Next Article in Journal
DBU-Intercalated α-Zirconium Phosphates as Latent Thermal Catalysts in the Reaction of Hexamethylene Diisocyanate and Phenol
Next Article in Special Issue
Improving Photoelectrochemical Activity of ZnO/TiO2 Core–Shell Nanostructure through Ag Nanoparticle Integration
Previous Article in Journal
Photocatalysis: Activity of Nanomaterials
Previous Article in Special Issue
Oxygen Evolution Activity of LaNbN2O-Based Photocatalysts Obtained from Nitridation of a Precursor Oxide Structurally Modified by Incorporating Volatile Elements
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Catalysts
Volume 11
Issue 5
10.3390/catal11050612
catalysts-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Effects of RhCrOx Cocatalyst Loaded on Different Metal Doped LaFeO3 Perovskites with Photocatalytic Hydrogen Performance under Visible Light Irradiation

by
Tzu Hsuan Chiang
*,
Gujjula Viswanath
and
Yu-Si Chen
Department of Energy Engineering, National United University, No. 2, Lienda, Nan-Shi Li, Miaoli 36006, Taiwan
*
Author to whom correspondence should be addressed.
Catalysts 2021, 11(5), 612; https://doi.org/10.3390/catal11050612
Submission received: 8 April 2021 / Revised: 5 May 2021 / Accepted: 7 May 2021 / Published: 11 May 2021
(This article belongs to the Special Issue Advances in Water-Splitting Activity of Highly Efficient Cocatalyst/Photocatalyst Composites)
Browse Figures
Review Reports Versions Notes

Abstract

:
Photocatalytic hydrogen (H2) production by water splitting provides an alternative to fossil fuels using clean and renewable energy, which gives important requirements about the efficiency of photocatalysts, co-catalysts, and sacrificial agents. To achieve higher H2 production efficiencies from water splitting, the study uses different metals such as yttrium (Y), praseodymium (Pr), magnesium (Mg), Indium (In), calcium (Ca), europium (Eu), and terbium (Tb) doped lanthanum iron oxide (LaFeO3) perovskites. They were synthesized using a co-precipitate method in a citric acid solution, which was loaded with the rhodium chromium oxide (RhCrOx) cocatalysts by an impregnation method along with a detailed investigation of photocatalytic hydrogen evolution performance. Photoluminescence (PL) and UV–Vis diffuse reflectance spectra (DRS) measured the rate of electron–hole recombination for RhCrOx/Pr-LaFeO3 photocatalysts, and X-ray powder diffraction (XRD), scanning electron microscope (SEM), high resolution transmission electron microscope (HRTEM), and X-ray photoelectron spectra (XPS) analyzed their characteristics. The experimental results obtained show that the samples with 0.5 wt.% RhCrOx loading and 0.1 M Pr-doped LaFeO3 calcined at a temperature of 700 °C (0.1Pr-LaFeO3-700) exhibited the highest photocatalytic H2 evolution rate of 127 µmol h−1 g−1, which is 34% higher photocatalytic H2 evolution performance than undoped LaFeO3 photocatalysts (94.8 μmol h−1 g−1). A measure of 20% of triethanolamine (TEOA) enabled a high hole capture capability and promoted 0.1-Pr-LaFeO3-700 to get the highest H2 evolution rate.

1. Introduction

Hydrogen is an ideal clean fuel of the future and is compatible with current fuel storage and transportation infrastructures, along with suitability for extended periods of storage [1]. Hydrogen gas is supplied to the anode compartment of proton exchange membrane fuel cells (PEMFCs), and is then oxidized to form two protons and two electrons [2]. Compared to conventional fossil fuels used for electricity generation, the PEMFC is considered to be one of the most efficient energy converters and is widely utilized in automotive vehicles [3] due to the low operating temperature, quick start-up capability, rapid response to load changes, and high efficiency [4]. PEMFCs can also be used as a grid-connected electrical generator [2], enabling the clean and efficient production of power and heat from a range of primary energy sources.
The development of efficient processes to utilize naturally available solar energy is an important research direction, and generating hydrogen by splitting water with solar energy has emerged as a strong contender [5]. Photocatalytic hydrogen production by water splitting provides an alternative to fossil fuels using clean and renewable energy. A big challenge faced by the development of a cost-effective and energy-efficient photocatalyst for water splitting is the efficiency of the hydrogen evolution rate. To increase the large-scale production of hydrogen using photocatalytic water splitting, a decrease in catalyst decay and recombination of electron–hole pairs during operation is required, in addition to reducing the process production cost by developing a cost-effective and energy-efficient photocatalyst [6]. It is also necessary to improve the light absorption, surface reactions, and the hydrogen evolution rate when using a lower percentage of noble metals in the photocatalyst.
Perovskites, used as photocatalysts, have a smaller band gap, and the band edge potentials can be tuned to absorb more of the visible light spectrum [7]. The perovskite materials can be modified by the addition of metal elements to their structure, which can provide the ability to produce hydrogen via photocatalysis in accordance with the needs of specific photocatalytic reactions. LaFeO3 is a small band gap perovskite material that absorbs light in the visible region and demonstrates a reasonably good photocatalytic hydrogen generation [8]. When LaFeO3 is modified with A or B site elements that act as substituents, the general formulas of the resulting chemical compounds are La1−xAxFeO3 (A = Ca, Sr, Ba, Ce, or other rare earth elements) and LaFe1−xBxO3 (B = Mn, Co, Cr, or other transition metals), respectively [9]. In addition, LaFeO3/g-C3N4 heterostructures have been successfully prepared and their photocatalytic hydrogen evolution performance under visible light irradiation was reported [10]. An effective strategy for the fabrication of visible-light-responsive photocatalyst materials for photocatalytic water splitting is to introduce a transition metal dopant into the matrix of the photocatalyst [11]. Suitable transition metal ions doped into photocatalysts can be used to easily tune the electron concentration, mobility, and lifetime of the charge carriers, and effectively alter the electronic structure and band levels of the photocatalyst via the localized or delocalized nature of the doping-induced states [12]. To enhance the photocatalytic H2 evolution, Ru [12] and Rh [13] doped LaFeO3 in glucose aqueous matrices have been studied.
Increasing the activity of a photocatalyst that contains loading metals or metal oxides as cocatalysts dispersed on the surface of the photocatalysts is extremely important. The photogenerated electrons migrate through the photocatalyst to the interface between the cocatalyst and the photocatalyst where they are entrapped by the cocatalyst [14], a process which strongly determines the adsorption and activation abilities of the photocatalytic reactions [15]. In addition, the cocatalysts prevent the adverse electron–hole recombination and accelerate the surface chemical reactions by inhibiting the backward reaction [16]. Noble metals, such as Pt [17], Au [18], Pd [19], and Rh [20] with a higher redox potential and work function and a weaker metal–hydrogen bond strength, are historically the most favorable for photocatalytic hydrogen evolution activity. These noble metal cocatalysts capture the photogenerated electrons to suppress electron–hole recombination and reduce the activation energy for hydrogen production [21]. Similarly, transition metals and their oxides have been employed as cocatalysts to enhance the rate of oxidation [22]. For example, Liu et al. [23] prepared transition metal oxide clusters, including MnOx, FeOx, CoOx, NiOx, and CuOx, that were loaded in situ into TiO2 nanosheets through an impregnation method, which significantly promoted the photocatalytic oxidation of water to O2 compared to RuO2/TiO2 and IrO2/TiO2 nanosheets. To achieve higher H2 production efficiencies from water splitting, electron donors are usually required to act as sacrificial reagents to consume holes and prevent the recombination of photoinduced electrons and holes on the photocatalyst surface [24]. The sacrificial agents methanol [25], ethanol [26], triethanolamine [27], disodium ethylenediaminetetraacetic acid [26], and sodium sulfide/sodium sulfite [15] have been used as hole scavengers for photocatalytic H2 evolution from water splitting.
To date, there remains little information on the effects of different metal dopants, cocatalysts, and sacrificial agents on the photocatalytic H2 production of LaFeO3 perovskites. In this study, the novelty of this work lies in the use of Y, Pr, Mg, In, Ca, Eu, and Tb metals doped into LaFeO3, while RhCrOx was used as the cocatalysts for the photocatalytic H2 evolution from water splitting. The effects of the doped photocatalysts on different sacrificial agents were studied and were found to improve the efficiency of photocatalytic hydrogen evolution under visible light irradiation. The highest hydrogen productivity obtained 0.5 wt.% RhCrOx loading and 0.1 M Pr-doped LaFeO3 calcined at a temperature of 700 °C in 20% TEOA solution.

