# Tuning CH4 Productivity from Visible Light-Driven Gas-Phase CO2 Photocatalytic Reduction on Doped g-C3N4/TiO2 Heterojunctions

Leila Hammoud, Clément Marchal, Christophe Colbeau-Justin, Joumana

Toufaily, Tayssir Hamieh, Valérie Caps, Valérie Keller

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## ► To cite this version:

Leila Hammoud, Clément Marchal, Christophe Colbeau-Justin, Joumana Toufaily, Tayssir Hamieh, et al.. Tuning CH4 Productivity from Visible Light-Driven Gas-Phase CO2 Photocatalytic Reduction on Doped g-C3N4/TiO2 Heterojunctions. Energy Technology, 2023, 11, 10.1002/ente.202201363. hal-04245794

HAL Id: hal-04245794

https://hal.science/hal-04245794v1

Submitted on 17 Oct 2023

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# Tuning CH₄ Productivity from Visible Light-Driven Gas-Phase CO₂ Photocatalytic Reduction on Doped g-C₃N₄/TiO₂ Heterojunctions

Leila Hammoud, Clément Marchal, Christophe Colbeau-Justin, Joumana Toufaily, Tayssir Hamieh, Valérie Caps, and Valérie Keller*

Herein, visible light-driven gas-phase photocatalytic CO₂ reduction into CH₄ is tuned by designing optimized three-component Au/doped C₃N₄/TiO₂ composite photocatalysts. The key point strategy consists in the formation of high-quality C₃N₄/TiO₂ heterojunction by associating low containing doped graphitic carbon nitride to commercially available TiO₂ UV-100. Those heterojunctions result in both visible light sensitization and increased charge-carrier separation. Further deposition of small Au nanoparticles (≈3 nm), quite exclusively onto TiO₂ surfaces, mainly acts as electron trapping/cocatalytic functions without excluding surface plasmonic effects. The resulting doped g-C₃N₄ material exhibits enhanced visible light harvesting properties, especially in the case of C-doping. In addition, it is assumed that B- and C-C₃N₄ doping, leading to a more or less lower conduction band position, is the impacting factor toward total CH₄ selectivity achievement. The (0.77 wt%)Au/(0.59 wt%)C-C₃N₄/TiO₂ composite photocatalyst, exhibiting the best compromise between the various impacting factors, leads to a continuous productivity rate of CH₄ of 8.5 µmol h⁻¹ g⁻¹ under visible light irradiation over at least 10 h. To the best of knowledge, this level of performance is unprecedented under continuous gas-phase flowing CO₂ in the presence of water as reducing agent, without addition of any sacrificial agent.

## 1. Introduction

In the frame of CO₂ transformation into clean fuels which could be an interesting alternative both to global warming limitation and depletion of fossil fuels, CO₂ photocatalytic reduction is considered as being a promising technology for converting CO₂ into

light-driven alcohols and hydrocarbons, as reported in several reviews summing up the state-of-the-art on that topic.[1-3] Indeed, this green alternative has received great and increased attention since the work dealing with photoelectrocatalytic reduction of CO₂ by Inoue et al.[4] However, CO₂ is one of the most stable molecules. Inspired by the natural photo-synthesis process in which reduction of CO₂ occurs with H₂O using solar irradiation, water is considered as being the most suitable hydrogen donor (mainly implied as H⁺ donor in CO₂ reduction mechanism) for CO₂ artificial photocatalytic reduction. Its main drawback is that it easily recombines into H₂, consuming photogenerated electrons thus lowering carbon-based valuable compounds yields formation.[5] Indeed, considering that the redox potential for CO₂ reduction is close to that for H₂O reduction to H₂ (however slightly more favorable), that the activation of H₂O is easier than that of CO₂, and that the formation of CH₄ needs eight electrons

(instead of two for H₂ production), the reduction of CO₂ is kinetically less favored. Consequently, different strategies can be carried out in order to drive selective production of CH₄, compared to H₂, such as suitable cocatalyst deposition,[6] use of plasmonic metal-based nanostructures, use of semiconductors able to generate electrons with appropriate reduction potential, working with

L. Hammoud, C. Marchal, V. Caps, V. Keller

ICPEES

Institut de Chimie et des Procédés pour l'Energie, l'Environnement et la Santé

CNRS/Université de Strasbourg

UMR 7515 (CNRS)

25 rue Becquerel, 67087 Strasbourg, Cedex, France

E-mail: vkeller@unistra.fr

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/ente.202201363.

© 2023 The Authors. Energy Technology published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/ente.202201363

