# Heterostructured g-CN/TiO 2 Photocatalysts Prepared by Thermolysis of g-CN/MIL-125(Ti) Composites for Efficient Pollutant Degradation and Hydrogen Production

Batukhan Tatykayev, Bilel Chouchene, Lavinia Balan, Thomas Gries, Ghouti Medjahdi, Emilien Girot, Bolat Uralbekov, Raphael Schneider

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Batukhan Tatykayev, Bilel Chouchene, Lavinia Balan, Thomas Gries, Ghouti Medjahdi, et al. Heterostructured g-CN/TiO 2 Photocatalysts Prepared by Thermolysis of g-CN/MIL-125(Ti) Composites for Efficient Pollutant Degradation and Hydrogen Production. Nanomaterials, 2020, 10 (7), pp.1387. 10.3390/nano10071387. hal-02900959

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Submitted on 16 Jul 2020

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Article

# Heterostructured g-CN/TiO₂ Photocatalysts Prepared by Thermolysis of g-CN/MIL-125(Ti) Composites for Efficient Pollutant Degradation and Hydrogen Production

Batukhan Tatykayev <fnref n="1" />,<fnref n="2" />, Bilel Chouchene ¹ID, Lavinia Balan ³ID, Thomas Gries ⁴ID,
Ghouti Medjahdi <fnref n="4" />, Emilien Girot <fnref n="1" />, Bolat Uralbekov <fnref n="2" /> and Raphaël Schneider <fnref n="1" />,*ID

Received: 18 June 2020; Accepted: 13 July 2020; Published: 16 July 2020

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**Abstract:** Photocatalysts composed of graphitic carbon nitride (g-CN) and TiO₂ were efficiently prepared by thermolysis of the MIL-125(Ti) metal organic framework deposited on g-CN. The heterojunction between the 12 nm-sized TiO₂ nanoparticles and g-CN was well established and the highest photocatalytic activity was observed for the g-CN/TiO₂ (3:1) material. The g-CN/TiO₂ (3:1) composite exhibits high visible light performances both for the degradation of pollutants like the Orange II dye or tetracycline but also for the production of hydrogen (hydrogen evolution rate (HER) up to 1330 µmolh⁻¹g⁻<fnref n="1" /> and apparent quantum yield of 0.22% using NiS as a cocatalyst). The improved visible light performances originate from the high specific surface area of the photocatalyst (86 m²g⁻<fnref n="1" />) and from the efficient charge carriers separation as demonstrated by photoluminescence, photocurrent measurements, and electrochemical impedance spectroscopy. The synthetic process developed in this work is based on the thermal decomposition of metal organic framework deposited on a graphitic material and holds huge promise for the preparation of porous heterostructured photocatalysts.

**Keywords:** graphitic carbon nitride; MIL-125(Ti); titanium dioxide; heterostructured photocatalysts

## 1. Introduction

The assembly of semiconductors into heterostructures has recently attracted considerable attention due to the numerous photocatalytic applications of these materials like detoxification of water or solar-to-chemical energy conversion [1–5]. In heterostructured photocatalysts, both the efficient spatial separation of charge carriers and the enhanced light absorption allow to boost the catalytic activity compared to the individual components due to a synergistic effect.

Titanium dioxide TiO₂ is one of the most important photocatalyst and has been widely used for environmental applications or water splitting [6–11]. However, the photocatalytic performances of TiO₂ are limited by its wide bandgap energy (ca. 3.2 eV) which restricts its photoactivation to UV light and by its modest charge transport efficiency. Graphitic carbon nitride (g-CN) is an organic layered

<footnote n="1">1 CNRS, LRGP, Université de Lorraine, F-54000 Nancy, France; b.tatykayev@gmail.com (B.T.); bilel.chouchene@univ-lorraine.fr (B.C.); emilien.girot@univ-lorraine.fr (E.G.)</footnote>

<footnote n="2">2 Department of General and Inorganic Chemistry, Al-Farabi Kazakhstan National University, Al-Farabi Av., 71, Almaty 050040, Kazakhstan; bulat.ural@gmail.com</footnote>

<footnote n="3">3 Conditions Extrêmes et Matériaux: Haute Température et Irradiation, UPR 3079 CNRS, Site Haute Température, 1D avenue de la Recherche Scientifique, 45071 Orléans, France; lavinia.balan@cnrs-orleans.fr</footnote>

<footnote n="4">4 CNRS, IJL, Université de Lorraine, F-54000 Nancy, France; thomas.gries@univ-lorraine.fr (T.G.); ghouti.medjahdi@univ-lorraine.fr (G.M.)</footnote>

<footnote>* Correspondence: raphael.schneider@univ-lorraine.fr; Tel.: +33-3-78-74-37</footnote>
semiconductor composed of carbon and nitrogen with an energy bandgap of ca. 2.7 eV and exhibiting
high chemical, thermal and photostability [1,12–14]. As TiO₂, bulk g-CN suffers from several drawbacks
including the fast recombination of photogenerated charge carriers and a low specific surface area
(less than 10 m²g⁻¹), which decrease its catalytic performance [1,12–14]. However, the conduction
band of g-CN is located at ca. –1.3 eV at pH 7 vs. NHE and is more negative than that of TiO₂ [15,16],
which allows photogenerated electrons to be efficiently transferred from the conduction band of g-CN
to the conduction band of TiO₂. This decrease of the photogenerated electron-hole pair’s recombination
allows significant improvements in the photocatalytic performance of g-CN/TiO₂ composites.

Various methods have been developed for the preparation of g-CN/TiO₂ heterostructured photocatalysts. The most commonly used process is the ultrasonication or the mechanical mixing of pre-formed TiO₂ and g-CN followed by a heat treatment at temperatures varying from 300 to 500 °C to hybridize their energy levels [17–21]. To improve the interfacial connection between TiO₂ and g-CN and thus the delocalization of charge carriers, synthetic methods involving the deposition of Ti precursors like Ti(On-Bu)₄ at the surface of exfoliated g-CN followed a hydro- or a solvothermal reaction and a calcination have been reported [22–25]. The reverse process including the generation of g-CN by calcination of N-rich precursors (melamine and/or cyanuric acid) in the presence of TiO₂ nanoparticles or titanate nanotubes has also been investigated [26–32]. Finally, the vapor phase deposition of g-CN at the surface of TiO₂ [33] or the surface modification of g-CN with chloroacetic acid to increase the association with TiO₂ have also been reported [34].

In recent years, the preparation of porous metal oxides derived from metal organic frameworks (MOFs) has gained high interest for environmental and energy-related applications. After thermolysis, the textural properties of MOFs are partially transferred to the porous metal oxide exposing a high density of active sites and in which charge carriers easily diffuse to the surface of the photocatalyst [35]. Using the NH₂-MIL-125(Ti) as starting MOF, this strategy has successfully been used to prepare photocatalysts associating TiO₂ nanoparticles deposited on carbon or associated to MoS₂ [36–38]. MIL-125(Ti) is a MOF responding to the formula Ti₈O₈(OH)₄[O₂C-C₆H₄-CO₂]₆ and exhibiting a well-defined porosity and high specific surface area (ca. 1700 m²g⁻¹) [39]. MIL-125(Ti) is a good precursor of TiO₂ nanocrystals [40,41] that demonstrated a good photocatalytic activity for CO₂ reduction, As(III) oxidation or pollutant degradation [42–48].

Herein, a novel synthetic process using in the key step the thermolysis of g-CN/MIL-125(Ti)
assemblies was developed to prepare highly active g-CN/TiO₂ photocatalysts. The mass ratio of g-CN
to TiO₂ was varied and the highest photocatalytic activity was obtained for the g-CN/TiO₂ (3:1) material.
The g-CN/TiO₂ photocatalyst was not only demonstrated to be highly efficient for the degradation of
pollutants in aqueous phase but also for H₂ photoproduction using NiS as co-catalyst.

**2. Materials and Methods**

**2.1. Chemicals**

Terephthalic acid (98%, Sigma), titanium (IV) propoxide (98%, Sigma), melamine (99%, Sigma), dimethylformamide (DMF) (99.9%, Sigma), Orange II sodium salt (microscopy grade, Sigma), tetracycline hydrochloride (Bioreagent, Sigma), glycerol (>99.5%, Sigma), methanol (99.9%, Sigma) and ethanol (anhydrous, Sigma) were used as received.

## 2.2. Synthesis of MIL-125(Ti)

Terephthalic acid (3 g, 18.28 mmol) was dissolved in 54 mL of anhydrous DMF and 6 mL MeOH by magnetic stirring for 15 min. To the obtained solution was added titanium (IV) propoxide (3.6 mL, 13.08 mmol) and the mixture was stirred for 30 min under argon. Then, the solution was transferred into a 100 mL Teflon-lined stainless autoclave and heated in an oven at 150 °C for 48 h. After cooling to room temperature, MIL-125(Ti) particles were collected by centrifugation (4000 rpm for 15 min),
washed with DMF (3 × 30 mL), with MeOH (3 × 30 mL) and finally dried at 70 °C overnight. A typical synthesis affords 1.86 g of a white powder.

