Amine-Functionalized ZnO Nanosheets for Efficient CO₂ Capture and Photoreduction

Abstract

Amine-functionalized ZnO nanosheets were prepared through a one-step hydrothermal method by using monoethanolamine, which has a hydroxyl group, for covalent attachment on ZnO and a primary amine group to supply the amine-functionalization. We demonstrate that the terminal amine groups on ZnO surfaces substantially increase the capability of CO₂ capture via chemisorption, resulting in effective CO₂ activation. As a result, the photogenerated electrons from excited ZnO can more readily reduce the surface-activated CO₂, which thereby enhances the activity for photocatalytic CO₂ reduction.

1. Introduction

Nowadays, the increasing level of CO₂ concentration from the use of fossil fuels has become the most serious environmental concern related to global warming and climate change. Researchers have realized that photocatalytic conversion of CO₂ into hydrocarbons by means of solar energy would be an ideal way to lower down the CO₂ concentration in the atmosphere and generate sustainable chemical fuels in the meantime. A pioneering study on the reduction of CO₂ was demonstrated by Inoue et al. in 1979 by using semiconductor photocatalysts [1]. Afterwards, many scientific studies have been devoted to the development of efficient photocatalysts for CO₂ photoreduction [2,3,4,5,6,7,8,9,10,11,12,13,14]. Among them, ZnO-based nanostructures with different sizes and morphologies have been extensively reported as photocatalysts since they are stable, non-toxic, and low cost [15,16,17,18,19,20]. However, the efficiencies for CO₂ photoreduction of these ZnO-based photocatalysts were usually unsatisfactory [21,22], which might be ascribed to the low affinity between CO₂ and ZnO surfaces [23].

As is well known, the amine functionalization can effectively assist immobilization of CO₂ and has been extensively used for CO₂ capture in industry [24,25]. Tremendous efforts have been made to introduce amine groups onto solid material surfaces to promote CO₂ capture [26,27]. The chemical interactions between amine groups and CO₂ lead to the formation of carbamate (or bicarbamate) that can transform into carbonate upon hydrolysis [28]. In this work, we report a straightforward method to prepare amine-functionalized ZnO nanosheets by using monoethanolamine (MEA) that possesses a hydroxyl (-OH) group for covalent attachment on ZnO and a primary amine (-NH₂) group to endow an amine-functionalized surface. We demonstrate that the terminal amine groups on ZnO surfaces substantially improve the CO₂ adsorption, which consequently results in significantly enhanced photocatalytic activity for CO₂ reduction as compared to the clean ZnO without surface amine groups.

2. Results and Discussion

Figure 1a shows the X-ray diffraction (XRD) patterns of the prepared MEA–ZnO sample and clean ZnO sample. These two samples exhibit almost the same diffraction lines which can be indexed to wurtzite ZnO. This indicates that the involvement of MEA into the reaction did not alter the crystal structure of ZnO. The absorption spectra of both samples also appear to have almost no difference, as shown in Figure 1b. The SEM image (Figure 2a) of the MEA–ZnO sample confirms the nanosheet morphology, and the high-resolution TEM (HRTEM) image (Figure 2a inset) shows a lattice fringe of d = 0.28 nm, corresponding to the (100) interplanar spacing of wurtzite ZnO [29]. In comparison, the clean ZnO sample prepared without MEA appears as irregular particles, as shown in Figure 2b. The sheet-like growth of ZnO is probably due to attached MEA molecules that suppress the intrinsically anisotropic growth of ZnO along the (0001) direction. As such, the ZnO crystal growth would intend to proceed sideways towards a nanosheet [15].

Figure 1. (a) XRD patterns of MEA–ZnO and clean ZnO. Both samples can be indexed as wurtzite ZnO; (b) UV-Vis spectra of MEA–ZnO and clean ZnO.

Figure 1. (a) XRD patterns of MEA–ZnO and clean ZnO. Both samples can be indexed as wurtzite ZnO; (b) UV-Vis spectra of MEA–ZnO and clean ZnO.

Figure 2. (a) SEM images of MEA–ZnO; (b) SEM image of clean ZnO; (c) TEM image of MEA–ZnO nanosheets; (d) HRTEM image of a representative MEA–ZnO nanosheet.

Figure 2. (a) SEM images of MEA–ZnO; (b) SEM image of clean ZnO; (c) TEM image of MEA–ZnO nanosheets; (d) HRTEM image of a representative MEA–ZnO nanosheet.

