Chemical Synthesis of ZnO:Er Nanorods for Photocatalytic H<sub>2</sub> Evolution

Poornaprakash, B.; Puneetha, Peddathimula; Reddy, A. Subba; Prasad, P. Reddy; Reddy, M. Siva Pratap; Tighezza, Ammar M.; Rosaiah, P.; Sangaraju, Sambasivam; Park, Si-Hyun; Reddy, B. Purusottam; Roy, Soumyendu; Kim, Y. L.

doi:https://doi.org/10.1155/2023/6674910

International Journal of Energy Research

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Research Article | Open Access

Volume 2023 | Article ID 6674910 | https://doi.org/10.1155/2023/6674910

Chemical Synthesis of ZnO:Er Nanorods for Photocatalytic H₂ Evolution

B. Poornaprakash,¹Peddathimula Puneetha,²A. Subba Reddy,³P. Reddy Prasad,⁴M. Siva Pratap Reddy,⁵Ammar M. Tighezza,⁶P. Rosaiah,⁷Sambasivam Sangaraju,⁸Si-Hyun Park,⁹B. Purusottam Reddy,⁹Soumyendu Roy,¹⁰and Y. L. Kim¹

Academic Editor: Arun Thirumurugan

Received14 Jun 2023

Revised08 Aug 2023

Accepted25 Aug 2023

Published12 Sept 2023

Abstract

The growth of one-dimensional (1D) ZnO systems has been attaining ample engrossment because of their unique photocatalytic and optoelectronic properties. In this context, we synthesized the high-quality ZnO and ZnO:Er nanorods through an inexpensive and facile solvothermal route. Morphological studies revealed that the fabricated samples correspond to nanorods. Erbium ion substitution into the host ZnO matrix was affirmed via XRD and Raman analyses. XPS analysis proposed that Er ion substitution in the Zn (II) site occurred with trivalent Er ions. The trivial diminishing of the optical band gap with the incorporation of Er into the host matrix was assessed by diffuse reflectance spectroscopy (DRS) studies. The photoluminescence spectra of both fabricated NRs display two nodes at ultraviolet and red luminescence. The fluorescence efficiency of the ZnO NRs diminished after Er doping. The ZnO and ZnO:Er NRs were measured for H₂ evolution by water splitting beneath the UV light illumination. The ZnO:Er illustrated the efficient H₂ evolution ability (21311 μmol h⁻¹ g⁻¹) in 5 h, which is higher than that of ZnO. To analyze the attained engrossing H₂ outcomes, electrochemical studies were also employed. Fortunately, this is the first-ever endeavor in H₂ production of the ZnO:Er NRs.

1. Introduction

One-dimensional semiconductor compounds have enticed enormous research interest in the past one decade owing to their distinctive properties in diverse applications [1, 2]. Specifically, in the field of materials science, ZnO nanostructures have been recognized as a benign photocatalyst material due to its desirable oxidative capacity, chemical solidity, and efficient catalytic activity [3, 4]. Conversion of solar energy into hydrogen (H₂) gas is an ecological way to defeat the traditional energy impediment [5, 6]. Nevertheless, the massive recombination possibility of photogenerated carriers and their relatively large band gap hamper its real-time practice to some extent [7, 8]. To conquer this difficulty, it is requisite to broaden its photoresponse from the ultraviolet to the visible light zone. Thus, it is crucial to surface alterations or doping/codoping, which have been believed to be a potent way to effectively hamper the recombination of photogenerated carriers and mold them as an efficient photocatalyst. The doping of transition metals/rare earth ions into the ZnO compound could amend the coordination environment of Zn in the ZnO lattice and alter the electronic energy band structure of the ZnO system. On the other hand, ample endeavors have been studied to modify ZnO via doping with several cations. Lately, doping of Ga, Co, and V ions in the ZnO system can engender a notable red shift in the band gap, promoting electron transfer and tailoring of the Fermi level of the ZnO system [9–11]. In addition, TM/RE-doped ZnO compounds also come beneath the diluted magnetic semiconductors (DMSs). The preparation parameters and techniques play a vital role in ZnO nanostructure development. To synthesize the ZnO nanorods (NRs)/nanowires (NWs), diverse chemical and physical routes have been investigated; however, physical techniques are complicated and costly. More recently, ZnO nanostructures have been effectively prepared and utilized for the design and development of gas sensors. To attain optimal sensing performance, diverse methods have been used to produce ZnO nanostructures of different properties. Among them, thin film techniques have been proven to be beneficial, unique, and economical methods to produce high gas sensing performances due to their ultrahigh surface-to-volume ratio [12, 13]. Amid all the chemical techniques, solvothermal is an inexpensive and promising method to fabricate superior ZnO NRs. Based on our previous results, erbium has been proven to be the best dopant/codopant to enhance the photocatalytic activity of II-VI semiconductor compounds [14, 15]. In the present investigation, we made a promising strive to synthesize the ZnO and ZnO:Er NRs through the solvothermal route and analyzed their structural, photoluminescence, and H₂ production properties. Fortunately, this is the first H₂ production study on the ZnO:Er NRs.

