Novel CuO/Mn3O4/ZnO nanocomposite with superior photocatalytic activity for removal of Rabeprazole from water

In this work, a nanohybrid of CuO/Mn3O4/ZnO was generated through a simple hydrothermal based procedure. The CuO/Mn3O4/ZnO nanohybrid has been characterized using X-ray diffraction, transmission electron microscopy high resolution transmission electron microscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and energy dispersive X-ray analysis. UV–visible spectrophotometry and photoluminescence techniques allowed evaluation of optical properties that additionally suggested the prevalence of strong interfacial interaction between the three moieties of the nanohybrid and suppressed electron–hole recombination. The hybrid photocatalyst brought on ~ 97.02 ± 1.15% disintegration of rabeprazole when illuminated with visible light. The progress of the photodegradation was in conformity with pseudo-first order kinetic model and had a velocity constant of 0.07773 min−1. Additionally, ~ 84.45% of total organic carbon removal was achieved while chemical oxygen demand was reduced by ~ 73.01%. Using high resolution liquid chromatograph mass spectrometry technique, identification of the degraded products was made and accordingly the mechanistic route of the aforesaid degradation was proposed.

There has been a great rise in the consumption of pharmaceutical drug over the last few decades and consequentially their occurrence in aquatic regions and drinking water supplies have also spiraled up without precedence. The presence of such biologically non-degradable organic contaminants has increasingly posed threat to the aquatic fauna and flora as well as terrestrial organisms as they carry the potential to cause severe disruption of all sorts of biological activities. Even human beings have failed to escape the adverse effects generated from the presence of such xenobiotic substances in the ecosystem 1,2 .
Among many other emerging organic water pollutants, pharmaceutical waste is of great concern 3,4 . Pharmaceutical substances often get into water bodies by human excretion and their inappropriate disposal of hospital waste 5,6 . Because of the ability to interact with living bodies, pharmaceutical wastes often impose threat to the aquatic ecosystem 7 . The increasing accumulation of pharmaceuticals marine biomes directly or indirectly affects the flora and fauna and in turn disturbs the ecological balance. Rabeprazole is a pharmaceutical drug that is widely recognized for its acid-inhibiting ability and is commonly employed for the treatment of a number of acid-related disorders [8][9][10][11] . The excessive use and indiscriminate disposal of rabeprazole lead to their accumulation in water bodies. Nevertheless, the drug can cause several side-effects that include headache, abdominal pain, constipation, dizziness, cramps, spasms and a great deal more 12,13 . The presence of even trace amount of rabeprazole in water bodies can wreak untold havoc on all forms of life in the aquatic ecosystem. Therefore, efforts have been made to rid water of such organic contaminants. In this regard, the fabrication of an efficient nano-scaled photocatalyst has drawn much attention 14,15 . This is owing to the fact that in comparison with other wastewater treatment techniques, photocatalytic degradation involves not just removal but complete breakdown of otherwise recalcitrant organic contaminants sans the generation of any harmful products [16][17][18][19][20] . This has led to the advent of a new set of oxidative procedures in the field of photocatalysis called Advanced Oxidation Process (AOP) aimed at total breakdown of persistent organic contaminant such as rabeprazole through generation of reactive oxygen species [21][22][23][24] .
