WO₃@Fe₂O₃ Core-Shell Heterojunction Photoanodes for Efficient Photoelectrochemical Water Splitting

1. INTRODUCTION

The purine (imidazo-[4, 5-d]pyrimidine) skeleton is an important structural motif which plays an important role in different life related processes^{[1, 2]}. During the wide range of biological activities, purine structure is considered as a privileged scaffold in medicinal chemistry. Many drugs containing purine fragment have been developed for the treatment of asthma, inflammation, cancer and gastrointe stinal diseases^[3-9]. In addition, some compounds with purine fragment, such as aureonuclemycin, are fungicides for plant disease control^[10]. As active substructures, heterocyclic ring structures with both S and N atoms^[11], especially 3, 4-dichlo roisothiazole^[12], showed good systemic acquired resistance and fungicidal activities in pesticide lead discovery.

The discovery of lead compounds is an important basis for novel pesticide development. Our group focused on agrichemical lead discovery, different pyrazole-thiazoles^[13], pyrazole-aromatics^[14], and thiadiazole derivatives^[15] were found to show various degrees of fungicidal activity. YZK-C22 is a highly active fungicidal lead^[16]. The research has shown that YZK-C22 does not act at traditional pesticide targets, but has a new potent target: pyruvate kinase (PK)^[17]. Based on the structure of the lead molecule YZK-C22 and its potent new target PK, 3, 4-dichloro-5-(6-chloro-9-(4-fluoro benzyl)-9H-purin-8-yl)isothiazole was rationally designed (Fig. 1) and synthesized (Scheme 1) by the combination of bioactive substructures of purine and isothiazole, and its crystal chemical structure and fungicidal activity were evaluated here.

Figure 1

Figure 1.  Design of the target compound

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2. EXPERIMENTAL

2.1 Instruments and reagents

Melting point was measured on an X-4 Digital Type Melting Point Tester (Gongyi, China) and uncorrected. ¹H NMR spectra were recorded on a Bruker AV400 spectro meter (400 MHz) (Wisconsin, United States of America) and chemical shifts were reported in ppm. ¹³C NMR spectra were recorded on a Bruker AV400 spectrometer (101 MHz) (Wisconsin, United States of America) with complete proton decoupling. ¹⁹F NMR spectra were recorded on a Bruker AV400 spectrometer (101 MHz) (Wisconsin, United States of America) with complete proton decoupling. High-resolution mass spectra (HRMS) were recorded with an Agilent 6520 Q-TOF LC/MS instrument (Agilent Technologies Inc. State of California, United States of America). Crystal structure was determined on a Rigaku Xtalab P200 diffractometer. All of the solvents and materials were of reagent grade and purified as required.

2.2 Synthetic procedure for the target compound

The procedure for the synthesis of compound 3 is shown in Scheme 1. As a key intermediate, pyrimidine amine 2 was synthesized according to the revision of the reported method^[5]. Triethylamine (1.00 mL, 7.37 mmol) was added to a suspension of compound 1 (98% content) (1.00 g, 6.14 mmol) in ethanol (10 mL), followed by the addition of 4-fluorobenzylamine (0.75 mL, 6.45 mmol). Then the reaction mixture was stirred for 18 h at 80 ℃. After the reactant was consumed, the reaction mixture was concentrated under reduced pressure to remove the solvent, and the intermediate 2 was obtained by purifying the crude residue using silica gel column chromatography with a mixture eluent of petroleum ether (60~90 ℃ fraction): ethyl acetate (2:1, v/v).

Scheme 1

Scheme 1.  Synthesis of the target compound 3

Reagents and conditions: (i) 4-fluorobenzylamine, EtOH, Et₃N, 80 ℃, 12 h (ii) 3, 4-dichloroisothiazole-5-carbonyl chloride, NH₄Cl, toluene, 100 ℃, 2 h; POCl₃, 100 ℃, 12 h

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Analytical data for intermediate 2. Yellow solid; yield, 25%; m.p.: 221~223 ℃. ¹H NMR (400 MHz, DMSO-d₆) δ 7.73 (s, 1H), 7.39~7.30 (m, 3H), 7.19~7.09 (m, 2H), 5.09 (s, 2H), 4.60 (d, J = 5.6 Hz, 2H). ¹³C NMR (101 MHz, DMSO-d₆) δ 161.2 (d, ¹J_F-C = 242.2 Hz), 151.7, 145.5, 136.9, 135.6 (d, ⁴J_F-C = 3.0 Hz), 129.3 (d, ³J_F-C = 8.2 Hz), 123.6, 115.0 (d, ²J_F-C = 21.2 Hz), 43.4. ¹⁹F NMR (101 MHz, CDCl₃) δ –114.59. HRMS (ESI) m/z calcd. for C₁₁H₁₁ClFN₄⁺ (M+H)⁺: 253.0651; found: 253.0649. Document^[18] reported its yield of 84% with the m.p. of 240~242 ℃.