2. Results and Discussion

2.1. Different Metal Doping in LaFeO3

To investigate the photocatalytic hydrogen evolution performance of LaFeO3 doped with different metals, 0.1 g of each doped photocatalyst was dispersed in 100 mL of an aqueous solution containing 10 vol% TEOA. The results are shown in Figure 1a, the Pr-LaFeO3 photocatalyst demonstrated a higher photocatalytic hydrogen evolution activity (127 μmol h−1 g−1) than the other metal doped and undoped LaFeO3 photocatalysts (94.8 μmol h−1 g−1). In addition, the Ca-LaFeO3 photocatalyst displayed no evidence of any hydrogen production.
The DSR curves of different metal doped LaFeO3 photocatalysts are presented in Figure 1b. The band gap (Eg) values were calculated are shown in Table 1. The Eg values for the various metal doped LaFeO3 photocatalysts were smaller than the value for the undoped LaFeO3, with the exception of Tb-LaFeO3, which suggests that metal doping creates a dopant energy level within the band gap of LaFeO3. The lowest band gap was measured for Pr doped LaFeO3 at 2.0 eV, which confirms that the higher activity. Combined with the band gaps calculated from the DSR curves, the valence band energies allowed the determination of the electronic structures and relative band positions of the prepared photocatalysts, as depicted in Table 1 and Figure 2a. Pr-doped LaFeO3 photocatalysts demonstrated a narrower band gap and a more positively shifted valence band as compared to undoped LaFeO3, which allows for more efficient visible-light utilization and charge excitation, while maintaining enough energy for the reduction of water. Consequently, these materials showed an improved visible-light photocatalytic performance for hydrogen production.
PL spectroscopy was used to examine the efficiency of trapping, separating, and transferring charge carriers in the semiconductors. In this study, all of the doped LaFeO3 photocatalysts demonstrated similar emission peaks around 574 nm, as shown in Figure 2b. In addition, the different metal doped LaFeO3 were found to have a lower PL intensity than undoped LaFeO3, except for In-LaFeO3, which can be attributed to the presence of slight defects and the introduction of impurity levels within the gaps, both of which encourage charge transfer and reduce the recombination rate of photoinduced electron–hole pairs [28].
Figure 3 displays the XRD patterns of the different metal doped LaFeO3 samples. The peaks at 2θ values of 22.66°, 32.22°, 39.74°, 46.19°, 57.46°, 67.42°, and 76.61° are in good agreement with the characteristic peaks for the (101), (121), (200), (202) (240), (242), and (204) planes of orthorhombic LaFeO3 (JCPDS no. 371493), respectively. A comparison of the crystallite sizes was accomplished by analyzing the width of the (121) peak as shown Table 1, and it was determined that all of metal had doped into the LaFeO3 structure due to the smaller crystallite size as compared to the undoped LaFeO3. SEM images and EDS analysis after loading with 0.5% RhCrOx cocatalyst on Pr-LaFeO3 (RhCrOx/Pr-LaFeO3) are shown in Figure 4a,b. The elemental mapping images indicate that the RhCrOx/Pr-LaFeO3 photocatalysts contain La, Fe, Pr, O, Rh, and Cr elements as shown in Figure 4c–h, indicating the successful incorporation of Pr and RhCrOx cocatalysts into the LaFeO3 photocatalyst. TEM and HRTEM images were utilized to further investigate the structure of Pr-LaFeO3, as shown in Figure 5. The HRTEM image shows lattice fringes with a spacing of 0.278 and 0.227 nm, which are attributed to the (121) and (220) planes of the cubic phase LaFeO3.

2.2. Photocatalytic Activity of Different Amounts of Pr Doping in LaFeO3

Figure 6 shows the photocatalytic H2 evolution rate as a function of the amount of Pr doping in LaFeO3, where Pr-mass doping in each sample was detected using ICP-MS analysis such as 3.5 ppm (0.05 M Pr), 7.9 ppm (0.1 M Pr), 13.1 ppm (0.15 M Pr), 47.8 ppm (0.5 M Pr), and 69.6 ppm (0.7 M Pr). The photocatalytic H2 evolution rate was significantly increased for samples containing Pr at a concentration less than or equal to 0.1 M. When the doping amount of Pr was larger than 0.1 M, the photocatalytic H2 production decreased. In addition, 0.1 M Pr doped in LaFeO3 gave a smaller Eg than at other molar ratios of Pr doping, as shown in Figure 7a. The PL emission spectra of undoped LaFeO3 and various extents of Pr doping in LaFeO3 are shown in Figure 7b. The PL emission intensity at 574 nm for the undoped LaFeO3 is weaker than for Pr doped LaFeO3, and the intensity gradually decreased with increasing amounts of Pr from 0.1 to 0.7 M. These results indicate that the doped more than 0.1 M of Pr may foam more defects such that facilitate the recombination of electrons and holes before redox reaction, thus decreasing photocatalytic activity [29].

2.3. Photocatalytic Activity of Pr-LaFeO3 with Loading of RhCrOx Cocatalyst

Figure 8 shows the photocatalytic H2 evolution rates obtained from varying amounts of a mixture of Rh and Cr precursor (a fixed 1:1 weight ratio of Rh:Cr) added to Pr-LaFeO3 photocatalysts under visible light irradiation. It was found that the photocatalytic H2 evolution rates increased with higher amounts of loaded Rh and Cr from 0 to 0.5 wt.%. The chemisorption of CO on Rh cocatalysts has been found to inhibit the recombination reaction while maintaining the rate of hydrogen evolution [30]. The results obtained 0.5 wt.% Rh and Cr is the optimal ratio loading on the Pr-LaFeO3 photocatalyst, which suppresses the back reaction lead to produce the highest rate of H2 evolution. However, at greater than 0.5 wt.% Rh and Cr, the H2 evolution rates decreased, presumably because the RhCrOx blocked active surface sites and shaded on the Pr-LaFeO3 photocatalyst. This was expected, as the loading of an excess of a cocatalyst generally decreases the activity of the photocatalyst. Ran et al. [31] report that the loading of an excess of a cocatalyst generally decreases the activity of photocatalysts by some factors such as covering the surface active sites of the photocatalysts and hindering its contact with sacrificial reagents or water molecules, shielding the incident light caused inhibition photogenerated electrons and holes inside the photocatalysts, and could act as charge recombination centers. Therefore, 0.5 wt.% RhCrOx loading on Pr-LaFeO3 was found to give the largest photocatalytic H2 evolution rate, which indicates that 0.5 wt.% of the RhCrOx cocatalyst is able to capture the photogenerated electrons and suppress the electron–hole recombination.

