A synergetic effect of surface plasmon and ammoniation on enhancing photocatalytic activity of ZnO nanorods

Abstract

A synergetic effect of surface plasmon (SP) and ammoniation on the enhancements of both ultraviolet and visible photocatalytic activities of ZnO nanorods is reported. SPs were supported by Ag nanoparticles attached to the surface of ZnO nanorods that were prepared by hydrothermal method. Ammoniation was conducted under 20 atmospheres at 400 °C in an autoclave. Photocatalysis was evaluated in terms of the degree of degradation of methylene blue by ZnO. It is found that ammoniation can dope both nitrogen and hydrogen ions for bandgap width narrowing and dangling bond passivation, respectively, while Ag nanoparticles may facilitate the separation and generation of electrons and holes in ZnO via SPs for redox reaction at the surface. This synergism of the two different approaches to processing ZnO enhances its photocatalytic ability significantly.

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1. Introduction

Photocatalytic applications such as water splitting for hydrogen acquisition and decomposition of organic molecules for environmental protection have aroused broad interest.^1–5 However, the requirement of wide bandgap width of photocatalyst for redox reaction at the surface limits the range of light available for photocatalysis to ultraviolet (UV) regime. The proportion of UV regime is unfortunately rather low in the solar emission spectrum on the ground.^2–5 To expand the photoactivity to the visible regime, bandgap engineering of photocatalyst has been proposed by doping the photocatalyst with metal^6,7 or non-metal ions,^8,9 aiming at introducing intermediate levels and/or narrowing the bandgap width. Although numerous results have been obtained so far, this type of approach is not sufficiently efficient either due to the difficulty of non-metal ion incorporation or to the adverse effect of excessive doping of metal ions on photocatalysis.^7,10 Recently, a new approach that applies surface plasmons (SPs) to enhance photocatalysis has attracted increasing attention.^11–13 The SPs are excited by the incident light and supported by Au or Ag nanoparticles (NPs) that are attached to the photocatalyst. It has been proposed that the attached metal NPs may suppress the recombination of electron and hole and/or facilitate the creation of electron–hole pairs for photocatalysis with the help of SPs.^11–14 Nevertheless, this approach of SP has also been inefficient so far after years of study. It is therefore speculated that if one applies these two methods together, a synergetic effect may work to significantly enhance the photocatalysis. In this work, we study the synergism of ion doping (nitrogen and hydrogen ion doping via ammoniation) and SP-enhancement to enhance both the UV and visible photocatalytic activities of ZnO nanorods (NRs). Our results show the availability and effectiveness of the proposed synergetic application of ion doping and SP.

2. Experimental

2.1. Materials

ZnO NRs were grown on a silicon (100) substrate (one-side polish, 10 × 10 × 0.2 mm³ in size). Prior to the growth, a nanocrystalline ZnO thin film (∼20 nm in thickness) was deposited on a clean silicon substrate by pulsed laser deposition. The nanocrystalline ZnO film served as a buffer layer for ZnO NRs to grow on the lattice-mismatched silicon substrate and as a seed layer to guide the nucleation of ZnO and the growth of ZnO NRs. ZnO NRs were then grown on the nanocrystalline ZnO seeded silicon substrate by hydrothermal reaction using a mixture of 0.04 M hexamethylenetetramine (HMT) and 0.04 M zinc nitrate hexahydrate [Zn(NO₃)₂·6H₂O] resolved in 1 L de-ionized water as the hydrothermal precursor solution. The hydrothermal growth was performed at 90 °C in a thermostatic water bath for 6 h. The samples were then taken out, leaving ZnO NRs on the substrate after natural volatilization.

