# MOF-Derived Multi-heterostructured Composites for Enhanced Photocatalytic Hydrogen Evolution: Deciphering the Roles of Different Components

Mian Zahid Hussain, Zhuxian Yang, Bart van der Linden, Werner R Heinz, Mounib Bahri, Ovidiu Ersen, Quanli Jia, Roland A Fischer, Yanqiu Zhu, Yongde Xia

<figure><img src="image_1.png" /></figure>

To cite this version:

Mian Zahid Hussain, Zhuxian Yang, Bart van der Linden, Werner R Heinz, Mounib Bahri, et al. MOF-Derived Multi-heterostructured Composites for Enhanced Photocatalytic Hydrogen Evolution: Deciphering the Roles of Different Components. Energy & Fuels, 2022, 36 (19), pp.12212 - 12225. 10.1021/acs.energyfuels.2c02319 . hal-04908262

HAL Id: hal-04908262

https://hal.science/hal-04908262v1

Submitted on 23 Jan 2025

**HAL** is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_2.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# MOF-Derived Multi-heterostructured Composites for Enhanced Photocatalytic Hydrogen Evolution: Deciphering the Roles of Different Components

Mian Zahid Hussain,* Zhuxian Yang, Bart van der Linden, Werner R. Heinz, Mounib Bahri, Ovidiu Ersen, Quanli Jia, Roland A. Fischer, Yanqiu Zhu, and Yongde Xia*

<figure><img src="image_3.png" /></figure>

Cite This: Energy Fuels 2022, 36, 12212–12225

<figure><img src="image_4.png" /></figure>

Read Online

## ACCESS

<figure><img src="image_5.png" /></figure>

Metrics & More

<figure><img src="image_6.png" /></figure>

Article Recommendations

<figure><img src="image_7.png" /></figure>

**ABSTRACT:** Bimetal-organic-framework (Bi-MOF) NH₂-MIL-125(Ti/Cu)-derived nanocomposites are systematically investigated to elucidate the role of individual species TiO₂, CuₓO and the porous carbon matrix in photocatalytic activity. Among the studied samples, the TiO₂/CuₓO/C nanocomposite derived from heat processing NH₂-MIL-125(Ti/Cu) under Ar/H₂O vapor demonstrates the highest photocatalytic H₂ evolution performance due to the formation of a phasejunction between the well-crystallized anatase/rutile TiO₂ polymorph, the optimized and codoped nitrogen/carbon in the composites, the formation of p-n heterojunctions between the TiO₂ and CuₓO nanoparticles, as well as their uniform distribution in a hydrophilic porous carbon matrix decorated with N and carboxylic functional groups. These

parameters enable the in situ-formed multi-heterostructures in these nanocomposites to not only possess relatively narrower energy band gaps and improved spatial charge separation due to the formed type-II staggered p-n heterojunctions but also offer multiple pathways for charge diffusion, resulting in lower charge-transfer resistance, suppressed bulk charge recombination, and consequently, much improved visible-light absorption. Therefore, the Bi-MOF NH₂-MIL-125(Ti/Cu)-derived TiO₂/CuₓO/C nanocomposite provides easily accessible active sites with an excellent photocatalytic H₂ evolution activity of 3147 µmol gcat⁻¹ h⁻¹, 99 times higher than that of bare TiO₂. This work provides a simple one-step approach to producing tunable novel nanocomposites for efficient photocatalytic H₂ evolution without using expensive noble metals as cocatalysts.

<figure><img src="image_8.png" /></figure>

## INTRODUCTION

Renewable energy sources are regarded as the most plausible alternatives to fossil fuels and nuclear energy to deal with the global challenges of climate change and environmental pollution.¹ The generation of green hydrogen (H₂) from photocatalytic water splitting is one of the viable solutions to produce an environmentally sustainable and cost-effective clean fuel to be directly used in automobiles, domestic heating, fuel cells, and industrial applications.²,³ Currently, H₂ is largely produced by economically expensive and environmentally unsustainable processes such as methane steam reforming.⁴ Since Fujishima demonstrated in 1972 that the solar-light-driven H₂ evolution imitates the photosynthesis process, much effort has been dedicated to searching for chemically stable and highly efficient photocatalysts.²,⁵

Most of the semiconducting photocatalysts possess wide energy band gaps (EBGs), which leads to poor photocatalytic performance due to the limited absorption of light and the higher recombination of photogenerated charge. Various strategies, such as defect generation, and doping of cationic (metal atoms) and/or anionic (C, N, S, P) species into the

photocatalyst, which narrows down the EBGs, have been adapted to improve the photocatalytic performance.⁶⁻⁸ Alternatively, photoactive multimetal compounds can be combined to generate heterostructures as well as metal and/or nonmetal composites that can synergistically improve the photocatalytic activities.⁹⁻¹¹ An ideal photocatalyst for hydrogen evolution reaction (HER) should absorb maximum light in the UV-visible and IR regions of the electromagnetic spectrum and produce the corresponding number of photo-generated charge carriers to the redox reaction with minimum charge recombination.⁹,¹⁰ In addition, it should exhibit long-term chemical and physical stability for constant H₂ evolution.⁹

Received: July 11, 2022

Revised: August 17, 2022

Published: September 1, 2022

<figure><img src="image_9.png" /></figure>
Since the early demonstrations of photoelectrochemical performance of TiO₂-based materials, TiO₂ has been one of the most promising semiconducting materials due to its photocatalytically suitable optoelectronic properties.²,⁹ Although the wider EBG (3.2 eV) and the limited absorption of incident light in the UV region restrict TiO₂ from being directly employed in photocatalytic reactions,⁶ the EBG of TiO₂ can be easily tuned by controlled doping of anionic and/or cationic species for better light absorption in the visible region of the light spectrum. However, charge recombination in such single metal oxide photocatalysts is still a challenge since photocharge generation/recombination takes place at the time scale of femtoseconds to picoseconds, whereas the catalytic reactions usually occur at the time scale of microseconds to milliseconds.⁹,¹² Therefore, to develop a successful photocatalyst, it is necessary to prolong the lifetime of the photogenerated charges, which can be realized by spatially separating the photogenerated charge carriers to maximize the interaction between the catalytically reactive species and photogenerated electrons/holes at the respective active sites for enhanced quantum yield. Together with the optoelectronic properties, the chemical composition and the morphology of the photocatalyst play important roles in determining the lifetime and diffusion lengths of the photogenerated charge carriers.⁹ The spatial separation of charge carriers, their diffusion path (length), and the catalytic activity in the semiconducting photocatalysts can be modified through various methods, including rational design of structures, atomically precise control of chemical compositions, optimized morphologies, and precise construction of interfaces of (multi)metal compound heterostructures.⁶ Many approaches to synthesizing such photocatalytically efficient systems have been reported; however, these approaches generally require multistep, complex, and relatively expensive processes, which make them practically unaffordable.⁶,⁷

Metal-organic frameworks (MOFs) are excellent materials due to their structural, morphological, and textural properties, offering chemically active inorganic and organic moieties. They possess extraordinarily high surface areas, modifiable textural properties, and accessible photocatalytic active sites.¹³,¹⁴ Despite these unique properties, the relatively poor semiconducting properties restrict the pristine MOFs from being directly employed as photocatalysts.¹⁵ However, MOFs are excellent sacrificial templates or precursors to derive an unlimited variety of photocatalytically efficient nanocomposites, which offer great flexibility to rationally design and modify the morphology, dimensionality, chemical composition, textural, optoelectronic, and semiconducting properties.¹⁶⁻²¹ All of these properties play very important roles in determining the photocatalytic activity of a catalyst. In this context, some fundamentally important questions need to be addressed. For instance, where does the photocatalytic activity in such chemically complex, MOF-derived nanocomposites originate from? What role do the derived metal oxides and carbon species play in the absorption of incident light and photocatalytic H₂ evolution? Based on some in situ characterizations and in-depth analyses of nanocomposites derived from NH₂-MIL-125(Ti/Cu), this work offers useful insights into optical and physicochemical properties of the derived composites to decipher the role of metal oxides (TiO₂, CuₓO) and porous carbon in photocatalytic H₂ evolution.

## EXPERIMENTAL SECTION

**Synthesis of NH₂-MIL-125(Ti).** The NH₂-MIL-125(Ti) was synthesized by following a previously reported method with some modifications for scaling up.²²,²³ In a typical synthesis, 30 mmol of 2-aminoterephthalic acid was dissolved in 75 mL of dimethylformamide (DMF) and methanol (MeOH) (1:1) in a screw jar under stirring (800 rpm) for 0.5 h. Once the organic linker was fully dissolved, 10 mmol of titanium butoxide [Ti(OBu)₄] was added slowly followed by placing it at 130 °C in an oil bath. After 40 h, the NH₂-MIL-125(Ti) yellow precipitate was collected, centrifuged (7800 rpm for 15 min), and washed several times with methanol for the removal of unreacted species and complete solvent exchange. The synthesized NH₂-MIL-125(Ti) was dried in a vacuum oven at 80 °C overnight.

**Synthesis of NH₂-MIL-125(Ti/Cu).** As a second metal, Cu was loaded into NH₂-MIL-125(Ti) via a postsynthesis modification method²⁴,²⁵ rather than the conventional one-pot synthesis.²⁶ Typically, 2 g of dried NH₂-MIL-125(Ti) was dispersed in 200 mL of MeOH and DMF and ultrasonicated for 3 h. A Cu(NO₃)₂·3H₂O (0.86 g) solution in MeOH (20 mL) was poured into the NH₂-MIL-125(Ti) dispersion followed by heating at 110 °C for 12 h. The dark-green powder of Cu-loaded NH₂-MIL-125(Ti/Cu) was centrifuged and washed multiple times in MeOH for complete removal of unreacted Cu species followed by drying in a vacuum oven at 80 °C.

