Development of Ag0.04ZrO2/rGO heterojunction, as an efficient visible light photocatalyst for degradation of methyl orange

Methyl orange (MO) is mutagenic, poisonous, and carcinogenic in nature, hence, effective methods are required for its degradation. We have synthesized pure ZrO2, Ag-doped ZrO2, and Ag-doped ZrO2/rGO as hybrid photocatalysts by facile hydrothermal method. These photocatalysts were characterized by powder XRD, scanning electron microscopy, EDX, FTIR, photoluminescence, UV–Vis diffuse reflectance (DRS), and Raman spectroscopy. The photodegradation of MO (10 ppm) was studied with pure ZrO2, Ag-doped ZrO2, and Ag-doped ZrO2/rGO (10 mg/100 mL catalyst dosage) photocatalysts at 100 min irradiation time under UV–Visible light. The pH effect and catalyst dosage on photodegradation of MO was investigated. Ag0.04ZrO2/rGO photocatalyst exhibited the maximum photocatalytic degradation of MO (87%) as compared to Ag0.04ZrO2 (60%) and pure ZrO2 (26%). Reusability experiments ensured the excellent stability of photocatalyst after five consecutive experiments. To the best of our knowledge, this is the first report on the facile hydrothermal synthesis of Ag0.04ZrO2/rGO photocatalyst for photocatalytic degradation of methyl orange.