2. Experimental and Characterizations

Zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O), erbium chloride hexahydrate (ErCl₃·6H₂O), and sodium hydroxide (NaOH) were used as starting chemicals, and methyl alcohol and milli-Q water were exploited as solvents. All the precursors were MERC company products. For the fabrication of ZnO NRs, 0.025 M of (Zn(NO₃)₂·6H₂O) was dissolved in 10 mL of methyl alcohol under magnetic stirring. Concurrently, 0.025 M of NaOH was dissolved in 10 mL of milli-Q water. Both solutions were mixed under magnetic stirring at 80°C. Eventually, this slurry was transferred into a 50 mL capacity autoclave and kept at 170°C for 2 days. The sediment was centrifuged and dried at 120°C for 10 h. For the fabrication of ZnO:Er NRs, 0.025 M of Zn(NO₃)₂·6H₂O and ErCl₃·6H₂O (2 at%) was dissolved in 20 mL of methyl alcohol beneath magnetic stirring. Next, 0.025 M of NaOH was dissolved in 20 mL of milli-Q water. Both solutions were mixed under magnetic stirring at 80°C. The further steps were the alike as those of the ZnO NRs.

The structural parameters were done utilizing a Seifert 3003 TT X-ray diffractometer with Cu Kα radiation with a wavelength of 1.540 Å, and the system was operated at 30 keV. Diffuse reflectance studies were employed using the Jasco V-670 double-beam spectrometer. A Phillips TECHNAI FE 12 transmission electron microscope (TEM) was employed for particle size and morphology confirmation. Photoluminescence studies were carried out using a JOBIN YVON Fluorolog-3 spectrometer with a 450 W Xenon arc lamp as an excitation source. An X-ray photon spectrometer (XPS), Model SPECS GmbH (Phoibos 100 MCD Energy Analyser) with Al Kα radiation (1486.6 eV) was utilized to find the existence of foreign phases in the nanoparticles. Raman spectra were recorded on a confocal Raman spectrometer (Thermo Fisher Scientific, Nicolet 6700) via utilizing a 532 nm laser source.

The photocatalytic hydrogen evolution tests were carried out in a 150 ml quartz reactor at room temperature with atmospheric pressure. The 10 mg of nanopowder was mixed in 100 ml of aqueous solution containing 0.25 mol L⁻¹ of Na₂SO₃ and Na₂S as electron donors. A 300 W (MaX 303 model) Xe lamp was utilized as a solar light (intensity: 50 mW cm⁻²) source. Prior to irradiation, the reactor was evacuated through a vacuum pump and bubbled with N₂ for 20 min to eliminate the air inside the reactor. The produced H₂ gas was appraised via an offline gas chromatograph (GC, YL-6500 instrument) equipped with a thermal conductivity detector and a 5-A molecular sieve column.

3. Results and Discussions

Figure 1 depicts the TEM images and EDS spectra of ZnO (Figures 1(a) and 1(b)) and ZnO:Er NRs (Figures 1(c) and 1(d)). It is visible that both samples were one-dimensional (1D) nanostructures without agglomeration. The prepared ZnO:Er NRs have a bigger diameter when correlated to that of the pristine NRs, and the length remains constant. It is evidence that the large grain size and surface area play an essential role in the enriched photocatalytic activity. The EDS spectra illustrate the existence of erbium, zinc, and oxygen in the fabricated NRs. From the EDS, the erbium concentration is found to be 1.41 at% for 2 at% of ZnO:Er NRs.

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(b)