2.2. Synthesis of different ZnO samples

For ammoniation, nitrogenation or hydrogenation of ZnO NRs, the ZnO NRs were placed in the chamber of autoclave, and then subjected to a 20.0 bar NH₃, N₂, or H₂ atmospheres at 400 °C for 1 day, respectively. Then, to incorporate Ag NPs on their surfaces, an Ag thin film with apparent thickness of 5 nm was firstly deposited on the ZnO NRs at 1 × 10⁻³ Pa atmosphere by thermal evaporation, followed by heating in nitrogen at 400 °C for 5 min. By doing so, Ag NPs were formed on ZnO NRs.¹⁵

2.3. Characterization methods

The surface morphology was examined using a scanning electron microscopy or SEM (Hitachi, S-4800). Optical absorption spectra were recorded at room temperature with a UV-Vis-NIR spectrophotometer (Tianjin Gangdong, WGD-8/8A) in an incident-light scanning mode. To measure the inter-band PL emission of ZnO, the ZnO samples were excited with λ = 325 nm laser beam from a CW He-Cd laser (Melles Griot, Series 74). The photoluminescence (PL) spectra were recorded with an intensified charge-coupled device (ICCD) (Andor Technology, iStar DH720) which was attached to the exit port of a spectrograph (Acton Research, SpectraPro-500i). The crystal structures of ZnO were examined by X-ray diffraction (XRD) with a Bruker D8 X-ray diffractometer using Cu Kα irradiation. Chemical compositions of the samples were analyzed using X-ray photoelectron spectroscopy (Ulvac Japan, PHI 5000 Versaprobe). All binding energies were referenced to the C 1s peak (284.6 eV) arising from adventitious carbon. To measure the photocurrent, two Al ribbons 5.0 mm apart were thermally evaporated on the sample. Photocurrent tests were then recorded under dark, UV and visible illumination conditions by using a source meter (Keithley, 2400).

2.4. Evaluation of photocatalytic activity

To evaluate the photocatalytic activity of ZnO NRs, photo-decomposition of methylene blue (MB) was used. Absorption spectra of MB solutions loaded with or without ZnO NRs were recorded. The concentration of MB was 0.01 mmol L⁻¹. A 400 W mercury lamp (Heraeus, MDQ 401SE) equipped with various filters served as a light source. After filtering, the lights with wavelengths centered at λ = 365 and 435 nm were used as UV and visible light sources, respectively. Photo-decompositions of MB by ZnO NRs were performed in a cuboid quartz vessel. In a photocatalysis test, 2.0 mL MB solution was added into the quartz vessel, which was aerated with an oxygen flow of 6 mL min⁻¹. The amount of volatilization of aerated MB solution was less than 5% after 8 h UV or visible illuminations. During the photo-decomposition process, MB solution was kept at 20 °C by using a homemade water cooler. The light powers were changed by using attenuation plates to measure the evolutions of UV and visible photocatalytic activities of ZnO NRs as functions of illumination light powers. To test the photostability of the sample, cycling tests of UV and visible photocatalysis were performed. Once a cycle was completed, the MB solution that had been decomposed would be replaced by another 2.0 mL MB solution for the subsequent cycling test.

3. Results and discussion

3.1. Morphology

Fig. 1(a) shows an SEM image of ZnO NRs. The sample is termed “ZnO”. The average diameter of NRs is 52.9 ± 18.3 nm. After loading Ag NPs, the whole morphology of ZnO NR arrays has no evident change as shown in Fig. 1(b). The sample is termed “Ag/ZnO”. It is known that with the decreasing dimensionality, the melting temperature of Ag drops considerably.^16–19 The melting of Ag thin film and the re-nucleation of Ag atoms lead to the formation of Ag NPs. Ag NPs are discernible as highlighted by dotted circles, with an average diameter of 20.5 ± 4.7 nm. Fig. 1(c) gives an SEM image of ZnO NRs after ammoniation, which is termed “ZnO + NH₃”. The average length and diameter of ZnO NRs change slightly, but the surfaces of the NRs are a little bit roughened due to the incorporations of nitrogen and hydrogen ions by high pressure pressing. In Fig. 1(d), an SEM of ZnO NRs after ammoniation and Ag NP loading sequentially is presented. This sample is termed “Ag/ZnO + NH₃”.

Fig. 1 SEM images of ZnO NRs that have been untreated (a), loaded with Ag NPs (b), ammoniated (c) and ammoniated and then loaded with Ag NPs (d).

