Nanophase-photocatalysis: loading, storing, and release of H₂O₂ using graphitic carbon nitride

Abstract

A blue light mediated photochemical process using solid graphitic carbon nitride (g-C₃N₄) in ambient air/isopropanol vapour is suggested to be linked to “nanophase” water inclusions and is shown to produce approx. 50 μmol H₂O₂ per gram of g-C₃N₄, which can be stored in the solid g-C₃N₄ for later release for applications, for example, in disinfection or anti-bacterial surfaces.

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Graphitic carbon nitride (g-C₃N₄, see Fig. 1A, first synthesised by Berzelius and named by Liebig¹) has a layered structure with many defects that can be produced at low cost and is employed in a number of important applications.^2,3 Here, we explore the potential of g-C₃N₄ for the formation, storage, and release of hydrogen peroxide (H₂O₂). The photocatalytic formation of H₂O₂ has been widely reported⁴ and is useful to disinfect,⁵ bleach,⁶ or clean environments.⁷ In addition to effective production of H₂O₂ based on photo-catalysis⁸ from ambient oxygen and a hole quencher, there are reports of piezo-catalysis,⁹ which is indicative of a broader range of molecular activation processes for g-C₃N₄ at the microscopic scale. Singlet oxygen formation with g-C₃N₄ has been observed.¹⁰

Fig. 1 (A) Molecular structure (idealised) for g-C₃N₄. (B) Scanning electron micrograph for g-C₃N₄ powder. (C) X-ray diffraction data for g-C₃N₄ (i) before and (ii) after blue light treatment in the presence of isopropanol vapour.

Previously, when exploring the photochemical reactivity of g-C₃N₄ (as a solid powder or when immobilising g-C₃N₄ particles into an intrinsically microporous host material PIM-1)¹¹ immersed in a liquid phase, we observed H₂O₂ production in the aqueous phase in the presence of hole quenchers such as glucose or Triton X-100. Here, isopropanol vapour is employed in order to provide a quencher during the corresponding solid-state photochemical process. In contrast to previous studies, here the photocatalytic process is performed with the g-C₃N₄ powder exposed to light in ambient air. H₂O₂ is released only later in the absence of light upon contact to water (eqn (1) and (2)).

g-C₃N₄(H₂O) + hv/quencher/O₂ → g-C₃N₄(H₂O₂) (1)

g-C₃N₄(H₂O₂) → g-C₃N₄ + H₂O₂(aq) (2)

In a typical experiment,‡ 50 mg of yellow g-C₃N₄ powder, possessing a flaky layered morphology (Fig. 1B), with surface area = 36.4 m² g⁻¹ (based on nitrogen adsorption data);¹¹ was placed into a Petri dish with a transparent cover and illuminated with a blue light source (Thorlabs M385LP1 with typically 80 mW cm⁻² 385 nm light in approx. 2 cm distance).¹¹ A small vial containing liquid isopropanol was placed into the same Petri dish to maintain an atmosphere of isopropanol vapour as a quencher for holes that are generated by photoexcitation. After a 30 minute treatment, the yellow powder was recovered and then stored in a glass vial. Data from X-ray diffraction analysis (Fig. 1C) show essentially the same features before and after photochemical treatment, which is consistent with literature data for layered g-C₃N₄ (for disordered heptazine-based polymer organized in layers with a separation of 0.326 nm).¹² The main (002) peak occurs at 27.5°. Although literature reports indicate that under long term photo-irradiation conditions endo-peroxide signals should be seen,¹³ the data reported here for Raman analysis, surface enhanced Raman, and XPS surface analysis (see Fig. S2–S8, ESI†) all confirm that there is no significant structural change in the bulk, or on the surface, of the g-C₃N₄ after blue light treatment. Most likely, the endo-peroxide content is too low to be detected under these reaction conditions, although a shift from C–N to C–O at% in XPS data (see ESI,† Fig. S2) appears to be linked to the formation of products from the photochemical process on the g-C₃N₄ surface.