For the MEA–ZnO sample, the nature of covalent attachment of MEA on the ZnO surface is verified by Fourier transform infrared (FT-IR) spectroscopy. Prior to the measurement, the as-prepared MEA–ZnO sample was annealed at 250 °C under Ar atmosphere to remove the physisorbed MEA since the boiling point of MEA is ~170 °C. The FT-IR spectrum of the MEA–ZnO sample after annealing (Figure 3) still shows most of the signature peaks of MEA. The bands at 2964, 2919, and 1399 cm⁻¹ can be assigned to the C–H stretch of MEA. The peaks at 1073 and 1028 cm⁻¹ are attributed to the C–O stretch and C–N stretch, respectively, and the band at 1560 cm⁻¹ is owing to the N-H stretch of MEA on the ZnO surface [28,30]. These IR results indicate that MEA molecules are successfully covalently binding on ZnO surfaces.

Figure 3. FT-IR spectra of clean ZnO, MEA–ZnO, and the pure MEA molecule.

Figure 3. FT-IR spectra of clean ZnO, MEA–ZnO, and the pure MEA molecule.

The effect of amine functionalization by MEA is examined through the CO₂ adsorption test. As shown in Figure 4, the MEA–ZnO exhibits much higher (approximately six times) CO₂ uptake capability (0.87 cm³·g⁻¹) than the clean ZnO (0.13 cm³·g⁻¹), while in comparison, the difference in their surface area is quite minor (15.1 m²·g⁻¹ for MEA–ZnO vs. 9.6 m²·g⁻¹ for clean ZnO). This result proves that the amine-functionalized ZnO surface can greatly enhance CO₂ adsorption. Moreover, the MEA–ZnO shows a dramatic rise in CO₂ uptake along with elevated CO₂ pressure, while the clean ZnO shows almost linearly increased CO₂ adsorption. This observation suggests that specific interactions between CO₂ and MEA–ZnO occur during the CO₂ adsorption process.

Figure 4. CO₂ adsorption isotherms of MEA–ZnO and clean ZnO.

Figure 4. CO₂ adsorption isotherms of MEA–ZnO and clean ZnO.

We further carried out photocatalytic CO₂ reduction by irradiating the prepared sample (MEA–ZnO or clean ZnO) with UV-Vis light in the presence of CO₂ and water vapor. As shown in Figure 5, the clean ZnO sample exhibits very low generation rates of methane (2.1 μmol/g) and CO (14.8 μmol/g). In comparison, the amine-functionalized sample (MEA–ZnO) showed a much higher production rate for methane (4.4 μmol/g) and CO (25.3 μmol/g). This comparison indicates that the amine functionalization by MEA can not only enhance the capture of CO₂ molecules but also facilitates their photoreduction into CO and CH₄. Control experiments were carried out in the absence of light or CO₂ (in O₂ atmosphere), which did not lead to generation of CO or CH₄. This suggests that excitation of ZnO is required to obtain the products (CO and CH₄) which come from CO₂ photoreduction rather than the oxidation of MEA by the photogenerated holes of ZnO.

To measure the amount of O₂ formed during the photocatalytic reaction, we performed a 12 h photocatalytic reaction for the MEA–ZnO catalyst under the reaction conditions described above. The detected amount of H₂, O₂, CO, and CH₄ were 81 ppm, 1145 ppm, 1520 ppm, and 146 ppm, respectively. According to the stoichiometry of the involved reactions (1) to (4) and the amount of gaseous products (H₂/CO/CH₄), the O₂ evolution amount should be around 1092 ppm, which is slightly lower than the actual detected O₂ amount (1145 ppm). This might be due to the possible generation of liquid hydrocarbon products (e.g., CH₃OH, HCOOH) from the reactions (5) to (7), which could not be effectively analyzed by the TCD (thermal conductivity detector) of GC (Gas Chromatography). Nevertheless, this result still can prove that the attached MEA on ZnO surfaces are stable against oxidation by the photogenerated holes from ZnO.

2H₂O + 4h⁺ → 4H⁺ + O₂

(1)

2H⁺ + 2e⁻ → H₂

(2)

CO₂ + 2H⁺ + 2e⁻ → CO + H₂O

(3)

CO₂ + 8H⁺ + 8e⁻ → CH₄ + 2H₂O

(4)

CO₂ + H⁺ + 2e⁻ → HCOO⁻ + H₂O

(5)

CO₂ + 4H⁺ + 4e⁻ → HCHO + H₂O

(6)

CO₂ + 6H⁺ + 6e⁻ → CH₃OH+ H₂O

(7)

Figure 5. The amount of generated (a) CH₄ and (b) CO as a function of irradiation time over the sample of MEA–ZnO or clean ZnO. Control experiments without light irradiation (control 1) or without CO₂ (control 2) showed no generation of CO and CH₄.

Figure 5. The amount of generated (a) CH₄ and (b) CO as a function of irradiation time over the sample of MEA–ZnO or clean ZnO. Control experiments without light irradiation (control 1) or without CO₂ (control 2) showed no generation of CO and CH₄.