## 2.3. Synthesis of Bulk g-CN

Melamine (5 g) was placed into a covered ceramic crucible and heated under air at 500 °C at a rate of 10 °C min⁻¹ and then maintained at 500 °C for 2 h. The yellow powder obtained (1.75 g) was ground in an agate mortar.

## 2.4. Exfoliation of g-CN

Bulk g-CN (0.5 g) was dispersed in 80 mL water and ultrasonicated for 60 min using a Sonic Dismembrator 550 (Fisher Scientific, Illkirch, France). Next, the obtained white-yellow dispersion was centrifuged for 30 min at 5000 rpm, the precipitate collected, washed with EtOH (30 mL) and dried at 70 °C overnight.

## 2.5. Preparation of g-CN/TiO₂ Photocatalysts

A representative synthesis of the g-CN/TiO₂ (3:1) photocatalyst is described. Exfoliated g-CN (180 mg) and MIL-125(Ti) (120 mg) were dispersed in 5 mL water and the mixture sonicated for 30 min. After centrifugation (4000 rpm for 15 min), the powder was washed with EtOH (30 mL) and dried at 70 °C for 3 h. Then, the powder was placed into a covered ceramic crucible, heated under air at 450 °C at a rate of 10 °C min⁻¹ and maintained at 450 °C for 2 h before cooling. The g-CN/TiO₂ composites were used without further treatment for photocatalytic experiments.

## 2.6. Photocatalytic Degradations

A 300 W Osram lamp coupled with UV cut-off filter (λ > 420 nm) was used as light source for photocatalytic degradation experiments. In a typical photocatalytic experiment, 20 mg of photocatalyst was dispersed in 40 mL of the pollutant aqueous solution (Orange II or tetracycline hydrochloride, 10 mg/L) and the mixture was stirred in the dark for 60 min to reach the adsorption-desorption equilibrium. Next, visible light was turned on and at regular irradiation time intervals, 2 mL of the dispersion were withdrawn, centrifuged for 5 min at 15,000 rpm and analyzed by UV-visible absorption to monitor the degradation of the pollutant (485 nm for Orange II and 357 nm for tetracycline).

Scavenging experiments were conducted using the same synthetic protocol except that *tert*-butanol (*t*-BuOH), *p*-benzoquinone (*p*-BQ), ammonium oxalate (AO), sodium azide (NaN₃) and dimethylsulfoxide (DMSO) were added to the aqueous dispersions before turning on light.

## 2.7. Photocatalytic H₂ Production

For photocatalytic H₂ production, the surface of nanocomposites was decorated with NiS nanoparticles using Ni(NO₃)₂ and Na₂S as the precursors for NiS. In a typical experiment, 400 mg of the g-CN/TiO₂ (3:1) catalyst was dispersed in 45 mL of deionized water and 1.3 mL of a 0.05 M Ni(NO₃)₂ was added. The mixture was stirred for 20 min to absorb Ni²⁺ ions at the surface of the g-CN/TiO₂ (3:1) composite. Next, 1.5 mL of a 0.05 M Na₂S solution was added to the solution to form NiS. The obtained g-CN/TiO₂ (3:1)/NiS composite contains ca. 1.5 wt.% NiS and is noted g-CN/TiO₂ (3:1)/1.5%NiS. The association of NiS with the other g-CN/TiO₂ photocatalysts was conducted using the same synthetic protocol.

The photocatalytic hydrogen production was conducted in an outer irradiation type quartz reactor using a 300 W Xe lamp as the light source. A circulation of water with an external cooling coil was used to maintain the temperature of the dispersion at 25 °C. In order to remove dissolved oxygen, the dispersion was first bubbled with nitrogen for 60 min before light irradiation. The reactor was connected to a pure nitrogen gas flow (100 mL min⁻¹) used to displace hydrogen from the photoreactor toward the micro gas chromatograph (Varian 490-GC). In a typical experiment, 25 mg of photocatalyst was dispersed
in 50 mL aqueous solution containing 10 vol.% of glycerol used as sacrificial substrate and the mixture was stirred to ensure a uniform irradiation of the catalyst dispersion.

## 2.8. Apparent Quantum Efficiency (AQE)

The apparent quantum efficiency (AQE) was determined using the previously described experimental setup and with the Equation (1):

$$AQE = \frac{n \times \Delta G}{I} \times 100\% = \frac{n \times \Delta G}{W \times S \times t} \times 100\% \quad (1)$$

where n is the number of H₂ molecules evolved, ΔG = 237 kJmol⁻¹ is the energy needed for one water molecule to entirely split into H₂ and O₂, W is the power of the lamp, S is the irradiated area, and t is the reaction time [33].

## 2.9. Photocatalyst Characterization

The morphology and the microstructure were investigated by scanning electron microscopy (SEM, Scanning Electron Microscope JSM-6490 LV, JEOL, Croissy, France) and transmission electron microscopy (TEM, Philips CM200 instrument operating at 200 kV, Philips, Suresnes, France). TEM images were acquired by placing a drop of the particles in water onto a gold grid.

X-ray powder diffraction (XRD) patterns were recorded using a Panalytical X'Pert Pro MPD diffractometer using Cu Kα radiation (λ = 0.15418 nm).

X-ray photoelectron spectroscopy (XPS) analyses were carried out on a Gammadata Scienta (Uppsala, Sweden) SES 200-2 spectrometer under ultra-high vacuum (P < 10⁻⁹ mbar).

Thermogravimetric analysis (TGA) was conducted under O₂ atmosphere using a TGA/DSC1 STAR equipment (Mettler-Toledo). Measurements were conducted from room temperature to 1000 °C at a heating rate of 10 °C min⁻¹.

The Brunauer–Emmett–Teller (BET) specific surface areas were measured with a Micromeritics ASAP 2420 instrument at liquid nitrogen temperature. Before analysis, the samples were out-gassed overnight in vacuum at 40 °C followed by 4 h out-gassing on the analysis port. The micropore volume was determined using the Barrett–Joyner–Halenda (BJH) method.

A Total Organic Carbon analyzer (Shimadzu TOC-VCSH, Marne-la-Vallée, France) was used to evaluate the amount of dissolved organic carbon before and after the photodegradation reactions.

Fourier transform infrared (FT-IR) spectra were recorded using a Brucker ALPHA spectrometer in the frequency range of 4000–450 cm⁻¹. UV-visible absorption spectra of liquid samples were measured using a Thermo Scientific Evolution 220 UV-visible spectrophotometer. The UV-visible diffuse reflectance spectra (DRS) were recorded on a Shimadzu 2600-2700 UV-visible spectrophotometer. Photoluminescence (PL) emission spectra were recorded at room temperature on a Horiba Fluoromax-4 Jobin Yvon spectrofluorimeter. Raman spectra were measured on a Horiba Scientific Xplora spectrometer using 532 nm wavelength incident YAG laser light.

The photoelectrochemical performance of the catalysts was evaluated in a three electrode cell using FTO glass coated with g-CN, TiO₂ or g-CN/TiO₂ composites with a surface area of 1 cm² as the working electrode, a Pt wire as the counter electrode and a saturated Ag/AgCl electrode as reference electrode. The electrolyte used was a 0.1 M Na₂SO₄ aqueous solution. The photocurrent densities were determined by measuring the current under 100 mW cm⁻² light illumination provided by a 300 W Xe arc lamp equipped with an AM 1.5G filter. A SP150 BioLogic potentiostat was used to measure the photocurrent response at a constant potential of +0.30 V vs. the reference electrode.
## 3. Results

### 3.1. Photocatalysts Synthesis and Characterization

MIL-125(Ti) particles were produced from Ti(On-Pr)₄ and terephthalic acid in a DMF/MeOH mixture [39] and associated to exfoliated g-CN under sonication. Next, the g-CN/MIL-125(Ti) composites were calcined at 450 °C for 2 h to decompose MIL-125(Ti) into TiO₂ (Figure 1). A weight loss of ca. 50% was observed during the thermolysis of pure MIL-125(Ti). g-CN/MIL-125(Ti) composites with g-CN:MIL-125(Ti) weight ratios of 1:2, 2:2, 3:2, 4:2, and 8:2 were prepared to engineer g-CN/TiO₂ photocatalysts with g-CN:TiO₂ ratios of 1:1, 2:1, 3:1, 4:1, and 8:1, respectively.