2.4. Photocatalytic Activity of Pr-LaFeO3 Prepared at Different Calcination Temperatures

To investigate the effect of calcination temperature on the photocatalytic activity, Pr-LaFeO3 was calcined at 500, 600, 700, 800, and 900 °C for 4 h with 0.5 wt.% RhCrOx cocatalyst, and the results are displayed in Figure 9a. It is evident that the photocatalytic H2 evolution rate significantly increases with the calcination temperature from 500 to 700 °C. The highest photocatalytic H2 evolution and the smallest Eg (Figure 9b) were found when Pr-LaFeO3 was calcinated at 700 °C. Unfortunately, the calcination temperature of 500 °C generated a lower photocatalytic H2 evolution rate and a higher PL emission intensity than higher calcination temperatures, as shown in Figure 9c. In Figure 10, the XRD data confirm that the structure of the photocatalyst generated by a calcination temperature of 500 °C did not match with the LaFeO3 structure, leading to the rapid recombination of photo-generated electron–hole pairs. In addition, the intensity of the (121) peak increased with higher calcination temperatures from 600 to 900 °C, which indicates that the crystalline quality increased with higher calcination temperatures, as shown in Figure 10. Generally, the crystalline quality of a photocatalyst affects the charge separation and migration of photogenerated carriers [32]. A high degree of crystallinity usually indicates a smaller quantity of defects and a corresponding increase in catalytic activity. Given that the density of defects impacts the availability of trapping and recombination centers between photogenerated electrons and holes, more defects result in a decrease in the photocatalytic activity [32]. However, in the current study, the photocatalytic H2 evolution rates decreased when the calcination temperature was greater than 800 °C. Therefore, XPS was used to measure the chemical composition of the RhCrOx/Pr-LaFeO3 photocatalysts, including the typical XPS survey spectra of Pr(3d), La(3d), Fe(2p), O(1s), Rh(3d), and Cr(2p), as shown in Figure 11. The binding energies obtained in the XPS analysis were corrected for specimen charging by referencing the C 1s line at 284.5 eV. The binding energies in the XPS spectra for Pr 3d and La 3d did not change when Pr-LaFeO3 was treated at various calcination temperatures. The XPS spectra for Pr 3d can be deconvoluted into three peaks, where the strong peak with a binding energy of 933.4 eV for Pr 3d5/2 corresponds to Pr3+, and the other two peaks are attributed to Pr4+ (937.5 eV) with a shake-off satellite (928 eV), as shown in Figure 11a. The La 3d doublet was located at 832.5 and 836.1 eV, which was ascribed to La3+ and La6+ of the 3d5/2 (Figure 11b). The spin−orbit splitting gap of ~16.8 eV between the 3d3/2 and 3d5/2 peaks was indicative of the La3+ and La6+ state. However, the Fe2+ and Fe3+ peak areas for Fe 2p varied depending on the calcination temperature, as shown in Figure 11c and Table 2. Two distinct peaks at binding energies of 710.2 and 712.6 eV for Fe 2p3/2 were observed, which correspond to the characteristic Fe2+ and Fe3+ ions in the oxide form. The smallest area for Fe2+ and the largest area for Fe3+ were found at a calcination temperature of 700 °C, which indicates that more Fe2O3 exists compared to FeO in Pr-LaFeO3. It has been reported that the Fe2O3 structure has a band gap of 2.17 eV, which can absorb a wider range of the solar spectrum and enhance the photocatalytic H2 evolution [33]. At temperatures greater than 800 °C, the area of Fe2+ increased and the area of Fe3+ decreased, which indicates that Fe2O3 was oxidized to FeO and resulted in a decrease in the photocatalytic H2 evolution.
When the Pr-LaFeO3 photocatalysts were calcinated at temperatures greater than or equal to 600 °C, the deconvolution of the O 1s binding energy spectrum results in three peaks that are attributed to lattice oxygen (O2−) at 529.4 eV, chemisorbed oxygen (O−/O2−) at 531.6 eV [34], and physically adsorbed oxygen at 533.6 eV [34]. However, the physically adsorbed oxygen peak was absent when the calcination temperature was 500 °C for Pr-LaFeO3, which might indicate a lower crystallization at this temperature.
A similar calcination temperature study was performed after loading RhCrOx cocatalysts onto the Pr-LaFeO3 photocatalysts at temperatures from 500 to 900 °C. The Rh 3d and Cr 2p XPS binding energies were similar, as shown in Figure 11e,f. The fitting of the data indicates the presence of Rh3+ attributable to Rh2O3 at 308.7 and 313.7 eV, for Rh 3d5/2 and 3d3/2, and a satellite peak of Rh2O3 at 311.1 and 316.1 eV, for Rh 3d5/2 and 3d3/2 [35]. In addition, the Cr 2p spectra for all samples were comparable, where the Cr 2p3/2 signal can be divided into two peaks at 577.4 eV and 580.3 eV, corresponding to Cr3+ and Cr6+, respectively. The relative amounts of Cr3+ and Cr6+ ions were analogous, indicating their similar Cr state [36].