**Synthesis of the Derived Nanocomposites.** As-prepared NH₂-MIL-125(Ti) and NH₂-MIL-125(Ti/Cu) were heat-treated in a flow-through quartz tube furnace for 2 h with a ramp rate of 5 °C min⁻¹ at 700 °C under air to derive TiO₂ and TiO₂/CuO, respectively, and under argon saturated with water vapor (Ar/H₂O) to obtain TiO₂/CuO and TiO₂/CuₓO/C nanocomposites, respectively. When the tube furnace reached the target temperature of 700 °C, the water vapor (as a mild oxidizing agent) was carried by argon gas passing through a water buffer bottle at room temperature with the flow rate of 50 mL·min⁻¹ during the dwell time only. The sample prepared from NH₂-MIL-125(Ti/Cu) under Ar/H₂O was named TiO₂/CuₓO/C₂h. For simplicity, this sample is denoted as TiO₂/CuₓO/C in the manuscript. Two more samples were prepared under an identical temperature and gas atmosphere but with different dwell times of 0.5 and 4 h to study the effect of the pyrolysis time on the derived nanocomposites. The resulting products were labeled as TiO₂/CuₓO/C₀.₅h and TiO₂/CuₓO/C₄h, respectively.

**Material Characterizations and Photocatalytic HER Measurements.** Powder X-ray diffraction (PXRD) patterns of the sacrificial templates MOFs and the derived nanocomposites were recorded by Bragg-Brentano geometry in a PANalytical Empyrean diffractometer (Cu Kα λ₁ = 1.5406 Å, λ₂ = 1.5444 Å, I₂/I₁ = 0.5, V = 45 kV, and I = 40 mA) equipped with a PANalytical PIXcel 1D detector. The morphologies and surface texture of the samples were recorded by an xT Nova Nanolab 600 FIB coupled with in situ scanning electron microscopy (SEM) all in one unit. High-resolution images and elemental maps of the samples were recorded with transmission electron microscopy (TEM) equipped with energy-dispersive X-ray spectroscopy (EDX) (JEM 2100 LeB6 200 kV). For in situ scanning transmission electron microscopy (STEM), a Jeol 2100 F FEG (200 kV) with a spherical aberration corrector, operating in bright-field (BF)-STEM or high-angle annular dark (HAADF)-STEM modes with a resolution of 0.11 nm, was used. The Mettler Toledo thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) 3 connected to mass spectrometry (MS) was employed for thermogravimetric analysis of activated MOFs and their derived composites in synthetic air and under Ar. To record the Raman spectra of the samples, Renishaw inVia Reflex Raman System RL532C, Class 3B (50–3000 cm⁻¹), was used. The X-ray photoelectron spectroscopy (XPS) data was collected by employing the Kratos Axis Ultra DLD spectrometer with a monochromated Al Kα X-ray source operating at 168 W (12 mA × 14 kV). The collected XPS data from all of the samples were analyzed using CasaXPS software (v2.3.23). XPS depth profiling of NH₂-MIL-125(Ti/Cu) was carried out to confirm the uniform atomic distribution of each element including loaded Cu. The measurement was performed using
<figure><img src="image_10.png" /><figcaption>Figure 1. (a) PXRD patterns, (b) Raman spectra, (c) TGA profiles in air, and (d) N₂ sorption isotherms of NH₂-MIL-125(Ti)- and NH₂-MIL-125(Ti/Cu)-derived TiO₂ (gray), TiO₂/CuO (red), and TiO₂/CuₕO/C (black) nanocomposites. The solid and empty symbols in (d) represent the adsorption and desorption branches of N₂ isotherms, respectively.</figcaption></figure>

a 110 µm-diameter spot after etching over an area of 2 × 2 mm² by Ar (1 × 10⁻⁶ Torr) at 4 kV. The UV-vis absorption spectra of the samples were measured using a ThermoScientific Evolution 200 spectrophotometer. The collected XPS spectra were calibrated with respect to adventitious carbon peaks at 284.8 eV (C 1s). The Brunauer–Emmett–Teller (BET) surface areas and the distribution of pore sizes of the samples were measured by the 3Flex instrument (Micromeritics). The samples were degassed for 3 h at 180 °C under a vacuum. The pore size was calculated by nonlocal density functional theory (NLDFT) applying the Tarazona model (cylindrical pores) for nanoporous materials. The electrochemical impedance spectra (EIS) and transient photocurrent density at room temperature were recorded by the CHI660E electrochemical workstation (a three-electrode setup). A carbon rod and Ag/AgCl were used as counter and reference electrodes, respectively. A glassy carbon (GC) (2.8 cm², finely polished) electrode was used as a working electrode. To prepare the ink for the working electrode, 1 mg of photocatalyst was dispersed in 500 µL of ethanol/water (1:4 v/v) solution and 5 µL of Nafion solution (5 wt %) by ultrasonication. A 0.1 M HClO₄ solution was used as an electrolyte for all EIS measurements. For transient photocurrent density, the linear sweep voltammetry (LSV) was recorded at a constant potential of -0.4 V (in 0.5 M H₂SO₄) under a simulated light source (Newport, 400 W Xe/Hg lamp). The electron paramagnetic resonance (EPR) spectrum of a selected sample was recorded by an X-band CWEPR Bruker EMX spectrometer (Oriel Instruments, 600 W xenon lamp with an IR water filter) at 100 kHz field modulation.

Photocatalytic H₂ evolution reaction (HER) tests of the derived composites were performed in a Pyrex-glass reactor (volume of 42.1 mL including a headspace of 17.1 mL for collection of generated gases) coupled with a CP 9001 gas chromatograph. All HER tests were performed under the isothermal condition (at 30 °C) using a

500 W Xe/Hg lamp (66 983, Newport) with a 285 nm cutoff optical filter. For each measurement, 10 mg of the photocatalyst was dispersed in a solution of deionized water (17.5 mL), and methanol (7.5 mL) was used as the sacrificial agent for hole compensation and put in the reactor under magnetic stirring and purged for 30 min under a constant Ar flow of 30 mL min⁻¹ to ensure complete deoxygenation of the system. The apparent quantum yield (AQY %) of the measured samples was calculated by the following equation²⁷

$$AQY_{\lambda} = \frac{2 \times \text{Number of evolved H}_2 \text{ molecules}}{\text{Total number of incident photons}} \times 100\% \quad (1)$$

## RESULTS AND DISCUSSION

**Chemical Composition, Crystallinity, and Textural Properties.** PXRD patterns (Figure S1a) confirm the synthesis of the precursor MOF NH₂-MIL-125(Ti) and the loading of Cu²⁺ into NH₂-MIL-125(Ti/Cu). After the coordination of Cu²⁺ species with the amino functional group (NH₂) attached to the organic linker, no additional peaks of Cu were observed. The N₂ sorption of activated precursors NH₂-MIL-125(Ti) and NH₂-MIL-125(Ti/Cu) exhibits typical type I isotherms (Figure S1b), confirming the microporous nature of these MOF materials with well-defined micropores (Figure S1c). It is inevitable that NH₂-MIL-125(Ti/Cu) shows a slight decrease in the surface area due to the loading of Cu species. Table S1 summarizes their BET surface area, pore diameter, and pore volume. To further confirm the homogeneous distribution of Cu atoms into NH₂-MIL-125(Ti), spherical aberration-corrected STEM-HAADF
<table><caption>Table 1. Textural Properties, Crystallinity, Energy Band Gap, and Photocatalytic HER Activity of the Derived Nanocomposites</caption><thead><tr><th rowspan="2">Sample</th><th colspan="2">Textural properties</th><th rowspan="2">A (101)/R (110) phase percentage (%)</th><th rowspan="2">EBG (eV)</th><th rowspan="2">H<sub>2</sub> evolution (μmol g<sub>cat</sub><sup>-1</sup> h<sup>-1</sup>)</th><th rowspan="2">AQY at 435 nm (%)</th></tr><tr><th>Surface area (m<sup>2</sup> g<sup>-1</sup>)</th><th>Total pore volume (cm<sup>3</sup> g<sup>-1</sup>)</th></tr></thead><tbody><tr><td>TiO<sub>2</sub></td><td>56.4</td><td>0.094</td><td>73.5/26.5</td><td>2.96</td><td>31.8</td><td>0.08</td></tr><tr><td>TiO<sub>2</sub>/CuO</td><td>37.2</td><td>0.075</td><td>63/37</td><td>2.64</td><td>2340</td><td>5.52</td></tr><tr><td>TiO<sub>2</sub>/C</td><td>378</td><td>0.33</td><td>44/56</td><td>2.91</td><td>426</td><td>2.45</td></tr><tr><td>TiO<sub>2</sub>/Cu<sub>x</sub>O/C<sub>0.5h</sub></td><td>250</td><td>0.365</td><td>72.5/27.5</td><td>2.33</td><td>1227</td><td>2.90</td></tr><tr><td>TiO<sub>2</sub>/Cu<sub>x</sub>O/C<sub>2h</sub></td><td>144</td><td>0.224</td><td>51/49</td><td>2.45</td><td>3147</td><td>7.66</td></tr><tr><td>TiO<sub>2</sub>/Cu<sub>x</sub>O/C<sub>4h</sub></td><td>155</td><td>0.245</td><td>40/60</td><td>2.38</td><td>2338</td><td>5.52</td></tr></tbody></table>

images (Figure S2) of NH₂-MIL-125(Ti/Cu) (before and after Cu loading) were recorded. In Figure S2b, Cu species loaded into NH₂-MIL-125(Ti) can be observed as white dots. The high-resolution STEM (Figure S2c) image of NH₂-MIL-125(Ti/Cu) and the selected area electron diffraction pattern (Figure S2d) confirm the nonagglomerated and uniformly distributed Cu species. Moreover, the XPS depth profiling of the precursor NH₂-MIL-125(Ti/Cu) was carried out to quantify the uniform distribution of the elemental species in crystals, including Ti, Cu, N, O, and C. The depth profiling (etched depth up to 240 nm) (Figure S3) confirms that the averaged values of atomic concentrations (at. conc. %) of Ti 2p, Cu 2p, C 1s, O 1s, and N 1s were 10.6, 4, 61, 19, and 5.4%, respectively.²³,²⁴ In addition, a detailed TGA-MS analysis of thermal decomposition of the precursor NH₂-MIL-125(Ti/Cu) under air and under an inert gas atmosphere is provided in Figure S4.²⁴