Data in Fig. 2A demonstrate the production of H₂O₂ (determined with para-nitrophenol and LC/MS detection; ESI†) when placing the photoactivated g-C₃N₄ powder into pure water. For the activated g-C₃N₄, typically 100–200 μM H₂O₂ in 2 cm³ water are released, whereas only traces of H₂O₂ are released without prior photoactivation. The release of H₂O₂ occurs immediately upon immersion and does not continue with time. Adding more g-C₃N₄ increases the H₂O₂ concentration in an approximately linear fashion. The number of moles of H₂O₂ released suggests approx. one H₂O₂ molecule for every 300 heptazine units in the g-C₃N₄ powder (or 0.06 wt% H₂O₂) assuming a bulk reaction. Fig. 2B shows data for the release of H₂O₂ (i) for g-C₃N₄ powder and (ii) for g-C₃N₄@PIM-1 composite (employing 17 wt% intrinsically microporous polymer PIM-1 to give a film on filter paper; see ESI†). As in previous studies,^11,14 PIM-1 is employed as molecularly rigid material to make the photocatalyst easily accessible to the aqueous phase and allows easy recovery for re-use of the catalyst. Fig. 2B shows that both g-C₃N₄ powder and polymer-embedded g-C₃N₄ exhibit similar H₂O₂ release.

Fig. 2 (A) Production of H₂O₂ with g-C₃N₄ powder suspended in 2 mL water (after 5 minutes) (i) as prepared catalyst and (ii) blue light/isopropanol vapour treated g-C₃N₄ (30 minutes). (B) H₂O₂ release (i) with g-C₃N₄ and (ii) with g-C₃N₄@PIM-1 dispersed in 2 mL water (5 minutes) both after blue light treatment. (C) Repeat measurements with g-C₃N₄@PIM-1 (5 mg) comparing the effects of (i) just intermittent drying in ambient light and (ii) intermittent treatment with blue light in isopropanol vapour. Error bars estimated ±10%.

The data in Fig. 2C demonstrate the re-use of g-C₃N₄@PIM-1 with/without additional blue light treatment for up to five cycles. The intermittent blue light treatment clearly enhances the H₂O₂ release so that 0.5 mM H₂O₂ can be produced with only a few reaction cycles. The formation and release of H₂O₂ under blue light conditions can be discussed in terms of the following reaction steps:

g-C₃N₄ + hv → g-C₃N₄* (3)

g-C₃N₄* + isopropanol → g-C₃N₄⁻ + H⁺(aq) + products (4)

2 g-C₃N₄⁻ + O₂ + 2 H⁺ → 2 g-C₃N₄(H₂O₂) (5)

g-C₃N₄(H₂O₂) → g-C₃N₄ + H₂O₂(aq) (6)

Photoexcitation of g-C₃N₄ leads to an excited state (eqn (3)), which after charge separation produces a hole and an electron. The hole reacts with the isopropanol quencher to give protons and products (eqn (4)). The electrons, which have been reported to generate a strong electron paramagnetic resonance and blue coloration,¹⁵ can combine with ambient oxygen and protons to give trapped H₂O₂ (eqn (5)). The g-C₃N₄(H₂O₂) material can be stored for several weeks without significant loss of H₂O₂ content (Fig. 3B), and its eventual immersion into water releases H₂O₂ (eqn (6)). Thermogravimetric analysis of g-C₃N₄ before and after photoreaction (see ESI,† Fig. S9) reveals the presence of water (approx. 2.5 wt%), which is likely to be present as a “nanophase” in between layers and likely to play a crucial role (see eqn (4) during the solid state photochemical formation of H₂O₂. Heating the g-C₃N₄(H₂O₂) sample to 150 °C removes H₂O and immediately destroys the H₂O₂. Intriguingly, a vacuum treatment (2h, approx. 10 mTorr) at room temperature also removes the H₂O₂.

Fig. 3 (A) Test solutions containing 2.5 mL water, 0.5 mL DMSO, 0.1 M CH₃COOH, 0.5 mM TMB, with (i) no addition, with (ii) 50 μM H₂O₂ as a reference, and with (iii) 0 mg, (iv) 5 mg, (v) 10 mg, (vi) 15 mg, (vii) 20 mg, (viii) 25 mg of photoactivated g-C₃N₄. (B) Plot of release of H₂O₂ (10 mg g-C₃N₄ in 2 mL water) versus time after blue light/isopropanol treatment (error bars based on one standard deviation and triplicate measurements).

