Influence of Atomic Hydrogen, Band Bending, and Defects in the Top Few Nanometers of Hydrothermally Prepared Zinc Oxide Nanorods
© The Author(s). 2017
Received: 7 August 2016
Accepted: 28 November 2016
Published: 6 January 2017
We report on the surface, sub-surface (top few nanometers) and bulk properties of hydrothermally grown zinc oxide (ZnO) nanorods (NRs) prior to and after hydrogen treatment. Upon treating with atomic hydrogen (H*), upward and downward band bending is observed depending on the availability of molecular H2O within the structure of the NRs. In the absence of H2O, the H* treatment demonstrated a cleaning effect of the nanorods, leading to a 0.51 eV upward band bending. In addition, enhancement in the intensity of room temperature photoluminescence (PL) signals due to the creation of new surface defects could be observed. The defects enhanced the visible light activity of the ZnO NRs which were subsequently used to photocatalytically degrade aqueous phenol under simulated sunlight. On the contrary, in the presence of H2O, H* treatment created an electronic accumulation layer inducing downward band bending of 0.45 eV (~1/7th of the bulk ZnO band gap) along with the weakening of the defect signals as observed from room temperature photoluminescence spectra. The results suggest a plausible way of tailoring the band bending and defects of the ZnO NRs through control of H2O/H* species.
Zinc oxide (ZnO) is a wide band gap semiconductor material with a band gap of about 3.4 eV and a large exciton binding energy at room temperature (60 meV) [1, 2]. It has unique optical and electrical properties and can be grown in various morphologies using low-cost synthesis techniques . It has been reported that well-ordered ZnO grains with fewer defects show better optical properties compared to the large discrete islands or structure less overgrowth based on flat continuous layers . Microshape dependency shows that ZnO nanorods (NRs) have the best optical properties among nanoshells and nanoneedles . In contrast, it has been also reported that enhanced surface defects play a crucial role in ZnO nanostructures when it is used as a visible light photocatalyst [6–9].
The method to enhance the optical properties of ZnO NRs has involved annealing in air , hydrogen treatment , and annealing in various environments [12, 13]. Yanob et al. showed that hydrogen is responsible for the near-band edge enhancement and drastic increment in the conductivity of the ZnO nanowires . Ching-Ming Hsu et al. reported that the conductivity of thin films of molybdenum-doped zinc oxide was increased by a factor of 4 when treated by hydrogen over a period of 30 min . Furthermore, hydrogen treatment of ZnO NRs can be used to control the electronic and optical properties of ZnO by creating defects within the ZnO crystal [15, 16]. These defects can be in the form of oxygen vacancies which act as deep donors, zinc vacancies as deep accepters, zinc interstitials as shallow donors, oxygen interstitials as deep accepters at the octahedral site, oxygen anti-sites as deep acceptors, and zinc anti-sites as shallow donors . However, existence of a certain type of defect depends on the surrounding environment during the preparation or post-treatment of the ZnO NRs. Additionally, the formation and density of these defects is partly dependent on the chemical composition of the samples, wherein recent studies have shown that the presence of H2O can play a part in directing the defect formation . A recent study by Gutmannet et al.  demonstrated the effect of annealing, ambient exposure, and photon flux-induced artifacts on work function Φ measurements of the nanocrystalline ZnO surfaces. The authors used ultraviolet photoemission spectroscopy (UPS) and low intensity X-ray photoemission spectroscopy (LIXPS) to determine the absolute Φ values and to confirm the hypothesis that surface hydroxylation by photo-induced H2O dissociation is most likely responsible for 0.30–0.35 eV Φ reduction of the band gap observed during UPS measurements. Their results suggest that any UPS measurements on ZnO surfaces exposed to ambient or H2O should consider 0.30–0.35 eV correction factor to determine the Φ accurately. Another study by Kumar Kumarappan  reported on the effect of H* cleaning on single ZnO (0001) crystal and associated upward band bending after partial removal of the surface contaminants at elevated temperature. Heinhold et al.  showed the influence of polarity and hydroxyl termination on the band bending at ZnO surfaces. Their results indicated how the Fermi level (E f) could be reversibly cycled between the conduction band and the band gap (E g) by controlling the surface H coverage using simple ultrahigh vacuum (UHV) heat treatments up to 750 °C, dosing with H2O/H2 and atmospheric exposure. In addition, they demonstrated the upward and downward surface band bending (V sbb) upon annealing of the H2O/H2 dosed O and Zn-polar faces of ZnO single-crystals.