2.5. Photocatalytic Activity of RhCrOx/Pr-LaFeO3 with Different Sacrificial Reagents

Sacrificial agents can be used to increase the photocatalytic activity for water splitting when a photocatalytic reaction is carried out in an aqueous solution containing electron donors or hole scavengers, which prevent electron–hole recombination [24]. The photocatalytic hydrogen evolution of RhCrOx/Pr-LaFeO3 was evaluated with the addition of TEOA, methanol, and ethanol as sacrificial agents with a working volume of 100 mL (90 mL distilled water and 10 mL sacrificial agent), respectively, and the results are shown in Figure 12a. It is clear that RhCrOx/Pr-LaFeO3 has the best photocatalytic H2 production with TEOA compared to the other sacrificial agents. In fact, no H2 evolution was measured for methanol and ethanol, indicating that they are poor hole scavengers for RhCrOx/Pr-LaFeO3. Jones et al. [37] reported that Pd supported on Titania also demonstrated a lower H2 evolution rate in methanol as compared to TEOA. A mechanism for the photo-reforming of methanol is described in Equations (1)–(4) [37]. Methanol has been shown to decarbonylate at ambient temperatures and leave behind CO adsorbed onto the surface of RhCrOx (Equation (1)), and when CO saturation is achieved, no further reaction (Equation (2)) occurs by band gap excitation in Pr-LaFeO3 to generate electron–hole pairs. The CO is then removed from the RhCrOx by the highly electrophilic oxygen species (the hole) in Equation (3). The final step involves the filling of the vacancy (Vo) in Pr-LaFeO3 by water. Thus, the photocatalytic H2 evolution was lower with methanol and ethanol.
CH3OH → COa + 2H2
O2− + hv → O− + e−
COa + O− → CO2 + e− +Vo
e− +Vo +H2O→ O2− + H2
Therefore, the choice of TEOA as hole scavenger leads to a higher hole capture efficiency due to the recombination of fewer electrons and holes [38,39] and a lower probability for photooxidation of the semiconductor material [39]. The mechanisms for the photochemical reactions of TEOA are summarized as follows in Equations (5)–(8) [27]:
C6H15NO3 → C6H15NO3+ + e−
C6H15NO3+ → C6H14NO3● + H+
C6H14NO3● → C6H14NO3+ + e−
C6H14NO3+ + H2O → C4H11NO3 + CH3CHO + H+
Figure 12b shows the H2 evolution rate over RhCrOx/Pr-LaFeO3 photocatalysts at different concentrations of TEOA. The H2 evolution rate was found to be the highest at 20% of TEOA. The results indicate that at 20% TEOA, the high hole capture capability could effectively accelerate the separation of photo-generated electron–hole pairs upon light irradiation, leading to the highest photocatalytic hydrogen evolution performance. However, when the photocatalysts were tested at 30% TEOA, it is likely that the excessive adsorption of hole scavengers occupied the active sites at the surface of the photocatalyst, leaving no available sites for H2 evolution [40] and generating a corresponding decrease in H2 evolution rates.
In addition, cycling experiments were carried out to determine the stability and reusability of RhCrOx/Pr-LaFeO3 for photocatalytic H2 evolution under visible light irradiation. The results shown in Figure 12c reveal that the photocatalytic activity of RhCrOx/Pr-LaFeO3 did not suffer any significant loss after four cycles. These results suggest that RhCrOx/Pr-LaFeO3 is reasonably stable and could be reused as a photocatalyst with considerable activity.
According to Figure 13, the Pr-LaFeO3 photocatalysts under visible light irradiation generate electron–hole pairs, where the electrons migrate to the RhCrOx cocatalysts and participate in the reduction of protons to evolve hydrogen gas. The holes are consumed by the oxidation of the sacrificial agent TEOA, reducing the recombination of the photogenerated charges and improving the photocatalytic H2 evolution.
Table 3 shows that the H2 evolution rates of various LaFeO3-based [8,12,13,41,42,43,44] and this study of Pr-LaFeO3. However, they are difficult to compare because the gas evolution rates depend on the different used sacrificial agents, co-catalyst, light source, and the distance between the light source and photocatalysts. Domen’s group [45] reported that the activities of photocatalysts to gas evolution rates in photocatalytic water splitting are extremely difficult to compare the activities measured in different reaction systems because of differences in the photocatalytic reactor systems and in the irradiance of different light sources. The apparent quantum yield (AQY) or apparent quantum efficiency (AQE), can be used as a standard measure of activity. Therefore, the experimental results obtained that the samples with 0.5 wt.% RhCrOx loading and 0.1 M Pr-doped LaFeO3 calcined at a temperature of 700 °C (0.1-Pr-LaFeO3-700) exhibited the highest photocatalytic H2 evolution rate of 127 µmol h−1 g−1, which is 34% higher photocatalytic H2 evolution performance than undoped LaFeO3 photocatalysts (94.8 μmol h−1 g−1). We believe that the idealization of this study such as Pr doping and RhCrOx loading on other LaFeO3-based photocatalysts for previous reports increases their H2 evolution rate at the same of their experimental environment.

3. Materials and Methods

3.1. Materials

Lanthanum nitrate (La(NO3)3·6H2O), praseodymium acetate hydrate (Pr(CH3CO2)3·xH2O), yttrium (III) nitrate hexahydrate (Y(NO3)3·6H2O), europium (III) acetate hydrate (Eu(CH3CO2)3·xH2O), and sodium hexachlororhodate (III) dodecahydrate (Na3RhCl6·12H2O, Rh 17.1 wt.%) were purchased from Alfa Aesar (Ward Hill, MA, USA). Iron nitrate and citric acid were supplied by Showa Corporation (Gyoda, Japan). Calcium chloride dihydrate (CaCl2·2H2O) and magnesium chloride hexahydrate (MgCl2·6H2O) were supplied by Panreac Química SLU (Barcelona, Spain). Terbium (III) nitrate hexahydrate (Tb(NO3)3·6H2O) and chromium (III) nitrate nonahydrate (Cr(NO3)3·9H2O, 99%) were purchased from Acros organics™ (Carlsbad, CA, USA). Triethanolamine (HOCH2CH2)3N, TEOA) was supplied by Sigma-Aldrich (St. Louis, MO, USA).

3.2. Synthesis of Different Metal Doped LaFeO3 Powders

The lanthanum ferrite perovskites were prepared following the method reported by Tijare et al. [8], which involved combining the starting materials, the different doping metals (Pr, In, Mg, Ca, Tb, Eu, and Y), La(NO3)3·6H2O, Fe(NO3)3·9H2O, and citric acid in a molar ratio of 0.1:1:1:4 in 100 mL of water with constant stirring for 60 min, respectively. The mixture was processed in an ultrasonicator at 40 kHz for 120 min. Yellowish-brown precipitates were collected and dried in an oven at 90 °C for 5 h, followed by calcination at 500 °C for 2 h and at 700 °C for 4 h.

3.3. RhCrOx/Pr Doped LaFeO3 Preparation

Different molar concentrations (0.1, 0.2, 0.4, 0.5, 0.6, 0.8, and 1.0 M) of Pr doped LaFeO3 photocatalysts were loaded with 0.5 wt.% of the RhCrOx cocatalyst prepared by combining Na3RhCl6·12H2O and Cr(NO3)3·9H2O in a 1:1 weight ratio by an impregnation method.

3.4. Characterization of Photocatalysts

The crystallite size of the different metal doped LaFeO3 samples was determined by X-ray powder diffraction (XRD) using a Rigaku ultimate IV desktop X-ray diffractometer with a Cu Kα radiation source at 30 kV and 15 mA and a scanning rate of 5° min−1. The crystallite sizes (Dhkl) of the prepared samples were estimated from the line broadening using the Debye–Scherrer equation [46] applied to the (121) peak: Dhkl = Kλ/Bhkl cosθ, where Dhkl is the crystallite size in the direction perpendicular to the lattice planes, λ is the X-ray wavelength of the Cu Ka radiation (λ = 1.5406 Å), θ is Bragg’s angle, Bhkl is the pure full width of the diffraction line at half of the maximum intensity, hkl are the Miller indices of the analyzed planes, and K is a numerical factor frequently referred to as the crystallite-shape factor and is typically 0.9. The morphological properties of the photocatalysts were studied via a scanning electron microscope (SEM) model JOEL JSM-6700F and a high resolution transmission electron microscope (HRTEM) model JOEL JEM-F200 operated at 300 kV. Furthermore, X-ray photoelectron spectra (XPS) were collected to determine the oxidation states and other details of the Pr-LaFeO3 photocatalysts on an ESCA Lab spectrometer with a sigma probe and a mono-chromate Al Kα source. The deconvolution of the core-level spectra was performed with the XPS peak fit software. The photoluminescence (PL) spectra were measured at room temperature using a micro-Raman spectrophotometer (UniRaman, ProTrusTech Co., LTD, Tainan, Taiwan) equipped with a laser that operates at an excitation wavelength of 532 nm. The work functions were measured using a photoelectron spectrometer (Model: AC-2) that is an open counter equipped with a UV source manufactured by Riken Keiki Co., LTD, Tokyo, Janpn. Pr elemental analysis was performed using by high-resolution inductively coupled plasma-mass spectrometer (HR-ICP-MS) using Thermo Scientific (Waltham, MA, USA) ELEMENT XR analyzer.
UV–Vis diffuse reflectance spectra (DRS) were measured on a UV–Vis spectrophotometer (U-3900, Hitachi Hight-Tech, Tokyo, Japan) using BaSO4 as a reference. The bandgap energy in the Pr-LaFeO3 photocatalyst was calculated using the formula [47]:
αhυ = B(hv − Eg)n
where α is the optical absorption coefficient, B is a constant, Eg is the optical band gap, and n is 1/2 or 2 for direct or indirect band gap semiconductors, respectively.
The photocatalysts present on the surface can serve as an electron capture center, providing more active sites. Therefore, it is necessary to determine the conduction band (CB) and valence band (VB) potentials of the photocatalysts. The energy levels were calculated using the following empirical equations [48], and may give a rough estimate of the relative positions of the normal hydrogen electrode (NHE), which could provide a reference to future experimental studies:
ECB = X − Ee − 1/2 Eg
EVB = ECB + Eg
where EVB and ECB are the VB and CB potentials, respectively. Moreover, Ee is the energy of free electrons versus hydrogen (4.5 eV).
X is the absolute electronegativity of a pristine semiconductor, and it was calculated using the following equation:
X = [χ (A)aχ (B)bχ (C)c](1/a+b+c)
where a, b, and c are the number of atoms in the compounds. Here, given that 0.1 mol of metals are doped in the LaFeO3 and the number of atoms is 0.1, the relative number of atoms for La, Fe, and O is 1, 1, and 3, respectively. For example, for Pr-LaFeO3, X = [χ (Pr)0.1χ (La)1χ (Fe)1χ (O)3](1/0.1+1+1+3). χ is Mulliken’s definition of the electronegativity of a neutral atom [49,50], defined as χ = 1/2(A + I). A is the atom’s electron affinity and I is the first ionization energy. A list of the electron affinities and first ionization energies for the various metals used in this study is shown in Table S1.