Compound 3 was synthesized according to the revision of the reported method^[19]. To a suspension of compound 2 (0.20 g, 0.79 mmol) in toluene, ammonium chloride (0.25 g, 4.74 mmol) and 3, 4-dichloroisothiazole-5-carbonyl chloride (0.10 mL, 0.79 mmol) were added successively. The reaction mixture was heated at 100 ℃ for 2 h. After cooling the mixture to room temperature, phosphorus oxychloride (8.0 mL) was added. Then, the mixture was slowly heated to 100 ℃ again and kept for 12 h. After the reaction completed, the reaction mixture was slowly dropwise added to ice water. Then, the pH of the mixture was adjusted to 7~8 using ammonia water (25%~28%) carefully, and compound 3 in the mixture was extracted with ethyl acetate (15 mL × 3). The combined organic layers were washed with saturated sodium chloride solution (20 mL) for 3 times and dried over anhydrous sodium sulfate. After the solvent evaporation under reduced pressure, the residue of the target compound 3 was purified by silica gel column chromatography with a mixture of petroleum ether: ethyl acetate (5:1, v/v) as eluent.

Analytical data for compound 3. Yellow solid; yield, 81%; m.p.: 133~134 ℃. ¹H NMR (400 MHz, CDCl₃) δ 8.82 (s, 1H), 6.99~6.92 (m, 2H), 6.92~6.85 (m, 2H), 5.53 (s, 2H). ¹³C NMR (101 MHz, CDCl₃) δ 162.7 (d, ¹J_F-C = 248.9 Hz), 153.1, 152.9, 152.0, 149.4, 148.8, 143.7, 131.5, 130.0 (d, ⁴J_F-C = 3.2 Hz), 129.3 (d, ³J_F-C = 8.2 Hz), 124.2, 116.2 (d, ²J_F-C = 21.8 Hz), 47.2. ¹⁹F NMR (101 MHz, CDCl₃) δ –111.9. HRMS (ESI) m/z calcd. for C₁₅H₈Cl₃FN₅S⁺ (M+H)⁺: 413.9545; found: 413.9549.

2.3 Structure determination

The colorless crystal of the title compound 3 with dimensions of 0.18mm × 0.16mm × 0.13mm was cultured from n-hexane/dichloromethane and selected for X-ray diffraction analysis. The data were collected on a Rigaku Xtalab P200 Single Crystal diffractometer equipped with mirror-monochromatic CuKα radiation (λ = 1.54184 Å) with an ω scan mode at 294.15 K. In the range of 4.22≤θ≤79.05°, a total of 22315 reflections were collected with 3532 unique ones (R_int = 0.0311), of which 3238 were observed with I > 2σ(I) for refinements. Using Olex2^[20], the structure was solved by the ShelXT^[21] structure solution program using Intrinsic Phasing and refined with the ShelXL^[22] refinement package using Least Squares minimization. All of the non-hydrogen atoms were located with successive difference Fourier syntheses. The hydrogen atoms were added according to theoretical models. The final full-matrix least-squares refinement converged at R = 0.0310, wR = 0.0842 (w = 1/[σ²(F_o)² + (0.0393P)² + 0.5096P], where P = (F_o² + 2F_c²)/3), S = 1.075, (Δρ)_max = 0.24, (Δρ)_min = –0.25 e/ų and (Δ/σ)_max = 0.001.

2.4 Fungicidal activity determination

The fungicidal activities of intermediate 2 and target compound 3 were evaluated at 50 mg/L according to the previously reported procedures^[23-25]. Seven representative fungi, A. s: Alternaria solani; B. c: Botrytis cinerea; C. a: Cercospora arachidicola; G. z: Gibberella zeae; P. p: Physalospora piricola; R. s: Rhizoctonia solani and S. s: Sclerotinia sclerotiorum, were tested. The commercially available pyrimidinamine fungicide diflumetorim and lead molecule YZK-C22 were selected as positive controls. Inhibitory rates (%) = (D_control – D_test)/(D_control – 4) × 100, where D_control was the average diameter (mm) of mycelia in the absence of any compounds and D_test was the average diameter (mm) of mycelia treated with the test compound. All experiments were tested in triplicates. Data were presented as the mean ± standard deviation. EC₅₀ of the target compound 3 and corresponding positive controls against R. solani were evaluated, too^[16].