C. Colbeau-Justin

ICP

Institut de Chimie Physique, CNRS UMR 8000

Université Paris-Saclay

Bâtiment 349, 91405 Orsay Cedex, France

J. Toufaily, T. Hamieh

MCEMA, Laboratory of Materials, Catalysis, Environment and Analytical Methods

Lebanese University Hadath

Campus Rafic Hariri, Bayrouth, Lebanon

T. Hamieh

Faculty of Science and Engineering

Universiteit Maastricht

P.O. Box 616, 6200 ED Maastricht, The Netherlands
continuous CO₂ gas-phase flow with increased contact time to
allow pathways involving several electron and H⁺ transfers, using
high surface area materials increasing CO₂ adsorption and
bandgap engineering for visible light harvesting, as well as heter-
ojunction formation for enhanced charge-carrier separation.[7-9]
Since a few years a large and increased number of semiconductors
and some metal sulfides have been reported for photocatalytic CO₂
reduction with water. However, the photocatalytic activity, stability,
and product selectivity are far away from interesting consideration.
Among a large variety of semiconductors investigated for photo-
catalytic CO₂ reduction, it was concluded that TiO₂-based photo-
catalysts exhibit the best activity, taking into consideration
chemical stability, and mainly moderate cost.[10] TiO₂-heterojunc-
tion photocatalysts appear as a promising strategy. Numerous
studies are dealing with TiO₂-SC₂ heterojunctions. Several of them
relating the successful formation the TiO₂/g-C₃N₄ show that the
photoactivity for environmental or solar fuels applications can be
successfully promoted, thanks to enhanced visible light absorption
and charge-carrier separation.[11-14] In the last years, g-C₃N₄ has
been shown as being an effective semiconductor to facilitate pho-
tocatalytic CO₂ reduction because of its high conduction band
energy, thus enabling the CO₂ reduction half-reaction.[15]
However, it has been observed that the selectivity toward a desired
reduction product is affected by the nanostructure design and the
choice of the associated photoactive component in the g-C₃N₄-
based nanocomposites.[16,17] Despite its visible light sensitization
effect, and appropriate positions of CB and VB in respect to H₂O
oxidation and CO₂ reduction into CH₄, g-C₃N₄ suffers from poor
utilization of the photoexcited charge carriers and often from low
CO₂ adsorption, resulting mainly from deficiency of electron
donation sites and unappropriate electronic structure.[18]
Furthermore, low efficient interlayer charge transfer also results
in increased recombination rate.[19,20] To overcome some of these
limitations and improve the photocatalytic performance, several
protocols are proposed, such as nanostructuring,[21] doping by
metals and nonmetals,[12,22] exfoliation,[23] controllable morpholo-
gies with different geometric shapes,[24] and copolymerization.[25]
Doping of graphitic carbon nitride with nonmetal element
modulates its surface morphology, electronic structure, size of
particles, and optical and physiochemical properties.[13] This non-
metal incorporation has an essential effect in tuning the band
structure, extending absorption of visible irradiation and creating
more active sites.[26]

One approach to solar light harvesting, namely, deposition of
metal nanoparticles (NPs) exhibiting surface plasmon-induced
effect (SPIE), has attracted great attention in the recent last
years.[27-29] Among several metals used for driving SPIE effects
in photocatalytic applications, Au and Ag are the most used due
to their high-quality factors in the visible light.[27] Furthermore,
Au NPs present the advantages of high inertness in numerous
environments and easy surface modification. These interesting
optical properties, coupled with catalytically active and redox-
mediator properties, might yield beneficial effects on CO₂ pho-
toreduction reactions.[3,30,31] Specifically, Au NPs can exhibit a
broad surface plasmon band in the visible depending on several
parameters such as average particles size and shape, interaction
with the support, dielectric constant of the environment, etc.
Due to earlier interest in Au NPs for catalysis applications,
several methods for Au/TiO₂ and Au/C₃N₄ preparation are well

described in the literature, such as deposition-precipitation, col-
loidal deposition, and impregnation-reduction.[32-35]

Here, we report a facile and one-step method for the prepara-
tion of (S, C, B)-doped g-C₃N₄/TiO₂ high surface area heterojunc-
tions. A second-step deposition of Au NPs (<1 wt%) resulted in a
three-component photocatalytic materials. Playing with the
g-C₃N₄ doping element, the quality of the g-C₃N₄/TiO₂ hetero-
junction and the Au deposition specificity, allowed to tune CH₄
selectivity and productivity. The optimized Au/gC₃N₄-B/TiO₂
and Au/gC₃N₄-C/TiO₂ led, respectively, to total CH₄ selectivity
and to a mean productivity rate of 8.5 µmol h⁻¹ g⁻¹ over at least
10h. To the best of our knowledge, this level of performance is
unprecedented under continuous gas-phase flowing CO₂ in the
presence of water vapor and under visible light irradiation, with-
out addition of any sacrificial agent.

2. Results and Discussion

2.1. Thermal Stability (TGA)

TGA profiles of CN(−X), represented in **Figure 1**, top, show first a
slight weight loss observed at roughly 200 °C, attributed to

<figure><img src="image_3.png" /><figcaption>**Figure 1.** TGA analysis of (top) CN–X. (Bottom) CN(–X)/TiO₂ UV-100 compared to reference TiO₂ UV-100 (550 °C).</figcaption></figure>
<table><caption>**Table 1.** TiO₂ anatase mean crystallite size (from XRD), doping element (B, S) content (wt%)*, and Au content (from ICP analyses).</caption><thead><tr><th></th><th>Anatase mean<br>cryst. size [nm]</th><th>B [wt%]</th><th>S [wt%]</th><th>g-C<sub>3</sub>N<sub>4</sub><br>[wt% (exp)]</th><th>Au<br>[wt% (exp)]</th></tr></thead><tbody><tr><td>TiO<sub>2</sub> UV-100</td><td>9 ± 1</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Au/TiO<sub>2</sub> [550 °C]</td><td>18 ± 1</td><td>-</td><td>-</td><td>0.77 ± 0.01</td><td>-</td></tr><tr><td>Au/CN/TiO<sub>2</sub></td><td>15 ± 1</td><td>-</td><td>-</td><td>0.72 ± 0.01</td><td>0.77 ± 0.01</td></tr><tr><td>Au/CN-C/TiO<sub>2</sub></td><td>14 ± 1</td><td>-</td><td>-</td><td>0.59 ± 0.01</td><td>0.77 ± 0.01</td></tr><tr><td>Au/CN-B/TiO<sub>2</sub></td><td>15 ± 1</td><td>0.06 ± 0.01</td><td>-</td><td>0.55 ± 0.01</td><td>0.74 ± 0.01</td></tr><tr><td>Au/CN-S/TiO<sub>2</sub></td><td>15 ± 1</td><td>-</td><td>0.03 ± 0.01</td><td>0.54 ± 0.01</td><td>0.77 ± 0.01</td></tr></tbody></table>

elimination of adsorbed water,[36] followed by a classical and rapid one-pot decomposition starting at 550 °C and achieved at 650 °C, due to s-triazine or tris-s-triazine units burning.[37] Doping results in a slightly higher water elimination contribution and to a shift to lower temperatures for the beginning of decomposition (this phenomenon is a little bit more pronounced for B-doping for which the corresponding temperature is decreased to ≈460 °C), whereas the end of decomposition is quite not affected. These slight differences in terms of thermal stability may be attributed to different crystallinity/crystal thickness.[38]

