Influence of Cocatalysts (Ni, Co, and Cu) and Synthesis Method on the Photocatalytic Activity of Exfoliated Graphitic Carbon Nitride for Hydrogen Production [original]

Citation: Rana, A.G.; Schwarze, M.;

Tasbihi, M.; Sala, X.; García-Antón, J.;

Minceva, M. Influence of Cocatalysts

(Ni, Co, and Cu) and Synthesis

Method on the Photocatalytic

Activity of Exfoliated Graphitic

Carbon Nitride for Hydrogen

Production. Nanomaterials 2022,12,

4006. https://doi.org/10.3390/

nano12224006

Academic Editor: Joon Ching Juan

Received: 13 July 2022

Accepted: 11 November 2022

Published: 14 November 2022

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nanomaterials

Article

Influence of Cocatalysts (Ni, Co, and Cu) and Synthesis Method

on the Photocatalytic Activity of Exfoliated Graphitic Carbon

Nitride for Hydrogen Production

Adeem Ghaffar Rana 1,2 , Michael Schwarze 3,*, Minoo Tasbihi 3, Xavier Sala 4, Jordi García-Antón4and

Mirjana Minceva 1,*

1Biothermodynamics, TUM School of Life Sciences, Technical University of Munich,

Maximus-Von-Imhof-Forum 2, 85354 Freising, Germany

2Department of Chemical, Polymer and Composite Materials Engineering, University of Engineering and

Technology (UET), Lahore 39161, Pakistan

3Department of Chemistry, Technische Universität Berlin, Straße des 17. Juni 124, 10623 Berlin, Germany

4Departament de Química, Unitat de Química Inorgànica, Universitat Autònoma de Barcelona,

08193 Bellaterra, Barcelona, Spain

*Correspondence: [email protected] (M.S.); [email protected] (M.M.);

Tel.: +49-8161716170 (M.M.)

Abstract:

Exfoliated graphitic carbon nitride (ex-g-CN) was synthesized and loaded with non-noble

metals (Ni, Cu, and Co). The synthesized catalysts were tested for hydrogen production using a

300-W Xe lamp equipped with a 395 nm cutoff filter. A noncommercial double-walled quartz-glass

reactor irradiated from the side was used with a 1 g/L catalyst in 20 mL of a 10 vol% triethanolamine

aqueous solution. For preliminary screening, the metal-loaded ex-g-CN was synthesized using the

incipient wetness impregnation method. The highest hydrogen production was observed on the

Ni-loaded ex-g-CN, which was selected to assess the impact of the synthesis method on hydrogen

production. Ni-loaded ex-g-CN was synthesized using different synthesis methods: incipient wetness

impregnation, colloidal deposition, and precipitation deposition. The catalysts were characterized by

X-ray powder diffraction, X-ray photoelectron spectroscopy, nitrogen adsorption using the Brunauer–

Emmett–Teller method, and transmission electron microscopy. The Ni-loaded ex-g-CN synthesized

using the colloidal method performed best with a hydrogen production rate of 43.6

mol h

−1

By contrast, the catalysts synthesized using the impregnation and precipitation methods were less

active, with 28.2 and 10.1

mol h

−1

, respectively. The hydrogen production performance of

the suspended catalyst (440

mol m

−2

−1

) showed to be superior to that of the corresponding

immobilized catalyst (236 µmol m−2g−1).

Keywords:

graphitic carbon nitride; water splitting; hydrogen production; nickel; cocatalyst deposition

1. Introduction

Energy and environmental challenges are emerging with industrial and economic

development [

–

]. Industrialization and increasing population are the main reasons

behind the energy and environmental crises. Most of the world’s energy demand is fulfilled

by nonrenewable sources (petrol, diesel, and coal), which are becoming depleted [

Additionally, during the past few decades, the increased consumption of fossil fuels has led

to severe environmental issues, such as global warming and climate change. As a result,

researchers are putting more effort into developing renewable energy sources, the only

alternative to ensure sustainable development. Solar light, wind, biomass, hydro, and

geothermal energy sources are eco-friendly sources of renewable energy [

–

]. Among

them, sunlight energy can be stored in the chemical bonds of a fuel (e.g., hydrogen, H

)

through artificial photosynthesis [

–

]. Hydrogen, an alternative renewable energy

source, can release considerable energy without emitting greenhouse gases; water is the

Nanomaterials 2022,12, 4006. https://doi.org/10.3390/nano12224006 https://www.mdpi.com/journal/nanomaterials

Nanomaterials 2022,12, 4006 2 of 13

only by-product of hydrogen combustion. Hence, photocatalytic H

generation through

solar light-driven water splitting using semiconductor materials that convert solar energy

to chemical energy has become a promising approach [

]. Consequently, the development

of a stable, effective, and inexpensive catalyst for the hydrogen evolution reaction (HER)

has become a challenging and important research topic [14].

