The escalating depletion of fossil fuels and the concomitant environmental degradation have intensified the global pursuit of sustainable and clean energy alternatives. Photoelectrochemical (PEC) cells, which directly convert solar energy into chemical fuels or electricity, represent a promising technology in this endeavor. Since the pioneering work on dye-sensitized TiO2 solar cells, metal oxide semiconductors such as TiO2, ZnO, and SnO2 have been extensively investigated as photoanode materials. However, their utility in harvesting the broad solar spectrum is fundamentally limited by their wide bandgap, which confines light absorption primarily to the ultraviolet region.
A prevalent strategy to circumvent this limitation involves coupling these wide-bandgap oxides with narrow-bandgap semiconductor sensitizers. This approach not only extends the spectral response into the visible light region but also can facilitate more efficient separation of photogenerated electron-hole pairs. Historically, II-VI compounds like CdS and PbS have been favored due to their optimal band alignment and good PEC performance. Nevertheless, the inherent toxicity of elements like cadmium and lead raises significant environmental and health concerns, hindering their large-scale, sustainable application. Therefore, the exploration of non-toxic, earth-abundant alternative sensitizers is of paramount importance. Indium sulfide (In2S3), a member of the III-VI semiconductor family, emerges as a highly promising candidate. It possesses a suitable bandgap (~2.0-2.3 eV), excellent photoconductivity, good stability, and high transmission in the visible spectrum, making it an ideal sensitizer for **thin film solar panel** architectures and related photoelectrochemical devices.
In this work, we demonstrate the fabrication and comprehensive characterization of a novel heterostructured photoanode based on In2S3-sensitized ZnO nanosheet arrays. The primary objective is to engineer a non-toxic, efficient photoelectrode by leveraging the synergistic effects between the vertically aligned ZnO nanostructure and the visible-light-absorbing In2S3 layer. We detail a facile, solution-processable synthesis route and provide an in-depth analysis of the structural, morphological, optical, and most critically, the photoelectrochemical properties of the resulting heterostructure. Our findings confirm a substantial enhancement in photocurrent generation, paving the way for more environmentally benign materials in **thin film solar panel** technologies.
Experimental Methodology: Synthesis and Fabrication
The fabrication of the In2S3/ZnO heterostructure was achieved through a sequential two-step chemical process, as outlined below and summarized in Table 1.
| Step | Process | Key Parameters / Solutions | Condition / Cycle |
|---|---|---|---|
| 1. Substrate Preparation | Ultrasonic Cleaning | Acetone, Ethanol, Deionized (DI) Water | 30 min each, dried at 50°C |
| 2. ZnO Seed Layer | SILAR Deposition | 0.005 M Zinc Acetate in Ethanol, Ethanol Rinse | 5 cycles (Dip-Dry per cycle) |
| Thermal Annealing | — | 350°C for 20 min (after each 5 SILAR cycles) | |
| Final Crystallization | — | 500°C for 30 min (Ramp: 5°C/min) | |
| 3. ZnO Nanosheet Growth | Hydrothermal Reaction | 0.5 M Zinc Acetate, 0.01 M NaOH, 0.05 g Sodium Citrate in DI Water | 95°C for 12 hours |
| 4. In2S3 Sensitization | SILAR Deposition | 0.00625 M InCl3 (30s) → DI Water Rinse (30s) → 0.009375 M Na2S (30s) → DI Water Rinse (30s) | 3 to 9 cycles |
| 5. Post-treatment | Annealing | Nitrogen (N2) atmosphere | 300°C for 30 min |
1. Growth of Vertically Aligned ZnO Nanosheet Arrays
The process began with the deposition of a ZnO seed layer on fluorine-doped tin oxide (FTO) substrates using the successive ionic layer adsorption and reaction (SILAR) technique. The cleaned FTO was subjected to multiple cycles of dipping in a zinc acetate precursor solution followed by an ethanol rinse and drying. This was intermittently annealed at 350°C to decompose the precursor and form crystalline ZnO nuclei. A final high-temperature anneal at 500°C ensured a robust and well-crystallized seed layer. Subsequently, vertically aligned ZnO nanosheets were grown via a hydrothermal method. The seeded substrate was immersed in an aqueous solution containing zinc acetate, sodium hydroxide, and sodium citrate (as a structure-directing agent) and maintained at 95°C for 12 hours. The growth mechanism can be described by the following reactions:
$$ \text{Zn(CH}_3\text{COO)}_2 + 2\text{OH}^- \rightarrow \text{Zn(OH)}_2 + 2\text{CH}_3\text{COO}^- $$
$$ \text{Zn(OH)}_2 \rightarrow \text{ZnO} + \text{H}_2\text{O} $$
The citrate ions selectively adsorb on certain crystal facets, promoting two-dimensional growth and leading to the nanosheet morphology.