Despite of the aforementioned efforts, hydrogenation of ZnO leads to effects which are not yet fully understood and explored. For example, in addition to the ambiguous electronic effects attributed to the hydrogen treatment of ZnO, it is unclear how the intrinsic H2O content due to the ZnO NRs soft chemistry preparation interacts with H* in the top few nanometers at room temperature—this is different from the abovementioned studies which were based on annealing of the H2O/H2 dosed O and Zn-polar faces of ZnO single-crystals. In addition to that and to the best of our knowledge, there are no reports on the effects of the electronic structure variation—such as band bending—in pristine- and hydrogen-treated ZnO NRs.
In this work, we report on the preparation of ZnO NRs on glass substrate by a simple hydrothermal process. The effect of trapped H2O molecules on the band bending at the surface of the ZnO NRs is probed using XPS and UPS. The effect of H* treatment on the samples with and without intrinsic H2O molecules is also explored in terms of band bending and defect-induced photoluminescence (PL) intensities. The correlation between oxygen vacancies with PL intensities and valence band maximum (E VBM) of ZnO NRs with and without trapped H2O molecules are further discussed.
The ZnO NRs were synthesized using the following materials: Analytical grade zinc acetate dihydrate (Zn (CH3COO)2·2H2O) was purchased from MERCK, Germany, and zinc nitrate hexahydrate (Zn (NO3)2·6H2O) and hexamethylenetetramine ((CH2)6N4) were obtained from Sigma-Aldrich, USA. All the chemicals were used without further purification. Standard microscope glass slides were used as substrates for the growth of ZnO NRs, which were cleaned in ultrasonic water bath using soap water, acetone, ethanol, and deionized (DI) water prior to the growth of the nanorods.
Substrate Seeding with ZnO Nanocrystallites
ZnO nanocrystallites were seeded on glass substrates using 10 mM solution of zinc acetate dihydrate in 20 mL dissolved in DI water followed by spraying on pre-heated (350 °C) substrates [22, 23]. After spraying, the samples were annealed at 350 °C for 5 h in the ambient and stored in an oven at 90 °C until further use. The purpose of ZnO seeding was to augment nucleation sites for ZnO NRs growth [24, 25].
Hydrothermal Synthesis of ZnO NRs
Briefly, the seeded glass substrates were placed in a solution with equimolar concentrations (20 mM) of zinc nitrate hexahydrate and hexamethylenetetramine (precursor solution) and then kept in an oven at 90 °C. The growth process was carried out for 10 h, and precursor solution was replenished after 5 h to maintain a constant growth rate for the nanorods . Then, the samples were thoroughly rinsed with DI water and annealed at 350 °C in ambient air to remove any un-reacted chemicals on the surface. The positioning of the glass slide inside the furnace and annealing temperature were crucial to engineer portions of ZnO NRs with and without H2O on the same glass slide simultaneously. These portions were identified by XPS before commencing of any experiment.
Atomic Hydrogen Treatment
Surface morphology of ZnO NRs on glass substrates was characterized by JEOL JSM-7800F (Japan) field emission scanning electron microscope (FESEM) working at 30 kV. X-ray photoemission spectroscopy (XPS) (Omicron Nanotechnology, Germany) with a monochromatic Al Kα radiation (hν = 1486.6 eV) working at 15 kV was used for surface, sub-surface, and bulk analysis of ZnO NR samples before and after the hydrogen treatment. Figure 1a shows experimental geometry used for XPS investigations. The obtained XPS spectra were deconvoluted to individual components using Gaussian Lorentzian function with Casa XPS software and calibrated with respect to the C 1s feature at 284.6 eV. During the XPS experiments, all the measured samples were flooded with electrons to neutralize surface charging effects. Ultraviolet photoelectron spectroscopy (UPS) (Omicron Nanotechnology, Germany) with radiation energy of (21.2 eV) was used for surface and sub-surface density of state analysis of ZnO nanorods samples before and after hydrogen treatment by changing the sample tilt angle as depicted in Fig. 1b. In order to measure changes in the ZnO Φ due to hydrogen treatment, UPS was calibrated and tested using ITO thin film following the procedure adopted by Park et al. . X-ray diffraction (XRD) of ZnO NRs were obtained by using a Rigaku MiniFlex600 X-ray diffractometer with Cu Kα X-ray radiation (wavelength = 1.54 Å). Room temperature photoluminescence (PL) of the NRs were recorded in a Perkin Elmer LS55 fluorescence spectrometer with an excitation wavelength of 325 nm. Photocatalytic activity test was carried out on H* untreated and H* ZnO samples soaked in 10 ppm phenol solution. The phenol degradation was done under simulated solar light (AM 1.5 radiation, 1 kW/m2), and the phenol degradation kinetics were studied by using ultra performance liquid chromatography (UPLC, LC-30AD, Shimadzu, Tokyo, Japan) technique.
