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Article

Enhanced Photoelectrochemical Water Oxidation Using TiO2-Co3O4 p–n Heterostructures Derived from in Situ-Loaded ZIF-67

by
Chau Thi Thanh Thu
1,†,
Hyo Jeong Jo
2,†,
Ganesh Koyyada
1,*,
Dae-Hwan Kim
2 and
Jae Hong Kim
1,*
1
Department of Chemical Engineering, Yeungnam University, 214-1, Daehak-ro 280, Gyeongsan 712-749, Republic of Korea
2
Division of Energy Technology, Daegu-Gyeongbuk Institute of Science and Technology (DGIST), Daegu 42988, Republic of Korea
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Materials 2023, 16(15), 5461; https://doi.org/10.3390/ma16155461
Submission received: 12 June 2023 / Revised: 22 July 2023 / Accepted: 26 July 2023 / Published: 4 August 2023
(This article belongs to the Special Issue Novel Approaches to Photoelectrochemical and Electrochemical Nanomaterials)
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Versions Notes

Abstract

:
Exposing catalytically active metal sites in metal–organic frameworks (MOFs) while maintaining porosity is beneficial for increasing electron transport to achieve better electrochemical energy conversion performance. Herein, we propose an in situ method for MOF formation and loading onto TiO2 nanorods (NR) using a simple solution-processable method followed by annealing to obtain TiO2-Co3O4. The as-prepared TiO2-ZIF-67 based photoanodes were annealed at 350, 450, and 550 °C to study the effect of carbonization on photo-electrochemical water oxidation. The successful loading of ZIF-67 on TiO2 and the formation of TiO2-Co3O4 heterojunction were confirmed by XRD, XPS, FE-SEM, and HRTEM analyses. TiO2-Co3O4-450 (the sample annealed at 450 °C) showed an enhanced photocurrent of 2.4 mA/cm2, which was 2.6 times larger than that of pristine TiO2. The improved photocurrent might be ascribed to the prepared p–n heterostructures (Co3O4 and TiO2), which promote electron–hole separation and charge transfer within the system and improve the photoelectrochemical performance. Moreover, the preparation of Co3O4 from the MOF carbonization process improved the electrical conductivity and significantly increased the number of exposed active sites and enhanced the photoresponse performance. The as-prepared ZIF-67 derived TiO2-Co3O4 based photoanodes demonstrate high PEC water oxidation, and the controlled carbonization method paves the way toward the synthesis of low-cost and efficient electrocatalysts.