3. RESULTS AND DISCUSSION

As shown in Scheme 1, the target compound 3 was synthesized in good yield by cyclization of pyrimidine amine 2 with 3, 4-dichloroisothiazole-5-carbonyl chloride. Its structure was characterized by ¹H NMR, ¹³C NMR, ¹⁹F NMR and HRMS. The crystal structure of compound 3, crystallizing from a mixed solvent of dichloromethane and n-hexane (1:2, v/v), is shown in Fig. 2.

Figure 2

Figure 2.  X-ray crystal structure of compound 3

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The selected bond lengths, bond angels and torsional angels of compound 3 are shown in Tables 1 and 2. The bond lengths and angles of the isothiazole ring agreed well with the values reported^[26]. Meanwhile, bond lengths and angles of the purine substructure appeared to be normal relative to the closely related compounds in literature^[27]. The sum of C(4)–N(5)–C(9), C(8)–N(5)–H(9) and C(8)–N(5)–H(5) angles was 359.96°, indicating the sp² hybridization state of N(5) atom. The torsion angle of N(2)–C(5)–C(8)–N(4) is –178.75°, indicating that the whole purine was coplanar. The torsion angles of C(2)–C(3)–C(4)–N(2) and C(8)–N(5)–C(9)– C(10) are –67.7° and 121.03°, which means that both the isothiazole and benzene rings were nonplanar with the purine ring. As shown in Table 3, the intermolecular hydrogen bonds C(9)–HA(9)⋅⋅⋅F(1)ⁱ, C(9)–HA(9)⋅⋅⋅Cl(2)ⁱⁱ and C(9)– HB(9)⋅⋅⋅N(2)ⁱⁱⁱ were found in compound 3, which lead to the position of benzene ring close to the isothiazole ring rather than the purine ring. These intermolecular hydrogen bonds stabilize the crystal packing (Fig. 3). In addition, the intermolecular C–H···π interaction of C(12)–H(12)···C(15)^iv (H(12)⋅⋅⋅C(15)^iv 2.676 Å) was also observed in the crystal packing of compound 3, which is two-dimensional. No π-π interaction was observed due to the large distance between adjacent benzene ring and isothiazole ring or purine ring.

Table 1

Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) for Compound 3

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Bond	Dist.		Bond	Dist.		Bond	Dist.
Cl(1)–C(1)	1.7125(17)		N(3)–C(7)	1.347(3)		C(5)–C(6)	1.391(2)
Cl(2)–C(2)	1.7046(17)		N(4)–C(7)	1.334(2)		C(5)–C(8)	1.392(2)
Cl(3)–C(6)	1.7244(19)		N(4)–C(8)	1.333(2)		C(9)–C(10)	1.509(2)
S(1)–N(1)	1.6466(15)		N(5)–C(4)	1.3753(19)		C(10)–C(11)	1.382(2)
S(1)–C(3)	1.7064(16)		N(5)–C(8)	1.3720(19)		C(10)–C(15)	1.385(2)
F(1)–C(13)	1.3618(19)		N(5)–C(9)	1.4697(19)		C(11)–C(12)	1.381(2)
N(1)–C(1)	1.301(2)		C(1)–C(2)	1.414(2)		C(12)–C(13)	1.362(3)
N(2)–C(4)	1.3164(19)		C(2)–C(3)	1.362(2)		C(13)–C(14)	1.360(3)
N(2)–C(5)	1.378(2)		C(3)–C(4)	1.466(2)		C(14)–C(15)	1.390(3)
N(3)–C(6)	1.314(2)
Angles	(°)		Angles	(°)		Angles	(°)
N(1)–S(1)–C(3)	95.25(8)		C(2)–C(3)–S(1)	108.45(11)		N(4)–C(8)–C(5)	126.88(15)
C(1)–N(1)–S(1)	109.11(12)		C(2)–C(3)–C(4)	124.94(15)		N(5)–C(8)–C(5)	105.65(13)
C(4)–N(2)–C(5)	103.45(13)		C(4)–C(3)–S(1)	126.29(12)		N(5)–C(9)–C(10)	112.86(12)
C(6)–N(3)–C(7)	117.55(15)		N(2)–C(4)–N(5)	114.21(13)		C(11)–C(10)–C(9)	120.87(14)
C(8)–N(4)–C(7)	111.53(16)		N(2)–C(4)–C(3)	124.18(14)		C(11)–C(10)–C(15)	118.91(14)
C(4)–N(5)–C(9)	128.24(13)		N(5)–C(4)–C(3)	121.24(13)		C(15)–C(10)–C(9)	120.21(14)
C(8)–N(5)–C(4)	105.51(12)		N(2)–C(4)–C(6)	134.17(16)		C(12)–C(11)–C(10)	120.81(15)
C(8)–N(5)–C(9)	126.21(13)		N(2)–C(4)–C(8)	111.16(13)		C(13)–C(12)–C(11)	118.53(17)
N(1)–C(1)–Cl(1)	120.31(13)		C(6)–C(5)–C(8)	114.63(15)		F(1)–C(13)–C(12)	118.51(18)
N(1)–C(1)–C(2)	116.91(15)		N(3)–C(6)–Cl(3)	118.19(13)		C(14)–C(13)–F(1)	118.62(17)
C(2)–C(1)–Cl(1)	122.77(14)		N(3)–C(6)–C(5)	121.36(17)		C(14)–C(13)–C(12)	122.87(16)
C(1)–C(2)–Cl(2)	124.57(13)		C(5)–C(6)–Cl(3)	120.45(15)		C(13)–C(14)–C(15)	118.27(16)
C(3)–C(2)–Cl(2)	125.10(13)		N(4)–C(7)–N(3)	128.05(17)		C(10)–C(15)–C(14)	120.61(16)
C(3)–C(2)–C(1)	110.26(15)		N(4)–C(8)–N(5)	127.46(15)