Results and Discussion
To better understand the surface composition of the NRs, the oxygen (O 1s) peaks of the as prepared samples with and without chemisorbed H2O molecules were studied using XPS. As shown in Fig. 2d, asymmetric O 1s peak of the sample without H2O on the surface could be coherently fitted by two Gaussian components, centered at 529.7 eV (O a) and at 531.0 eV (O b). An additional component at 533.5 eV (O c) is found for the sample with H2O as shown in Fig. 2e, attributed to chemisorbed H2O species on NR surface. The O a peak is attributed to lattice oxygen in wurtzite ZnO forming Zn–O bonding, while O b is attributed to O2− ions in oxygen-deficient regions within the ZnO matrix (oxygen vacancies) and the surface adsorbed loosely bonded oxygen like hydroxyls (OH) bonds, i.e., ZnO(OH) . O c can be ascribed to the specific chemisorbed oxygen, from adsorbed CO2, O2, or H2O . The peak positions correspond well with literature, which show that the O b and O c peaks lie at approximately 1.35 and 3.8 eV to the right of the lattice oxygen peak in ZnO crystal . In order to observe the effect of the chemisorbed H2O species on the electronic properties of ZnO NRs, ultraviolet photoelectron spectroscopy (UPS) technique was employed as shown in Fig. 2f. An enhancement in the intensity of band structure features (i.e., the valence band maximum (E VBM), Zn 4s-O 2p (~6–8 eV), and Zn 3d (~10.5–11.5 eV)) is observed for the ZnO sample which contains the chemisorbed species. Consequently, the E VBM was found to be high (3.5 ± 0.1 eV) as estimated from the linear extrapolations of the foremost edges of the UPS data graphs depicted from Fig. 2g. It is noteworthy that the O 2p (~4.5 eV) ZnO-related surface state is absent in the UPS of the sample with H2O but visible for the samples without H2O with reduced E VBM value of (3.0 ± 0.1 eV) as shown in Fig. 2h.
Auxiliary experiments (not shown here) of Ar ion beam sputtering of 5 keV or annealing at 500 °C for 1 h of the ZnO NRs lead to the removal effect of the bulk H2O content. A careful comparison between O 1s peaks for the sample without H2O after hydrogen treatment and before (i.e., comparison between Figs. 4a and 2d) reveals that H* treatment has an effect in the reduction of O b (from 64.4 to 59%) and increase of the binding energy of the O 1s (from 529.7 to 530.8 eV for O a and from 530.9 to 531.0 eV for O b), therefore supporting the cleaning effect to take place due to the H* treatment.
Despite an increase in the binding energy of O 1s obtained for the sample which contains H2O, O b tends to increase as seen from the comparison between Figs. 2e and 4d. The increase in binding energy observed for O 1s (0.3–0.4 eV) and Zn 2p3/2 (0.2 eV) peaks after hydrogen treatment suggests weakening of the Zn–O bond in the crystal lattice, which can increase the nuclear attraction force experienced by the electron, resulting in the increase of binding energy for both lattice oxygen and zinc.
where Σ = (kT/q)ln(N C/n)  is the energy difference between E F and the conduction band minimum (E CBM) in the bulk of the sample (n is the bulk carrier concentration 2 × 1017 cm−3 and N C is the conduction band effective density of states = 2.94 × 1018 cm−3 for ZnO). Using these values, the Σ was found to be 0.064 eV and V sbb = 0.51 eV. This positive V sbb value is a sign of upward band bending which generates an electron depletion layer on the ZnO NRs surface and is comparable to the 0.53 eV value found by Kumarappan  upon H* cleaning of ZnO (0001) single crystal at high annealing temperatures. The UPS band structure features and the decrease in E VBM values are supported by the XPS valence band spectra presented in Fig. 5b. Due to the glazing angle used for XPS investigation, it is observed that the Zn 3d XPS core level—not to be confused with the secondary cutoff peak of UPS shown in Fig. 5a and E VBM shift to lower binding energy value of 1.6 eV as evident in Fig. 5b. This value is attributed to surface contaminants covering the un-treated surface. As depicted in the inset of Fig. 5c and upon hydrogen treatment, the UPS O 2p peak gets attenuated and the Zn 4s-O 2p appears with enhanced intensity as the main feature related to the hydrogen treatment of the ZnO sample without H2O. The trend of band structure features after hydrogen treatment is clearly supported by XPS valence band data shown in Fig. 5d. For example, E VBM values estimated from Fig. 5d show a decrease from 3.3 to 2.8 eV moving from bulk to surface geometry. The large E VBM value of 3.3 eV confirms an upward band bending and is close to the band gap (3.37 eV) of ZnO. Furthermore, this value is expected from the maximum XPS sampling depth (d) of ~10 nm estimated from 3λ max = d, where λ max is the maximum electron mean free path value equaling to 3.5 nm for the XPS Al Kα radiation used in this study. The decrease of O b, attenuation of the UPS O 2p peak, intensity enhancement of Zn 4s-O 2p, and enhanced smoothness of all UPS and XPS spectra after hydrogen treatment strongly strengthen the conclusion that hydrogen treatment does have a cleaning effect on the ZnO NR sample in the absence of H2O.