3.2. Optical properties and structure

Fig. 2(a) shows the absorption spectra of the four samples of “ZnO”, “Ag/ZnO”, “ZnO + NH₃” and “Ag/ZnO + NH₃”. The inset gives the corresponding Tauc plots according to the direct bandgap feature of ZnO. It is seen that as compared to “ZnO”, the absorption edge of “ZnO + NH₃” is redshifted, and the bandgap becomes narrowed. This suggests a substitutional incorporation of nitrogen ions via the decomposition of NH₃, as will be shown by the XPS result in the following. Due to the overlap of N 2p level with the O 2p one at the maximum of valence band (VB) of ZnO, the bandgap width of ZnO decreases.^20–23 On the other hand, after loading Ag NPs, the apparent bandgap width of ZnO is also decreased for “Ag/ZnO”. It is pointed out that the bandgap width of ZnO here does not change, but only the Fermi level of Ag NP acts like an intermediate level, which makes visible absorption possible.^24–26 For “Ag/ZnO + NH₃”, the absorption edge is further redshifted, indicating a synergism of ammoniation and Ag NP loading on the photo-absorption. Fig. 2(b) depicts the difference curve between the absorption spectra of “Ag/ZnO” and “ZnO”, and that between “Ag/ZnO + NH₃” and “ZnO + NH₃”. They represent the absorptions arising from Ag NPs on “ZnO” and “ZnO + NH₃” substrates, respectively. The SP resonances (SPRs) occur at 415 and 446 nm for the substrates of “ZnO” and “ZnO + NH₃”, respectively. The different positions of SPR could be due to the different indices of “ZnO” and “ZnO + NH₃”,^27,28 as the index depends on the environment sensitively.^15,28

Fig. 2 (a) Optical absorption spectra and Tauc's plots (inset). (b) Difference curve between the absorption spectra of “Ag/ZnO” and “ZnO”, and that between “Ag/ZnO + NH₃” and “ZnO + NH₃”. (c) PL spectra for the ZnO NRs under the excitations of λ = 325 nm. (d) XRD patterns of ZnO NRs. (e) PL spectra for different ZnO NRs that have been treated in N₂, NH₃ and H₂, respectively, termed “ZnO + N₂”, “ZnO + H₂” and “ZnO + NH₃”. (f) Absorption spectra of MB solution and those loaded with different ZnO NR samples after UV (λ = 365 nm) illumination for 1 h, “MB-virginal” is for MB solution alone.

In Fig. 2(c), the PL spectra of the four samples are presented. The 389 nm peak arises from the inter-band emission of ZnO, while the broader one ranging in ∼450 to 750 nm is from the emissions of defects within ZnO.^29,30 The inset shows the details of the four spectra. It is seen that after ammoniation, the inter-band emission is enhanced drastically. This PL intensity enhancement could be attributed to the passivation of defects by the hydrogen doping during the ammoniation.²³ On the other hand, sample annealing at 400 °C during the ammoniation may also reduce the density of defect;^29,30 the decrease in the intensity of defect emission is consistent with this attribution. After loading Ag NPs, both the PL intensities of ZnO and defects drop. The reason could be that Ag NPs that are in close contact with ZnO act as a quencher of the light emission.^24,29,31 The change of PL intensity of “Ag/ZnO + NH₃” is in between.

Fig. 2(d) shows the XRD spectra for the four samples. All the four samples have wurtzite structures and their XRD peaks were in good agreement with the Powder Diffraction Standards data (JCPDS card no. 076-0704) for ZnO. It is clear that basically, the main crystallographic structures (002) and (100) of ZnO are maintained after ammoniation and/or Ag NP loading.

High pressure and high temperature ammoniation or hydrogenation can cause hydrogen ion doping.^32,33 To further identify this point, “ZnO + H₂” and “ZnO + N₂” were prepared for comparison as in Fig. 2(e) and (f). It is clear that the intensities of inter-band PL emission of both ZnO + H₂ and ZnO + NH₃ are greater than that of ZnO + N₂, and the photocatalytic abilities of ZnO + H₂ and ZnO + NH₃ are also greater than that of ZnO + N₂. This indicates the incorporation of hydrogen ions, which passivates defects in ZnO, therefore, both the inter-band PL intensity and photocatalysis are enhanced.