Heterogenous photocatalysis is a class of AOP that has proved effective in the removal of persistent organic contaminants from aquatic environment 25 . Among metal oxide semiconductors, ZnO with a band gap of ~ 3.37 eV, superior electrochemical stability, high electron mobility, favourable isoelectric point of ~ 9 Synthesis. Synthesis of the nano-scaled materials. To aqueous solution of 25 mmol of copper acetate monohydrate, 1 mL of glacial acetic acid was added and the solution was heated to 100 °C. This was followed by drop-wise addition of an aqueous solution of NaOH under magnetic agitation. The solution color gradually changed from blue to black, and a considerable amount of black precipitate (CuO) was formed 40 . To this was added 25 mmol of zinc nitrate hexahydrate (Zn(NO 3 ) 2 .6H 2 O) followed by addition of drops of aqueous solution of NaOH. The reaction mixture was magnetically agitated and then to it was added 25 mmol of manganese chloride tetrahydrate (MnCl 2 . 4H 2 O) followed by a dropwise addition of an aqueous NaOH solution. After thorough magnetic agitation of this reaction mixture, it was next shifted to an autoclave. The autoclave was then put inside an oven maintained at 100 °C. The reaction mixture was kept in this condition for 24 h. The dark brown residue thus formed was collected, rinsed repeatedly with ethanol and then dried. The dried sample was calcined at 300 °C in a muffle furnace for 4 h. The calcined sample was given the tag CuO/Mn 3 4 and CuO were obtained with Hitachi F4600 equipment. For recording the absorbance spectra of the samples, GENESYS 10S UV-visible spectrophotometer was used. Photocurrent (PC) measurements were carried out on a Biologic SP-200 electrochemical workstation with a standard three-electrode cell at room temperature. Electrochemical impedance spectroscopy (ESI) was performed using a potentiostat with a sinusoidal perturbation voltage of 2 mV and the frequency range of 0.01 Hz to 1 MHz. The concentration of leached metal ions was determined using a Hitachi 180-70 atomic absorption spectrometer.
Evaluation of photocatalytic activity. The photocatalytic activity of the designed nano-hybrid photocatalyst was put to evaluation by monitoring its ability to cause disintegration of rabeprazole in its aqueous medium underneath an LED illumination and the whole experiment was executed in a chamber that had in it a Philips white LED bulb of 23 W. A luxmeter was used to measure lux and radiation intensity. Illuminance measured 11,830 lx The drug solution alongside the photocatalyst was mechanically agitated for a span of 30 min in order to attain adsorption-desorption equilibrium. Thereafter, the absorbance was recorded. Next, the reaction mixture was subjected to LED irradiation for the attainment of photocatalytic degradation. The photodegradation was monitored by tracking the maximum absorbance of rabeprazole at 290 nm at an interval of 10 min for a total span of 60 min.
The following equation was employed to evaluate the efficiency of degradation: where C 0 and C stand for the respective concentrations of rabeprazole at t = 0 and t = t. For the assessment of kinetics of the photodegradation the equation underneath was used: where C 0 and C designate rabeprazole concentrations at t = 0 and t = t and k is the velocity constant of the pseudo-first order reaction in min −1 . The error bars in the diagrams displaying photodegradation represent minor standard deviations and are indicative of remarkable reproducibility over five repetitions of the experiment under a given set of conditions. Trapping experiments using various scavengers were carried out for ascertaining the involvement of reactive species in the photodegradation of rabeprazole. Furthermore, the photodegradation capacities were also evaluated in terms of parameters such as chemical oxygen demand and total organic carbon.

Results and discussion
XRD studies. XRD analysis was done for determination of the crystalline phases of the compounds prepared. Figure 1 (1) Degradation efficiency(%)     Table 1 are suggestive of the successful synthesis of CuO/Mn 3 O 4 /ZnO nanostructures with substantial physical integration among the individual components. The atomic percentages further suggest the presence of CuO, Mn 3 O 4 and ZnO in the ratio ~ 1:1:1 in the nanohybrid. Further the absence of impurity peaks in EDAX spectrum was fairly consistent with the neat XRD plot obtained for the ternary nanocomposite.