3.5. Calibration Curves for Hydrogen

The photocatalytics of water splitting have been carried out in a photocatalytic reactor. The total hydrogen and oxygen evolution rates were calculated by using these calibration curves. Before making the hydrogen calibration line, the hydrogen peak position and area must be determined using the GC detection signal. The calibration curves for hydrogen and oxygen have been carried out in a photocatalytic reactor (Labsolar-6A, Beijing Perfectlight Technology Co., Ltd., Beijing, China). The reactor was fixed with a condenser, which was further attached to a gas collector and gas chromatograph (GC6890N, Agilent, Santa Clara, CA, USA) with molecular sieve 5A column and thermal conductivity detector (TCD). Argon gas and N2 gas were used as a carrier gas and make-up gas. The H2 calibration equation as Y = −11.60 + 81.09 × X as shown in Figure S1.

3.6. Photocatalytic H2 Evolution

First, 0.1 g of the photocatalyst was dispersed in 100 mL of an aqueous solution containing 10 vol % TEOA. The reaction temperature was maintained at 20 °C. The hydrogen concentration was monitored using a photocatalytic reactor (Labsolar-6A). A condenser was fixed onto the reactor, which was then attached to a gas collector and a gas chromatograph (GC6890N, Agilent) equipped with a 5 Å molecular sieve column and a thermal conductivity detector (TCD). The entire experimental setup was placed under vacuum using a vacuum pump to remove air. The photocatalytic activity of the photocatalysts was then monitored for hydrogen generation through the photocatalytic water splitting reaction with TEOA as the sacrificial agent, under varying conditions with feasible parametric changes using a visible light source (350 W Xe lamp) with a 400 nm long-pass cut-off filter.

4. Conclusions

In summary, RhCrOx/Pr-LaFeO3 photocatalysts were fabricated, characterized, and found to exhibit a significantly improved photocatalytic performance compared to undoped LaFeO3. Pr-doped LaFeO3 photocatalysts demonstrated a narrower band gap and a more positively shifted valence band as compared to undoped LaFeO3, which allows for more efficient visible-light utilization and charge excitation, while maintaining enough energy for the reduction of water. The photocatalysts with 0.1 M Pr doped into LaFeO3 and calcinated at 700 °C (0.1Pr-LaFeO3-700) gave the smallest Eg of 2.0 eV, and a low PL emission intensity due to the presence of a high proportion of Fe2O3 as demonstrated by the Fe 2p XPS data. In addition, 0.5 wt.% RhCrOx cocatalysts loaded on 0.1Pr-LaFeO3-700 in 20% TEOA solution showed the highest photocatalytic H2 evolution rate of 127 μmol h−1 g−1. These results indicate that this photocatalyst possesses a higher visible light absorption capacity and a lower photogenerated electron–hole recombination rate. Reusability tests indicated that the as-prepared RhCrOx/0.1-Pr-LaFeO3-700 photocatalysts are stable and reusable. The choice of 20% of TEOA enabled a high hole capture capability and was found to effectively accelerate the separation of photo-generated electron and hole pairs upon light irradiation, leading to the highest photocatalytic hydrogen evolution performance.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/catal11050612/s1, Table S1: Electronegativities and the number of atoms for different metals, Figure S1: H2 calibration curve.

Author Contributions

Writing—original draft preparation, writing—review and editing, supervision, project administration, conceptualization, and funding acquisition, T.H.C.; investigation, formal analysis and data curation, G.V.; software, Y.-S.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Ministry of Science and Technology, grant number MST 108-2221-E-239-013.

Data Availability Statement

The data presented in this study and supporting information are available.