From TGA analyses of CN(–X)/TiO₂ samples (Figure 1, bottom), one can deduce the g-C₃N₄ content (wt%) of the composite materials (Table 1). The first region of weight loss can be attributed to water molecules desorption and/or to TiO₂ dehydroxylation. One can, however, note that this first mass loss is more pronounced on CN–C-based composite. It must also be underlined that the total graphitic carbon nitride contents are low, between 0.54 and 0.72 wt%, whatever the samples. Although the presence of TiO₂ NPs on g-C₃N₄ generally lowers its temperature of combustion,[39] in our case, the association of TiO₂ and C₃N₄ somehow resulted in a drastic shift of the beginning of C₃N₄ decomposition toward higher temperatures, starting at ≈680 and 700 °C, respectively. The presence of TiO₂ thus prevents the 2D g-C₃N₄ structure from combustion. One can assume that this shift toward higher temperatures is due to strong interactions between TiO₂ NPs and graphitic carbon nitride sheets.

## 2.2. Structural Characterization (XRD)

The X-ray diffraction (XRD) pattern of the bare (uncalcined) TiO₂ UV-100 (Figure S2, top, Supporting Information) exhibited a major peak at 25.4° assigned to (101) planes of the anatase phase in agreement with JCPDS file 21-1276. Further treatment at 550 °C in air did not lead to the appearance of other crystallographic phases. However, an increase in crystallite mean particle size is observed (from 9 to 18 nm), due to sintering phenomena. One can mention that this sintering effect of TiO₂ crystallites is hindered within the composite materials during the synthesis and the formation of g-C₃N₄ nanosheets upon polycondensation at 550 °C, confirming strong interactions between both semiconductors even at those low graphitic carbon nitride contents, as previously observed.[40] As a result of the minor g-C₃N₄ content (max. 0.72 wt%), XRD patterns showed no obvious diffraction peak (Figure S2, Supporting Information) attributable to crystallized g-C₃N₄. This observation is in good agreement with

other studies showing that at least 30 wt% of g-C₃N₄ is necessary to observe the corresponding diffraction lines in the XRD pattern of TiO₂-gC₃N₄ composites.[1,13] In the same manner, no diffraction peak related to crystallized Au was detected, resulting from too small Au loading (0.76 ± 0.02 wt%) and/or crystallite size (Figure S2, bottom, Supporting Information).

## 2.3. Surface Characterizations

### 2.3.1. BET Surface Area and Porosity Measurements

From BET measurements, one can observe that CN(–X) materials all exhibited type-II adsorption–desorption isotherms profiles (Figure S3, top, Supporting Information) related to macroporous solids with strong interactions between adsorbed nitrogen and the pores walls. They showed a large but monomodal pore size distribution (Figure 2) (as derived from Barrett, Joyner et Halenda (BJH) methods) centered at ≈33–38 nm, except for S-doped g-C₃N₄, which exhibited a somewhat larger mean pore diameter of ≈51 nm (Figure 3, top, Supporting Information). Bare g-C₃N₄ revealed the highest surface area of 171 m² g⁻¹, among all the studied samples, and the highest pore volume. Both surface areas and pore volume decreased upon doping

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

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Mean pore size distribution of (top) CN–X samples. (Bottom) CN–X/TiO₂ and Au/TiO₂ UV-100 (550 °C) samples.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 3.** UV-vis absorbance spectra of (top) CN-X/TiO₂ and reference TiO₂ UV-100 (550 °C) (bottom) Au/CN-X/TiO₂ and Au/TiO₂ UV-100 (550 °C).</figcaption></figure>

(Table 2). Among the different doped g-C₃N₄, S-doping yielded the largest pore volume and the most important surface area.

For the heterojunction materials, the coexistence of meso-pores and macropores is observed (Table 2). Indeed, pore size distributions of all CN-X/TiO₂ composites exhibited a bimodal

feature close to that observed by TiO₂ (550 °C). Adsorption-desorption isotherm profiles of the composites (Figure S3, bottom, Supporting Information) are also similar to that of TiO₂ (550 °C), showing that the morphology of the low loaded CN-X/TiO₂ heterojunction materials is determined by the major TiO₂ component. Variations in surface areas and pore volumes may thus be attributed to modifications in the principal TiO₂ component. It is interesting to highlight that surface areas and pore volumes of the heterojunctions are significantly higher than those expected from a mere linear combination of TiO₂ 550 °C and CN-X considering their respective weight contents in the composite. This is consistent with inhibition of TiO₂ crystallite growth during CN-X polycondensation (as compared with the growth observed upon raw treatment of TiO₂ UV-100 in air at 550 °C), as shown by XRD.

More specifically, surface areas and pore volumes varied from 85 ± 1 m² g⁻¹ and 0.30 cm³ g⁻¹ for the B- and S-doped heterojunctions up to 112 ± 7 m² g⁻¹ and 0.35 ± 0.01 cm³ g⁻¹ for the undoped and C-doped heterojunctions. This suggests that crystallization and growth of titania are impacted differently by the different conditions of polycondensation, i.e., the presence of the various dopants, and that the B and S-dopants are less efficient in controlling the growth of anatase during the polycondensation process.

### 2.3.2. UV-Vis Absorption Properties

CN-X/TiO₂ composite materials showed photon absorption contribution of both TiO₂ anatase and g-C₃N₄ counterparts, despite the very low content of this second semiconductor (Figure 3 Top). One can observe that the second contribution is more or less pronounced depending on the nonmetal dopant. Doping of g-C₃N₄, except for B element, led to a displacement of the g-C₃N₄ absorption edge toward higher wavelengths, as confirmed by the lowering of bandgap value (Table 2), calculated from Tauc plot using Kubelka-Munk formula (Figure S4, Supporting Information). Furthermore, it is worthy to underline that C-doping resulted in a more intense redshift, and narrowing of the band gap (2.28 eV), followed by S- and B-doped-based C₃N₄. The presence of Au NPs is evidenced by the characteristic surface plasmon resonance (SPR) contribution centered at ≈550 nm (Figure 3, bottom).