Optical properties. The UV-visible spectra ( Fig. 5a)  The red shift in the absorbance edges of the nanohybrids with the ternary photocatalyst undergoing the maximum shift further confirmed the formation of integrated photocatalysts with strong interfacial interactions that ensured photocatalysis within the visible range of light. Besides, a possible intermixing of the orbitals in the valence shells of Zn, Mn and Cu species leading to the generation of conduction band at lower magnitude of energy in the nanohybrids could also be inferred 50 . PL spectra of a semiconductor material chiefly arise due to downward electronic transition from conduction to valence band and the associated intensity can be a measure of the rate of electron-hole recombination. The nanohybrids were therefore compared with the pristine metal oxide nano-scaled samples. PL spectrum of ZnO   Fig. 5d). The radiative recombination was intense in the pristine sample and weakened in binary nanocomposites while it was the weakest in CuO/Mn 3 O 4 /ZnO. All these data suggest that a wide separation of photogenerated electrons and holes could be achieved in the ternary nanohybrid.

Study of the operating parameters of photodegradation of rabeprazole
Effect of photocatalyst dosage. The test for optimality of photocatalyst loading was performed to avoid its redundant use. As a result, the designed photocatalyst could be used at its best with greatest efficiency. The rate of rabeprazole photodegradation in presence of different loadings of CuO/Mn 3 O 4 /ZnO is shown in Fig. 6(b). To find out the optimum amount of the designed photocatalyst required for degradation of rabeprazole, a 50 mL of 25 ppm rabeprazole solution was taken, and with an initial pH of 7, the dosage of catalyst was regulated in the range 100-1000 ppm. Maximum photodegradation yield was attained at a catalyst concentration of 700 ppm (Fig. 6a). The efficiency of photodegradation underwent a slender decline past this concentration. Although additional use of photocatalyst implies introduction of yet greater number of active sites on its surface, there occurs severe solution opacity that leads to a decline in photocatalytic activity. The velocity constant reached the greatest value (6.35 × 10 -2 min −1 ) for 25 ppm of photocatalyst dose (Fig. 6b).
Effect of initial rabeprazole concentration. For   www.nature.com/scientificreports/ of 700 ppm and initial pH of 7 at varying rabeprazole concentrations. Maximum degradation was achieved at 30 ppm of rabeprazole concentration (Fig. 6c). Rabeprazole concentration higher than this optimal concentration demonstrated a slight fall in the degradation efficiency. This could be ascribed to the shortening of photon path length at higher rabeprazole concentrations 51 . Furthermore, higher rabeprazole concentrations would need greater photocatalyst surface area for degradation and this could be only achieved by the addition of extra amount of photocatalyst that would invariably raise the solution opacity 52 . Therefore, optimization of rabeprazole dose enabled superlative yield of photodegradation. The velocity constant assumed the highest magnitude (6.65 × 10 -2 min −1 ) at optimal rabeprazole concentration (Fig. 6d).

Effect of pH.
The effect of pH on the photocatalytic breakdown of the aqueous rabeprazole solution was studied. For this purpose, the catalyst concentration was fixed at 700 ppm and the concentration of rabeprazole was maintained at 30 ppm while the initial pH was varied from 4 to 8 during the experiments. There was a marginal increase in photodegradation yield till pH 5 and then a minor decrease was registered (Fig. 7a). The point of zero charge (pH pzc ) of the catalyst was found to be ~ 4.8 pH. Thus with increasing the pH of the solution, large number of hydroxide ions can be accumulated over the nanocatalyst surface 53 . Hydroxide ions gathering over the photocatalyst surface at higher pH might lead to repulsion of electron-rich rabeprazole species by the catalyst. As evident from Fig. 7(b), it was at pH 5; the velocity constant acquired its maximum value (7.77 × 10 -2 min −1 ).
