(80a) On the Relative Effect of Nanorod Versus Oval-Shaped ZnO Nanoparticles in Their Photocatalytic Activity

Pharmaceutical industries generate waste, that contain a large number of contaminants, which are released into the atmosphere/environment after partial treatments. Another type of waste is municipal waste, where large amounts of antibiotics and other solvents are released because of their low removal efficiency. Generally, these wastes include different drugs that are used in the treatment of various human diseases. The presence of these contaminants in the environment can cause direct effect on human health and other living bodies. Their long-term presence in the open atmosphere can create drug resistance among the microorganisms and as a result, such drugs may not work in a particular health issues. Various methods of removal/degradation have been reported to minimize these contaminants below the permissible limits. Methods such as advanced oxidation processes, catalytic ozonization, photocatalytic degradation, UV-H₂O_2, are majorly reported. Photocatalytic degradation was found to be promising, due to its ease of use, handling a large number of wastes, and being a stable method. In the past, various types of nanoparticle-based photocatalysts such as, TiO₂, ZnO, ZnS, ZnIn₂S₄ and goldwere reported. Among these, TiO₂ and ZnO are frequently investigated. Their efficiencies can be tuned/tested by their size, shape and surface interactions.

In the present work, ZnO nanoparticles have been used as a photocatalyst, which has attracted researchers, due to their unique physical and chemical characteristics. ZnO has a wide bandgap (E_g = 3.37 eV), high photosensitivity, higher electron mobility, as well as has high quantum efficiency. In addition to that, biocompatibility and eco-friendliness of ZnO, makes it an ideal candidate for the photocatalytic degradation process. Here we have used nanorod (NR) and oval-shaped (OS) ZnO as a photocatalyst, for the degradation of rifampicin (RIF). RIF is a pharmaceutical drug that is mostly used in the treatment of tuberculosis as a first-line drug. These drugs are recalcitrant and therefore their removal is a necessary task for the safety of humans.

For the NR synthesis, equimolar concentration (10 mM) of each of ZnCl₂ precursor and hexamethylenetetramine (HMTA), as a reducing agent, were mixed and stirred at 85 °C, for 5 hours. Similarly, for the OS synthesis, the 10 mM ZnCl₂ precursor and 40 mM ammonia solution (25 % w/v), as reducing agent, were mixed and stirred at 85 °C for 1 hour. ZnO was formed due to Zn²⁺ ion from the precursor and OH^-- ion from the reducing agent and at a particular temperature leading the ZnO formation [eq. 1]. X-ray diffraction analysis confirms the hexagonal wurtzite, crystalline ZnO structure. In the case where NH₃ was used as a reducing agent, within a short time (10 second), all the free Zn²⁺ ions were utilized and resulted in ZnO oval-shaped nanoparticles, whereas slow degradation of HMTA lead to formation of ZnO nanorods. Due to this reason, the reaction time is different in both cases. Further, these properties were examined in detail using different characterizing tools. The sizes of NR are found to be- 2190 nm and 367 nm in length and diameter, respectively. while for OS, it is found- 314 nm and 160 nm respectively.

Zn²⁺ + 2OH^- --> Zn(OH)₂ --> ZnO_(s) + H₂O Eq. 1

To determine the efficacy of the ZnO catalyst, we have used both NR and OS shaped nanoparticles, in the photocatalyst degradation of the drug. Our results show that, the degradation rate in NR is high compared to OS. In case of NR, the degradation rate is (0.04 hr^-1 mg^-1), whereas in OS the rate is only (0.013 hr^-1 mg^-1). The reason for obtaining a higher degradation rate in NR can be due to its higher surface area, lower energy E_g, and higher concentration of oxygen vacancies. The primary reason for the difference in the degradation rate is the BET surface area (SA), in which NR (67.9 m² g^-1) has 25 times higher area than OS (2.7 m² g^-1). Also, the BET pore volume of NR is six-fold higher than OS (NR - 0.06 cm³ g^-1; OS - 0.01 cm³ g^-1). Hence, NR has a large number of active sites on its surface, by which easily dissociable constituents were easily degraded. Using DRS, we found that the E_g of NR (3.21 eV) is lower than OS (3.24 eV), so the electronic transitions will use less energy for excitation. With XPS, we found that, the concentration of oxygen vacancies (V_o) is higher in NR compared to OS, and these V_o act as a donor. The electrons are trapped by these donorsand due to this, trapping decreases the charge recombination and thereby enhancing the charge separation efficiency in NR. OS shows higher zinc interstitial (Zn_i) defects than NR, but these defects were insignificant in enhancing degradation efficiency.

The valence band (VB) and conduction band (CB) potentials of the photocatalysts NR and OS were calculated theoretically using the Mulliken electronegativity parameter. To calculate it, the standard redox potentials of 2.38 eV (OH*/H₂O) and -0.33 eV (O₂/O₂*) were used, as VB and CB positions respectively. We have observed that the VB of both NR and OS is 2.86 and 2.87 eV respectively and is more positive than the standard redox potential. Therefore, the degradation in both could be occurring due to the oxidation of H₂O to OH* radicals (Fig. 1). The CB potential of NR and OS is -0.35 eV and -0.36 respectively, lower than standard potential. Therefore, electron in CB of both can reduce O₂ and generate the O₂* radicals. The generated radicals, OH* and O₂* are taking part in RIF degradations (Fig. 1). Overall, the higher SA, high V_o, and lower E_g in NR compared to OS, leads to the higher photocatalytic activity, while rest of the parameters are less effective.

Hence, the effect of physicochemical and optical properties of photocatalysts that lead to enhancement of the photocatalytic activity of the drug, RIF, has been investigated.

Figure 1: Photo-degradation of RIF with NR versus OS