<table><caption>**Table 2.** Bandgap values, surface area, total pore volume, and mean pore size diameter.</caption><thead><tr><th></th><th>E<sub>g1</sub> [eV] (TiO<sub>2</sub>)</th><th>E<sub>g2</sub> [eV] CN(-X)</th><th>S<sub>BET</sub> [m<sup>2</sup> g<sup>-1</sup>]</th><th>V<sub>pore</sub> [cm<sup>3</sup> g<sup>-1</sup>]</th><th>Mean pore diameter [nm]</th></tr></thead><tbody><tr><td>TiO<sub>2</sub> UV-100</td><td>3.25 ± 0.02</td><td>—</td><td>315 ± 10</td><td>0.33 ± 0.05</td><td>2–3</td></tr><tr><td>TiO<sub>2</sub> [550 °C]</td><td>3.20 ± 0.02</td><td>—</td><td>65 ± 2</td><td>0.24 ± 0.02</td><td>7–21</td></tr><tr><td>CN</td><td>—</td><td>2.83 ± 0.02</td><td>171 ± 3</td><td>0.94 ± 0.04</td><td>33</td></tr><tr><td>CN/TiO<sub>2</sub></td><td>3.17 ± 0.02</td><td>2.35 ± 0.02</td><td>119 ± 2</td><td>0.36 ± 0.02</td><td>5–21</td></tr><tr><td>CN-C</td><td>—</td><td>2.58 ± 0.02</td><td>115 ± 2</td><td>0.56 ± 0.02</td><td>33</td></tr><tr><td>CN-C/TiO<sub>2</sub></td><td>3.12 ± 0.02</td><td>2.28 ± 0.02</td><td>105 ± 2</td><td>0.34 ± 0.02</td><td>6–21</td></tr><tr><td>CN-B</td><td>—</td><td>2.78 ± 0.02</td><td>104 ± 2</td><td>0.54 ± 0.03</td><td>38</td></tr><tr><td>CN-B/TiO<sub>2</sub></td><td>3.15 ± 0.02</td><td>2.34 ± 0.02</td><td>84 ± 2</td><td>0.29 ± 0.02</td><td>7–21</td></tr><tr><td>CN-S</td><td>—</td><td>2.65 ± 0.02</td><td>136 ± 2</td><td>0.62 ± 0.02</td><td>51</td></tr><tr><td>CN-S/TiO<sub>2</sub></td><td>3.12 ± 0.02</td><td>2.30 ± 0.02</td><td>86 ± 2</td><td>0.30 ± 0.02</td><td>7–21</td></tr></tbody></table>
The position of the SPR peak is not really affected by the
nature of the g-C₃N₄ support. This is consistent with the similar
size ranges of Au NPs produced on the different supports

(≈3 nm, **Figure 4**) and their supposed similar interactions with
the support. Only the Au/CN–C/TiO₂ sample seemed to show
slight shift in the position of the SPR. Moreover, one can also

<figure><img src="image_7.png" /><figcaption>**Figure 4.** TEM micrographs of (top) Au/CN/TiO₂, (middle) Au/CN-S/TiO₂, (bottom left) Au/CN-B/TiO₂, and (bottom right) Au/CN-C/TiO₂.</figcaption></figure>
mention that the intensity of the SPR peak is the same whatever the samples, confirming the same Au loading. It is worthy to underline the overlap between the g-C₃N₄ and Au SPR absorption windows that might be beneficial for increased visible light photons absorption by the former SC, due to energy transfers related to electromagnetic field enhancement around Au NPs.[41]

### 2.3.3. TEM Characterization

Figure 4 presents TEM images of Au/CN(–X)/TiO₂ nanocomposites. One can highlight that on Au/CN–X/TiO₂ composites (Figure 4, middle and bottom), Au NPs are quite exclusively deposited onto TiO₂ surfaces, scarcely on CN ones and none on CN–X ones. This observation can easily be explained considering the very low concentration of CN–(X) sheets, consequently leading to very low probability of contact between Au precursor and those surfaces. In all cases, the mean Au NPs size is ≈3.0–3.5 nm, noting, however, that the distribution is the narrowest on Au/CN–C/TiO₂ material, confirming probably the slight shift in SPR position observed from UV–vis absorbance measurements. One can also mention that B–gC₃N₄ showed particular morphology of rolled up nanosheets (≈10 sheets) well interfaced with Au/TiO₂ material.

### 2.3.4. Photocatalytic Performances

Under artificial visible light irradiation, exclusively H₂ and CH₄ were produced (Table 3) in the gas phase. First, one can mention that quite no activity is observed without Au NPs deposition, clearly meaning that the visible light CO₂ photoreduction activity is induced by the presence of Au. Only the CN–C/TiO₂ heterojunction led to the formation of some hydrogen, showing some photocatalytic activity in the absence of Au, but no methane production. Au NPs deposition resulted in increasing the photoreduction activity in a more or less pronounced way, and increased the selectivity of CN–C/TiO₂ toward methane. It must be underlined that the reference Au/CN photocatalyst was inactive toward CO₂ photoreduction under visible light activation conditions (not shown), whereas the reference Au/TiO₂ exhibited low activity, mainly selective toward H₂ production. Looking at the Au/CN/TiO₂ undoped composite, its activity was enhanced compared to its reference counterparts, but its selectivity for methane production remained low. Contrarily, the doped Au/CN–X/TiO₂ composite photocatalysts appeared highly selective toward CH₄ production. Both Au/CN–B/TiO₂ and Au/CN–S/TiO₂ samples reached total methane selectivity. Furthermore, Au/CN–B/TiO₂ composite led to the highest methane productivity (4.49 µmol h⁻¹ g⁻¹), which is, to the best of our knowledge, one of the best performances reported up to now in the literature under visible light irradiation under similar experimental conditions. The Au/CN–C/TiO₂ nanocomposite led both to the highest activity and CH₄ formation rate of 8.49 µmol h⁻¹ g⁻¹ under visible light activation. Hence, the Au/CN–X/TiO₂ three-component system allowed to considerably increase CH₄ selectivity and to reach total selectivity with the boron and sulfur dopants. From Figure 5 showing the evolution of CH₄ (top) and H₂ (bottom) products as a function of time under continuous CO₂/H₂O flow, it can be observed that