Acknowledgments

We thank Chen, Ching-Shiun at Chang Gung University in Taiwan for helping with PL data measuring.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Acar, C.; Dincer, I.; Naterer, G.F. Review of photocatalytic water-splitting methods for sustainable hydrogen production. Int. J. Energy Res. 2016, 40, 1449–1473. [Google Scholar] [CrossRef]
  2. Ahmed, K.; Farrok, O.; Rahman, M.M.; Ali, M.S.; Haque, M.M.; Azad, A.K. Proton exchange membrane hydrogen fuel cell as the grid connected power generator. Energies 2020, 13, 6679. [Google Scholar] [CrossRef]
  3. Barbir, F.; Yazici, S. Status and development of PEM fuel cell technology. Int. J. Energy Res. 2008, 32, 369–378. [Google Scholar] [CrossRef]
  4. Wu, B.; Matian, M.; Offer, G.J. Hydrogen PEMFC system for automotive applications. Int. J. Low Carbon Technol. 2012, 7, 28–37. [Google Scholar] [CrossRef] [Green Version]
  5. Li, R. Latest progress in hydrogen production from solar water splitting via photocatalysis, photoelectrochemical, and photovoltaic-photoelectrochemical solutions. Chin. J. Catal. 2017, 38, 512. [Google Scholar] [CrossRef]
  6. Lin, W.C.; Yang, W.D.; Huang, I.L.; Wu, T.S.; Chung, Z.J. Hydrogen production from methanol/water photocatalytic decomposition using Pt/TiO2-xNxcatalyst. Energy Fuels 2009, 23, 2192–2196. [Google Scholar] [CrossRef]
  7. Tasleem, S.; Tahir, M. Recent progress in structural development and band engineering of perovskites materials for photocatalytic solar hydrogen production: A review. Int. J. Hydrogen Energy 2020, 47, 19078–19111. [Google Scholar] [CrossRef]
  8. Tijare, S.N.; Joshi, M.V.; Padole, P.S.; Mangrulkar, P.A.; Rayalu, S.S.; Labhsetwar, N.K. Photocatalytic hydrogen generation through water splitting on nano-crystalline LaFeO3 perovskite. Int. J. Hydrogen Energy 2012, 37, 10451–10456. [Google Scholar] [CrossRef]
  9. Lin, Q.; Xu, J.; Yang, F.; Yang, X.; He, Y. The influence of Ca substitution on LaFeO3 nanoparticles in terms of structural and magnetic properties. J. Appl. Biomater. Func. Mater. 2018, 16, 17–25. [Google Scholar] [CrossRef] [Green Version]
  10. Xu, K.; Xu, H.; Feng, G.; Feng, J. Photocatalytic hydrogen evolution performance of NiS cocatalyst modified LaFeO3/g-C3N4 heterojunctions. New J. Chem. 2017, 41, 14602–14609. [Google Scholar] [CrossRef]
  11. Ismael, M. Highly effective ruthenium-doped TiO2 nanoparticles photocatalyst for visible-light-driven photocatalytic hydrogen production. New J. Chem. 2019, 43, 9596. [Google Scholar] [CrossRef]
  12. Iervolino, G.; Vaiano, V.; Sannino, D.; Rizzo, L.; Palma, V. Enhanced photocatalytic hydrogen production from glucose aqueousmatrices on Ru-doped LaFeO3. Appl. Catal. B Environ. 2017, 207, 182–194. [Google Scholar] [CrossRef]
  13. Vaiano, V.; Iervolino, G.; Sannino, D. Enhanced photocatalytic hydrogen production from glucose on Rh-doped LaFeO3. Chem. Eng. Trans. 2017, 60, 235–240. [Google Scholar]
  14. Ni, M.; Leung, M.K.H.; Leung, D.Y.C.; Sumathy, K. A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production. Renew. Sustain. Energy Rev. 2007, 11, 401. [Google Scholar] [CrossRef]
  15. Zhai, Q.; Xie, S.; Fan, W.; Zhang, Q.; Wang, Y.; Deng, W.; Wang, Y. Photocatalytic conversion of carbon dioxide with water into methane: Platinum and copper(I) oxide co-catalysts with a core–shell structure. Angew. Chem. Int. Ed. 2013, 52, 5776. [Google Scholar] [CrossRef] [PubMed]
  16. Chen, S.; Shen, S.; Liu, G.; Qi, Y.; Zhang, F.; Li, C. Interface engineering of a CoOx/Ta3N5 photocatalyst for unprecedented water oxidation performance under visible-light-irradiation. Angew. Chem. Int. Ed. 2015, 54, 3047. [Google Scholar] [CrossRef]
  17. Hoffmann, M.R.; Martin, S.T.; Choi, W.; Bahnemannt, D.W. Environmental applications of semiconductor photocatalysis. Chem. Rev. 1995, 95, 69–96. [Google Scholar] [CrossRef]
  18. Daskalaki, V.M.; Antoniadou, M.; Puma, G.L.; Kondarides, D.I.; Lianos, P. Solar light-responsive Pt/CdS/TiO2 photocatalysts for hydrogen production and simultaneous degradation of inorganic or organic sacrificial agents in wastewater. Environ. Sci. Technol. 2010, 44, 7200–7205. [Google Scholar] [CrossRef] [PubMed]
  19. Jin, Z.; Zhang, X.; Lu, G.; Li, S. Improved quantum yield for photocatalytic hydrogen generation under visible light irradiation over eosin sensitized TiO2-Investigation of different noble metal loading. J. Mol. Catal. A Chem. 2006, 259, 275. [Google Scholar] [CrossRef]
  20. Sayama, K.; Yase, K.; Arakawa, H.; Asakura, K.; Tanaka, A.; Domen, K.; Onishi, T. Photocatalytic activity and reaction mechanism of Pt-intercalated K4Nb6O17 catalyst on the water splitting in carbonate salt aqueous solution. J. Photochem. Photobiol. A Chem. 1998, 114, 125–135. [Google Scholar] [CrossRef]
  21. Jang, J.S.; Ham, D.J.; Lakshminarasimhan, N.; Choi, W.Y.; Lee, J.S. Role of platinum-like tungsten carbide as cocatalyst of CdS photocatalyst for hydrogen production under visible light irradiation. Appl. Catal. A Gen. 2008, 346, 149–154. [Google Scholar] [CrossRef]
  22. Matsumura, M.; Saho, Y.; Tsubomura, H. Photocatalytic hydrogen production from solutions of slufite using platinized cadmium sulfide powder. J. Phys. Chem. 1983, 87, 3807–3808. [Google Scholar] [CrossRef]
  23. Liu, L.; Ji, Z.; Zou, W.; Gu, X.; Deng, Y.; Gao, F.; Tang, C.; Dong, L. In situ loading transition metal oxide clusters on TiO2 nanosheets as cocatalysts for exceptional high photoacitivity. ACS Catal. 2013, 3, 2052–2061. [Google Scholar] [CrossRef]
  24. Wang, M.; Shen, S.; Li, L.; Tang, Z.; Yang, J. Effects of sacrificial reagents on photocatalytic hydrogen evolution over different photocatalysts. J. Mater. Sci. 2017, 52, 5155–5164. [Google Scholar] [CrossRef]
  25. Kawai, T.; Sakata, T. Photocatalytic hydrogen production from liquid methanol and water. J. Chem. Soc. Chem. Commun. 1980, 15, 694–695. [Google Scholar] [CrossRef]
  26. Huerta-Flores, A.M.; Torres-Martinez, L.M.; Moctezuma, E.; Singh, A.P.; Wickman, B. Green synthesis of earth-abundant metal sulfides (FeS2, CuS, and NiS2) and their use as visible-light active photocatalysts for H2 generation and dye removal. J. Mater. Sci. Mater. Electron. 2018, 29, 11613–11626. [Google Scholar] [CrossRef]
  27. DeLaive, P.J.; Foreman, T.K.; Giannotti, C.; Whitten, D.G. Photoinduced electron transfer reactions of transition-metal complexes with amines. Mechanistic studies of alternate pathways to back electron transfer. J. Am. Chem. Soc. 1980, 102, 5627–5631. [Google Scholar] [CrossRef]
  28. Du, C.; Yan, B.; Lin, Z.; Yang, G. Enhanced carrier separation and increased electron density in 2D heavily N-doped ZnIn2S4 for photocatalytic hydrogen production. J. Mater. Chem. A 2020, 8, 207–217. [Google Scholar] [CrossRef]
  29. Jafari, T.; Moharreri, E.; Amin, A.S.; Miao, R.; Song, W.; Suib, S.L. Photocatalytic water splitting-The untamed dream: A review of recent advances. Molecules 2016, 21, 900. [Google Scholar] [CrossRef]
  30. Berto, T.F.; Sanwald, K.E.; Byers, J.P.; Browning, N.D.; Gutiérrez, O.Y.; Lercher, J.A. Enabling overall water splitting on photocatalysts by CO-covered noble metal co-catalysts. J. Phys. Chem. Lett. 2016, 7, 4358–4362. [Google Scholar] [CrossRef] [PubMed]
  31. Ran, J.; Zhang, J.; Yu, J.; Jaroniecc, M.; Qiao, S.Z. Earth-abundant cocatalysts for semiconductorbased photocatalytic water splitting. Chem. Soc. Rev. 2014, 43, 7787. [Google Scholar] [CrossRef]
  32. Chou, H.L.; Hwang, B.J.; Sund, C.L. Catalysis in fuel cells and hydrogen production. In Catalysis in Fuel Cells and Hydrogen Production in Batteries, Hydrogen Storage and Fuel Cells; Suib, S.L., Ed.; Elsevier: Amsterdam, The Netherlands, 2013; pp. 217–270. [Google Scholar]
  33. Lin, Z.; Du, C.; Yan, B.; Yang, G. Amorphous Fe2O3 for photocatalytic hydrogen evolution. Catal. Sci. Technol. 2019, 9, 5582–5592. [Google Scholar] [CrossRef]
  34. Li, X.; Zhang, H.; Liu, X.; Li, S.; Zhao, M. XPS study on O(ls) and Fe(2p) for nanocrystalline composite oxide LaFeO3 with the perovskite structure. Mater. Chem. Phys. 1994, 38, 355–362. [Google Scholar] [CrossRef]
  35. Pagliaro, M.V.; Bellini, M.; Bevilacqua, M.; Filippi, J.; Folliero, M.G.; Marchionni, A.; Miller, H.A.; Oberhauser, W.; Caporali, S.; Innocenti, M.; et al. Carbon supported Rh nanoparticles for the production of hydrogen and chemicals by the electroreforming of biomass-derived alcohols. RSC Adv. 2017, 7, 13971. [Google Scholar] [CrossRef] [Green Version]
  36. Yu, S.; Xu, S.; Sun, B.; Lu, Y.; Li, L.; Zou, W.; Wang, P.; Gao, F.; Tang, C.; Dong, L. Synthesis of CrOx/C catalysts for low temperature NH3-SCR with enhanced regeneration ability in the presence of SO2. RSC Adv. 2018, 8, 3858. [Google Scholar] [CrossRef] [Green Version]
  37. Jones, W.; Martin, D.J.; Caravaca, A.; Beale, A.M.; Bowker, M.; Maschmeyer, T.; Hartley, G.; Masters, A. A comparison of photocatalytic reforming reactions of methanol andtriethanolamine with Pd supported on titania and graphitic carbonnitride. Appl. Catal. B Environ. 2019, 240, 373–379. [Google Scholar] [CrossRef]