We have also evaluated the long-term stability of MEA–ZnO for CO₂ photoreduction through a cycling test. After each cycle of the four-hour test, the reactor was degassed by CO₂ and the sample was re-used for the next test. As shown in Figure 6, no obvious decrease of the yield of CH₄ and CO was observed after three cycles, suggesting that the MEA–ZnO sample is very stable during the process of CO₂ photoreduction. After the cycling test, the MEA–ZnO sample was examined again by FT-IR spectroscopy, and no change was observed in the IR spectrum, which further confirmed the good stability of MEA–ZnO during the reaction.

Figure 6. The cycling test of CO₂ photoreduction over 12 h by using the MEA–ZnO sample.

Figure 6. The cycling test of CO₂ photoreduction over 12 h by using the MEA–ZnO sample.

Based on the above results, we conclude that the amine functionalization of ZnO can effectively facilitate the capture of CO₂ as well as its further photoconversion into CH₄ and CO. The principle can be illustrated in Figure 7. The terminal amine groups enable the creation of “C–N” bonding with CO₂ by forming carbamate, which acts like an activation process of CO₂ due to higher reactivity of carbamate than that of linear CO₂ [31,32,33,34]. Further, these active carbamates are close to the ZnO surface and tend to establish direct interactions with Zn²⁺, which allows for receiving electrons from excited ZnO to implement reduction reactions towards CO and CH₄ production. By this method, the overall CO₂ photoreduction process can be promoted.

Figure 7. Schematic illustration of adsorption and photoreduction of CO₂ on the MEA-functionalized ZnO nanosheet.

Figure 7. Schematic illustration of adsorption and photoreduction of CO₂ on the MEA-functionalized ZnO nanosheet.

3. Experimental Section

3.1. Preparation of Amine-Functionalized ZnO (MEA–ZnO)

The amine-functionalized ZnO sample was prepared by a hydrothermal method. In a typical procedure, a mixture aqueous solution (20 mL) containing 0.1 M Zn(Ac)₂, 0.2 M NaOH, and 0.005 M MEA was put in a Teflon-lined autoclave and heated at 90 °C for 12 h. Then the solid product was collected and annealed at 250 °C under Ar atmosphere for 3 h to remove physisorbed MEA on ZnO surfaces. For comparison, the clean ZnO sample was also prepared as the control sample under the same condition without using MEA.

3.2. Characterizations

X-ray powder diffraction (XRD) analysis was carried out on a Shimadzu XRD-6000 X-ray diffractometer (40 KV, 40 mA, Shimadzu Corp., Kyoto, Japan). The sample morphologies were examined by scanning electron microscopy (SEM, JEOL JSM-7600F, JEOL Ltd., Tokyo, Japan) and transmission electron microscopy (TEM, JEOL JEM-2100F, JEOL Ltd.). UV-Vis diffuse reflectance spectra (DRS) were recorded over the spectral range 320–800 nm on a Lambda 750 UV/Vis/NIR spectrophotometer (Perkin Elmer, Waltham, MA, USA). BaSO₄ was used as a reflectance standard. Fourier transform infrared spectra (FT-IR) were collected on a Frontier FT-IR/NIR spectrometer (Perkin Elmer). The Brunauer–Emmett–Teller (BET) surface areas and CO₂ adsorption test were measured on a Micro-meritics ASAP 2020M⁺C system (Micromeritics Instrument Corporation, Norcross, GA, USA).

3.3. Photoreduction of CO₂

The photocatalytic CO₂ reduction was performed in a 100 mL gastight reactor with a quartz window and two side-sampling ports. In a typical process, the MEA–ZnO sample (20 mg) was suspended in an ethanol solution and the suspension was dispersed on a 6.25 cm² glass slide upon drying. The glass slide containing MEA–ZnO was then put into the gastight reactor. Prior to the irradiation, the reactor was purged with high purity CO₂ for 30 min to remove the residual air and then 0.1 mL ultrapure water was injected into the reaction system. A xenon lamp (MAX-302, Asahi Spectra Co. Ltd., Tokyo, Japan) was used as the light source for the photocatalytic reaction. During the reaction, the gas product was analyzed periodically through a gas chromatograph (GC-7890A, Agilent Technologies Inc., Santa Clara, CA, USA) with a TCD detector.

4. Conclusions

In summary, we have successfully prepared amine-functionalized ZnO nanosheets through a hydrothermal process. The amine functionalization on ZnO surfaces substantially increases the capability of CO₂ capture via chemisorption with effective CO₂ activation. As such, amine-functionalized ZnO nanosheets exhibit significantly enhanced photocatalytic activity for CO₂ reduction into CH₄ and CO as compared to the clean ZnO.