-type behaviour. The VB onset of TiO₂ and CdS-TiO₂ were respectively measured at 2.6 eV and 1.0 eV (See Fig. 6). However, to have a clear picture of the interface energetics at the TiO₂ | CdS, the VB onset of the bare CdS is missing, since the synthesis of control CdS samples under the same conditions without TiO₂ support cannot be realised or will necessarily lead to different sizes, morphologies and surface states and consequently to different electronic properties.

We have identified the vectorial method as an innovative way to extract the valence band spectrum of these CdS NPs.⁶⁵ A linear combination of the spectra of pure TiO₂ and CdS-TiO₂ has been used to deduce the VB XPS spectrum of pure CdS. A comparison of the shape of the spectra, between the deduced CdS spectra and the spectrum of a reference CdS powder (bulk material), concludes that the method is reliable (See Fig. S6). The VB onsets of the CdS NPs were then deduced from the extracted spectra and measured at 1.0 eV for CdS-TiO₂. It shows that the onsets of the CdS are not altered by the presence of the oxide. It is also interesting to note that the A, B, and C states observed in the CdS-TiO₂ VB spectrum (See Fig. 6a) can be assigned to only TiO₂ for C, only CdS for A and a combination of both for B. C (7.0 eV) corresponds to the hybridized Ti-3d and O 2p bonding orbitals (σ).⁶⁶ A (2.9 eV) corresponds to the low energy state of the CdS valence band (X₅,ₗ – W₂,ₗ) made of S 3p and CdS.⁶⁷,⁶⁸ B (5.4 eV) is the sum of the high energy CdS valence band state (W₁,ₗ) and the non-bonding (π) hybridized orbital as well as the non-bonding O 2p orbital of TiO₂. The VB onset gives the energy difference between the valence band maximum and the Fermi level (i.e. binding energy at 0 eV). This information, combined with the optical band gaps measured by UV-Vis absorption spectroscopy (3.2 eV for TiO₂ and 2.4 eV for CdS), gives the electronic structure diagram depicted in Fig. 6.

<figure><img src="image_7.png" /><figcaption>**Fig. 6** a) Valence band spectra measured by XPS of TiO₂, CdS-TiO₂, and the deduced CdS with the vectorial method, b) electronic structure diagram for the TiO₂ | CdS heterojunction.</figcaption></figure>

Based on the energy diagram extracted from XPS measurements and the many reports on CdS-sensitized TiO₂,⁸,¹⁵ the most plausible operation mechanism for H₂ photoproduction begins with the initial absorption of a photon by the CdS QD. This absorption is followed by the injection of an electron into the conduction band of TiO₂ (Scheme 1). Indeed, from the above XPS measurements, there is a significant exergonic driving force for this process (ΔG<sub>inj</sub> = - 0.8 eV). Although TiO₂ is not inherently an efficient catalyst for H₂ evolution, the

electrochemical potential of the electrons in conduction band of TiO₂ is sufficiently negative to spontaneously reduce the protons into H₂. Finally, the holes residing in the VB of CdS oxidize the present alcohol molecules probably through a surface state activation mechanism.⁶⁹⁻⁷¹ Naturally, the adsorption on TiO₂ of a hydrogen evolution catalyst such as a non-noble molecular catalyst⁷² and/or an alcohol oxidation catalyst on CdS QD (anchored *via* a thiol derivative) such as TEMPO⁷³ should certainly enhance the overall photocatalytic performances of the above described system.