SEM and EDX study. The Fig. S1d. Pure ZrO 2 shows the large-sized cavities of thick rod-like structure 29,30 . The SEM micrographs of Ag 0.04 ZrO 2 are shown in Fig. S2a-d which shows that the crystallinity has decreased with increasing concentration of Ag and EDX is in Fig. S2e. The crystallinity is regained when the heterostructure with rGO is formed. The SEM micrographs of Ag 0.04 ZrO 2 /rGO photocatalyst are shown in Fig. 2a-d having dense nanorods which are aligned vertically. The average diameter of the nanorods is 100 nm. The large network structure of Ag 0.04 ZrO 2 / rGO nanorods may not only increase the active sites for a photocatalytic reaction but also provide channels for solution diffusion during the intercalation/de-intercalation process toward photocatalyst.
Elemental analysis of Ag 0.04 ZrO 2 /rGO nanorods was done by EDX which is shown in Fig. 2e. The spectrum shows the O, Ag, Zr, and C with an atomic percentage of 49.68, 3.19, 21.27, and 25.86, respectively 31 . Fourier transform infrared spectroscopy. FTIR spectroscopy was used to identify chemical bonds as well as functional groups of synthesized material by producing an infrared absorption spectrum. ZrO 2 , Ag 0.04 ZrO 2, and Ag 0.04 ZrO 2 /rGO photocatalysts were characterized with FTIR spectroscopy. In Fig. S3a, a comparison of the FTIR spectra of pure ZrO 2 and Ag 0.04 ZrO 2 is presented. Both the FTIR spectra show the band around 559 cm −1 which arises due to the Zr-O vibration in zirconia. Figure S3b shows the FTIR spectrum of Ag 0.04 ZrO 2 /rGO (1:1). In this spectrum, the band at 561 cm −1 is due to the Zr-O vibrations in the photocatalyst. This spectrum also shows the bands of rGO 32 .  (1) (hνα)1/n = A hν − Eg  www.nature.com/scientificreports/ where, h is Planck's constant, ν is the vibrational frequency, α is the absorption coefficient, Eg is the bandgap energy (eV), A is a proportionality constant, and n refers to the type of electron transition (for directly allowed transitions, n = 1/2). The value of α is directly proportional to the Kubelka-Munk function (F(R∞)) 35 : The Tauc plot shows the bandgap energy by the projection of the tangent on the x-axis to the turning point of curvature. The result is shown in Fig. 3b.
The bandgap energies of ZrO 2 , Ag 0.04 ZrO 2 , and Ag 0.04 ZrO 2 /rGO are 3.48, 3.11, and 2.99 eV, respectively. The incorporation of Ag as dopant has lowered the bandgap energy of Ag 0.04 ZrO 2 (3.06 eV) while the addition of rGO has further lowered the bandgap of Ag 0.04 ZrO 2 /rGO thus increasing the photocatalytic activity 36-38 . Photoluminescence analysis. Photoluminescence (PL) spectroscopy is used to observe the separation and transfer of photogenerated electrons and holes in the photocatalyst/heterojunctions. Figure 4 shows the PL spectra of ZrO 2 , Ag 0.04 ZrO 2 , and Ag 0.04 ZrO 2 /rGO photocatalysts with an excitation wavelength of 325 nm. The shorter and longer wavelength emission of ZrO 2 and Ag 0.04 ZrO 2 /rGO photocatalysts could result from nearband-edge transitions and oxygen vacancies respectively 39 .
The redshift in the spectrum of Ag 0.04 ZrO 2 /rGO could be attributed due to interfacial charge transfer from Ag 0.04 ZrO 2 to rGO. This charge transfer decreases the PL intensity of Ag 0.04 ZrO 2 /rGO photocatalyst 40,41 . The intensity is observed in the following order: ZrO 2 > Ag 0.04 ZrO 2 > Ag 0.04 ZrO 2 /rGO. The electron/hole pairs are well separated in Ag 0.04 ZrO 2 /rGO, which exhibits higher photocatalytic activity.
(2) (hνF(R∞)) 2 = A hν − Eg  www.nature.com/scientificreports/ Specific surface area analysis (BET). Figure 5a shows the nitrogen adsorption-desorption studies of ZrO 2, and Ag 0.04 ZrO 2 /rGO photocatalysts. These studies are conducted to measure the specific BET surface area and pore structure of the photocatalysts. The BET surface area of Ag 0.04 ZrO 2 /rGO photocatalyst was calculated as 142.441 m 2 /g which is higher than ZrO 2 which is 37.3996 m 2 /g. An increase in the pore diameter presented in Fig. 5b from 0.08026 cm 3 /g for ZrO 2 to 0.98852 cm 3 /g for Ag 0.04 ZrO 2 /rGO is also observed. This suggests that the higher surface area and pore volume of Ag 0.04 ZrO 2 /rGO can be achieved by the modification of ZrO 2 with Ag and rGO. The higher specific BET surface area partly justifies the better adsorption and faster removal of pollutants interacting with the surface of the photocatalyst. Because of higher specific BET surface area, Ag 0.04 ZrO 2 / rGO shows best photocatalytic activity. The specific surface area, mean pore diameter, pore volume, and BHJ pore diameter are summarized in Table 1.
Degradation study of methyl orange (MO). The degradation of MO was evaluated under visible irradiation with ZrO 2 , Ag 0.04 ZrO 2 , and Ag 0.04 ZrO 2 /rGO (1:1) photocatalysts are shown in Fig. S4. Figure 6a,b shows the A/A° of MO using ZrO 2 , Ag x ZrO 2 (x = 0.01-0.05), and Ag 0.04 ZrO 2 /rGO (1:1, 1:2 and 1:3) photocatalysts under visible radiations. Figure S5 shows the comparison of A/A° of degradation of MO ZrO 2 , Ag 0.04 ZrO 2, and Ag 0.04 ZrO 2 /rGO. Figure 6c shows the % degradation of MO with pure ZrO 2 , Ag 0.04 ZrO 2, and Ag 0.04 ZrO 2 /rGO. The Ag 0.04 ZrO 2 /rGO exhibits 87% degradation while Ag 0.04 ZrO 2 and ZrO 2 show 60% and 26% degradation of MO in 100 min . The degradation of MO is highest with Ag 0.04 ZrO 2 /rGO photocatalyst as compared to Ag 0.04 ZrO 2 and ZrO 2 due to lower bandgap energy and a lower rate of recombination of e − /h + in Ag 0.04 ZrO 2 /rGO. Figure  Kinetic studies. The photocatalytic degradation follows a pseudo-first-order kinetic reaction; its kinetics can be expressed as follows: where k is the reaction rate constant and t is the reaction time. Figure 7 shows the reaction kinetics of degradation of MO by ZrO 2 , Ag 0.04 ZrO 2, and Ag 0.04 ZrO 2 /rGO (1:1) photocatalysts. These results illustrate that MO is degraded by Ag 0.04 ZrO 2 /rGO more efficiently than pure ZrO 2 or Ag 0.04 ZrO 2 . The degradation rate constant (k) is calculated from the slope of the straight line. The degradation rate constant of Ag 0.04 ZrO 2 /rGO with 1:1 (0.0204) is higher than that of doped Ag 0.04 ZrO 2 (0.00871) and pure ZrO 2 (0.00289).    www.nature.com/scientificreports/ Effect of pH on the photocatalytic performance. The pH is a major factor that affects the surface charge of the photocatalyst, the nature of the dye, and the ability of the dye to absorb into the photocatalyst surface. The degradation of MO was performed at pH 1,3,5,7,9 and 11 at a fixed dose of Ag 0.04 ZrO 2 /rGO (Fig. 8a,b). The degradation of MO is higher in acidic pH and is less in basic pH. However, under acidic conditions, MO change to a quinone structure. A visible color change, along with an absorbance peak shift was observed at lower pH values, further supporting the existence of a quinone structure of MO. The quinone structure is more prone to oxidation over the azo structure due to the sulfonic groups (-SO 3 − ) aiding in capturing hydrogen and further enhancing the hydrophobicity of the catalyst surface 42 . The enhanced degradation of MO at lower pH 03 is due to the formation of hydroxyl radicals during the reaction (OH − + h + → OH · ), the hydroxyl radicals are scavenged more slowly at a lower pH allowing them to react more readily with the dye.
Effects of dosage of the catalyst. To examine the effect of dosage of photocatalyst, different experiments were performed at 10 ppm MO concentration and pH 3, by varying the dose of Ag 0.04 ZrO 2 /rGO photocatalyst between 5 and 15 mg/100 mL. It can be seen in Fig. 8c,d that the degradation rate of MO increases with the increasing dosage of Ag 0.04 ZrO 2 /rGO. However, it is interesting to find that the degradation rate first increased with the increased dosage of catalyst (5-10 mg), then decreased with the further increase of catalyst (15 mg). The reason is that by increasing the catalyst dosage the surface area of the catalyst for the adsorption of MO increases which increases the MO degradation. But when the catalyst dosage is increased to 15 mg, a blockage of the light penetration occurs, which decreases the degradation of MO 43 .