3.3. XPS analysis

Fig. 3(a)–(d) give the XPS spectra of Zn 2p, O 1s, Ag 3d and N 1s, respectively, for the four samples. From “ZnO” down to “Ag/ZnO + NH₃”, the Zn and O signals change slightly in both intensity and line-shape. Fig. 3(a) shows the Zn 2p binding energies for Zn 2p_3/2 and Zn 2p_1/2 are at 1021.3 eV and 1044.4 eV, respectively.^21,26,34,35 Decompositions of O 1s spectra reveal two components peaking at 530.0 eV and 531.4 eV, respectively. The former component is from ZnO,^21,34,36,37 while the latter is due to the abundant surface hydroxyl groups/adsorbed water molecules.^21,34,36,37 Nitrogen signals were detected for the samples after ammoniation. Also two components are derived from N 1s spectra, which peak at 397.8 eV and 399.9 eV, respectively. It has been known that the 397.8 eV component arises from the substitutional nitrogen ions,^20,21,38,39 while the other is from surface adsorbed amines.^20,21,38,40 These NH₃ atoms are adsorbed on the surface of ZnO, which could lead to the decrease in degradation of MB due to the decrease in area of ZnO exposed to MB. However, our experimental results show that after ammoniation, the photocatalytic activity of ZnO is enhanced. Therefore, the enhancement of photocatalysis as achieved by nitrogen and hydrogen dopings via ammoniation overwhelms the unbeneficial factor brought about by the residual NH₃ adsorption. Similar results were also found in ref. 22, 40 and 41. In Fig. 3(d), signals of Ag 3d_5/2 and Ag 3d_3/2 are found, which are centered at 367.3 and 373.3 eV, respectively.^24–26,35 As compared to their bulk counterparts, the binding energies decrease by ∼1 eV due to the charge transfer between Ag and ZnO.^24,25

Fig. 3 XPS spectra of (a) Zn 2p, (b) O 1s, (c) N 1s and (d) Ag 3d spectra.

3.4. Photocatalytic activity

We now examine the photocatalytic activities of the four samples. The UV light (λ = 365 nm) power is 15.5 mW and the visible one (λ = 435 nm) is 87.8 mW. Fig. 4(a) gives the MB absorption spectra of the four samples after illumination by the UV light for 1 h. It is seen that after ammoniation or Ag NP loading, the UV photocatalysis is enhanced, and further enhancement is achieved by the synergism of ammoniation and Ag NP loading. Fig. 4(b) shows the degradation curves of MB for the four samples. We define the degree of MB degradation as I₁/I₀, where I₁ is the absorbance peak intensity at λ = 660 nm of the MB loaded with photocatalyst, and I₀ is that without photocatalyst. The absorbance intensity is proportional to the MB concentration. The enhancement of UV photocatalysis after ammoniation could be attributed to the passivation of dangling bonds in ZnO by hydrogen via decomposition of NH₃. These dangling bonds usually play a role of electron trap and make redox reaction at the surface difficult. The sample of “Ag/ZnO”, however, offers an example of SP-enhanced photocatalysis. One mechanism suggests that photo-excited electrons can jump from the conduction band (CB) of ZnO to the Fermi level of Ag NPs, and Ag NPs act as electron sinks, which reduce the recombination of photo-excited electrons and holes, and prolong the lifetime of photo-generated pairs.^24,26,31,42 Another mechanism states that it is mainly the SP electromagnetic field from the Ag NP as induced by the UV illumination, as will be shown below, that excites the electron from the VB of ZnO to the Fermi level of Ag NP; this electron can be further injected into the CB of ZnO by the SP. This non-linear process of generating electron–hole pairs also helps to enhance the photocatalysis.¹⁴ For “Ag/ZnO + NH₃”, SP and ammoniation work together, leading to a synergetic effect on the enhancement of photocatalysis. Fig. 4(c) and (d) are the visible-light counterparts to Fig. 4(a) and (b), respectively. No visible photocatalysis in “ZnO” is observed as expected for its wide bandgap width. However, after ammoniation or Ag NP loading, visible photocatalysis becomes available. For “ZnO + NH₃”, the visible photocatalysis is due to the bandgap narrowing as indicated by Fig. 2(a). For “Ag/ZnO”, the second mechanism¹⁴ aforementioned holds for the SP-enhanced photocatalysis. Again, the synergetic effect of SP and ammoniation on the enhancement of visible photocatalysis is observed.