<table><caption>**Table 3.** Average production rate and electronic selectivity (over 10 h of continuous reaction) in the presence of water vapor (CO₂/H₂O = 96/4 mol. ratio) at 0.3 mL min⁻¹ continuous flow under visible light irradiation.</caption><thead><tr><th></th><th>Products</th><th>r<sub>10h</sub> [µmol h<sup>-1</sup> g<sup>-1</sup>]</th><th>Selectivity [%]</th></tr></thead><tbody><tr><td rowspan="2">TiO<sub>2</sub> [550 °C]</td><td>H<sub>2</sub></td><td>0</td><td>0</td></tr><tr><td>CH<sub>4</sub></td><td>0.06</td><td>100</td></tr><tr><td rowspan="2">Au/TiO<sub>2</sub> [550 °C]</td><td>H<sub>2</sub></td><td>0.81</td><td>87</td></tr><tr><td>CH<sub>4</sub></td><td>0.25</td><td>13</td></tr><tr><td rowspan="2">CN/TiO<sub>2</sub></td><td>H<sub>2</sub></td><td>0</td><td>0</td></tr><tr><td>CH<sub>4</sub></td><td>0.09</td><td>100</td></tr><tr><td rowspan="2">CN–C/TiO<sub>2</sub></td><td>H<sub>2</sub></td><td>0.38</td><td>100</td></tr><tr><td>CH<sub>4</sub></td><td>0.00</td><td>0</td></tr><tr><td rowspan="2">CN–B/TiO<sub>2</sub></td><td>H<sub>2</sub></td><td>0.00</td><td>0</td></tr><tr><td>CH<sub>4</sub></td><td>0.03</td><td>100</td></tr><tr><td rowspan="2">CN–S/TiO<sub>2</sub></td><td>H<sub>2</sub></td><td>0.00</td><td>0</td></tr><tr><td>CH<sub>4</sub></td><td>0.25</td><td>100</td></tr><tr><td rowspan="2">Au/CN/TiO<sub>2</sub></td><td>H<sub>2</sub></td><td>2.09</td><td>95</td></tr><tr><td>CH<sub>4</sub></td><td>0.04</td><td>5</td></tr><tr><td rowspan="2">Au/CN–C/TiO<sub>2</sub></td><td>H<sub>2</sub></td><td>1.93</td><td>5</td></tr><tr><td>CH<sub>4</sub></td><td>8.49</td><td>95</td></tr><tr><td rowspan="2">Au/CN–B/TiO<sub>2</sub></td><td>H<sub>2</sub></td><td>0</td><td>0</td></tr><tr><td>CH<sub>4</sub></td><td>2.01</td><td>100</td></tr><tr><td rowspan="2">Au/CN–S/TiO<sub>2</sub></td><td>H<sub>2</sub></td><td>0</td><td>0</td></tr><tr><td>CH<sub>4</sub></td><td>0.42</td><td>100</td></tr></tbody></table>

Au/CN–C/TiO₂ sample followed a typical trend with an increase of production in the first 150 min to reach a maximum, followed by deactivation, as observed before on solar Au/TiO₂ photocatalysts.[42] However, the fully selective Au/CN–B/TiO₂ photocatalyst did not exhibit this deactivation phenomenon; it produced continuous high-level CH₄ production over at least the 10 h of tests.

### 2.3.5. TRMC

The lifetime of charge carriers is completely described by the whole TRMC decay (I(t)).[31,32] This decay is complex and may be due to different mechanisms that governs free electron mobility (considered as the most mobile photogenerated charge carriers on TiO₂- and CN-based materials), like recombination and trapping. Different features can be observed at the different excitation wavelengths. From Figure 6, top left, one can see, comparing the two commercially available TiO₂ P25 and TiO₂ UV-100, that the last one allowed slightly larger initial charge-carrier formation but more rapid free electrons loss of mobility. Contrarily to what is generally observed, the deposition of Au NPs maintains this large density of charge-carrier generation while enhancing considerably electron mobility, probably by acting as electron traps and thus hindering charge-carrier recombination. Exciting the metal-free TiO₂ and CN (CN–C) reference supports at 450 nm (Figure 6, middle left) did not lead to significant charge-carrier production. However, one can underline that the association of TiO₂ (UV-100) to CN or CN–C boosted the
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Specific CH₄ (top) and (bottom) H₂ production rates in presence water vapor (CO₂/H₂O = 96/4 mol. ratio) at 0.3 mL min⁻¹ continuous flow under visible light irradiation.</figcaption></figure>

generation of “visible light-activated” charge carrier, assuming carbon nitride-driven visible light photosensitization. More interestingly, the decay of free electron mobility is more important in the case of carbon doping, suggesting a less effective e-hole pairs separation onto the CN–C/TiO₂ heterojunction. Au NPs deposition (Figure 6, middle right) damps the initial TRMC signal in a more or less pronounced way depending on the support, probably due to electron trapping effects of Au NPs. Nevertheless, it is worthy to highlight that this initial signal drastically decreased on TiO₂ support. It decreased less on Au/CN/TiO₂ and even less on the CN–C/TiO₂ heterojunction, for which damping of the TRMC signal by Au NPs was quite limited. One may assume in this last case that this most efficient charge-carrier generation may be associated to possible favorable field enhancement effect that counterbalances the negative electron trapping effect by Au NPs. Furthermore, this positive effect is enhanced onto the CN–C/TiO₂ heterojunction, thanks to its higher absorbance in the 450-500 nm range (Figure 3). At 550 nm, as shown in Figure 6, bottom left, one can see small charge-carrier generation but only on TiO₂ and CN(CN–C)/TiO₂ heterojunction, nothing is detected on CN or (CN–C) reference samples. One may also have