Depending upon the gaseous atmosphere during the heating process, MOFs transform into metal oxides or metal oxide/carbon composites at higher temperatures.¹⁶,²⁸,²⁹Figure 1a shows the PXRD patterns of NH₂-MIL-125(Ti) and NH₂-MIL-125(Ti/Cu)-derived TiO₂, TiO₂/CuO under air and TiO₂/CuₐO/C under Ar/H₂O at 700 °C. In all three derived samples, the signature XRD peak of TiO₂ appeared at 25.3° for the anatase (101) and at 27.4° for the rutile (110) phase. However, compared to the samples derived under Ar/H₂O, the anatase phase is dominant in samples derived in the air. The anatase-to-rutile-phase ratio (A/R %) values of the TiO₂ polymorph are determined to be 73.5/26.5, 63/37, and 51/49 for samples TiO₂, TiO₂/CuO, and TiO₂/CuₐO/C, respectively. This is because the thermal decomposition of the precursor NH₂-MIL-125(Ti/Cu) under an inert atmosphere proceeds slowly compared to the thermal decomposition in air (Figure S4a,b). Moreover, the in situ formation of carbon from the organic linker (NH₂-BDC) of MOFs under Ar/H₂O also influences the localized chemical environment, resulting in the relatively higher growth of the rutile phase of TiO₂ compared to the anatase TiO₂ phase. The sample TiO₂/CuₐO/C derived under Ar/H₂O for 2 h of dwell time exhibits an almost 1:1 A/R phase ratio of TiO₂ polymorphs, which is ideal for the formation of a phasejunction.¹⁰ The particle sizes of the A/R TiO₂ polymorph (by the Scherrer equation) were calculated to be 19.3/19.5, 27.3/24.9, and 8.3/13.9 nm for the samples TiO₂, TiO₂/CuO, and TiO₂/CuₐO/C, respectively. The formation of relatively smaller TiO₂ nanoparticles under an Ar/H₂O atmosphere is directly influenced by the presence of carbon (from the organic linker), which prevents the smaller nanoparticles from sintering/agglomerating, resulting in the immobilization and uniform distribution of TiO₂ species embedded in the porous carbon matrix.²² These smaller

particle sizes are likely to offer increased photocatalytic active sites.

To investigate the effect of pyrolysis time on the crystallinity and phase ratios of TiO₂ and CuₐO species in the derived composites, two more samples, TiO₂/CuₐO/C<sub>0.5h</sub> and TiO₂/CuₐO/C<sub>4h</sub>, at different dwell times of 0.5 and 4 h, respectively, were prepared. PXRD results of those samples (Figure S5) suggest that in the sample TiO₂/CuₐO/C<sub>0.5h</sub> TiO₂ remains poorly crystalline with a dominant anatase phase (A/R ratio of 72.5/27.5), and the CuₐO species (highlighted in the inset) exhibit a broader peak with a lower intensity, indicating that they are mainly amorphous. However, in the sample TiO₂/CuₐO/C<sub>4h</sub>, TiO₂ shows a significant improvement in crystallinity, with the rutile phase outgrowing the anatase phase (A/R ratio of 40/60) due to the longer dwell time at the target temperature. In addition, CuₐO also exhibits relatively better crystallinity in this sample. The phase ratios and particle sizes of TiO₂ nanoparticles in the derived composites are summarized in Table 1. Obviously, in NH₂-MIL-125(Ti/Cu)-derived multi-heterostructured TiO₂/CuₐO/C nanocomposites, a heating time of 2 h at 700 °C is the optimum condition to form a good crystalline phasejunction between the TiO₂ polymorph with an A/R ratio of 1:1, which is consistent with previous observations.²²,²⁴

In the composite derived from NH₂-MIL-125(Ti/Cu) under air, Cu is fully oxidized to CuO at 700 °C, which is confirmed by the signature PXRD peak, appearing at 2θ of 35.5°. However, under a water vapor atmosphere, the Cu species are partially oxidized to Cu₂O accompanied by a small amount of CuO as well as a significant amount of Cu⁰.¹⁷,²⁴ This is due to the fact that the reduction potential of Cu²⁺ is above -0.27 V, which results in the Cu²⁺ species in the MOF precursor transforming into pure Cu⁰ nanoparticles upon pyrolysis under inert conditions.¹⁷ In Ar/H₂O, a mildly oxidizing atmosphere is established by introducing regulated water vapor at the target temperature of 700 °C, which partially oxidizes the Cu species into Cu₂O. The signature PXRD peak for Cu₂O (highlighted in the inset) appeared at 2θ of 36.4°, and characteristic PXRD peaks of Cu⁰ are observed at 2θ of 43.3, 50.4, and 74.1°. It can be anticipated that the optimized anatase/rutile ratio of TiO₂ benefits the formation of a phasejunction between TiO₂ and p-n heterojunctions between TiO₂ and CuₐO during the pyrolysis of NH₂-MIL-125(Ti/Cu) at 700 °C.

As discussed above, heat treatment of MOFs and Bi-MOFs in air results in metal oxides, while metal oxide/carbon composites are obtained under Ar/H₂O. To investigate the nature of carbon, Raman spectra of these derived samples were recorded. As expected, no characteristic Raman peaks of carbon are observed in TiO₂ and TiO₂/CuO derived under air; however, signature carbon peaks of D and G bands appeared at
<figure><img src="image_11.png" /><figcaption>**Figure 2.** XPS spectra of elements (a) Ti 2p, (b) O 1s, (c) Cu 2p, (d) Cu LMM, (e) C 1s, and (f) N 1s of the derived TiO₂, TiO₂/CuO, and TiO₂/CuₓO/C.</figcaption></figure>

around 1349 and 1595 cm⁻¹ in TiO₂/CuₓO/C (Figure 1b), which correspond to the defects/disorders in carbon and the graphitic/nanocrystalline carbon, respectively.³⁰,³¹ The Iₔ/I>G ratio indicates the formation of various defects in carbon. In the sample TiO₂/CuₓO/C, the Iₔ/I>G ratio is calculated to be 0.92, which signifies the formation of defects and the presence of nitrogen- and oxygen-related surface species on the carbon matrix.²⁴ The TGA profiles (Figure 1c) of the derived composites in the air atmosphere also confirmed that no carbon is present in the derived TiO₂ and TiO₂/CuO samples. However, the sample TiO₂/CuₓO/C obtained under Ar/H₂O exhibits a 14% weight loss between 300 and 400 °C, indicating

the presence of a significant amount of carbon, from the organic linker.³²,³³

The N₂ sorption of derived samples TiO₂, TiO₂/CuO, and TiO₂/CuₓO/C exhibits type IV isotherms with a small hysteresis above the relative pressure (p/p₀) of 0.4 (Figure 1d), indicating the emergence of mesopores due to the collapse of the MOF framework. The specific surface areas of the derived samples TiO₂, TiO₂/CuO, and TiO₂/CuₓO/C are 56, 37, and 144 m² g⁻¹, respectively. The relatively low surface areas in the derived metal oxides are because all of the carbon from the organic linker (NH₂-BDC) is oxidized in the form of NOₓ, CO, and CO₂ (confirmed by TGA-MS). However, the
TiO₂/CuₓO/C composite, derived under the Ar/H₂O atmosphere, exhibits a moderate BET surface area due to the presence of 14 wt % porous carbon, as confirmed by TGA under air in Figure 1c. Generally, in MOF-derived composites, a higher amount of porous carbon provides a higher BET surface area, which leads to a better distribution of metal oxide species throughout the carbon matrix. Consequently, it prevents the agglomeration of metal oxides and provides additional photocatalytic active sites. The textural properties of NH₂-MIL-125(Ti/Cu)-derived composites under Ar/H₂O at 700 °C for variable dwell times were also investigated. The N₂ sorption isotherms of TiO₂/CuₓO/C₀.₅h and TiO₂/CuₓO/C₄h are shown in Figure S6; the surface areas and pore volumes of the derived composites are summarized in Table 1. The sample TiO₂/CuₓO/C₀.₅h exhibits a relatively higher surface area of 250 m² g⁻¹ compared to the samples TiO₂/CuₓO/C₂h (144 m² g⁻¹) and TiO₂/CuₓO/C₄h (155 m² g⁻¹), which may be due to the fact that the sample TiO₂/CuₓO/C₀.₅h is prepared at a shorter dwell time (0.5 h), and there is a relatively higher amount of porous carbon present in this sample. On the contrary, in the sample prepared for a longer dwell time (4 h), most of the carbon is partially oxidized into CO/CO₂, leaving only a small amount present in the sample. Despite the lower C content, the sample TiO₂/CuₓO/C₄h exhibits a relatively high surface area because the controlled and partial oxidation of carbon prevents the TiO₂ and CuₓO nanoparticles from agglomeration and retains the inherited morphology, which results in a slightly higher N₂ sorption. TGA combined with MS spectra of the sample TiO₂/CuₓO/C₄h (Figure S7a,b) confirms the release of CO₂ along with H₂O and NO. Interestingly, the weight loss between 300 and 500 °C due to the oxidation of carbon species (releasing in the form of CO₂) coincides with the weight gain due to the oxidation of Cu⁰ into CuO.