  38. Berr, M.J.; Wagner, P.; Fischbach, S.; Vaneski, A.; Schneider, J.; Susha, A.S.; Rogach, A.L.; Jäckel, F.; Feldmann, J. Hole scavenger redox potentials determine quantum efficiency and stability of Pt-decorated CdS nanorods for photocatalytic hydrogen generation. Appl. Phys. Lett. 2012, 100, 223903. [Google Scholar] [CrossRef]
  39. Wei, Y.; Cheng, G.; Xiong, J.; Zhu, J.; Gan, Y.; Zhang, M.; Li, Z.; Dou, S. Synergistic impact of cocatalysts and hole scavenger for promoted photocatalytic H2 evolution in mesoporous TiO2 NiS hybrid. J. Energy Chem. 2019, 32, 45. [Google Scholar] [CrossRef]
  40. Zhang, G.; Zhang, W.; Crittenden, J.; Minakata, D.; Chen, Y.; Wang, P. Effects of inorganic electron donors in photocatalytic hydrogen production over Ru/(CuAg)0.15In0.3Zn1.4S2 under visible light irradiation. J. Renew. Sustain. Energy 2014, 6, 033131. [Google Scholar] [CrossRef]
  41. Iervolino, G.; Vaiano, V.; Sannino, D.; Rizzo, L.; Ciambelli, P. Production of hydrogen from glucose by LaFeO3 based photocatalytic process during water treatment. Int. J. Hydrogen Energy 2016, 41, 959–966. [Google Scholar] [CrossRef]
  42. Iervolino, G.; Vaiano, V.; Sannino, D.; Rizzo, L.; Galluzzi, A.; Polichetti, M.; Pepe, G.; Campiglia, P. Hydrogen production from glucose degradation in water and wastewater treated by Ru-LaFeO3/Fe2O3 magnetic particles photocatalysis and heterogeneous. Int. J. Hydrogen Energy 2018, 43, 2184–2196. [Google Scholar] [CrossRef]
  43. Chen, Z.; Fan, T.; Zhang, Q.; He, J.; Fan, H.; Sun, Y.; Yi, X.; Li, J. Interface engineering: Surface hydrophilic regulation of LaFeO3 towards enhanced visible light photocatalytic hydrogen evolution. J. Colloid Interface Sci. 2019, 536, 105–111. [Google Scholar] [CrossRef]
  44. Li, J.; Pan, X.; Xu, Y.; Jia, L.; Yi, X.; Fang, W. Synergetic effect of copper species as cocatalyst on LaFeO3 for enhanced visible-light photocatalytic hydrogen evolution. Int. J. Hydrogen Energy 2015, 40, 13918–13925. [Google Scholar] [CrossRef]
  45. Hisatomi, T.; Takanabe, K.; Domen, K. Photocatalytic water-splitting reaction from catalytic and kinetic perspectives. Catal. Lett. 2015, 145, 95–108. [Google Scholar] [CrossRef] [Green Version]
  46. Holzwarth, U.; Gibson, N. The Scherrer equation versus the Debye–Scherrer equation. Nat. Nanotechnol. 2011, 6, 534. [Google Scholar] [CrossRef] [PubMed]
  47. Chrysicopoulou, P.; Davazoglou, D.; Trapalis, C.; Kordas, G. Optical properties of very thin (<100 nm) sol–gel TiO2 films. Thin Solid Films 1998, 323, 188–193. [Google Scholar] [CrossRef]
  48. Xu, Y.; Schoonen, M.A.A. The absolute energy positions of conduction and valence bands of selected semiconducting minerals. Am. Mineral. 2000, 85, 543–556. [Google Scholar] [CrossRef]
  49. Mulliken, R.S. A new electroaffinity scale; together with data on valence states and on valence ionization potentials and electron affinities. J. Chem. Phys. 1934, 2, 782–793. [Google Scholar] [CrossRef]
  50. Butler, M.A.; Ginley, D.S. Rediction of flatband potentials at semiconductor-electrolyte interfaces from atomic electronegativities. J. Electrochem. Soc. 1978, 125, 228–232. [Google Scholar] [CrossRef]
Catalysts 11 00612 g001 550
Figure 1. (a) The hydrogen evolution and (b) DRS curves of different metal doped LaFeO3.
Figure 1. (a) The hydrogen evolution and (b) DRS curves of different metal doped LaFeO3.
Catalysts 11 00612 g001
Catalysts 11 00612 g002 550
Figure 2. (a) Band structures and (b) photoluminescence spectra of different metal doped LaFeO3 and undoped LaFeO3.
Figure 2. (a) Band structures and (b) photoluminescence spectra of different metal doped LaFeO3 and undoped LaFeO3.
Catalysts 11 00612 g002
Catalysts 11 00612 g003 550
Figure 3. XRD patterns of different metal doped LaFeO3 photocatalysts.
Figure 3. XRD patterns of different metal doped LaFeO3 photocatalysts.
Catalysts 11 00612 g003
Catalysts 11 00612 g004a 550Catalysts 11 00612 g004b 550
Figure 4. (a) SEM images, (b) EDX, and (c–h) elemental mapping images of RhCrOx/Pr-LaFeO3 photocatalysts.
Figure 4. (a) SEM images, (b) EDX, and (c–h) elemental mapping images of RhCrOx/Pr-LaFeO3 photocatalysts.
Catalysts 11 00612 g004aCatalysts 11 00612 g004b
Catalysts 11 00612 g005 550
Figure 5. (a) TEM-associated SAED pattern and (b) HRTEM imageof Pr-LaFeO3.
Figure 5. (a) TEM-associated SAED pattern and (b) HRTEM imageof Pr-LaFeO3.
Catalysts 11 00612 g005
Catalysts 11 00612 g006 550
Figure 6. Photocatalytic activity of different molar concentrations of Pr doping in LaFeO3 under visible light irradiation.
Figure 6. Photocatalytic activity of different molar concentrations of Pr doping in LaFeO3 under visible light irradiation.
Catalysts 11 00612 g006
Catalysts 11 00612 g007 550
Figure 7. (a) UV–Vis DRS spectrum and (b) photoluminescence spectra of different concentrations of Pr doping in LaFeO3.
Figure 7. (a) UV–Vis DRS spectrum and (b) photoluminescence spectra of different concentrations of Pr doping in LaFeO3.
Catalysts 11 00612 g007
Catalysts 11 00612 g008 550
Figure 8. Photocatalytic H2 performance of Pr-LaFeO3 photocatalysts modified with different amounts of Rh and Cr cocatalyst.
Figure 8. Photocatalytic H2 performance of Pr-LaFeO3 photocatalysts modified with different amounts of Rh and Cr cocatalyst.
Catalysts 11 00612 g008
Catalysts 11 00612 g009a 550Catalysts 11 00612 g009b 550
Figure 9. (a) Photocatalytic H2 evolution; (b) UV-DRS spectrum; (c) photoluminescence spectra for Pr-LaFeO3 treated at different calcination temperatures with 0.5 wt.% RhCrOx cocatalyst.
Figure 9. (a) Photocatalytic H2 evolution; (b) UV-DRS spectrum; (c) photoluminescence spectra for Pr-LaFeO3 treated at different calcination temperatures with 0.5 wt.% RhCrOx cocatalyst.
Catalysts 11 00612 g009aCatalysts 11 00612 g009b
Catalysts 11 00612 g010 550
Figure 10. XRD patterns of Pr-LaFeO3 after calcination at different temperatures.
Figure 10. XRD patterns of Pr-LaFeO3 after calcination at different temperatures.
Catalysts 11 00612 g010
Catalysts 11 00612 g011a 550Catalysts 11 00612 g011b 550
Figure 11. XPS spectra of RhCrOx/Pr-LaFeO3 photocatalysts after calcination at different temperatures: (a) Pr 3d, (b) La 3d, (c) Fe 2p, (d) O 1s, (e) Rh 3d, and (f) Cr 2p, and the green, pink and blue fitted curves for different metal.
Figure 11. XPS spectra of RhCrOx/Pr-LaFeO3 photocatalysts after calcination at different temperatures: (a) Pr 3d, (b) La 3d, (c) Fe 2p, (d) O 1s, (e) Rh 3d, and (f) Cr 2p, and the green, pink and blue fitted curves for different metal.
Catalysts 11 00612 g011aCatalysts 11 00612 g011b
Catalysts 11 00612 g012a 550Catalysts 11 00612 g012b 550
Figure 12. (a) Photocatalytic activity of RhCrOx/Pr-LaFeO3 with different sacrificial reagents and (b) different concentrations of TEOA. (c) Cyclic water splitting using RhCrOx/Pr-LaFeO3 photocatalysts.
Figure 12. (a) Photocatalytic activity of RhCrOx/Pr-LaFeO3 with different sacrificial reagents and (b) different concentrations of TEOA. (c) Cyclic water splitting using RhCrOx/Pr-LaFeO3 photocatalysts.
Catalysts 11 00612 g012aCatalysts 11 00612 g012b
Catalysts 11 00612 g013 550
Figure 13. Process of photocatalytic hydrogen evolution over RhCrOx/Pr-LaFeO3 under visible light irradiation with TEOA as the sacrificial reagent.
Figure 13. Process of photocatalytic hydrogen evolution over RhCrOx/Pr-LaFeO3 under visible light irradiation with TEOA as the sacrificial reagent.
Catalysts 11 00612 g013
Table
Table 1. Different photocatalysts and their Eg, ECB, EVC, and crystallite sizes.
Table 1. Different photocatalysts and their Eg, ECB, EVC, and crystallite sizes.
PhtotcatalystsEg
(eV)		ECB
(eV)		EVC
(eV)		Crystallite Sizes
(nm)
LaFeO32.31−0.112.2028.79
Eu-LaFeO32.08−0.072.0119.38
Ca-LaFeO32.17−0.102.0721.39
Pr-LaFeO32.00−0.011.9916.04
Mg-LaFeO32.20−0.102.1027.85
Tb-LaFeO32.30−0.172.1318.31
In-LaFeO32.05−0.042.0120.10
Y-LaFeO32.00−0.011.9915.60
Table
Table 2. Area ratios of Fe2+ and Fe3+ in the XPS spectra of Pr-LaFeO3 photocatalysts treated at different calcination temperatures.
Table 2. Area ratios of Fe2+ and Fe3+ in the XPS spectra of Pr-LaFeO3 photocatalysts treated at different calcination temperatures.
Calcining Temperature (°C)Fe2+ (Area %)Fe3+ (Area %)
50075.924.1
60027.972.1
70011.488.6
80037.063.0
90081.019.0
Table
Table 3. Comparison of H2 evolution rate of LaFeO3-based photocatalysts reported.
Table 3. Comparison of H2 evolution rate of LaFeO3-based photocatalysts reported.
PhtotcatalystsSacrificial AgentsCocatalystLight SourceH2 Evolution RateReference
Pr-doped LaFeO320% TEOA solutionRhCrOx350 W Xe lamp127 μmol h−1 g−1This work
Rh-doped LaFeO3Glucose-UV-LEDs 10 W, light intensity: 57 mW/cm2) 375–380 nm3822 μmol h−1 g−1 L−1[13]
Ru-doped LaFeO3Glucose-UV-LEDs 10 W, light intensity: 57 mW/cm2) 375–380 nm875 μmol h−1 g−1[12]
LaFeO3Glucose-UV-LEDs 10 W, light intensity: 57 mW/cm2) 375–380 nm400 μmol h−1 g−1[41]
Ru-LaFeO3/Fe2O3Glucose Visible LEDs, 440 nm1000 μmol h−1 g−1 L−1[42]
Nano LaFeO3Ethanol solution19 μL Pt400 W tungsten light sources3315 μmol h−1 g−1[8]
Polyaniline-coverd LaFeO310% TEOA solution3% Pt300 W Xe lamp92.4 μmol h−1[43]
Ternary LaFe0.8/LaCu0.2 catalysts12.5% HCHO solution-UV light (cut-off λ < 400 nm) 125 W Xe lamp250 μmol h−1 g−1[44]
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Share and Cite