<figure><img src="image_4.png" /><figcaption>**Figure 1.** Schematic illustration of the preparation of the graphitic carbon nitride (g-CN)/TiO₂ photocatalysts from g-CN/MIL-125(Ti) composites.</figcaption></figure>

The morphology, the structure and the composition of g-CN/MIL-125(Ti) and g-CN/TiO₂ composites were first investigated by SEM and TEM (Figure 2). After exfoliation by sonication, g-CN sheets have sizes varying from a few hundred of nanometers up to several micrometers and exhibit a crumpled layered morphology with a relatively smooth surface (Figure 2a). Using g-CN/MIL-125 (3:2) and g-CN/TiO₂ (3:1) samples as representatives, MIL-125(Ti) and TiO₂ particles are bound to g-CN sheets and uniformly distributed at their surface as indicated by SEM, TEM, TEM-associated EDS analyses and by the corresponding elemental mappings (Figure 2a–d, Figures S1 and S2). A marked exfoliation of g-CN can also be observed by SEM during the thermolysis of MIL-125(Ti) particles, which will be confirmed by BET specific surface measurements (vide infra). The SEM images show that MIL-125(Ti) particles decompose into TiO₂ particles with an average diameter of ca. 400 nm (Figure 2a,b) but TEM analyses demonstrate that TiO₂ is actually made up of clusters composed of TiO₂ nanoparticles with an average size of 12 ± 6 nm (Figure 2e). The HR-TEM image of these TiO₂ nanocrystals shows a lattice spacing of 0.35 nm, which corresponds to the (101) plane of anatase TiO₂ (Figure 2f).

The FT-IR spectrum of MIL-125(Ti) exhibits the typical vibrational bands between 1715 and 1300 cm⁻¹ corresponding to the C=C and C-H bonds of the aromatic ring and to the asymmetric (1648 cm⁻¹) and symmetric (1386 cm⁻¹) stretching vibrations of the carboxylate functions (Figure 3a). The signals observed at 807, 739, and 657 cm⁻¹ can be assigned to Ti-O-Ti-O vibrations [49]. For pure g-CN, the peaks located from 1658 to 1236 cm⁻¹ correspond to the stretching modes of the carbon-nitrogen (C=N and C-N) bonds, while the sharp peak located at 806 cm⁻¹ can be attributed to the out-of-plane breathing vibration of the s-triazine units [15,16,50]. The broad absorption bands at 3235 and 3154 cm⁻¹ correspond to the stretching modes of NH₂ and NH functions. All these signals can be observed in the FT-IR spectra of g-CN/MIL-125(Ti) composites, indicating the successful deposition of MIL-125(Ti) at the surface of g-CN. Pure TiO₂ generated by the thermal decomposition of MIL-125 only exhibits a strong signal at 410 cm⁻¹ corresponding to the vibration mode of Ti-O-Ti bonds in TiO₂ (Figure 3b) [51]. No signals of organics could be observed indicating that MIL-125 was efficiently decomposed into TiO₂ by heating at 450 °C for 2 h. The shift observed for the Ti-O bond from 412 to
460 cm⁻¹ when increasing the g-CN:TiO₂ ratio from 1:1 to 8:1 suggests a strong interaction between TiO₂ and g-CN, which should favor the photocatalytic activity of these materials.

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Scanning electron microscopy (SEM) images of (a) g-CN/MIL-125 (3:2) and (b) g-CN/TiO₂ (3:1), transmission electron microscopy (TEM) images of (c) g-CN/MIL-125 (3:1), and (d–f) g-CN/TiO₂ (3:1) composites.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Figure 3.** Fourier transform infrared (FT-IR) spectra of (a) MIL-125(Ti), g-CN, and of g-CN/MIL-125(Ti) composites and (b) TiO₂ and g-CN/TiO₂ composites.</figcaption></figure>

XRD and Raman spectroscopy were used to analyse the structure of g-CN/TiO₂ composites. As previously observed after the pyrolysis of MIL-125(Ti) at a relatively mild temperature, the pure anatase phase of TiO₂ is formed (Figure 4a) [42,44]. The peaks at 2θ values of 25.35, 36.83, 37.93, 38.72, 48.16, 54.19, 55.16, and 62.75° belong to the (101), (103), (004), (112), (200), (105), (211), and (204) lattice planes of anatase (JCPDS No 21-1272). For g-CN, the diffraction peaks located at 13.03° and 27.51° are related to the (100) interplanar structural packing (repeated s-triazine units) and to the (002) interlayer stacking structure [15,16]. Compared to bulk g-CN, a slight decrease of the cristallite size from 7.4 to 6.9 nm (Figure S3) and a slight shift to higher angles of the (002) peak (Figure S4) were observed
for exfoliated g-CN. The intensity of the g-CN related peaks increases with the g-CN content of the nanocomposites. Moreover, a shift of the (002) reflection from 27.51° to 27.66° is observed after the thermolysis of MIL-125(Ti), which confirms the strong interaction of TiO₂ nanoparticles with g-CN sheets. The Raman spectrum of pure TiO₂ shows peaks at 395, 518, and 639 cm⁻¹ corresponding to the B1g, A1g + B1g and Eg modes of anatase TiO₂, respectively (Figure 4b) [52]. Despite the modest resolution of the g-CN Raman spectrum using the 532 nm laser, three strong peaks corresponding to the vibration of s-triazine units can be observed at 472, 705, and 1234 cm⁻¹ [53]. The Raman signals of TiO₂ and g-CN can all be observed in g-CN/TiO₂ composites, further confirming the structure of these materials.

<figure><img src="image_7.png" /><figcaption>**Figure 4.** (a) X-ray powder diffraction (XRD) patterns and (b) Raman spectra of TiO₂, g-CN, and g-CN/TiO₂ composites.</figcaption></figure>

The actual weight ratio of TiO₂ in g-CN/TiO₂ composites and the thermal stability of these photocatalysts were determined by TGA. The TGA profiles of the g-CN/TiO₂ photocatalysts recorded in air atmosphere with a heating rate of 10 °C min⁻¹ are given in Figure 5. The weak weight loss (ca. 2%) observed below 100 °C originates from the removal of water molecules adsorbed at the surface of the catalysts. As can be seen from Figure 5, g-CN starts to decompose at ca. 550 °C into CO₂ and NO₂. A sharp weight loss is observed at ca. 580 °C for all composites. The content in g-CN was determined from the remaining TiO₂ weight and was found to be significantly lower than the theoretical ratio (Table 1). The relative content in g-CN is the lowest for composites with a high loading in TiO₂ (g-CN/TiO₂ ratios of 1:1 and 2:1), suggesting that TiO₂ promotes the thermal decomposition of g-CN.

<figure><img src="image_8.png" /><figcaption>**Figure 5.** Thermogravimetric curves of g-CN/TiO₂ composites.</figcaption></figure>
<table><caption>**Table 1.** Theoretical and actual g-CN:TiO₂ ratios determined by thermogravimetric analysis (TGA) analyses.</caption><tr><td>Theoretical g-CN:TiO<sub>2</sub> Ratio</td><td>1:1</td><td>2:1</td><td>3:1</td><td>4:1</td><td>8:1</td></tr><tr><td>Actual g-CN:TiO<sub>2</sub> Ratio</td><td>0.28:1</td><td>0.91:1</td><td>1.59:1</td><td>2.12:1</td><td>4.41:1</td></tr></table>

X-ray photoelectron spectroscopy (XPS) was used to investigate the composition and the surface chemical states of C, N, Ti and O elements present in the g-CN/TiO₂ (3:1) composite (Figure S5). The major signal in the XPS high resolution C 1s spectrum is located at 288.08 eV and can be assigned to sp²-hybridized C atoms in nitrogen heterocycles (Figure S6a). The weaker signal observed at 284.93 eV originates from sp²-hybridized C-C bonds [54,55]. The N 1s spectrum can be deconvoluted into three peaks at 401.15 eV (N-N bonding structure), 400.09 eV (tertiary nitrogen atoms, N-(C)₃) and 398.54 eV (sp²-hybridized N in triazine rings, C-N=C) (Figure S6b) [55]. The weak signal observed at 404.51 eV can be attributed to charging effects. For Ti 2p, two signals located at 458.87 eV and 464.62 eV corresponding to Ti 2p₃/₂ and Ti 2p₁/₂, respectively, can be observed (Figure S6c). The doublet splitting energy of Ti 2p is of 5.75 eV, which confirms that Ti is in the +4 oxidation state [56]. Finally, the O 1s spectrum can be deconvoluted into two peaks at 530.11 and 532.30 eV corresponding to lattice oxygen atoms (Ti-O bonds) and to carbonate species adsorbed at the surface of the photocatalyst, respectively (Figure S6d).