Table 2

Table 2.  Selected Torsional Angles (°) for Compound 3

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Angle	(°)		Angle	(°)
Cl(1)–C(1)–C(2)–Cl(2)	–0.2(2)		C(4)–N(5)–C(9)–C(10)	–56.3(2)
Cl(2)–C(2)–C(3)–C(4)	4.5(2)		C(5)–N(2)–C(4)–C(3)	172.27(15)
S(1)–N(1)–C(1)–Cl(1)	–177.56(10)		C(6)–C(5)–C(8)–N(4)	–0.7(2)
S(1)–C(3)–C(4)–N(5)	–67.76(19)		C(6)–C(5)–C(8)–N(5)	178.55(14)
F(1)–C(13)–C(14)–C(15)	–179.32(18)		C(7)–N(3)–C(6)–Cl(3)	179.48(15)
N(1)–S(1)–C(3)–C(4)	172.91(14)		C(8)–N(5)–C(4)–C(3)	–172.18(14)
N(1)–C(1)–C(2)–Cl(2)	–178.53(13)		C(8)–N(5)–C(9)–C(10)	121.03(16)
N(2)–C(5)–C(6)–Cl(3)	–1.5(3)		C(9)–N(5)–C(4)–N(2)	178.92(14)
N(2)–C(5)–C(8)–N(4)	–178.75(15)		C(9)–N(5)–C(4)–C(3)	5.6(2)
N(2)–C(5)–C(8)–N(5)	0.48(18)		C(9)–N(5)–C(8)–N(4)	0.5(3)
N(5)–C(9)–C(10)–C(11)	–49.3(2)		C(9)–C(10)–C(11)–C(12)	–178.15(16)
N(5)–C(9)–C(10)–C(15)	131.96(16)		C(9)–C(10)–C(15)–C(14)	177.84(17)
C(2)–C(3)–C(4)–N(2)	–67.7(2)		C(11)–C(12)–C(13)–F(1)	179.01(18)
C(2)–C(3)–C(4)–N(5)	104.98(18)		C(13)–C(14)–C(15)–C(10)	0.6(3)
C(4)–N(5)–C(8)–N(4)	178.30(15)		C(15)–C(10)–C(11)–C(12)	0.6(3)

Table 3

Table 3.  Hydrogen Bond Lengths (Å) and Bond Angles (°) for Compound 3

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D–H···A	d(D–H)	d(H···A)	d(D···A)	∠(DHA)
C(9)–H(9A)···F(1)i	0.97	2.59	3.422(2)	144
C(9)–H(9A)···Cl(2)ii	0.97	2.94	3.555(2)	123
C(9)–H(9B)···N(2)iii	0.97	2.68	3.612(2)	162
Symmetry codes: i: –1/2 + x, 1/2 – y, –1/2 + z; ii: –x, 1 – y, – z; iii: –1.5 + x, 1/2 – y, –1/2 + z