The Φ values calculated using ΔE obtained from Fig. 5a, c are 3.6 ± 0.1 and 3.7 ± 0.1 eV before and after hydrogen exposure for the sample without adsorbed H2O.
To illustrate the effect of H* treatment on the work function (Φ) for the sample with adsorbed H2O, the UPS spectra in Fig. 6b are plotted with respect to the kinetic energy. Therefore, the extrapolated lines fitted on the secondary cutoff peaks to the energy scale directly correspond to the Φ values. Clearly, large Φ values (compared to that found in the sample without water) decreasing from 5.9 ± 0.1 to 5.1 ± 0.1 eV are found from the bulk to the surface, respectively. Considering the aforementioned correction factor (0.3–0.35 eV) following the work function of Gutmann et al.  attributed to the effect of UV exposure during the UPS experiments, the corrected surface Φ values after H* treatment turn to be 4.0 ± 0.1 and 5.4 ± 0.1 eV for the sample without H2O and with H2O, respectively.
It is interesting to compare the measured E VBM and Φ values with those reported before. The E VBM (2.8–3.3 eV) and Φ (4.1 eV) values found for the sample without water after H* treatment agree very well with published values by Kim et al.  (Φ = 4.08 eV) after Ar+ ion sputtering/heating ZnO single crystal at 700 °C, Gutmann et al.  (E VBM = 3.0 eV, Φ = 4.1 eV) on nanocrystalline ZnO surfaces after annealing at 400 °C in UHV environment, and Heinhold et al.  (E VBM = 3.41 eV) after annealing ZnO single crystal at 750 °C for 15 min. This agreement is not surprising since annealing or Ar+ ion sputtering has similar effect of partial cleaning of ZnO as H* treatment.
PL R factor, E VBM and V sbb UPS parameters, and XPS Zn binding energies
No H* treatment
With H* treatment
Sample without H2O
R = 4.1, E VBM = 3.0 eV
R = 2.3, E VBM = 2.8 eV, V sbb = 0.51 (upward)
Zn (BE) = 1020.5 eV
Zn (BE) = 1020.8 eV
Sample with H2O
R = 2.2, E VBM = 3.5 eV
R = 5.7, E VBM = 3.76 eV, V sbb = 0.45 (downward)
Zn (BE) = 1021.1 eV
Zn (BE) = 1021.5 eV
The surface band bending phenomenon seen from V sbb is correlated to the estimated PL R values. The upward band bending reflects small R value (2.3) (i.e., enhancement of PL intensity) for the sample without H2O. However, the downward band bending induced the negative accumulation layer in the sample with H2O, causing attenuation of PL (i.e., large R value 5.7).
Zinc oxide nanorods were synthesized on glass substrates using a hydrothermal process, and the surface defects were modulated by H* treatment at room temperature. XPS and UPS revealed the surface, sub-surface, and bulk chemical composition and electronic band structures of the ZnO samples with and without H2O in their structure. The H* treatment had the effect of cleaning the ZnO NRs, enhancement of PL signals, upward band bending, and improved phenol degradation for the sample without H2O. Downward band bending and attenuation of PL signal were the main features for the sample with H2O. The reported results show that the surface, sub-surface, and bulk chemical oxygen vacancies can be correlated to the observed defects and the H2O/H* species can be used to tailor the band bending of the ZnO NRs which might be required for several applications.
The authors would like to thank Mr. Jamal Al- Sabahi from Chair in Nanotechnology, Water Research Center, Sultan Qaboos University, for helping in the photocatalytic activity experiment.
MJA-S conducted the experiment and prepared the manuscript. SA-H, JD, and AA-H designed the experiment and reviewed the manuscript. HHK, MTZM, TB, and KL prepared and reviewed the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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