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Figure 2(a) depicts the XRD prototypes of the ZnO and ZnO:Er NRs. In this figure, the diffraction nodes can be demonstrated in the hexagonal phase of the ZnO matrix. The lack of foreign peaks belonging to the metal clusters/secondary phases designating the dopant does not alter the internal structure of the host matrix. But in the ZnO:Er NRs, a major peak was trivially shifted towards the lower 2θ region, evidencing the incorporation of Er at the ZnO lattice. At the same time, the intensity of the diffraction nodes is mildly diminished via Er ion substitution. The diminishing could be attributed to the bigger ionic radius of the Er (III) dopant ion as compared to that of the Zn (II). The average crystallite sizes were assessed by employing Scherrer and Bragg’s law. The calculated lattice constant values were nm and nm for the ZnO and nm and nm for the ZnO:Er NRs, respectively. Furthermore, the assessed average crystallite size is from 41 to 48 nm. The acquired assessments agreed thoroughly with those of recently published articles [16–18]. Recently, our research group also successfully prepared Ga- [9], Co- [10], and V- [11] doped ZnO NRs through various methods. Moreover, to explore the additional structural analysis and defect sites of the fabricated ZnO and ZnO:Er NRs, Raman analysis was employed, as depicted in Figure 2(b). The spectra possess nodes at the wave number regions of 333, 389, and 441 cm⁻¹. Mostly, the nodes at 389 cm⁻¹ and 441 cm⁻¹ urge the indirect optical as well as longitudinal phonon modes, respectively. Moreover, the node in the zone of 333 cm⁻¹ is ascribed to the E_2H to E_2L phonon–phonon interaction activity. Genuinely, the E_2H mode is ascribed to the creation of lattice distortions and the presence of oxygen defects in the ZnO matrix. The E_2H phonon sort of ZnO:Er NRs does not notably change their venue as compared to that of the ZnO NRs, perhaps due to the vibrating of oxygen atoms. Furthermore, while the activity of Er incorporation amends the mass, the strain is stabilized at the oxygen core site. Thus, the presence of local strain and resulting stress on the surface of the NRs because of imperfections does not produce a solid peak shift in E_2H. Similar results were found by a few researchers in doped ZnO nanostructure [10, 19].

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(b)

To probe the valence states, chemical compositions, and impurity phases, XPS measurements were employed for all the ZnO and ZnO:Er NRs. Figure 3 represents the XPS (Figure 3(a)) survey scan and narrow scans of zinc (Figure 3(b)), sulfur (Figure 3(c)), and erbium (Figure 3(d)) of the ZnO:Er NRs. Figure 3(a) portrays the existence of Zn, O, and Er in the fabricated NRs. Figure 3(b) demonstrates two wide familiar peaks in the binding energy regions of 1022.10 eV and 1044.91 eV, which belong to Zn 2p_3/2 and Zn 2p_3/2 peaks, respectively [10]. The distance between these two peaks was 22.81 eV, confirming the divalent state of zinc in the host lattice. Further, Figure 3(c) displays the narrow scan of oxygen by the peaks at the binding energy regions of 528.95 eV and 532.01 eV [10]. The major peak in the lower binding energy zone of 528.95 eV is ascribed to the presence of O atoms at the systematic crystal site of the hexagonal ZnO NRs. A tiny hump at the higher binding energy zone of 531.46 eV is ascribed to the absence of O atoms in the wurtzite ZnO matrix. Figure 3(d) illustrates the Er peak at 170.21 eV, which corresponds to the binding energy of the Er 4d, confirming the trivalent chemical state of Er ions [14, 15]. From the XPS, the Er content is found to be 1.73 at%, which is very close to the targeted value and similar to the obtained EDS value. No foreign peaks belong to the other phases/clusters identified, evidencing the high purity of the fabricated NRs.

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(b)

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Figure 4(a) exhibits the reflectance spectra of the ZnO and ZnO:Er NRs. The absorption bands at 486, 521, 541, 654, and 797 nm designate the existence of trivalent Er ions in the ZnO NRs, which could be ascribed to the 4f-4f transition of trivalent Er ions from the ground state (⁴I_15/2) to diverse excited states of ⁴G_11/2, (²G, ⁴F, ²H)_9/2, ⁴F_3/2, ⁴F_7/2, ²H_11/2, ⁴S_3/2, ⁴F_9/2, and ⁴I_9/2 [20]. Furthermore, the absorption edge of the ZnO NRs depicted a trivial red shift upon Er ion doping, confirming the decline in the optical band gap. The optical band gaps of the synthesized NRs were appraised from DRS spectra by plotting the square of the Kubelka–Munk function () versus energy and extrapolating the linear part of the curve , as represented in Figure 4(b), which ranged from 3.21 to 3.30 eV. The decline could have occurred due to the sp-d exchange interaction between Er ions and the host [15]. The exchange interactions between dopant ions and band electrons produce a positive and negative correction to the energy of conduction and valance bands, respectively, and result in a decline in the optical band gap, which is an often-occurring issue in doped II-VI compounds. Figure 4(c) manifests the PL spectra of ZnO and ZnO:Er NRs. Both samples have two wide peaks in the wavelength zones of 405 nm and 648 nm belonging to UV and red luminescence, respectively. At the same time, the fluorescence efficiency is significantly diminished in ZnO:Er NRs as compared to ZnO NRs. The diminishing of fluorescence efficiency is perhaps attributed to the nonrecombination of holes and electrons/separation of charge carriers. Moreover, a red shift occurred after doping of Er into the ZnO matrix, confirming the authentic incorporation of Er ions in the place of Zn in the parent lattice.