Graphical Abstract

1. Introduction

Photoelectrochemical (PEC) water splitting is considered one of the most promising strategies for mitigating global fossil fuel shortages and addressing environmental issues [1,2,3]. The choice of semiconductor is critical in enhancing the PEC performance. However, semiconductors employed in PEC water splitting suffer from drawbacks, such as the recombination of photogenerated electron–hole pairs and a comparatively higher band gap, which limit the solar energy conversion efficiency. In this context, titanium dioxide (TiO2) has gained considerable attention as a prospective semiconductor for PEC water splitting owing to its perfect band-edge position, environmentally benign features, desired photocorrosion resistance, and cost-effectiveness [1,4]. Despite these advantages, the comparatively larger bandgaps of the rutile (3.0 eV) and anatase (3.2 eV) phases, slow oxygen evolution reaction (OER) kinetics, and a higher charge recombination have limited the use of TiO2 in PEC [4]. Several strategies have been proposed to overcome these limitations, including the use of metal oxides [5], heterojunctions [6], surface modification [7], introduction of defects [8], and quantum dots [4].
Among the aforementioned strategies, combining TiO2 photoelectrodes with a particular metal oxide is considered an effective method to enhance the PEC performance [9]. A suitable metal oxide with TiO2 heterostructure can improve the interfacial electric field and charge carrier migration [10]. Zhang et al. reported that the distinct rod-like structure of TiO2 is beneficial for fabricating heterojunctions owing to the large operating surface area of TiO2 [11]. Pan et al. found that the photocurrent density of the ZnO/TiO2 heterostructure was 3.8 times higher than that of pristine TiO2 owing to improved electron–hole separation [12]. Compared to ZnO, Co3O4 exhibits excellent photocatalytic properties owing to its absorption properties in the visible range [13,14,15]. Several strategies have been used to prepare Co3O4/TiO2 nanostructures. Xian et al. reported a solvothermal technique to prepare 0D/2D Co3O4/TiO2 nanostructures that exhibited enhanced photocatalytic performance [16]. Wang et al. reported a hydrothermal method for preparing Co3O4 QD/TiO2 nanobelts capable of improved water oxidation [17].
Metal–organic frameworks (MOFs) have become promising sacrificial templates for preparing metal oxide-based nanomaterials [18,19] because of the opportunity for property tenability with careful selection of functionalized organic linkers and metal ions [20,21]. Ordered MOF structures at the molecular level with uniform metal ion and organic linker distribution can avoid the regrowth and aggregation (inactivation) of metal/metal oxides during PEC water oxidation [5,22]. In particular, metal oxides derived from hollow MOF structures have porous walls; the large cavities allow an efficient diffusion of the ions and electrolytes, thereby exposing the active sites and minimizing the electron transfer resistance. In addition, the improved conductivity and O2 adsorption capacity of MOF-derived metal oxides further increase their electrocatalytic performance [18,23].
Zeolitic imidazolate frameworks (ZIFs) are a large MOF subclass, among which ZIF-67 is one of the most characteristic members [23] with self-assembled Co and imidazole links [24] Du et al. successfully prepared PtCo@NC via the thermal decomposition of Pt@ZIF-67, demonstrating its high catalytic activity and excellent durability in ORR [25]. Sun et al. successfully prepared Ni2P/CoN-PCP catalysts from the carbonization of MOF materials, which exhibited remarkable catalytic activity owing to excellent electrical conductivity, exposure of a large number of active sites, improved surface area, and porosity [26]. Recently, Qijia et al. prepared Co3O4 loaded photoanodes using a GIF-67 templating process by changing the soaking time of TiO2 based photoanode in MOF solution. It showed 1.65 higher photocurrent than the pristine TiO2 [27]. These previous studies suggest that the fabrication of MOF-derived Co3O4/TiO2 heterostructures is a viable option to enhance PEC water splitting.
Herein, we successfully prepared in situ ZIF-67 loaded TiO2 nanorod (NR)-based photoanodes through a simple solvothermal strategy and studied the effect of carbonization on the PEC water oxidation performance by subjecting the photoanodes to carbonization at 350, 450, and 550 °C. The prepared photoanodes were characterized, and their PEC water oxidation performances were systematically compared. Among the four photoelectrodes, the highest photocurrent (2.4 mA·cm−2) was observed for TiO2-Co3O4-450 (the sample subjected to carbonization at 450 °C), which was 2.6 times higher than that observed for pristine TiO2. The preparation of photoelectrodes is illustrated in Scheme 1.

2. Experimental Section

2.1. Preparation of Rutile TiO2 Film (TiO2)

Fluorine-doped tin oxide (FTO)-coated glasses (1.5 mm × 2.5 mm, 8 Ω/cm2) were ultrasonically cleaned using detergent, Milli-Q water, ethanol, and acetone, respectively. The TiO2 film was synthesized via a previously reported hydrothermal method, with some modifications [28]. In particular, 1.32 mL of TBOT was added dropwise to an 80 mL solution containing equal volumes of HCl (35%) and Milli-Q water under continuous and vigorous magnetic stirring until the solution turned transparent. The solution was then transferred into a 100 mL Teflon-lined stainless steel autoclave. Eight pieces of the FTO glass were held in an upright position using a custom-made Teflon holder and were immersed in the solution and heated to 150 °C in an oven for 4 h. The FTO glass containing the TiO2 film was cooled and subsequently rinsed thoroughly with Milli-Q water and ethanol. It was then sintered at an elevated temperature of 450 °C for 1 h in air.

2.2. Preparation of TiO2-ZIF-67

ZIF-67 was prepared via a previously reported procedure [29]. Briefly, 1.164 g of Co(NO3)2.6H2O was dissolved in 200 mL of methanol and stirred at room temperature to form a “pink A” solution, and the TiO2 films were soaked in the pink A solution. Similarly, 0.985 g of Melm was dissolved in 100 mL methanol to form a “transparent B” solution. The transparent B solution was added dropwise to the pink A solution containing the TiO2 films, and the mixture was vigorously stirred at room temperature for 24 h. The TiO2-ZIF-67 films were thoroughly rinsed with methanol to remove inorganic residues and dried at 50 °C for 1 h in an oven.

2.3. Preparation of TiO2-Co3O4

The TiO2-ZIF-67 thin films were heated at a ramping speed of 3 °C/min and sintered at 350, 450, and 550 °C in air for 3 h. The three Co3O4 samples thus obtained are denoted according to the respective sintering temperatures as TiO2-Co3O4-350, TiO2-Co3O4-450, and TiO2-Co3O4-550.