Effect of irradiation time. The effect of irradiation time was investigated by measuring the degradation yield at different time periods under optimal operating conditions of photocatalyst dosage, initial pharmaceutical concentration and pH. The photocatalyst loading used for this purpose was 700 ppm while initial rabeprazole concentration of 30 ppm and initial pH of 5 was employed. A maximum rabeprazole decomposition of ~ 97.02 ± 1.15% was attained by 60 min past which no significant photodegradation was registered (Fig. 8a,b). The near complete exhaustion of active sites on the photocatalyst surface culminated in the apparent stoppage of the light driven degradation reaction after 60 min.    (Fig. 8a,b). An outstanding velocity constant of 0.07773 min −1 was achieved during photodegradation over CuO/Mn 3 O 4 /ZnO. Furthermore, this ternary composite achieved 84.45% of TOC removal and 73.01% COD reduction. (Fig. 8c,d). Going by the photodegradation percentage, the ternary nanohybrid proved ~ 1.46 times better than Mn 3 O 4 /ZnO, ~ 1.81 times better than CuO/ZnO and ~ 1.65 times better than CuO/Mn 3 O 4 . Also, the ternary photocatalyst proved remarkably more efficient than CuSnO 3 that took 120 min to cause ~ 87% of rabeprazole degradation at a velocity constant of 0.063 min −154 . A comparative account of the photocatalytic performance of the synthesized nanocatalyst with the previously reported catalysts has been presented in Table 2.
The quantum efficiencies of the aforementioned photocatalyst systems have been calculated in Table S2 (ESI).
Reusability of the nano-scaled hybrid photocatalyst. The designed hybrid photocatalyst was retrieved and could be used for five cycles in succession (Fig. 8e,f). The photocatalyst was collected by centrifugation. It was then repeatedly washed with deionized water and acetone and dried at 70 °C. The dried sample was again used for the next cycle of experiment. Degradation yield and pseudo-first order velocity constant showed a minor decrease that could be attributed to a small-scale dislodgement of the coupled photocatalyst system. Besides, XRD pattern obtained for the recycled photocatalyst showed retention of the characteristic crystallographic planes found in CuO/Mn 3 O 4 /ZnO before use (Fig. 9). This established the overall durability of the fabricated photocatalyst. The XPS of the recycled photocatalyst was also performed and as evident from Supplementary Fig. S1, the surface composition of the recycled photocatalyst and the electronic states of Zn, Mn, Cu and O remained unaltered indicating sustenance of co-existing ZnO, Mn 3 O 4 and CuO phases after five consecutive runs of the experiment. This was additionally supported by the results of elemental analysis from EDAX spectrum ( Supplementary Fig. S2) of the recycled catalyst that showed little difference from that obtained for the unused catalyst. These data revealed the stability of the ternary nanocomposite suggesting substantial interfacial interaction between the moieties and lent considerable evidence in favour of minimal leaching of metal ions during photocatalysis. The concentrations of leached metal ions were determined during photocatalysis using pristine metal oxide components and the ternary nanocomposite (Fig. 10). During this investigation, it emerged that at total of 7.38 ppm of Zn 2+ and 6.91 ppm of Cu 2+ were respectively released from pristine samples of ZnO and CuO while 6.21 ppm and 4.96 ppm of Mn 2+ and Mn 3+ ions leached at the end of photocatalysis. This leaching dropped to 3.28 ppm, 2.77 ppm, 1.44 ppm and 0.78 ppm of Zn 2+ , Cu 2+ , Mn 2+ and Mn 3+ when the ternary nanocomposite was used in the experiment indicating that a greater stability was rendered to the ternary hierarchical structure. The fabrication technique so employed in the preparation of the hierarchical structure could have imparted good elemental contact and high dispersion as corroborated by TEM, XPS, EDAX and optical data leading to low aggregation of individual metal oxides within the ternary nanocomposite. Further, it has been reported that a hierarchical structure of semiconductor leads to the higher stability against aggregation 55,56 . Thus, the leaching of metal ions triggered by aggregation will be hindered by hierarchical configuration.

Effect of scavengers.
