Elsevier

Chemical Engineering Journal

Volume 426, 15 December 2021, 131298
Chemical Engineering Journal

Near-infrared, eco-friendly ZnAgInSe quantum dots-sensitized graphene oxide-TiO2 hybrid photoanode for high performance photoelectrochemical hydrogen generation

https://doi.org/10.1016/j.cej.2021.131298Get rights and content

Highlights

  • Near-infrared, eco-friendly ZnAgInSe QDs were used for PEC H2 generation.

  • Hybrid GO-TiO2 photoanodes showing efficient charge transfer were developed.

  • In-depth device mechanism of ZAISe QDs/TiO2-GO photoelectrode was investigated.

  • Optimized ZAISe QDs/TiO2-GO sample exhibits a photocurrent density of ~6.7 mA/cm2.

Abstract

Colloidal semiconductor quantum dots (QDs) are promising building-blocks for the manufacture of cost-effective photoelectrochemical (PEC) cells towards efficient solar-to-hydrogen conversion. Nevertheless, the state-of-the-art QDs-based PEC systems still suffer from the frequent utilization of highly toxic elements in QDs (Cd and Pb), hindering their future practical applications and potential commercialization. Here, we report a PEC device fabricated using eco-friendly, near-infrared (NIR) ZnAgInSe (ZAISe) QDs and hybrid TiO2/graphene oxide (GO) film. Based on the synergistic effect of QD’s broad light absorption and excellent charge extraction/transport properties of TiO2/GO film, as-assembled QDs-photoanode exhibits an outstanding saturated photocurrent density of ~6.7 mA/cm2 with good stability under standard 1 sun illumination. The introduction of functional GO can lead to the reduced charge transfer resistance, suppressed charge recombination, and enhanced electron transport within the QDs-TiO2 photoanodes. The results offer a facile and effective method to enhance the performance of environmentally friendly QDs-based PEC devices and shed light on the development of low-cost, “green” and high-efficiency solar-to-hydrogen conversion system.

Introduction

The growing worldwide energy requirements in the near future has become one of the greatest challenges to be addressed. It is significantly urgent to investigate clean, safe and sustainable energy resources to replace non-renewable energies and reduce the environment pollution. Among various sustainable energy resources, solar energy is regarded as the ideal energy source with unexhausted and environment-friendly features. However, to date, it is still challenging to efficiently convert the solar energy into “green” and efficient fuels such as hydrogen [1]. Solar-driven photoelectrochemical (PEC) hydrogen (H2) production, which directly uses solar energy to produce H2 from water, is one of the most attractive strategies to address the global energy demand issues [2], [3], [4], [5]. Generally, a classical PEC system is assembled by a semiconductor photoanode, electrolyte, and a counter electrode [6]. Up to now, various semiconductor materials with different morphologies, including Fe2O3, TiO2, Cu2O, Si and InP, etc. [7], have been investigated as photoanodes for PEC H2 generation. Because of its stability and appropriate energy band position, TiO2 nanoparticles are one of the most widely applied materials in classical PEC cells [8]. Unfortunately, the wide bandgap (~3.2 eV) of TiO2 renders the absorption region limited in the ultraviolet (UV) part (~5% of solar photons), which seriously impedes the solar-to-H2 efficiency [9]. Various strategies including narrow band gap semiconductor sensitization, doping, surface passivation and interface engineering etc. have been demonstrated as effective techniques to broaden the light absorption, optimize the charge dynamics and enhance the solar-to-H2 efficiency of TiO2-based photoelectrodes [10], [11], [12], [13], [14], [15], [16].

Colloidal semiconductor quantum dots (QDs) with unique size-, composition-, and shape-tunable optoelectronic properties are promising building blocks to sensitize TiO2 and as-formed heterostructures have demonstrated enhanced light absorption, tailored band structure and improved charge separation and transfer, which are very beneficial to achieve high efficiency solar-driven PEC systems [17], [18], [19], [20]. To date, QDs-based PEC systems have demonstrated outstanding light absorption, suitable band alignment and high solar energy conversion efficiency. Examples include CdS/CdSe and PbS/Mn-CdS QDs-based photoanodes with record photocurrent density of ~ 22 mA/cm2 [21], [22]. However, for most of the high performance QDs-PEC devices, the presence of hypertoxic heavy metals (e.g. Cd and Pb) can hamper their further commercialization and practical applications [22], [23]. Therefore, it is significant to investigate heavy metal-free and eco-friendly QDs for next-generation PEC H2 production systems. Unfortunately, the current reported eco-friendly near-infrared region (NIR) QDs-sensitized PEC devices still suffer from low efficiency [24] and there are two major approaches to improve the PEC capability of QDs-sensitized photoanode: (i) optimizing the optical properties of QDs including absorption region, photoluminescence (PL) lifetime, PL quantum yield (QY), etc.; (ii) designing the structure of photoanode for enhanced charge transfer.

In terms of the QDs, ternary colloidal I-III-VI group QDs are emerging as building-blocks in optoelectronics due to their low toxic nature and excellent optical properties [25], [26], [27]. Among them, environment-friendly AgInSe QDs possess a narrow band gap (~1.24 eV) and broad light absorption range reaching up to NIR region, which are regarded as the desired candidates for solar technologies [28]. As one of the most commonly used shell materials, the wide bandgap ZnS was generally grown on the naked QDs to passivate the surface defects, thus optimizing the optical properties of QDs for suppressed non-radiative recombination [29]. Nevertheless, a large lattice mismatch (~11%) of AgInSe and ZnS leads to a mass of strain-induced defects between the interface of core and shell materials [30]. Developing quaternary Zn-I-III-VI nanostructures is a more convenient and feasible approach than the traditional shell overcoating methods to obtain improved optical properties [31], [32]. In particular, eco-friendly quaternary ZnAgInSe (ZAISe) QDs have demonstrated a high QY over 50% for bioimaging [33], thus exhibiting the efficient generation of photo-excited charge carriers with potential application in solar-driven PEC cells.