The release of H₂O₂ into aqueous media can be visualised with the 3,3′,5,5′-tetramethylbenzidine (TMB) colour reaction.¹⁶ The g-C₃N₄ treated with blue light and isopropanol was added 2 cm³ of deionized water. After 5 minutes, the solution was filtered with a 0.2 μm filter. The solution is added to the vial containing TMB and 0.1 M CH₃COOH. Fig. 3A shows photographs of the test solutions (i) without and (ii) with 50 μM H₂O₂ intentionally added as reference. Samples (iii) to (viii) correspond to added weights (0, 5, 10, 15, 20, 25 mg) of g-C₃N₄ photochemically charged g-C₃N₄. A Nafion-impregnated filter paper was employed to bind the cationic oxidised form of TMB to amplify the blue colour signal. The colour reaction is consistent with the analysis of H₂O₂ concentration by mass spectrometry and suggests H₂O₂ release in the 50 to 500 μM range.

In conclusion, active sites for photo-generated H₂O₂ binding/storage in the solid g-C₃N₄ material appear to exist (approx. one bound H₂O₂ per 300 heptazine units), which are probably associated with nanophase water. Once bound, H₂O₂ in g-C₃N₄ remains stable over prolonged periods of time (Fig. 3B). However, a short vacuum treatment at room temperature or a heat treatment at 150 °C removes both the liquid nanophase H₂O and any trapped H₂O₂.

Coatings of g-C₃N₄ could be employed to release H₂O₂ upon contact with water, for example as a disinfectant in hospital environments.²⁰ Further studies of the nature of the H₂O₂ binding interaction, the effects of humidity and the nature of the hole quencher, and further g-C₃N₄ structural engineering provide future potential to achieve higher levels of H₂O₂ to be bound, stored, and released. Other types of photochemical gas conversions could be performed under illumination in solid state and with aqueous nanophase entrapment.

Akalya Karunakaran: data curation; formal Analysis; investigation; methodology; writing – review & editing. Katie J. Francis: data curation; investigation. Chris R. Bowen and Richard J. Ball: conceptualization; methodology; supervision; writing – review & editing. Yuanzhu Zhao and Lina Wang: data curation; investigation; writing – review & editing. Neil B. McKeown and Mariolino Carta: conceptualization; supervision; writing – review & editing. Philip J. Fletcher, Remi Castaing, and Mark Isaac: data curation; investigation; writing – review & editing. Laurence J. Hardwick: formal analysis; supervision; writing – review & editing. Gema Cabello and Igor V. Sazanovich: data curation; formal analysis; investigation; methodology; writing – review & editing. Frank Marken: conceptualization; project administration; resources; supervision; writing – original draft.

A. K. thanks the National Overseas Scholarship-India. L. J. H. and G. C. were supported by the UK Faraday Institution (EPSRC EP/S003053/1) through the Degradation Project (grant numbers FIRG001 and FIRG024). STFC is acknowledged for beam time on the ULTRA facility to carry out Kerr-gated experiments. We thank Alexander Cowan (University of Liverpool) for helpful discussion.

Conflicts of interest

There are no conflicts of interest to declare.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3cc01442h

‡ Experimental: g-C₃N₄ was prepared as reported previously.¹⁷ PIM-1 was prepared following a literature method.^18,19 Films of g-C₃N₄@PIM-1 were obtained by drop casting. An amount of 5 mg of graphitic carbon nitride (measured with a Ohaus PX224 analytical balance) was added to the 1 cm³ solution of 1 mg PIM-1 in chloroform and sonicated for 15 minutes. The solution was drop-casted on filter paper with a size of 2 × 2 cm². The sample was air dried to leave a layer of g-C₃N₄@PIM-1 on the filter paper (thickness approx. 0.25 mm). Further experimental details are provided in the ESI.†

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