<figure><img src="image_8.png" /><figcaption>**Scheme 1.** Proposed photocatalytic mechanism for dual H₂ evolution and alcohol oxidation in the hybrid CdS/TiO₂ system.</figcaption></figure>

## Conclusions

In conclusion, this study demonstrates, for the first time, the successful utilization of the SILAR methodology for the growth of photocatalytic CdS quantum dots on TiO₂ isolated nanoparticles. The structural characterization of CdS-TiO₂ hybrids was conducted using Transmission Electron Microscopy (TEM), Energy-Dispersive X-ray Spectroscopy (EDS), and X-ray Photoelectron Spectroscopy (XPS). The photocatalytic performance of CdS-TiO₂ NPs was evaluated under visible light irradiation for dual alcohol oxidation into aldehydes with concomitant H₂ evolution. The system operates without any catalyst and does not require sacrificial electron donor. Remarkably, the photocatalytic system exhibited excellent stability, retaining over 95 % of its initial efficiency after three recycling cycles. Overall, this facile preparation approach for hybrid QD-TiO₂ NPs opens up exciting prospects for the development of QD-based dye-sensitized photocatalysis utilizing the many potential QD materials compatible with the SILAR technique. Expanding on these promising discoveries, our current research endeavors are directed towards developing QD-based sensitized systems using benign metals such as Copper, Indium, Gallium and others. This represents a pivotal advancement towards realizing highly efficient photocatalytic systems without any sacrificial reagent.

## Data availability

The data supporting this article have been included as part of the Supplementary Information.

## Conflicts of interest

There are no conflicts to declare.
## Acknowledgements

This project was supported by HORIZON TMA MSCA Postdoctoral Fellowships - European Fellowships (HORIZON-TMA-MSCA-PF-EF) action program under the call MSCA Postdoctoral Fellowships 2021 ((HORIZON-MSCA-2021-PF-01) to V.N., grant agreement No. 101064765, SOLAR-CAT. This study also received financial support under the EUR LUMOMAT project and the Investments for the Future program ANR-18-EURE-0012. C. Maheu thanks the Marie Skłodowska-Curie action for the project OMATSOLFUEL (n° 101105640). Measurements were performed using the CEISAM's and IMN's characterization platforms, AMaCC and PLASSMAT, Nantes, France.

## References

1. A. Rahman, O. Farrok and M. M. Haque, *Renew. Sust. Energ. Rev.*, 2022, **161**.

2. European Commission, https://climate.ec.europa.eu/eu-action/climate-strategies-targets/2030-climateenergy-framework_en).

3. N. S. Lewis and D. G. Nocera, *Proc. Natl. Acad. Sci.*, 2007, **104**, 20142-20142.

4. S. Martinez-Villarreal, M. Kammoun and A. Richel, *Curr. Opin. Green Sust.*, 2023, **39**.

5. D. Nocera, *Photosynth. Res.*, 2007, **91**, 133-133.

6. J. Radhakrishnan, S. Ratna and K. Biswas, *Inorg. Chem. Commun.*, 2022, **145**.

7. A. Fujishima and K. Honda, *Nature*, 1972, **238**, 37-38.

8. N. T. K. Chung, P. T. Nguyen, H. T. Tung and D. H. Phuc, *Molecules*, 2021, **26**, 2638.

9. I. Hod and A. Zaban, *Langmuir*, 2014, **30**, 7264-7273.

10. G. Shilpa, P. M. Kumar, D. K. Kumar, P. R. Deepthi, V. Sadhu, A. Sukhdev and R. R. Kakarla, *Mater. Sci. Energy Technol.*, 2023, **6**, 533-546.

11. A. Cerdán-Pasarán, T. López-Luke, I. Zarazúa, E. De la Rosa, R. Fuentes-Ramírez, K. C. Sanal and A. Alatorre-Ordaz, *J. Appl. Electrochem.*, 2019, **49**, 475-484.

12. Y.-L. Chen, Y.-H. Chen, J.-W. Chen, F. Cao, L. Li, Z.-M. Luo, I.-C. Leu and Y.-C. Pu, *ACS Appl. Mater. Interfaces*, 2019, **11**, 8126-8137.

13. H. Guo, B. Luo, J. Wang, B. Wang, X. Huang, J. Yang, W. Gong, Y. Zhou and X. Niu, *J. Mater. Chem.A*, 2020, **8**, 24655-24663.