Figure 3

Figure 3.  Crystal packing of compound 3

Symmetry codes: ⁱ: –1/2 + x, 1/2 – y, –1/2 + z; ⁱⁱ: –x, 1 – y, –z; ⁱⁱⁱ: –3/2 + x, 1/2 – y, –1/2 + z; ^iv: 1/2 – x, –1/2 + y, 1/2 –z

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Fungicidal bioassay of intermediate 2 and the target compound 3 against seven phytopathogenic fungi at a concentration of 50 mg/L was compared with commercially pyrimidinamine fungicide diflumetorim and lead compound YZK-C22 as positive controls. As shown in Table 4, the intermediate 2 showed weak effects at 50 mg/L, the target compound 3 showed over 50% of inhibitory activities against B. cinerea, C. arachidicola, G. zeae, R. solani, S. sclerotiorum at 50 mg/L with inhibition of 58%, 53%, 55%, 67% and 59%. Most of them were better than diflumetorim but less than YZK-C22. To further assess the fungicidal potency, the EC₅₀ values of target compound and positive controls with inhibition over 60% at 50 mg/L were measured. The results shown in Table 5 indicated that compound 3 exhibited good fungicidal activities with EC₅₀ value of 25.06 mg/L or 60.44 µmol/L against R. solani. It was active at the same level of that of the positive control diflumetorim (19.76 mg/L or 60.29 µmol/L) and less active than the positive control YZK-C22 (4.21 mg/L or 12.32 µmol/L)^[16]. Docking studies showed that the target compound had larger binding energy with pyruvate kinase than the positive control YZK-C22 because of the effecting of absorption, transduction and metabolism. Our studies indicated that isothiazolopurin derivative could be a fungicidal lead deserving for further study.

Table 4

Table 4.  Fungicidal Activities of Compounds Synthesized (Inhibition Rate/%)^a

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Compd.	A.s b	B.c	C.a	G.z	P.p	R.s	S.s
2	27 ± 0	14 ± 1	43 ± 1	25 ± 2	18 ± 1	24 ± 0	24 ± 1
3	38 ± 1	58 ± 0	53 ± 2	55 ± 1	34 ± 0	67 ± 1	59 ± 2
Diflumetorim	55 ± 1	44 ± 1	67 ± 1	48 ± 1	39 ± 1	74 ± 0	44 ± 2
YZK-C22	60 ± 2	71 ± 3	77 ± 2	77 ± 1	55 ± 2	82 ± 2	63 ± 1
a Values are the average of three replicates, tested at a concentration of 50 mg/L.bA.s: Alternariasolani; B.c: Botrytis cinerea; C.a: Cercosporaarachidicola; G.z: Gibberellazeae; P.p: Physalosporapiricola; R.s: Rhizoctoniasolani; S.s: Sclerotinia sclerotiorum.

Table 5

Table 5.  EC₅₀ of the Target Compounds with Inhibition over 60% at 50 mg/L in Vitro

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Fungi	Compd.	Regression equation	R2	95% confidence interval(mg/L)	EC50(mg/L)	EC50(µmol/L)
R. solani	3	y = 3.0674 + 1.3814x	0.9543	17.70~35.47	25.06	60.44
	Diflumetorim	y = 3.0814 + 1.4806x	0.9969	18.07~21.61	19.76	60.29
	YZK-C22[16]	y = 4.2367 + 1.2237x	0.9766	2.97~5.95	4.21	12.32

References

1. [1]

   Fu, J.; Fan, Z.; Nakabayashi, M.; Ju, H.; Pastukhova, N.; Xiao, Y.; Feng, C.; Shibata, N.; Domen, K.; Li, Y. Interface engineering of Ta₃N₅ thin film photoanode for highly efficient photoelectrochemical water splitting. Nat. Commun. 2022, 13, 729-735.  doi: 10.1038/s41467-022-28415-4
   

2. [2]

   Seabold, J.; Choi, K. Effect of a cobalt-based oxygen evolution catalyst on the stability and the selectivity of photo-oxidation reactions of a WO₃ photoanode. Chem. Mater. 2011, 23, 1105-1112.
   

3. [3]

   Liu, Y.; Yu, F.; Wang, F.; Bai, S.; He, G. Construction of Z-scheme In₂S₃-TiO₂ for CO₂ reduction under concentrated natural sunlight. Chin. J. Struct. Chem. 2022, 41, 2201034-2201039.
   