A wide variety of semiconductor-based photocatalysts, such as TiO

[

], ZnO [

Bi [

], CdS [

], and g-CN [

], have been investigated in the last few decades. The more

conventional (i.e., TiO

and ZnO) have advantages and disadvantages regarding stability

and nontoxicity [

]. Conversely, they typically have large bandgaps and a high rate

of electron–hole recombination, which results in low solar-to-fuel efficiency [

]. Among

semiconductor materials, graphitic carbon nitride (g-CN) has attracted considerable atten-

tion since Wang et al. first reported its use for water splitting [14,24]. g-CN, a visible-light

photocatalyst composed of N, C, and H, has attracted interest because of its extensive ap-

plication in CO

reduction [

], pollutant degradation [

], organic synthesis reactions [

and water splitting [

]. These nitrogen-rich materials are inexpensive, abundant, and

easy to synthesize [

]. The thermal decomposition of nitrogen-rich precursors, such

as melamine, urea, thiourea, cyanamide, or dicyanamide, is used to synthesize g-CN in

the form of tri-s-triazine sheets. Moreover, the nontoxic g-CN, which can be activated by

visible light because of the low bandgap energy (2.7 eV), possesses chemical, electronic, and

thermodynamic stability [

]. However, the photocatalytic performance of bulk g-CN

is low because of the low specific surface area (10 m

/g), low availability of active sites, low

adsorption and absorption, and rapid recombination of photogenerated electron-hole pairs.

Several strategies can be adopted during the photocatalyst design to improve the surface

and optical properties of g-CN to overcome these problems. These include metal and

nonmetal doping, morphology control, compositing with other semiconductor materials,

and exfoliation [

]. From these strategies, exfoliation is a fast, efficient, and easy method

to improve a given material and its optical properties by increasing its surface area [

Moreover, doping can help improve the electronic properties of materials by introducing

more electron-hole pairs and minimizing charge recombination [8,22,24,30].

Modifying the catalyst with a cocatalyst enhances the performance of materials sig-

nificantly. Immobilization of a cocatalyst on the surface of the semiconductor material is

one of the efficient and essential methods to accelerate the separation efficiency of electron–

hole pairs generated in the process, consequently enhancing the overall photocatalytic

performance of the semiconductor material. Therefore, adding cocatalysts enhances hy-

drogen production in a water-splitting reaction. Noble and non-noble metals can play this

role [1,6,13,21,31].

Noble metal cocatalysts typically enhance the kinetics of the reaction at low overpoten-

tials and help induce charge separation from the semiconductor to the cocatalyst [

]. Noble

metals, such as platinum (Pt) [

], gold (Au) [

], ruthenium (Ru) [

], silver (Ag) [

and palladium (Pd) [

], are usually the most common cocatalysts for improving the

photocatalytic performance [

]. However, noble metals are scarce, which prevents their

practical implementation on a large scale. Hence, researchers are trying to find a cocatalyst

that can replace these noble metals, such as non-noble metals, metal oxides, and metal

sulfides [

]. From all the non-noble metals available for HER, nickel-based cocatalysts

have attracted attention because of their low price, stability, and high activity [1].

Recently, Ni metal and Ni-based compounds (Ni

P, Ni(OH)

, NiN

, NiB, NiS, and

C) have shown significant HER performance when used as cocatalysts in g-CN-based

photocatalytic systems. Ni can play a similar role to noble metals, improving the separation

efficiency of electron–hole pairs [

]. The cocatalyst loading on the support material

can be achieved using diverse synthetic methods. The synthesis methods determine the

structure, dispersion, and size distribution of the cocatalyst, which affects the activity of the

final hybrid material. In recent years, various methods have been proposed to synthesize

supported nanoparticles. However, few studies have compared different synthesis methods

Nanomaterials 2022,12, 4006 3 of 13

for a given support and a given photocatalytic processes. Therefore, rationally choosing

the most suitable method remains a challenge [37].