The X-ray photoelectron spectroscopy (XPS) peaks of Ti 2p₃/₂ (Figure 2a) in the derived samples TiO₂, TiO₂/CuO, and TiO₂/CuₓO/C appear at 458.5, 458.3, and 458.7 eV, respectively, which confirm that Ti⁴⁺ oxidation state exists in all three samples. A small shift of 0.2 eV in the core level (Ti 2p₃/₂) in samples TiO₂/CuO and TiO₂/CuₓO/C compared to the bare TiO₂ indicates the band-bending at the interfacial surface due to the formation of a p-n heterojunction between TiO₂ and CuO.³⁴ Additionally, the presence of doped N and C species in the sample TiO₂/CuₓO/C also modifies the local electronic and chemical environment of Ti atoms due to the formation of Ti-N and Ti-C bonds and oxygen-related defects present in the TiO₂ crystal lattice.²²,²⁴,³⁵ This is further confirmed by the C 1s and N 1s spectra. The deconvoluted O 1s peaks (Figure 2b) of the derived sample TiO₂ are observed at 529.7, 531.1, and 533.0 eV, which can be assigned to O_lattice (Ti-O and Cu-O), surface-absorbed OH, and C=O groups, respectively. A small shift of 0.2 eV in the O_lattice peak in samples TiO₂/CuO and TiO₂/CuₓO/C (consistent with Ti 2p₃/₂) confirms the formation of p-n heterojunction, as well as the presence of defects and vacancies due to the oxygen vacancies and doped N/C species.⁶⁻⁸ The peak observed at 533.4 eV in the sample TiO₂/CuₓO/C composite can be ascribed to the carboxylic (O=C-O) groups functionalized on the surface of the porous carbon matrix.³⁶

In TiO₂/CuₓO/C, two main deconvoluted peaks of the core-level Cu 2p₃/₂ appearing at 932.9 and 934.9 eV (Figure 2c) are assigned to the Cu(I)/Cu(0) and Cu(II) oxidation states, respectively. Usually, during the pyrolysis of Cu-MOFs

under an inert atmosphere, Cu is reduced to Cu⁰ particles.¹⁷,²⁸ However, the oxidation state of Cu species can be controlled by optimizing the pyrolysis conditions including the gas atmosphere, the pyrolysis temperature, and the dwell time. In the sample TiO₂/CuO, the Cu 2p₃/₂ peak observed at 934.4 eV (and a strong satellite peak at 941.6 eV) confirms the Cu(II) oxidation state.³⁴,³⁷ The Cu species are fully oxidized to CuO during the heating of NH₂-MIL-125(Ti/Cu) up to 700 °C in air; however, under Ar/H₂O, the Cu species are partially oxidized to Cu₂O/CuO (with a dominant Cu(I) state) due to the limited exposure to oxygen. Therefore, a significant amount of Cu is found in the Cu(0) state in TiO₂/CuₓO/C. The shift in binding energies (B.E.) and the intensities of the Cu 2p₃/₂ in these two samples (obtained under Ar/H₂O and under air, respectively) clearly indicates the critical role of the gaseous atmosphere in MOF derivation,²⁴ as also confirmed by PXRD patterns in Figure 1a. The high-resolution XPS spectra of Cu LMM can better distinguish the oxidation states of the Cu species. As shown in Figure 2d, the high-resolution XPS Cu LMM peaks observed at 565.1, 576.3, and 578.6 can be assigned to Cu(I), Cu(II), and Cu(0), respectively.³⁸,³⁹ These observations confirm that Cu(II) is the primary oxidation state in the sample TiO₂/CuO obtained under air, whereas Cu species predominantly form Cu(I) and Cu(0) oxidation states in the sample TiO₂/CuₓO/C under Ar/H₂O.

The C 1s XPS peaks (Figure 2e) appearing at 284.8, 285.7, and 288.5 eV in TiO₂/CuₓO/C are assigned to the sp² bonded C=C, sp³ bonded C-C, and surface-functionalized carboxylate (O-C=O) groups, respectively.³⁵,⁴⁰ These (OH/COOH) functional groups present on the porous carbon matrix in samples derived under an Ar/H₂O atmosphere enhance the hydrophilic behavior of the sample and improve the water-catalyst interaction, resulting in better photo-catalytic activity.²⁴,³⁶ The N 1s XPS spectrum (Figure 2f) of TiO₂/CuₓO/C is deconvoluted into three peaks observed at B.E. values of 398.7, 400.5, and 402.4 eV, representing pyridinic N, pyrrolic N species, and oxidized N species (N-O), respectively.³³,⁴¹,⁴² This observation shows good consistency with recently published works.²²,²⁴ During the pyrolysis of the precursor NH₂-MIL-125(Ti/Cu) under Ar/H₂O, the N atoms from the cleaved-off organic linker (2-aminoterephthalic acid) replace O atoms, resulting in situ interstitial N atoms doping into TiO₂. It also creates defects and oxygen vacancies in the TiO₂ crystal lattice. Due to the oxygen-saturated atmosphere in the Ar/H₂O atmosphere, oxygen-rich N-O-like interstitial/intraband states are formed.²²,⁴³ In the sample TiO₂/CuₓO/C, the deconvoluted N 1s peak observed at 402.4 eV can be assigned to these oxygen-rich N-O-like intraband/interstitial states formed above the valence band (V.B.) of TiO₂. This observation is further supported by XPS valence band edge spectra as well as UV-vis absorption spectroscopy. In samples TiO₂ and TiO₂/CuO prepared in air, a broad, low-intensity N 1s peak is observed at around 400 eV, indicating the presence of doped interstitial N sites into TiO₂ (N-Ti-O).⁸,⁴³ However, in these samples, no additional peaks for surface-functionalized N species are observed. The low intensity of the N 1s peaks is due to the fact that on heat processing of MOF precursors at high temperatures, most of the N species (from the organic linker, 2-aminoterephthalic acid) is oxidized and released in the form of NOₓ. Only a trace amount of N doped into TiO₂ is left behind. However, when the sample TiO₂/Cu₂O/C was prepared under Ar/H₂O (water steam), the organic linker only
<figure><img src="image_12.png" /><figcaption>**Figure 3.** (a, b) In situ STEM images of selected particles of NH₂-MIL-125(Ti/Cu) obtained at RT and 700 °C under an Ar atmosphere. Representative high-resolution transmission electron microscopy (HRTEM) images of the derived samples (c) TiO₂, (d) TiO₂/CuO, and (e) TiO₂/CuₓO/C. The EDX elemental maps of samples (f) TiO₂, (g) TiO₂/CuO, and (h) TiO₂/CuₓO/C. The color codes representing the elements are as follows: green for Ti, gray for O, yellow for C, cyan for N, and magenta for Cu.</figcaption></figure>

partially oxidizes, leaving a small amount of N-functionalized carbon (Figures 1c and 2e).

**Morphology.** As shown in Figure S8, scanning electron microscopy (SEM) images suggest that after pyrolyzing NH₂-MIL-125(Ti/Cu) at 700 °C, the disc-like morphology is retained in these derived composites. However, compared to the precursors, the particle surface turns rougher with cracks and cleavages. It is worth noting that in the sample TiO₂/CuO (Figure S8c) prepared by heat treatment of the precursor NH₂-MIL-125(Ti/Cu) at 700 °C under air, the surface is the roughest compared to other derived samples because a higher diffusion of Cu takes place at higher temperatures under air, and the absence of the carbon matrix causes a relatively higher mobilization of metal species, which consequently results in the deterioration of the disc-like morphology. However, under Ar/H₂O, the organic linker forms a porous carbon matrix; therefore, the formed carbon can immobilize the TiO₂ and CuₓO nanoparticles, preserving the inherited morphology (Figure S8d).

In situ STEM images of NH₂-MIL-125(Ti/Cu) during the heating process from room temperature to 700 °C under inert conditions were recorded to monitor the morphology changes. The in situ STEM images (Figure 3a,b) show that during the

heating of NH₂-MIL-125(Ti/Cu) under Ar up to 700 °C, the precursor NH₂-MIL-125(Ti/Cu) loses the periodicity of the isoreticular structure due to the cleaving off of coordination bonds between the organic linker and the Ti oxo-cluster. As confirmed by the TGA-MS results (Figure S4), the emission of gas species (COₓ, NHₓ, and NOₓ) initiates at above 350 °C, which causes the disintegration of the Ti oxo-cluster [Ti₈O₈(OH)₄] and the organic linker [O₂C-C₆H₃(NH₂)-CO₂]₆, inevitably resulting in the shrinkage of particle sizes and the formation of micro-/mesoporous carbon from the organic linker, simultaneously recrystallizing the metal species into anatase/rutile TiO₂ and CuₓO/Cu⁰, as confirmed by XPS and PXRD. At 700 °C, the particles shrink approximately 35%, with an estimated width of 0.65 µm and the formation of mesopores in the carbon matrix (Figure 3b). In situ STEM clearly demonstrates the shrinkage of the particles, the formation of the mesoporous carbon matrix with uniformly immobilized TiO₂ and CuₓO nanoparticles, and the preservation of the inherited morphology.²³

High-resolution TEM images of derived TiO₂, TiO₂/CuO, and TiO₂/CuₓO/C samples are shown in Figure 3c–e. The interfaces between particle boundaries of TiO₂, CuₓO, and Cu⁰ confirm that they are physically in contact to form
<figure><img src="image_13.png" /><figcaption>**Figure 4.** (a) Photocatalytic H₂ evolution. (b) Comparison of HER activities of derived composites TiO₂ (gray), TiO₂/CuO (red), and TiO₂/CuₓO/C (black). (c) Recyclability study of the best-performing sample TiO₂/CuₓO/C₂h.</figcaption></figure>

heterojunctions. In samples derived under air (Figure 3d), anatase TiO₂ and CuO are the dominant crystalline phases (confirmed by PXRD) with measured interplanar spacings of 0.32 and 0.21 nm, respectively. However, in the sample TiO₂/CuₓO/C (Figure 3e), anatase (101) and rutile (110) TiO₂ polymorphs as well as Cu₂O and Cu⁰ nanoparticles can be clearly seen, embedded in the porous carbon matrix. Energy-dispersive X-ray (EDX) elemental mappings of the derived samples TiO₂, TiO₂/CuO, and TiO₂/CuₓO/C (shown in Figure 3f–h) confirm that all of the elemental species (Ti, Cu, O, N, and C) are homogeneously distributed throughout the nanocomposite.³,³⁴,³⁷ The p–n heterojunction formation between TiO₂ and CuₓO, as well as the change in V.B. positions of the derived TiO₂, is further investigated by XPS and UV–vis absorption spectroscopy analyses.