Reusability.
To check the reusability of the catalyst, Ag 0.04 ZrO 2 /rGO photocatalyst was washed with deionized water several times and dried in the oven after every experiment. Ag 0.04 ZrO 2 /rGO photocatalyst was used for the degradation of MO in five repeated experiments. In every experiment, the irradiation time was 100 min. The Ag 0.04 ZrO 2 /rGO photocatalyst exhibited a high visible light photostability after five repeated experiments, although a slight decrease of photocatalytic activity is observed compared to the first-run result from 87 to 78% degradation, respectively as shown in Fig. 8e,f. Phooelectrochemical measurements. Figure 9a shows the electrochemical impedence spectroscopic measurements of the pure ZrO 2 , Ag 0.04 ZrO 2 and Ag 0.04 ZrO 2 /rGO photocatalysts under visible light irradiation. The smallest semicircle is observed for the photocatalyst Ag 0.04 ZrO 2 /rGO, showing the lowest charge transfer resistance in the as prepared photocatalyst.
Chronoamperometric response is shown in Fig. 9b at a potential of 0.8 V under the chopped light illumination. The photocurrent increases immediately from OFF to ON state proving that the present system is sensitive to light illumination and efficient in the generation and separation of electron-hole pairs through p-n junction.

Mechanisms of photocatalytic degradation of MO.
The potential of the valence band and conduction band, as well as the band gap energy, are important factors to determine the mechanism. The potential of the conduction band was calculated from Mott-Schottky plots which is be − 0.12 eV vs RHE for Ag 0.04 ZrO 2 and − 0.62 eV vs SHE for rGO. The bandgap energies calculated by using the Tauc plot are 3.11 eV for Ag 0.04 ZrO 2 and 1.69 eV for rGO. The potential of the valence band of Ag 0.04 ZrO 2 (2.99 eV) and rGO (1.07 eV) was calculated by using this formula: VB = CB + Eg.
The detailed mechanisms of photocatalytic degradation of MO by Ag 0.04 ZrO 2 /rGO are shown in Fig. 10. This mechanism shows that when light falls on the photocatalyst, the elctrons from the valence band of Ag 0.04 ZrO 2 and rGO get excited and move to the conduction bands. The holes from the valence band of Ag 0.04 ZrO 2 move the valence band of rGO. The electrons from the conduction band of rGO move to the conduction band of Ag 0.04 ZrO 2 hence reducing the elecectron-hole recombination as shown by the PL spectra. These photoexcited electrons react with the adsorbed oxygen and convert it to superoxide radicals which react with methyl orange and immediately decompose the dye to water and CO 2 . A possible mechanistic rout is given below: Holes on the other hand react with the water molecules and produce OH . radicals and react with methyl orange and immediadely decompose it to H 2 O and CO 2 . Therefore, this photocatalyst system provides active sites which shows the ability to harvest large amount of light hence better degradation efficiency. Many researchers have reported the degradation of methyl orange till now and a comparison table with the present study is shown in Table 2.   Characterization. X-ray diffractometer (DRONE-8, Russia), using Cu Kα radiation as the X-ray source, operated at 45 kV and 100 mA was utilized to study the crystalline structure and phase composition of photocatalysts. Scanning electron microscopy (MAIA3 TESCAN) was employed to determine the morphology of the photocatalysts. The absorbance of the photocatalysts was determined by utilizing the ultraviolet-visible (UV-Vis) diffuse reflectance spectroscopy (Lambda 365S, Perkin Elmer, Massachusetts, USA) in the wavelength range of 200-800 nm. Fourier transform infrared spectrometer (Alpha, Bruker) with range 550 to 4000 cm −1 was used to obtain IR spectra of the compound. Perkin Elmer spectrophotometer (Massachusetts, USA FL 6500/8500) with 150 W Xe lamp (200-900 nm) was used to measure the PL of photocatalysts.
Degradation studies of methyl orange. The degradation studies of MO were performed with all prepared photocatalysts. The prepared photocatalysts (10 mg) were added in 100 mL of the aqueous solution of MO (10 ppm) and stirred initially for 30 min in the dark to attain adsorption-desorption equilibrium. Then the mixture was then exposed to UV-Visible light using a 500 W UV-Vis lamp. The 5 mL aliquot was taken every 20 min. and analyzed with UV-Vis spectrophotometer. The photocatalytic degradation treatment was studied at www.nature.com/scientificreports/ pH 3 and 100 min. irradiation time. The photocatalytic degradation efficiencies of photocatalysts were calculated using the following formula 51 : where A 0 is the initial absorbance of MO solution and A is the absorbance after irradiation.

Data availability
All data generated or analyzed during this study are included in this article and its supplementary information file.