2.4. Preparation of Photoanodes for PEC Tests

Copper wires were attached to the as-prepared photoanodes using silver paint for connectivity. The samples were dried in air for 3 h. Finally, the boundaries of the samples were encased using non-conducting epoxy resin, leaving behind an illuminated area of 1 cm2. The samples were then dried in a desiccator for at least 3 h.

3. Results and Discussion

TiO2-Co3O4 was prepared via a three-step process as illustrated in Scheme 1. Initially, rutile-phase TiO2 nanorods (NR) were grown uniformly on an FTO glass plate via a hydrothermal method. Next, ZIF-67 was directly grown in situ on the TiO2 NRs via a simple solution-processable wet chemical method. Finally, TiO2-Co3O4 was prepared by calcination. Three composites, namely TiO2-Co3O4-350, TiO2-Co3O4-450, and TiO2-Co3O4-550, were synthesized by varying the calcination temperature. The photo-electrochemical water oxidation capabilities of the as-synthesized photoanodes were investigated.

3.1. XRD Characterization of the Prepared Nanostructures

The crystalline phase and the effect of heating on TiO2 and ZIF-67 were investigated using XRD analysis, and the corresponding XRD patterns are depicted in Figure 1 and Figure S1. The diffraction patterns shown in Figure S1 reveal that the TiO2 NR was in the rutile phase (JCPDS No. 21-1276), and SnO2 was in the tetragonal phase (FTO) (JCPDS. No. 46-1088). The XRD patterns of the TiO2-ZiF-67 (Figure 1) photoanode showed new peaks at 10.3°, 12.7°, 17.9°, 22.1°, and 26.6°, corresponding to ZIF-67, thereby confirming that the ZIF-67 was successfully loaded on TiO2 [28]. For the electrodes annealed at temperatures 350, 450, and 550 °C, XRD peaks were observed at 19.0°, 31.2°, 36.9°, 41.3°, 54.4°, 62.8°, and 69.9°, corresponding to the planes of (111), (220), (311), (400), (422), (511), and (440), respectively. The observed peaks are all in good agreement with those of Co3O4 (JCPDS 42-1467) and are well-matched with previous reports [30]. The XRD results indicate that Co3O4 was formed by the calcination of ZIF-67. Moreover, the characteristic peaks of CoO or other impurities were not observed (Figure 1). The strong intensity peak of the Co3O4 at 36.9° indicated the (311) plane is the preferred growth orientation.