Fig. 4 Absorption spectra of MB solution and those loaded with different ZnO NR samples after UV (λ = 365 nm) illuminations for 1 h (a) and visible (λ = 435 nm) illuminations for 5 h (b). MB degradation of MB solutions as functions of UV illuminations time (c) and visible illuminations time (d). “MB-virginal” is for MB solution alone.

3.5. Photocurrent tests

Fig. 5(a)–(d) give the I–V curves for the four samples of “ZnO”, “ZnO + NH₃”, “Ag/ZnO” and “Ag/ZnO + NH₃” under dark, UV and visible illumination conditions. It is seen that the current evolutions are consistent with those of photocatalysis as shown in Fig. 4. This indicates that the ammoniation, surface plasmon and combination of these two do help to generate photocharges, which leads to the enhancement of both photocurrent and photocatalysis. Synergetic effect of ammoniation and surface plasmon also works.

Fig. 5 Photocurrent tests for “ZnO” (a), “ZnO + NH₃” (b), “Ag/ZnO” (c), and “Ag/ZnO + NH₃” (d) under UV (λ = 365 nm) and visible (λ = 435 nm) illuminations.

3.6. Cycling tests

To investigate the reusability of photocatalyst, the experiment of MB degradation is repeated for “Ag/ZnO + NH₃” under UV (λ = 365 nm) and visible (λ = 435 nm) light illuminations. From Fig. 6, it is seen that the MB degradation rate keeps steady after five photocatalytic cycles. This indicates that the photoactivity of photocatalyst could be stable in repeated use.

Fig. 6 Cycling tests of photodecomposition of MB by “Ag/ZnO + NH₃” under UV (λ = 365 nm) (a) and visible (λ = 435 nm) (b) light illuminations.

3.7. FDTD simulations

In Fig. 7(a) and (b), we show the results of simulated electromagnetic field distributions around semi-spherical Ag NPs (ϕ = 20 nm) on ZnO substrates under illuminations of lights with λ = 365 and 435 nm, respectively, by using a finite difference time domain (FDTD) software.¹⁴ It is seen that SPs are generated in both cases, and the SP fields with maximal intensities occur at the interface between ZnO and Ag NP. Note that since the frequency of SP electromagnetic field is intrinsically determined by the size and shape of Ag NP and its environment, for both cases, the SP resonance frequencies or wavelengths are the same, i.e., 415 nm in wavelength or 2.99 eV in photo-energy.

Fig. 7 FDTD simulation of SP field distribution around an Ag NP 20 nm across on the surface of ZnO with excitation wavelengths of 365 (a) and 435 nm (b). The white dotted line stands for the boundary between ZnO and Ag NP.

3.8. Mechanisms in enhancing photocatalytic activity

In Fig. 8, a schematic drawing is given for “Ag/ZnO + NH₃” to describe the synergism of bandgap narrowing and SPs for enhancing photocatalysis in terms of the non-linear transition mechanism.¹⁴ The bandgap narrowing may enable direct excitation of electrons from the VB to the CB of ZnO by visible light. On the other hand, an extra channel of excitation opens, i.e., electrons from the VB can be also excited by the SP field to the Fermi level of Ag NPs, which are then injected from the SP into the CB.^14,43,44 The visible excitation source is mainly from the surface plasmon field as its intensity is much larger than the incident one. We discussed this issue in our previous publication.¹⁴ Consequently, both electron and hole are created in the CB and VB of ZnO, respectively, and the requirements for a full redox catalytic process and charge balance are met.