14. Y.-L. Lee, C.-F. Chi and S.-Y. Liau, *Chem. Mater.*, 2010, **22**, 922-927.

15. D. Zhao and C.-F. Yang, *Renew. Sust. Energ. Rev. Reviews*, 2016, **54**, 1048-1059.

16. J.-C. Hu, J. Wu, B.-Y. Deng and F. Wang, *Mater. Chem. Front.*, 2023, **7**, 514-522.

17. G. Zhang, Y. Zhou, X. Fan, J. Zou, W. Dong and X. Xu, *J. Hydrogen Energy*, 2017, **42**, 19877-19884.

18. G. H. Carey, A. L. Abdelhady, Z. Ning, S. M. Thon, O. M. Bakr and E. H. Sargent, *Chem. Rev.*, 2015, **115**, 12732-12763.

19. P. V. Kamat, K. Tvrdy, D. R. Baker and E. J. Radich, *Chem. Rev.*, 2010, **110**, 6664-6688.

20. D. Ma, J. Chen, J. Li, X. Ji and J.-W. Shi, *J. Mater. Chem.A*, 2024, **12**, 12293-12324.

21. G. Sun, B. Xiao, H. Zheng, J.-W. Shi, S. Mao, C. He, Z. Li and Y. Cheng, *J. Mater. Chem.A*, 2021, **9**, 9735-9744.

22. A. Sahu, A. Garg and A. Dixit, *Sol. Energy.*, 2020, **203**, 210-239.

23. N. Gao, L. Huang, T. Li, J. Song, H. Hu, Y. Liu and S. Ramakrishna, *Journal of Applied Polymer Science*, 2020, **137**, 48443.

24. O. Abdelkarim, A. Mirzaei, G. S. Selopal, A. Yurtsever, G. Bassioni, Z. M. Wang, M. Chaker and F. Rosei, *Chem. Eng. J.*, 2022, **446**, 137312.

25. H. B. Yang, J. Miao, S.-F. Hung, F. Huo, H. M. Chen and B. Liu, *ACS Nano*, 2014, **8**, 10403-10413.

26. T. P. A. Ruberu, Y. Dong, A. Das and R. Eisenberg, *ACS Catal.*, 2015, **5**, 2255-225.

27. E. Benazzi, V. Cristino, R. Boaretto, S. Caramori and M. Natali, *Dalton Trans.*, 2021, **50**, 696-704.

28. J. Huang, B. Xu, L. Tian, P. B. Pati, A. S. Etman, J. Sun, L. Hammarström and H. Tian, *Chem. Commun.*, 2019, **55**, 7918-7921.

29. X. Su, Y. Chen, L. Ren, Y. He, X. Yin, Y. Liu and W. Yang, *ACS Sustain. Chem. Eng.*, 2019, **7**, 11166-11174.

30. B. Chon, S. Choi, Y. Seo, H. S. Lee, C. H. Kim, H.-J. Son and S. O. Kang, *ACS Sustain. Chem. Eng.*, 2022, **10**, 6033-6044.

31. H. Su, W. Wang, R. Shi, H. Tang, L. Sun, L. Wang, Q. Liu and T. Zhang, *Carbon Energy*, 2023, **5**, e280.

32. P. Sun, Z. Xing, Z. Li and W. Zhou, *Chem. Eng. J.*, 2023, **458**, 141399.

33. D. Romito, C. Govind, V. Nikolaou, R. J. Fernández-Terán, A. Stoumpidi, E. Agapaki, G. Charalambidis, S. Diring, E. Vauthey, A. G. Coutsolelos and F. Odobel, *Angew. Chem. Int. Ed.*, 2024, **63**, e202318868.

