Solar Production of Hydrogen Peroxide with Carbon Quantum Dots

By Samudrapom DamReviewed by Susha Cheriyedath, M.Sc.Apr 27 2022

A team of researchers recently published a paper in the journal Advanced Science that demonstrated an efficient and eco-friendly hydrogen peroxide (H₂O₂) generation method based on piezoelectric semiconductors.

Study: Efficient Production of Solar Hydrogen Peroxide Using Piezoelectric Polarization and Photoinduced Charge Transfer of Nanopiezoelectrics Sensitized by Carbon Quantum Dots. Image Credit: Danijela Maksimovic/Shutterstock.com

Existing Ways to Synthesize H₂O₂

H₂O₂ is used extensively as an oxidant in several applications such as wastewater treatment, disinfection, and textile processing. H₂O₂ is traditionally produced on an industrial scale by anthraquinone oxidation. However, the production method leads to environmental problems due to the generation of large amounts of wastewater and solid waste.

Direct production of H₂O₂ by mixing oxygen (O₂) and hydrogen (H₂) is unsafe, owing to the risks of explosion. Thus, new methods must be developed to produce H₂O₂ in a sustainable, cost-effective, and safe manner.

Schematic preparation processes for BaTiO₃:Nb with different morphologies and modified with carbon quantum dots. Image Credit: Hedin, N et al., Advanced Science

The conversion of O₂ and water (H₂O) into H₂O₂ through the semiconductor-based photocatalysis method represents a feasible way to industrially produce H₂O₂. Although several semiconductor photocatalysts were developed to produce H₂O₂, such catalysts often produced very small amounts of H₂O₂ owing to the slow reaction kinetics, which necessitated the identification of new methods for photocatalytically induced and efficient synthesis of H₂O₂.

Recently, piezoelectrics gained considerable prominence as catalysts in redox reactions for H₂O₂ production and water splitting. Piezocatalysts are typically referred to the noncentrosymmetric crystals, such as bismuth ferrite and barium titanate (BaTiO₃). However, the reaction rates of piezocatalysts are low due to the periodic application of mechanical force and failure of related piezopotentials to initiate the critical Gibbs free energy threshold required for redox reactions.

New Method Proposed for H₂O₂ Generation

The coupling of photoelectric and piezoelectric effects in piezoelectric semiconductors is a proven and powerful approach for improving the catalytic activities for a range of solar-driven reactions. The piezopotential generated when the piezoelectric semiconductor is subjected to light irradiation and mechanical energy, such as ultrasound activation, can modulate the separation and migration of photoinduced charge carriers, which considerably enhances the catalytic abilities compared to photocatalysis without piezoelectric polarization, leading to higher H₂O₂ production rate.

However, the energy-band structure of piezoelectric semiconductors partially determines the piezoelectric photocatalytic efficiency.

Structural and electronic properties of various catalysts. a) Crystal structure of tetragonal BaTiO₃. b) X-ray diffraction patterns of pure carbon, BaTiO₃, BaTiO₃:Nb, and BaTiO₃:Nb/C. c) Raman spectra of BaTiO₃, BaTiO₃:Nb, and BaTiO₃:Nb/C. d) Diffuse reflectance ultraviolet–visible absorption spectra of the catalysts. e) Electrochemical Mott–Schottky curves of BaTiO₃, BaTiO₃:Nb, and BaTiO₃:Nb/C. f) Band structures of BaTiO₃ and BaTiO₃:Nb. Image Credit: Hedin, N et al., Advanced Science

BaTiO₃ has gained attention as a piezocatalyst owing to its exceptional piezoelectric properties. However, the wide bandgap of the semiconductor makes it unsuitable as a photocatalyst. Although doping bulk BaTiO₃ with niobium (Nb) and manganese (Mn) cations can reduce the bandgap width, the method also reduces the piezoelectric polarization intensity to a considerable extent.

Moderate dopant concentrations are typically effective in enhancing both piezophotocatalytic and piezocatalytic activities. However, few studies have focused on catalytic production of H₂O₂ under the piezophototronic effect using Nb-doped piezoelectrics.

Carbon-based nanoparticles, such as carbon quantum dots (CQDs) that are 2-10 nanometers in size, possess robust optical absorption properties in the near-visible region. CQDs are used almost exclusively for organic dye photodegradation and as photosensitizers. Although the role of CQDs in photocatalysis remained underexplored, the nanoparticles demonstrated their effectiveness in the photoreduction of carbon dioxide to formic acid.    