The BET specific surface areas and the porosities of g-CN, TiO₂, and g-CN/TiO₂ photocatalysts were investigated by nitrogen adsorption-desorption experiments. Figure 6a shows that all materials exhibit isotherms of type IV according to the Brunauer-Deming-Deming-Teller (BDDT) classification, indicating the presence of mesopores. The hysteresis loops observed at high relative pressure (P/P₀ > 0.8) suggest the presence of slit-like pores, which is consistent with the morphology of the photocatalysts determined by SEM and TEM (Figure 2). Pure g-CN exhibits a low BET specific surface area (9.8 m²g⁻¹), which is in accordance with previous reports (Table 2) [15,16]. The BET specific surface area of pure TiO₂ is significantly higher (56.3 m²g⁻¹), indicating that the textural properties of MIL-125(Ti) were partly transferred to TiO₂ after the calcination step. The BET specific surface areas of g-CN/TiO₂ composites increase with the content in g-CN until the g-CN:TiO₂ ratio of 2:1 and then decrease, which suggests than an optimal amount of MIL-125(Ti) favours the exfoliation of g-CN during the heating at 450 °C and thus the increase of the specific surface area. This may originate from the production of gases like CO₂ during the thermolysis of MIL-125(Ti) that generate more larger pores that allow to increase the BET specific surface area. The gradual decrease of S<sub>BET</sub> when increasing the g-CN:TiO₂ ratio from 3:1 to 8:1 may originate from a pore blocking of TiO₂ by g-CN. The S<sub>BET</sub> values measured for g-CN/TiO₂ composites prepared via annealing of Ti(On-Bu)₄ in the presence of g-CN or by calcination of melamine in the presence of TiO₂ crystals typically range from 34 to 77 m²g⁻¹ [19,22,23,26,28]. The specific surface areas of g-CN/TiO₂ (2:1 and 3:1) photocatalysts are significantly higher (ca. 102 and 86 m²g⁻¹) suggesting that during the calcination step, g-CN:MIL-125(Ti) ratios of 2:2 and 3:2 are optimal to simultaneously exfoliate g-CN and to generate porous TiO₂. Finally, the pore volumes and pore sizes were determined according to the Barrett-Joyner-Halenda (BJH) method (Figure 6b, Table 2). As previously, the higher pore volume values were obtained for the g-CN/TiO₂ 2:1 and 3:1 materials (up to 0.29 cm³g⁻¹). The large size of these pores further confirms the presence of mesopores. The high specific surface areas of g-CN/TiO₂ 2:1 and 3:1 materials associated to their mesoporosity will provide more surface-active sites and thus should improve their photocatalytic performance.
<figure><img src="image_9.png" /><figcaption>**Figure 6.** (a) Nitrogen adsorption-desorption isotherms of TiO₂, g-CN, and g-CN/TiO₂ photocatalysts and (b) the corresponding pore-size distributions.</figcaption></figure>

<table><caption>**Table 2.** Brunauer–Emmett–Teller (BET) specific surface areas, pore volumes and pore sizes of g-CN, TiO₂, and g-CN/TiO₂ composites.</caption><thead><tr><th>Sample</th><th>BET<br>(m<sup>2</sup>g<sup>-1</sup>)</th><th>Pore Volume<br>(cm<sup>3</sup>g<sup>-1</sup>)</th><th>Pore Size<br>(nm)</th></tr></thead><tbody><tr><td>g-CN</td><td>9.8 ± 0.2</td><td>0.07</td><td>17.75</td></tr><tr><td>TiO<sub>2</sub></td><td>56.3 ± 0.4</td><td>0.18</td><td>10.19</td></tr><tr><td>g-CN/TiO<sub>2</sub> (1:1)</td><td>59.8 ± 0.3</td><td>0.20</td><td>11.15</td></tr><tr><td>g-CN/TiO<sub>2</sub> (2:1)</td><td>102.2 ± 0.4</td><td>0.29</td><td>9.42</td></tr><tr><td>g-CN/TiO<sub>2</sub> (3:1)</td><td>86.2 ± 0.3</td><td>0.23</td><td>9.53</td></tr><tr><td>g-CN/TiO<sub>2</sub> (4:1)</td><td>63.8 ± 0.1</td><td>0.20</td><td>11.19</td></tr><tr><td>g-CN/TiO<sub>2</sub> (8:1)</td><td>48.8 ± 0.2</td><td>0.21</td><td>16.05</td></tr></tbody></table>

UV-visible absorption spectra of TiO₂, g-CN, and g-CN/TiO₂ composites are shown in Figure 7a and the bandgap energy of the materials were determined using the relation $\alpha h\nu = A(h\nu - E_g)^2$ where $\alpha$, $h$, $\nu$, A, and $E_g$ at the absorption coefficient, the Plank's constant, the frequency of light, a constant, and the bandgap energy, respectively. Pure anatase TiO₂ and g-CN exhibit bandgap energies of 3.02 and 2.70 eV, respectively, values in good accordance with those described in the literature (Figure 7b) [15,16,57]. With the increase of the g-CN loading in the photocatalysts, the UV-visible absorption in the 400–470 nm range increases and simultaneously the bandgap energy decreases from 3.02 to 2.73 eV. This extension of TiO₂ absorption should improve its photocatalytic performance in the visible range.

<figure><img src="image_10.png" /><figcaption>**Figure 7.** (a) UV-visible absorption spectra of TiO₂, g-CN, and g-CN/TiO₂ composites and (b) plots of $(\alpha h\gamma)^2$ vs $h\gamma$ used to determine the energy bandgap of the photocatalysts.</figcaption></figure>
## 3.2. Photodegradation of Pollutants under Visible Light Irradiation

First, the surface charges of g-CN and of the g-CN/TiO₂ (3:1) catalysts in water were determined. For pure g-CN, the point of zero charge (pzc) was determined to be 4.23, value in good agreement with previous reports (Figure S7) [58]. As the pzc of pure TiO₂ is usually between 3 and 4 [59], the pzc of the g-CN/TiO₂ composite slightly decreases to 3.70 after deposition of TiO₂ at the surface of g-CN sheets. These results show that the g-CN/TiO₂ photocatalyst exhibits a positive charge at neutral pH and should associate via electrostatic interactions with negatively-charged pollutants in aqueous solution.

In a first set of experiments, the photocatalytic activity of g-CN/TiO₂ materials was evaluated in the bleaching of the anionic diazo Orange II dye (concentration of 10 mgL⁻¹) under visible light irradiation (intensity of 15 mWcm⁻²). Figure 8a depicts the variation of the relative concentration (C/C₀) of Orange II vs irradiation time with respect to the changes of the dye absorption at 485 nm. The g-CN/TiO₂ (3:1) composite exhibits the highest degradation efficiency for Orange II (ca. 95% bleaching after 180 min irradiation) while control experiments using TiO₂ or g-CN showed significantly lower degradation efficiencies (22 and 50%, respectively) after the same illumination period. The photocatalytic activity increases with the increase of the g-CN/TiO₂ ratio until 3:1 and then slightly decreases for composites prepared with g-CN:TiO₂ ratios of 4:1 and 8:1, which suggest that the charge-carrier transfer efficiency is the highest when using the 3:1 g-CN:TiO₂ ratio. The decrease of the photocatalytic efficiency for g-CN/TiO₂ 4:1 and 8:1 composites is likely related to the decrease of their specific surface area as shown in Table 2 and to the increased charge carrier recombination (see below). Figure S8 shows the time dependent changes in the UV-visible absorption spectrum of Orange II during its photodegradation by the g-CN/TiO₂ (3:1) catalyst. In the meantime, the orange color of the solution gradually disappeared, indicating that the structure of the dye was decomposed. The total organic carbon (TOC) decreased from 5.48 to 1.13 gL⁻¹ after 180 min irradiation, which confirms the efficient mineralization of the dye. The first-order kinetics of Orange II degradation over the different photocatalysts are plotted in Figure S9. The apparent first-order rate constants k are 0.0033, 0.0058, 0.0156, 0.0137, and 0.0106 min⁻¹ for g-CN/TiO₂ catalysts prepared with g-CN:TiO₂ ratios of 1:1, 2:1, 3:1, 4:1, and 8:1, respectively, further confirming that the g-CN/TiO₂ (3:1) composite exhibits the highest photocatalytic activity.

The g-CN/TiO₂ (3:1) photocatalyst was also successfully used for the degradation of tetracycline hydrochloride, an antibiotic commonly used in human and veterinary medicine [60] and which poses serious threats on various eco-systems due to its poor biodegradability [61]. The photocatalytic activity of the g-CN/TiO₂ (3:1) composite was compared to that of pure TiO₂ and g-CN (Figure 8b). A marked absorption of tetracycline at the surface of g-CN/TiO₂ and TiO₂ catalysts (ca. 40 and 20%, respectively) was observed. Although tetracycline exists in its zwitterionic form at neutral pH [62], these results suggest that the enolate form of tetracycline strongly binds to the positively-charged surface of TiO₂ and g-CN/TiO₂ catalysts. The photocatalytic activity of the g-CN/TiO₂ composite under visible light irradiation is significantly higher than that of TiO₂ and g-CN, further confirming that the association of g-CN and TiO₂ improves the separation of electron-hole pairs. The apparent first-order rate constant k determined for TiO₂, g-CN, and g-CN/TiO₂ photocatalysts are 0.0035, 0.0078, and 0.0093 min⁻¹, respectively, and confirm that the kinetic of degradation is the highest for the g-CN/TiO₂ (3:1) composite (Figure S10). As can also be seen from Figure S11, almost 95% of tetracycline is decomposed after 180 min visible light irradiation. Moreover, the TOC value decreased from 5.48 to 1.63 g L⁻¹, further demonstrating that the g-CN/TiO₂ (3:1) catalyst is of high interest for environmental applications. The performances of the g-CN/TiO₂ (3:1) composite are higher than those of g-CN based catalysts recently developed for the degradation of TC (P- and S-doped g-CN, C-doped g-CN, or WO₃/g-CN/Bi₂O₃, 70–85% degradation after ca. 1 h using a light intensity of 100 mWcm⁻²) when considering that a much lower intensity irradiation was used in our experiments (25 mWcm⁻²) [63–65].
<figure><img src="image_11.png" /><figcaption>**Figure 8.** Photocatalytic degradation profiles of (a) Orange II and (b) tetracycline over TiO₂, g-CN, and g-CN/TiO₂ catalysts under visible light irradiation, and (c) photodegradation stability of Orange II using the g-CN/TiO₂ (3:1) catalyst.</figcaption></figure>

The photocatalytic stability of the g-CN/TiO₂ (3:1) composite was evaluated by five consecutive tests using Orange II as model pollutant (Figure 8c). A slight decrease of the dye degradation efficiency is observed after the first cycle (91%) but after the catalyst activity remains almost stable (88% after the fifth reuse), which indicates that the g-CN/TiO₂ (3:1) composite shows a good stability as a photocatalyst. In addition, XRD analyses indicate that the crystallinity of the material is not affected by the reuses (Figure S12).