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Figure 5(a) displays the consequences of H₂ production over the ZnO and ZnO:Er NRs beneath UV light irradiation. The obtained H₂ production values are 17086 μmolg⁻¹ h⁻¹ and 21311 μmolg⁻¹ h⁻¹ after 300 min for ZnO and ZnO:Er NRs. The improved H₂ evolution of the ZnO:Er NRs could be ascribed to the solar energy capability of synthesized NRs, the emergence of charge carriers, including electron and hole defects or trapping sites, and the transition of charge carriers towards the surface of the ZnO:Er NRs. Moreover, the nanorods possess a high surface area to permit the creation and transition of charge carriers to the surface of the ZnO:Er NRs. From the PL studies, the PL intensity also diminished in the ZnO after Er doping into the host matrix, indicating the mitigation of electron and hole recombination, which is an additional outcome of improving the H₂ evolution in the ZnO NRs after Er doping. Figure 5(b) illustrates the transient photocurrent results of the ZnO and ZnO:Er NRs. It is visible that the ZnO:Er NRs displayed greater photocurrent efficiency than the ZnO NRs. The generation of a large current in ZnO:Er NRs stipulates the presence of a few charge carriers and their migration. Moreover, to probe the charge migration rate, impedance measurement was employed, and the outcomes are represented in Figure 5(c). Figure 5(c) depicts the electroimpedance spectroscopic measurement of the ZnO and ZnO:Er NRs. This result indicated that the ZnO:Er NRs possessed an immense charge migration potential compared to the ZnO NRs. Figure 5(d) illustrates the durability measurement for H₂ evolution via the synthesized ZnO:Er NRs over 60 min of equal intervals. It is evident that the rate of H₂ production gradually improves in ZnO:Er NRs for each equal interval of times up to 360 min, and it is saturated at the 7^th cycle. However, after the inclusion of lactic acid as a hole scavenger in reaction media, H₂ evolution gently improves, and it stipulates that the fabricated ZnO:Er NRs possess additional durability. The improvement in the H₂ production rate could be attributed to the formation and existence of plentiful electrons in the parent matrix. Thus, ZnO:Er NRs are promising candidates for photocatalytic H₂ evolution beneath UV light irradiation. As per the literature, no H₂ evolution reports are available for ZnO:Er nanorods to compare our outcomes with others. Khataee et al. [21] reported enhanced dye degradation properties of ZnO:Er nanoparticles through the sonochemical route. Divya and Pradyumnan [22] observed enhanced photocatalytic activity in Er-doped ZnO nanoparticles via solid-state synthesis. Recently, our research group also achieved enhanced H₂ evolution outcomes in the ZnO nanorods after V doping [11]. Moreover, our research group also achieved enhanced photocatalytic dye degradation activity in ZnO nanorods after Ga and Co doping [9, 10].

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4. Conclusions

1D nanorods of ZnO and ZnO:Er NRs were synthesized by a facile solvothermal technique. TEM images confirmed the successful formation of nanorods of the prepared samples without agglomeration. Raman and XRD studies indicate that the hexagonal structure and impure-free behavior of the fabricated NRs and the Er doping could not disturb the intrinsic structure of the host matrix. A slight decline in the optical band gap was achieved in ZnO NRs after Er doping. Two PL peaks in the wavelength regions of 405 nm and 648 nm belong to UV and red luminescence and have been found for both samples with diminished fluorescence efficiency. The ZnO:Er portrayed the maximum H₂ evolution ability (25188 μmol h⁻¹ g⁻¹) in 300 min, which is higher than that of ZnO. The plausible causes behind the enhanced H₂ evolution have been discussed in detail.

Data Availability

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Authors’ Contributions

B. Poornaprakash and Peddathimula Puneetha contributed equally to this work.

Acknowledgments

This work was supported by the Technology Development Program (S3038568) funded by the Ministry of SMEs and Startups (MSS, Korea). This work was supported by Researchers Supporting Project number (RSPD2023R765), King Saud University, Riyadh, Saudi Arabia. This work was partially supported by the National Research Foundation Korea funded by the Ministry of Science, ICT and Fusion Research (Grant Nos: NRF-2022R1I1A1A01064248 and 2019R1A2C1089080).

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International Journal of Energy Research

Chemical Synthesis of ZnO:Er Nanorods for Photocatalytic H2 Evolution

Abstract

1. Introduction

2. Experimental and Characterizations

3. Results and Discussions

4. Conclusions

Data Availability

Conflicts of Interest

Authors’ Contributions

Acknowledgments

References

Copyright

Chemical Synthesis of ZnO:Er Nanorods for Photocatalytic H₂ Evolution