3.2. Morphology Characterization of the Prepared Nanostructures

The surface morphologies and structural patterns of TiO2-ZIF-67 and the derived structures were examined via SEM and TEM analyses. Figure 2 shows the NR structure of TiO2, which had an average width of ~77.31 nm and a length of ~904.25 nm (Figure S4). Moreover, the top and cross-sectional images shown in Figure 2a confirm that the thickness of the TiO2 NR layer was 1.2 μm and that the dodecahedral structure of ZIF-67 was successful loaded [29]. Figure 2b–d show the morphologies of the ZIF-67 after calcination at 350 °C, 450 °C and 550 °C, respectively. As the carbonization temperature increased from 350 to 450 °C, the crystal began to expand at different interfaces, and the interaction with TiO2 also was observed to increase (Figure 2b,d). Upon further increasing the carbonization temperature to 550 °C, the MOF framework began to misalign, as shown in Figure 2d. Moreover, the EDS analysis was executed for the TiO2-ZIF-67 and TiO2-Co3O4-450 samples, and the results are discussed in Figure S2 and Table S1. Compared to TiO2-ZIF-67, the decreased carbon content observed for TiO2-Co3O4-450 might be due to the burning of organic matter lost in the form of CO2.
High-resolution transmission electron microscopy (HRTEM) was used to obtain a better understanding of the structural changes due to thermolysis; Figure 3 shows the corresponding HRTEM images. The HRTEM images show the NR and dodecahedral structures of TiO2 and ZIF-67 [31], respectively, and are consistent with the FESEM images. As observed in Figure 3b,c, upon calcination of TiO2-ZIF-67 at 350 and 450 °C, the hollowed nature inside the dodecahedral increased due to the release of gas like H2O and CO2 during the calcination process, and the actives sites were exposed. However, further increasing the calcination temperature to 550 °C induced decomposition or misalignment of the nanostructure, as observed in Figure 3d. This decomposition or misalignment can be ascribed to the variance in the thermal expansion coefficients: the contact between dissimilar metals (Ti, Co) increased with an increase in the temperature. The HRTEM image of TiO2-Co3O4-450 (Figure 4a) showed lattice spacing of 0.239 and 0.285 nm attributed to the (311) and (101) planes of TiO2 and Co3O4, respectively [32,33]. Moreover, the selected area electron diffraction (SAED) pattern, displayed in Figure 4b, indicated the multi-crystalline nature of TiO2-Co3O4-450, which correlated with the XRD results. The HAADF spectra of TiO2-Co3O4-450 (Figure 4c–f) illustrate the presence of Co, Ti, and O elements. The obtained results further support the successful formation Co3O4 metal oxide.
XPS analysis was conducted to further elucidate the elemental surface composition and electronic state alteration due to thermolysis, and the obtained results are depicted in Figure 5. XPS survey scans of pristine TiO2, TiO2-ZIF-67, and TiO2-Co3O4-450 are presented in Figure 5a. The presence of Co and Ti in the TiO2-ZIF-67 sample indicates the successful decoration of ZIF-67. Furthermore, upon carbonization at 450 °C (TiO2-Co3O4-450), the N element peak disappeared from the TiO2-Co3O4-450 survey scan spectrum, possibly due to ligand loss. These results are consistent with the SEM-EDX analysis results. The XPS spectra of Ti 2p of the as-prepared samples are described in Figure 5b. The binding energies (BE) at 464.44 eV and 458.72 eV were ascribed to the Ti 2p1/2 and Ti 2p3/2, respectively. The change among these peaks was 5.8 eV, which confirmed the presence of the Ti+4 electronic state in TiO2 [27,34]. As revealed in Figure 5c, the Co 2p peaks of the TiO2-ZIF-67 and TiO2-Co3O4-450 samples showed four peaks attributed to the 2p1/2 and 2p3/2 doublets and their respective satellite peaks positioned at higher binding energies. The characteristic fitting peaks of TiO2-ZIF-67 at 780.32 and 796.20 eV related to Co+3, and the 781.86 and 797.25 eV peaks were ascribed to Co+2. The Co 2p Co+3 and Co+2 peaks of the TiO2-Co3O4-450 sample appeared at 779.07/794.44 eV and 780.72/796.20 eV, respectively. The ratio of Co3+/Co2+ in TiO2-Co3O4-450 was higher compared to the ZIF-67. It can be attributed to the partial ligand damage due to carbonization, which destroys the coordination bond of Co−N and boosts the creation of Co3+. The improved Co3+ intensity in TiO2-Co3O4-450 indicated better electrochemical activity [35,36]. The O1s XPS spectra of pristine TiO2, TiO2-ZIF-67, and TiO2-Co3O4 450 are shown in Figure 5d. All of these samples’ O 1s spectra showed two fitted peaks [37]. The peak appearing at ~529.91 eV was recognized as the lattice oxygen (Olat), and the 531.67 eV peak was recognized as the adsorbed oxygen (Oads). In general, Olat is more active and significant in redox reactions, whereas Oads depends strongly on the oxidative properties of the catalyst [38]. Additionally, the Oads species concentration is related to the density of oxygen vacancies. Hence, the relative ratios of the Oads peak to the Olat of TiO2, TiO2-ZIF-67, and TiO2-Co3O4-450 were estimated as 0.26, 0.56 and 0.32, respectively. The higher relative ratio of TiO2-Co3O4-450 further supported the higher catalytic water oxidation property [35].