### 3.3. *Active Species Involved in the Photodegradation, Photoelectrochemical Measurements, and Mechanism*

To identify the active species responsible for the degradation of Orange II, trapping experiments were conducted using *tert*-butanol (*t*-BuOH), *p*-benzoquinone (*p*-BQ), ammonium oxalate (AO), sodium azide (NaN₃), and DMSO as hydroxyl radicals (•OH), superoxide radicals (O₂•−), holes (h⁺), singlet oxygen (¹O₂), and electron (e⁻) scavengers, respectively [15,16]. Figure 9a shows the influence of these scavengers on the photocatalytic performance of the g-CN/TiO₂ (3:1) composite after 180 min irradiation. Weak decreases of the Orange II photodegradations were observed when adding *t*-BuOH and DMSO (88.5 and 83.3%, respectively) indicating that •OH radicals and electrons only play a minor role in the mechanism. The photodegradation efficiencies more significantly declined upon addition of AO and NaN₃ (67 and 43.7%, respectively) indicating that h⁺ and ¹O₂ are involved in the degradation.
Finally, O₂•- radicals play the key role in the photodegradation as the most deleterious effect was observed using p-BQ (7.5% after 180 min irradiation).

<figure><img src="image_12.png" /><figcaption>**Figure 9.** (a) Photodegradation of Orange II under visible light irradiation using the g-CN/TiO₂ catalyst in the presence of reactive species trapping compounds, (b) band structure of the g-CN/TiO₂ photocatalyst, and (c) schematic illustration of the delocalization of charge carriers and of the formation of reactive species involved in the photodegradation of Orange II.</figcaption></figure>

The band edge position of the valence band (VB) of TiO₂ at the point of zero charge can be calculated using the empirical Equation (2):

$$E_{CB} = \chi - E^{e} - 0.5 E_{g} \quad (2)$$

where E<sub>CB</sub> is the CB potential, χ is the absolute electronegativity of TiO₂ (5.90 eV), E<sup>e</sup> is the energy of free electrons on the hydrogen scale (ca. 4.5 eV), and E<sub>g</sub> is the bandgap energy (3.02 eV) [66]. The band edge positions of the VB and of the CB of TiO₂ nanoparticles were estimated to be +2.91 and -0.11 eV, respectively. For g-CN with a χ value of 4.42 eV, E<sub>CB</sub> and E<sub>VB</sub> values were determined to be -1.43 and +1.27 eV, respectively. A diagram showing the band structure of g-CN/TiO₂ nanocomposite is presented in Figure 9b along with the redox potentials of the reference reactions.

On the basis of these data, a scheme for the photogenerated electron/hole transfer steps under visible light irradiation at the interface of the g-CN/TiO₂ catalyst can be proposed (Figure 9b,c). Under visible light irradiation (λ > 420 nm), only g-CN is able to absorb photons, which results in the transfer of electrons from the VB to the CB of TiO₂. Due to the electrostatic field at the junction, photoexcited electrons can easily transfer to the CB while holes remain in g-CN, thereby hindering the charge carriers recombination and increasing the lifetimes of photogenerated electrons and holes. Then, separated electrons and holes can initiate reduction and oxidation reactions with O₂ and H₂O molecules adsorbed on the catalyst surface. The VB potential of g-CN (+1.27 eV) is higher than that of the •OH/H₂O couple (+2.32 eV) and holes in the VB g-CN cannot react with H₂O to generate •OH
radicals. However, these holes can oxidize Orange II into Orange II⁺. In the meantime, electrons accumulated in the CB of TiO₂ can easily react with O₂ to generate O₂•⁻ radicals. These radicals can directly oxidize the dye or transfer electrons to the holes in the VB of g-CN to give ¹O₂. These data are in good accordance with scavenging results described in Figure 9a where O₂•⁻, holes and ¹O₂ were demonstrated to play the major role in the photodegradation mechanism.

To support the enhanced photocatalytic performance of the g-CN/TiO₂ (3:1) composite under visible light irradiation, the recombination process of photogenerated electron-hole pairs in g-CN, TiO₂ and g-CN/TiO₂ composites was first investigated by PL spectroscopy. Upon excitation at 350 nm, g-CN exhibits a strong PL emission with the main signal located at 439 nm while TiO₂ is non-fluorescent (Figure 10a). The shape of the PL peak for g-CN/TiO₂ composites is similar to that of g-CN but its intensity is weaker indicating that the electron-hole pair's recombination is hampered. There is no correlation between the g-CN loading in g-CN/TiO₂ composites and the PL intensity gradually decreases when increasing the g-CN:TiO₂ ratio from 1:1 to 3:1 before re-increasing for the 4:1 and 8:1 ratios. The separation of electron-hole pairs is the most efficient in the g-CN/TiO₂ (3:1) material, which is consistent with photocatalytic results.

<figure><img src="image_13.png" /><figcaption>**Figure 10.** (a) Room temperature photoluminescence (PL) emission spectra of TiO₂, g-CN, and g-CN/TiO₂ materials (λex = 350 nm), (b) Photocurrent responses of TiO₂, g-CN, and g-CN/TiO₂ (3:1) composite under visible light irradiation and (c) EIS Nyquist plots of TiO₂, g-CN, and g-CN/TiO₂ materials.</figcaption></figure>
The electronic interactions in the various g-CN/TiO₂ photocatalysts were also studied by measuring the photocurrent responses and by using electrochemical impedance spectroscopy (EIS). Figure 10b shows the I-t curves of g-CN, TiO₂, and g-CN/TiO₂ (3:1) materials during ten on-off cycles of visible light irradiation. The photocurrent quickly increases when light is turned on and decreases to zero when the light is turned off, indicating that the materials respond to light with a good reproducibility. An improved charge carrier separation efficiency and a fastest charge transfer through the electrode interface are observed for the g-CN/TiO₂ (3:1) composite as the photocurrent response is ca. 1.8-fold higher than that of g-CN. Due to its bandgap energy of 3.02 eV (410 nm), pure TiO₂ responds to visible light irradiation. However, the photocurrent continuously increases during the illumination and does not reach a steady state contrary to g-CN and to the g-CN/TiO₂ (3:1) composite. A similar behavior was previously observed and may originate either from photo-excited states exhibiting a short lifetime and/or from a better contact between TiO₂ and the electrode during photoelectrochemical measurements [19].

The transfer resistance of electrons on the electrode surface plays also a key role on the electron transfer efficiency. The Nyquist plots of the EIS for TiO₂, g-CN, and g-CN/TiO₂ photocatalysts are shown in Figure 10c. The smallest diameter of the semi-circular Nyquist plots is observed for the g-CN/TiO₂ (3:1) composite which further confirms that the electron transfer is the fastest in this material.

### 3.4. Hydrogen Photoproduction

The hydrogen production activity of g-CN/TiO₂/NiS catalysts was evaluated using a 300 W Xenon lamp as light source and an aqueous solution containing 10 vol.% glycerol as sacrificial substrate. Pure TiO₂ and g-CN were used as references in these experiments. The efficient and cheap NiS co-catalyst which has shown its effectiveness for H₂ photoproduction using g-CN and TiO₂ was used instead of noble metals like Pt or Pd [67,68]. After deposition of NiS at the surface of g-CN/TiO₂ (4:1), (3:1), and (2:1) composites, photogenerated electrons likely transfer from the g-CN/TiO₂ photocatalyst to the valence band of the NiS semiconductor (Z-scheme transfer) which promotes H₂ evolution according to Equations (3) and (4):

$$NiS + e^{-} + H^{+} \rightarrow HNiS \quad (3)$$

$$HNiS + e^{-} + H^{+} \rightarrow NiS + H_{2} \quad (4)$$

Meanwhile, electrons are reinjected in the g-CN/TiO₂ photocatalyst via the oxidation of the sacrificial substrate.