For appraisal of the roles played by reactive species such as O 2 −· , · OH, h + and e − , photodegradation experiments were performed in presence of quenchers. 4-hydroxy-2,2, 6,6-tetramethylpiperidinyloxy (TEMPOL), benzoic acid (BA), triethanolamine (TEOA) and silver nitrate (AgNO 3 ) were used as scavengers of O 2 −· , · OH, h + and e −57-60 . The photodegradation in presence of TEMPOL and BA suffered obvious retardation (Fig. 11)     ). This took place because the standard reduction potential of (O 2 / O 2 −· ) is 0.13 eV which is higher than the CB edge potential of ZnO [65][66][67][68] . The holes, on the other hand, reacted with water molecules (H 2 O) to introduce hydroxyl radicals ( • OH). Generation of · OH radicals might follow routes described underneath [69][70][71][72] . The synergistic effect among the moieties of the integrated photocatalyst leading to its photocatalytic abilities could be best delineated by the following equations:  www.nature.com/scientificreports/ To further throw light on the aforesaid charge transfer mechanism, the separation efficiency of the photogenerated charge carriers was investigated using photocurrent response and electrochemical impedance spectra (EIS). The photocurrent densities versus the irradiation time curves of the different samples under chopped illumination are shown in Fig. 13(a). The photocurrent densities dramatically rose when the lamp was turned on and went on to sustain nearly stable values for the time the lamp was kept on. However, the photocurrents rapidly decreased to zero as soon as the lamp was turned off. The photocurrent generated by the ternary nanocomposite was much higher than the binary nanocomposites and the pristine samples. This suggested a smaller recombination and a more efficient separation of photo-generated electron-hole pairs at the interface between the moieties in the ternary nanocomposite. In other words, the remarkable improvement of the photocurrent of the ternary nanocomposite is a consequence of a longer life-span and more efficient separation of charge carriers than in individual metal oxide moieties or their binary combinations. The EIS Nyquist plots of the photocatalysts are shown in Fig. 13(b). The arc radius of the ternary nanocomposite was smaller than that of the binary nanocomposites and the pristine metal oxides suggesting a higher efficiency of charge separation and charge transfer across the interfaces of metal oxides in the ternary nanocomposite than in the binary nanocomposites and the pristine samples. These data corroborate the results of photoluminiscence investigations.
The identification of transformation products is essential for proper determination of the route of degradation for which HRLCMS was performed at an intermediate stage of the rabeprazole photodegradation. Supplementary (11)  www.nature.com/scientificreports/ Figure S3 represents the liquid chromatogram resulting from HRLCMS and Supplementary Fig. S4 displays the various mass spectra emerging from this analysis, which bore out the formation of various transformation moieties that appear in the depiction of a plausible route of rabeprazole photodegradation in Fig. 14. The most probable degradation pathway involves three steps. In the first step, the parent rabeprazole molecule may undergo cleavage of C-S bond to generate two daughter fragments of mass 118 (compound I) and mass 243 (compound II). The compound II eventually may break into two parts to form compound III (mass 171) and a smaller fragment of mass 74 gets eliminated. The resultant compound III may further undergo disintegration to generate smaller degradation intermediates of mass 105 and mass 72. All these intermediates on prolonged degradation eventually get fragmented to smaller parts.

Brief investigation on the influences of foreign species
The performance of the photocatalyst was monitored in presence of foreign species such as inorganic ions and organics. Inorganic ions of 0.05 M were added to the reaction system and the photocatalytic degradation yield was estimated in each case. There was suppression of photocatalytic activity in presence of chloride, nitrate, carbonate and phosphate (Fig. 15a,b). This was because all of them quenched hydroxyl free radicals and also displayed pronounced tendency to get adsorbed on the surface of the photocatalyst resulting in the blockage of active sites [72][73][74][75][76] . Usually, potassium and magnesium do not exert appreciable influences on photocatalysis. However, since their chloride salts were used, photocatalytic behaviour diminished in presence of potassium and magnesium (Fig. 15c,d) 76 . Aluminum cations are immensely adsorbed on the surface of photocatalyst thereby suppressing photodegradation yield (Fig. 15c,d) 76 . Copper (II) cations induced short-circuit by a cyclic interaction with electrons and thereafter holes and in the course impeded generation reactive species thereby exerting sizeable negative influence on photocatalysis (Fig. 15c,d) 72 .