As for traditional TiO2 photoanode in PEC systems, considering the inherent morphology of mesoporous TiO2, the photoexcited electrons have to traverse the grain boundaries of stacked TiO2 during the device operation, thus giving rise to the undesired recombination of photoexcited electron-hole pairs [34]. Therefore, the fabrication of a novel structured photoanode with suppressed charge recombination and outstanding charge transport properties is an effective way to acquire high-efficiency PEC devices [35]. In view of the traits of low-cost and simple synthetic technique, the incorporation of two-dimensional (2D) materials [such as MoS2, WS2, graphene, and graphene oxide (GO)] shows great promise to enhance electron transport in optoelectronic devices [36], [37], [38], [39]. Specifically, GO with unique structural and optoelectronic properties, including large specific surface area, outstanding mechanical strength, conductivity, and optical transparency, have been widely applied in solar-to-fuel conversion devices [40], [41]. Kusuma et al. used GO as the electron-conducting network to boost the power conversion efficiency (PCE) of QDs-sensitized solar cells (QDSSCs) [42]. Xu et al. reported a novel perovskite/GO photocatalyst for CO2 reduction and obtained a high conversion rate by improving electron extraction and transport [43]. Furthermore, the oxygen-containing group of GO is beneficial to anchor TiO2 nanoparticles during synthetic process and form a homogeneous film, which is conducive to mutual contact between the two components, thereby increasing the charge transfer efficiency [44], [45]. Therefore, the incorporation of GO as the high-efficiency charge transmission path in the eco-friendly QDs-sensitized TiO2 PEC device is promising for boosting the performance, while the related investigation is still lacking.

In this work, we demonstrate the manufacture of a PEC device via employing environmentally friendly NIR ZAISe QDs-sensitized TiO2-GO hybrid structure as photoanode for H2 generation. As-synthesized ZAISe QDs exhibit broad NIR light absorption (over 840 nm) and suitable band alignment with TiO2-GO hybrid films for efficient charge carriers’ separation/transfer. The introduction of moderate amount of GO in QDs-sensitized TiO2 leads to enhanced charge collection/transport efficiency and suppressed charge recombination as compared to QDs/pure-TiO2 sample. As a result, such NIR, eco-friendly ZAISe QDs/TiO2-GO photoanodes with optimized GO amount (0.015 wt%) can deliver a saturated photocurrent density of ~ 6.7 mA/cm2 with good stability under standard one sun illumination (AM 1.5G, 100 mW/cm2). Our results indicate that the rational structural design of NIR, eco-friendly QDs-sensitized photoelectrode is promising to realize high-efficiency solar-to-fuel conversion PEC systems.

Section snippets

Materials

GO was prepared by a modified Hummers method [46]. Zinc acetate (Zn(Ac)2·2H2O), Silver (I) acetate (Ag(Ac)), Indium (III) acetate (In(Ac)3), Selenium (Se), Sodium Sulfide (Na2S), Sodium Sulfite (Na2SO3·9H2O), 1-Octadecene (ODE), 1-Dodecanethiol (DDT), Oleic acid (OA), Oleylamine (OLA), Hexadecyltrimethylammonium bromide (CTAB), Toluene, Acetone, Ethanol, and Methanol were purchased from Sigma-Aldrich. Zirconium dioxide (ZrO2) was obtained from Aladdin. Ti-Nanoxide Blocking-Layer/Spin-Coating

Fabrication and structure of ZAISe QDs/TiO2-GO photoanodes

The fabrication process of ZAISe/TiO2-GO hybrid photoanode is briefly shown in Scheme 1. ZAISe QDs were first prepared via a wet-chemical synthesis method (Scheme 1a) reported in the literature [33]. The TiO2 blocking layer (BL) was then prepared on the FTO substrates by spin-coating and the TiO2-GO mesoporous films were further prepared by tape-casting the TiO2-GO mixture paste (Scheme 1b), wherein the film thicknesses can be controlled by varying the tape thickness. Afterwards, the ZAISe/TiO2

Conclusion and perspectives

In conclusion, eco-friendly NIR ZAISe QDs are prepared and used to sensitize TiO2 photoanodes with different GO concentration for high-efficiency PEC H2 production. As-fabricated ZAISe QDs/TiO2 photoanode exhibited an outstanding saturated photocurrent density of ~ 5.4 mA/cm2 under one sun illumination (AM 1.5G, 100 mW/cm2) due to the broad NIR light absorption of ZAISe QDs and appropriate band alignment with TiO2. The PEC performance is further optimized by introducing different concentrations

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

X.T. acknowledges support from the National Key Research and Development Program of China (Grant No. 2019YFE0121600) and the Sichuan Science and Technology Program (Grant No. 2021YFH0054). Z.M.W. acknowledges the National Key Research and Development Program of China (Grant No. 2019YFB2203400) and the “111 Project” (Grant No. B20030). Z.M.W. is also grateful to the Universiti Kebangsaan Malaysia for a distinguished international Professor appointment. A.V. acknowledges the Kempe Foundation, the

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