MDPI and ACS Style

Chiang, T.H.; Viswanath, G.; Chen, Y.-S. Effects of RhCrOx Cocatalyst Loaded on Different Metal Doped LaFeO3 Perovskites with Photocatalytic Hydrogen Performance under Visible Light Irradiation. Catalysts 2021, 11, 612. https://doi.org/10.3390/catal11050612

AMA Style

Chiang TH, Viswanath G, Chen Y-S. Effects of RhCrOx Cocatalyst Loaded on Different Metal Doped LaFeO3 Perovskites with Photocatalytic Hydrogen Performance under Visible Light Irradiation. Catalysts. 2021; 11(5):612. https://doi.org/10.3390/catal11050612

Chicago/Turabian Style

Chiang, Tzu Hsuan, Gujjula Viswanath, and Yu-Si Chen. 2021. "Effects of RhCrOx Cocatalyst Loaded on Different Metal Doped LaFeO3 Perovskites with Photocatalytic Hydrogen Performance under Visible Light Irradiation" Catalysts 11, no. 5: 612. https://doi.org/10.3390/catal11050612

APA Style

Chiang, T. H., Viswanath, G., & Chen, Y.-S. (2021). Effects of RhCrOx Cocatalyst Loaded on Different Metal Doped LaFeO3 Perovskites with Photocatalytic Hydrogen Performance under Visible Light Irradiation. Catalysts, 11(5), 612. https://doi.org/10.3390/catal11050612

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop