Regular Article
Intermediate excited state suppression and upconversion enhancement of Er3+ ions by carbon-doping boosting photocarrier separation in bismuth oxychloride nanosheets

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Abstract

Low luminescence efficiency of rare-earth ions doped upconversion (UC) nanomaterials is still a major limitation for their applications. Here, based on bismuth oxychloride nanosheets that show efficient photocarriers separation due to combining spontaneous polarization and layered semiconductor, we report a new carbon heterovalent doping strategy for efficient UC luminescence enhancement by suppressing the intermediate excited states of Er3+ ions. The first-principles calculations and photoelectrochemical characterizations provide evidences that the replacement of C ions for Cl strengthen the spontaneous polarization and inter electric field (IEF) of bismuth oxychloride nanosheets, which further improve the photocarriers separation efficiency. Under 808 or 980 nm excitation, the emission intensity of 4I13/2 energy level of Er3+ ions (1550 nm) increase slightly with C doping, but the its decay time and the visible UC emission are improved tremendously at the same time. We show that the recombination rate of intermediate excited state electrons of Er3+ ions with the ground state is inhibited by the enhanced IEF, which promotes the energy reabsorption transition to upper energy levels, thus enhancing the visible UC emission. This work not only may provide a new insight into the method for engineering of UC emissions but also deepen the understanding for layered semiconducting material to modify the transition of Lanthanide ions.

Graphical abstract

The spontaneous IEF inside the BiOCl was improved significantly by C doping, which can suppress the recombination of photoelectron-hole pairs. The stronger IEF inhibits the recombination rate of electrons on the intermediate state 4I13/2 of Er3+ ions, which promotes the energy reabsorption transition to the upper energy levels to enhance the visible UC emission.

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Introduction

In recent years, upconversion luminescence (UCL) of trivalent rare-earth ions (RE3+) doped nanophosphors has drawn considerable interest, due to their potential applications in lasers, [1], [2] solar cells, [3], [4], [5] biological fluorescence imaging and near infrared (IR) detection, [6], [7] IR quantum counters, [8] and display technologies [9]. However, the insufficient luminescence intensity still constitutes the main limitation for practical applications of nanosized UC materials because of more defects and impurities [10]. Up to now, various methods have been attempted to improve UC luminescence efficiency, such as modulating energy transfer, [11], [12] broadening the absorption region [13] and manipulating host lattice [14], [15].

As for the UC process, the nonradiative relaxation of RE3+ ions place restrictions on UCL efficiency for nanomaterials [10]. Traditionally, selecting suitable host of low phonon energy and fabricating core-shell structure have been used to eliminate the nonradiative relaxation of RE3+ in the UC nanophosphors [16], [17]. Conversely, Yang reported a different strategy for the enhancement of the UC emissions. They showed that using the photonic crystals band gap to suppress the spontaneous emission from intermediate excited state of RE3+ ions, where the electrons occupation possibility of the excited energy level could be improved, is effective in the enhancement of visible UC emissions similarly [18]. Although the complexity of structural design and the wavelength limitation of photonic crystals may restrict the practice applications, it brings a good inspiration that we can design the inherent structure or component for host materials to suppress the spontaneous relaxation of electrons at intermediate excited state, thus promoting the energy reabsorption to enhance the UC emissions.

Over the last few years, the rapid increase in graphene has led to the development of an active research field focusing on two-dimensional (2D) layered materials. It is worth noting that these materials have higher carrier mobility and separation efficiency of electron-hole pairs due to their wide range of anisotropic properties [19], [20], [21]. Especially, illustrated by the example of Bi-based polarized semiconductors, such as BiOX (X = F, Cl, Br or I), BiOIO3, Bi2O2[BO2(OH)], Bi4NbO8Cl and Bi3O4Cl, a layered semiconductor having spontaneous electric polarization shows important applications in electronic information technology, which have been reported as efficient photocatalysts in organic pollutants degradation, H2 evolution and CO2 photoreduction [22], [23], [24], [25], [26], [27]. These materials have mostly been favored as the photocatalysis since they present the excellent wide-band photocatalysis performance over their band gap [28], [29]. The phenomena show such a fact that, in these layered semiconductors, the photoelectrons generated from the intermediate levels, which is caused by impurities, defects or other factors, might be separated and utilized efficiently to improve the photocatalytic ability [30], [31], [32]. One important reason is that they all possess the spontaneous inter electric field (IEF) as efficient inhibition of photoelectron-hole pair recombination [33], [34], [35].

Inspired by the inter crisscross application of the knowledge in different field, we predicate that when the f-f energy level of RE3+ dopants act as the impurity levels in the layered host, the IEF may decrease the recombination rate of intermediate excited state electrons similarly. As a result, when the IEF is high enough or is enhanced to an enough level, it may offer the possibility to inhabit the spontaneous relaxation rate of intermediate excited state electrons of RE3+ ions and prolong its decay time. This will increase the probability of transitioning to the higher energy level and improve the UC emission consequently.

To confirm our idea, we prepared C-Er3+ co-doped BiOCl single-crystalline nanosheets and investigated the influence of IEF on the UC emission properties via heterovalent doping of carbon. The reason that we chose this strategy is that the separation of photoelectrons inside the BiOCl nanocrystals can be tailored via C impurity doping; [36] meanwhile, it has been found that the polarization enhancement of other layered semiconductors via heterovalent doping has the ability to promote the separation of charge carriers [7]. On the other hand, we recently observed that the lightly-doped C into the BiOCl nanosheets will quench the downshifting emission intensity of Eu3+ ions, but can modify the emission behaviors of electric dipole transition of Eu3+ ion dopants significantly via photoferroelectric effect, as the C doping enhance the IEF [37]. In this work, in a much higher doping level, we show that due to boosting the spontaneous IEF more significantly, the C dopants not only can increase the carrier effective mass along [0 0 1] direction, but also inhibit the recombination rate of photoelectrons and holes in the BiOCl nanosheets. As a result, the decay time of the intermediate state of Er3+ ions (4I13/2 level) is prolonged significantly by more efficient hole separation and transfer inside nanosheet host, thus promoting the energy reabsorption transition to upper energy levels and the visible UC emission greatly.

Section snippets

Synthesis details

Samples of BiOCl:3%Er/xC (x = 0, 1, 2, 3 mmol) were prepared via a hydrothermal processing and a subsequent thermal treatment with glucose as the carbon dopants source, [36] which were called as 0C, 1C, 2C, 3C, respectively, specific synthesis details refer to supplementary information.

The details of characterizations and theoretical calculations for the sample are shown in the Electronic supplementary information.

Phase identification of C doped BiOCl nanosheet

Fig. 1 shows the X-ray diffraction (XRD) patterns of the BiOCl:3%Er/x C (x = 0, 1, 2, 3 mmol) nanocrystals. The XRD patterns of the prepared samples can be clearly indexed to the tetragonal BiOCl phase (JCPDS: 06-0249). The intense and sharp diffraction peaks show that the as-synthesized product was well-crystallized and no other diffraction peaks of carbon atoms is detected, indicating the reliability and the feasibility of this C doping strategy.

The surface element compositions and chemical

Discussion

To confirm the reason that C doping prolong the intermediate excited state of Er3+ ions, namely the enhanced IEF rather than the conventional lattice modification effect, the UC enhancement mechanism caused by the lattice modification was discussed as a distinction. In briefly, according to the Judd-Ofelt (J-O) theory, as the symmetry of the crystal sites around the RE dopants is changed by heterovalent doping, the intensity of UCL can be calculated by the following steady-state Eq. [15]:I0=β0(τ

Conclusions

In summary, via a hydrothermal processing and a subsequent thermal treatment, the influence of heterovalent C doping on the UC luminescence properties of Er3+ doped BiOCl nanosheets were investigated in detail. The theoretical calculations and SPV tests provide evidences that the spontaneous IEF at the direction of (0 0 1) of BiOCl nanosheets was improved significantly with doping C, suppressing the recombination of photoelectron-hole pairs more efficiently. Under the stronger IEF, the separated

CRediT authorship contribution statement

Jiajun Han: Writing - original draft. Taizhong Xiao: . Jiajing Wang: Formal analysis. Tong Liu: Data curation. YongJin Li: Formal analysis, Software. : . Yuehong Peng: Formal analysis, Software. Zhaoyi Yin: Conceptualization. Jianbei Qiu: Supervision, Validation. Zhengwen Yang: Supervision, Validation. Zhiguo Song: Funding acquisition, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work is supported by the National Natural Science Foundation of China (No. 11874186) and Foundation of Yunnan Province 2019HC016.

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