Fig. 8 Schematic diagram of the synergism of ammoniated ZnO and Ag NP to excite electron–hole pairs on ZnO under UV or visible light illuminations.

Fig. 9 shows the photocatalytic activities of the four samples under visible (λ = 435 nm) illumination for 5 h as functions of the normalized illumination light powers. The visible light power without attenuation is 87.8 mW. We define photocatalytic activity of ZnO as ΔI/I₀, where ΔI = I₀ − I₁; I₁ and I₀ are absorbance intensities of MB loaded with and without ZnO, respectively. No photocatalytic activity of “ZnO” exits under λ = 435 nm light illumination, or ΔI/I₀ = 0. For “ZnO + NH₃”, the activity under λ = 435 nm illumination evolves linearly with the increasing light power. For “Ag/ZnO” and “Ag/ZnO + NH₃”, however, nonlinear evolutions appear for visible light illuminations, with the exponents being 2.55 and 1.70, respectively. The “Ag/ZnO” evolves more nonlinearly than the “Ag/ZnO + NH₃”. This is because that direct excitation from the VB to CB of “ZnO + NH₃” is allowed for visible light illumination, which is linearly dependent on the light power, therefore, a linear component is added to the photocatalytic process for “Ag/ZnO + NH₃”. The results lend a support for the availability of the synergetic mechanism as proposed in Fig. 8.

Fig. 9 Photocatalytic activities of ZnO samples as functions of normalized excitation visible light powers (λ = 435 nm).

The visible photocatalytic reaction processes on “Ag/ZnO + NH₃” are summarized by the following equations, where e_Ag⁻ denotes the electron trapped by Ag NPs, and e_SPR⁻ denotes the electron to be injected into the CB of ZnO from Ag NPs via SPR effect.^44,45

ZnO + hν → e⁻ + h⁺ (1)

e⁻ → e_Ag⁻ (2)

e_Ag⁻ + hν (or SP) → e_SPR⁻ (3)

e_SPR⁻ → e_CB⁻ (4)

O₂ + e_CB⁻ → O₂⁻ (5)

O₂⁻ + MB → products (6)

h⁺ + MB → products (7)

h⁺ + H₂O → H⁺ + ·OH (8)

·OH + MB → products (9)

The main points for decomposing MB are the creation of photoelectrons and holes on ZnO, with the help of Ag NPs or SP when the excitation photon energy is in the visible regime. Both the electrons and holes take part in the process of decomposition of MB, as indicated by the equations above.

Furthermore, measurement of N₂ adsorption and desorption isothermal on ZnO nanoparticles as prepared with a similar method as used here⁴⁶ suggests a type V adsorption–desorption isothermal structure of the surface of nanostructured ZnO. Finally, it needs to be addressed that Ag NPs alone do not contribute to the photocatalysis. We prepared Ag NPs on quartz substrate that is not photoactive, and tried to decompose MB with this sample of “Ag/SiO₂” under visible illumination. From Fig. 10, it is seen that although SP was excited, no decomposition happened as shown in the inset. Therefore, the SP-enhancement of photocatalysis should be assisted by the photocatalyst.

Fig. 10 Absorption spectra of quartz and Ag/SiO₂. The inset shows those of MB solution and Ag/SiO₂ after visible (λ = 435 nm) illumination for 3 h.

4. Conclusions

We report a synergetic effect of surface plasmon and ammoniation on the enhancements of both ultraviolet and visible photocatalysis in ZnO NRs. It is found that ammoniation can offer nitrogen doping for bandgap narrowing and hydrogen doping for dangling bond passivation, while Ag nanoparticles that support surface plasmons may help to suppress the recombination of electron and hole and/or excite extra electrons and holes within ZnO NRs. The synergism of the two effects could overcome their intrinsic inefficiencies and lead to a significant enhancement of photocatalysis.

Acknowledgements

This work is supported by the National Natural Science Foundation of China under grant no. 61275178 and 51472051. We thank Professor Jiada Wu for experimental assistances.

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