assigned this observation to carbon nitride-driven sensitization, which phenomenon was, however, more attenuated at 550 nm than at 450 nm, possibly due to the negligible absorbance of the heterojunctions above 550 nm (Figure 3, top). Under these conditions, addition of Au NPs decreased the signal intensity, certainly resulting from trapping effect of Au NPs. Comparing the signals of the photocatalytic composite material Au/CN–C/TiO₂, which exhibited the most important CH₄ productivity, at 450 and 550 nm (after normalization, taking into account the energy received at different wavelength pulses) showed that even if a large signal is observed at 450 nm, no charge-carrier production occurred at 550 nm. This observation supports the assumption that the electron trap effect is predominant compared to the SPIE one due to the fact that the possible near-field intensification around Au NPs cannot be used by the heterojunction at this wavelength.

Summarizing the TRMC behaviors in correlation with the abovementioned performed characterizations (TGA, XRD, UV–vis, TEM) allows us to explain the best performance of the Au/CN–C/TiO₂ composite photocatalyst and to propose schematic charge-carrier and reaction pathways (Figure 7). First, the more extended visible light harvesting (absorption tail up to 550 nm, evidenced by UV–vis absorbance measurements) resulted in more extended visible light sensitization, due to strong and high-quality interactions between both kinds of nano-components. Second, it has already been reported that citric acid-added urea synthesis for obtaining g-C₃N₄ yielded the formation of few loaded N-doped graphitic carbon units in g-C₃N₄ matrix, but too small to be detected with certainty using ICP, XRD, UV–vis, and TEM analyses.[43] This kind of carbon doping may allow enhanced electron delocalization onto some N-doped graphitic carbon units and thus better charge-carrier separation. Third, Au NPs mainly acted as electron trapping/cocatalytic function for CO₂ reduction; however, plasmonic effects cannot be excluded for Au/CN–C/TiO₂; indeed, the limited electron trapping effect observed for Au/CN–C/TiO₂ composite, compared to undoped CN-containing photocatalyst, could result from a counterbalancing field enhancement effect. In addition to the aforementioned impacting criteria, one can also assume that CN doping, along with the lowering of band gap energy, may also modify CO₂ adsorption. Optimized design of the band edge(s) position probably lowered the one of the conduction band. As a consequence, the electrons located onto the conduction band of doped CN became less energetic toward H₂ reduction half-reaction. In parallel, as CO₂ adsorption most probably takes place onto Au/TiO₂ surfaces, CO₂ reduction half-reaction can thus easily be carried out, thanks to the electrons initially photogenerated onto doped CN and transferred from its conduction band to that of TiO₂.[44] CO₂ adsorption on the CN–X component can, however, not be ruled out. CO₂ adsorption has indeed been reported to be facilitated on, e.g., B-doped C₃N₄ (as compared to undoped CN) which may also contribute to the observed optimized performances of the Au/CN–X/TiO₂ composites. Nevertheless, the BET surface areas, in the 90–120 m² g⁻¹ range, seemed not to be an impacting factor for visible light-driven gas-phase photocatalytic CO₂ reduction in presence of H₂O.

The most performing spent photocatalysts (Au/CN–C/TiO₂ and Au/CN–B/TiO₂) have also been characterized by means
<figure><img src="image_9.png" /><figcaption>**Figure 6.** TRMC measurements obtained at (top left) 360 nm, (middle) 450 nm, (bottom) 550 nm, and (bottom right) 550 nm. (Top right) Normalized signals compared at 450 and 550 nm.</figcaption></figure>

of structural (XRD, Figure S5, Supporting Information), surface (X-ray photoemission spectroscopy (XPS), Figure S6, Supporting Information), and optical (UV-vis absorbance, Figure S7, Supporting Information) analyses and compared to the fresh samples. It is clearly evidenced that no structural modification occurred during photocatalysis. Looking at XPS surface analyses, mainly dominated by typical TiO₂ behaviors, one can observe that neither carbon, titanium, oxygen, gold, nor boron surface species

were affected by the photocatalytic tests. Considering light absorption properties, one can, however, mention that the Au/CN-C/TiO₂ samples, exhibiting initially the more extended visible light absorption properties, were slightly affected by reducing its visible light absorption capacity; one can thus assume that carbon-doping species, mainly located inside the porosity of the material, slightly lose their stability under photocatalytic conditions.
<figure><img src="image_10.png" /><figcaption>**Figure 7.** Schematic charge-carrier and reaction pathways on (top left) Au/CN/TiO₂, (top right) Au/CN–C/TiO₂, and (bottom) Au/CN–B/TiO₂.</figcaption></figure>

**3. Conclusion**

In this article, we succeeded in tuning visible light-driven gas-phase photocatalytic CO₂ reduction into CH₄ by designing optimized three-component Au/doped C₃N₄/TiO₂ composite photocatalysts. The strategy consisted in associating low containing doped graphitic carbon nitride to commercially available TiO₂ UV-100, leading to the formation of high-quality heterojunction with strong interactions between the two components. Namely, B- and C-doping were obtained by introducing the corresponding doping element precursors with urea, in the presence of suspended TiO₂, before thermal polycondensation. The resulting doped g-C₃N₄ material exhibited enhanced visible light harvesting properties, especially in the case of C-doping. Further deposition of small Au NPs (≈3 nm), quite exclusively onto TiO₂ surfaces, acted as electron trapping/cocatalytic functions. Depending on the activation wavelength, surface plasmonic effects could also contribute to the photocatalytic activity, especially in the 400-500 nm range. In addition, it was assumed that B– and C–C₃N₄ doping, leading to a more or less low conduction band position, was the impacting factor toward total CH₄ selectivity. The (0.77 wt%)Au/(0.59 wt%)C–C₃N₄/TiO₂ composite photocatalyst, exhibiting the best compromise between those various impacting factors, led to a continuous productivity rate of CH₄ of 8.5 µmol h⁻¹ g⁻¹ over at least 10 h. To the best of our knowledge, this level of performance is unprecedented under continuous gas phase flowing CO₂.