Control experiments show that H₂ is not produced in the absence of the photocatalyst or light irradiation. As can be seen from Figure 11, H₂ is produced immediately after turning on light and the hydrogen evolution rate (HER) remains almost stable along the 3 h of irradiation. The highest HER values were obtained for the g-CN/TiO₂ (3:1) catalyst (1330, 1256, and 1223 µmolh⁻¹g⁻¹ after 1, 2, and 3 h of irradiation, respectively). These HERs are ca. 1.65- and 5.8-times higher than those measured for TiO₂ and g-CN. The apparent quantum efficiency (AQE) of the g-CN/TiO₂ (3:1) photocatalyst is of 0.22%, value 1.7-fold and 5.8-fold higher than those determined for pure TiO₂ and g-CN, respectively. These results further demonstrate that the heterojunction constructed between TiO₂ and g-CN combined to the high specific surface of the catalyst contribute to the enhancement of photogenerated electron transfer and thus to the photocatalytic activity. Finally, the photocatalytic activity for H₂ production is at least 2.6-times higher than that of binary g-CN/TiO₂ catalyst recently described in the literature (the highest HER values vary from 446 to 500 µmolh⁻¹g⁻¹) [18,25,32,33]. Noteworthy is also that some higher values were reported (up to 8931 µmolh⁻¹g⁻¹) but using Pt as co-catalyst [22].
<figure><img src="image_14.png" /><figcaption>**Figure 11.** Photocatalytic hydrogen production rates after 1, 2, and 3 h illumination of g-CN, TiO₂, and g-CN/TiO₂ catalysts associated to the NiS cocatalyst (1.5 wt.%) using a 300 W Xe lamp as irradiation source and water containing 10 vol.% glycerol as sacrificial substrate.</figcaption></figure>

**4. Conclusions**

Photocatalysts associating TiO₂ nanoparticles with an average size of ca. 12 nm and g-CN sheets were efficiently prepared by thermolysis of MIL-125(Ti) particles in the presence of g-CN. The high specific surface area of MIL-125(Ti) is transferred to TiO₂ nanoparticles and g-CN/TiO₂ composites exhibiting specific surface area up to 102 m²g⁻¹ could be engineered using the synthetic protocol developed in this work. The g-CN/TiO₂ (3:1) composite was demonstrated to exhibit the highest photocatalytic activity both for the degradation of pollutants like Orange II or tetracycline and for hydrogen production (HER of 1330 µmolh⁻¹g⁻¹ after 1 h irradiation without using Pt as co-catalyst). The close interfacial connection between TiO₂ and g-CN allows the efficient charge carriers separation with photo-excited electrons transferred from g-CN to TiO₂ while holes remain in g-CN. The design of heterostructured materials using the thermal decomposition of metal organic frameworks to generate porous metal oxide like TiO₂ was demonstrated to be an efficient strategy to improve the solar utilization for photocatalytic applications.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/10/7/1387/s1, Figure S1: (a) SEM image, (b) EDX analysis, and (c-f) EDX mapping of the g-CN/MIL-125(Ti) (3:2) composite. Figure S2: (a) SEM image, (b) EDX analysis, and (c-g) EDX mapping of the g-CN/TiO₂ (3:1) composite. Figure S3. XRD analysis of the (002) peak for (a) bulk g-CN and (b) exfoliated g-CN using the DIFFRAC.EVA software from Bruker. Figure S4. XRD patterns of bulk and exfoliated g-CN. Figure S5: XPS survey spectrum of the g-CN/TiO₂ (3:1) photocatalyst. Figure S6: High resolution XPS spectra of (a) C 1s, (b) N 1s, (c) Ti 2p, and (d) O 1s for the g-CN/TiO₂ (3:1) photocatalyst. Figure S7: Zeta potentials of g-CN and g-CN/TiO₂ (3:1) photocatalysts as a function of pH. Figure S8: UV-vis spectrum changes of Orange II during its photodegradation by the g-CN/TiO₂ (3:1) composite. Figure S9: Pseudo-first-order kinetics fitted curves of Orange II degradation over TiO₂, g-CN, and g-CN/TiO₂ composites under visible light irradiation. Figure S10: Pseudo-first-order kinetics fitted curves of tetracycline degradation over TiO₂, g-CN, and the g-CN/TiO₂ (3:1) composite under visible light irradiation. Figure S11: UV-vis spectrum changes of tetracycline during its photodegradation by the g-CN/TiO₂ (3:1) composite. Figure S12: XRD patterns of the g-CN/TiO₂ (3:1) catalyst after synthesis (black line) and after 5 reuses for the degradation of the Orange II dye (red line).

**Author Contributions:** Conceptualization, R.S. and B.U.; methodology, R.S., B.T., and B.C.; validation, B.T., B.C., T.G., L.B., E.G. and R.S.; investigation, B.T., B.C., L.B., T.G., E.G., and G.M.; writing—original draft preparation, B.T. and B.C.; writing—review and editing, B.T., B.C., and R.S.; project administration, R.S. and B.U.; funding acquisition, B.U. and R.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors thank the Bolashak International Scholarship of JSC "Center for International Programs" for financial support.

**Conflicts of Interest:** The authors declare no conflict of interest.
References

1. Fu, J.; Yu, J.; Jiang, C.; Cheng, B. g-C₃N₄-Based Heterostructured Photocatalysts. *Adv. Energy Mater.* **2018**, *8*, 1701503. [CrossRef]

2. Li, H.; Zhou, Y.; Tu, W.; Ye, J.; Zou, Z. State-of-the-Art Progress in Diverse Heterostructured Photocatalysts toward Promoting Photocatalytic Performance. *Adv. Funct. Mater.* **2015**, *25*, 998–1013. [CrossRef]

3. Wang, Y.; Wang, Q.; Zhan, X.; Wang, F.; Safdar, M.; He, J. Visible light driven type II heterostructures and their enhanced photocatalysis properties: A review. *Nanoscale* **2013**, *5*, 8326–8339. [CrossRef] [PubMed]

4. Zhang, P.; Lou, X.W. Design of Heterostructured Hollow Photocatalysts for Solar-to-Chemical Energy Conversion. *Adv. Mater.* **2019**, *31*, 1900281. [CrossRef] [PubMed]

5. Li, X.; Yu, J.; Low, J.; Fang, Y.; Xiao, J.; Chen, X. Engineering heterogeneous semiconductors for solar water splitting. *J. Mater. Chem. A* **2015**, *3*, 2485–2534. [CrossRef]

6. Shayegen, Z.; Lee, C.-S.; Haghighat, F. TiO₂ photocatalyst for removal of volatile organic compounds in gas phase–A review. *Chem. Eng. J.* **2018**, *334*, 2408–2439. [CrossRef]

7. Low, J.; Cheng, B.; Yu, J. Surface modification and enhanced photocatalytic CO₂ reduction performance of TiO₂: A review. *Appl. Surf. Sci.* **2017**, *392*, 658–686. [CrossRef]

8. Kumaravel, V.; Mathew, S.; Bartlett, J.; Pillai, S.C. Photocatalytic hydrogen production using metal doped TiO₂: A review of recent advances. *Appl. Catal. B Environ.* **2019**, *244*, 1021–1064. [CrossRef]

9. Ge, M.; Cai, J.; Iocozzia, J.; Cao, C.; Huang, J.; Zhang, X.; Shen, J.; Wang, S.; Zhang, S.; Zhang, K.-Q.; et al. A review of TiO₂ nanostructured catalysts for sustainable H₂ generation. *Int. J. Hydrog. Energy* **2017**, *42*, 8418–8449. [CrossRef]

10. Fagan, R.; McCormack, D.E.; Dionysiou, D.P.; Pillai, S.C. A review of solar and visible light active TiO₂ photocatalysis for treating bacteria, cyanotoxins and contaminants of emerging concern. *Mater. Sci. Semicond. Process.* **2016**, *42*, 2–14. [CrossRef]

11. Shwetharani, R.; Sakar, M.; Fernando, C.A.N.; Binas, V.; Balakrishna, R.G. Recent advances and strategies to tailor the energy levels, active sites and electron mobility in titania and its doped/composite analogues for hydrogen evolution in sunlight. *Catal. Sci. Technol.* **2019**, *9*, 12–49. [CrossRef]

12. Wen, J.; Xie, J.; Chen, X.; Li, X. A review on g-C₃N₄-based photocatalysts. *Appl. Surf. Sci.* **2017**, *391*, 72–123. [CrossRef]

13. Masih, D.; Ma, Y.; Rohani, S. Graphitic C₃N₄ based noble-metal-free photocatalyst systems: A review. *Appl. Catal. B Environ.* **2017**, *206*, 556–588. [CrossRef]

14. Zhang, S.; Gu, P.; Ma, R.; Luo, C.; Wen, T.; Zhao, G.; Cheng, W.; Wang, X. Recent developments in fabrication and structure regulation of visible-light-driven g-C₃N₄-based photocatalysts towards water purification: A critical review. *Catal. Today* **2019**, *335*, 65–77. [CrossRef]

15. Moussa, H.; Chouchene, B.; Gries, T.; Balan, L.; Mozet, K.; Medjahdi, G.; Schneider, R. Growth of ZnO Nanorods on Graphitic Carbon Nitride gCN Sheets for the Preparation of Photocatalysts with High Visible-Light Activity. *ChemCatChem* **2018**, *10*, 4973–4983. [CrossRef]

