Pharmaceutical pollutant as sacrificial agent for sustainable synergistic water treatment and hydrogen production via novel Z- scheme Bi₇O₉I₃/B₄C heterojunction photocatalysts

Highlights

• Novel Z-scheme Bi₇O₉I₃/B₄C heterojunction synthesized by hydrothermal route.

• Antibiotic as sacrificial agent- H₂ evolution and antibiotic removal simultaneously.

• Antibiotic degradation route and photocatalytic mechanism probed.

• Waste to clean energy concept.

Abstract

The dual-function photocatalytic systems with a promising capability for hydrogen evolution and simultaneous pollutant degradation are surely a significant step towards waste-to-energy conversion goals. However, the performance of such photocatalysts is often limited by poor visible-light activity, charge separation and surface reverse reaction involving photogenerated electrons and radicals/intermediates. In this work, we report hydrothermal synthesis of novel Bi₇O₉I₃/B₄C (BIBC) heterojunction photocatalyst for advanced Norfloxacin antibiotic degradation with simultaneous hydrogen evolution under visible light. In particular, BIBC-30 heterojunction shows H₂ evolution rate of 812 μmol g⁻¹h⁻¹ with simultaneous 94.2% NFN removal which are much higher than bare B₄C (∼6 times) and Bi₇O₉I₃ (∼4 times). Under oxic/aerobic conditions too, a high 456.3 μmol g⁻¹h⁻¹H₂ evolution with nearly complete norfloxacin degradation was achieved. The low band gap of Bi₇O₉I₃ and presence of metallic Bi^o extends the absorbance to NIR region and B₄C enlarges the surface area of junction along with suppression of the back reaction. It was observed that BIBC heterojunction exhibits manifolds H₂ evolution rate with NFN as sacrificial agent (18.3% apparent quantum efficiency) is manifolds higher than pure water, methanol, triethanolamine and rhodamine B. An effective Z-scheme charge transfer facilitated by Bi^o is active in the intimately coupled heterojunction with suitably placed energy bands. This work shows that waste to energy conversion can be promisingly achieved by performing H₂ evolution and pollutant removal simultaneously.

Introduction

The realization of finite fossil fuel scarcity and environmental pollution has led the research community and environmentalists to search for alternative renewable and environmentally friendly green fuel [1], [2], [3]. Conversion of solar energy to storable chemical fuel from a clean source such as water is the key technology to the carry out the future energy demands [4]. Hydrogen has emerged as the most promising green fuel because of its high energy emission, zero hazard and most important can be produced from the water [5], [6], [7]. Electrocatalytic hydrogen evolution has been most popular route for hydrogen generation [8]. There are numerous issues including cost-effectiveness related to these techniques. Sustainable photocatalytic H₂ production has been a promising and solution to tackle dual issues of the energy crisis and environmental pollution [9], [10], [11]. However, for efficient photocatalytic hydrogen generation, some external sacrificial agents or donors as alcohols, organic acids etc are generally employed for scavenging holes and diminishing recombination [12], [13]. Such addition is economically viable and practically suitable as the external sacrificial agents act as an energy supply or input and thus raise the cost of hydrogen evolution eventually [14].

Regarding sustainable and efficient hydrogen evolution, there are two important requisites. Firstly, the photocatalyst must be able to separate photogenerated electrons and holes efficiently along with numerous requisite reaction sites and high visible light activity [15], [16].The second challenge lies in recovery or generation of hydrogen energy from waste water for implementation of a environmental friendly and sustainable energy production-cum-water treatment with promising scale-up.

Among numerous contaminants of emerging concern, antibiotics and their residues have been most widespread because of their resistance to biological oxidation and other traditional methods [17], [18], [19]. Norfloxacin (NFN) belongs to class of fluoroquinlones which are one of the most common antibiotics [20], [21]. These are not completely metabolized by body and about 70% is excreted out [22]. Being reported in ng L⁻¹ to μg L⁻¹ in environment, their presence is surely a threat to aquatic ecosystem and humans [23] and lead to antibiotic resistance in microbes in longer run [24]. Thus utilizing these antibiotics as sacrificial agent in photocatalytic hydrogen evolution serves both the purposes of clean energy generation and wastewater treatment.

The semiconductor heterojunctions especially Z-schemes formed by choosing materials with suitable band structures are highly beneficial in the charge separation [25] which is an essential prerequisite for such purpose. Recently, bismuth oxyhalides have garnered extensive attentions owing to their significant sunlight response, built-in electric field and adequate chemical stability, such as (Bi_aO_bX_c, X  = Cl, I, Br) [26], [27], [28]. Among them, a new bismuth-rich Bi₇O₉I₃ has been widely reported as one of most promising photocatalysts in the field of photocatalytic energy production and environmental remediation [29], [30]. It possesses excellent photocatalytic activity and high charge transfer ratio owing to the increased Bi and O content, deeply anisotropic p or hybridized sp states, and highly dispersed conduction band (CB) and valence band (VB) [31]. Nevertheless, the photocatalytic efficiency of Bi₇O₉I₃ remains far from suitable for practical applications because the quantum yield of Bi₇O₉I₃ is rather poor due to the rapid recombination of photogenerated electron–hole pairs and slow rate of photogenerated charge transfer [26], [32].

Boron carbide (B₄C), is a stable and low-cost metal-free semiconductor with a strong absorption in solar light owing to mid-gap states and various structural distortions [33], [34]. It has low band gap, low density with an excellent resistance to oxidation resistance and tunable electrical properties [35]. Feng and co-workers (2013) utilized B₄C in photocatalytic hydrogen evolution, with a decent hydrogen evolution activity originating from structural defects and distortions in [36]. Very few works have reported heterojunctions based on B₄C for photocatalytic applications as B₄C/TiO₂ [35], B₄C/C₃N₄ [37] etc.

As compared to Bi₄O₉I₃, boron carbide has more strength and active sites for a robust heterojunction formation. In the current work, synthesis of Bi₇O₉I₃/B₄C heterojunction is reported for simultaneous photocatalytic hydrogen evolution and fluoroquinolone antibiotic degradation under visible light. The unique hierarchical structure of the heterojunction shows high visible light capture, high electron transport, highly suppressed electron-hole recombination with an effective Z-scheme mechanism. A 456.3 μmol g⁻¹h⁻¹H₂ evolution with almost complete norfloxacin removal was achieved under anoxic conditions with retaining of high activity under oxic conditions too. An effective Z-scheme transfer facilitated by Bi^o (as found in XPS findings) was active for a higher charge separation and effective hydrogen evolution with NFN molecules acting as sacrificial agent by capturing directly holes as well as indirectly ^●OH radicals. The activity was also tested with other pollutants as rhodamine B and tetracycline. This work paves way for converting different waste-water sources for a sustainable hydrogen evolution, water treatment and resource utilization. Utilizing pharmaceutical waste for hydrogen generation is essentially an important step towards realizing sustainable development and circular economy.