4. [4]

   Li, C.; Li, T.; Jing, M.; Yuan, W.; Li, C. M. Remarkably promoted photoelectrochemical water oxidation on TiO₂ nanowire arrays via poly-mermediated self-assembly of CoO_x nanoparticles. Sol. Energ. Mat. Sol. C 2020, 207, 110349.
   

5. [5]

   Zou, J.; Liao, G.; Jiang, J.; Xiong, Z.; Bai, S. In-situ construction of sulfur-doped g-C₃N₄ defective g-C₃N₄ isotype step-scheme heterojunction for boosting photocatalytic H₂ evolution. Chin. J. Struct. Chem. 2022, 41, 2201025-2201033.
   

6. [6]

   Yuan, W.; Yuan, J.; Xie, J.; Li, C. M. Polymer-mediated self-assembly of TiO₂@Cu₂O core-shell nanowire array for highly effcient photoelectrochemical water oxidation. ACS Appl. Mater. Inter 2016, 8, 6082-6092.  doi: 10.1021/acsami.6b00030
   

7. [7]

   Zhu, P.; Wang, Y.; Sun, X.; Zhang, J.; Waclawik, E. R.; Zheng, Z. Photocatalytic-controlled olefin isomerization over WO_3-x using low-energy photons up to 625 nm. Chin. J. Catal. 2021, 42, 1641-1647.  doi: 10.1016/S1872-2067(21)63815-9
   

8. [8]

   Ran, L.; Qiu, S.; Zhai, P.; Li, Z.; Gao, J.; Zhang, X.; Zhang, B.; Wang, C.; Sun, L.; Hou, J. Conformal macroporous inverse opal oxynitride-based photoanode for robust photoelectrochemical water splitting. J. Am. Chem. Soc. 2021, 143, 7402-7413.  doi: 10.1021/jacs.1c00946
   

9. [9]

   He, J. S.; Liu, P. Y.; Ran, R.; Wang, W.; Zhou, W.; Shao, Z. P. Single-atom catalysts for high-efficiency photocatalytic and photoelectro-chemical water splitting: distinctive roles, unique fabrication methods and specific design strategies. J. Mater. Chem. A 2022, 10, 6835-6871.  doi: 10.1039/D2TA00835A
   

10. [10]

    Gaikwad, M. A.; Suryawanshi, U. P.; Ghorpade, U. V.; Jang, J. S.; Suryawanshi, M. P.; Kim, J. H. Emerging surface, bulk, and interface engineering strategies on BiVO₄ for photoelectrochemical water splitting. Small 2022, 18, 2105084.  doi: 10.1002/smll.202105084
    

11. [11]

    Li, C.; Chen Z.; Yuan, W.; Xu, Q. H.; Li, C. M. In situ growth of α-Fe₂O₃@Co₃O₄ core-shell wormlike nanoarrays for a highly efficient photoelectrochemical water oxidation reaction. Nanoscale 2019, 11, 1111-1122.
    

12. [12]

    Pinto, F.; Wilson, A.; Moss, B.; Kafizas, A. Systematic exploration of WO₃/TiO₂ heterojunction phase space for applications in photoelectro-chemical water splitting. J. Phys. Chem. C 2022, 126, 871-884.
    

13. [13]

    Wang, Y.; Wang, Y.; Zhao, J.; Chen, M.; Huang, X.; Xu, Y. Efficient production of H₂O₂ on Au/WO₃ under visible light and the influencing factors. Appl. Catal. B 2021, 284, 119691-119702.
    

14. [14]

    Liu, X.; Wanga, F.; Wanga, Q. Nanostructure-based WO₃ photoanodes for photoelectrochemical water splitting. Phys. Chem. Chem. Phys. 2012, 14, 7894-7911.
    

15. [15]

    Wang, Z.; Zhu, H.; Tu, W.; Zhu, X.; Yao, Y.; Zhou, Y.; Zou, Z. Host/guest nanostructured photoanodes integrated with targeted enhancement strategies for photoelectrochemical water splitting. Adv. Sci. 2022, 9, 103744.
    

16. [16]

    Francisco, F.; Dias, P.; Ivanou, D.; Santos, F.; Azeyedo, J.; Mendes, A. Synthesis of host-guest hematite photoelectrodes for solar water splitting. Chemnanomat 2019, 5, 911-920.
    

17. [17]

    Yu, W.; Chen, J.; Shang, T.; Chen, L.; Gu, L.; Peng, T. Direct Z-scheme g-C₃N₄/WO₃ photocatalyst with atomically defined junction for H-2 production. Appl. Catal. B Environ. 2017, 219, 693-704.
    