Synthesis, Characterization, and Evaluation of CQD-loaded Nb-doped BaTiO₃

In this study, researchers synthesized Nb-doped BaTiO₃ (BaTiO₃:Nb) with CQD photosensitizers and evaluated its effectiveness for the piezophotocatalytic production of H₂O₂ under ultrasound and visible light co-irradiation.

The hydrothermal method, followed by the post-annealing method, was used to fabricate both Nb-doped BaTiO₃ and undoped BaTiO₃ samples. The only difference during the synthesis of undoped BaTiO₃ samples was the removal of niobium pentachloride (NbCl₅). Another type of Nb-doped BaTiO₃ was also prepared by the same process using sodium oleate in place of poly (ethylene glycol) (PEG). 

CQDs were loaded to the as-synthesized BaTiO₃:Nb samples through the in-situ deposition method. BaTiO₃:Nb catalysts synthesized using PEG as additive were designated as BaTiO₃:Nb/C after CQD loading, while the catalysts synthesized using sodium oleate as additive were designated as BaTiO₃:Nb-C.

Field-emission scanning electron microscopy and transmission electron microscopy (FE-SEM and FE-TEM), X-ray diffraction (XRD), piezoresponse force microscopy (PFM), dynamic light scattering (DLS), electron paramagnetic resonance (EPR) spectroscopy, and Raman spectroscopy were used to characterize the synthesized samples. Finite method calculations were performed using COMSOL Multiphysics 5.4 with a steady-state study-based model of a piezoelectric device.

Finite element method simulation for the strain and piezopotential distribution on the surface of BaTiO₃ a,b) nanorod and c,d) nanoparticle with the cavitation pressure of 10⁸ Pa. Image Credit: Hedin, N et al., Advanced Science

Catalytic Production of H₂O₂

The synthesized BaTiO₃:Nb/C and BaTiO₃:Nb-C catalysts were mixed with a water-ethanol solution in a glass bottle and stirred in the dark for 10 minutes. The catalytic reactions were then performed under ultrasound activation with 40-kilohertz frequency, visible light irradiation with a wavelength of more than 420 nanometers, and both ultrasound and visible light co-irradiation to study piezo-, photo-, and piezophoto-catalysis reactions for the generation of H₂O₂, respectively. Subsequently, researchers evaluated the photocatalytic reactions for overall water splitting.  

Significance of the Study

BaTiO₃:Nb sensitized by CQDs were synthesized successfully. The catalyst was effective for the production of H₂O₂ from H₂O and O₂ in an ethanol solvent under visible light and ultrasound co-irradiation. The CQDs successfully absorbed visible light at wavelengths greater than 420 nanometers.

The H₂O₂ generation rates attained using BaTiO₃:Nb/C catalysts during piezophotocatalysis were 1360 micromole per gram per hour, with 0.14 percent of solar-to-chemical conversion (SCC) efficiency in 120 minutes of piezophotoreaction. The production rate was significantly higher than sole photo- and piezo-catalysis due to the synergistic coupling of photocatalysis and piezoelectric polarization.

The H₂O₂ generation rates were higher in piezocatalysis compared to photocatalysis as the piezoelectric polarization field was mediated by BaTiO₃:Nb, where the charge carriers from CQDs were injected into the BaTiO₃:Nb conduction band. These carriers can migrate to the catalyst particle surface without recombination and induce redox reactions at the liquid-solid interface. The attribution was verified when BaTiO₃:Nb/C with highly anisotropic geometries demonstrated substantially higher piezophotocatalytic activity compared to the BaTiO₃:Nb-C with spherical structures.

Equation-driven calculations demonstrated that the anisotropic geometry was primarily responsible for the high piezopotential. The CQD deposition improved the polarized charge transfer and conductivity of the synthesized catalysts, which facilitated the polarized charge-induced migration of surface-screening charges and thereby improved the piezocatalytic activities.

Taken together, the findings of this study demonstrated a new and effective approach based on nanopiezoelectrics for large-scale production of H₂O₂ using mechanical energy and visible light that holds the potential to replace the environmentally toxic synthesis method of anthraquinone oxidation.

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