**Photocatalytic Hydrogen Evolution Performance.**

The photocatalytic HER performances of the derived TiO₂, TiO₂/CuO, and TiO₂/CuₓO/C nanocomposites were evaluated in a methanol/water solution under UV–vis light (285 nm cutoff filter). No noble metals (Pt, Pd, or Au) were used as electron-mediating cocatalysts. The HER reactor setup is shown elsewhere.⁴ As shown in Figure 4a,b, under UV–vis light, the HER performance of derived TiO₂, TiO₂/CuO, and TiO₂/CuₓO/C nanocomposites increases with reaction time. After exposing to UV–vis light for 5 h, the derived TiO₂, TiO₂/CuO, and TiO₂/CuₓO/C composites generated 159, 11700, and 15735 µmol gₐₗ⁻¹ H₂, respectively. To systematically investigate the role of CuₓO species and the porous carbon matrix in the photocatalytic HER activity, the H₂ evolution of the TiO₂/C composite (without loaded Cu)

under identical experimental conditions was also tested. This TiO₂/C sample (carbon-containing) demonstrates an HER activity of 426 µmol gₐₗ⁻¹ h⁻¹, 13 times higher compared to the bare TiO₂ (carbon-free). Moreover, the samples TiO₂/CuO (carbon-free) and TiO₂/CuₓO/C (carbon-containing) exhibit HER activities of 2340 and 3147 µmol gₐₗ⁻¹ h⁻¹, respectively, indicating the photocatalytic HER activity for the composite TiO₂/CuₓO/C is 34% higher than that for the composite TiO₂/CuO. These observations clearly suggest that the presence of porous carbon in the composites offers media for charge migration and helps distribute the photocatalytic active sites on the porous carbon matrix without agglomerations. With more photocatalytic active sites being exposed, and the improved absorption of UV–vis light, TiO₂/CuₓO/C results in improved HER performance of the composites. On the other hand, the sample TiO₂/CuO (Cu-containing) exhibits 2340 µmol gₐₗ⁻¹ h⁻¹ H₂ generation, 74 times higher than bare TiO₂ (Cu-free). Interestingly, the H₂ evolution of 3147 µmol gₐₗ⁻¹ h⁻¹ for the sample TiO₂/CuₓO/C (Cu-containing) is 7.3 times for the sample TiO₂/C (Cu-free) under the same condition, which is also 99 times higher than that for the bare TiO₂. These observations clearly suggest that Cu species in the composites play a fundamental role in decreasing the EBGs by forming p–n heterojunctions between TiO₂ and CuₓO species, increase the range of UV–vis light absorption, and consequently enhance the photocatalytic HER activity of TiO₂/CuₓO/C composites.

Built on the observed photocatalytic HER performance and the physical as well as chemical characterizations of these different TiO₂-based composites obtained under similar
<figure><img src="image_14.png" /><figcaption>**Figure 5.** (a) Tauc plots (inset shows UV-vis absorption spectra) and (b) XPS valence band edge spectra of the derived samples TiO₂, TiO₂/CuO, and TiO₂/CuₓO/C.</figcaption></figure>

experimental conditions, it is clear that both porous carbon and Cu species are indispensable components in the TiO₂/CuₓO/C composite for photocatalytic application. However, the functionalized porous carbon matrix provides a platform for charge migration and offers a high surface area to better disperse the photocatalytic active sites. Additionally, codoped N and the CuₓO species in TiO₂/CuₓO/C significantly contribute to enhancing the HER activity via the formation of multi-heterojunctions between TiO₂ and CuₓO species.

To further elucidate the effect of the pyrolysis time on HER activity, two other samples derived from NH₂-MIL-125(Ti/Cu) with dwell times of 0.5 and 4 h, respectively, were tested under identical conditions. As shown in Figure S9, these samples (TiO₂/CuₓO/C₀.₅h and TiO₂/CuₓO/C₄h) exhibit HER activities of 1227 and 2338 µmol gcat⁻¹ h⁻¹, respectively, which are only 39 and 74% of the best sample TiO₂/CuₓO/C₂h. This is due to the fact that the pyrolysis time of the precursor determines the crystallinity of the metal oxides and the textural properties of the porous carbon in the resulting composites, which consequently affects their photocatalytic HER performance. The apparent quantum yields (AQY %) of the derived samples are summarized in Table 1, and the detailed calculation of the AQY % for the selected sample is provided in the Supporting Information. Under the irradiation of visible light with a wavelength of 435 nm (in the range of EBGs of the photocatalysts), the apparent quantum yield of the most active nanocomposite TiO₂/CuₓO/C₂h is 7.66%, which is 38% higher than that of TiO₂/CuO and TiO₂/CuₓO/C₄h. The recyclability of the best sample TiO₂/CuₓO/C₂h was also evaluated by running the photocatalytic HER test under identical conditions for 5 consecutive cycles. As shown in Figure 4c, this sample exhibits good recyclability and stability with a slight decrease of 12% in photocatalytic HER performance after 5 cycles, possibly due to the photocorrosion of the composite catalyst.⁴⁴ In these NH₂-MIL-125(Ti/Cu)-derived samples, TiO₂ and CuₓO nanoparticles are distributed in the porous carbon matrix. Since CuₓO is prone to photocorrosion in aqueous media under illumination, when the TiO₂/CuₓO/C catalyst is exposed to irradiated UV-vis light in an aqueous solution for a longer time, the photogenerated electrons will trigger self-reduction of CuₓO species in an aqueous media, causing photocorrosion of the composite in a long-term recyclability test. Consequently, these partial electronic/structural changes in CuₓO may gradually alter the nature of p-n heterostructures between

TiO₂ and CuₓO, which influences their EBG, as well as gradually change the decay or recombination of photo-generated charges. As a result, a slow decrease in photocatalytic efficiency is observed in the recyclability test.⁴⁴,⁴⁵ Further experimental studies are required to investigate how to improve the chemical and physical stability of TiO₂ and CuₓO against photocorrosion. Figure S10 presents a bar graph comparing the photocatalytic HER activities of some recently reported noble-metal-free, state-of-the-art TiO₂/CuO nano-materials.⁴⁶⁻⁵² The TiO₂/CuₓO/C composite in our work is one of the best-performing composites for photocatalytic HER activity.

**Mechanistic Insights into the Role of Different Components in TiO₂/CuₓO/C Composites for Photocatalytic HER Performance.** UV-vis absorption spectra and XPS valence band edge (VBE) spectra were recorded to investigate the optical properties (Figure 5) of the derived composites. Estimated from the Tauc plots, the energy band gaps (EBGs) of the derived composites TiO₂, TiO₂/CuO, and TiO₂/CuₓO/C are 2.96, 2.64, and 2.45 eV, respectively. The narrowing of the EBGs in the MOF-derived samples compared to the pure TiO₂ (3.2 eV) can be attributed to the doped N and C species above the V.B. of TiO₂, the forming of the p-n heterojunction between TiO₂ and CuₓO, and the presence of defects.⁹,⁵³ Additionally, in the sample TiO₂/CuₓO/C, multi-heterostructures are formed due to the optimal ratio of anatase/rutile (A/R ratio of 1:1) TiO₂, as well as the presence of CuO and Cu₂O nanoparticles.³,¹⁰ The UV-vis absorption spectra of the NH₂-MIL-125(Ti/Cu)-derived samples under Ar/H₂O at various pyrolysis times are shown in Figure S11a. The energy band gaps of samples TiO₂/CuₓO/C₀.₅h and TiO₂/CuₓO/C₄h (from Tauc plots, Figure S11b) are estimated to be 2.33 and 2.38 eV, respectively. The relative shift in wavelength and peak intensity of the UV-vis absorption spectra and the subsequent change in energy band positions clearly indicate that the presence of the carbon matrix has a significant influence on the light absorption capabilities of these derived composites.

To investigate the shift in the energy band positions of the derived composites, high-resolution XPS spectra of the VBE were recorded. The VBE of the NH₂-MIL-125(Ti)-derived TiO₂ (Figure 5b) is observed at 2.51 eV, whereas the VBE spectra of the NH₂-MIL-125(Ti/Cu)-derived TiO₂/CuO under air show two shoulder-like bands at around 1.53 eV and 0.8 eV, respectively, assigned to the TiO₂ and CuO
<figure><img src="image_15.png" /><figcaption>**Figure 6.** (a) Transient photocurrent density measured under simulated solar light with on-off cycles at the fixed potential (−0.4 V). (b) EIS Nyquist plots with the equivalent circuit diagram. (c) Schematic illustration of the energy band alignments and formation of multi-heterostructures for the TiO₂/CuₓO/C composite and the proposed mechanism of photocatalytic H₂ evolution.</figcaption></figure>

nanoparticles.²⁴,⁵⁴ In the sample TiO₂/CuₓO/C, the VBEs of TiO₂ and CuₓO shifted to 1.34 and 0.5 eV, respectively. A noticeable difference in the B.E. of Cu species from 0.8 to 0.5 eV in these samples also confirms that the Cu species are present predominantly in the Cu(I)/Cu(0) state in TiO₂/CuₓO/C. A relatively bigger shift in the VBE of Ti species, from 2.51 eV for bare TiO₂ to 1.53 eV for TiO₂/CuO, can be attributed to the formation of a p-n heterojunction between TiO₂ and CuO, which causes band-bending at the interfaces (also confirmed by HRTEM).²⁴,³⁴,³⁷ A further shift in VBE of Ti toward lower B.E., 1.34 eV in the sample TiO₂/CuₓO/C, is caused by the doped N and C species above the V.B. of TiO₂.⁸,⁴³ Supported by the PXRD patterns and the core-level XPS spectra, in the sample TiO₂/CuₓO/C, the shift in VBE of TiO₂ and CuₓO toward lower B.E. is attributed to the phasejunction formation between anatase and rutile TiO₂ with an optimized phase ratio of A/R ca. 1:1 and p-n heterojunction formation between TiO₂ and CuₓO.³,³⁷,⁵²,⁵⁵

The transient photocurrent density under an incident light and the subsequent charge transfer are the determining semiconducting properties of photocatalytic HER performance of a nanomaterial. The photocurrent densities (as a function of applied voltage) and the transportation of the photogenerated electrons on these MOF-derived materials were measured under simulated solar light. As shown in Figure 6a, samples TiO₂, TiO₂/CuO, and TiO₂/CuₓO/C exhibit photocurrent densities of 15, 26, and 27 µA cm⁻², respectively. An up to 2-fold increase in photocurrent density of the TiO₂/CuO composite compared to the bare TiO₂ clearly indicates that

the p-n heterojunction formed between TiO₂ and CuO benefits the enhancement of the light absorption and the generation of photoelectrons. It is interesting to observe that the transient photocurrent density of the carbon-containing sample TiO₂/CuₓO/C is similar to the sample TiO₂/CuO. However, the shapes of their photocurrent density curves are different. Upon turning the light on, the transient photocurrent density in the sample TiO₂/CuO/C instantly reaches the maximum value and remains relatively stable until the incident light is switched off. However, the TiO₂/CuO composite (and also TiO₂) shows a gradual increase in the photocurrent density before reaching the maximum value. Comparing the intensities and shapes of the curves for transient photocurrent densities of these samples, it is likely that the photogenerated of electrons and holes primarily takes place on metal oxides, but the charge recombination and charge transport depend on the available charge migration pathways within the sample.⁶,⁵⁶ The instantaneous increase in photocurrent density up to the maximum value in the composite TiO₂/CuₓO/C upon light exposure indicates that the presence of porous carbon improves the electrical conductivity and provides additional charge migration pathways and, consequently, slows down the decay/recombination of photogenerated charges. In addition, the functionalized hydrophilic porous carbon matrix also provides enhanced interaction between adsorbed water molecules and the photogenerated electrons.²²,²⁴