18. [18]

    Li, H.; Zhao, F.; Zhang, J.; Luo, L.; Xiao, X.; Huang, Y.; Ji, H.; Tong, Y. A g-C₃N₄/WO₃ photoanode with exceptional ability for photoelectrochemical water splitting. Mate. Chem. Front. 2017, 1, 338-342.
    

19. [19]

    Pinto, F.; Wilson, A.; Moss, B.; Kafizas, A. Systematic exploration of WO₃/TiO₂ heterojunction phase space for applications in photoelectrochemical water splitting. J. Phys. Chem. C 2022, 126, 871-884.
    

20. [20]

    Wei, P.; Lin, K.; Meng, D.; Xie, T.; Na, Y. Photoelectrochemical performance for water oxidation improved by molecular nickel porphyrinintegrated WO₃/TiO₂ photoanode. Chemsuschem 2018, 11, 1746-1750.
    

21. [21]

    Sun, W.; Wang, D.; Rahman, Z. U.; Wei, N.; Chen, S. 3D hierarchical WO₃ grown on TiO₂ nanotube arrays and their photoelectrochemical performance for water splitting. J. Alloys Compd. 2017, 695, 2154-2159.
    

22. [22]

    Khare, C.; Sliozberg, K.; Meyer, R.; Savan, A.; Schuhmann, W.; Ludwig, A. Layered WO₃/TiO₂ nanostructures with enhanced photocurrent densities. Int. J. Hydrogen Energy 2013, 38, 15954-15964.
    

23. [23]

    Zhang, Y. F.; Zhu, Y. K.; Lv, C. X.; Lai, S. J.; Xu, W. J.; Sun, J.; Sun, Y. Y.; Yang, D. J. Enhanced visible-light photoelectrochemical performance via chemical vapor deposition of Fe₂O₃ on a WO₃ film to form a heterojunction. Rare Metals 2020, 39, 841-849.
    

24. [24]

    Kim, E.; Kim, S.; Choi, Y. M.; Park, J. H.; Shin, H. Ultrathin hematite on mesoporous WO₃ from atomic layer deposition for minimal charge recombination. ACS Sustain. Chem. Eng. 2020, 8, 11358-11367.
    

25. [25]

    Memar, A.; Phan, C. M.; Tade, M. O. Photocatalytic activity of WO₃/Fe₂O₃ nanocomposite photoanode. Int. J. Hydrogen Energy 2015, 40, 8642-8649.
    

26. [26]

    Sadhasivam, S.; Gunasekaran, A.; Anbarasan, N.; Mukilan, N.; Jeganathan, K. CdS and CdSe nanoparticles activated 1D TiO₂ hetero-structure nanoarray photoelectrodes for enhanced photoelectrocatalytic water splitting. Int. J. Hydrogen Energy 2021, 46, 26381-26390.
    

27. [27]

    Qiu, Y.; Pan, Z.; Chen, H.; Ye, D.; Guo, L.; Fan, Z.; Yang, S. Current progress in developing metal oxide nanoarrays-based photoanodes for photoelectrochemical water splitting. Sci. Bull. 2019, 64, 1348-1380.
    

28. [28]

    Pu, Y.; Wang, G.; Chang, K.; Ling, Y.; Li, Y. Au nanostructure-decorated TiO₂ nanowires exhibiting photoactivity across entire UV-visible region for photoelectrochemical water splitting. Nano Lett. 2013, 13, 3817-3823.
    

29. [29]

    Luo, Z.; Wang, T.; Zhang, J.; Li, C.; Li, H.; Gong, J. Dendritic hematite nanoarray photoanode modified with a conformal titanium dioxide inter-layer for effective charge collection. Angew. Chem. Int. Ed. 2017, 56, 12878-12882.
    

30. [30]

    Peng, G.; Lu, H.; Liu, Y.; Fan, D. The construction of a single-crystalline SbSI nanorod array-WO₃ heterostructure photoanode for high PEC performance. Chem. Commun. 2021, 57, 335-338.
    

31. [31]

    Gimenes, D. T.; Nossol, E. Effect of light source and applied potential in the electrochemical synthesis of Prussian blue on carbon nanotubes. Electrochim. Acta 2017, 251, 513-521.
    

32. [32]

    Mao, G.; Li, C.; Li, Z.; Xu, M.; Wu, H.; Liu, Q. Efficient charge migration in TiO₂@PB nanorod arrays with core-shell structure for photoelectrochemical water splitting. CrystEngComm 2022, 24, 2567-2574.
    