34. N. S. Mohamed Mustakim, C. A. Ubani, S. Sepeai, N. Ahmad Ludin, M. A. Mat Teridi and M. A. Ibrahim, *Sol. Energy.*, 2018, **163**, 256-270.

35. M. I. Aziz, F. Mughal, H. M. Naeem, A. Zeb, M. A. Tahir and M. A. Basit, *Mater. Chem. Phys.*, 2019, **229**, 508-513.

36. F. U. Hassan, U. Ahmed, M. Muhyuddin, M. Yasir, M. N. Ashiq and M. A. Basit, *Mater. Res. Bull.*, 2019, **120**, 110588.

37. Y.-J. Yuan, D. Chen, Z.-T. Yu and Z.-G. Zou, *J. Mater. Chem.A*, 2018, **6**, 11606-11630.

38. J. A. Nasir, Z. u. Rehman, S. N. A. Shah, A. Khan, I. S. Butler and C. R. A. Catlow, *J. Mater. Chem.A*, 2020, **8**, 20752-20780.

39. L.-f. Hong, R.-t. Guo, Y. Yuan, X.-y. Ji, Z.-d. Lin, Z.-s. Li and W.-g. Pan, *ChemSusChem*, 2021, **14**, 539-557.

40. C. Prasad, N. Madkhali, J. S. Won, J. E. Lee, S. Sangaraju and H. Y. Choi, *Mater. Sci. Eng. B.*, 2023, **292**, 116413.

41. M.-Y. Qi, M. Conte, M. Anpo, Z.-R. Tang and Y.-J. Xu, *Chem. Rev.*, 2021, **121**, 13051-13085.

42. X. M. C. Ta, R. Daiyan, T. K. A. Nguyen, R. Amal, T. Tran-Phu and A. Tricoli, *Adv. Energy Mater.*, 2022, **12**, 2201358.

43. F. Niu, W. Tu, Y. Zhou, R. Xu and Z. Zou, *EnergyChem*, 2023, **5**, 100112.

44. D. Tang, G. Lu, Z. Shen, Y. Hu, L. Yao, B. Li, G. Zhao, B. Peng and X. Huang, *J. Energy Chem.*, 2023, **77**, 80-118.

45. A. Maldotti, A. Molinari and R. Amadelli, *Chem. Rev.*, 2002, **102**, 3811-3836.

46. J. C. Colmenares, W. Ouyang, M. Ojeda, E. Kuna, O. Chernyayeva, D. Lisovytskiy, S. De, R. Luque and A. M. Balu, *Appl. Catal. B Environ.*, 2016, **183**, 107-112.

47. A. L. Flourat, J. Combes, C. Bailly-Maitre-Grand, K. Magnien, A. Haudrechy, J.-H. Renault and F. Allais, *ChemSusChem*, 2021, **14**, 118-129.

48. Z. Zheng, F. Han, B. Xing, X. Han and B. Li, *J. Colloid Interface Sci.*, 2022, **624**, 460-470.

49. P. Berton and J. L. Shamshina, *Pharmaceuticals-Base*, 2023, **16**.

50. S. R. Joshi, K. L. Kataria, S. B. Sawant and J. B. Joshi, *Ind. Eng. Chem. Res.*, 2005, **44**, 325-333.
51. Q. He, P. J. Miedziak, L. Kesavan, N. Dimitratos, M. Sankar, J. A. Lopez-Sanchez, M. M. Forde, J. K. Edwards, D. W. Knight, S. H. Taylor, C. J. Kiely and G. J. Hutchings, *Faraday Discuss.*, 2013, **162**, 365-378.

52. X. Xiang, B. Zhu, J. Zhang, C. Jiang, T. Chen, H. Yu, J. Yu and L. Wang, *Appl. Catal. B Environ.*, 2023, **324**, 122301.