In presence of organics such as isopropanol, humic acid sodium salt (HAS) and sodium dodecyl sulfate (SDS) as well, the diminishing influence was noticeable (Fig. 16a,b). Isopropanol quenches hydroxyl radicals and involves itself in preferential adsorption on the surface of photocatalyst 77 . HAS attenuates light through solution thereby slowing down photocatalysis 78 . The negative impact of SDS was chiefly due to the retarding influence of photogenerated sulfate moiety which has excellent hydroxyl scavenging properties 79 . Acetone, on the other hand, is a remarkable photosensitizer and generates hydroxyl radicals in abundance under visible light illumination (Fig. 16a,b). This explains the enhancement in photocatalytic activity upon its addition 80 . Influence of water samples. The grab samples of three different environmental waters were collected and their TOC and pH measured before application. The TOC of mineral water, river water and tap water was ~ 0.27 ppm, ~ 5.38 ppm and ~ 3.56 ppm while their pH was ~ 7.17, ~ 8.34 and ~ 7.75. All these water samples www.nature.com/scientificreports/ reduced photodegradation efficiency (Fig. 16c,d). One important reason could be light attenuation by different species present in these waters. Also, these waters, in all likelihood, abound in photocatalysis-inhibiting inorganic species and organics.

Conclusion
In the current work, a facile fabrication of CuO/Mn 3 O 4 /ZnO nanohybrid photocatalyst was carried for an efficient photodegradation of rabeprazole, a pharmaceutical drug considered to be an emerging organic water contaminant. The structure, morphology and composition were evaluated from XRD, TEM, HRTEM, SAED, SEM and EDAX data. All these data clearly suggested the formation of the nanocomposite. The optical band gap of the nanocomposite was found to be ~ 2.32 eV and the PL, photocurrent and EIS data suggested a fair separation of light induced charge carriers and their efficient extraction as compared to binary nanocomposites and pristine metal oxides. Both these moieties functioned as principal visible light harvesting units and with electronic structures compatible to each other and to ZnO, they also assisted in outright decline in the rate of recombination of photoinduced charge carriers. These charge carriers with effectively prolonged lifetime engendered reactive species that eventually led to the disintegration of the target organic water contaminant. Besides, scavenger experiment bore out the active roles played by superoxide and hydroxyl free radicals whose very generation is premised on the electron transfer mechanism so proposed thereby confirming the formation of heterojunction interfaces between the moieties of the hybrid photocatalyst. Additionally, the photocatalyst was observed to demonstrate reasonably sustained photocatalytic activity up to five runs. The lower leaching of metal ions in the ternary nanocomposite when compared to pristine samples of metal oxides further suggested considerable stability of the hybrid photocatalyst. The stability was further confirmed from XRD, XPS and EDAX investigations of the recycled catalyst www.nature.com/scientificreports/ that displayed retention of the crystallinity and composition of the unused catalyst. This could be attributed to stronger interfacial interactions between the moieties of the hybrid photocatalyst and high of nanoparticles of the individual metal oxides in the nanocomposite. Investigation of the photocatalyst's performance in presence of co-existing foreign species showed that co-anions like chloride, nitrate, carbonate, and phosphate exerted a negative impact, while co-cations like potassium and magnesium ions had little bearing on photocatalysis, although aluminum and copper (II) ions offered great hindrance to photocatalytic activity. Organics save acetone too showed inhibiting influence. Acetone being a photosensitizer augmented photocatalysis through generation of additional hydroxyl radicals. Grab water samples induced retardation of photocatalysis due to suppressing actions of the various components present in them.