To further understand the effect of carbonization temperature on the conductivity of TiO2-Co3O4, an electrochemical double-layer capacitance (Cdl) experiment was performed, and the electrochemical active surface area (ECSA) was estimated for all of the materials [39,40]. Figure 6a–e display the cyclic voltammetry of TiO2, TiO2-ZIF-67, TiO2-Co3O4-350, TiO2-Co3O4-450 and TiO2-Co3O4-550, respectively, with changing scan rate from 30 mVS−1 to 250 mVS−1. This series of experiments was performed to extract the linear relationship of difference in current density (Δj/2) with scan rate. As observed from Figure 6f, TiO2, TiO2-ZIF-67, TiO2-Co3O4-350, TiO2-Co3O4-450 and TiO2-Co3O4-550 photoelectrodes showed 0.03. 0.031, 0.036, 0.041 and 0.039 mFcm−2, respectively. The obtained results suggest that the TiO2-Co3O4-450 photoanode has a larger active electrochemical surface area; it thereby exposes more active sites compared to its counterparts. Hence, the catalytic properties of TiO2-Co3O4-450 could be ascribed to the higher specific surface area.
PEC water oxidation performances of the as-prepared photoelectrodes (TiO2, TiO2-ZIF-67, TiO2-Co3O4-350, TiO2-Co3O4-450, and TiO2-Co3O4-550) were analyzed using a three-electrode setup under an illumination of 1.5 G at 100 mW/cm−2 with 0.5 M Na2SO4 solution. To study the effect of carbonization temperature on the TiO2-ZIF-67 photoanodes, we set the calcination temperatures to 350, 450, and 500 °C, and the prepared photoanodes were named TiO2-Co3O4-350, TiO2-Co3O4-450, and TiO2-Co3O4-550, respectively. Figure 7a demonstrates the linear sweep voltammetry (LSV) results for the as-prepared photoanodes. The TiO2-ZIF-67 photoanode showed a photocurrent of 1.8 mA/cm2 at 1.85 V vs. RHE, which was twice the photocurrent of the pristine TiO2-based photoanode (0.9 mA/cm2 at 1.85 V vs. RHE). The increase in photocurrent upon the introduction of ZIF-67 might be due to the improved interaction with molecules of water and its intermediates, which increases the charge transfer between the electrolyte and photoanode interfaces. Upon calcination at 350 and 450 °C, the photocurrent further increased, respectively, to 2.1 and 2.4 mA/cm2 at 1.85 V vs. RHE. This increase in the photocurrent can be ascribed to the formation of p-type porous Co3O4 metal oxide. As observed in the XRD and HRTEM studies, the temperature treatment at 350 and 450 °C resulted in the enhanced particle size along with an increase in the size of the cavity, which facilitated the efficient diffusion of ions and electrolytes. Therefore, the active sites are exposed, thereby promoting interactions with water and its intermediates. Consequently, the electron–hole separation increases at the catalyst interface, leading to increased water oxidation. The photocurrent decreased to 2.2 mA/cm−2 upon further heating to 550 °C, which can be attributed to the damaged morphology, as observed in the TEM analysis [6,41]. Moreover, the observed onsite potentials of the as-prepared photoelectrodes of TiO2, TiO2-ZIF-67, TiO2-Co3O4-350, TiO2-Co3O4-450, and TiO2-Co3O4-550 were 0.59, 0.56, 0.50, 0.46 and 0.55 V vs. RHE, respectively. The enhancement in the photocurrent and the decline in the onsite potential of TiO2-Co3O4 compared to those of TiO2-ZIF-67 further demonstrate the advantage of the controlled carbonization process of ZIF-67 in achieving higher PEC water oxidation [42].
Chronoamperometry (CA) analysis was performed with chopped illumination at 1.85 V vs. RHE to better understand the effect of the MOF and its derivatives on the stability and photoresponse of TiO2 with respect to time. As shown in Figure 7b, the photocurrent was immediately improved after irradiation and suddenly fell to zero after the irradiation was stopped, suggesting a reproducible current for the prepared photoanodes. These results demonstrated the reproducible nature of the as-prepared photoanodes. The photoresponse of TiO2-Co3O4-based photoanodes was higher than those of TiO2-ZIF-67 and pristine TiO2, suggesting reduced recombination after carbonization. The observed photocurrent trend in the CA analysis is consistent with the LSV results. Moreover, we performed a one-hour continuous illuminated CA analysis of the pristine TiO2 and TiO2-Co3O4-450 photoanodes to assess the durability of the prepared electrodes. As shown in Figure 7c, TiO2-Co3O4-450 photoanodes exhibited 99% of their initial performance even after 1 h of continuous illumination, which is comparable to the stability of TiO2. Further, using SEM, we analyzed the morphological changes in TiO2-Co3O4-450 photoanode after 1 h of a stability experiment, and the corresponding SEM images are included in Figure S3.