53. W. W. Yu, L. Qu, W. Guo and X. Peng, *Chem. Mater.*, 2003, **15**, 2854-2860.

54. E. Nikoloudakis, P. B. Pati, G. Charalambidis, D. S. Budkina, S. Diring, A. Planchat, D. Jacquemin, E. Vauthey, A. G. Coutsolelos and F. Odobel, *ACS Catal.*, 2021, **11**, 12075-12086.

55. F. Su, J. Lu, Y. Tian, X. Ma and J. Gong, *Phys. Chem. Chem. Phys.*, 2013, **15**, 12026-12032.

56. B. Ningzhong, S. Liming, T. Tsuyoshi, L. Daling and D. Kazunari, *Chem. Lett.*, 2006, **35**, 318-319.

57. M. Sathish and R. P. Viswanath, *Catal. Today*, 2007, **129**, 421-427.

58. N. Bao, L. Shen, T. Takata and K. Domen, *Chem. Mater.*, 2008, **20**, 110-117.

59. L. Jia, D.-H. Wang, Y.-X. Huang, A.-W. Xu and H.-Q. Yu, *J. Phys. Chem. C*, 2011, **115**, 11466-11473.

60. C. Wang, L. Wang, J. Jin, J. Liu, Y. Li, M. Wu, L. Chen, B. Wang, X. Yang and B.-L. Su, *Appl. Catal. B Environ.*, 2016, **188**, 351-359.

61. M. Mitronika, C. Villeneuve-Faure, F. Massol, L. Boudou, W. Ravisy, M. P. Besland, A. Goullet and M. Richard-Plouet, *Appl. Surf. Sci.*, 2021, **541**, 148510.

62. V. Pfeifer, P. Erhart, S. Li, K. Rachut, J. Morasch, J. Brötz, P. Reckers, T. Mayer, S. Rühle, A. Zaban, I. Mora Seró, J. Bisquert, W. Jaegermann and A. Klein, *J. Phys. Chem. Lett.*, 2013, **4**, 4182-4187.

63. T. Schulmeyer, R. Kniese, R. Hunger, W. Jaegermann, M. Powalla and A. Klein, *Thin Solid Films*, 2004, **451-452**, 420-423.

64. T. Fang, H. Hu, J. Liu, M. Jiang, S. Zhou, J. Fu, W. Wang and Y. Yang, *J. Phys. Chem. C*, 2021, **125**, 18734-18742.

65. S. Béchu, B. Humbert, V. Fernandez, N. Fairley and M. Richard-Plouet, *Appl. Surf. Sci.*, 2018, **447**, 528-534.

66. C. Maheu, L. Cardenas, E. Puzenat, P. Afanasiev and C. Geantet, *Phys. Chem. Chem. Phys.*, 2018, **20**, 25629-25637.

67. M. Gansukh, Z. Li, M. E. Rodriguez, S. Engberg, F. M. A. Martinho, S. L. Mariño, E. Stamate, J. Schou, O. Hansen and S. Canulescu, *Sci. Rep.*, 2020, **10**, 18388.

68. A. Zunger and A. J. Freeman, *Phys. Rev. B*, 1978, **17**, 4850-4863.

69. X. Ning, S. Meng, X. Fu, X. Ye and S. Chen, *Green Chem.*, 2016, **18**, 3628-3639.

70. S. Zhang, W. Huang, X. Fu, G. Chen, S. Meng and S. Chen, *J. Hazard. Mater.*, 2018, **360**, 182-192.

71. W. Zhang, C.-C. Song, J.-W. Wang, S.-T. Cai, M.-Y. Gao, Y.-X. Feng and T.-B. Lu, *Chin. J. Catal.*, 2023, **52**, 176-186.

72. K. E. Dalle, J. Warnan, J. J. Leung, B. Reuillard, I. S. Karmel and E. Reisner, *Chem. Rev.*, 2019, **119**, 2752-2875.

73. J. E. Nutting, M. Rafiee and S. S. Stahl, *Chem. Rev.*, 2018, **118**, 4834-4885.