16. Ouedraogo, S.; Chouchene, B.; Desmarets, C.; Gries, T.; Balan, L.; Fournet, R.; Medjahdi, G.; Bayo, K.; Schneider, R. Copper octacarboxyphthalocyanine as sensitizer of graphitic carbon nitride for efficient dye degradation under visible light irradiation. *Appl. Catal. A Gen.* **2018**, *563*, 127–136. [CrossRef]

17. Obregon, S.; Colon, G. Improved H₂ production of Pt-TiO₂/g-C₃N₄-MnOx composites by an efficient handling of photogenerated charge pairs. *Appl. Catal. B Environ.* **2014**, *144*, 775–782. [CrossRef]

18. Elbanna, O.; Fujitsuka, M.; Majima, T. g-C₃N₄/TiO₂ Mesocrystals Composite for H₂ Evolution under Visible-Light Irradiation and Its Charge Carrier Dynamics. *ACS Appl. Mater. Interfaces* **2017**, *9*, 34844–34854. [CrossRef]

19. Troppova, I.; Sihar, M.; Rehi, M.; Ritz, M.; Praus, P.; Koci, K. Unconventionally prepared TiO₂/g-C₃N₄ photocatalysts for photocatalytic decomposition of nitrous oxide. *Appl. Surf. Sci.* **2018**, *430*, 335–347. [CrossRef]

20. Sadanandam, G.; Zhang, L.; Scurrell, M.S. Enhanced photocatalytic hydrogen formation over Fe-loaded TiO₂ and g-C₃N₄ composites from mixed glycerol and water by solar irradiation. *J. Renew. Sustain. Energy* **2018**, *10*, 034703. [CrossRef]
21. Hafeez, H.Y.; Lakhara, S.K.; Bellamkonda, S.; Rao, G.R.; Shankar, M.V.; Bahnemann, D.W.; Nappolian, B. Construction of ternary hybrid layered reduced graphene oxide supported g-C₃N₄-TiO₂ nanocomposite and its photocatalytic hydrogen production activity. *Int. J. Hydrog. Energy* **2018**, *43*, 3892–3904. [CrossRef]

22. Han, C.; Wang, Y.; Lei, Y.; Wang, B.; Wu, N.; Shi, Q.; Li, Q. In situ synthesis of graphitic-C₃N₄ nanosheet hybridized N-doped TiO₂ nanofibers for efficient photocatalytic H₂ production and degradation. *Nano Res.* **2015**, *8*, 1199–1209. [CrossRef]

23. Huang, Z.; Sun, Q.; Lv, K.; Zhang, Z.; Li, M.; Li, B. Effect of contact interface between TiO₂ and g-C₃N₄ on the photoreactivity of g-C₃N₄/TiO₂ photocatalyst: (0 0 1) vs (1 0 1) facets of TiO₂. *Appl. Catal. B Environ.* **2015**, *164*, 420–427. [CrossRef]

24. Ma, J.; Tan, X.; Yu, T.; Li, X. Fabrication of g-C₃N₄/TiO₂ hierarchical spheres with reactive {001} TiO₂ crystal facets and its visible-light photocatalytic activity. *Int. J. Hydrog. Energy* **2016**, *41*, 3877–3887. [CrossRef]

25. Pan, J.; You, M.; Chi, C.; Dong, Z.; Wang, B.; Zhu, M.; Zhao, W.; Song, C.; Zhang, Y.; Li, C. The two dimension carbon quantum dots modified porous g-C₃N₄/TiO₂ nano-heterojunctions for visible light hydrogen production enhancement. *Int. J. Hydrog. Energy* **2018**, *43*, 6586–6593. [CrossRef]

26. Hao, R.; Wang, G.; Tang, H.; Sun, L.; Xu, C.; Han, D. Template-free preparation of macro/mesoporous g-C₃N₄/TiO₂ heterojunction photocatalysts with enhanced visible light photocatalytic activity. *Appl. Catal. B Environ.* **2016**, *187*, 47–58. [CrossRef]

27. Giannakopoulo, T.; Papoilias, I.; Todorova, N.; Boukos, N.; Liu, Y.; Yu, J.; Trapalis, C. Tailoring the energy band gap and edges' potentials of g-C₃N₄/TiO₂ composite photocatalysts for NOx removal. *Chem. Eng. J.* **2017**, *310*, 571–580. [CrossRef]

28. Li, J.; Zhang, M.; Li, X.; Li, Q.; Yang, J. Effect of the calcination temperature on the visible light photocatalytic activity of direct contact Z-scheme g-C₃N₄-TiO₂ heterojunction. *Appl. Catal. B Environ.* **2017**, *212*, 106–114. [CrossRef]

29. Liu, C.; Wang, F.; Zhang, J.; Wang, K.; Qiu, Y.; Liang, Q.; Chen, Z. Efficient Photoelectrochemical Water Splitting by g-C₃N₄/TiO₂ Nanotube Array Heterostructures. *Nano Micro Lett.* **2018**, *10*, 37. [CrossRef]

30. Wang, Y.; Yang, W.; Chen, X.; Wang, J.; Zhu, Y. Photocatalytic activity enhancement of core-shell structure g-C₃N₄@TiO₂ via controlled ultrathin g-C₃N₄ layer. *Appl. Catal. B Environ.* **2018**, *220*, 337–347. [CrossRef]

31. Sheng, Y.; Wai, Z.; Miao, H.; Yao, W.; Li, H.; Zhu, Y. Enhanced organic pollutant photodegradation via adsorption/photocatalysis synergy using a 3D g-C₃N₄/TiO₂ free-separation photocatalyst. *Chem. Eng. J.* **2019**, *370*, 287–294. [CrossRef]

32. Li, C.; Lou, Z.; Yang, Y.; Wang, Y.; Lu, Y.; Ye, Z.; Zhu, L. Hollow sphere Nanoheterojunction of g-C₃N₄@TiO₂ with High Visible Light Photocatalytic Property. *Langmuir* **2019**, *35*, 779–786. [CrossRef]

33. Tan, Y.; Shu, Z.; Zhou, J.; Li, T.; Wang, W.; Zhao, Z. One-step synthesis of nanostructured g-C₃N₄/TiO₂ composite for highly enhanced visible-light photocatalytic H₂ evolution. *Appl. Catal. B Environ.* **2018**, *230*, 260–268. [CrossRef]

34. Yu, S.; Wang, Y.; Sun, F.; Wang, R.; Zhou, Y. Novel mpg-C₃N₄/TiO₂ nanocomposite photocatalytic membrane reactor for sulfamethoxazole photodegradation. *Chem. Eng. J.* **2018**, *33*, 183–192. [CrossRef]

35. Zhan, W.; Sun, L.; Han, X. Recent progress on engineering highly efficient porous semiconductor photocatalysts derived from Metal–organic frameworks. *Nano-Micro Lett.* **2019**, *11*, 1. [CrossRef] [PubMed]

36. Ma, B.; Guan, P.-Y.; Li, Q.-Y.; Zhang, M.; Zang, S.-Q. MOF-derived flower-like MoS₂@TiO₂ nanohybrids with enhanced activity for hydrogen evolution. *ACS Appl. Mater. Interfaces* **2016**, *8*, 26794–26800. [CrossRef]

37. Gu, Y.; Cheng, K.; Wu, Y.-N.; Wang, Y.; Morlay, C.; Li, F. Metal–organic framework-templated synthesis of bifunctional N-doped TiO₂–carbon nanotablets via solid-state thermolysis. *ACS Sustain. Chem. Eng.* **2016**, *4*, 6744–6753. [CrossRef]

38. Yan, B.; Zhang, L.; Tang, Z.; Al-Mamun, M.; Zhao, H.; Su, X. Palladium-decorated hierarchical titania constructed from the metal-organic frameworks NH₂-MIL-125(Ti) as a robust photocatalyst for hydrogen evolution. *Appl. Catal. B Environ.* **2017**, *218*, 743–750. [CrossRef]

39. Dan-Hardi, M.; Serre, C.; Frot, T.; Rozes, L.; Maurin, G.; Sanchez, C.; Férey, G. A New Photoactive Crystalline Highly Porous Titanium(IV) Dicarboxylate. *J. Am. Chem. Soc.* **2009**, *131*, 10857–10859. [CrossRef]

40. Dou, J.; Li, Y.; Xie, F.; Ding, X.; Wei, M. Metal–Organic Framework Derived Hierarchical Porous Anatase TiO₂ as a Photoanode for Dye-Sensitized Solar Cell. *Cryst. Growth Des.* **2016**, *16*, 121–125. [CrossRef]
41. Xiu, Z.; Alfaruqi, M.H.; Gim, J.; Song, J.; Kim, S.; Thi, T.V.; Duong, P.T.; Baboo, J.P.; Mathew, V.; Kim, J. Hierarchical porous anatase TiO₂ derived from a titanium metal-organic framework as a superior anode material for lithium ion batteries. *Chem. Commun.* **2015**, *51*, 12274–12277. [CrossRef] [PubMed]

42. Guo, Z.; Cheng, J.K.; Hu, Z.; Zhang, M.; Xu, Q.; Kang, Z.; Zhao, D. Metal-organic frameworks (MOFs) as precursors towards TiOx/C composites for photodegradation of organic dye. *RSC Adv.* **2014**, *4*, 34221–34225. [CrossRef]