This study evaluated the effect of non-noble metal (Ni, Cu, and Co) cocatalyst loading

on exfoliated g-CN (ex-g-CN) for hydrogen production. The hybrid photocatalyst was

initially prepared using the incipient wet impregnation (IWI) method and the superior

photocatalytic activity of Ni (over Cu and Co) was observed. After this preliminary

screening, the best-performing Ni-based hybrid photocatalyst was prepared using one

of the following methodologies: (i) precipitation deposition method (PRDM), (ii) IWI

method, and (iii) colloidal deposition method (CM). The effects of the synthesis method

on the HER photocatalytic performance were studied with triethanolamine (TEOA) as the

sacrificial agent under simulated solar light irradiation using a 300-W xenon lamp with a

395-nm cutoff filter. Moreover, the effects of the catalyst exposure to the reaction medium

(immobilization or suspension) were studied for practical applications.

2. Materials and Methods

2.1. Chemicals

Melamine (C

, 99%) and dihydrogen hexachloroplatinate(IV) hydrate, 99.9% (metal

basis) (H

PtCl

6·

O) were purchased from Alfa Aesar (Haverhill, MA, USA). Nickel

sulfate hexahydrate (NiSO

4·

O), copper sulfate pentahydrate (CuSO

4·

O), and cobalt

sulfate monohydrate (CoSO

4·

O) (Merck, Darmstadt, Germany), sodium hydroxide

(NaOH), ethanol (C

OH), methanol (CH

OH), cyclohexane (C

), 1-butanol (C

O),

and ascorbic acid (C

) were obtained from VWR (Radnor, PA, USA). Triton X-100

O(C

(n = 9–10)) was supplied by Sigma-Aldrich (Darmstadt, Germany).

TEOA (C

) was acquired from Sigma-Aldrich (Darmstadt, Germany). All chemicals

were used as received.

2.2. Synthesis of ex-g-CN

Bulk carbon nitride (g-CN) was synthesized with prethermal decomposition of melamine

using the procedure established in a previous study [

]. Briefly, a closed crucible with

melamine was placed into a muffle furnace (Carbolite Gero, GPC 1200, Derbyshire, UK)

for thermal decomposition. The heating program consisted of two steps: heating to

450 ◦C

with a gradient of 2

◦

C min

−1

, and this temperature was kept for 2 h. The sample was then

heated to 550

◦

C at a rate of 2

◦

C min

−1

, maintaining this temperature for 4 h. Bulk g-CN

was crushed in a mortar and pestle, rinsed with ultrapure water, and dried overnight at

◦

C. Carbon nitride (ex-g-CN) was exfoliated from g-CN by placing g-CN in an open

crucible inside a muffle furnace for 2 h at 500

◦

C using a heating ramp of 2

◦

C min

−1

. The

ex-g-CN was obtained after thermal treatment.

2.3. Cocatalyst Loading onto ex-g-CN

Incipient Wet Impregnation

The IWI method was used to synthesize Ni, Cu, Co, and Pt-loaded ex-g-CN catalyst

(

2 wt.%

theoretical loadings) for the preliminary screening of hydrogen production. The Ni-

based catalyst synthesized using this method was called Ni

IWI

/ex-g-CN [

]. According

to stoichiometric calculations, the required amount of a respective salt precursor was

added dropwise to the ex-g-CN. The beaker containing the catalyst powder was placed

in a sonicator for better dispersion. The material was dried overnight at 80

◦

C before

further use.

2.4. Precipitation Deposition Method

PRDM

/ex-g-CN (2 wt.% theoretical Ni loading) was synthesized using the PRDM [

The respective amount of the nickel salt precursor was added to 300 mL of milli-Q water.

The solution was maintained at pH 9 using 0.1M NaOH. At a stable pH, ex-g-CN was

added with continuous stirring. The deposition–precipitation procedure was conducted at

Nanomaterials 2022,12, 4006 4 of 13

◦

C at a constant pH for 2 h, and the slurry was stirred and dried overnight at the same

temperature. The catalyst was washed and dried at 80 ◦C under vacuum.

2.5. Colloidal Deposition Method

/ex-g-CN (2 wt.% theoretical Ni loading) was synthesized using the CM. The

microemulsion system for synthesizing Ni nanoparticles consists of two phases, e.g., the

water and oil phases. The nickel salt precursor, surfactant (Triton X-100), and co-surfactant

(1-butanol) were present in the water phase. The oil phase was cyclohexane. Two mi-

croemulsions were prepared for the synthesis: (1) with Ni salt and (2) with a reducing

agent (ascorbic acid). The synthesis process was conducted in a glass reactor with stirring.