Electrochemical impedance spectroscopy (EIS) was performed to determine the electric charge-transfer resistance ($R_{ct}$) of these derived nanocomposites. As shown in Figure 6b,
two semicircles were observed, which can be assigned to the formation of a passivation layer at the solid electrolyte interface (R<sub>SEI</sub>), the charge-transfer resistance (R<sub>ct</sub>) as well as the Warbung element (Z<sub>w</sub>) in the low-frequency region. The R<sub>ct</sub> value can be estimated from the diameter of the arc/semicircle in the high-frequency region of the Nyquist plots. The R<sub>ct</sub> values of TiO<sub>2</sub>, TiO<sub>2</sub>/CuO, and TiO<sub>2</sub>/Cu<sub>x</sub>O/C are estimated to be 40, 33, and 25 Ω, respectively. A decrease in the R<sub>ct</sub> values of TiO<sub>2</sub>/CuO and TiO<sub>2</sub>/Cu<sub>x</sub>O/C compared to the bare TiO<sub>2</sub> confirms that the in situ-formed multi-heterojunctions (anatase-rutile phasejunction and TiO<sub>2</sub>-Cu<sub>x</sub>O p-n heterojunction) offer multiple pathways for charge migration, resulting in the improvement of the charge-transfer efficiencies of the derived composites. It is worth highlighting that the R<sub>ct</sub> value of the carbon-containing sample TiO<sub>2</sub>/Cu<sub>x</sub>O/C is smaller compared to TiO<sub>2</sub>/CuO, which clearly indicates that the porous carbon matrix in these derived composites contributes to the electrical conductivity by minimizing the charge-transfer resistance. The measured photocurrent responses and R<sub>ct</sub> values of these composites are well consistent with their photocatalytic HER performances. To further investigate the charge generation mechanism in these composites, electron paramagnetic resonance (EPR) spectrum of a representative sample TiO<sub>2</sub>/Cu<sub>x</sub>O/C was measured under UV-vis light for 10 min at 77 K in vacuum conditions. Figure S12 shows that a sharp signal appears at the g factor of 1.97, which represents the presence of Ti<sup>3+</sup>. Another high-intensity signal observed at the g factor of 2.002 (closer to the g factor for the free electrons, 2.0023) is due to the formation of carbon-centered radicals (C°). In pure TiO<sub>2</sub>, this signal is not observed.<sup>57</sup> The formation of C° upon irradiation of UV-vis light suggests that the underlying paramagnetic species might be related to the photocatalytic reaction in the carbon-doped TiO<sub>2</sub>.<sup>57,58</sup>

Based on the various characterizations and in-depth analysis, the photocatalytic mechanism (as proposed in Figure 6c) and the origin of photocatalytic H₂ evolution activities in the bimOF-derived nanocomposites can be elucidated. The best-performing sample, TiO₂/CuₓO/C, consists of the TiO₂ polymorph and CuₓO/CuO and Cu⁰ species, uniformly distributed in the porous carbon matrix. Due to the optimal ratio (A/R 1:0.96, close to 1:1) and the staggered (type-II) valence/conduction band positions, the TiO₂ polymorph forms a phasejunction between A (101) and R (110) phases.¹⁰ Photogenerated electrons in the conduction band (C.B.) preferably migrate from the rutile to the anatase phase of TiO₂ due to its higher electron affinity.⁵⁵ Simultaneously, the photogenerated holes (h⁺) in V.B. can migrate to the opposite direction, from anatase to the rutile phase of TiO₂. In TiO₂/CuₓO/C, in situ-doped N and C species elevate the VBEs of anatase and rutile TiO₂ closer to the Fermi level by forming additional carbon- and oxygen-rich intraband/interstitial (N-O-like) electronic states. This further narrows down the overall energy band gap of TiO₂ to 2.45 eV compared to that of the pure TiO₂ (3.2 eV) and consequently enhances the visible-light absorption and charge generation.²²,⁴³

The second significant factor in photocatalytic HER activity of these nanocomposites is the formation of the heterostructure between Cu₂O and CuO nanoparticles. Confirmed by the PXRD patterns and high-resolution XPS spectra, the VBEs of Cu₂O and CuO lie at 932.9 and 934.9 eV (core levels), respectively, which can form the heterojunction between Cu₂O and CuO nanoparticles.²⁴,³⁴,⁵⁹ More impor-

tantly, the VBE estimated from the XPS of the Cu₂O/CuO species lies at around 0.8 eV below the Fermi level. Such an energy band alignment between the C.B. of TiO₂ and the V.B. of CuₓO nanoparticles (respecting the Fermi level) favors the p-n heterojunction formation between the highly crystalline TiO₂ polymorph and CuₓO particles. Moreover, the Cu⁰ nanoparticles (evidenced by PXRD) present in this sample can further promote a faster photogenerated electron transfer from the C.B. of Cu₂O to CuO, the anatase TiO₂ nanoparticle, or the porous carbon. The availability of multiple charge-transfer pathways can significantly suppress the possibilities of charge recombination. The presence of a functionalized carbon matrix (with -N and -OH/-COOH groups) keeps the metal oxide species homogeneously distributed, preventing bulk agglomeration. Such spatial distribution of the metal compounds ensures the maximum exposure of the photocatalytic active sites and the reactant species, hence improving the efficiency of the photocatalytic HER performance. On the other hand, it is obvious that no heterostructure is formed in NH₂-MIL-125(Ti)-derived TiO₂ under air. However, in NH₂-MIL-125(Ti/Cu)-derived TiO₂/CuO under air, the photocatalytic activity largely originates from the p-n heterojunction formed between the highly crystalline TiO₂ polymorph and CuO nanoparticles. In other NH₂-MIL-125(Ti/Cu)-derived samples, the poorly crystalline metal oxide species in the sample TiO₂/CuₓO/C<sub>0.5h</sub>, the absence of the optimized ratios of the TiO₂ polymorph and dominant CuO species, and the negligible amount of carbon in the sample TiO₂/CuₓO/C<sub>4h</sub> are the primary reasons behind their relatively poor HER performance. It can be inferred that electron-rich anatase TiO₂ and CuO nanoparticles act as photocatalytic active sites where the photocatalytic reaction takes place to reduce H₂O into H₂, while the V.B.s of rutile TiO₂ and Cu₂O nanoparticles act as hole-scavenging platforms to complete the other half-reaction.

This work is an attempt to elucidate the origin of photocatalytic HER activities of NH₂-MIL-125(Ti/Cu)-derived TiO₂/CuₓO/C nanocomposites by systematic investigation of the contribution of each component, including TiO₂, CuₓO, and porous carbon species in composites. Employing the bi-MOF as a precursor, in situ tunable multiheterostructures uniformly distributed in a selectively functionalized carbon matrix can be readily produced via a simple one-step method under appropriate pyrolysis conditions. This method offers a simple route to in situ tune the compositions, structures, morphologies, and the semiconducting properties of the derived composites to achieve high HER performance photocatalysts without noble metals.

## CONCLUSIONS

The TiO₂/CuₓO/C nanocomposite derived from NH₂-MIL-125(Ti/Cu) under Ar/H₂O at 700 °C for 2 h exhibited highest photocatalytic H₂ evolution activity. Supported by various characterizations and in-depth analysis, high photocatalytic activity is due to the formation of phasejunctions between well-crystallized anatase/rutile TiO₂ polymorph with an optimized ratio, N/C codoping, p-n heterojunction between the TiO₂ and CuₓO nanoparticles, and their uniform distribution in a functionalized porous carbon matrix. These nanocomposites enable increased visible-light absorption due to the relatively narrower energy band gaps and the improved spatial separation of photogenerated charges resulting from the simultaneous formation of type-II staggered phasejunction and p-n
heterojunction. The optimized nanocomposites with in situ-
formed multi-heterostructures offer multiple pathways of
charge migration, resulting in lower charge-transfer resistance
and suppressed bulk charge recombination. Therefore, such
metal oxide multi-heterojunctions homogeneously dispersed in
the functionalized porous carbon matrix provide easily
accessible active sites for enhanced photocatalytic HER activity
without expensive noble metal cocatalysts.

■ ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.energyfuels.2c02319.

XRD, gas sorption, textural properties, TEM, XPS depth profile and TGA data of precursors as well as XRD, gas sorption, TGA, SEM, UV-vis spectra, EPR, and photocatalytic HER data of different TiO₂/CuₓO/C samples (PDF)

■ AUTHOR INFORMATION

Corresponding Authors

Mian Zahid Hussain — College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter EX4 4QF, U.K.; School of Natural Sciences and Catalysis Research Center, Technical University of Munich (TUM), 85748 Garching, Germany; Email: ge68muq@tum.de

Yongde Xia - College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter EX4 4QF, U.K.; orcid.org/0000-0001-9686-8688; Email: Y.Xia@ exeter.ac.uk

Authors

Zhuxian Yang – College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter EX4 4QF, U.K.