IPCE analyses were performed in the wavelength range of 350–550 nm following eq S1 to better understand the energy conversion efficiency of the prepared photoanode. As shown in Figure 8a, all photoanodes showed the highest IPCE at ~350 nm. The observed highest IPCE values of TiO2, TiO2-ZIF-67, and its derivatives at 350, 450, and 550 °C were 25, 42, 52, 53, and 57%, respectively. The maximum quantum yield was observed for TiO2-Co3O4-450 and was 2.25 times greater than that of the pristine TiO2-based photoanode. The results demonstrate the advantage of TiO2 and Co3O4 heterojunctions in improving the visible light harvesting ability and the separation/transportation of photogenerated charge species.
In addition, we calculated the applied bias potential to the current conversion efficiency (ABPE) from the LSV analysis data using eq S2. As found in Figure 8b, the maximum ABPE of pristine TiO2, TiO2-ZIF-67, and its derivatives at 350, 450, and 550 °C were 0.42, 0.81, 1.20, 1.48, and 1.30%, respectively. The ABPE of TiO2-Co3O4-450 was 3.5 and 1.8 times higher than pristine that of TiO2 and TiO2-ZIF-67, respectively, indicating efficient charge separation by the introduction of MOF and calcination [4].
EIS was executed to evaluate the carbonization temperature effects on the characteristics of the interfacial charge transfer resistance and carrier transport capacity. In general, the radius of the semicircle in the EIS fitted plots represents the interfacial charge transfer resistance (Rct), that is, the lower the radius, the lower the Rct. As shown in Figure 8c, the Rct decreasing order was TiO2 > TiO2-ZIF-67 > TiO2-Co3O4-350 > TiO2-Co3O4-450 > TiO2-Co3O4-550. The lower Rct of the TiO2-Co3O4 based photoanodes compared to the TiO2–ZIF-67 and pristine TiO2 photoanodes demonstrates the advantage of the carbonization process in improving charge transportation. In particular, TiO2-Co3O4-450 showed the lowest Rct, indicating higher charge separation/migration; this could be a possible reason for the higher photocurrent of the TiO2-Co3O4-450 photoanode compared with other photoanodes. Further, the charge injection efficiency (ηinjection) was estimated for the prepared photoanodes by the hole trapping method using H2O2 as a sacrificial agent [27]. The formula used for the calculation of ηinjection is as follows.
η injection = J photocurrent / J H 2 O 2
Jphotocurrent and JH2O2 are the photocurrent density with and without H2O2 in the 0.5 M Na2SO4 solution. As shown in Figure 8d, ηinjection of TiO2, TiO2-ZIF-67, TiO2-Co3O4-350, TiO2-Co3O4-450 and TiO2-Co3O4-550 photoanodes were 44.05, 53.42, 60.21, 94.31 and 77.13% at 1.85 V vs. RHE, respectively. The higher ηinjection was observed for the TiO2-Co3O4-450 compared to its counterpart photoanodes. The higher ηinjection means lower recombination and a fast charge transfer process.
The energy levels of the prepared photoanodes are crucial to understanding the enhanced PEC performance of TiO2-CO3O4 composite material. The valence band (VB) XPS analyses of TiO2 and MOF derived Co3O4 materials were performed, and the obtained plots are illustrated in Figure 9. The valence band maxima (VBM) of individual materials were determined by the extrapolation method. The VBM of TiO2 and Co3O4 were found to be 2.01 and −0.19 eV, respectively, under the Fermi level. The VBM value of Co3O4 indicated the p-type nature of Co3O4. Further, the estimated bandgaps from the absorption experiments of TiO2 and Co3O4 were 3.03 and 2.91 eV, respectively. Using the obtained VBM results, the conduction band (CB) edges of TiO2 and Co3O4 were estimated as −1.02 and −3.1 eV, respectively (Figure 9c). Based on the reported p–n junction-based PEC water oxidation mechanism, a space charge layer (SCL) is formed at the interface of p- and n-type semiconductor materials [43,44,45,46]. The generation of the SCL drives the migration of the maximum of charge carriers in opposite directions in p- and n-type materials, which causes the formation of the electric potential at the contacts of the conductors and the p–n junction. Figure 9c illustrates the expected charge transfer methods in the TiO2/Co3O4 based photoanode. The Co3O4 CB is higher than that of TiO2, whereas the Fermi level is less negative (lower). When a heterojunction is formed between TiO2 and Co3O4, the Fermi level will rearrange and reach the equilibrium level by the diffusion of electron–hole pairs from each material, which leads to the creation of SPL, as mentioned earlier (Figure 9c). Upon irradiation, the generated holes in the VB of TiO2 are transferred to Co3O4 and then moved to water oxidation, while the electrons (photogenerated) from the CB of Co3O4 are transported to the TiO2 and then transferred to the Pt electrode through FTO and back contacts, where H+ is reduced and generates H2. The electric field formed at the p–n junction of TiO2 and Co3O4 materials could enhance the effective electron–hole pair separation and dramatically lower the rate of recombination [43,47]. This outcome was crucial in improving the photocurrent density observed for TiO2/Co3O4 photoanodes in comparison to TiO2 photoanodes. Moreover, the acquired photoelectrochemical performances were compared with the previously reported most similar composites and are listed in Table S2.