**4. Experimental Section**

**Materials:** Urea (NH₂CONH₂, 99%, Sigma-Aldrich), citric acid monohydrate (HOC(COOH)CH₂COOH)₂·H₂O, ≥99%, Sigma-Aldrich), boric acid (H₃BO₃, ≥99.5%, Sigma-Aldrich), chloroauric acid (HAuIIICl₄·3H₂O) (≥99.9% trace metal basic, Alfa Aesar), sodium

borohydride (≥98%, Sigma-Aldrich), ethanol (CH₃CH₂OH, ≥96%, Sigma-Aldrich), triethanolamine (TEOA, ≥ 99%, Sigma-Aldrich), and 2-thiobarbituric acid (C₄H₄N₂O₂S, ≥98%, Sigma-Aldrich) were used without further purification. TiO₂ Hombikat UV-100 (anatase > 250 m² g⁻¹) was purchased from Sachtleben Chemie GmbH and used without further purification.

**Synthesis of (Doped) g-C₃N₄ and TiO₂/(Doped) g-C₃N₄-Based Materials:** Pure g-C₃N₄ was synthesized via thermal polycondensation of urea at 550 °C for 3 h, with a heating ramp rate of 15 °C min⁻¹. 10 g of urea was put into an alumina crucible with a top cover, resulting in ≈0.25 g of g-C₃N₄. The obtained product is denoted as CN. The nonmetal doped g-C₃N₄ was synthesized by mixing urea (10 g) and 10 mg of the precursor of nonmetal elements (H₃BO₃ for element B, HOC(COOH)CH₂COOH)₂·H₂O (acetic acid) for element C and C₄H₄N₂O₂S (2-thiobarbituric acid) for element S. After grinding, the mixture was annealed at 550 °C for 3 h in a muffle furnace. The samples were named as CN–X, where X is the dopant element (C, B, or S) in g-C₃N₄.

The TiO₂/g-C₃N₄ heterojunctions were prepared by wet impregnation of 10 g of the urea precursor onto 2 g of TiO₂ in suspension in 20 mL of distilled water at 40 °C, followed by evaporation of the solvent under magnetic stirring. The obtained materials were named CN/TiO₂. Alternatively, the TiO₂/(doped)g-C₃N₄ heterojunctions were obtained by mixing at the same time 10 mg of the abovementioned precursors of nonmetal elements with the 10 mg of urea during the wet impregnation step, and named CN–X/TiO₂ (with X = C, B, or S). For reference, TiO₂ UV-100 was submitted to the same experimental conditions (at 550 °C) than for polycondensation reaction; the resulting material was labeled TiO₂ (550 °C).

**Deposition of Au NPs Onto TiO₂/(Doped) g-C₃N₄ Materials:** Au (0.86 wt% theoretical value) was loaded onto the surface of CN(–X)/TiO₂ samples by an impregnation-reduction method[35,42,45] using HAuIIICl₄ as the precursor of gold. In a 100 mL flask, 40 mL of H₂O, 80 mL of HAuIIICl₄·3H₂O (0.22 M in water), and 400 mg of C₃N₄ were mixed. The whole is left under a magnetic stirrer (1000 rpm) for 45 min while the precursor is impregnated. Then 1 mL of a solution of NaBH₄ (0.1 M) was added (with NaBH₄/metal precursor = 5) to the freshly prepared mixture. A gray coloration which highlights the reduction of Au NPs is observed. After the reduction, the mixture is stirred for 15 min and then the obtained material is filtered and washed with 1 L of distilled water. During filtration, the filtrate
appears clear and colorless, which suggests that a good deposit has been obtained. Finally, the composite is placed in an oven at 100 °C for 24 h; a gray powder of Au-based material is obtained. By inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis, it was confirmed that the deposition yields of Au NPs were around 89%, leading to an actual Au loading of 0.76 ± 0.02%. The resulting samples are labeled Au/CN-X/TiO₂.

**Materials Characterization:** Elemental analyses of the composites were performed using ICP-AES. The limit of detection of the instrument is 0.1 mg L⁻¹ for Au. Analysis of the Au component of the nanocomposites allowed us to calculate deposition yields which are defined as the ratio between the real deposited mass of Au (deduced from ICP-AES analysis) and the mass of Au introduced during deposition step.

Thermogravimetric (TGA) analyses were performed using a model Q 5000 IR apparatus from TA Instrument equipped with a sample changer and a microbalance with a sensitivity of around 0.1 mg. 3 mg of each sample was placed in a platinum crucible, and then heated from room temperature to a temperature of 800 °C with a temperature rise ramp of 15 °C min⁻¹ under an air flow rate of 25 mL min⁻¹. Standard deviation on the decomposition temperature of the as-prepared samples is ±10 °C.

The X-Ray diffraction apparatus used is a Bruker D8 Advance diffractometer equipped with a Lynxeye XE detector operating at 40 kV and 40 mA in θ/θ mode. The source of the X-rays is a copper anticathode using the Kα line at 1.5418 Å. In order to obtain normalized diffractograms, a fixed amount of 50 mg of the sample was placed on a plastic holder. The acquisition of diffractograms was performed in scanning mode in steps of 2θ = 10° to 2θ = 38° with a step of 0.041 and a counting time of 5 s per step. The mean crystal thickness is calculated from the Debye–Scherrer equation based on the width at half maximum and the position of the most intense peak.

The nitrogen adsorption–desorption isotherms were obtained using a Micromeritics Asap 2420 porosimeter. The analysis is carried out under atmospheric pressure at the nitrogen liquefaction temperature (–196 °C). The materials were degassed at 150 °C under primary vacuum for 8 h with a temperature rise of 10 °C min⁻¹ in order to desorb water and adsorbed molecules from their surfaces. The specific surfaces of the materials were calculated from the adsorption isotherm by the Brunauer–Emmett–Teller (BET) method. From the desorption, the pore size distribution can be calculated by the BJH method specifically for mesoporous solids.