Emulsion 2 contained ascorbic acid, cyclohexane, butanol-1, triton X-100, and water. Emul-

sion 1 contained the nickel salt precursor, cyclohexane, butanol-1, triton X-100, and water.

Emulsion 2 was added slowly to the reactor containing emulsion 1. The mixture was stirred

at 700 rpm for 30 min at room temperature to form the colloidal stabilized Ni nanoparticles

and then stirred for 2 h at room temperature. The ex-g-CN support was then added to the

mixture with vigorous stirring to deposit the Ni nanoparticles onto the surface of ex-g-CN.

The suspension was stirred further for 2 h at 55

◦

C. After the process, the reactor was cooled

to room temperature. The solid was centrifuged and washed several times with acetone.

Finally, the catalyst was dried at 80 ◦C under vacuum.

2.6. Characterization

The Brunauer–Emmett–Teller (BET) surface area was determined from a nitrogen

adsorption–desorption experiment at 77 K (quadrasorp, Quantachrome, Boynton Beach,

FL, USA). The crystalline phases were examined using X-ray diffraction (XRD, Mini Flex

600C, Rigaku, Tokyo, Japan) was conducted using CuK

radiation at a voltage, current,

and spin speed of 40 kV, 15 mA, and 80 rpm, respectively, in the range between 3

◦

and 60

◦

with a step size of 0.0075

◦

. Transmission electron microscopy (TEM, JEM-ARM300F2)

images were obtained using a probe corrected with a cold FEG emitter (JEOL Ltd., Tokyo,

Japan) operated at 300 kV with a camera length of 10 cm. The acquired and evaluated high-

angle annular dark-field images represent a detection angle of 54–220 mrad. The image

contrasts were formed mainly by Rutherford scattering and were correlated to the atomic

number. X-ray photoelectron spectroscopy (XPS, Phoibos 150 analyzer, SPECS GmbH,

Berlin, Germany) was conducted under ultra-high vacuum conditions (base pressure

5×10−10 mbar

) using a monochromatic Al K

X-ray source (1486.74 eV). The energy

resolution measured from the FWHM of the Ag 3d

5/2

peak for a sputtered silver foil

was 0.62 eV at the Catalan Institute of Nanoscience and Nanotechnology in Barcelona.

Ni loading in Ni-loaded ex-g-CN samples was determined using inductively coupled

plasma optical emission spectrometry (Perkin-Elmer Optima 4300DV model system) in the

Chemical Analyses Service of the Universitat Autònoma de Barcelona.

2.7. Photocatalytic Experiments

The photocatalytic experiments were performed in a noncommercial side-irradiated

double-walled quartz-glass reactor (shown in a previous work [

]) with a maximum

volume of 35 mL. In general, 20 mg of the prepared photocatalyst was placed into the

reactor, and 20 mL of an aqueous TEOA solution (10 vol% TEOA) was added. The solution

was flushed with Ar for 15 min to remove oxygen, and the reactor was closed with a

rubber septum. The reactor was connected to a thermostat (ministat 125, Huber, Germany,

Tset = 19 ◦C

) and placed onto a magnetic stirrer in front of a 300-W Xe lamp equipped with

a 395-nm cutoff filter. The distance between the lamp and reactor was 10 cm. The lamp was

switched on, and after irradiating the intensively stirred photocatalyst suspension for the

desired time, a gas phase sample was taken with a gas-tight syringe. The gas sample was

analyzed by gas chromatography (Agilent Technologies 7890 A) equipped with a thermal

conductivity detector.

Nanomaterials 2022,12, 4006 5 of 13

The hydrogen production photocatalytic activity was calculated as follows:

H2µmol g−1h−1=(V0−VL)·H2(GC)

mcat ·tR·Vm(1)

where V

is the total volume of the reactor, V

is the volume of the solution, H

is the

amount of H

detected using GC, m

cat

is the amount (g) of catalyst, t

is the irradiation

time, and Vmis the molar volume of hydrogen.

3. Results

3.1. Catalyst Characterization

Before assessing the as-prepared photocatalysts for the HER, they were characterized

using the different techniques. Optical properties were characterized using UV-Vis and

photoluminescence (PL) spectroscopy (Figures S1 and S2). The bandgap energy for bulk

and exfoliated g-CN were 2.42 and 2.62 eV showing light absorption in the visible range.

After loading with Co, Ni, and Cu, the bandgap energy was 2.6, 2.5, and 2.4 eV, respectively.