4. Conclusions

In conclusion, we successfully prepared in situ-loaded TiO2-ZIF-67 and TiO2-Co3O4 photoanodes by varying the carbonization temperature from 350 to 550 °C. The effect of carbonization temperature on prepared photoanodes was systematically studied to explore the PEC water oxidation process. Different characterization methods, such as SEM, TEM, XRD, and XPS, disclosed the successful formation of hollow Co3O4 metal oxides upon thermal treatment. The TiO2-Co3O4-based photoanodes showed higher photocurrent densities and lower onsite potentials than the TiO2-ZIF-67 and pristine TiO2 photoanodes. In particular, an improved photocurrent of 2.4 mA·cm−2 was observed for the TiO2-Co3O4-450 photoanode, which was 2.6 times larger than that of the pristine TiO2 photoanode and 1.33 times larger than that of the TiO2-ZIF-67 photoanode. The XRD and HR-TEM analyses revealed that the derived Co3O4 has a larger crystalline size and cavity size than those of ZIF-67, and the crystalline size and cavity size were increased by raising the carbonization temperature from 350 to 450 °C. The increased porous surface and cavity inside the Co3O4 particles allow for efficient diffusion of ions and electrolytes. Therefore, the number of exposed active sites increases, leading to increased electron–hole separation and transportation. Moreover, impedance analyses revealed that TiO2-Co3O4 has a lower charge transfer resistance than TiO2-ZIF-67, which further supports the increased electron separation and transport due to the formation of porous Co3O4 metal oxide. The increased electron–hole separation, decreased charge transfer resistance, and improved interaction with water molecules and their intermediates are the possible reasons for the increase in the photocurrent of the TiO2-Co3O4 photoanodes. The obtained results indicate the advantage of controlled carbonization of ZIF-67 in improving its catalytic properties and pave the way for the synthesis of better PEC catalysts.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/ma16155461/s1, General Procedures, Materials, Characterizations, PEC analysis procedure, XRD of pristine TiO2 and FTO films, SEM-EDS analysis [27,48,49,50,51,52,53].

Author Contributions

Conceptualization, C.T.T.T., G.K. and J.H.K.; methodology, C.T.T.T. and G.K.; software, D.H.K.; validation, G.K. and J.H.K.; formal analysis, C.T.T.T. and G.K.; investigation, C.T.T.T.; resources, J.H.K.; data curation, G.K. and H.J.J.; writing—original draft preparation, C.T.T.T. and H.J.J.; writing—review and editing, G.K. and J.H.K.; visualization, G.K. and J.H.K.; supervision, G.K. and J.H.K.; funding acquisition, J.H.K. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the 2022 Yeungnam University Research Grant.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data will be made available on request.

Conflicts of Interest

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.

Abbreviations

MOFs, metal-organic frameworks; NR, nanorods; PEC, Photoelectrochemical; ZIFs, Zeolitic imidazolate frameworks; TBOT, titanium butoxide; 2-Melm, 2-Methylimidazole; FTO, Fluorine-doped tin oxide; FESEM, field-emission scanning electron microscopy; FETEM, field-emission transmission electron microscopy; EDS, energy dispersive spectrometry; XRD, X-ray diffraction; XPS, X-ray photoelectron spectroscopy; LSV, Linear sweep voltammetry; EIS, Electrochemical impedance spectroscopy; IPCE, incident photon-to-current conversion efficiency; HRTEM, High-resolution transmission electron microscopy; SAED, selected area electron diffraction; CA, chronoamperometry; ABPE, applied bias potential to the current conversion efficiency.