2. Deposition of In2S3 Sensitizer Layer
The as-grown ZnO nanosheet arrays were then sensitized with In2S3 using the SILAR method to ensure uniform and conformal coating. The sample was sequentially immersed in cationic (indium chloride, InCl3) and anionic (sodium sulfide, Na2S) precursor solutions, with intermediate DI water rinsing steps to remove loosely adsorbed ions. Each complete sequence (In3+ → rinse → S2- → rinse) constitutes one SILAR cycle. The deposition reaction is based on the ionic adsorption and subsequent reaction:
$$ 2\text{In}^{3+} + 3\text{S}^{2-} \rightarrow \text{In}_2\text{S}_3 $$
The number of SILAR cycles (n) was varied from 3 to 9 to optimize the sensitizer layer thickness. Finally, the heterostructure films were annealed at 300°C in a N2 atmosphere to improve the crystallinity and interfacial contact of In2S3, a critical step for enhancing charge transport in the final **thin film solar panel** electrode.
Structural and Morphological Characterization
The crystalline structure of the synthesized films was examined using X-ray diffraction (XRD). The diffraction pattern for the heterostructure, as shown in a representative analysis, confirms the successful formation of a composite material. Peaks corresponding to the hexagonal wurtzite phase of ZnO (JCPDS No. 36-1451) are prominently observed, with the (110) reflection showing particularly high intensity, indicating a preferred growth orientation of the nanosheets. Concurrently, distinct diffraction peaks are indexed to the cubic phase of β-In2S3 (JCPDS No. 65-0459), such as the (311), (440), and (620) planes. The coexistence of these phases without detectable impurity peaks validates the formation of the In2S3/ZnO heterostructure.
The evolution of morphology is crucial for the performance of a **thin film solar panel** photoanode. Scanning electron microscopy (SEM) images reveal the transformation. The pristine ZnO film consists of well-defined, vertically aligned nanosheets with widths of ~100 nm and lengths of ~500 nm, creating a highly porous and interconnected network with an approximate thickness of 3 μm. This three-dimensional architecture offers a large surface area for subsequent sensitizer deposition. After the SILAR deposition of In2S3, the fundamental nanosheet framework is preserved. However, the In2S3 layer uniformly coats the surface of individual ZnO nanosheets and infiltrates the inter-sheet spaces. This coating increases the apparent thickness of the nanosheets to about 200 nm and bridges the gaps between them, while still maintaining a porous structure essential for electrolyte penetration in a PEC cell.
Optical Absorption Properties
The light-harvesting capability of a photoanode is a decisive factor for its efficiency. Ultraviolet-visible (UV-Vis) absorption spectroscopy was employed to study the optical properties. The absorption spectrum of bare ZnO nanosheet arrays shows a sharp absorption edge near 370 nm, characteristic of its wide bandgap (~3.3 eV). Sensitization with In2S3 induces a profound change. All In2S3/ZnO heterostructures exhibit significantly enhanced absorption across the measured spectrum (300-550 nm) with a noticeable red-shift of the absorption onset into the visible region. This extension is directly attributable to the narrower bandgap of In2S3. The absorption intensity generally increases with the number of SILAR cycles (n) from 3 to 7, due to the increased loading of the light-absorbing In2S3 material. The bandgap energy (Eg) can be estimated from the Tauc plot using the relation for direct semiconductors:
$$ (\alpha h\nu)^2 = A(h\nu – E_g) $$
where α is the absorption coefficient, hν is the photon energy, and A is a constant. The analysis confirms a reduction in the effective optical bandgap of the heterostructure compared to pure ZnO, which is highly beneficial for designing a broadband **thin film solar panel** absorber.
Photoelectrochemical Performance Evaluation
The PEC performance of the fabricated electrodes was evaluated in a standard three-electrode configuration under simulated AM 1.5G solar illumination (100 mW/cm²). Aqueous sodium thiosulfate (Na2S2O3) was used as a sacrificial electrolyte to scavenge photogenerated holes.
Current Density-Voltage (J-V) and Transient Photocurrent (I-t) Analysis
The linear sweep voltammetry (J-V) curves reveal the photocurrent generation capability. The bare ZnO nanosheet electrode produces a modest photocurrent density. In stark contrast, all In2S3/ZnO heterostructure electrodes demonstrate dramatically enhanced photocurrents. The performance shows a clear dependence on the In2S3 deposition cycles, optimizing at n=7. This sample achieves a maximum photocurrent density approximately four times greater than that of the pristine ZnO electrode. The transient photocurrent response (I-t) measured at a constant bias under chopped light illumination provides further insight. All heterostructure electrodes show rapid and reproducible photocurrent spikes upon illumination, indicating efficient and swift separation of photogenerated carriers at the In2S3/ZnO interface. The steady-state photocurrent values from the I-t curves corroborate the trend observed in the J-V measurements, with the n=7 sample yielding the highest value.