Bart van der Linden – Catalysis Engineering, Chemical Engineering Department, Delft University of Technology, 2629 HZ Delft, Netherlands; orcid.org/0000-0003-1384-7457

Werner R. Heinz – School of Natural Sciences and Catalysis Research Center, Technical University of Munich (TUM), 85748 Garching, Germany

Mounib Bahri – Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504 CNRS, Université de Strasbourg, 67034 Strasbourg, France

Ovidiu Ersen – Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504 CNRS, Université de Strasbourg, 67034 Strasbourg, France; orcid.org/0000-0002-1553-0915

Quanli Jia – Henan Key Laboratory of High Temperature Functional Ceramics, Zhengzhou University, Zhengzhou 450052, China

Roland A. Fischer – School of Natural Sciences and Catalysis Research Center, Technical University of Munich (TUM), 85748 Garching, Germany; orcid.org/0000-0002-7532-5286

Yanqiu Zhu – College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter EX4 4QF, U.K.; orcid.org/0000-0003-3659-5643

Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.energyfuels.2c02319

Author Contributions

M.Z.H.: Investigation, methodology, data curation, writing—original draft. Z.Y.: Data curation, validation, writing—review & editing. B.v.d.L.: Methodology, data curation, validation. W.R.H.: Investigation, validation, data curation. M.B.: Investigation, validation, data curation. O.E.: Methodology, writing—review & editing. Q.J.: Investigation, data curation. R.A.F.: Supervision, writing—review & editing. Y.Z.: Supervision, writing—review & editing. Y.X.: Supervision, conceptualization, methodology, validation, writing—review & editing.

Notes

The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The authors are thankful to TUM Innovation Network Artificial Intelligence in Material Science (ARTEMIS), EPSRC CDT in Metamaterials at the University of Exeter, Leverhulme Trust (RPG-2018-320), and Royal Society (IEC \NSFC\201121) for financial support.

■ REFERENCES

(1) Fayomi, G. U.; Mini, S. E.; Fayomi, O. S. I.; Oyeleke, O.; Omole, D. O.; Akinwumi, I. I. Overview of Industrial Pollution Activities and its Curbing Mechanisms. *IOP Conf. Ser.: Mater. Sci. Eng.* **2019**, *640*, No. 012097.

(2) Cao, S.; Piao, L.; Chen, X. Emerging Photocatalysts for Hydrogen Evolution. *Trends Chem. 2020*, *2*, 57–70.

(3) Low, J.; Yu, J.; Jaroniec, M.; Wageh, S.; Al-Ghamdi, A. A. Heterojunction Photocatalysts. *Adv. Mater.* **2017**, *29*, No. 1601694.

(4) Barelli, L.; Bidini, G.; Gallorini, F.; Servili, S. Hydrogen Production through Sorption-Enhanced Steam Methane Reforming and Membrane Technology: A Review. *Energy* **2008**, *33*, 554–570.

(5) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. *Nature* **1972**, *238*, 37–38.

(6) Kumar, S. G.; Devi, L. G. Review on Modified TiO₂ Photocatalysis under UV/Visible Light: Selected Results and Related Mechanisms on Interfacial Charge Carrier Transfer Dynamics. *J. Phys. Chem. A* **2011**, *115*, 13211–13241.

(7) Rahimi, N.; Pax, R. A.; Gray, E. M. Review of Functional Titanium Oxides. I: TiO₂ and its Modifications. *Prog. Solid State Chem.* **2016**, *44*, 86–105.

(8) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides. *Science* **2001**, *293*, 269.

(9) Schneider, J.; Matsuoka, M.; Takeuchi, M.; Zhang, J.; Horiuchi, Y.; Anpo, M.; Bahnemann, D. W. Understanding TiO₂ Photocatalysis: Mechanisms and Materials. *Chem. Rev.* **2014**, *114*, 9919–9986.

(10) Su, R.; Bechstein, R.; Sø, L.; Vang, R. T.; Sillassen, M.; Esbjörnsson, B.; Palmqvist, A.; Besenbacher, F. How the Anatase-to-Rutile Ratio Influences the Photoreactivity of TiO₂. *J. Phys. Chem. C* **2011**, *115*, 24287–24292.

(11) Fu, J.; Yu, J.; Jiang, C.; Cheng, B. g-C₃N₄-Based Heterostructured Photocatalysts. *Adv. Energy Mater.* **2018**, *8*, No. 1701503.

(12) Takanabe, K. Photocatalytic Water Splitting: Quantitative Approaches toward Photocatalyst by Design. *ACS Catal.* **2017**, *7*, 8006–8022.

(13) Zhou, H.-C.; L, J. R.; Yaghi, O. M. Introduction to Metal–Organic Frameworks. *Chem. Rev.* **2012**, *112*, 673–674.

(14) Li, H.; Eddaoudi, M.; O'Keeffe, M.; Yaghi, O. M. Design and Synthesis of An Exceptionally Stable and Highly Porous Metal-Organic Framework. *Nature* **1999**, *402*, 276–279

(15) Nasalevich, M. A.; van der Veen, M.; Kapteijn, F.; Gascon, J. Metal–Organic Frameworks as Heterogeneous Photocatalysts: Advantages and Challenges. *CrystEngComm* **2014**, *16*, 4919–4926.
(16) Luo, H.; Zeng, Z.; Zeng, G.; Zhang, C.; Xiao, R.; Huang, D.; Lai, C.; Cheng, M.; Wang, W.; Xiong, W.; et al. Recent Progress on Metal-organic Frameworks Based- and Derived-Photocatalysts for Water Splitting. *Chem. Eng. J.* **2020**, 383, No. 123196.

(17) Das, R.; Pachfule, P.; Banerjee, R.; Poddar, P. Metal and Metal Oxide Nanoparticle Synthesis from Metal Organic Frameworks (MOFs): Finding the Border of Metal and Metal Oxides. *Nanoscale* **2012**, *4*, 591–599.

(18) Song, F.; Li, W.; Sun, Y. Metal–Organic Frameworks and Their Derivatives for Photocatalytic Water Splitting. *Inorganics* **2017**, *5*, 40.

(19) Zeng, L.; Guo, X.; He, C.; Duan, C. Metal–Organic Frameworks: Versatile Materials for Heterogeneous Photocatalysis. *ACS Catal.* **2016**, *6*, 7935–7947.

(20) Hussain, M. Z.; Yang, Z.; Huang, Z.; Jia, Q.; Zhu, Y.; Xia, Y. Recent Advances in Metal–Organic Frameworks Derived Nanocomposites for Photocatalytic Applications in Energy and Environment. *Adv. Sci.* **2021**, *8*, No. 2100625.

(21) Hussain, M. Z.; Yang, Z.; Khalil, A. M. E.; Hussain, S.; Awan, S. U.; Jia, Q.; Fischer, R. A.; Zhu, Y.; Xia, Y. Metal-organic Framework Derived Multi-functionalized and Co-doped TiO₂/C Nanocomposites for Excellent Visible-light Photocatalysis. *J. Mater. Sci. Technol.* **2022**, *101*, 49–59.

(22) Hussain, M. Z.; Yang, Z.; van der Linden, B.; Huang, Z.; Jia, Q.; Cerrato, E.; Fischer, R. A.; Kapteijn, F.; Zhu, Y.; Xia, Y. Surface Functionalized N-C-TiO₂/C Nanocomposites Derived from Metal-organic Framework in Water Vapour for Enhanced Photocatalytic H₂ Generation. *J. Energy Chem.* **2021**, *57*, 485–495.

(23) Hussain, M. Z.; Bahri, M.; Heinz, W. R.; Jia, Q.; Ersen, O.; Kratky, T.; Fischer, R. A.; Zhu, Y.; Xia, Y. An In Situ Investigation of the Thermal Decomposition of Metal-organic Framework NH₃-MIL-125 (Ti). *Microporous Mesoporous Mater.* **2021**, *316*, No. 110957.

(24) Hussain, M. Z.; van der Linden, B.; Yang, Z.; Jia, Q.; Chang, H.; Fischer, R. A.; Kapteijn, F.; Zhu, Y.; Xia, Y. Bimetal–organic Framework Derived Multi-heterostructured TiO₂/CuₓO/C Nanocomposites with Superior Photocatalytic H₂ Generation Performance. *J. Mater. Chem. A* **2021**, *9*, 4103–4116.

(25) Huang, Z.; Yang, Z.; Jia, Q.; Wang, N.; Zhu, Y.; Xia, Y. Bimetallic Co-Mo Sulfide/carbon Composites Derived from Polyoxometalate Encapsulated Polydopamine-decorated ZIF Nanocubes for Efficient Hydrogen and Oxygen Evolution. *Nanoscale* **2022**, *14*, 4726–4739.

(26) Huang, Z.; Yang, Z.; Hussain, M. Z.; Jia, Q.; Zhu, Y.; Xia, Y. Bimetallic Fe-Mo Sulfide/carbon Nanocomposites Derived from Phosphomolybdic Acid Encapsulated MOF for Efficient Hydrogen Generation. *J. Mater. Sci. Technol.* **2021**, *84*, 76–85.

(27) Yuan, L.; Han, C.; Yang, M.-Q.; Xu, Y.-J. Photocatalytic Water Splitting for Solar Hydrogen Generation: Fundamentals and Recent Advancements. *Int. Rev. Phys. Chem.* **2016**, *35*, 1–36.

(28) Oar-Arteta, L.; Wezendonk, T.; Sun, X.; Kapteijn, F.; Gascon, J. Metal Organic Frameworks as Precursors for the Manufacture of Advanced Catalytic Materials. *Mater. Chem. Front.* **2017**, *1*, 1709–1745.

(29) Xia, W.; Mahmood, A.; Zou, R.; Xu, Q. Metal–organic Frameworks and Their Derived Nanostructures for Electrochemical Energy Storage and Conversion. *Energy Environ. Sci.* **2015**, *8*, 1837–1866.

(30) Ferrari, A. C.; Robertson, J. Interpretation of Raman Spectra of Disordered and Amorphous Carbon. *Phys. Rev. B* **2000**, *61*, 14095–14107.

(31) Bokobza, L.; Bruneel, J.-L.; Couzi, M. Raman Spectra of Carbon-Based Materials (from Graphite to Carbon Black) and of Some Silicone Composites. *C* **2015**, *1*, 77–94.

(32) Hussain, M. Z.; Pawar, G. S.; Huang, Z.; Tahir, A. A.; Fischer, R. A.; Zhu, Y.; Xia, Y. Porous ZnO/Carbon Nanocomposites Derived from Metal Organic Frameworks for Highly Efficient Photocatalytic Applications: A Correlational Study. *Carbon* **2019**, *146*, 348–363.