References

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Materials 16 05461 sch001
Scheme 1. (I) Preparation of the TiO2 nanorod film (II) Preparation of the TiO2-Co3O4 nanostructure photoanode for photoelectrochemical water splitting.
Scheme 1. (I) Preparation of the TiO2 nanorod film (II) Preparation of the TiO2-Co3O4 nanostructure photoanode for photoelectrochemical water splitting.
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Figure 1. XRD diffraction patterns of (a) TiO2-ZIF-67, (b) TiO2-Co3O4-350 °C, (c) TiO2-Co3O4-450 °C, (d) TiO2-Co3O4-550 °C.
Figure 1. XRD diffraction patterns of (a) TiO2-ZIF-67, (b) TiO2-Co3O4-350 °C, (c) TiO2-Co3O4-450 °C, (d) TiO2-Co3O4-550 °C.
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Figure 2. FE-SEM images of the Co3O4-TiO2 nanostructures. (a) TiO2- ZIF-67, (b) TiO2-Co3O4-350, (c) TiO2-Co3O4-450, (d) TiO2-Co3O4-550.
Figure 2. FE-SEM images of the Co3O4-TiO2 nanostructures. (a) TiO2- ZIF-67, (b) TiO2-Co3O4-350, (c) TiO2-Co3O4-450, (d) TiO2-Co3O4-550.
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Figure 3. HRTEM images of (a) TiO2-ZIF-67, (b) TiO2-Co3O4-350, (c) TiO2-Co3O4-450, (d) TiO2-Co3O4-550.
Figure 3. HRTEM images of (a) TiO2-ZIF-67, (b) TiO2-Co3O4-350, (c) TiO2-Co3O4-450, (d) TiO2-Co3O4-550.
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Figure 4. (a) HR-TEM. (b) The selected area electron diffraction (SAED) pattern. (c) HAADF image of the TiO2-Co3O4-450 elemental mapping, (d) Ti, (e) O, (f) Co.
Figure 4. (a) HR-TEM. (b) The selected area electron diffraction (SAED) pattern. (c) HAADF image of the TiO2-Co3O4-450 elemental mapping, (d) Ti, (e) O, (f) Co.
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Figure 5. (a) XPS survey scan; (b) Ti 2p, (c) Co 2p, and (d) O 1s of TiO2-ZIF-67 and TiO2-Co3O4-450.
Figure 5. (a) XPS survey scan; (b) Ti 2p, (c) Co 2p, and (d) O 1s of TiO2-ZIF-67 and TiO2-Co3O4-450.
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Figure 6. Electrochemical active surface area test (ECSA) of (a) TiO2, (b) TiO2-ZIF-67, (c) TiO2-Co3O4-350, (d) TiO2-Co3O4-450, and (e) TiO2-Co3O4-550; (f) electrochemical double layer capacity (Cdl).
Figure 6. Electrochemical active surface area test (ECSA) of (a) TiO2, (b) TiO2-ZIF-67, (c) TiO2-Co3O4-350, (d) TiO2-Co3O4-450, and (e) TiO2-Co3O4-550; (f) electrochemical double layer capacity (Cdl).
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Figure 7. (a) LSV plots. (b) Transient photocurrent density plots at 1.85 V vs. RHE of all the photoelectrodes. (c) Stability test of pristine TiO2 and TiO2-Co3O4-450 at 1.85 V vs. RHE.
Figure 7. (a) LSV plots. (b) Transient photocurrent density plots at 1.85 V vs. RHE of all the photoelectrodes. (c) Stability test of pristine TiO2 and TiO2-Co3O4-450 at 1.85 V vs. RHE.
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Figure 8. (a) Incident photon-to-current conversion efficiency (IPCE), (b) ABPE, (c) EIS of TiO2, TiO2-ZIF-67, TiO2-Co3O4- 350 °C, TiO2-Co3O4-450 °C and TiO2-Co3O4-550 °C (upper-right insert figure is the enlargement of high-frequency impedance, and the other insert figure is the equivalent circuit model). (d) Charge injection efficiencies of the samples.
Figure 8. (a) Incident photon-to-current conversion efficiency (IPCE), (b) ABPE, (c) EIS of TiO2, TiO2-ZIF-67, TiO2-Co3O4- 350 °C, TiO2-Co3O4-450 °C and TiO2-Co3O4-550 °C (upper-right insert figure is the enlargement of high-frequency impedance, and the other insert figure is the equivalent circuit model). (d) Charge injection efficiencies of the samples.
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Figure 9. (a,b) VB-XPS spectra of TiO2 and Co3O4. (c) Schematic energy level diagram and charge transfer process for Co3O4-TiO2 heterostructures.
Figure 9. (a,b) VB-XPS spectra of TiO2 and Co3O4. (c) Schematic energy level diagram and charge transfer process for Co3O4-TiO2 heterostructures.
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Thanh Thu, C.T.; Jo, H.J.; Koyyada, G.; Kim, D.-H.; Kim, J.H. Enhanced Photoelectrochemical Water Oxidation Using TiO2-Co3O4 p–n Heterostructures Derived from in Situ-Loaded ZIF-67. Materials 2023, 16, 5461. https://doi.org/10.3390/ma16155461

AMA Style

Thanh Thu CT, Jo HJ, Koyyada G, Kim D-H, Kim JH. Enhanced Photoelectrochemical Water Oxidation Using TiO2-Co3O4 p–n Heterostructures Derived from in Situ-Loaded ZIF-67. Materials. 2023; 16(15):5461. https://doi.org/10.3390/ma16155461

Chicago/Turabian Style

Thanh Thu, Chau Thi, Hyo Jeong Jo, Ganesh Koyyada, Dae-Hwan Kim, and Jae Hong Kim. 2023. "Enhanced Photoelectrochemical Water Oxidation Using TiO2-Co3O4 p–n Heterostructures Derived from in Situ-Loaded ZIF-67" Materials 16, no. 15: 5461. https://doi.org/10.3390/ma16155461

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