The initial increase in photocurrent with SILAR cycles (up to n=7) is attributed to improved light absorption and increased interfacial area for charge generation. However, beyond the optimum cycle number (n=9), the photocurrent declines. This is likely due to the formation of an excessively thick In2S3 layer, which increases the charge transport distance and the probability of charge carrier recombination before reaching the ZnO collector. Furthermore, a thicker, less-ordered layer may introduce higher series resistance, impeding electron injection and transport—a key consideration in the design of efficient **thin film solar panel** modules where charge collection is paramount.
Electrochemical Impedance Spectroscopy (EIS)
To elucidate the charge transfer dynamics at the electrode/electrolyte interface, EIS was performed. The Nyquist plots typically consist of a semicircle in the high-frequency region, the diameter of which corresponds to the charge transfer resistance (Rct) at this interface. The EIS data for the champion In2S3/ZnO (n=7) heterostructure exhibits a significantly smaller semicircle diameter compared to the bare ZnO electrode. This reduction in Rct indicates a more efficient charge injection process from the photoexcited In2S3 into the ZnO backbone and subsequently to the external circuit via the FTO substrate. The enhanced interface kinetics, facilitated by the type-II band alignment between In2S3 and ZnO, is a direct contributor to the superior photocurrent generation. The equivalent circuit used for fitting the EIS data often includes solution resistance (Rs), charge transfer resistance (Rct), and a constant phase element (CPE), modeling the interfacial capacitance. The simplified relation for the characteristic frequency (fmax) of the charge transfer process is given by:
$$ f_{\text{max}} = \frac{1}{2\pi R_{ct} C} $$
where C is the interfacial capacitance. The observed shift to higher fmax for the heterostructure further confirms faster charge transfer kinetics.
Mechanism of Enhanced Performance
The superior PEC performance of the In2S3/ZnO heterostructure can be understood through its energy band diagram and the ensuing carrier dynamics, as illustrated in Figure 1. Under visible light illumination, photons with energy greater than the bandgap of In2S3 are absorbed, exciting electrons from its valence band (VB) to its conduction band (CB). The favorable band alignment at the In2S3/ZnO interface creates a type-II heterojunction. Specifically, the CB edge of In2S3 is positioned at a more negative potential than that of ZnO, while its VB edge is more positive. This thermodynamic driving force promotes the injection of photogenerated electrons from the CB of In2S3 into the CB of ZnO. Simultaneously, the photogenerated holes remain in the VB of In2S3 and are consumed by the hole scavenger (S2O32-) in the electrolyte.
This spatial separation of electrons and holes across the hetero-interface effectively suppresses their recombination, thereby increasing the lifetime and collection efficiency of the photogenerated carriers. The vertically aligned ZnO nanosheet array serves as an excellent electron highway, providing a direct and rapid conduction path for the injected electrons to reach the FTO current collector. This synergistic combination—visible light absorption by In2S3, efficient charge separation at the interface, and unidirectional electron transport through the ZnO nanoarchitecture—is the cornerstone of the enhanced photoresponse. This principle is directly applicable to the engineering of sensitized nanostructures for next-generation **thin film solar panel** technologies.
Conclusion and Perspectives
In summary, we have successfully fabricated a non-toxic In2S3/ZnO heterostructured photoanode via a facile and scalable two-step chemical synthesis. The heterostructure comprises a vertically aligned ZnO nanosheet array uniformly sensitized with a cubic-phase In2S3 layer. This design capitalizes on the large surface area and direct electron transport path of the ZnO nanostructure and the visible-light-harvesting capability of In2S3. Comprehensive characterization confirms the extended optical absorption range and, more importantly, a quadrupling of the photocurrent density in the optimized heterostructure compared to pristine ZnO. Electrochemical impedance analysis reveals significantly reduced charge transfer resistance, indicating highly efficient interfacial kinetics.
This work underscores the potential of using environmentally benign In2S3 as a highly effective sensitizer for wide-bandgap metal oxide semiconductors. The demonstrated performance enhancement stems from the rational design of a type-II heterojunction that facilitates superior charge separation and transport. Future work could focus on further optimizing the interface quality through surface passivation, exploring the integration of this heterostructure into full solar cell devices, or coupling it with other narrow-bandgap materials to form tandem absorbers for even broader spectral utilization. The insights gained from this study contribute valuable knowledge towards the development of efficient, sustainable, and cost-effective materials for advanced **thin film solar panel** applications and solar fuel generation.