The optical properties remained after cocatalyst loading. In the case of different methods

to load Ni onto ex-g-CN, bandgap energy was constant at about 2.5 eV. PL spectroscopy

showed that g-CN materials absorbed the maximum in the range of 430–460 nm. The

chemical composition was studied using FTIR (Figure S3) and spectra are typical for g-CN

materials. Peaks at 806 cm

−1

can be attributed to triazine units, whereas the strong bands

between 1636 and 1242 cm

−1

belong to the C=N and C–N bonds of heterocyclic rings.

There were no peak alterations and new peaks after the addition of the cocatalyst.

3.2. X-ray Diffraction

The phase structures of g-CN, ex-g-CN, and Ni-loaded ex-g-CN were examined using

XRD and the diffractograms are shown in Figure 1. All samples showed the characteristic

peak at 27.2

◦

and a weaker peak at 13

◦

. The strong peak was indexed to the (002)

plane, a characteristic interlayer stacking peak of aromatic g-C

systems. Moreover, the

weak peak was assigned to the (100) crystal plane, which was attributed to the repeated

tri-s-triazine units. The decrease in intensity and the slight shift in the peaks were attributed

to exfoliation and Ni loading. There were no peaks of Ni due to the low Ni content. The

XRD patterns of cobalt and copper loaded ex-g-CN showed similar peaks with no visible

peak of respective metal (Figure S4).

Nanomaterials 2022, 12, x FOR PEER REVIEW 5 of 13

395-nm cutoff filter. The distance between the lamp and reactor was 10 cm. The lamp was

switched on, and after irradiating the intensively stirred photocatalyst suspension for the

desired time, a gas phase sample was taken with a gas-tight syringe. The gas sample was

analyzed by gas chromatography (Agilent Technologies 7890 A) equipped with a thermal

conductivity detector.

The hydrogen production photocatalytic activity was calculated as follows:

𝐻 (μmol gh)= (𝑉−𝑉

)⋅𝐻

(𝐺𝐶)

𝑚 ⋅𝑡

⋅𝑉



(1)

where V0 is the total volume of the reactor, VL is the volume of the solution, H2 is the

amount of H2 detected using GC, mcat is the amount (g) of catalyst, tR is the irradiation

time, and Vm is the molar volume of hydrogen.

3. Results

3.1. Catalyst Characterization

Before assessing the as-prepared photocatalysts for the HER, they were characterized

using the different techniques. Optical properties were characterized using UV-Vis and

photoluminescence (PL) spectroscopy (Figures S1 and S2). The bandgap energy for bulk

and exfoliated g-CN were 2.42 and 2.62 eV showing light absorption in the visible range.

After loading with Co, Ni, and Cu, the bandgap energy was 2.6, 2.5, and 2.4 eV, respec-

tively. The optical properties remained after cocatalyst loading. In the case of different

methods to load Ni onto ex-g-CN, bandgap energy was constant at about 2.5 eV. PL spec-

troscopy showed that g-CN materials absorbed the maximum in the range of 430–460 nm.

The chemical composition was studied using FTIR (Figure S3) and spectra are typical for

g-CN materials. Peaks at 806 cm−1 can be attributed to triazine units, whereas the strong

bands between 1636 and 1242 cm−1 belong to the C=N and C–N bonds of heterocyclic rings.

There were no peak alterations and new peaks after the addition of the cocatalyst.

3.2. x-Ray Diffraction

The phase structures of g-CN, ex-g-CN, and Ni-loaded ex-g-CN were examined us-

ing XRD and the diffractograms are shown in Figure 1. All samples showed the charac-

teristic peak at 27.2° 2θ and a weaker peak at 13° 2θ. The strong peak was indexed to the

(002) plane, a characteristic interlayer stacking peak of aromatic g-C3N4 systems. Moreo-

ver, the weak peak was assigned to the (100) crystal plane, which was attributed to the

repeated tri-s-triazine units. The decrease in intensity and the slight shift in the peaks were

attributed to exfoliation and Ni loading. There were no peaks of Ni due to the low Ni

content. The XRD patterns of cobalt and copper loaded ex-g-CN showed similar peaks

with no visible peak of respective metal (Figure S4).

Figure 1.

XRD patterns of g-CN, ex-g-CN, and Ni-loaded ex-g-CN synthesized using the incipient wet

impregnation (IWI) method, colloidal method (CM), and precipitation deposition method (PRDM).

3.3. Brunauer–Emmett–Teller

The specific surface areas were measured to reveal any change in the structural features

of carbon nitride before and after exfoliation. The surface area of ex-g-CN, obtained from

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