(33) Huang, Z.; Yang, Z.; Hussain, M. Z.; Chen, B.; Jia, Q.; Zhu, Y.; Xia, Y. Polyoxometallates@zeolitic-imidazolate-framework Derived Bimetallic Tungsten-cobalt Sulfide/Porous Carbon Nanocomposites

as Efficient Bifunctional Electrocatalysts for Hydrogen and Oxygen Evolution. *Electrochim. Acta* **2020**, *330*, No. 135335.

(34) Shi, Q.; Ping, G.; Wang, X.; Xu, H.; Li, J.; Cui, J.; Abroshan, H.; Ding, H.; Li, G. CuO/TiO₂ Heterojunction Composites: An Efficient Photocatalyst for Selective Oxidation of Methanol to Methyl Formate. *J. Mater. Chem. A* **2019**, *7*, 2253–2260.

(35) Chen, X.; Burda, C. The Electronic Origin of the Visible-Light Absorption Properties of C-, N- and S-Doped TiO₂ Nanomaterials. *J. Am. Chem. Soc.* **2008**, *130*, 5018–5019.

(36) Hussain, M. Z.; Schneemann, A.; Fischer, R. A.; Zhu, Y.; Xia, Y. MOF Derived Porous ZnO/C Nanocomposites for Efficient Dye Photodegradation. *ACS Appl. Energy Mater.* **2018**, *1*, 4695–4707.

(37) Zhang, Y.; Hu, W.; Wang, D.; Reinhart, B. J.; Huang, J. Electron Shuttle in the MOF Derived TiO₂/CuO Heterojunction Boosts Light Driven Hydrogen Evolution. *J. Mater. Chem. A* **2021**, *9*, 6180–6187.

(38) Yano, T.; Ebizuka, M.; Shibata, S.; Yamane, M. Anomalous Chemical Shifts of Cu₂P and CuLMM Auger Spectra of Silicate Glasses. *J. Electron Spectrosc. Relat. Phenom.* **2003**, *131-132*, 133–144.

(39) Hiraba, H.; Koizumi, H.; Kodaira, A.; Takehana, K.; Yoneyama, T.; Matsumura, H. Effects of Copper Surface Oxidation and Reduction on Shear-Bond Strength Using Functional Monomers. *Materials* **2021**, *14*, 1753.

(40) Pels, J. R.; Kapteijn, F.; Moulijn, J. A.; Zhu, Q.; Thomas, K. M. Evolution of Nitrogen Functionalities in Carbonaceous Materials during Pyrolysis. *Carbon* **1995**, *33*, 1641–1653.

(41) Bhadra, B. N.; Vinu, A.; Serre, C.; Jhung, S. H. MOF-derived Carbonaceous Materials Enriched with Nitrogen: Preparation and Applications in Adsorption and Catalysis. *Mater. Today* **2019**, *25*, 88–111.

(42) Amreetha, S.; Dhanuskodi, S.; Nithya, A.; Jothivenkatachalam, K. Three Way Electron Transfer of a C–N–S Tri doped Two-phase Junction of TiO₂ Nanoparticles for Efficient Visible Light Photocatalytic Dye Degradation. *RSC Adv.* **2016**, *6*, 7854–7863.

(43) Di Valente, C.; Finazzi, E.; Pacchioni, G.; Selloni, A.; Livraghi, S.; Paganini, M. C.; Giamello, E. N-doped TiO₂: Theory and Experiment. *Chem. Phys.* **2007**, *339*, 44–56.

(44) Xing, H.; E, L.; Guo, Z.; Zhao, D.; Li, X.; Liu, Z. Exposing the Photocorrosion Mechanism and Control Strategies of a CuO Photocathode. *Inorg. Chem. Front.* **2019**, *6*, 2488–2499.

(45) Wang, K.; Bielan, Z.; Endo-Kimura, M.; Janczarek, M.; Zhang, D.; Kowalski, D.; Zielińska-Jurek, A.; Markowska-Szczupak, A.; Ohtani, B.; Kowalska, E. On the Mechanism of Photocatalytic Reactions on CuₓO@TiO₂ Core–shell Photocatalysts. *J. Mater. Chem. A* **2021**, *9*, 10135–10145.

(46) Karthik, P.; Shaheer, A. R. M.; Vinu, A.; Neppolian, B. Amine Functionalized Metal–Organic Framework Coordinated with Transition Metal Ions: d–d Transition Enhanced Optical Absorption and Role of Transition Metal Sites on Solar Light Driven H₂ Production. *Small* **2020**, *16*, No. 1902990.

(47) Kampouri, S.; Ireland, C. P.; Valizadeh, B.; Oveisi, E.; Schouwink, P. A.; Mensi, M.; Stylianou, K. C. Mixed-Phase MOF-Derived Titanium Dioxide for Photocatalytic Hydrogen Evolution: The Impact of the Templated Morphology. *ACS Appl. Energy Mater.* **2018**, *1*, 6541–6548.

(48) Hou, H.; Shang, M.; Gao, F.; Wang, L.; Liu, Q.; Zheng, J.; Yang, Z.; Yang, W. Highly Efficient Photocatalytic Hydrogen Evolution in Ternary Hybrid TiO₂/CuO/Cu Thoroughly Mesoporous Nanofibers. *ACS Appl. Mater. Interfaces* **2016**, *8*, 20128–20137.

(49) Xiang, Q.; Yu, J.; Jaroniec, M. Enhanced photocatalytic H₂-production Activity of Graphene-modified Titania Nanosheets. *Nanoscale* **2011**, *3*, 3670–3678.

(50) Zhang, Y.-H.; Li, Y.-L.; Jiu, B.-B.; Gong, F.-L.; Chen, J.-L.; Fang, S.-M.; Zhang, H.-L. Highly Enhanced Photocatalytic H₂ Evolution of Cu₂O Microcube by Coupling with TiO₂ Nanoparticles. *Nanotechnology* **2019**, *30*, No. 145401.

(51) Li, R.; Wu, S.; Wan, X.; Xu, H.; Xiong, Y. Cu/TiO₂ Octahedral-shell Photocatalysts Derived from Metal–organic Framework@semiconductor Hybrid Structures. *Inorg. Chem. Front.* **2016**, *3*, 104–110.
(52) Liao, Y.-T.; Huang, Y.-Y.; Chen, H. M.; Komaguchi, K.; Hou, C.-H.; Henzie, J.; Yamauchi, Y.; Ide, Y.; Wu, K. C. W. Mesoporous TiO₂ Embedded with a Uniform Distribution of CuO Exhibit Enhanced Charge Separation and Photocatalytic Efficiency. ACS Appl. Mater. Interfaces **2017**, *9*, 42425–42429.

(53) Kubacka, A.; Fernández-García, M.; Colón, G. Advanced Nanoarchitectures for Solar Photocatalytic Applications. *Chem. Rev.* **2012**, *112*, 1555–1614.

(54) Serpone, N.; Emeline, A. V.; Ryabchuk, V. K.; Kuznetsov, V. N.; Artem'ev, Y. M.; Horikoshi, S. Why do Hydrogen and Oxygen Yields from Semiconductor-Based Photocatalyzed Water Splitting Remain Disappointingly Low? Intrinsic and Extrinsic Factors Impacting Surface Redox Reactions. ACS Energy Lett. **2016**, *1*, 931–948.

(55) Scanlon, D. O.; Dunnill, C. W.; Buckeridge, J.; Shevlin, S. A.; Logsdail, A. J.; Woodley, S. M.; Catlow, C. R. A.; Powell, M. J.; Palgrave, R. G.; Parkin, I. P.; et al. Band Alignment of Rutile and Anatase TiO₂. *Nat. Mater.* **2013**, *12*, 798–801.

(56) Peter, L. M.; Wijayantha, K. G. U.; Tahir, A. A. Kinetics of Light-driven Oxygen Evolution at α-Fe₂O₃ Electrodes. *Faraday Discuss.* **2012**, *155*, 309–322.

(57) Qiu, B.; Zhou, Y.; Ma, Y.; Yang, X.; Sheng, W.; Xing, M.; Zhang, J. Facile Synthesis of the Ti³⁺ Self-doped TiO₂-graphene Nanosheet Composites with Enhanced Photocatalysis. *Sci. Rep.* **2015**, *5*, No. 8591.

(58) Chiesa, M.; Paganini, M. C.; Livraghi, S.; Giamello, E. Charge trapping in TiO₂ polymorphs as seen by Electron Paramagnetic Resonance spectroscopy. *Phys. Chem. Chem. Phys.* **2013**, *15*, 9435–9447.

(59) Zhen, W.; Jiao, W.; Wu, Y.; Jing, H.; Lu, G. The Role of a Metallic Copper Interlayer during Visible Photocatalytic Hydrogen Generation over a Cu/Cu₂O/Cu/TiO₂ Catalyst. *Catal. Sci. Technol.* **2017**, *7*, 5028–5037.

<figure><img src="image_16.png" /></figure>

Recommended by ACS

Carbon Vacancy-Modified Carbon Nitride Allotropic Composite for Solar Hydrogen Generation Coupled with Selective Oxidation of 5-Hydroxymethylfurfural

Chih-Chia Chen, Jih-Jen Wu, et al.

APRIL 07, 2023

ACS SUSTAINABLE CHEMISTRY & ENGINEERING

READ

<figure><img src="image_17.png" /></figure>

Photocatalytic and Photoelectrocatalytic Water Splitting by Porous g-C₃N₄ Nanosheets for Hydrogen Generation

Amir Mehtab, Tokeer Ahmad, et al.

AUGUST 18, 2022

ACS APPLIED NANO MATERIALS

READ

<figure><img src="image_18.png" /></figure>

A Geometrically Well-Defined and Systematically Tunable Experimental Model to Transition from Planar to Mesoporous Perovskite Solar Cells

Dirk Döhler, Julien Bachmann, et al.

SEPTEMBER 22, 2022

ACS APPLIED ENERGY MATERIALS

READ

<figure><img src="image_19.png" /></figure>

Internal Electric-Field-Driven CoAl-LDH Coupled N-Rich Carbon Nitride of C₃N₅ for Improved Photocatalytic Performance

Lixu Wu, Feipeng Jiao, et al.

JANUARY 26, 2023

INDUSTRIAL & ENGINEERING CHEMISTRY RESEARCH

READ

<figure><img src="image_20.png" /></figure>

Get More Suggestions >