High-resolution transmission electron microscopy (HRTEM) was performed on a Jeol 2100 F microscope equipped with a lanthanum hexaboride cathode (LaB6) operating at 200 kV. The sample was sonicated in ethanol and then a drop of the suspension is placed on a carbon membrane placed on a copper grid. HRTEM is used to determine the size of Au NPs and consequently the distribution in size is obtained by measuring the particles size for more than 100 particles using the ImageJ software [W. Rasband, http://rsb.info.nih.gov/ij/]. UV–vis absorption spectra were recorded on a Perkin Elmer 950 spectrophotometer fitted with a Labsphere RSA ASSY 100 nm integrating sphere. The spectra are acquired in reflection mode (diffuse reflectance). In order to determine the absorbance coefficient, the diffuse reflectance spectra were converted to Kubelka–Munk units via the equation: $F(R) = (1-R^2)^{1/2}$. To obtain the bandgap (Eg) of the semiconductor (SC), the Tauc equation ($F(R)*\hbar\nu$)S = (hν – Eg) was used where h is the Planck constant, ν is the frequency, and S is a coefficient which is 1/2 in the case of an SC indirect gap (TiO₂) and 2 in the case of a direct band gap SC (for bulk g-C₃N₄). However, nano-sheet-based g-C₃N₄ has been reported as having an indirect gap with the corresponding S coefficient being 1/2.[46]

The charge-carrier lifetimes in TiO₂ after UV illumination were determined by microwave absorption experiments using the time-resolved microwave conductivity method (TRMC).[42,47] The TRMC technique is based on the measurement of the change of the microwave power reflected by a sample, ΔP(t), induced by its laser pulsed illumination. The relative difference ΔP(t)/P can be correlated, for small perturbations of conductivity, to the difference of the conductivity Δσ(t) considering the following equation

$$ \frac{\Delta P(t)}{P} = A\Delta\sigma(t) = Ae\sum_{i} \Delta n_{i}(t)\mu_{i} \quad (1) $$

where Δnᵢ(t) is the number of excess charge-carrier i at time t and μᵢ their mobility. The sensitivity factor A is independent of time, but depends on different factors such as the microwave frequency or the dielectric constant. Considering that the trapped species have a small mobility, which can be neglected, Δnᵢ is reduced to mobile electrons in the conduction band and holes in the valence band. And in the specific case of TiO₂ and g-C₃N₄, the TRMC signal can be attributed to electrons because their mobility is much larger than that of the holes. The incident microwaves were generated by a Gunn diode of the Kα band at 30 GHz. Pulsed light source was an OPO laser (EKSPLA, NT342B) tunable from 225 to 2000 nm. It delivers 8 ns full width at half maximum pulses with a frequency of 10 Hz. The light energy densities received by the sample were 1.6, 11, and 7.2 mJ at 360, 450, and 550 nm, respectively. Each measurement has been repeated two times. The main data provided by TRMC are the maximum value of the signal (Iₘₐₓ), which indicates the number of the excess charge carriers created by the pulse, including decay processes during the excitation by the laser (10 ns), and the decay (I(t)) due to the decrease of the excess electrons, either by recombination or trapping processes.

**Photocatalytic Tests:** Photocatalytic setup has already been described in detail elsewhere.[48] During the test, continuous flowing reaction mixture (CO₂ + H₂O, CO₂/H₂O = 96/4) flow (0.3 mL min⁻¹) passes through a light-transparent photoreactor (6 mL) equipped with a Hg lamp simulating artificial solar light (150 W ceramic–metal–halide Hg lamp). Total photon flux for the whole lamp spectrum is 0.028 and 0.024 mol s⁻¹ m⁻² for visible light irradiance after applying a filter cutting λ < 400 nm (Figure S1, Supporting Information). The irradiated surface is 19.65 cm². For photocatalytic tests, 50 mg of materials are dispersed in ethanol and deposited on a 50 mm diameter glass disk by evaporation at 100 °C. The surface concentration of photocatalyst on the glass disc is 25 g m⁻². The glass disk is then put in the reactor and the pilot is purged for 5 min with CO₂ flow (>100 mL min⁻¹) to remove air and other gas impurities. As a blank test, the setup is running without CO₂ gas flow under illumination to control the absence of gaseous products that may be the result of carbon-based contaminants conversion.[49]. Products from the reactor are analyzed by an online micro-GC (Agilent 3000A SRA instrument, Molsieve MS5A columns). Finally, the lamp is switched on and the duration of test is 10 h. Production rates were calculated according to the following equations

$$ r_x(\text{mol h}^{-1}\text{g}^{-1}) = \frac{[X] \times 10^{-6} \times (\text{flow rate}) \times 60}{V_m \times m_{\text{photocat}}} \quad (2) $$

where [X] is the concentration in ppm, m<sub>photocat</sub> is the mass of photocatalyst, S<sub>irradiated</sub> is the irradiated surface (0.001965 m²), and the flow rate is 0.0003 L min⁻¹ and V<sub>m</sub> = 24.79 L mol⁻¹ (STP conditions). Electronic selectivity was calculated by these equations

$$ \text{CH}_4 \text{ Selectivity} = \frac{8 [\text{CH}_4]}{8 [\text{CH}_4] + 2 [\text{H}_2]} \quad (3) $$

$$ \text{H}_2 \text{ Selectivity} = 1 - \text{CH}_4 \text{ Selectivity} \quad (4) $$

## Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

## Conflict of Interest

The authors declare no conflict of interest.
Data Availability Statement

The data that support the findings of this study are available from the
corresponding author upon reasonable request.

Keywords

CH₄ productivity, CO₂ reduction, doped gC₃N₄/TiO₂ heterojunctions, photocatalysis

Received: March 26, 2023

Revised: June 15, 2023

Published online: August 28, 2023

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