33. [33]

    Wu, H.; Liu, Q.; Zhang, L.; Tang, Y.; Wang, G.; Mao, G. Novel nano-structured WO₃@Prussian blue heterojunction photoanodes for efficient photoelectrochemical water splitting. ACS Appl. Energy Mater. 2021, 4, 12508-12514.
    

34. [34]

    Cao, L.; Liu, Y.; Zhang, B.; Lu, L. In situ controllable growth of Prussian blue nanocubes on reduced graphene oxide: facile synthesis and their application as enhanced nanoelectrocatalyst for H₂O₂ reduction. ACS Appl. Mater. Inter. 2010, 2, 2339-2346.
    

35. [35]

    Li, Y.; Hu, J.; Yang, K.; Cao, B.; Li, Z.; Yang, L.; Pan, F. Synthetic control of Prussian blue derived nano-materials for energy storage and conversion application. Mater. Today Energy 2019, 14, 100332.
    

36. [36]

    Hu, M.; Belik, A. A.; Imura, M.; Mibu, K.; Tsujimoto, Y.; Yamauchi, Y. Synthesis of superparamagnetic nanoporous iron oxide particles with hollow interiors by using Prussian blue coordination polymers. Chem. Mater. 2012, 24, 2698-2707.
    

37. [37]

    Zakaria, M. B.; Belik, A. A.; Liu, C. H.; Hsieh, H. Y.; Liao, Y. T.; Malgras, V.; Yamauchi, Y.; Wu, K. C. W. Prussian blue derived nano-porous iron oxides as anticancer drug carriers for magnetic-guided chemotherapy. Chem. Asian J. 2015, 10, 1457-1462.
    

38. [38]

    Wang, Y.; Wang, Y.; Zhao, J.; Chen, M.; Huang, X.; Xu, Y. Efficient production of H₂O₂ on Au/WO₃ under visible light and the influencing factors. Appl. Catal. B 2021, 284, 119691-119702.
    

39. [39]

    Ma, M.; Zhang, K.; Li, P.; Jung, M. S.; Jeong, M. J.; Park, J. H. Dual oxygen and tungsten vacancies on a WO₃ photoanode for enhanced water oxidation. Angew. Chem. Int. Ed. 2016, 128, 11998-12002.
    

40. [40]

    Ma, J.; Mao, K.; Low, J.; Wang, Z.; Xi, D.; Zhang, W.; Ju, H.; Qi, Z.; Long, R.; Wu, X.; Song, L.; Xiong, Y. Efficient photoelectrochemical conversion of methane into ethylene glycol by WO₃ nanobar arrays. Angew. Chem. Int. Ed. 2021, 133, 9443-9447.
    

41. [41]

    Iandolo, B.; Wickman, B.; Zoric, I.; Hellman, A. The rise of hematite: origin and strategies to reduce the high onset potential for the oxygen evolution reaction. J. Mater. Chem. A 2015, 3, 16896-16912.
    

42. [42]

    Chai, H.; Gao, L.; Wang, P.; Li, F.; Hu, G.; Jin, J. In₂S₃/F-Fe₂O₃ type-II heterojunction bonded by interfacial S-O for enhanced charge separation and transport in photoelectrochemical water oxidation. Appl. Catal. B Environ. 2022, DOI 10.1016/j.apcatb.2021.121011.

43. [43]

    Zhang, M.; Luo, W.; Li, Z.; Yu, T.; Zou, Z. Improved photoelectro-chemical responses of Si and Ti codoped α-Fe₂O₃ photoanode films. Appl. Phys. Lett. 2010, 97, 042105-042105.
    

44. [44]

    Mei, B. A.; Munteshari, O.; Lau, J.; Dunn, B.; Pilon, L. Physical interpretations of nyquist plots for EDLC electrodes and devices. J. Phys. Chem. C 2018, 122, 194-206.
    

45. [45]

    Sivula, K.; Formal F. L.; Gratzel, M. WO₃-Fe₂O₃ photoanodes for water splitting: a host scaffold, guest absorber approach. Chem. Mater. 2009, 21, 2862-2867.
    

46. [46]

    Wu, Q.; Bu, Q.; Li, S.; Lin, Y.; Zou, X.; Wang, D.; Xi, T. Enhanced interface charge transfer via n-n WO₃/Ti-Fe₂O₃ heterojunction formation for water splitting. J. Alloys Compd. 2019, 803, 1105-1111.