43. Khaletskaya, K.; Pugin, A.; Medishetty, R.; Rösler, C.; Wiktor, C.; Strunk, J.; Fischer, R.A. Fabrication of Gold/Titania Photocatalyst for CO₂ Reduction Based on Pyrolytic Conversion of the Metal–Organic Framework NH2-MIL-125(Ti) Loaded with Gold Nanoparticles. *Chem. Mater.* **2015**, *27*, 7248–7257. [CrossRef]

44. Ye, F.; Li, H.; Yu, H.; Chen, S.; Quan, X. Hydrothermal fabrication of few-layer MoS₂ nanosheets within nanopores on TiO₂ derived from MIL-125(Ti) for efficient photocatalytic H₂ evolution. *Appl. Surf. Sci.* **2017**, *426*, 177–184. [CrossRef]

45. Liu, Z.; Wu, Y.; Chen, J.; Li, Y.; Zhao, J.; Gao, K.; Na, P. Effective elimination of As(iii) via simultaneous photocatalytic oxidation and adsorption by a bifunctional cake-like TiO₂ derived from MIL-125(Ti). *Catal. Sci. Technol.* **2018**, *8*, 1936–1944. [CrossRef]

46. Yuan, S.; Gao, Y.; Bao, J.; Zhang, Y.; Chen, W.; Fang, J.; Zhou, Y.; Wang, Y.; Zhu, W. Preparation of disk-like Pt/CeO₂-p-TiO₂ catalyst derived from MIL-125(Ti) for excellent catalytic performance. *Appl. Organomet. Chem.* **2018**, *32*, e4395. [CrossRef]

47. Gao, K.; Chen, J.; Liu, Z.; Li, Y.; Wu, Y.; Zhao, J.; Na, P. Intensified redox co-conversion of As(III) and Cr(VI) with MIL-125(Ti)-derived COOH functionalized TiO₂: Performance and mechanism. *Chem. Eng. J.* **2019**, *360*, 1223–1232. [CrossRef]

48. Valero-Romero, M.J.; Santaclara, J.G.; Oar-Arteta, L.; van Koppen, L.; Osadchii, D.Y.; Gascon, J.; Kapteijn, F. Photocatalytic properties of TiO₂ and Fe-doped TiO₂ prepared by metal organic framework-mediated synthesis. *Chem. Eng. J.* **2019**, *360*, 75–88. [CrossRef]

49. Hu, S.; Liu, M.; Li, K.; Zuo, Y.; Zhang, A.; Song, C.; Zhang, G.; Guo, X. Solvothermal synthesis of NH₂-MIL-125(Ti) from circular plate to octahedron. *CrystEngComm* **2014**, *16*, 9645–9650. [CrossRef]

50. Pawar, R.C.; Kang, S.; Park, J.H.; Kim, J.-h.; Ahn, S.; Lee, C.S. Room-temperature synthesis of nanoporous 1D microrods of graphitic carbon nitride (g-C₃N₄) with highly enhanced photocatalytic activity and stability. *Sci. Rep.* **2016**, *6*, 31147. [CrossRef]

51. Music, S.; Gotic, M.; Ivanda, M.; Popovic, S.; Twkovic, A.; Trojko, R.; Sekulic, A.; Furic, K. Chemical and micro structural properties of TiO₂ synthesized by sol-gel procedure. *Mater. Sci. Eng. B* **1997**, *47*, 33–40. [CrossRef]

52. Ohsaka, T.; Izumi, F.; Fujiki, Y. Raman spectrum of anatase, TiO₂. *J. Raman Spectrosc.* **1978**, *7*, 321–324. [CrossRef]

53. Jiang, J.; Ou-yang, L.; Zhu, L.; Zheng, A.; Zou, J.; Yi, X.; Tang, H. Dependence of electronic structure of g-C₃N₄ on the layer number of its nanosheets: A study by Raman spectroscopy coupled with first-principles calculations. *Carbon* **2014**, *80*, 213–221. [CrossRef]

54. Raziq, F.; Qu, Y.; Humayun, M.; Zada, A.; Yu, H.; Jing, L. Synthesis of SnO₂/B-P codoped g-C₃N₄ nanocomposites as efficient cocatalyst-free visible-light photocatalysts for CO₂ conversion and pollutant degradation. *Appl. Catal. B Environ.* **2017**, *201*, 486–494. [CrossRef]

55. Qiu, Y.; Xin, L.; Jia, F.; Xie, J.; Li, W. Three-Dimensional Phosphorus-Doped Graphitic-C₃N₄ Self-Assembly with NH₂-Functionalized Carbon Composite Materials for Enhanced Oxygen Reduction Reaction. *Langmuir* **2016**, *32*, 12569–12578. [CrossRef]

56. Sun, P.; Xu, R.; Zhang, W.; Zada, I.; Liu, Q.; Gu, J.; Su, H.; Zhang, Z.; Zhang, J.; Zhang, D. Photocatalyst of organic pollutants decomposition: TiO₂/glass fiber cloth composites. *Catal. Today* **2016**, *274*, 2–7. [CrossRef]

57. Irie, H.; Watanabe, Y.; Hashimoto, K. Carbon-doped Anatase TiO₂ powders as a visible-light sensitive photocatalyst. *Chem. Lett.* **2003**, *32*, 772–773. [CrossRef]

58. Zhu, B.; Xia, P.; Ho, W.; Yu, J. Isoelectric point and adsorption activity of porous g-C₃N₄. *Appl. Surf. Sci.* **2015**, *344*, 188–195. [CrossRef]

59. Mandzya, N.; Grulke, E.; Druffel, T. Breakage of TiO₂ agglomerates in electrostatically stabilized aqueous dispersions. *Powder Technol.* **2005**, *160*, 121–126. [CrossRef]

60. Leong, S.; Li, D.; Hapgood, K.; Zhang, X.; Wang, H. Ni(OH)₂ decorated rutile TiO₂ for efficient removal of tetracycline from wastewater. *Appl. Catal. B Environ.* **2016**, *198*, 224–233. [CrossRef]
61. Cetocioglu, Z.; Ince, B.; Gros, M.; Rodriguez-Mozaz, S.; Barcelo, D.; Orhon, D.; Ince, O. Chronic impact of tetracycline on the biodegradation of an organic substrate mixture under anaerobic conditions. *Water. Res.* **2013**, *47*, 2959–2969. [CrossRef] [PubMed]

62. Hsu, L.-C.; Liu, Y.-T.; Syu, C.-H.; Huang, M.-H.; Tzou, Y.-M.; Teah, H.Y. Adsorption of tetracycline on Fe (hydr)oxides: Effects of pH and metal cation ($Cu^{2+}$, $Zn^{2+}$ and $Al^{3+}$) addition in various molar ratios. *R. Soc. Open sci.* **2018**, *5*, 171941. [CrossRef] [PubMed]

63. Jiang, L.; Yuan, X.; Zeng, G.; Chen, X.; Wu, Z.; Liang, J.; Zhang, J.; Wang, H.; Wang, H. Phosphorus- and Sulfur-Codoped $g-C_3N_4$: Facile Preparation, Mechanism Insight, and Application as Efficient Photocatalyst for Tetracycline and Methyl Orange Degradation under Visible Light Irradiation. *ACS Sustain. Chem. Eng.* **2017**, *5*, 5831–5841. [CrossRef]

64. Panneri, S.; Ganguly, P.; Mohan, M.; Nair, B.N.; Peer Mohamed, A.A.; Warrier, K.G.; Hareesh, U.S. Photoregenerable, Bifunctional Granules of Carbon-Doped $g-C_3N_4$ as Adsorptive Photocatalyst for the Efficient Removal of Tetracycline Antibiotic. *ACS Sustain. Chem. Eng.* **2017**, *5*, 1610–1618. [CrossRef]

65. Jiang, L.; Yuan, X.; Zeng, G.; Liang, J.; Chen, X.; Yu, H.; Wang, H.; Wu, Z.; Zhang, J.; Xiong, T. In-situ synthesis of direct solid-state dual Z-scheme $WO_3/g-C_3N_4/Bi_2O_3$ photocatalyst for the degradation of refractory pollutant. *Appl. Catal. B Environ.* **2018**, *227*, 376–385. [CrossRef]

66. Xu, Y.; Schoonen, M.A.A. The absolute energy positions of conduction and valence bands of selected semiconducting minerals. *Am. Miner.* **2000**, *85*, 543–556. [CrossRef]

67. Xu, F.; Zhang, L.; Cheng, B.; Yu, J. Direct Z-scheme $TiO_2/NiS$ Core-Shell Hybrid Nanofibers with Enhanced Photocatalytic $H_2$-Production Activity. *ACS Sustain. Chem. Eng.* **2018**, *6*, 12291–12298. [CrossRef]

68. Liu, M.; Chen, Y.; Su, J.; Shi, J.; Wang, X.; Guo, L. Photocatalytic hydrogen production using twinned nanocrystals and an unanchored $NiS_x$ co-catalyst. *Nat. Energy* **2016**, *1*, 16151. [CrossRef]

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